Metal oxides like Ga2O3, In2O3, SnO2 and ZnO are transparent conducting oxides (TCO) of high industrial relevance which have been widely applied for fabrication of transparent electrodes in photovoltaic devices, liquid crystal displays, light emitting diodes and chemical sensors. For such applications the thin oxide films just need to be highly conductive and transparent but in principle they do not require good structural properties. Indeed they have an amorphous or polycrystalline structure and a relatively high defect density.
Recently, there is a great interest in considering these metal oxides from another point of view, namely as wide band gap transparent semiconducting oxides (TSO&’s). That means there are numerous attempts to obtain single-crystalline thin oxide films with good crystallographic properties and controlled carrier concentration. When grown as single crystalline epitaxial films, as it is normally done for classical semiconductors (e.g. Si, GaAs, InP), they could offer the great potential of a new class of semiconductors, with application in transparent microelectronics, optoelectronics, power electronics, short wavelength photonics and chemical and biological sensor devices.
In this presentation we report on the growth of high quality, single-phase β-Ga2O3 layers on (100) β-Ga2O3 substrates. The high quality n-type β-Ga2O3 single crystals for the preparation of (100) β-Ga2O3 substrates were grown by the Czochralski method at the Leibniz-Institute for Crystal Growth (IKZ). Crucial layer deposition parameters were varied in a wide range: substrate temperatures between 750°C and 850°C, chamber pressures between 5 mbar and 100 mbar and the molar ratio between gallium and oxygen. Trimethylgallium (TMGa) was used as gallium precursor while as oxygen precursor either pure oxygen, water or carbon dioxide was employed. The influence of the different oxygen precursors on the initial stage of the layer growth and the properties of the grown layers were studied by different characterization methods (HRTEM, TEM, SEM, AFM, Ellipsometry, C-V- and Hall- measurements). The presence of hydrogen or carbon in the growth atmosphere had a significant influence on the kinetic conditions at the β-Ga2O3 substrate surface during the initial stage of the layer growth. Our MOVPE grown homoepitaxial layers are insulating due to of the high purity and the large band gap of Ga2O3.

Ordered nanocrystal assemblies in 1D, 2D and 3D are being considered as building blocks for novel ‘nanocrystal solids&’ which can exhibit tunable electronic and optical properties useful for optoelectronics and photonics applications. Both physical properties and functionalities of such nanocrystal solids are highly dependent of the local environment of comprising individual nanocrystals. To gain detailed insights on how the electronic and excitonic transports occur through individual nanocyrstals, we systematically study local transport phenomena using the simplest nano-building block assembly of a known geometry. In this study we use colloidal Tin doped Indium Oxide nanocrystals (ITO) self-assembled into a well-defined 1D structure assisted by physical templating and capillary forces. We use cathodoluminescence spectroscopy to examine non-radiative Forster resonance energy transfer along the chain of nanocrystals by locally exciting a single ITO nanocrystal at a known position and monitoring luminescence relative to the location of excitation. We also perform Scanning Kelvin Probe Microscopy to determine electrical charge transport potential barriers of individual nanocrystals from one nanocrystal to the next assembled in a 1D array. Results of this study should aid creating guiding principles for building nanocrystal solids with properties by design.

Transparent conducting oxides (TCOs) are essential to many technologies from solar cell to transparent electronics. While n-type TCOs (using electrons as carriers) are widespread in current applications (e.g., indium tin oxides or ITO), their p-type counterparts have been much more challenging to develop and still exhibit carrier mobilities an order of magnitude lower.
In this talk, we will report on a high-throughput computational search for oxides with low hole effective mass, wide band gap and p-type dopability. Screening thousands of binary and ternary oxides in the Materials Project Database using state of the art ab initio techniques, we will present several unsuspected compounds with very promising properties. Beyond the description of those novel TCOs candidates, we will discuss and chemically rationalize our findings, highlighting several design strategies towards the development of future high mobility p-type TCOs.

Wide band-gap semiconductors such as ZnO have been extensively studied due to the commercial desire e.g. for blue and ultraviolet light emitting devices. Some reasons to prefer ZnO are its non-toxicity and high biocompatibility in contrast to nitride-based materials. Especially low dimensional structures like nanowires (NW) have attracted considerable attention recently. The large surface-to-volume ratio helps to analyze the influence of geometrical aspects of impurities and structural defects.
Presumably caused by hydrogen passivation ZnO exhibit usually n-type conductivity[1]. Doping with group-V elements such as Sb on the O site would in general lead to p-type conductivity. First-principles calculations indicate that a complex with induced Zn vacancies and low formation energy is the reason for p-type ZnO rather than the deep acceptor-level produced by the substitution of oxygen[2]. However, there is also evidence that Sb acts as a shallow donor in ZnO NW so that the doping mechanism remains controversial[3].
To investigate the doping behavior we performed low temperature photoluminescence (PL) and Raman measurements on ensemble and single Sb doped ZnO NW grown by metalorganic vapor phase epitaxy (MOVPE) on c-sapphire with different doping concentrations. The PL spectra is dominated by the transitions of the donor bound excitons in the range between 3.355 and 3.375 eV and also exhibit a transition at 3.3641 eV with a FWHM of approx. 200 µeV. The relative intensity is related to the Sb concentration. This can be interpreted as a Sb related neutral donor bound exciton based on prior results[3].
Raman spectroscopy can be utilized as a technique to identify particular dopant incorporations. Besides the Raman active modes of ZnO we identified additional local vibrational modes in the spectra which are related to the Sb doping. The observed modes at around 531 and 685 cm-1 appear exclusively in Sb doped ZnO[4]. Other modes also appear e.g. in N doped ZnO and originate from the formation of complexes related to the doping. To investigate the spatial distribution of the Raman modes we also performed tip-enhanced Raman spectroscopy (TERS) and found an inhomogeneous enhancement of the local vibrational modes.
[1] C.G. Van de Walle, Phys. Rev. Lett. 85, 1012 (2000);
[2] S. Limpijumnong et. al., Phys. Rev. Lett. 92, 155504 (2004);
[3] E. Senthil Kumar et al., Appl. Phys. Lett. 102, 132105 (2013);
[4] C. Bundesmann et al., Appl. Phys. Lett. 83, 1974 (2003).

Bismuth vanadate (BiVO4) is used as a photoelectrode for the oxidation of water, and for these applications, it is of fundamental importance to understand the electrical and photoelectrochemical properties of this material. To this end, the electronic transport properties of BiVO4 single crystal doped with Mo and W were characterized using resistivity measurements and Hall effect measurements over temperatures ranging from 250 K to 300 K.
In metal oxides, the electronic transport is described by the small polaron model in which the resistivity varies with temperature as ρ (T)prop;Te^((E_a/(k_B T))), where Ea is the hopping activation energy, kB is the Boltzmann constant and T is the absolute temperature. Resistivity measurements confirm that small polaron hopping dominates in temperature ranges from 250 K to 300 K.
Limited by a large misalignment voltage, measurements of the Hall effect using a static magnetic field (DC field Hall) in the doped samples did not provide a clear signal above the instrument noise. AC field Hall effect measurements were used to obtain values for carrier mobility, mu; and carrier concentration, n for the doped single crystals. This technique applies an oscillating magnetic field to the sample, making the resulting Hall voltage time dependent while the misalignment voltage remains a DC voltage. This method facilitates the measurement of the Hall effect in low mobility materials. AC field Hall effect measurements showed both W:BiVO4 and Mo:BiVO4 crystals were n-type with electron mobility of ~0.2 cm2 V-1 s-1 at 300 K. In the temperature range of 250 K to 300 K, mobility was effectively constant. For an n-type semiconductor, carrier concentration is given by n=1/(ρemu; ), from which n was determined as ~5 × 1015 cm-3 at 300 K, significantly lower than the impurity concentrations from ICP-MS measurements, which were between 4-8 × 1019 cm-3 for 0.3% W and 0.6% Mo doping respectively. This unexpected result suggests a large degree of charge compensation in these single crystals. These measurements of the mobility and charge density of BiVO4 demonstrate the advantage of AC field Hall measurements over DC field Hall method for low mobility materials.

Transparent conductive oxides (TCO's) are of interest in many optoelectronic devices including photovoltaics, low emissivity windows, and displays. There is growing interest in utilizing ZnO to replace indium tin oxide (ITO) due to the availability, cost, and relative low toxicity of ZnO. The basic requirements for adopting a suitable thin film TCO include; high transparency to visible light, high conductivity, high carrier mobility, excellent stability, low cost, and scalable synthesis. In an effort to meet many of these requirements, we have grown Ga-doped ZnO by pulsed laser deposition (PLD) in mixtures of H₂ and Ar with varying average growth rates. We find that 100 nm thick Ga-doped ZnO films grown by PLD at 200 °C in an ambient of 67%H₂/33%Ar had resistivities (ρ < 1.8 x 10^-4 Omega;-cm) nearly identical to thicker (620 nm) films. The high conductivity 100 nm (Ga,H)-doped ZnO thin films grown on quartz also show a high transmittance of tau; > 92% at 473 nm. These films are also stable up to 600 °C. The microstructure, high conductivity, and high transmittance of these ZnO thin films grown by PLD are independent of average growth rate and therefore meet many of the requirements needed for technology incorporation.

Thin films of Ga- and Al-doped ZnO (GZO and AZO) are being proposed as replacements for Sn-doped In₂O₃ (ITO) in transparent electrodes (TEs), and as replacements for lossy metals in IR-plasmonics (IRPs). For the TE application, ITO has three disadvantages: higher cost, higher toxicity, and lower transparency; and three advantages: higher conductivity, independence of thickness d, and better stability in hot, wet environments. Here we show that ITO advantages of higher conductivity and independence of thickness can be effectively nullified.
Regarding conductivity, we can now consistently prepare GZO with ρ = 1.2 x 10^-4 Omega;-cm. The key breakthrough in this achievement was showing that the dominant acceptor was the Zn vacancy, and then reducing its concentration with anneals on Zn foil.¹ Such a low resistivity has seldom been reported in ITO, or indeed, any other semiconductor material.
Regarding thickness dependence, we have shown that the electrical parameters µ and n for AZO or GZO on a mismatched substrate can often be modelled as: µ(d) = µ(infin;)/[1 - d*/(d - δd)] and nsheet = n(d - δd), where n is actually independent of thickness when properly normalized to the electrical thickness (d - δd). Here δd is a dead layer at the interface, and µ(infin;) and d* serve as figures of merit for the bulk and interface, respectively. For AZO/quartz with d = 25 - 150 nm, we find that n = 8.36 x 1020 cm2/V-s, µ(infin;) = 25.5 cm2/V-s, d* = 23 nm, and δd = 14 nm. However, by inserting a thin (5-nm) ZnON buffer layer, most of the thickness dependence represented by these high values of d* and δd can be eliminated, giving the same bulk parameters n and µ(infin;), but much better interface parameters, d* = 7 nm, and δd = -4 nm. This very useful buffer layer was developed by Itagaki et al.^2,3
Regarding plasmonics, our GZO with ρ = 1.2 x 10^-4 Omega;-cm also has n = 1.5 x 10²¹ cm^-3 and µ = 34 cm²/V-s, giving a bulk plasmon resonance at lambda;_res = 1.05 µm. Metals tend to be very lossy at such long wavelengths. Moreover, specific values of lambda;_res > 1.05 µm can easily be produced with simple air anneals that decrease n.⁴ For example, we have achieved the telecom wavelengths of 1.3 and 1.55 µm. Optimum values of n and µ to produce a desired lambda;_res can be conveniently accomplished with a plasmonic resonance phase diagram, µ vs n, based only on m* and εinfin;, the high-frequency dielectric constant.
¹D.C. Look, K.D. Leedy, L. Vines, B.G. Svensson, A. Zubiaga, F. Toumisto, D.R. Doutt, and L.J. Brillson, Phys. Rev. B 84, 115202 (2011).
²N. Itagaki, K. Kuwahara, K. Nakahara, D. Yamashita, G. Uchida, K. Koga, and M. Shiratani, Appl. Phys. Exp. 4, 011101 (2011).
³D.C. Look, K.D. Leedy, A. Kiefer, B. Claflin, N. Itagaki, K. Matsushima, and I. Surhariadi, Optical Engineering 52, 033801 (2013).
⁴D.C. Look and K.D. Leedy, Appl. Phys. Lett. 102, 182107 (2013).

The theoretical prediction of the electrical properties of semiconductors relies on the ability to calculate accurate defect formation energies ΔH. The methodology of supercell calculations for ΔH is a topic that continues to receive great interest. A major issue that has plagued such calculations for a long time is the “band-gap problem” of the local density and generalized gradient approximations (LDA and GGA) within density functional theory (DFT). Particularly for charged defects, such as electrically active dopants, for which ΔH depends on the Fermi level, an unambiguous prediction of the formation energy requires the correct band gap energy. A major paradigmatic change in the field took place when the implementation of Fock exchange into plane wave DFT codes allowed band-gap corrected hybrid functional calculations of supercells, albeit at the cost of a considerably increased computational overhead that sometimes invites compromises on convergence parameters. A considerable number of works have since been devoted to revisit previously studied cases, often finding quantitatively different results or even a qualitatively changed physical picture. Such revisions have raised concerns about the validity of previous density functional results and about the accuracy of ΔH calculations in general.
Considering a set of 6 semiconductor materials (Cu3N, Cu2O, Zn3N2, ZnO, AlN, and Al2O3) and a total of 24 defects and dopants in their fully ionized charge states, we show here that the difference between standard density and hybrid functionals reflects systematically the dependence of ΔH on the reference energies for the electronic and atomic chemical potentials, i.e., the energy of the valence band maximum and the energies of the elemental phases of the constituent atoms, respectively. Using GW quasi-particle energy calculations to determine the VBM energy and the fitted elemental reference energies (FERE) approach [1] to determine the elemental-phase energies, we obtain a close agreement between standard GGA(+U) and hybrid functionals. Thus, the dependence of the calculated defect formation energies on the type of the underlying functional is dramatically reduced when using appropriate values for the electronic and atomic reference energies. Existing results without such corrections can be easily re-evaluated. For the hybrid functionals, the present approach further provides an option to avoid using different exchange mixing parameters for different materials.
[1] S. Lany, Phys. Rev. B 78, 245207 (2008); V. Stevanovic, S. Lany, X. Zhang, and A. Zunger, Phys. Rev. B 85, 115104 (2012).
Supported by the US Department of Energy, Office of Energy Efficiency and Renewable Energy.

The rare-earth orthovanadates, RVO₃, offer a fascinating phase diagram with both orbital- and spin-ordering phenomena, owing to their two-fold electron occupations of the triply degenerate V t_2g orbitals. Their spin-orbital phase diagram was found to evolve with temperature, R-site ionic size, and R-site substitution, and exhibits switching between different patterns of spin and orbital order. In order to understand these effects of the rare-earth ionic size and the orbital ordering, we report a soft x-ray spectroscopic study of the electronic structure of Lu_1-xLa_xVO₃ single-crystals, which approach both the smallest and largest rare-earth ionic radii. The x-ray absorption and emission spectra at the O K-edge, which probe both the unoccupied and occupied O 2p partial density of states, closely measure the O 2p-V 3d hybridization and reveal the strong dependence of the O 2p-R 5d and 4f hybridization with temperature and R-site cation. Moreover, the resonant inelastic x-ray scattering spectra at the V L₃-edge, which probe the intra-site dd^* transitions, directly measure the 3d orbital splitting of single vanadium-site as a function of temperature and R-site ionic size. Together, these combined spectroscopic techniques provide a detailed study of the band structure and the crystal-field excitations of Lu_1-xLa_xVO₃ with varied R-site ionic radius during phase transitions.

Recently, resistance-switching effect of oxide semiconductors has driven lots of attention. Many oxides like SrTiO3 and other single crystal pervoskite oxides, ZnO, and amorphous In-Ga-Zn-O and so on, present a metal-insulator-transition like procedure which is reversible, controllable and non-volatile. Both of these contribute to the possibility of applying the oxide semiconductors to the field of memory device, for example, a non-volatile resistance switching random access memory (ReRAM) or a biological synaptic device. In this report, we show that in transparent conductive oxide of In2O3, the change of resistance takes place at grain boundary.
The ~90 nm thick indium oxide film is grown on conductive silicon substrate (ρ<0.01 Omega;*cm) by metal organic chemical vapor phase deposition (MOCVD) and is characteristic with a grain size of 79±24 nm. The transparency of the thin film is higher than 75% with an optical band gap of 3.49 eV. Its electrical property is measured by conductive atomic force microscope (CAFM), which is grain/grain boundary resolve and allow us to investigate the micro-conductance of the In2O3. The conductive tip scans a 700 nm x 700 nm area for several times continuously and found that : (1) the core of the nano-grain presents almost uniform resistance with the grain boundary; (2) as the scan continues, some highly resistive network appears, which corresponds to electron capture of grain boundaries; (3) the charging effect of grain boundary gradually depletes the grain core and finally turn the whole grain into a high resistance state.
The current-voltage (I~V) measurement is also carried out in our CAFM system. The experiment setup allows us to investigate the electrical property of a single-grain. It is found that the I~V characteristics of a single-grain presents very similar evolution to that of the CAFM scanning. The resistance of the single-grain increase gradually during the negative voltage sweeps. Our results indicate that: (1) in certain application where high mobility is needed, the grain boundary in In2O3 brings negative effect since it traps carriers; (2) however, such property of grain boundary could also be used to realize nano-scale auto modulation of device resistance.

One of the major challenges in the area of transparent conducting and semiconducting oxides concerns understanding of the defect mechanisms responsible for carrier generation in these wide-bandgap materials. Ternary and quaternary oxides are highly appealing technologically due to a possibility of controlling the electrical and optical properties over wide ranges. At the same time, the structural and compositional complexity of multi-cation oxides makes it challenging to determine the origin of conductivity and the role of each constituent cation in carrier generation.
In this work, first-principles density functional approach is employed to systematically calculate the formation energies of possible acceptor and donor point defects as well as the implied defect complexes in InGaZnO4, InGaZn3O6 as well as in light-metal containing InAlZnO4, InMgZnO4, and InAlCaO4. We determine the equilibrium defect and electron densities as a function of growth temperature and oxygen partial pressure. The results reveal that the oxygen vacancies, long believed to be the carrier source in these oxides, are scarce, and cation antisite defects are the major electron donors in the conductive oxides. The proposed carrier generation mechanism helps explain the observed intriguing behavior of the conductivity in In-rich vs Ga-rich oxides of the In-Ga-Zn-O system. The results of the defect formation analysis not only agree with the observed dependence of the conductivity on the oxygen partial pressure in InGaZnO4, but also explain why InAlZnO4 samples are unstable under a wide range of growing conditions.

Interest in ZnO is fueled by its prospects in optoelectronics owing to its direct wide band gap and large exciton binding energy. A typical low-temperature photoluminescence (PL) spectrum of undoped ZnO contains sharp and intense excitonic lines in the UV range and one or more broad bands in the visible part of the spectrum. In spite of numerous reports on red, orange, yellow, and green broad bands observed mostly at room temperature and attributed to variety of native defects such as V_O, V_Zn, and Zn_i, very little is known about luminescence properties of defects causing these and other bands in ZnO, and their likely geneses are speculative at this time. In this presentation, the luminescence associated with defects in ZnO will be briefly reviewed, and the most interesting cases will be discussed in more detail.
We studied PL in undoped, Li-, Ga-, Na-, and Cu-doped high-quality ZnO samples grown by different techniques in various laboratories. By studying PL excited with a cw and pulsed UV lasers in a wide range of excitation power density, sample temperature, and decay time after a laser pulse, we were able to classify and analyze in detail more than 10 broad bands with unique luminescence properties. Among these bands, Cu-related green band with a characteristic fine structure and Li-related orange luminescence band peaking at 1.96 eV at 10 K are well-studied and reliably identified. Some other PL bands exhibit unusual properties. Phenomenological models are used to explain temperature dependences of PL intensity.

Oxygen vacancies in oxide semiconductors have been a widely debated topic over the past decades. The properties attributed to oxygen vacancies include the inherent n-type conduction, poor p-type dopability, coloration (absorption), deep level luminescence and non-radiative recombination. However, the only direct experimental evidence of their existence has been obtained on the crystal surface, e.g., on the TiO₂ (110) surface [1].
Positron annihilation spectroscopy is a method sensitive to neutral and negatively charged open volume defects. The decreased electron density in a vacancy manifests itself as an increase of positron lifetime and the narrowing of the 511 keV photo-peak in the annihilation gamma spectrum, compared to a defect-free crystal. Positron lifetime spectroscopy provides information on the atomic structure, the charge state and often the concentration of the vacancies while the Doppler broadening of the photopeak allows the identification of atoms surrounding the vacancy defects. Positron annihilation spectroscopy has been efficiently used to identify and quantify technologically important vacancy-related defects in, for example, group IV semiconductors, III-nitrides and ZnO [2].
We present developments leading to the possibility of detecting oxygen vacancy-related defects in oxide semiconductors. The examples cover such materials systems as ZnO, In₂O₃, SnO₂, and spinel MgAl₂O₄. We show that O vacancies may be directly detected in these oxides, in cases where they are complexed with cation vacancies. The detection is based on the modification of the positron annihilation signal of the cation vacancies when one or more oxygen vacancies are attached, similar as observed for nitrogen vacancies in InN [3]. In ZnO evidence of indirect positron annihilation -based detection of isolated O vacancies has also been found.
[1] R. Schaub et al., Science 299, 377 (2003).
[2] F. Tuomisto and I. Makkonen, Defect identification in semiconductors with positron annihilation: experiment and theory, Rev. Mod. Phys., to be published.
[3] C. Rauch, I. Makkonen, and F. Tuomisto, Phys. Rev. B 84, 125201 (2011).

The properties of native defects in ZnO have been investigated and debated for years, yet questions still remain regarding their fundamental properties. These species are often blamed for causing unintentional n-type conductivity, for giving rise to deep luminescence signals, and for acting as compensating centers. In this work, we re-examine the properties of native donors and acceptors in ZnO using hybrid density functional calculations, which allow for the quantitative prediction of defect transition levels and formation energies. We focus on the optical and electrical properties of these defects, and calculate both their optical and thermodynamic transition levels. In agreement with previous work, we find that native donors are highly unlikely to act as shallow donors in as-grown material, due to large formation enthalpies or high ionization energies. Our calculations indicate that the zinc vacancy will be the most stable native defect for relevant chemical potentials in n-type ZnO, and the most likely to influence electronic conductivity. We then examine how interaction with hydrogen affects the properties of these defects, and find that hydrogen interstitials bind strongly with zinc vacancies, significantly altering electronic and optical transition energies. Finally, we compare our theoretical results with published experimental studies, in an effort to reconcile observed optical spectra with our calculations.

ZnO is a wide bandgap (~3.4eV) semiconductor with potential applications in short-wavelength optoelectronic devices such as UV light emitting diodes and lasers. However, the difficulty to achieve stable and reliable p-type doping in ZnO has hindered this material from being used widely as a semiconductor to fabricate optoelectronic devices. The main source of the n-type conductivity, which is found to persist in as grown ZnO, is still widely debated. Hydrogen, which is commonly found to be present during ZnO growth, has been shown to behave as a shallow donor in ZnO according to first principles calculations [1]. Intentional incorporation of hydrogen in ZnO has also been found to increase the free electron concentration [2-4].
In this work, we study the thermal evolution of defects and the optical properties in high dose H implanted hydrothermally grown ZnO. We also investigate the thermal stability of implanted H in ZnO single crystals that are grown by different techniques. 100 keV H^- was implanted into the (0001) Zn face of hydrothermally grown and melt grown ZnO single crystals with a dose of 1x10¹⁷ cm^-2. The samples were subsequently annealed from 600^oC to 950^oC for 1 h in air. The samples were then characterized by secondary ion mass spectrometry (SIMS), Fourier transform infrared spectroscopy (FTIR), cathodoluminescence (CL), transmission electron microscopy (TEM) and X-ray diffraction (XRD).
SIMS results show that after annealing at 950^oC, about 0.15% of the original implanted H concentration can still be detected in the implanted region of the hydrothermally grown samples. This is quite surprising as Ip et al. [5] showed that implanted H in ZnO will outdiffuse completely from the implanted region to a concentration below the SIMS detection limit at >700^oC. However, the implanted H dose used by Ip et al. [5] is 2 orders of magnitude lower than the dose we used in our experiment. This shows that the thermal stability of implanted H in ZnO may be dose dependent. From TEM analysis, H implantation is also found to create vacancy type defects in the implanted region of the samples after annealing. The shape of the defects is found to evolve with annealing temperature. From CL measurement, the UV emission of the samples is found to drop in the implanted region. This could be caused by the creation of a large amount of non-radiative centers in the implanted region by H implantation. The defect band of the as implanted samples is also found to skew towards higher energy compared to the one in as grown ZnO. The energy dependent CL emission from the samples will be compared with the defect distribution in the samples obtained from TEM and XRD.
[1] C. G. Van de Walle., Phys. Rev. Lett. 85, 1012 (2000)
[2] B. Theys et al., J. Appl. Phys. 91, 3922 (2002)
[3] K. Ip et al., Appl. Phys. Lett. 82, 385 (2003)
[4] A. Y. Polyakov et al., J. Appl. Phys. 94, 400 (2003)
[5] K. Ip et al., Appl. Phys. Lett. 81, 3996 (2002)

The effect of defects on the optoelectronic properties of zinc oxide
(ZnO) thin-films is well studied. And yet, the origin of these defects
remains a matter of intense investigation. As thin-film processing
increasingly includes solution-processed methods, the effect of
defects becomes even more pronounced. This is because, unlike vacuum
processed films, nanoparticle-derived films have an abundance of
nanometer-sized grains with a large amount of grain boundaries and
surface area. In ZnO, this gives rise to the well-known defect
emission in the photoluminescence spectra. In some instances, methods
can be developed to minimize this emission, but the high concentration
of these defects presents an opportunity to study the optical
properties of such defects directly.
Using nanoparticle-derived thin-films of ZnO, we investigated the
optical properties of ZnO defects using confocal microscopy and
contactless microwave conductivity methods. Samples were studied after
having been annealed at temperatures above and below the coalescence
point of nanoparticle-derived films. The abundance and optical
stability of defects were found to vary greatly with annealing
temperature. Using confocal microscopy, single defects were probed and
both emission spectra and the second-order correlation function of
individual defects were measured. Two different optical emission
characteristics were observed, with different emission pathways. The
emission lifetime of these defects was determined to be ~ 4 ns,
shorter than the commonly identified oxygen vacancy (typical lifetimes
are on the order of hundreds of ns). Of prime importance to this work
is the fact that sub-bandgap light was used to illuminate the sample.
In the absence of an established model for sub-bandgap light
absorption and charge dynamics, microwave conductivity measurements
were employed to monitor free-charge formation and lifetime in
nanoparticle-derived ZnO films. The measured lifetimes of free charge
carriers are compared with photoluminescence measurements of the same
samples-on the order of hundreds of nanoseconds. Though, the average
charge lifetime when illuminated with either sub-bandgap or above
bandgap light is found to be extremely sensitive to annealing
temperature-increasing rapidly above the coalescence temperature of
ZnO nanoparticle films. Finally, a comprehensive charge kinetics model
will be presented that shows that free-charges in ZnO are not quickly
trapped as others have suggested.

Molecular beam epitaxy of bixbiyte In2O3 on (001) oriented YSZ typically results in {111} faceted surfaces whereas that on (111) YSZ results in smooth films [1]. This behavior has been explained by low surface free energy (or high stability) of the (111) bixbiyte surface: Theory calculations found the surface free energies of low index bixbiyte surfaces to increase from (111) to (011) to (001) surfaces [2]. The highest stability of the (111) surface together with growth conditions that impede nucleation [4] has been applied to the formation of box-shaped or wire-shaped In2O3 structures on the micro and nanoscale [3]. On the other hand, it was found that In-rich growth conditions [1] or high Sn-doping [6] lead to the formation of smooth, unfaceted (001) In2O3 films on YSZ(001). These results are in-line calculations of stoichiometric dependent surface free energy [5].
Here we present experimental study of In2O3 on YSZ(001), (011), (111) grown by MBE under O-rich, In-rich and high Sn-doping conditions. The resulting surface and crystal orientation were investigated by RHEED, AFM, SEM, and XRD. Facets were identified and allowed us to draw the following conclusions on relative surface free energies: (111) is lowest under all conditions, (001) is lowered by In-rich conditions and Sn-doping. Interestingly, a flat (011) surface was not observed. During and after growth, reconstructions were visible on (001) In2O3 surface under In-rich conditions. Our experimental results compare well to theory of [5].
[1] O. Bierwagen, M.E. White, M.Y. Tsain, and J.S. Speck, Appl. Phys. Lett. 95, 262105 (2009).
[2] A. Walsh and C.R.A. Catlow, J. Mater. Chem. 20, 10438 (2010).
[3] K.H.L. Zhang, A. Walsh, C.R.A. Catlow, V.K. Lazarov, and R.G. Egdell, Nano Letters 10, 3740 (2010).
[4] O. Bierwagen and J.S. Speck, J. Appl. Phys. 107, 113519 (2010).
[5] P. Agoston and K. Albe, Phys. Rev. B 84, 045311 (2011).
[6] N. Taga, M. Maekawa, M. Kamei, I. Yasui, and Y. Shigesato, Jpn. J. Appl. Phys. 37, 6585 (1998).

Doping is necessary to achieve full potential of oxide semiconductors for active electronic applications. However, in these wide-bandgap materials n-type and p-type extrinsic dopants are often counteracted by formation of intrinsic compensating defects of opposite charge. In this contribution we experimentally demonstrate that even degenerate levels of both electrons and holes in oxide semiconductors can be enabled by non-equilibrium phenomena. The examples are (a) p-type doping in ZnCo2O4, and (b) n-type doping of ZnO. In both cases degenerate doping occurs only by virtue of an appropriate combination of non-equilibrium effects and suitable defect physics.
In thermodynamic equilibrium, ZnCo2O4 is a p-type oxide with rather small conductivity (0.001 - 0.01 S/cm) at 300 - 400C. Growth of ZnCo2O4 thin films by conventional physical vapor techniques such as RF sputtering or pulsed laser deposition (PLD) at these relatively low temperatures increase p-type conductivity by a factor of 100-1000 [1] due to two distinct types of non-equilibrium effects: (a) Co/Zn site disorder and subsequent ionization of additional Zn-on-Co antisite acceptors [2], and (b) possibility of deviation from the nominal ZnCo2O4 composition towards the Zn-rich side in excess of the solubility limit, with further increase in concentraton of Zn-on-Co acceptors and the hole concentration [3]. These results demonstrate the importance of choosing oxides with suitable defect chemistry (e.g. Co-on-Zn donor-like antisites are electrically benign) in order to enable non-equilibrium enhancement of p-type doping.
Doping of ZnO with group-III elements such as Al, Ga and In, is known to produce high levels of n-type doping. High conductivities (1,000-10,000 S/cm) that are typically achieved during the growth at 250 - 350C around 10^-8 atm exceed by a factor of 100,000 the values expected from thermodynamic equilibrium considerations based on theoretical calculations. In contrast, temperature (T) and oxygen partial pressure (pO2) dependence of conductivity in gallium zinc oxide annealed at high temperature in oxidizing atmosphere show good agreement with the ab initio theoretical thermodynamic model. These results indicate that high levels of Ga doping in ZnO thin films deposited on various substrates are enabled by the highly non-equilibrium state of these samples [4].
In summary, both ZnO and ZnCo2O4 examples demonstrate that non-equilibrium phenomena can be used as a means to enable doping in oxide semiconductors for various active applications. This understanding necessitates development of new methods to quantify and control non-equilibrium processes in oxide semiconductors.
This work is supported by the U. S. Department of Energy under Contract No. DE-AC36-08GO28308 to NREL
[1] A. Zakutayev et al, PRB 85, 085204 (2012)
[2] T. R. Paudel et al, AFM 21, 4493 (2011)
[3] J. D. Perkins et al, PRB 84, 205207 (2011)
[4] A. Zakutayev et al, submitted

Zinc oxide (ZnO) is a wide band gap semiconductor (E_g ~ 3.4 eV ) that has received considerable attention the past few years due to its potential applications in light emitting devices and photovoltaics. However, the technological advances of ZnO have been hindered by the difficulty in controlling and understanding the electrical behavior of intrinsic and impurity related defects. For instance, a defect level around E_c - 0.3 eV (E_c denotes the condction band edge), normally labeled E3, is found in all ZnO samples irrespective of the growth method used. E3 was first studied by Simpson and Cordaro [1] and later characterized and labeled E3 by Auret et al. [2, 3], where it was shown that E3 is not influenced by electron irradiation. The origin of the E3 defect is very much a subject of ongoing debate and possible explanations include a relation to interstitial zinc [4], oxygen vacancy or even transition metals [5].
Here, we show that hydrogen implantation can increase the E3 defect level concentration. Box profiles and mono-energy protons with doses ranging from 5e10 to 2e11 cm^-2 were implanted into hydrothermally grown ZnO samples with original concentrations of E3 below 5x10^14 cm^-3. For the characterization, we used junction capacitance spectroscopy with Pd-Schottky contacts.
Both, the charge carrier concentration and the absolute E3 concentration show an increase of more than one order of magnitude compared to the non-implanted samples for mono-energy as well as box-profile proton implantations. Since this increase is not observed in electron irradiated samples [3] or implantations with Zn and O [6] this strongly indicates that hydrogen is involved in the formation of E3.
The origin of E3 will be further discussed in relation to implantation damage and associated complex formation with hydrogen.
[1] J. C. Simpson and J. F. Cordaro, J. Appl. Phys 63, 1781 (1988).
[2] F. D. Auret, S. A. Goodman, M. Hayes, M. J. Legodi, H. A. van Laarhoven, and D. C. Look, Appl. Phys. Lett. 79, 3074 (2001).
[3] F. D. Auret, S. A. Goodman, M. J. Legodi, W. E. Meyer, and D. C. Look, Appl. Phys. Lett. 80, 1340 (2002).
[4] H. Frenzel, H. v. Wenckstern, A. Weber, H. Schmidt, G. Biehne, H. Hochmuth, M. Lorenz, and M. Grundmann, Phys. Rev. B 76, 035214 (2007).
[5] H. V. Wenckstern, G. Biehne, M. Lorenz, M. Grundmann, F. D. Auret, W. E. Meyer, M. Hayes, and J. M. Nel, J. Kor. Phys. Soc. 53, 2861 (2008).
[6] L. Vines, J. Wong-Leung, C. Jagadish, E.V. Monakhov, B.G. Svensson , Physica B, 407, 1481 (2012)

The wide-bandgap semiconductor, zinc oxide (ZnO), is known to be extremely useful for many electrical applications including transparent conducting oxides, thin film transistors and light emitting diodes. By using the Kelvin probe method, certain properties of ZnO can be more fully understood. Our Kelvin probe setup has the ability to change ambients (gas/ vacuum/ air), illumination energy and intensity, and the temperature of the sample in the range of 80 - 650 K. As in gallium nitride (GaN) and other semiconductors, we expect negative charge at the surface of n-type ZnO which should cause upward band bending. A surface photovoltage (SPV) caused by the presence of UV illumination can be measured by the Kelvin probe. These measurements provide important information about the surface band bending, type of conductivity, and other electrical and optical properties of the near-surface layer. In this work, we investigate Ga-doped ZnO by using the Kelvin probe technique. The concentration of free electrons at room temperature varies from 3×10¹⁷ to 9×10¹⁹ cm^-3 for samples with different concentrations of Ga doping. For the SPV measurements, above-bandgap UV light was used to generate a SPV, and the change in the surface potential was monitored by a Kelvin probe. Under UV illumination, the SPV immediately increases by about 0.3 eV and then begins to rise logarithmically with time. After long (1 hr) exposures to UV, a gradual decrease of the SPV in air ambient and a slow increase in vacuum is observed. Such behavior can be explained by photo-induced adsorption of acceptor-like species from air or desorption of such species in vacuum. In contrast to our results for undoped or moderately, Si-doped n-type GaN, the restoration of the SPV signal in dark after illumination is very slow and cannot be explained with a thermionic model. Moreover, the SPV transients for Ga-doped ZnO resemble those for degenerate Si-doped GaN samples. We assume that a thin native oxide layer plays a significant role in both cases. Apparently, fast changes in the band bending are masked by a potential drop inside the insulating oxide layer, and the observed slow changes in SPV are caused by a slow penetration of charge carriers through the oxide layer. This would explain the non-thermionic behavior of the SPV transients.

TiO₂ is the most widely used oxide for photocatalytic applications due to its low cost and high activity. The discovery of the photolysis of water on the surface of TiO₂ in 1972 launched four decades of intensive research on the underlying chemical and physical processes involved. Although the individual polymorphs of TiO₂ have received considerable attention, mixed phase samples have been shown to display much higher photoactivity. The origin of the improved performance of mixed phase TiO₂, however, was never truly understood, in particular the fundamental band alignment between rutile and anatase.
The general consensus for the past two decades has been that the conduction band of anatase sits 0.2 eV above that of rutile, however, this alignment motif could not explain the superior photocatalytic properties of the mixed phases. Instead, a complicated mechanism of interfacial trapping and charge transfer was put forward as one explanation, albeit within the constraints of the assumed band alignment model.
We have demonstrated, through a combination of state-of-the-art materials simulation techniques and X-ray photoemission experiments, that a type-II (staggered) band alignment of ~0.4 eV exists between anatase and rutile, with anatase possessing the higher electron affinity. This decreases the effective band gap at the interface to ~2.8 eV, increasing its visible light activity. Our results help to explain the robust separation of photo-excited charge carriers between the two phases, and it is expected that our new rationalization of the anatase/rutile alignment will play a vital role in the future development of improved TiO₂ based photocatalysts.
[1] D. O. Scanlon, C. W. Dunnill, J. Buckeridge, S. A. Shevlin, A. J. Logsdail, S. M. Woodley, C. R. A. Catlow, M. J. Powell, R. G. Palgrave, I. P. Parkin, G. W. Watson, T. W. Keal, P. Sherwood, A. Walsh and A. A. Sokol, “Band alignment of rutile and anatase TiO₂ from theory and experiment”, Nature Materials, In Press (2013)

We have carried out extensive computational studies of atomic-scale defects and interfaces in oxides, including transition-metal oxides ZnO, SnO₂ and In₂O₃ and materials in the perovskite family. Special attention is paid to accurate description of electronic levels in the bandgap region, using hybrid exchange-correlation functionals and Hubbard on-site corrections. In particular, the role of vacancy defects in the optical, transport and magnetic properties is elucidated in view of recent experimental findings.

ZnO is a promising electronic material for a wide variety of applications ranging from sensing to light emission. So far, full scale exploitation of this material in device applications has been hindered by difficulties in realizing reproducible p-type doping. First of all, unintentionally doped ZnO is an n-type conducting material though the origin of donors responsible for this n-type conductivity remains highly controversial. Secondly, the electrical activity of dopants is affected by intrinsic defects. Here, the key point defects to be considered include zinc vacancies (V_Zn), zinc interstitials (Zn_i), oxygen vacancies (V_O), and oxygen interstitials (O_i), as well as related complexes. Thus, a reliable spectroscopic identification of intrinsic defects is one of the key prerequisites for resolving these issues.
A powerful technique to investigate chemical identity, local structure and energy level positions of defects is electron paramagnetic resonance (EPR) spectroscopy. In this work we employ this technique to investigate defects created in bulk ZnO by electron irradiation with energies of 0.45, 0.6, 0.8 and 1.2 MeV. The EPR spectra of the as-grown reference sample as well as of the 0.45 MeV- irradiated sample are dominated by a shallow donor (SD) signal with g _#8741; = 1.957 and g_perp; = 1.956. With increasing electron irradiation energy the SD signal intensity decreases due to the lowered Fermi level position. Starting from irradiation energies of 0.6 MeV, a defect with g_#8741; = 1.9945 and g_perp; = 1.996 can be detected under light illumination with photon energies exceeding 2 eV. Previously this defect has been assigned to V_O. The deduced V_O concentration increases with increasing electron irradiation energy. This allows us to estimate the threshold energy for creation of defects in the oxygen sublattice of ZnO as being between 0.45 and 0.6 MeV. When the electron irradiation energy is raised to at least 0.8 MeV, three sets of six equidistant signals with g-values close to that for V_Zn are observed under light illumination. The observation of six EPR lines for each signal implies resolved hyperfine interaction between an electron with an effective spin 1/2 and a nucleus with a nuclear spin 5/2 and 100% natural abundance. There are only three chemical elements fulfilling this requirement, namely Al, Mn and I. According to the performed secondary ion mass spectrometry (SIMS) measurements, only Al, however, is present in a concentration high enough to account for the deduced concentrations of the EPR-active centers. From detailed angular-dependent EPR studies, the revealed centers are non-axial and show a rather similar behavior to the so-called A-centers observed previously in other II-VI materials. In our case, at least one of the centers likely involves a twofold negatively charged V_Zn and an Al atom located in the next nearest Zn site. Such a defect complex has recently been used to explain compensation effects in Al-doped ZnO.

Zinc oxide (ZnO) and related compounds are a potential material system for optoelectronic devices working in the near UV and a viable alternative to gallium nitride (GaN). Defects strongly influence material properties, such as carrier lifetime or mobility. In this contribution we investigate native defects in pulsed-laser deposited ZnO thin films introduced by argon implantation. To obtain a preferably homogeneous argon depth profile four Ar-implantations with acceleration energies of 270keV, 135keV, 70keV, and 30keV were carried out resulting in a box-like argon distribution in the ZnO film. Defects were studied by means of deep level transient spectroscopy (DLTS).
We found two deep levels introduced by the argon implantation, namely E950 and E1300. While E950 was already found by Nel et al. in n-type single crystal ZnO after 5.4MeV-particle bombardment [1], the deeper level E1300 is to the best of our knowledge unreported for ZnO so far. In order to study the annealing behaviour of these defects, the implanted samples were annealed under an oxygen ambient of 700mbar at temperatures ranging from 120°C to 900°C.
We observed a decrease of the concentration of E950 below the detection limit for temperatures above 660°C. The annealing behavior could not be described by first or second order kinetics. This implies that the annealing process is either 'diffusion-assisted' or that the peak of E950 is not due to a single defect but that more than one defect contribute to the DLTS signal and cause thus the unusual annealing behavior.
[1] J M Nel, F.D Auret, L Wu, M J Legodi, W E Meyer, M Hayes, Sensors and Actuators B 100, 270 (2004).

Alloying MgO into ZnO substantially increases the band-gap energy but reduces the conductivity and makes it harder to be doped with donors. While the former makes it attractive as energy-level-tunable transparent conducting contacts, the latter limits its applications. By combining high quality material synthesis, characterization and theory, we attribute the major limitation on the electrical conductivity of epitaxial Ga-doped Zn0.7Mg0.3O to intrinsic acceptors, such as zinc vacancies (VZn), which both trap carriers and increase the ionized impurity scattering (IIS). In addition, it shows that VZn can form defect complexes with Ga substituted on a Zn site (GaZn) during annealing, and this can increase the mobility by reducing the number of IIS centers. This work establishes that enhancing defect pairing is an effective strategy to increase mobility in semiconductors where IIS limits the transport.
Epitaxial Zn0.7Mg0.3O:Ga(1%) thin films were deposited on c-sapphire substrates by pulsed laser deposition. Temperature-dependent Hall effect measurements results indicate that the samples are degenerately doped and the mobility is limited by IIS. The detailed analysis of electrical properties suggests that ionized acceptor defects are present. TEM imaging and composition analysis indicate that neither extended defects nor impurities are present at sufficient levels to account for the low ionization efficiency. Thus, we consider intrinsic defects. First-principles calculations of formation enthalpies find that zinc vacancies (VZn) have the lowest formation enthalpy among intrinsic acceptors. Considering a simple model with just four kinds of defects (isolated GaZn, isolated VZn, GaZn-VZn pairs and 2GaZn-VZn clusters), we can derive their concentrations from four independent equations with measured electron concentration, mobility and Ga concentration as inputs. Analysis performed on samples with different annealing times shows that as-deposited films have more isolated VZn than predicted for thermodynamic equilibrium. Upon annealing, the samples approach equilibrium as indicated by a decrease in the concentration of isolated VZn and a corresponding increase in the concentration of GaZn-VZn pairs and 2GaZn-VZn clusters. This clustering of oppositely charged defects reduces the net number of ionized impurities consistent with the observed increase in mobility from 12 cm2/Vs to 18 cm2/Vs upon annealing. In particular, a Zn0.7Mg0.3O:Ga(1%) sample annealed for 2 hours in vacuum (pO2 = 10-7 Torr) at 400C has an electrical conductivity σ = 475 S/cm, a 50% increase over σ = 320 S/cm, our best as-deposited conductivity to date.
This work was supported through the Solar Energy Technology Program of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy.

Zinc oxide (ZnO) is a wide band gap semiconductor (Eg ~ 3.4 eV) that has received considerable attention during the past few years due to its potential applications in light emitting devices and photovoltaics. However, the technological advances of ZnO have been hindered by the difficulty in controlling and understanding the electrical behavior of dopants and impurities. In particular, controlling the diffusion of dopants and impurities, and understanding the main impurity configurations after in-diffusion or ion implantation, remains a major challenge for ZnO based devices. While hydrogen, lithium and sodium are relatively mobile in ZnO [1-3], other impurity elements like N and Al appear to be rather immobile [4-5]. Moreover, with the exception of possibly hydrogen, diffusion studies of ZnO are scarse in the literature.
Here, we explore the diffusion mechanisms of a range of impurity elements in ZnO after ion implantation and subsequent thermal treatments. Samples of hydrothermally grown ZnO (HT-ZnO) were implanted at room temperature with H, N, Na, P, K, Ni, Ga and Sb using using energies in the 1.20 keV - 1.44MeV range and to doses of 2 × 1014 - 1× 1016 cm-2. After implantation, isochronal annealing series (30min) were conducted in a tube furnace from 700 up to 1300oC, depending on the implanted element. Firstly, N shows a high thermal stability, but the diffusivity depends strongly on the Fermi level position in accordance with Fairs vacancy model [6], where the doubly charged configuration makes a substantial contribution to the overall diffusivity. Secondly, the diffusion of P and Sb appear to be limited by the stability of their atomic configuration in the implanted region, rather than by the barrier for migration. Thirdly, the diffusion of typical n-type dopants (Ga, Al) show a distinct dependence on the charge state of the zinc vacancy, implying a vacancy-mediated mechanism in accordance with Fairs model [6]. Finally, other typical impurity elements, like H, Na, K, and Ag, show a trap and solubility limited diffusion process and their effective diffusivity hinges on the material used.
[1] K. M. Johansen, J. S. Christensen, E. V. Monakhov, A. Yu. Kuznetsov, and B. G. Svensson,
Appl. Phys. Lett. 93, 152109 (2008).
[2] P. T. Neuvonen, L. Vines, A. Yu. Kuznetsov, B.G. Svensson, X. Du, F. Tuomisto, and
A. Hallén, Appl. Phys. Lett. 95, 242111 (2009).
[3] K. E. Knutsen, K. M. Johansen, P. T. Neuvonen, B. G. Svensson, and A. Yu. Kuznetsov, J.
Appl. Phys. 113, 023702 (2013).
[4] T. M. Boslash;rseth, F. Tuomisto, J. S. Christensen, E. V. Monakhov, B. G. Svensson, and A. Yu.
Kuznetsov, Phys. Rev. B 77, 045204 (2008).
[5] T. M. Boslash;rseth, J. S. Christensen, K. Maknys, A. Hallén, B. G. Svensson, and A. Yu.
Kuznetsov, Superlatt. and Microstruct. 38, 464 (2005).
[6] R.B. Fair and J.C.C. Tsai, J. Electrochem. Soc., 124, 1107 (1977)

We are working to develop a first-principles based theoretical and computational framework that is both accurate enough to predict material properties and efficient and economic enough to be used to prescreen optoelectronic materials with suitable band gap, structural and thermal stability to realize material design in virtual space. The structural and electronic properties of various ZnO-based binary semi-conductoring alloys (such as Zn1minus;xBexO, ZnOxS1minus;x) have been studied. Recently, we have also investigated Zn1minus;x(LiGa)0.5xO by first-principles methods. It is found the (Li, Ga) atoms prefer to enter into the Zn sites along the non-polar m-plane, and further they tend to enter into the Zn sites in the non-polar a-plane for the composition x = 0.25. For x = 0.5, the preferable occupations of (Li, Ga) atoms are the Zn atoms sites on the semi-polar (10-1-1) azimuth orientation. Our results reveal these solid solutions are energetic stable and certain ordered structures of Zn2LiGaO2 (x=0.5) are more stable than parent materials. The calculated bandgaps of low-energy alloys all have direct gaps with magnitude increase with the increasing doping concentrations.

Cadmium oxide (CdO) is an intrinsically n-type semiconductor. Owing to its native carrier concentration and the Moss-Burstein shift, it simultaneously possesses optical transparency and electrical conductivity. Materials with these properties are classified as transparent conducting oxides and are commonly used as transparent electrodes in solar cells and flat screen TVs. Experimental evidence demonstrates an increase in both carrier concentration and carrier mobility with certain dopants, with a corresponding increase in electrical conductivity. To investigate the effect of point defects on these properties, we employed first principles calculations based on density functional theory. We calculated band structures for bulk CdO using variations on the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, including PBE+U as well as the screened exact exchange approach of Heyd-Scuseria-Ernzerhof (HSE06). Our results demonstrate that HSE06 most closely matches experimental values for the direct and indirect band gaps, the lattice parameter, the width of the valence band, and the d-state energies. We have calculated defect formation energy as a function of Fermi level and growth conditions for intrinsic O and Cd vacancies as well as extrinsic substitutional defects in a variety of charge states. Some debate has been made in the literature over whether the native n-type conductivity in CdO is due to O vacancies or Cd interstitials. Our results indicate a significant equilibrium concentration of O vacancies in undoped CdO even in O-rich growth conditions, consistent with findings in the literature that suggest O vacancies are the main intrinsic contributor of n-type carriers. The formation energies as a function of Fermi level together with self-consistent solutions to the mass balance equation to determine defect concentrations will be presented. Results from these calculations will be used to discuss the impact of doping on the concentrations of intrinsic and extrinsic defects. In addition, the strain from each defect and its potential role in the physical properties of CdO will be discussed. The work presented in this abstract is supported through grant DMR-1151568 from the National Science Foundation.

Doping the intrinsic n-type semiconductor ZnO with nitrogen was initiated by the search to overcome the shortcoming of p-type conductivity in ZnO. Due to the complex behavior of nitrogen in ZnO this has not been achieved yet. Reasons might be manifold like passivation by unintentional donors or intrinsic defects and the formation of nitrogen molecules.
The structure of the vibrational properties of doped ZnO can give further insight into the underlying physics of the incorporation of the nitrogen impurity and might help to understand the discrepancy between N incorporated into the material and p-conductivity achieved. Several additional vibrational features have been observed by means of Raman spectroscopy in nitrogen doped ZnO samples almost independent of the growth methods employed, i.e. by sputtering, chemical vapor deposition, or molecular beam epitaxy. Attributing these signals to localized nitrogen related modes or to the effect of the symmetry reduction within the crystal lattice caused by the point defect nitrogen on an oxygen site does not explain all observed features neither their properties. More complicated structures, like nitrogen-hydrogen complexes, have been considered but do not give a satisfactory explanation of the observed results. However, the Raman spectra show further evidences for a more complicated defect, N dominated clusters, as being the source of the additional Raman signals. Such an observation is in agreement with cathodoluminescence measurements and density functional theory calculations.

Carbon has been variously reported to produce ferromagnetism[1] and p-type conduction[2] in ZnO grown by various techniques yet there is surprisingly little understanding of its microscopic properties. High concentrations of unintentional carbon are reported in ZnO grown using metalorganic vapor phase epitaxy (MOVPE) at low growth temperatures. In this work we investigate the intentional doping of ZnO nanowires (NW) with carbon tetrachloride. NWs are an excellent platform for spectroscopy studies such as low temperature photoluminescence (PL) since they decouple the substrate induced strain from the NWs. ZnO is very rich in sharp luminescence lines, corresponding to excitons bound to various donor impurities, in the near UV region [3]. 4.2 K PL spectra of undoped NWs grown by MOVPE exhibit sharp donor bound exciton (DCarbon has been variously reported to produce ferromagnetism[1] and p-type conduction[2] in ZnO grown by various techniques yet there is surprisingly little understanding of its microscopic properties. High concentrations of unintentional carbon are reported in ZnO grown using metalorganic vapor phase epitaxy (MOVPE) at low growth temperatures. In this work we investigate the intentional doping of ZnO nanowires (NW) with carbon tetrachloride. NWs are an excellent platform for spectroscopy studies such as low temperature photoluminescence (PL) since they decouple the substrate induced strain from the NWs. ZnO is very rich in sharp luminescence lines, corresponding to excitons bound to various donor impurities, in the near UV region [3]. 4.2 K PL spectra of undoped NWs grown by MOVPE exhibit sharp donor bound exciton (D⁰X) transitions I₈ and I₉, associated with excitons bound to residual neutral Ga and In donors respectively, at 3359.9 meV and 3356.8 meV. A set of 4 sharp line peaks at 3361.3, 3361.7, 3361.9 and 3362.3 meV are observed in all of our undoped MOVPE samples and they seem to be donor bound excitons specific to the MOCVD growth technique possibly involving carbon and some other unidentified constituents. Increasing the carbon concentration increases the intensity of these lines. For heavily C doped NWs the overall PL intensity approaches zero, indicating the onset of significant point defect concentrations. At the highest doping levels, Raman spectroscopy indicated the presence of strong graphitic D and G modes coincident with the onset of structural defects in transmission electron microscopy. Carbon doping also has a strong effect on NW morphology and even a small amount of carbon decreases the axial growth and causes an unusual inverse tapering and the formation of highly perfect flat topped hexagonal c-plane facets.
References:
1. H. Pan et al., Phys. Rev. Lett, 99, 127201 (2007)
2 S. T. Tan and X. W. Sun, Appl. Phys. Lett. 91, 072101 (2007)
3. B.K. Meyer et al. Phys. Stat. Sol. (b) 241, 231 (2004).

The quest to produce p-type conductivity in ZnO via nitrogen doping has been of high scientific and technological interest throughout the past two decades. In this presentation I will review what has been learned recently about the nature of the compensating centers in nitrogen-doped ZnO from defect-sensitive magnetic resonance techniques. In particular, detailed electron paramagnetic resonance (EPR) at 9.5 GHz and optically-detected magnetic resonance (ODMR) at 24 GHz were performed on ZnO:N bulk crystals grown by seeded chemical vapor transport in an ammonia ambient. A variety of nitrogen-related and other defect centers (of unknown microscopic origin) were revealed in these studies with magnetic resonance parameters that are best described as associated with both deep donor and acceptor states. The existence of such centers significantly compromises the ability to achieve high hole densities via conventional N-doping methods and suggests that alternative routes to obtain reliable p-type doping in ZnO are warranted.
We would like to thank N.T. Son (Linkoping University) for many helpful discussions.

The success of spintronics technology depends on the development of suitable room temperature ferromagnetic (RTFM) semiconductors. ZnO is a promising candidate, which exhibits RTFM [1] while preserving its semiconducting properties (band gap ~3.3 eV). The role of intrinsic defects, mainly cation vacancy plays a vital role for such magnetic phenomena. Recently RTFM has also been reported in MgO [2] which is an insulator with a band gap ~7.2 eV. We thus have a flexibility to tailor the band gap over a wide range and still maintain the magnetic properties over a wide composition range of Zn@)MgO alloys. Research efforts in these oxides focus on an interdisciplinary investigation of multi-functional materials for variety of applications such as renewable energy, and sensing and optical communication applications.
In this paper, to be specific we report the preparation of the appropriate composite target material, and deposition of Zn0.95Mg0.05O films on Si and glass substrates by pulsed laser deposition (PLD). We have systematically study the effects of deposition conditions (deposition time and oxygen pressure) on RTFM in the Zn0.95Mg0.05O films. Two sets of samples were deposited by 1) by varying the film thickness depending on deposition times of 10, 20, 30 and 40 minutes respectively maintaining O2 paartial pressure at 20µBar, and 2) by varying oxygen pressure of 9, 20, 50, 100 and 150 µBar for a fixed deposition time of 30minutes, respectively. The growth rate of the thin films was found to be ~ 10nm/minute. RTFM was confirmed by using superconducting quantum Interface Device (SQUID). In our experiments, the sample deposited at 30 minutes and O2 pressure of 20µBar (thickness: 306nm) showed the highest magnetization of 7.89emu/cm3. We have also extensively studied structural properties using X-diffractometer (XRD) and the morphology by means of high resolution scanning electron microscope (HRSEM). The composition and thickness of the thin films were determined using energy dispersive spectrometer (EDS) and Focused Ion Beam techniques using Nova 600 Nanolab. Room temperature (RT) optical absorption measurements were performed using an UV-visible-near infrared spectrophotometer to determine the thickness dependence of the band gaps in these films. The RT photoluminescence was measured using the He-Cd laser line of 325 nm as the excitation source to discuss the relationship between RTFM and defects in Zn(1-x)MgxO films. The results, not only demonstrated an approach to obtain high quality RTFM Zn@MgO films, but also helped to identify the role of defects on the observed ferromagnetism and how they differ in their roles between sputtered and PLD deposited samples.
[1] M. Kapilashrami et al. Appl. Phys. Lett. 95, 033104 (2009).
[2] C. Moyses Araujo et al. Appl. Phys. Lett. 96, 232505 (2010).

Dilute magnetic semiconductors (DMS) are envisioned as sources of spin-polarized carriers for future semiconductor devices which simultaneously utilize spin and charge of the carriers. The hope of discovering a DMS with ferromagnetic order up to room temperature (RT) still motivates research on suitable DMS materials such as doped oxide semiconductors. The combination of x-ray absorption near edge spectroscopy (XANES) and x-ray linear dichroism (XLD) allows to separately determine the valence state and the local structural environment of dopant atoms, cationic and anionic sublattice in doped oxides as demonstrated for a range of magnetically doped oxide semiconductors like Co-doped ZnO [1], Co/Al-codoped ZnO [2], Co/N-codoped ZnO [3], Gd-doped ZnO [4], and Fe/N-codoped TiO2 [5] to assure phase pureness. Especially for the Co-doped ZnO system a comprehensive set of quality indicators could be established which assure that Co substitutes for Zn and the formation of menot;talnot;lic Co precipitations can be ruled out [6]. In addition, x-ray magnetic circular dichroism (XMCD) allows to study the subnot;lattice magnetization of the dopant separately which can be compared to integral SQUID magnetometry and by that establishing that Co-doped ZnO is an anisotropic paramagnet [7]. Signs of apparent ferronot;magnetic order could be directly attributed to the presence of metallic Co precipitations which could be evidenced by a combination of XANES and transmission electron microscopy [8]. Finally, unprecedented high field XMCD measurements up to 17 Tesla have allowed to quannot;tify the antinot;ferronot;magnetic next cation neighbor exchange interaction [9].
Here the resulting magnetic properties of highly (up to 35%) Co doped, Co/Al, Co/Cu, and Co/N co-doped and Cu doped ZnO thin films will be correlated with the dopant incorporation, its valence and the presence of defects. In all cases, either a coexistence of para- and antinot;ferronot;magnot;netism is found, or the existence of metallic Co-containing secondary phases is evinot;denced. Only in the latter case SQUID magnetometry reveals a sizable superparamagnetic resnot;ponse at room temperature beyond the artifact-level of the SQUID magnetometer [10,11].
[1] A. Ney et al., Phys. Rev. Lett. 100, 157201 (2008)
[2] A. Ney et al., Phys. Rev. B 82, 041202(R) (2010)
[3] D. Schauries et al., Phys. Rev. B 87, 125206 (2013)
[4] V. Ney et al., Phys. Rev. B 85, 235203 (2012)
[5] T.C. Kaspar et al., Phys. Rev. B 86, 035322 (2012)
[6] A. Ney et al., New. J. Phys. 12, 013020 (2010)
[7] A. Ney et al., Phys. Rev. B 81, 054420 (2010)
[8] A. Ney et al., New. J. Phys. 13, 103001 (2011)
[9] A. Ney et al., Phys. Rev. B 85, 245202 (2012)
[10] A. Ney et al., J. Magn. Magn. Mater. 320, 3341 (2008)
[11] M. Sawicki et al., Semicond. Sci. Technol. 26, 064006 (2011)

The worldwide research at nanoscale mainly focuses on new functional materials in which surface and interface effects play an important role. The possibility of combining ferromagnetism and semiconductor properties in a single material is an attractive issue because of its applications in novel magneto-optoelectronic devices. Among the semiconductor systems, ZnO is the semiconductor of choice due to its magnetic, electric and optoelectronic properties which can be optimized via surface functionalization with different organic molecules. Until recent times, the doping of transition metal such as V, Cr, Fe and Co into ZnO system is the conventional method to make the system ferromagnetic. Despite some initial promising results on dilute magnetic semiconductors (DMSs), ferromagnetism in such systems is still under debate. Recently, Garcia et al have demonstrated that it is possible to induce room temperature ferromagnetism (RTFM) in ZnO nanoparticles by capping with different organic molecules such as dodecanethiol, dodecylamine and trioctylphosphine oxide.
The present study investigates the structural, optical, morphological and magnetic properties of pure and V doped ZnO (Zn0.95V0.05O & Zn0.90V0.10O) films by surface functionalization with thiol and amine. The unfunctionalized and surface functionalized pure ZnO, Zn0.95V0.05O and Zn0.90V0.10O films have been systematically characterized using X- ray diffraction (XRD), atomic force microscopy (AFM), contact angle measurement (CA), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), temperature programmed desorption (TPD) and vibrating sample magnetometer (VSM) measurements. XRD patterns show slightly weak reflection on thiol and amine functionalization as compared to the corresponding unfunctionalized ZnO films. AFM measurements before and after surface functionalization reveal no disorder in the ZnO surface on surface functionalization with thiol and amine. The formation of chemical bondings of thiol and amine with ZnO surface has been confirmed by XPS measurements. Room temperature PL spectra of thiol/amine functionlized ZnO films show the quenching of oxygen vacancies relative to the unfunctionlized ZnO films. The TPD measurements show the desorption of sulphur completely occur from the surface of thiol functionalizaed ZnO films about 550 K. The magnetic properties of films has been investigated at three different stages viz., (i) as prepared pure and V doped ZnO films (ii) functionalized with thiol and amine and (iii) after desorption of thiol respectively. The sulphur (or nitrogen) atoms of the thiol (or amine) adsorbed on O vacancies causes charge redistribution in ZnO films resulting magnetic moment as evidenced from VSM measurements.

The possibility of achieving ferromagnetism in semiconductor and semiconductor based devices has propelled an emerging field of electronics known as spintronics and identifying suitable materials for spintronics is pursued vigorously. Spintronics aims at using both spin and charge of the electron to bring about novel devices such as spin transistor, spin polarized light emitting diode and other spin based devices. The most popular approach to incorporate magnetic degrees of freedom into semiconductors is by substituting magnetic ions into lattice sites of host which is known as diluted magnetic semiconductor (DMS). The room temperature ferromagnetism (RTFM) has been reported in ZnO doped with Co, Mn, Ni and Fe. The stability and robustness of RTFM in magnetic ion doped ZnO remains controversial. As in case of magnetic ion doped III-V semiconductors, it is debated whether it is intrinsic or due to defects or due to clusters of the magnetic ions. Moreover, there are several problems with magnetic ions induced DMS, such as, (i) large difference in size of the magnetic ions with Zn, (ii) clustering of magnetic ions, (iii) poor control on dopoant distribution and (iv) difficulty to achieve higher dopant concentration. To overcome these problems due to magnetic dopant induced RTFM, it is being attempted to dope ZnO with nonmagnetic ions for achieving ZnO based DMS. Cu and Ag are most suitable candidates for this purpose, because (i) neither metallic Cu and Ag nor any compound with Ag,Zn,O and Cu, Zn,O is ferromagnetic and (ii) Cu and Ag at Zn site (AgZn and CuZn) will have extra hole which is responsible for RTFM in Cu and Ag doped ZnO. Essentially substitution of Zn2+(3d9) with Cu2+(3d9) and Ag2+(4d9) should give rise to RTFM. Here we present the observation of robust RTFM in transparent ZnO:Cu and ZnO:Ag thin films and how to achieve high saturation magnetic moment by optimizing different growth conditions. The detailed microstructural characterization has been carried out to reveal the origin of RTFM in the ZnO:Cu and ZnO:Ag which were grown by RF magnetron sputtering. Optical absorption studies of thin films revealed decrease in the bandgap energy with dopant content indicating strong p-d hybridization between O and Cu/Ag in ZnO. Rutherford backscattering spectrometry was used to quantify the dopant content in ZnO host. X-ray diffraction was used to ascertain the mixing of dopant atom with the host insulating matrix and homogeneity of thin films. X-ray photoelectron spectroscopy was used to ascertain the bonding characteristics and oxidation states of dopant inside the ZnO host. Finally we show that it is extremely important to optimize growth conditions and concentration of Cu/Ag to have maximum magnetization in ZnO.

Mn doped ZnO thin films, prepared by pulsed laser deposition, exhibited ferromagnetism at 300 K and superparamagnetism at 5 K. The temperature dependent magnetization did not exhibit any distinctive blocking phenomena or phase transition corresponding to any secondary phase(s). X-ray diffraction and high resolution transmission electron microscopic analyses carried out at various locations near the interface were too indicative of absence of any secondary phase(s). X-ray photoelectron spectroscopy and X-ray absorption spectroscopy confirmed the iso-valent substitution of Mn2+ for Zn2+ ions. Magnetic and magneto-transport properties showed that of s-d exchange induced intrinsic ferromagnetism. From the bound magnetic polaron percolation model [1] and the variable range of hopping conduction below a metal to insulator transition [2], we found that the observed superparamagnetism can be due to randomly distributed point defects. Such defects could trap electrons and form non-interacting polaron clusters at low temperatures [3], which can result in a large positive magnetoresistance (MR) (~ 40%). Such large positive MR was not observed in other diluted magnetic ZnO films that did not exhibit superparamagnetism [4]. We have obtained a qualitative agreement of our experimental data to the percolation model and hopping conduction. This analysis may support the conflicting results reported on the presence or absence of ferromagnetism in un-doped and transition metals doped ZnO.
References:
1) A. Kaminski and S. Das Sarma, Phys. Rev. Lett. 88, 247202 (2002)
2) M. Sawicki, D. Chiba, A. Korbecka, Y. Nishitani, J. A. Majewski, F. Matsukura, T. Dietl and H. Ohno, Nat. Phys. 6, 22, (2010)
3) H. Chou, C. P. Lin, J. C. A. Huang, and H. S. Hsu, Phys. Rev. B 77, 245210 (2008)
4) S. Venkatesh et al, (to be communicated)

Tin oxide is an interesting semiconductor, in that two different phases are readily grown in thin film form: SnO₂, with a band gap energy of approximately 3.5 eV, and metastable SnO, with a somewhat narrower, less well characterized band gap energy. SnO₂ has many similarities with ZnO, another ultraviolet band gap semiconductor, including a strong tendency to be n-type as grown, as well as a surface electron accumulation layer. Both of these characteristics are related to a charge neutrality level near the conduction band edge, and together frustrate both pn junction and Schottky based device fabrication. In contrast, the narrower band gap compound SnO is intrinsically p-type. Due to the metastable nature of this material it must be grown at lower temperature, above which mixed phase films or purely SnO₂ may result. Here we describe the characteristics of a series of films grown by plasma-assisted molecular beam epitaxy under different conditions. SnO is detected by x-ray diffraction, x-ray photoelectron spectroscopy and Raman spectroscopy for some but not all films, reaffirming the need for careful control of both substrate temperature and flux ratio if single-phase films are desired. We will also describe results of prototype Schottky device fabrication experiments.

Binary wide bandgap oxides including tin-doped indium oxide (ITO) are widely used as transparent conductors. The deposition methods range from solution growth through laser ablation and rf magnetron sputtering. However, several wide bandgap oxides, namely In2O3, SnO2, and β-Ga2O3 offer rich semiconductor behavior and potential novel device applications - particularly if grown with techniques such as molecular beam epitaxy (JMBE) or metalorganic chemical vapor deposition (MOCVD).
In this presentation, we review our work on the heteroepitaxial growth by plasma-assisted MBE of SnO2 and In2O3 and initial homoepitaxial growth studies of In2O3 and β-Ga2O3.
For SnO2 heterepitaxial MBE growth on r-plane sapphire yields (101) oriented films. The growth shows novel behavior, for constant oxygen flux, increasing metal flux in the oxygen-rich regime leads to increased growth rate. Surprisingly metal rich growth leads to a subsequent decrease in growth rate which is attributed to volatile suboxide formation. Systematic doping studies show that Sb is a shallow donor in SnO2 and group III elements, including In, are deep acceptors. By systematically changing the intentional donor or acceptor concentration, the conductivity of SnO2 could be changed by eight orders of magnitude.
In2O3 has a a bixbyite structure which can be considered as a 2x2x2 fuorite structure with 1/4 of the anion sites missing and associated local anion and cation site relaxation. Yttria stabilized zirconia (fluorite structure)is a natural growth substrate for In2O3. Heteroepitaxy studies were performed on (001) and (111) YSZ substrates. In the fuorite and bixbyite sturctures, (001) is an unstable crystal plane due to its high charge. In contrast, (111) is a very stable crystal plane. Here, we developed a multistep growth procedure to realize smooth (001) films under metal-rich growth conditions. In contrast, smooth (111) films could be realized for either oxygen-rich or metal-rich growth conditions. Systematic doping studies show that Sn, as expected, is a shallow donor in In2O3 whereas Mg is a deep acceptor. Systematic doping studies show that the conductivity can be controlled by nine orders of magnitude.
Structural studies by high resolution x-ray diffraction, conventional scattering contrast and high resolution TEM and HAADF STEM, show that the In2O3 films on YSZ have anitphase domains and rotation domains correspond to the Volmer-Weber island size at coalescence.
Both SnO2 and In2O3 have a strong propensity to form a surface electron accumulation layer (SEAL). Unlike semiconductors such as InN, in which the SEAL is nearly impossible to avoid, light ion processing of SnO2 and In2O3 can eliminate the SEAL. We show systematic XPS studies to support these conclusions.
Finally, we discuss systematic processing and transport studies to identify the origin of unintentional donors in SnO2 and In2O3.

We have studied the optoelectronic properties of SnO grown by pulse laser deposition on YSZ(001) substrates. Our films display an extrapolated optical gap of ~ 2.6 eV, with a Hall measured p-type hole concentration of 1 x10¹⁷ cm^-3 and mobility of mu; = 7 cm²V^-1s^-1. X-ray photoelectron spectroscopy (XPS) revealed a thin SnO₂ overlayer capping the SnO bulk. We employed a combination of bulk-sensitive O K-edge x-ray emission/absorption spectroscopy with hard (hv = 4 keV) XPS to determine the (partial) density of states (PDOS) above and below the Fermi level. We directly compared our experimental results with density functional theory calculations (DFT) incorporating van der Waals corrections. The band structure calculations confirmed the combination of a small (fundamental) indirect band gap of 0.61 eV and a larger (optical) direct band gap of 2.71 eV. We note excellent agreement between our experimental spectra and the DFT calculated PDOS (cross-section weighted and convoluted with instrumental broadening). We confirm that the valence band maximum (VBM) has significant Sn 5s orbital character, with a small fundamental band gap. The natural band alignment between the SnO and SnO₂ overlayer reveals a large VBM offset, which should facilitate both n & p-type doping. We conclude that the origin of the high hole mobility, bipolar doping ability, and reasonable optical transparency of SnO is a result of: 1) significant Sn 5s character at the VBM (due to O 2p-Sn 5s anti-bonding character associated with the lone pair distortion); 2) the combination of a small indirect band gap of 0.61 eV with a much larger direct band gap of 2.71 eV; and, 3) the location of band edges with respect to the vacuum level.

The anisotropic dielectric function of rutile SnO₂ is presented spanning phonon frequencies up to interband transitions (from 0.04 eV up to 20 eV). The results were obtained on several molecular-beam epitaxy grown films grown on (110) TiO₂ and on r-plane Al₂O₃ substrates. The SnO₂ layers have their optical axis in plane or inclined, respectively. After modeling the optical response of the layer stack and taking surface roughness into account, the components of the dielectric tensor were extracted and compared to recent theoretical calculations. It turns out that electron-hole interaction influences the dielectric function up to 20 eV resulting in a pronounced redshift and redistribution of the oscillator strength of features related to van Hove singularities. The data emphasize a band gap value for SnO₂ of approx. 3.5 eV at room temperature.
On a series of n-type doped SnO₂:Sb layers, the plasmon-longitudinal optical phonon coupling was studied as a function of carrier concentration. The rutile crystal structure allows three phonon-polariton pairs for the electric field vector perpendicular to the optical axis, yielding four plasmon-phonon coupled mode branches. In contrast, for the electric field vector parallel to the optical axis only two plasmon-phonon coupled modes are observed. Carrier concentration dependent effective electron masses are obtained and will be discussed in detail.

Indium oxide ( In₂O₃ ) is a wide bandgap semiconducting oxide material with the band gap around 2.8 eV. By heavy tin (Sn) doping, In₂O₃ becomes indium tin oxide (ITO), a transparent conducting oxide (TCO) which is widely used as transparent contact. For semiconductor device applications, In₂O₃ is used in gas sensors due to a surface electron accumulation layer (SEAL). However, the SEAL hampers other semiconductor device applications such as Schottky diodes and field effect transistors, as also observed in other wide bandgap semiconducting oxides such as SnO₂ and ZnO. Understanding the origins of the SEAL may contribute largely to putting these oxides in practical realization of these semiconductor devices. In this report we investigate the SEAL by hard x-ray photoelectron spectroscopy (HX-PES) with in-situ cleaved, melt-grown bulk In₂O₃ single crystals. The spectroscopy was performed at room and low temperatures, which enabled us to separate semiconducting behavior from the SEAL and the bulk region (probing depth ~20nm). The in-situ sample cleaving was performed in an interior chamber under vacuum below 1 × 10^-5 Pa. After cleaving, the samples were loaded to the measurement chamber with the vacuum level below 1 × 10^-6 Pa. The surface electronic states were measured with HX-PES in the SPring-8 BL15XU undulator beamline at the temperatures of 300 and 50 K. The incident x-ray energy and the total energy resolution were 5.95 keV and 240 meV, respectively. At the uncleaved surface, a SEAL was confirmed by the downward band bending toward the surface and a well-defined conduction band feature (CBF, associated with conduction band states including donor states) at the onset of the Fermi energy. In addition, in-gap states, likely related to defects, were detected. Even at the in-situ cleaved In₂O₃, a SEAL was confirmed with a weaker CBF and weaker in-gap states. These results suggest the SEAL to be an intrinsic feature of In₂O₃, which can be enhanced by near surface defects. The details of the temperature dependence of HX-PES and the effect of oxygen annealing will be discussed.
The authors are grateful to HiSOR, Hiroshima Univ. and JAEA/SPring-8 for the development of HX-PES at BL15XU of SPring-8. The HX-PES measurements were performed under the approval of the NIMS Beamline Station (Proposal No. 2012A 4614 and 2012B 4606).

Charge ordering is a real space ordering of valence electrons and the charge ordering phase transition is accompanied by an abrupt change in resistivity. This change in resistivity is a promising feature that can be taken advantage of and used as a basis for electronic devices. Lanthanum strontium ferrite, LaxSr1-xFeO3, (LSFO) thin films were deposited on STO (001) substrates using molecular beam epitaxy and have been found to charge order through resistivity and synchrotron x-ray measurements. Ionic liquids are becoming a widespread means of forcing charge accumulation at an interface. Charge accumulation in a biased ionic liquid could be used to mediate the charge ordered state in LSFO and help establish the basis of a charge ordered device. Ionic liquids have been used to gate a variety of materials; however, their use is controversial because electrochemical reactions can occur between the film and the ionic liquid under certain biasing conditions. Spatially resolved observations of the thin film / ionic liquid interface have not previously been made. Using transmission electron microscopy (TEM) we analyze the system at the nanoscale and employing electron energy loss spectroscopy in the scanning transmission electron microscope we can probe the local chemical and electrical properties of the thin film / ionic liquid interface to assess the extent and character of charge accumulation. Finally, by examining these samples with a cryo-holder in the TEM we observe the effect on the charge ordering transition at varying temperatures. Our findings indicate that while a change in properties may appear to be due to charge accumulation, an electrochemical reaction between the thin film and ionic liquid can also occur and result in the observed change.
This research was supported by the Office of Naval Research under grant N00014-11-1-0664. This research was supported in part by Oak Ridge National Laboratory's Shared Research Equipment (ShaRE) User Program (JCI), which is sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy.

The unusual properties of ZnO surfaces have fueled a number of debates concerning their role in catalysis, gas sensing, and electronic devices. In the case of polar ZnO surfaces, an impressive amount of literature has been devoted to their very existence, given the high ionicity of the Zn-O bond. Furthermore, many of the electrical and optical properties of ZnO, such as Schottky barrier height, work function, x-ray photoemission, and photoluminescence are now known to vary significantly with crystallographic polarity. To add even more interest, ZnO is a member of a relatively small class of materials that includes In₂O₃, InN, and CdO whose surfaces show evidence of carrier accumulation in quasi-2D near surface potential wells and whose charge neutrality level unusually lies outside of the band gap in the first conduction band [1-3].
Under most laboratory and device preparation conditions, the ZnO surface is saturated with hydroxyl groups which appear to strongly influence the band bending in the near-surface region. In this paper, we investigate the relationship between this hydroxyl coverage and the electronic nature of the near-surface region and show that in certain situations these can be carefully controlled with important implications for device fabrication.
[1] King et al., Phys. Rev. B 79, 035203 (2009).
[2] Allen et al., Phys. Rev. B 81, 075211 (2010).
[3] Ozawa et al., Phys. Rev. B 83, 125406 (2011).

Since a ZnO crystal has a wurtzite-type structure, it shows spontaneous polarization along the c-axis and thereby has polar surfaces corresponding to the O-terminated face (c(-)-face) and Zn-terminated face (c(+)-face). Various properties of ZnO depend on its polarity such as surface electronic structure, chemical stability of the surface, and impurity incorporation. Therefore, when designing devices using ZnO, it is important to develop a crystal growth technology to enable the polarity selective growth of ZnO films. Recently, we found that heavily doping Al in ZnO causes a polarity change. This finding indicates that impurity doping affects the polarity of ZnO films. In this presentation, the effects of doping with Ga, In, Sb and F on the polarity and properties of ZnO films will be reported.

Ab initio based surface phase diagrams, which depict the stability of surface phases at given environmental conditions as specified by temperature and pressure [1], have proven to be an important tool in various areas of application, such as semiconductor physics or catalysis. Obtaining surface phase diagrams showing the stability of phases in an electrochemical environment appears quite desirable, but modelling electrochemical system presents a challenge to first-principles methods.
We have developed a method which links ab initio calculations quite naturally to experimental observables, such as the pH-scale and the electrode potential, which determine and characterise the state of an electrochemical system. It is based on the formation energies of ions in solution and is similar to the approach used in the defect chemistry of semiconductors [2]. We utilise our recently developed method to explore the stability of the polar Zn terminated ZnO(0001) surfaces in an electrochemical environment.
[1] M. Valtiner, M. Todorova, G. Grundmeier, J. Neugebauer, Phys. Rev. Lett. 103, 065502 (2009).
[2] C.G. Van de Walle and J. Neugebauer, J. Appl. Phys. 95, 3851 (2004).

Recently, the transparent electronics have attracted great attention for the next generation display industry. Transparent oxide thin film transistors (TFTs) have been extensively investigated as the solution of transparent TFTs due to their excellent electrical and optical characteristics. In particular, amorphous indium-gallium-zinc oxide (a-IGZO) TFTs have shown excellent electrical properties, including high field effect mobility, high optical transparency and low processing temperature, which make these transistors very promising alternatives to amorphous silicon (a-Si) TFTs, especially in a backplanes for flexible displays or transparent active matrix organic light emitting diode (AMOLED) displays.
In this study, we investigated the electrical behaviours of Au nanoparticles embedded in the channel of a-IGZO TFTs. The Au NPs were embedded at each position from 20 nm above the front channel to the back channel of a-IGZO. In case of the Au nanoparticles (NPs) on the back channel surface of a-IGZO, these TFTs showed the enhanced stability of threshold voltage (Vth). On the other hand, as the Au NPs are embedded close to the front channel of a-IGZO, the Vth values of a-IGZO TFTs with Au NPs were gradually shifted in the positive direction compared to that without Au NPs. This can be understood by considering that the electron trapping into Au NPs seems to be dominant rather than the electron injection into a-IGZO, due to the reduction of electric field in a-IGZO below the Au NPs. Therefore, it is believed that the Au NPs embedded close to the front channel of a-IGZO can act as electron trap sites. The Au NPs embedded in the a-IGZO channel layer caused the increase of off-current. In addition, as the size of Au NPs decreased from about 3.6 to 2.2 nm, the off-current of a-IGZO TFTs with Au NPs was decreased without change of the Vth values. We supposed that the embedded Au NPs can form the electron conduction path causing the current leakage of a-IGZO TFTs. When the size of Au NPs is about 2.2 nm, the electron transport between Au NPs seems to be rarely occurred because of the distance among Au NPs. On the basis of above results, the Vth values of a-IGZO TFTs with Au NPs can be controlled without increase of the off-current using the Au NPs smaller than about 2.2 nm.

Amorphous oxide semiconductors (AOS) are considered prime candidates as channel material for thin-film transistors for display backplanes, because of their superior charge carrier mobility compared to amorphous silicon or organic semiconductors, combined with their low-temperature processability [1]. When processed at low temperatures, however, amorphous IGZO has a high concentration of subgap states. Depending on their energy level, these states have a negative impact on the TFTs performance, to cite a few: deterioration of subthreshold slope, shift of the threshold voltage, reduction of the mobility and bias stress instability. Even though significant work has been devoted to modeling and understanding charge trapping and related instability [2,3], the relationship between trap density of states and material preparation still needs to be determined. This discordance can be attributed to the wide range of employed characterization techniques and relatively complex device structures. Deep Level Transient Spectroscopy (DLTS) and transient capacitance measurements have the great advantage over other techniques of indicating directly the trap profile without the need of complex calculations.
Here, we employed for the first time a Schottky diode [4,5], the simplest semiconductor device, to characterize the trap distribution in amorphous IGZO. DLTS and low-frequency noise measurements were complementary employed to extract the trap density and its energy distribution. Transient capacitance measurements at different temperatures indicate an acceptor-like exponential distribution of states described as:
N_t=N_TA exp[((-(E_C-E))/W_TA )],
where E is the state energy, E_C is the conduction band edge, N_TA is the intercept energy at E_C, and W_TA is the characteristic decay energy. Our measurements reveal W_TA and N_TA equal to 90meV and 10¹⁹cm^-3eV^-1, respectively. At E_C-E> 0.7eV, the trap density saturates at ~10¹⁵cm^-3eV^-1. Moreover, by varying the bias and reverse voltage pulses and keeping the pulse height fixed, we found out that the traps are not uniformly distributed within the IGZO film. The same Schottky diodes shows a 1/f behavior at low frequencies (f<200Hz) and 1/f^γ (with γ < 1) at higher frequencies. It was shown [6,7] that the presence of 1/f^γ is related to the presence of the exponential tail states and the factor γ can be easily used to calculate W_TA, which gave a value of 110meV, in reasonable agreement with the DLTS results.
[1]K. Nomura et al., Nature 432, 488 (2004).
[2]H. Hsieh et al., Appl. Phys. Lett. 92, 133503 (2008).
[3]M. Kimura et al., Appl. Phys. Lett. 92, 133512 (2008).
[4]A. Chasin et al., Appl. Phys. Lett. 101, 113505 (2012).
[5]A. Chasin et al., IEDM Tech. Dig., 12.4.1-12.4.4 (2012).
[6]J.I.Lee et al., Solid-State Electron. 43, 2181 (1999).
[7]C. A. Dimitriadis et al., J. Appl. Phys. 83, 1469 (1998).

In recent years, a few researches about germanium doping in oxide semiconductors have been reported. Germanium involved conducting oxide is first reported by Hosono et al. at 2011 [1]. They formed Strontium-germanium-oxide (SrGeO_3), and converted structure of SrGeO_3. Therefore, a band gap of SrGeO_3 is reduced and a conductivity is increased. In similar approach, Gong et al. shows the results about germanium-doped indium-zinc-oxide (Ge:InZnO) as conducting materials [2]. In certain germanium ratio greater than 10%, Ge:InZnO compound exhibits 2 times higher conductivity than pristine InZnO.
In this research, we investigated solution-processed Ge:InZnO thin-film transistors (TFTs) for the first time. We prepared the Ge:InZnO solution with different content of germanium from 0% to 15%. The bottom-gate and top-contact structure was used and Ge:InZnO active layer was spin-coated on the heavily doped p-type Si wafer with thermally oxidized SiO2 of 1200 Å, and those samples were annealed at 500°C for 3 hours. And aluminum electrodes were deposited each sample by sputter. We investigated the chemical and electrical properties of solution-processed Ge:InZnO TFTs. In 5% germanium content of Ge:InZnO thin-films, on/off ratio was ~10^6. As the germanium content was increased, the electrical conductivity of Ge:InZnO thin-film was increased. The added germanium affected the band gap of Ge:InZnO thin-films narrower. These results imply that germanium could perform the role of carrier supplier in InZnO: significant increase in the electrical conductivity of Ge:InZnO thin-films.
[1] Hiroshi Mizoguchi, Toshio Kamiya, Satoru Matsuishi, and Hideo Hosono, Nature Communications 2, 470 (2011).
[2] Jian Sun, Weng Soon Lai, Weifeng Yang, and Hao Gong, EPL (Europhysics Letters) 100 (1), 17003 (2012).

As an amorphous transparent semiconducting oxide (TSO) compatible with low temperature deposition and with a relatively high mobility (> 10 cm²/Vs) a-IGZO is of high technological importance. The stability of thin film transistors (TFTs) with a-IGZO active layers is known to be degraded under negative bias illumination stress by the presence of sub bandgap states near the valance band maximum. The elimination of these states while maintaining good semiconducting properties is highly desirable. We explore the defect state related origin of the subgap states in amorphous indium gallium zinc oxide (a-IGZO) thin films grown by dc-magnetron sputtering. We employed a combination of x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), transmission electron microscopy, (TEM) and optical absorption spectroscopy to fully explore the possible origins of the subgap states. The subgap state free nature of films grown under oxygen rich conditions was verified by comparing hard x-ray photoelectron spectroscopy (HAXPES) and soft XPS of the valance band as well as by optical absorption spectroscopy. Comparison of the In 3d core regions showed no correlation between In charge state and the presence of the subgap feature. Argon sputtering introduced a subgap feature in samples that originally were free of this feature. The origin of the subgap feature is attributed to unrelaxed oxygen environments, specifically under-coordination of oxygen by indium, induced by preferential sputtering. Subsequent low temperature vacuum annealing (200 °C for ~20 minutes) eliminated the induced subgap feature. Another crucial device parameter is the work function of the active material. We determined the work function of our a-IGZO films (carrier concentration ~10¹⁹cm^-3) to be ~4.48 eV by measuring the energy difference between the Fermi level and the onset of the secondary electron peak in the UPS spectra.

The electronic structures of the crystalline InGaZnO (IGZO) system were investigated using ab initio density functional theoretical calculations and soft x-ray absorption spectroscopies. Detailed analyses on the orbital characters revealed that a strong orbital hybridization between the In 5s and sd orbital states occurs in the energy range near the conduction band minimum (CBM). The tail-like features at the CBM, which can significantly influence the electrical properties, are contributed mainly by the s-d(3z2-r2) mixed state. This anisotropic hybridized orbital state endows the IGZO system an untra-sensitivity to the local structural change along the c-axis. Results of our calculations further suggest that the non-centrosymmetric distortion of the ZnO5 coordination can induce an elongation of the InO6-O bonds along c-axis lifting up the energy of the s-d(3z2-r2) mixed state. Therefore, the difference in the electronic structure between InGaZnO4 and In2Ga2ZnO7 should be attributed to the abundance of the distorted ZnO5 near the InO6 clusters. In amorphous IGZO, the tendency in the electronic structural evolution upon the compositional change is similar but weakened due to the weaker anisotropy in the s-d mixed states.

The wide band gap semiconductor β-Ga₂O₃ (E_g = 4.9 eV at room temperature) is a promising material for the realization of solar blind photodetectors[1-3]. To ensure photodetection in the full solar blind spectral range (> 4.43 eV) it is necessary to lower the band gap of binary β-Ga₂O₃[4,5].
In this contribution we present the structural and optical properties of (Ga_1-xIn_x)₂O₃ thin film grown by pulsed laser deposition and the properties of Schottky contacts (SCs) and photodetectors (PDs) thereon.
The thin film was deposited on a 2 inch c-plane sapphire substrate using a continous composition spread (CCS) technique. The growth temperature was 650 °C and the oxygen partial pressure 3×10^-4 mbar. Our CCS approach results in x values of 0.01 on the one side and 0.87 on the other side of the wafer. XRD wide angle scans clarified that the thin films are (-2 0 1)-orientated in the monoclinic crystal structure similar to binary β-Ga₂O₃ up to an In concentration of x=0.2. For higher In content a cubic phase corresponding to the crystal structure of In₂O₃ is observed besides the monoclinic phase. The optical transmission of our thin film shows a strong shift of the absorption edge to lower energies with increasing In incorporation. This results in a variation of the optical band gap from 4.98 eV (lowest In content) to 3.59 eV (highest In content).
On the (Ga_1-xIn_x)₂O₃ thin films SCs were realized by reactive direct current sputtering of platinum. They typically show an effective barrier height of about Phi;_B=0.95 V and an ideality factor of eta;=3.5. Annealing the contacts for 10 min at 400 °C in nitrogen atmosphere increases the barrier height to Phi;_B=1.20 V and decreases the ideality factor to eta;=2.2. The current on/off ratio determined at +2V and -2V is above 10⁵ for the best SCs.
To investigate the photoresponse a metal-semiconductor-metal contact structure was reactively sputtered on thin films with In concentrations up to x=0.2. According to the absorption edge in the transmission spetra the onset of the photoresponse shifts to lower energies with increasing In content. The maximum of the responsivity of the investigated MSM-PDs is between 4.93 eV (lowest In content) and 4.46 eV (x=0.2). We observed a gain mechanism based on the trapping of photogenerated holes at the metal-semiconductor interface[6,7] which is responsible for quantum efficiencies exceeding the maximum theoretical value of 1.
[1] Y. Kokubun et al., Appl. Phys. Lett. 90, 031912 (2007)
[2] R. Suzuki et al., Appl. Phys. Lett 94, 222102 (2009)
[3] J. J. Delaunay et al., Adv. Funct. Mater. 20, 3972-3978 (2010)
[4] T. Oshima et al., Phys. Stat. Sol. 9, 3113-3115 (2008)
[5] Y. Kokubun et al., Phys. Stat. Sol. 7, 1741-1745 (2010)
[6] O. Katz et al., Appl. Phys. Lett. 79, 1417 (2001)
[7] Z. Zhang et al.,Appl. Phys. Lett. 99, 083502 (2011)

A promising oxide semiconductor for application in high power electronics and transparent photonic devices is β-gallium oxide (Ga₂O₃) with a band gap of about 4.9 eV.
In this contribution we present investigations on the preparation and the properties of Schottky contacts (SCs) on Ga₂O₃ thin films. Using pulsed laser deposition (PLD), the thin films were grown from a Ga₂O₃ target with 1% SiO₂ on c-plane sapphire or (100) magnesium oxide (MgO) substrates. Further, we have grown samples on a highly conducting Ga-doped zinc oxide (ZnO:Ga) layer on an a-plane sapphire substrate, acting as an ohmic back contact in order to reduce the series resistance of the SCs as shown previously for SCs on ZnO thin films [1]. The ohmic contacts were fabricated by thermal evaporation of Ti and Al as reported in [2]. Subsequently, we prepared SCs by reactive and inert DC sputtering of different metals like Cu, Ag or Nb.
The I-V characteristics of such SCs showed a rectifying behaviour with rectification ratios at V = ±2 V larger than 8 orders of magnitude for some of the Cu contacts. The dominant current transport mechanism is thermionic emission. By fitting the I-V characteristics, we determined ideality factors n as low as 1.2 and effective barrier heights Phi;_B up to 1.06 eV for the best inert sputtered Cu and Ag contacts at room temperature. Reactive sputtering of the SCs increases the barrier height further, but it also increases the series resistance. Temperature dependent I-V measurements between T = 100 K and T = 320 K yielded a linear dependence of Phi;_B and n^-1minus;1 on the inverse temperature in accordance to the theory of thermionic emission in the presence of a laterally inhomogeneous barrier. The mean barrier height Phi;_B^m for the Cu contacts is determined to be 1.3 eV with a standard deviation σ_F of 0.13 eV. The same value of 1.3 eV was determined for the effective barrier height at a temperature of 550 K, were the ideality factor goes down to 1.04 and is therefore close to the ideality factor expected for a homogeneous barrier due to image charge effects. By applying high voltages in the reverse direction, we estimated the breakdown field of the reactively sputtered Nb contacts to be about 1.2 MV/cm. Further, by using the back contact layer, the series resistance could be reduced by almost 2 orders of magnitude to values of about 100 Omega;. This enabled us to perform C-V and admittance spectroscopic measurements up to high frequencies in order to investigate the doping profile and shallow defects of the Ga₂O₃.
We performed the deposition of the Ga₂O₃ at an oxygen partial pressure of p = 10^-3 mbar on the c-plane sapphire substrates and the ZnO:Ga layer or at p = 6 × 10^-4 mbar on the (100) MgO substrates in order to obtain single phase (-201)- or (100)-oriented thin films, respectively.
[1] H. von Wenckstern et al., Appl. Phys. Lett. 88, 092102 (2006)
[2] E. G. Villora et al., Appl. Phys. Lett. 92, 202118 (2008)

To obtain an oxide thin film with optimal electronic properties, it is often desirable to minimize the number of dislocations. An important parameter in controlling the dislocation density in thin film is the critical thickness, which is the film thickness beyond which the nucleation of misfit dislocations becomes energetically favorable to relieve the epitaxial strain in a process called relaxation. Exceeding this thickness will greatly increase the dislocation density of a film and degrade its electronic, magnetic and dielectric properties. Here we focus on determining the experimental critical thickness of SrTiO₃ on (La0.3Sr0.7)(Al0.65 Ta0.35 )O3 (001) (LSAT) substrate using high resolution x-ray diffraction (HRXRD) and reflection high energy electron diffraction (RHEED) and investigating the effect of the substrate temperature, and oxygen pressure on the rate of strain relaxation. Because the lattice parameter of LSAT (3.868 Å) is smaller than that of SrTiO₃ (3.905 Å), the SrTiO3 films will be under a biaxial compressive strain of -0.94%. Using elastic constants of SrTiO3, an out-of-plane lattice parameter of 3.929 Å is calculated for a coherently strained SrTiO3 (001) film on LSAT (001).
In this presentation, we will present a detailed study of strain relaxation in SrTiO₃ films grown on LSAT (001) substrate by hybrid molecular beam epitaxy (MBE). The out-of-plane and the in-plane lattice parameter measurements (measured in high resolution x-ray diffraction) were used as probes to determine the critical thickness. While Mathew-Blakeslee analysis predicts a critical thickness of 12 nm beyond which lattice parameters of a fully coherent film begin to relax towards the bulk values, remarkably no measurable changes in the lattice parameters were observed even for films with thickness of 180 nm (measured in grazing incidence x-ray diffraction), indicating a critical thickness larger than 180 nm. It was found that the starting substrate morphology (roughness) could have significant influence on the strain relaxation and mosaicity of the film. A detail discussion on the origin of such a large critical thickness will be presented.
Finally we will discuss the influence of strain on low temperature transport properties of oxygen vacancydoped and Nd-doped SrTiO₃ films. We will also present the consequences of high dislocation density on their transport properties, particularly for the insight they provide between the interplay between carrier mobility and structural defects.

Topological insulators (TI) have drawn widespread attention1 due to their unique transport properties with potential applications in spintronics and quantum computing. These materials are characterized by conducting surface states while the bulk shows insulating nature. It has been theoretically predicted2 that Sr3SnO (SSO) exhibits strong topological insulating behavior as it reveals band inversion at the gamma point, but there has not been experimental confirmation. In this work, we report SSO grown epitaxially on Si (001) by pulsed laser deposition. The epitaxial integration with Si (001) was achieved by using epitaxial cubic yttria-stabilized zirconia (c-YSZ) as an intermediate buffer layer, which was grown by in situ single camber process. An optimized alternating target laser ablation deposition technique has been developed to grow SSO single crystal thin films from strontium monoxide SrO and tin dioxide SnO2 targets. Detailed X-ray theta;-2theta;, and phi;-scans were performed for structural characterization of the SSO/c-YSZ/Si(001) heterostructure. The electron diffraction patterns of these films confirmed the single crystal nature of the films with cube-on-cube epitaxy. All the constituent layers of the heterostructure were found to be epitaxial with orientation relationship: (001)[100]SSO aligned with (001)[100] c-YSZ aligned with (001)[100]Si. The resulting depth profiles by time-of-flight secondary ion mass spectrometry (ToF-SIMS) confirms homogeneity in the film composition. The resistivity of SSO thin films were found to decrease with increasing temperature, indicating semiconductor behavior. The Ultraviolet photoelectron spectroscopy (UPS) study on the SSO films show the existence of surface states extending up to the Fermi energy, indicating the existence of a conducting charge layer at the surface, such states could be “topological surface states (TSS)” on PLD-grown SSO films.
1H. Chen et,al ., Phys. Rev. B 85, 195113 (2012).
2M Klintenberg, arXiv:1007.4838v1 [cond-mat.mtrl-sci].

Pb(Zr,Ti)O3 (PZT) thin films have been widely used as piezoelectric materials because of their excellent piezoelectric properties. However, the lead contained in the PZT is harmful. Thus, the development of lead-free piezoelectric materials with excellent piezoelectric properties has attracted much attention from an environmental viewpoint. (K0.5Na0.5)NbO3 (NKN) is known to a promising candidate for lead-free piezoelectric materials, since high Curie temperature of 420 C and piezoelectric constant of 160 pC/N. In particular, 0.92(Na0.5K0.5)NbO3-0.06BaZrO3- 0.02(Bi0.5Li0.5)TiO3 (NKN-BZ-BLT) ceramics developed in our group in 2007 showed high Curie temperature of 243 C and piezoelectric constant of 420 pC/N comparable with those of commercial PZTs. Previously we had reported epitaxial growth of (Na1-xKx)NbO3 (x=0.3-0.7) films on (100) SrTiO3 substrates as the first step to the deposition of films with a complex composition of BaZrO3 and (Bi,Li)TiO3 containing (Na,K)NbO3.1) In this study, NKN-BZ-BLT films were epitaxially grown on SrTiO3 substrates by a pulsed laser deposition (PLD). Effects of oxygen pressure during the deposition on crystallinity and various electric properties were discussed.
Substrate temperature for the epitaxial growth was fixed to 800 C according to the results obtained previous work.1) The FWHM (full-width at half maximum) estimated from rocking curve measurements ranged from 0.25 deg. to 0.55 deg., which were larger than those obtained for the NKN films, suggesting lower crystallinity than epitaxial NKN films. A minimum FWHM of 0.25 deg. was found for the films deposited in an oxygen pressure of 60 Pa. A reciprocal space map of the films around 103 spot of perovskite structure showed two spots assigned to the film and SrTiO3 substrate, indicating that the films were epitaxially grown on substrate. Lattice constants of the film deposited in an oxygen pressure of 60 Pa were a=0.400 nm and c=0.402 nm, which were larger than those of NKN films. The larger constant was explained by the substitution of large ions such as Ba2+ and Zr4+ into Na+ or K+ and Nb5+ sites, respectively. Electric properties such as relative permittivity and loss tangent as a function of frequency and piezoelectric constant will be discussed.
1) K. Sakurai et al., 2012 MRS Fall Meeting, Z13. 35

We report direct evidence of conduction band filling in the cubic transparent pervoskite BaSnO₃ by La-doping, along with determining the orbital character of the conduction band minimum (CBM). Thin (100 nm) films of 3% La-doped BaSnO₃ were grown on SrTiO₃ by pulsed laser deposition. The La doping and corresponding filling of the conduction band were confirmed using hard x-ray photoelectron spectroscopy (HAXPES). The corresponding laboratory-based x-ray photoelectron spectroscopy reported the same Fermi level to valence band maximum (VBM) energetic separation of 3.48 eV as with HAXPES, but no band filling could be observed. The enhancement of the Sn 5s cross-section using HAXPES suggests the CBM has signicant Sn 5s and weak O 2p orbital character. Conduction band filling is confirmed by direct comparison with hybrid density functional theory (DFT) calculations and supports a 3.2 eV indirect band gap. The use of hybrid DFT is verified by excellent agreement between our photoemission spectra and O K-edge x-ray emission & absorption spectra. Our experimental and computational results demonstrate that the conduction band is primarily of Sn 5s orbital character with little O 2p contribution, which is a prerequisite for designing a perovskite-based n-type transparent conducting oxide (TCO). Our results highlight the potential of La-doped BaSnO₃ as a suitable earth abundant TCO alternative to Sn-doped In₂O₃ (ITO), especially in the emerging realm of perovskite-dominated "oxide electronics" where ITO cannot compete.

A self-consistent phase-field model has been proposed to study the current-voltage (I-V) characteristics in a ferroelectric BaTiO₃ single crystal. We modeled the Ag/BaTiO₃/Ag sandwiched structure assuming a [001] oriented single tetragonal BaTiO₃ domain at room temperature. The capacitor is subject to a sweep bias along [001] direction from -1.0V to 1.0V. Our simulation shows switchable asymmetric I-V response and rectification ratios up to 10² due to the strong ferroelectric polarization in BaTiO₃. The net polarization induced charges result in the inhomogeneous redistribution of ionic and electronic charge carriers in equilibrium state. And the enhancement and inhibit of electronic carrier concentration under forward and reverse bias are considered to account for the diode-like I-V behaviors. The dependences of I-V asymmetry and rectification ratios on the amount of polarization bound charges at metal/ferroelectric interfaces have also been studied.

LaNiO3 thin films were deposited on SrLaAlO4 (100) and SrLaAlO4 (001) single crystal substrates by a chemical solution deposition method and heat-treated in oxygen atmosphere at 700oC in tube oven. Structural, morphological, and electrical properties of the LaNiO3 thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), and electrical resistivity as temperature function (Hall measurements). The X-ray diffraction data indicated good crystallinity and a structural preferential orientation. The LaNiO3 thin films have a very flat surface and no droplet was found on their surfaces. Samples of LaNiO3 grown onto (100) and (001) oriented SrLaAlO4 single crystal substrates reveled average grain size by AFM approximately 15-30 and 20-35 nm, respectively. Transport characteristics observed were clearly dependent upon the substrate orientation which exhibited a metal-to-insulator transition. The underlying mechanism is a result of competition between the mobility edge and the Fermi energy through the occupation of electron states which in turn is controlled by the disorder level induced by different growth surfaces.

Doped peroviskite-type oxides have been extensively investigated because of their optical, ferroelectric, and semiconducting properties. However, most of the prior studies have been carried out with broad beam techniques such as X-ray diffraction. Therefore, very small local compositional changes and nanoscale phase precipitates could be not fully detected or understood. In the particular case of a commercially available phosphor, (Pr,Al)-doped SrTiO3 powders, the occurrence and enhancement of its red emission has been attributed to the modified local crystal field around activated Pr dopants based on a theoretically structural model where Al dopants replace Ti atoms around Pr dopants sitting at Sr-sites.
Using an aberration-corrected transmission electron microscope (TEM), we report the first atomic-level two-dimensional elemental maps of Pr dopants within a Pr-implanted SrTiO3 crystal using the electron energy loss spectroscopy (EELS) technique. We observe cuboidal-shaped nano-clusters of about 0.8-2.4 nm in size based on atomic resolved high-angle annular dark-field (HAADF) images. In the simultaneously acquired hybrid annular bright-field (ABF)- low-angle dark-field (LADF) images, these nano-clusters have enhanced intensity. The contrast is attributed to the local compositional changes including the enrichment of Al dopants. The local changes in composition also leads to the distortion of Ti-O groups, which is confirmed by numerically extracted Ti L2,3-edge spectra of these nano-clusters. The methods of this numerically extraction was reported and demonstrated on the surfaces of SrTiO3 crystals [1].
As a result of the local structural changes, the neighboring Al dopants around a Pr dopant appear to blue-shift the red emission peak of this material by introducing low-lying defect states. The presence of this defect states is further suggested by the low-loss spectra of the implanted crystals and the ellipsometric spectra. Based on the above experimental evidences we provide the local defect configurations within (Pr, Al) implanted SrTiO3 and thus provide insight into its mechanism of the red light emission.
Reference:
[1] G.-z. Zhu, G. Radtke and G. A. Botton, Nature, 2012, 490, 384.
[2] G.-z. Zhu et. al., Phys. Chem. Chem. Phys., 2013, 15(27), 11420-11426.
[3] We are grateful to NSERC for Discovery and Strategic Grants supporting this work.

Transparent solid-state circuit systems have been extensively studied as the promising technology for the application in large area electronic systems by stacking clear display, electronic paper or other transparent devices into clear spaces[1],[2]. As one of the critical devices of the circuit systems, the transparent memory devices are needed but it is difficult to realize by the traditional memory technology. Oxide-based resistive random access memory (RRAM), which presents the excellent device performances and data storage functions, shows the great potential in transparent memory application. Although the transparent oxide thin films could be deposited by sputtering, chemical vapor deposition, atomic layer deposition, and pulsed laser deposition, the solution process technology shows the great advantages for the low-cost and large-area film application.
In this work, a solution-processed method is developed to fabricate fully transparent RRAM devices with a configuration of FTO/ZrO2/ITO, where the ZrO2 layer was firstly deposited on FTO substrate by sol-gel and then ITO films were deposited on ZrO2 layer by sol-gel as the top electrodes. Following a furnace annealing treatment at 500°C for 2h, the FTO/ZrO2/ITO based RRAM devices were fabricated. The solution processed FTO/ZrO2/ITO based RRAM devices show the fully transparent and excellent bipolar resistance switching behaviors. No forming process, low operation voltage less than 2V and programming current less than 1mA have been observed. The resistance ratio between high and low resistance states was more than 10, and more than 100 switching cycles and robust data retention have been demonstrated in the fully transparent FTO/ZrO2/ITO based RRAM devices.
[1] J. F. Wager, “Transparent Electronics”, Science, Vol. 300 No.5623, p. 1245, May 2003.
[2] J. Yao, J. Lin, Y.H. Dai, G.D. Ruan, Z. Yan, L. Li, L. Zhong, D. Natelson, J.M. Tour, “Highly transparent nonvolatile resistive memory devices from silicon oxide and grapheme”, Nature Communications Vol.3, No.1101, Oct 2012

Boron Suboxide (B6O) is a compound semiconductor possessing an α-rhombohedral Boron type structure (R3m) and is the second most studied material of the boron-rich solids after B4C. The allowed phase width of B6O is expressed as B12(O2-xBx) where 0 le; x le; 1. In the stoichiometric unit cell of B6O two oxygen atoms are positioned along the hexagonal <111> (c-axis) at the boron-icosahedral interstitial sites 6c, with the single 3b central position left unoccupied. The oxygen atoms donate one electron each to the boron icosahedra and compensate for the electron deficiency of pure α-B. The resulting short interatomic bond lengths and strong covalent bonding establish the bulk properties of B6O, which possess the smallest unit cell volume of the α-boron derivates. This structure is different from the semi-metallic structure of B4C, where the 3b site is occupied and three interstitial atoms form a chain along the c-axis. When stoichiometric (x = 0) B6O is an intrinsic high-resistivity semiconductor with an expected hole mobility at 300K approaching 100 cm2V-1s-1 , a band gap of about 2.7 eV, and a melting point of 2092oC. It is also a Super-Hard class material with a bulk modulus of 230 GPa and a Vickers hardness exceeding 40 GPa. Sub-stoichiometric crystals of B6O contain boron atoms replacing interstial oxygen atoms and possess diminished properties. We will present a low pressure method for producing stoichiometrically correct B6O macroscopic crystals, contrasting this method with alternate approaches in the literature, and discuss applications for B6O which include nuclear detectors and cutting technologies.

One central aspect of perovskite oxides is their strong structure - property coupling. This is also true for epitaxial interfaces, where structural as well as electronic reconstructions are expected to result in novel functional properties. Recently there has been an increased interest in (111)-oriented epitaxial systems, having triangular cation surface symmetry and interfaces sharing three corners of the oxygen octahedral. Here we report on the synthesis of epitaxial step-and-terrace La0.7Sr0.3MnO3 thin films deposited by pulsed laser deposition on (111)-oriented SrTiO3. The thin film surface morphology can be controlled by the choice of substrate preparation technique. By relying on DI-water treatment of the SrTiO3, step-bunched surfaces with faceted step-and- terrace structures following the <1-10> in plane directions are observed. However, by relying on HF-etched substrates, coherent step-and-terrace structures along the [1-10] direction are obtained. In order to investigate how the symmetry of the (111)-oriented surface affects the magnetic properties, as compared to the cubic (001)-surface, we image magnetic domains on the micron scale by X-ray photoemission electron microscopy (X-PEEM). By studying the magnetic domain structure as a function of applied magnetic fields we show that the [11-2] direction, one of the low-index surface directions, is an easy axis. Moreover, in-plane magnetization reversal is accomplished by a non-continuous step-wise switching process with intermediate states. In the presentation we will discuss how the magnetization of these intermediate states relates to the low-index directions, especially the <1-10> and <11-2> directions of the (111)-oriented surface, and correlate with angle dependent macroscopic VSM investigations.

Ga₂O₃ is a semiconductor with a wide band gap of 4.8 eV, which is much larger than those of SiC and GaN. The unique properties of this material, along with the feasibility of simple and low-cost melt growth methods for mass production of bulk single-crystals, have positioned Ga₂O₃ as a strong candidate for new power devices [1]. Recently, we showcased the great potential of Ga₂O₃ electron devices by demonstrating Ga₂O₃ metal-oxide-semiconductor field-effect transistors (MOSFETs) on single-crystal β-Ga₂O₃ (010) substrates [2]. In-depth understanding of the MOS characteristics concerning the Al₂O₃/Ga₂O₃ system in [2] is paramount to the realization of high performance Ga₂O₃ power devices. Moreover, the chemical similarity between Al₂O₃ and Ga₂O₃ may present unique MOS properties unseen in other conventional non-oxide semiconductor systems. In the present study, the structural and electrical properties of the Al₂O₃/Ga₂O₃ interface were investigated by cross-sectional transmission electron microscopy (TEM), current-voltage (I-V) and capacitance-voltage (C-V) measurements on an Al₂O₃/Ga₂O₃ MOS diode structure fabricated on a Ga₂O₃ (010) substrate.
We fabricated the Al₂O₃/Ga₂O₃ MOS diode structure in the following sequence. A 300-nm-thick Sn-doped n-Ga₂O₃ layer was grown on an Fe-doped semi-insulating β-Ga₂O₃ (010) substrate by molecular-beam epitaxy [3]. The Sn doping concentration was 7×10¹⁷ cm^-3. An annealed Ti/Au ohmic contact (cathode) was fabricated on a highly Si-doped region formed in the n-Ga₂O₃ layer by selective-area ion implantation. A 20-nm-thick Al₂O₃ layer was subsequently deposited on the Ga₂O₃ layer by plasma atomic layer deposition (ALD) at 250°C. The anode electrode was formed on the Al₂O₃ with a Ti/Pt/Au metal stack.
The cross-sectional TEM micrograph of the MOS structure indicated the presence of a 3~4-nm-thick polycrystalline interfacial layer on Ga₂O₃ followed by an amorphous layer with a thickness of about 16 nm. This is in contrast to typical ALD Al₂O₃ films that are entirely amorphous. Selective area electron diffraction analysis confirmed that the crystalline structure was γ-Al₂O₃. The I-V characteristics showed an extremely small leakage current, which was less than the lower limitation of the measuring instrument of 0.9 nA/cm2. The C-V curves showed a small hysteresis of 0.8 V and were stretched out, which can be attributed to charge traps in the Al₂O₃ layer and/or at the Al₂O₃/Ga₂O₃ interface.
We have analyzed the structural and electrical properties of Al₂O₃/Ga₂O₃ MOS diodes fabricated on a Ga₂O₃ (010) substrate. The formation of a characteristic polycrystalline γ-Al₂O₃ film on Ga₂O₃ at the initial stage of deposition was observed.
This work was partially supported by NEDO, Japan.
[1] M. Higashiwaki et al., Appl. Phys. Lett. 100, 013504 (2012), [2] M. Higashiwaki et al., Device Research Conference, late news paper (2013), [3] K. Sasaki et al., The 40th Int. Symp. Compound Semicond., TuB3-2, pp.34 (2013).

Electrostriction in general can be found in any piezoelectric material, where the application on an electric field causes a distortion of the material; however, usually hydrostatic pressure is applied in practical experiments. It was shown, e.g., that the photoluminescence in ZnO is shifted to smaller energies upon applying a hydrostatic pressure [1]. The dependence of the structure of ZnO has also been directly studied by x-ray diffraction and extended x-ray absorption fine structure spectroscopy (EXAFS) upon applying hydrostatic pressure [2]. A comparable effect upon applying an electric field and thus indirectly applying pressure through piezoelectric distortion has not been observed yet, although ZnO is of interest as piezoelectric material for surface acoustic wave devices and electrostriction coefficients for ZnO have so far been only derived by theory [3].
Here we will strudy the effect of an external electrical field on epitaxial layers of ZnO and doped ZnO with element specificity by means of x-ray absoption near edge spectroscopy (XANES). We can show that upon applying an electrical field there is aridigd band shift of the entire absoption spectrum which depends linearly on the electrical field. In addition, the magnitude of the shift is shown to be different for the host cation site (Zn) copared to dopants (e.g. Co). This observed effect is discussed in terms of electrostriction and opens new possibilities to study piezoelectricity via XANES, i.e. with element specificity.
[1] G. H. Li et al., phys. stat. solidi (b) 244, 87 (2007)
[2] F. Decremps et al. Phys. Rev. B 68, 104101 (2003)
[3] I. Kornev et al., proceedings 29th Int. Conf. Phys. Semicond., p. 71 (AIP, 2009)

Superconducting circuits find widespread use including high energy-resolution cryogenic radiation detectors, sensitive magnetic detectors, nanomechanical motion sensors and quantum-limited parametric amplifiers. In particular, they have recently attracted much interest for implementation of quantum bits for quantum information processing, communication, and computing architectures. The performance of these devices, which essentially consist of patterned metal on an insulating substrate, is limited by unknown noise sources leading to quantum decoherence in time scales below a practical threshold for measurement. Typical devices are comprised of aluminum superconductor deposited on sapphire (alumina, α-Al2O3) substrates. In this work, we employ first-principles density functional theory to understand paramagnetic noise sources that can couple directly to the external magnetic field on α-Al2O3 (001) surface. Both intrinsic and extrinsic surface defects are investigated. Our study shows that various environmental chemical species and adsorbates can introduce paramagnetic noise into the superconducting circuits. The fundamental understanding of this important physical origin of quantum decoherence presents an opportunity to develop unique fabrication techniques and designs for minimizing noise. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Memristive devices have the created excitement in the scientific community because of their potential applications in computer memories, hybrid CMOS-memristor architectures and artificial neural networks. An area of intensive research has been trying to understand the physical mechanisms behind the resistive switching phenomena happening in these devices. The most widely accepted mechanism is migration of oxygen ions forming a conductive channel or filament. Though there is strong evident for the formation of filaments and the presence of oxygen vacancies, the migration of a filamentary interface has not been reported. The planar configuration offers a convenient way to observe switching in these devices. In this work, we report the fabrication of TiO2 based planar memristive devices with Pt electrodes and a gap of 300nm. After electroformation, the switching in these devices is nonvolatile, reproducible and stable under UV light. AFM, SEM, and other techniques are applied to understand the underlying physics of the switching.

Strongly correlated electrons in transition-metal oxides produce a rich variety of electronic phases including solid and liquid states of charges, spins, and orbitals. These phases often compete with each other in a delicate balance, giving rise to dramatic phase changes upon application of external stimuli. Among such phase change functions, the current switching effect associated with the dielectric breakdown of the insulating charge-ordered state has been the subject of intensive research because of its potential applications in next-generation switching and sensing devices. However, achieving the effect at room temperature is difficult because charge ordering (CO), in general, is feasible only at very low temperatures. In this context, the room-temperature switching effect that was recently discovered in layered ferrites RFe2O4 (R=Dy-Lu, Y) is attracting a great deal of attention [1,2].
The electronic state of RFe2O4 is characterized by a charge frustration in the two-dimensional Fe triangular lattices. Since the Fe ions have an average valence of 2.5+, equal amounts of Fe2+ and Fe3+ ions coexist on the geometrically frustrated triangular lattices and show unique long-period two-dimensional CO below ~500 K. Further, below ~320 K, fully localized, three-dimensional CO arises as a result of interlayer correlation. To investigate the nature of charge transport and switching behavior in detail, we have fabricated c-axis oriented films of LuFe2O4 [3] and YbFe2O4 on YSZ(111) substrates by pulsed-laser deposition. Current switching was observed in the three-dimensional state of both the compounds. High-field transport measurements revealed that collective depinning of localized charge carriers is essential to induce switching. However, in these compounds, when the ordering pattern changed to two-dimensional on heating, the switching effect became significantly weak. We speculate that the lack of the interlayer CO correlation and the resulting dephasing effect could probably be the reason for weak switching behavior in the two-dimensional state.
[1] C. Li et al., Appl. Phys. Lett. 93, 152103 (2008).
[2] L. Zeng et al., Europhys. Lett. 84, 57011 (2008).
[3] K. Fujiwara et al., J. Phys. D: Appl. Phys. 46, 155108 (2013).

In the last years, the electro-optical industries have searched to minimize the size of electronic systems, increase the efficiency of optical devices, employ materials not harmful to the human life or environment, and use synthesis techniques with low costs. These factors are essential for the development of electronic components, for example: solid state lasers, light-emitting diodes, displays and scintillators. The materials belonging to the scheelite group are promising candidates for these technological purposes, specially the strontium tungstate (SrWO4). In this study, SrWO4 powders were synthesized by the co-precipitation (CP) and polymeric precursor method (PPM) at different heat treatment temperatures (500 °C, 600 °C and 700 °C) in resistive furnace and microwave oven. A high concentration of organic matter was detected by the thermogravimetric curves for the samples formed by the PPM. This result is related to the citric acid and ethylene glycol employed in this synthesis route. Independent of the conventional or microwave heating, the X-ray diffraction patterns revealed that all powders have a scheelite-type structure as well as the existence of crystallographic orientation, demonstrating the importance of the synthesis methods in the preparation of SrWO4 phase. The Raman spectra indicated that the powders obtained by the CP have well-defined vibrational bands, suggesting a matrix structurally ordered at short-range. The Raman-active modes of other powders synthesized by the PPM showed a structural transition from disordered to ordered, retarding the interaction between the [SrO8] and [WO4] clusters. The micrographs showed that both synthesis routes induce the formation of distinct particle shapes. While the samples prepared by the CP are composed of rice-like microparticles, the others synthesized by the PPM are formed by irregular particles (disordered assembly). According to the X-ray absorption near-edge structure spectra, all the powders exhibited the tungsten atoms bonded to four oxygens, confirming the existence of [WO4] clusters into the structure. The ultraviolet-visible absorption spectra revealed distinct band gap energies, indicating that the powders have different kinds and/or distributions of structural defects, independently of temperature or furnace. The photoluminescence (PL) emissions exhibited an inverse behavior with the temperature evolution between the samples in both synthesis methods, respectively. The reduction of PL intensity for the powders obtained by the CP was attributed to the relaxation of structural defects with the temperature, while the increase of PL intensity in the others formed by the PPM was explained by the phenomenon of structural order-disorder into the lattice.

Crystalline perovskites, ABO₃-type compounds, can be insulating or metallic, with many also being ferromagnetic, ferroelectric, multiferroic, or superconducting. This wide range of properties, combined with possibilities for lattice-matching to Si (001) during growth, allows for multi-functional heterostructures to be engineered. Epitaxial oxide films and heterostructures integrated with silicon have numerous potential applications as high-k insulators, nonvolatile field-effect transistors, electro-optics, integrated sensors, etc.
Molecular beam epitaxy (MBE) has been the primary means of integrating epitaxial oxides with silicon, with several groups achieving a clean interface between strontium titanate, SrTiO₃ (STO), and Si (001). However, there is often difficulty maintaining a clean interface with the relatively high temperature of MBE growth during the subsequent growth of oxide films and heterostructures on the STO template. Alternative low-temperature growth methods, such as atomic layer deposition (ALD), offer both practical and economic benefits for the integration of crystalline oxides on silicon.
Integration of metallic La-doped SrTiO₃, La_xSr_1-xTiO₃ (La:STO), films with Si (001) has been achieved by using a thin SrTiO₃ (< 2 nm) buffer layer. The STO buffer layer grown by MBE serves as a crystalline seed layer for growth of La:STO using ALD. The relatively low temperature of ALD (~200°C) maintains a clean interface between STO and Si. Reflection high-energy electron diffraction and X-ray diffraction patterns reveal that the La:STO grown on STO-buffered Si (001) at 205°C, with post-deposition annealing at 500-600°C, are c-axis oriented and fully crystalline. Electrical characterization, including resistivity and carrier concentrations, of the La:STO thin films will also be discussed.
The degree of La incorporation in the film is controlled through the ALD cycle ratio. La incorporation is analyzed through in situ XPS and independently confirmed through electron energy loss spectroscopy. In addition, the ALD process allows for layered La:STO structures to be grown. For example, a thin La:STO (5% La-doping) was grown on the STO-buffered Si (001) substrate. The next layer was a region of undoped STO, while the topmost layer was heavily doped La:STO (20% La-doping). Cross-sectional transmission electron microscopy of this structure shows the high degree of crystallinity, with clean interfaces between STO-Si and La:STO-STO. Replacing the undoped STO layer with a ferroelectric layer (e.g., BaTiO₃) is a promising material candidate for realization of a negative capacitance device, enabling ultra-low power switching.

Light emission from semiconductors depends not only on the efficiency of carrier injection and recombination but also extraction efficiency. For ultraviolet emission from high band gap materials such as ZnO and GaN, nanowires have higher extraction efficiencies than thin films, but conventional approaches for creating a p-n diode result in low efficiency. We exploited the noncentral symmetric nature of vertical aligned n-type ZnO nanowire/p-type GaN film to create a piezoelectric potential within the nanowire by applying stress. Because of the polarization of ions in a crystal that has noncentral symmetry, a piezoelectric potential (piezopotential) is created in the crystal under stress. The piezopotential acts as a “gate” voltage to tune the charge transport and enhance carrier injection, which is called the piezo-phototronic effect. We propose that band modification traps free carriers at the interface region in a channel created by the local piezoelectric charges.
The emission intensity at a fixed applied voltage have been enhanced by a factor of 6 after applying a 0.15% compressive strain. This huge enhanced performance is suggested arising from an effective increase in the local “biased voltage” as a result of the band modification caused by piezopotential and the trapping of holes at the interface region in a channel created by the local piezoelectric charges near the interface. Our study can be extended from ultraviolet range to visible range for a variety of optoelectronic devices that are important for today&’s safe, green, and renewable energy technology.

Using ultra-high vacuum scanning tunneling microscopy, we investigated the conditions under which switching at perovskite surfaces, namely barium titanate, is governed by polarization and oxygen migration. An analysis of the local density of states revealed that switching is initially dominated by polarization switching and then after repeated switching cycles, oxygen vacancy migration becomes dominant. Next we deposited gold nanoparticles onto strontium titanate substrates and measured local conductivity with an atomic force microscope. In this way, we were able to investigate how charge transport is affected by contact size. As the size decreased, transport properties became dominated by tunneling over thermionic emission. Finally nanoparticles were deposited onto barium titanate surfaces and switching was performed using an atomic force microscope. Our results show that the switching voltage decreased as size decreased and thus showed that increasing the tunneling contribution of charge transport decreased the voltage required to induce polarization switching at perovskite interfaces.

We report on the structural and electrical characteristics of bulk and thin film of ternary oxide SmGdO₃. Bulk sample of SmGdO₃ was prepared by palletising and sintering the calcined mixture of predetermined amount of Sm₂O₃ and Gd₂O₃ powders. The crystalline structure of the sample was studied by X-ray diffraction measurements and Raman spectroscopy. Capacitance and leakage current measurements on bulk sample revealed a high and linear dielectric constant of ~ 19 with low dielectric loss and leakage current which is suitable for gate dielectric application in CMOS logic devices and high-k MIM capacitors. In addition, the non-volatile resistive memory switching phenomenon was studied in thin films of SmGdO₃ which were deposited by pulsed laser deposition using sintered pallet of SmGdO3 as target. Commercially available Pt/TiO₂/SiO₂/(100) Si was used as substrate and top Pt electrode of lateral dimension 40×40 mu;m² were deposited by sputtering to construct Pt/SmGdO₃/Pt MIM devices. After initial forming process which occurred at comparatively higher voltage, the Pt/SmGdO₃/Pt devices showed repeatable unipolar switching between high and low resistance states with resistance ratio of ~ 10⁴, and low & well defined switching voltages with narrow dispersion. These properties indicate suitability of this material for the emerging logic and memory device applications.

Recently, electrical-pulse-induced resistance switching has been reported in thin films of metal oxides such as Pr_1-xCa_xMnO₃ (PCMO) [1, 2]. This effect provides a possibility of one of the next-generation nonvolatile memories, called a resistance random access memory (ReRAM). In this work, the correlation of resistance switching behavior with dielectric functions was discussed on the basis of the current-voltage (I-V) characteristics and ellipsometric spectra of PCMO films to understand an underlying mechanism of the resistance switching.
Pr_0.5Ca_0.5MnO₃ films were deposited on LaAlO₃ (100) single crystalline substrates by RF magnetron sputtering. The process pressure was set from 20 to 40 mTorr to investigate the deposition pressure dependence of electrical and optical properties of PCMO films. In order to measure the electrical properties of the deposited films, metallic electrodes were deposited on top of PCMO films by vacuum evaporation. I-V measurements were performed to investigate the deposition pressure dependence of the resistance switching behavior in PCMO-based devices. Resistance switching was identified only in the devices composed of the PCMO films deposited under the low pressure of 20 mTorr. Non-switching behavior was observed in the devices with the PCMO films deposited under the high pressure of 40 mTorr.
The optical properties of the PCMO films deposited under different gas pressures were revealed by spectroscopic ellipsometry. We succeeded in fitting the experimentally observed spectra by the theoretically simulated ones constructed on the basis of the triple Lorentz oscillator model. The deposition pressure dependence of the electronic structure of PCMO films was detected as differences in dielectric functions by spectroscopic ellipsometry. The ellipsometric data indicate that the PCMO films exhibiting resistance switching have large oscillator strength of the electric dipole charge transfer transition in the (MnO₆)^9- and (MnO₆)^8- octahedral complexes, small oscillator strength of d-d transitions in the Mn³⁺ and Mn⁴⁺ ions, and large high frequency dielectric constant. The formation of (MnO₆)^9- and (MnO₆)^8- octahedral complexes and oxygen vacancies may be required for obtaining large resistance switching.
References
[1] S. Q. Liu, N. J. Wu, and A. Ignatiev, Appl. Phys. Lett., 76 (2000) 2749.
[2] T. Nakamura, K. Homma, K. Tachibana, Nanoscale Res. Lett., 8 (2013) 76 and references therein.

Na0.5Bi0.5TiO3 (NBT) ceramic has been considered a promising material to replace the lead-free ceramics. Effect of Ca addition on the structure and electrical properties of NBT ceramic has been studied in the present work. Polycrystalline (Na0.5Bi0.5)1-x-CaxTiO3 (x=0.05, 0.1 and 0.15) (abbreviated as NBCT1, NBCT2 and NBCT3 respectively) ceramics were synthesized by conventional solid state reaction technique. The crystal structure of the prepared ceramics was examined by X-ray diffraction technique. The information about crystal structure of the prepared ceramics was extracted from Rietveld refinement using GSAS software with EXPGUI interface. The crystal structure for NBCT1 sample was found to be rhombohedral with space group R3c. While for NBCT2 and NBCT3 samples coexistence of rhombohedral and orthorhombic phases was observed. A good agreement was observed between experimental and simulated patterns, well supported by the low values of chi;2 (1.86, 2.27 and 2.17 for NBCT1, NBCT2 and NBCT3 respectively). The electrical properties were investigated by using impedance spectroscopy in the frequency range from 10 Hz to 5 MHz from 573 K to 723 K. Impedance plots were studied to get an insight of the electrical processes occurring in the samples. Nquist diagrams for all the compositions at different temperatures indicated a decrease in resistance with increase in temperature suggesting thermally activated process. The semicircular arcs had a depressed center (below the real axis) suggesting a non Debye type of relaxation mechanism of the charge carriers. Frequency dependent electrical conductivity obeyed the universal Jonscher power law. At lower frequencies conductivity is nearly constant resulting in dc conductivity. At a particular frequency conductivity starts increasing; this frequency is termed as hopping frequency. With increase in Ca content conductivity decreases for NBCT2 and then with additional substitution of Ca, conductivity increases for NBCT3. This variation of conductivity can be related to presence of two phases for NBCT2 composition in significant proportion (R3c 70.6% and Pbnm 29.4%) and for NBCT3 orthorhombic phase is main space group (R3c only 4.6%). The conductivity spectra also depicted an increase in conductivity with rise in temperature. Normalized impedance and modulus plots were employed to distinguish whether the relaxation process is due to long range or localized movement of charge carriers. The peaks of normalized imaginary electric modulus and imaginary impedance spectra were not found to coincide at the same frequency suggesting a localized motion of charge carriers in all the compositions. The dc conductivity as a function of reciprocal of temperatures followed the Arrehenius law. The activation energies obtained from Arrehenius plots were as 1.14 eV, 1.06 eV and 1.06 eV for NBCT1, NBCT2 and NBCT3 respectively. The values of conduction activation energy decreased with increase in Ca content.

SrTiO₃ is a wide band-gap perovskite oxide semiconductor that is widely investigated in bulk form, in part due to its remarkable electronic properties when doped. These properties arise from its quantum paraelectric nature which enables such unique features as a high-mobility low-density metallic state, quantum transport in an unusual limit, and the most dilute superconducting state thus reported. Recent progress with thin films and heterostructures of doped SrTiO₃ has further demonstrated record high mobilities, 2D electron gases at interfaces, and even simultaneous observation of quantum oscillations and superconductivity. Such progress has been possible only by precise control of stoichiometry and defect density in SrTiO₃ using techniques such as oxide/LASER MBE or high-temperature PLD. In this work, we demonstrate that high pressure oxygen RF sputtering from a ceramic target is similarly capable of growth of high-quality, precisely stoichiometric thin films of SrTiO₃. By employing homoepitaxy on SrTiO₃(001) substrates, we show that optimization of deposition temperature (above 750 ^omicron;C), oxygen pressure (above 2.5 mBar) and deposition rate (below 2 Å/min) leads to films that are indistinguishable from the substrate via grazing incidence and wide-angle x-ray scattering. We reiterate the importance of pre-annealing of substrates in oxygen above 900 ^omicron;C to obtain such properties and eliminate interfacial scattering contrast. Detailed transport measurements on reduced films grown on a variety of substrates will be discussed, particularly for the insight they provide into the interplay between carrier mobility and structural perfection. Finally, we will also show that certain forms of post-deposition heat treatment can remarkably improve the stoichiometry and defect density of films deposited under sub-optimal conditions.
Work supported by NSF DMR and NSF MRSEC.

Ohmic contact formation is of central importance in the fabrication of devices involving wide-gap semiconductors. Most metals create Schottky barriers when deposited on wide-gap oxides. Ohmic contacts are rare, particularly when considering single-element metals. We have explored the detailed behavior of epitaxial Cr on Nb-doped STO(001), and have found that it creates an ideal Ohmic contact [1]. Moreover, both the epitaxial stabilization of the metal film and the desirable electrical properties stem from limited interface reactivity. A fraction of the first few monolayers of Cr diffuse into the STO and occupy interstitial sites, thereby metalizing the near-surface region and transferring charge to Ti cations at B sites [2]. First-principles calculations suggest that Fe should have a similar effect. Accordingly, we have commenced an analogous study of the Fe/STO interface. A comparison of the two material systems will be presented.
[1] C. Capan, G. Y. Sun, M. E. Bowden, S. A. Chambers, Appl. Phys. Lett. 100, 052106 (2012).
[2] S. A. Chambers, M. Gu, P. V. Sushko, H. Yang, C. Wang, N. D. Browning, Adv. Mat. 25, to be published 19 July 2013.

Resistance switching phenomena in metal oxides can be described by changes in local stoichiometry. These changes are assumed to explain the change in the interfacial Schottky barrier heights, that in the tunnel barrier heights, or conducting filament formation. The changes in local stoichiometry can be induced by migration of ionic species, that is, oxygen ions and vacancies. However, the physical origins for the migration of each ionic species in resistance switching are not clearly understood yet. The purposes of this work are (1) to understand the different effects and physical origins of each ionic migration and (2) to address the effect of space charge redistribution in TiN/TiOx/AlOx/Ir resistance switching element.
In our model, internal electric field (Eint) enhances the generation of mobile oxygen ions, while it suppresses generation of mobile vacancies because of opposite charge polarity of each ionic species. The distinctive characteristics between set and reset or between unipolar and bipolar operations are consistently explained by changes in Eint.
As one of the conclusions of this study, I suggest a switching strategy for multiple resistance states. To attain multiple resistance states by controlling reset operation, we need to control applied voltage to adjust reset resistance (Rreset). The decrease of set resistance (Rset, with the increase of Eint) induces the increase of the slope of negative differential resistance regime (SNDR). The regime of negative differential resistance is that of voltages for reset operation, which means that the voltage range for reset operation becomes narrower when Rset decreases. Therefore, there is a lower bound for Rset in implementing reset voltage controlled multiple resistance states. To sum up, multiple resistance states with narrow state distribution would be possible in following cases: (1) multiple states by controlling set operation via two different switching methods according to the distribution of Rreset, and (2) multiple states by controlling reset operation with moderate Rset.

We investigated the wide bandgap (E_g = 3.1 eV) p-type semiconductor γ-copper(I)-iodide (γ-CuI) as an alternative candidate to p-type transparent semiconducting oxides like SnO or ZnCO₂O₄.
Two fabrication methods of CuI have been used. One way is exposure of Cu thin films on a substrate to iodine vapor which transforms Cu to CuI. The other method is vacuum evaporation of CuI.
The electrical properties of these films are similar since they have a hole mobility around 6 cm²/Vs, a hole density of 5-10×10¹⁸cm^-3 and a resistivity of about 0.2 Omega;cm. The surface rms roughness of the iodized Cu films on glass in the order of 30-40 nm is much higher than for the CuI films from vacuum evaporation having rms roughness of about 2nm. This finds expression in the transmittance measurements where only the evaporated CuI films show Fabry-Perot interferences. They also exhibit high transmittance of T_VIS >75% in the visible spectral range whereas T_VIS <64% for iodized Cu films. The latter one also have an opal glass appearance due to strong light scattering.
Further distinction of differently fabricated samples was made by X-ray diffraction and scanning electron microscopy. Additionally the epitaxial relationship of CuI on c-ZnO was investigated.
Heterodiodes consisting of p-CuI and pulsed-laser deposited n-ZnO were fabricated on a-sapphire substrates and characterized electrically [4].
The transparent diodes showed rectification ratios I_f/r > 10⁷ at ±2V. This value is one of the best compared to similar wide bandgap heterojunctions with at least one oxidic part as reported by Alivov et al. (1×10⁴ at ±8V, p-6-H-SiC/nZnO [1]), Lee et al. (7×10⁵ at ±2V, p-TiO_x/n-TiO_x [2]) or ourselves (1×10⁸ at ±2V, p-ZnCO₂O₄/n-ZnO [3]). We determined diode ideality factors down to 1.5. A small hysteresis is probably due to imperfections at the interface.
[1] Schein et al., IEEE Electron Device Letters 33, 676 (2012)
[2] Alivov et al., Applied Physics Letters 86, 241108 (2005)
[3] Lee et al., Advanced Materials 19, 73 (2007)
[4] Schein et al., Appl. Phys. Lett. 102, 092109 (2013)

Tailoring the physical properties of semiconducting oxides usually requires of appropriate doping. In particular, Cr doped TiO₂ exhibits enhanced photocatalytic activity and optimal luminescence properties for optoelectronic applications. The incorporation of chromium into the TiO₂ is a subject of present interest in the case of small dimensional structures.
In this work microtubes and microrods, with Cr contents in the range 1.2 - 2.8 % cationic fraction, grown at temperatures between 1250 - 1300 °C by a vapor-solid process using Cr doped TiO₂ nanoparticles as precursors have been studied.[1] The microstructures are tens of microns length and exhibit flat lateral surfaces formed by {110} planes and nearly square cross- sections of few microns width. An analysis of the Cr related luminescence on the basis of how chromium is incorporated into the TiO₂ lattice has been carried out by means of photoluminescence (PL), Raman spectroscopy and x-ray absorption spectroscopy (XAS).
PL measurements at low temperatures (T ge; 10 K) show the presence of sharp intense peaks characteristic of Cr³⁺ at around 1.7 eV (1.70, 1.74 eV), corresponding to intraionic transitions (²E-⁴A₂) known as R-lines. The intensity of these peaks, which depends on the amount of Cr, is drastically reduced as the temperature increases over 100 K. Moreover some of these peaks are shifted to lower energies than those reported for other Cr-doped semiconductor oxides, which can be explained due to doping-induced variations in the crystal field at the Cr³⁺ ions and the resulting structure of defects. The characteristic luminescence from TiO₂, with an intense emission related to Ti³⁺ interstitials at 1.5 eV is discussed in relation with the Cr doping. Variations in the Raman spectra related to the incorporation of Cr in the rutile TiO₂ lattice have been observed. In particular the B_2g mode (826 cm^-1) is shifted and increases its relative intensity as the concentration of Cr in the microstructures is raised. This mode is related to an antisymmetric (Ti-O) stretching vibration of the TiO₆ octahedra, therefore it is sensitive to the local environment of Ti ions which can be distorted by the presence of chromium in the rutile TiO₂ lattice. Experimental XAS measurements and theoretical calculations (CTM4XAS) are carried out for several x-ray absorption edges (O K, Ti L_2,3 and Cr L) in order to gain new insight in the local structural variations induced by Cr doping. Variations in the crystal field and distortion of the Cr³⁺ octahedral symmetry in TiO₂ can be related to the photoluminescence measurements.
[1] G. C. Vásquez, M.A. Peche-Herrero, D. Maestre, A. Cremades, J. Ramírez-Castellanos, J.M. González-Calbet and J. Piqueras. Cryst. Eng. Comm. (2013) DOI: 10.1039/c3ce40513c

Amorphous oxide semiconductors have attracted great attention as channel layers for thin film transistors (TFTs) due to characteristics such as high mobility, device stability, and transparency [1,2]. Several Indium-Zinc based ternary In-α-Zn0 metal oxides have been synthesized with sol-gel methods and a moderate thermal budget (le; 450°C) compatible with plastic substrates. We investigated, for a given In/Zn ratio of 0.5 in weight, the influence of the additional material, α, on TFT parameters such as charge carrier mobility, threshold voltage, turn-on voltage, and the on/off current ratio. Materials such as α = Ga, Sn, Ag, Al, Y, Ca, Ni, Mo, Mn, Cu, Pd have been evaluated and best results have been obtained using Ga and Sn with 5% in weight. We obtained respectively a charge carrier mobility of 8 cm2v-1s-1 with gallium and 7 with Sn Ioff current as low as 10-11A for both and turn-on voltage V0 = 0V and -3V and a subthreshold swing of 0.4 and 0.6 V.dec-1, and a threshold voltage Vt = 3 and 5V. Then, we performed additional experiments on the influence of In/Zn ratio (from 0.5 to 4) on the TFT electrical performances. We found that the highest mobility is obtained by increasing the In concentration up to In/Zn =. However, in these conditions, the off current (Ioff) and turn-on voltage (V0) tend to degrade to values of respectively 10-4A and -8V [3]. The best suitable composition for optimal TFTs electrical parameters is obtained for In/Zn = 2.5, with a charge carrier mobility of 12 cm2V-1s-1 for α = Ga, Ioff current as low as 10-11A and V0 = -2V. For all the α materials investigated here, we established a general trends for the TFT parameters as function of the composition of the ternary metal oxide.
References:
[1] Dongjo Kim et al, App. Phys. Lett. 95, 103501 (2009)
[2] Sunho Jeong et al, J. Mater. Chem., 2012, 22, 1243
[3] J. Kang et al., Applied Surface Science 258, 3509- 3512 (2012).

Oxide semiconductor thin film transistors (TFTs) have been studied extensively as switching devices in active-matrix (AM) liquid crystal displays (LCDs), organic light emitting diodes (OLEDs) and flexible displays on account of their high field effect mobility (>10 cm2/V-s), high electrical uniformity over a large-size substrate, and low processing temperature. Indium zinc oxide (IZO) is one of the most promising channel materials, which has a very high mobility exceeding 30 cm2/V-s. In our previous work, it was found that the photo-bias instability of IZO TFTs strongly depended on crystal structure and oxygen vacancy concentration of the channel layer by a cation combinatorial approach. The effect of crystal structure on the photo-bias instability was particularly a quite interesting result. For the further insight study on the parameters to affect the photo-bias instability, in this research, a stretched exponential fitting analysis was introduced to the IZO TFTs.
The fabricated IZO TFTs have a bottom gate and top contact configuration. The 16-nm-thick IZO channel layers with different In fractions were prepared by a radio-frequency (rf) magnetron co-sputtering using ZnO and In2O3-ZnO (90:10 wt.%) targets. Prior to electrical measurements, poly-methyl-methacrylate (PMMA) passivation layer was formed for excluding ambient effect in the back channel region. To apply a stretched exponential fitting analysis, negative bias stress (NBS) tests were performed in four temperatures of 25, 50, 70 and 90 degrees C for the IZO TFTs with different In contents.
The threshold voltage (Vth) shift behavior during the NBS test was well fitted by a stretched exponential equation, indicating that Vth degradation of IZO TFTs was attributed to the charge trapping mechanism. Hole charges in a channel layer, such as doubly-ionized oxygen vacancies (Vo2+), are attracted toward a channel/insulator interface by a negative gate bias and trapped to the interface or bulk states in an insulator. The 52 at.% In IZO TFT with a poly-crystalline structure showed severe instability, a large Vth shift of -7.3 V, whereas the 73 at.% In IZO TFT with an amorphous structure showed a small Vth shift of -3 V under negative illumination bias stress of 2000s. By stretched exponential fitting for the two IZO TFTs, activation energies for hole trapping process were obtained. Activation energies of the 52 and 73 at.% In IZO TFTs are 0.746 and 1.345 eV, respectively. The hole migration by a negative gate bias easily occurs through grain boundaries in a poly-crystalline structure, compared to an amorphous network structure. As the In content increased from 52 to 73 at.%, the amorphous region of the IZO channel layer was expanded, resulting in restricting the hole migration. Consequently, the difference between two activation energy values, ~0.6 eV, can be considered to be the barrier difference for a hole migration between the crystalline phase and the amorphous phase.

The interface between LaAlO₃ and SrTiO₃, two good band insulators, which was found in 2004 to be conducting [1], and in some doping range, superconducting with a maximum critical temperature of about 200 mK [2] is attracting worldwide attention.
In this presentation, I will discuss recent magnetotransport experiments in nanostructures that reveal a remarkable tuning of the electronic properties and allow weak localization and weak anti-localization as a function of doping and temperature to be followed. I will then discuss superconductivity, field-effect experiments, the phase diagram of the system [3] and the possible role of the strong spin-orbit [4] on superconductivity. I will finally mention our recent work towards functional heterostructures based on several conducting interfaces.
[1] A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004).
[2] N. Reyren, S. Thiel, A. D. Caviglia, L. Fitting Kourkoutis, G. Hammerl, C. Richter, C. W. Schneider, T. Kopp, A.-S. Ruetschi, D. Jaccard, M. Gabay, D. A. Muller, J.-M. Triscone and J. Mannhart, Science 317, 1196 (2007).
[3] A. D. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart, and J.-M. Triscone, Nature 456, 624 (2008).
[4] A. D. Caviglia, M. Gabay, S. Gariglio, N. Reyren, C. Cancellieri, and J.-M. Triscone, Physical Review Letters 104, 126803 (2010).

Hole-doped perovskite oxides have stimulated considerable interest because of the richness of their properties. An interesting but lightly explored set of materials are the mixed-valent Cr-based perovskites. We have studied La1-xSrxCrO3-delta (LSCO) epitaxial films deposited on SrTiO3(001) for 0 le; x le; 1 by molecular beam epitaxy. The structure and composition, as well as the dependence of electronic and optical properties on composition, were systematically investigated using in situ XPS, RBS, XRD, STEM/EELS, optical absorption, photoconductivity and transport measurements. We have also carried out DFT and hybrid functional calculations for structurally perfect, as well as defected LSCO. Pure LaCrO3 (d3) is a Mott-Hubard insulator with charge transfer gap of ~4.8 eV. Sr doping introduces holes at the top of the LaCrO3 valence band, which is of predominantly Cr 3d character, leading to a Cr4+ (d2) configuration. The chemical potential shifts toward the valence band with increasing x. A finite density of states at the Fermi level was observed for x ge; 0.50, suggestive of a insulator-to-metal transition. This trend is supported by DFT calculations and is ascribed to an increasing valence band width arising from strong hybridization between Cr 3d2 and O 2p states. However, transport measurements reveal that the films exhibit non-metallic behavior, marked by dρ/dT < 0 and good fits to a hopping conductivity model, albeit with very low activation energy (< ~0.1 eV). Materials characterization reveals that these unexpected electronic properties are correlated with the presence of ordered oxygen vacancies which result in the formation of patches of insulating material in which donor electrons from O vacancies locally compensate holes from Sr doping. These O vacancies, which we tentatively conclude stem from the instability of Cr4+ in octahedral coordination, can be at least partially removed by gentle heating in air.

The resistance switching (RS) phenomena observed in Schottky junctions of metal/[niobium-doped strontium titanate (Nb-STO)], such as SrRuO3/Nb-STO and Pt/Nb-STO junctions, have attracted considerable attentions due to the potential application for the next-generation random access memory.[1-2], and numerical investigations have been conducted to understand RS mechanism and improve RS properties. In spite of making many efforts, even now the RS mechanism has not been perfectly understood and a further effort is obviously required. One reason why the RS mechanism has not been understood in detail is that the properties at the metal/Nb-STO junction cannot be explained by entering the physical parameters of bulk STO into an ordinary model of metal/semiconductor Schottky junctions. At ordinary metal/semiconductor junctions, the relative dielectric permittivity (εr) of the semiconductor is a constant that is identical to that of bulk. However, for metal/Nb-STO junctions, the εr strongly depends on the electric field,[3] and the potential distribution at the metal/Nb-STO junction cannot be uniquely determined solely from the capacitance-voltage and current-voltage measurements.
In this study, the interface band structures of of PtOx/Nb-STO Schottky junctions, which exhibit the excellent RS properties compared with conventional Pt/Nb-STO junctions, were investigated by the soft and hard x-ray photoemission (XPE) spectroscope techniques for an understanding of RS mechanism in metal/Nb-STO junctions. The broadening and shift of the XPE peaks for Nb-STO, which resulted from the formation of a potential barrier at the interface, were also quantitatively analyzed by fitting simulations. [4] As a result, we have clearly observed the change in Schottky barrier profiles with the partial oxidation of Pt electrode; the partial oxidation of electrode causes the increase in Schottky barrier height from 1.0 eV to 1.3 eV and the inhomogeneity in Schottky barrier distribution. The XPE spectra also indicate that the Ti3+ forms near the electrode interface with the partial oxidation of electrode, and it possibly causes the inhomogeneous Schottky barrier distribution. These results suggest that the reduction of Nb-STO surface and oxidation of Pt electrode near the electrode interface and homogeneity in Schottky barrier distribution should strongly contribute to the RS phenomena in metal/Nb-STO Schottky junction devices.
[1] H. Sim et al., IEDM Tech. Digest. IEEE, 758 (2005).
[2]T. Fujii et al., Phys. Rev. B 75, 165101 (2007).
[3]S. Suzuki et al., J. Appl. Phys. 81(10), 6830 (1997).
[4]N. Ohashi et al., Appl. Phys. Lett. 101, 251911 (2012).

Two-dimensional electron gases (2DEGs) at interfaces between two insulating oxides have attracted significant attention because they can exhibit unique properties, such as strong electron correlations, superconductivity and magnetism. In this presentation, we will discuss emergent properties at interfaces and quantum wells formed between strongly correlated Mott insulating rare earth titanates (SmTiO₃ and GdTiO₃), and the band insulator SrTiO₃. Such interfaces exhibit a high-density 2DEG, of approximately 1/2 electron per surface unit cell, providing ~ 3×10¹⁴ cm^-2 mobile charge per interface. We show that electron correlation effects due to short-range Coulomb interactions can be induced at strong confinements, and that the properties of the correlated electron system can be tuned by proximity effects. In particular, we report on magnetism, mass enhancement and a metal-insulator transition at extreme 3D densities, in high carrier density SrTiO₃ quantum wells that can be obtained using these interfaces. We show that ferromagnetism in the high-density 2DEG can be induced by proximity to the ferrimagnetic GdTiO₃, and that the spin transfer depends on the magnetization direction in the GdTiO3. Antiferromagnetic coupling is obtained at interfaces with SmTiO₃. We also report on resonant tunneling studies that probe the subbands in the high-density 2DEG.

In this presentation new approaches to metal oxide and hybrid materials for unconventional electronic applications are presented. Particularly, we will discuss our latest results in developing amorphous and polycrystalline In-Y-X-O (Y, X = none, Ga, Sn, Zn, Y, La) formulations for combustion synthesis in which the corresponding films can be annealed at temperatures < 300 C. For instance, solution-processed amorphous IGZO films for TFT fabrication at temperatures <300 C can be achieved by controlling film precursor molar ratio resulting in electron mobilities > 2-5 cm2 Vminus;1sminus;1 and Ion:Ioff > 106 for TFTs using SiO2 as the gate dielectric. Furthermore, we demonstrate that hybrid integration of solution-processed IYXO semiconductor films with self-assembled dielectrics (SAND) enable > 20 cm2 Vminus;1sminus;1. Furthermore, we will discuss structural evolution with the annealing temperature and film thickness using combustion synthesis to develop the metal oxide lattice. Finally, a solution-processed IGO-In2O3 bilayer approach is presented.

Despite promising starting mass-production of oxide TFT for the mobile TFT-LCD and AMOLED TV, there still remain several technical issues such as high mobility, TFT structure with low parasitic capacitance, and low resistance electrode to achieve high resolution and/or large area display. Oxide semiconductors suitable for high mobility are known to be ZITO, AZITO, IGO, In rich IGZO, and ZnON. Self-aligned TFT and Cu based electrode have been studied for TFT with low parasitic capacitance and low resistance electrode, respectively. The common bottle neck for these three issues of oxide TFT is the selection of source-drain material. It has been reported that the metallization process make effective channel length shorten from the origin of increased carrier amount under the source-drain region. According to our results, effective channel length of high mobility TFT becomes more shorten and it seems to be very hard to control the uniformity of Vth in the case of high mobility TFT.
Here, we investigate the effect of metallization process of high mobility ESL AZITO TFT with Mo, Mo-Ti source-drain materials. Mo and Mo-Ti are well known Cu barrier material of oxide TFT. With considering the possibility of use Cu/Mo-Ti or Cu/Mo layer for large area oxide TFT, we select Mo and Mo-Ti for the contact layer of high mobility oxide TFT.
We will report the effect of source-drain metal on the high mobility oxide TFT in the view point of material and electrical property of TFT by investigating source-drain/active interface, measurement of effective channel length and TFT performance.

In active matrix display, the resolution is getting higher while the frame rate is being increased to reduce the motion blur. As the display resolution and frame rate increase, the charging time becomes too short to fully charge the storage capacitance. With conventional amorphous silicon thin film transistors (TFTs) with field effect mobility of around 1cm2/Vs, it is not possible to charge the pixel capacitance within this charging time. Therefore, new TFT technology with high mobility is necessary.
In this paper, zinc oxynitride (ZnON) thin film transistors were suggested as the switching elements for next generation active matrix displays. ZnON film was formed by reactive sputtering with the use of zinc metal target and the mixture gas of argon, oxygen and nitrogen. By varying the oxygen gas ratio, the electrical characteristics such as mobility and carrier concentration were controlled. Optimized ZnON TFTs show the superior field effect mobility over 50cm2/Vs. The hysteresis in transfer characteristics and threshold voltage (Vt) shifts under repeat Id-Vg sweep were investigated. In addition, the instability of threshold voltage under gate bias temperature stress (BTS) was examined. Briefly, it is observed that the negative Vt shift under negative gate bias is more problematic than the positive Vt shift under positive bias. Since ZnON film shows small band gap of 1.3eV, it is estimated that a large number of holes exist in the ZnON semiconductor. Therefore, the hole trapping near the gate insulator interface is the critical issue to the Vt instability of ZnON TFT. Also, it is presumed that the hole trapping is the main reason why ZnON TFTs show the poor hole conductivity even under high negative gate voltage. Discussions will be made on the material&’s microstructure, density of states (DOS) of ZnON film and the associated electrical properties.

Oxide semiconductors, such as titanium dioxide (TiO2), tungsten trioxide (WO3) and zinc oxide (ZnO), are of relevance for a wide range of applications such as solar energy conversion, electrochromic displays, gas sensing.
Moreover, thin films of oxide semiconductors offer the exciting opportunity to fabricate transistors operating at relatively low operating voltages (0.5 - 2V) making use of the electrolyte-gating approach [1].
Electrolyte-gated thin film transistors, where electrolytes are used as gating media to change the conductivity of semiconductor thin films, have been intensively investigated for low power electronic applications. In the 1950s, electrolyte gating was used in the development of the first transistor. [2]
Electrolytes such as ionic liquids are interesting as gating media for their low-volatility, non-flammability, relatively high ionic conductivity. Furthermore, they can have electrochemical stability windows compatible with the operating voltage of transistors.
At present, the effectiveness of the gating obtained using ionic liquids as the electrolytes it is not easily predictable because factors governing the formation of the electrical double layer at the ionic liquid/semiconductor interface are largely undiscovered. Therefore, there is a need to investigate different ionic liquid/semiconductor interfaces to understand how factors such as ionic conductivity, size and shape of the ions affect the gating process.
We report on electrolyte-gated transistors making use of solution processed WO3, TiO2, and ZnO nanostructured thin films making use of ionic liquids such as imidazolium- and phosphonium-based as the gating medium. The characteristics of the semiconductor/ionic liquid interfaces were investigated by electrochemical impedance spectroscopy, which was also used to deduce the mobility of the charge carriers in the films. The transients characteristics of the devices (transistor current over time) were also measured to gain insight on the stability of the devices and to shed light onto the mechanism of doping (electrostatic vs electrochemical) governing the transistor behavior.
[1] a) S. H. Kim, K. Hong, W. Xie, K. H. Lee, S. Zhang, T. P. Lodge, and C. D. Frisbie, Adv. Mater. 25, 1822 (2013); b) G. Tarabella, F.M. Mohammadi, N. Coppedè, F. Barbero, S. Iannotta, C. Santato, and F. Cicoira, Chem. Sci. 4, 1395 (2013).
[2] W. H. Brattain and J. Bardeen, Bell Sys.Tech. 32, 1 (1953).

The rare-earth nickelates (RNiO3, R = rare-earth element), as correlated oxides with sharp thermally-driven insulator-metal transition, have attracted attention in areas spanning physics to electronics. SmNiO3, as the first nickelate with its insulator-metal transition temperature (400 K) above room temperature, is of great interest to explore integration of correlated oxides with conventional circuits. Thermodynamic instability however places extreme limitations on synthesis of such oxides in the absence of epitaxial strain. Here, we report the synthesis of SmNiO3 thin films on various gate oxides by sputtering followed by ultrahigh pressure oxygen annealing at intermediate temperatures. The temperature - pressure boundaries for stable phase formation are first calculated and apparatus constructed to reach these conditions. X-ray diffraction and transmission electron microscopy both show that the SmNiO3 phase is orthorhombic. In-plane resistance-temperature measurements demonstrate a stable insulator-metal transition at ~120-140 oC, depending on the underlayer. Single-crystalline oxide substrates (sapphire and LaAlO3) were also used as templates to study the crystallinity and growth orientation of SmNiO3 film and will be discussed in detail. The successful synthesis of stable SmNiO3 on various oxides layers creates opportunities to study the fundamental electronic properties of correlated oxide semiconductors in gated transistor structures for logic switches and memory.

For over three decades, the infrared spectroscopy peaks of around 3500 cm^-1 observed in hydrogen-doped perovskite oxides, namely SrTiO₃ and BaTiO₃ samples, have been assigned to an interstitial hydrogen (H_i) attached to a lattice oxygen with two possible configuration models: the octahedral edge (OE) and the cubic face (CF) models. In this talk, I will show that, based on first-principles calculations, both OE and CF configurations are not energetically stable. Starting from either configuration, H would spontaneously relax into an off axis (OA) site with the calculated vibrational frequency significantly lower than the observed 3500 cm^-1 peak. In addition, the calculated diffusion barrier is low, suggesting that H_i can be easily annealed out. We propose that the observed peaks around 3500 cm^-1 are associated with defect complexes. An A-site vacancy (V_Sr for SrTiO₃ or V_Ba for BaTiO₃) can trap H_i and form a H-vacancy complex which is both stable and has the frequency in agreement with the observed main peak. The complex can also trap another H_i and form 2H-vacancy complexes; consistent with the observed additional peaks at slightly higher frequencies. The study of H in SrTiO₃ and BaTiO₃ forms a foundation to better understand H in perovskite oxides in general.

Perovskite oxide semiconductors are an attractive system for light harvesting applications, including alloys with bandgaps across the visible spectrum, made from materials that are inexpensive and abundant. As LaCrO₃ is a wide bandgap semiconductor and SrCrO₃ conducting, the (La,Sr)CrO₃ alloy system can potentially be tuned to absorb in the visible. However, the degree of conductivity in SrCrO₃ films seems to depend upon defects in the material and in particular oxygen vacancies. The structure and resulting properties of SrCrO₃ films grown on LaAlO₃ by molecular beam epitaxy (MBE) are investigated using conventional and scanning transmission electron microscopy (TEM/STEM). As-grown films having poor conductivities (1.4 ohm cm²), are found to have ordered oxygen vacancies aligned along (111) planes. The precise oxygen deficiency is measured using resonant Rutherford backscattering spectrometry (RRBS) and films are found to be 15% O-deficient. There is a marked improvement in the conductivity after annealing in oxygen. TEM analysis of the samples before and after the oxygen annealing will be shown, demonstrating the role of defects and oxygen vacancies on the resulting conductivity. The local structure before oxygen annealing best matches a 15R prototype cell similar to previous reports for Ba₅MnNa₂V₂O₁₃. Electron energy loss spectroscopy (EELS) of the Cr L and O K-edges are used to determine the cation oxidation state in the film, nominally Cr⁴⁺, and in the vicinity of the ordered defects, both before and after annealing.

The combination of electrical conductivity and optical transparency in a single material gives transparent conducting oxides (TCOs) an important role in modern optoelectronic applications such as in solar cells, flat panel displays and smart coatings.[1-3] The current industry standard n-type TCO is In₂O₃:Sn (ITO) which demonstrates conductivities of ~10⁴ S cm^minus;1, whilst retaining > 90% transparency.[3] The overwhelming demand for ITO, coupled with the low natural abundance of indium has made indium an increasingly expensive commodity, which has led to a large research drive to replace ITO as the industry standard n-type TCO.[4]
Perovskite structured oxides, with the formula ABO₃, are key materials in the field of “oxide electronics”, possessing many diverse physical properties including high-transition-temperature superconductivity, optical transparency, ferroelectricity, piezoelectricity, and photocatalytic activity.[5] The emergence of all-perovskite multilayer heterostructures has refocused interest on the development of perovskite materials with improved functionalities.[6,7] Very recently, the transparent perovskite BaSnO₃ has been reported to possess electron mobilities and conductivities comparable with the best TCOs when La-doped.[8,9]
In this presentation we will examine the electronic structure of BaSnO₃ using hybrid density functional theory. We present a computational analysis of the intrinsic and extrinsic defect chemistry of BaSnO₃, pinpointing optimum growth conditions and identifying promising donor dopants.[10] It is expected that these results will serve as a guide to experimentalists attempting to optimize BaSnO₃ for high performance solar cell applications.
[1] G. Thomas, Nature, 1997, 389, 907.
[2] K. Hayashi et al, Nature, 2002, 419, 462.
[3] T. Minami, Semicond. Sci., Tech. 2005, 20, S35.
[4] L. Wang et al, J. Appl. Phys. 2010, 107, 043103.
[5] T. Wolfram and S. Elliatioglu, Electronic and Optical Properties of d-Band Perovskites (Cambridge University Press, 2006).
[6] W. Meevasana et al, Nature Mater., 2011, 10, 114.
[7] P. D. C. King, et al.,Phys. Rev. Lett., 2012, 108, 117602.
[8] X. Luo et al., Appl. Phys. Lett., 2012, 100,172112.
[9] H. J. Kim et al, Appl. Phys. Exp., 2012, 5, 061102.
[10] D. O. Scanlon, Phys. Rev. B, 2013, 87, 161201(R).

Transition-metal oxides host some of the most diverse, and attractive, materials properties known to date, including high temperature superconductivity, magnetism, and metal-insulator transitions. Understanding how these correlated electronic phases can be rationally manipulated at an atomic scale promises enormous potential for utilizing their emergent properties. Here we directly observe a thickness-controlled metal-insulator transition in ultrathin epitaxial films of the correlated metal LaNiO₃ using angle-resolved photoemission. We discover an abrupt and dramatic suppression of the quasiparticle coherence at a critical thickness of 2 unit cells, accompanying the onset of an insulating phase as measured by electrical transport. Our spectroscopic results reveal how the metal-insulator transition is driven by an instability to an incipient order of the underlying quantum many-body system, and demonstrate the power of artificial confinement to harness control over competing phases in complex oxides with atomic scale precision.

Recently, there is a growing interest in amorphous oxide semiconductors amongst the display and electronics industries for application in thin film transistors. The technique of choice for depositing these materials has been sputtering from an oxide target, and has been adopted by industry to produce the first commercial display products. In this work, we present spatial atomic layer deposition (S-ALD) as an alternative method to deposit these materials in an industrially scalable process at atmospheric pressure. S-ALD combines the advantages of temporal ALD (superior control of layer thickness and composition, large-scale uniformity and unparalleled conformality inherent to self-limited layer-by-layer growth) with high deposition rates, eliminating the need for vacuum processing.
S-ALD films were grown at 200 oC using pre-mixed vapors of metal precursors with water vapor as the oxygen source, spatially separated and confined by inert gas curtains. Diethylzinc, trimethylindium, triethylgallium, and tetrakis(dimethylamino)tin are used as Zn, In, Ga, and Sn precursors, respectively. By adjusting the partial pressures of the vaporized metal precursors and the exposure time, the stoichiometry of the films could be varied in the range 0 < In/Zn < 0.3, 0 < Ga/Zn < 0.1 and 0 < Sn/Zn < 0.25. The metal content in the final films has been measured via EDX. The amorphous nature of the films was confirmed via XRD. Growth rates varying from 0.3 to 0.03 nm/s have been determined via spectroscopic ellipsometry, with a substantial nucleation period (up to 300 cycles) observed for IZO and IGZO films before a constant growth rate is measured.
We have fabricated thin film transistor devices with S-ALD indium zinc oxide (IZO), indium gallium zinc oxide (IGZO), and zinc tin oxide (ZTO) as the active layers in the channel. The performance of these devices has been quantified for varying stoichiometries, post-deposition anneals, and exposure times. Mobilities up to 10 cm2/Vs have been achieved for IZO devices with thin (20 nm) semiconducting channels deposited on an SiO2 gate dielectric.

Transparent electronics has arrived and is contributing for generating a free real state electronics that is able to add new electronic functionalities onto surfaces, which currently are not used in this manner and where silicon cannot contribute [1]. The already high performance developed n- and p-type TFTs have been processed by physical vapour deposition (PVD) techniques like rf magnetron sputtering at room temperature which is already compatible with the use of low cost and flexible substrates (polymers, cellulose paper, among others). Besides that a tremendous development is coming through solution-based technologies very exciting for ink-jet printing, where the theoretical limitations are becoming practical evidences. In this paper we will review some of the most promising new technologies for n- and p-type thin film transistors based on oxide semiconductors and its currently and future applications.

Probing an internal potential distribution using a gated-four-probe (GFP) method is a key technique to characterize intrinsic parameters such as parasitic resistance and intrinsic field-effect mobility, not only in conventional amorphous silicon thin-film transistors (TFTs) but in oxide-based TFTs. In this talk, we introduce the applications of the GFP method in amorphous indium-gallium-zinc-oxide (a-IGZO) TFTs with amorphous indium-zinc-oxide source/drain electrodes. First, we extracted parasitic resistance and density of states distribution using the GFP method. Then, temperature dependent intrinsic characteristics were analyzed with a wide temperature range from 93 to 373 K. We found that temperature independent transfer characteristics and low activation energy of the contact resistance originated from low defect density of states and a formation of Ohmic contact between source/drain electrodes and an a-IGZO active layer, respectively. Therefore, the a-IGZO TFTs can be used in the condition of the wide temperature range with small variations of electrical properties.

Amorphous oxide is an attractive option for thin film transistors (TFTs) for their relatively high mobility, transparency and low temperature deposition allowing integration on flexible substrates. They are heavily investigated for large area flexible display such as AMOLED [Adv. Mater. 24, 2945 (2012)]. This application requires scaling down the TFT to allow the high resolution aspect while maintaining decent drive current and carrier mobility that allow the required switching speeds for this application [ECS Trans. 22, 227 (2009)]. Thus maximizing the drive current per chip area is a critical aspect. Integrating devices on plastics is limited by low thermal budget (<150 °C) which typically affects the quality of the semiconductor-dielectric interface especially in terms of trap densities [IEEE LEOS 2, 533 (2003). That limits effective gating to modulate carrier density in the channel. Also, patterning plastic substrate for making 3D TFTs or for making smaller gate length TFTs is challenging compared to silicon substrate. So, we report an area efficient flexible thin film transistor architecture that allows expansion of the device width through wavy channel (WC) architecture [Appl. Phys. Lett. 102, 134109 (2013)], which is integrated on flexible bulk mono-crystalline silicon (100) substrate. This architecture expands the transistor width in the vertical direction using fin type features. That gives rise to a higher normalized drain current compared to planar devices for two reasons: (i) expansion of the device width in the vertical direction and (ii) enhanced effective electric field in the channel due to the fin architecture. We used an atomic layer deposition (ALD) based Zinc Oxide (ZnO) as channel material and ALD aluminum oxide (Al₂O₃) as gate dielectric to fabricate WCTFT that gave 3.5x drain current of a planar counterpart consuming the same chip area for the maximum number of fins incorporated in the design. We optimized the ALD ZnO film as a function of deposition temperature to achieve the best resistivity value for TFT operation [Semicond. Sci. Technol. 26, 085007 (2011)]. With our study we show that the WCTFT architecture has additional advantages such as increased control over the device threshold voltage (V_t) as a function of the number of fins in the channel. We observed linear decrease of V_t as a function of the number of fins. Hence, through this design, the positive V_t shift issue, reported to flexible ZnO TFT as a function of the bending radius, can be mitigated [IEEE Electron Device Lett. 31, 1254 (2010)]. This phenomenon consequently lowers power consumption as lower Vdd needs to be used compared to planar devices with the same chip area. Finally, wavy channel architecture TFT holds promise to provide output current boost-up in an area efficient way and compatible with most of the known TFT materials (oxide, organic, solid state like poly, mono-crystalline and hydrogenated amorphous silicon).

Amorphous oxide semiconductors and in particular, indium-gallium-zinc oxide (a-IGZO) thin films have gained considerable interest for display and flexible electronics applications. In this paper we present fabrication and characterization of RF sputtered a-IGZO TFTs having a modified etch stop structure with source/drain contact windows on Corning LOTUSTM Glass. The effect of annealing time, channel length and metal oxide thickness on device performance in terms of mobility, on/off current ratio, threshold voltage, and sub threshold slope was investigated.

TFTs are fabricated by first depositing 140 nm of AlNd onto 150mm glass substrates and patterning the aluminum film by lift-off in order to form gate electrodes. A 100 nm thick SiO2 layer is then deposited by PECVD to serve as the gate dielectric. RF sputtered 50 nm of IGZO thin film forms the active layer of the TFTs. A 50nm RF-sputtered SiO2 serves as a first passivation layer which protects the underlying IGZO film. The SiO2 and IGZO layers are patterned by dry and wet etching processes, respectively. Then, a second 50 nm SiO2 passivation layer is deposited, followed by two different patterning and dry etching steps in order to form the contact windows on top of the source and drain regions and the gate pads. Finally, a double layer of Mo/AlNd electrode forms the source and drain electrodes through a lift-off process. The device fabrication is then completed by a thermal annealing at 300 C in nitrogen ambient; various annealing times were investigated.
Device transfer characteristics and gate leakage current were measured at VDS = 0.1 V and +10 V for a VGS in the range of -10 to +20 V. Mobility was extracted from the maximum transconductance while the threshold voltage was obtained using the linear extrapolation method from the characteristics obtained at VDS = 0.1 V. For each wafer a total of 41 dies were tested and multiple TFTs with different geometries were characterized per die. Some TFTs showed an average field effect mobility of 15.5 cm2V-1s-1(with standard deviation of 2.5) and threshold voltage of 4.8 volts (with standard deviation of 1) even after 1 hour of annealing time. Increasing the annealing time resulted in a relatively small improvement in average mobility and threshold voltage but it had a big improvement on the number of functional devices. TFTs with a thicker IGZO layer, located around the center of a substrate, required more annealing time with respect to thinner ones located around the edge. A dependency between channel length and required annealing time was also observed with longer devices requiring more annealing time to exhibit good characteristics.

Tin dioxide is a well known metal oxide compound used in many electronic applications, such as transplant conductor for optical and electronic devices, and chemical sensor for gas detection. It shows native n-type conductivity and it has been believed that non-stoichiometry is the reason for its n-type conductivity. However, density functional theory predicted that non-stoichiometry, e.g., oxygen vacancy, is not a reason for conductivity of nominally undoped tin dioxides. Moreover, hydrogen is predicted to be a shallow donor in tin dioxide by simulation.
In this study, intentional hydrogen doping was performed to evaluate mobility in tin dioxide. Tin dioxide films, doped and undoped, were deposited by molecular beam epitaxy and bulk tin dioxide was synthesized by sintering. We employed heavy water as the hydrogen source and secondary ion mass spectrometer was used to measure the isotope tracer distribution in the sample. As a result, we realized that the hydrogen concentration as well as hydrogen mobility strongly depend on the dopant concentration. Density functional calculation was also performed to explain the local structure around hydrogen impurity in tin dioxide.

The semiconducting properties of Zinc Oxide (ZnO) have in the past decade been studied quite extensively and several different potential applications have been explored. One of the most successful applications so far is as a transparent conductive oxide (TCO), where ZnO is typically doped with Aluminium (AZO) to produce highly conductive and transparent layers. AZO is an environmentally friendly and abundant alternative to Indium doped tin oxide as a TCO. Although Al is one of the most important donors in ZnO, fundamental studies related to configuration, solubility and diffusivity are scarce in the literature.
In this study, secondary ion mass spectrometry (SIMS) has been employed to study the diffusion properties of Al in single crystalline ZnO. A thin film of AZO was deposited onto a single bulk crystal of ZnO (nominally undoped) using magnetron sputtering. The sample was then heat treated at temperatures in the range of 900-1300°C to enable diffusion of Al into the bulk material. The diffusion of Al is suggested to be Zn-vacancy mediated and can be described by employing the phenomenological Fair's vacancy model for diffusion. For further understanding, we have also undertaken hybrid density functional calculations. The calculations show that the complex between Al on Zn-site (Al_Zn) and the Zn-vacancy is highly stable with a dissociation energy of 2.8 eV. The charge state of this complex is effectively -1, and it thus acts as a compensating defect, limiting full “usage” of the Al_Zn donor as an n-type dopant. Furthermore, these calculations also predict a high formation energy for both the Al on O site and interstitial Al donors, indicating that they do not contribute to long range diffusion of Al in ZnO.

Modern photonic devices such as microresonators, ultraviolet detectors and electrically driven nanolasers are often based on ZnO micro- and nanostructures with high crystalline quality. However, the presence of defects has great impact on material properties, such as carrier lifetime or mobility. Understanding the incorporation of defects is of no doubt an essential step for the development of future applications.
We have investigated the electrical properties of a ZnO microwire grown by carbo-thermal evaporation, a pulsed laser deposition ZnO thin film on a-plane sapphire substrate and a hydrothermally grown Zn-face ZnO single crystal (Tokyo Denpa Co. Ltd.). The samples were investigated by means of current-voltage (IV) measurements, capacitance-voltage (CV) measurements, and deep-level transient spectroscopy (DLTS).
This first investigation of a ZnO microwire revealed the presence of the defects T2 [1] and E3 [2] in all samples. Additionally, in the single crystal and in the thin film sample E64 [3] and E4 [2,4] were detected.
The DLTS-scan of the ZnO thin film shows two additional peaks stemming from the electron emission of T3 [5] and E1 [6], respectively. The bulk sample as well as the microwire sample show a positive peak in the DLTS scan, which is usually a hint for emission of minority carrier traps. We attribute this signal to the freeze-out of free carriers and therefore an increase of the series resistance of the samples [7,8]. Hall effect measurements on a similar microwire showed that freeze-out of carriers occurs at 200K. Due to freeze-out defect levels beeing more shallow than T2 could not be characterized by DLTS.
Our investigations support the common opinion that T2 and E3 are intrinsic defects since these levels are found in all samples and their occurrence is not related to the growth technique.
[1] M. Schmidt, M. Ellguth, R. Karsthof, H. von Wenckstern, R. Pickenhain, M. Grundmann, G. Brauer, and F. C. C. Ling, Phys. Status Solidi B 249, No. 3, 588 (2012).
[2] T. Frank, G. Pensl, R. Tena-Zaera, T. Ohshima, H. Itoh, D. Hofmann, D. Pfisterer, J. Sann, B. Meyer, Appl. Phys. A 88, 141-145 (2007).
[3] H. von Wenckstern, PhD thesis (2008).
[4] H. von Wenckstern, R. Pickenhain, H. Schmidt, M. Brandt, G. Biehne, M. Lorenz, M. Grundmann, and G. Brauer, Appl. Phys. Lett. 89, 092122 (2006).
[5] M. Schmidt, M. Ellguth, F. Schmidt, T. Lüder, H. von Wenckstern, R. Pickenhain, M. Grundmann, G. Brauer, and W. Skorupa, Phys. Status Solidi B 247, No. 5, 1220-1226 (2010).
[6] F. D. Auret, S. A. Goodman, M. J. Legodi, W. E. Meyer, and D. C. Look, Appl. Phys. Lett. 80, 1340 (2002).
[7] A. Broniatowski, A. Blosse, P. C. Srivastava, and J. C. Bourgoin, J. Appl. Phys. 54, 2907 (1983).
[8] S. Anand, S. Subramanian, and B. M. Arora, J. Appl. Phys. 72, 3535 (1992).

Vanadium oxide bronzes (M$_{x}$V$_{2}$O$_{5}$ where M = Ag, K, Na, Pb, Sr) exist in layered or tunnel structures depending on the concentration (x) and ionic size of the dopants. Dopant induced in-gap states create electronic instability that can be utilized to dramatically alter the electronic properties of these materials in the nanoscale, under various external stimuli. We report the results of electrical transport measurements in single crystalline, individual nanowires of Ag$_{x}$V$_{2}$O$_{5}$ ($\beta$ $\&$ $\delta$ phases) in two terminal device configuration. The current-voltage characteristics in these semiconductor-like materials are highly nonlinear thereby making them potential candidates as nanoscale switching materials (at T = 150 K, OFF state: V$_{DS}$ = 5 mV, I$_{DS}$ = 0.5 nA , ON state: V$_{DS}$ = 1.5 V), I$_{DS}$ =0.5 $\mu$A; at T = 300 K, OFF state: I$_{DS}$ = 50 nA , ON state: I$_{DS}$ = 30 $\mu$A). Repeatability of the switching behavior, the speed of switching and the role of different dopants on the electronic properties will be discussed.

Transparent semiconducting oxides are recently investigated with large effort due to their tremendous potential for applications in transparent (opto-)electronics [1]. Nowadays ultraviolet (UV) photodetector (PD) arrays based on wide bandgap semiconductors find applications for surveillance purpose, remote sensing and chemical/biological sensors [2]. The bandgap of the ternary semiconductor (Mg,Zn)O can be tuned between 3.3-4.8 eV for wurtzite modification [3]. We have reported previously on wavelength-selective PD arrays using (Mg,Zn)O heterostructures [4], in which the layer with higher Mg-content acts as an optical filter blocking high energy radiation and the active layer with lower Mg-content contributes to photo response.
Here we demonstrate visible-blind narrow-bandwidth wavelength-selective monolithic multichannel PD arrays based on a novel design, in which we have separated the filter and active layer. The PDs are fabricated on a (Mg,Zn)O thin film with a continuous composition spread (CCS) serving as the active layer. Its absorption edge changes continuously across the wafer. This layer was deposited by pulsed-laser deposition (PLD) on a double-sided polished a-plane sapphire 2 inch wafer using a multi-segmented PLD-target to achieve a linear CCS of the (Mg,Zn)O thin film. The multichannel detection can be enabled with an integrated optical filter layer with a single Mg-content, which is deposited on the other side of the wafer, enabling the tuning of the cutoff energies and varying bandwidth. Therefore, a PD array with narrow bandwidths can be realized by compositionally graded filter layer, which has the same target constituents, but with a CCS perpendicular to that of the active layer.
The change of the Mg-content of the CCS can be correlated with a variation of the emission energy of cathodoluminescence (CL) spectra and is also characterized by energy dispersive X-ray spectroscopy (EDX). The linearity of continuously changing bandgap and the chemical composition of Mg across the spread are observed by CL- and EDX-measurements. The difference of the maximum emission energy is about 400 meV and the Mg-content increases from 7% to 21%. This PD array has the ability to detect and process signal at different photon energy simultaneously with designate spectral resolution. An internal gain mechanism within the multichannel PD arrays caused by trapping of photo-exited holes at metal/semiconductor interface was also found [4,5].

[1] M. Grundmann et al., Phys. Stat. Sol. (A) 207,1437 (2010)
[2] M. Razeghi and A. Rogalski, J. Appl. Phys. 79, 7433 (1996)
[3] H. von Wenckstern et al., The (Mg, Zn)O alloy, Chap. 10, Taylor and Francis/CRC Press, Florida, USA, (2012)
[4] Z. P. Zhang et al., Appl. Phys. Lett. 99, 083502 (2011)
[5] O. Katz et al., Appl. Phys. Lett. 84, 4092 (2004)

Gallium oxide (Ga2O3) has attractive materials for its large wide band gap. There are five types of crystal polymorph in Ga2O3 that is alpha, beta, gamma, delta, and epsilon [1]. Among them, beta phase is most stable, therefore being widely-used in researches. According to recent progress, Higashiwaki et al demonstrated of Schottky diodes and field-effect transistors with high breakdown voltage by using of beta-Ga2O3 substrate [2,3]. It indicated a possibility to realize power devices beyond SiC or GaN because of larger band gap with 4.9 eV. However, in terms of industrial applications, there are some demands to realize Ga2O3 power devices. Beta-Ga2O3 substrate is not commercially available, in addition large-scale beta-Ga2O3 wafer has not realized yet. One of the solutions to overcome these problems is using of commercially available substrate. Sapphire (alpha-Al2O3) is the most candidate materials because of being widely-used, high thermal stability, and relatively high thermal conductivity. In addition, there are many reports about alpha-Ga2O3: growth of high-crystalline alpha-Ga2O3 on c-plane sapphire substrates using of Mist Chemical Vapor Deposition (CVD) technique [4,5], clarification of crystal growth mechanism of alpha-Ga2O3 on sapphire [6], and conductivity about Sn doped alpha-Ga2O3 thin films on sapphire substrates [7,8]. It was ready to grow alpha-Ga2O3 thin films on large-scale sapphire substrate for realizing commercially available wafers aiming to power devices. In this symposium, we report the high quality alpha-Ga2O3 thin film on c-cut 4-inch sapphire substrate by using of Mist-CVD method. We conducted X-ray diffraction measurements at 10 points in the 4-inch sapphire wafer. XRD 2theta;/theta; scanning profile indicated oriental growth of alpha-Ga2O3 thin film aligned along c-axis. Beta phase or other oriented crystal phase could not be identified. In order to estimate crystalline quality of the film, XRD rocking curve (omega;-scanning) measurements were also conducted. The omega-scan rocking curve full width at half maximum (FWHM) values are from 46.44 to 167.04 arcsec, so we had successfully grown highly-crystalline alpha-Ga2O3 thin film on 4-inch sapphire substrate. Moreover, cross-sectional TEM images support the evidence of crystallinity: there were a little remarkable dislocation lines along c-axis at the entire field of small samples cut out of large-scale samples. At the symposium, we will report electrical properties of Schottky diodes based on alpha- Ga2O3.
[1] R. Roy et al: JACS, 74 (1952)719. [2] M. Higashiwaki et al: APL, 100 (2012) 013504.
[3] K. Sasaki et al: APEX, 5 (2012) 035502. [4] D. Shinohara et al: JJAP, 47 (2008) 7311.
[5] T. Kawaharamura et al: JJAP, 47 (2008) 4669. [6] K. Kaneko et al: JJAP, 51 (2012) 020201. [7] T. Kawaharamura et al: JJAP, 51 (2012) 040207. [8] K. Akaiwa et al: JJAP, 51 (2012) 070203.

Resistance switching phenomena observed in metal oxide thin films has recently attracted a great deal of attention to develop next generation low power, low cost, high speed, and nonvolatile resistive random access memory (RRAM) devices. The memory effect in these materials is realized through the switching of the resistance of their thin films between two states of high and low resistances. Among other metal oxides currently being explored for the development of RRAM, ZnO has been demonstrated as a potential candidate. In this paper we report nonvolatile unipolar resistive switching characteristics of ZnO thin films deposited by thermal ALD.
About 100 nm thick film of ZnO were grown on Pt/TiO₂/SiO₂/Si substrates at 200°C by thermal ALD. Diethyl Zinc (DEZn) and water (H₂O) were used as precursors for zinc and oxygen respectively. To construct MIM devices, the top electrodes of ~ 70 nm thick Pt film with a diameter of ~ 40 µm were deposited using DC Sputtering. The switching characteristics and conduction mechanisms of these devices were studied through current-voltage (I-V) measurements in the top-bottom configuration both at room temperature and at varying temperatures in the range of 300-500K. The devices were found to be initially in high resistance state (HRS) ~ 0.7 MOmega; (measured at ~0.1 V) and did not show any resistance switching until the applied bias voltage was increased to ~ 3.2 V (initial forming voltage) with a current compliance of 5 mA at which the resistance of device dropped suddenly to a low value of ~ 40Omega; (measured at ~ 0.1 V). After this initial forming process, which rendered the devise in LRS, as the voltage was swept again form 0 to 1 V the device switched from LRS to HRS (ON state) at a voltage of ~ 0.5 V. The RRAM device again switched to LRS (OFF state) at ~ 1.8 V as the voltage was swept from 0 to 2 volt. The repeatable nonvolatile switching of the resistance of RRAM device between LRS and HRS was obtained with nearly constant resistance ratio ~ 10⁴ and well defined and non-overlapping switching voltages in the range of 0.45-0.60 V and 1.8-2.2V respectively for up to 10² test cycles. The current conduction mechanism of the device in LRS and HRS were found to be dominated by the Ohmic behavior and Poole-Frenkel emission respectively. The temperature dependent measurements of the resistance of the device indicated metallic and semiconducting conduction behavior in LRS and HRS respectively. The resistance of LRS and HRS of the device read at 0.1 V, showed no obvious degradation for up to ~ 5×10⁴ s indicating good data retention. A similar switching characteristic was also observed at elevated temperatures up to 500K. Further optimizations and studies on the device structure are underway to achieve better endurance and switching characteristics and understand the associated resistance switching mechanisms.

Many researches on amorphous oxide semiconductor (AOS) based thin-film transistors (TFTs) have been vigorously investigated because of their superior mobility and transparency compared with amorphous Si TFTs. Furthermore, solution process has many advantages compared to conventional vacuum process: reducing manufacturing cost and easiness of modifying composition ratio or adding additives. Recently, our group reported solution-processed indium gallium zinc oxide TFTs with multi-stacked active layers (MSALs) for enhancement of electrical performances and stability under bias stress conditions.[1][2] However, the effects of MSALs on process temperature have not been figured out precisely. In this study, we compared the performances between MSAL TFTs and single layer TFTs in various process temperatures.
We prepared the indium oxide solution with different molarity of 0.05 and 0.15 M. Each solution was spin-coated on the heavily doped p-type Si wafer with thermally oxidized SiO2 of 1200 #8491;, and those samples were pre-annealed at 150°C for 5 minutes. The 0.15 M indium oxide samples were post-annealed at 150, 200, and 250°C for 2 hours. On the other hand, the 0.05 M indium oxide samples were post-annealed at 150, 200, and 250°C for 40 minutes. For the 0.05 M samples, the spin-coating, pre-, and post-annealing processes were repeated 2 times more to make three MSALs.
The 0.05 M MSAL TFTs annealed at 250°C showed following TFT characteristics: 1.05 cm2/Vs of field effect mobility, 1.32×107 of on/off current ratio, 8.75 V of threshold voltage, and 0.75 V/decade of subthreshold swing. The 0.15 M single layer TFT annealed at 250°C, however, showed not switching characteristic but only leakage current regardless of gate voltage. The results show that the MSALs have possibility to decrease process temperature in solution-processed AOS TFTs.
[1] D.J. Kim, D.L. Kim, Y.S. Rim, C.H. Kim, W.H. Jeong, H.S. Lim and H.J. Kim: Improved electrical performance of an oxide thin-film transistor having multi-stacked active layers using a solution process. ACS applied materials & interfaces 4, 4001 (2012).
[2] D.J. Kim, Y.S. Rim and H.J. Kim: Enhanced electrical properties of thin-film transistor with self-passivated multistacked active layers. ACS applied materials & interfaces 5, 4190 (2013).

As oxide semiconductors such as amorphous In-Ga-Zn-O (a-IGZO) have higher electron mobility than materials used for conventional thin-film transistors (TFTs) like amorphous Si, they are considered to be one of the most promising materials for next generation flat panel display [1]. The critical issue is that the performance of TFTs is known to be influenced by the fabrication process conditions, which implies that sub-gap states in a-IGZO. In the present study, we have evaluated the sub-gap states of a-IGZO thin films treated with various process conditions by photoinduced current transient spectroscopy (PITS) [2].
A series of a-IGZO thin films with a thickness of 50 nm was deposited by DC magnetron sputtering on glass substrates at room temperature. Then, the films were annealed in the temperature range between 250 and 350 °C for 1 h in air. The PITS measurements were performed in the temperature range between 80 and 400 K. A laser with a wavelength of 357 nm was sued for excitation.
According to PITS spectra from the a-IGZO thin films without thermal annealing, three broad peaks were mainly observed. The spectra are quite similar to those for ZnO [2]. The features of the spectra were drastically changed depending on the annealing temperature. The three broad sub-gap states slightly decreased after 250°C annealing, and furthermore, only broad band around 120 K was detected after 350°C annealing. Considering the temperature dependence, the formation of the sub-gap states could be originated from oxygen and/or hydrogen related centers.
[1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, Nature 488, 432 (2004).
[2] Ch. Hurtes, M. Boulou, A. Mitonneau, and D. Bois, Appl. Phys. Lett. 32, 15 (1978).
[3] V. Quemener, L. Vines, E. Monakhov, and B. G. Svensson, Appl. Phys. Lett. 100, 112108 (2012).

In this work, the transparent n-type semiconductor SnO₂ is self-assembled on p-type tin sulfide thin film via thermal oxidation of p-type tin sulfide layer in oxygen atmosphere. Various parameters of thermal oxidation were applied to form n-SnO₂ and the p-n junction (p-SnS/n-SnO2) was also formed.
The oxygen defect of n-SnO₂ is controllable by changing the oxidation temperature and partial pressure. The band structure, therefore, was changed with the oxygen defect. Furthermore, the turn-on voltage of p-n junction can be tuned by controlling the oxidation state of n-SnO₂. It shows the potential on band gap engineering for the applications of optical and electrical devices, like solar cells and gas sensors.
The X-ray Photoelectron Spectroscope (XPS) was used to identify the valence state of tin and oxygen. Transmission Electron Microscope (TEM) and Energy Dispersed X-ray Spectroscopy (EDX) were utilized to characterize the structure and element composition of n-SnO₂. The electrical properties of the p-n junction were measured.

Herein, we report a novel and easy strategy for fabricating solution-processed metal oxide thin film transistors by controlling the dielectric constant of H2O as a function of the metal precursor solution temperatures. As a result, indium zinc oxide (IZO) TFTs fabricated with the IZO precursor solution at 4°C had the highest mobility of 12.65 cm2/Vs while the IZO TFTs fabricated with the IZO precursor solution at 25 and 65°C had a field effect mobility of 5.39 and 4.51 cm2/Vs, respectively. In the case of the IZO precursor solution at 4°C, metal cations such as indium (In3+) and zinc ions (Zn2+) possibly are fully surrounded by H2O molecules due to their higher dielectric constant. These chemical speices in the IZO precursor solution at 4°C are advantageous for thermally-driven hydrolysis and condensation reactions. Because they can have high potential energy for thermally-driven hydrolysis and condensation reaction to form metal oxide lattices. The highest mobility of the IZO TFTs fabricated with the IZO solution at 4°C is due to the formation of many metal-oxygen-metal (M-O-M) bonds. These ns-orbitals of metal cations overlap each other and form electron conduction pathways. Thus, it is advantageous for electron conduction to form high proportion of M-O-M bonds in the IZO thin films since oxide lattices allow electrons to transport through the IZO.

How correlated electrons respond to electric fields is a fundamental problem that needs to be understood for eventual applications in solid state devices. Semiconductor device junctions are one classical way to probe this rigorously where electrostatic effects can be systematically studied in the low field limit. It is not trivial though to fabricate pn junctions with correlated oxides due to challenges in material synthesis with good control over interface properties and their high carrier density even in the insulating phase. In this presentation, we will discuss our recent studies on fabrication correlated insulator (VO₂) - band semiconductor (GaN) np junctions and complete electrostatic characterization as well as dynamics of the minority carriers. The junctions show rectifying current-voltage behavior from room temperature to 100 °C, spanning across the metal-insulator transition of VO₂. The barrier height was extrapolated from electrical measurements such as current-voltage and capacitance-voltage measurements. The role of crystal symmetry breaking on the ideality factor of the junctions has been explored. We have also observed strong frequency dependence in the capacitance-voltage measurements and attribute the observations to the effects of interfacial states, series resistance and/or minority carriers depending on the applied dc voltage. The response of the junction to electric fields allow us to build a model for electron thermodynamics at the interfaces of vanadium dioxide and have broad significance to understanding and modeling physics of correlated insulator devices.

ZnO is one of the emerging UV optical materials due to its environmentally friendly chemical nature, resistivity to harsh environments, and deep excitonic level. Furthermore, engineering the band gap of ZnO via alloying ZnO with certain atomic constituents provides materials with tunable optical and electronic properties, which is essential for the potential applications of ZnO in deep UV region and visible region. Alloying ZnO and ZnS is an alternate of decreasing the band gap of ZnO, which typically involves using toxic material. ZnS is quite chemical friendly and widely found in nature as the mineral sphalerite. While both ZnO and ZnS have bandgaps in the UV range, 3.3 and 3.5 eV respectively, the ternary compound ZnS_xO_1-x shows a large bowing parameter due to the large electronegativity difference and size mismatch between O and S, which allows the bandgap of ZnS_xO_1-x to reach into the visible range. In this work we report the growth of ZnS_xO_1-x films (0le;xle;0.28) on c-plane quartz substrates by a radio frequency magnetron sputtering system. The elemental composition was confirmed by Energy-dispersive X-ray spectroscopy (EDS). The Bandgap energy of the alloyed films was determined by optical transmission and show composition dependence. It was found that as sulfur composition x increases from 0 to 0.28, the Bandgap energy decreases from the UV range of 3.20 eV to the blue range of 2.57 eV. Phonon mode behavior, X-ray diffraction and morphology will be discussed.
The authors gratefully acknowledge the National Science Foundation, DMR-1202532

Control of Opto-electronic properties of wide band gap II-VI semiconductor ZnO by using electric double layer (EDL) gate is an attractive proposition. In FET configuration where the EDL makes the gate, high carrier concentration (~1014 cm2) can be achieved by applying a moderate gate voltage. In this paper we report a new effect where a synergy between the carriers created by UV illumination and those created by the gate has been achieved leading to enhanced optical response in ZnO in the EDL-FET configuration. The effect shows that the illumination can have a large control on the FET characteristics leading significant enhancement of trans-conductance under illumination with energy more than the band gap energy of ZnO. The effect has been seen in high quality single crystal, epitaxial films made on Sapphire by pulsed laser deposition and in films formed on fused quartz using chemical deposition. It has been found that the photocurrent enhancement under simultaneous presence of illumination and gate voltage is more than the photocurrent (in absence of gate voltage) and the current in the FET (drain-source current) in dark. We suggest (based on supporting experiments) that an enhancement of the field effect mobility under UV illumination leads to the observed synergetic effect.
To support our hypothesis we did an independent experiment which shows that nano capacitance formed at the gate, is unchanged under band gap illumination but the illumination leads to large creation of charge in the channel region. This leads to passivation of the ionized oxygen vacancy (Vo+) (which is a scattering centre in ZnO) leading to decrease in the scattering in the channel region and a significant enhancement of the field-effect mobility under illumination. We also find that the intensity of photo-luminescence arising from Oxygen vacancy can be changed by a factor of nearly 2 by applying gate voltage in EDLFETs, showing a direct connection of the oxygen vacancy defect and the gate effect in the EDL-FET.

Rhombohedral α-AgGaO2 has a reported band gap of 2.4 eV [1] which make it a good candidate for visible light sensitive photocatalyst. The reported synthesis methods of α-AgGaO2 are based on cation exchange reactions, hydrothermal technique and ultrasound assisted method.[2-6] However, no direct synthesis from metal oxides has been reported in the literature.
In this work, we demonstrate for the first time, the synthesis of α-AgGaO2 directly from a mixture of Ag2O and Ga2O3 powders. Synthesis experiments are conducted under extreme pressures (~ 10 GPa) and high temperature (~600oC) conditions using diamond anvil cell (DAC). The synthesized materials have the form of 100-200 nm nanocrystals, with well-developed facets and single crystallinity, as confirmed by high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). Energy dispersive x-ray (EDX) spectra measured from these nanocrystals confirm elemental composition expected for AgGaO2. Furthermore, detailed analysis of electron diffraction indicates rhombohedral delafossite crystal structure with lattice parameters a asymp; 2.99 #8491; and c asymp; 18.4 #8491;. Raman spectra obtained for these samples show several peaks between 350 cm-1 and 700 cm-1. The strongest two of them agree with the Raman-active vibrational modes calculated using ab initio DFT method. The appearance of additional Raman peaks seems to originate from the relaxation of selection rules caused by high density of structural defects (stacking faults, etc.) observed in the TEM study.
References:
1. Maruyama, Y.; Irie, H.; Hashimoto, K. J. Phys. Chem. B 2006, 110, 23274-23278.
2. Vanaja, K. A.; Ajimsha, R. S.; Asha, A. S.; Jayaraj, M. K. Appl. Phys. Lett. 2006, 88, 212103
3. Ouyang, S.; Kikugawa, N.; Chen, D.; Zou, Z.; Ye, J. J. Phys. Chem. C 2009, 113, 1560-1566
4. Sheets, W. C.; Mugnier, E.; Barnabe, A.; Marks, T. J.; Poeppelmeier, K. R. Chem. Mater. 2006, 18, 7-20
5. Shannox, R. D.; Rogers, D. B.; Prewitt, C. T. Inorganic Chemistry, Vol. 10, No. 4, 1971, 713-718
6. Nagarajan, R.; Tomar N. Journal of Solid State Chemistry, 182, 2009, 1283-1290

Pb(Zr,Ti)O3 (PZT) thin films have been widely used for various electronic devices because of their excellent piezoelectric properties. However Pb-contain materials will be restricted its use by various regulations such as RoHS and ELV. Pb-free (Na,K)NbO3 (NKN)-systems are promising candidates for lead-free piezoelectric materials because of their piezoelectricity comparable to that of PZT. Especially, 0.92(Na0.5K0.5)NbO3-0.06BaZrO3-0.02(Bi0.5Li0.5)TiO3 (NKN-BZ-BLT) ceramics reported by Wang et al. showed the piezoelectric constant (d33) of 420pC/N as high as those of commercial PZT. In this study, preferentially oriented NKN and NKN-BZ-BLT films were prepared by r.f. sputtering on Si substrate. Preferential orientation is favorable for the piezoelectric application because orientation axis aligns parallel to the polarization axis. Preparation of the films on Si substrate is good for various device applications.
(Na0.5K0.5)NbO3 and 0.92(Na0.5K0.5)NbO3-0.06BaZrO3-0.02(Bi0.5Li0.5)TiO3 films were deposited on (111)Pt/TiO2/SiO2/(100)Si substrates by r.f. magnetron sputtering. X-ray diffraction (XRD) measurements were carried out using the Bragg-Brentano method. Ratio of O2 gas in the mixture of Ar and O2 (gamma= O2/(Ar + O2)) was varied from 0 to 0.5. Substrate temperature during deposition was varied from r.t. (without intentionally heating) to 600 C.
Diffraction peaks was found for the films deposited at ge;500 C, while a halo peak was seen for the films at le;400 C, indicating that the films deposited at le;400 C showed an amorphous structure. The diffraction peaks found for the films deposited at 500 C and 600 C were assigned to the perovskite structure. Though a few peaks assigned to impurity phases such as K4Nb6O17 4.5H2O and niobate were observed for the films deposited at both 500 C and 600 C, their peak intensities for the films deposited at 500 C were smaller than those for the films at 600 C. It was also found that the weak peaks assigned to the impurity phases found for the films at 500 C decreased with decreasing gamma Optimum conditions for deposition of the NKN films were a substrate temperature of 500 C and gamma=0.05. Relative permittivity of the obtained NKN films showed 600 (at 1 kHz). This value was lower than that reported in literature. This may be originated from a variation of chemical composition, since chemical composition measured by the wavelength dispersive X-ray fluorescence spectrometer indicated CNa / CK =0.4, where CNa and CK are measured Na and K content and CNa / CK = 1 is a nominal composition.

As we become an increasingly wireless society, there is a large demand for widening the range of frequencies accessible to devices. At gigahertz frequencies, the thin film dielectric material, Ba_xSr_1-xTiO₃, responsible for tuning between frequencies in microwave circuits, experiences large loss. This is thought to be due to point defects. Oxide molecular-beam epitaxy (MBE) provides a controlled way to fabricate a new generation of thin-film, high-frequency tunable dielectrics via atomic layering and epitaxial strain. We recently demonstrated record tunable dielectric performance at gigahertz frequencies for strained Sr_n+1Ti_nO_3n+1 phases. In this work we use MBE to build Ba_xSr_1-xTiO₃ into a Ruddlesden-Popper superlattice, Sr_n+1Ti_nO_3n+1-(BaTiO₃)₁ in hopes of accommodating point defects by the (SrO)₂ faults, leaving the rest of the dielectric material pristine. The first of this series, Sr₂TiO₄-(BaTiO₃)₁, is grown on (001) LSAT using MBE, and characterized by x-ray diffraction. We are making and measuring this superlattice series to identify a tunable dielectric with properties that could surpass the best materials used in microwave applications today.

Transition metal oxides are known for manifesting a variety of opto-electronic and ionic phenomena that are of interest from the perspective of both fundamental physics and applications. One such class of phenomena linking ionic (oxygen vacancies) and electronic transport is resistive switching and metal-insulator transitions (MIT). Governed by several different mechanisms, they not only expand our understanding of the correlation effects, ionics, interaction between charge, spin and orbital degrees of freedom, but also find use in memristive electronics, non-volatile memories, smart glasses, Mott transistors etc. Calcium-doped bismuth ferrite (CaBFO) is one of the prospective materials for these applications as at doping level of ~ 10% it exhibits a bias-driven two orders of magnitude insulator-metal transition. The reversible change in the electronic conductivity is attributed to the dynamics of the oxygen vacancies introduced by Ca-doping, and thus this material presents an interesting case of electrochemically-controlled resistive switching. Here we report on the investigation of the MIT in epitaxial CaBFO thin film at the nanoscale using scanning probe microscopy techniques (first order reversal curve current-voltage spectroscopy and time-resolved kelvin probe force microscopy). Varying temperature, ambient gas humidity and oxygen content, we demonstrate the influence of these parameters on switching and map CaBFO surface out in terms of local electrochemical activity and activation energy of the electronic hole transport. It is demonstrated that nanoscopic surface defects possess intrinsically high conductance, which may indicate lower oxygen vacancy content. Theoretical modeling of the transport phenomena confirmed the link between the ionic and electronic dynamics and allowed for estimation of the oxygen vacancy diffusivity.
Research was supported (E.S., S.J., S.V.K., I.K.) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. A portion of this research was conducted at the Center for Nanophase Materials Sciences (E.S., S.J., S.V.K., I.K.), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

We introduce a simple, single-piece analytic approach for enhancing field-effect mobility (MFE) by engineering gate-insulator capacitance (CGI) in solution-processed, disordered oxide-semiconductor (OS) thin-film transistors (TFTs). Recently, a remarkable increment of performances for solution-processed OSs TFTs has been achieved by developing high-k dielectric materials for gate insulators. With increasing CGI, MFE for solution-processed OS TFTs drastically increases in low-voltage operations. However, conventional metal-oxide-semiconductor field-effect-transistors (MOSFETs) theory cannot explain this CGI-dependent MFE, because MFE is independent factor to CGI. Furthermore, there is still no clear explanation or physical implementation why MFE strongly depends on CGI, while verifying CGI-dependent MFE is a key to realize high performance, transparent OS TFTs which cannot be achieved by organic- or silicon- based TFTs.
Our simple analytic expression reveals that MFE increases by increasing CGI with following power-law dependence, which has been clearly verified by an experimental manner. Solution-processed ZnO TFTs with various GCI was fabricated to confirm the analytic prediction, where the devices are based on conventional bottom-gate top-contact structure with channel-length of 1000 micron and -width of 50 micron. As gate insulators, dielectric SiO2/HfLaOx double layers are applied, where the thicknesses of SiO2/HfLaOx layers were controlled to change CGI; the CGI varies from 13 to 203 nF/cm2. Consequently, using the analytic expression, we successfully demonstrated that the MFE of the ZnO TFTs is notably increased by ~100 times to 6.01 cm2/Vs (at 203 nF/cm2) from 0.066 cm2/Vs (at 13 nF/cm2).

Due to its stability and wide ranging applications, TiO2 (110) is one of the most studied oxide surfaces. Semiconductor photocatalysis with a primary focus on TiO2 as a durable photocatalyst has been applied to a variety of problems of environmental interest in addition to water and air purification. Unraveling the photochemistry of molecular oxygen interaction with TiO2 is a vital step toward understanding fundamental reactions that take place on TiO2-based materials. We use hybrid DFT/TDDFT approach and a time-dependent wavepacket dynamics method to investigate adsorption and photoinitiated dissociation of O2 on reduced TiO2 nanocluster. We have identified the most favorable spin state for O2 adsorbed at a defect site on TiO2 surface. We find that O2 dissociation at the defect site in the electronic ground state involves a spin forbidden intersystem crossing, and therefore involves a large barrier along the reaction pathway. Time-dependent wavepacket calculations reveal photoinitiated O2 dissociation on TiO2, which is in agreement with recent experiments. Photoinitiated dissociation is found to be very fast via a direct mechanism on the excited states. Non-adiabatic effects among the singlet electronic states play an important role in the O2 dissociation whereas the spin-orbit effect is negligible. In addition, we have identified a stable configuration for adsorption of two O2 molecules on the surface. Adsorption of the second O2 molecule further stabilizes the system. Bader charge analysis and natural transition orbital analysis have been performed to gain insight into the charge transfer from substrate to absorbate. Theoretical electronic spectra and density of states for various configurations will be presented. The present work provides a novel approach for studying the O2 dynamics on semiconductor surfaces. Implications of the present findings on the photochemistry on oxide semiconductors will be discussed.
This work was performed at the Brookhaven National Laboratory under Contract No. DE-AC02-98CH10886, and used resources of the National Energy Research Scientific Computing Center (NERSC) under Contract No. DE-AC02-05CH11231, with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences.

The large bandgap (3.37 eV) and exciton binding energy (60 meV) of ZnO [1] has recently stimulated intensive research into optoelectronic device applications, such as light-emitting diodes and laser diodes in the ultraviolet region [2-6]. Although metal-organic chemical vapor deposition (MOCVD) has many advantages for industrial applications over methods such as molecular beam epitaxy [2,3] and pulsed laser deposition [4,5], ZnO films produced by MOCVD tend to be low quality, due to incomplete reaction between the metalorganic and oxygen source gases in the gas phase. We have developed a new CVD method for ZnO film growth using a reaction between an alkylzinc (DMZn) gas and high-temperature H2O generated by a Pt-catalyzed exothermic H2-O2 reaction. The resulting ZnO films grown on a-plane (11-20) sapphire (a-Al2O3) substrates exhibit excellent optical and electronic properties [7]. In this study, we have investigated the effect of the N2O gas supply during film growth on the properties of the resultant ZnO films.
The CVD apparatus and growth procedure employed in this study have been previously reported [7]. Epitaxial ZnO films (6-8 mu;m thick) were grown directly on a-Al2O3 substrates at a substrate temperature of 773 K for 60 min without a buffer layer. Although the N2O gas supply pressure to the reaction zone was varied from 3.2×10-3 to 9.7×10-2 Pa, all films exhibited n-type characteristics.
The electron mobility of N2O-doped (3.2×10-3 Pa) film at 290 K was 234 cm2/Vs, while that of non-doped ZnO film was 207 cm2/Vs. The electron mobilities of ZnO films produced with various N2O pressures increased significantly with decreasing temperature to 100 K, but decreased at temperatures less than 100 K. The mobility of the N2O doped film (234 cm2/Vs at 290 K) increased to 1100 cm2/Vs at 100 K. The carrier concentrations of all films were 4-6×1016 cm-3 at RT, while those of the N2O-doped ZnO films became smaller than the non-doped film from 290 to 80 K. Activation energies for the N2O-doped ZnO films obtained from Arrhenius plots were in the range of 23.0 to 23.7 meV, while that for the non-doped ZnO film was 21.8 meV. Although a part of the donor impurities are compensated by the nitrogen acceptor, the intrinsic donor impurities caused by the defects are considered to be reduced by the doping of nitrogen, in view of the large electron mobility for the N2O-doped ZnO films.
Acknowledgements
This work was supported in part by Tokyo Electron Ltd. and a Grant-in-Aid for Scientific Research (No. 24360014) from the Japan Society for the Promotion of Science.
References
[1] B. K. Meyer et al., Phys. Stat. Sol. B, 241 (2004) 231.
[2] M. Sano et al., Jpn. J. Appl. Phys., 42 (2003) L1050.
[3] H. Tampo et al., Appl. Phys. Lett., 84 (2004) 4412.
[4] A. Tsukazaki et al., Nat. Mater., 4 (2005) 42.
[5] E. M. Kaidashev et al., Appl. Phys. Lett., 82 (2003) 3901.
[6] J. Dai et al., Appl. Phys. A, 89 (2007) 645.
[7] K. Yasui et al., MRS Symp. Proc., 1315 (2011) 21.

Zinc oxide (ZnO) has the potential to be used for advanced optoelectronic and spintronic applications. Gd doped ZnO thin films were prepared using pulse laser deposition. The effect of Gd concentration (0.01-0.15 at.%), oxygen pressure (P(O2)) during deposition (5-500mTorr), and thermal annealing at different conditions on the magnetic properties of Gd doped ZnO films were investigated using optical spectroscopy: photoluminescence (PL), PL excitation (PLE) and time-resolved spectroscopy. Low temperature PL spectra revealed a green band emission centered at 500nm (2.48 eV) for GdxZn1-xO thin films deposited under low oxygen conditions (P(O2) le; 25mTorr). As the P(O2) increases we observe a redshift and those GdxZn1-xO films deposited at P(O2) ge; 50 mTorr show an emission at 700nm (1.77 eV) that vanishes and the emergence of green emission observed after annealing under vacuum. We will show how changes in the magnetic behavior are dependent on the nature of the point defects in the films. All films that show the green band emission exhibit similar ferromagnetic behavior. However, we observe a change in the magnetic response for the films showing red band emission. We investigate the origin of this behavior using time-resolved spectroscopy and PLE measurements to identify the point defects (oxygen interstitials, oxygen vacancies and Zn vacancies), which result in the observed properties.

Nickel Oxide (NiO) is one of the promising candidates for a constituent material of visible-light-transparent (VLT) devices because of its p-type conductivity and large bandgap energy (3.7 eV). The fundamental step in realizing VLT electronic / optoelectronic devices using p-NiO is a fabrication of transparent p-n junctions with n-type semiconductors such as ZnO, MgZnO, and ITO.
Determination of the interfacial band-offset is very important for the fabrication of semiconductor devices, particularly solar cells, because band-offsets determine the current normal to the junction and various interface properties. However, there have been few reported results on band-offsets of heterojunctions containing NiO. In this presentation, band-offsets of the heterostructures composed of NiO and various n-type semiconductors and metals will be shown to eventually demonstrate the photovoltaic action of a layered structure with soda-lime glass / ITO / NiO / n-type layer / ITO.
Polycrystalline NiO films were deposited on ITO-coated glass substrates by the RF reactive sputtering method. Then, Ni, Mo, ZnO, MgZnO, or ITO film was deposited as a back-contact or a window layer. The valence band offset at the n-type layer / NiO heterointerface was evaluated by the photoelectron yield spectroscopy (PYS) measurement.
The valence band offsets relative to NiO were determined to be approximately -1.0 and -1.5 eV for the ITO and ZnO, respectively. The energy band diagrams drawn using these values indicate that both of them form TYPE-II heterostructures. The relationship between the band-offsets and current density - voltage characteristics of the representative NiO-based solar cells will be presented.

Tin oxide is one of the popular metal oxide semiconductor used in solar cells, sensors, and catalysts. The surface modification by organic self assembled monolayer is one of the promising techniques to tune and to control the surface work function. In our study, we investigated the work function change of the SnO2 (110) surface which was modified with various benzoic acid derivatives using density functional theory (DFT). All calculations were carried out on Quantum Espresso program. Electron correlation and exchange parts were treated by local density (LDA), generalized gradient approximation (GGA) with Hubbard U methods. To improve band structure calculation we used LDA+U method. The results of the calculation with LDA method indicated that the work functions of the pure and modified (by C6H4-COOH) surfaces of SnO2 (110) were 7.40 eV and 7.03 eV, respectively. As the experimental value of work function of SnO2 (110) surface is about 7.74 eV, these results indicate that the DFT calculation for SnO2 (110) surface modification by benzoic acid derivatives is feasible.

The back-gated field-effect transistor (FET) has been considered as one of the most promising devices applied for low power consumption logic circuits, non-volatile memories and sensors. Among various gate dielectric materials for back-gated FETs, ferroelectric materials have been widely utilized due to their high dielectric constant and their switchable remnant polarization. A back-gated ferroelectric FET with Metal-Semiconductor-Insulator-Metal (MSIM) structure is reported, in which an Al-doped zinc oxide (ZAO) channel layer with optimized dopant concentration of 1% was applied as a natural n-type semiconductor in order to increase the carrier density and reduce the channel resistance of the channel layer and thus to guarantee large enough load capacity of the transistor. Different ferroelectric thin films were considered for the MSIM device, such as lead zirconate titanate (PZT), bismuth ferrite without (BFO) and with the dopants of 5% lanthanum (BLFO) or 5% manganese (BFMO). The electron depletion and accumulation switching operation was conducted due to the modulation of the ferroelectric layer on ZAO channel resistance caused by the switchable remnant polarization of the ferroelectric layer, which could be proved by the measurement of capacitance-voltage characteristics and would result in different stable storage states with different channel conductivity. In this work, the ferroelectric thin films were directly deposited on metal gate electrodes exhibited better hysteresis characteristics, which also could reduce the limitations of high-temperature thermal treatment in the choices of materials and processes applied in the device. The drain current versus gate voltage transfer characteristics of the back-gated FETs in this work was analyzed and investigated. The structural characterization of the as-prepared device fabricated on Pt/Ti/silicon dioxide/Si substrate was carried out. The switching properties of the ZAO channel layer due to the modulation effects of switchable remnant polarization of ferroelectric thin film in the gate were observed and confirmed by the capacitance-voltage and the drain current versus gate voltage transfer curves. As a result, the back-gated FET in this work exhibited an eminent counterclockwise loop in the curve of drain current versus gate voltage and a drain current switching ratio up to two orders of magnitude.

Metal oxide semiconductors (MOSs) have been widely researched for the channel materials of the thin-film transistors (TFTs) due to having high electrical uniformity in large area, low temperature process, and high field-effect mobility. Among MOSs, indium oxide(In2O3) have high optical transparency and conductivity. However, as the channel materials, In2O3 is required to add a suppressor material with a high oxygen attraction such as Gallium, Zirconium, Aluminum, and Hafnium for having device modulation. Recently, the researches of solution-processed metal oxide TFTs not only have been focused on material selections or temperature issues but also studied functional process. Previously, our research group reported self-patterning process for metal oxide TFTs using UV irradiation [1]. By selective deposition of passivation material and thermal diffusion, one-step channel definition and passivation processed TFT was fabricated. The TFT was consisted of two layers by UV assisted self-patterning process. (In2O3/Al2O3). The In2O3 was used for an electrode and Al2O3 for a selective passivation. Here, we have studied the simultaneous behavior of Al2O3 which showed both a passivation material and a suppressor material for the indium oxide to become a channel layer. As a result, the on/off ratio of aluminum (Al) diffused indium oxide TFT varied from 3.85 x 104 to 2.75 x 106. Furthermore, turn-on voltage of the TFT varied from -8 V to -6.4 V. This phenomenon indicates that the diffusion of Al was occurred so that Al was defined the channel region of the indium oxide bottom layer during the thermal annealing process. Accordingly, the formation of aluminum oxide layer was attributed for both a passivation and channel dopant layer.

[1] Y. S. Rim, H.S Lim and H. J. Kim, ACS Appl. Mater. Interfaces 5(9), 3565 (2013).

Resistive switching phenomenon is studied in TiO₂-based metal-dielectric-metal structures for its potential as next-generation non-volatile memory. The resistance of such devices can be switched between a non-volatile high and a low resistance state by appropriately setting the voltage. Conduction is thought to occur as a result of the formation of conductive filamentary structures of ordered phases of titanium sub-oxides with long-range order - such as Magnéli phases - that lie between the top and bottom metal electrodes.
In this study, bipolar resistive switching is investigated in Ti-TiO₂-Ag trilayers fabricated by the anodization of Ti. By progressively varying compliance current, the typically abrupt SET and RESET switching processes are mapped out. The subsequent current-voltage (I-V) curves are modeled by assuming that an oxygen vacancy-rich filament extending from one of the electrodes acts as a virtual electrode for conduction over a barrier. A Schottky model is employed to extract the geometric and electronic properties from the I-V curves. Application of the Schottky model leads to the determination of the effective filament length as well as the contact cross-section, dielectric barrier height and permittivity, leading to a more complete characterization of the growth mechanism of conducting filaments during the SET and RESET cycles. Previous studies focused only on changes in the filament length¹, here we additionally report on changes in the dielectric barrier height. The observed changes in Schottky barrier height and filament length are attributed to a field-driven migration of oxygen vacancies. Both parameters are seen to behave non-monotonically as a function of compliance current, indicating that multiple effects - either chemical or thermal - come into play during the SET and RESET processes.
Common trends observed in the SET and RESET processes are discussed, providing insight into the growth dynamics of conducting filaments and mechanisms underlying TiO₂ and other vacancy-driven resistive switching systems.
This work is supported by the National Science Foundation under the Grants DMR-0906608 and DMR-0907007.
¹ Y-E. Syu, et al., Appl. Phys. Lett. 102, 172903 (2012).

A comprehensive study of the room temperature electrical and electrothermal transport of single-crystalline indium oxide (In2O3) and indium tin oxide (ITO) films over a wide range of electron concentrations is reported. We measured the room temperature Hall mobility Seebeck coefficient S of unintentionally doped and Sn-doped high-quality, plasma-assisted molecular-beam-epitaxy-grown In2O3 for volume Hall electron concentrations from 7e16 cm^-3 (unintentionally doped) to 1e21cm^-3 (highly Sn-doped, ITO). The mobility and Seebeck coefficient were modeled by a numerical solution of the Boltzmann transport equation. Ionized impurity scattering and polar optical phonon scattering were found to be the dominant scattering mechanisms. Acoustic phonon scattering was found to be negligible. Fitting the Hall-scattering-factor corrected concentration-dependent Seebeck coefficient S(n) for non-degenerate samples to the numerical solution of the Boltzmann transport equation and to widely-used, simplified equations allowed us to extract an effective electron mass of m*=0.30+/-0.03 times the free electron mass. The modeled mobility and Seebeck coefficient based on polar optical phonon and ionized impurity scattering describes the experimental results very accurately up to electron concentrations of 10^{19} cm^{-3} , and qualitatively explains a mobility plateau or local maximum around 10^{20} cm^{-3} for which screening of polar optical phonons plays a role.

ZnO-based heterostructures have been widely studied for growth in the polar orientations, i.e. along the c axis of the wurtzite structure. However non-polar surfaces are of interest since, in this case, the c-axis of the layer lies in the growth plane. It is expected that QW structures can be grown without any reduction of the exciton binding energies.
First we show the feasibility of growing high quality nonpolar quantum wells (QWs) on m plane (10-10) ZnO substrates by molecular beam epitaxy. The layers exhibit no residual strain, smooth interfaces, no extended defects, surface roughness of a few angstroms, and X-Ray FWHM of a few tens of arc seconds. The FWHM of photoluminescence (PL) from the QWs is below 5meV for QWs larger than 3 nm with a stable exciton up to room temperature. In addition, a very low residual doping level can be achieved on m-plane ZnO after growth optimization (n~10^14 cm-3 or below).
Then the non polar ZnO films were doped with Nitrogen, which was activated in an rf-plasma cell. The N concentration can be tuned as a function of the growth temperature from 3.10^17 /cm3 to 10^20 /cm3. Note that neither the growth temperature nor the N incorporation seems to affect the growth process. Indeed RHEED measurements exhibit streaky patterns in both doped and undoped films with RMS roughness measured by AFM below 1 nm all over the temperature range studied here. Low temperature PL spectra taken from ZnO:N layers exhibit a broad donor acceptor pair (DAP) emission around 3.24 eV which is an unambiguous optical signature of shallow acceptor levels.
Finally a series of light emitting devices based on non polar QWs were fabricated. Even after the growth of the n-type ZnMgO and the top nitrogen doped ZnMgO, the structural quality does not deteriorate (AMF RMS<0.5nm, X-Ray FWHM of several tens of arc seconds). After processing, the devices exhibit a rectifying behavior of 4 orders of magnitude. Under forward bias a clear emission from the 4nm QWs is observed at 373nm, very close to the PL from the QWs (371nm).

Zinc oxide (ZnO) has been widely used in a variety of optoelectronic applications due to its wide and adjustable band gap. The photocatalytic activity of ZnO can be enhanced by adding various ions enabling control of intrinsic electrical/optical properties. In this study, highly crystalline silver-doped ZnO nanoparticles (ZnO:Ag) were synthesized by aqueous precipitation method at room temperature without any post thermal treatment. Analytical characterization of precipitation products was performed by XRD, XPS, SEM, TEM and UV-Vis spectroscopy. The effect of silver content (0.3-8 at.%) on structural and optical properties of resultant ZnO nanoparticles has been reported. Detailed crystallographic investigation was accomplished through Rietveld refinement. It was found that silver doping causes an expansion in the ZnO lattice accompanied by a modification in its&’ optical properties. The analytical findings indicate substitution of Ag+ ions into Zn2+ sites within the ZnO lattice. The silver incorporation also affected the ZnO nanoparticle size in the range of 15±2 to 20±2 nm. The photocatalytic activity of the ZnO:Ag nanoparticles was determined by methylene orange degradation studies and compared to that of undoped ZnO. Improved photocatalytic activity was obtained for ZnO:Ag nanoparticles as compared to undoped counterparts. This doping strategy can be applied to a variety of other dopants to impart important properties to ZnO nanoparticles.

We report the influence of the substrate temperature in the range of 300 - 500°C on the structural and optical properties of ZnO thin films grown by low-cost chemical spray pyrolysis method using zinc-acetate precursor solutions. The ZnO films were deposited onto soda-lime glass. As the substrate-temperature increases, the intensity (002) sharp peaks showed by XRD indicate a perpendicular crystalline growth to the substrate surface. The grain size increases between 12.5 nm and 21.6 nm as the substrate-temperature increases. Images by SEM showed acicular grains from 350°C The ZnO films have an average transmittance > 80% in the visible region and an optical bandgap (Eg) between 3.18 and 3.27 eV, depending on the substrate-temperature. Optical bandgap is increased almost linearly with the substrate-temperature where low stress compressive in the ZnO films was observed

Semiconducting metal oxide sensors have been widely studied due to their small dimensions, low cost, and low power consumption. Among semiconductor oxide materials, zinc oxide (ZnO) is a very interesting one due to its chemical and thermal stability, its large exciting binding energy and band gap as n-type semiconductor, and, especially, to its high response to toxic and combustible gases. ZnO is a potential material for gas sensor applications because of its high piezoelectric coupling, greater stability of its hexagonal phase and its pyroelectric property. Hydrothermal synthesis is one of the most useful method among the other synthesis methods such as direct strike and homogeneous precipitation, microemulsion, sol-gel, gel combustion etc. to produce homogeneous, nanosized ZnO powders with high purity, controlled particle size and morphology.This method provides an important advantage is that the purity of hydrothermally synthesized powders significantly exceeds the purity of the starting materials. Moreover ZnO with various morphologies can be synthesized via simple hydrothermal reactions. The research objectives of this study were to understand formation and growth process of ZnO particles with various morphologies and to investigate role of starting materials, (i.e., zinc nitrate hexahydrate (Zn(NO3)2.6H20) and zinc chloride (ZnCl2) on the particle morphology. In the present study, ZnO particles with various morphologies were synthesized via an unstirred hydrothermal method. When using Zn(NO3)2.6H20 as precursor, after 12 h at 100°C, final morphology of the produced ZnO particles was flower like (lenght of the flower 5-10 mu;m). On the other hand, the final morphology was rod like (lenght of the rod 0.5-1mu;m ), when the using ZnCl2 starting materials under the same synthesis conditions.

Transparent oxides based thin film transistors (TFTs) have attracted remarkable attention because of their high field effect mobility, small sub-threshold swing, good uniformity and high electrical reliability. Especially, amorphous In-Ga-Zn-O (a-IGZO) is one of the most promising candidates acting as semiconductor channel layer in display fields, such as active matrix liquid crystal displays (AMLCDs) and organic light emitting diode displays (OLEDs). But one of problems is the electrical stability of a-IGZO TFTs, including defect creation by prolonged gate bias, reaction with ambient, conditions of channel deposition process, and the influence of illumination or temperature stress. To improve this stability, many research groups have investigated methods to protect channel from ambient and improve the variation of carrier and trap density with illumination, thickness, oxygen concentration and temperature. But, most of results needed extra process like doping, treatment, passivation besides a-IGZO deposition process.
In this study, we will present how to improve the instability of a-IGZO TFT channel by Ti thin film on the back-channel surface. Ti layer on the a-IGZO channel was formed as thin film on the a-IGZO surface by evaporation and was easily transformed into Ti oxide layer due to oxidation reaction with a-IGZO. As a result, oxygen vacancy increased by Ti/Ti oxide transformation in the a-IGZO back-channel contributed to increase of carrier density leading to negative shift of threshold voltage. Ti oxide formed on the a-IGZO surface conserved back-channel from reaction with ambient molecules and reduced the influence of illumination, severely affecting electrical stability under bias condition, by partial absorption of light. To analyze property of Ti oxide film, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and UV visible spectrophotometer were utilized.

Medical science and biotechnology are making great progress in improving human health and longevity. Because many human illnesses have a hereditary component, it is necessary to investigate genetic information. In this regard, deoxyribonucleic acid (DNA) detection technology which is one of the most powerful methods for gene investigation has become increasingly important for diagnosis of genetic diseases, drug discovery, and viral and bacterial identification. For quick and simple DNA detection, oxide thin-film transistor (TFT)-based DNA biosensors which fall within a class of label-free methods have been developed recently [1-3]. In this work, we present the comprehensive DNA detection mechanism and the ability to sense various DNA molecules using solution-processed oxide TFTs. DNA was physically immobilized on the channel surface and directly affected the electrical response of the oxide TFTs such as a significant decrease in field-effect mobility and a progressive shift of threshold voltage in the positive direction. From these results, the DNA detection mechanism turned out that it was attributed to electron trapping and electrostatic interactions caused by negatively charged phosphate groups on the DNA backbone. Moreover, by depositing oxide thin-films using a solution process, many advantages such as simple and low cost fabrication could be obtained. Therefore, these solution-processed oxide TFTs, a label-free method for DNA detection, could be used to realize low-cost portable DNA sensing systems.
[1] S. J. Kim, B. Kim, J. Jung, D. H. Yoon, J. Lee, S. H. Park, and H. J. Kim, Appl. Phys. Lett. 100, 103702 (2012).
[2] S. J. Kim, J. Jung, D. H. Yoon, and H. J. Kim, J. Phys. D: Appl. Phys. 46, 035102 (2013).
[3] J. Jung, S. J. Kim, D. H. Yoon, B. Kim, S. H. Park, and H. J. Kim, ACS Appl. Mater. Interfaces 5, 98 (2013).

Indium oxide (In2O3) has recently attracted much attention due to its high optical transparency with wide band-gap values and high single crystal mobility (160 cm2/Vs), although In2O3 has already been widely used as a transparent conducting material in liquid crystal displays, a sensing material in gas sensors, as well as a high mobility transparent thin-film transistors (TFTs)[1, 2]. Unfortunately, In2O3 film grown by MOCVD presents high electron carrier density, usually, to be difficult to apply to TFTs [3]. However, the origin of conduction electron, carrier transport mechanism, and its relationship between structural and electrical properties are still unclear. To better understand above things, in this work, polycrystalline In2O3 films were grown by metal-organic chemical vapor deposition (MOCVD) at various growth conditions using TMIn and O2 vapor as the sources. The structural and electrical properties are studied by scanning electron microscopy (SEM), Conductive Atomic Force Microscope (CAFM), X-ray diffraction (XRD) and Hall Effect measurement. Body centered cubic (bcc) Bixbyite (222) and (400) peaks were found in all XRD spectrums. Interestingly, positive correlation were observed between carrier concentrations and intensity ratio of (222) and (400) XRD peak. It is presumably resulted from the different level of surface energy on (100) and (111) facet, which may lead to different defects. The carrier density as low as 9x1017/cm3 was obtained when the intensity ratio of (222) and (400) peak is at 1/28. Moreover, the temperature dependence hall mobility and the relationship between hall mobility and carrier concentration were investigated. We confirmed that the carrier transport process is dominated by ionized impurity scattering model. The origin of donor was discussed in this work.
Reference
[1] J. Am. Chem. So. 2011, 133, 5166-5169.
[2] J. Appl. Phys. 2010, 107, 033514.
[3] J. Appl. Phys. 2011, 110, 093712.

ZnO is a large band gap (~3.3 eV), high exciton binding energy (~60 meV), non-stoichiometric semiconductor presenting several important optical and electronic properties which concede it applications in several areas in chemistry, physics and biology, as well as day-to-day utilities. These properties can be enhanced even further through doping, and the functionality, or multifunctionality, required for nowadays applications may call for a necessity of doping with compounds with different properties. Among several possibilities are those with luminescent lanthanides such as Eu(III), Tb(III) and Pr(III), which are strong candidates for combination with ZnO&’s electroluminescence properties, but represent a challenge towards their insertion into ZnO&’s structure because of differences of ionic radii, charge, and coordination number. In this work, systems of pure and doped ZnO nanoparticles anchored in a mesoporous silica host were synthesized using the impregnation-decomposition cycles (IDC) method. This method consists of a layer-by-layer growth based on sequential cycles of impregnation of the given host with a metallo-organic single-source precursor, followed by its thermal decomposition. Through this, Eu(III)-doped ZnO precursor layers were intercalated with pure ZnO layers, in a way to force the ion into ZnO&’s structure. The IDC method allowed for linear mass increase and alternation between precursors, where ZnO precursors doped with Al(III), Sr(II), Eu(III), Pr(III) and Yb(III) were used for intermediary layers. HR-TEM results together with XRD diffraction spectra have shown that these nanoparticles are of wurtzite structure, presenting an average size of 4.8 nm for 10 IDC. UV-Vis diffuse reflectance spectra have shown that pure ZnO grows predominantly as monodisperse sites, with absorption at ~245 nm, and no band gap is observed. With the inclusion of other ions in the system, of both transition metals and lanthanides, ZnO&’s growth is favored and the band gap appears and is red-shifted with increasing IDC, presumably due to quantum confinement effects. Converting these band gap values to nanoparticle diameter through means of a theoretical equation, the obtained average sizes were compatible with the obtained through HR-TEM. Photoluminescence spectra of Eu(III)-containing systems present intra-4f emissions 5D0→7FJ (J = 0, 1, 2, 3, 4) and for systems where ZnO&’s band gap was observed through DRS, the excitonic emissions were observed, being also red-shifted with the number of IDCs, but no defect emissions were observed. Lifetime measures of Eu(III) 5D0→7F2 have given long lifetimes, ranging from 2.0 to 3.0 ms, and these values corroborate with the layer-by-layer growth proportionated by the method. Excitation spectra have shown both ZnO monodisperse sites and nanoparticles transfer energy to the host, and even though superimposed by Eu(III) bands a minor transfer to Eu(III) could be present, suggesting its successful insertion into ZnO.

We report room temperature ferromagnetism (RTFM) in hydrogenated Zn_0.96Co_0.04O bulk samples synthesized via a standard solid state reaction route. In a previous work, we reported the absence of intrinsic ferromagnetism in high-quality bulk samples synthesized in oxygen atmosphere at 1400 °C. We concluded that the presence of Co is not a sufficient condition to induce a ferromagnetic behavior. In this work, we submitted the Zn_0.96Co_0.04O samples to a hydrogenation annealing process in order to introduce defects, as an attempt to induce RTFM.
Paramagnetic Zn_0.96Co_0.04O samples were annealed in a gaseous mixture of argon 95% and hydrogen 5%. The effects of hydrogenation on the structural properties were investigated by x-ray diffraction (XRD). The microstructure and composition distributions were characterized by transmission electron microscopy (TEM) and energy dispersive x-ray (EDS) measurements. Raman scattering was used to study of the incorporation of dopants and the resulting lattice disorder of the host lattice. Co K-edge x-ray absorption near-edge structure (XANES) was used to determine the valence state of Co in the ZnO lattice. Changes in the density of defects were estimated by Hall and Photoluminescence (PL) measurements. Magnetic characterizations were performed using a superconducting quantum interference device (SQUID) magnetometer.
The structural characterization confirmed the absence of secondary phases or Co clusters. Hall Effect showed an increase on the carrier concentration associated to the induction of defects due to the hydrogenation process. PL revealed the presence of an acceptor-like band associated to zinc vacancies and a second band of donor-like character associated to zinc interstitial defects, confirming the Hall assumptions. The magnetic measurements revealed a two-phase behavior, with the coexistence of paramagnetic and ferromagnetic phases at room temperature. The magnetization saturation increased from 0.0046 emu/g to 0.515 emu/g with the hydrogenation process corresponding to an average magnetic moment of 3.55 mu;_B per cobalt atom.
We discuss the origin of the RTFM characteristic observed solely for hydrogenated Zn_0.96Co_0.04O samples, which we attribute to the presence of defects at zinc sites.
The authors are grateful to Capes FAPEMIG, FAPESP and CNPq for financial support.

Effect of Gd doping in ZnO towards achieving the key properties, such as room temperature ferromagnetism for spintronic applications, has been the scope since from anomalously high magnetic moment reported upon Gd doping in structurally similar GaN. In the present study we have prepared Gd (1-5 at%) doped ZnO thin films by pulsed laser deposition at various oxygen pressures. We would show how heavy amount of Gd doping in ZnO thin films exhibit ferromagnetism at room temperature and superparamagnetism at low temperature (5 K). Few of the heavily Gd doped ZnO thin films exhibited the signatures of secondary phases (GdZn2 and pure Gd) from their respective Curie transitions (at 70 K and 293 K) or from the blocking phenomena in the temperature dependent magnetization measurements. High resolution transmission electron microscopy analyses revealed segregation of secondary phase. Such segregated ferromagnetic particles (Gd), act pseudo multilayers of ferromagnetic-semiconductor layers that were resulting in a large positive magneto resistance (MR ~76%) at 5 K. We attribute the room temperature ferromagnetism due to an intrinsic origin from the Gd doping and the formation of polarons due to point defect clusters. Gd3+ doping for Zn2+ ions was confirmed by the synchrotron based X-ray absorption spectroscopy. Superparamagnetism was attributed to the Gd clustering that was revealed by HR-TEM and positive MR in the heavily Gd doped ZnO thin films. More investigations on the role of point defects to the observed magnetic and transport properties through various spectroscopic techniques are underway.
References:
1) K. Sato, L. Bergqvist, J. Kudrnovský, P. H. Dederichs, O. Eriksson, I. Turek, B. Sanyal, G. Bouzerar, H. Katayama-Yoshida, V. A. Dinh, T. Fukushima, H. Kizaki, and R. Zeller, Rev. Mod. Phys. 82, 1633 (2010)
2) S. Dhar, O. Brandt, M. Ramsteiner, V. F. Sapega, and K. H. Ploog, Phys. Rev. Lett. 94 037205 (2005).
3) Y. R. Ryu, S. Zhu, D. C. Look, J. M. Wrobel, H. M. Jeong and H. W. White J. Crystal Growth 216, 330 (2000)
4) A. Janotti and C. G. Van de Walle, Phys. Rev. B 76, 165202 (2007)

Tin oxide (SnO2) is an important electronic material which has been widely used in an extensive range of applications such as gas sensor, catalyst, heat mirror, varistor, transparent thin film electrode, optoelectronic device etc. In gas sensor applications, SnO2 - based systems are most promising materials which are capable of sensing a large number of pollution species, offer high sensitivity, simpler design, and relatively inexpensive components. One of the most important factors affecting sensitivity of the sensors is the actual grain or crystallite size that should be less than 10 nm for the sensor materials. Thus, hydrothermal synthesis is one of the most useful methods among the others such as direct strike and homogeneous precipitation, microemulsion, sol-gel, gel combustion etc. to produce homogeneous, nanosized SnO2 powders with high purity, controlled particle size and morphology. The research objective of this study was to develop an understanding about the formation and growth mechanism of SnO2 particles via hydrothermal synthesis as a function of cation concentration (0.01-1.0 M) and treatment time (1-24 h). SnO2 powders were successfully synthesized at 200°C from hydrous tin oxide, which was prepared by direct strike precipitation of SnCl4 solution by adding NH4OH solution at pH 9. As initial cation concentration increases 0.01 to 0.05 M, crystallite size of synthesized powder increases from 3.1 to 4.6 nm. However, as initial cation concentration increases 0.2 to 1.0 M, crystallite size of synthesized powder decreases from 9.3 to 8.5 nm. While Ostwald type growth mechanism was observed at certain cation concentration, classical nucleation and growth mechanism was observed after this certain point during hydrothermal synthesis. Evaluation of tin oxide (SnO2) particles was also investigated by altering the treatment time from 1 to 24 h. Diffusion controlled growth behavior was observed as a function of synthesis time.

Resistive switching devices have the potential to have many applications in computer engineering from analog memory to hardware driven neuromorphic networks. Such systems require that the device resistance be tuned as accurately as possible and as quickly as possible. In the widely studied Pt\TiO2\Pt bipolar memristors, it&’s been demonstrated that the devices can be switched at nanosecond speeds. Less well known, however, is that the devices will continue to switch for several minutes after the initial write pulse before reaching a stable state. These switching transients impose a serious limitation on the bandwidth for precisely programming the device states, potentially limiting the accuracy to which the devices can be speedily programmed.
To study this effect, this work uses methods similar to those in Laplace Deep Level Transient Spectroscopy. The switching transient time constants are resolved by numerically solving for the Inverse Laplace transform using FTIKREG. The switching transient is probed as a function of temperature as well as exposure to Ultra Violet radiation. It&’s proposed that the switching transient is likely caused by electronic traps in the TiO2 film.

Resistive switching memories have attracted intense interest for potential use in next-generation non-volatile memories. Mostly, resistive switching functions with formation and annihilation of the conducting filaments through a dielectric. In oxide dielectrics, in response to a high applied electric field, the redistribution of intrinsic charged point defects or extrinsic point defects play the main role in formation of the conducting filaments. To understand the mechanism of the filament formation, direct characterizing of the “electroforming” process is essential. In TiO₂, oxygen vacancies, V_Ö, and titanium interstitials, Ti_i, are the main as the charged ionic carries. The present work aims to understand the origin and the mechanism of conductive filaments formation at the mesoscopic length scale.
TiO₂ single crystals are equilibrated at specific oxygen partial pressures and temperatures to define the initial-state point defect concentrations. Platinum is deposited on the annealed samples utilizing a DC sputtering system to form Schottky contacts. The Pt/TiO_2-x/Pt is degraded by applying DC bias voltage. During the electroformation process, the leakage current as a function of time is monitored until the conducting filaments are formed. The electroformed TiO_2-x and the distribution of point defects are characterized by utilizing electron microscopy techniques. Specifically, cathodoluminescence (CL) is utilized for identifying and quantifying the specific point defect species and responsible for the filament formation. CL spectra taken from conductive filaments and non-filament regions indicate a noticeable increase in the concentration of the titanium interstitials in the filament regions. The time evolution of the degradation process, resulted from the point defect redistribution, influenced by external parameters such as bias voltage and high temperature, is modeled and compared with experimental results.

The diffusion or migration of vacancy defects under driving forces, especially of oxygen vacancies VO, are of high interest for understanding and controlling oxide semi-conductors. While especially true in their use as memristors, the control of VO is also very important at much lower concentrations, e.g. in n-type doped TiO2.
The memristive effect in TiO2 is governed by the accumulation of oxygen vacancy defects (VO) and subsequent transformation of the insulating TiO2 into TinO2n-1 Magnéli phases, and vice versa. Similar redox type mechanisms occur in many metal oxides and may cause severe problems when their application is as a semiconductor rather than memristive material. To understand VO diffusion, we develop a general method for simulating vacancy dynamics and perform detailed computational studies in rutile titania.
We employ the charge self-consistent Densi-ty-Functional based Tight-Binding (SCC-DFTB) method implemented in the DFTB+ code, together with the tiorg parameter set. We calculate free energy profiles for the VO diffusion using meta dynamics, employing a modified version of the PLUMED code, coupled to DFTB+, which implements a permutation invariant vacancy tracking (PIVOT) collective variable. For comparison with existing literature, we also perform nudged elastic band calculations. We assume the vacancy to be double positively charged (VO2+) in all calculations. We follow the nomenclature of Iddir et al. for the symmetry inequivalent hopping events.
We find a strong dependence of the absolute and relative barrier heights on the size of the employed models. Therefore great care must be taken to draw meaningful conclusions.
Our results clearly show that the migration of O vacan-cies in TiO2 is governed by hops from the equatorial plane to the pole of a single Ti octahedron.
Acknowledgements
JMK is grateful for funding from a DFG research grant.

Vanadium oxide 1D nanostructures have attracted the interest of many researches in the last decade due the wide variety of applications which are strongly related to its crystalline structure and morphology. It is well know that technological application of nanostructured materials are strongly related to their crystalline structure, crystal size and morphology.The main objective of this work was to study the morphology evolution of V2O5nH2O nanostructures under different temperatures and treatment times in hydrothermal conditions. The synthesis of V2O5nH2O nanostructured materials was made by using the H2O2-V2O5 route. The as-synthesized samples consisted of stable colloidal suspensions of V2O5nH2O nanostructures in water with 0.06M of vanadium. In this work, the morphological evolution of V2O5nH2O 1D nanostructures obtained by hydrothermal assisted peroxocomplex degradation method was followed using mainly the small-angle X-ray scattering (SAXS), scanning transmission electron microscope (FE-STEM) and a high-resolution transmission electron microscope (HRTEM) techniques. The importance of intrinsic parameters such as the temperature and time of synthesis on the nanostructure evolution was analyzed based mainly on the dehydration process and on the oriented attachment (OA) mechanism.

In order to fabricate ZnO-based quantum structures (e.g. quantum wells (QWs) and quantum Hall fraction) for advancing their application in devices such as UV-lasers (LDs), light emitting diode (LED) and high electron mobility transistors, it is required to realize tunable energy gap on ZnO as band gap engineering. Among the possible ternary alloys, MgxZn1-xO, CdxZn1-xO, and BexZn1-xO, based on the wurtzite ZnO system, BexZn1-xO has recently attracted interest due to the structural identity between ZnO and BeO, unlike in the other alloy systems. In 2006, Ryu et al. first reported the possibility of tuning the ZnO-band gap energy (from 3.3 to 10.6 eV) by substituting Be into Zn sites, following that a BexZn1-xO-based quantum structure would induce UV emission in a conventional LED. [Appl. Phys. Lett. 88, 052103] However, the physical and electronic properties of the BexZn1-xO alloy system have yet to be fully investigated, either theoretically or experimentally. Here, we demonstrated the influence of Be incorporation into the host lattice of ZnO as a function of Be composition (x = 0 to 0.77) under the alloying process. All the crystalline BexZn1-xO thin films (thicknesses of approx. 200 nm) were prepared on c-axis Al2O3 (0001) substrates, with different Be concentrations using radio-frequency (RF) magnetron co-sputtering. Structural and optical properties, as well as the chemical bonding environment in the alloy films were investigated using various characterization techniques (XRD, UV-visible spectroscopy, AFM, and monochromatic XPS). Our results demonstrate band gap modulation of the BexZn1-xO alloys, while maintaining a hexagonal crystalline nature over the entire Be composition range, as indicated by a continuous shift in the out-of-plane lattice parameter [c = 5.29 (x = 0) to 4.57 Å (x = 0.77)] and optical energy gap [Eg: 3.18 (x = 0) to over 4.41 eV (x = 0.41)]. Moreover, structural fluctuations (e. g. phase-separation) observed during the band gap modulation of the alloy films are explained by the correlation between Be pinning and grain boundary movement on the formation of the alloy film grain structure.
Presenting author: [email protected]
Corresponding author: [email protected]

β-Ga₂O₃ is naturally an n-type semiconductor having a wide-gap of 4.8 eV, which has attracted much attention towards realizing power device applications. The doping control in high-quality epitaxial films is crucially required, which has been previously attempted only by using molecular-beam epitaxy technique. To investigate various device architectures including modulation doping, pulsed-laser deposition technique (PLD) would be much convenient, where parallel synthesis method can be easily implemented [1]. Here we have studied homoepitaxial growth of β-Ga₂O₃ by using PLD to achieve tuning of the donor concentration in a wide range with keeping atomically flat surface. Using temperature-gradient method, β-Ga₂O₃ films were grown on (-201) single crystal substrates at growth temperature (T_g) ranging from 700 to 830°C by ablating a Ga₂O₃ ceramic target (4N purity) with KrF excimer laser pulses. We found that the growth rate (10 ~ 80 nm/h) was strongly dependent on T_g due to significant sublimation of highly volatile precursor species such as Ga₂O. The surface morphology was also influenced by T_g, where three-dimensional islands were observed at T_g = 700°C, while atomically flat surface was obtained at T_g = 750°C and higher temperatures. These films showed semi-insulating behavior as verified by the capacitance-voltage measurement. Then we carried out Si doping at T_g = 750°C by using the Ga₂O₃ ceramic target partially covered with a Ga₂O₃:Si single crystal, in which Si concentration is 3×10¹⁸ cm^-3. The carrier density of the doped films increased from 5×10¹⁷ to 1×10¹⁹ cm^-3 with increasing the areal fraction of the Ga₂O₃:Si, indicating successful substitutional doping of Si on the Ga site. Figure1 shows carrier depth profiles measured for the obtained films. Moreover, the atomically flat surface remained intact regardless of the doping concentration. Our results suggest that PLD is suitable to production of the device grade β-Ga₂O₃ heterostructures. [1] H. Koinuma, and I. Takeuchi, Nat. Mater. 3, 429 (2004).

Tungsten trioxide (WO3) is a semiconductor material with undisputable applications in gas sensors, batteries, pigments and catalysis [1]. As such WO3 has been studied extensively [2] and in particular its different crystal structures [3]: monoclinic, triclinic, orthorhombic, tetragonal and hexagonal [4] as various phases are sought for different applications. For example, epsilon WO3 has been proven as a highly selective sensor of acetone [5], a tracer for diabetes type-1, in the human breath. Here, up to 20 g/h of WO3 nanoparticles [6] with closely controlled crystal/grain size and phase composition have been made by scalable flame spray pyrolysis (FSP) by varying its process parameters. Then the formation of different WO3 phases is promoted by different heat treatments (controlling temperature, time and heating/cooling rate). Most notably, the dynamic phase transition of WO3 has been investigated in detail by in situ XRD analysis during heating and cooling of such nanomaterials revealing how metastable WO3 phases can be captured stably. Finally the effect of dopants is explored since it is routinely used in practice to control crystal growth and phase transition during WO3 synthesis and processing.
[1] Hammond C, Straus J, Righettoni M, Pratsinis S E and Hermans I 2013 Nanoparticulate Tungsten Oxide for Catalytic Epoxidations, ACS Catal. 3 321-327.
[2] Salje E and Viswanathan K 1975 Physical-Properties and Phase-Transitions in WO3, Acta Crystallogr. Sect. A A 31 356-359.
[3] Woodward P M, Sleight A W and Vogt T 1997 Ferroelectric tungsten trioxide, J. Solid State Chem. 131 9-17.
[4] Gerand B, Nowogrocki G, Guenot J and Figlarz M 1979 Structural Study of a New Hexagonal Form of Tungsten Trioxide, J. Solid State Chem. 29 429-434.
[5] Wang L, Teleki A, Pratsinis S E and Gouma P I 2008 Ferroelectric WO3 nanoparticles for acetone selective detection, Chem. Mater. 20 4794-4796.
[6] Righettoni M, Tricoli A and Pratsinis S E 2010 Thermally-stable, silica doped ε-WO3 for sensing of acetone in the human breath, Chem. Mater. 22 3152-3157.

Study and fabrication of nanoscale non-volatile memory is an important subject of current research as the conventional memory is approaching its physical limits. As resistive switching phenomenon observed in various transition metal oxides is highly localized in nature, these resistive switching devices seem to be ideally suited for improving scalability and miniaturization, both in lateral and vertical dimension. In this context it may be mentioned that conductive atomic force microscopy (CAFM) technique provides a direct means to visualize the conducting filaments, ascertain their spatial distribution and measure their I-V characteristics along with nanoscale switching from low resistance state (LRS) to high resistance state (HRS) or vice versa. The study demonstrates a simple and effective methodology based on loading force variation during conductive atomic force microscopy (CAFM) to realize multilevel resistive switching effect in cuprous oxide (Cu2O) based memory cell, in the direction of improving device density and switching speeds. An important aspect of the proposed method is that it is easily extendible and can be adapted to resistive switching devices based on materials other than Cu2O. On account of the fact that resistive switching is a phenomenon occurring at nanoscale level, it is evident that understanding of the underlying mechanisms necessitates fabrication of ultra low dimension devices. In this direction, CAFM which has an excellent lateral (5 nm) and vertical (1 pA) resolution has proven to be a significant tool for simultaneously probing the memory cells at a localized scale both morphologically and electrically.
One of the significant findings of this study is that multilevel switching states can be achieved not only by voltage and current programming of the device but also by an alteration in tip-sample loading force. In the study, the tip-sample contact area has been modified by varying the loading force during the CAFM experiment from 16 to 80 nN. This is a convenient means to alter the top electrode dimension, i.e., the device size, while simultaneously ascertaining the resultant change in switching nature and parameters. For instance, a change in the loading force from 80 to 16 nN results in a decrease in current in low resistance state from 590 to 40 nA. As the CAFM operates in spreading resistance mode, current crowding leads to electric field enhancement at lower tip-sample contact area, which results in scaling of the switching parameters with loading force. It may be noted that the switching type can be altered from unipolar to bipolar by changing the tip-sample contact area. The reported approach provides new opportunities for the preparation of multilevel nonvolatile memory devices with continued device scaling as a function of tip-sample contact area.

Titanium dioxide (TiO2) is used in a wire range applications such as photocatalysis and sensor device [1-3]. TiO2 is excellent for its stability, nontoxicity, and low cost. Since Brian O&’Regan and Michael Grätzel [4] reported on highly efficient TiO2 based dye-sensitized solar cells, much attention has been given to the synthesis of TiO2 nanoparticles. In this work is shown a new and effective method for the preparation of TiO2 nanoparticles in the crystallographic forms, anatase and rutile. The method involves dissolving the TiOSO4 powder in H2O2 solution [5,6] and thermal treatment of amorphous precipitate. The technique of X-ray diffraction was used to follow the structure evolution of the amorphous precipitate. Additionally, the microstructure analysis was realized with Rietveld refinement. Pure anatase structure and rutile were obtained at 600oC and 1000oC with a grain size estimated around 30 and 580 nm, and band gap of 3.12 and 2.89 eV, respectively. We believe that the chemical method of obtaining TiO2 nanoparticles is a promising alternative of low cost whose potential for solar cells deserve a careful evaluation, especially in hybrid solar cells that employs TiO2 as electron acceptor and as transport channels.
References
[1] Hagfeldtt A. and Gratzel M. Chem, Rev.. 95, 49-68 49 (1995).
[2] Linsebigler A.L., Guangquan L., and Yates J.T., Chem. Rev., 95, 735-758 735 (1995).
[3] Chen X. and Mao S.S. Chem. Rev., 107, 2891-2959 (2007).
[4] Regan B. O, Grätzel M., Nature (London) 353 (1991) 737.
[5] Karuppuchamy S., Nonomura K., Yoshida T., Sugiura T., Minoura H., Solid State Ionics 151 19 (2002).
[6] Campos, C.S.; Spada, E.R.; Reis, F.T.; Sartorelli, M.L.; Paula, F. R.; Faria R.M., Journal of Raman Spectroscopy, 43,433-438, (2012).

Transition-metal doped ZnO thin films have a chance to enhance the piezoelectricity resulting from boosting the asymmetric dipole moment by the introduction of noncollinear bonds. Among them, vanadium-doped ZnO (VZO) is one of the promising materials which are expected to apply to both piezoelectric and transparent conductive layer in the photoacoustic tomography sensor. This is because incorporation of V also creates high-density carrier to obtain low resistivity. In this study, improvement of c-axis orientation and reduction of resistivity in VZO thin films were investigated by introducing thin ZnO buffering layer on a quartz substrate.
VZO and ZnO films were deposited by RF magnetron sputtering. V of about 1 at.% was doped by co-sputtering ceramic ZnO target with V chips. Sputtering conditions of RF power and Ar gas pressure were 150 W and 1 Pa, respectively. Thin ZnO buffering layer was deposited at 600^oC and VZO films were formed at 300^oC. VZO films at the initial growth stage have, in general, a variety of crystalline orientations other than c-axis orientation due to amorphous quartz substrate. The buffering ZnO acts to align VZO effectively to c-axis direction. Using this stacking growth, thin VZO of only 50 nm thick showed strong diffraction peak from (100) and (110) planes on in-plane XRD. The peak intensity of thin VZO was higher than that of buffering ZnO film. This indicates the stacking growth enables to enhance c-axis orientation of upper VZO layer assisted by the buffering ZnO film. From the VZO crystalinity dependence on VZO thickness, crystallinity deteriorated in thick VZO. This means that noncollinear bonds caused by V doping affected crystalline orientation even in this stacking growth. Resistivity of VZO on the buffering ZnO has successfully decreased by more than half compared to single-layer VZO. Furthermore, resistivity in stacked VZO/ZnO was constant down to 50 nm thick, even though it is well known that resistivity increases with thinning film down to less than 100 nm thick due to the interface scattering. Therefore, this stacking growth is very effective to achieve low-resistivity and thin TCO films.

Because of its large direct band gap of 3.37 eV and high exciton binding energy (~60 meV), which offer efficient excitonic emission at room temperature and above, ZnO nanostructures in the würtzite polymorph are an ideal choice for electronic and optoelectronic applications. As such, numerous investigations have been conducted in recent years to develop applications of ZnO, such as light-emitting diodes and lasers. Some of the important parameters in this regard are free carrier concentration, doping compensation, minority carrier lifetime, and luminescence efficiency, which are directly or indirectly related to the defects that, in turn, depend on the method of synthesis. We report the synthesis of undoped ZnO nanorods from the solution medium, through microwave irradiation of an aqueous solution of zinc acetate dehydrate [Zn(CH3COO)2. 2H2O] and KOH in 1:15 molar ratio. In this process, zinc acetate dihydrate acts as both the precursor to ZnO and as a self-capping agent. Upon exposure of the solution to microwaves in a domestic oven, ZnO nanorods 80 nm in diameter and measuring ~1.5 µm - 3 µm in length are formed in minutes. These ZnO structures have been characterised in detail by X-ray diffraction (XRD), selective area electron diffraction (SAED) and high-resolution scanning and transmission microscopy, which reveal that each nanorod is single-crystalline. Optical characteristics of the nanorods were investigated through photoluminescence (PL) and cathodoluminescence. These measurements on a collection of nanorods reveals that defect state-induced emission is prominent, with a broad greenish yellow emission. CL measurements were made on a number of individual nanorods, at different accelerating voltages for the electron. It is found that CL intensity increases with increasing accelerating voltage. A red shift is observed in the CL spectra as the accelerating voltage is raised, implying that emission due to oxygen vacancies dominates under these conditions, and that interstitial sites can be controlled with the accelerating voltage of the electron beam. We have investigated the time resolved fluorescence (TRFL) life time (tau;) measurement of zinc oxide (ZnO) nanorods. The measured life time is tau; = 9.88 picoseconds which reveals that rapid switching, making such nanorods excellent candidates for optoelectronic devices. Current vs. voltage and capacitance vs. voltage characteristics of single ZnO nanorod devices fabricated with focused ion beams support the claim for optoelectronic device applications. Our measurements - which will be presented in detail - lead to the inference that ZnO nanorods synthesised by the present process are suitable for optoelectronic devices, such as room temperature LEDs and laser diodes.

ZnO is a wide band gap II-VI semiconductor with extensive applications such as room temperature lasing, optoelectronics, photocatalysis and sensors. Various methods have been devoted to synthesize the ZnO nanostructures such as vapour phase transport, hydrothermal and wet chemical method. Hydrothermal method has realized a feasible route to fabricate micro and nano ZnO building blocks with variety of nano architectures. Several research works have been reported on the hydrothermal growth of ZnO nanostructures. However, the growth mechanisms of 3 dimensional nanostructures by solution methods are assumed as an oriented attachment growth under hydrothermal conditions. Moreover, there is no report for the evidence of such a growth behavior. In the present work, we report the formation mechanism of 3 dimensional ZnO nanoflowers from 1 dimensional nanorod under hydrothermal growth conditions and experimentally evidenced the oriented attachment growth of nanostructures. FESEM image of 15 h grown sample shows the morphology of nanorods and the monodispersity in size and morphology. The diameter of an individual nanorod was 130 nm and the length was 450 nm. Bottom view of an individual nanorod was visualized by TEM and exhibits the hexagonal morphology. The nanorods were grown with the diameter of 110 nm and size of 450 nm at the initial stage. The size of primary grown part of nanorods and secondary grown area were highly monodispersed. In addition to that the triethylamine molecules acted as surface passivating ligand and thus restricted the agglomerations and initialized the growth along a particular plane. FESEM image of the sample grown for 25 h on FTO substrates shows the distribution of ZnO nanorods with monodispersity. However, some of the places (white circle marked) had ball-like nanostructures with the diameter of 8 mu;m. The morphology of nanostructure resembles a ball-like three dimensional architecture. Moreover, the ball-like nanostructure consisted of ZnO nanorods. In the hydrothermal growth process, the nucleation and growth depend on the growth temperature and the pressure inside the autoclave. FESEM images of ZnO nanostructures grown for 40 h shows sheet-like morphology due to the oriented attachment growth behavior for longer growth time of 40 h. FESEM images of ZnO nanostructures grown for 48 h shows the porous 3 dimensional flower-like morphology. This growth was observed in the entire substrate with monodispersity in size and morphology. The change of growth behavior of ZnO nanorods to 3 dimensional porous ZnO flower was observed by hydrothermal growth. Triethylamine molecules effectively facilitated the monodispersed ZnO nanorods for the 15 h growth. The higher growth time of 40 and 48 h and high temperature of 200 degree C yielded the 3 dimensional morphology of ZnO. The other results such as XRD pattern, UV visible and photoluminescence spectra will be presented at the conference.

ZnO:In thin films were deposited by a homemade ultrasonic spray pyrolysis system (USP). The effect of In concentration on the electrical, optical, structural, and morphological properties have been studied. These films are found to show (002) preferential growth at low indium concentrations. An increase in the indium concentration causes a change in the structure and morphology of the films as confirmed by X-ray diffraction technique and scanning electron microscopy studies. Indium concentration also significantly increases the electron concentration, making the films heavily n type. However the crystallinity and surface roughness of the films decrease with increase in the doping concentration as a result of the formation of smaller grain size which is clearly displayed in AFM images. Typical optical transmittance values were in the order of 80% for 3 at% films deposited at 430 °C. The lowest resistivity value of 1.5 × 10^-3 Omega; cm was obtained for films with [In/Zn]=3 at % at 430 °C.

Amorphous InGaZnO (a-IGZO) has been heralded as a promising channel material because of impressive properties such as a high mobility, low subthreshold swing, and low threshold voltage (Vth) [1]. Nevertheless, a-IGZO TFTs especially unpassivated bottom gate TFTs suffer from instability because the exposed backchannel is vulnerable to ambient effects [2]. Passivation layers such as SiOx [3], and Al2O3 [4] have been used, although they are fabricated using complicated vacuum processes.
In this work, we report an effective polysilsesquioxane-based passivation layer that is fabricated using a simple solution process. Methylsilsesquioxane and a copolymer of methylsilsesquioxane and phenylsilsesquioxane were used as passivation for samples Me 100 and Me 60/Ph 40, respectively. A simple 3-step process was used to coat the passivation onto a-IGZO bottom gate TFTs. First, the passivation was spin-coated on a-IGZO TFT at 3000 rpm for 15 s. Second, prebaking was done at 130 oC for 90 s. Lastly, post-baking in air was performed at 300 oC for 1 h. Contact holes were formed by dry etching using inductively coupled reactive ion etching method. Post-annealing treatment was then performed at 300 oC for 2 h under O2 atmosphere.
Stability of passivated TFTs against positive bias stress (PBS; Vgs=20 V), negative bias stress (NBS; Vgs=-20 V), and negative bias illumination stress (NBIS; Vgs=-20 V, illumination with Halogen lamp) was tested for 10000 s. Both samples showed remarkable stability under PBS with a Vth shift (ΔVth) of 0.49 V and 0.1 V for Me 100 and Me 60/ Ph 40 samples, respectively. Similarly, both samples had a minimal ΔVth of less than -0.1 V under NBS and a small ΔVth of ~ -2.4 V under NBIS.
To understand the mechanism of stability improvement, the H, C, and O concentration in the samples&’ a-IGZO layer was measured using Quadrupole-Secondary Ion Mass Spectrometry (Q-SIMS). Additionally, the change in chemical bonding states of oxygen (O 1s) in the a-IGZO layer was studied using X-ray Photoelectron Spectroscopy (XPS). Q-SIMS results show an increase in the C, H, and O content in the a-IGZO layers of passivated samples. Deconvolution of O 1s profiles showed the emergence of a 3rd peak in passivated samples at ~ 531.6 eV which is attributed to oxide lattice with hydroxides [5]. Both passivated and unpassivated samples had two peaks, OI and OII, attributed to oxide without Vo and with Vo, respectively [5]. We believe that the improvement in stability especially in NBIS can be attributed to the increased H and C content in the a-IGZO layer. These results show the high potential of polysilsesquioxane-based passivation layers as effective passivation layers.
[1] T. Kamiya, et al.: Sci. Technol. Adv. Mater. 11 044305 (2010).
[2] D. Kang, et.al.: Appl. Phys. Lett. 90 192101 (2007).
[3] S. Choi, and M. Han: IEEE Elect. Dev. Lett. 33 396 (2012).
[4] Y. Ko, et.al.: Phys. Status Solidi RRL 5 403 (2011).
[5] S. Jeong, et. al.: Adv. Mater. 22 1346 (2010).

Carrier doping into semiconductor or insulator in field effect transistor configuration has been established as powerful methodology to induce or control novel functionalities such as superconductivity and ferromagnetism. In order to apply high electric field, sufficiently thin gate insulators with high electric field endurance and high relative dielectric constant (κ) are strongly demanded. However, such thin gate insulators usually suffer from dielectric breakdown and reduced κ.
Recently, high-κ titania nanosheet with a thickness of only ~1 nm was synthesized by exfoliation of K0.8Ti1.73Li0.27O4 layered compound. The densely packed ten-layer nanosheets maintained the large κ up to 125 with low leakage current density [1]. However, since the multilayer still contains intralayer grain boundaries that serve as significant leakage current paths, an individual titania nanosheet is expected to show further superior properties as a gate insulator. Here, we report insulating and dielectric properties of an individual titania nanosheet.
The nanosheets were deposited by dropping the colloidal suspension of the nanosheet on the array of bottom electrodes that are patterned with electron beam lithography. Top electrode was deposited on the single nanosheet to form a capacitor structure. The leakage current density of the individual titania nanosheet was less than 10^-7 A/cm2 for the voltage below ~2.2 V, corresponding to the electric field of ~20 MV/cm. From the result of capacitance measurement κ of an individual titania nanosheet was estimated to be from 2.4 to 3.1. The reduced κ might be originated from the presence of organic impurities and/or air gap between nanosheet and electrodes.
[1] K. Akatsuka et al., ACS Nano 3, 1097 (2009).

The alignment of band energies in transparent conductive oxides (TCO) to semiconductors is crucial for the further development of oxide contacting layers in electronic devices. While tools to describe the offsets of tetrahedral semiconductors are well developed [1], metal oxides represent a more challenging case [2,3].
The importance of the offset at interfaces between TCOs and semiconductors is becoming increasingly recognised in the process of device design. Often two otherwise excellent materials will not work in conjugation, due to incommensurate carrier energy levels. Engineering of the TCO band energies offers a solution.
The growth of ultra-thin films on the surface of an oxide material can be used to introduce a dipole moment at that surface, due to charge differences. The dipole, in turn, alters the electrostatic potential - and hence the band energies - in the substrate oxide. We demonstrate the potential for the application of thin-films in this context, applying analytical and numerical simulations. The simulations highlight the different parameters that can affect the band energy shifting potential of a given thin-film layer. In particular we highlight the effect of formal charge, layer orientation, layer thickness and surface coverage, with respect to their effect on the electrostatic potential. The results allow us to propose some important design principles, informing further development and application of thin-films for band energy engineering in TCOs. Furthermore, we present the results of ab initio calculations of the effect of monolayer rutile oxide structures on the energy levels of SnO2. These results demonstrate how the atomic arrangement of the surface monolayer affects the potential energy shift, with oxides possessing the same formal charge resulting in opposite shifts of the SnO2 workfunction.
1. “Revised ab initio natural band offsets of all group IV, II-VI and III-V semiconductors” Y.-H. Li, A. Walsh, S. Chen, W.-J. Yin, J.-H. Yang, J. Li, J. L. F. Da Silva, X. G. Gong and S.-H. Wei, Applied Physics Letters 94, 212109 (2009).
2. “Multi-component transparent conducting oxides: progress in materials modelling” A. Walsh, J. L. F. Da Silva and S.-H. Wei, J. Phys.: Condens. Matter 23, 334210 (2011).
3. “Prediction of Electron Energies in Metal Oxides” K. T. Butler and A. Walsh, Acc. Chem. Res. In Press (2013).

The deposition of transparent conducting metal oxides generally requires elevated temperatures or processes that generating high-energy particles .i.e. sputtering. Consequently these materials have not been exploited in many applications, particularly those where deposition on sensitive substrates are required i.e. top contacts in organic optoelectronic devices. Here, we report on our recent studies detailing a novel pulsed laser deposition (PLD) technique that supports the growth of crystalline, highly conductive (<50 Omega;/sq) and transparent (90 %) aluminium-doped ZnO (AZO) and indium tin oxide (ITO) thin films. The low growth temperature (150 °C) and optimized growth conditions enable growth directly onto organic (polymer and small-molecule) layers without optical or electronic degradation.

Our technique allows precise control of the oxide film thickness, crystalline orientation and stoichiometry. Microstructural and morphological data of oxide and organic layers are presented and are supported by photovoltaic characterization of transparent top contact devices. Our methodology and the range of materials accessible by the PLD technique opens possibilities into unexplored photovoltaic architectures - including the incorporation of transparent interlayers and the ability to fabricate device from the metal electrode up.

ZnO nanomaterials with p-type majority charge carrier are of great research interest for opto-electronic and blue laser diode applications. Recently, it has been reported that Li doped ZnO thinfilms show stable p-type characteristics. Here we report on the synthesis and room temperature ferromagnetic, optical properties and high pressure Raman studies of 4% Li doped ZnO nano-flowers. The room temperature SQUID measurement confirms ferromagnetic nature of the sample with high saturation magnetization value. A strong peak at about 400 nm was observed in photoluminescence spectra which could be associated with dominate LO phonon-replicas in ZnO. Raman spectra shows well defined peaks of E2 low and E2 high at about 98, 437 cm-1, respectively. A pressure dependent (~ 7 GPa) Raman studies of this sample was carried out using a diamond anvil cell in hydrostatic medium. It was observed that the frequency of the E2 high Raman peak increases with increases in applied pressure. On the other hand, the frequency of E2 low decreases with increase in pressure. The pressure dependent frequency shifts of both these Raman modes are compared with bulk ZnO.

Tin oxide (SnO2) powders were synthesized by two different routes by using the homogenous precipitation method. In both routes tin chloride (SnCl45H2O) was used as Sn precursor. First route consisted in preparing, separately, aqueous solutions of tin chloride and urea (CH4N2O) at equal molar concentrations, namely, from of 0.05 to 1.2 M. The starting solutions were prepared from a 1:2 mix of tin chloride and urea solutions (volume proportion), and then stirred and heated at 87±5°C, until a white powder is completely precipitated (the precipitation started from 80°C). In the second route the same tin chloride starting solutions were used, but the precipitation was activated with ammonia (NH4OH) instead of urea. The ammonia was added drop by drop until the pH reached a value of 12. In this route, the molarity values were varied from 0.05 to 1.3 M. In both routes, the precipitates were filtered and dried in air at 100°C, for 24 hr. For some trials the repeatability of the process was tested, in both routes. Finally, all synthesized powders were calcined in a normal atmosphere at different temperatures, 600, 800, and 1000 0C. Structural analysis, before and after calcination, of all synthesized powders was performed by X-Ray diffraction (XRD) to confirm the SnO2 phase. The morphological characteristics of the powders were analyzed by Scanning Electron Microscopy (SEM) technique. In this work, the effects of precipitation agent (urea and ammonia) and the calcination temperatures on structural and morphological properties of SnO2 powders are reported.

Colloidal Ga-doped zinc oxide nanoparticles (NPs) are receiving attention to their novel opto-electronic properties with applications as transparent thermal barriers for smart windows, dye-sensitized solar cells or transparent conducting films. Whatever the application, one important goal is to get NPs-based inks with high transparency in the visible range. Two ways of optimization can be distinguished: (i) optimizing the print ink formulation: NPs dispersion with prevention of aggregation; (ii) optimizing the NPs transparency. We investigated both routes: (i) a low-cost and low-temperature (soft chemistry) route for the preparation of the Ga-doped ZnO NPs by forced hydrolysis in polyol ; (ii) the encapsulation of each single NPs by a low refractive-index shell serving as a self-supported anti-reflecting layers.
(i) The synthesis process developed for Ga-doped zinc oxides is forced co-hydrolysis at 170°C of zinc and doping ions acetates in Di-Ethylene-Glycol solvent (DEG); particles are constituted of spherical aggregates of few 100 nm with primary crystallite size of about 10 nm. The reductive strength of the solvent allows avoiding p-type intrinsic defects acting like electron killers. For the first time, direct correlation law between the doping ion concentration and the electron densities was established via the infrared absorption spectra. The bell-shaped curve correlating doping concentration and electron densities originated from the compensation of two doping ions while they share one ligand (oxygen anion) by one bulk electron killer (one zinc vacancy).
(ii) ZnO@MgF2 core-shell particles were synthesized by deposition of fluoride sols on ZnO particles through a vacuum slip casting process like. Then, particle films were elaborated by a screen printing process which ensured direct transmission measurements. The encapsulation of ZnO particles with a coating shell of 1.3 wt.% of MgF2 improves significantly the visible light transmission : 48% of transmission for ZnO film, 80% for ZnO@MgF2 particles films of roughly same thickness.
In a second step the 10 nm size NPs with the best infrared properties were dispersed into chloroform suspensions with the help of various thiol additives. Visible transparent - Infrared absorbent suspensions are so obtained. These suspensions can be directly deposited on glass substrates by dip or spin coating in order to get thin films or they can be dispersed in various polymers in order to obtain infrared barrier composites.

The copper oxide system receives a renewed interest for photovoltaic applications due to the suitability of its band gap for achieving high efficiencies as well as to it consisting of earth-abundant elements. By means of Raman spectroscopy it is possible to study the crystal structure, defects as well as non-stoichiometry.
Three different copper oxide phases are known: Two stable phases cubic Cu₂O (cuprous oxide) and monoclinic CuO (cupride oxide) and a metastable Cu₄O₃ tetragonal phase (paramelaconite). Each reveals unique Raman spectra due to their differences in crystal structure. According to group theoretical considerations for a perfect crystal the number of Raman signals and their frequencies for the cupride oxide phase are well understood. For Cu₄O₃ the number of single phonon Raman signals agrees with the number expected for a perfect crystal.
An exception remains: the cuprous oxide Cu₂O. A perfect Cu₂O crystal should exhibit one Raman signal (T_2g) only according to symmetry considerations. However, cuprous oxide Raman spectra reveal a rich structure of silent and infrared phonon modes independently of the growth mechanisms, i.e. molecular beam epitaxy, chemical vapor deposition, or sputtering as well as natural bulk crystals. The discrepancy between the observed number of modes and the single active Raman mode expected for a perfect crystal can be understood by an analysis of possible intrinsic point defects in Cu₂O leading to a symmetry reduction and a breakdown of the selection rules. We demonstrate that the number and the type of the phonons becoming visible in the Raman spectra depend on the type of point defect.

The effect of annealing on structural defect and d0 ferromagnetism in SnO2 nanoparticles prepared by solution combustion method is investigated. The as-synthesized SnO2 nanoparticles were annealed at 400 - 800 oC for 2 hours, in ambient conditions. The crystallinity, size and morphology of the samples were studied using XRD and TEM studies. The annealing results in grain growth due to coarsening as well as reduction in oxygen vacancies as confirmed by Raman spectroscopy, photoluminescence spectroscopy and x-ray photoelectron spectroscopy. All the as synthesized and annealed samples exhibit room temperature ferromagnetism (RTFM) with distinct hysteresis loops and the saturation magnetization as high as ~0.02 emu/g in as-synthesized samples. However the saturation magnetization is drastically reduced with increasing annealing temperature. Further the presence of singly charged oxygen vacancies (Vo- signal at g-value 1.99), is confirmed by Electron Paramagnetic Resonance (EPR) studies, which also diminish with increasing annealing temperature. The observed diminishing RTFM and simultaneous evidences of diminishing O vacancies clearly indicate that RTFM is driven by defects in oxide lattice and confirms primary role of oxygen vacancies in inducing ferromagnetic ordering in metal oxide semiconductors. The study also provides improved fundamental understanding regarding the ambiguity in the origin of intrinsic RTFM in semiconducting metal oxides and projects their technological application in the field of spintronics.
[1] R.B. A. Sundaresan, N. Rangarajan, U. Siddesh, C. N. R. Rao, Physical Review B, 74 (2006) 161301-161304.
[2] J.K. Furdyna, Journal of Applied Physics, 64 (1988) R29-64.
[3] S.A. Ahmed, Solid State Communications, 150 (2010) 2190-2193.

By performing voltage ramp stress measurements on organic and inorganic (oxide and nitride) dielectrics and semiconductors, the current limiting conduction mechanisms were identified. The organic thin films were fabricated using thermal evaporation under high vacuum conditions, silicon oxides and nitrides were processed by plasma enhanced chemical vapor deposition (PECVD). To achieve a better long time stability the samples containing organic materials were encapsulated in N₂-atmosphere by glass lids directly after fabrication. Therefore the current voltage characteristics were analyzed regarding mechanisms such as space charge limited current (SCLC), thermionic emission, tunneling processes (Fowler-Nordheim and direct tunneling) and reemission from traps (Poole-Frenkel effect). It is shown that by controlled doping of organic and inorganic dielectrics, the limiting properties of the different conduction mechanisms can be tailored by choice. The electrical transport phenomena were extracted depending on the dopant concentration of nitrides in SiO₂ and transition metal oxides in organic hole transporting materials. Additionally, electrical breakdown properties of the different types of thin films with focus on a shift of the breakdown electric field towards higher values were compared. By performing mobility measurements via admittance spectroscopy it was observed that the charge mobility in the different types of doped materials is linked to the limiting conduction mechanisms.

Oxide glasses in the system xTiO2. (60-x)Bi2O3.40 SiO2 have been prepared by a normal melt quench technique in a wide range of compositions. The present study includes optical characterization of these glasses by using UV-visible spectroscopic technique. The optical absorption spectra have been recorded at room temperature in the wavelength range from 400 to 1200 nm. The optical band gap energy (Eopt) has been evaluated from the Tauc's plots, which shows direct forbidden transitions in these glasses. The optical band gap energy (Eopt) values were observed to increase with increase in TiO2 concentration while the reverse trend was observed for refractive index (n).The increase in optical band gap (Eopt) from optical absorption analysis indicates that the glass network becomes more compact and degree of disorder decreases with increase of concentration of TiO2. The origin of Urbach&’s energy seems to be associated with phonon-assisted indirect transitions and its value varies with the random internal electric field either due to lack of long range order or presence of defects.

Vanadium oxides are identified as potential candidates in thermal sensing because of their relatively large temperature coefficient of resistance, favorable electrical resistance, low noise and compatibility with the machining processes of the sensing layer. They exhibit semiconductor to metal transition, which is associated with hysteresis. It is reported in the literature that both the bandwidth of the hysteresis and transition temperature can be tuned for defense applications by doping vanadium oxides with Mo and W. These films show reduced transition temperatures which could be exploited for room temperature thermal sensing applications. Precise control of synthesis parameters is required in stabilizing pure phase in thin film form.
In this work, a novel technique called, “Ultrasonic nebulized spray pyrolysis of aqueous solution combustion mixture” was adopted to deposit films of V2O5 and Mo-doped V2O5. Subsequently they were reduced to VO2 and Mo- doped VO2 respectively. Structural characterization of the synthesized oxides was done using X-ray diffraction. XRD was used to investigate the monophasic nature, lattice parameters and crystallite size of the films. Morphology of the synthesized powders was found to be plate like and bimodal using SEM with their sizes varying between 70 nm and 2 µm. Particle size measured using SEM matched with the crystallite size measured using XRD. A four-probe electrical resistivity set up was built in-house and variation in the metal-insulator transition temperature of VO2 and doped VO2 were investigated using the same. The resistance of the VO2 pellet before and after the transition were 106 and 103 #8486; and the hysteresis ~ 10 0 C.

Oxygen sensors are widely used in various fields such as biology, medicine, transportation, agriculture, life protection, chemical processing, and environmental monitoring. Compared with traditional potentiometric or amperometric oxygen sensors, nanowire-based oxygen sensors offer many advantages such as high sensitivity, short Debye length comparable to the target molecules, and low power consumption. However, traditional nanowire sensors which require using silicon substrates are not flexible and often need to work at high temperature. These disadvantages limit their use in real flexible room temperature environment. In this work, we presented a novel flexible room temperature oxygen sensor based on individual ZnO nanowire. This oxygen sensor utilizes Metal-Semiconductor-Metal structure, which forms two back to back Schottky diodes between metal and ZnO nanowire. Due to the exponential relationship of current and barrier height, the Schottky diodes have a much better sensitivity than the traditional Ohmic contact formed between metal and semiconductor nanowire. In addition, the pretreatment by UV light can enhance the sensitivity as well. The UV light can generate electron-hole pairs at the ZnO nanowire surface. The holes can combine with oxygen anions and form an oxygen molecule. Therefore, the ZnO nanowire surface which is treated by UV light is more favorable for oxygen to adsorb. More importantly, by introducing the strain to this structure, the already high sensitivity is even further enhanced by the piezotronic effect. ZnO is a piezoelectric material. The piezoelectric potential introduced by strain can tune the barrier height of the Schottky diodes in the Metal-Semiconductor-Metal structure. A proper direction of strain can increase the Schottky barrier height. The increasing Schottky barrier height could influence the transport property of the device. In addition, it can also widen the depletion region of the Schottky junction and increase the effective area of junction to electrostatically adsorb the oxygen anions. Therefore, the sensitivity of oxygen sensor can be enhanced by piezotronic effect. This distinct piezotronic property provides a new approach in controlling the single nanowire sensor performance by coupling the gas molecule adsorption and piezoelectric properties. [1]
Reference:
1. Advanced Materials (2013), DOI: 10.1002/adma.201301262

The magnetic and electric properties of impurities in semiconductors are strongly dependent on the lattice sites which they occupy. While the majority site can often be predicted based on chemical similarities with the host elements and is usually simple to assess experimentally, minority sites are far more complicated to predict, detect and identify. This is particularly the case in dilute magnetic semiconductors (DMS). Although DMS materials are ideally described as purely cation-substituted alloys [1], even small fractions of the magnetic dopant ions occupying other lattice sites may significantly affect all the magneto-electric properties, for example, by acting as compensating defects. The canonical example is Mn-doped GaAs, where interstitial Mn compensates for substitutional Mn both electrically and magnetically [1,2]. In oxide-based DMS systems such as transition-metal doped ZnO, it is generally accepted that transition metal impurities occupy only cation (Zn) substitutional sites [1].
We present recent electron emission channeling studies on the lattice location of transition metals in ZnO. The technique is based on channeling and blocking effects acting on the β^- particles emitted during decay of radioactive ⁵⁶Mn [3], ⁵⁹Fe, ⁶¹Co [3] or ⁶⁵Ni, implanted at the ISOLDE facility at CERN. Although majority cation (Zn) substitution was observed for all the transition metals, as expected, minority fractions (20-30%) of Mn, Co and Ni in anion (O) sites were also identified. Both cation and anion substitutional fractions were virtually unaffected by thermal annealing up to 900°C. Such anion-site configuration, which had never been observed before for any transition metal in ZnO and related metal-oxides, challenges the current understanding of impurity lattice location and defect formation in compound semiconductors. We discuss potential mechanisms driving anion substitution, in the context of similar findings on transition-metal doped nitrides (e.g. Mn-doped GaN [4]). In particular, we examine the observed chemical trends across the four transition metals in ZnO and compare it to the behavior of the same impurities in GaN. In addition, we discuss potential implications of the observed anion substitution on the magnetic behavior of DMS materials in terms of electrical compensation.
[1] T. Dietl, Nat. Mater. 9, 965 (2010).
[2] T. Jungwirth, K. Y. Wang, J. Masek, K. W. Edmonds, J. Konig, J. Sinova, M. Polini, N. A. Goncharuk, A. H. MacDonald, M. Sawicki, A. W. Rushforth, R. P. Campion, L. X. Zhao, C. T. Foxon, and B. L. Gallagher, Phys. Rev. B 72, 165204 (2005).
[3] L.M.C. Pereira, U. Wahl, S. Decoster, J.G. Correia, L. M. Amorim, M.R. da Silva, J.P. Arauacute;jo, and A. Vantomme, Phys. Rev. B 84, 125204 (2011).
[4] L.M.C. Pereira, U. Wahl, J.G. Correia, S. Decoster, L. M. Amorim, M.R. da Silva, J.P. Arauacute;jo, and A. Vantomme, Phys. Rev. B 86, 195202 (2012).

By transmission electron microscopy, the density of basal plane stacking faults (BSFs) of nonpolar m-plane ZnO epilayers grown by metalorganic chemical deposition on a γ-LiAlO2 (100) substrate is estimated to be ~105 cmminus;1. The luminescence of BSFs in m-plane ZnO is analyzed using spatially resolved cathodoluminescence (CL) spectroscopy. Complementary features observed in CL images for detection energies of 3.379 (near-band-edge) and 3.324 eV confirm the latter to be a typical BSF-associated luminescence in ZnO. A clear blue shift of the BSF-associated luminescence is observed when the electron beam approaches the BSF. This experimental result proves the existence of a polarization field in the BSFs along the c-axis of the ZnO epilayer. Furthermore, the BSF-related transition is clearly identified from low temperatures to room temperature. This consequence indicates that an asymmetric near-band-edge luminescence of ZnO exhibiting a low-energy tail at room temperature may contain a significant contribution of BSF-related emission. For an m-plane Zn0.988Mg0.012O epilayer grown on γ-LiAlO2 (100) substrate, both the near-band-edge and BSF-related luminescence are observed to be blue shifted compared with those of ZnO. The BSF-associated emission of an m-plane ZnO epilayer grown on a β-LiGaO2 (100) substrate dominates the CL spectrum at low temperature. Correspondingly, a higher density of BSFs is observed in m-plane ZnO epilayer grown on β-LiGaO2 compared with that grown on γ-LiAlO2, which can be attributed to the different growth behavior.

Blue laser recording has been proposed as a promising candidate for storage of large amount of information at high definition resolution. Optical recording systems based on write-once discs are becoming most popular storage media due to their low cost and good compliance with read-only-memory (ROM). Inorganic thin films are increasingly studied as an information storage medium to replace the currently adopted organic layers to overcome their detrimental issues related to optical absorption, thermal stability, and moderate solubility in organic solvents. Towards this end, bismuth oxide thin films have been proposed as an excellent candidate due to their attractive optical and electrical properties such as wide band gap, high refractive index and dielectric permittivity, and remarkable photosensitivity and photoluminescence. Surprisingly, the mechanical properties have received little or no attention.
In this study, we report on the preparation of bismuth oxide thin films via a relatively novel Biased Target Ion Beam Deposition method and present a detailed report relating the deposition parameter space to mechanical (hardness and elastic modulus), optical (n and k), and structural (composition, crystallinity, surface roughness) characteristics for as-deposited and annealed material. This extensive library of material data was generated via nanoindentation, optical transmission, AFM, SIMS, XRD, SEM, EDS, and WDS.
Subsequently, we use a focused blue laser (405nm) to write an array of dots in the bismuth oxide thin film and demonstrate clear and circular recording marks in form of “bubbles” or “little volcanos” (FWHM ~500nm). Results indicate excellent static recording characteristics, writing sensitivity and contrast. The recording mechanism is investigated and is believed to be related to laser-induced morphology change.

Zinc oxide is a wide band gap material, with a high chemical stability, and good photoelectric and piezoelectric properties. It is used in various technological domains such as transparent electrodes, solar cells, gas sensors and surface acoustic wave devices. Its low price compared with other material makes it a good candidate for industrial applications. Many techniques, such as laser ablation, spray pyrolysis, sputtering, electron beam evaporation and metal-organic chemical vapor deposition have been developed and used to grow semiconductor ZnO thin films on a variety of substrates.
Undoped semiconductor zinc oxide thin films were grown at room temperature using two techniques: plasma deposition and electron beam evaporation in an argon atmosphere. Plasma deposition offers some advantages, such as low ion damage and low deposition temperature. The optical transmittance of the films deposited by both methods was higher than 80% in the near UV-VIS range; the energy band gap and index of refraction agree with values reported in the literature. The resistivity of films grown by plasma deposition was , lower than the value found for plasma assisted e-beam evaporated films.

Zirconium dioxide (ZrO2), called usually zirconia, is a frequently studied ceramic material with many practical applications, mainly after its stabilization. It is well established experimentally that zirconia has three phases at ambient pressure: monoclinic, tetragonal and cubic, in dependence on temperature. Here we report on the results of ab initio calculations which reveal possible existence of another two phases, one of them being tetragonal and the other one cubic. Both phases are similar to their already known counterparts and differ mainly in the arrangement of oxygen atoms in the lattice. Though such structures were not observed experimentally, it is not impossible that they exist in reality as there are several reports in the literature mentioning unidentified zirconia phases. The new tetragonal and cubic phases have their energies between those for known tetragonal and cubic phases. In addition to energies, we give detailed crystallographic description of the phases including the space group. Ab initio calculations are carried out using the pseudopotential and full potential all electron codes which give similar results. Electron exchange-correlation effects are treated within the local density and generalized gradient approximations.

VO₂ has been a textbook example of the metal-insulator transition (MIT) for the last 50 years. Despite this, it is only in recent years that a general consensus between experiment and theory has been reached as to the driving mechanism behind the transition. On the one hand, the large dimerizing (Peierls-like) structural distortion reinforces the relevance of the lattice, as well as imposing a limit on the timescale of the transition. On the other hand, much of the phase diagram requires the consideration of electron-electron correlations (i.e Mott-like). Here, we present experimental photoemission and soft x-ray spectroscopic evidence that the transition can be driven from a Peierls-like transition to a Mott-like transition by varying the applied strain of the lattice. Moderately-strained thin films of VO₂, grown on oriented TiO₂(001) and TiO₂(110) substrates, exhibit signatures that are characteristic of the structural Peierls distortion (much like bulk VO₂), principally owing to the large change in orbital occupation that accompanies the transition. For highly-strained VO₂/TiO₂(100), however, these hallmarks are absent, and the MIT resembles an electron driven Mott-like transition, i.e. without the large structural distortion. We suggest that such highly strained films may be valuable in circumventing the structural bottleneck in the timescale of the MIT in VO₂.

Metal-insulator transitions are a fascinating topic of fundamental and applied materials physics research. NbO2 is a 4d rutile-based transition-metal oxide that exhibits a temperature-induced metal-insulator transition. Since NbO2 is a 4d1 system, one may expect electron correlation effects to be weak; however, it remains insulating up to 800 oC, considerably higher than 68 oC for VO2, a 3d1 system. We present the first demonstration of epitaxial growth of NbO2 films on various substrates of different crystal structures, including corundum, perovskite, spinel, and rock salt. When the film and substrate planes have different rotational symmetries, structural variants of thin-film grains are formed. Rotational variants can be eliminated with proper choice of substrate crystal orientation; we show this for films on m-plane and r-plane Al2O3. A comprehensive study of the growth of this new thin-film material will be presented. We will also compare the optical spectral features of NbO2 and VO2, a model correlated oxide, in their insulating states. We find that indeed the optical gap of NbO2 is larger, and electrical measurements show that NbO2 is far more insulating. Higher-energy optical features will be discussed in the context of a one-electron band-structure picture. Our experimental optical spectra should be of great utility for future investigation of the excited-state properties and ultimately lead to more satisfying many-electron descriptions of the optical response of NbO2 as well as related correlated materials.

ZnO-based heterostructures have been widely studied for growth in the polar orientations, i.e. along the c axis of the wurtzite structure. Several reports have already shown the interest of the non polar orientations along which the electric field can be suppressed. Surprisingly, there is no report on ZnO based quantum heterostructures grown along a semipolar orientation.
In this communication, we demonstrate the feasibility of growing high quality semipolar (Zn,Mg)O/ZnO quantum wells (QWs) and multiple quantum wells on a (10-12) ZnO bulk substrate. A series of quantum wells was grown with different thickness (from 0.7 to 10 nm) by molecular beam epitaxy. The Mg content was varied from 10 to 35%. Wide atomically flat terraces with clear atomic steps are observed by atomic force microscopy after the optimization of the substrate preparation and the growth conditions. The X-ray diffraction confirms the high quality of the interfaces since clear Pendellösung fringes and superlattice peaks are evidenced in symmetrical and asymmetrical reflections. The full width at half maximum of the ZnMgO layers is in the range of several tens of arc seconds in every direction. No relaxation is detected from the reciprocal space maps for layers thinner than 200nm. The photoluminescence from the QWs was measured at 10K and room temperature. The full width at half maximum is as low as 5meV for a 3.1 nm QW at low T. The confinement is clearly observed for QW thinner than 3 nm. The emission of wider QWs is below the ZnO band gap owing to the presence of an electric field, which can be estimated around 400kV/cm for a barrier with 17% Mg. The emission is strongly polarized perpendicular to the projection of the c axis on the (10-12) plane, in agreement with the selection rules.
Finally N was incorporated into the ZnMgO barriers via a plasma cell and a light emitting device based of semipolar QWs was fabricated.

Solar-blind (200-280 nm) ultraviolet (UV) detectors have recently attracted a great deal of attention owing to a variety of potential applications, including missile warning, secure communications, flame sensing, etc [1-4]. Quaternary ZnO-BeO-MgO material system is ideally suited for achieving devices operating in this spectral range and allows one to overcome limitations of ternary ZnO-BeO and ZnO-BeO compounds related to insufficient solubilities.

In this contribution, we report on the growth of quaternary alloy BeMgZnO layers with a bandgap tunable over a wide wavelength range from 250 to 380 nm by using plasma-enhanced molecular-beam epitaxy. Epitaxial films were grown on a c-plane sapphire substrates at various substrate temperatures and Mg fluxes, while the ratio of incorporated Zn and Be species was kept constant. Crystal structure of the layers was studied by high resolution X-ray diffraction (HRXRD) and Reflection High Energy Electron Diffraction (RHEED) analysis, and optical properties were evaluated from optical absorption/transmission measurements and photoluminescence.
Substrate temperature of around 400oC was found to be optimum for achieving the best structural and optical qualities. Both increase and decrease in substrate temperature gave rise to broadening HRXRD rocking curves and broadening of the absorption edge.
The analysis of HRXRD/RHEED data and bandgap values derived from the optical measurements suggests that adding of Mg to ZnBeO solid solution substantially improves the incorporation of Be on lattice sites. The finding has been attributed to strain compensation, because alloying of ZnO with BeO and MgO results in the lattice deformation of opposite signs.
Bandgap values assessed from optical absorption measurements were compared with those computed with hybrid density functional theory.
1. D. H. Zhang and D. E. Brodie, Thin Solid Films 238, 95 (1994).
2. Y. Takahashi, M. Kanamori, A. Kondoh, H. Minoura, and Y. Ohya, Jpn. J. Appl. Phys. 33, 6611 (1994).
3. S. A. Studenikin, N. Golego, and M. Cocivera, J. Appl. Phys. 87, 2413 (2000).
4. P. Sharma, A. Mansingh, and K. Sreenivas, Appl. Phys. Lett. 80, 553 (2002).

The p-type conducting Copper-oxide compound semiconductors (Cu2O, CuO) provide a unique possibility to tune the band gap energies from 2.1 eV to the infrared at 1.40 eV into the middle of the efficiency maximum for solar cell applications. By a pronounced non-stoichiometry the electronic properties may vary from insulating to metallic conduction. They appear to be an attractive alternative absorber material in terms of abundance, sustainability, non-toxicity of the elements, and numerous methods for thin film deposition that facilitate low cost production. In the talk the synthesis and characterisation of Cu2O thin films used as p-type absorbers in heterojunction solar cells will be reported. We discuss i) controlled p-type doping by nitrogen ii) the role of the possible n-type dopant Zn, and iii) tuning of the energy gap by alloy formation.

We present properties of phonon modes, electronic band-band transitions and refractive index for crystalline (In_xGa_1-x)₂O₃ thin films in a wide composition range. Therefor we studied (Ga,In)₂O₃ thin films with a continuous variation of the indium concentration between almost 0% and 85% grown by pulsed laser deposition. This gives us the opportunity to investigate the physical properties of this ternary system at any composition within these limits.
The results which we present in this contribution were obtained by means of spectroscopic Raman scattering and ellipsometry measurements. These optical measurements on the one hand are non-destructive and yield a fast characterization of the material. Also they are of fundamental physical interest, since there are none reported in the literature for this compound material so far.
Micro-Raman line scans along the composition gradient unveil the expected continuous redshifting of the phonon modes with increasing indium concentration as well as an abrupt phase transition between the monoclinic crystal structure of β-Ga₂O₃ and the cubic structure of In₂O₃. For several modes with sufficiently high scattering intensity, we present their composition dependent phonon energies. These allow to determine the incorporated indium concentration from the Raman spectra. No differences for phonon energies and broadenings are observed between the x=0 end of the graded sample and our high-quality, pure β-Ga₂O₃ thin films substantiating the application potential.
Carrying out local ellipsometric measurements along the chemical gradient, the optical properties of the thin film in dependence on the In concentration were deduced in the wide spectral rage from 0.5eV - 9eV. Using mathematical inversion and a B-Spline model the evolution of the optical properties and band gap energy has been evaluated. By applying model dielectric functions we have established a parametric dielectric function valid for a wide range of indium concentrations.
The wide band gap of β-Ga₂O₃ of 4.9 eV at room temperature makes it interesting for a variety of applications. Due to its large breakdown field, it is promising for the use in high-power devices such as field-effect transistors [1]. It can also be used in optoelectronics where it allows to build solar-blind photodetectors [2,3]. These applications further benefit from tuning the band gap, which is possible by alloying gallium oxide with indium covering a band gap range of over 1eV.
[1] M.Higashiwaki, K.Sasaki, A.Kuramata, T.Masui, S.Yamakoshi, Appl. Phys. Lett. 100, 013504 (2012).
[2] Y. Kokubun, K. Miura, F. Endo, S. Nakagomi, Appl. Phys. Lett. 90, 031912 (2007)
[3] T. Oshima, T. Okuno, N. Arai, N. Suzuki, S. Ohira, S. Fujita, Appl. Phys. Exp. 1, 011202 (2008).

The growing trend to develop not only transparent but also flexible consumer electronic devices, such as smartphones with foldable displays or wearable electronics spurs significant research activities in particular in the areas of transparent conducting oxides such as ZnO. Owing to its wide band gap, high exciton binding energy and a wide array of synthesized nanostructure morphologies, ZnO is the preferred candidate for applications in transparent electronics. Due to the fact that many ZnO nanostructures are realized using low-temperature chemical-bath-based approaches, it is possible to grow them on cheap flexible polymer substrates. On the other hand, wet chemical approaches often lack the scalability and purity control of the more refined vacuum-based technologies.
Recently we presented a method of fabrication of nanoporous ZnO films using the vacuum technique of magnetron sputtering with postdeposition oxidation annealing [1] and showed the applications of the material in gas [2] and biochemical sensors [3]. Since after deposition the films are nanostructured Zn which is transferred to ZnO only after thermal oxidation at 400°C or more, the application of this material on flexible substrates was until now impossible.
In this communication we report on the method of fabrication of nanoporous ZnO films on flexible PVC substrates by means of a three-step approach: (1) the deposition of a nanostructured Zn film on a carrier Si substrate; (2) oxidation of the Zn into ZnO at T > 400°C in an oxygen flow and (3) transfer of the nanoporous ZnO to a flexible PVC foil covered by an adhesive with the subsequent removal of the Si substrate. This approach relies on strain engineering between the Si carrier substrate and the ZnO during the zinc oxidation phase as well as on the geometry of the applied heat flux. In order to detach the films from the carrier substrate, a slow heating ramp using a resistive heater had to be applied, as compared to the regular RTP process. Due to the high level of control of the vacuum based processes as well as their scalability this method is possible to be applied in industrial-scale coating, including roll-to-roll processes.
The nanostructured character of the films and their structural properties are retained after the transfer as evidenced by Scanning Electron Microscopy imaging, X-ray Diffraction studies and photoluminescence measurements under He-Cd laser excitation at 5K. Image processing is applied for the qualitative description of the resulting porous film, yielding the porosity of 2.7% with the mean pore diameter of 22.3 nm.
This study was partially supported by the European Union within European Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-108/09, "MIME").
[1]. M.A. Borysiewicz et al., phys. stat. sol. A 209, 2463-2469 (2012)
[2]. M.A. Borysiewicz et al., MRS Proc. 1494 (2013) doi:10.1557/opl.2013.34
[3]. M.A. Borysiewicz et al., MRS Proc. 1552 (2013) doi:10.1557/opl.2013.580

Transparent thin film transistors (TTFTs) based on metal oxide semiconductors become one of significant issues for the next generation display. Specially, as metal oxide semiconductor TFT fabricated by solution process can solve serious issues such as cost problem, metal dopants which can improve electrical performance have been great interest for the solution process.[1,2] In addition, the mechanism of alkali metal doping for enhanced mobility is one of main issues in metal oxide semiconductors. Most studies have only focused on the fluctuation of crystallinity , morphology or the oxygen vacancy content of alkali metal-doped metal oxide semiconductors. However, the correlation between doping amount and improvement of electrical performance of metal oxide semiconductors were not completely explained by the analysis of the fluctuation of the crystallinity, morphology or the variation of oxygen vacancy content. For the reason, a technique for monitoring on the electrical properties of the metal oxide semiconductors is necessary to apply the metal oxide TFTs to industry.
Here, we introduced the alkali metals, Na and Li, as dopants toward solution amorphous zinc tin oxide (ZTO) thin film transistor (TFT) with good electrical performances. In addition, we analyzed the effects of alkali metal doping by analyzing the crystallinity, oxygen vacancy, morphology and optical properties. In addition, through practical approach using optical band gap, we successfully demonstrated that the shift in the optical band gap supported by the Burstein-Moss theory can directly show the enhancement of mobility related to the interstitial doping of alkali metals.
[1] S. Y. Park, B. J. Kim, K. Kim, M. S. Kang, K. H. Lim, T. Il Lee, J. M. Myoung, H. K. Baik, J. H. Cho, Y. S. Kim, Adv. Mater. 24, 834 (2012)
[2] K. H. Lim, K. Kim, K. Kim, S. Kim, S. Y. Park, Y. S. Kim, Adv. Mater. 25, 2994 (2013)

Amorphous transition-metal oxides are a new class of semiconductors recently highlighted in transparent electronics due to their superior electronic transport properties and high optical transparency [1,2]. Recently, we reported a 2.65% efficient thin-film Cu₂O-ZnO heterojunction solar cell [3], and a key enabler for this achievement is the introduction of a ultra-thin (~5nm) amorphous zinc-tin-oxide (a-ZTO) layer at the Cu₂O/ZnO interface. The buffer layers are deposited by atomic layer deposition (ALD) between the ZnO and Cu₂O layers; these layers strongly enhance energy-conversion efficiency by increasing the open-circuit voltage and fill-factor. Detailed measurements demonstrate that Zn-rich a-ZTO layer can enhance solar cell performance and reduce interfacial recombination by acting as an electron-blocking layer through a more favorable band alignment to Cu₂O. On the other hand, Sn-rich a-ZTO films are detrimental for solar cell performance despite having a favorable band alignment to Cu₂O. In addition, we also reported that thin-film transistors fabricated with Zn-rich a-ZTO channels performed better than the ones that have Sn-rich a-ZTO channels [4]. We hypothesize that these phenomena are due to deep-level defects in Sn-rich a-ZTO films that enhance recombination and reduce mobilities, as supported by preliminary capacitance-frequency measurements [3].
In this contribution, we use X-ray absorption fine structure (EXAFS) to identify a possible origin of the deep-level defects observed in capacitance experiments. We perform EXAFS at the K-edges of Zn and Sn in a-ZTO films with varying Zn and Sn compositions. We report that EXAFS confirms the amorphous nature of all ZTO films. In addition, quantitative modeling of EXAFS data highlights a strong correlation between solar cell efficiency and the peak magnitudes of the first coordination shell in the Fourier-transformed EXAFS signal at the Sn K-edge. This result suggests that oxygen vacancies located near the Sn atoms may play a role in enhancing recombination and reducing mobilities in Sn-rich a-ZTO. No similar correlation was observed at Zn K-edge. Our observations are consistent with a density-functional theory study that predicts the origin of these deep-level defects to be under-coordinated Sn-O atom complexes instead of under-coordinated Zn atoms [5]. Lastly, we discuss how this study can provide a framework to investigate the defect structure of similar class of amorphous transition-metal oxide semiconductors.
[1] E. Fortunato, P. Barquinha, and R. Martins, Adv. Mater. 24, 2945, (2012).
[2] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, Nature 432, 488 (2004).
[3] Y. S. Lee, J. Heo, S. C. Siah, J. P. Mailoa, R. E. Brandt, R. G. Gordon, and T. Buonassisi, Energy Environ. Sci. (in press, DOI:10.1039/C3EE24461J).
[4] J. Heo, S. B. Kim, and R. G. Gordon, Appl. Phys. Lett. 101, 113507 (2012).
[5] W. Korner, P. Gumbsch, and C. Elsässer, Phys. Rev. B 86, 165210 (2012).

Due to the wide range of applications in the electronics industry, transparent amorphous oxide semiconductors have been attracting considerable attention by research groups and the industry around the globe. This interest is fueled by the high mobility that these transparent semiconducting materials exhibit despite of their amorphous morphology. An example of these semiconducting oxides is the wide band gap n-type semiconductor zinc tin oxide (ZTO) whose mobility can exceed 25 cm2/Vs in the amorphous state [1,2]. In addition to the high mobility and transparency of these amorphous thin films, it is desirable to employ fabrication techniques that operate at low temperatures and with techniques that are compatible with a large-scale production. One way to accomplish these two goals is to deposit the semiconducting layer with a sol-gel method.
We will show that we are able to fabricate fully functional field-effect transistors (FETs) whose dielectric (zirconium dioxide) and semiconducting (ZTO) layers were deposited with sol-gel methods. From temperature-dependent FET characteristics, the temperature-dependent mobility and consequently information about the nature of the charge carrier motion in these devices are obtained. There is a clear difference between low-mobility and high-mobility samples.
At sufficiently high mobilities, a mobility edge that separates thermally activated and band-like transport is observed. For low-mobility samples, charge transport is dominated by thermal excitation. Transport based on thermal excitation seems to be well described by the multi-trap release model. This model is used to determine the trap release energies involved. Since the traps are very shallow, the conventional Boltzmann statistics cannot be used. Instead, Fermi-Dirac statistics was employed.
Furthermore, we will show data where the 2-terminal linear, 2-terminal saturation and 4-terminal linear mobilities are compared. The importance of the 4-terminal configuration for the determination of the linear mobility in FETs is pointed out as contact resistance at the electrode-semiconducting oxide interface can be factored out.
[1] B. N. Pal, B. M. Dhar, K. C. See and H. E. Katz, Nature Materials 8, 898 (2009).
[2] C.-G. Lee and A. Dodabalapur, Appl. Phys. Lett. 96, 243501 (2010).

In order to provide cost-efficient and homogeneous oxide thin films for electronic devices it is desirable to use amorphous materials which can be deposited at room temperature (RT). In comparison to amorphous silicon their electron mobilities are more than one order of magnitude higher [1,2]. Zinc-tin-oxide (ZTO) is a promising n-type semiconducting material in which only abundant materials are contained which is not the case for the well explored amorphous transparent semiconducting oxide (a-TSO) indium-gallium-zinc oxide IGZO [2]. High electron mobilities in the order of 10 cm ²/Vs allow usage in pixel drivers for active matrix displays. Rectifying contacts are needed for the fabrication of such pixel drivers. They could be realized with metals or with zinc-cobalt-oxide (ZCO) since it is one of the rare p-type amorphous oxide semiconductors (AOS) [3].
We present electrical properties of highly rectifying contacts on amorphous ZTO layers. The ZTO films produced by pulsed laser deposition at room temperature on glass substrates are X-ray amorphous. Our first metal-semiconductor structures showed only weakly rectifying behavior due to trap-assisted tunnel currents. In order to improve rectification, we introduced a thin insulating ZTO layer between the metal and the thin film. Optimized Schottky contacts with gold on this structure exhibit current on/off-ratios up to 10⁷ at ± 2 V. Using the p-type AOS ZCO we fabricated all amorphous oxide heterodiodes. Again, we introduced an insulating ZTO layer on the n-side of the heterointerface and obtained diodes having current on/off ratios up to 5 × 10⁶ at ± 1.6 V. These diodes outperform the best fully amorphous TSO pn-junctions having on/off ratios of 10³ at ± 4 V (p-ZnRh₂O₄/n-InGaZnO [3]) and 10² at ± 7 V (p-ZnCo₂O₄/n-InGaZnO [4]). Temperature dependent current-voltage characteristics and the current transport mechanism across the heterointerface will be discussed.
[1] K. Nomura et al., Jpn. J. Appl. Phys. 45, 4303 (2006)
[2] P. Görrn et al., Adv. Mater. 18, 738 (2006)
[3] F.-L. Schein et al., IEEE Electron Device Letters 33, 676 (2012)
[4] S. Kim et al., J. Appl. Phys. 107, 103538 (2010)
[5] T. Kamyia et al., Adv. Funct. Mater. 15, 968 (2005)

We present our latest results in the development of high performance Al-doped Zn-Sn-O (AZTO) amorphous thin films. Novel optoelectronic technologies, such as flexible solar cells and organic light emitting devices, require Transparent Conductive Oxide (TCO) electrodes which are not only highly conductive and transparent, but also mechanically flexible. Indium Tin Oxide (ITO) is the most used TCO up to date for these demanding applications, as it combines good electrical and optical properties, while being compatible with rigid and flexible substrates. However, there is a need from the market to reduce or eliminate the use of Indium as it is scarce and expensive. ITO also leads to contamination issues in organic devices. The preferred candidate to replace ITO is ZnO. However, ZnO is not ideal for flexible applications due to its polycrystalline nature and large thickness required to achieve good conductivity. Amorphous Transparent Oxide Semiconductors are a relatively new class of materials, which could fulfill the requirements for replacing ITO. These amorphous materials have attracted much attention due to their large electron mobility (as compared to amorphous Si) and compatibility with flexible substrates. By combining ZnO:Al and SnO₂ we have grown transparent amorphous thin films, with good mechanical (flexible) and optical properties. Subsequently, to optimize the electrical properties, we have studied the effect of Al and H₂ doping on this multi-compound TCO.
We have investigated the effect of the sputtering growth parameters and low temperature post annealing (< 200°C) on the morphological, electrical and optical properties of AZTO thin films. The AZTO films were deposited by co-sputtering from separate ZnO:Al and SnO₂ targets at 50°C. The ideal Sn/Zn ratio to achieve a conductive, transparent and amorphous film has been determined from a compositional gradient study and tuning the sputtering power for each of the targets. A clear variation of the electrical and optical properties was observed at different O₂ and Ar sputtering partial pressures. To further improve the electrical properties, the as-fabricated films were annealed at 200°C under different conditions: in air, N₂ and H₂ atmospheres, and for different annealing times. Samples annealed under H₂ atmosphere present an increase in carrier concentration, while maintaining an almost invariable mobility. For the air and N₂ annealed samples, the effect of the annealing depends strongly on the initial oxygen content of the film. The as-deposited AZTO films present a remarkably low surface roughness of less than 1 nm RMS. The films were also submitted to damp heat and bending tests after deposition on PET substrates. Finally, we compared the performance of this In-free TCO, with other similarly co-sputtered amorphous thin films like InGaZnO and Al:InZnO. With these results we have moved one step closer to the development of a competitive abundant material TCO.

Thin films and heterostructures of perovskite oxides have received much interest due to the remarkable properties they possess, such as multiferroic behavior, metal-insulator transitions, superconductivity, and two-dimensional electron gases. While these phenomena are now well established at low temperature, realization of this staggering range of functionality at room temperature remains a challenge, perhaps one of the most important one in the field. Recently, a step forward in this direction came with the demonstration of relatively high electron mobility (~ 300 cm²/Vs), at 300 K in BaSnO₃. This was achieved at a carrier density sim;10²⁰cm^-3 and with the added bonus of direct relevance to transparent conductors due to the wide band-gap [1].
In first part of this presentation, we will present detailed structural characterization of undoped and La-doped BaSnO₃ grown on MgO(001) and SrTiO₃(001) substrates using the high pressure oxygen sputtering. High-resolution XRD confirms phase-pure, epitaxial (001) BaSnO₃ on both substrates. Upon variation of oxygen pressure in the range of 2-3 mbar, the out-of-plane lattice parameter (used as a sensitive probe to stoichiometry and strain relaxation) of the undoped films on SrTiO₃ varies between 4.126 and 4.132 Å (in the thickness range of 80-220 Å), deviating only 0.4% from the bulk value of 4.116 Å. Growth rate (from GIXR) and rms roughness (from AFM) show similar downward trends with increasing pressure and varied between 1.8-0.66 Å/min and 15.7-1.73 Å, respectively. Furthermore, the out-of-plane lattice parameters of 2% La-doped BaSnO₃ films are 4.125 and 4.128 Å on MgO(001) and SrTiO₃(001), respectively, at a thickness of 950 Å. Irrespective of the substrate, the measured out-of-plane lattice parameters of Ba_0.98La_0.02SnO₃ films are similar, being slightly more compared to bulk BaSnO₃. This most likely indicates relaxed films, but with expanded lattice parameter due to the combined effect of La-doping and non-stoichiometry/defects. AFM of Ba_0.98La_0.02SnO₃ film reveals an atomically smooth surface morphology with root mean square roughness of 8.77 Å and 2.1 Å on MgO(001) and SrTiO₃(001), respectively.
In the second part, we will present a detailed electronic transport study of La-doped BaSnO₃ grown on the above mentioned two substrates, which will be compared with those of reduced BaSnO₃ grown on MgO(001). Resistivity and Hall measurements on Ba_0.98La_0.02SnO₃/SrTiO₃(001), reveal room temperature resistivity of 11 mOmega;cm, n-type carriers at a concentration of 6 × 10¹⁹ cm^-3, and a Hall mobility of about 15 cm²V^-1s^-1. Temperature dependent transport and magnetotransport studies will be described in detail as a function of doping, and will be related to the influence of substrate temperature, oxygen gas pressure, strain relaxation, and growth rates of doped Ba_1-xLa_xSnO₃ films.
Work supported by NSF through the UMN MRSEC.
[1] H. J. Kim et al., Applied Physics Express 5, 061102 (2012)