Single crystals of β-Ga₂O₃ and ZnO are promising materials for use in the fabrication of high-power electronic devices [1] and blue light emitting devices [2] owing to their wide bandgap energy and availability of large diameter high-quality bulk substrates. However, these bulk substrates generally show n-type conductivity due to impurities and vacancies. Hence, development of homo-epitaxial growth methods of high-purity and high-quality films of β-Ga₂O₃ and ZnO are important to explore device fabrication.
After the success of preparation of high-quality and high-purity freestanding nitride substrates of GaN [3] and AlN [4] by halide vapor phase epitaxy (HVPE), our group have been investigating HVPE of β-Ga₂O₃ and ZnO films on bulk β-Ga₂O₃ and ZnO substrates, respectively. The advantages of employing HVPE are high-purity crystal growth at high temperatures and high growth rates due to chloride chemistry. We first examined possible precursors for HVPE as well as influence of growth parameters by means of thermodynamic analysis. Then, we constructed HVPE reactors based on it.
In HVPE of β-Ga₂O₃, GaCl and O₂ can be used as precursors, and the growth is expected up to 16000C under an atmospheric pressure using N₂ carrier gas. In our HVPE system, GaCl generated in the upstream region maintained at 8500C by the reaction between Ga metal and Cl₂ gas was mixed with O₂ in the downstream region to grow β-Ga₂O₃. Growth was performed on bulk β-Ga₂O₃ substrates with various orientations at temperatures ranging from 800 to 10500C with various growth rates (0-20 mu;m/h). X-ray diffraction (XRD) and secondary ion mass spectrometry (SIMS) measurements revealed high structural quality of the grown layer almost identical to the substrate and low concentrations of impurities below their background levels.
In HVPE of ZnO, both ZnCl₂-H₂O-N₂ and ZnCl₂-O₂-N₂ systems were investigated. In both cases, ZnO can be grown even at high temperature of 10000C on ZnO (0001) substrate. XRD measurements of the grown layers revealed high structural quality almost identical to the substrate. SIMS measurements also revealed low impurity concentrations of H, C, Si and Cl below the background levels. High-frequency C-V measurements for metal-oxide-semiconductor (MOS) diodes using the grown layer revealed n-type conductivity with a concentration of N_D-N_A of 1x10¹⁵ cm^-3, which gives a platform to investigate growth of p-type ZnO layers by HVPE.
[1] M. Higashiwaki et al., Appl. Phys. Lett. 100 (2012) 013504.
[2] K. Nakahara et al., Appl. Phys. Lett. 97 (2010) 013501.
[3] K. Motoki et al., Jpn. J. Appl. Phys. 40 (2001) L140.
[4] Y. Kumagai et al., Appl. Phys. Express 5 (2012) 055504.

β-Ga₂O₃ exhibits large band gap energy of ~4.8 eV, which implies the material is a promising candidate for UV-sensors and next-generation power device applications [1]. Compared with SiC and III-nitrides from the viewpoint of device fabrication, it is advantageous that single crystal substrates with large-diameter and high quality can be fabricated by melt growth. Nowadays, 2 inches in diameter β-Ga₂O₃ wafers with low dislocation densities (~10⁴ cm^-2) have been reported [2]. Although high purity (low impurity concentration) crystals are needed to exploit the superior potential of β-Ga₂O₃ for power devices, it is difficult to control the impurity levels as low as 10¹⁶ cm^-3 by melt growth. One promising approach to obtain a high purity β-Ga₂O₃ crystal is using halide vapor phase epitaxy (HVPE). Recently, our group has theoretically predicted the possibility of HVPE growth of β-Ga₂O₃ by thermodynamic analysis [3]. In this study, we report the first demonstration of homo-epitaxial β-Ga₂O₃ growth by HVPE.
Sn-doped (001) β-Ga₂O₃ substrate was used. The source gases were gallium chloride (GaCl) and oxygen (O₂). GaCl was generated in the upstream region of the reactor by the reaction between high-purity Ga metal and chlorine (Cl₂) gas at 8500C. GaCl and O₂ were separately introduced to the β-Ga₂O₃ substrate by N₂ carrier gas, and growth of β-Ga₂O₃ was performed. Growth temperatures were ranging from 800 to 10500C. The input partial pressure of GaCl was fixed at 5.0x10^-4 atm and the input VI/III ratio was varied from 5 to 25 by changing the input partial pressure of O₂.
Single crystalline (001) β-Ga₂O₃ layers could be obtained at the growth temperatures of 800-10500C, which was confirmed by 2theta;-omega; X-ray diffraction. It was revealed that the growth rate of β-Ga₂O₃ was almost constant at around 5 mu;m/h in this temperature range under the condition of VI/III=5. Thus, the growth process is limited by mass transportation in these temperatures. Furthermore, the growth rate increased to 10 mu;m/h by increasing VI/III ratio to 25. These results agree well with the thermodynamic analysis, which implies that the HVPE growth of β-Ga₂O₃ is thermodynamically controlled [3]. Impurity concentration in HVPE-grown β-Ga₂O₃ was measured by secondary ion mass spectrometry (SIMS). The impurity concentrations of Si, C, H and Sn (dopant of substrate) were below the background levels of the SIMS chamber. Therefore, HVPE growth of β-Ga₂O₃ is quite promising approach to achieve a high purity and high quality crystal.
[1] M. Higashiwaki et al., Appl. Phys. Lett. 100 (2012) 013504.
[2] H. Aida et al., Jpn. J. Appl. Phys. 47 (2008) 8506.
[3] K. Nomura et al., J. Cryst. Growth, to be published.

β-Ga₂O₃ is a semiconductor with a wide bandgap of 4.8eV and is expected to become high-power device materials [1]. However, defects in β-Ga2O3 crystal are not well understood. Here, we performed synchrotron light X-ray topography observation of β-Ga₂O₃ single crystals.
β-Ga₂O₃ single crystals were grown by the Edge-defined Film-fed Growth (EFG) method at Tamura Corporation, where pulling direction was the [010] direction. We observed (-201) and (010)-orientated plates cut from the single crystal. X-ray topography was performed where the incident monochromatic X-ray with a photon energy of about 9 keV was used and the sample was set in reflection geometry.
We found that defects run in the [010] direction, and some of them seem low angle grain boundaries, and noticed twinning on the (100) plane.
1) M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, and S. Yamakoshi, Appl. Phy. Lett. 100 (2012) 013504.

The nature of the surface species formed by an atomic layer deposition (ALD) precursor is intrinsic in understanding the chemistry of thin film deposition, and is a key element in designing precursor compounds. We have recently adapted a modified model for fitting ²⁹Si signals in solid-state NMR to allow us to determine the fraction of #9567;Si-OH surface species (both single and geminal) that have been occupied after exposure to a pulse of precursor vapour.¹ N,N'-diisopropylacetamidinatediethylgallium(III) (1) was found to have excellent volatility with no decomposition during a ramped thermogravimetric analysis experiment. Stepped-isotherm experiments showed a 1 Torr vapour pressure at 64°C. The initial chemisorption of a new gallium(III) alkyl amidinate compound was investigated on high-surface area silica for NMR analysis.
To study the resulting surface species, 1 was exposed to a pre-treated high-surface area silica substrate at 100°C, 200°, and 300°C. Stable, persistent chemisorbed surface species were observed at all three temperatures. Substrates were analyzed by ²⁹Si and ¹³C solid-state nuclear magnetic resonance spectroscopy (SS-NMR) and ¹H high-resolution NMR. At 100°C and 200°C the reactivity of compound 1 to geminal and lone hydroxyl surface sites varied slightly: either one or both ethyl groups were eliminated to produce an alkylated (or non-alkylated) gallium acetamidinate on the silica surface, producing fractional coverages of 0.17 - 0.18. At 300°C there was a larger degree of reactivity producing a minor amount of the same surface species as at 100°C and 200°C. Additional chemisorbed products were also produced at 300°C, likely arising from the decomposition of the ligand framework but ultimately giving a fractional coverage of 0.31 on hydroxyl-terminated silica.
This presentation will discuss surface coverage and surface chemistry in the context of the deposition of gallium oxide thin films from 1, and the general thermal characteristics of this precursor (melting point, volatility, thermal stability). The details of subsequent experiments using 1 and water for atomic layer deposition of Ga₂O₃ thin films will also be presented.
[1] F. Zaera, Coord. Chem. Rev. 257 (2013) 3177-3191

β-Ga₂O₃ has attracted attention as a material for the fabrication of high power devices owing to its wide band-gap of approximately 4.9 eV. One of the important feature of β-Ga₂O₃ is that the bulk single crystal grown by melt growth [1] is available as a substrate for homoepitaxial growth. However, the bulk crystal generally exhibits n-type conductivity due to impurity incorporation and formation of oxygen vacancy. Therefore, in order to fabricate the high-voltage power devices, the growth technique of β-Ga₂O₃ with high quality and high purity is required. Our group proposes β-Ga₂O₃ growth by halide vapor phase epitaxy (HVPE) which is possible to grow high-quality and high-purity crystal at high temperatures and high growth rates. In our previous studies, suitable growth conditions for the HVPE growth of GaN [2], AlN [3] and ZnO [4] were estimated by thermodynamic analyses, which results were in good agreement with the experimental results. Therefore, thermodynamic analysis of β-Ga₂O₃ growth by HVPE is also useful for selecting precursors and growth conditions.
In this study, the prospect of HVPE growth of β-Ga₂O₃ is investigated by thermodynamic analysis and results are compared with experimental results. GaCl and O₂ gases were selected as precursors for HVPE growth of β-Ga₂O₃ by comparing equilibrium constants of the reactions between Ga- and O-precursors. The GaCl and O₂ gases with input partial pressure (denoted as P0_GaCl and P0_O2, respectively) were supplied to the HVPE system by carrier gases of H₂ and/or inert gas (IG). In the calculation, the following eleven gaseous species were chosen as neccesary gaseous species over Ga₂O₃ crystal: GaCl, GaCl₂, GaCl₃, (GaCl₃)₂, Ga₂O, Cl₂, O₂, H₂, HCl, H₂O and IG. The equilibrium partial pressures of each gaseus species and driving force for the β-Ga₂O₃ growth (ΔP_Ga2O3) were calculated with given growth conditions.
When H₂ is not included in the carrier gas, the ΔP_Ga2O3 is larger than zero up to 16000C, which indicates that the β-Ga₂O₃ growth is expected. With increase of H₂ in the carrier gas, however, the ΔP_Ga2O3 decreases. Therefore it is preferable to use non-hydrogenous carrier gas for the growth of Ga₂O₃ by GaCl-O₂ system. In the non-hydrogenous carrier gas at 10000C, the ΔP_Ga2O3 increases with increase of P0_O2 below input VI/III ratio (2P0_O2/P0_GaCl) of 1. The equilibrium partial pressures drastically change at input VI/III ratio of arround 1, and the ΔP_Ga2O3 is almost constant above input VI/III ratio of 1. The calculated growth rates from the ΔP_Ga2O3 are in good agreement with the experimental growth rates, which indicates the growth of β-Ga₂O₃ by HVPE can be thermodynamically controlled.
[1] M. Higashiwaki et al., Phys. Status Solidi A 211 (2014) 21.
[2] A. Koukitu et al., Jpn. J. Appl. Phys. 37 (1998) 762.
[3] Y. Kumagai et al., Phys. Status Solidi (b) 243 (2006) 1431.
[4] T. Fujii et al., J. Cryst. Growth 314 (2011) 108.

β-gallium oxide (β-Ga₂O₃) possesses material properties suitable for power device applications, such as an extremely large bandgap of 4.8-4.9 eV and large breakdown electric field strength of 8 MV/cm [1]. The electron mobility is estimated to be around 300 cm²/(Vmiddot;s) for electron densities of 10¹⁵-10¹⁶ cm^-3. Baliga&’s figure of merit, a basic parameter showing how suitable a material is for power devices, of Ga₂O₃ is several times larger than that of 4H-SiC or GaN. Another important feature is that large single-crystal β-Ga₂O₃ substrates can be fabricated with melt-growth methods such as floating-zone and edge-defined film-fed growth. This fact would directly lead to easy and low-cost mass production. This is a big advantage of Ga₂O₃ over 4H-SiC, GaN, and diamond substrates. For these reasons, we consider that Ga₂O₃ is a very attractive new semiconductor for power device applications because of its material properties and ease of mass production.
For power devices, the growth technique has to be capable of fabricating film a few microns thick in which the carrier concentration can be precisely controlled in the range of 10¹⁵-10¹⁷ cm^-3. There have been a few reports on epitaxial growth of Ga₂O₃ films on β-Ga₂O₃ (100) substrates by MBE. These films had atomically flat surfaces because of the step-flow growth mode. However, there was a fatal problem with the growth on the (100) plane in that the rate was only a few tens of nanometers per hour.
To overcome the above problem, we studied the relation between the growth rate and the Ga₂O₃ substrate orientation and succeeded in increasing the rate to several micrometers per hour by changing the orientation from (100) to (010) [2]. Next, we investigated the growth temperature dependence of Sn-doped Ga₂O₃ homoepitaxial films on the (010) plane. Smooth surfaces were obtained at growth temperatures of 550-650°C. On the other hand, a doping delay that was probably due to segregation of Sn occurred at higher than 600°C. Therefore, it was found that the optimum growth temperature for obtaining high-quality Sn-doped Ga₂O₃ homoepitaxial films is in a narrow range between 540°C and 570°C. By optimizing the growth temperature, we succeeded in precisely controlling the carrier concentration in the range of 10¹⁶-10¹⁸ cm^-3 [3].
In this paper, we describe the Ga₂O₃ MBE growth technique for power device applications. We show that a high growth rate and precise carrier concentration control are possible by changing the substrate orientation and optimizing the growth temperature. Part of this work was supported by NEDO, Japan.
[1] M. Higashiwaki, K. Sasaki, et al.: Appl. Phys. Lett. 100 (2012) 013504. [2] K. Sasaki et al.: Appl. Phys. Express 5 (2012) 035502. [3] K. Sasaki et al: J. Cryst. Growth 392 (2014) 30-33.

Gallium oxide is a wide band gap (4.9 eV) semiconductor with potential applicability as a transparent semiconducting oxide. Homoepitaxy of β-Ga₂O₃ thin film has been performed in the last years mainly by molecular beam epitaxy (MBE), realizing first Schottky barrier diodes and field-effect transistors (MESFETs) [1-3]. In contrast, the possibilities using MOCVD, which is more close to thermodynamical equilibrium and suitable for large scale production, are less explored.
In this presentation we study homoepitaxial growth of β-Ga₂O₃ on on-axis and off-axis (100) substrates. Trimethylgallium (TMGa) and H₂O are used as precursors for Ga and O, respectively. Growth is performed in the temperature range between 750°C and 850°C and chamber pressures between 5 and 100 mbar [4]. Structural studies are performed by aberration corrected TEM as well as by Z-contrast STEM using an HAADF detector.
TEM analyses reveal that in case of on-axis substrates and at low chamber pressures the layer is characterized by a high density of planar defects, primarily stacking faults and stacking mismatch boundaries, while the surface is rough and exhibits elongated islands of several nm height. We conclude that the stacking faults result from possible double positioning of 2D nuclei, which form stacking mismatch boundaries upon coalescence. This is caused by low surface diffusion of the ad-atoms and cannot be overcome by intentional miuscuts up to 1.5°. This shows that the diffusion length is typically smaller than approximately 20 nm even at the highest growth temperatures of 850°C. Addition of Indium however enables perfect step flow growth for a miscut angle of 1 °. The layers are free of planar defects and in some cases In₂O₃ is found at the growth surface. We explain these findings by In segregation to the surface where it is present as a thin metallic film increasing the mobility of the ad-atoms. This behavior is promoted by the lower thermal stability of In₂O₃ compared to that of Ga₂O₃ and resemble the surfactant behavior of In the case of III-Nitrides.
[1] T. Oshima et al. Thin Solid Films 516 5768-5771 (2008)
[2] K. Sasaki et al. Appl. Phys. Express 5 035502 (2012)
[3] M. Higashiwaki et al. Appl. Phys. Lett. 103 123511 (2013)
[4] G. Wagner et al. Phys. Status Solidi A 211 27 (2013)

Epitaxial growth of semiconducting α-gallium oxide on insulating aluminium oxide is potentially of high interest for device applications. The moderate misfit and high difference in the band gap (4.9 eV for gallium oxide, 8.7 eV for aluminium oxide) make the system interesting for two-dimensional electron gases and intersubband devices. Unfortunately α-Ga₂O₃ is thermodynamically metastabe under ambient conditions. The formation of α-Ga₂O₃ has, however been reported at 1000°C and at pressures as high as 44kbar [1].
In this talk, we will show by means of transmission elctron microscopy that the α phase of Ga₂O₃ in a heteroepitaxial growth process on a sapphire substrate (0001) can be stabilized by strain up to a thickness of 4 monolayers. Similar is known from conventional semiconductors previously. An example is the epitaxial stabilization of α-Sn on InSb.
The α-Ga₂O₃ layers were grown onto c-oriented sapphire substrates by meteal organic vapour deposition, molecular beam epitaxy and pulsed laser deposition at temperatures of 800°C. The samples were analysed by Z-contrast STEM in FEI Titan at 300 keV. Contrats simulations were done by frozen phonon calculations. Independent on the growth method applied we find a closed coherent 3 monolayer thick layer of α-Ga₂O₃ on top of the sapphire. Because of the lack of experimental data on the elastic constants we derive the pressure induced by the lattice mismatch of 4.8% based on the compression modulus derived in high pressure experiments by Lipinska et al. [2]. We find a value of 14GPa, which is good agreement with literature data on the phase transitions between α-Ga₂O₃ and β-Ga₂O₃. We show further that strain relaxation beyond a critical thickness leads to formation of the β-phase and leads in consequence to the domain like structure found in epitaxial growth of β-Ga₂O₃ on sapphire.
[1] J. P. Remeika et al. Appl. Phys. Lett. 8, 87 (1966)
[2] Lipinska-Kalita et al. Phys. Rev. B 77, 094123 (2008)

We report on Schottky barrier diodes (SBD) on binary bixbyte In₂O₃ and monoclinic Ga₂O₃ and SBDs on alloyed (In_xGa_1-x)₂O₃ thin films. Highly rectifying Schottky contacts are routinely obtained for various metals on b-Ga₂O₃ realized by thermal evaporation or a reactive sputtering process. The diodes are highly stable at elevated temperatures. This allows studying such diodes at temperatures at which non-idealities induced by a laterally varying Schottky barrier potential are negligible [1]. The ideality factor of the diode is then determined by image force lowering, the barrier height obtained from current-voltage and capacitance-voltage measurements are the same.
Barriers on In₂O₃ show up to now rectification up to 4 orders of magnitude; lowering the free electron concentrations in such thin films by compensation with Mg will be explored to increase the rectifying properties.
Schottky contacts on ternary (In_xGa_1-x)₂O₃ have best properties for low Indium contents. The rectification and the barrier height decrease systematically with increasing x.
The group-III sesquioxides find potential application in high power applications, as deep-UV photodetectors or QWIPs. For these applications the realization of rectifying metal-semiconductors is a prerequisite. Realization of Schottky diodes on b-Ga₂O₃ was already reported and is rather straightforward. However, the commonly observed surface electron accumulation layer on In₂O₃ [2] has hindered realization of high-quality Schottky contacts thereon. Recently a promising reactive sputtering approach was used to create SBDs with current rectification of four orders of magnitude on In₂O₃ thin films grown by molecular beam epitaxy. We used this reactive sputtering process to realize SBDs on In₂O₃ thin films grown by pulsed-laser deposition and show that the rectification of such diodes exceeds that reported in the literature if the contact metal is deposited In₂O₃:Mg layer having a reduced free electron concentration due to compensation.
The SBDs on ternary (In_xGa_1-x)₂O₃ were realized on a single 2” wafer having a lateral variation of the In content x. The thin film was created employing a recently introduced facile approach to grow films with a continuous composition by pulsed-laser deposition [4]. This approach enables us in principle, to investigate thin film properties for any In admixture x.
[1] Daniel Splith et al., phys. stat. sol. (a) 211, 40 (2014)
[2] P. King et al., Phys. Rev. 101, 116808 (2008).
[3] H. von Wenckstern et al. , APL Mater. 2, 046104 (2014)
[4] H. von Wenckstern at al., CrystEngComm 15, 10020-10027 (2013) #8232;

Development of quantum devices, such as quantum wells and topological insulators, has been getting more active. Thin film fabrication processes, such as MBE and MOCVD, which enable control of precursor materials in an atomic scale, are normally employed in order to prepare the quantum devices. Especially, migration of precursor materials on the surface is one of the most important issues for fabrication of highly crystalline and homogeneous thin films using these processes. For example, highly oriented GaAs thin films of satisfactory crystallinity can be prepared at temperature of 600°C by MBE [1,2]. However, high temperature is required to increase migration length since migration length decreases with temperature [3,4]. In order to prepare highly crystalline thin films at low temperature, usage of surfactants such as bismuth (Bi) or antimony (Sb) has been developed. However, high growth temperature above 400°C is still required.
On the other hand, mist CVD is a technique for functional thin film growth under open-air atmospheric pressure using solution precursors. Therefore, this method is an environmental friendly process. It would be very innovative (or “interesting”), if the quantum devices can be fabricated by such a simple cost-effective technology.
Recently, high quality thin films can be grown by mist CVD [5]. This is because precursor materials react in vapor phase as a result of Leidenforst effect, which enables “super migration” over 10 cm in the reaction field [6]. Thus, growth of multiple quantum well Fe₂O₃/Ga₂O₃ by mist CVD was attempted. As a result, a multiple quantum well of 20 periods of alternating 6 nm Fe₂O₃ and 14 nm Ga₂O₃ layers was successfully fabricated by mist CVD at temperature of 400°C. TEM images and XRD measurement show a highly ordered structure with both Fe₂O₃ and Ga₂O₃ layers of highly single crystallinity. This indicates that high quality thin films and the high performance devices can be prepared not only by the MBE and MOCVD but also by an environmental friendly technique such as mist CVD.
We would like to report in detail and discuss it in the meeting.
[1] S. Nagata and T. Tanaka, J. Appl. Phys., 48 (1977) 940
[2] J. H. Neave, et al., Appl. Phys. Lett., 47 (1985) 100
[3] F.A.Trumbor, Bell Syst. Tech. J., 39 (1960) 205
[4] K. Sakamoto, et al., Jpn. J. Appl. Phys., 32 (1993) L204
[5] T. Kawaharamura, et al., Phys. Status Solidi C 10 (2013) 1565
[6] T. Kawaharamura, Jpn. J. Appl. Phys., 53 (2014) 05FF08

Currently, Zinc Oxide (ZnO) films can be obtained by several techniques, where spray pyrolysis offers low-cost, simplicity, is useful for large area applications and no need of high vacuum. These advantages make of great potential the use of ZnO by spray pyrolysis as active layer of low-cost thin-film transistors (TFTs) applications. Low-cost TFTs applications are possible by the use of low-cost plastic substrates. Although, ZnO TFTs fabricated by spray pyrolysis have already been demonstrated, the temperature of deposition to obtain high performance devices is still high enough to be compatible with low-cost electronics.
In this work, the annealing effects at 180°C in Aluminum-ZnO contacts as function of time were studied. Also, the characterization and application in TFTs of ZnO films obtained at low-temperature (200°C) are presented. The ZnO films obtained by ultrasonic Spray Pyrolysis at 200 °C were deposited over Aluminum contacts on SiO2/Si wafers to demonstrate the use of active layer in thin-film transistors.

Organic-inorganic hybrid materials can be deposited from organic precursors diluted with oxidants using plasmas technology. The main advantage for this technology is the property modulation capability through the dilution ratio of the oxidants during the deposition process. The film can be polymer-like (organic-like) or oxide-like (inorganic-like) depending on the amount of oxidant in the source gases. The most representative organic-inorganic hybrid material that is deposited from Hexamethyldisiloxane and oxygen by plasma-enhanced chemical vapor deposition (PECVD) has been applied as corrosion protection coatings for metals, interfacial medium coatings for biomaterials, encapsulation materials for solar cells and organic light emitting diodes, and gate dielectrics for thin-film transistors. In this study, we investigate the organic-inorganic hybrid film deposited from a newly-emerging organic Bis-amino-silane precursor (Bis-DiethylAminoSilane, Si(NEt₂)₂H₂) diluted with oxygen by PECVD. We further demonstrate its usage as the gate dielectric for ZnO thin-film transistors (TFTs).
The deposited films reveal >90% optical transmittance from ~400 to ~2600 nm in wavelength. The films become more inorganic-like (SiO₂-like) when the [O₂]/[Bis-amino-silane] flow rate ratio increases, as verified by the Fourier transform infrared spectroscopy and contact angle measurements. The hybrid material is then used as the gate dielectrics for ZnO TFTs. An on/off current ratio of >10⁶, threshold voltage of 3 V, and subthreshold swing of 0.48 Vdec^-1 are achieved for the TFT using the gate dielectric deposited at an [O₂]/[Bis-amino-silane] flow rate ratio of 50 and a process power of 55 W. In comparison with commonly used gate dielectric material SiO₂ for TFTs, the appealing features of this organic-inorganic hybrid material are its flexibility offered by organic components along with adjustable interfacial properties. Further results will be reported in the symposium.

Amorphous oxide semiconductors (AOSs) such as indium gallium zinc oxide (a-IGZO) has been widely applied to flexible electronics due to the advantages of high carrier mobility, good film uniformity, low processing temperature and production cost. The first transparent a-IGZO thin-film transistor (TFT) was prepared on flexible polyethylene terephthalate (PET) substrate by using pulse laser deposition (H. Hosono, J. Non-Cryst. Solids, 35(2006), 851). In Hosono&’s work, the device with the field-effect mobility about 10 cm²/V#8729;sec was achieved. Afterward, sputtering, chemical vapor deposition (CVD) and the so-called “wet” processes were extensively studied in order to realize the large-area TFT production.
This work demonstrates the preparation of fully transparent a-IGZO TFT with bottom-gate structure on flexible polyethylene naphthalate (PEN) substrate. First, the double-layer SiN_x/SiO_y gate dielectrics with various thickness ratios (20 nm/180 nm, 35 nm/165 nm and 50 nm/150 nm) were deposited on PEN substrates containing indium tin oxide (ITO) electrodes by employing the plasma-enhanced CVD at temperature below 150°C. The 40-nm thick a-IGZO serving as the active channel layer and 400-nm thick indium zinc oxide (IZO) serving as the source/drain electrodes of TFT were then deposited in sequence by radio-frequency sputtering at room temperature to complete the sample preparation. UV-visible spectroscopy measurement indicated that the average transmittance of such a TFT is about 85 % in visible wavelength range. At a low operating voltage of 5 V, the field-effect mobility of 46.8 cm²/V#8729;sec, threshold voltage of 1.5 V, subthreshold swing of 1.3 V/decade, and on/off current ratio about 10⁵ were achieved in the TFT containing SiN_x/SiO_y gate dielectric with thickness ratio of 35 nm/165 nm as revealed by the electrical characterizations.

Within the class of amorphous oxide semiconductor devices, c-axis aligned crystal InGaZnO₄ (CAAC IGZO) transistors have been shown to possess dramatically improved device stability and reduced leakage currents. This work explored the RF sputtering conditions required to form CAAC IGZO thin films, and characterized the long-range crystallographic texturing. Strong c-axis alignments, with large in-plane domains, were obtained under a broad range of deposition conditions.
Films were deposited on thermally oxidized Si wafers by reactive RF sputtering in varying O₂/Ar plasmas (0-100% O₂) with substrates heated from room temperature to 400^°C. Crystallinity was quantified by X-ray diffraction and the crystallographic texture determined using a Bruker General Area Detector Diffraction System (GADDS). Selected samples were further characterized using high-resolution transmission electron microscopy (HRTEM) with an FEI Tecnai F20 TEM to image in plan-view and cross-section.
For substrate temperatures below 190°C, amorphous IGZO characterized by a broad X-ray peak at 2theta;=32° was observed. Above 190°C, a transformation to nano-crystalline films with an X-ray peak at 2theta;=30° (009 planes) occurs. GADDS shows increasingly strong c-axis texture up to deposition temperatures near 300°C. Under optimal conditions at 310°C and with 10% O₂, films deposited exhibited a c-axis texture full width at half maximum (FWHM) of 20°. At higher temperatures, increasingly random orientation was observed. CAAC formation was also found to depend on the O₂/Ar ratio during sputtering. Formation requires reactive sputtering, with loss of alignment at O₂ fractions above 70%. At the optimal temperature, sputtering in 100% O₂ increases the width of the c-axis texture peak to 35° FWHM.
HRTEM of films deposited under optimal conditions show a high degree of crystallographic alignment. Cross-section images show alignment of crystal planes perpendicular to the substrate with alignment variation of ±9° over a 15nm x15 nm field of view, consistent with the GADDS measurements. Plan-view images with a 15nm x 15nm field of view exhibit expected hexagonal symmetry with FFT diffractograms suggesting a single orientation with ±15° angular variation. These results indicate formation of much longer alignment of grains than previously reported, and suggest near single crystal structure over 15 nm length scales. With loss of the c-axis alignment, HRTEM show films that appear increasingly “polycrystalline”. These results will be discussed in light of nucleation and growth models for CAAC films.

Morphology of indium-Gallium-Zinc oxide (IGZO) thin films is an important factor affecting the mobility. We have observed a drastic improvement of the mobility of the IGZO thin film attributed to reduction of the roughness. The aqueous IGZO-precursor solution was carefully spin-coated on a gate insulator layer in the air. In order to discuss the relationship between the morphology and the annealing process, the coated IGZO-precursor films were annealed under the condition of various temperatures less than 300 0C and times in the air. Thin film transistor (TFT) structure was fabricated by rf-sputtering of aluminum source and drain electrodes on the annealed IGZO thin films for estimating the mobility. The surfaces of the IGZO thin films were observed with an atomic force microscope (AFM). When grain aggregation is observed on the surface, the mobility tends to be lower. This is because carrier transportation would be interrupted due to the grain boundary. The appropriate condition of annealing prevents shrinkage and discontinuous parts of the coated film. The resulting continuous IGZO films give the high mobility (> 2 cm² V^-1 s^-1). We also discuss the morphology of the solution-processed IGZO thin films prepared onto the plastic substrate.

Metal-oxide thin-film transistors (TFTs) have attracted considerable attention over the past decade due to their high carrier mobility and excellent uniformity. Besides, the solution processability and transparency have opened new horizons for low-cost printable and transparent electronics on flexible substrates. My thesis is focused on the interface engineering of solution-processed oxide thin-film transistors
Firstly, a simple passivation method is developed to overcome the water susceptibility of solution-processed InZnO thin-film transistors (TFTs) by utilizing octadecylphosphonic acid (ODPA) self-assembled monolayers (SAMs). The unpassivated InZnO TFTs exhibit large hysteresis in their electrical characteristics due to the adsorbed water at the semiconductor surface. Formation of a SAM of ODPA on the top of InZnO removes water molecules weakly absorbed at the back channel and prevents water diffusion from the surroundings. Therefore, the passivated devices exhibit significantly reduced hysteretic characteristics.
Secondly, we developed a simple and environmentally friendly spin-coating method for high-k dielectrics (AlOx, ZrOx,YOx and TiOx). These materials were used as gate dielectrics for solution-processed nanocrystalline In2O3 or amorphous InZnO TFTs with a maximum processing temperature of 300 C. The role of high-k dielectrics in device performance was systematically studied. Among the high-k dielectrics, the AlOx based devices showed the best performance with mobilities of 21.7 cm2 V1 s1 in an In2O3 TFT and 11.6 cm2 V1 s1 in an InZnO TFT with the on/off current ratio exceeding 106. Furthermore, the devices exhibited ultra-low operating voltages (<3 V) and negligible hysteresis. A comprehensive study suggests that the high performance of the AlOx-based devices could be attributed to the smooth dielectric/semiconductor interface and the low interface trap density besides its good insulating properties. Therefore, the solution-processed AlOx can be used as a promising high-k dielectric for low cost, low voltage, high-performance oxide electronic devices.
Future research related to oxide TFTs based biosensors and Oxide/2D material heterostructures will also be discussed.
Reference:
1 Xu, Wangying, et al. "Facile passivation of solution-processed InZnO thin-film transistors by octadecylphosphonic acid self-assembled monolayers at room temperature." Applied Physics Letters 104.17 (2014): 173504.
2 Xu, Wangying, et al. "Role of solution-processed high-κ gate dielectrics in electrical performances of oxide thin-film transistors." Journal of Materials Chemistry C (2014).DOI: 10.1039/c4tc00334a

Amorphous oxide semiconductors (AOSs) as channel layers in thin film transistors (TFTs) have emerged as promising materials in display industry due to its excellent electrical properties that include high field-effect mobility (above 5cm²/Vbull;s) and optical transparency compared with amorphous Si-based TFTs. Although most previous researches on the fabrication of AOSs were mainly investigated via vacuum deposition technique, solution coating method, especially ink-jet printing system offer opportunity for direct patterning in large area and enable the fabrication of low-cost electronics. However, coarse printing resolution and low performance of ink-jet printed AOSs electronics restrict their potential use in the manufacturing processing of TFT backplanes.
In this talk, we present high-resolution electrohydrodynamic inkjet (e-jet) printing system for fabrication of AOS-TFTs with high performances. E-jet printing enables to achieve line width from 2 µm in minimum to above with uniform thickness, and also to easily tune the thickness by repeating printing times. E-jet printed AOS-TFTs show superior mobility value exceeding 100cm²/Vbull;s with high on/off ratios (> 10⁷). Also, we fabricated all-printed AOS-TFTs composed of e-jet printed S/D/G electrode, channel, interconnects and passivation layers. These all-printed devices exhibited excellent improvement under bias stability test without any degenerative changes in electrical performances. Finally, ultra flexible AOS-TFT arrays demonstrated with e-jet printing provide the possibility in fields of flexible display and wearable devices.

Oxide heterostructures and interfaces exhibit many interesting properties, which attract much attention to both academic society and industry. Highly conductive interfaces or two-dimension electron gases (2DEGs) have been identified in many oxide heterostructures such as LaTiO₃/SrTiO₃, LaAlO₃/SrTiO₃, and MgZnO/ZnO. Usually, high quality crystals are essential for the existence of 2DEGs. However, highly conductive interfaces also have been noted in amorphous SrTiO₃-based oxide interfaces [1] and defective rf-sputtered MgZnO/ZnO heterostructures fabricated on foreign substrates (glass, polyimide, and stainless steel substrates) [2-4]—the former is attributed to the oxygen vacancies generated during thin film growth, whereas the latter can be achieved with thermal annealing or atmospheric pressure plasma jet treatment on ZnO prior to the MgZnO deposition [2-4]. This initiates the possibility for fabricating oxide heterostructure electronic devices using low-cost large-area compatible processes with which high quality crystals are difficult to grow.
We first investigate HfZnO films fabricated using low-cost large-area compatible rf-sputtering processes. Room-temperature sputter-deposited HfZnO tends to become amorphous as the Hf content increases. The Hf/Zn ratios of the deposited films double those of the targets. The built-in stresses of as-deposited films are compressive; the stresses are relaxed and even become tensile after thermal annealing. All the films exhibit high transmission of 80 % in the visible wavelength region. The optical bandgap increases with the Hf content but decreases with the annealing temperature. Hf doping increases the resistivity of ZnO owing to the disorder of the material structure and the higher bandgap, which result in more carrier traps and less thermally excited carriers in the conduction bands [5].
We then fabricate HfZnO/ZnO heterostructures on glass substrates using rf-sputtering technique. The resistance drops around 1-3 orders of magnitude after HfZnO thin films deposited on thermally annealed ZnO thin films. This implies the presence of highly conductive interfaces at the HfZnO/ZnO heterojunctions. The thermal annealing on ZnO prior to the depositions of HfZnO is critical for the formation of the highly conductive interface in this HfZnO/ZnO heterosturcture. With a fixed annealing duration of 30 min, this drop of resistance becomes more apparent as the annealing temperature exceeds 500 °C. After depositing Hf_0.05Zn_0.95O (target composition) on ZnO, the carrier mobility and carrier concentration increase from 0.47 to 14.3 cm²V^-1s^-1 and from 2.27'10¹² to 1.93'10¹³ cm^-3, respectively.
Reference
[1] Y. Chen et al., Nano Lett. vol.11 p.3774 (2011).
[2] H. Chin et al., J. Appl. Phys. vol.108 p.054503 (2010).
[3] S. T. Lien et al., J. Phys. D: Appl. Phys. vol.46 p.075202 (2013).
[4] T. H. Wu et al., J. Phys. D: Appl. Phys. vol.47 p.255102 (2014).
[5] C. H. Li et al., J. Appl. Phys. vol.114 p.084503 (2013).

SrZnO films, with SrO/(SrO+ZnO) le; 0.25, were grown by pulsed laser deposition using different SrO ratios, x. The stoichiometry of the films was achieved by employing targets with the desired film stoichiometry. The effects of temperature and oxygen pressure during growth on the films&’ structural properties were investigated. XRD results indicate that the film crystallinity was improved as the temperature and O₂ pressure increased up to 650 ^oC and 0.5 Torr, respectively. However, the crystallinity decreased as the SrO content in the films increased. No results obtained for the crystalline films showed any orientation of significant peaks besides the peak attributed to the (0001) plane, suggesting epitaxial growth. The magnitude of the c-lattice constants for films grown at 650 ^oC and an Oshy;₂ pressure of 0.5 Torr showed a slight increase as the SrO content increased, which is attributed to the larger atomic radius of strontium compared to that of zinc. Growth at a higher temperature, 750 ^oC, resulted in a sharp (0002) peak at a low SrO content (5%), whereas growth at a higher SrO content (10%) resulted in a non-crystalline film with minute crystallites with a (1120) orientation. This orientation was attributed to the fact that the film underwent a transition from a wurtzite to a rock-salt structure, leading to a reduction in the (1120)/(0001) surface energy ratio. Funding was provided by the Saudi National Plan for Science and Technology (project # 10-NAN1197-02 )

Zinc oxide (ZnO) is a transparent oxide semiconductor (TOS) with a wide band gap. It is expected that the light emission diode (LED) using ZnO shows the high luminance efficiency, because ZnO has a large exciton binding energy (60meV) compared with GaN (25meV). However ZnO tend to be n-type property because of O vacancy and naturally carrier doping. Therefore it is very difficult to achieve p-type property in ZnO. The main reasons for the difficulty of achieving p-type ZnO are the formation of native donor defects such as O vacancies and Zn interstitial impurities, and unstable acceptor doping. Currently, the deposition methods of p-type ZnO are almost MBE and pulse laser deposition (PLD), but it is little reported about easier deposition method like sputtering. In a reactive sputtering process, controlling the O₂ gas flow is generally important, which is difficult because of occurring frequently the oxygen desorption during deposition. Therefore, doping N as acceptors without self-compensation by oxygen desorption is poorly reported in sputtering.
In this paper we report about fabrication method of p-type ZnO and ZnO p-i-n junctions using sputtering. We focused silver (Ag) as acceptor. Ag is a group-IB element, which is found to substitute for Zn(#8545;) site in the würtzite ZnO lattice. Ag doped ZnO was grown by Rf-spuettering in vacuum. By optimizing the deposition parameter such as substrate temperature, sputtering pressure and target-sample distance, we designed to synthesize intrinsic ZnO films without O vacancy of oxygen desorption. As the result of measurement of electron spin resonance (ESR), ZnO films deposited in the optimized sputter condition were not appeared the peak derived from oxygen adsorption. Thus sputtering parameters and conditions without oxygen absorption were obtained from above and other experimental results, which is used in fabrication of p-type ZnO. Ag doped ZnO is deposited by co-sputtering of ZnO and Ag under the oxygen-rich condition. As the result, ZnO:Ag films showed high crystallization from the results of X-ray diffraction and cross sectional SEM. In order to observe the band structure around Fermi level in ZnO:Ag, it is measured by UPS. The band structure of ZnO:Ag films around the VBM showed the signature electronic states derived from p-type, which presented that ZnO:Ag films is p-type semiconductor. It was found by hole measurement that the hole mobility and carrier concentration are 0.72 cm²/V#12539;s and 1×10¹⁶ cm^-3, respectively.
#12288;Next, we produced the ZnO p-i-n junction formed by above method. The p-i-n junction is deposited on fused quartz substrate, which is consisted by multilayer structure of n-type ZnO:Al/ intrinsic-ZnO/ p-type ZnO:Ag. The result of I-V measurement showed a nonlinear curve like a diode, which appeared the success of fabricating the p-i-n junctions.Since this device showed sharp electronic reaction with respect to UV light, it is assumed that ZnO p-i-n junction can be used as the UV sensor.

Ga doped ZnO (GZO) films have wide potential application as transparent electrodes in optoelectronic devices such as thin films solar cells, organic light emitting diodes and flat panel displays because of their high transparency, low electrical resistivity, non-toxicity and abundance in nature. GZO films deposited on heated substrates at around 200°C showed better electrical and optical properties compared to those deposited at room temperature (RT) without heating substrates. In order to improve RT deposition, it is needed to understand the effect of heating substrates on growth of polycrystalline GZO films. It has been pointed out that some kinds of defects could be annealed out during the deposition of Al doped ZnO on heated substrates. On the other hand, it is well known that polycrystalline ZnO films deposited on heated glass substrates commonly shows high orientation in c-axis normal to the substrate surface. However, the effects of c-axis orientation on transport properties have not yet well understood. In this study, we investigated the effects of heating substrates on electrical and optical properties of GZO films from the view points of defects and structural properties by combining control of microstructure and post deposition annealing.
GZO films with thickness about 200 nm were deposited on glass substrates at RT by RF magnetron sputtering system using a ceramic target of ZnO:Ga₂O₃ 5.7 wt%. The structural properties were controlled by inserting self-buffer layer of GZO deposited at 200°C. The GZO films deposited on bare glass substrates at RT showed poor c-axis orientation, while we obtained high c-axis orientation by inserting a 40 nm thick GZO film deposited at 200°C as a buffer layer. It was found that the electrical and optical properties cannot be improved by controlling microstructures. The GZO films were annealed at 200°C in the deposition chamber under flowing Ar gas environment after deposition of GZO films. Carrier concentration and optical transmittance increased regardless of presence of a buffer layer after the annealing process. These observations can be understood as annealing out of defects. However, significant improvement in hall mobility was only observed for highly c-axis oriented GZO films deposited on the self-buffer layer. We evaluated the contribution of grain boundary scattering on hall mobility by analyzing transmission and reflection spectra based on the conventional Drude model. The results show that the contribution of grain boundary clearly exists in the GZO films without a buffer layer, while it is nearly negligible in the case of well oriented GZO films deposited on the buffer layer. We conclude that the independent effects of heating substrates for growth of GZO films on glass substrates are improvement of c-axis orientation and annealing out of defects. These two contributions are necessarily discussed separately.

Transparent conductive oxide (TCO) films have been used for various optoelectronic devices because of their high visible transmittance and low electrical resistivity. Al-doped ZnO (AZO) films should be promising potential alternatives to indium-based TCO films, such as ITO. They are potential candidates as Indium-based TCO films for the applications of transparent electrodes.
In this study, AZO films have been prepared by rf magnetron co-sputtering using two ceramic targets because of its advantage of uniform deposition. AZO films were deposited on various substrates under incident angles of sputtered particles at approximately 30°.The distance (d) between the target and the substrate is set within the range of 0.5-1.5 times the mean free path of the constituent element in a sputtering gas. A distance d were adjusted in the range of 190 mm from the 105 mm, by forming AZO films, electron mobility and the relation between the distance d is examined.
High Radio frequency power of ZnO target (phi;3 inch in size) 200W on the power, the power of the Al target(phi;3 inch in size) 0-22.5W discharging simultaneously. In an experiment in advance, and the ratio of input power and Al is doped, ZnO film formed between the electric resistivity since that is the minimum value, and the optimum auto doping conditions in the device.
All the XRD patterns showed a strong peak at 2theta;= 34.5° corresponding to ZnO (002) indicating that the films were polycrystalline structure with the <001> preferred orientation. The SEM images show columnar structure of ZnO. Carrier density increased with increasing Al concentration and Hall mobility decreased with increasing Al concentration. The lowest electric resistivities of AZO films were 7.7×10 ^-4Omega;cm for the films deposited on unheated glass substrate. The substrate temperature during the deposition was lower than 50 ^o C for the unheated substrate. The carrier density and Hall mobility were 5.6×10 ²⁰ cm ^-3 and 14.3 cm ²/V#12539;s, respectively. We could obtain low resistivity and low absorption coefficient AZO films on unheated substrates. Normally, during deposition, the substrate is heated to promote crystallization of the film. However, in this case, such as an acrylic resin substrate, is deformed by heat, the substrate cannot be used are apt to deteriorate. AZO films deposited on an acrylic resin were showing a same degree or more of high Hall mobility.
On the other hand, the lowest electric resistivity of AZO films were 4.5×10 ^-4Omega;cm for the films deposited on heated glass substrate at 260°C.The carrier density and Hall mobility were 5.0×10 ²⁰ cm ^-3 and 27.5 cm ²/V#12539;s, respectively. Transmittances in visible region for these AZO films were more than 85%.

ZnO is a wide band gap (3.3 eV) semiconductor with numerous optical and electrical applications, such as optoelectronics, transparent electronics, spintronic devices, sensors, and varistors. All of these applications are either affected by, or depend on, defects and grain boundary properties according to various dopants (Bi, Pr, Co, Mn, Ni, Cr, Cu, Li, H, N, P, Sbhellip;). ZnO is strictly a non-stoichiometic material of Zn_1+xO (n-type) in which Zn interstitials are too mobile to be stable at room temperature. The deep or shallow defects commonly found in nano and bulk ZnO are intrinsic zinc interstitial and oxygen vacancy, and extrinsic donors and acceptors. These defects controlled by doping elements and/or post-heat treatments also have improved the electrical and optical properties of ZnO. It is known that cobalt oxide added to ZnO varistors improves the nonlinear current-voltage characteristics, which is presumably due to the formation of the Schottky-type double barriers at the grain boundaries. Also it is found that the presence of a large Bi or Pr ions induces the concentration of native defects of acceptor type in the grain boundary of doped ZnO. However, it is not known about the relationship between the defects and grain boundary properties in the simple two component system of Co₃O₄-doped ZnO polycrystalline revealed a good varistor property.
In our study, we investigated the effects of Co₃O₄ doping in ZnO on the defects and grain boundary properties using dielectric functions (Z*, M*, ε*, Y*, and tanδ) including admittance and impedance spectroscopy and the first-principles calculations. It will provide information about the relationship between the formations of donor or acceptor defects at the bulk or grain boundary for the double Schottky barrier theoretically and experimentally. Experimentally Co-doped ZnO sintered at 1000#8451; in air have two kinds of defects (Zn interstitial and O vacancy) according to its doping levels, a single grain boundary (apparent activation energy E_a=0.6~1.0 eV), and a good varistor properties (nonlinear coefficient α=35). Also the first-principles calculations are conducted to analyze the bulk defects and interface states in Co-doped ZnO. The origin of varistor behavior in ZnO-Co₃O₄ system has been discussed in relation to grain boundary traps and defect chemistry by the experimental results and the first-principles calculations.

Polycrystalline ZnO is a typical n-type ceramic semiconductor with the optical band gap of 3.3 eV, and thus has been applied to various electrical and optical applications such as gas sensors, piezoelectric transducers, phosphors, transparent conducting films, and varistors. To control the figure of merits of these devices it is essential to understand the behavior of defects and interface states in doped ZnO, especially in ZnO varistors. Up to day many researchers have studied to improve the ESD (Electrostatic Discharge) and surge characteristics for a ZnO chip varistor compare with TVS-diode in order to extend market share. Recently it is reported that the difference in the ESD withstand capability between the Bi-based ZnO and the Pr-based ZnO chip varistors is only to differences in their distributions of high-resistance regions and the existence of the Schottky barrier. It is necessary to characterize the dynamics of the potential barrier after the application of ESD pulses and identify the degraded region such as gran boundaries, electrode interfaces, defects, and secondary phases systematically. Few studies have been carried out on the ability to withstand ESD and the degradation mechanism in ZnO varistor.
In the present study, we investigated the characterization of the local electrical properties in the grain and grain boundary of ZnO chip varistor before and after the application of ESD. Bi-based and non Bi-based ZnO chip varistors were prepared by a multilayered chip device process to analyze the effects of ESD. We have applied 15 kV ESD pulses for 10 times on chips and measured I-V, C-V, and admittance and impedance changes with frequency (100 Hz to 10 MHz) and temperature (-170 ~ 500#8451;). Bi-based ZnO chip samples were degraded dramatically but non Bi-based ones which a new varistor composition developed by us not changed in I-V characteristics with ESD. In the study of impedance and modulus spectroscopy, the activation energy of grain boundary of Bi-based ZnO was changed from 0.67 eV to 0.46 eV accompanied by the change of capacitances. From admittance spectroscopy it was revealed that the defect levels in Bi-based samples slightly changed from 0.20 eV to 0.18 eV but Bi-free samples not changed. From these results we will discuss the factors of improving the electric and dielectric properties on the ESD withstand capability of the Bi-based ZnO chip varistors.

Zinc oxide is a n-type semiconductor with a direct band gap of 3.3 eV. Its high optical transparency and its electrical conductivity make it one of the promising candidates for several optoelectronic applications such as liquid crystal displays, transparent electrodes in solar cells, thin film-based transistors. Moreover its radiation hardness, much higher than other widegap semiconductors, can be exploited for applications in environments with potential radiation hazard (space satellites, large scales facilities as particle accelerators or nuclear reactors).
Here we present a study on the effect of irradiation on ZnO thin films both with heavy and light ions. The aim is twofold. Heavy ion irradiation is used as a mean to tune/enhance the optical and optoelectronic properties of the material. On the other hand, light particles irradiation is a powerful means to create in the material defects like those expected for permanence in harsh environment.
ZnO films are 500 nm thick, deposited on (0001) sapphire, and are patterned both in Van der Paw and in Hall-bar geometry by optical lithography and chemical wet etching. The contacts on ZnO are defined by sputtering of 100 nm of Al.
Heavy ion irradiations are performed using 250 MeV Au-ions at several fluences up to 2×10¹³ cm^-2. This ion energy is expected to introduce defects mainly by electron scattering and can guarantee a homogeneous damage all along the film, since the ions implant deeply in the substrate. Light particle irradiations are carried out using 5.5 MeV He-ions. In order to evaluate both the radiation hardness threshold as well as to investigate the possible creation of new electron traps, irradiation at fluences ranging over five orders of magnitude are performed.
The irradiation is performed after protecting half sample by a Ta mask and electrical resistivity and Hall voltage are then measured. This allows the direct comparison of the Hall-effect measurements, performed both on the irradiated region and in the non-irradiated region. After irradiation with heavy ions, the resistivity was found to decrease with increasing the ion fluence, with an increase of the carrier concentration of about one order of magnitude for fluence of about 10¹³ cm^-2. This behavior could be ascribed to the creation of oxygen vacancies and zinc interstitials. The mobility increased too, of about 60%. By optical analysis, it has been noticed a red shift of the absorption edge and an increasing of optical absorption at wavelengths of about 600 nm.
A radiation hardness threshold higher than 1×10¹⁴ cm^-2, corresponding to an average displacement per atom (dpa) of 4×10^-5 is detected after helium particle irradiation, dpa higher than that expected for a ten year long permanence in spacecraft environment. Here the zinc vacancies are expected to be the major irradiation-induced defects. Above the threshold, the resistivity is found to increase with the fluence, twice in the film irradiated with 1×10¹⁵ He/cm^-2.

ZnO is an excellent luminescent material in the UV-range of ~ 3.4 eV. However, many as-grown ZnO samples require certain annealing treatment in order to eliminate the luminescence due to its inherent intrinsic defects, which is in the visible range. Under the proper annealing conditions, the intrinsic defect-related luminescence can be minimized and the desirable UV light emission can be significantly enhanced. In this research, we present a route to achieve these objectives via annealing in different environments. The ZnO samples were grown on sapphire substrates utilizing a magnetron DC-sputtering technique. The films were grown at 250 C for 2hr using a Zn metallic target in an Ar-O₂ environment at a pressure of 11 mT and at 30 W of delivered power. The SEM images show that the morphology of the films consists of nano-grains and the XRD indicate that the films have the Wurtzite structure with preferred c-axis orientation. The optical properties of the ZnO samples were analyzed via photoluminescence (PL). The as-grown samples were found to exhibit a prominent visible PL at 2.80 eV, attributed to Zn interstitials, and weak UV-PL at 3.25 eV. In order to understand the route toward enhanced UV-PL, one set of the as-grown ZnO films were annealed under O₂ atmosphere and the other set in Ar atmosphere both at ~ 900 C for one hour. The PL spectra of O₂ annealed films indicated that the light emission due to Zn interstitials was diminished, while the UV-PL was enhanced. However, an O₂ -related defect emission peak at ~ 2.18 eV was evolved. These defect dynamics can be attributed to the out-diffusion of Zn interstitials which is a result of their low migration barrier energy and to the formation of oxygen-rich surface. The ZnO films that were annealed under Ar atmosphere were found to exhibit significantly enhanced UV emission with nearly completely quenched visible emissions. Like in the case of the O₂ annealed samples, Zn interstitials were removed from the crystal lattice during the annealing process. From both experiments it can be concluded that annealing under Ar environment conditions is essential for achieving strong UV-PL, and Zn interstitials are non-radiative center defects responsible for quenching the UV light emission. In order to gain an insight into the origin of the UV light emission of the Ar annealed sample, PL was performed at 77 K. The optical analysis indicated that the PL emission was dominated by the donor-bound exciton at 3.35 eV and by structural-defect related PL at 3.31 eV. We acknowledge the US Department of Energy, Office of Basic Energy Science, Division of Materials Science and Engineering under Grant No. DE-FG02-07ER46386. J.H. who contributed in discussion acknowledges the National Science Foundation under Grant No. DMR-1202532.

We use electron paramagnetic resonance (EPR) and gamma spectroscopy to observe the transmutation of ⁶⁴Zn to ⁶⁵Cu in a ZnO bulk crystal grown by the seeded-vapor-transport method at Eagle-Picher (USA). The same transmutation effects are observed in a ZnO crystal grown by the hydrothermal method at Tokyo Denpa (Japan). Nuclear transmutation results in uniform copper doping of ZnO at precisely controlled levels.¹ Our crystals were irradiated for 20 hours in the nuclear reactor at Ohio State University; the total neutron flux was ~2.1 x 10¹³ neutrons cm^-2s^-1 and the thermal neutron flux was ~1.3 x 10¹³ neutrons cm^-2s^-1. During the irradiations, ⁶⁴Zn nuclei absorb thermal neutrons and form unstable ⁶⁵Zn nuclei. These ⁶⁵Zn then slowly decay to ⁶⁵Cu via electron capture or positron emission (the half-life of ⁶⁵Zn is 244 days). Major features in the gamma spectrum of a neutron-irradiated crystal include the primary 1115.5 keV gamma rays from the ⁶⁵Zn decay and a positron annihilation peak at 511 keV. Our observation of these peaks confirms the successful transmutation of ⁶⁴Zn nuclei to ⁶⁵Cu. Additional direct evidence for transmutation is provided by the EPR spectra of Cu²⁺ (3d⁹) ions. The ⁶³Cu and ⁶⁵Cu hyperfine lines are well resolved in these EPR spectra and the relative concentrations of ⁶³Cu and ⁶⁵Cu nuclei in a crystal are easily determined. EPR spectra from isolated Cu²⁺ ions acquired approximately 180 days after the irradiation with neutrons show only hyperfine lines from ⁶⁵Cu nuclei. The absence of ⁶³Cu lines in these spectra demonstrates that the observed ⁶⁵Cu signals are due to transmuted ⁶⁵Cu nuclei resulting from the neutron irradiation (⁶³Cu nuclei are not produced by transmutation). Copper impurities are ubiquitous in nearly all ZnO materials and must be accounted for when investigating the transmutation of ⁶⁴Zn to ⁶⁵Cu. In our as-grown ZnO crystals, these copper ions are present as Cu⁺-H complexes. The Cu⁺-H complexes are not affected by the neutron irradiation, but they dissociate when a crystal is heated to 900 °C. This thermal behavior of the as-grown copper complexes allowed us to use EPR to distinguish between the copper that was present before the neutron irradiation and the copper that was produced as a result of the neutron irradiation. Besides the transmutation of Zn to Cu, a second major effect of the neutron irradiation was the formation of zinc and oxygen vacancies as a result of momentum-conserving collisions initiated by the high-energy neutrons. Singly ionized charge states of the vacancies are monitored at low temperature with EPR.
¹ F. A. Selim, M. C. Tarun, D. E. Wall, L. A. Boatner, and M. D. McCluskey, Appl. Phys. Lett. 99, 202109 (2011).

Cu-doped p-type ZnO films were grown on c-sapphire substrate by plasma-assisted molecular beam epitaxy (MBE). The effect of Cu doping on ZnO films was investigated by low temperature photoluminescence (PL) experiments. An acceptor bound exciton (A⁰X) emission was observed at 3.353 eV at 14 K. From temperature dependent PL study, the acceptor energy level for the Cu dopant is estimated to be 0.14 eV above the valence band edge. Both Hall bar and van der pauw Hall effect measurements were performed on Cu-doped ZnO samples. Hall effect results indicate that a growth window was found for the formation of p-type ZnO thin films and the best conductivity was achieved at the substrate temperature of 500 °C, Cu cell temperature of 600 °C, Zn cell temperature of 340 °C and oxygen flow of 2 sccm. The sample exhibited a high hole concentration of 1.54×10¹⁸ cm^-3_, a low resistivity of 0.6 Omega; cm and a moderate mobility of 6.60 cm²/V s at room temperature. Seebeck measurements was performed by a homemade Seebeck system on Cu-doped ZnO samples to further study the p-type behavior. A positive slope (i.e. positive Seebeck coefficient) determined that the samples were p-type. A metal oxide semiconductor (MOS) capacitor device was also fabricated by using our MBE grown Cu-doped ZnO film as semiconductor layer, atomic layer deposition of aluminum oxide as oxide layer and e-beam evaporated Au as metal layer. The characteristics of capacitance-voltage (C-V) measurements of the fabricated MOS capacitor device also demonstrated that our MBE grown Cu-doped ZnO thin film was p-type. Other measurement results such as XRD, XPS, Raman, transistor characteristics will also be presented to further elucidate the p-type mechanism. This study suggests that Cu should be an excellent dopant for reliable and reproducible p-type ZnO.

Over the last decade, ZnO has attracted much attention as a semiconductor material and has been extensively investigated for applications in ultraviolet light emitting diodes [1]. Since wurtzite-type semiconductors such as ZnO and GaN are polar semiconductors, ZnO-based junction properties have to be considered in the relation to the polarization. In our previous work, we found that the valence band spectra of single crystalline ZnO obtained by hard X-ray photoemission spectroscopy (HAXPES) depend on the polarity of the surface [2,3], although HAXPES technique is a bulk-sensitive probe [4]. To further investigate the electronic structures of ZnO with polar and non-polar surfaces, the detail analysis on the band symmetry in the valence band is needed. It is known that the atomic orbital cross-section depends on the polarization of photons. In this work we have performed polarization dependent HAXPES for various single crystalline ZnO surfaces to identify the band symmetry in the valence band electronic states. The HAXPES measurements at room temperature were performed at the undulator beamline BL15XU of SPring-8 [5,6]. Total energy resolution was set to 240 meV at the photon energy of 5.95 keV. A diamond phase retarder was used to switch the polarization of X-rays. In the hard X-ray region, the polarization of X-rays can be controlled by a diamond phase retarder. The polarization dependent valance band spectra of ZnO with polar and non-polar faces clearly showed that the Zn 3d and 4s states were spread over the whole valence band regions and the spectral shapes of the valence band strongly depend on the polarity of the ZnO surfaces. The detail analysis on the band symmetry in the valence band of ZnO with polar and non-polar surfaces will be presented.
References
[1] D. C. Look and B. Clafin, Phys. Stat. Sol. B241 (2004) 624.
[2] T. Osawa et al., Appl. Phys. Lett. 92 (2008) 232108.
[3] N. Ohashi et al., Appl. Phys. Lett. 94 (2009) 624.
[4] Y. Takata et al., Nucl. Instru. Methods Phys. Res., Sect. A547 (2005) 50.
[5] S. Ueda et al., AIP Conf. Proc. 1234 (2010) 403.
[6] S. Ueda, J. Electron Spectrosc. Rel. Phenom. 190 (2013) 235.

Two dimensional materials, graphene or silicene, have attracted considerable interest due to their high carrier mobility. The semi-metallic nature with zero-bandgap, resulting from the linear dispersion relation, is however considered as a barrier to overcome for optoelectronic applications. Meanwhile, two dimensional graphene-like ZnO, having a direct bandgap, was experimentally discovered from few monolayer thick ZnO (0001) grown on Ag (111), showing the depolarizing effect that compensates the spontaneous polarization of ZnO. An earlier density functional study reported a bandgap of 1.68 eV, implying the potential application for optoelectronic devices. However, understanding of structural effect (strain and film thickness) and point defects in graphene-like ZnO are still limited.
Through first-principles calculations within density functional theory, we investigate the effects of strain and film thickness on the atomic structure and electronic properties of graphene-like ZnO. The calculations were carried out using a screened hybrid functional implemented in VASP, providing us the accurate atomic structure and electronic properties of oxides. We attempt to identify stable graphene-like ZnO that relies on film thickness and strain perpendicular to [0001]. We find that the band structure of graphene-like ZnO strongly correlates with film thickness and strain. The electronic density of states has considerable correlation with the angle of O-Zn-O within ZnO sheet; in other words, graphene-like ZnO is found to have sp² hybridized orbitals rather than sp³ hybridized orbitals. Native defects (e.g., antisite defects, vacancies) and hydrogen in the oxide are also investigated. The results will be discussed in terms of the orbital overlapping of each defect or atom.

Solution-processed metal oxide semiconductors are advantageous for application in large-area electronics; the exploitation of materials is feasible and the low-temperature deposition on flexible substrates is possible. We previously reported that hydrogen peroxide (H₂O₂) improved the negative bias stress stability (NBS) of oxide thin-film transistors (TFTs) by reducing electron trap sites.¹ For a further study, here we describe how the formation of H₂O₂ species can be controlled for the alteration of metal oxide chains in indium oxide (In₂O₃) TFTs. Variations of device characteristics were attained through photolysis (UV) and ozonation (O₃) of H₂O₂ , herein called advanced oxidation process, as the consequence of different concentration of H₂O₂ species generated.² For generation of H₂O₂ species and optimization of its effect, the spin-coated H₂O₂ treated In₂O₃ films were irradiated with UV and O₃ before annealing. After the activation, deviations of threshold voltage (V_th), sub-threshold slope (S.S), on-current (I_on), and field-effect mobility (µ_FE) were observed that is distinguishable from non-activated film. By employing the advanced oxidation process, In₂O₃ TFTs exhibited the best performance with values as following: V_th = 12.65 V, S.S = 0.45 V/decade, I_on = 3.88 x 10⁷, µ_FE = 1.57 cm²/Vs. Compared to our previous work, this study enhanced the device characteristics of TFTs processed at a much lower temperature of 240^oC. This change is, firstly, accredited to a controlled production of OHmiddot; (oxidation potential, E^o = 2.31 V), which scavenges and attacks carbon chains to chemically breaks them down via redox process, eliminating organic chemical-induced defect sites and residuals in In₂O₃ film. Following that, secondly, different oxidation pathways oxidize In(OH)₃ to In₂O₃ and substitutes oxygen deficient sites for O₂, which originates from molecular ozone reaction (O₃ + H₂O₂ -> OHmiddot; + O₂ + OHmiddot;₂), resulting in increased metal oxide chains. Thus, these results represent a facile method towards development of inexpensive fabrication for large-areas.
[1] J. M. Kwon, J. Jung, Y. S. Rim, D. L. Kim, and H. J. Kim, ACS Appl. Mater. Interfaces, 3371-3377 (2014).
[2] C. W. Jones, Applications of Hydrogen Peroxide and Derivatives, The Royal Society of Chemistry, United Kingdom, p. 44 (1999).

Indium tin oxide (ITO) is a solid solution composed by Sn doped In₂O₃ (or vice-versa) and it is a very studied material due to its excellent optical and electrical properties which allows a wide range of applications, such as resistive films, opto-electronics devices, photothermal converters, among others. However, in order to achieve the best performance of ITO it is very important to control parameters such as chemical composition, morphology and dimensions. Microwave-assisted solvothermal (MAS) method has been one important route to prepare ITO nanoparticles allowing good control of size and morphology of resulting material. In this context, we proposed in this work a new way to manufacture transparent and conductive thin film based on ITO nanoparticles obtained by MAS method. In the first step, ITO synthesis was conducted by dissolution of 0.7 g of InCl₃ .4H₂O and 0.042 g of SnCl₄ .5H₂O in 25 ml of ethyleneglycol, in a molar ratio of Sn⁴⁺/In³⁺ around 5%. This solution was homogenized by constant stirring at 80 0C followed by dripping of 2.5 mol.L^-1 N(CH₃)₄OH.5H₂O base aiming the pH control of the solution at 6.5. The final solution was placed into a sealed Teflon autoclave which was taken into a microwave system using 2.45 GHz MW frequency with a maximum power of 800 W. A rate of 50 °C.min^-1 was imposed to the system until the solution temperature reached 200 °C, which was kept for 30 min. After the synthesis, the precipitated powder was collected and washed with ethanol several times to eliminate residuals, and then the material was dried at 80 °C. In the second step, a thin film was obtained by preparation of an ITO nanoparticles dispersion in a polyester matrix (with viscosity around 20 cP), followed by spin-coating deposition of the suspension on amorphous quartz substrate. The organic film was heat treated at 400 °C for one hour in pure oxygen atmosphere to remove any organic material. X-ray and electron diffraction results indicated that the powder and thin film present the cubic In₂O₃ phase and the tin doping was observed by EDS analyses. By using FEG-SEM and TEM analyses, we observed that ITO powders were composed by spherical particles with average diameter of 4 nm resulting in very dense thin films with thickness around 200 nm. Optical and electrical measurements showed that the films present a transmittance of up to 98,5% in the visible range and optic band gap around 3,55 eV, which is very close to the ones reported on the literature. The films presents a resistivity of 3,6.10^-3 #8486;.m confirming the formation of the ITO phase, since the resistivity of In₂O₃ phase is much higher than that observed in our films. These results showed that this two-step route is very efficient in a preparation of dense ITO films with high transparency and conductivity indicating that these films can be applied as solar cell electrode or transparent resistive heater.

Indium tin oxide (ITO) is one of the well known n-type degenerate semiconductors possessing high electric conductivity and optical transparence at visible light spectrum. The imaginary part of the dielectric function of ITO is about a factor of 3 or 4 smaller than the silver which is widely used in visible light plasmonics. Inherent great loss in metallic materials motivates people to search for alternative low loss plasmonics materials while ITO was found favorable with relative low loss [1]. Thin films and nanorod arrays of ITO have been used in infrared plasmonics for further extend the range of plasmonics applications in communication and sensor [2]. Based on photovoltaic effect induced electrostatic modification at ITO/LiNbO₃ interface, charge density in ITO at interface vicinity increases to a higher level leading to effective plasma resonance frequency up to visible regime. With the help of photorefractive phase grating, surface plasmon polaritons can be stimulated which can be demonstrated by several novel phenomena. The loss reduction is highly desirable for broad plasmonics applications.
150 nm ITO films were deposited onto the two optically polished surfaces of 0.1wt% Fe₂O₃ doped c-cut thin LiNbO₃ slabs. The two surfaces were intentionally polished with a small wedge angle about 0.400. Upon exposed to a p-polarized continuous incident light at 561 nm (1.2 mm in diameter, 63.0 mW in power) along +c axis, strong photovoltaic electric field as high as 10⁴ V/cm in Fe: LN was built, which drove charge carrier to ITO/LN interface, thus modulated the charge carrier density of ITO adjacent thin layer from 3.55×10¹⁹ cm^-3 to 2.12×10²⁰ cm^-3 by a rough estimation, shifting the ITO plasma resonance frequency 1.58 µm in infrared regime to visible light regime at 648 nm [3]. In addition, multiple light beams were overlapped within the slabs and photorefractive phase gratings with grating spacing Λ=17.5 µm were written in samples after exposed to laser beam for 10 minute, corresponding to the internal beam crossing angle 0.410. In assistance with phase gratings diffracted incident light into various high orders, quasi-wavevector m(2π/Λ) was added to the x-component of incident light k_xin, SPPs would be excited when wavevectors were matched. Multiple reflection and transmission light patterns, as high as 89% total reflected power and lower transmission spectra supported this explanation. Reconfigurable waveguide was formed on the slab surface while considering α=11.24 cm^-1 in LiNbO₃, indicating the low loss SPPs propagation.
The plasmonics application of electrostatic modification induced by PV effect at interface has a bright future and will be further discussed.
[1] P. R. West, S. Ishii, G. V. Naik, et al. Laser Photon. Rev. 4, 795 (2010).
[2] S Q Li, P J Guo, L X Zhang, et al. ACS Nano, 5, 9161 (2011).
[3] C. H. Ahn, A. Bhattacharya, M. Di Ventra, et al. Rev. Mod. Phys. 78, 1185 (2006).

The optical band gap is of fundamental importance for selecting semiconductors for optoelectronic or solar energy conversion applications. While analysis of optical absorption data in the form of a Tauc plot is widely used to determine the band gap of conventional semiconductors, whether this approach is accurate or appropriate for complex oxides remains an open question. Here, we use a combination of variable angle spectroscopic ellipsometry (VASE) and density functional theory (DFT) to investigate the nature of optical transitions in LaFeO₃. Epitaxial LFO films were deposited on a variety of substrates using molecular beam epitaxy. The optical absorption spectrum of LFO was measured with VASE and computed at the single-particle wave function level. The absorption spectrum obtained from DFT with the Perdew-Burke-Ernzerhof exchange-correlation functional, in which the band gap and band dispersions are well-defined, was fit with various Tauc models to assess the most accurate fitting protocol. We find that the Tauc model for a direct, forbidden transition provides the optimal fit for determining the band gap, consistent with the near energetic degeneracy of the calculated direct and indirect transitions owing to the weakly dispersive conduction bands and d-d nature of the transition. This model is then applied to the experimental data for extraction of accurate transition energy values in thin film LFO. Using the direct-forbidden Tauc model, we obtain a band gap energy of ~2.4 eV. The implications of this work extend beyond LFO in pointing to a general approach for analyzing optical absorption spectra from various oxide films.
MS and SM were supported by the National Science Foundation (DMR-1151649); AC and JR were supported by the Army Research Office (W911NF-12-1-0133). MS acknowledges support from the Department of Education (GAANN-RETAIN, Award No. P200A100117).

Nb₂O₅ is a n-type semiconductor with a ~3.5 eV band gap semiconductors, has a wide range of applications due to their excellent electrical and optical properties. Various Nb₂O₅ morphologies such as nanowires, nanorods, and nanoporous have been widely investigated through different routes including hydrothermal, anodization and thermal oxidation method. Nb₂O₅ nanostructures have been studied in diverse applications such as in solar cells, electrochromic smart windows, catalyst, batteries and gas sensor.
Nano-vein like porous Nb₂O₅ was synthesized via an anodization process that was conducted at 50 °C in the electrolyte of NH₄F/ethylene glycol, containing a small amount of water. Films were then annealed at 440 °C for 30 min at the ramp up/down of 2 °C/min for crystallization. 30 minutes of anodization result in ~1 mm thick of Nb₂O₅ film made of fairly organized pores with inner diameters ranging from 30 to 50 nm. In this work, the Nb₂O₅ Schottky diode based sensor, integrated with Pt catalytic metal, was developed and investigated. The performances of the Schottky diode based sensors under few types of gases/vapours such as hydrogen, methane and ethanol are fully studied and discussed.

Aerogels are open-celled highly porous materials with remarkably low densities and high surface areas. These properties make aerogels very appealing for a set of applications such as dielectrics for supercapacitors. Among all materials that aerogels can be made of, tin oxide (SnO₂) is particularly interesting for being a direct bandgap (3.6 eV) semiconductor. This can lead, for instance, to the development of extremely light and transparent optoelectronic devices. In this work we correlate a set of theoretical investigations on their networks to some structural, optical and chemical experimental characterizations.
On the theoretical side we show that depending on the concentration of precursors and the preparation temperature it is possible to obtain gels whose interconnecting networks of nanocrystals are either Erdös-Rényi or Zipf scale-free. This leads to different levels of localization varying from one limited by fractons up to a general Anderson localization.
Experimentally, monolithic tin oxide aerogels are often prepared by an epoxide assisted sol-gel route. We show that, although it is a versatile technique, this route unintentionally produces an electrically semi-insulating and opaque material. X-ray diffraction (XRD), diffusive reflectance spectroscopy (DRS), particle induced X-ray emission (PIXE), transmission electron microscopy (TEM) and photoluminescence (PL) were used to deeply investigate the origin of this behavior.
TEM shows a set of crystalline regions embedded on an amorphous matrix. XRD measurements revealed that the crystalline regions are composed of ~2.5 nm wide Cassiterite crystallites. DRS measurements indicate a pseudo-bandgap of ~4.6 eV caused by a Burstein-Moss shift. We found from PIXE that Cu, Zn and Fe ions are naturally occurring impurities in the gel matrix substituting Sn⁺⁴ ions. Finally, we found from PL measurements that the impurity ions form positively charged complexes with oxygen vacancies causing high electron trapping.
Since unreacted species strongly limit the electrical conduction, novel chemical routes related to the epoxide strategy have recently been studied by our group in order to produce purer SnO₂ monolithic aerogels. Preliminary results based on a peroxide technique will be presented showing a significant reduction in the density of positively charged complexes.

The typical transparent conductive oxide Sn-doped In₂O₃ (ITO) is one of the most widely used as thin-film electrodes in photovoltaic (PV) cells because of its low resistivity (~10^minus;4Omega;middot;cm) and high optical transparency (ge;80% over the visible to near infrared wavelengths). However, ITO films are unfavorable for use as transparent conductive thin-film electrodes in pervoskite-based PV cells because of a distinct mismatch of ITO films with pervoskite-structured materials, such as ~60% lattice mismatch with BiFeO₃. Thus, an interface formed between the pervoskite active layer and the ITO top electrode hinders the effective transport of the photogenerated electrons and holes, resulting in a low solar-to-electric conversion efficiency. Nowadays, with the emergence field of ferroelectric perovskite based photovoltaic devices, doped Strontium titanate (SrTiO₃; STO) based TCO becomes an extremely attractive as a transparent conductive layer in such devices owing to its chemically stable structure, smooth surface and low lattice mismatch with most ferroelectric perovskite oxides. Integration of crystalline doped STO based TCO in ferroelectric photovoltaic heterostructures will open an alternative way to achieve highly efficient devices in various technological applications including photovoltaics. Fully understand of the electrical conduction and optical transparency behavior in doped STO films is required for designing high-efficiency perovskite-based PV cells. Here I will review the recent progress of our group in the exploration of TCO based on p-type In-doped SrTiO₃ films (SrIn_xTi_1minus;xO₃). We will present, the controlled growth and characterization of thin films via pulsed laser deposition and molecular beam epitaxy. The optimization of optical and electrical properties of such system will be also discussed.

Oxide semiconductor thin-film transistors (TFTs) have attracted much attention owing to their high field-effect mobilities in comparison to the current industrial standard, hydrogenated amorphous silicon (a-Si:H) TFTs. The development of n-type oxide-semiconductor TFTs is more mature; high-performance TFTs made of indium gallium zinc oxide (IGZO) and zinc tin oxide (ZTO) have been demonstrated. However, the development of p-type oxide TFT is much lagged, especially for those fabricated at low processing temperature. P-type oxide TFTs with satisfactory performance are desired because they are essential for the development of oxide CMOS technology.
In this study, SnO thin films were first characterized, and SnO thin-film transistors (TFTs) were then demonstrated. The SnO thin films were sputter-deposited at room temperature, followed by the thermal annealing at 245°C in air using infrared rapid thermal furnace. The X-ray diffraction pattern reveals the amorphous nature of the as-deposited film. After 2.5-min thermal annealing, SnO crystalized and the grain size increased to 23 nm, whereas the Tauc bandgap increased from 1.8 to 2.73 eV. Hall measurement indicates a resistivity of 15.7 Omega;-cm, hall mobility of 0.53 cm²V^-1s^-1, and hole concentration of 7.45×10¹⁷ cm^-3. Next, we investigated the performance of inverted-staggered bottom-gate SnO TFTs. The best achieved TFT shows a linear mobility of ~1 cm²V^-1s^-1, threshold voltage of 2.75 V, subthreshold swing of 1.37 V dec^-1, and on/off ratio of >10⁴. The output characteristics of the TFT show clear linear and saturation regions without current crowding at low source-drain voltage. Gate-bias stability was tested under gate-bias voltage of +10 V with source and drain grounded. The threshold voltage shift is 0.81 V after 10000 s stressing. The threshold voltage shift can be fitted using a stretch-exponential equation, suggesting that the dominant mechanism for the instability is charge trapping at the interface or in the gate dielectric. The TFT characteristics with SnO thin films under corresponding rapid thermal annealing conditions will also be presented in the conference.

The two widely known tin oxide phases SnO₂ and SnO are easily distinguished by examining their Raman spectra. Such Raman spectra contain information about the crystal structure and orientation as well as its quality, impurities, etc. Ion beam sputtered samples of SnO₂ and SnO with well-defined orientations are investigated using rotational Raman spectroscopy, a technique where the sample is rotated in plane with respect to the incident laser polarization. The intensity of the scattered light is analyzed for different polarizations with respect to the incident light. The data obtained is used to confirm the assignments of Raman modes appearing in the spectra of SnO₂ and SnO and to determine the values of the corresponding Raman tensor elements. Samples grown in the regime between the formation of these two phases yield completely different Raman spectra implies the formation of a third tin oxide phase in this intermediate regime. We present the third phase a first identification of the Raman modes of this additional Sn_xO_y phase. Furthermore, naturally grown crystals are investigated and compared to the samples grown by ion beam sputtering and chemical vapor deposition.
In addition to angle dependent measurements, pressure dependent measurements were performed using a diamond anvil cell. The results of these measurements are presented up to more than 10 GPa for the three examined phases. This is the first report of pressure dependence of the third phase&’s Raman spectra and an extension of the previous work conducted on SnO₂ and SnO under pressure.

Rare earth compounds have been extensively studied due to their outstanding properties which are employed in various fields, including phosphors, laser, catalysis and other functional materials based on their electronic, optical, and chemical characteristics. Silver Tungstate has many applications including a novel ozone gas sensor, antibacterial agent, photoluminescence and photocatalytic properties. Recently, a novel process of Ag growth on α-Ag₂WO₄ surface when it is bombarded with an electron beam has been discovered. However, there are no records on the luminescence of rare earth ions doping Silver Tungstate. Emission bands of europium ions are easy to study in the visible region due to the structure of the energy levels, such as fine lines of absorption and emission. It occurs because of electron transitions within the partially filled 4f layer which is protected from the environment by the 5s and 5p external electrons, and enables their use as a spectroscopic probe for site symmetry determination. This dopand can enhance the efficiency and purity of this material color or even produce new LEDs phosphors besides structural modification due to vacancies in Ag⁺ sites. Silver Tungstate (Ag₂WO₄) and Europium doped Silver Tungstate (Ag₂WO₄:Eu³⁺) powders were synthesized by co-precipitation method at 80 °C for 15 minutes using Sodium tungstate dehydrate (Na₂WO₄.2H₂O), Silver nitrate (AgNO₃) and Europium Oxide (Eu₂O₃) as precursors. Trivalent rare earth ions presenting in different molar ratio concentrations were introduced into the Ag₂WO₄ lattice aiming to study their structural and photoluminescence properties. This methodology was used because it is simple, cheap, does not need high production temperatures, is industrially favorable, as well as yields materials with homogeneous shape and size without deleterious phases. X-ray diffraction presented a single phase indexed as orthorhombic structure with space group Pn2n (PDF#34-0061), showing crystallinity at long-range. The optical properties of these microcrystals were investigated by ultravioletminus;visible (UVminus;vis) spectroscopy and photoluminescence (PL) measurements. The emission spectra excited at 350.7 nm (krypton-ion laser) showed the characteristic ⁵D₀→⁷F_J (J=0, 1, 2, 3 and 4) transitions of Eu³⁺ resulting in a red luminescence emission corresponding to the stronger ⁵D₀→⁷F₂ at around 615 nm, and a broad band in the blue region (~450 nm) related to the structural defects and electronic structure of the Ag₂WO₄. Micro-Raman were used to analyze the degree of structural orderminus;disorder at short-range and revealed the presence of at least 5 high intensity Raman-active vibrational modes. The FE-SEM micrograph showed hexagonal rod-like elongated α-Ag₂WO₄ microcrystals with agglomerate nature and average height of 1 mu;m. These results show novel and interesting properties for these new materials.

There is great demand for the development of the energy-saving technology in the worldwide. Many intensively researches for power electronic devices in the high voltage switching applications based on wide-bandgap semiconductors such as SiC and GaN have been studied to deliver on the properties of the higher breakdown voltage and lower loss than those of Si devices.
From the viewpoint of the properties of bandgap, an oxide semiconductor, β-gallium oxide (β-Ga₂O₃) is a strong contender for the power electronic device materials. And we achieved the large single crystal synthesis of β-Ga₂O₃ by the melt-growth methods and the first demonstration of Ga₂O₃ transistors.
It is important to clarify the carrier density of materials for the crystal growth process. We measured THz transparent spectra of doped β-Ga₂O₃ samples using with THz time-domain spectroscopy which has an advantage that the spectra can be measured without electrical contacts process and estimated the carrier density from the plasma frequency. This is the first report on THz transmittance measurement of various doped β-Ga₂O₃ crystals to the authors' best knowledge.
Heavily-Si-doped β-Ga₂O₃ sample did not transmit THz wave and well-compensated, semi-insulating sample transmitted THz wave. Comparing with the results obtained by Hall-effect measurements, our THz wave transmission results are explained by the reflection caused by plasmon and by its cut-off frequency which depends on the carrier density.

Cupric oxide (CuO), an intrinsic p-type semiconductor, possesses a narrow bandgap (1.2 eV) that enables strong light absorption across the visible portion of the solar spectrum, suggesting its great potential for solar energy conversion. A growing number of reports have demonstrated interest in utilizing CuO in photovoltaic devices. However, few efforts have been made to grow high quality CuO films on transparent, conductive substrates (e.g. tin-doped indium oxide (ITO)-coated glass) controllably and efficiently. In addition to photovoltaic devices, CuO has also been used as a pseudocapacitor electrode material; in this application, CuO possessing a low degree of crystallinity is preferred. It remains a substantial challenge to prepare high quality, adhesive, and morphology-/crystallinity-controllable CuO thin films on ITO in an efficient manner.
In this work, we report the preparation of high quality CuO thin films on ITO/glass substrates using a novel seed-layer assisted chemical bath deposition (SCBD) technique, which is a safe and environmentally friendly process involving an aqueous solution at moderate temperatures (~100 °C). A CuO seed-layer is first formed by the electrodeposition of Cu₂O on ITO/glass substrates for 10 sec, followed by thermal annealing in air (15 min) to convert Cu₂O to CuO. The seed layer is found to be essential to promote the growth of adhesive, high quality CuO films on ITO/glass substrates, since no substantial CuO films form without the seed layer present. We have found that the morphology and relative crystallinity of the CuO films can also be tuned by adding sodium lactate to the SCBD solution. CuO films deposited from a solution without sodium lactate are highly crystalline, exhibiting a resistivity of 3.3 × 10⁵ Omega;middot;cm. CuO films grown from a solution with a lactate:copper molar ratio equal to 1.0 are largely amorphous, and possess a larger resistivity (7.2 × 10⁵ Omega;middot;cm). While CuO films grown via SCBD from a solution without lactate show higher crystallinity and electrical conductivity than the CuO films deposited from lactate-containing solution, the latter CuO films demonstrate much greater performance as a pseudocapacitor electrode material because of their more amorphous character.

Ga₂O₃ is a wide band gap III-oxide semiconductor which is attractive for applications in optoelectronic and electronic devices. Ga₂O₃ is known to have five polymorphs. The stable β-phase of Ga₂O₃ has cubic structure whereas the metastable α-phase has corundum-like structure.
Single crystalline α-Ga₂O₃ samples have been grown on c-oriented sapphire substrates by means of mist Chemical Vapor Deposition (mist CVD). Ga acetylacetonate was used as source material. The acetylacetonate was solved in deionized water with a small amount of hydrochloric acid. The solution was atomized using an ultrasonic transducer at 2.4 MHz and the formed aerosols were transferred to a quartz tube using a carrier gas. The samples were grown at 460 0C on the substrate holder in the quartz tube.
The samples were characterized by means of XRD and EDAX. The XRD 2theta;-theta; scan profiles show, in addition to the diffraction peak corresponding to the sapphire of the substrate, a single peak corresponding to the (0006) reflection of a α-Ga₂O₃ crystal with corundum structure, which indicates that single crystalline α-Ga₂O₃ films without inclusion of any other crystal structures were successfully grown on sapphire substrate by using mist CVD.
The space group of the α-Ga₂O₃ crystals is D_3d⁶, with two formula units per unit cell. The irreducible representations of the optical modes in the crystal are 2A_1g + 2A_1u + 3A_2g + 2A_2u + 5E_g + 4E_u. Among these optical modes, seven are Raman active modes (2A_1g + 5E_g) whereas six are infrared active modes (2A_2u + 4E_u).
The c-oriented α-Ga₂O₃ crystals, with a thickness of about 300 nm, were analyzed by means of Raman spectroscopy by using a T64000 Jobin-Yvon spectrometer. The Raman measurements were performed by exciting with the 514.5 nm laser line and using a 100x objective in backscattering configuration. Since the α-Ga₂O₃ crystals are transparent to the 514.5 nm laser line, the Raman spectra obtained also contain the modes corresponding to the sapphire of the substrate. To clearly distinguish the Raman peaks corresponding to the α-Ga₂O₃ crystals from those corresponding to the substrate, we have also measured the Raman spectrum of a sapphire crystal.
Thus, we have been able to detect most of the Raman active modes of the α-Ga₂O₃ crystals, which span over a 200 - 800 cm^-1 frequency range. The width of the Raman peaks obtained proves the good quality of the grown samples. A discussion about the various peaks of the Raman spectra is carried out to unambiguously identify their nature and the contribution of the respective atomic motions to the Raman scattering of the compound. Since the sapphire substrate presents the same crystalline structure (corundum structure) as α-Ga₂O₃, a comparison between the respective modes of both compounds allows one to discuss the variations that take place in the lattice dynamics of these compounds when a Ga atom replaces an Al atom in the crystal structure.

Wurtzite β-CuGaO₂ (energy band gap, Eg = 1.47 eV) and β-AgGaO₂ (Eg = 2.2 eV) and their alloys with ZnO are the promising semiconductors that are applicable to optoelectronic devices working in visible-NIR wavelength region [1,2]. The knowledge of electronic structures of the materials enables us to understand their properties. In present study, therefore, we calculated the electronic structures of β-CuGaO₂ and β-AgGaO₂ by first principle calculations.
The geometry optimizations of β-CuGaO₂ and β-AgGaO₂ crystals and the calculations of their band structures were conducted using density functional theory (DFT) method. For comparison, we also calculated them for the delafossite α-CuGaO₂ and α-AgGaO₂. LDA+U was used as functional. A 5×4×5 Monkhorst-Pack k-point sampling and the norm-conserving pseudopotential were used. The cut-off energy for the plane-wave basis set, Ecut, was 840 eV. In order to evaluate the appropriateness of the calculations, we compared the calculated valence band DOS with their XPS spectra. The XPS measurements were performed using monochromated Ag Lα X-rays (2984.2 eV).
In the energy band structure of β-CuGaO₂, both valence band#12288;maximum (VBM) and conduction band minimum (CBM) were at the Γ point. This indicates that β-CuGaO₂ is a direct. The VBM was dominantly consisted of Cu 3d electrons and had very large DOS. This implies that the absorption coefficient of β-CuGaO₂ near the absorption edge is very high similar to CuInSe₂ [3]. Taking 1.47 eV of the band gap and p-type electronic conduction into account, β-CuGaO₂ is an appropriate material for the absorber of thin film solar cell. In the case of β-AgGaO₂, the calculation indicated that it is an indirect semiconductor as already reported in previous paper [4].
We evaluated the effective masses of the hole and electron, m_h*/m₀ and m_e*/m₀, for wurtzite-type β-CuGaO₂ and β-AgGaO₂ and delafossite-type α-CuGaO₂ and α-AgGaO₂. The m_h*/m₀ of the wurtzite phases were several times larger than those of delafossite phases while m_e*/m₀ of both phases were comparable.
[1] T. Omata, H. Nagatani, I. Suzuki, M. Kita, H. Yanagi, N. Ohashi, J. Am. Chem. Soc. 136, 3378(2014).
[2] I. Suzuki, H. Nagatani, Y. Arima, M. Kita, T. Omata, Appl. Phys. Lett., 103, 222107(2013).
[3] T. Maeda, T. Takeichi, T. Wada, phys. status solidi (a), 203, 2634 (2006).
[4] Y. Maruyama, H. Irie, K. Hashimoto, J. Phys. Chem. B, 110, 23274(2006).

Band gap engineering of ZnO by alloying with β-NaFeO₂ type complex oxides that is one of the wurtzite-derived structures has been recently demonstrated. In the LiGaO₂-ZnO system, we found out new quaternary wurtzite-derived Zn₂LiGaO₄ with a band gap of 4.0 eV at room temperature. An incommensurately modulated ordering was pointed out in this compound. In the present study, high temperature state of Zn₂LiGaO₄-ZnO alloys were studied by using an in-situ high temperature X-ray diffraction (XRD) as well as room temperature XRD up to 1673 K. The XRD results were discussed with the selected area electron diffraction (SEAD) of the samples that were quenched in liquid nitrogen after annealing.
Zn₂LiGaO₄ exhibited three phases, low temperature form (phase I), intermediate temperature form (phase II) and high temperature form (phase III). These three phases possess wurtzite-derived structure and they are hardly distorted as compared to ideal wurtzite structure in entire temperature range. However, the incommensurate modulations of phase I and II were different from each other and it was determined to be 0.26(a*+b*) and 0.27(a*+b*), respectively from SAED. On the other hand, only diffused superlattice diffraction was observed in XRD and SEAD of phase III. This indicates that degree of cation ordering in phase III could be lower than that in phase I and II. A short range cation ordering remained in high temperature form.
A phase diagram of x(Zn₂LiGaO₄)_1/4(1-x)ZnO alloy system in the Zn₂LiGaO₄ rich side was proposed based on the XRD results. For 0.96le;xle;1, phase I and II appeared from room temperature to 1070 K and from 773 K to 1373 K respectively. Phase III were observed for 0.96le;xle;1 above 1373 K and for 0.10le;xle;0.96 above room temperature. It was found that the incommensurately modulated phase I and II were stable in the limited composition and temperature region.

The metal insulator transition in VO₂ has been studied extensively. In the linear chain of Vanadium atoms a Peierl&’s instability causes the formation of V dimers which leads to the metal insulator transition. This transition is invariably associated with a monoclinic VO₂ (M) to rutile VO₂ (R) phase transition. The VO₂ (A) phase by itself is insulating while the VO₂ (B) phase shows a broad metal insulator transition at 150 K. In this paper we show that a composite film of VO₂ (A) and VO₂ (B) phases also exhibits a metal insulator transition similar to the VO₂ (M/R) phase transition. However, extensive TEM and temperature dependent XRD studies reveal that the film is mainly comprised of VO₂ (A) and VO₂ (B) phases and very little of M phase. The A phase is under compressive stress while the B phase is under tensile stress and we believe this stress leads to the dimer induced metal insulator transition in this system presumably triggered by the small amount of M phase present. This raises the question “Is a structural phase transition necessary for the MIT in VO₂?”

Metal-to-insulator transition (MIT) behavior in Anderson-Mott insulators maintained long standing interest ranging over several decades in both the physics and materials science communities. In particular, strongly correlated oxides, cuprates and nicklates are of major interest due to their convenient transition temperature above room temperatures. Among correlated oxides, transition temperature of vanadium dioxide (VO₂) is close to room temperature (68°C) in bulk crystals. Over the last decade metal-to-insulator transition in VO₂ has been demonstrated to be triggered by electrical, optical, magnetic, strain and thermal energy.^[1] The MIT behavior in VO₂ is attributed to insulating monoclinic phase to metallic tetragonal rutile phase transition.
Ultrafast nature of the phase transition in VO₂ makes it attractive for applications in electronics and optical devices, ^{[2, 3]} however utilization of corresponding drastic change in thermo-physical properties are rarely reported.^[4] In this study, we investigate thermal and electronic properties of VO₂ thin films on various substrates across the transition temperature to seek possibility of utilizing VO₂ based thermal switches for applications in thermal devices. All films are deposited by radio frequency (RF) sputtering on various substrates with varying lattice mismatch. The sputtering is performed in Ar environment from a V₂O₅ target at 550-600°C. Thermal properties are measured by time domain thermo-reflectance (TDTR) method. Electrical conductivity and carrier concentration of the films are measured by a temperature varying four-probe and Hall-Effect measurements. We will discuss interplay of phononic and electronic component to thermal conductivity in the light of Wiedemann-Franz law across MIT of VO₂ films deposited under various conditions. This work sheds light on practicality of thermal switches, which is a key step towards realization of advanced thermal devices such as thermal logic gates and thermal memory.
References
[1] Z. Yang, et al., Annu. Rev. Mater. Res. 41, 337 (2011)
[2] D. Ruzmetov, et al., J. Appl. Phys. 107, 114516 (2010)
[3] T. Driscoll, et al., Appl. Phys. Lett. 93, 024101(2008)
[4] D-W. Oh, et al., Appl. Phys. Lett. 96, 151906 (2010)

Gravure is an attractive patterning technique for printing high-resolution features (< 5µm) at high throughput (speeds ge; 1m/s). Here, for the first time, we extend gravure printing to a new class of inorganic materials by developing sol-gel inks and printing methods for forming transparent metal oxide electrodes on flexible Willow glass substrates. This work presents the development of gravure-printed transparent conductors from Sb-doped tin-oxide (ATO) sol-gel precursors; using this precursor, we are able to realize printed lines with excellent transparency and conductivity on par or better than comparable reports.
Viscous sol-gel precursors were prepared from SnCl₂2H₂O and SbCl₃ in a binary solvent system (ethanol/ethylene glycol), with viscosities ranging from 4 cP to 20 cP. ATO Mass loading from 10-40% was achieved using ethylene glycol as a stabilizer. Printed-lines of width 30 mu;m-40 mu;m were patterned by an engraved gravure roller with 20 mu;m nominal cell-size. Drying at 200^oC (10 mins) followed by a 15 minute anneal at 500^oC in compressed dry-air yields conductive lines with good conductivity and transparency. The resistivity (7X10^-3 ~ 1X10^-2 ohm cm) of the printed ATO compares favorably to reports of spin-coated SnO₂:Sb sol-gel conductors. Optical transmission through the ATO/glass substrate exceeds 90% in the visible range. The resulting lines also show good mechanical stability; the sheet resistance of ATO lines is stable at bending strains at least up to the mechanical limits of the 100 mu;m thick glass substrate (up to 0.15% tensile and compressive strain). The bending endurance of gravure-printed ATO is examined, showing negligible degradation in sheet resistances up to 10⁴ bending cycles.
To our knowledge, this is the first successful report of high-throughput patterning of transparent sol-gel conductors at this resolution. The mechanical stability of printed ATO lines during moderate bending strain supports the viability of high-throughput roll-to-roll processing for gravure-printed sol-gel conductors. The conductivity of the gravure-printed transparent lines may make them suitable electrodes for flexible and transparent devices such as touch screens and fully transparent thin-film transistors.

We investigate the vibrational properties of the three copper oxide phases Cu₂O, Cu₄O₃, and CuO employing first principles density functional theory calculations using the VASP code. Phonon bandstructure and density of states for all three phases are derived from a supercell small displacement method. The splitting of the LO and TO modes at the Γ point is obtained by properly taking into account the non-analytical contributions to the dynamical matrix in the limit q → 0. We examine Raman properties by calculating Raman susceptibilities and derived Raman spectra and compare the results to our recent experiments [1]. As the Raman spectrum of Cu₂O is dominated by Cu defect-activated IR-active and silent modes, special attention is paid to the particular influence copper split vacancies (V_Cu^split) and copper vacancy (V_Cu) on the Raman spectrum. Further, IR absorbance is also investigated by calculating the low frequency dielectric tensors.
[1] T. Sander, C. T. Reindl, M. Giar, B. Eifert, M. Heinemann, C. Heiliger, and P. J. Klar, Phys. Rev. B, accepted (2014)

Electron paramagnetic resonance (EPR) is used to identify a new and unique silicon-related point defect in nominally undoped single crystals of rutile TiO₂. Commercially available oxidized c plates of TiO₂ were obtained from Crystec in Germany. At 20 K, exposure to 442 nm laser light produces the new S = 1/2 center and also produces neutral and singly ionized oxygen vacancies. The principal g values of the new center are 1.9159, 1.9377, and 1.9668 and the corresponding principal axes are along the [-110], [001], and [110] directions. Hyperfine lines associated with the new EPR spectrum show that the unpaired spin interacts equally with two Ti nuclei and unequally with two silicon nuclei. The two Ti ions are at regular lattice sites while one of the silicon ions is substituting for a Ti ion and the other silicon occupies an adjacent interstitial position. The silicon is present in the TiO₂ crystals as an unintentional impurity. Principal values for the larger of the two silicon hyperfine interactions are 91.4, 95.4, and 316.4 MHz and the principal axes are also along the [-110], [001], and [110] directions, respectively. We propose that an interstitial silicon atom (Si⁰) is adjacent to a silicon ion (Si⁴⁺) substituting for a Ti⁴⁺ ion in the as- grown crystals and initially forms a neutral nonparamagnetic complex [Si_int-Si_Ti]⁰. During the laser illumination at 20 K, a hole is trapped by this complex and a [Si_int-Si_Ti]⁺ defect is formed with S = 1/2. Our silicon-related EPR signal disappears in two steps during a thermal anneal. The first step is between 22 and 30 K and coincides with the release of an electron from neutral oxygen vacancies. The second step is between 32 and 40 K and coincides with the release of an electron from singly ionized oxygen vacancies. The electrons released from the oxygen vacancies recombine with the holes at the silicon complexes.

Recently, comparable transparency and sheet resistance values were obtained from silver nanowire networks to that of commercial standard indium tin oxide (ITO) thin films [1]. Therefore, this novel transparent conductor is promising for various optoelectronic device applications, such as solar cells, organic light emitting diodes (OLEDs) and photodetectors. However, problems such as high roughness and oxygen instability remain yet to be resolved. So far, some n- type metal oxides were investigated to both decrease the surface roughness and increase stability of silver nanowire networks. In this study, p-type molybdenum oxide (MoO₃) thin film is deposited onto silver nanowire networks and optoelectronic properties of formed nanocomposite electrodes were investigated. All solution based routes for the synthesis of silver nanowires [2] and fabrication of the nanocomposite electrode capable of large area deposition was used. Depending on the oxide thickness, surface roughness of the nanocomposite and stability of silver nanowires were monitored. Lastly, an OLED was fabricated where the nanocomposite served both as an anode and hole injection layer, eliminating the need for a separate hole injection layer.
[1] S. Coskun et al. Nanotechnology 24 (2013) 125202.
[2] S. Coskun et al. Cryst. Growth Des. 11 (2011) 4963.

ZnO and related materials have excellent optical and electronic properties, and recent advancements in epitaxial growth technique have spurred the investigation of these materials for applications such as ultraviolet light-emitting diodes and invisible transistors. Although reliable technique for p-type doping into ZnO is not yet completed, the crystalline quality and materials purity is now approaching ultimate limitsminus; the fractional quantum Hall effect has been observed in ultraclean ZnO/MgZnO interfaces. The majority of the applications presently envisaged can essentially be realized if single-crystalline layers glow on low-cost glass and polymer substrates in a fashion of graphoepitaxy. The notion of graphoepitaxy has been applied for many classes of materials, including semiconductors, metals, molecular compounds, and alkali halides. A rational mechanism of crystal orientations is induced by the fact that nucleation occurs under geometric constraint in a grating lithographically fabricated on the glass surface. Here we study this for the ionic oxide semiconductor, hexagonal ZnO on a quartz substrate with a square-wave relief grating structure. The epitaxial structure was obtained by two-step growth; pulsed-laser deposition (PLD) of a c-axis oriented, homogeneous nucleation layer, followed by mist chemical vapor deposition (Mist CVD) of an in-plane oriented, a few microns domains (a few microns). Surprisingly, rotational symmetry of the domains was found to be twisted by 900 from right angles commonly found in graphoepitaxy (i.e., crystal facet m-planes parallel to the side walls). We investigated surface morphology for the growth-interrupted films to identify that the m-plane facets indeed appeared perpendicular to side walls. The details of novel selection rule of the domain formation and the electronic properties of the obtained films will be presented.

Anti-Stokes process is expected for an energy up-conversion process of solar cells for example. The processes of ZnO and GaN are thought to occur via intermediate trap states but the details are still obscure. Recently, a very weak and very long lifetime photoluminescence was found in ZnO. This trap is probably related to the anti-Stokes process because of its luminescent process. The detailed properties of the anti-Stokes photoluminescence and intermediate trap state are discussed here.
The near the band edge (NBE) spectra of the Stokes (excited by 325.0 nm) and anti-Stokes (excited by 441.6 nm) photoluminescence using continuous wave (cw) excitation were almost the same except for its self-adsorption near the band gap. The deep luminescence had the same 2.4 eV peak (GL) for both Stokes and anti-Stokes process under cw excitation. The luminescence with 2.25 eV peak (YL) was, however, observed after the excited light turned-off only with the excitation of smaller energy than that of the band gap (441.6 nm). The photoluminescence intensity of NBE (cw), GL (cw), and YL (0.06 ms after excited light turn-off) dependent on the excited light intensity of 441.6 nm were observed to be proportional to the n-th power of the excitation intensity and the n was not 1. This shows the luminescence had some effects like carrier loss or two-step process. The n for NBE, GL and YL were 1.41, 1.12, and 0.78. The order n for NBE is almost twice of that for YL, and the n for GL is in-between. This indicates the YL is related to the anti-Stokes process and the NBE is two-step process using the trap related to YL as the intermediate state. The excited states of the deep trap was estimated to be two pathways, that is, one is the direct recombination of GL and the other is falling much deeper trap with lattice relaxation related to YL.
The YL had the characteristics of donor acceptor (D-A) pair luminescence from the peak energy dependency of excitation light intensity. The luminescence was a highly compensated D-A pair luminescence with tunneling effect summarized from the peak energy dependence on the exciting energy and the intensity decay after the excitation turned-off. Considering with the condition of electric dipole transition like light emission and adsorption, the parity of initial state and final state has to change when the dipole transition is allowed. The trap state related to YL requires having s- and p-orbital-like characteristics in order to allow the transition from/to valence and conduction bands. The trap related to the YL is considered to be an exciton-like D-A pair with p-orbital-like envelope function distorted from the crystallographic strain field due to the highly compensated donor and acceptor considering with the experimental results.

The main drawback for the development of successful ZnO optoelectronics is the lack of reliable p-type doping. This has remained a controversial topic that has been tackled from many viewpoints. Various elements have been proposed, being the most promising one N, although its electrical behavior and incorporation are still unknown. Recent studies have suggested that the formation of complex levels is a promising path towards efficient p-type doping. However, other studies have reported that N may have a very low solubility, creating deep levels instead of efficient shallow acceptor levels.
Six ZnO films have been grown by molecular beam epitaxy, homoepitaxially on ZnO substrates, in order to minimize the defect concentration. First, three undoped samples, oriented in the m-, a-, and r- planes were grown. Among them, the orientation that showed the lower residual carrier concentration was the m-plane, on which three samples were then grown with an increasing exposure to N.
PL measurements indicated a good crystalline quality, except for the highest N concentration sample. Semitransparent Schottky diodes were processed, showing good rectification properties. With the N incorporation the carrier concentrations decreased, becoming highly compensated for high N contents. Since all of the samples were highly resistive, deep level optical spectroscopy (DLOS) has been used in order to determine the presence of traps throughout the entire bandgap. In the N-doped samples the total capacitance variation (#8710;C/C₀) was slightly larger than in the undoped ones. Most interestingly, the evolution of this change along the scanned energy range is highly dependent on the presence of N. In the undoped samples, only two onsets can be clearly observed. The first one, very deep, at E_V+0.92 eV, and a shallower one at E_V+0.25 eV, which is responsible for practically the total capacitance change. This trap distribution changes dramatically with the incorporation of N. New levels can be observed both near and far from the valence band (VB) edge. Two new shallow levels appear, with energies E_V+0.21 eV and E_V+0.29 eV, where the first one is responsible for nearly half of the total #8710;C/C₀ change. The rest of the capacitance variation is divided among many new levels farther from the VB, with the most prominent ones located at E_V+1.23 eV and E_V+1.39 eV.
From these results, it is clear that N does not easily incorporate as a shallow acceptor in ZnO. Although some shallower levels arise, most of the capacitance variation appears in the mid-gap region, indicating that no efficient acceptor level has been incorporated with the addition of N in ZnO.

Donor doped zinc oxide has garnered great interest as an earth-abundant, lower cost alternative to tin-doped indium oxide (ITO) in transparent conducting oxide applications. In order to reach the highest levels of conductivity, dopant levels on the order of a few atomic percent are needed. At these high dopant concentrations, the electron mobility in zinc oxide is found to be lower than predicted, perhaps due to inhomogeneous dopant distribution. In this study, pulsed laser deposition was performed under various deposition conditions to produce thin films of zinc oxide with varying aluminum concentration. These films were analyzed via atom probe tomography (APT), a method which allows for 3-dimensional compositional mapping of specimens. Using this method, the degree of clustering of dopant atoms was explored, revealing that clustering does not seem to occur to a measureable degree in aluminum doped zinc oxide. APT measurements also revealed significant segregation of aluminum to the film-substrate interface, the nature of which was explored through multiple depositions under varying experimental conditions. The measurements also revealed zinc rich areas lying at specific orientations with respect to the film-substrate interface.

ZnO doped with Aluminum (ZnO:Al, AZO) is one of the most common used TCOs due to cost issues and characteristics (60meV exciton binding energy and Eg~3,3eV band gap). Assessment of the AZO conductivity is relevant because of its impact in the performance of the devices. However, this typically requires the use of specific test structures with suitable electrical contacts. Optical tools do not require specific sample preparation, are non-destructive, contactless, compatible with hazardous and chemically aggressive environments and allow also for analysis of uniformity with high (micrometric) spatial resolution. In this context, UV Resonant Raman spectroscopy is well suited to perform this assessment. It provides information of the chemico-physical and electrical relevant properties of the material, and can be applied directly on the TCO layer from finished devices. Raman scattering based techniques can fulfill the requirements needed in industry for on-line quality control and process monitoring.
In this work, an analysis on ZnO layers with different Al doping concentrations (resistivity from 25 Omega;/#61487; to >50 kOmega;/#61487;) has been performed by Hall Effect, absorption characterization and pre-resonant UV-Raman spectroscopy under 325nm excitation wavelength. The layers were grown on glass substrates by sputtering, and the doping ratio has been controlled by modifying the Ar/O₂ mixture. UV Raman scattering spectra measured with UV excitation show a main peak centered about 570 cm^-1 that is identified with the A₁(LO) ZnO mode and a broad band peaking at about 510 cm^-1 attributed to a defect induced band related to intrinsic defects in the ZnO lattices. The relative intensity of this band shows a direct correlation with the carrier density as determined by Hall effect measurements. In contrast, the Raman spectra do not show a clear dependence on the carrier mobility. This correlation allows to perform a quantitative estimation of the carrier concentration and the AZO conductivity in the AZO layers from the analysis of the Raman spectra.
This methodology has also been applied to analyze the impact of soft annealing processes of the AZO layers on their electrical conductivity, observing the possibility to achieve significant dopants activation by an annealing step at 250^oC in air, optimizing doping introduced during sputtering deposition at 120^oC. This is also accompanied by a significant improvement in the AZO transparency. An analysis of the impact of the carrier content on the spectral features of the Raman active vibrational modes will be presented, and the nature of the defects responsible of these effects (that are relevant because of their involvement in the AZO conduction processes) will be discussed.

ZnMgO/ZnO Quantum well heterostructures (QWs) have attracted much attention due to their opportunity of combining band gap engineering, with large excitonic binding energies. So far studies on ZnO have mainly focused on films grown in c-(0001) orientations. The wurtzite ZnO layers exhibit built-in electric fields along the c-axis, affecting the electronic properties. Non-polar surfaces are an alternative route to the fabrication of wide QWs with no reduction of the exciton binding energies as compared to bulk in wide QWs. This property will be demonstrated in QWs grown on sapphire. Then we will show a drastic improvement of the optical and structural properties when the QWs are grown on ZnO substrates. A comparison between the different nonpolar orientations (m- or a-planes) will be given pointing out the interest of the m-plane heterostructures for their optical properties. In addition we will show that it is possible to grow high quality semipolar QWs on (10-12) ZnO bulk substrates with wide atomically flat terraces and no plastic relaxation. The photoluminescence emission of wide QWs is below the ZnO band gap owing to the presence of an electric field, which can be estimated. 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 we will present the differences of these orientations in terms of residual and intentional doping as well as their potentialities in terms of optoelectronics devices.

Ga₂O₃ is a wide band gap semiconductor that has recently attracted great interest for its potential application in high power transistors, transparent coatings, and resistive switching devices. In these applications, the behavior of charge carriers and defects strongly influence the material&’s performance. Using first-principles calculations based on hybrid density functional we study the electronic properties of native defects and impurities in Ga₂O₃. In this talk we will discuss the impact of oxygen, gallium vacancies, and hydrogen impurities on the electrical and optical properties of Ga₂O₃, focusing on defect related luminescence, the interaction of hydrogen with Ga vacancy, and the effect of self-trapping holes on the optical properties of Ga₂O₃.

Cadmium Oxide (CdO) is a well known transparent conducting oxide (TCO), used in a number of opto-
electronic applications including IR detectors, displays, and photovoltaics. Given the variably conducting nature of CdO, understanding the role and contributions of electron, phonon and defect interactions on the thermal transport properties are of utmost importance to further advance this material system in a wide array of technologies. In this study, thin films of CdO were doped with Dysprosium to modulate electronic and thermal carrier mobilities, as well as oxygen vacancy concentrations within the oxide. We use measurements of thermal conductivity via time domain thermoreflectance to analyze the effects of increasing Dysprosium impurities and decreasing oxygen vacancies on thermal transport and phonon scattering in CdO. A twin peaked maximum in thermal conductivity was observed with up to a 60% increase over undoped CdO at the 10¹⁹ minus;10²⁰cm^minus;3 doping range. This optimum doping range limits vacancy concentration without inducing overwhelming lattice strain or impurity scattering to achieve the maximum in overall thermal mobility. The two peaks in the maximum are indicative of the superposition of separate maxima in phonon and electron contributions to thermal conductivity. The phonon contribution is being impacted at a lower Dysprosium concentration than the electron thermal conductivity, since the dopants are increasing the population of carriers for the electron contribution. These findings lend significant insight into efforts in thermal management of conducting oxides in general.

Our previous work has shown that CdO intentionally doped with Ga or In can have electron concentration in excess of 10²¹ cm^-3 with mobility higher than 100 cm²/Vs providing a transparent conducting oxide (TCO) with a record low resistivity of mid 10^-5 Omega;bull;cm and a wide transparency window extending from 400 to 1500 nm [1]. We have also demonstrated that the intrinsic gap of CdO can be further increased by alloying with up to ~30% of ZnO [2]. In this work we carried out a systematic investigation of the electrical and optical properties of In doped Cd_1-xZn_xO across the full composition range. The films were synthesized with RF magnetron sputtering. Since CdO and ZnO crystallize in rocksalt (RS) and wurtzite (WZ) structures, respectively, Cd_1-xZn_xO alloys undergo a phase change from RS to WZ at a composition x~0.35-0.45. We have found that properties of Cd_1-xZn_xO:In thin films depend critically on the crystal structure. The intrinsic bandgap of RS Cd_1-xZn_xO alloys increases from 2.2 to 2.7 eV with increasing Zn content, while that of the WS alloys decreases from 3.3 eV for ZnO to 1.7 eV for Cd_0.7Zn_0.3O with increasing Cd content. In both cases the change in the band gap originates from the shift of the valence band with the conduction band edge remaining constant with respect to the vacuum level. There is a significant difference in the electrical properties of the two crystallographic phases. The RS alloys (x<0.3) have ~3 times higher mobility than the WZ alloys (x>0.45). In doped RS Cd_1-xZn_xO thin films (x<0.3) can have an electron concentration as high as 1x10²¹/cm³ with resistivity in the range of 10^-4~10^-5 Omega;bull;cm. The Burstein-Moss shifted absorption edge of these alloys is larger than 3.2 eV. Due to the high mobility, these RS alloys have good transmission at wavelengths as high as 1500 nm. Compared with a commercial Flourine doped Tin Oxide (FTO) TCO with sheet resistance of 7 Omega;, our optimally In doped Cd_1-xZn_xO thin films with similar sheet resistance have four times smaller film thickness. In addition the higher IR transparency allows ~10% more of solar spectrum photons to be transmitted through the Cd_1-xZn_xO. The superior IR transparency offers a unique potential of using RS Cd_1-xZn_xO:In for photovoltaics utilizing the near infrared part of the solar spectrum including Si PV technology.
1. K. M. Yu, M. A. Mayer, D. T. Speaks, H. He, R. Zhao, L. Hsu, S. S. Mao, E. E. Haller, and W. Walukiewicz, J. Appl. Phys. 111, 123505 (2012).
2. D. M. Detert, S. H.M. Lim, K. Tom, A. V. Luce, A. Anders, O. D. Dubon, K. M. Yu, and W. Walukiewicz, Appl. Phys. Lett., 102, 232103 (2013).

It is well known that group 3-d transition metals introduce highly localized donor and acceptor states in II-VI compound semiconductors. Thus, vanadium whose localized d-donor level lies at about 5 eV below the vacuum level act as efficient donor in CdO. Interesting case is represented by ZnO in which vanadium d-level is located close the conduction band minimum (CBM). Therefore vanadium doped ZnO offers an opportunity to investigate effects of the interaction between the localized d-states of vanadium and the extended states of the ZnO host matrix. We have synthesized Zn_1-xV_xO thin films with 0 le; x le; 0.08 using magnetron sputtering. Polycrystalline ZnO:V films were deposited on glass substrates at 250 ^oC. Rapid thermal annealing of the films were carried out in N and O atmosphere. The composition of the films was determined using Rutherford Backscattering Spectroscopy (RBS). Electrical and optical properties of as grown and annealed samples were studied with Hall effect, optical absorption and photoluminescence (PL). Unexpectedly, in as grown and annealed samples the absorption edge energy increases with increasing vanadium content by as much as 0.13 eV for x = 0.08. In contrast the PL peak energy is decreasing with the vanadium content. To understand this unusual behavior we use the Band Anticrossing (BAC) model to consider the effects of close proximity of fully occupied vanadium d-donor level and the CBM of ZnO. The interaction results in an upward shift of the mostly unoccupied conduction band states and broadening of the occupied donor d-levels into a narrow band. The optical transitions from the VB to the CB are responsible for the absorption edge whereas transitions from the d-level derived narrow band to the VB are the primary origin of the PL. The composition dependence is explained by the BAC model with the d-level of vanadium at 0.12 eV below CBM of ZnO and the coupling constant of 0.67 eV.

TiO₂ is a widely used transparent conducting oxide with applications in solar cell technology as well as photocatalysis, for which mixed anatase- and rutile-phase samples have been found to outperform the individual polymorphs, a consequence of their energy band alignment. In this work we use a state-of-the-art hybrid quantum mechanical/molecular mechanical (QM/MM) embedded cluster approach to determine the bulk ionization potentials of eight TiO₂ polymorphs, four of which are naturally occuring, and four of which are 'high pressure' phases. We also perform plane-wave hybrid-density functional theory (DFT) calculations to determine their energy band gaps, thereby providing the band alignments for a range of TiO₂ polymorphs. We predict that improved photocatalytic activity should result from mixing brookite and anatase, or, if achievable experimentally, hollandite and brookite. We justify our results in the context of the effect of local oxygen coordination on the Madelung potential.

Titanium dioxide is one of the most thoroughly studied photocatalytic materials with numerous proposed applications, ranging from water photooxidation for hydrogen production to cleanup of environmental pollutants and self-cleaning coatings. Photocatalytic methoxy splitting on rutile TiO₂ (110) surfaces that leads to formation of formaldehyde has been previously observed in STM and TPD experiments [1]. In this work we theoretically study non-adiabatic excited-state dynamics of methoxy splitting on TiO₂ surfaces. Because such simulations are computationally demanding we developed an efficient methodology based on time-dependent density functional theory (TDDFT) and localized basis set. We use this methodology to study the excited-state trajectories that lead to the splitting of methoxy and examine in detail electron and hole motion that leads to the reaction. We compare electronic density during the excited-state dynamics to reference densities of the electronic ground state computed along the same trajectory. Interestingly while the excited-state time evolution is smooth, the ground state electronic density is observed to have a distinct jump that corresponds to C-H bond breaking and O-H bond formation. We discuss the importance of topology of the ground and excited state potential energy surfaces for the reaction.
[1] Phillips, K. R., Jensen, S. C., Baron, M., Li, S.-C. & Friend, C. M. Sequential photo-oxidation of methanol to methyl formate on TiO2 (110). Journal of the American Chemical Society 135, 574-577 (2013).

TiO₂ is a prototypical metal oxide and used in (photo)catalysis, photosensitized solar cells, and transparent conductive oxides. Industrially two forms of TiO₂ are used, rutile and anatase. The behavior of charge carriers is of key importance in virtually all applications of these materials. When excess electrons are added to the conduction band of an oxide, the electron-phonon interaction may favor electron trapping, which can result in the formation of either localized (small) or delocalized (large) polarons depending on the strength of the electron-phonon coupling. By combining first principles calculations within the DFT+U method (with the U computed entirely ab initio through the constrained random phase approximation) and experimental STM and STS techniques we investigate the nature of electron polarons in rutile (110) and anatase (101) surfaces. The excess charge were provided by either oxygen vacancies or by Nb-doping. In rutile the excess electrons can localize at any lattice Ti atom, forming a small polaron. The polarons in rutile can hop rapidly among lattice Ti sites. In stark contrast to rutile, electrons in a perfect anatase lattice prefer delocalized (band-like) solution, while small and immobile polarons can only be formed near surface oxygen vacancies or at the step edges. When electrons are introduced via a dopant (Nb) that modifies the lattice structure only slightly small polaron remains favorable in rutile, whereas more delocalized electronic solutions are preferred in anatase which can be interpreted as large polarons. Our results illustrate the basic principles of electron localization in the model oxide TiO2 and help to understand the different behavior of TiO₂ rutile and anatase in applications.
[1] M. Setvin, C. Franchini, X. Hao, M. Schmid, A. Janotti, M. Kaltak, C. Van de Walle, G. Kresse, U. Diebold, A direct view at polarons in TiO₂ rutile and anatase, arXiv:1401.7817 (2014)
[2] M. Setvin, X. Hao, B. Daniel, J. Pavelec, Z. Novotny, G. Parkinson, M. Schmid, G. Kresse, C. Franchini, and U. Diebold, Charge Trapping at the Step Edges of TiO₂ Anatase (101), Angew. Chem. Int. Ed. 53, 4714 (2014).

The redistribution of intrinsic charged point defects under applied voltage bias leads to spatial heterogeneities in the stoichiometry. Under direct-current (DC) biasing, the boundary conditions of the electrodes define the transport behavior of point defects near the interface. When the electrodes are impermeable to mass transport, an accumulation of point defects in the near-electrode region leads to the local electronic carrier concentration enhancement at the interface. Such defect redistribution is responsible for the time-dependent increase in the leakage current in many devices. While this leakage current enhancement is detrimental in capacitor devices, the phenomenon of lattice defect migration can be utilized to form novel functional behaviors, such as resistive switching in metal-oxides via modulation of the interface Schottky barrier at the reverse-biased cathode.
This research combines electrical characterization measurements with electron microscopy analyses to understand the mesoscopic redistribution of point defects as a function of electric field and time. Rutile TiO₂ single crystals are equilibrated at specific oxygen partial pressures and temperatures to define the initial defect chemistry state, and platinum electrodes are deposited to establish Schottky contacts. The samples are then subjected to electric fields up to 200V/cm, while the leakage current is continuously monitored.
During the degradation process the migration and accumulation of the positively charged defects, e.g. oxygen vacancies and titanium interstitials, results in Schottky barrier modulation at the reverse-bias electrode, also known as field-programmable rectification switching. As the electric field is increased to higher levels, the concentration of point defects at the interface increases to the point that rutile TiO_2-x is thermodynamically unstable. The severe nonstoichiometry in the near-electrode regions induces microstructural defects such as dislocations, planar defects and eventual Magnéli phase formation. Using diffraction contrast and high-resolution phase-contrast TEM imaging, the microstructure of severely reduced regions are investigated. Aberration corrected STEM microscopy from the interface region confirms that Magnéli phases form by the long range ordering of point defects into crystallographic shear planes. The Ti valence state and oxygen stoichiometry near the cathode are determined using monochromated electron energy loss spectroscopy (EELS). The implications of this defect redistribution processes and its reversibility are discussed within the context the overall electrical transport characteristics.
This work was supported by the National Science Foundation under grant number DMR-1132058.

Among the three polymorphs of titanium oxide, brookite is the least studied. Recent studies has been demonstrated that anatase/brookite composites has higher photocatalytic properties than single phase anatase or rutile. Nowadays, only a few works have reported the properties of anatase/brookite composites. The properties of this composites depends of many factors such as synthesis method, surface specific area, cristal sizes and content of each crystalline phase [1#8209;3].
In this research, titanium isopropoxide was used as Ti precursor to synthesized anatase/brookite composites under ultrasonic irradiation at 50°, 60° and 70 °C without the addition of any acid and any surfactant. The obtained samples were labeled S50, S60 and S70. To study the phase transformation the materials were subjected at different heat treatments under ambient conditions to 250-750 °C.
Selected materials were characterized by TEM and SEM. All samples were characterized by XRD diffraction. XRD data were refined by Rietveld method to determine the crystal sizes and content of each phase before and after heat treatments. The nitrogen adsorption was performed to determinate the specific surface area by the BET method and pore size distribution by BJH method. The band gap was determined by UV-Vis spectroscopy by means of Kubelka-Munk method. The vacancies in fresh materials were determined by XPS.
Anatase/brookite composites were succefully synthesized by ultrasonic irradiation. Fresh materials S50, S60 and S70 have a specific surface area of 260 m² g^-1. At 750 °C anatase and brookite are transformed to rutile. In general, the sample S70 has the best physico#8209;chemical properties, such as narrow pore size distribution and lowest band gap energy.
The authors acknowledge to CONACyT 174689 and PAPIIT IN104714 projects. For techical support to Israel Gradilla, Francisco Ruiz, David Domínguez and Ricardo Rosas.
References
[1] A. Di Paola, M. Bellardita, L. Palmisano. Brookite. Catalyst 2013; 3: 36#8209;76.
[2] H. Zhao, L. Liu, J. M. Andino, Y. Li. Journal of materials Chemistry A 2013; 1: 8209#8209;16.
[3] N. Tzikalos, V. Belessi, D. Lambropoulou. Environmental Science and Pollution Research 2013; 20: 2305#8209;20.

The rutile TiO₂ lattice is well known for its ability to “trap” photoinduced electrons at Ti⁴⁺ ions and form Ti³⁺ ions with an unpaired d¹ electron. This has been shown experimentally to result in a large family of similar, yet slightly different, Ti³⁺-related centers that include both intrinsic small polarons and donor-bound small polarons. In these latter centers, the Ti³⁺ ion is located next to an oxygen vacancy or an impurity such as fluorine, lithium, or hydrogen. The small polarons are easily formed in commercially available bulk crystals of rutile TiO₂ by illuminating the nominally undoped and oxidized samples at temperatures between 10 and 30 K with sub-band-gap laser light (e.g., 442 nm). Once formed, the ground state of the defects can be readily studied using electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) techniques as long as the sample is not warmed. Detailed information about the ground-state model of each electron trap is extracted from the g matrices and the hyperfine matrices. Differences and similarities of the various Ti³⁺ centers will be described and illustrated in this presentation. The oxygen vacancy is particularly interesting in rutile TiO₂, as two of its possible charge states are paramagnetic. The singly ionized oxygen vacancy with one trapped electron is an S = 1/2 defect and hyperfine lines from ⁴⁷Ti and ⁴⁹Ti nuclei show that the unpaired electron is localized on only one of the three titanium ions adjacent to the vacancy (i.e., the spin is not shared by two or more of these titanium neighbors). The neutral oxygen vacancy with two trapped electrons has a triplet (S= 1) ground state and consists of two nearest-neighbor, exchange-coupled Ti³⁺ ions aligned along the [001] direction and equidistant from the vacancy. Illumination at 20 K forms the neutral oxygen vacancies. As the crystal is slowly warmed in the dark, the neutral vacancies (S = 1) thermally decay between 25 and 30 K and convert to singly ionized vacancies (S = 1/2) by releasing an electron. The singly ionized vacancies then thermally decay between 35 and 40 K. An estimate of the activation energy for the neutral oxygen vacancy is 63 meV. Another important result is related to hydrogen in rutile TiO₂. The EPR signal presently assigned to the hydrogen-related small polaron was initially reported in 1961 (labeled Center A at that time) and the spectrum has often been erroneously attributed to an interstitial Ti³⁺ ion in the otherwise regular lattice. Recent studies, however, show that this Center A is the neutral hydrogen donor (i.e., small polaron) with the unpaired spin localized primarily on a titanium ion adjacent to an OH^- molecule. Thus far, there is no EPR evidence that interstitial Ti³⁺ ions are present in slightly reduced rutile TiO₂ crystals.

Titanium dioxide (TiO₂) is an emerging material for integrated optics due to its wide transparency and high refractive index; however, unlocking TiO₂&’s potential demands large-area single-crystal films. We have identified magnesium fluoride (MgF₂) as a nearly ideal substrate on which to grow single-crystal rutile TiO₂ for photonic applications, having a lattice match of 1.1%-3.3% (depending on direction) and both the wide transparency and low refractive index necessary to compliment rutile&’s optical properties. In this presentation, we will report on the first growth of epitaxial rutile TiO₂ thin-films on MgF₂ substrates using reactive molecular-beam epitaxy (MBE). Using in situ reflective high-energy electron diffraction (RHEED) and ex situ four-circle x-ray diffraction, we demonstrate the epitaxial growth of TiO₂ on multiple orientations of MgF₂ single crystals. We optimize growth parameters using (001) oriented MgF₂ and control for substrate temperature, titanium flux, as well as oxidant type (ozone or oxygen) and partial pressure. Epitaxial TiO₂ films with the highest crystalline quality were achieved at a growth temperature of 500°C, a titanium flux of 1×10¹³(±10%) atoms/(cm²sec), and using pure oxygen as the oxidant. We will discuss how to adapt these growth parameters for epitaxial growth on other MgF₂ orientations. Our optimized conditions enable the growth of crack-free TiO₂ films that are over 100 nm thick with excellent crystallinity (rocking curve full width at half maximum of 30 arc sec). Lastly, we will analyze how the high refractive index (n=2.45, ordinary axis, at lambda;=800 nm), low surface roughness (down to 2.3 nm) affect planar waveguiding losses and discuss the future of rutile TiO₂ integrated optics.

Titanium silicon mixed oxide thin films (TiSiO) are attractive for a large range of applications including high-k materials, optical coatings and waveguides as far as they benefit from the homogeneity and insulating properties of silicon oxide and high values of refractive index and permittivity of titanium oxide [1]. PECVD is a low temperature deposition method which allows the accurate tuning of film properties like titanium dioxide [2]. Here we investigate the deposition of TiSiO films in O₂/TTIP/HMDSO (oxygen/titanium tetraisoproxide / hexamethyldisiloxane) diffusion plasma of an rf ICP source (3 mTorr, 400 W) which can be operated in continuous (CW) or pulsed mode. This study is devoted to the investigation of inorganic titanium silicon mixed oxide film properties for optical and electrical applications. The optical properties and the thickness of the films were investigated by in and ex situ UV-Visible phase modulated ellipsometry. The surface temperature was evaluated by irreversible temperature sensitive labels stuck at the substrate backside.
By varying the relative Ti and Si precursor ratio, XPS results show that Ti_1-xSi_xO₂ films with tunable fraction of Si can easily be obtained. Increasing the silicon content (x) from 0.5 up to 1 leads to homogeneous and amorphous films as evidenced on SEM views and XRD spectra. The optical index (@ 633nm) and low frequency permittivity evolves from 2.2 and 90 (for pure TiO₂, x=0) to 1.46 and 4.6 (for pure SiO₂, x=1). Coupling ellipsometry and XPS allowed us to propose schematic energy diagrams, showing that the films are n-type wide band gap (Eg) semiconductors for which Eg increases as a function of x.
In addition, to obtain a process compatible with sensitive substrates like polymers, pulsed plasmas were investigated in order to limit the thermal budget. Thus by applying a frequency and duty cycle in the 0.25-1 kHz and 10-100% range, respectively, we could successively decrease the deposition temperature from 130°C to 40°C without degrading the film optical properties.
The electrical performance is currently under evaluation. The use of pulsed plasmas, which allows to reduce the of ion bombardment, is expected to improve the electrical performance of the material as it was previously shown in the case of SiO₂ films deposited in pulsed O₂/HMDSO plasmas [3] .
[1] F. GRACIA, F. YUBERO, J.P. HOLGADO, J.P.ESPINOS, A.R. GONZALEZ-ELIPE, T. GIRARDEAU, Thin Solid films 500, 19-26 (2006)
[2] D. LI, M. CARETTE, A. GRANIER, J.P. LANDESMAN, A. GOULLET, Thin Solid Films 522, 366-371 (2012)
[3] A. BOUSQUET, A. GOULLET, C. LETEINTURIER, N. COULON, A. GRANIER, The European Physical Journal Applied Physics 42, 3-8 (2008)

Titanium dioxide thin films are good candidates for designing optical integrated waveguides or optical devices such as filters or resonators due to their high refractive index and low absorption in the visible range. However, they grow in columnar structure and have a relatively low band gap. Mixed TixSiyOz materials open new possibilities to overcome some of the limitations imposed by TiO2.
In the present work, we investigate the compositional variation of optical properties of Si-doped TiO2 by means of Density Functional Theory. Special Quasi-random Structures method is used to generate structural models of SixTi(1-x)O2 disordered solid solutions for x=0.0625, 0.125, 0.1875, 0.25, 0.5 and 0.75 for anatase and rutile phases. These initial supercells are structurally optimized (i.e. optimized with respect to the cell shape, size, and atomic positions) using the Vienna Ab initio Simulation Package. Electronic structure and optical constants of the resulting structures are calculated by the linearized augmented plane wave method as implemented in the Wien2k full potential all electrons code together with the recently developed modified Becke-Johnson exchange-correlation potential allowing precise prediction of the band gap.
Our calculations show that the rutile phase is favoured over the anatase structure up to ~0.8 mole fraction of SiO2. At these high Si contents are, however, other phases (or amorphous structure) expected as is the case of SiO2. Hence we conclude that alloying Si into TiO2 preserves its thermodynamically preferred phase. The calculated dielectric function is compared to the experimental data obtained by fitting the optical measurements (ellipsometry, spectrophotometry) carried out on TiO2 films containing rutile and anatase phases and TixSiyOz films prepared by plasma enhanced chemical vapor deposition. This comparison reveals very good agreement for pure TiO2 phase, while for higher concentrations of Si our calculations suggest drop of the optical band gap. We show that this drop is related to the presence of Si induced states on O sites. Nevertheless, the optical response as measured by the imaginary part of the dielectric function is very low in the range ~0.5eV above the calculated optical band gap, thus rationalising the experimental data where this drop is not observed (instead almost no change of the optical band gap is suggested up to ~0.4 SiO2 mole fraction).

Solid state chemical sensors, often in a MEMS approach, became a mature technology and create product innovations in building, medical, automotive, consumer and industrial technologies. The key feature of each chemical sensor is the sensitive layer that undergoes a reversible interaction e.g. with gas species to be detected. The result is finally translated into an electrical output signal. Metal oxides, due to their chemical robustness and longevity are very frequently used as sensitive layers at few 100 °C. However, since surface reactions occur with a large variety of gases, these sensors respond similar to various gases with similar chemical reactivity. Certain applications, like measuring the overall amount of smell in room air are demanding this. Applications like the detection of explosive natural gas / toxic CO or breath ethanol require one gas to be selectively detected. Applications like the recognition of fire related gas patterns or the smell pattern in human breath for medical diagnosis require a differentiated detection of quite complex gas patterns.
Surface engineering, for a long time, has been the answer of tailoring the gas reactivity of metal oxide surfaces to the needs of the application, keeping the oxides surfaces at a constant temperature. Options are the choice of the preferred chemical reaction at a stable oxide surface by usage of the proper temperature, the application of catalytic filters that oxidize interfering gases before they reach the surface and coating the surface with material that add the desired chemical reactivity - these may be other oxides, but a lot of catalytically active noble materials showed nice results here. Nanosized materials have been successfully used to improve the benefits. The research on these materials however recently revealed, that pre-adsorbed species from former measurements play an increasingly important role here. One example is the intermitted usage of higher temperatures, temporarily creating activated oxygen species that then convey nice gas reactions even at room temperature.
This leads to a recent paradigm change, the trend here is called transient operation of metal oxides, usually performed by rapid changes of the operation temperature. This technique distinctly employs non equilibrium conditions of the surface and plays with adsorption/desorption time constants in the range of 0.1-10s by changing temperatures with a similar rate. This is made feasible by sensor constructions where with oxides placed on temperature cycled membrane structures. The sensor signal is monitored as a function of time and multivariate approaches or functional data analysis is employed to reveal the chemical information. It is astonishing to see which differentiated analysis of multi-component gas mixtures can be obtained with a single oxidic surface, employable e.g. to distinguish between the smells of different fire states or to distinguish between the smells of different human activities.

Nanotechnology and the different methods for the preparation of nanostructures are in continuous evolution. Easy and cheap growth techniques for the production of nanostructures in a variety of morphologies are constantly proposed by the research community. Key features are the capability to control the composition, the particle shape and size distribution since in chemical sensing and many other applications these nanostructures exploit properties related to crystallographic features.
In 2002, the field of metal oxide nanowires underwent a significant expansion and became one of the most active research areas in nanoscience. Stimulating advances have been made at an extraordinarily fast rate in different laboratories all over the world, following curiosity, discovery or hypothesis driven research. Nowadays it is more than a decade from the first presentation of metal oxide nanowires as chemical sensors. Significant advances have been made both in terms of preparation procedures and their integration into functional sensing devices, while progress in fundamental understanding of their functional properties is slow-moving. In fact, the full integration still remains a challenge that has been wisely approached in different ways.
Metal oxides in forms of nanowires are interesting materials for chemical sensors. Their peculiar morphology assures a high surface to volume ratio necessary to maximize surface related properties like the ones governing chemical sensing transduction principles. Their exceptional crystalline features guarantees stable crystalline and therefore electrical properties over long-term operation, a required quality for an industrial application of any kind of sensor or device in real environments. We have thoroughly studied the deposition using evaporation and condensation from powder in controlled environment using different experimental set up. Metal oxide nanowires were integrated in functional devices for chemical sensing and then tested towards a wide range of chemicals.
The most recent developments in bottom up and top down approaches for chemical sensing application will be reviewed, and the recent achievements obtained at SENSOR laboratory will be presented. N-type metal oxide such as titanium, tungsten, niobium and zinc oxide and p-type metal oxide such as copper oxide were prepared using different techniques starting from metal/ metal oxide powders and films and their chemical sensing properties were studied in different operating conditions.
Acknowledgements
The research leading to these results has received funding from the European Community&’s FP7-ICT-2013-10, MSP— Multi-Sensor-Platform for Smart Building Management under the project n° 611887.

Metal oxide (MOX) materials, like SnO₂, In₂O₃, ZnO, hellip;, are employed in the development and fabrication of solid state gas sensors due to the well-known interaction between the MOX surface and the different gases, which leads to a variation in the electrical characteristics of the metal oxides. In fact, several commercial gas sensors of different providers worldwide are based on them. Generally, the gas sensing properties are a consequence of the adsorption-desorption processes that take place at the surface of the MOX in which the oxygen-poor surface plays a key role in the sensing mechanism. Different accepted models for the sensing have been presented and are commonly accepted to interpret the observed gas sensing behavior.
MOX nanowires (NWs) have emerged in the last two decades as a step forward in the development of gas sensors, when compared to thin or thick film MOX materials, due to their high aspect ratio. Especially monocrystalline materials have become important to achieve effective and known interactions of their surface in gas sensors.
Among the different MOX materials, In₂O₃ is receiving relatively little attention due to the fact that its synthesis window is much more reduced than that of SnO₂ or ZnO, and that it leads to either semiconducting or metallic behavior, being its reproducibility of the material complicated. For sensing applications, In₂O₃, however, shows some interesting features like a low level of detection of NO_x compounds in air and a reduced sensitivity to CO, which gives rise to an enhanced selectivity as compared to the 2 above-mentioned compounds.
In this work we will present the synthesis of In₂O₃, as well as the fabrication and characterization of gas sensors based on them. The synthesis is carried out in a horizontal furnace using the carbothermal reduction process from In₂O₃ and graphite powders, and using Ar and O2 as carrier gases. Depending on the concentration of O₂ in the gas phase, on the flow used, on the vacuum level in the furnace and on the furnace and substrate temperatures, either continuous layers or bundles of NWs have been synthesized on Au-seeded Si or Al₂O₃ substrates. NWs have been removed from the surface where they grew, have been dispersed on prepatterned Si substrates and have been contacted using Focused Electron and Focused Ion Beam techniques. Characterisation towards the different gases has been carried out on a self-constructed gas testing chamber.
The material&’s growth, structural and electrical characteristics will be presented as a function of the growth conditions and the correlation with the gas sensing properties discussed with the view on published sensing mechanisms.

Nanoscale field effect transistors (FETs) are one of the most promising methods for chemical and biological sensing based on solid-state devices. In fact, many two- and three-dimensional materials have been implemented in sensing applications, but quasi 1-dimensional nano structures hold the potential of significantly improving the sensitivity and speed of the sensor responses. Here we focus on optimizing the current-voltage relationship and gating response of tin dioxide (SnO₂) nanobelt (NB) FETs, a promising material that has been demonstrated as effective for channel-limited gas [1], pH [2] and protein [3] sensors. FETs are fabricated with individual SnO₂ NBs. The NBs are grown by a physical vapor-liquid-solid process in a horizontal tube furnace in an argon environment at 300 torr and 1000 °C over 2 hrs. The growth recipe utilized produces NBs with nominal thicknesses down to 30 nm and width to thickness ratios typically approaching 10:1. The lengths are up to millimeters on the growth substrate. The dimensions are desirable for FET application, however the electrical characteristics of the as-grown materials are often not optimum for high-performance FETs. We have developed a multistep thermal annealing procedure in low vacuum ranging from 150 to 250 °C and oxygen environment at atmospheric pressure and 600 °C to increase the conductivity by more than 10³. The multistep annealing process is necessary to consistently obtain FETs with low resistance, Ohmic contacts as determined via comparison of 4- and 2-probe measurements which differ by <5%. Utilizing a typical backgate geometry the device is transitioned from the on state to the off state over a gate voltage range of less than 30 V through a thick 250 nm SiO₂ dielectric layer. The On/Off ratio is as large as 10⁴. We believe the oxygen annealing effectively activates the NBs while the vacuum annealing both helps clean the material and tune the carrier density at the surface, affecting metallization. The reactivity of the surface makes oxides ideal for chemical functionalization of the surface enabling specific biodetection. We use a proven method of surface functionalization through binding of a self-assembled monolayer of a common amine-terminated silane, APTES, by immersion. We can utilize the amine group to attach a chain consisting of biotin and streptavidin leaving the device functional for further biological specification. We verify the process by observation of fluorescently tagged streptavidin compared to the background. Potential applications of these devices as highly portable, real-time biomedical sensors will be discussed.
[1] L.L. Fields et al., Appl. Phys. Lett. 88, 263102 (2006).
[2] Yi Cheng et al., Nano Lett. 8, 4179-4184 (2008).
[3] Yi Cheng et al., Biosensors and Bioelectronics 26, 4538-4544 (2011).

Recent advances in the fabrication of oxide and transparent p-conducting thin films are reported. In conjunction with n-type epitaxial ZnO and In₂O₃ on Al₂O₃ and amorphous ZTO (zinc tin oxide) on glass thin films, bipolar diodes are reported. We will present three materials approaches: (i) p-type amorphous material derived from ZnCo₂O₄ spinel (a-ZCO), (ii) p-type zincblende cuprous iodide (CuI), and (iii) p-type amorphous nickel oxide (a-NiO).
We report the (by far) highest rectification (r>10¹⁰) for bipolar oxide diodes (a-ZCO/c-ZnO, a-NiO/c-ZnO), fully amorphous ZTO/ZCO pn and pin-diodes (r>10⁶), fully transparent CuI/ZnO diodes (r>10⁶), and a-ZCO/In₂O₃ and a-NiO/In₂O₃ diodes with rectification of 10⁴.
We will compare different fabrication methods (pulsed laser deposition, sputtering, evaporation) on the diode properties, discuss the nature of current in type-II bipolar heterostructures (NiO/ZnO and CuI/ZnO) and demonstrate applications in electronics and photovoltaics.

A₂BO₄ spinel oxides can be classified into four Doping Types depending on the relative energy ordering and level within the gap of the cation anti-site defect acceptor and donor levels. Doping Type II (DT-2) spinels where the acceptor lies above the donor and, in addition, the donor lies in valance band, are natural host materials for p-type conduction. High-throughput theoretical screening finds Co₂ZnO₄ to be the prototype DT-2 material. In particular, the Co_Td site defect level is resonant in the valence band, making it electrically neutral thus allowing the electrically active Zn_Oh acceptor to yield p-type conductivity, independent of the concentration of Co_Td defects. Resonant elastic x-ray diffraction (REXRD) site occupancy measurements on bulk ceramic samples grown in air at 800 °C confirm this basic prediction. Further, intentional non-equilibrium growth to increase the Zn_Oh concentration due to either quenched-in cation-site-occupancy disorder or incorporation of excess Zn should be an effective doping strategy. Experiments using combinatorial co-sputtered Co_2-xZn_1+xO₄ thin film “libraries” with intentional composition gradients on 2”x2” glass substrates support this. More definitive REXRD site occupancy measurements of as-deposited and annealed Co₂ZnO₄ and Co₂NiO₄ epitaxial films grown on SrTiO₃ by pulsed laser deposition confirm this. Finally, 17 candidate extrinsic dopants were evaluated theoretically yielding Li_, Mg and Ni as the most promising. Ni doping was tested via combinatorial co-sputtering and found to be effective. In fact, the conductivity of Co₂Zn_1-xNi_xO₄ increases monotonically with increasing Ni all the way to Co₂NiO₄ for which σ asymp; 100 S/cm. In addition to presenting the specific materials results discussed above, we will also describe our Inverse Design approach to theory-guided Materials-by-Design.

Conducting oxides are key materials in nowadays electronic devices. Especially transparent conducting oxides (TCOs) are astonishing materials due to their unique optical and electrical properties and their wide range of possible applications. One of these systems is the ZnCo₂O₄ spinel. Although known for some time as dye and under recent discussion as anode material for lithium batteries [1] and as gate material in transistors [2], only very little effort has been made to investigate the magnetic properties [3].
As a known p-type material, ZnCo₂O₄ is a promising candidate for ferromagnetism in oxide semiconductors, since the lack of p-type dopability is one of the major limitations in the related system Co:ZnO, which was proposed as a possible room temperature dilute magnetic semiconductor [4]. As secondary phase ZnCo₂O₄ in Co:ZnO has furthermore been suspected as the reason for the observed ferromagnetism [5].
We present the successful growth of crystalline ZnCo₂O₄ thin films by means of magnetron sputtering on c-plane sapphire substrates. Additional samples were fabricated to study the transition from highly cobalt doped n-type wurtzite ZnO to the cubic p-type ZnCo₂O₄ in dependence of preparation conditions. Standard characterization using SQUID magnetometry, XRD, AFM and TEM has been done and the transport properties of the thin films have been measured. Additionally, the samples were investigated using element specific synchrotron techniques like x-ray absorption near edge structure spectroscopy (XANES) and x-ray linear dichroism (XLD) to determine the valence of the Co and to exclude metallic aggregations. High field x-ray magnetic circular dichroism (XMCD) was used to quantify the behavior of the Co sublattice magnetization up to 17 T. Magnetic ordering below ~12 K was observed in SQUID and the low temperature M(H)-hysteresis show history dependent behavior reminiscent of an exchange bias.
The combination of its semiconducting, magnetic and optical properties makes ZnCo₂O₄ an interesting system for possible spintronic applications.
[1] Sharma, Y., et al. Advanced Functional Materials 17 (2007): 2855
[2] Schein, F., et al. Electron Device Letters, IEEE 33 (2012): 676
[3] Kim, H. J., et al. physica status solidi (b) 241 (2004): 1553
[4] Dietl, T., et al. Science 287 (2000): 1019
[5] Liu, Y. and MacManus-Driscoll, J.L. Applied Physics Letters 94 (2009): 022503

CuMO₂ semiconductors crystalize naturally in the delafossite mineral structure, and have been shown to possess large optical band gaps and to display p-type conductivity. These attributes led to an explosion of interest in these materials for p-type transparent conducting oxide (TCO) applications, but also for water-splitting applications. It is now realised that these materials are limited by indirect band gaps and deep acceptor levels, which limits their range of utility.
Recently, however, β-CuGaO₂ which crystalizes in a wurtzite-derived orthorhombic β-NaFeO₂ structure has been synthesized, displaying a direct band gap of 1.5 eV, ideal for visible light conversion. The discovery of a direct band gap, highly absorbing oxide is extremely exciting, as at present, the only current oxide solar absorber is Cu₂O, which currently displays photovoltaic conversion efficiencies of ~5%. In this presentation we investigate the electronic structure of a range of possible β-CuMO₂ materials. We analyse the structure-property relationships between the delafossite and β-NaFeO₂ structure, and rationalise the experimentally observed change in properties between β-CuGaO₂ and delafossite CuGaO₂. Finally, we identify other β-CuMO₂ compounds that could have interesting technological applications.

Delafossite-type CuAlO₂ ternary oxides have been widely studied as p-type transparent conducting oxides for applications in various optoelectronics devices. However, so far there have been few reports investigating CuAlO₂ as a potential phosphor material. In light of developments in field emission displays and other flat display technologies, a new generation of phosphors exhibiting both good luminescence and conductivity are required. CuAlO₂ is an intrinsic p-type transparent conducting oxide which may serve as a host material to various trivalent rare earth dopants at the Al site. Such an alteration does not change the hole transport along the O-Cu-O path. Therefore both luminescent and electrical properties could be sustained by the doped CuAlO₂. We hereby used the substitution of trivalent Eu³⁺ into the Al³⁺ sites of the delafossite structure to prepare CuAl_1-xEu_xO₂ (x=0.001, 0.003, 0.01, 0.03, 0.05 and 0.1) fibers via a cost-effective electrospinning method. This method could reduce the annealing temperature so that the single-phase Eu-doped fibers could be be obtained via one-step annealing in air. Photoluminescence measurements indicated that under the excitation wavelengths of 390nm and 420nm, the CuAl_1-xEu_xO₂ exhibited emission behaviors from both intrinsic near-band-edge violet emission and Eu^{3+ 5}D₀ → ⁷F_J red emission. The variation in intensity between the five ⁵D₀ → ⁷F_J emissions channels resulted from changes in the Eu³⁺ local symmetry. At dopant levels above 3 at% Eu, luminescence was absent due to concentration quenching. Further electrical characterization confirmed the p-type conductivity and the temperature-resistance dependence of this semiconductor-like material. Delafossite CuAlO₂ materials may develop to be promising options in dual channel light emitting and other optoelectronic applications.

Amorphous InGaZn oxide (IGZO) is of great interest as a higher mobility large area electronic oxide for display applications. It has a higher mobility than a-Si:H. However, it suffers from an instability, the negative bias illumination stress instability, whose cause is widely debated. Oxygwen vacancies are often given as the cuase. Here, a model of the instability is presented, based on the photo-excitation of electrons from oxygen interstitials. The O interstitials are present to compensate accidental hydrogen donor impurities. The O interstitials are found to spontaneously form in O-rich conditions for Fermi energies at the conduction band edge, much more easily than in related oxides. The excited electrons give rise to a persistent photoconductivity due to an energy barrier to recombination. The formation energy of the O interstitials varies with their separation from the H donors, which leads to the observed voltage stress dependence on the compensation.

Amorphous InGaZnO₄ (a-IGZO) is a promising semiconductor for transparent and flexible thin film transistors (TFTs), and exhibits significantly improved performance compared to conventional a-Si:H based TFTs. Post-deposition annealing of a-IGZO is critical to achieve the high mobility required for applications in high performance displays. Previous studies have focused on furnace annealing under controlled ambient, or under nanosecond time scale excimer laser annealing. This work explores the use of post-TFT fabrication millisecond timeframe anneals using a scanned CO₂ laser, with TFTs exhibiting saturation field-effect mobilities above 70 cm²/Vs.
IGZO films (50 nm thick) were RF sputtered from an oxide target (In₂O₃:Ga₂O₃:ZnO = 1:1:1) onto thermally oxidized Si wafers (100 nm SiO₂), with substrates heated to 260^oC during deposition. TFTs were formed by wet chemical etch followed by liftoff patterning of Ti/Au source/drain contacts; back channels were not intentionally passivated. TFTs were subsequently annealed either in a furnace at 350^oC in dry air for 30 minutes, or by exposure to a scanned CO₂ laser spike annealing (LSA) beam. Under LSA, a line-focused continuous wave CO₂ laser beam (lambda;=10.6 µm) is scanned over the substrate, rapidly heating the surface to a peak temperature which subsequently cools by thermal conduction into the substrate as the beam passes. Laser and scan conditions were established for a 1 ms dwell at a peak temperature of ~600^oC. In addition to the shorter anneal time, LSA also has a much faster quench to room temperature enabling formation of metastable structures. Furnace annealed samples typically cooled at 2 K/s while LSA samples cooled at ~350,000 K/s.
IGZO films remained X-ray amorphous after both furnace and LSA annealing. The saturation mobility of devices as deposited was typically 10-12 cm²/Vs with a large negative V_on. With furnace annealing, the mobility increased to typically 16 cm²/Vs with V_on shifting to +5 V. Anneals under LSA, in contrast, resulted in devices with saturation mobilities above 70 cm²/Vs and turn on voltages near -5V. Annealing is not complete, however, as the sub-threshold slope is high indicating a large trap density remaining after the high temperature quench.
These high mobility devices remained stable for extended time (24 days) even without back channel passivation. However, subsequent furnace annealing erases the LSA annealing benefit and results in devices comparable to furnace annealing only. This suggests that LSA anneals result in a metastable structure that can be later thermally relaxed. These results are discussed within a model of metastable hydrogen donor activation, from traps near the conduction band edge, during a fast thermal quench.

This work investigates the quality of back-channel passivation applied to sputter-deposited IGZO bottom-gate TFTs. B-staged bisbenzocyclobutene-based (BCB) resins, alumina and SiO₂ are used as passivation materials. Sputtered quartz and PECVD SiO₂ rendered the IGZO channel material highly conductive (ρ < 0.01 Omega;#903;cm) due to defect states created at the back-channel during deposition. The BCB spin-coat deposition and curing processes enhanced the material conductivity slightly, while subsequent annealing in air at 250 °C resulted in TFTs with a threshold voltage V_T ~ 0 V and improved stability compared to unpassivated devices. Devices passivated with electron-beam evaporated alumina followed by annealing in air at 400 °C for 30 min also demonstrated improved stability, with V_T ~ 1 V, channel mobility µ_ch ~10 cm²/V#903;s and subthreshold swing SS ~ 250 mV/dec. Compositional changes of the IGZO layer induced by the passivation and annealing processes are quantified using XPS and EDX analysis. These are correlated with electrical measurements obtained from TFTs and interdigitated capacitors, with an emphasis on multifrequency C#8209;V and C-time analysis for evaluation of defect states at the back-channel interface between the IGZO and passivation material.

Amorphous InGaZnO (a-IGZO) is a promising candidate material for thin film transistors in display devices. One major challenge for mass application is its instability under illumination. Previous reports show a large and slow photoconductivity response in a-IGZO thin films. By identifying the energetic distribution of defect states from transient photoconductivity, we can optimize the fabrication process and improve device stability.
In this work, we measured transient photoconductivity of a-IGZO thin films prepared under various conditions over an extended day-long time scale. The first set of samples studied was 50 nm InGaO₃(ZnO)₂ thin films deposited using pulsed laser deposition. The initial conductivity of each sample was tuned by varying the oxygen pressure during deposition (P_O2). With increasing P_O2, the initial conductivity strongly decreased over 3 orders of magnitude. The samples were then illuminated using an array of 385 nm wavelength UV LED for 24 hours. Amplitude of photoconductivity response depended strongly on P_O2, with the relative amplitude of photoconductivity increasing from less than two-fold to over 200-fold with increasing P_O2. For all samples tested, dark relaxation of the conductivity after turning off the LED array was slower and weaker compared to photo-excitation, implying different relaxation mechanisms.
From the measured transient photoconductivity, we extracted its time constant distribution, and thus the activation energy and density of states distribution. In a-IGZO thin films, conductivity changed very rapidly right after the LED array was switched on/off, but was still changing slowly after a whole day of excitation/relaxation, suggesting that multiple time constants were involved. We fitted each transient curve with both stretched exponential fitting and multi-exponential fitting for comparison. Density of states (DOS) distribution of the activation energies responsible for photoconductivity was then extracted through inverse Laplace transform. DOS spectrum with a wide continuous peak was observed in all samples regardless of fitting methods. The peak position and line-width of DOS showed little dependence on P_O2, and only the amplitude decreased with increasing P_O2.
Stretched exponential fitting of transient photoconductivity provides a powerful tool to study deep level DOS in a-IGZO. It allows one to extrapolate the long-time photoresponse with relatively short measurement. The peak position, line-width, and amplitude of DOS spectrum can be directly predicted from the three fitting parameters of stretched exponential fitting. This analysis method will be applied to data from additional thin films prepared by sputtering, spin-coated combustion method, and spray-coated combustion method. Comparison of DOS spectra from samples using different fabrication methods will be presented.

Room-temperature fabrication of semiconducting thin films is promising due to low cost and the possibility to deposit on flexible substrates. Zinc-tin oxide (ZTO) fullfills the working hypothesis of Hosono [1] and has with that a high conductivity even in the amorphous state. Up to now, only few different Sn/Zn ratios were realized and studied experimentally [2,3,4]. In order to tune the film properties as desired more information of the dependence of the properties on the Sn/Zn ratio are needed.
We present ZTO thin films deposited by pulsed laser deposition (PLD) using a continuous composition spread method [5]. Within this approach the materials were deposited from a segmented ceramic PLD target onto a 50×50 mm² glass wafer. Using energy dispersive X-ray analysis the spatial dependence of the Sn/Zn ratio was mapped. The amorphous phase was confirmed with X-ray diffraction measurements. After that the sample was divided along the compositional gradient into pieces. Absorption edge was obtained by transmission and reflection measurements and shows a systematic dependence on the Sn/Zn ratio. Electrical properties were determined by both room-temperature and temperature dependent Hall-effect measurements. For selected Sn/Zn ratio the results were compared to those of thin films fabricated by a conventional PLD approach.
[1] Hosono, H., Kikuchi, N., Ueda, N., and Kawazoe, H. Journal of Non-Crystalline Solids 198-200,165 (1996)
[2] Chiang, H. Q., Wager, J. F., Ho man, R. L., Jeong, J., and Keszler, D. A. Applied Physics Letters 86, 013503 (2004)
[3] Erslev, P. T., Chiang, H. Q., Hong, D., Wager, J. F., and Cohen, J. D. Journal of Non-Crystalline Solids 354, 2801 (2008)
[4] Schlupp, P., von Wenckstern, H., and Grundmann, M. MRS Proceedings 1633, 101 (2014)
[5] von Wenckstern, H., Zhang, Z., Schmidt, F., Lenzner, J., Hochmuth, H., and Grundmann, M. CrystEngComm 15, 10020 (2013)

In recent years, there is an increasing shift towards the use of wide bandgap materials in their amorphous form owing to several technological advantages and the fact that amorphous oxides exhibit similar or even superior properties to those observed in their crystalline counterparts. To understand how the structural properties change upon amorphization and how chemical composition affects the local and long-range structure of the amorphous oxides, we employ first-principles molecular dynamics to generate amorphous In-O and In-X-O where X=Zn, Ga, Sn, Ge, Y, Sc, or La with 4:1 ratio of In:X, and compare their structural features to those obtained for amorphous indium oxide and relevant crystalline binary oxides. We find that the Metal-Oxygen polyhedra are generally well preserved upon amorphization, and the mobility in the amorphous state is governed by how the MO polyhedra are connected into a network. In particular, the In-In distance distribution exhibits strong asymmetry towards longer distances near the crystalline-to-amorphous transition where the observed mobility is highest. Furthermore, in a-IO and all a-IXO, Indium remains undercoordinated with little dependence on X, whereas all X atoms reach their natural coordination. This finding suggests that the carrier generation is primarily governed by In atoms, in agreement with transport measurements in In-based amorphous oxides. At the same time, the presence of X affects the number of six-coordinated In atoms which correlates well with the observed variation in the carrier concentration as a function of the substitution fraction in a-IXO.

Amorphous oxide semiconductors (AOSs) based on In₂O₃ have found recent applications as channel layers in thin-film transistors (TFTs) for active-matrix displays. Additional metal oxide components in the In₂O₃ system are selected based on contribution to mobility, free carrier control, and amorphous phase stability. Common constituents such as SnO₂ and ZnO are incorporated to stabilize an amorphous phase, while sustaining high electron mobility. Free carrier generation is proposed to be suppressed via addition of insulating (large band-gap) metal oxides, with the majority of focus on Ga₂O₃. Targeting field-effect mobility > 20 cm² V^-1 s^-1 we report findings on AOS materials with rarely investigated carrier suppressing additives to the In₂O₃ system, comparing two distinct thin-film deposition methods. TFTs were fabricated on p⁺⁺ Si substrates with 100 nm of SiO₂ dielectric and Al S/D contacts in a staggered, bottom-gate structure. Solution processed AOS thin-films were hindered by densities of < 90 % (relative to crystalline bulk density) and thus resulting TFTs exhibited field-effect mobility < 7 cm² V^-1 s^-1, while sputtered thin-films possessed densities ge; 95 % and field-effect mobility as high as 20 cm² V^-1 s^-1 was attained. In this presentation we present results on a side-by-side comparison of solution processed and sputtered In₂O₃-based AOS thin-films, to uniquely highlight the influence of processing and film structure on TFT performance.

We have fabricated oxide semiconductor thin film transistors (TFTs) and circuits on thin flexible substrates using a lamination and transfer free process. Oxide semiconductor TFTs have high mobility and good stability and have found commercial application in active matrix displays. Low processing temperatures for some oxide semiconductors makes them of particular interest for flexible substrate applications, including flexible displays. Here we report ZnO TFTs fabricated on thin polyimide substrates. The polyimide substrates were spin cast from Hitachi Dupont MicroSystems PI-2611 polyimide precursor on Si wafers and cured at 300 #730;C in air. The thickness of the cured polyimide was about 5 mu;m. Staggered bottom gate structure TFTs were used in this work. First, chromium was deposited and patterned by wet etching as the gate layer. Next, Al₂O₃ and ZnO layers were deposited by plasma enhanced atomic layer deposition (PEALD) at a substrate temperature of 200 #730;C as gate dielectric and semiconductor active layers. Aluminum was then deposited and patterned by lift off to form source and drain contacts. Finally, a 30 nm Al₂O₃ layer was deposited by atomic layer deposition (ALD) as a passivation layer. Passivated ZnO TFTs on thin flexible polyimide substrates typically have mobility of 14 cm²/V#8729;s, sub-threshold slope < 300 mV/dec, and on/off ratio of > 10⁸. The device characteristics on plastic substrates are very similar to those of devices fabricated on glass substrates. Greater than 99% device yield was obtained for samples with 80 TFTs. After device fabrication and initial testing, the thin polyimide layer was removed from the Si wafer carrier. Fortuitously, the adhesion of the polyimide layer to the Si wafer is sufficiently good to allow device fabrication, but sufficiently low to allow simple mechanical stripping. The TFTs on released substrates had excellent stability against bending, with no noticeable degradation under bending radii as small as 5 mm. Inverters and ring oscillators fabricated on thin polyimide also functioned properly and had characteristics similar to circuits fabricated on glass.

Globally there has been a strong interest in both metal oxide semiconductor thin-film transistors and in printed electronics. In our lab we use selective area deposition of metal oxides as an alternative approach to printed electronics. In this “patterned-by-printing” approach, an inhibiting polymer ink is printed and the active materials are deposited via spatial atomic layer deposition (ALD). This approach separates the printable ink requirements from the active materials requirements, while maintaining the advantages of additive device fabrication.
In this talk we will discuss how the use of selective area deposition has opened up the design space for ZnO thin-film electronics, enabling the facile fabrication of devices and circuits. By using digital printing techniques to print a polymer inhibitor, we can fabricate unique device architectures and rapidly screen variations on a design theme. With the “patterned-by-printing” approach there is no need to be concerned about relative etch rates of the material layers or solvent compatibility when printing multiple material layers. Device isolation and vias between layers of conductor are accomplished by simple changes in the printed pattern. Additionally, by careful consideration of the process flow, device interfaces can be controlled and device defects mitigated. For illustration, we will provide example architectures that are easily achievable by “patterned-by-printing” and that are, in many cases, correspondingly difficult to achieve through subtractive processing methods. Data from a variety of ZnO transistor geometries will be presented, including top gate and bottom gate transistors, as well as information on circuits using these transistors.
We will briefly review the fundamentals of the “patterned-by-printing” approach and provide a high level overview of our current understanding of the process space, including process time and temperature. We will use this as background for the discussion of the potential breadth of the “patterned-by-printing” design space.

Complementary TFT inverter technology, requiring both n-channel and p-channel TFTs, is an essential building block for realizing integrated logic circuits for large-area or flexible electronics with low power consumption. In the past decade, n-channel oxide-based TFTs have been demonstrated with many attractive properties, such as high carrier mobility, high transparency, and low-temperature process compatibility. However, to develop p-channel oxide-based TFTs with comparable performance via low-temperature process is still a challenge.
In the past few years, p-channel oxide TFTs based on tin monoxide (SnO) have made a great progress. SnO TFTs with high hole mobility, good reliability in air ambient, small transfer hysteresis, and low-temperature process compatibility (<185°C) have been demonstrated. However, up to the present time, only few inverters based on SnO TFTs have been implemented.
In this work, we report a complementary oxide-based TFT inverter comprising a p-channel SnO TFT and an n-channel ZnO TFT fabricated at a maximum process temperature of 250°C. The SnO TFT exhibits a threshold voltage of 3.5 V, hole field-effect mobility of 0.33 cm²/V-s, sub-threshold swing of 2.5 V/dec and an on/off current ratio of ~10³. The corresponding values for the ZnO TFT are 6.22 V, 3.5 cm²/V-s, 350 mV/decade, and >10⁶. The inverter has balanced noise margins and shows a dc gain of ~17 at a supply voltage of 10 V, which is larger than that of the complementary TFT inverters using p-channel SnO TFTs reported previously. The relatively high gain may be attributed to the improvement in the sub-threshold swing and on/off ratio of the p-type SnO TFT.

ZnO a typical n-type semiconductor has been applied to various electrical and optical applications such as gas sensors, piezoelectric transducers, phosphors, transparent conductors, and varistors. ZnO-based varistors have been widely used for voltage stabilization or transient surge suppression in electronic circuits and electric power systems. Excellent non-ohmic current-voltage characteristics are attributed to grain boundary properties in relation to phase formation in sintering and dopant-induced defects. The bulk trap levels and interfacial state levels formed by adsorbed oxygen and transition ion dopants are the key to the clarification of the improvement of ZnO varistor properties. Both ZnO chip varistor and TVS diode have the same function to protect the device from overvoltage or surge. However the ESD (electrostatic discharge) withstand capability, performance, and reliability of ZnO chip varistor is not good than TVS diode. A lot of efforts to overcome these inferior characters in ZnO chip varistor are still required.
In the present study, to find out the factors improving the performance of the chip varistor while varying the temperature, voltage, and frequency has been studied using capacitance-voltage characteristics (dc bias of 0~15 V, frequency of 1 kHz~1 MHz), admittance and impedance spectroscopy (20~480#8451;, 100 Hz~10 MHz, zero bias), and I-V characteristics analysis. The specimens doped with Cr₂O₃, Co₃O₄, La₂O₃, and CaCO₃ were prepared by a typical chip manufacturing process to make 1005 mm chip sintered at 1150#8451; for 2 hours in air. As a result, the breakdown voltage of the chip (Vn) of ~12 V, non-linear coefficient (α) of 14, and the dielectric constant (1 MHz) of 509 were calculated with excellent ESD properties. At room temperature, Z&’&’ and M&’&’ peaks increasing the applied voltage were shifted to a high frequency regions and the grain boundary capacitance measured with applied voltage and temperature decreased from 60 pF to 72 pF. And also donor concentration, barrier voltage, interface state density, and depletion layer width were calculated with temperature. On the basis of these various results, we will discuss the factors of improving the ESD withstand capability of the ZnO chip varistors.

In commercial scale application of aqueous solution fabricated transparent electrode, it is critical to improve its polycrystalline film conductance without hurting transparency. Here we demonstrated that grain boundary modification results in dramatically high Hall mobility, which strengthen AZO thin film conductance without any transparency compromise like increasing film thickness or carrier concentration. This grain boundary modification includes grain boundary density decrease and electron traps passivation at grain boundaries, which are accomplished by ultraviolet laser crystallization of the nanocrystals-ink. This technique enables a low cost method to produce AZO films with outstanding structural and optoelectronics properties, suggesting a potential for large-scale manufacturing. The AZO film exhibits low resistivity of 1×10^-3 Omega; cm, high mobility of 18.1 cm² V^-1 s^-1, and low sheet resistance of 75 Omega;/sq. The high transmittance (T) of 88%-96% @550nm is obtained. The sheet resistance and transparency are compared with those in other techniques. The performance in this work is the best among all the solution based deposition and comparable to vacuumed based deposition techniques. In addition, HAZE measurement shows AZO film scattering transmittance as low as 1.8%, which is superior over most transparent electrode alternatives. This technique of grain boundary modification and physical analysis of polycrystalline film inner scattering minimization can be applied to other polycrystalline metal oxide thin films.

ZnO is a n-type semiconductor extensively studied because of its optical and electrical properties. Its applications involve the development of gas sensors, solar cells, light emitting diodes and transistors. The suitable doping of ZnO with Al can cause modifications on its electric conductivity and optical transparency. Successive ionic layer adsorption and reaction (SILAR) is a simple and flexible method that offers an easy way to dope films. Here, we show preliminary results on the deposition of ZnO:Al thin films by the SILAR method which is implemented with an automatized system. The growing of the films was performed by sequentially immersing glass and SiO2/Si substrates in water at temperatures close to the boiling point, a precursor reaction solution, water at room temperature and ultrasonic water bath. The precursor reaction solution was prepared with solutions of Zn(CH₃COO)₂ 1 M, Al(NO₃)₃ 1M (3, 5 and 7%) and a mixture of TEA-NH₄OH. The resulting ZnO:Al films were transparent and well adhered to the substrates. From X-ray diffraction analysis was determined that the ZnO:Al films have hexagonal wurtzite structure with a preferential orientation along the c-axis. Changes in the morphology of the films were obtained: spike-shaped aggregates for the undoped ZnO films and spherical-shaped aggregates for the ZnO:Al films. The optical transparency of about 80% and wide optical bandgap (3.25-3.28) suggest the application of the ZnO:Al films in transparent semiconductor devices. Thin film transistors were fabricated with ZnO:Al films as active layers which showed a transfer characteristic with good saturation.

We have observed emissions of blue and white light of wide range of intensities from ZnO nanorods under high DC [1] and pulsed electrical stress. These nanorods have been grown on 3 different substrates - fluorine doped tin oxide (FTO) glass, poly-crystalline silicon and bulk p-type silicon. The measured optical spectra show fine and consistent lines corresponding to various atomic electron transitions (AETs), mostly in the blue range. In certain cases, a broad spectrum white light is observed. The non-linear DC I-V characteristics suggest dielectric breakdown of air and ZnO, sublimation of ZnO and subsequently initiation of a plasma. The characteristics of the emitted light exhibit strong dependency on substrate choice and pulse parameters. With a probe separation of ~20 µm, the breakdown has been triggered even with a pulse duration of 1 µs on FTO glass substrate while it took at least 100 ms to do that on other two substrates. Moreover, it was possible to control the light emission from the nanorods on FTO glass by adjusting the pulse amplitudes and durations. Longer pulses (of several milliseconds) with amplitudes above 20 V on this substrate lead to high intensity blue-white light and cause severe damage of the tungsten probe tips, indicating a high gas temperature associated with a thermal plasma. In contrast, shorter pulses (1-100 µs) with moderate amplitude (20 - 50 V) result in low intensity blue light emission on this substrate comprising AETs of excited neutral zinc atoms, and cause minimal damage of the probe tips suggesting existence of non-thermal equilibrium state inherent in cold plasmas.
References
[1] L. Lucera, L. Adnane, K. Cil, V. Manthina, A. Agrios, H. Silva and A. Gokirmak, "Light emission from electrically stressed ZnO nanorods," Bulletin of the American Physical Society, vol. 57, 2012.

This presentation will describe results recently obtained with pulsed electron beam deposition (PED) of ZnO on c-plane sapphire and silicon (100) substrates. The PED technique is potentially useful for growth of wide bandgap materials at lower substrate temperatures, a capability that can allow use of new buffer layer materials, introduction of chemically dissimilar lattice-matched materials, help solve wafer bowing and cracking problems during growth, and allow for the high energy deposition of materials that are transparent to standard laser wavelengths. The introduction of this technique could lead to improvements in device quality and fabrication of vertical device structures. In this study, ZnO thin films were deposited on sapphire and Si (100) at substrate temperatures of 300°, 500°, and 700°C in an Ar (7 mTorr) atmosphere. A high power electron gun pulse (Neocera Inc.) was used to ablate the ZnO target (1” dia. x 0.250” thick, 99.99% pure) stationed at 5 cm vertical distance from the substrate. The electron pulses were generated at 12KV, 0.3 J/pulse at 1 Hz for initial few nm of growth, and then increased to a 3 Hz pulse rate. Higher pulse rates (5 and 7 Hz,) were explored to study the impact of pulse rate on the film properties. Scanning electron microcopy (SEM), x-ray diffraction (XRD), Rutherford backscattering (RBS), photoluminescence (PL) and Hall Effect measurement were performed to evaluate structural, morphological, optical and electrical properties. SEM imaging confirmed a rough surface morphology with the presence of 30 nm to 100 nm scaled ZnO crystallites (for the ZnO/Sapphire samples), while smaller but more coalesced crystallites of 30-70 nm size were observed for ZnO/Si (100) samples. We observed increased thickness of deposited film for increment in substrate temperature (52, 94, and 120 nm for 300°, 500°, and 700°C growth temperature, respectively, for the ZnO/sapphire samples), for a fixed number of pulses in a specific chamber pressure. XRD theta;-2theta; scans from 2theta; = 0° to 2theta; = 70° on the ZnO on sapphire showed only two other peaks near 2theta; = 34.3°, besides the peaks from the sapphire. The peaks consist of a stronger peak at 34.320° and a much weaker peak at 35.951° corresponding to the (0002) and (10-11) orientations for ZnO, respectively. XRD for the ZnO on Si (100) samples showed of a strong peak at 2theta; = 34.402° corresponding to polar ZnO (0002) plane and a very weak peak at 2theta; = 61.67° corresponding to ZnO (103) plane, besides the Si substrate peaks. Room temperature PL measurements showed near band edge (NBE) emission in the region of 3.3 eV. Hall measurements confirmed growth of n-type ZnO films with mobility in the range of 10-15 cm²/V-sec. These results demonstrate that PED-grown ZnO is highly c-axis oriented and has the potential for the initial growth of ZnO on various substrate materials for initial buffer layer growth for future optoelectronic and device applications.

ZnO is a wide band-gap semiconductor with a number of outstanding properties which are promising for many applications. In spite of obvious advantages, an industrial application of ZnO is still suppressed by the absence of large diameter ZnO wafers at the market. Several ways of attack could be explored to overcome this hurdle. One consists in further development of ZnO crystal growth technology to produce ZnO wafers of large diameter. Another strategy aims at a development of methods providing deposition of ZnO onto the cheap Si substrates of large diameter. Fabrication of ZnO-on-Si structures will make possible direct integration of ZnO based devices with Si electronic circuitry on the same substrate. Direct deposition of ZnO on the surface of Si substrate is not a trivial task, because ZnO and Si have different types of crystal lattices and different thermal expansion coefficients which lead to high strain between the Si substrate and ZnO layer. To decrease the strain the mesoporous silicon (PS) buffer layer can be used.
In this paper we restrict ourselves to the main technological problems that were solved on the way from making an idea to use PS as buffer layer to the pilot line experiments. We show that with respect to cost and material minimization it is very important to deposit ZnO materials onto the Si substrate by cheap wet chemical and/or electrochemical techniques. To deposit ZnO onto the Si wafers three wet techniques were investigated: (a) electrochemical; (b) hydrothermal chemical; (c) chemical displacement of Al sacrificial layer to ZnO.
We show that ZnO films of good uniformity, adhesion and appropriate quality can be deposited by electrochemical method onto the PS buffer layer. The fabricated deposits were highly textured polycrystalline ZnO films with an intensive visible luminescence. The influence of process parameters on the characteristics and morphology of the ZnO deposits is analysed.
The features of hydrothermal chemical deposition that allowed to obtain arrays of high-quality ZnO nanocrystals are presented. Deposited ZnO nanocrystals are characterized by an intensive UV photoluminescence at room temperature.
Chemical growth of thin nanostructured ZnO films using Al sacrificial layer is also presented. Fabricated ZnO films are shown to have polycrystalline nature, consisting of particles with sizes from few to several nanometers, having stoichiometric composition and demonstrating strong photoluminescence in a near-UV region. They show high adhesion to the substrate.
Possible applications of ZnO materials deposited onto Si with PS buffer layer are discussed.
This work is part of the Belarus Government Research Programs “Electronics and Photonics” and “Nanomaterials and Nanotechnologies” and is financially supported by the Grants No 1.1.03 and 1.09.

ZnO nanostructures are typically grown via Chemical Vapor Deposition at temperatures higher than 400 °C or in aqueous solution between 70 and 100 °C. Application of ZnO in efficient energy harvesting requires to lower the associated growth costs. We propose a reliable aqueous solution and a vapor process to grow ZnO nanostructures at 50 °C. A 1:1 molar aqueous solution of Zinc Nitrate Hexahydrate and Hexamethylenetetramine at a temperature of 75 °C was used to grow nanowires with a typical diameter of 50 nm and a length of 300 nm. The vapor method uses the same aqueous solution as a precursor for the reactant, while the sample temperature is maintained at 50 °C. In this case two dimensional nanoplatelets were grown with a diameter less than 750 nm, and a length of 20 nm. By varying the temperature of the aqueous solution, either nanowires, interwoven nanoplatelets, or single nanoplatelets can be grown. Only single nanoplatelets have been observed in the vapor growth method. The substrate used for the seeding layer was an Al doped ZnO thin film deposited on glass or crystalline Silicon via physical vapor deposition. The grown nanoplatelets appear to be the first stage growth of large diameter nanowires, but are limited in the growth of the C-crystal plane due to the available concentration of reactants and the limited thermal energy of the solution. Scanning electron microscope images show the same hexagonal structure between the nanowires and nanoplatelets, and the transmission electron microscopy (TEM) diffraction patterns and X-Ray diffraction (XRD) patterns of the two structures confirms they share a Wurtzite crystal structure. Analysis of the lattice parameters via TEM and XRD, transmissivity via a spectroscope, and conductivity using contact conductive atomic force microscopy for single nanoplatelets, nanowires, and interwoven nanoplatelets has been studied. The structures have large bandgap of approximately 3.2-3.3 eV depending upon the growth conditions. Both the nanoplatelets and the nanowires present a very low resistivity found as low as 2x10^-4 ohm/cm and a very high transitivity in the ultraviolet/visible range to be approximately 85-91%. We propose a mechanism of the growth of the nanoplatelets and the aspect ratio dependence of the transmissivity. Integration of the ZnO nanostructures as Transparent Conductive Oxide (TCO) with hybrid solar cell and thermoelectric devices shows a very low defect density at the interface of the inorganic material and the ZnO nanostructures, resulting in a promising low cost solution for energy harvesting systems.

Gd-doped ZnO thin films were prepared by pulsed laser deposition technique with Gd concentrations varying from 0.0224 at% to 0.225 at% and at different oxygen pressures (5 - 500 mTorr). We demonstrate that room temperature ferromagnetism can be controlled by background oxygen pressure during deposition. Only ZnO films with low Gd dopant concentrations (<0.12 at%) prepared at low oxygen pressure (<25 mTorr) exhibited ferromagnetic behaviour at room temperature. These samples display negative-magnetoresistance and electric transport properties; we propose that the exchange is mediated by a spin-split defect band, formed by localized defects related to oxygen deficiency (such as oxygen vacancies), near the Fermi level. The density of localized carriers at those defect states were estimated by variable range of hopping conduction. SQUID magnetometry and in-situ vacuum annealing X-ray magnetic circular dichroism studies show that the ferromagnetic exchange could be mediated by complexes of these defects with Gd³⁺ ions. Furthermore, anisotropic magnetization loops at room temperature and variation of c-lattice parameter with Gd doping suggest that the exchange is strongly influenced by inter-atomic distance. From our findings, we propose that low Gd concentrations (le;0.0224 at%), a high density of oxygen deficiency related defects and localized carriers are prerequisites to achieve room temperature ferromagnetism in Gd-doped ZnO thin films. The elucidation of such controlled synthesis criteria permits the design of Gd-doped ZnO as a practically usable diluted magnetic semiconductor for spintronic applications.

We present studies of the role of defect structure, and the oxygen ambience dependence on the room temperature Photoluminescence and Magnetization of Band gap engineered Thin films of Mg_0.05Zn_0.95O deposited on quartz and sapphire substrates by pulse laser deposition. The structure and surface morphology of the films were investigated by x-ray diffraction and our focused ion beam/SEM Nanolab set up. The band gap emission for the films grown on sapphire and quartz substrates was measured as 357.5 and 357.4 nm respectively. Furthermore, Mg doped ZnO films exhibit a blue shift as compared to undoped ZnO (~368 nm) on both types of substrates. This could be a contribution arising from Mg incorporation into ZnO since Mg increases the ZnO band gap. In addition, a dominant broad blue defect band centered around ~ 385 nm is found to dcrease in intensity as the oxygen pressure in the ambience during deposition is increased. There are weaker green and red bands that are due to oxygen and zinc vacancies, respectively. A comparison of the differences due to the sapphire and quartz substrates observed as the oxygen pressure increases for the films grown will be discussed. The band edge disappears completely for the film deposited in 89.5% O_pp indicating the dependence of band crystal quality compared to sapphire samples in agreement with XRD results. The magnetic properties of the films were measured at room temperature. The saturation magnetization (Ms) of MgZnO films on sapphire and quartz substrates increased from 0.23-0.86 and 0.1-0.37 emu/cc respectively for oxygen partial pressures from 74-89.5%. However, an abrupt change in the values of Ms has been observed in 89.5% of O_pp for both types of films. These results indicating the sensitivity of the film properties and the substrate effects will be discussed. Finally the effect of nitrogen ambience during deposition on the blue band will also be presented.

Metal-oxide (MO) semiconductors have emerged as the next generation thin film electronics, owing to their high carrier mobility, even in the amorphous state, large-area uniformity, low-cost, optical transparency, which can be utilized for flat-panel displays, sensor arrays, and flexible circuitry. Impressive progresses in solution-processed MO electronics have been made using several methodologies including “sol-gel”, “DUV” irradiation, pre-formed nanostructures, and “combustion synthesis”. However, due to the gaseous production during the condensation and densification, high-quality solution-processing MO films with technologically-relevant thicknesses achievable in one step still be a challenge. Therefore, the thickness has to be controlled thin enough to form high quality oxide films. While referring to MO TFTs application in circuits and active-matrix display backplanes, a technologically relevant oxide semiconductor thickness is about 50 nm or even thicker. Here we report a new route to high performance solution-processed MO electronics with thickness control by using spray-coating deposition technique, which demonstrates for the first time indium gallium zinc oxide (IGZO) thin-film transistors with semiconductor thicknesses, electron mobilities, stability, and process conditions compatible with conventional sputtering deposition operations.

Spintronics is an emerging field where not only charge but spin degree of freedom of the electrons or holes can be utilized. Diluted magnetic semiconductors are potential candidate for Spintronics applications. Wideband gap semiconductors, like ZnO and GaN, are predicted to have Curie temperature above room temperature, so diluted magnetic semiconductors based on these materials can be readily used for practical applications.
There are many studies on the thin film growth of magnetic transition metal Co, Mn, Cr, doped ZnO reporting ferromagnetic behavior in these materials. The origin of ferromagnetism in these materials is controversial because ferromagnetism can originate either from ZnO based dilute magnetic semiconductor or from magnetic nano-clusters of transition metal compounds.
Here we are reporting the thin film growth of Ti doped ZnO. Titanium oxide clusters are not know to be ferromagnetic.
We deposited Ti doped ZnO by RF and DC magnetron sputtering. Growth was performed at different Ti/Zn flux ratio ranging from 3% to 20%. After the growth surface morphology was observed by atomic force microscopy. Energy dispersive x-ray spectroscopy was performed to quantify the presence of Ti in the film. We also performed photospectrometry of the grown films to study any change in the bandgap as a function of Ti concentration in the film. There is a slight change in the band gap with the increase in the Ti concentration. We also performed X-ray diffraction to determine the crystallinity of the film. We found all the films are highly crystalline and found no secondary phases of Ti_xO_y. We have also observed that a shift in the ZnO peak with the increase in the Ti concentration indicating change in the lattice constant. Effect of Ti/Zn flux ratio on the magnetic properties of the films will be presented.
This work was supported by deanship of scientific research at KFUPM through internal research grant IN111035.

Zinc oxide, a n-type semiconductor with wide band gap ~ 3.34 eV and large (60 meV) exciton binding energy has applications in blue light emitting diodes (LED), UV lasers and solar cells. Cation alloying at Zn-sites are used to tailor device specific properties, such as Ga or Al for high n-type conductivity in solar cells and Mg and Cd for wide spectrum LED. Anion doping by substituting O-atoms with chalcogens (S, Se) is relatively more important since it significantly alters electronic and optical properties of ZnO due to electronegativity and ionic radii differences relative to oxygen. Anions (S) at O-sites create gap-states and strong valence band offset bowing due to overlapping of Zn-S like bonds. This could facilitate p-type ZnO by shallow acceptor doping which would be an important milestone in futuristic transparent electronics based on p-n junction components. Only limited studies on S-alloyed by sputtering of ZnS in O₂ or by laser ablation of ZnO-ZnS target in small composition range are reported. Major challenge is ZnO - Zn(S) phase stability and secondary phase prevention. In this work, we report on the formation of ZnO_1-xS_x by sulfurization of ZnO and investigate structural phases, O-substitution kinetics and optical properties in ZnO converted into ZnO_1-xS_x and bilayer ZnO_1-xS_x/ZnO structural forms.
ZnO_1-xS_x and ZnO_1-xS_x/ZnO bilayer structures were synthesized by surface conversion of ZnO thin films by S radicals derived from pyrolysis of di-tert-butyl disulfide (TBDS). Chemical vapor sulfurization of sputter deposited 35-75 nm thick ZnO film is done at 400°C under 100 sccm flow of TBDS vapor preheated to 100°C. S radical by diffusion in ZnO substitute at O-sites to produce S-alloyed layer ZnO_1-xS_x at the surface. Controlled anion (S) doping was achieved by TBDS flow rate and diffusion time to achieve S-alloying with x=0.1-0.9. X-ray diffraction study at variable sulfurization times show systematic diminishing of ZnO(002) and evolution of ZnS(111) diffraction and at x=0.9, only ZnS peaks confirming S-diffusive-alloying of ZnO. S-diffusion profile was established by x-ray photoelectron spectroscopy and ion etching across depth of ZnO_1-xS_x (x=0.9) film which is modeled to expound S-diffusion kinetics. Raman spectra of ZnO_1-xS_x films show distinctive changes compared with ZnO and ZnS spectra and interpreted for structural phase by S-substitution at O-sites. Band gap of ZnO_1-xS_x and ZnO_1-xS_x/ZnO bilayer structures was studied by optical absorption spectroscopy and a systematic shift of absorption edge with varying x in ZnO_1-xS_x was observed. Using Tauc relation for direct band gap and absorption coefficient, decrease in optical band gap from ZnO value 3.21 eV to 2.4 eV was determined consistent with variation in x in the time evolution of the S-alloying process from ZnO to ZnO_1-xS_x. This paper will report the results and interpretations of these investigations.

Amorphous oxide semiconductor (AOS) thin film transistors (TFTs) are candidates for the next generation of silicon TFTs. The manufacturing process of AOS TFTs is not different from the amorphous silicon (a-Si) TFTs. The AOS TFTs provide high mobility (>10cm²/Vs, larger than the conventional a-Si TFTs with a low mobility of 1cm².Vs), great uniformity and visible light permeability. Moreover the device fabrication has the advantage which can be applied to the transparent flexible display in the low temperature. Since most of the proposed uses of the a-IGZO TFT will expose the TFT to a backlight or ambient light during operation, the stability of a-IGZO TFTs under light illumination is critical.
The photosensitivity issue caused by the backlight or ambient light illumination, particularly, ultraviolet (UV) light has hindered a-IGZO TFTs from being more applicable for transparent electronic devices. It was reported that the threshold voltage would shift to negative direction under negative bias illumination stress (NBIS) with UV-light, since the photo-induced holes are either trapped at the interface between gate insulator (GI) and active layer or injected into the gate dielectric bulk.
Therefore, we approached two types of method conditions. In the first instance, employing the gate dielectric/channel interface engineering in the bottom-gate, RF-sputtered a-IGZO TFTs. a-IGZO is grown under low oxygen partial pressure condition to minimize the damage from highly energetic oxygen anion bombardment into the substrate during sputtering. Because high oxygen patial pressure is employed during the bulk growth of active layer to reduce the oxygen vacancy related defects in active film.
Second, The plasma treatment has been employed to enhance the film quality, suppress the back-channel current and reduce the metal-semiconductor contact resistance. For practical application, it is necessary for oxide transistors to exhibit a fine reliability, suffering from the stability phenomena known as gate-bias stress and visible-light illumination.
In this paper, we propose the fabrication of high performance a-IGZO TFTs by low oxygen partial pressure condition and post treatment.

Device physics concerning about amorphous oxide-based thin-film transistors (AOS TFTs) has been tremendously investigated in order to analyze various electrical and optical properties and to improve device performance of the TFTs. To detect the internal potential distribution in the channel region of TFTs, gated-four-probe method has been widely used not only for conventional amorphous silicon TFTs but for oxide-based TFTs. By increasing the number of probe electrode, additional electrical and physical properties can be studied. Here, we demonstrate the novel gated-multi-probing method to detect the electrical properties using amorphous indium-gallium-zinc-oxide (a-IGZO) TFTs which is the representative oxide-based TFTs. By comparing conventional gated-four-probe method, there are several advantages for the gated-multi-probe method. First, mapping a channel potential can be possible as a function of the position. This enables the studied of channel potential variations depending on gate-to-source (V_GS) and drain-to-source (V_DS) voltages similar with scanning Kelvin probe microscope. Second, the effect of back electrode can be studied for different probe electrode. We found that additional probe electrodes induce channel shortening effect depending on the workfunction probe electrode, which improves effective field-effect mobility. All the experimental results were compared with ATLAS (from Sivaco Inc.) simulation results.

We have measured and simulated the unity power gain f_max of ZnO thin-film transistors (TFTs) in order to design transistor oscillators that will robustly operate above f_t. These TFTs are to be integrated into hybrid systems combining large-area electronics and CMOS ICs. Previously demonstrated systems based on a-Si TFTs and CMOS have enabled applications in energy harvesting, wireless sensing, and communication [1]. MHz-frequency oscillators based on resonant circuit topologies are key components in such systems because they permit operation at frequencies above the f_t of low-mobility a-Si TFTs. Transistor oscillators can operate above f_t so long as the transistors can provide power gain that is greater than unity, i.e., before the f_max frequency is reached. Hence, transistor f_max is a critical performance metric that must be evaluated to determine the high-frequency potential of a TFT technology.
Our TFTs are based on plasma-enhanced atomic layer deposition (PEALD) ZnO [2], and have mobilities >10cm²/Vs, V_th = 3V, and <200mV/decade subthreshold slope. TFTs on glass substrates employ a 100nm thick Cr gate metal, 32nm PEALD Al₂O₃ gate dielectric, 10nm PEALD ZnO active layer, 28nm PEALD Al₂O₃ back-channel passivation, and 100nm Ti/150nm Au source and drain metal. F_max was extracted directly from the 0dB crossing of the maximum available gain, which was determined from S-parameter measurements. Fabricated devices with W/L ratios of 600um/30um and gate-source/drain overlaps Xov = 15um yield f_max values of 930kHz, in excellent agreement with predictive models we developed with extracted device parameters.
There are several parasitic factors that limit f_max. Analytically, f_max = f_t/radic;((2π*f_t*Cgd*Rg) + Rg/ro), where Cgd ~ W*(L+2*Xov), Rg is the gate resistance, and ro is the output resistance. Because our ZnO TFTs have ro values >100kohms, the Cgd*Rg term becomes the dominant parasitic to minimize to boost f_max. To address the high Cgd resulting from 15um overlaps, we developed a self-aligned process that reduces Xov to 0.7um [3]. To reduce high Rg values (~3kohm) in our TFTs, we replaced the 100nm thick Cr gate metal with a 30nm Cr/ 50nm Al/ 20nm Cr stack. Self-aligned devices with W/L = 600um/30um and Rg = 700ohms have measured f_max values of 5MHz, a dramatic 5X improvement over our Cr gate, non-self aligned devices. In a further effort to reduce Cgd, shorter-channel (400um/20um) self-aligned devices with Rg = 925ohms were made, which exhibit f_max values of 6.8MHz, an additional improvement.
F_max can be further improved by shrinking the channel length and reducing Rg with higher conductivity metals, thicker gates, and/or multifinger layouts. Oscillator work is also ongoing--to date, PEALD ZnO TFTs have yielded LC oscillators that operate >3MHz.
[1] Y. Hu et al., VLSI Symp., June 2013 [2] T. Moy et al, DRC, June 2014. [3] D. A. Mourey et al., IEEE TED, Feb. 2010. [4] Y. Afsar et al, ITC, Jan. 2014.

Cu_xO, a semi-transparent metal oxide, has become popular due to its promising electrical and optical properties. Especially, due to their p-type electrical conductivities, Cu₂O and Cu_xO phases of this material have been employed as p-channel layer in thin film transistors. In addition, use of this material has also been demonstrated in optical applications including metal-semiconductor-metal (MSM), and p-n heterojunction photodetectors. Though being investigated intensively in several applications, the thermistor properties of this material has not been reported up to date. In this study, we investigate the temperature dependent resistivity of Cu_xO, for the first time. In addition, structural properties, as well as optical characteristics of this material have been studied, in a detailed manner.
Resistors with interdigitated contact structures are fabricated on thermally-grown oxide-coated silicon wafer. Fabrication starts with the standard RCA cleaning of the substrate. Next a 22-nm-thick Cu_xO layer is sputtered at room temperature. This layer is patterned via optical lithography and BCl₃ based dry etching. Ohmic contact formation is achieved via the sputtering and lift-off of the Ni/Au metal stack. Chemical compositions and bonding states are determined by X-ray photoelectron spectroscopy (XPS) using Thermo Scientific K-Alpha spectrometer with a monochromatized Al Kα X-ray source. In order to obtain crystal properties, grazing-incidence X-ray diffraction (GIXRD) measurements are carried out in a PANalytical X'Pert PRO MRD diffractometerusing Cu Kα radiation.
Structural properties obtained by using the linear profile analysis (LPA), in the XRD spectra of the Cu_xO thin films reveal the polycrystalline nature of as-deposited layers. As the result of the elemental characterization, performed with the XPS using depth profile, % 43.78 Oxygen, % 51.5 Copper, and % 1.13 Silicon are detected in the film. Temperature-dependent resistance values are obtained on resistor structures formed on such layers at temperature levels ranging from 15#730;C to 35#730;C. Obtained results demonstrate that the temperature coefficient of resistance (TCR) of this material is obtained to be as high as -4%/K. In conclusion, with its promising electrical and structural properties, Cu_xO can be a strong candidate to be used in next generation uncooled microbolometers.

Nickel Oxide (NiO) thin film material has useful characteristics (e.g., wide energy band gap, high work function, and high transmittance) and has become of interest in many areas such as sensors, solar cells, antiferromagnetic layers, LEDs, and oxygen evolution catalysts. In solar cell applications, the aforementioned material properties are very critical for both optical buffer layer and hole transport/electron blocking (HT/EB) layer functions. The former function can enhance the trapping of impinging light, and the latter enables selective photo-generated carrier collection at the device anode. Many techniques are employed to deposit NiO thin films including pulsed laser deposition (PLD), sputtering, evaporation, sol-gel deposition, chemical bath deposition, and metal organic chemical vapor deposition (MOCVD). In addition, a new deposition approach using atomic layer deposition (ALD) has rapidly emerged due to its attributes of precise thickness control, relatively low process temperature, stoichiometry control, and conformality on irregular and complex surface topographies.
In this report, we present characterization results for 20nm ALD NiO thin films deposited by applying alternating pulses of Nickel Alkyl Amidinate (Ni-amd) and water at 200C. The as-deposited and annealed films were characterized using UV-Vis spectrometry, XPS, XRD, AFM, spectroscopic ellipsometry, and FESEM. Annealing was performed on a hotplate at 435C in air, and was found to significantly modify transmittance and resistivity. The transmittance of an annealed film was about 20% better than that of the as-deposited film. The electrical conductivity was also enhanced dramatically after the annealing step. This latter, surprising behavior is very different from that of post annealed NiO thin films deposited by reactive sputtering. For that deposition process, both optical transmittance and electrical resistivity are reported to increase with annealing. We will present our characterization data as well as a model based on those results to explain these interesting differences between ALD and reactively sputtered NiO.

Thin films with a combination of high electrical conductivity and optical transparency are crucial for many modern devices such as touch panels, organic light-emitting diodes, liquid-crystal displays, or solar cells. In this respect, polycrystalline fluorine-doped SnO₂ (FTO) thin films have lately received increasing interest due to their very good electrical and optical properties as well as interesting thermal and chemical stability. In this work, the interplay between structural, electrical and optical properties of FTO thin films grown by ultrasonic spray pyrolysis is investigated with a special emphasis on texture and electron scattering mechanisms [1-3]. It is found that texture transitions from <110> or <101> to <100> and <301> crystallographic orientations occur as film thickness is increased [1] or growth temperature is raised [2]. The texture effects are described in the framework of the thermodynamic approach considering the minimization of total free energy as the driving force for grain growth and texture maps are deduced as a function of elastic strain, film thickness and surface energy [1,2]. It is further shown that the predominant electron scattering mechanisms include ionized impurity scattering as well as grain and twin boundary scattering [3]. Importantly, in order to enhance the Haze factor/light scattering of these FTO thin films, we additionally demonstrate the simple and low-cost fabrication of ZnO/FTO nanocomposite thin films by integrating ZnO nanoparticles (NPs) in FTO thin films [4]. By readily modulating their initial concentration, these nanocomposites exhibit controllable high haze factor from 0.4 to 64.2% in the visible range, while maintaining low sheet resistance (i.e., ~10 Omega;/sq) and high average optical transmittance (i.e., >80%) [4]. The physical properties of these nanocomposites are discussed in detail and related to their structural properties.
[1] V. Consonni et al., J. Appl. Phys. 111, 033523 (2012).
[2] V. Consonni et al., Acta Materialia 61, 22 (2013).
[3] G. Rey et al., J. Appl. Phys. 114, 183713 (2013).
[4] G. Giusti et al., submitted (2014).

The development of new chemical sensors made from semiconductor nanostructures has attracted great interest in recent years. It is expected that these devices may be able to detect pollutant gases at ppm, and even ppb levels, with high sensitivity, selectivity and response speed in order to satisfy a wide range of requirements in several areas. SnO₂ is the most studied and best known gas sensing material, however the gas sensor properties of tin oxides with unusual stoichiometries have been reported only recently. In this work, the chemical sensor performance of tin oxide nanobelts with different stoichiometries (SnO₂, SnO and Sn₃O₄), synthesized by a carbothermal reduction method, is presented. Analysis by X-ray diffraction (DRX), field emission scanning electron microscopy (FEG-SEM) and transmission electron microscopy (TEM) showed that these nanomaterials are single-crystalline with homogeneous size along the growth direction. The results also indicated that SnO₂, SnO and Sn₃O₄ nanobelts grow in the tetragonal, orthorhombic and triclinic structures, respectively. Gas sensor measurements showed that the sensor based on Sn₃O₄ nanobelts exhibits the highest sensor response to 50 ppm NO₂ at 200 °C with an approximately 155-fold increase in electrical resistance. Moreover, at this operating temperature, Sn₃O₄ nanobelts were found to display the highest selectivity to NO₂ relative to CO while SnO nanobelts exhibited the highest selectivity to NO₂ relative to H₂ and CH₄. We discuss the unusual surface electronic structure of these novel tin oxides that may contribute to their enhanced sensitivity. Overall, these results show that tin oxide semiconducting nanomaterials, with the unusual oxidation states of SnO and Sn₃O₄, show great promise as alternatives to SnO₂ for use in high performance gas sensor devices.

We report on the electrical properties of SnO_x thin films made by reactive sputtering of a Sn metal target on glass and r-plane sapphire substrates. We investigated the electrical properties of SnO_x films as a function of the oxygen pressure and the growth temperature. The mobility of the SnO_x films on glass substrates was as high as 16.1 cm²/Vs after post-deposition annealing at 400 C. Its carrier density was 2.91 × 10¹⁸ cm^-3. By x-ray diffraction we have found that the films are mixture of SnO and SnO₂ phases, suggesting possibility of further enhancement of the electrical properties. We will present our efforts to produce SnO₂ single phase films for higher mobility as well as the performance of their field effect transistors using the atomic-layer-deposited Al₂O₃ as the gate oxide.

Recently, it was found that La-doped BaSnO₃ (BLSO) has high electrical mobility at room temperature and excellent oxygen stability even at high temperatures. By taking advantage of its atomically flat surface, we demonstrated field effect transistors, the most fundamental device for oxide electronics. We fabricated an n-type accumulation-mode field effect transistor on undoped BSO buffer layers on SrTiO₃ (STO) substrates using 0.14% La-doped BSO as the semiconducting channel and amorphous Al₂O₃ as the gate insulator, grown by pulsed laser deposition (PLD) and atomic layer deposition (ALD), respectively. 4% La-doped BSO was used as the source and the drain electrodes and Sn:In₂O₃ (ITO) as the gate electrode. At room temperature, we achieved the field effect mobility value of 14.73 cm²/Vs and the on/off ratio value higher than 10⁵ for V_DS = 1 V. These values are consistent with the known material parameters of BLSO and Al₂O₃, providing further evidence for the excellent surface quality of BLSO and its stability.

Epitaxial thin films of NiFe₂Oshy;₄ (NFO), CoFe₂O₄ (CFO) and NiCo₂O₄ (NCO) grown by direct liquid injection chemical vapor deposition technique (DLI-CVD)¹_shy; are studied by Electron Magnetic Circular Dichroism (EMCD)² technique to analyze the site specific quantitative spin and orbital magnetic moments. EMCD is analogous to the widely known X-ray magnetic circular dichroism (XMCD) performed using synchrotron source. XMCD allows one to extract the individual spin and orbital magnetic moments by using the sum rules first proposed by Thole and Carra et al.³ The EMCD technique is a recently developed novel technique in a transmission electron microscope to study magnetism in materials without the availability of spin polarized electrons. This technique is based on the principle of simultaneous momentum transfer occurring at two different locations on the Thales circle where the momentum transfer is perpendicular to each other from the π/2 phase shifted direct and diffracted beams generated by the crystal planes. This technique so far has been utilized to determine the magnetic orbital and spin moments ratio values (m_L/m_S) using the sum rules⁴ but individual components determination is not reported yet. Individual magnetic moments calculation needs evaluation of the parameter ‘K&’ in the EMCD sum rule equation that depends on various experimental conditions like the sample thickness, orientation, detector positions, excitation error etc.⁴ We have attempted to determine ‘K&’ parameter for a given sample thickness and reciprocal lattice vector (g) to show for the first time that dichroic signal over individual L₃and L₂ edges varies with the corresponding change in the ‘K&’ parameter. We also present the role of particular momentum transfer direction on the imbalance of dichroic signal on the respective L₃ and L₂ edges and thereby demonstrate the difficulty in quantifying magnetic moments using the EMCD sum rules. Possible approaches to overcome this difficulty are also outlined.
References:
¹ N. Li, Y. H. A. Wang, M. N. Iliev, T. M. Klein, A. Gupta, Chemical Vapor deposition 17 (2011) 261
² P. Schattschneider, S. Rubino, C. Hébert, J. Rusz, J. Kunes, P. Novák, E. Carlino, M. Fabrizioli, G. Panaccione, G. Rossi, Detection of magnetic Circular Dichroism using a Transmission Electron Microscope, Nature 441 (2006) 486
³ B. T. Thole, P. Carra, F. Sette, G. V. Laan, X-ray Circular Dichroism as a probe of Orbital Magnetization, Physical Review Letters 68 (12) (1992) 1943
⁴ J. Rusz, S. Rubino, P. Schattschneider, First-principles theory of chiral dichroism in electron microscopy applied to 3d ferromagnets, Physical Review B 75 (21) (2007) 214425

Solution-processed metal oxide thin-film transistors (TFTs) have been widely studied for flexible, large-area, printed-electronics applications. The research effort has focused on improving the semiconductor materials, devising dielectric material formulations with optimized structures, and on using low-temperature processing in order to achieve compatibility with plastic substrates. For the successful realization of printed or plastic electronics, high quality dielectrics, deposited via a solution-based process, are also required with desirable requirements for gate insulators include large permittivities, low leakage current densities, an optimized structure with smooth interfaces and also the possibility of low temperature processing. In this work, inorganic nitrate salts such as zirconium oxynitrate and aluminum nitrate were used to form ZrO₂ and Al₂O₃ oxide dielectric layers below 150 °C utilizing the decomposition property of nitrate under UV-assisted thermal annealing. The dielectric constants for ZrO₂ is 14.6 at 1 kHz, which is within the range of previously reported values. In the case of Al₂O₃, the dielectric value of our sample is 7.04 at 1 kHz, which is at the lower end range of the literature values. Using ellipsometric analysis, the averaged void concentrations are estimated to be about 15% for the ZrO dielectrics and 25% for the Al₂O₃ dielectrics. The leakage current densities of ZrO₂ and Al₂O₃ dielectrics are less than about 10^-9 A/cm² at 1 MV/cm and are comparable to those formed by annealing at high temperature. To test the performance of formed oxide dielectric layer, solution-based ZnO TFT was fabricated using the ZrO₂ gate insulator. This device could be operated at less than 3 V and exhibited excellent electrical characteristics with a mobility of 1.37 cm²/V.s and an off-current density of 10^-12 A/cm². Moreover, since the highest process temperature used was around 150 °C, our device fabrication process is compatible with plastic substrates. These results augur very well for the commercial development of metal oxide thin film transistors.

The major challenge in current ultraviolet light emitting diode (UV LED) technology employing group III nitride based semiconductors is the reduction of dislocation densities which reduce radiative recombination efficiencies [1]. Low defect density single crystalline wide band gap semiconductor thin films are required for use in UV-LEDs, but so far, no material system has been developed which meets this criterion. Therefore, this work investigates the use of Ni_xMg_shy;1-xO alloys, which are closely lattice matched to inexpensive MgO substrates. NiO is an inherently p-type direct band gap semiconductor with a band gap energy of 3.7 eV which can be readily tuned by alloying with MgO to form homogeneous Ni_xMg_shy;1-xO solid solutions [2] reaching direct band gaps around 4.8 eV (260 nm) appropriate for the application of UV light emitters in water disinfection.
Compositionally homogeneous, single crystalline Ni_xMg_shy;1-xO thin films with 0 < x < 1 were grown epitaxially on MgO substrates by pulsed laser deposition. High resolution X-ray diffraction 3D reciprocal space maps of the Ni_xMg_shy;1-xO (200) diffraction peak confirmed the high crystalline quality of all Ni_xMg_shy;1-xO thin films as well as the absence of any strain gradients or composition gradients within the specimens. The PLD-prepared Ni_xMg_shy;1-xO thin films showed atomically smooth surfaces with a mean-square surface roughness in the order of 5 Å, making them well suited for integration in nano-structured multilayer devices.
UV-visible absorption measurements indicated strong optical band gap bowing for Ni_xMg_shy;1-xO thin films, despite the close lattice matching of NiO and MgO. A linear increase in band gap from 3.7 eV to 4.5 eV for pure NiO up to Ni_0.23Mg_shy;0.77O. The band gap increases sharply up to the MgO band gap for alloys with higher MgO contents. This indicates that Ni_xMg_shy;1-xO alloys with appropriate band gaps for device applications can be grown successfully.
The electrical resistivity rises steeply from 10³ Omega; cm for pure NiO to 10⁹ Omega; cm for Ni_0.41Mg_shy;0.59O, indicating a reduction in the densities of (or compensation of) native point defects including Ni³⁺ cations. The achievement of highly resistive Ni_xMg_shy;1-xO alloy films is an essential starting point for future device applications.
We also report results from X-ray photoelectron spectroscopy studies of Ni_xMg_shy;1-xO thin films and from density functional theory calculations to offer further insight into the physics of this semiconductor alloy system.
[1] M. Kneissl. Advances in group III-nitride-based deep UV light-emitting diode technology. Semicond. Sci. Technol. 26, 014036 (2011).
[2] J. W. Mares, R. C. Boutwell, M. Wei, A. Scheurer and W. V. Schoenfeld. Deep-ultraviolet photodetectors from epitaxially grown Ni_xMg_1-xO. Appl. Phys. Lett.97, 161113 (2010)

In the last decades, oxide semiconductors at micro and nanoscale with controlled sizes and well-defined shapes have received a special attention in the area of materials science because of their technological applications in catalysts, optical devices, magnetorecording, energy storages, and sensors. In terms of morphological studies, the great challenge found by scientists is the development of general synthetic approaches capable to control, manipulate and stabilize in aqueous medium, the wide diversity of architectures presented by the inorganic materials. Particularly, these scientific aspects have the strategic interest of the electro-optical industries for the improvement or development of optical devices of the next generation, for example: light-emitting diodes, scintillators, lamps, solid state lasers and displays. Based on this purpose, the barium tungstate (BaWO₄) can be considered a promising candidate because of their blue and green luminescence emissions. In our study, the influence of different reaction times (from 0 h to 24 h) was investigated in the growth processes and optical properties of BaWO₄ powders synthesized by the rapid injection-based route in aqueous solution at room temperature and 90 °C. The X-ray diffraction patterns indicated that all samples have a scheelite-type tetragonal structure with a high degree of preferred orientation along the (200) crystallographic plane. In addition, the Raman spectra revealed the presence of intense and well-defined vibrational bands, suggesting a matrix structurally ordered at short-range. The scanning electron microscopy micrographs showed that the powders obtained at room temperature resulted in the formation of rice-like microparticles, while those synthesized at 90 °C with different reaction times promoted the origin of spine-like microstructures. In these microstructures, the existence of a spontaneous self-organization of nanocrystals in a common crystallographic orientation suggests a growth process governed by oriented-attachment mechanism. The increase of reaction time in the synthesis performed at 90 °C also resulted in a surface densification of these microstructures, indicating the occurrence of a matter transport phenomenon. Differences in the band gap energies were also verified by means of ultraviolet visible spectra and related to the modifications in the distribution and organization of intermediary energy levels within the forbidden region. These energy levels are arising from the high concentration of structural defects into the matrix. According to the particle shape evolution, the photoluminescence spectra demonstrated a significant shift of the maximum emission band from blue to yellow in the visible electromagnetic spectrum. In this case, the morphological change can be considered a key parameter to control the final PL response of BaWO₄ phase.

Platinum catalyst assisted tungsten trioxide (Pt/WO₃) is well known as a gasochromic material. Gasochromism means a reversible color change phenomenon by exposing to reactive gas. Actually, Pt/WO₃ is colored to blue usuing hydrogen gas and expected as a visible hydrogen gas leakage sensor. Protons and electrons arose on loaded Pt particles worked as electrical carrier by injecting to WO₃ lattice when Pt/WO₃ was exposed to hydrogen gas. From this mechanism, Pt/WO₃ can detect hydrogen gas. And, protons and electrons move to surface and contact to oxygen gas in air when the atmospheric condition changed to air. Generally, WO₃ hydrogen gas sensor is fabricated as polycrystalline thin film on glass substrate and Pt particles are deposited on the film surface. These thin film sensors take long time to recover from blue color to transparent. Therefore, it is difficult to use at room temperature. Thus, we have to use Pt/WO₃ hydrogen sensors around 200 - 300 0C.
In this study, we developed the WO₃ single crystal hydrogen gas sensor with Pt electrode. The WO₃ single crystals were prepared by sublimation method. And, Pt electrodes were deposited on a part of WO₃ crystal surface by DC sputtering. The Pt electrode also worked as a catalyst for gasochromism. The single crystals obtained were approximately 0.5 mm in size, and the crystal phase showed orthorhombic from X-ray diffraction pattern. The electrical conductivity in hydrogen gas increased 100 times compared with the atmospheric condition.
The highly important point was that the conductivity reached equilibrium state within 3 seconds at room temperature. It means that the WO₃ single crystal hydrogen gas sensor shows very quick response and recovery. On the other hand, Pt/WO₃ thin film prepared by sol-gel process showed the large electrical conductivity and quick response by exposing to hydrogen gas. However, the recovery of the conductivity by exposing to dry air was very slow.
From these results, WO₃ single crystal is promising as novel hydrogen gas sensor compared with commercial sensors at room temperature.

In this study, very thin layers of TiO₂ nanoparticle films were deposited on glass and ITO coated glass substrates by sputtering method. These films were sputtered by different RF powers from 60 W up to 120 W, for one and two successive runs, while the substrate temperature was adjusted to 200 ⁰C.
In order to determine the diffraction patterns, the phases and orientations of the deposited TiO₂ films, XRD measurements were used. The films coated on glass substrates all showed amorphous behavior while the films sputtered on ITO coated glass substrates showed polycrystalline natures with high intensity diffraction angle 2Θ about 35°. The thicknesses of the samples were measured by using a Dektak 6M thickness profilometer, which shows that while increasing the RF power cause an incensement in thickness of the films, the second run time does not have any significant change on the thickness of the produced film compared to one run time TiO₂ sputtered films for the same RF powers. Optical measurements of the deposited TiO₂ films, which were determined to be n-type by hot-probe technique, exhibited direct band-gap energy of around 3.4-3.5 eV with absorption coefficient in the order of 10⁵ at room temperature.

Among the metal-oxide nanostructures, doped titanium dioxide (TiO2) nanostructures have been widely investigated because of its unique physicochemical properties [1, 2]. A facile and environmentally friendly synthesis approach for the production of vanadium doped titanium dioxide (V_xTi_1minus;xO₂) nanostructures was demonstrated via hydrothermal decomposition of vanadium and titanium peroxo-complexes. The effect of vanadium addition on the structural and morphological properties of V_xTi_1minus;xO₂ (x=0, 5, 10, 20 mol %) nanostructures was investigated by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and electron microscopy techniques. XRD analysis showed that all V_xTi_1minus;xO₂ samples presented only the TiO₂ anatase crystalline phase and, despite the different amounts of vanadium ions, the single crystalline nature was preserved. Transmission electron microscopy (TEM) shows that increasing V contents resulted in morphological evolution, from anisotropic to isotropic structures. EELS and XAS results demonstrated that the nature and the distributions of defects strongly depend on the concentrations of the dopants. In fact the dopants are localized mainly on crystal surface by STEM-EELS done with atomic resolution. XAS measurements at the K-edges (for V and Ti) revealed that V ions occupy the Ti ⁴⁺ -site, which confirms the doping effect. Furthermore, high-angle annular dark-field (HAADF) imaging, combined with EELS mapping, indicated that the vanadium ions were homogeneously distributed in the structure without any kind of segregation. These morphological and compositional modifications upon vanadium addition led to evolution of the TiO2 optical properties. The as-obtained nanostructures also were found to be active for photocatalytic degradation of Methilene Blue (MB) solution, under visible-light irradiation (lambda;>420 nm).
[1] T. M. Inerbaev, J. D. Hoefelmeyer and D. S. Kilin, Journal of Physical Chemistry C 117 (2013), p. 9673.
[2] W. Kim, T. Tachikawa, H. Kim, N. Lakshminarasimhan, P. Murugan, H. Park, T. Majima and W. Choi, Applied Catalysis B-Environmental 147 (2014), p. 642.

Bi₂Ti₂O₇ (BTO) is a photocatalyst with a band gap of 2.8eV, and its catalytic activity can be enhanced by cation manipulations. Recently, iron doped BTO (Fe-BTO) has been synthesized by using a surfactant assisted wet chemical reverse micelle-based approach. Upon activation by visible light, a peaked H₂ evolution was seen around 1% Fe concentration and then declined at higher concentration. Subsequent observation discovered coexistence of Fe₂O₃ at Fe loading around 2%. We sought to understand the fundamental reason for the occurrence of this phase and how to prevent this secondary phase from forming. The theoretical model determined that Fe substitution to be more stable at Bi sites and identified the doping configurations which lead to the formation of the secondary phase (Fe₂O₃). In addition, by analyzing free energy landscape, a suitable chemical environment has been predicted for the synthesis of single phase Fe-BTO.

Although perovskite oxides are a promising material system for solar energy conversion applications due to their visible band gaps, chemical stability, abundance, non-toxicity, and tunability, there is a lack of basic understanding regarding carrier dynamics in these materials despite the critical role that such an understanding plays. Ultrafast spectroscopy can rectify this limited understanding by providing a direct method to measure photoexcited carrier dynamics.
In this work, ultrafast pump-probe transient reflectance (TR) spectroscopy was used to quantify lifetimes and clarify recombination mechanisms in epitaxial perovskite oxide thin films of LaFeO₃ (LFO) grown by molecular beam epitaxy on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (LSAT). TR measures the change in reflectance, #8710;R, with time following photoexcitation with an optical pump. Because the reflectance is modulated by changes in free carrier density, TR is an ideal technique to track the relaxation of photoexcited carriers in epitaxial thin films on the femtosecond to nanosecond time scales relevant to carrier recombination in semiconductors. Recombination mechanisms were clarified by manipulating the pump fluence to control the free carrier density and by comparing kinetics at different energetic transitions,
This work reveals two negative transients in reflectance with local maxima at ~2.5 eV and ~3.5 eV which correspond to two optical transitions in LFO as determined by ellipsometry; the former being the band gap transition. The kinetics at these transients were best fit with an exponential decay model with fast (5 - 40 ps), medium (~200 ps), and slow (3 ns) components that we attribute mainly to recombination of photoexcited carriers. The fast component (5-40 ps) decreased with increasing pump fluence, indicating that carrier-carrier interactions are important, as is the case with Auger and radiative recombination. The medium and slow time constants were both essentially independent with pump power and probe energy, suggesting that these time constants could represent Shockley-Read-Hall and/or surface recombination. Regardless of the mechanisms, more than 10% of carriers remain for longer than 3 ns. This work establishes the approach of using TR to quantify carrier dynamics in thin (< 20 nm) epitaxial oxide films and will enable future studies that can probe the effects of strain, thickness, cation ordering, and the presence of interfaces on carrier lifetimes and recombination mechanisms in epitaxial perovskite films. The ability to use these strategies accessible in oxide heterostructures to engineer ultrafast carrier dynamics in perovskite films may provide new means for enhancing their applicability as photovoltaic and photocatalytic materials.

Vanadium dioxide (VO₂) has been heavily researched in recent years due to its metal-insulator transition near room temperature, which is accompanied by unique changes in optical and electronic properties (e.g. large change in the electrical conductivity and optical transmission in the infrared). These unique properties allow VO₂ to be tailored for use in various smart material applications such as sensors and switches. A detailed study on the growth of VO₂ by oxygen plasma-assisted pulsed laser deposition and radio frequency reactive sputter deposition is presented with a focus on the optimization, control, and correlation of its structural, optical, and electronic properties. These properties can then be exploited for technological applications such as smart windows by integrating VO₂ with other materials into specific structures (e.g. multilayer stacks).

Despite remaining questions regarding the nature of the transition, the metal-semiconductor transition in VO₂ at 68 C was one the first and best-studied correlated electron systems and has been applied as electronic switches, thermochromic smart windows, and bolometers in infrared cameras. In this paper, we report the depression of the transition temperature by transition metal substitution in the range of < 1 % to 15 % for the vanadium. The transitions in the polycrystalline thin films were characterized by near infrared reflectance over temperatures from 5 C to 85 C from 1 mm² spot. While the technique is neither fast enough nor sensitive enough to distinguish the electronic and structural phase transitions, the switch in near infrared reflectance is of particular interest for smart window or bolometer applications, and provides a rapid, low-cost, and easily interpretable probe for the transition. The films studied were of the general composition V_1-xM_xO₂, with M = W, Nb, Mo, Hf, or another transition metal, and x varying continuously across the 76.2 mm diameter silicon substrate. These ‘combinatorial&’ thin film libraries permitted a high-throughput experiment in which > 100 unique film compositions could be probed for the phase transition in a single one day experiment. We find that W, Nb, or Mo substitutions at less than 15 % depress the VO₂ transition temperature to less than 40 C. Higher impurity rates can stabilize a crystalline phase structure that does not undergo a phase transition in the investigated temperature range. Other transition metal substitutions, such as Hf, do not depress the transition temperature. High throughput x-ray diffraction of these libraries reveal that while the (011) VO₂ peak is shifted to higher d-spacings by incorporation of metals that depress the transition (W, Nb, Mo), it is unchanged by Hf.

We demonstrate a three terminal zinc oxide biosensor on flexible substrates for the detection of target agents in human sweat. Pulsed laser deposition method was used to deposit zinc oxide thin films on flexible substrates. Electrodes were patterned and fabricated to form a zinc oxide device on sensor substrates. A gate-source voltage (V_GS) specific to this sensor configuration was applied to ensure the formation of the conduction channel between the source to drain at the interface of the zinc oxide and sensor substrate. ZnO surface was functionalized with Dithiobis succinimidyl propionate (DSP) linker and α-cortisol antibody to setup the biosensor immunoassay. Current flow between source and drain was modulated by binding of the target agent α-cortisol antibody immobilized onto the ZnO surface and through the flexible substrate. As a control, we performed testing with α-cortisol without DSP linker. When human sweat containing target agent (cortisol) comes in contact with the ZnO surface, an electrical change in charges at the ZnO-solution interface was observed. This was as a result of the binding of target agent to the antibody at the ZnO-solution interface. This change due to the binding of specific concentrations of cortisol was subsequently characterized by the current flow between source and drain. Specificity of cortisol binding was validated using electrochemical impedance spectroscopy (EIS). DC measurements were performed on this device in the presence of varying gate voltages. AC measurements were carried out to evaluate the C-V characteristics of the sensor device. The molecular binding events at the ZnO-solution interface were validated through EIS. Further, we performed experiments to demonstrate the changes in (I_DS) for different concentrations of cortisol for calibration of the sensor over the clinically relevant concentration range of 8 ng/mL - 140 ng/mL.

SrTiO₃ is an important component for oxide based two dimensional (2D) free electron gas systems [1,2,3]. It is also frequently used as a substrate for strain engineered dielectric and multiferroic materials (i.e. at least two of ferroelectric, ferromagnetic and ferroelastic phases co-exist) and high temperature superconductors. For these applications, the crystal quality is critical. It has been shown that edge dislocations in SrTiO3 destroy interfacial conductivity of the 2D free electron gas system in a 5 nm radius around them [4]. Commercially available substrates lack the required quality. They typically have etch pit densities in the 10⁵ (optical floating zone method) to 10⁶ cm^-2 (Verneuil method) range.
Crystal quality can be improved by growing from stoichiometric [5] or non-stoichiometric melts [6]. To optimize such growth methods, it is important to understand the high temperature optical properties since these determine most of the heat transport through the crystal during growth.
For that reason, the optical absorption edge and IR absorption of SrTiO₃ were measured at temperatures from 4 to 1703 K. The absorption edge decreases from 3.2 eV at room temperature to 1.9 eV at 1703 K and the extrapolated value at the melting point (2350 K) is 1.3 eV. The IR transmission decreases rapidly above 1400 K and it is about 50% of the room temperature value at 1675K. This absorption is caused mainly by free carriers. These are generated by thermal excitation of electrons over the band gap. Estimates of the free carrier density and absorption based on the temperature dependent optical gap are in good qualitative agreement but predict the rise in absorption at a much lower temperature. The red shift of the optical absorption edge and the rising free carrier absorption impede radiative heat transport through the crystal, which can cause growth instabilities or spiral formation and affect the design of SrTiO3 based devices operating at elevated temperatures.
[1] A. Ohtomo, D. A. Muller, J. L. Grazul, and H. Y. Hwang, Nature419 (2002) 378-380.
[2] A. Ohtomo and H. Y. Hwang, Nature427 (2004) 423-426.
[3] Y. Hotta, T. Susaki, and H. Y. Hwang, Phys. Rev. Lett. 99 (2007) 236805.
[4] S. Thiel, C. W. Schneider, L. Fitting Kourkoutis, D. A. Muller, N. Reyren, A. D. Caviglia, S. Gariglio, J.-M. Triscone, and J. Mannhart, , Phys. Rev. Lett.102 (2009) 046809.
[5] V. Aleksandrov, M. Vishnyakova, Y. Voron'ko, V. Kalabukhova, E. Lomonova, V. Myzina, and V. V. Osiko, Izv. Akad. Nauk SSSR19 (1983) 104-107
[6] Guguschev, C.; Klimm, D.; Langhans, F.; Galazka, Z.; Kok, D.; Juda, U. & Uecker, R, Cryst.Eng.Com 16 (2014) 1735-1740.

Transparent thin-film transistors composed of wide-bandgap p-type NiO were fabricated by a scalable reactive RF sputtering method using an inexpensive metallic Ni target.
Thin-film transistors (TFTs) composed of metallic oxides have long been attracting much interest.#12288;In addition, transparent conducting oxide (TCO) films are widely used in flat-panel displays, solar cells, and touch panels. Most of the current research activities have been focused on n-type TFT and n-type TCO because of their controllability of transparency, resistivity, and mobility. To realize complementary circuits, both n-type and p-type transistors are required. However, fabrication of high-performance p-type oxide semiconductors and their p-type transistors is still a challenging task.
Binary NiO shows a p-type conductivity with a bandgap of 3.7 eV. Both Ni and O are cheaper and less toxic than any other element that constitutes semiconductor materials. In addition, both elements are safe to handle and do not pollute the environment. Nowadays, NiO thin films have been obtained by various methods, including sputtering, pulsed laser deposition, oxidation of Ni film, and chemical deposition. Among these methods, sputtering is the most suitable for economically depositing large-area films with well-controlled compositions. There have been few reports to date on a p-type NiO TFT, although some works have reported on TFTs using p-type oxides, such as copper oxide and tin monoxide. In this meeting, we will present “invisible” thin-film transistors composed of wide-bandgap p-type NiO fabricated using RF sputtering and an inexpensive metallic Ni target.
Approximately 300-nm-thick NiO thin films were deposited ^[1] on an insulating SiO₂ layer and several conducting substrates (back-gate channel) by reactive RF sputtering without the intentional substrate heating. The deposition was carried out at a pressure of 3.8 Pa. The O₂ fraction in the sputtering gas was 0.5%. The source and drain electrodes (50-nm-thick Au) were deposited onto the sample through a metal mask.
The fabricated NiO-based p-channel TFT was operated using p-type NiO, which shows an optical transmittance of ge;70% and a carrier concentration of 10¹⁶ cm^-3. The gate capacitance decreased with decreasing the negative gate voltage (10 - 0V). By using NiO, we may be able to easily fabricate invisible and scalable TFTs using conventional RF sputtering equipment and cheap Ni metal targets.
[1] M. Warasawa, Y. Watanabe, J. Ishida, Y. Murata, S. F. Chichibu, and M. Sugiyama, Jpn. J. Appl. Phys. 52 (2013) 021102.

We have grown high quality non-polar m-plane ZnO/ (Zn,Mg)O quantum well (QW) heterostructures on ZnO substrates. The photoluminescence of the structures display narrow and well resolved lines from the QW emission. They are identified as the free exciton line, X, and the trion singlet ground state line, X_S. We have performed the optical spectroscopy of the structures. Under external magnetic field, we could identify for the first time in ZnO QWs, the first excited states of the trions, X_T, the first state having 2 electrons ferromagnetically aligned. We will report on the electronic structure and properties of the singlet, X_S, and triplet states, X_T.
In addition, ZnO QW heterostructures containing a thin (Zn,Mg,Mn)O spin aligner were grown, the ZnO QW acting as a spin detector. We will show that there is no possibility to analyze the spin injection in nonpolar ZnO QWs by studying the circular polarization of the emitted light, as traditionally done, for example, in spin LEDs. Instead, we propose to monitor the spin injection using the X_T/X_S intensity ratio. For 50 nm thick spin aligners containing 2% of Mn, this ratio is found to increase by a factor of ~20 at low fields, which demonstrates a successful spin injection in ZnO QW.

As suggested by their name, transparent conducting oxides provide the technologically desirable combination of nearly metallic conductivity with a high degree of optical transparency over the entire visible spectrum. Despite being so widespread in applications ranging from electronics to sensors and photovoltaics, the fundamental properties of these materials remain quite poorly understood. In order to explore their suitability for novel applications, e.g. in the context of plasmonic materials, a thorough understanding of their optical properties is necessary. For TCOs some of the open questions can be traced back to the intricate interplay of free carriers (free electrons in the case of In₂O₃ and Ga₂O₃) and excitonic effects.
Here, in order to understand their optical properties, we shed light on excitonic effects and important features of the optical absorption spectra of ideal, undoped In₂O₃ and Ga₃O₃. We use modern theoretical-spectroscopy techniques based on the solution of the Bethe-Salpeter equation for the optical polarization function to compute the dielectric functions of these materials over a large photon energy range. This allows us to achieve an accurate description of optical properties (including excitonic and local-field effects) from first principles.
Our ab-initio calculations clearly point to a strong influence of excitonic and local-field effects on the complex frequency-dependent dielectric functions. We find that optical features near the absorption onset can be attributed to bound excitonic states. In addition, we predict the absorption spectra across a large photon energy range and discuss the strong optical anisotropy of Ga₂O₃ both at low as well as high photon energies.

Ga₂O₃ and In₂O₃ are n-type oxide semiconductors used in gas sensors and in their highly doped variety as transparent contacts. Beyond these rather conventional applications, Ga₂O₃ is a potential material for high-performance UV sensors and power electronics. The band gaps of In₂O₃ (2.8eV) and beta-Ga₂O₃ (4.8eV) provide the potential for band-gap engineering by alloying and even the formation of heterostructures. For conventional III-V and III-N semiconductors molecular beam epitaxy (MBE) is an established method to grow high-quality layers, alloys, and heterostructures. Likewise, high-quality Ga₂O₃ and In₂O₃ has been demonstrated by MBE.
This contribution provides insight into the MBE growth kinetics of Ga₂O₃ and In₂O₃. In-situ laser reflectometry was used to measure the growth rates of both materials as a function of oxygen flux, metal flux, and growth temperature. In addition, in-situ line-of-sight quadrupole mass spectrometry was used to identfy the desorbing species from the growth surface. All these information allowed us to develop a detailed understanding of the processes at the substrate surface.
These processes include the formation of volatile Ga₂O (but not GaO) that results in decreasing growth rates above the stoichiometric O/Ga-ratio, and even in etching of an existing Ga₂O₃ film under a Ga-flux. Comparison of the growth rate (with oxygen flux present) and etching rate (without oxygen flux) perfectly matches the chemical reactions: 2Ga+3O -> Ga₂O₃ and 4Ga+Ga₂O₃ -> 3Ga₂O. In addition, there is evidence for an attenuated direct formation of Ga₂O from the Ga and oxygen flux.
In₂O₃, in comparison, forms easier (likely also from O-species that cannot oxidize Ga) and does not suffer from a growth-rate reduction due to suboxide formation.
Beyond the metal and oxygen flux the growth temperature influences the growth rate by desorption of the incoming species which differs for Ga and In due to their difference in vapor pressure.
Understanding the growth kinetics of the Ga₂O₃ and In₂O₃ will help controlling and understanding the growth of Ga₂O₃-In₂O₃ alloys.

The optical properties of cubic (bixbyite) In₂O₃ are under intense discussion. There is not even a consensus about the direct or indirect nature of the fundamental band gap. Here, I will present recent results obtained by spectroscopic ellipsometry from the phonon region in the far infrared up to 40 eV using several different instruments including synchrotron radiation. As samples a bulk (111) In₂O₃ crystal and several epitaxial (001) thin films on (001) yttria stabilized zirconia grown by plasma-assisted molecular beam epitaxy are investigated.
These studies yield the dielectric function of In₂O₃ and by analyzing certain peculiarities of ellipsometric data the fundamental band gap energy is found (2.75 ± 0.02 eV). The data hint towards an indirect nature of the fundamental band gap. Post-growth treatments of Sn doped heteroepitaxially grown samples leads to free electron concentrations in the region between 10¹⁷ and 10²¹cm^-3 as confirmed by Hall experiments. The optical response in the infrared spectrum yields up to 8 infrared active transverse optical phonon modes. Additionally, the free-carrier density dependent screened plasma frequency of In₂O₃:Sn is detected and the effective electron mass as a function of the doping concentration is established.
A pronounced nonparabolic curvature is found in the degenerately filled conduction band. Implications on the absorption onset on Burstein-Moss effect and band gap renormaliztaion are discussed.
In the last part, a comprehensive survey on results obtained from different metal oxide semicondcutors is presented.

Wide bandgap oxide semiconductors show in general a doping asymmetry and therefore bipolar devices can only be realized via heterojunctions. Several approaches to bipolar diodes comprising ZnO have already been investigated. Heterostructures with p-Si, p-GaAs, p-GaN have been widely studied but also all-oxide bipolar heterodiodes, e.g. p-ZnCo₂O₄/n-ZnO or p-NiO/n-ZnO with extraordinary high rectification of more than 10 orders of magnitude are feasible. In this contribution we adopt the latter approach to create bipolar heterojunctions with In₂O₃ as n-type oxide semiconductor and have achieved rectification of about 4 orders of magnitude using p-NiO or p-ZnCo₂O₄ as p-conducting layer.
Recently, the interest on In₂O₃ extends beyond application as transparent electrode and properties of semiconducting In₂O₃ thin films were reported [1,2]. Transport measurements, IR reflectivity in combination with photoemission studies revealed that In₂O₃ tends to form a surface electron accumulation layer [3,4]. This high free electron concentration at the surface makes the formation of rectifying contacts non-trivial. So far, an ex-situ plasma cleaning step prior to contact deposition enabled weakly rectifying Schottky barrier diodes [5]. A somewhat better rectification was recently reported for diodes for which a reactive sputtering process of the Schottky contact metal was employed [6]. However, highest rectification at room temperature is below four orders of magnitude for best diodes and not sufficient for most applications (e.g. field-effect transistors). Therefore, the exploitation of pn-heterodiodes presents an additional, interesting approach towards higher rectification.
We investigated nominally undoped In₂O₃ thin films with a compensated In₂O₃:Mg surface layer with different thickness (0 nm, about 10 nm and about 100 nm). The thin films were deposited by pulsed-laser deposition at 600°C and 0.016 mbar. On top, NiO (ZnCo₂O₄) was deposited at room temperature and 0.1 mbar (0.03 mbar) through a photolithographic mask, defining circular pillar-like pn-diodes having diameters between 150 µm and 750 µm. Arrays of such diodes were investigated by current-voltage measurements (IV) at room temperature. Best rectifications of p-NiO/n-In₂O₃ and p-ZnCo₂O₄/n-In₂O₃ are with 4 orders of magnitude better than that of best Schottky barrier diodes on In₂O₃ thin films grown by molecular beam epitaxy [6]. Further, temperature-dependent IV and the breakdown behavior of the diodes will be discussed.
¹ A. Bourlange, et al.,J. Appl. Phys. 106, (2009).
² O. Bierwagen, M.E. White, and M.Y. Tsai, Applied Physics hellip; 95, 262105 (2009).
³ P. King, T.D. Veal, D.J. Payne, and A. Bourlange, Phys. Rev. 101, 116808 (2008).
⁴ P.D.C. King et al., Phys. Rev. B 79, (2009).
⁵ O. Bierwagen, J.S. Speck, T. Nagata, and T. Chikyow, Applied Physics hellip; 98, 172101 (2011).
⁶ H. von Wenckstern, D. Splith, F. Schmidt, M. Grundmann, O. Bierwagen, and J.S. Speck, APL Mater. 2, 046104 (2014).

Hollandite-type compounds exhibit one-dimensional channels of octahedrally coordinated metal atoms, which was originally studied by mineralogists for nuclear waste immobilization. The structure also provides an intriguing target for catalysis, energy storage in batteries, and energy conversion applications. Due to mixed valence state of the octahedrally coordinated metal atoms, unique properties such as metal-insulator transition in V-based, ferromagnetism in Cr-based, and high ionic conductivity in Ti-based hollandites were observed. To harness and employ the unique properties of hollandites in electronic devices, understanding the electronic structure would be the key study for interface and device engineering.
In this work, the electronic structure of a new family of oxide semiconductors, potassium titanate hollandite series, were studied. The growth of metastable Hollandite-type K_y(M_xTi_8-x)O₁₆ (M = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) single crystals was achieved using a molten salt evaporation technique. X-ray diffraction and energy dispersive spectroscopy were employed for structural characterization and stoichiometry determination, respectively. Temperature dependent electrical properties showed insulating behavior for the whole series. X-ray photoelectron spectroscopy, and electron energy loss spectroscopy techniques were used to map the valence and conduction band density of states, respectively. Electronic structure calculations were performed and the results were matched to that of the experimental data, which allowed for band-gap and respective band alignment determination of the whole series. UV-Vis spectroscopy was employed to further confirm the optical band-gap. The results showed that for M = Sc, Cr, Fe, Co, and Ni the band-gap is above 1.2 eV with Sc having the largest gap, while for M = Ti, V, and Mn the band-gap is smaller than 1.0 eV making UV-Vis spectroscopy inefficient in determining the optical band-gap.

β-Ga₂O₃, In₂O₃, and SnO₂ are attractive transparent semiconducting oxides (TSOs) with wide bandgaps of 4.8, 2.8, and 3.5-3.8 eV, respectively. They are however, chemically unstable at elevated temperatures and tend to decompose, therefore growing such single crystals from the melt is a very challenging task. Bulk single crystals of β-Ga₂O₃ (melting point, MP=1820°C) were grown from the melt by the Czochralski method; In₂O₃ (MP=1950°C) - from the melt with the use of our novel crystal growth technique under the name Levitation-Assisted Self-Seeding Crystal Growth Method (LASSCGM); while SnO₂ (MP>2100°C) - from the gas phase by the physical vapor transport (PVT), since melt growth of SnO₂ is rather not possible due to its very high instability at high temperatures. In addition to these TSOs a novel, melt-grown wide band gap (5 eV) transparent semiconducting oxide will be introduced. This report will focus on basic aspects of crystal growth, crystal quality and electrical / optical properties of both as-grown and annealed crystals. An extensive characterization of the crystals showed that their structural quality is fully compatible with their use as substrates for epitaxial deposition. Availability of bulk TSO single crystals combined with their unique properties offers opportunities for developing novel devices, such as for high power electronics, transparent electronics and light detectors.

p-type transparent conducting oxides show remarkably lower properties in comparison to their n-type counterpart. In order to overcome this shortcoming, it is important to understand the fundamental properties of the band structure and how this is affected by the presence of defects. As a case study, we chose Cr₂O₃ and its further modification by Mg doping to assess the evolution of the electronic, electrical and optical properties and correlate them to the presence of defects. High quality epitaxial films have been deposited by molecular beam epitaxy (MBE) or pulsed laser deposition (PLD). Defects have been introduced in a controlled and selective manner and both undoped and Mg-doped films have been grown and characterized. For undoped Cr₂O₃, the energetic position and composition of both the valence band maximum and conduction band minimum has been determined by spectroscopic techniques, namely x-ray photoelectron spectroscopy (XPS), x-ray emission (XES) and x-ray absorption (XAS). Following native stoichiometric defects, were introduced and their presence was probe by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and x-ray emission (XES).
Finally doping with Mg was performed. Conductivity of the films was observed to change over several orders of magnitude depending both on the technique used for the deposition (PLD vs MBE), on the material used as a source (Cr versus Cr₂O₃) and the oxygen partial pressure. The origin of these differences was investigated by a combination of structural measurements (XRD), electrical measurements and x-ray photoelectron spectroscopy. Investigating the fundamental properties of epitaxial films will allow us to add to our understanding of the role of defects in p-type TCOs, helping to improve the growth of this class of material

Positron annihilation spectroscopy is a very powerful technique for the detection, identification and quantification of vacancy-type defects in semiconductors and oxides [1]. This statement is unfortunately rather strongly limited to cation vacancies that possess significant open volume and suitable charge (negative of neutral). On the other hand, oxygen vacancies in oxide semiconductors are a widely debated topic: 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 [2]. Small subsitutional cation defects, such as Li on Zn site in ZnO have been predicted to trap holes in polaronic states on O sites surrounding the impurity [3], but experimental detection and identification of such defect states is difficult. Interestingly, positrons are trapped at Li_Zn in ZnO [4].
Through combining state-of-the-art positron annihilation experiments and ab initio computational approaches, we show that O vacancies may be directly detected in semiconducting oxides, in cases where they are complexed with cation vacancies. Examples cover such materials systems as ZnO, In₂O₃, and SnO₂. Theoretical modeling shows that the very same defects that are predicted to exhibit polaronic hole trapping also trap positrons in an unexpected manner. We will compare and discuss the physical origins of the trapping of these positively charged particles.
[1] F. Tuomisto and I. Makkonen, Rev. Mod. Phys. 85, 1583 (2013)
[2] R. Schaub et al., Science 299, 377 (2003).
[3] S. Lany and A. Zunger, Phys. Rev. B 80, 085202 (2009)
[4] K. M. Johansen et al., Phys. Rev. B 83, 245208 (2011).

Bismuth vanadate (BiVO₄) is a champion photoanode material for water splitting, but fundamental studies of this material are lacking. Electronic transport studies are especially uncommon, likely due to difficulties in synthesizing high quality samples and measuring high resistivity, low mobility materials. Recently, we reported the synthesis of both undoped and tungsten doped BiVO₄ single crystals using the floating zone technique. We showed that electrons in this material move via thermally activated small polaron hopping transport between 250 and 400 K, and that the AC Hall mobility is low at room temperature: ~0.2 cm² V^-1 s^-1. Unlike in conventional, wide-band semiconductors, in small polaron conductors the Hall and drift mobility may be vastly different and therefore require independent verification. To achieve this, the resistivity, Seebeck coefficient and AC Hall effect were measured in these crystals as functions of temperature and orientation. Comparison with small polaron theory will yield information about the energy barriers, hopping type (adiabatic or non-adiabatic), hop geometry and any anisotropy. The goal of our study is to gain insight into the nature of small polarons in this material, as well as reporting a true drift mobility and hopping activation energy. We hope our approach will be applicable to elucidating transport in many low-mobility metal oxides.

β-NaFeO₂ structure is an orthorhombic wurtzite-derived structure, in which divalent zinc ions in wurtzite ZnO are regularly replaced by monovalent sodium and trivalent iron ions similar to the structural relationship between the zincblende and chalcopyrite structures. The oxides with this structure are very attractive materials because of their wide variety of energy band gap, such as β-LiGaO₂ (E_g=5.6 eV), β-AgAlO₂ (E_g=2.8 eV) and β-AgGaO₂ (E_g=2.2eV). The compounds containing monovalent copper usually shows isostructure with the compounds containing monovalent silver, such as delafossite α-AgInO₂ and α-CuInO₂. However the β-NaFeO₂ type copper oxides had not been reported. Recently, we successfully synthesized the β-NaFeO₂ type β-CuGaO₂ [1]. In the present paper, we report its structural, optical and electrical properties.
β-CuGaO₂ was synthesized by the ion-exchange from Na⁺ to Cu⁺ in the β-NaGaO₂ precursor that possesses β-NaFeO₂ type structure. Rietveld analysis of the obtained powder after the ion-exchange indicated that β-CuGaO₂ with the β-NaFeO₂ structure was successfully obtained. The pseudohexagonal wurtzite lattice parameters of β-CuGaO₂, a= 3.231 Å and c = 5.278 Å, are very close to those of ZnO, a =3.250 Å and c = 5.207 Å; that is, the lattice mismatch between β-CuGaO₂ and ZnO is only 0.6% in the ab-plane and 1.4% along the c-axis. This is an advantage of the β-CuGaO₂ in order to fabricate multilayered thin films with ZnO.
Optical absorption spectrum of β-CuGaO₂ indicated that its energy band gap was 1.47 eV; this is the energy band gap realizable maximum conversion efficiency of the single junction solar cell [2]. The electrical conductivity at room temperature of the β-CuGaO₂ sinter without intentional doping was very small, 1×10^-6 Scm^-1, but the thermoelectromotive force was +400 mu;VK^-1. This indicates that β-CuGaO₂ exhibits a p-type semiconductor.
The calculated electronic band structure indicated that β-CuGaO₂ is a direct semiconductor and shows intense absorption of light near the band edge. These properties render this new material promising as an absorber in solar cells.
[1] T. Omata, H. Nagatani, I. Suzuki, M. Kita, H. Yanagi, N. Ohashi, J. Am. Chem. Soc. 136, 3378(2014).
[2] W. Shockley, H. J. Queisser, J. Appl. Phys. 32, 510(1961).

The study of the dynamics of photo-excited carriers provides valuable information on the interaction mechanisms and relaxation processes. These studies immensely improve our understanding of time scales of carrier recombination, relaxation and transport in semiconductor materials and devices which lead to optimizing the operation of optoelectronic devices, more specifically, emitters and detectors. The main goal of this work is to investigate the dynamics of photo-excited carriers in ZnO. Specifically, we study the luminescence spectrum and the effect of various dephasing mechanisms on the photo-excited carrier densities. At the same time, experimental studies on ZnO samples are performed. These results were obtained in Army Reseach Laboratory using time-resolved photoluminescence method on a Zn-faced ZnO sample grown along the c-axis. In the theoretical model, interaction with a photo-excitation laser pulse is treated coherently and a generalized Monte-Carlo simulation is used to account for scattering processes. The scattering mechanisms included are carrier interactions with polar optical phonons and acoustic phonons, and carrier-carrier Coulomb interactions. The screening effects are also considered within the simulation. We consider the electronic structure of bulk ZnO as a two band model that includes a non-parabolic conduction band and a parabolic valence band which is accurate enough for the purpose of this work. The results of the normalized luminescence intensity obtained from the simulation are in very good agreement with those of experiments. Furthermore, on the theoretical side, we also present the effect of temperature on the luminescence spectra. According to these results, the luminesence rise time decreases as the temperature increases due to the stronger effect of polar optical scattering mechanism at higher temperatures. We also show the effect of various interaction mechanisms on the carrier densities at different excitation energies obtained by the simulation. These results help understand the role of interaction mechanisms in relaxing the photo-excited material.

ZnO thin films were synthesized by RF magnetron sputtering of high purity ZnO solid targets on sapphire substrates. Depositions were carried out at selected temperatures between 20⁰C and 900⁰C, and post-deposition annealing was performed at 900⁰C for 3 min in an oxygen atmosphere. Samples for electron microscopy investigations were prepared by lift-out technique in a multi beam FIB/SEM instrument. The ZnO thin film shows uniform thickness (about 1 µm), as determined by dark-field scanning transmission electron microscopy imaging. Irrespective of the deposition temperature, the ZnO thin films are polycrystalline. Individual grains have a columnar morphology with the long axis oriented perpendicular to the ZnO/sapphire interface. The grain size varies with the deposition temperature, and a direct correlation between grain size and photoluminescence have been observed. The intensity of 3.317 eV peak decreases with increasing grain size. Beside grain boundaries, dislocations have been observed in the investigated materials by atomic resolution TEM. The dislocations/photoluminescence relationship is not clear. The purpose of this study is to quantify the observed crystallographic imperfections and understand their role on the photoluminescence of undoped ZnO thin films deposited on sapphire substrates.

There is now tremendous interest in the II-VI semiconductor ZnO for next generation opto- and microelectronics. On a fundamental level, the high sensitivity of ZnO&’s properties to growth and processing provides an excellent test bed for probing physical phenomena that may be generic to semiconductors in general. Likewise, as the scope of ZnO device applications continues to expand, there is increasing interest in controlling not only its bulk but, just as important, its surface and interface properties - particularly those related to surface charge transfer and electrical contacts. We now know that oxygen and zinc vacancies, V_O and V_Zn, respectively, are present in most ZnO crystals, that they can segregate to surfaces and interfaces, that they are electrically-active, and that their densities can reach values sufficient to dominate charge transport across interfaces. Depth-resolved cathodoluminescence spectroscopy (DRCLS) and self-compensation studies in degenerate n-type Ga-doped ZnO confirm their electrical activity. ZnO defect segregation to surfaces and interfaces extends to other oxide semiconductors as well including (Mg,Zn)O alloys and the complex oxide SrTiO₃, for which the extent of interface defect segregation depends sensitively on overlayer growth conditions. Mechanisms that can drive these phenomena include surface states, piezoelectric fields, electric fields within surface space charge regions, and the free energy change across band bending regions.
Recent studies of isostructural Mg_xZn_1-xO alloys over a wide (0<x<0.56) range reveal how the energies, densities, and spatial distributions of these native point defects vary systematically. DRCLS results reveal similar segregation of both V_Zn and V_O, independent of piezoelectric fields. Both DRCLS defect densities exhibit pronounced minima at ~44% Mg corresponding to similar a and c parameter minima at ~52%. The corresponding 3.36% cell volume decrease may serve to inhibit defect formation due to electrostatic repulsion as observed in SrTiO₃ and other complex oxides. DFT calculations confirm these defect/alloy composition variations. Surface segregation decreases with decreasing native defect density to zero at the defect density minimum. We established absolute densities of defects in these materials using both positron annihilation spectroscopy and transient surface photovoltage spectroscopy, which provides a non-contact technique to measure bulk trap densities.
Applying electric fields, we found major changes in oxide defects and their distributions. Similarly, hyperspectral imaging of ZnO nanostructures revealed qualitatively the same defect segregation as in bulk ZnO crystals. Because these native point defects can be charged and mobile, the free energy associated with surfaces, interfaces, and lattice unit cell dimension can be a major factor in carrier transport and doping of these oxide semiconductors.
Supported by NSF Grant DMR-1305193 (Charles Ying and Haiyan Wang).

P-type doping in zinc oxide (ZnO) is the holy grail for the optoelectronic applications of ZnO. Though there have been numerous reports of p-type doping in ZnO, the achievements have mainly been limited to thin films though ZnO nanowires (NWs) have been major building ingredients for nanotechnology. P-type doping in ZnO NWs have been achieved by ion implantation and dopant diffusion followed by thermal annealing. However, the conventional methods are not suitable to fulfill the requirements of doping in nanoscale, conformal formation of p-type layer and precise control of dopant concentration, since ion implantation has inherent line-of-sight problem on incorporating dopants and diffusion method is difficult to control the dopant profile. Thus, depositing p-type ZnO layer is one of the most powerful approach to form p-type ZnO region in nanoscale. Chemical vapor deposition (CVD) can be a proper approach to prepare p-type ZnO layer since CVD for crystalline material is compatible with conventional semiconductor processing in wafer scale.
Here we present the epitaxial growth of phosphorus doped ZnO shell layer on the surfaces of n-type ZnO nanotubes (NTs) via metalminus;organic chemical vapor deposition (MOCVD). The vertically aligned and position controlled n-type ZnO NTs were prepared by MOCVD with the precursors of diethylzinc, oxygen, and triethylgallium. Subsequently ZnO:P layers were grown on the surfaces of ZnO NTs. After the ZnO:P shell growth the top-openings of core ZnO NTs were capped. To activate the p-type dopants thermal annealing under nitrogen atmosphere was performed. The p-type conductivity was confirmed by Hall measurements at room temperature. The doping concentration and optical characteristics of ZnO:P layer were investigated by Hall measurement and low-temperature photoluminescence spectroscopy. The ZnO:P growth technique was employed to form ZnO radial p-n junction arrays in position controlled manner. The electroluminescence of ZnO radial p-n junctions prepared by chemical vapor deposition will be also discussed.

Control of native oxygen defects in transition metal oxides like ZnO plays an important role in photovoltaic, catalytic, photonic and electronic applications. The interaction chemistry between bulk point defects and active sites on clean semiconductor surfaces is comparable in richness to the reaction of gases with surfaces. Surfaces of metal oxides can be used to manipulate the concentrations and spatial distributions of oxygen defects, particularly oxygen vacancies. Such surface-based defect engineering methods should play an especially important role in nanostructured devices where the surface to volume ratio is high. The present work discusses a surface-based mechanism for injection of O interstitials (O_i) into the bulk from the c-plane ZnO under O-rich conditions. The injected O_i subsequently annihilate O vacancies that are harmful for electronic and photonic applications. By employing a complementary combination of isotopic oxygen gas-solid exchange experiments and ab initio density functional theory calculations, we show that the clean ZnO (c-plane) polar surfaces are capable of injecting highly mobile O_i into the bulk. Measured oxygen self-diffusivities are several orders of magnitude higher than that reported in the literature so far. First principles calculations predict the preponderance of O_i under O-rich conditions. The present work also offers much more definitive experimental evidence to support this prediction. The kinetic barrier of ~1.7 eV for injection of O_i calculated using climbing-image nudged elastic band method is in good agreement with the experimentally-measured activation energy of O injection. The defect injection rate exhibits a stark contrast in its temperature-dependence on the Zn-terminated and O-terminated surfaces, suggesting differences in the nature of active sites on the polar surfaces. The tunability of defect injection rates by varying polar surface orientation allows us to develop design principles for defect engineering using polar oxide surfaces.

With a large band gap of 3.4 eV, a large exciton binding energy of 60 meV, and a wide availability of single crystals, zinc oxide has often been regarded as a promising material for light-emitting diodes and lasers. However, recent theoretical and experimental has indicated that p-type conductivity may not be achievable due to a lack of effective acceptor dopants in this material [1]. While this development seems to preclude the production of all-ZnO pn junctions, zinc oxide is still a useful electronic material for many devices. For such applications, determining which species affect electrical conductivity is still a crucial task. In this work, we examine the properties of native defects and impurities that may influence electrical conductivity in ZnO. We employ hybrid density functional calculations, which overcome the band-gap problem of traditional density functional theory and which allow for the quantitative prediction of defect transition levels, formation energies, and optical transition levels. By analyzing the stability of these defects as a function of Fermi level and chemical potential, we are able to predict which species are most likely to be present under realistic growth conditions. Finally, by predicting optical transition energies of deep defects, we are able to compare our results with experiment.
Work performed in collaboration with Anderson Janotti and Chris G. Van de Walle.
[1] J. L. Lyons, A. Janotti, and C. G. Van de Walle, “Theory and modeling of oxide semiconductors,” Semiconductors and Semimetals, volume 88 (Academic Press, 2013).

In this study, we investigated films of BaZnO alloys grown by pulsed laser deposition. Different concentrations of Ba were tried. X-Ray diffraction showed that increasing the temperature and pressure lead to better Crystalline films. Most orientations were 0001, yet 1120was observed for certain growth temperature. Optical properties, mainly transmittance, showed transparent windows of BaZnO in the visible and infrared region (lambda; > 400 nm), and for films of 10% and 25% Ba the window was wider. Ellipsometry results provided evidence of void incorporation in film with Ba of 25%. This result was verified by scanning electron microscopy as well. Moreover, the index of refraction (extracted by Cauchy's model) of samples grown at low temperature and pressure were less than ZnO; this reduction in (n) was attributed to the porous nature of films, so air contributed to the integrated index of refraction. One last observation was that films of higher Ba were thicker, and this was attributed to: 1) ablation enhancement, 2) higher sticking coefficient; and 3) voids and pores incorporation in the film. Funding was provided by the Saudi National Plan for Science and Technology (project # 10-NAN1197-02 )

We investigated impurities, defects, and associated electronic transport properties in ZnO. Possible origins of n-type onductivity in ZnO are still in debate, even though they have been long studied with experimental and theoretical points of view. One of possibilities for such difficulties is unintentional impurity doping into ZnO, in particular Li and Al, both of which are often incorporated during the crystal growth. In addition, these concentrations can be changed before/after thermal annealing. To deeply examine the effect of these impurities on the conductivity of ZnO, several ZnO crystals with different Al/Li ratios were employed in this study. Annealing them in ultrahigh vacuum and subsequent hydrogen doping in a humid environment varied dramatically the carrier concentration (N_e), especially for a quasi-stoichiometric (Al/Li~1.0) ZnO (QS-ZnO). These results indicate that N_e in the QS-ZnO can be determined as a result of Zn interstitial and hydrogen donors. We discuss such mechanism can be applicable to other ZnO crystals.

Tin monoxide (SnO) has attracted great attention due to its p-type characteristic for the applications of complementary metal oxide semiconductor, gas sensor and other transparent oxide devices. Although lots of works were devoted for the growth of p-type SnO film, physical vapor deposition methods such as sputtering, evaporation, and pulsed laser deposition were only viable techniques to obtain stoichiometric p-type SnO. [1-2] Meanwhile, for uniform and conformal growth over complex substrate, it is required to fabricate SnO film using chemical vapor deposition or atomic layer deposition (ALD). In this work, we successfully developed the ALD process for pure SnO growth in a low temperature range of 120-210 ^oC from dimethyamino-2-methyl-2propoxy-tin(II) (Sn(dmamp)shy;₂) and H₂O. The growth behavior of SnO film showed genuine ALD self-limiting reaction as a function of Sn(dmamp)₂ and H₂O pulse time. The growth per cycle of SnO films were varied from 0.03 to 0.008 nm/cycle as increasing the growth temperature from 120 to 210 ^oC. The SnO ALD exhibited excellent conformallity inside a hole pattern where an aspect ratio is 10 (opening diameter : 100 nm, depth : 1000 nm). GAXRD patterns of the films grown at 150-210 ^oC showed the characteristic diffraction peak for crystalline SnO, whereas amorphous SnO film was deposited below 150 ^oC. From room temperature hall measurement, it can be confirmed that as-deposited films showed p-type conductivity and carrier concentrations of <1.6×10¹⁸. Further detail growth characteristics and film properties analyzed by XPS, AES, and TEM will be discussed in the presentation. [1] H. Yabuta, N. Kaji, R. Hayashi, H. Kumomi, K. Nomura, T. Kamiya, M. Hirano, and H. Hosono, Appl. Phys. Lett., 97, (2010) [2] W. Guo, L. Fu, Y. Zhang, K. Zhang, L. Y. Liang, Z. M. Liu, H. T. Cao, and X. Q. Pan, Appl. Phys. Lett., 96, (2010)

Tin forms two stable oxides, a dioxide and a monoxide. Both are of great interest for applications ranging from electrochemistry to optoelectronics. Tin dioxide (SnO₂) is a wide-bandgap n-type semiconductor, while tin monoxide (SnO) is a small-bandgap p-type semiconductor with an indirect gap. SnO₂ is already fairly well-investigated both theoretically and experimentally, but SnO is far less well-examined. In the present work, we gain new insights into some of the open questions concerning SnO in particular, but also in comparison to SnO₂. We present calculations for electronic and optical properties using density functional theory (DFT). We also investigate the influence of the crystal geometry and the DFT exchange-correlation functional on the indirect and direct (optical) bandgap of SnO.

Tin oxide thin films were grown on sapphire substrates by using ion beam sputter deposition (IBSD) of a pure Sn metallic target under variation of gas mixture and substrate temperature. Schemes of the dependence of the films crystal phase on the oxygen pressure and the substrate temperature will be presented. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) served as procedures to identify composition and stoichiometry.
Tin monoxide thin films were fabricated at low temperatures and low oxygen fluxes. X-ray diffractometry and Raman scattering revealed that these films contained only SnO, whereas SnO₂ was present in films grown at higher temperatures, suggesting the decomposition of SnO. Transmittance spectroscopy revealed an indirect fundamental gap of 0.7 eV and a direct gap of 2.8 eV, respectively. Thermoelectric and Hall measurements consistently indicated p-type conductivity for the films grown at temperatures below 450°C. Morphology was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively.

Complementary metal oxide semiconductor (CMOS) is the most fundamental building block in many integrated circuits (ICs), large due to its low power consumption. In the field of transparent electronics, there is a need to develop thin film transistor (TFT) CMOS devices using transparent metal oxide semiconductor based n- and p-type TFTs. However, the application of TFT-based CMOS in ICs is limited by the incompatible processing procedures required for the n- and p-type oxide TFTs. P-type tin monoxide (SnO) and n-type tin dioxide (SnO₂) are promising candidate oxides for next generation transparent displaying technology, each of them with a decent electrical performance and also a large optical band gap and, accordingly, a good transparency in the visible range. Yet, different deposition parameters and post deposition annealing conditions lead to a complicated fabrication process to integrate these two oxides in the same device.
Here, we report a novel approach of transforming SnO to SnO₂ at a temperature as low as 190 #8451;, which facilitates the fabrication of both n- and p-type TFTs in one step. Such a low temperature was achieved using a dual active layer structure (SnO layer / capping layer). The capping layer could oxidize SnO to form n-type SnO₂ at a temperature much lower than normally required to form SnO₂ by annealing single layer of SnO in air. The performance of bilayer n-type TFT was tunable and optimized by controlling the thickness of SnO layer and also the thickness of capping layer placed on top of SnO. Finally, a transparent CMOS-like inverter is successfully built by combining the p-type SnO TFT and n-type bilayer TFT, with a maximum temperature of 190 #8451; for the post annealing process.

Electronic structure theory plays an important role as a computational tool helping to understand the phenomenology of semiconductors in terms of microscopic models, e.g., for doping, defects, interfaces, or alloy compositions. Increasingly, it is becoming instrumental for the design and discovery of novel materials. The availability of quasi-particle energy calculations in the GW approximation allows now for a fairly robust band gap prediction, which is important for an accurate prediction of defect and surface/interface properties combination with density functional theory (DFT) supercell calculations [1, 2].
In SnO₂, the group V dopants are considered as a viable alternative to fluorine doping. However, the group V elements (P, As, Sb, Bi) are multivalent and can assume both a +V and +III oxidation state, which implies an ambipolar doping character and the possibility to act as both electron donors and as compensating acceptors. The electrical activity of these dopants is determined by the energy difference between the conduction band minimum and the transition level between the two oxidation states. For the accurate prediction of this energy difference, we employed an approach that combines HSE06 hybrid functional calculations with GW quasi-particle energy calculations for both the band-edge energies and the defect state [3]. We find that Bi shows a clear deep-level behavior with Fermi level pinning inside the band gap, thereby preventing doping and leading to insulating behavior. As is a borderline case, and only P and Sb act as electron-donors without being affected by multivalency.
As an important materials class for TCO and contact materials, the II-VI family represents a remarkably diverse range of materials properties, allowing for great opportunities to optimize functional contacts specifically for the active layer, such as, e.g., the solar absorber material in photovoltaics. In order to support this design process, we studied the structural and chemical trends of the oxides and sulfides on Mg, Zn, and Cd, considering both the zinc-blende (ZB) and the rock-salt (RS) structures representing the tetrahedral and octahedral coordination environment, respectively [4]. Including the results for polymorphism, band-gaps and -structures, the band line-up relative to the vacuum level, as well as alloy mixing enthalpies, these results provide a basis for the rational design of tailored contact and electrode materials. An interesting observation is that there are counteracting contributions to the band-lineup arising from the atomic orbital energy ordering and the surface dipole, raising the question of design rules for band-offsets at non-epitaxial interfaces.
[1] H. Peng et al., Phys. Rev. B 88, 115201 (2013).
[2] V. Stevanovic et al., Phys. Chem. Chem. Phys. 16, 3706 (2014).
[3] H. Peng and S. Lany. (unpublished)
[4] S. Lany, Proc. of SPIE Vol. 8987, 89870K (2014), DOI: 10.1117/12.2043587

Computational science has changed the way in which new materials are discovered. Metal oxides have been at the forefront of the computational materials design movement, with numerous novel materials and doping schemes proposed and realised in recent years. However, bulk band structures are only a part of the story; for true holistic design of devices from materials up, it is necessary to be able to match the band energies of the relevant components. Here, we present three critical concepts allowing for the prediction, understanding and control of these levels: a) Absolute binding energies and band structures, b) Bulk, surface and interface contributions to band energies, c) Crystallite nanostructure design.
The concept of absolute band energies is crucial to the matching of energy levels across devices, nonetheless traditional periodic density functional theory (DFT) calculations - the workhorse of materials design - provide only the relative electron energy levels of a system, thus forbidding the matching of levels between device components [1]. We have developed several schemes for overcoming this limitation, which will be presented [2,3].
By applying a multi-scale embedding scheme we have been able to decouple the surface and bulk contributions to band energies [4]. This understanding of the importance of the surface contribution allows us to propose methods for the tuning of band energies (on the absolute scale) through the application of ultra-thin heteroepitaxial films [4]. Finally we will explore the possibility of manipulating energy levels in oxide materials through the controlled growth of nano-crystals. In these structures the confluence of surface effects gives rise to a rich new space of tuneable energy levels.
The ability to predict absolute energy levels, coupled with knowledge of how to exploit surface and interface effects in tuning them is the next step in rational materials design. The concepts discussed in this presentation will help to open the door to the application of metal oxides in a range of new scenarios.
References:
[1] A. Walsh and K. T. Butler; Acc. Chem. Res. 47, 364 (2013).
[2] K. T. Butler, C. H. Hendon and A. Walsh; J. Am. Chem. Soc., 136, 2703 (2014).
[3] D. O. Scanlon et al;Nat. Mater., 12, 798 (2013).
[4] K.T. Butler, J. Buckeridge, C.R.A. Catlow and A. Walsh; Phys. Rev. B, 89, 115329 (2014).

Many technologies require oxides with high electronic conductivity or mobility (e.g., transparent conducting oxides, oxide photovoltaics or photocatalysis). In this talk, we will report on a high-throughput ab initio computational search for low electron effective mass oxides.
We have screened with high-throughput computing more than 4,000 binary and ternary oxides to identify the compounds with the lowest electron effective mass. We will report on the results of this computational search, focusing on the compounds presenting exceptionally low electron effective masses. Among those, a few candidates combining large band gaps and low effective masses are highlighted as potential new n-type transparent conducting oxides (besides the already known In2O3 or ZnO). In addition, we will present chemical rules leading to low electron effective masses in oxides. Those confirm some previously known trends but also offer more surprising findings.

Transparent conducting oxides (TCOs) are an integral part to many technological applications, ranging from front electrodes in flat panel displays to window layers in solar photovoltaics. Many n-type TCOs are well known, for example, indium tin oxide and zinc oxide. However, their p-type counterparts have struggled to achieve similar conductivities. We hypothesize that this is partly due to the high effective mass of the valence band, which results in the low mobility, even in the impurity-scattering-free limit. To guide the discovery of novel materials with low hole mass, we use the result from a recent high-throughput material computation to understand strategies for delocalizing the valence band. We find that good candidates can be found by introducing metal cations with energy levels similar to the O2p, which can be understood in the context of the covalency. In this contribution, we will present the synthesis, the thin-film deposition, and the structural, optical and electrical characterization of the candidate material. Band gap and hall measurement to assess the optical transparency and carrier type will be presented. We will discuss how to further engineer the electronic structure for the design and development of future p-type TCOs.

Boron suboxide (B₆O) is a member of icosohedral boron-rich solids known for their physical hardness and stability under irradiation bombardment, but it has also recently emerged as a promising high mobility p-type transparent conducting oxide. Using a combination of hybrid functional and many-body perturbation theory calculations, we report on the electronic structure and defect properties of this material. Our calculations identify B₆O has a direct band gap in excess of 3.0 eV and possesses largely isotropic and low effective masses for both holes and electrons. Of the native defects, we identify no intrinsic origin to the reported p-type conductivity and confirm that p-type doping is not prevented by intrinsic defects such as oxygen vacancies, which we find act exclusively as neutral defects rather than hole-killing donors. We also investigate a number of common impurities and plausible dopants, finding that isolated acceptor candidates tend to yield deep states within the band gap or act instead as donors, and cannot account for p-type conductivity. Our calculations identify the only shallow acceptor candidate to be a complex consisting of interstitial H bonded to C substituting on the O site, (CH)_O. We therefore attribute the origins of p-type conductivity to these complexes formed during growth or more likely via isolated C_O which later bind with H within the crystal. Lastly, we identify Si as a plausible n-type dopant, as it favorably acts as a shallow donor and does not suffer from self-compensation as may the C-related defects. Thus in addition to the observed p-type conductivity, B₆O exhibits promise of n-type dopability if the stoichiometry and both native and extrinsic sources of compensation can be sufficiently controlled.
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.