Oxide materials are often characterized by strong electronic correlations, complex phase diagrams and competing ground states. This competition makes these materials very sensitive to external parameters. An interface, which naturally breaks inversion symmetry, is a major perturbation and one may thus expect that electronic systems with unusual properties can be generated at oxide interfaces [see reviews 1-4]. In this talk, I will discuss the fascinating interface between LaAlO₃ and SrTiO₃, two good band insulators, which was found in 2004 to be conducting [5], and, in some doping range, superconducting with a maximum critical temperature of about 200 mK [6,7]. I will describe recent experiments aiming at determining the origin of the electron gas [7]. I will then discuss, in standard samples and in nanostructures, how superconductivity [8] and spin-orbit [9] evolve as the doping level of the system is changed using the electric field effect. I will also discuss results on high mobility samples displaying Shubnikov-de-Haas oscillations that can be tuned by a gate voltage and link the observed properties to the complex sub-band electronic structure induced by the interfacial confinement of the electron gas.
[1] J. Heber, Nature 459, 28 (2009).
[2] J. Mannhart and D. Schlom, Science 327, 1607 (2010).
[3] P. Zubko, S. Gariglio, M. Gabay, P. Ghosez, and J.-M. Triscone, Annual Review : Condensed Matter Physics 2, 141 (2011).
[4] H. Y. Hwang, Y. Iwasa, M. Kawasaki, B. Keimer, N. Nagaosa, Y. Tokura, Nature Materials 11, 103 (2012).
[5] A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004).

[6] N. Reyren, S. Thiel, A. D. Caviglia, L. Fitting Kourkoutis, G. Hammerl, C. Richter, C. W. Schneider, T. Kopp, A.-S. Ruetschi, D. Jaccard, M. Gabay, D. A. Muller, J.-M. Triscone and J. Mannhart, Science 317, 1196 (2007).
[7] M.L. Reinle-Schmitt, C. Cancellieri, D. Li, D. Fontaine, S. Gariglio, M. Medarde, E. Pomjakushina, C.W. Schneider, Ph. Ghosez, J.-M. Triscone, and P.R. Willmott, Nature Communications, 3, 932 (2012).
[8] A. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart, and J.-M. Triscone, Nature 456, 624 (2008).
[9] A.D. Caviglia, M. Gabay, S. Gariglio, N. Reyren, C. Cancellieri, and J.-M. Triscone, Physical Review 104, 126803 (2010).

Epitaxial oxide heterointerface plays a crucial role to explore unusual and novel properties of oxide-based complex material systems. One of the remarkable phenomena gives the recent discovery of the formation of quasi 2-dimensional electron gas (2DEG) at the interface between two insulating materials, LaAlO₃ and SrTiO₃. This polar/non-polar interface has exhibited multifunctionality such as superconducting and magnetically-ordered ground states which originates from electronic phase separation. It was reported that the conductivity of the interface can be modulated by various external parameters such as electric field, surface adsorbates, light exposure, magnetic field, and biaxial strain. In this study, we demonstrate a chemical alloying method to tune 2DEG conductivity using epitaxial LaAlO₃/Sr_xCa_1-xTiO₃ heterostructures on SrTiO₃ substrates(0 le; x < 1) . With the Sr content in the Sr_xCa_1-xTiO₃ films, the heterostructures exhibited gradual increases in the conductivity over a range of 6 orders of magnitude. Such a tunable conductivity was attributed to the control of TiO₆ octahedral rotations by changing the A-site cation composition, suggesting the creation of new functionalities in oxide heterointerfaces via the control of octahedral rotations. Therefore, we suggest that the use of pseudosubstrates with chemical substitution or alloying is a promising route to finely tune conductivity at oxide heterointerfaces

Since the discovery of the fascinating interfacial electronic properties at the LaAlO₃/SrTiO₃ heterointerface [1], there have been intense investigations of this system. Exploiting the large dielectric constant of SrTiO₃ at low temperatures, a dramatic tunability of the interfacial conductivity has been demonstrated using external electric fields, attracting considerable attention for fundamental studies, as well as device applications [2,3]. In detail it has been shown that the change of the electron mobility dominates the conductivity change, rather than changes in the carrier density [4], suggesting qualitatively that the potential profile at the interface is strongly modulated by the gate electric field. However it is still unclear experimentally what the interface band structure in this system is, especially the potential profile as a function of depth under the application of electric fields.
In this study, we have performed a photoemission spectroscopy (PES) analysis with an applied electric field on the SrTiO₃ to directly determine the potential depth profile. The ability to vary the PES probing depth in the range 0.1 nm to 10 nm enables us to obtain information about the potential profile in the band-bending region at the interface.
By applying a negative back gate voltage at room temperature, the energy difference between the core-level spectra of LaAlO₃ (La 4d and Al 2p) and SrTiO₃ (Ti 2p and Sr 3d) increases by about 0.2 eV for soft x-ray PES (shallower probing depth), while there are no shifts for hard x-ray PES (deeper probing depth). Analysis of these data suggest the existence of an accumulation layer with thickness 2-3 nm, concomitant with an abrupt potential change close to the interface, associated with a collapse of the permittivity of SrTiO₃ by the confining electric field between LaAlO₃ and SrTiO₃.

Pulsed laser deposition (PLD) has served as one of the most common modes of materials synthesis for modern condensed matter research of complex oxide thin films. Recently, this has included the synthesis and observation of high-mobility two-dimensional electron gases at LaAlO3/SrTiO3 heterointerfaces. In this presentation, we will discuss how variations in the PLD growth process of LaAlO3 films can produce dramatic stoichiometric deviations which directly impact the electrical and thermal properties of LaAlO3/SrTiO3 interfaces. By varying the laser fluence and growth pressure, we have observed systematic variations in the cation stoichiometry of LaAlO3 films. Films of LaAlO3 from 1 to 300 nm were grown by reflection high-energy electron diffraction-assisted PLD on SrTiO3 (001) substrates at a range of oxygen pressures (1e-6-1e-3 Torr) and laser fluences (1.2-2.2 J/cm2). X-ray diffraction and atomic force microscopy reveal the ability to achieve single-phase, epitaxial films with atomically smooth and terraced surfaces in all cases. Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy indicate that the film cation stoichiometry varies dramatically as a function of laser fluence and growth pressure. These variations, however, do not readily reveal themselves in structural studies and thus make it difficult to detect unless directly probed. The observations reveal that lower and higher laser fluence promotes La-excess and La-deficiency, respectively, that can be as much as 5% off of ideal stoichiometry. The interfacial electrical conductance of LaAlO3/SrTiO3 heterointerfaces will LaAlO3 films ranging between 1 and 30 unit cells in thickness has proved to be highly sensitive to the stoichiometry of the LaAlO3 films. Varying the relative La:Al cation stoichiometry by a few atomic percent in films grown at 1e-3 Torr O2 results in 7 order-of-magnitude change in the 2K sheet resistance, with highly conducting states occurring only in La-deficient films. Further reduction of the growth pressure results in an increase of the sheet carrier density and a dramatic change in the carrier mobility indicative of a transformation from two- to three-dimensional conduction in the substrate. We will also discuss frequency dependent time-domain thermoreflectance (TDTR) measurements of the LaAlO3/ SrTiO3 heterointerfaces. Prior work has shown that thermal conductivity of these perovskite oxides is highly sensitive to material stoichiometry with deviation from bulk-like thermal properties corresponding to deviations in material chemistry. Using these TDTR measurements, we have probed the evolution of thermal conductivity in the underlying SrTiO3 layer and observe diminished thermal conductivity indicative of potential nonstoichiometry that could play a role in the observed electronic transport. This work is supported by the Department of Energy under grant number DEFG02-07ER46459.

The extremely high carrier density at the conducting interface between the Mott-insulator GdTiO₃ and the band insulator SrTiO₃ makes the system a promising candidate for information processing, power switching, and tunable plasmonic devices. Approximately ½ electron per interface unit cell, that is 3.5 × 10¹⁴ cm^-2 are distributed among a few tightly confined and several shallow, loosely bound subbands in the SrTiO₃ - this itinerant carrier density is an order of magnitude higher than is achieved in the nitrides, for example. However, the low temperature electron mobility in these MBE grown heterostructures is limited to only about 300 cm² V^-1 s^-1, in contrast to the 120,000 cm² V^-1 s^-1 that may be achieved in lightly doped SrTiO₃ grown by the same technique, and the room temperature mobility is strongly reduced due to the electron-phonon interaction. In this study, we seek to understand the relative importance of electron-electron interactions, electron-phonon interactions, and disorder in determining the interface carrier mobility. Using terahertz time-domain and FTIR spectroscopies, we characterize the carrier mass and scattering times and the mid-IR polaron absorption as a function of the thickness of SrTiO₃ quantum wells in GdTiO₃. For the electron-phonon interaction, we consider the competing effects of screening at high densities and the increase of the polaron mass enhancement with 3D to 2D crossover. We anticipate electron-electron correlations to become very important upon approaching an effective 3D carrier density of one-electron per unit-cell in the SrTiO₃, as this density is associated with the Mott transition in the rare-earth titanates. Interestingly, at the extreme limit of a single SrO plane in the GdTiO₃, we observe insulating behavior with an optical gap substantially less than that of the surrounding GdTiO₃, suggesting the existence of a novel 2D Mott insulating phase.

Polar interfaces between insulating perovskite materials have been the subject
of special attention in the past few years, following the discovery of two-dimensional
conductivity in LaAlO3/SrTiO3. In the first part of this talk I will introduce the problem by
using general concepts of macroscopic electrostatics, in the framework of the modern
theory of polarization. Based on these ideas, I will show how we can understand
the origin and the spatial distribution of the metallic electron gas in terms of a self-consistent tight-binding model, whose physical parameters are extracted from ab-initio calculations of
bulk SrTiO3. This model allows the efficient and accurate description of the carrier confinement within an arbitrary applied voltage, either within a front-gating or a back-gating configuration.
In the second part of the talk, if time permits, I will move on to more advanced topics. These include the electrical properties of capacitors based on quasi-2D electrodes (e.g. Pt/LaAlO3/SrTiO3 heterostructures), the orientation dependence of the interface polarity [e.g. (001) versus (011)], ferroelectric superlattices based on charged-mismatched perovskite materials (e.g. BiFeO3/PbTiO3), and carrier confinement at polar domain walls.

We examine the possibility of multi-band superconductivity in SrTiO3 interfaces by investigating the effects of a two-dimensional two-band model and by making comparisons to the behavior of bulk SrTiO3. In undoped SrTiO3, one of the bands is occupied, while the upper band is empty. As the chemical potential shifts, due to doping by negative charge carriers or application of an electric field, the second band becomes occupied, giving rise to a strong enhancement of the transition temperature and a sharp feature in the gap functions, which is manifested in the local density of states spectrum. By comparing our results with tunneling experiments in Nb-doped SrTiO3, we find that intra-band pairing dominates over inter-band pairing, unlike other known multi-band superconductors. Given the similar transition temperature and band structure of LaAlO3/SrTiO3 heterostructures, we speculate that the superconductivity observed in SrTiO3 interfaces may be similar in nature to that of bulk SrTiO3, involving multiple bands with distinct electronic occupations. Work was carried out under the help and support of the National Nuclear Security Administration of the U.S. Department of Energy at Los Alamos National Laboratory under Contract No. DE-AC52-06NA25396.

A heterostructure consisting of two perovskite band insulators, LaAlO₃ (LAO) and SrTiO₃ (STO) have received much attention due to the presence of high mobile two-dimensional electron gas (2DEG) at the interface. In this study, electron transport properties at n-type LAO/STO interfaces have been investigated numerically. Carrier distributions, band structures and electron sheet density have been calculated by solving Schrödinger equations with Poisson equation in a self-consistent manner for various LAO/STO interfaces, including the A-site, B-site and AB-site atom interdiffused interfaces and the sharp interface (i.e., no interdiffusions). It has been found that the interface with A-site atom interdiffusion has the critical thickness of 4 unit cells below which it remains insulating. The other interfaces have the critical thickness above 4 unit cells. For the A-site interdiffused interface, most electrons are localized within 10 nm from the interface forming 2DEG with multi-band occupations. Along the A-site atom interdiffused interface the electron mobility has been calculated using the linearized Boltzmann transport equation including scattering mechanisms of acoustic phonon, polar optical phonon, interface roughness and net charged layers. The calculated mobility is compared with available experimental data. At low temperature, the mobility is limited by the interface roughness and net charged layers. At room temperature, the polar optical phonon is the dominant electron scattering mechanism and the mobility is almost independent from the thickness of LaAlO₃ film.

Recent observations of high mobility, two dimensional electron gases (2DEGs) at ABO₃ oxide interfaces have sparked numerous efforts to explore emergent phenomena (such as metal-insulator transitions, novel magnetic effects and superconductivity) arising from electron confinement at these interface. In particular, theory and experiments have sought to address the overwhelming challenge of identifying the origin of 2DEGs at insulating oxide interfaces and surfaces as well as to develop rules or concepts by which interfacial charge carrier densities and mobilities can be tuned. In this presentation, I will discuss the use of first principles calculations to explore how chemical identity, fractional doping and interface orientation can be used to tune both the density and mobility of carriers at an interface. For example, 2DEGs at III/I-V oxide interfaces exhibit a significant increase in the intrinsic limit of interfacial carrier densities (relative to the prototypical SrTiO₃ heterointerfaces). In KNbO₃ and KTaO₃ derived interfaces, the electronic reconstruction at the interface leads to much lower band electron effective masses which are indicative of possibly higher mobilities. Furthermore, fractional doping of SrTiO₃ at an interface can also have meaningful consequences for the mobility of electrons at a heterointerface. Collectively, these results represent a framework for chemically modulating 2DEGs, thereby providing alternative pathways through which the underlying physics of electron confinement can be explored. This work was supported by the Materials Sciences and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Metal-oxide semiconductors (ZnO, IGZO) have been extensively investigated recently as channel materials for thin film transistors (TFTs). For low-cost flexible electronics, low temperature techniques are of critical importance. TFTs were demonstrated using ZnO channels deposited by atomic layer deposition (ALD) [1-3]. Recently, we demonstrated a ZnO based charge trapping memory cell using a single ALD step [4]. ALD techniques are promising due to low temperature growth, large area uniformity, precise thickness control, highly conformal deposition and scalability to roll-to-roll processes.
In this work, ~14nm ZnO is grown by ALD at 80°C on heavily doped (5x1018/cm3 Boron) Si substrates. The ZnO layer is n-type due to native crystallographic defects such as interstitial zinc and oxygen vacancies, which behave as electron donors [5-6]. Using conductive AFM, the current vs. voltage curve is measured. The IV results confirm an n-p diode characteristic with turn on voltage around ~3.4V consistent with the bandgap of ZnO. This confirms a ZnO(n)/Si(p+) heterojunction diode is obtained. In addition, the reverse bias breakdown voltage of ~-4.46V is obtained. For the 14nm ZnO layer this corresponds 3.18 MV/cm breakdown electric field. These ZnO based n-p junctions can be used for future low cost thin film photovoltaic cells. Finally, the results confirm that ZnO is a promising material for future low cost, flexible and transparent electronic and photonic applications.
References:
[1] R.L. Hoffman, B.J. Norris, and J.F.Wager, “ZnO-based transparent thin-film transistors,” Appl. Phys. Lett., vol. 82, no. 6, pp. 733-735, 2003.
[2] R.L. Hoffman, “ZnO-channel thin-film transistors: Channel mobility,” J. Appl. Phys., vol. 95, no. 10, pp. 5813-5819, 2004.
[3] E.M. C. Fortunato, P. M. C. Barquinha, A. C. M. B. G. Pimentel, A. M. F. Goncalves, A. J. S. Marques, R. F. P. Martins, and L. M. N. Pereira, “Wide-bandgap high-mobility ZnO thin-film transistors produced at room temperature,” Appl. Phys. Lett., vol. 85, no. 13, July 2004.
[4] Feyza B. Oruccedil;, Furkan Cimen, Ayman Rizk, Mohammad Ghaffari, Ammar Nayfeh Member, and Ali K. Okyay, “Thin-Film ZnO Charge-Trapping Memory Cell Grown in a Single ALD Step” IEEE Electron Device Letters (Accepted for Publication)
[5] S.H. K. Park, C.S. Hwang, H.S. Kwack, J.H. Lee, and H.Y. Chu, “Characteristics of ZnO Thin Films by Means of Plasma-Enhanced Atomic Layer Deposition,” Electrochem. Soild-State Lett., vol. 9, no. 10, July 2006.
[6] Y.T. Shih, M.K. Wu, M.J. Chen, Y.C. Cheng, J.R. Yang, and M. Shiojiri, “ZnO-based heterojunction light-emitting diodes on p-SiC(4H) grown by atomic layer deposition,” Appl Phys B, vol. 98, pp. 767-772, 2010.

Homoepitaxial growth of α-Al₂O₃ thin films on the atomically stepped sapphire (10-12) substrates has been achieved at room-temperature (RT) by pulsed laser deposition (PLD) process. The present low-temperature film process could be applied to nanometer-scale surface patterning of the sapphire substrate.
Single crystal aluminum oxide (α-Al₂O₃, sapphire) substrates have been regarded as attractive and important ones due to their excellent properties, and have been widely used as insulating substrates for epitaxial growth of semiconductors such as blue-light emitting GaN or Si (Silicon-on-sapphire; SOS). From the point of high-performance mechanical, optical and microelectronic applications, it is strongly desired to pattern the sapphire surface with feature size of micro- and nanometer-scale. So far, nanometer-scale sapphire processings have been studied extensively using by top-down methods such as focused-ion beam, dry etching, laser ablation, and so on. Previously we reported RT homoepitaxial growth of α-Al₂O₃ films on sapphire (10-12) substrates using an electron-beam assisted process for fabrication of micro-patterns on sapphire substrates [1], in which the α-Al₂O₃ thin films were found to grow homoepitaxially on the substrate at RT in the electron-beam irradiated region during film deposition, while in the non-irradiated region the film was amorphous. In this study, we have succeeded in homoepitaxial PLD growth of α-Al₂O₃ thin films with neither substrate heating nor electron-beam irradiation by controlling the substrate surface morphology such as atomic step arrangement in an atomic scale.
In experimentals, the thin films were grown at RT on the atomically stepped sapphire (10-12) substrates which have atomic steps of 0.36 nm height and terraces width of 20-500 nm [2]. There was observed good correlation between the crystallinity of these thin films and atomic terrace widths of the sapphire substrates, that is, crystallinity was increased with decreasing of terrace width. The RT epitaxial growth of the α-Al₂O₃ thin film was confirmed by reflection high energy electron diffraction (RHEED) on the substrate with the narrow terrace width of 20 nm. Furthermore, nanometer-scale patterning of the sapphire substrates surface was examined by applying the present RT homoepitaxial thin film growth as well as wet-etching at RT. Selective RT homoepitaxial film growth using pattern-mask was found to be applicable to RT-surface patterning of the sapphire substrate.
[1] A.Sasaki, H. Isa, J.Liu and M. Yoshimoto, Jpn. J. Appl. Phys., vol. 41, (2002) 6534.
[2] M.Yoshimoto et al., Appl. Phys. Lett. 67, 2615 (1995)

We prepared In2O3 semiconductor by inkjet-printing technique for low temperature and high performance oxide TFT. The In2O3 formulation, various substrate temperatures during inkjet process and post-annealing effects were thoroughly investigated. An optimum substrate temperature and an annealing temperature were 50oC and 200oC, respectively. The inkjet-printed In2O3 thin film was characterized by analytical methods. Annealing temperature decreased to obtain TFT behavior, a saturation mobility of 0.1 cm2/Vs was obtained after post annealing at 150oC. When the annealing temperature was 200oC, a saturation mobility of 1.83 was obtained. The best electrical properties after post annealing at 200oC were obtained, a mobility of 3.98 cm2/Vs, a threshold voltage of 1.83 V, a subthreshold slope of 0.4 V/dec, and an on-to-off current ratio of 108, the best properties by inkjet process so far. Positive and negative bias stability with relaxation behavior were also investigated.

As flexible electronics have expanded their applications to displays, biomedical devices, photovoltaic devices such as solar cells, etc., there has been an increasing need for high quality polymer substrates. Although the polymer substrates are highly flexible, several layers of functional coatings of inorganic materials are required to compensate the poor functional properties and mechanical resistances of the commercial polymers. As an electrode among the functional coatings, compound materials such as indium tin oxide and aluminum doped zinc oxide have been exclusively used due to their superior electrical and optical properties to metallic materials. However, ironically, the inherent brittle nature of the oxides limits the mechanical reliability including flexibility of the substrates. TCO becomes more brittle on the polymer substrates because it is likely to grow as an amorphous film. Due to the low glass transition temperature of the polymer substrates, both the substrate heating during deposition and post-deposition thermal annealing to improve the crystallinity of TCO are highly limited. In this study, we investigate the primary factors that determine the microstructure of TCO growing on the polymers, focusing on the effect of the physical and chemical status of the polymer surfaces on the film microstructures. Varying the surface morphology and chemical status, the effect of surface conditions on the microstructure of growing TCO was analyzed by transmission electron microscopy (TEM). Finally, the functional and mechanical properties of TCO were measured and explained with relation to the microstructures. Based on our analysis, the density of functional groups on the polymer substrate surface is one of the major factors that determine the microstructure of TCO. As the interfacial bond density between the polymers and the add-atoms of TCO is too low, heterogeneous nucleation of crystalline TCO at the interface is suppressed; hence, the degree of crystallinity of the TCO films is governed by the homogeneous nucleation during growth. Based on our analysis, we proposed surface treatment methods for growing crystalline TCO with sound texture at low temperatures.

There has been a growing interest in amorphous oxide semiconductors (AOS) represented by amorphous In-Ga-Zn-O (a-IGZO) as the most promising materials for thin-film transistors (TFTs). A distinctive feature of a-IGZO is high field effect mobilities (>10 cm²V^-1s^-1) of a-IGZO TFTs fabricated even at room temperature (RT). [1, 2] On the other hand, smaller band gap AOSs are required for solar cell application because the band-gap energy (E_g ~3.1 eV) of a-IGZO is too large for this purpose.
In this study, we fabricated amorphous Cd-Si-O thin films as a new AOS with narrow E_g (~2 eV). Cd-Si-O thin films were deposited on silica glass substrates by RF magnetron sputtering method without intentional substrate heating. 2”-diameter CdO and 1”-SiO₂ sintered ceramic disks were used as targets. RF power for CdO and SiO₂ targets were fixed at 5 W and 60 W, respectively. Processing pressure was varied from P_Ar = 1.5 to 10 Pa with pure Ar gas (99.999%). The X-ray diffraction patterns of the films deposited at P_Ar < 5 Pa showed a small but distinguish peak originated from NaCl-type crystalline CdO, however, the films deposited at higher processing pressure did not show any peaks, suggesting that these films are possibly amorphous. Hall mobilities of these amorphous films were ~1 cm²V^-1s^-1. The optical band gap energy of the film prepared at 7.5 Pa estimated from the Tauc&’ plot was ~1.8 eV.
[1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, Nature, 432, 488 (2004).
[2] T. Kamiya, K. Nomura, and H. Hosono, Sci. Technol. Adv. Mater., 11, 044305 (2010).

The energy band alignment of semiconductors describes how the valence and conduction band energies of two materials are aligned when the two materials are brought into direct contact. In the case of semiconductor/metal contacts, the discontinuities are known as Schottky barriers. These determine the rate of charge injection at the contact, which can become a determining factor for thin film devices.
We have determined the energy band alignment at a large set of interfaces of functional oxides using photoelectron spectroscopy (XPS). The materials (ZnO, In2O3, SnO2, BaxSr1-xTiO3 (x=0-1), PbZrxTi1-xO3 (x=0-1), BiFeO3, (Bi,Na)TiO3, (K,Na)NbO3, Cu2O, Fe2O3, Bi2O3, Al2O3, SiO2, Ta2O5, Nb2O5) are either prepared in-situ in the form of thin films by magnetron sputtering or obtained in the form of thin films, polcrystalline or single crystalline ceramics from different partners.
The results give insights into the fundamental properties which govern energy band alignment of such materials and how the band alignment relates to electrical properties of the materials. The barrier heights at oxide/metal interfaces can be modified by chemical oxidation and reduction and, in the case of ferroelectric materials, also by polarization. Furthermore, Fermi level pinning in materials with a high defect concentration can also lead to a modification of energy band alignment.

Hematite (α-Fe2O3) doped with WO3 was grown on FTO substrates using physical vapor deposition to form translucent WO3/α-Fe2O3 nanocolumnar thin films. These films have much high absorption in the near UV region, which provides additional photovoltage for photoelectrochemical (PEC) water splitting. Compared with pure hematite photoanodes, these WO3-doped thin films exhibited a photocurrent onset potential as low as 0.43 V vs RHE. This result represents one of the lowest onset potentials discovered for hematite-based PEC water oxidation systems. Compositional analysis by XPS and EDS shows that there are different W concentrations at the surface and in the bulk, and suggests that additional tuning should be possible.

Iridium oxide (IrO2) is one of a few naturally highly conductive transitional metal binary oxides and has very unique material properties, such as single stoichiometry, high thermal and chemical stability, and excellent diffusion barrier for oxygen, which make it a very interesting material from both fundamental and applications. Here we show that IrO2 can be epitaxial grown on commercially available oxide substrates, e.g. SrTiO3, LaAlO3, MgO, and Al2O3, by pulsed laser deposition. X-ray diffraction demonstrates that crystallographic structure of epitaxial IrO2 thin films are strongly affected by both lattice mismatch and crystal symmetry mismatch between film and respective substrates. Consequently, IrO2 grown on MgO substrates, with the rock salt structure, exhibits a single domain structure, while epitaxial film on perovskite and corundum substrates exhibit complicated in-plane domain configuration with film lattice rotating and tilting with respective to substrate lattices. Electronic structure of epitaxial IrO2 film is determined by x-ray photoelectron spectroscopy and first-principle calculations, where both measured and calculated valence band spectrum shows a distinct inflection of the density of state right at Fermi level, indicating a high electron density. Further electrical transport measurement demonstrates various conducting mechanisms as a function of temperature. The obtained highly conductive epitaxial IrO2 thin film is a promising electrode material for application of oxide electronic devices.

Thin film heterostructures constitute backbone of solid state device technology. Next generation solid state devices will require integration of various functions on practical substrates, particularly on Si(100). This integration will necessitate epitaxial growth of materials of different functions on a single chip where total misfit could range from less than 1% to over 25% or even more. In this study, we grew ZnO epitaxial thin films on Si(001) substrates using cubic yttria-stabilized zirconia (YSZ) buffer layer and demonstrated how crystallographic alignment across the ZnO/YSZ interface changes at different growth temperatures. The epitaxial relationship at the ZnO/YSZ interface was found to be (0001)[-2110]ZnO||(001)[110]YSZ and (0001)[-12-10]ZnO||(001)[100]YSZ at 535 and 750 oC, respectively. These epitaxial relationships are interpreted using the framework of the domain matching epitaxy paradigm. The misfit strain between a-ZnO and [110] c-YSZ is around 10% and it is around 25% for a-ZnO and [100] c-YSZ. We show that a competition between strain free energy and chemical free energy finally determines the epitaxial orientation. At other temperatures in between, a combination of these epitaxial relationships was observed. The epitaxial relationship between the YSZ film and Si(001) substrate was shown to be cube-on-cube: {001}<100>YSZ||{001}<100>Si. It was revealed that the epitaxial relationships are determined by the surface termination characteristics of the YSZ layer across ZnO/YSZ interface.

Zinc oxide (ZnO) with a wurtzite-type structure is a well-known wide band gap oxide semiconductor, whose energy band gap is 3.37 eV. Its energy band gap is generally controlled by alloying with the MgO for widening and with the CdO for narrowing. However, their alloying regions are limited, because the crystal structure of the MgO and CdO is the rock salt type and it is quite different from that of ZnO. We recently proposed a pseudo-binary alloying system of ZnO with β-LiGaO₂ as an alternative system for the ZnO-MgO system; the terminal β-LiGaO₂ possesses wurtzite-derived β-NaFeO₂ structure. The band gap of ZnO is widened up to ~4.0 eV by the alloying while keeping its electrical conductivity. In the present study, we demonstrated band gap narrowing of ZnO by alloying with wurtzite-derived β-NaFeO₂ type β-AgGaO₂, whose energy band gap is reported as 2.2 eV.
The x(AgGaO₂)_1/2-(1-x)ZnO alloyed thin films were fabricated by the conventional rf-magnetron sputtering. Mixed powders of β-AgGaO₂ and ZnO with various mixing ratios were used as target materials. The β-AgGaO₂ powder was prepared by the ion-exchange of precursor β-NaGaO₂ phase from Na⁺ to Ag⁺. The chemical compositions of the alloyed films fabricated were determined by EDX analysis as x = 0, 0.28, 0.45, 0.70 and 1 in the x(AgGaO₂)_1/2-(1-x)ZnO formula. Their optical band gaps were studied based on their transmission spectra.
The ZnO was alloyed with the β-AgGaO₂ in the whole alloying region; i.e., 0<x<1, and the phases appeared were identified as simple wurtzite-type phase based on the XRD profiles. The situation that the phase of the alloyed film was not the β-NaFeO₂ type but the simple wurtzite type was very close to our previous report on the x(LiGaO₂)_1/2-(1-x)ZnO alloyed films. The optical band gap of ZnO decreased with the increasing concentration of β-AgGaO₂ component in the films; the optical band gap of ZnO was successfully narrowed down to ~2.4 eV by the alloying with the β-AgGaO₂. We also evaluated the bowing parameter, b, in E_g(x)= xE_g(β-AgGaO₂) + (1-x) E_g(ZnO) - b(1-x)x. The obtained value of b was 1.4 eV. Such a small b value should be attributable to the small lattice mismatch of ~5 % and small difference of the valence and conduction band energies between ZnO and β-AgGaO₂.

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

In this contribution, we present two industrially scalable, high-throughput processes at atmospheric pressure for the deposition of transparent and conductive ZnO thin films, i.e.: plasma enhanced chemical vapor deposition (AP-PECVD) and Spatial Atomic Layer Deposition (Spatial ALD).
AP-PECVD: ZnO films have been deposited by using de-ionized water as oxidant for diethylzinc. Along the H2O-delivering nozzle, RF plasma (0 - 200W) has been generated by the use of a commercial PTB LF-30 power source. Films are grown at a deposition rate of 7 nm/s and a temperature of 200 °C on a moving glass substrate (60 mm/min). As deposited films are highly transparent in the visible range (~ 90 %) and resistive (ρ= 0.1 Ohm cm). A short (4 minutes) exposure to near UV/visible light results in a very low resistivity value of 1.5 E-3 Ohm cm (n = 1E20 cm^-3, mu; = 38 cm^2/Vs), which is independent of the film thickness in the range from 150 to 1200 nm. The photo-enhanced conductivity is stable in time at room temperature when ZnO films are coated by an 60 nm thick Al2O3 film. The Al2O3 film was applied with a Spatial ALD process. ZnO/Al2O3 films have been used as front electrode and barrier respectively, in CIGS test cells on glass substrate, resulting in efficiencies up to 16 %, comparable to reference cells with sputtered ZnO:Al front electrodes.
Spatial ALD: ZnO films have been deposited by using de-ionized water as oxidant for diethylzinc. Precursors are dosed in different zones of the reactor and a moving substrate is sequentially exposed to each of these zones, so that, as compared to the conventional ALD, a purge step is no longer needed. This results in a growth rate up to 1 nm/s. At a deposition temperature of 250 C, the electrical properties of ZnO can be controlled, ranging from heavily n-type conductive (with ρ = 4E-3 Ohmcm for 250 nm thickness, n = 7E19 cm^-3 and mu; = 30 cm^2/Vs) to insulating. The content of metal dopants (Al, In) in ZnO films can be tuned by pre-mixing Zn and dopant precursors (DEZ with TMA and TMIn, respectivelly). The film&’s carrier density is accurately controlled, ranging from 7E18 cm^-3 (i-ZnO at 200 C) to 5E20 cm^-3 (Al/Zn asymp; 0.10) and to 6E20 cm^-3 (In/Zn asymp; 0.02). Intrinsic and doped ZnO films are highly transparent in the visible range (~ 90 %). The electrical properties of i-ZnO and ZnO:In degrade when films are exposed for 1000 hours to a 85 % humidity atmosphere at 85 C. The resistivity (ρ) increases in time (t) according to: ρ(t) ~ ρ(0) exp(t)^0.5, which is derived by extending the Seto&’s model in order to account for the diffusion of atmospheric gasses along the grain boundaries, where carrier traps are created. Thin (75 nm) Al2O3 films deposited by S-ALD are used as moisture barrier (WVTR: 10E-5 g/m^2/day) to prevent the degradation of ZnO thin films.

Phase pure, dense Cu2O thin films are grown on glass and polymer substrates at 150-225 C by a novel atmospheric atomic layer deposition (AALD) approach. Carrier mobilities of 5 cm2V-1s-1 and carrier concentrations of ~1016 cm-3 are achieved in films 50 - 120 nm thick, over a >10 cm2 area. Growth rates are ~1 nm min-1, which is two orders of magnitude faster than for Cu2O growth via conventional ALD.. We show that mobility values decrease with increasing deposition temperature, due to an increase in particle size. Carrier concentration does not show a dependency on temperature. The high mobilities achieved using the atmospheric, low temperature method represent a significant advance for flextronics and flexible solar cells which require growth on plastic substrates.

High efficiency, flexible CdTe solar cells have enormous potential for use in applications as a low cost device in a flexible form factor with a low weight. One limitation of the current flexible CdTe device technology is the polymer superstrate, which has high optical absorption and poor thermal and environmental stability. Here, we present the initial results of our project developing CdTe superstrate devices on flexible glass. Corning® Willowtrade; Glass is a uniquely appealing substrate for the deposition of high efficiency superstrate CdTe devices because of its high strain point, high hermeticity, excellent dimensional stability, and superior optical properties. These attributes allow the flexible glass to be used in conventional high-temperature CdTe processing steps.
Our initial focus in this project was on the deposition of high-performance transparent conducting oxides and buffer layers with good thermal and environmental stability that would survive the subsequent high-temperature processing steps. We deposited SnO2:F, SnO2, Cd2SnO4, and In2O3:Sn on 100 mu;m thick Willow Glass. The SnO2:F and SnO2 were grown by metal-organic chemical vapor deposition (MOCVD) at 550oC. The sheet resistance ranged from 5-15 Omega;/sq with a transmittance > 85 % in the visible (transmittance includes the glass plus TCO film). Cd2SnO4 was rf sputtered at room temperature and annealed at 650oC to yield films with sheet resistances ranging from 4-12 Omega;/sq and transmittance in the visible > 90%. Finally, In2O3:Sn films were deposited by rf sputtering at 300oC to yield sheet resistances of ~ 12 Omega;/sq and transmittance in the visible of > 90%. We will present additional electrical and optical data on these films, along with information on their reliability and integration into flexible CdTe devices.

Cu₂O is a promising earth abundant alternative to traditional photovoltaic materials (CIGS, CdTe, Si etc.) because of its low cost, high availability, and straightforward processing. Cu₂O has potential as an absorber in photovoltaics with a direct gap of 2.1 eV and a detailed balance efficiency for a homojunction of eta; ~20%. It is a native p-type semiconductor with relatively high absorbance in the visible region above its energy gap. Long minority carrier diffusion lengths and high hole mobilities have also been reported. However, the current energy conversion record for a Cu₂O device is only 4%. One of the major challenges to Cu₂O photovoltaics has been the difficulty of extrinsic doping, and all recent Cu₂O devices have been intrinsically doped by point defects. This means devices have high parasitic series resistance, thus development of thin (< 50 µm) Cu₂O substrates is essential. Thinner substrates also represent a more cost effective use of material. Furthermore, Cu₂O also shows chemical instability at the ZnO/Cu₂O interface, thus controlling the interface stoichiometry is crucial to making high performance devices.
We report a method to fabricate ultrathin Cu₂O substrates by thermal oxidation of ultrathin Cu foils yielding absorber thicknesses of less than 20 microns which may be handled and processed into devices. X-ray diffraction spectra shows that we have grown phase pure Cu₂O with no elemental Cu or CuO inclusions. Hall measurements indicate that the substrates have bulk mobilities of 10-20 cm²V^-1s^-1 and carrier concentrations on the order of 10¹⁴ cm^-3. Current-voltage characteristics of these Cu₂O substrates were derived from ZnO/Cu₂O heterojunction device measurements which indicate open circuit voltages of V_OC ~ 530 mV.
We also report on the ZnO/Cu₂O interface, particularly the conduction band offset and interface composition. The conduction band offset is an important parameter in a heterojunction because it limits the maximum achievable open circuit voltage for a heterostructure. Capacitance voltage measurements indicate a conduction band offset of .4 eV. This number is smaller than previously reported in literature from X-ray photoelectron spectroscopy experiments, but more in line with experimentally determined V_OC&’s for Cu₂O/ZnO heterostructures.
We have also demonstrated improved control of oxide stoichiometry at the ZnO/Cu₂O interface, supported by XPS measurements used to characterize interface chemical reactions . The core level and Auger peak positions were analyzed to determine the oxidation state of the copper at the interface. By altering the partial pressure of O₂ in the atmosphere during ZnO sputtering, we can either produce a very copper rich interface or a stoichiometric interface. We will also show how the density of interface defects changes for different deposition conditions, and how device performance is affected by the local stoichiometry of Cu₂O at the ZnO/Cu₂O interface.

Amorphous metal oxide semiconductors have drawn much attention due to their high mobility, high optical transparency in the visible light regime, and low processing temperature. In particular, indium gallium zinc oxide has been considered as the most promising material for TFT application in the next generation active-matrix organic-light-emitting displays or transparent displays. However, indium and gallium are relatively expensive and toxic; therefore, indium-free materials have been investigated widely in recent years.
In this work, we focus on the zirconium-doped zinc tin oxide (ZZTO) ternary compound material system. The effects of Zr and Sn on the electrical performance of the ZZTO TFTs are investigated. The ZZTO TFTs, which have an inverted staggered bottom-gate structure, are fabricated on 1” × 1” Corning Eagle 2000 glass substrates. First, a 100nm-thick Cr is e-beam evaporated and patterned as the gate electrode. Next, a 400nm-thick silicone-SiO2 hybrid layer is deposited by plasma-enhanced chemical vapor deposition at 100°C as the gate dielectric, followed by the deposition of 40nm-thick ZZTO channel layer from single target by reactive rf magnetron sputtering in O₂/Ar ambient without any intentional thermal heating. Finally, a 100nm-thick ITO is deposited by e-beam evaporation as the source/drain contacts.
The result shows that the field-effect mobility decreases but the electrical bias-stress stability improves as the Zr doping concentration increases. On the contrary, the incorporation of Sn can enhance the field-effect mobility but degrade the electrical stability of the TFT. Among the parameters studied, Zr_0.03Zn_0.32Sn_0.65O achieves the best balance between the mobility and stability. The threshold voltage, field effect mobility, and sub-threshold swing of the as-fabricated Zr_0.03Zn_0.32Sn_0.65O TFTs are 5.9V, 3.2cm²/V-s and 1.3V/decade, respectively. Further results will be reported in the symposium.

Recently, thin film solar cells have attracted considerable attention due to low cost and large area fabrication. Transparent conducting oxide (TCO) has been widely used for various solar cells including amorphous silicon solar cells. Low resistive and highly transparent TCO is required for transparent electrodes. The stability of TCO&’s resistivity under long term use in a harsh atmospheric environment is important for applications of front electrodes of thin film solar cells. However, it is well known that widely used ZnO-based TCOs including the Al-doped ZnO (AZO) exhibited the increase of electrical resistivity under oxidizing atmosphere [1]. It is very important that the resistivity of the TCO should not be degraded under the long term use at atmospheric air.
The purpose of this paper is to report boron (B) and aluminum (Al) codoped ZnO (BAZO) TCOs of which electrical resistivity are rather stable.
The BAZO films with various B content (1.9-4.9 at. %) were prepared on glass substrate by employing DC (AZO) and RF (BZO) magnetron cosputtering at relatively low temperature of 130 oC which may not any thermal damages to glass substrates.
The accelerated reliability tests were performed through thermal annealing at300 oC in atmospheric air and the H2O vapor annealing at specially designed chamber. After the annealing for 4 hour, the resistivity of BAZO (4.9 at. % B) film increased by less than 1.5 times while that of typical AZO film increased by over 8 times in atmospheric air. The resistivity variation of the AZO and BAZO films were indicated in the Table 1. Under the H2O vapor annealing for 8 hour test, the resistivity of BAZO films was increased about only 4 times, which was significantly less than that of AZO film (110 times).
To investigate the electrical resistivity variation of the BAZO, X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) were performed. XPS analysis confirmed that the peak related with chemisorbed oxygen at 100 nm-inside of the film was suppressed in BAZO film compared to AZO film. HRTEM indicated that the grain structure of BAZO films were much less deteriorated after the thermal annealing compared with AZO films. The B atoms suppressed the oxygen permeation into the BAZO film, which resulted in the inactive oxygen chemisorption.
Our experimental results suggest that BAZO film can be a strong candidate for highly reliable TCO applications, which can replace the widely used AZO films.
Reference
1. P. Nunes, E. Fortunato, and R. Martins, Thin Solid Films, 383, 277 (2001).

Many people interested in flexible and light devices using plastic base electronics such as flexible cell phone and unbreakable touch panel display. Now, the most touch based smart phone and touch devices are used rigid substrate as glass and transparent electrode is applied with indium tin oxide (ITO) layer.
Therefore, we experimented on ITO thin film on polyethylene terephthalate (PET) sheet for flexible and light devices via ionized physical deposition (IPVD) method. The IPVD technique is combined with RF magnetron sputtering and modified induction coupled plasma (ICP). The IPVD carry out high quality ITO thin film, which has a high transmittance and a low resistance, simultaneously.
Then, the IPVD deposited ITO film were investigated in terms of the power of ICP and RF power, the ratio of O2/Ar gas flow, and deposition pressure. And their structural, electrical and optical properties were characterized by X-ray diffraction, 4 point probe, and UV-vis. spectrometer, respectively. After 130oC vacuum annealing, The sheet resistance of ITO film was 85 Omega;/sq. and its transmittance in 350~750nm was 83% at thickness of 30nm. In addition, we simulated index matched structure and deposited index matching layer on PET sheet. Finally, we compared the properties of ITO film with/without index matched structure. We present the detailed experiment results and other analysis results of ITO films with deposition conditions.

Nickel oxide (NiO) is a promising candidate for p-type transparent oxide semiconductors (TOSs) because of its wide bandgap energy (3.7 eV). It is a suitable material as transparent devices with an n-type TOS such as ZnO. In fact, NiO has been used in transistors, UV-visible light emitting diodes, and UV detectors. Therefore, NiO might be used in a “multifunctional window”, which is integrated of visible-light-transparent solar cells, sensors, electronic circuits, and so on. Visible-light-transparent devices much more attractive because optical transparency permits more installation locations.
A highly transparent undoped NiO film exhibits the insulating property, and low resistivity NiO films tend to show low transmittance. Its resistivity should be controlled by composition of O atoms and Ni3+ ions, for instance, by the addition of monovalent atoms or by the appearance of Ni vacancies and/or interstitial oxygen in NiO crystallites. However, its electrical properties have not been fully evaluated experimentally. Recently, NiO films have been deposited by various methods including sputtering, pulsed laser deposition, and spray pyrolysis. Among these, sputtering is the most suitable method because it enables the economical deposition of large-area films of well-controlled composition.
In this presentation, the critical importance of O2 pressure control for the deposition of visible-light-transparent p-type NiO films will be shown. In addition, a NiO-based solar cell fabricated for a realizing multifunctional window will be described.
Undoped and Cu-doped NiO films with a thickness of approximately 200-500 nm were deposited by RF reactive sputtering on soda-lime glass substrates. The fraction of O2 in the sputtering gas [O2/(Ar + O2)] was varied from 0.1-15 %.
The fabricated NiO-related solar cells exhibited photovoltaic effect under illumination. Optical transmittance of greater than 70% was obtained in the wavelength range of 400 to 800 nm. These results indicate that NiO films are effective for use in multifunctional window.

Magnesium oxide (MgO) thin films have attracted great scientific and technological interest in recent decades. Because of its distinguished properties such as a wide band gap (7.2 eV), a low dielectric constant (9.8), a low refractive index, an excellent chemical and thermal stability (melting point=2900°C), it is widely used as inorganic material in diverse areas such as fire resistant construction materials, optical materials, protective layers in plasma display panels, buffer layers of multilayer electronic/photonic devices, and perovskite ferroelectric thin films. Precursor used in the ALD requires volatility, stability, and low deposition temperature. Precursors using a heteroleptic ligands with different reactivity have advantage of selective reaction of the heteroleptic ligands on substrate during ALD process. In this study, we have synethesized new heteroleptic magnesium precursors by the reaction of the β-diketonate and aminoalkoxide on the basis of molecular design. These two ligands have been widely used for the development of precursor because of the excellent volatility, chelating effects by increasing the coordination number of the metal, and advantageous to synthesize a single precursor. A newly-synthesized Mg(II) precursor was adopted for growing MgO thin films through ALD. The MgO films have been characterized by X-ray photoelectron spectroscopy, Auger electron spectroscopy, atomic force microscopy, X-ray diffraction, and transmission electron microscopy.

We consider the response of the electrons within wurtzite zinc oxide to extremely high, ultra-short applications of an electric field. In particular, we examine how the electrons within this material respond to the application of applied electric field strengths in excess of 5 MV/cm, for durations of the order of 100 fs. First, we examine whether or not impact ionization is likely to play a role in influencing the results, noting that the breakdown field corresponding to this material is less than the applied electric field strengths that are being considered. Then, we examine how an ensemble of electrons responds to the application of such a field using a Monte Carlo simulation approach. The dependence of the electron drift velocity on the time elapsed since the onset of the applied electric field strength will be the primary focus of this analysis. The device implications of these results are then explored.

In a heterostructure device, it is often the case that a two-dimensional electron gas forms within the channel. In this paper, we review some recent progress that has been made in understanding the nature of the electron transport that occurs within a zinc oxide based two-dimensional electron gas. Monte Carlo simulations of the electron transport that occurs within this electron gas are employed for the purposes of this analysis. Steady-state electron transport results are first presented. Then, transient electron transport results are considered. For the purposes of this transient electron transport analysis, how electrons, initially in thermal equilibrium, respond to the sudden application of a constant electric field, is considered. A comparison with the case of bulk zinc oxide is then provided. Finally, the device implications of these results will be explored.

A three-valley Monte Carlo simulation approach is used in order to probe the transient electron transport that occurs within bulk wurtzite zinc oxide. For the purposes of this analysis, we study how electrons, initially in thermal equilibrium, respond to the sudden application of a constant applied electric field. We find that for applied electric field strength selections in excess of 300kV/cm, that an overshoot in the corresponding electron drift velocity is observed. An undershoot in the electron drift velocity is also observed for applied electric field strength selections in excess of 700kV/cm, this velocity undershoot not being observed for other compound semiconductors, such as gallium arsenide and gallium nitride. The sensitivity of the results to variations in some of the, as yet unknown material parameters, is then probed. Finally, we employ a means of rendering transparent the electron drift velocity enhancement offered by the transient electron transport, and then use the calculated dependence of the peak transient electron drift velocity on the applied electric field for the design optimization of short-channel high-frequency electron devices.

Room temperature ferromagnetism (RTFM) in semiconducting materials is one of the indispensable tools for spintronic applications. For this application, the possibilities for RTFM have been explored by doping transition metals (TMs) in oxide semiconductors. ZnO is a wide-band gap II - VI semiconductor material with 3.4 eV band gap and large exciton binding energy of ~60 meV at room temperature, predicted to be a promising host material to achieve room temperature ferromagnetism [1]. Furthermore, its band gap can be mediated by Mg doping. By increasing the Mg content in ZnO, P. Bhattacharya et al. even increased the band gap up to 6.2 eV [2]. Y. Li et al. found that RTFM can be tuned in Mg_xZn_1minus;xO films by band gap and defect engineering [3].
In this work, MgZnO thin films prepared by DC/ RF magnetron co-sputtering method in (N₂+O₂) ambient conditions. We studied the effect of film thickness, various O₂ contents in working gas and annealing temperature on the structural, optical and magnetic properties. RTFM was observed in the films and the saturation magnetization (M_S) increases at first as the film&’s thickness increases and then decreases. The film with higher thickness becomes diamagnetic. The M_S value as high as ~14.6 emu/cm³ was achieved for the MgZnO film of thickness 52 nm and Mg content 7.33 at%. After annealing, the films became more crystalline and the M_S values reduced suggesting the ferromagnetism may be due to oxygen vacancies. We also observed that the O₂ content in working gas plays an important role in determining the magnetic properties of as-grown MgZnO films. The optical band gap of MgZnO films determined to be in the range 3.8 to 4.2 eV is found to decrease with the increase of films thickness. The decrease in the optical band gap with the increase of film thickness may be due to decrease of lattice defects.
References:
[1] M. Kapilashrami et al. Appl. Phys. Lett. 95, 033104 (2009).
[2] P. Bhattacharya et al. Thin Solid Films 447, 564 (2004).
[3] Y. Li et al. Appl. Phys. Lett. 97, 102506 (2010).
‘ project supported by Swedish Agency VINNOVA, Carl Tryggers Foundation, and ERASMUS program.

Amorphous In-Ga-Zn-O (a-IGZO) used for active materials in TFT is suitable for low-temperature and large-area processes. However, a clear understanding on the thermal instability of a-IGZO TFT characteristics is still needed for its successful commercialization. Many instabilities such as drift of mobility generally occur at much lower temperature than crystallization temperature (~600 °C), mainly due to its unstable amorphous structure. When an amorphous material cooled down to glass transition temperature (T_g), it is not in internal equilibrium so that experiences self-stabilization in a process called structural relaxation which results in time-dependent properties. Structural changes have been examined by a spectroscopic ellipsometry, but it only detected limited symptoms because it couldn&’t observe the changes in mechanical properties. In this study, we investigated thermally-induced structural changes in a-IGZO film with continuous heating by mechanical stress analysis, in order to detect the changes in density and mechanical properties associated with structural changes.
Mechanical stress was determined by the wafer curvature measurement and thermal cycling is conducted from RT to 640 °C in the N₂ atmosphere at pressure of 10 Torr to avoid oxidation. A-IGZO films were deposited on Si wafer by RF magnetron sputtering at RT using In₂Ga₂Zn₁O_7-x target (50 W, 5 mT, P_O2=0.01 %).
The stress measurements clearly detected the evidence of significant amount of structural relaxation started at quite low temperature as well as the symptoms of glass transition and crystallization. Heating of 300-nm-thick a-IGZO film from 170 to 568 °C, irreversible change of stress from -103 MPa (compressive) to +500 MPa (tensile) were detected, which means the significant amount of densification associated with the structural relaxation. Heating above 568 °C, tensile stress relaxed toward stress-free direction. This is the T_g and here the significant drop of viscosity as glass transition occur leading to stress relaxation. The T_g has dependence on the film thickness, for 15, 50, 100, and 200 nm, T_gs are 399, 503, 517, and 560 °C, respectively. From the ramping rate dependence of T_g, fragility (m) can also be obtained which indicates the degree of structural relaxation near T_g. For thick films of 300 nm, m=15.3 indicates a-IGZO is covalent-like stable glass, while for thinner films of 50 nm, m=28.8 indicates ionic-like unstable glass. The distribution of T_g and fragility on the film thickness shows thinner film exhibits less stable structure than thicker film due to the effects of free-surface. Such large surface effects indicate the instability in a-IGZO could be originated from the instability in surface rather than in bulk. In further work, the determination of defect annihilation and effects of oxygen vacancies in a-IGZO will be conducted using the stress analysis.

More than half of the energy utilized in the automobile exhaust, industrial factories and power stations is consumed in the form of waste heat. Thermoelectric devices are strong candidates for scavenging this wasted heat and converting it to useful electrical energy, because of its environmental friendliness, no moving parts operation, and scalable size. Potential thermoelectric materials need to meet the requirements of high Seebeck coefficient, low thermal conductivity and high electrical conductivity.
Oxides thermoelectric materials are very promising in the thermoelectric study attributed to its stability at high temperature. Among them, Zinc oxide (ZnO) based semiconductor materials have attracted much attention due to its well-known high mobility and reasonably good Seebeck coefficient. One issue affecting the thermoelectric performance of this system is its relatively high thermal conductivity. Recently, significant effort has been directed towards reducing the thermal conductivity while maintaining the power factor of this system. Successful methods include nano-structured the polycrystalline, introducing a nanovoid structure to the densely sintered ceramic and inducing second phase by co-doping Ga with Al in ZnO. The phonon transport has been suppressed by these methods. Furthermore, it is known that because of the structure disorder in amorphous solids, the greater phonon scattering leads to considerably smaller phonon mean free path than the crystalline analogue. So the thermal conductivity of amorphous material is lower than that of its crystalline form. Thus, amorphous ZnO-based material with high mobility, good Seebeck coefficent, and low thermal conductivity becomes an attractive target system for thermoelectric applications.
In this work, we studied amorphous Indium Zinc Oxide prepared by dc magnetron sputtering under different oxygen partial pressure. Rich Indium oxide in this oxide system improves the mobility, while the amorphous structure will decrease the phonon thermal conductivity. The electrical property of as-deposited films was monitored during and after the heat treatment. Seebeck coefficient was measured over the range room temperature to 700K. Thermal conductivity of the films was measured by the 3-omega method. Seebeck coefficient, resistivity and thermal conductivity of films deposited under different oxygen partial pressure were compared, along with annealing under controlled oxygen partial pressure. The temperature dependent thermoelectric figure of merit, ZT, of amorphous Indium Zinc Oxide was obtained from these measurements.

Zinc oxide (ZnO) is a wide band gap semiconductor with applications ranging from opto-electronics, spintronics and memristor based devices. The physical properties of zinc oxide films are highly sensitive to the growth parameters such as oxygen partial pressure and substrate temperature. Titanium nitride (TiN) is mechanically hard with a reasonable electrical conductivity to be used as electrode in electronic devices. We deposited (200) oriented TiN thin films on Si(001) substrate by pulsed laser deposition (PLD) technique. We used this as a buffer layer to deposit (0002) oriented epitaxial ZnO thin films. The growth conditions were optimized to be 650^oC substrate temperature and 1x10 ^-4 torr vacuum for TiN and, 500^oC substrate temperature and 5x10 ^-2 torr oxygen partial pressure for ZnO. With platinum (Pt) top electrode deposited by PLD using metal shadow mask on ZnO/TiN/Si(001), we observed Schottky diode type I-V characteristics in the grown heterostructures. Our XRD 2theta; scans indicated that our films are single crystalline with fixed out of plane orientations as ZnO<0002> // TiN<002> // Si<001>. We will present XRD-phi scan and TEM images to explain the details of epitaxy and show the quality of interfaces between various layers, respectively. We will further investigate whether Pt/ZnO/TiN structure can be used as resistance switching (or) memristor device.

Highly transparent composite electrodes made of multilayers of In- and Ga-doped ZnO and Cu (IGZO/Cu/IGZO) thin films (30/3-9/30 nm thick) are deposited onto flexible substrates at room temperature and with the use of radio frequency magnetron sputtering. The effect of Cu thickness on the electrical and optical properties of the multilayer stack has been studied in accordance with the Cu morphology. The micro-structural, optical and electrical properties of the multilayers are studied with the use of atomic force microscopy, UV-Vis spectrophotometry, Hall measurement and four point probe analyses. Results are compared with those from a single IGZO layered thin film. The average optical transmittance and sheet resistance both decreases with increase of copper thickness and have been optimized with a 6 nm thick Cu middle layer. The Haacke figure of merit (FOM) has been calculated to evaluate the performance of the films. The highest FOM achieved is 6 x 10-3 Omega;-1 for a Cu thickness of 6 nm with a sheet resistance of 12.2 Omega;/sq and an average transmittance of 77%. The multilayered thin films are annealed upto 150 oC in vacuum, forming gas and O2 environments and the optical and electrical properties are studied and compared against the as-deposited samples. Thus IGZO/Cu/IGZO multilayer is a promising flexible electrode material for the next-generation flexible optoelectronics.

The fabrication of a thin film optoelectronic device involves the exposure of the transparent conductive oxide (TCO) to a high process temperature. Indium Gallium doped zinc oxide (IGZO) is a well known TCO with high optical transparency, moderate conductivity and high mobility. However, its electrical properties deteriorate after subsequent high temperature processes in air atmosphere. On the other hand indium tin oxide (ITO) has higher conductivity than IGZO and better thermal stability. Therefore, IGZO/ITO bilayers have been deposited on glass by radio frequency magnetron sputtering at room temperature and subsequently annealed at high temperatures in order to study their thermal stability. In the present work, IGZO layers with a thickness ranging from 10 nm to 100 nm were deposited over a 50 nm thick ITO layer. Results are compared with those from a single IGZO layered thin film without the ITO bottom layer. The structural, optical and electrical properties of the multilayers are studied with the use of scanning electron microscopy, UV-Vis spectrophotometry and Hall measurement. An IGZO optimal thickness of 50 nm is found to improve the bilayer thermal stability at temperatures upto 400 °C keeping good opto-electrical properties. The electrical resistivity for the optimized IGZO/ITO composite films is about 4.4 x 10-4 Ohm-cm, and the transmittance in the visible range is about 80%.

Vanadium dioxide (VO₂) undergoes an insulator-metal transition (IMT) accompanied by drastic changes in its electrical and optical properties at a critical temperature T_c of 67°C. Across the IMT, VO₂ switches from being transparent to reflective in the infrared region. In addition, the electrical resistance reduces by several orders of magnitude. The thermal IMT occurs via percolation with the different phases coexisting and one phase gradually dominating over the other. In the presence of impurities or dopants, the T_c and percolation speed of the VO₂ change. Erbium (Er³⁺), with stimulated emission lines in the 1.55 µm telecommunications band, has been used extensively in fiber-optic communication systems. Since the IMT in VO₂ can be triggered on an ultrafast time scale when excited by an optical pulse, the combination of VO₂ and Er³⁺ could make an ultrafast optical switch that is capable of simultaneous signal amplification. In this paper, we describe experimental investigations into the possibilities of making such a device.
VO₂ thin films were grown on Si substrates by electron-beam evaporation to a nominal thickness of 100nm. The samples were then implanted with Er at various fluences and energies. Annealing procedures in flowing N₂ gas were conducted to repair the implantation damage and activate the Er. We tested the optical switching via reflectivity measurement and performed PL spectroscopy on the various samples.
Reflectance measurements on different samples showed that incorporation of Er³⁺ into VO₂ hinders the relaxation process of the IMT, as indicated by the concentration-dependent low-temperature tails of the IMT hysteresis curve. This feature was only seen on samples that were annealed at high temperatures, and was most prominent for samples annealed at 800C. PL was observed on the samples annealed at temperatures at or above 800C; hence this can also be considered as the critical temperature for self-coordination of the Er³⁺ ion with the surrounding O atoms, which makes Er³⁺optically active in VO₂. PL yield increased with Er³⁺ concentration.
Because the Er 4f shell is shielded from external fields, the intense fluorescence in Er³⁺ has a lifetime of a few milliseconds. Given the structure of VO₂, Er in VO₂ is probably six-fold coordinated with O atoms, with compatible point symmetries C_2v, C_4v, and O_h. While the point symmetry O_h cannot form the electronic states for the intra-4f transition, those optical transitions are not restricted by polarization selection rules restrict in a system with C_2v or C_4v symmetry. In our experiments, PL of VO₂:Er was successfully observed, suggesting the possibility of utilizing VO₂:Er as an optical switch and amplifier. Furthermore, relaxation of the polarization selection rules implies that VO₂:Er would be fully compatible with wavelength-division multiplexing technology, with signal amplification independent of the polarization of the input signals.

Hysteretic metal-insulator transitions (MIT) mediated by ionic dynamics or ferroic phase transitions underpin emergent applications for non-volatile memories and logic devices. In virtually all cases to date, the MITs are controlled by applied electric or magnetic fields, giving rise to a broad set of existent and emergent information technology applications. However, multiple classes of MIT are intrinsically coupled to strain and therefore will be affected by mechanical stimulus. Here, we explore mechanically driven MIT in oxides at the nanoscale and demonstrate creation of remanent conductive and non-conductive states, i.e. electroresistive switching. To demonstrate this effect, we have chosen a NiO thin film as a model system, resistive switching in which closely resembles MIT. Scanning the surfaces with an electrically grounded tip significantly alters their electronic conductance changing it to high-resistance state. This strongly suggests that the mechanical control modifies the chemical composition of the sample surface. Furthermore, scanning the surfaces with a grounded tip at different pressure levels reveals that both surface potential and current are controlled by the contact force. The additional theoretical results, based on the density functional theory and 3D finite element modeling, illustrate the fidelity of proposed mechanism. Although there can be a couple of different mechanisms, e.g. triboelectric and electrostatic effects, for the observed phenomena, these effects can be excluded based on the experimental results. In conclusion, we have theoretically predicted and experimentally observed the pressure-induced changes in NiO. Since the coupling between strain and electrochemical reactions can be used to tune material properties, the present pressure-controlled physics of oxides can open up new opportunities for device applications.
Research was supported (S.V.K., Y.K.) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. A portion of this research was conducted at the Center for Nanophase Materials Sciences (S.V.K., S.J., M.D.B.), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. This work was also supported by WCU program through the NRF funded by the Korea MEST (Grant No. R31-2008-000-10057-0) and Basic Science Research Program through the NRF funded by the Korea MEST (Grant No. 2011-0025607). V.B.S. gratefully acknowledges the support of the Army Research Office through Contract W911NF-11-1-0171.

For over a century, electrochemical phenomena in solids were intensively explored in the context of energy storage and conversion devices, electrochromic systems, corrosion, and metallurgy. Recently, much attention has been attracted to coupling between ionic and electronic transport phenomena, as exemplified by electroresistive materials and devices. The processes involved in formation and operation of electroresistive oxide memories in oxides are very complex and sensitively depend on bias levels, boundary conditions, and forming currents. However, much less is known on electronic and ionic phenomena that precede resistive switching behavior, including ionic polarization, possible gas-solid reactions, and nucleation of conductive phase. In this presentation, we explore the interplay of electronic and ionic transport in NiO using scanning probe microscopy. The electrochemical strain microscopy demonstrates the presence of strong hysteretic electromechanical response, indicative of bias-induced ionic transport. We further combine the first-order reversal curves measurements in electrochemical strain microscopy and in I-V measurements to decouple reversible and irreversible processes. Insight into NiO memristive applications and universal method to study ionic systems can be can be extended to other system, most notably ferroelectric, tunneling barriers, fuel cells, etc.
Research was supported (S.V.K., Y.K.) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. A portion of this research was conducted at the Center for Nanophase Materials Sciences (S.V.K., S.J., M.D.B.), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. This work was also supported by WCU program through the NRF funded by the Korea MEST (Grant No. R31-2008-000-10057-0) and Basic Science Research Program through the NRF funded by the Korea MEST (Grant No. 2011-0025607).

Tin oxide (SnO2) is one of the most commonly used Transparent Conductive Oxides (TCOs), successfully implemented in various fields of application, such as gas sensors, architectural windows, flat panel displays or thin-film solar cells. In industry, Atmospheric Pressure Chemical Vapor Deposition (APCVD) is often used and in spite of the commercial success, fundamental insight in this kind of processes are lacking. As an example, alcohols are commonly reported to be used as additives in the APCVD process of SnO2 in order to improve the film electrical properties. The addition of alcohols appears to improve both the initial film formation (in the nucleation stage) as well as the subsequent bulk film electrical, structural and morphological properties. However, the exact mechanism which leads to these improvements is, however, still largely unknown.
This work presents a detailed study on the initial and bulk growth of undoped tin oxide thin films by APCVD and the influence of methanol addition during deposition on the electrical properties and morphology. Relatively high carrier mobilities were observed for very thin films (~18 cm2/Vs for 10 nm thick films!), which indicate very good nucleation process, confirmed also by the large ~35 nm crystallites observed by HR-SEM. Methanol has significant effects on both film morphology and electrical properties. These effects are most distinct in the bulk growth regime, where an increase in electron concentration of almost an order of magnitude was observed. Remarkably high mobility values of up to 55 cm2/Vs were obtained. This was in part attributed by densification of the layer as shown by SEM and ellipsometry measurements, though structurally no changes could be observed by XRD.
For a better understanding of the initial nucleation and crystal formation, TEM images were taken from the top surface of tinoxide deposited on a Si wafer with 20 nm thick windows. The 20 nm thick window is transparent for the electron beam and therefore images of the SnO2 crystals and subsequent film formation were obtained for different tin precursors. A non-continuous layer of separated SnO2 crystals as well as continuous films could be deposited and their morphology was investigated. Due to the high magnification available, features such as stacking faults and defects as twinning could be discerned in a top view.
Finally, a hydrolization mechanism is proposed as an alternative to the previously assumed radical chemistry mechanism. This is based on the difference in reactivity of SnCl4 and tBu-SnCl3 and XPS studies that show that the longest surviving species on the surface is the tBu-Sn bond and not the stronger Sn-Cl bond. A special butyl-Sn-heptaisobutyl-POSS species was synthesised in order to obtain a reference material for the XPS studies. Using the setup, chemical vapour deposition was obtained at temperatures above 200°C, while atomic layer deposition was demonstrated at room temperature.

Efficient conversion of the CO2 emitted from vehicles, power generation plants, and industrial processes into useful compounds would lead to an enormous reduction in greenhouse gas emissions without the need for expensive storage solutions. This approach is currently impractical due to the high temperatures and pressures -- and thus large amounts of energy -- generally required for the synthesis of compounds using the highly stable CO2 molecule as a precursor. To make direct CO2 capture and conversion economically viable, new materials able to catalyze the conversion reactions at significantly lower temperatures will be essential. In this work, we use density function theory computations to design a dynamically tunable ferroelectric oxide-supported thin film catalyst that can capture CO2 directly from the emission stream and convert it into methanol. We demonstrate that switching the polarization of the underlying ferroelectric substrate substantially changes the surface atomic and electronic properties of the heterostructure, thereby encouraging strong CO2 adsorption in one state while activating CO2 and thus significantly lowering the energy barrier for conversion to methanol in the other state. Our approach may lead not only to new technologies for reducing emissions, but also to novel catalysts that could decrease energy consumption for industrial-scale synthetic processes.

We present a study of the atomic and chemical structure of the surface of a fully strained, TiO2-terminated, ferroelectric BaTiO3 (BTO) (001) epitaxial film on a SrTiO3 substrate after controlled exposure to water. The epitaxial quality was checked by atomic force microscopy and X-ray diffraction. Quantitative low-energy electron diffraction compared with multiple scattering simulations was used to measure the structure of the first few atomic layers of BTO surface. The surface chemistry was investigated using high-resolution X-ray photoelectron spectroscopy. Finally, temperature-programmed desorption measured the desorption energies. We find that water undergoes mainly dissociative adsorption on the polarized BTO(001) surface. There are two competing sites for dissociative adsorption: oxygen vacancies and on-top Ti surface lattice atoms. The Ti on-top site is the dominant site for OHminus; chemisorption. One fifth of the surface Ti atoms bind to OHminus;. The concentration of surface oxygen vacancies acts mainly to favor initial physisorption. Before exposure to water, the outward pointing polarization in the BTO film is stabilized by atomic rumpling in the TiO2 termination layer. After exposure to water, the chemisorbed OHminus; species provide the screening, inverting the surface dipole layer and stabilizing the bulk polarization. Molecular adsorption is observed only for high water coverage.

Constructing stable and high-capacitance nanostructured electrode materials are under current research focus in the field of electrochemical energy storage. Metal sulfides are an emerging class of high-performance pseudocapacitive (as well as solar cells) materials. Only a few reports are available on the hydrothermal synthesis of metal sufide nanostructures. Here we report a facile ion exchange reaction method to construct self-supported metal sulfide nanoarrays on carbon cloth and nickel foam substrates. Different CoS and NiS nanoarrays, such as nanowires and nanowalls, are successfully achieved from their corresponding metal oxide and hydroxide pre-forms. The sufide CoS and NiS nanoarrays exhibit superior redox reactivity for electrochemical energy storage. As cathode materials for supercapacitors, the CoS and NiS nanoarrays show remarkable pseudocapacitive performance with high capacitance and energy density, pretty good rate capability and cycle life. The CoS nanowire arrays exhibit a specific capacitance of 1040 F g-1 at 2 A g-1, and noticeable capacitance retention of 92 % after 3000 cycles. The NiS nanowall arrays show a specific capacitance of 718 F g-1 at 2 A g-1 and capacitance retention of 82 % after 3000 cycles. The enhancement is partly due to the integrated porous nanoarrays architecture with fast ion/electrons transfer path and good strain accommodation. This research may also be extended to the fabrication of other metal sulfide nanoarrays for applications in catalysis and lithium ion battery.
Acknowledgement
This research is supported by SERC Public Sector Research Funding (Grant number 1121202012), Agency for Science, Technology, and Research (A*STAR).

As components of the solar energy harvesting and conversion devices, nanostructured assemblies with well-defined geometrical shapes have emerged as high efficiency and economically viable alternate over planar junction thin film architectures. By exploiting phase-separated self-assembly, we present a new approach towards nanostructured thin film solar cells. Through a single step process, we demonstrate growth of a composite film matrix formed as self-assembled, well ordered, phase segregated, oriented p-n type interfacial nanopillars of TiO₂ and Cu₂O , which are epitaxial and single crystalline in both phases. The composite films were structurally characterized to atomic resolution by a variety of analytical tools, and evaluated for preliminary optical properties using absorption measurements. We show nearly complete atomic order at the TiO₂- Cu₂O interface (i.e., p-n junction), and an absorption profile that captures a wide range of the solar spectrum extending from ultraviolet to visible wavelengths. The preliminary current-voltage characteristics of the TiO₂- Cu₂O nanopillar arrays will also be presented.

Oxygen electrocatalysis is central to the efficiencies of direct solar and electrolytic water-splitting devices, fuel cells, and metal-air batteries. Epitaxial oxides have shown high activity for the oxygen reduction and oxygen evolution reactions (ORR/OER).[1-3] However, the lack of fundamental understanding of the oxide surfaces limits mechanistic understanding at the molecular level. In this study, we will discuss the changes on the surface of (La,Sr)CoO3 (100) films grown epitaxially on yttria-stabilized zirconia using in situ ambient-pressure X-ray photoelectron spectroscopy[4-5], in situ X-ray absorption spectroscopy as a function of temperature, potential and oxygen partial pressure. Such information will be correlated with the activity of oxygen electrocatalysis from which surface characteristics that greatly influence the activity will be discussed.
References
1. Crumlin, E. J.; Mutoro, E.; Ahn, S. J.; la O', G. J.; Leonard, D. N.; Borisevich, A.; Biegalski, M. D.; Christen, H. M.; Shao-Horn, Y. J Phys Chem Lett2010, 1, 3149.
2. la O', G. J.; Ahn, S. J.; Crumlin, E.; Orikasa, Y.; Biegalski, M. D.; Christen, H. M.; Shao-Horn, Y. Angewandte Chemie-International Edition2010, 49, 5344.
3. Y.-L. Lee, J. Kleis, J. Rossmeisl, Y. Shao-Horn, D. Morgan. Energy & Environ. Sci.2011, 4, 3966.
4. E. Mutoro, E.J. Crumlin, H. Pöpke, B. Luerssem, M. Amati, M.K. Abyaneh, M.D. Biegalski, H.M. Christen, L. Gregoratti, J. Janek and Y. Shao-Horn, JPCL2012, 3, 40.
5. E.J. Crumlin, E. Mutoro , Z. Liu , M. E. Grass , M. D. Biegalski , Y.L. Lee , D. Morgan , H. M. Christen , H. Bluhm and Y. Shao-Horn, EES2012, 5, 6081.

The efficiency of solid oxide fuel cells (SOFCs) is mainly limited by the rate limiting oxygen reduction reaction (ORR) activity at the cathode. Recently we have demonstrated that La0.8Sr0.2CoO3-δ (LSC113) thin films and (La0.5Sr0.5)2CoO4+δ (LSC214) decorated LSC113 thin films show enhanced ORR activity ~1 and 2-3 orders of magnitude higher, respectively, than bulk LSC113.[1,2] To understand the physical origins of this enhancement, we carried out in situ X-ray reflectivity and ambient pressure X-ray photoelectron spectroscopy studies[3]. We have observed different surface structural and composition changes for LSC113 films with and without LSC214 decorations at elevated temperatures. Combined with ex situ Auger spectroscopy, we identified those surface species are mainly due to changes at the top several nanometers of the films. An explanation will be provided to explain the enhanced ORR activity in these thin film SOFC cathode materials.
(1) Crumlin, E. J.; Mutoro, E.; Ahn, S. J.; la O', G. J.; Leonard, D. N.; Borisevich, A.; Biegalski, M. D.; Christen, H. M.; Shao-Horn, Y. J Phys Chem Lett 2010, 1, 3149.
(2) la O', G. J.; Ahn, S. J.; Crumlin, E.; Orikasa, Y.; Biegalski, M. D.; Christen, H. M.; Shao-Horn, Y. Angewandte Chemie-International Edition 2010, 49, 5344.
(3) Crumlin, E. J.; Mutoro, E.; Liu, Z.; Grass, M. E.; Biegalski, M. D.; Lee, Y. L.; Morgan, D.; Christen, H. M.; Bluhm, H.; Shao-Horn, Y. Energy & Environmental Science 2012, 5, 6081.

Multivalent transition metal oxide SrCoOx (SCO) has rich oxygen content (x)-dependent magnetic and electronic phases, ranging from antiferromagnetic insulator to ferromagnetic metal. The Co valence state change due to the modification of x in SCO mainly governs the material&’s property. In particular, the redox reaction of the material itself and the resultant changes in the physical properties are essential ingredient in applications such as solid oxide fuel cells, gas sensors, and many other devices that exploits ionic (oxygen) transport property.
Using optical spectroscopy complemented by first principles calculation, we studied optical properties and electronic structures of SCO epitaxial thin films. In particular, brownmillerite SrCoO2.5 (BM-SCO) and perovskite SrCoO3 (PV-SCO) thin films have been studied, where they have distinct crystal structures and valence states. BM-SCO has a one-dimensional oxygen vacancy ordered structure with a common Co3+ valence state. On the other hand, PV-SCO has a typical perovskite structure, but the Co4+ valence state is rather unstable, and therefore, difficult to synthesize [1]. Both experimental and theoretical results agreed that these two films have drastically different electronic ground states as well. While BM-SCO showed an insulating ground state with an optical band gap of ~0.5 eV, PV-SCO showed a clear Drude response suggesting a metallic ground state. Despite such large discrepancy in the physical properties, however, we found that a topotactic transformation between two structurally distinct phases could be readily achieved in high quality epitaxial thin films. The temperature dependent, ambient controlled real-time ellipsometry conspicuously showed that these two topotactic phases could be reversibly obtained at relatively low temperatures. Our study suggests that the electronic structure of SCO can be switched reversibly through oxygen insertion and extraction, simultaneously with the crystal structure and Co valence state. Thus, it provides a valuable insight in studying the link between the fundamental physical properties and its technological applications of transition metal oxide epitaxial thin films.
[1] H. Jeen, W. S. Choi, M. D. Biegalski, I. C. Tung, J. W. Freeland, D. Shin, H. Ohta, M. F. Chisholm, and H. N. Lee, “Reversible redox reactions in epitaxial strontium cobaltites” submitted (2012).
The work was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division.

Functional perovskite oxides and their interfaces offer opportunities to overcome the current limits of energy storage and conversion systems related to water dissociation and formation. However, current understanding of the electrochemical reaction mechanisms on complex oxide surfaces and interfaces is far from complete, particularly with regard to time-dependent phenomena occurring in aqueous solutions under applied electric fields. Here we employ in-situ synchrotron techniques to probe the real-time behavior of model oxide surfaces and elucidate the mechanisms governing electrocatalytic processes. We utilize x-ray scattering, spectroscopy, and electrochemical characterization to study epitaxial SrRuO3 (SRO) and LaNiO3 (LNO) thin films grown on Nb-doped SrTiO3 (001) substrates. We find that SRO and LNO exhibit large activities for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), respectively, but their surfaces are not stable under certain electrochemical conditions. For the case of SRO, the c-axis (out-of-plane) lattice parameter changes reversibly with varying electric field at potentials lower than 1.25 V vs. RHE, but several irreversible changes are observed at the higher potentials necessary for OER. These changes are connected to the chemical dissociation of SRO during the reaction. LNO also shows irreversible structural changes near the potentials at which oxygen reduction occurs. The structural instability correlates with changes in oxidation state of the active sites (Ru and Ni) connected to the loss of perovskite structure. With our in-situ methodology, we can determine the stability of active sites on complex oxide surfaces during water dissociation and formation. The results will offer much needed insight and strategies for the creation of advanced electrocatalysts designed at the atomic level.

Point defects determine and control transport and other basic properties in many oxide materials, e.g. oxygen vacancies are responsible for fast oxygen transport in perovskite materials used as SOFC cathodes. However, defect properties can change considerably in nano-sized materials and (ultra)thin films. Prediction of defect properties under such confinement conditions is important for numerous applications.
We discuss the confinement effects through investigation, how the electronic and energetic properties of an oxygen vacancy are changed (in a comparison with the bulk properties) when this defect is confined in an ultrathin insulating film. We have chosen SrTiO3 as a prototypical crystal for a wide class of ABO3 perovskite-structured materials with partly covalent chemical bonding and an oxygen vacancy (Vo, called also the color F center) therein as a typical ionic carrier in this type of solids. The F center is a combination of missing oxygen ion (oxygen vacancy, i.e. an ionic defect) and two electrons trapped in/around the vacancy site (polaron or the electronic defect). As ionic and electronic effects here should be of different extension. Thus, energetic confinement effects could arise due to both restricted ionic relaxation around the vacancy and localization of an electronic wave function of the defect in ultrathin films. Of special importance is a role of change in phonon properties in such confined and defective systems.
One-dimensional confinement effects were modeled within the hybrid HF-DFT LCAO approach considering defects in the center and on the surface of ultrathin films (slabs) consisting of 3-13 crystalline planes. We predict considerable oxygen vacancy migration towards the surfaces. The pronounced decrease of the defect formation energy (ca.1 eV), a much deeper defect band level and a noticeable change in the electronic density redistribution at the near-surface vacancy site with respect to that in the bulk were observed. The results also show that the size effect pertains to the interactions between the oxygen vacancy and two neighboring titanium atoms and orientation (parallel or perpendicular to the surface) of the Ti-Vo-Ti complex. The role of phonons in the free enthalpy of oxygen vacancy formation and its temperature dependence is discussed. Lastly, we discuss anti-ferrodistortive phase transition in SrTiO3 thin films with and without defects.
[1]. E. Kotomin et al, PCCP 13, 923 (2011).
[2] V.E.Alexandrov et al, Eur. Phys. J. B 72, 53 (2009).
[3] R.A. Evarestov et al, Phys. Rev. B 85, 174303 (2012).

Oxide materials are known to be active in a variety of redox reactions, making them important for many energy technologies. Unfortunately, the complex interactions between such reactions and the structural/chemical evolution of the oxide surface are not well-understood. This has hindered progress in many areas, including solid oxide fuel cells, corrosion, and the development of new heterogeneous catalysts. With the advent of high precision growth techniques, epitaxial oxide heterostructures can now be synthesized with controlled strain, orientation, and surface termination. When combined with the unique capabilities of the APS, this permits in situ studies of redox reactions on model oxide surfaces and can therefore lead to the development of much needed structure-reactivity relationships for this important class of materials.
In this presentation, I will discuss studies of both ferroelectric and non-ferroelectric oxide surfaces in reactive environments. For ferroelectric BaTiO3, we find that reducing environments (in carbon monoxide or low oxygen pressures) favor downward polarization, in agreement with a recent computational prediction. Due to the small effect of polarization on spontaneous strain in coherently strained BaTiO3, resonant scattering at the Ba K-edge was used to help determine the atomic displacements. Interestingly, we observe that this result depends on the nature of the bottom electrodes. I will also discuss recent results on epitaxial SrCoO2.5 films and their behavior during CO oxidation.

Mixed ionic and electronic conducting oxides (MIEC) have attracted attention for many applications in energy generation and storage devices. The key to succeed in these applications is achieving efficient oxygen transport, i.e. oxygen diffusion and oxygen reduction reaction (ORR). While restriction on the oxygen diffusion can be overcome by thinning the MIEC, the ORR depends on the surface chemistry of materials. Conventionally, the ORR is known to depend on vacancy concentration, surface electron concentration, and dissociation rate of dioxygen molecules. Thus, efforts have been focused on engineering those factors by a heavy doping of aliovalent elements to increase the catalytically active vacancy sites. In this regards, oxygen vacancy ordered brownmillerite-phase transition metal oxides are promising candidates due to the high concentration of oxygen vacancies, offered from the multivalency of transition metals. However, conventional wisdom on brownmillerite complex oxides is that oxygen transport is extremely low, which has been conjectured as immobile oxygen vacancies below vacancy order-disorder transition temperature. This low oxygen permeation below the temperature was mainly observed in polycrystalline brownmillerite samples. Here, we successfully synthesized ordered oxygen vacancy channels in brownmillerite SrCoO2.5 (SCO) thin films with various crystallographic orientations by using pulsed laser epitaxy. By using out-of-plane electrochemical impedance spectroscopy (EIS) techonology, we have observed that the ORR activity can be drastically improved (up to two orders of magnitude) when the oxygen vacancy channels are properly aligned. In this presentation, the epitaxial growth of SCO thin films with various orientations, and their EIS results will be systematically presented to demonstrate the role of multivalent transition metals or oxygen vacancies on the catalytic oxygen reduction reaction.
The work was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division.

Silicon dioxide is a well known insulator and widely used in modern electronics and information technology. In contrast to bulk SiO₂, silicon dioxide thin films have a low but nevertheless not practically negligible ionic conductivity for metallic cations e.g. Cu^z+ (Ag⁺) and electrode redox reactions at the metal/SiO₂ interface can be demonstrated. However, the kinetics of this reaction and the transport of mass and charge on the nanoscale is controversially discussed and an improvement of the models proposed in the literature is needed. In this study we investigated anodic reduction and oxidation processes of Cu/Cu^z+ (Ag/Ag⁺) at the interface with amorphous SiO₂. Special attention has been paid to the role of moisture and/or oxygen half cell reactions supplying counter charge(s) at the inert electrode. Detailed understanding of these nanoscale transport and redox processes is prerequisite for the development of various encouraging applications ranging from new concepts for logical and artificial neuromorphic devices to highly dense future non-volatile memories.

We report the epitaxial growth of an insulating ferromagnetic oxide on silicon based on SrTiO3. SrTi_1-xCo_xO_3-δ films (x = 0.1 to 0.5) were grown on Si(100) substrates using a three-unit cell buffer of undoped SrTiO₃ by molecular beam epitaxy (MBE). Magnetization as a function of magnetic field was performed for all samples at room temperature and at 10 K. Room-temperature ferromagnetism with a saturation moment of 3 mu;_B/Co is confirmed in single phase samples with composition 30-40% cobalt. Cross-sectional transmission electron microscopy confirms the single crystalline nature of the cobalt-substituted SrTiO₃. We also performed x-ray photoelectron spectroscopy (XPS) to determine the composition and chemical state of the samples. The measurements show that cobalt is in the +2 high spin state with no detectable cobalt metal. The XPS measurements also reveal that a near equivalent number of oxygen vacancies are created for every cobalt atom in the crystal. Resistivity vs. temperature measurements of samples exhibiting ferromagnetism show highly insulating behavior. In order to elucidate the origin of ferromagnetism while accounting for the experimental observations, we also performed first-principles calculations of cobalt-substituted SrTiO₃ with one oxygen vacancy per cobalt. The calculations show that ferromagnetism with high spin Co²⁺ can be stabilized above 25% Co concentration similar to experiment. The calculations also confirm the insulating nature of the material. The ability to integrate an insulating room-temperature ferromagnet on silicon in epitaxial form can have potential application in spin filtering spin injection contacts and overcome the problems of impedance mismatch and interface losses for spintronics applications using silicon as the spin host.

Cerium oxide (CeO2) has been widely used as the electrolyte materials for solid oxide fuel cells in recent years. Thin film is one of the most popular forms for the oxygen ionic conductivity investigation, especially after the huge increase of ionic conductivity was reported for multilayer heterojunction [1]. On the other hand, grain boundary (GB) in CeO2 is also known to be another key factor which greatly affects the ionic conductivities [2]. However, underlying mechanism for changing oxygen ionic transport is not yet well established due to the complex atomic and electronic structure. In our study, the atomic and electronic structures of CeO2-ZrO2 heterointerface and CeO2 GBs were investigated by atomic-resolution electron microscopy and spectroscopy with the aid of theoretical calculations.
CeO2 thin films were heteroepitaxially grown on the yttria-stabilized zirconia (YSZ) bicrystal substrates by pulsed laser deposition (substrate temperature: 900 °C, oxygen pressure: 3.0 × 10-3 Pa). This allows us to study CeO2-ZrO2 hetrointerface at the film/substrate interface as well as single GBs obtained in the CeO2 films. Further, various kinds of GBs were fabricated, which are selected from the geometrical coincidence viewpoint. Atomic and electronic structures observations were carried out with aberration (Cs) corrected scanning transmission electron microscopy (STEM, JEOL ARM200 equipped with CEOS Cs corrector) and electron energy loss spectrometer (Enfina, Gatan Inc.). Theoretical studies were also conducted for obtaining stable GB atomic and electronic structure, with density functional theory under local-spin density + U approximation.
STEM imaging show that atomically bonded CeO2-ZrO2 interface and CeO2 GBs were successfully fabricated. Formation of misfit dislocation was confirmed for the CeO2-ZrO2 interface, and peculier atomic arrangement was observed for the CeO2 GBs. EELS measurement around the interface revealed that Ce is partially reduced to 3+ state near the interface, and furthermore, is more reduced near misfit dislocations. This suggests that more oxygen vacancies would be formed around the interface. On the other hand, degree of Ce reduction and, therefore, oxygen nonstoichiometry depends on GB geometrical coincidence in the films, which is also supported from the atomic structure calculations of the GBs [3,4]. These results suggest that different GBs in CeO2 films would have different degrees of contribution to oxygen ionic conductivity.
[1] J. Garcia-Barriocanal et al. Science. 321, 676 (2008)
[2] X. Guo and R. Waser. Prog. Mater. Sci. 51, 151 (2006)
[3] H. Hojo et al. Nano Lett. 10, 4668 (2010)
[4] B. Feng et al. App Phys Lett. 100, 073109 (2012)

The interplay between charge, spin, and orbital degrees of freedom in transition-metal oxides leads to a wide range of physical properties including Mott insulator, charge and spin ordering, multiferroics, and superconductivity. A number of novel effects and functions that are inaccessible in the bulk form have been realized at abrupt interfaces created by means of atomic-scale epitaxy and advanced nano-characterization techniques. For example, it has been revealed that a superconducting state and ferromagnetism coexist at the interface between LaAlO₃ and SrTiO₃, whereas the ground states in bulk materials are nonmagnetic and insulating. Also, strongly correlated effects have been observed for a fractional quantum-Hall state at the ZnO/MgZnO interface. In this talk, we review the electronic properties and phenomena observed at these interfaces and related materials. We also discuss possibility to apply transition-metal oxides to solar-energy-harvesting photochemistry.

In the past decade or so, there has been a growing interest in amorphous oxide semiconductors (AOS) represented by amorphous In-Ga-Zn-O (a-IGZO) as the most promising materials for thin-film transistors (TFTs). A distinctive feature of a-IGZO is high field effect mobilities (>10 cm²V^-1s^-1) of a-IGZO TFTs fabricated even at room temperature (RT). [1, 2] These values are much higher than those of a-Si:H TFTs (<1 cm²V^-1s^-1). Meanwhile, the band gap energy (E_g ~3.1 eV) of a-IGZO is too large to apply to an active layer of a solar cell. In this study, we prepared amorphous Cd-Ga-O thin films as a new AOS with narrow band gap energy (< 3 eV) appropriate for a solar cell material.
Cd-Ga-O thin films were deposited on silica glass substrates by rf-magnetron sputtering method without intentional substrate heating. CdO (99.99%) and Ga₂O₃ (99.9%) mixed powders with various mixing ratios were employed as sputtering targets. Pressure during depositions was maintained at 1.5 Pa with pure Ar gas (99.999%).
The X-ray diffraction patterns of obtained thin films did not show any distinguish peaks, suggesting that these films are possibly amorphous. The atomic ratios of Cd to Ga in the films were varied from Cd:Ga = 0.1:0.9 to 0.8:0.2. The optical band-gap energies estimated from the Tauc&’ plots were decreased from 4.1 to 2.3 eV by increasing Cd concentration, however, Hall mobilities of all of these films were ~10 cm² V^-1s^-1: The Hall mobility, carrier concentration, and electrical conductivity of the film (E_g = 2.3 eV) were 24 cm² V^-1s^-1, 3×10²⁰ cm^-3 and 1.2×10³ S cm^-1, respectively.
[1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, Nature , 432 , 488 (2004).
[2] T. Kamiya, K. Nomura, and H. Hosono, Sci. Technol. Adv. Mater. , 11 , 044305 (2010).

Metal oxides with d-shells are increasingly important technologically, and ZnO, especially, is a prototypical d band semiconducting oxide with many important applications, including photovoltaics and photocatalysts. A fundamental understanding of the electronic structure of ZnO is important in improving the performance of ZnO in these applications. While theoretical electronic structure methods accurately predict the bandstructure of s-p materials, such as GaAs, active d-electron materials pose a challenge to these methods due to the presence of strong self-interaction energy. A common theoretical approach employing density functional theory (DFT) calculations with local density approximation and generalized gradient approximation are well-known to underestimate the band gap of ZnO, giving values of less than 1 eV, compared to the experimental value of 3.44 eV. In order to accurately predict band structure, it is necessary to go beyond DFT, which, for semiconducting periodic systems, is usually carried out through calculation of the electron self-energy in the GW approximation. Though recent GW calculations are able to produce band gap that is comparable to experiments, they still place the Zn-d band too high in energy by about 1 eV. Experimentally, angle-resolved photoemission spectroscopy (ARPES) probes the ZnO band structure, including the Zn-d band, which were not determined precisely in previously reported literature.
In our current approach, we employ high resolution ARPES of ZnO together with resonant photoemission as a stringent test of theoretical treatments of ZnO. We show that GW calculations based on various input wavefunctions lead to too-high Zn-d band energies. This issue is resolved by applying an on-site potential for Zn-d states to correct the d band energy. The calculated band gap from this theoretical approach agrees very well with experimental band gap and band structure, including the band position of Zn-d states,. The improved GW quasi-particle energies will be useful for improving the predictions of various material&’s properties, including defect formation energies, band-offsets, and ionization potentials.

The main constraint for photovoltaic energy conversion in conventional semiconductor pn junctions is that the chemical potential of photo-excited charge carriers is limited by the presence of an indirect bandgap of energy E_g. Transmission losses of photons with E < E_g and thermalization losses of photons with E > E_g determine the so-called Shockley-Queisser limit of photovoltaic energy conversion. It is proposed that the conversion efficiency can be strongly improved by quenching the rapid thermalization process of photo-excited electron-hole pairs and converting such “hot carriers” into a photo-voltage [1]. Consequently, finding mechanisms, where large lifetimes of photo-excited carriers are established would be highly desirable. Here, we present a case study on the relation between atomic level structure, electric transport and photovoltaic effect in a manganite-titanite pn-heterojunction [2]. For the p-doped material we have selected Pr_1-xCa_xMnO₃ (PCMO) which exhibit strong electron-electron as well as strong electron-phonon correlations. In the visible range, its absorption spectrum is dominated by a polaron band with lifetimes of the optically excited “hot polarons” up to ns. At room temperature an optical bandgap is absent whereas a charge bandgap is opening below the charge ordering temperature of T_CO ~240 K. The n-doped material is SrT_i1-yNb_yO₃ (STNO, y = 0:002 and 0.01) which has a bandgap of 3.2 eV. High-resolution electron- microscopy and -spectroscopy reveal a nearly dislocation-free, epitaxial interface and gives insight into the local atomic and electronic structure. The presence of a photovoltaic effect under visible light at room temperature suggests the existence of mobile excited polarons within the band gap-free PCMO absorber. Electron beam induced current (EBIC) which reveals a polaron diffusion length of about 20 nm. The temperature-dependent rectifying current-voltage characteristics prove to be mainly determined by the presence of an interfacial energy spike in the conduction band and are strongly affected by the colossal electro-resistance (CER) effect. The photocurrent can be strongly modified by the CER effect.
Combined atomic column resolved Electron Energy Loss Spectroscopy (EELS) and DFT calculation give detailed insights into the electron structure of the involved materials and the interface and confirm the presence of an energy spike. We discuss the results in the view of the single particle bandstructure. In addition, we use semi-empirical models for the analysis of the small polaron absorption.
[1] W. A. Tisdale, K. J. Williams, B. A. Timp, D. J. Norris, E. S. Aydil, and X.-Y. Zhu, Science 328, 1543 (2010).
[2] G. Saucke, J. Norpoth, D. Su, Y. Zhu and Ch. Jooss, Phys. Rev. B 85, 165315 (2012)

Mg_xZn_1-xO/ZnO provides extremely clean two-dimensional electron systems (2DESs) that exhibit the integer and fractional quantum Hall effects [1,2], as in GaAs-based heterostructures, and mobilities as high as 800,000 cm²/Vs [3,4], which is the highest among 2DESs in oxide heterostructures. The uniqueness of Mg_xZn_1-xO/ZnO, compared with the GaAs-based heterostructures, lies in the fact that such clean 2DESs emerge with effective mass of electrons in ZnO, which is over four times larger than that in GaAs, indicating that the effects of electron correlation are expected to be much more pronounced than their GaAs counterparts. In the low carrier density regime, transport measurements in high magnetic fields have indicated the presence of strong electron correlation in the 2DESs in Mg_xZn_1-xO/ZnO through the observations of enhanced effective electron masses and spin susceptibilities. However, other experimental probes, presumably optical techniques, are requisite for quantitative discussion and understanding of the correlation effects.
Here we show the results of combined magnetotransport and cyclotron resonance experiments, which can be unique tools of gauging the strength of the electron correlation. The prerequisites for this method are 2DESs with single valley occupation, so that the 2DESs in Mg_xZn_1-xO/ZnO heterostructures are the ideal systems, in contrast that most of 2DESs exhibit complex band structures.
Mg_xZn_1-xO/ZnO heterostructures with sheet carrier densities from 1.9 to 12 cm^-2 were grown by molecular beam epitaxy utilizing distilled pure ozone. The cyclotron resonance measurements were performed in the frequency range from 9 to 400 GHz using the same batch of samples that were used in the magnetotransport measurements.
We have observed a steep enhancement of transport masses (m*_tr) with decreasing carrier density, whereas the effective masses determined by the cyclotron resonance (m*_CR) are independent of the carrier density and are comparable to the effective mass of bulk ZnO. The discrepancies between m*_tr and m*_tr directly gauges the strength of the electron-electron interactions. Therefore, observed enhancement of m*_tr, which exceeds m*_tr by nearly 60 %, is a direct consequence the electron-electron interactions [5].
[1] A. Tsukazaki et al., Science 315, 1388 (2007).
[2] A. Tsukazaki et al., Nature Mater. 9, 899 (2010).
[3] D. Maryenko et al., Phys. Rev. Lett. 108, 186803 (2012).
[4] J. Falson et al., Appl. Phys. Express 4, 091101 (2011).
[5] Y. Kasahara et al., Phys. Rev. Lett., Accepted.

Meeting the growing demands for clean, renewable energy requires rapid development of energy conversion technologies that rely on complex, tailored functionality. For engineered functionality, oxide materials offer a wide spectrum of phenomena - such as magnetism, ferroelectricity, and surface reactivity - that provide a playground for materials design and integration. The direct coupling of material properties across a hetero-interface is a potentially transformative route to achieving new types of functionality inaccessible at bulk scales. For example, TiO₂ has been considered to be a strong candidate for photocatalytic water splitting reactions to produce hydrogen. It exhibits photoactivity under UV light irradiation and strong oxidation potential of photo-generated holes. Despite these benefits, TiO₂-based systems continue to exhibit low photocatalytic efficiency. One of the reasons for low efficiency is recombination of the photo-generated electron-hole pair before the water-splitting reaction can take place. Tuning the surface chemistry of TiO₂ layers supported by a polarizable perovskite underlayer may enhance the photocatalytic activity on the surface of a TiO₂-based system.
In this study, we use first-principles electronic structure methods based on density functional theory to explore the TiO₂/BaTiO₃ thin-film hetero-system comprising of a BaTiO₃ thin film sandwiched on both sides by several atomic layers of TiO₂ (TiO₂ - BaTiO₃ - TiO₂). The polarization of the BaTiO₃ is expected to affect the surface chemistry of TiO₂ thin film and adsorption energy of water molecules on the surface. However, thin-film perovskites such as BaTiO₃ are subject to depolarization fields, and do not display ferroelectricity as in the bulk form. Therefore, the interface between the perovskite and TiO₂ needs to be engineered to recover polarization. To recover polarization in our simulations, we vary the interfacial composition and number of monolayers of TiO₂ on either side of BaTiO₃ thin film. Our results demonstrate that breaking the in-plane symmetry at the two TiO₂/BaTiO₃ interfaces provides charge compensation and creates interface dipoles that allow the BaTiO₃ thin film to recover polarization. For appropriate interfacial compositions, the polarization can even be enhanced relative to the bulk ferroelectric polarization. Finally, we illustrate the application of our approach to photocatalytic water splitting by demonstrating the effect of this defect-induced polarization on water molecule adsorption energies on the TiO₂ surfaces.

The integration of functional oxides requires the use of complex heterostructures involving oxides of which the structure and properties strongly depend on the strain state and strain-mediated interface coupling. The experimental observation of strain-related effects of the individual components remains challenging. Here we report a Raman scattering investigation of complex multilayer BaTiO3/LaNiO3/CeO2/YSZ thin film structures on silicon [1]. It is shown that the Raman signature of the multilayers differs significantly for three different laser wavelengths (633, 442 and 325 nm). Our results demonstrate that Raman scattering at various wavelengths allows both the identification of the individual layers of a functional oxide multilayers and monitoring their strain state. It is shown that all layers of the investigated multilayer are strained with respect to the bulk reference samples, and that strain induces a new crystal structure in the embedded LaNiO3. Based on this, we demonstrate that Raman scattering at various wavelengths offers a well-adapted, non-destructive probe for the investigation of strain and structure changes, even in complex thin film heterostructures and more generally in oxide composite structures.
[1] J. Kreisel, M. C. Weber, N. Dix, F. Sánchez, P. A. Thomas, and J. Fontcuberta, Adv. Funct. Mater. , 10.1002/adfm.201201272 (2012).

The conversion of solar energy to produce hydrogen by means of water splitting is a potentially transformative approach to achieving a clean and renewable source of energy. Photo-produced hydrogen is a valuable fuel and energy carrier, which is easier to store than electricity or heat, and it is nonpolluting, inexhaustible, and flexible with respect to energy conversion in heat or electricity [1]. However, despite its promise, the photocatalytic splitting of water into hydrogen and oxygen using sunlight still suffers from low energy conversion efficiencies. In this study, we explore the possibilities of obtaining enhanced photocatalytic conversion by exploiting the properties of polar interfaces coupled with finite size effects. By tailoring the properties of a polar surface junction, it may be possible to realize tailored surface chemistries that are optimized for oxidation and/or reduction. To demonstrate this concept, we computationally explore the intersection of finite size effects and surface polarity compensation mechanisms in the anatase (TiO2) / cuprous oxide (Cu2O) heterojunction, a system which has recently been realized in experiment [2]. Due to its low valence band edge, anatase has a good oxidation potential; by contrast cuprous oxide has a good reduction potential due to its high conduction band edge. This makes the heterosystem an intriguing candidate for photocatalytic water splitting; however, the interface between the two materials is polar, unstable in the thermodynamic limit, and subject to polarity compensation mechanisms. Using the first principles techniques based on density functional theory, we model the polar interface between TiO2 and Cu2O and analyze the polarity compensation mechanisms that take place for finite-sized systems. In the thermodynamic limit, such polar heterojunctions have diverging potential (polar catastrophe). However, for thin-film systems composed of only several atomic layers on either side of the heterointerface the intersection of this polarity with finite size effects results in novel compensation mechanisms that can result in tunable surface chemistries suitable for enhanced photocatalysis. Thus, the goal of our study is to establish that polar thin-film heterointerfaces can be useful systems for catalysis, and to demonstrate that computational modeling of these metal oxide systems can help guide experimental work in order to deterministically control surface orientation and chemistry for optimized photocatalysis.
[1] C. A. Bignozzi, Topics in current chemistry: Photocatalysis (Springer, Heidelberg, 2011).
[2] S. Lee, C -W. Liang, and L. W. Martin, ACS Nano 5, 3736-3743 (2011)

Materials for efficient solar photocatalysis (including water-splitting) require absorption of visible light, spatial separation of reduction and oxidation sites, and minimal electron-hole recombination. Complex layered-oxide materials and co-catalysts provide reaction site and charge separation, but generally do not possess band gaps small enough to be useful under solar illumination. Traditional group IV and III-V semiconducting materials have better visible light absorption, but these materials are not stable in water under illumination. Our approach is to utilize anatase TiO₂, a well-studied material with strong catalytic activity and excellent chemical stability, grown epitaxially on a material with more efficient visible light absorption and charge separation. To achieve this goal, we select the multiferroic oxide BiFeO₃ which possesses a smaller direct band gap (2.7 eV) and a robust spontaneous polarization (95 mu;C/cm²). Epitaxial thin films of 5-100 nm anatase TiO₂ / 50-300 nm BiFeO₃ / 50 nm SrRuO₃ were grown by pulsed-laser deposition and characterized by X-ray diffraction, atomic and piezoresponse force microscopy (AFM-PFM), and scanning electron microscopy. Films were grown on SrTiO₃ (100), (110), and (111), DyScO₃ (110), and TbScO₃ (110) single crystal substrates to explore the effect of film orientation and domain structure on the optical and photo-electrochemical (PEC) properties. We have demonstrated the ability to write up- and down-polarized domains in the BiFeO₃ through the TiO₂ layer (up to 10 nm thick) using PFM, and have conducted photo-deposition experiments of Ag and PbO₂ from solution under simulated AM1.5 spectrum illumination to probe the role of polarization on the PEC activity. Using energy dispersive X-ray spectroscopy and AFM, we studied the deposition process and observe enhanced deposition on TiO₂/BiFeO₃ heterostructures compared to single-layer TiO₂ films. Additionally, photo-reduction occurs preferentially on down-poled regions and photo-oxidation on up-poled regions of the samples. In order to elucidate the mechanism of this preferential deposition, we measured the heterostructures&’ light/dark current-voltage response. These studies show that the magnitude of the short-circuit current density, J_SC, is dependent on the direction of the polarization in the film, as a result of the anomalous photovoltaic effect which varies with polarization direction. Using a low-signal gas chromatography (GC)/PEC reactor we explore the implications for water-splitting by probing the H₂ and O₂ evolution rates of the heterostructures and find that the deterministic control and spatial separation of reduction and oxidation sites significantly impact efficiency. As part of this work, we will discuss ongoing studies of a two-part photocatalytic device that produces hydrogen and oxygen at separate reaction surfaces thus eliminating the need for gas separation and making the entire photocatalytic process more efficient.

Perovskite oxides are perhaps the most widely studied and technologically important of all the ABO3 phases. The remarkable versatility of the perovskite structure (the A and B site can accommodate nearly every element of the periodic table) leads to a huge range of properties, including (but not limited to) ferroelectricity, ferromagnetism and colossal magnetoresistance, piezoelectricity, multiferroicity and metal-insulator transitions. One reason for this is that nearly all cubic perovskites are unstable to energy-lowering structural distortions and hence typically have rich structural phase diagrams. The most common distortions are those that give rise to ferroelectricity (usually an off-centering of the B-site cation) and tilts or rotations of the BO6 octahedra. Whereas there are many perovskites that are either ferroelectric or have rotated octahedra, there are very few perovskites that are both ferroelectric and have rotated octahedra. This observation has lead to the widespread assumption that ferroelectricity and octahedral rotations are fundamentally incompatible.
Using first-principles density functional theory calculations, in combination with crystal chemistry and symmetry principles, we have shown that ferroelectricity and octahedral rotations are not fundamentally incompatible. In fact, the most fertile place to search for new ferroelectrics may be the place that has thus far been considered the least likely to contain them: materials that are expected to have large rotations. I will show why and how ferroelectricity is suppressed in the most common space group adopted by perovskites (Pnma) and explain how we can use this knowledge to design new ferroelectrics and functional materials. Our results suggest that although BaTiO3 was the first known perovskite ferroelectric and has perhaps been the most intensively studied, it is an exception rather than the rule and is not representative of the majority of perovskite and perovskite-like ferroelectrics.

Although ferroelectric switching has been one of the most significantly researched fields in solid state physics, it is still an intriguing research topic for modern electronic devices. One of the most arguable comments reported recently is the involvement of the negative capacitance (NC) effect in the ferroelectrics in ferroelectrics (FE) - dielectrics (DE) stacked system (Salahuddin and Datta, Nano Lett. 2009), which appears to thermodynamically stabilized by the DE, but could be destabilized by the formation of FE domains. NC effect will revolutionize many nano-scale electronic devices if it could be stably achieved. Even though their experimental verification has been reported, direct observation of negative capacitance is yet to be made. This can be fundamentally difficult because of the very high tendency of domain formation in ferroelectric thin film system; especially the FE-DE system could involve a serious depolarization effect which largely favors the poly-domain structure. In this work, the underlying physics of the NC effect in ferroelectrics and methods to stabilize the NC effect will be introduced. In addition, recent finding on the observation of the NC effect from the switching domain of ferroelectric thin film in a FE-DE stacked system will also be discussed. The slow injection of compensating charge by the involvement of the thin DE layer made the observation feasible. The estimated transient capacitance density of the FE-DE system was much higher than that of FE or DE layer alone.
*Presenting author&’s e-mail: [email protected]

Electrostatic doping of correlated oxides is of interest for probing electronic phase transitions and exploring device applications. Recent developments of electric double layer transistors (EDLT) with ionic liquid gating have enabled large carrier accumulation density and stimulated the study of carrier induced phase transitions in several oxide materials. However, possible electrochemical reactions at the electrolyte/oxide interface could mimic the electrostatic effect and are therefore important in interpreting these field-effect phenomena. On a more optimistic front, such chemical reactions allow one to create new phases of matter far away from equilibrium, although not what one may be typically looking for when studying field effect response in a transistor device. Here, we use DEME-TFSI and VO₂ as a model system and focus on the understanding of (electro/)chemical stability of ionic liquid/correlated oxide interface in EDLT geometry. We show that the gate leakage current though the ionic liquid could provide critical information on the electrochemical reactions at the interface. In combination with the results from X-ray photoelectron spectroscopy, impedance spectroscopy, and the charging dynamics of the electric double layer capacitance, we map a phase space for gate voltage induced electrostatic versus electrochemical response. Discussion in a broader context of high carrier concentration correlated insulators will then be presented.

Multiferroic heterostructures combining ferromagnets and ferroelectric materials play a key role in Spintronics for their applications in the design of novel devices [1-3]. La0.7Ca0.3MnO3 (LCMO) optimally doped epitaxial films were grown on ferroelectric BaTiO3 (BTO) substrates. Electronic transport (magnetoresistance and electroresistance) and magnetic properties showed important anomalies in the temperature interval between 60 and 150 K, below the metal-insulator transition. Hysteresis loops display extremely unusual features: after switching at coercivity, the magnetization overshoots the eventual high-field value, so called Matteucci-cycles [4]. We study with x-ray diffraction the complex strain distribution in the film caused by the substrate. Scanning probe microscopy reveals changes in BTO surface morphology at low temperature. LCMO thickness is a critical factor: 120-A-thick films showed large anomalies sensitive to electric poling of the BTO, whereas the behavior of 150-A-thick films is closer to that of the reference LCMO samples grown on SrTiO3 [5]. Based on magnetic depth profiles from Polarized Neutron Reflectometry, magnetic anisotropy from Ferromagnetic Resonance, and a magneto-elastic anisotropy model, the following picture is proposed: through inhomogenous strain and electric polarization effects, the ferroelectric substrate induces an inhomogenous spin distribution in the magnetic layer, compatible with the magnetic granular behavior seen in magnetism and electronic transport. This implies the coexistence of two magnetic moment populations, in-plane and out-of-plane ferromagnetic patches in La0.7Ca0.3MnO3, possibly interspersed with antiferromagnetic regions, as predicted recently [6,7]. Substrate poling effects and magnetoelectric coupling are investigated.
[1] C. A. F. Vaz and C. H. Ahn, in Ferroelectrics—Physical Effects, edited by Mickaeuml;l Lallart (InTech, 2011), p. 329.
[2] H. Lu et al., Appl. Phys. Lett. 100, 232904 (2012).
[3] W. Eerestein, N. D. Mathur, and J. F. Scott, Nature 442, 759 (2006).
[4] A. Alberca et al., Phys. Rev. B 84, 134402 (2011)
[5] A. Alberca et al., Phys. Rev. B 86, 144416 (2012)
[6]: J. D. Burton and E. Y. Tsymbal, Phys. Rev. B 80, 174406 (2009).
[7]: S. Dong, X. Zhang, R. Yu, J. M. Liu, and E. Dagotto, Phys. Rev. B 84, 155117 (2011).

Recently, the important role of flexoelectric effects in ferroelectric nanostructures has been emphasized by demonstration of mechanically-induced polarization reversal in ultrathin ferroelectric films.[1] Demonstration of voltage-free switching of ferroelectric polarization by tip-induced strain gradient represents a paradigm shift in controlling electronic properties of ferroelectrics and functional oxide materials in general. Here, we report calibration of the flexoelectric effect by relating the mechanical stress generated by the scanning probe microscopy (SPM) tip and the threshold switching voltage in nanometer-thick epitaxial films of BaTiO3. The experimental data have been compared with results of finite element modeling, which allowed assessment of the flexoelectric coefficient and mechanically-induced switching behavior as a function of the films thickness.
[1] H. Lu, et al., Mechanical Writing of Ferroelectric Polarization, Science 336, 59 (2012).

In this talk we introduce a new thin film transistor (TFT) comprised exclusively of metal oxide thin films. The unique innovation is the use of tungsten oxide (WO₃) as the channel layer. In its oxidized stated, tungsten oxide is a transparent wide band gap (>3 eV) insulator. However, when exposed to a negative bias light ions (H⁺, Li⁺) can be intercalated into the material, providing it with mixed conductivity. This change in conductivity is accompanied by a change in transmission due to the well known electrochromic effect. In this talk we describe the fabrication and performance of 4 terminal TFTs. FTO-coated glass served as the gate electrode, and the alumina dielectric (~ 100 nm) was deposited by plasma-enhanced chemical vapor deposition. The WO_x channel is deposited by reactive sputtering, and it is shown that it is critical to control the stoichiometry of the deposited material through appropriate modulation of the Ar/O₂ ambient. Aluminum evaporation and conventional photolithography procedures were developed to pattern aluminum contacts that serve as the source/drain. Geometries explored included channel lengths ranging from 40 - 120 mu;m, and channel widths from 50 - 200 mu;m. The four terminal device was operated as follows. The device is placed in solution and ions were injected and removed in response to the application of an applied bias between the FTO gate and a platinum counter electrode in the solution (V_GC). This parameter turns the TFT on and off while modulating its transparency. The source/drain current was then measured at a function of V_GC. It was found that the devices have very low turn on voltages (V_on < 1 V). The contrast ratio in the source-drain current is significant (I_on/I_off > 10⁴). Moreover, the leakage current between source and gate was also ~7 orders less than the source-drain current, showing that alumina was an effective dielectric. Optical modulation was also observed in direct registry with the applied gate voltage. We will report on the sensitivity to film synthesis parameters and device geometry. This device introduces a new paradigm for solid state device technology, connecting the fields of optics, electronics, and solid-state ionics.

Memristors are continuously tunable resistors that emulate synapses. Conceptualized in the 1970s, they traditionally operate by voltage-induced displacements of matter, but the mechanism remains controversial. Purely electronic memristors have recently emerged based on well-established physical phenomena with albeit modest resistance changes. In this talk, we will show that voltage-controlled domain configurations in ferroelectric tunnel barriers [1] yield memristive behaviour with resistance variations exceeding four orders of magnitude and a 10 ns operation speed [2,3,4]. We will present results for various ferroelectric and electrode materials. Using models of ferroelectric-domain nucleation and growth we explain the quasi-continuous resistance variations and derive a simple analytical expression for the memristive effect. Our results suggest new opportunities for ferroelectrics as the hardware basis of future neuromorphic computational architectures.
Support from the ERC (grants n° 267579 and 259068) is acknowledged.
[1] V. Garcia et al, Nature 460, 81 (2009)
[2] M. Bibes et al, WO 2010142762-A1 (2010)
[3] A. Chanthbouala et al, Nature Nanotech. 7, 101 (2012)
[4] A. Chanthbouala et al, Nature Materials 11, 860 (2012)

Complex oxide heterostructures offer a fascinating platform for studying various physical properties and coupling of functionalities. In particular, the ferroelectric (FE) control of electronic transport is one of the emerging technologies in oxide heterostructures that utilizes the spontaneous electric polarization. The conventional wisdom on electroresistance in ultrathin FE tunnel junctions (FTJs) exploits solely the differences in the electrostatic potential across the FTJs that are induced by changes in the FE polarization direction. Here, we show that in practice the junction current ratios between the two polarization states can be abnormally larger than suggested by those simple argumentations, and that only FTJs with nanometer thin layers can effectively produce a high on/off electroresistance ratio. To understand these surprising results, an additional control parameter is employed which is related to the crossing of electronic and magnetic phase boundaries of the correlated electron oxide used as an electrode in the nanojunctions. This FE-induced phase modulation at the heterointerface ultimately results in a colossal electroresistance effect. We used both experimental and theoretical approaches to investigate the underlying mechanism of electroresistance in PbZr0.2Ti0.8O3/La1-xSrxMnO3 (x = 0.20, 0.33 and 0.50) oxide heterostructures. A comparative study of out-of-plane and in-plane transport properties on heterostructures with different LSMO hole-doping levels clearly attested to the relevance of the phase modulation described above. Thus, our study highlights that the strong coupling of degrees of freedom across heterointerfaces could yield versatile and novel applications in oxide electronics.
*The work was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division.

A ferroelectric field effect transistor (FE-FET) modulates carrier density in a non-volatile manner by electrostatically accumulating and depleting charge at the interface of the channel and ferroelectric gate. The rare earth nickelate LaNiO3 is metallic in bulk, but exhibits a thickness dependent metal-insulator transition (MIT) which can be controlled as a function of chemical substitution. Bulk NdNiO3 exhibits a MIT and anti-ferromagnetic behavior. When LaNiO3 and NdNiO3 are grown in thin film form, the MIT can be tuned as a function of thickness and strain. We use molecular beam epitaxy to grow single crystalline thin films of rare nickelates on LaAlO3 substrates with atomically abrupt interfaces. Here, we show that by coupling ferroelectric PbZrTiO3 (PZT) with an ultra-thin film of LaNiO3 or NdNiO3, we can control the metal-to-insulator transition in the rare earth nickelates. Thin films of LaNiO3 can be tuned from an insulating to conducting state that exhibits weak localization, and room temperature changes in resistivity of 80% as a function of ferroelectric polarization can be achieved for 3 u.c. films of LaNiO3. Large changes in the MIT transition temperature in NdNiO3 can be induced as a function of the ferroelectric polarization.

Control over band alignment at oxide-semiconductor interfaces is essential for improving the performance of devices such as thin film solar cells and energy efficient solid state lighting. However, despite its technological importance, the engineering of band alignment remains as much art as science when new oxide materials are introduced. We have previously described how the surface potential of a conducting transition metal oxide can be greatly affected by the oxygen stoichiometry of the film [1]. Here we will address the question of whether the same phenomena affect the barrier height of oxide-semiconductor contacts. We will present studies of Schottky junctions formed between Al-doped ZnO (AZO) conducting oxide thin films and lightly doped silicon. We synthesized AZO films with varying oxygen content, and performed transport measurements (I-V and C-V) on both n- and p-type metal-oxide-semiconductor (MOS) devices to study the degree to which oxide stoichiometry can be used to tune the junction characteristics. Our results suggest that, unlike conventional metals, metallic oxides may allow control over the barrier height in silicon MOS devices. We will interpret our results in the context of the Bardeen and Schottky models of metal-semiconductor junctions, and we attempt to formulate a design guideline for engineering oxide-semiconductor interfaces.
[1] R. Jaramillo and Shriram Ramanathan, “Electronic granularity and the work function of transparent condudcting ZnO:Al thin films”, Adv. Funct. Mater. 21, 4068 (2011).

Spin polarized transport in ferromagnetic tunnel junctions, characterized by tunnel magnetoresistance, has already proven a high application potential in the field of spintronics and in magnetic random access memories (MRAM). Until recently, in such a junction the insulating barrier played only a passive role keeping apart the ferromagnetic electrodes in order to allow electron tunneling. However, a new dimension was added to these devices by replacing the insulator with a ferroelectric material, which possesses permanent dielectric polarization switchable between two stable states. The obtained multiferroic tunnel junction (MFTJ) is a non-volatile memory device with four states, given by two possible ferroelectric polarization directions in the barrier and two different magnetization alignments of the electrodes.
Here, we will show that due to the coupling between magnetization and ferroelectric polarization at the interface between a magnetic electrode and the ferroelectric barrier of a MFTJ, the spin polarization of the tunneling electrons can be reversibly and remanently inverted by switching the ferroelectric polarization of the barrier.[1] Selecting the spin direction of the tunneling electrons by short electric pulses in the nanosecond range rather than by an applied magnetic field is highly relevant for spintronics, especially for spin-based information technology.
[1] D. Pantel, S. Goetze, D. Hesse and M. Alexe, Nature Materials 11, 289 (2012).

The functionality of energy storage and generation systems like Li-ion batteries or fuel cells is not only based on but also limited by the flow of ions through the device. To understand device limitations and to draw a roadmap to optimize device properties, the ionic flow has to be studied on relevant length scales of grain sizes, structural defects, and local inhomogeneities, i.e. over tens of nanometers. Knowledge of the interplay between the ionic flow, material properties, and microstructure can be used to optimize the device properties, for example to maximize energy density, increase charging/discharging rates, and improve cycling life for Li-ion batteries for applications in electric vehicles and aerospace. Until recently, existing solid-state electrochemical methods were limited to a spatial resolution of 10 micron meter or greater, well above the characteristic size of grains and sub-granular defects. Our development of Electrochemical Strain Microscopy (ESM) has reduced this resolution limit down to 100 - 10 nm which allows studies of the local Li-ion flow in electrode materials and across interfaces.
Here, we present how ESM can be used to measure ionic transport properties and extract spatially resolved maps of the activation energy and diffusivity in thin film cathodes for Li-ion batteries. We also explore how the ionic transport is influenced by the electrode microstructure such as grain orientation, step edges, and mechanically induced defects. We will provide insight in the application of ESM for various classes of materials with different Li-ion storage mechanisms. Additionally, the application of ESM to study ionic transport across solid-solid and solid-liquid interfaces will be presented and discussed. Theoretical calculations are shown to support the experimental data and to give insight into the signal generating mechanism.
Support was provided by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division through the Office of Science Early Career Research Program, by the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy, and by the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

Thin films of Li-La-Ti-O were fabricated by pulsed laser deposition as a model solid electrolyte for basic research on all solid-state Li-ion battery. The surface morphology of the grown films depends a great deal on substrate temperature, oxygen partial pressure, ablation conditions as well as target composition, and Li₄Ti₅O₁₂, La₂Ti₂O₇, Li₂La₂Ti₃O₁₀, and TiO₂ are precipitated as secondary phases in addition to the intended Li_3xLa_2/3-xTiO₃ (x ~ 0.11). The growth condition dependences suggest that Li tends to be lost at higher temperature by evaporation (> 900 °C range) and at higher oxygen partial pressure by gas scattering (> 5 Pa range), although lower oxygen partial pressure promotes Li evaporation at high temperature. Li-ion conductivity of the films correlates with the film surface morphology and kind of the secondary phases.

All solid-state Li-ion batteries (LIBs) are attractive for powering an emerging class of autonomous microsystems with applications in biology, medicine, environmental monitoring, agriculture, and intelligence gathering. The advantages of solid-state LIBs include compatibility with MEMS manufacturing, extremely long lifetimes, negligible self-discharge rates, and enhanced safety owing to the lack of flammable and/or toxic liquid-phase electrolytes. However, low areal energy density of thin film LIBs and the low ionic conductivity of solid electrolytes compatible with thin film LIB manufacturing severely limit the application space of autonomous microsystems. 3D solid-state LIBs could substantially increase the energy and power densities, but require an ultra-thin solid electrolyte. To address this need, we have developed thin film solid state LIBs with LiCoO2 cathode and Si anode and with a solid electrolyte only ~70 nm thick. These LIBs operate at 4 V without electrolyte breakdown and with negligible self-discharge. In our talk we will discuss how the electrolyte is stabilized, the effects of additional interlayers, and the integration of this electrolyte into an operating 3D solid-state LIB.

The search for materials that can absorb a large portion of visible light, present the appropriate electronic structure, and are stable in solution has been a major effort of the energy and photocatalysis community for years. Due to the absence of a single material possessing all the qualities desired for a potentially high-efficiency photocatalysts, researchers have explored composite systems that combine strong light absorption of one material with the chemical reactivity and stability of other materials such as TiO2. In this work, we will discuss a new manifestation of this concept and the unexpected observation of strong, broad-band light absorption in complex oxide “metals” previously unstudied for these applications. These complex oxide “metals” are not really metals but have complex electronic structures and properties that are the result of electron correlations, complex band structures, and degenerate doping effects. One example of such a correlated oxide material is strontium ruthenate (SrRuO3) which is known to be a metallic itinerant ferromagnet, to exhibit so-called bad metal behavior at high temperatures (while being a Fermi liquid at low temperatures), and is known for its widespread utility as a conducting electrode in many complex oxide heterostructures. Using pulsed-laser deposition, 5-100 nm SrRuO3 thin films have been synthesized on SrTiO3 substrates. Spectroscopic ellipsometry and photospectrometry studies have revealed that the complex electronic structure of SrRuO3 is also responsible for unexpected optical properties including high absorption (~69 % from 100 nm think films) across the visible spectrum (commensurate with a low band gap semiconductor, silicon) and remarkably low reflection (20-30 %) compared to traditional metals. Various TiO2/SrRuO3 heterostructures have also been synthesized and have been characterized by light/dark current-voltage and photocatalytic studies. Strong visible light absorption results in photovoltaic effects and large photocatalytic activities (Methylene blue decomposition tests for probing the activities show three-fold higher activities, ~1269 µmol/hr-g, from TiO2/SrRuO3 than nitrogen-doped TiO2). The devices function by photo-excited hot carrier injection from the SrRuO3 to the TiO2 across the Schottky junction at the interface and the effect is enhanced in thin films due to electronic structure changes that promote more efficient charge injection. We will also discuss the study of other candidate complex oxide “metals” (such as vanadates, manganites, cobaltates, and nickelate) including detailed optical and electronic characterization to examine their potentials in such applications. As part of this work we will examine optimized photovoltaic/-catalytic device structures by studying a range of important material parameters and proposed band diagrams. This work provides an exciting new approach to the challenge of designing visible-light photosensitive materials.

Visible light-driven photocatalysts have been attracting attention for harvesting solar energy. The transition-metal oxides with empty or filled 3d states have actively been studied upon tuning of their band-gaps by chemical doping. In contrast, those with partially filled 3d states have been poorly understood because of complex d-d transition. In this study, we have examined a model solid-solution composed of α-Fe₂O₃, a photocatalyst available from abundant minerals, and isostructral α-Cr₂O₃, whose band-gaps are 2.1 and 3.0 eV, respectively. The α-(Cr_xFe_1-x)₂O₃ thin films were prepared by using pulsed-laser deposition on c-sapphire substrates. The films were epitaxially grown along the c-axis orientation and lattice parameters were a good agreement with values of reference powder samples in the entire composition range. In addition, film surfaces were confirmed to be optically flat by inspections with atomic force microscopy. Moreover, new absorption band appeared above 1.7 eV in the intermediate composition films (0.2 < x < 0.9) [1]. To clarify the origin of the band-gap narrowing, electronic structures of the solid-solution films were characterized by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy using synchrotron radiation. It is found that the top of valence band consists of Cr 3d and O 2p states, whereas, the bottom of conduction band consists of Fe 3d state, exhibiting a type-II band alignment. Thus, the lowest transition level is attributed to interatomic charge transfers from occupied Cr t_2g and O 2p states to empty Fe t_2g* state. Our results indicate that the use of high-quality epitaxial films will be a suitable approach to explore visible light-driven photocatalyst based on complex transition-metal oxides. [1] H. Mashiko, T. Oshima, and A. Ohtomo: Appl. Phys. Lett. 99 (2011) 241904.

Thin film heteroepitaxy provides elegant routes to access functionalities restricted from bulk equilibrium phase diagrams through artificially imposed elastic and electronic boundary conditions in complex oxides. Established paradigms for functional control in ABO3 perovskites include chemical pressure (cation substitution) and epitaxial strain (altering B-O bond lengths and bond angles through heteroepitaxy). In this work, we describe how first-principles density functional theory (DFT) calculations and group theoretical methods are able to both predict new behavior in artificial materials and disentangle the microscopic mechanisms controlling the film&’s functionality, enabling materials by design. The principal strategies involves exploiting the well-defined atomic structure-property links in perovskites to achieve a targeted response. In this context, we describe how control over the octahedral rotations ubiquitous in perovskite thin films provides a new opportunity for tailoring structure-driven properties and phase transitions for device applications. We focus on three examples consisting of aluminate, gallate and scandate perovskites as prototypical oxides: isosymmetric structural transitions due to rotation-tilt frustration, large ferroelectric polarizations induced by octahedral rotations, and the combined effect, which leads to a large piezoelectric response at a ferroelectric to ferrielectric transition. We conclude by discussing additional avenues to exploit the octahedral framework structure in perovskites with correlated electrons for advanced electronic applications.

Complex coupling of charge, spin, orbital, and lattice degrees of freedom create emergent physical phenomena at interfaces in perovskite transition metal oxide (TMO) thin films. For ferroelectric materials, the distribution of ferroelectric domains and the structural and electronic parameters of the domain walls strongly influence their functional properties. Additionally, local chemical changes such as oxygen vacancy injection can play a major role. Therefore, atomic scale studies of the different structural, electronic, and chemical order parameters at domain walls and interfaces are vital for optimization and design of the ferroelectric-based thin film devices.
In this study, we use quantitative scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) (Nion UltraSTEM operated at 100 kV and equipped with Gatan Enfina EEL spectrometer) to study atomic-scale phenomena in a multiferroic thin film heterostructures of BiFeO3 (BFO) epitaxially grown on (La,Sr)MnO3 (LSMO) layer on a SrTiO3 (STO) substrate. At this interface, charge, spin, and polarization order parameters can all in principle be active. At the interface, we find multiple thin domains with out-of-plane polarization opposite to that of the rest of the film. These interface domains are quite small (le; 10 nm), and the domain walls separating them from the bulk of the film are associated with lattice expansion (~20 pm along out-of-plane direction). We also discovered that the LSMO/BFO interfaces corresponding to domains with opposite out-of-plane polarization directions are associated with a change in local valence state of near-interface Mn cations. One of these interfaces shows local lattice expansion similar to that found at a domain wall, which in combination with the data on Mn chemistry provides strong evidence for local accumulation of charge carriers driven by oxygen vacancy injection. Our precise observations at the interface give us an unprecedented look at ferroelectric interfaces and domain walls, elucidating atomic-scale underpinnings of the unusual interface properties revealed through macroscopic routes.
* Research supported by U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division (YMK, SJP, SVK, AYB), and through a user project supported by ORNL&’s Shared Research Equipment (ShaRE) User Program (YMK, AYB), which is also sponsored by DOE-BES.

The recent interest in ultrathin oxide films is driven by the desire to understand the fundamental limits of low dimension order, and to create novel electronic ground states by accurately controlling the local electronic and structural reconstructions[1]. In this context, low dimensional manganite systems are especially fascinating due to their wide range of electronic and magnetic phases. Here we study La_0.7Sr_0.3MnO₃ on low miscut SrTiO₃{100}, utilizing our established optimizations of high quality ultrathin films[2,3].
In a thickness range of 7-10 unit cells (uc), we find a strong anisotropy in the electric transport properties depending on the current flow direction with respect to the substrate steps. In particular the magnetoresistance (MR) showed dramatic orientation dependence. For example, the MR of a 9 uc sample at T = 5 K and mu;₀H = 12 T was ~ -12 % for current parallel to the steps, but close to -75 % in the perpendicular direction. This suggests the presence of an emergent insulating phase at the step edge, and arrays of intrinsic spin valves with alternating anti-parallel magnetic domains aligned with the terrace direction. These observations suggest opportunities to create and study self-organized mesoscopic phase separation more broadly in other complex oxides.
References:
1. H. Y. Hwang, Y. Iwasa, M. Kawasaki, B. Keimer, N. Nagaosa, and Y. Tokura, Nature Materials 11, 103 (2012).
2. B. Kim, D. Kwon, J. H. Song, Y. Hikita, B. G. Kim, and H. Y. Hwang, Solid State Commun. 150, 598 (2010).
3. B. Kim, D. Kwon, T. Yajima, C. Bell, Y. Hikita, H. Y. Hwang, and B. G. Kim, Appl. Phys. Lett. 99, 092513 (2011).

Transition metal oxides are nowadays pivotal in many technology and science areas. From energy production, harvesting and catalysis to advanced actuators and modern spintronic components, transition metal oxides have paved its way along decades of deep research efforts and developments. Whereas electronic properties of thin films materials are rather well understood, achieving a detailed understanding of the role of epitaxial strain on magnetic, electronic or other properties related to electron filling of 3d orbitals, the electronic properties of surfaces have remained elusive. The ultimate reason is that understanding surfaces requires appropriate tools to fabricate and control them at atomic level. Recent developments in oxide thin film growth now make it possible thus opening new opportunities for oxides, where their surface properties are relevant.
Here, taking the La2/3Sr1/3MnO3 manganite as a working example, we will show that the electronic occupancy of the 3d-eg orbitals, i.e. x2-y2 and 3z2-r2, at the film surface, can be tailored at wish. Epitaxial manganite films were grown on single crystalline substrates imposing either compressive or tensile strain and the electronic occupancy of 3d-eg orbitals was determined by x-ray linear dichroism at the Mn L2,3 edge. In addition to the well known influence of strain on the electronic orbital filling, an additional contribution was found favoring the 3z2-r2 occupancy and thus unbalancing (in some cases overriding) the strain-related contributions. This additional contribution is attributed to the symmetry-breaking free surfaces. The implications of these findings on surface-related properties such as catalytic activity or more generally surface reactivity of transition metal oxides, will be discussed.

Ferroelectric/dielectric superlattices have ferroelectric properties that depend on the coupling of polarization and atomic structure between adjacent component layers. In the case where the polarization in the ferroelectric layers is only weakly coupled across the dielectric layers, the system adopts a complex polarization configuration consisting of a nanoscale striped polarization domain pattern. In this case, in the absence of an applied electric field, the dielectric layers are weakly polarized compared to the ferroelectric layers. Under applied electric fields, however, the system transforms into a uniform polarization state in which the dielectric layers show the same polarization as the ferroelectric layers. As a consequence of this field-dependent interlayer distribution of the polarization, the piezoelectric distortion has been predicted to vary across different component layers. This prediction, however, has not yet been tested experimentally.
We report a nanosecond-resolution synchrotron time-resolved x-ray microdiffraction study of a PbTiO₃/SrTiO₃ superlattice thin film under applied electric fields. The diffuse x-ray scattering intensity arising from the striped domains decreases as the superlattice switches into a uniform polarization state. At early times after the application of electric fields where the striped domains are metastable, the piezoelectric expansion of striped domains is suppressed due to the mechanical clamping between adjacent domains. At long times after the application of electric fields the uniform polarization state dominates and the mechanical clamping is removed. Here, the intensities of the superlattice reflections show changes depending on the indices of the reflections. A kinematical x-ray simulation was conducted to calculate the intensity changes of superlattice reflections under different strain distribution conditions. The experimental observation agrees with the model in which the piezoelectric distortion in dielectric SrTiO₃ layers is larger than in ferroelectric PbTiO₃ layers. This distribution of piezoelectric strain is commensurate with the change of the polarization in each component layer expected under applied electric fields. The precise determination of the layer-dependent structure at an atomic scale is crucial to understand the origin of the field-dependent electronic properties of the superlattice.

Until now, the quasi-amorphous (QA) phase in BaTiO3 (BTO), SrTiO3 (STO) and BaZrO3 was achieved by pulling a thick film through a steep temperature gradient. Here we show that a room temperature deposited ultra-thin film, subsequently annealed in O2 can also produce a QA phase. The atomic, electronic and ferroelectric (FE) structure of a QA, ultra-thin BTO grown on STO were studied by X-ray diffraction (XRD), X-ray photoelectron diffraction (XPD), X-ray photoelectron spectroscopy (XPS) and piezo-force microscopy (PFM). The absence of long range order is confirmed by in and out of plane XRD, as well as Ti 2p XPD. FE polarized domains with good retention have been successfully written into the QA film and exhibit a clear P-E hysteresis loop. Substrate clamping frustrates volume expansion during annealing, leading to a QA film. Photoelectron spectroscopy confirms a similar overall electronic structure as for thicker films but with some significant differences. Simple charge transfer arguments are not sufficient to explain the high resolution core level spectra. Ba, Ti and O all show components associated with a surface region. The observation of such a component in the Ti 2p spectrum is most likely linked with the high dynamic charge tensor induced by the large off center displacement of the Ti ion.

In the field of spintronics, various approaches via spin-polarized currents have been proven effective in controlling the direction of magnetization in ferromagnetic layers. However, overheating problems are unavoidable in view of the high current densities involved. Alternatively, electric field control of magnetization in multiferroics, which couple the electric and magnetic order parameters, could be a promising approach due to the possibility of “zero-current” operations. In the present study, the phase field method is adopted to investigate ferroelectric control of local magnetism. Due to strong elastic coupling, a 90 degree in-plane domain pattern of a ferroelectric substrate can be fully transferred onto the as-deposited ferromagnetic thin film. Thus, under an applied electric field, the magnetic domain pattern could be maintained or erased depending on the field direction. Moreover, the simulation results demonstrate that the ferroelectric domain wall motion could be manipulated by a small external electric field, and concurrence of ferromagnetic domain wall motion is realized through the strong pinning effects between domain boundaries of the ferroelectric substrate and ferromagnetic thin film, which could be a new method for precision control of the ferromagnetic domain wall motion without applying an external magnetic field or spin-polarized currents. Thus, ferroelectric control of local magnetism is a promising new approach for producing electric-field controlled spintronics devices.

Ta2O5 has application in dynamic random access memory (DRAM), antireflective coatings, optical wavegudes, and electrochromic devices. Recently, N-doped tantalum oxide or tantalum oxynitride (TaOxNy) has been investigated as a photocatalyst the solar production of hydrogen gas. Also, tungsten oxides (WOx) have been intensively studied for a long time due to useful applications such as electrochromic devices, information storage, solar cells, gas sensors, counter electrode for DSCs, photoelectrochemical water splitting catalysts, and RRAM with MIM structure. So, we focused development of precursors for Ta and W precursors for nanomaterials or thin films. In our group, we have been studied novel precursors using aminoalkoxide ligands to substitute two alkyl groups on the alkoxide carbon of the ligands. The metal precursors have been applied to prepare metal and metal oxide thin films using CVD or ALD in our group and other research teams. We will present for our research results of novel Ta(V) and W(VI) tBu-imido/aminoalkoxide precursors, Ta(NtBu)(dmamp)2Cl, Ta(NtBu)(dmamp)2Me, W(NtBu)2(dmamp)2 [dmamp = 1-(dimethylamino)-2-methyl-2-propoxide], and W(NtBu)2(dmamb)2 [dmamb = 1-(dimethylamino)-2-methyl-2-butoxide], for Ta- and W-based nanomaterials or thin films, and their application to nanomaterials or thin films of Ta2O5 and WOx.

Studies on resistive switching behavior in well-known multiferroic material BiFeO₃ (BFO) and its variants have received tremendous attention recently due to their potential in developing futuristic multifunctional devices combining ferroelectricity, ferromagnetism, photo-voltaic, and charge conduction. There have been many reports on the multiferroic and photo-voltaic properties of BFO, however reports on resistive switching behavior in polycrystalline BFO is scanty. Although bipolar resistive switching has been reported for epitaxial BFO thin films grown on single-crystal substrates however the ON/OFF ratio was limited to ~100. In this paper we report unipolar resistive switching in Sm doped BiFeO₃ (Sm:BFO) thin films with ON/OFF ratio as high as ~10⁵. Photo-response in these films is also reported. For these experiments ~250 nm thick polycrystalline Sm:BFO thin films were grown by RF sputtering with argon and oxygen flow ratio of ~ 5:2 sccm at ~ 675°C substrate temperature on ~ 150 nm thick buffer layer of pulsed laser deposited Sr₂RuO₄ (SRO) on commercial Pt/TiO₂/SiO₂/ Si substrate. About 60 nm thick Pt top electrode of diameter ~ 100 µm was fabricated over the Sm:BFO film using DC sputtering. The Sm doping in BFO was carried out to improve leakage current behavior of BFO. The as grown Pt/Sm:BFO/SRO capacitor was initially in high resistance state (HRS) with resistance of ~2 M#8486; and switched to low resistance state (LRS) with resistance of ~ 20#8486; after applying a large forming voltage ~ 10 V keeping current compliance at 5 mA. The resistance of Sm:BFO again switched to HRS at ~ 0.7 V when voltage was swept from 0 to 1 V and to LRS at ~ 4.5 V as the voltage was swept from 0 to 6 volt. The nonvolatile switching of the resistance of Sm:BFO between LRS and HRS was obtained with nearly constant resistance ratio ~ 10⁵ and non overlapping switching voltages in the range of 0.7-1 V and 4-6 V respectively. In addition, the Pt/Sm:BFO/SRO structure showed efficient photo-response with increase in photocurrent by a factor ~10⁵ at zero bias when illuminated with solar spectrum using an incident light having power density of ~ 1kW/m². Further studies on resistive switching phenomenon and photo-response of Sm:BFO are currently underway.

The scarcity of the single-phase multiferroic materials with magnetism and ferroelectricity is mainly owing to the mutually exclusive nature of their physical origins. The attempts of designing new materials with room temperature multiferroic properties are strongly challenging. Here, we present the discovery of a new class of room temperature multiferroic phase Bi3Fe2Mn2O10+δ (BFMO322). The parent phases are two partially miscible phases of BiFeO3 and BiMnO3. However, the new phase presents a unique Super-Cell (SC) structure with the bismuth bilayer sublattice structure in commensurate with the Fe-O-Mn sublattice. The new structure possibly favors the spin canting effect, which could be responsible for the observed magnetism, and, the non-centrosymmetric nature of the new structure permits the ferroelectric behavior. The layered bismuth SC heterostructures exhibit a room temperature ferrimagnetism of ~ 110 emu/cc and a remnant polarization Pr of ~ 6 µC/cm2, simultaneously. More interestingly, the integration of two partial miscible multiferroic materials with strain confinement opens a new route to develop single-phase thin films with room temperature multiferroic properties. This demonstration will stimulate further work on exploring new single-phase multiferroic thin films by proper intermixing of two perovskite BiRO3 (R=Cr, Mn, Fe, Co, Ni) materials.

Highly strained BiFeO3 thin films have been found to exhibit a ‘tetragonal-like monoclinic&’ crystal structure different from the bulk rhombohedral structure. We grew highly strained BFO thin films by pulsed laser deposition and studied temperature dependent structural changes in epitaxially strained tetragonal-like BiFeO3 films using second harmonic generation (SHG) technique. We observed a strong SHG peak at ~370 K, corresponding to the structural transition between two T-like, MC and MA BFO phases for a thin (30-nm thick) strained BFO film grown on LaAlO3 substrate. Symmetry analysis based on rotational SHG revealed an existence of a non-tetragonal intermediate phase at the structural transition and it was mainly responsible for the enhanced SHG. For a thicker, 150 nm-thick BFO/LAO film, additional SHG features showed up above 400 K, associated with rhombohedral-like phase. We also observed a noticeable enhancement and symmetry change of SHG at the G-type to C-type anti-ferromagnetic transition, suggesting a possibility of a structural change coupled to the magnetic transition.

Achieving high and stable tunability, low dielectric loss and low temperature coefficient of capacitance are the crucial requirements for radio frequency and microwave device applications. We have created a self-assembled ferroelectric nanocomposite (Ba _0.6Sr_0.4TiO₃)_1-x(Sm₂O ₃ )_x containing Ba_0.6Sr_0.4TiO₃ (BSTO) and Sm₂O₃ (SmO) constituents which has a very high tunability (75% at 200kV/cm field) that scales inversely with loss (<0.01). Furthermore, the low temperature coefficient of tunability is evidenced by maintaining high values of tunability (>50%) even up to 160 °C. The presence of piezo-response domain switching up at 120 °C also proves the possibility of utilising ferroelectric in polar phase for electric-controllable microwave device despite of the addition losses associated with domain wall movements. Simultaneously, the leakage is significantly reduced and the desired dielectric constant is fixed by selection of the appropriate SmO fraction, x. This new paradigm in tunability control comes about because of a vertical strain control mechanism which yields a high tetragonality (c/a ration of 1.0216) in BSTO. The BSTO in the x=0.75 nannocomposite is comprised of highly strained nanopillars (~12 nm) in a matrix of interspersing stiff SmO. With nanoscaffolding effect exerted by SmO, the expected trade-off between tunability and dielectric loss of BSTO is revisited.

CoFe₂O₄ (CFO) offers unique properties as a magnetoelectric material due to its large magnetoelastic response when strained. Previous work has shown that when CFO is co-deposited with BiFeO₃ (BFO) nanostructured phase segregation occurs, with CFO pillars forming in a BFO matrix, and that electrical control of the magnetic anisotropy is possible.[1] The CFO-BFO nanocomposite system has been proposed as a possible electrically-controlled spintronic logic or memory scheme.[2] This work will discuss the patterning and growth of high quality CFO-BFO nanocomposites using the novel pulsed electron-beam deposition (PED) technique. We will present a novel technique developed to direct the self-assembly of the CFO pillars in CFO-BFO nanocomposites using electron-beam lithography. [4] Our recent results have demonstrated the ability to pattern epitaxial nanoscale CFO islands on the surface of a Nb-doped SrTiO₃ substrate using EBL. These island templates are then used to direct the growth of the nanocomposites into square arrays of pillars with spacings as small as 100 nm.
Scanning probe measurements examining annealing effects on the magnetic and structural properties of the CFO islands have been performed. Results will be presented showing how these properties affect the resulting nanocomposite. The magnetic properties of the composite films have been characterized using magnetic force microscopy (MFM) and are in good agreement with previous results from our group for unpatterned nanocomposites.[4] Cross-sectional transmission electron microscopy analysis of the films was used to quantify the strain in the CFO pillars and evaluate the magnetoelastic anisotropy and is in agreement with the MFM results. Piezoresponse force microscopy (PFM) analysis of the patterned composite will also be presented. Switching-spectroscopy PFM measurements have been performed to measure the voltage required to switch the ferroelectric BFO matrix. The conductive PFM tip has also been used for PFM lithography to pattern domains in the matrix. By demonstrating the ability to selectively switch the domains, we have taken the first steps towards a new multiferroic logic and memory architecture which makes use of CFO-BFO nanocomposites.
[1] F. Zavaliche, et al. Nano Lett. 7 (2007).
[2] S.A. Wolf, et al. Proc. IEEE 98 (2010).
[3] R. Comes, et al. Nano Lett. 12 (2012).
[4] R. Comes, et al. J. App. Phys. 111 (2012).

A renaissance of interest in multiferroics has taken place over the last decade with over 900 publications on magnetoelectrics and multiferroics last year[1], compared to around 45 in 2001[2]. This renaissance was driven by a desire to produce novel multiferroic devices[1], such as multiferroic memory, which requires room temperature operation. While remarkable advances towards devices have recently been made in multiferroic heterostructures [3][4] the equivalent advances in single phase materials have been rather less dramatic. In this context, the recent discovery of a new room temperature multiferroic by some of the authors[5], is potentially interesting.
Herein, we investigate the properties of the bulk multiferroic ceramics[5], as well as lamellae cut from single grains of bulk using a Focused Ion Beam Microscope. We take magnetic hysteresis measurements, using a SQUID, of the bulk ceramic at 300K and 4K, which confirms the presence of ferromagnetism. Such measurements also suggest the presence of a second phase in the microstructure. Electron microscopy confirms that a distinct grain boundary phase is present in the bulk ceramics.
Individual grains, however, appear to be single phase as confirmed by EDX mapping and selected area diffraction patterns. We go on to take capacitance voltage loops of individual lamellae, cut from single grains, demonstrate that these are ferroelectric. We also present some data on the coupling between the ferromagnetic and ferroelectric order parameters. We use this to get an order of magnitude estimate of the effective magnetoelectric coupling coefficient of ~1 x 10 7 sm-1.
[1] A. P. Pyatakov and A. K. Zvezdin, Physics - Uspekhi 55, 557 - 581 (2012)
[2] R. Ramesh and N. Spaldin, Nat. Matt. 6, 21-29 (2007)
[3] J. T. Heron et al., PRL 107, 217202 (2011)
[4] Tuomas H. E. Lahtinen et al., Scientific Reports 2, 258 (2012)
[5] D. Sanchez et al., AIP Adv. 1, 042169, 2011

Ferroelectric oxides are used in a wide variety of applications including capacitors, transistors, piezoelectric transducers, and RAM devices. The perovskite family has proven to be especially useful, with materials such as PZT and BT becoming the industry standards in dielectric and multiferroic applications. A special class of these materials, relaxor ferroelectrics such as PMN and PLZT, has extraordinarily high electrostrictive coefficients, making them useful in piezoelectric applications. Tuning the dielectric and piezoelectric properties of these materials can be achieved through compositional tuning near the morphotropic phase boundaries. It has been proposed that relaxor ferroelectrics achieve their electrostrictive properties through nanoscale phase separation, although further atomic scale chemical and structural characterization is required. Atom probe tomography (APT) is capable of analyzing the atomic scale, 3-D distribution of atoms in a solid, however, it has not been applied thoroughly to ferroelectric oxides.
In this study, laser-assisted APT coupled with transmission electron microscopy (TEM) pre- and post-APT analysis was performed on several different ferroelectric oxides. Specimens consisted of thin films deposited on metallic substrates as well as bulk sintered pellets. The APT parameters, such as laser pulse energy (below 1000 pJ/pulse), acquisition rate, and specimen base temperature (between 20K and 70 K) were optimized to obtain accurate cation and anion stoichiometry of the oxides. The changes in the Pb and Zr, or Pb and La, content could be correlated to segregation of species to form domains within the material. The combination of the APT and TEM techniques allowed for crystal structure analysis, 3D atomic scale chemical identification, and accurate reconstructions of the atom probe data [1].
[1] This research is supported by NSF Award Number 1040456.

While modern technology has relied extensively on exchange bias for applications such as magnetic read heads and hard drives, the fundamental reasons behind the existence of exchange bias are still a topic of intense debate.^1,2 Although exchange bias is typically studied in multilayer systems, we have found an alternative pathway for engineering the same behavior in a single thin film.³ We report on the observation of exchange bias in nanocrystalline (NC) spinel ferrite thin films that are not biased by another layer, and propose an explanation for this unusual result. We have chosen to use Mn-Zn soft ferrites, which have widespread use in high-power applications, and we show that exchange bias in this material can be induced on silicon substrates.³
We have grown NC films of the spinel ferrite (Mn,Zn,Fe)₃O₄ on Si, MgAl₂O₄ and MgO substrates at room temperature in vacuum by pulsed laser deposition. We also grew epitaxial films of (Mn,Zn,Fe)₃O₄ on MgAl₂O₄ and MgO at higher temperatures and pressures. XRD measurements show that samples grown at room temperature are textured out-of-plane and NC, regardless of whether an isostructural substrate is used. Those grown at higher temperatures on MgAl₂O₄ and MgO are epitaxial. SQUID magnetometry measurements reveal two unusual features in the magnetic hysteresis loop. First, the virgin curves of the NC samples lie outside of the hysteresis loop, and then cross back into the loop at higher fields. Second, despite the sample being composed of only a single film, an exchange field of approximately -400 Oe in NC samples and -30 Oe in epitaxial samples is observed when measured at 10 K and field-cooled in 50 kOe. The exchange bias vanishes above 200 K. The film thickness and degree of texturing is also investigated and correlated with the magnitude of the exchange field. With detailed x-ray absorption spectroscopy and x-ray magnetic circular dichroism studies, we found that compared to epitaxial films, the NC films exhibit different cation site distributions and greater Fe and Mn mixed valence states.
We postulate that magnetic coupling between the crystalline, magnetically ordered grains and structurally disordered, magnetically frustrated regions between grains leads to macroscopic exchange bias features. The presence of increased cation disorder further supports our hypothesis that single-layer exchange biasing in these films is the result of a magnetically disordered matrix. NC ferrimagnetic oxide films provide an interesting model system by which to study magnetic interactions in a system with mixed disorder; in our case, there is unexpected exchange bias. Furthermore, low-temperature synthesis of ferrite films on silicon can be technologically relevant for on-chip integration.
[1] Piramanayagam, S.N. J. Appl. Phys. 102, 011301 (2007).
[2] Nogués, J. and Shuller, I.K. J. Magn. Magn. Mater. 192, 203-232 (1999).
[3] Alaan, U.S. et al. J. Appl. Phys. 111, 07A337 (2012).

Multiferroic heterostructures consisting of alternate layers of the ferroelectric (FE) perovskite PbZr_0.52Ti_0.48O₃ (PZT) and the ferromagnetic (FM) half-metal La_0.7Sr_0.3MnO₃ (LSMO) have been under intense investigation for their potential applications in magneto-electric (ME) data storage devices [1]. In this work, we report on the high quality epitaxial growth of PZT/LSMO heterostructures on MgO (100) and SrTiO₃ (STO) (100) substrates using both single (KrF laser only) and dual-laser (both KrF and CO₂ lasers) ablation processes. The deposition conditions were optimized to overcome some of the challenges during the growth of stoichiometric PZT/LSMO multilayer thin films [2]. The major issues of the preferential evaporation of Pb during the ablation of PZT target [3] and the precipitation and surface segregation of Mn₃O₄ during LSMO film growth [4], both leading to the non-stoichiometric thin films with poor FE and FM properties, were investigated by studying the chemical composition of laser-target interaction sites and intensified charge-coupled detector (ICCD) imaging coupled with optical emission spectroscopy of the laser-ablated plumes. While X-ray diffraction revealed the single crystalline nature of the PZT/LSMO heterostructures, high resolution transmission electron microscope cross-sectional images of the interfaces showed atomically sharp and flat boundaries with cube-on-cube epitaxial growth. Atomic force microscope images showed uniform grain growth with surface roughness values as low as 1.6 nm. Magnetization measurements showed saturation of 254 and 283 emu/cm³ on MgO and STO substrates, respectively, and the presence of in-plane magnetic anisotropy in the PZT/LSMO heterostructures. LSMO/PZT/LSMO capacitors on MgO and STO substrates showed high remnant polarizations of 44-58 µC/cm² at coercive fields of 30-40 kV/cm. An in-depth X-ray strain compression relaxation mechanism was proposed to explain the structure-property relationships in PZT/LSMO heterostructures.
[1] Y. Wang et al., NPG Asia Mater. 2, 61 (2010).
[2] D. Mukherjee et al., J. Appl. Phys. 112, 064101 (2012).
[3] D. Mukherjee et al., J. Appl. Phys. 111, 064102 (2012).
[4] D. Mukherjee et al., J. Appl. Phys. 112, 083910 (2012).

We report demonstrating the feasibility of controlling the dielectric properties - high dielectric constant (k) and substantially extended relaxation frequency - of thin film nanolaminates (NLs) consisting of alternating TiO_x and Al₂O₃ sublayers with various sublayer thicknesses grown by atomic layer deposition. For 150 nm thick TiO_x/Al₂O₃ NLs with sub-nanometer thick sublayers, few Angstrom change in sublayer thickness dramatically increases relaxation cut-off frequency by more than 3 orders of magnitude (from kHz to MHz) with high dielectric constant ( > 800 ). This unusual phenomenon is discussed in the framework of two-phase Maxwell-Wagner relaxation.

We present direct measurements of the electrocaloric and pyroelectric coefficients of PbZr_0.2Ti_0.8O₃ (PZT) films using high frequency optical methods. The sample structure is a capacitor fabricated from heterostructures of 20 nm SrRuO₃/150 nm PZT /20 nm SrRuO₃ grown on DyScO₃ and GdScO₃ (110) substrates by pulsed-laser deposition. The SrRuO3 top electrodes are patterned by a photolithography process that uses a MgO mask process and are eventually capped with ~150 nm of V to produce an optical transducer that absorbs the incident laser power and provides a strongly temperature dependent optical reflectivity, i.e., a high thermoreflectance, for measurements of the electrocaloric coefficient.
For measurements of the pyroelectric coefficient, the PZT epitaxial layer is heated by a laser modulated at frequencies between 1 Hz and 10 MHz. The pyroelectric current is collected using lock-in amplifiers. We developed a multilayer thermal transport model to calculate the temperature change of the PZT films. For measurements of the electrocaloric coefficient, a sinusoidal voltage with frequencies between 100 kHz and 10 MHz is applied to the films to create a periodic electric field, which causes a periodic temperature change of the PZT films. We measure the temperature change of the V film by monitoring the intensity of the reflected laser beam. The entropy change of the PZT film is extracted from the data using a multilayer thermal transport model.
We find that the pyroelectric and electrocaloric coefficients of PZT are 190 ± 10 mu;C m^-2 K^-1 and 130 ± 13 mu;C m^-2 K^-1, respectively, at room temperature. A Maxwell relation states that these two quantities should be equal in the absence of mechanical effects. We show that the difference between the pyroelectric and electrocaloric coefficients is due to a suppression of the electrocaloric coefficient by a themoelastic contribution, in which an entropy change caused by the combination of the contraction of the film due to the inverse piezoelectric effect and the anharmonicity of the lattice vibrations opposes the entropy change created by the usual electrocaloric effect. The inverse piezoelectric coefficient needed to reconcile the pyroelectric and electrocaloric coefficients is d33 asymp;25 pm V^-1, in good agreement with typical values for PZT.

manganese oxide based nanoparticles were synthesized by sol-gel process. Methanol, ethanol, and propanol were used as alternative solvent during sol-gel process with manganese acetate as precursor for the preparation of pristine manganese oxide. Hybrid manganese oxide modified by additions of carbon nanotubes was further prepared. The effects of different solutions and post heat treatment temperatures on the morphology, characteristic, and electrochemical properties of the manganese oxide based materials were investigated. Particle size of pristine manganese oxide samples prepared from methanol, ethanol, and propanol were compared by SEM and TEM results. Smallest particle size was observed for manganese oxide prepared from propanol, with diameters range from 16 nm to 50nm. XRD results showed that the as prepared manganese oxide based samples at calcination temperature of 300oC and above were composed of Mn2O3 as dominant phase, with Mn3O4 as minor phase. Specific capacitance measured from two electrode systems of manganese oxide prepared from methanol, ethanol, and propanol at scan rate of 10 mV/s were 88.3, 66.0, 104.8 F/g and the result for the hybrid sample was 140.5 F/g. Results from electrochemical impedance spectroscopy (EIS) also showed superior electrochemical properties of the hybrid sample over pristine manganese oxide samples. It is evident that the addition of carbon nanotubes not only improved the specific capacitance but also the overall electrochemical properties of the manganese oxide supercapacitor.

Epitaxial complex oxide thin film heterostructures have attracted a great attention for their multifunctional properties, such as piezoelectricity, ferroelectricity, superconductivity and ferromagnetism. Besides of their bulk property, interface between two different oxides can provide a promising platform to explore novel properties where symmetry breaking is easily controlled. Two dimensional electron gas (2DEG) confined at the interface between two insulating perovskite oxides such aslanthanum aluminate (LaAlO3)-strontium titanate (SrTiO3) interface, provides opportunities to expand various electronic and memory devices in nano-scale. In order for 2DEG to be used in electronic devices, it is desirable to reproducibly control the conductance of 2DEG with a large range. Recently, it was reported that the conductivity of 2DEG could be controlled by external electric field. However, the switched conductivity of 2DEG was not stable with time, resulting in relaxation because of the reaction between charged surface on LaAlO3 layer and atmospheric conditions. In this report, we demonstrated a way to control the conductivity of 2DEG in non-volatile way integrating ferroelectric materials into LAO/STO heterostructure. We fabricated epitaxial Pb(Zr0.2Ti0.8)O3 films on LaAlO3/ SrTiO3 heterostructure by pulsed laser deposition. The conductivity of 2DEG was reproducibly controlled by switching the spontaneous polarization of Pb(Zr0.2Ti0.8)O3 layer. The controlled conductivity was stable with time without relaxation. . This is also consistent with robust polarization state of Pb(Zr0.2Ti0.8)O3 layer confirmed by piezoresponse force microscopy. This work demonstrates a model system to combine ferroelectric material and 2DEG, which guides a way to realize novel multifunctional electronic oxide based devices.

In the infinite-layer (IL) compound ACuO₂ (A = Ca, Sr or Ba) the CuO₂ sheets are separated only by alkaline-earth atoms, which makes them the simplest example for two-dimensional (2D) CuO₂-plane structures - the essential unit for cuprate-based high-temperature superconductors. A charge-reservoir layer containing apical oxygen is assumed to be a prerequisite for getting superconductivity in CaCuO₂ (CCO). Therefore, it is straightforward to grow CuO₂ planes directly on top of A-site terminated ABO₃ perovskite substrates to create a five-fold, pyramidally coordinated CuO₂ layer at the CuO₂/AO interface and, hopefully 2D superconductivity. To shed more light on this issue we report here on the growth and the interfacial structural properties of CCO films grown on various perovskite related substrates. The samples were prepared by pulsed laser deposition. The structural properties of the bulk films were characterized by x-ray diffraction, whereas the interfacial structural properties were analyzed in detail by transmission electron microscopy. Although epitaxial growth is able to stabilize highly c-axis oriented CCO films on various substrates, the interface between film and substrate was found to be strongly defective. With decreasing film thickness the sample resistivity increases indicating that the interfacial conductivity is more insulating than metallic in character as expected from hole-doping of CCO due to apical oxygen at the CuO₂/AO interface. Our experimental results suggest that the absence of superconductivity is likely caused by the defective interface.

In the family of A2BO4 layered Ruddlesden-Popper (RP) phases, there is little experimental evidence supporting the observation of noncentrosymmetric (NCS) crystals. In this work, we discuss novel pathways to engineer NCS phases in RP compounds based on the synergy of multiple-level computational materials science approaches, including group theory, electrostatic Madelung energy calculations and ab initio density functional theory (DFT) simulations. Our framework relies on building a complete atomic structure configurational space, and then evaluating the structure stability through energetic considerations. We use the group theoretical methods to evaluate how rigid octahedral rotations, which when combined with A-site cation ordering, give rise to structures without inversion symmetry. While group theory identifies the symmetry rules that govern the structural state and simplifies the intractable problem by limiting the configuration space to subgroups of the parent space group, the stability of the ground state structure remains to be determined. Here we describe how density functional theory (DFT) and electrostatic Madelung energy calculations are able to test the feasibility of candidate NCS RP structures. Examples based on RP phases with correlated electrons will be shown, where we have tested our computational approach, and identified new, previously unknown NCS phases that may be of use in advanced optoelectronic applications

Interface properties between magnetron sputtered paraelectric (Ba,Sr)TiO3 (BST) films with different Ba/Sr ratio and reactively magnetron sputtered (amorphous) Al2O3 films were investigated using X-ray photoelectron spectroscopy (XPS). Bilayer BST/Al2O3 capacitors were prepared on platinized silicon substrates with Pt bottom and top electrodes and were characterized by current-voltage and capacitance-voltage analysis up to the GHz frequency range. The Al2O3 film thickness, determined from the attenuation of substrate core levels in the XPS measurements, was varied between 0-15 nm. The leakage current first increases with Al2O3 film thickness despite the additional barrier for electrons imposed by the large Al2O3 energy gap. This can be explained by electron tunneling through the Al2O3 layer.
After insertion of the low permittivity Al2O3 layer, the tunability of the capacitance by an applied DC voltage is maintained. However, regardless of the Ba/Sr ratio, insertion of Al2O3 induces a pronounced hysteresis in the capacitance-voltage characteristic, which increases with Al2O3 film thickness. This can be explained by trapping of charges at the BST/Al2O3 interface. Due to the barrier formed by the Al2O3, the charges, and hence the electric field across the BST, remain at the interface after removal of the tuning voltage. Time dependent capacitance measurements indicate a retention time of the stored charge of more than 1 year. Compared to the well-known formation of a charged interface at epitaxial SrTiO3/LaAlO3 interfaces, the presented results suggest that both negative and positive charges can be present also at interfaces of polycrystalline materials without the need for polar surface termination of the dielectric.

Density-functional theory (DFT) within the local density approximation (LDA) + Hubbard U approach was used to study interface electronic structures in stoichiometric and oxygen-deficient LaAlO3/SrTiO3 superlattices with regularly spaced n-type and p-type interfaces. Fundamental asymmetric behaviors between the complementary n-type and p-type interfaces were revealed. Implication of the asymmetric interface is studied in the formation of 2DEG. In the oxygen deficient LaAlO3/SrTiO3 superlattices, oxygen vacancies are more electrostatically favorable at the p-type interface than at the n-type one. The extra electrons induced by the oxygen vacancies at the p-type interface easily spread to the n-type interface and occupy the Ti orbitals, while those induced by the vacancies at the n-type interface are strictly confined and reside in Ti and/or orbitals. The nature of the asymmetric interface and its interaction with oxygen vacancies are discussed in detail along with proposed physical properties.

Recently there is a surge of interest in searching for topological order in correlated electronic systems such as transition metal oxides. The strong spin-orbit interaction of 5d electrons and the geometric frustration in the crystal lattice make the pyrochlore iridate(A2Ir2O7) an ideal candidate to achieve this goal. Pioneer experiments on bulk polycrystalline and single crystal samples revealed a temperature dependent metal-insulator transition coupled to a long range magnetic order, and the transition temperature can be tuned by either A-site ionic radius or an external pressure [1,2]. It has been theoretically proposed that the pyrochlore iridate is a topological insulator or an Weyl semimetal, in which the low lying excitations possess a gapless Dirac spectrum [3,4]. The exotic surface properties predicted by these theoretical proposals are important for both fundamental physics and spintronics applications.
Epitaxial thin film is the ideal platform for studying the effect of surface/interface and reduced dimensionality. In this talk we present our efforts to understand and control the metal-insulator transition and the underlying electronic structure of pyrochlore iridates via epitaxial Bi2Ir2O7 thin films. Bulk Bi2Ir2O7 is located at the metallic side of the phase diagram. However as the film&’s thickness decreases the transport evolves from a metallic to a strongly localized character. Resonant X-ray spectroscopy suggests that the density of states near Fermi level is dominated by the Ir Jeff=1/2 states. Intriguingly, the magnetoresistance shows a linear field dependence over a wide range of fields at low temperatures, which is possibly consistent with the existence of Dirac nodes [5].
[1] K. Matsuhira, M. Wakeshima, R. Nakanishi, Y. Yamada, A. Nakamura, W. Kawano, S. Takagi, and Y. Hinatsu, J. Phys. Soc. Jpn. 76, 043706 (2007).
[2] F. F. Tafti, J. J. Ishikawa, A. McCollam, S. Nakatsuji, and S. R. Julian, Phys. Rev. B 85, 205104 (2012)
[3] D. A. Pesin and L. Balents, Nat. Phys. 6, 376 (2010).
[4] Xiangang Wan, Ari M. Turner, Ashvin Vishwanath, and Sergey Y. Savrasov, Phys. Rev. B 83, 205101 (2011)
[5] A. A. Abrikosov, Phys. Rev. B 58, 2788 (1998).

It is generally agreed upon that under appropriate growth conditions, the metallicity at the n-type TiO2/LaO interface between LaAlO3 and SrTiO3 can be attributed to interfacial electron reconstruction [1,2]. More recently, there have been a number of reports of ferromagnetism at this interface as well [3,4]. The origin of the long range magnetic order is still unclear. However there have also been attempts to generate spin polarized functionality at the LAO/STO interface via doping of the STO with small amounts of aliovalent Co and Mn ions in the Ti site [5]. However, the strongest effect appears to be the modification of carrier concentration instead of tuning magnetic scattering and/or interactions.
Here we report on the synthesis and characterization of LAO/STO interfaces where Tm+3 and Lu+3 dopants are introduced onto the La+3 site of the LAO film in order to investigate the role of the LAO in the metallic transport and to explore the insertion of strong spin-orbit interaction and magnetic moment into the LAO side of the LAO/STO system. X-ray diffraction confirmed epitaxial growth of 2-20 nm thin films of Lanot;0.98Tm0.02AlO3 and Lanot;0.98Lu0.02AlO3 grown on TiO2 terminated (001) STO substrates while Rutherford backscattering confirmed the dopant concentration. Furthermore, x-ray absorption spectroscopy of the M4 edge indicated the presence of Tm3+ ions but not Lu3+ ions as they have a full 4f shell. Likewise, x-ray magnetic circular dichroism demonstrated the presence of strong local Tm3+ moments in the corresponding doped films. We find that these dopants modify the carrier concentration and mobility to a limited extent compared to the undoped samples. The modest decreases in the carrier concentration and increases in mobility indicate that scattering in the quasi-two-dimensional electron gas is not affected significantly by the presence of magnetic or spin-orbit scatterers in the LAO.
[1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004).
[2] M. Huijben, G. Rijnders, D. H. Blank, S. Bals, S. van Aert, J. Verbeeck, G. van Tendeloo, A. Brinkman, and H. Hilgenkamp, Nature materials 5 (7), 556 (2006).
[3] J. A. Bert, B. Kalisky, C. Bell, M. Kim, Y. Hikita, and H. Y. Hwang, K. A. Moler, Nature Physics 7, 767 (2011).
[4] L. Li, C. Richter, J. Mannhart, and R. C. Ashoori, Nature Physics 7 (10), 762 (2011).
[5] T. Fix, J. L. MacManus-Driscoll, and M. G. Blamire, Applied Physics Letters 94 (17), 172101 (2009).

Superlattices composed of transition metal oxides can offer novel magnetic and electronic properties that are not found in the individual constituent materials. These properties arise from the complex interactions that occur at their interfaces, including electronic reconstruction, magnetic exchange interactions, chemical intermixing and structural effects[1]. Despite recent progress, many unanswered questions about the fundamental origin of these interfacial properties remains. In this study, the interfacial ferromagnetic (FM) exchange coupling between soft and hard magnetic oxide materials, La0.7Sr0.3MnO3 (LSMO) and La0.7Sr0.3CoO3 (LSCO), respectively, and the length scale of such interactions are investigated. These materials display properties such as colossal magnetoresistance, half metallicity, as well as nanoscopic magnetic phase separation into FM clusters within a non-magnetic matrix. The LSMO/LSCO superlattices were grown by pulsed laser deposition with the sublayer thickness ranging from 6 to 60 unit cells and detailed structural and magnetic characterization was carried out by x-ray diffraction, bulk magnetometry, and element-specific soft x-ray magnetic spectroscopy. At large sublayer thickness, the magnetic hysteresis loops showed two distinct switching behaviors, one with a large anisotropy field and a second with small coercivity, signatures of the hard and soft layers, respectively. X-ray magnetic circular dichroism (XMCD) on Co confirmed that LSCO was the hard layer. The coercivity and anisotropy field decreased gradually with decreasing sublayer thickness until a thickness of 12 unit cells. At this sublayer thickness, Mn/Co XMCD measurements showed that the two sublayers were intimately coupled with a single magnetic transition and relatively small coercive field. These findings suggest that a magnetic transition from two independent magnetic layers to an interface exchange-coupled state occurred when the sublayer thickness was below a critical thickness of 12 unit cells. The FM coupling between the two layers may be mediated by a correlated double exchange mechanism[2] through interfacial charge transfer between Co and Mn ions, as observed in x-ray absorption spectra. These results highlight that interface engineering is a powerful tool for tuning the magnetic properties of artificial perovksite oxide superlattices when their thicknesses are controlled to dimensions below 12 unit cells (~5 nm).
1. Zubko, P., S. Gariglio, M. Gabay, P. Ghosez, and J.M. Triscone, Annu Rev Conden Matt Phys., 2011. 2,141-165.
2. Tokura, Y. and N. Nagaosa, Science, 2000. 288(5465),462-468.

The family of crystal structures known as perovskites is able to accommodate nearly all elements as major constituents in a dense structure with numerous phase transitions. These phase transitions often accompany the many novel properties of perovskites including pyroelectricity, piezoelectricity, ferromagnetism, multiferroicity, non-linear optical effects, and superconductivity. The archetypal perovskite, SrTiO₃, exhibits many of these useful properties when suitably doped or strained, including superconductivity, a high dielectric constant up to 20,000, ferroelectricity, the highest mobility of any oxide (>30,000 cm²/V s), photocatalysis for water splitting, and transparent conductivity. In this work we show that the band gap of both (100) and (111) oriented SrTiO₃ films deposited by molecular-beam epitaxy (MBE) can be modified from the bulk value by 10% (0.3 eV) and 20% (0.6 eV) for the two orientations respectively using biaxial epitaxial strain of ±3%. Structural quality and strain state is verified by high-resolution x-ray diffraction (XRD). Film microstructure is examined by high-resolution transmission electron microscopy (HRTEM). Spectroscopic ellipsometry in the VIS-UV spectral range is used to determine the band gap of the strained SrTiO₃ films. The band gap behavior as a function of epitaxial strain for both film orientations follows trends predicted by theory and provides a new means to accomplish band gap engineering of SrTiO₃ that may then be expanded to other related perovskite systems. Such band gap manipulation is relevant to applications in solar cells, water splitting, transparent conducting oxides, superconductivity, two-dimensional electron liquids, and other emerging oxide electronics.

ZnO thin film transistors (TFTs) have advantages of high transparency in visible light range, high carrier mobility compared to conventional a-Si:H TFTs, and can be processed at relatively low temperature. In this research, we investigate the rf-sputtered hafnium-doped ZnO TFTs. Compared to ZnO TFTs, improvement of electrical performance has been observed in the HfZnO TFTs with Hf doping concentration of <1 at.%.
The HfZnO TFT has a coplanar top-gate, top-source/drain structure. First, a hafnium-doped ZnO layer is rf-sputtered on Corning Eagle 2000 glass substrate at room temperature, followed by a thermal anneal at 600°C to reinforce the grain formation. Next, an ITO layer is deposited and patterned as the source and drain contacts. Finally, Al₂O₃ and Ti are deposited and patterned to form the gate dielectric and gate contact. The HfZnO TFT exhibits a linear mobility of 47.6 cm²/Vs, a saturation mobility of 76.5 cm²/Vs, an ON/OFF current ratio of 4.2×10⁴, a subthreshold swing of 0.331 V/decade and a threshold voltage of -0.279 V. Compared to ZnO TFT (mu;_lin = 0.874 cm²/Vs, mu;_sat = 2.12 cm²/Vs, ON/OFF = 6.7×10⁴, SS = 1.68 V/decade and V_TH = 3.47 V), the field-effect mobility and the subthreshold swing are greatly enhanced by the incorporation of small amount of hafnium. The HfZnO TFTs are further used to implement the N-MOS inverter with the gate connected to the drain in the load transistor. For a beta ratio of 15, the inverter shows a peak gain magnitude of 14.2 and an undefined logic state region of 3 V when it is biased at 18 V.

Among the magnetic garnets, bismuth iron garnet (Bi3Fe5O12 or BIG) has the highest specific Faraday rotation in visible and infrared light. Using these remarkable magneto-optical properties, these materials has been used in optical isolator, optical fiber communication and magnetic field sensors. The improvement of the performance of bismuth iron garnet in these applications requires to understand the intrinsic magneto-optical properties and their temperature dependency.

In this work we investigate the temperature dependence of the magneto-optical (MO) Faraday properties of bismuth iron garnet Bi3Fe5O12 thin single-crystalline film. The Faraday rotation and ellipticity spectra were measured between 4.2K and 710K for a photon energies ranging from 1.6 and 3.6eV. Using a model based on two diamagnetic lines associated to tetrahedral and octahedral iron site, we successfully reproduce the magneto-optical spectra and extracted the tetrahedral and octahedral contribution for all temperature from the total Faraday rotation and ellipticity spectra.
This analysis of the magneto-optical spectra using this simple crystal electric field model allows accessing the magnetic properties of the individual sublattice (tetrahedral and octahedral) at specific photon energies. In particular, hysteresis loop measured through Faraday ellipticity at selected photon energies (ex: ~3eV) exhibit an anomalous behavior that gives access to magnetic response both each of the sublattices. We observed that the saturation field for the two sublattices is different.
Our measurements show the Faraday rotation and ellipticity to exhibit a very complex thermal dependence for certain photon energies. The analysis of the spectral response for each sublattice offers the opportunity to differentiate the tetrahedral and octahedral contribution in magneto-optical properties by selecting photon energies at which the contribution of one sublattice vanishes. In addition, the study of temperature dependency of tetrahedral and octahedral contribution allows to determinate the evolution with temperature of each sublattice for different photon energy. We show that, below 300K, the contribution of two sublattices to the magneto-optical signal impacts differently.

The rare-earth nickelates (RNiO₃) are correlated electron systems that exhibit an abrupt metal-insulator phase transition (MIT) and unusual up-up-down-down antiferromagnetic (AF) order in the insulating phase. The mechanisms behind these phase transitions are not well understood. The difficulty in understanding the full phase diagram arises due to the transition from weak (for lighter R) to strong (for heavier R) electron correlation with rare-earth substitution. Among RNiO₃, SmNiO₃ (SNO) is of interest because it falls in the intermediate regime between the limits of weak and strong electron correlation. It is also the first with an MIT (400 K) above room temperature, which may be useful for commercial technologies in the long term.
We present the first-reported Hall coefficient (R_H) measurements of SNO thin films from 30 - 400 K across both Néel and metal-insulator transitions. These are also the first Hall measurements of any RNiO₃ compound well into the insulating phase. Few Hall measurements are reported in the literature likely due to relatively high carrier concentrations leading to low Hall voltages, significant temperature drift due to the large temperature coefficient of resistivity in the insulating phase, and large overall resistivity. We observe distinct features in the temperature-dependence of R_H, particularly at the Néel transition. By relating the evolution of R_H with published band structure calculations of metallic RNiO₃, we propose a mechanism for the AF order in SNO that succinctly explains the connection between R_H and the Néel transition and that may unify the origin of magnetism across the RNiO₃ series. Our results provide support for recent theoretical results on density wave order driven by band structure (though not the Fermi surface per se) in materials between the limits of weak and strong electron correlation. In the longer term, our results could be of relevance in the design of correlated oxide devices exploiting their complex electronic/magnetic phase diagram.

Oxide heteroepitaxy integrating film/substrate and multilayer combinations with different crystal structures may lead to new opportunities in creating functional heterostructures from chemically, structurally, and electronically disparate constituents. There has been significantly less work on epitaxy of materials of different crystal structures compared to isostructural oxide heterostructures. Epitaxy of structurally dissimilar materials often requires matching film and substrate planes of different symmetries, giving rise to epitaxial variants. As a detailed case study, we will focus on highly oriented (010) VO2 thin films—with a distorted rutile structure—on (111) perovskite LaAlO3, (0001) corundum Al2O3 substrates, (111) spinel MgAl2O4, (111) rocksalt MgO, and (0001) and (000-1) wurtzite ZnO substrates. The substrates chosen possess 3m surface symmetry, while the [010] axis of rutile is a 2_1 screw axis; therefore, structural variants related by 120 degree rotations are required to enable “epitaxial” growth. We will present a comparison of the metal-insulator transition characteristics of the VO2 films, show that this approach can be extended to other rutile-based oxide thin films, and discuss the effects of interface structure and various crystal orientations.

With the development of atomically precise epitaxial growth techniques for complex oxides, there recently has been much research in complex oxide thin films and interfaces due to the wide range of electronic and magnetic properties that can be generated in these systems. One material that has been the subject of both experimental and theoretical studies over the past decade is CaMnO₃ (CMO), which is an antiferromagnetic insulator in bulk form[1]. When juxtaposed with a paramagnetic metal in a superlattice such as CaRuO₃, CMO exhibits ferromagnetic behavior[2][3]. While this effect has been purported to arise from the interface of CMO layers adjacent to the paramagnetic metal, bulk CMO samples have exhibited a weak ferromagnetism of ~0.04mu;B/Mn [4][5]. The origin of this ferromagnetism has previously been attributed to spin canting or more recently ferromagnetic clusters around oxygen defects [6][7][4]. Epitaxial CMO layers may exhibit similar ferromagnetic properties in addition to an enhancement of ferromagnetism from biaxial epitaxial strain. In any case, until now, there has not been a systematic study of the magnetic properties of thin film CMO.
We have grown CMO on (100) LaAlO₃ (LAO) and (100) SrTiO₃ (STO). X-ray diffraction measurements confirm epitaxial growth of (100) CMO films with excellent crystallinity characterized by Δomega;~ 0.27° for STO and Δomega;~ 0.54° for LAO. Samples on LAO are strained while samples on STO are fully relaxed. Cation stoichiometry of 1Ca:1Mn has been confirmed for LAO within the limits of Rutherford backscattering spectrometry. Superconducting quantum interference device magnetometry measurements indicate a magnetic moment of ~0.03mu;B/Mn for samples grown on LAO and ~0.01mu;B/Mn for samples on STO. Preliminary x-ray magnetic circular dichroism (XMCD) measurements confirm manganese as the source of ferromagnetism. A detailed analysis of the XMCD spectra indicates the presence of Mn³⁺ as well as Mn⁴⁺ consistent with the presence of oxygen vacancies. Together these results suggest that the ferromagnetism in the CMO layers is not sufficient to explain the ferromagnetism that has been observed in CaRuO₃/CaMnO₃ superlattices[3].
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Since the first demonstration of epitaxial growth of crystalline SrTiO3 on Si (001) by Mckee and co-workers [1], sub-monolayer Sr on Si(001) has been extensively investigated experimentally [2] and theoretically [3]. Charge transfer induced by half-monolayer (½ ML) of Sr has been shown to be a key element enabling wetting of Si by SrTiO3 [4]. However, a detailed understanding of the electronic structure reconstruction in sub-monolayer Sr/Si(001) is not complete. Such knowledge could be extended and applied to the other epitaxial crystalline oxides on semiconductors [4,5]. Recently, using in-situ x-ray core-level spectroscopy, we have studied the change in electronic structure of Si(001) induced by sub-monolayer Sr deposition in terms of surface core level shift (SCLS) [2]. One of the interesting features is shift of the Si 2p level toward the higher binding energy by 0.49 eV after Sr deposition. In this talk, we present a detailed theoretical investigation of the surface core level shifts in sub-monolayer Sr/Si(001). Using the final state theory [6,7], we calculate the bulk 2p binding energy to be increased by 0.42 eV when ½ ML Sr is deposited in excellent agreement with experiment. We are able to compare the calculated evolution of the surface band structure in sub-monolayer Sr/Si(001) to angle-resolved photoemission spectroscopy (ARPES) data.
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A group of vanadium dioxide (VO2) thin films were deposited on sapphire substrates by reactive magnetron sputtering at the same growth condition but with varied substrate temperature TS (575, 625, 675, 700 oC, respectively). Their structural and metal-insulator transition (MIT) properties were comparatively studied based on XRD, SEM, XPS, Raman spectroscopy, and electrical characterization. The room-temperature resistance can be tuned by up to 2-3 orders with increase of TS. Obvious resistance-temperature hysteresis curves can be observed in all four samples, but very different behaviors. Optical properties of these four samples were investigated by UV-VIS-NIR spectroscopy and Terahertz time domain spectroscopy. All four samples exhibit obvious modulation across MIT in NIR and THz range. It was found that their transmission in NIR range varied a lot, with the 575 oC sample shows the highest transmission in both insulating (81%)and metallic state (4%), while 700oC sample shows the lowest transmission(30% and 1.5%). These findings suggest that the electrical and optical properties of VO2 films can be tuned by adjusting growth temperature, which offers more material-related options for various applications employing MIT in VO2, such as optical modulation and electrical switching.

Vanadium dioxide (VO2) thin-film undergoes dramatic optical properties change across its metal-insulator transition (MIT), which renders it a promising material for optical modulation and switching applications, especially in the infrared and THz ranges. Particularly, intensity modulation depth as large as 90% was previously reported, suggesting its potential application to realize spatial light modulators (SLMs) for operation at THz frequencies. In this work, 2x2 THZ SLM prototypes were fabricated using 120 nm thick epitaxial VO2 thin films grown on c-cut sapphire by reactive magnetron sputtering. Plasma etching was employed to define the SLM independently addressed pixels. Metal wires were deposited on the top of each pixel to function as the heating elements which will trigger the MIT through current control. Transmission Terahertz time-domain spectroscopy measurements revealed that amplitude modulation depth of 3-4 folds can be achieved for each pixel under current control. Raster-scanning tests were also performed across the entire SLM device to verify the amplitude modulation uniformity, which yielded distinct transmission gray-scale patterns for the ‘on&’ and ‘off&’ pixels, demonstrating relatively small cross-talk between adjacent pixels.

Surface structure and stoichiometry of oxides are important for understanding the emergent physical phenomena of oxides. Scanning probe microscopy (SPM) has become a crucial tool for exploring surfaces down to the atomic level. Here, we report an approach for studying local surface chemistry and order parameter fields based on a local crystallographic analysis of SPM data of oxide surfaces. Local atomic distances and angles are analyzed and quantified from determined local indexes (locations) for atoms or alternatively larger scale grids for nanoscale objects. We obtained initial estimated atom locations by finding the centroid of the remaining isolated regions of pixels, after applying thresholds to the topographic image. With the initial estimated locations, we determined the refined positions by automatically fitting each atom individually using a shape function. Based on the refined locations, we can further derive and quantify properties that are not readily clear in the scanning probe microscopy images. This approach was applied to analyze scanning tunneling microscopy data for the surface of La5/8Ca3/8MnO3 (001) [1] and demonstrated distortion domains with different distortion orientations. These studies provide a new pathway to extract and quantify local properties for scanning probe microscopy images.

Research was supported (W.L., S.V.K.) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. This research was conducted at and supported by (Z.G., S.J., A.P.B.) the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

[1] K. Fuchigami, Z. Gai, T. Z. Ward, L. F. Yin, P. C. Snijders, E. W. Plummer, and J. Shen, “Tunable Metallicity at La5/8Ca3/8MnO3 Surface by Oxygen Overlayer” Phys. Rev. Lett. 102, 066104 (2009).

Knowledge of the electron sampling depth and related saturation effects is important for quantitative analysis of x-ray absorption spectroscopy data, yet for oxides with the perovskite structure no quantitative values are so far available. Here we study absorption saturation in films of two of the most-studied perovskites, La1xCaxMnO3 (LCMO) and YBa2Cu3O7 (YBCO), at the L2,3 edge of Mn and Cu. By measuring the electron-yield intensity as a function of photon incidence angle and film thickness, the sampling depth d, photon attenuation length lambda; and the ratio lambda;/d have been independently determined between 50 and 300 K. Over this temperature range the LCMO and YBCO resistivities change many orders of magnitude - LCMO crosses a metal-insulator transition temperature TMI ~ 150 K, and YBCO crosses a superconducting transition Tc ~ 90 K. The extracted sampling depth dLCMO asymp; 3 nm in insulating La1xCaxMnO3 at high temperatures (150 - 300 K) is not much larger than values reported for pure transition metals (dFe, Co or Ni asymp; 2-2.5 nm), but is smaller than dYBCO asymp; 4 nm for YBa2Cu3O7 that is in turn smaller than the value reported for Fe3O4 (dFe3O4 asymp; 4.5 nm). The measured dLCMO increases to 4.5 nm when La1 xCaxMnO3 enters the metallic state well below TMI, but remains constant for YBCO below Tc. These results indicate that a universal rule of thumb for the sampling depth in oxides cannot be assumed, and that it can be measurably influenced by electronic phase transitions that derive from strong correlations.

Effect of oxygen to nickel gas ratio (O2/Ni) on atmospheric pressure metalorganic chemical vapor deposition (APMOCVD) growth of NiO is reported. Single crystal cubic NiO films are grown on sapphire substrates by APMOCVD at 500 °C as a function of O2/Ni. X-ray diffraction analysis including grazing incident angle theta; of 0.6°, theta;-2theta;, rocking curve and Phi; scan are employed for crystallographic characterization. Furthermore, surface roughness is studied by atomic force microscopy (AFM). No evidence of diffraction peaks in X-ray grazing incident angle measurement confirms that all the grown NiO films are well oriented along a certain direction. theta;-2theta; scan results further indicate that the samples are highly oriented only along [111] direction on top of sapphire (0001). The analysis of full width at half maximum (FWHM) of Rocking curve of (111) plane shows that higher O2/Ni gas ratio results in higher crystallinity. The highest crystallinity is achieved with FWHM as low as 0.106°. AFM measurement shows that NiO films grown with higher O2/Ni gas ratio have smoother surfaces.

Wide-gap semiconductors, such as ZnO, have garnered interest in the realm of dilute magnetic semiconductors (DMSs) due to their potential applications to optoelectronic and magneto-optical devices. Another class of materials gaining interest are d⁰ ferromagnets, in which ferromagnetism arises despite the absence of magnetic elements.
Recent experiments on N-doped ZnO DMS have reported room-temperature ferromagnetism (RTFM) with a homogeneous distribution of N-dopants. However, the origin of RTFM observed in this system remains debated, since analogies to the previously studied transition-metal-doped ZnO systems suggest the observed RTFM results from heterogeneities in the system.
Through first-principles calculations, we show that heterogeneities are indeed the origin of the observed RTFM in N-doped ZnO. That is, Monte Carlo simulations indicate that self-organized N-rich nanostructures form under layer-by-layer growth conditions. Furthermore, our calculations show that these nanostructures have strong ferromagnetic coupling between N-atoms within each nanostructure in addition to high blocking temperature, assuming a homogeneous distribution of dopants within each nanocluster. These self-organized nanostructures have potential applications to high-density magnetic memory.

MgO based magnetic tunnel junction (MTJ) has attracted much research interests for its large tunneling magnetoresistance (TMR) and potential device applications. Previous studies showed that the symmetry-filter property of the MgO barrier is the origin of the huge TMR effect. However, TMR of the Fe/MgO/Fe single-crystal MTJ decreases rapidly with the applied bias voltage according to both experimental and theoretical studies. To our knowledge, study of bias dependence of the TMR of the Fe/MgO/Fe MTJ on bias voltage has not been reported yet.
In this paper, the electronic structure and quantum transport properties of Fe/MgO/Fe and Fe/Ag/MgO/Ag/Fe single-crystal MTJs with 13-layer MgO barrier were calculated with ATK code, which is based on density functional theory combined with non-equilibrium Green&’s function technique. For calculations, we used revised Perdew-Burke-Ernzerhof form of GGA exchange-correlation functions and standard norm-conserving pseudo-potentials. The LCAO basis sets, a single-zeta polarized for Fe, a double-zeta polarized for Mg and Ag and a double-zeta for O were adopted. A 16×16 k-point mesh was used to sample the transverse Brillouin zone (BZ). The tunneling current is evaluated with the Landauer-Büttiker formula.
From the results of BZ resolved transmission coefficients and Fe/MgO interface density of states (DOS), we found there are large transmission peaks near the Γ point as well as large interface DOS peaks. These interface states lead to significant increase of current for anti-parallel magnetic configuration (APC) with bias, then the drop of the TMR under bias of about 1.1 V.
The transmission coefficient of the two spin channels in APC and the spin down channel in PC increase more rapidly with energy than that of spin up channel in PC. This can be explained by the continuity of the spin down Δ1 band of the Fe electrode. Because the energy band changes gradually along k-point, the transmission will be enhanced when the energy approaches the spin down Δ1 band of the Fe electrode in APC.
According to our calculation, we also found that Fe/Ag/MgO/Ag/Fe MTJ showed higher TMR in a wide bias range than Fe/MgO/Fe. The inserted Ag mono-layer suppresses the transmission of the spin up channels and removes the interface states at 1.06eV. As a result, the transmission coefficient of spin down channel in PC and both channels in APC increase slower. This means that inserted Ag mono-layers at Fe/MgO/Fe interface can remarkably improves the TMR performance at high bias voltage.
As a summary, our research showed that the dependence of TMR on bias of Fe/MgO/Fe MTJ is determined by three factors: (1) interface states lies at 1.06 eV in spin down channel, (2) rapid increase of current in APC due to the continuity of the spin down Δ1 band of the Fe electrode and (3) the position of the spin down Δ1 band of the Fe electrode.

The rare-earth nickelates (RNiO₃) feature an insulator-metal transition (at T_IM) that can be tuned from 130 to 600 K with chemical substitution, and an electronic phase diagram that is remarkably sensitive to epitaxial strain. This malleability makes RNiO₃ appealing candidates for a growing number of proposed applications of multi-functional layers in oxide electronics. At the same time, the physical origin of the insulating state remains a puzzle. After two decades of research there remains disagreement over the nature of the insulating gap (Charge transfer? Hubbard? Polarons? Antiferromagnetism?), and no firm experimental measurements of either the insulating gap or the transport mechanism in the insulating phase have been reported. For condensed matter physics this is an important puzzle to solve, since the nickelates may hold the key to understanding an even broader class of oxide materials (including the colossal magnetoresistive manganites and the superconducting iron pnictides) that fall between the limits of strong and weak electron correlation. Understanding the nature of the insulating gap is also important for work on applications of RNiO₃ in oxide electronics. Much present-day research into phase transition oxide switches (“Mott transistors” and the like) relies on assumptions about the underlying electronic structure that may not be well supported in the case of RNiO₃. A better understanding of the insulator-metal transition would therefore help to better focus applied research efforts.
These outstanding questions over the nature of the insulating state are best addressed by a combination of DC characterization and AC spectroscopy. Here we report infrared spectroscopy, resistivity, and Hall coefficient measurements on epitaxial SmNiO₃ thin films synthesized with varying oxygen content. We find evidence that electron-lattice coupling is the principle mechanism responsible for the insulator-metal transition. By measuring with fine temperature steps across T_IM we track the evolution of spectral features associated with the metallic and insulating phases. We will discuss our results in the context of proposals that RNiO₃ are polaronic insulators [Medarde et al., Phys. Rev. Lett. 80, 2397 (1998); Mroginski et al., Phys. Rev. B 60, 5304 (1999)]. Furthermore, by varying oxygen content we observe how the metallic phase adjusts to an increase in disorder. Our results draw interesting connections between electron-phonon coupling and bad metallicity observed in many correlated electron oxides.

Recent developments in growth of atomically-abrupt interface of complex oxide heterostructure or superlattice have open up a new avenue for tailoring their electronic properties. The two dimensional electron gases (2DEGs) forming at the interface between a Mott insulator LaTiO₃ and a band insulator SrTiO₃ is one good example. At the LaTiO₃/SrTiO₃ interface, the 3d¹ electron in LaTiO₃ is transferred to the adjacent SrTiO₃, so that 2DEG is created at the interface with a few unit-cell widths. Previous electron energy loss spectroscopy (EELS) study showed a spectral broadening of Ti³⁺ edge, suggesting the charge transfer from the LaO layer of LaTiO₃ to the adjacent SrTiO₃ layers.
Here we show direct mapping of the 2DEGs forming at the heteroepitaxial interfaces of LaTiO₃/SrTiO₃ superlattices using inline electron holography. Inline electron holography can directly extract the electrostatic potential distribution across the LaTiO₃/SrTiO₃ interfaces from a through-focal series of bright-field transmission electron microscope (TEM) images. Compared to the other electron holographic techniques, the inline holography can offer better a spatial resolution (less than 1 nm) and a larger field-of-view (~0.5×0.5 mu;m²). By applying a Laplacian image filter to the obtained potential map, we were able to directly map the total charge density across the interfaces. Our charge density map clearly visualized the presence of free electrons with the density of ~3×10¹⁴ cm^-2 at the LaTiO₃/SrTiO₃ interfaces, corresponding to the transfer of 0.5 electron per a unit cell of LaTiO₃ to the adjacent SrTiO₃ layers. Moreover, the local strain map obtained from a through-focal series of dark-field images showed that not only the LaTiO₃ layers but also the SrTiO₃ layers in the superlattice were strained in a complementary manner to accommodate the lattice misfit strain, which was confirmed by finite element modeling. The effects of lattice strain on the spatial distribution of 2DEGs will be discussed in greater detail.

Functional materials based on complex oxides in thin film form offer new and exciting strategies for meeting many of our outstanding energy challenges. Unfortunately, synthesis of such oxide films can be a major challenge even when utilizing reactive molecular-beam epitaxy (MBE), a powerful deposition technique that is often regarded to allow the construction of materials atomic plane by atomic plane. To understand the fundamental physics of oxide growth by reactive MBE, we have developed world&’s first reactive MBE system with in-situ synchrotron x-ray scattering capability at the Advanced Photon Source. Here we present in-situ surface x-ray scattering results of the homoepitaxial growth of SrTiO₃ thin films on (001)-oriented SrTiO₃ substrates. We measured the in-situ x-ray specular reflectivity and surface diffuse x-ray scattering during co-deposition and shuttered-growth of Sr and Ti for SrTiO₃ homoepitaxy. The goal of this study was to have a direct comparison of shuttered growth vs co-deposition to understand the nature of the distinctly different approaches. An area detector recorded both the specular x-ray scattering connected to out-of-plane atomic positions and the diffuse x-ray scattering associated with in-plane correlations at the same time. During growth of SrTiO₃ by co-deposition, where the fluxes of Sr and Ti are roughly equal, the specular intensity at the half-order position is at a minimum while the diffuse intensity is at a maximum. This is consistent with a 2D island growth mode with unit-cell-high SrTiO₃ islands that nucleate/grow on the terraces and coalesce before the next layer starts. Interestingly, we observe that shuttered growth does not proceed by the nucleation and coalescence of single-layer-high SrO islands, followed by the same for TiO₂. Instead, the scattering indicates that the SrO grows as islands and then restructure into SrTiO₃ unit cells during the growth of the TiO₂ to form an atomically flat layer. This shows directly that the process of growth is quite distinct between the two approaches. Work at Argonne, including the Advanced Photon, is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Recently there has been an intense interest in studying 5d transition metal oxide due to the novel physical states caused by strong spin-orbital coupling and the possibility to realize the topological band/Mott insulator. SrIrO3 is a non-Fermi-liquid metal with hexagonal structure in the bulk. However, epitaxial growth of SrIrO3 on perovskite substrate can stabilize the perovskite phase of SrIrO3, which, otherwise, can only be stabilized by high-pressure synthesis. The induced metal-insulator transition at low temperature, combined with the antiferromagnetic insulating ground state of Sr2IrO4, indicates that the perovskite SrIrO3 would lie at the boundary of Mott metal-insulator transition due to the competition between the on-site coulomb interaction U and small bandwidth W caused by strong spin-orbital coupling. Therefore it is an ideal route to fine-tune the competition between the U and W by changing the epitaxial strain, which would lead to different phases such as the magnetic metal/ nonmagnetic metal/ magnetic insulator. In this talk, we present both the structural and physical characterization of SrIrO3 on different substrates. XRD confirms the perovskite structure in the strained state and reveals how the different phases evolves as the film becomes relaxed. The properties of different strain states are characterized by magnetometer and transport measurement. Our careful study of the different tuning parameters provides insight into understanding the ground state and the competing mechanisms in SrIrO3, which serves as the guideline to search for exotic physical phenomena in 5d TMO heterostructures.

Atomic-scale studies of structural and magnetic properties of two different oxide-based model systems will be presented: 1) ultrathin oxide films grown epitaxially on metal substrates and 2) surfaces of all-oxide samples. In both cases, novel structural phases of oxides can be stabilized, providing many opportunities for unexpected discoveries. A fundamental understanding of magnetic and spin-dependent phenomena requires the determination of spin structures and spin excitations down to the atomic scale. The direct visualization of atomic-scale spin structures [1-4] has first been accomplished for electrically conducting samples by combining the atomic resolution capability of Scanning Tunnelling Microscopy (STM) with spin sensitivity, based on vacuum tunnelling of spin-polarized electrons [5]. The resulting technique, Spin-Polarized Scanning Tunnelling Microscopy (SP-STM), nowadays provides unprecedented insight into collinear and non-collinear spin structures at surfaces of magnetic nanostructures [6] and has already led to the discovery of new types of magnetic order at the nanoscale [7]. More recently, the detection of spin-dependent exchange and correlation forces has allowed a first direct real-space observation of spin structures at surfaces of antiferromagnetic insulators [8]. This new type of scanning probe microscopy, called Magnetic Exchange Force Microscopy (MExFM), provides a powerful new tool to investigate different types of spin-spin interactions based on direct-, super-, or RKKY-type exchange down to the atomic level [9, 10]. By combining MExFM with high-precision measurements of damping forces [11] localized or confined spin excitations in magnetic systems of reduced dimensions now become experimentally accessible.
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Dislocation, which is one-dimensional lattice defect in crystals, is potentially used as atomic-scale nanowires inside crystalline environments. Functional properties appeared at dislocation core structures are, therefore, in great usage for miniaturizing devices. However, the properties appeared at such small volumes are still not well investigated. In this study, we investigated the magnetic properties of dislocations in NiO thin films. We show that the dislocations in antiferromagnetic NiO exhibit unique ferromagnetic properties at their cores.
NiO films are grown on Nb-doped SrTiO3 single crystal substrates by pulsed laser deposition (PLD). Lattice mismatch between substrate and film is about 7.04% and high density threading dislocations are self formed in the films by the mismatch. .
Magnetic force microscopy (MFM) observations revealed that the dislocations in NiO thin films show spontaneous magnetization although the bulk NiO is antiferromagnetic. The ferromagnetic Curie temperature of the dislocations are estimated to be around 525 K, which is very close to the Neel temperature of the antiferromagnetic NiO. Switching behavior of the spontaneous magnetizations is examined by ex-situ MFM measurements after applying magnetic field. These measurements showed very high coercive force higher than 4 T.
The atomic-scale structures of the dislocations are investigated by TEM and HAADF-STEM. The Burgers vector of the dislocations are determined to be b = a/2 [110], where the dislocation lines lie parallel to [001]. The atomic structures of the dislocations cores are observed by HAADF-STEM. The electronic state and local composition are investigated by electron energy-loss spectroscopy (EELS). EELS analysis revealed that the dislocations cores are locally non-stoichiometry with nickel less composition. The ferromagnetic property appeared at dislocations should be caused by the nickel less structure which breaks the balance of numbers of up spins and down spins.

Mott materials with strong spin-orbital coupling have emerged as a new playground for searching quantum many-body phases with exotic electronic and magnetic properties. In particular, 5d transition metal oxides have received special attention due to the intriguing opportunities to obtaining novel topological insulators, superconductivity, Weyl semimetals, quantum spin liquid, and so on. Realizing these fascinating phenomena is believed to lead to a new generation of electronic and spintronic devices based on functional oxides. While recent studies focus on the bulk by applying, e.g., chemical doping, pressure and magnetic field, we present here our investigation on using epitaxy to control perovskite iridates, a prototype of 5d complex oxides, as ultrathin films and heterostructures. The combination of strain, dimensionality and interfacial coupling offers a unique pathway to innovative functionalities by tuning the interplay between “Mott-ness” and spin-orbit coupling. The experimental results derived from transport, magnetometry and advanced resonant x-ray spectroscopy, including linear and circular dichroism, will be discussed.

Itinerant ferromagnets have provided intriguing quantum phenomena such as anomalous Hall effect (AHE) and topological Hall effect through a strong coupling between charge carrier transport and magnetism [1]. In this context, perovskite EuMO3 (M: transition metal) is promising because Eu2+ has large magnetic moment of 7mu;B. In this study, we aimed at fabricating perovskite EuMoO3 (EMO) in analogy with highly conducting SrMoO3. However, EMO has not been synthesized due to chemically incompatible valence states of Eu2+ and Mo4+. This difficulty was overcome by utilizing epitaxial stabilization from an isostructural substrate as well as highly nonequilibrium process of thin film growth by pulsed laser deposition (PLD) [2].
We fabricated EMO thin films by PLD on GdScO3 (110) substrate with SrTiO3 (STO) buffer layer. We used a growth temperature of 1023 K and Ar gas containing 3% H2. X-ray diffraction (XRD) showed peaks suggesting the perovskite structure and high crystallinity of the film. Additional support for the epitaxial growth of EMO was obtained by reciprocal space mapping of four-circle XRD and transmission electron microscopy. The valence states of Eu2+ and Mo4+ were confirmed by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy. The band structure of EMO was calculated by DFT, which was consistent with the XPS spectrum. The EMO films showed a ferromagnetic transition at a Curie temperature of 5 K and a saturated magnetization of about 7mu;B. The electrical resistivity decreased with reducing temperature and reached 30 mu;Omega;cm at 2 K. AHE was also observed, which suggested the coupling between carrier transport and magnetism.
The characteristics of this charge-spin coupling were further investigated by fabricating supperlattices composed of EMO and STO with controlling EMO/STO ratio, which resulted in the reduction of conductivity and disappearance of AHE signal by decreasing EMO layer thickness. These results indicated the existence of “dead layer” near the interface between EMO and STO, where Eu magnetic moment was canted even below the Curie temperature and affected carrier transport through the spin scattering.
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[2] Y. Kozuka, T. C. Fujita, M. Kawasaki et al., Chem. Mater. 24(19) 3746 (2012)

The generation of ferromagnetism (FM) at the interfaces of materials that do not exhibit FM in the bulk is of interest from both fundamental and technological perspectives. There are remarkably few examples of such interfacial FM phenomena. One example is the CaMnO₃ (CMO) / CaRuO₃ (CRO) system composed of paramagnetic (PM) metallic CRO and the antiferromagnetic insulating CMO. In this system, the FM has been attributed to an interfacial double exchange interaction although other effects such as defect or intermixing induced FM cannot be completely dismissed.[1] LaNiO₃ (LNO), a strongly correlated PM metal like CRO, provides an alternative material to study interfacial double exchange effects in CMO based superlattices. Unlike CRO, ultrathin films of LNO are known to undergo a thickness-induced metal-insulator transition, allowing us to study the role of electron mobility on any induced FM.[2] It has also been suggested that ultrathin LNO films may undergo a transition to an antiferromagnetic, spin glass, or spin density wave state.[3,4] These transitions would be expected to significantly alter magnetic coupling at the interface of an LNO/CMO interface.
To probe the interfacial double exchange in CMO-based superlattices, we fabricated superlattices of (LNO_m/CMO_n)₈ where m = 2-9 and n = 8, 14, and 20 and characterized their magnetic properties using a combination of SQUID magnetometry, X-ray magnetic circular dichroism (XMCD), and polarized neutron reflectometry (PNR). We report FM at the LNO/CMO interfaces. Samples with m ge; 4 were metallic while m < 4 samples were insulating. SQUID magnetometry and XMCD showed that only the superlattices with m ge; 4 were FM, indicating that the FM is coincident with the LNO metallicity. XMCD shows that the FM originates in the Mn ions only while PNR measurements indicate that the observed FM is periodic and is incompatible with a uniform magnetization of the CMO. In fact, the observed PNR spectrum can only be reproduced by a model with one magnetized unit cell of CMO at the interface. These observations are consistent with FM mediated through an interfacial double exchange interaction but not with FM resulting from intermixing or defects. Finally, we find that samples with m = odd (5, 7, 9) exhibit significantly higher saturated magnetic moments that samples for which m = even. This may be evidence of an oscillatory magnetic coupling through the LNO layer, the origins of which are unclear but may be explained in terms of either an RKKY interaction through the conducting LNO or by the formation of a spin density wave in the LNO layer.
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Magnetic semiconductors entwine two of the most successful and productive concepts in both fundamental physics and industrial applications. In this marriage ferromagnetic materials have played an undismissable role. Recently it has been proposed that antiferromagnets may become attractive alternative material systems in the spintronic research and applications, embracing as well the realm of semiconductors [1,2].
Antiferromagnetic spintronics have recently been demonstrated by the fabrication of tunnel devices [3,4], atomic-size proof-of concepts [5], even devices without auxiliary ferromagnetic layers [6]. On the other hand, spin-gating electrical currents has been shown in a ferromagnetic - normal metal capacitively-coupled device [7].
Here we present the control of the electrical conductivity of an antiferromagnetic semiconductor by manipulating the magnetic state of a contiguous ferromagnetic layer acting as a spin-based gate. We present an oxide-based fully epitaxial heterostructure, its structural characterization and the electrical measurements showing a direct link between state of the ferromagnetic gate and ohmic resistance of the semiconductor, even displaying distinct remnant resistance states.
.............................................................................................................
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Functional oxides based on the transition metal series display a wide spectrum of remarkable electronic properties including magnetism, superconductivity and metal-insulator transitions. These novel properties arise from the interaction between the charge, orbital, spin, and lattice degrees of freedom. The key to controlling these properties lies in the ability to control the underlying structure. By using epitaxial growth to strain oxide crystal structures, thin film synthesis offers novel route to control oxide structure in ways not attainable in the bulk counterparts. This allows one to access new regions of phase-space to explore emergent states not present in bulk form. Extending this to ultrathin heterostructures then offers the ability to harness dimensionality as an additional knob to control the interactions of strongly correlated electrons. Here I will highlight our recent work on complex oxide heterostuctures focused on using strain and confinement to manipulate orbital configurations in nickelates[1-4] and cuprates[5-6].
Polarized X-rays are a powerful probe to explore the splitting of the 3d states due to strain and interfacial confinement. For the case of nickelate films, these results show a very strong asymmetric response to strain due to the evolution of the electronic properties under tensile vs. compressive conditions[1]. Extending this into the regime of ultrathin superlattices finds a confinement driven metal to insulator transition[2,3] followed by interface chemistry driven orbital polarization in the single unit cell limit[4]. This will be contrasted to the case of manganite-curpate interfaces where the altered magnetic and electronic states result from the covalent bonding between Mn and Cu at the interface[5,6]. Overall, these examples highlight the opportunities to create new materials by direct control of the orbital degree of freedom with confinement and strain.
Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.
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[5] J. Chakhalian, J.W. Freeland et. al. Nature Physics 2, 244 (2006).
[6] J. Chakhalian, J.W. Freeland et. al. Science 318, 1114 (2007).

Progress in understanding and exploiting the properties of complex oxide heterostructures requires advances in state-of-the-art growth and characterization techniques for these materials, as atomic control of their synthesis is demonstrably inferior to that of their semiconductor counterparts. Here we report significant improvements in the atomic design of perovskite interfaces made possible by advances in in situ control of the surfaces distortions by reflection electron diffraction during growth that exploit the detection of the characteristic octahedral in the surface layer as it is being deposited. This allows growth optimization of the atomically-designed interface that eliminates cation intermixing and anomalous unit cell dilations that have previously been observed. As an example, careful analysis of the crystal structure in manganite interfaces shows an unusual evolution of the octahedral distortions that include both J-T type and rotations near the interface that are not seen in bulk. These new results should be included in electronic structure calculations modeling the properties of real heterointerfaces.

Epitaxial thin oxide films with perovskite structure (ABO3) has been extensively studied in the search of new properties and functionalities that can be enabled in the films and at their interfaces that are not possible in the bulk, including novel magnetic and electronic states. For example, LaCoO3 (LCO) thin films show ferromagnetic ordering under low temperatures, which differ from bulk (non-magnetic) characteristic. However, little is known about the local atomic and electronic structure in the vicinity of the interface.
For this study, LCO thin films were deposited by pulsed laser deposition method with different thicknesses (2, 5, 15 unit cell and 20 nm thickness) on SrTiO3 substrate. X-ray photoelectron spectroscopy studies of the grown films have demonstrated that Co 3p edges shift up to 2 eV for 15 u.c. and 20 nm films, indicating possible presence of 2D electron gas. Scanning transmission electron microscopy and electron energy loss spectroscopy with atomic resolution were then performed to elucidate possible sources of difference in properties between the films of different thickness, including discrepancies in local structure and chemical intermixing at the interface.
The structure of the 5 u.c and 15 u.c LCO films was examined in detail. Atomic position mapping from STEM HAADF and BF images can reveal lattice parameter and octahedral tilt behavior with atomic resolution; it is also possible to quantify local vacancy distribution from the associated lattice modulations.1 Out-of-plane lattice spacing in the 15 u.c. film shows an increase at the interface, while in-plane lattice spacing shows modulation usually indicative of octahedral tilts. Similar behavior was not observed in the 5 u.c. film, where neither the increase nor the modulation was observed. BF STEM imaging confirmed suggestions from lattice parameter mapping, showing that octahedral tilts were active in the 15 u.c. film but not in the 5 u.c. film.
Studies of the chemical and electronic structure of the interface were conducted with EELS. Some degree of Ti/Co intermixing was observed, which was similar for films of both thicknesses. A complex pattern of O K fine structure evolution at the interface was observed. O K edge contains multiple contributions, including that from the hybridized O 2p to Co 3d electronic states;2 using this data, different contributions to electronic structure could in principle be deconvolved via advanced simulations. The results of the simulations, as well as data on oxygen vacancy mapping, will also be presented.
* Research supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division, and through a user project supported by ORNL&’s Shared Research Equipment (ShaRE) User Program, which is also sponsored by DOE-BES.

Field-effect transistors of functional oxides attract considerable attention due to its capability to modulate electronic properties and charge carrier density by an external electric bias. However, breakdown of dielectrics always prevent us from accumulation of high density carriers. An electric double layer transistor (EDLT), which employs a liquid electrolyte as a gate dielectric, recently attracts considerable attention due to its capability to induce high charge carrier densities. Charge carrier density can be reversibly modulated with changing gate bias (V_G) from zero to above 10¹⁴ cm^-2 [1]. High carrier density enables us to induce phase transition such as insulator-to-superconductor and paramagnet-to-ferromagnet [2-4].
In this talk, we will present electric field induced superconductivities in two dimensional electron gases at electrolyte/insulator interfaces in EDLTs. The first example is superconductivity in insulating SrTiO₃. With inducing a sheet carrier density more than 10¹³ cm^-2, we could induce superconductivity purely by electric field for the first time [2]. Critical magnetic field showed a quite good agreement to the two dimensional Ginzburg-Landau theory, indicating that this system behaves as a typical two dimensional superconductor. As a second example, we will show inducing superconductivity in KTaO₃ that is known as a non-superconducting material down to 0.01 K even if making it metallic with chemical doping. Maximum charge carrier density in KTaO₃ EDLT is one order of magnitude larger than the density that can be achieved with chemical doping. Superconductivity appears in KTaO₃ at 50 mK for V_G of 5 V, and Tc shows bell-shaped dependence on V_G.[3] It shows the potential of electrostatic doping to achieve higher charge carrier densities than is possible with chemical doping.
This work was conducted in collaboration with H. Aoki, Y. Iwasa, M. Kawasaki, N. Kimura, S. Nakamura, T. Nojima, A. Ohtomo, H. Shimotani, H. T. Yuan.
[1] K. Ueno, et al., Appl. Phys. Lett. 96, 252107 (2010).
[2] K. Ueno, et al., Nature Mater. 7, 855 (2008) and supporting on-line material.
[3] K. Ueno, et al., Nature Nanotech. 6, 408 (2011) and supporting on-line material.
[4] Y. Yamada, K. Ueno, et al., Science 332, 1065 (2011).

There is a rapidly increasing trend to implant medical devices directly into the body. Currently, most implantable medical devices and systems are powered by batteries, but these power sources have serious limitations due to the lifetime and the size. In general, patients require a surgical operation just to replace the battery after a few years. Therefore, to supply durable and stable power to implantable biomedical devices is one of the most challenging issues in active medical implants.
The objective of this research is to develop a new class of biocompatible and flexible electronic devices and systems, with the emphasis on self-powering. Our research will address the critical need for autonomous power - implantable energy scavenging devices to recharge the batteries that power the implanted devices, along with the power conditioning electronics. We demonstrate the new approach for enlarging the sensing capacity and the power generation of implantable medical sensors and energy harvesters via cost effective chemical solution-derived bio-compatible piezoelectric films coupled with conducting flexible substrates. Our devices exploit natural energy sources of the human body to recharge lifetime-limited implanted batteries. While many research groups have demonstrated the bulk-scale prototypes of piezoelectric energy harvesters, only a few groups have fabricated thin film-type energy harvesters capable of generating useful power all using MEMS processes. We go one step further and for the first time demonstrate useful power generation with a new and simple-to-process flexible electronic system without any complicated MEMS process.

Piezoelectric thin films draw a lot of interest in MEMS heterostructures, such as, energy harvesting systems. The energy harvesting device performance is given by the figure of merit (e312/ ε), indicating that a high piezoelectric coefficient (e31) combined with a low dielectric constant (ε) is vital to achieve a higher piezoelectric voltage. In this study, epitaxial piezoelectric thin films (doped-PZT) were grown on both Silicon and SrTiO3 substrates by pulsed laser deposition (PLD). Here, Nb5+ and Fe3+ were chosen as donor dopant and acceptor dopant, respectively. By using controlled buffer layers, both (001) and (110) orientated doped-PZT thin films were grown. The piezoelectric coeffient (e31) increased significantly in Nb-PZT. Furthermore, giant imprint behaviors in polarization hysteresis loops were observed in epitaxial (001) doped-PZT thin film. For the first time, unique imprint behaviors are reported, which were induced by epitaxial strain and differ from the imprint effects in texture and sol-gel films. A huge build-in bias emerged only in preferred orientation, which indicated dipoles were aligned in a certain direction through the thin films. The horizontal shift in electric field also corresponded to the shift in dielectric constant (ε). A low dielectric constant is expected and the effect on the figure of merit for energy harvesting is discussed.

Perovskite oxides have become a fascinating class of materials because of the wide variety of electronic properties including intriguing ferroic (magnetic or ferroelectric) response for potential use in memory or sensor applications. At the same time, epitaxial strain has been demonstrated to massively change the fundamental properties of such oxides in particular affecting their electronic behavior [1].
An elegant way to form threedimensional structures based on the release and deterministic rearrangement of two-dimensional films has been established over the last years [2,3]: An inherently strained layer stack is deposited on top of a sacrificial layer (or substrate) and is released by selective removal of this sacrificial layer. Due to cunning strain design and patterning, the layer stack bends up forming cantilevers or rolls up into nano- and microtubes. The technique has been employed to form fluidic systems, optical resonators, microtube lasers, and metamaterial waveguides from various material systems. Due to the strain relaxation driving the bending and roll-up process, the three-dimensional microobjects exhibit a unique strain state, influencing the properties of the microtubes.
In this work [4], an approach for the fabrication of three-dimensional microobjects (freestanding cantilevers, rolled-up microtubes) from perovskite oxides, i. e. ferromagnetic SrRuO₃ (SRO) known for its chemical stability and antiferromagnetic Pr_0.7Ca_0.3MnO₃ (PCMO) is reported. The diameter of the obtained tubes varies between 6 and 8 mu;m and a preferred <100> rolling direction is observed. The etching selectivity between the SRO film and the SrTiO₃ (STO) substrate is estimated as 1:9100. X-ray diffraction (XRD) is carried out to evaluate the original and final strain states. Unlike our previous studies using mu;-focus XRD, diffraction is carried out for an ensemble of microtubes using a conventional single crystal diffraction beamline setup. Results clearly reveal the change in the strain state after roll-up, with the PCMO layer relaxing towards its bulk lattice parameter, whereas the upper SRO layer is compressed. Finally, mu;-XRD is carried out on the same beamline allowing for comparing the ensemble properties with a single object. We find that a single tube can represent the ensemble indicating a good overall homogeneity of the roll-up process.
[1] A. D. Rata, A. Herklotz, K. Nenkov, L. Schultz, and K. Dörr, Physical Review Letters 100,076401 (2008).
[2] O. C. Schmidt, N. Schmarje, C. Deneke, C. Muller, and N. Y. Jin-Phillipp, Advanced Materials 13, 756 (2001).
[3] C. Deneke, R. Songmuang, N. Y. Jin-Phillipp, and O. G. Schmidt, Journal of Physics D 42, 16 (2009).
[4] Ch. Deneke, E. Wild, K. Boldyreva, S. Baunack, P. Cendula, I. Mönch, M. Simon, A. Malachias, K. Dörr and O. G. Schmidt, Nanoscale Research Letters 6, 621 (2011).

Ferroelectricity is known to couple strongly to distortions of the crystallographic lattice, and in recent years the strain dependence of the ferroelectric polarization in epitaxial thin films has been successfully simulated and experimentally probed for some prototype ferroelectrics. It is natural to expect a strain dependence of the switching dynamics, but domain dynamics depend much more heavily on the presence of lattice defects than the polarization. Therefore, films grown in different strain states, which necessarily also contain different levels of defects, provide limited data on such strain dependencies. To overcome this limitation, we have introduced the application of piezoelectric substrates for reversible strain control in epitaxial ferroelectric films (M. D. Biegalski et al., APL 98, 142902 (2011)). This enables the study of switching times in capacitors comprised of epitaxial BiFeO₃ and PbZr_0.52Ti_0.48O₃ (PZT) films under reversibly controlled strain states using pulsed measurements of the switched polarization. We find a pronounced reversible strain-induced modification of the switching time, in fact, reaching a tenfold change triggered by only 0.1% of biaxial lattice compression. The strain effect is found to be fundamentally different, even changing its sign, in two ranges of low and high applied electric fields identified here as creep and depinning regimes of domain wall motion, respectively. Interestingly, switching in BiFeO₃ is slowed down in the creep regime and accelerated at high fields upon reversible biaxial compression, whereas the opposite response is observed in the PZT capacitors. Because such quantitative data has not been available previously, the physics of strain-dependent wall motion or domain nucleation has remained poorly understood. However, our experimental approach now provides the necessary access to meaningful data that will eventually help to identify the relevant intrinsic and defect-dominated parameters.

Lattice strain is a fundamental parameter for tuning the variety of physical properties in transition metal oxides such as high-T_c superconductivity and charge, orbital or spin ordering. The strain-clamping effect of the substrate on the electronic properties of epitaxial thin films can be tuned and studied using buffer layers. The buffer layer thickness [1] or the chemical composition [2] is chosen to match the desired in-plane lattice constant of the subsequent film of interest. The piezoelectric buffer layer is also employed to tune the in-plane lattice constant [3]. However, the tunable range of the lattice constant has been limited by the rigid structure of the buffer materials usually used.
Here, we present a non-perovskite oxide Sr₃Al₂O₆ (cubic, a = 1.5844 nm, space group Pa-3) as a strain-tunable buffer layer for the fabrication of oxide heterostructures. Sr₃Al₂O₆ consists of large covalently bonded [Al₆O₁₈]^18- clusters loosely ionically bonded to Sr²⁺ ions making the structure relatively flexible. In this study, we use SrTiO₃ (STO) {100} as the substrate, and the perovskite Nd_0.5Sr_0.5MnO₃ (NSMO) as the oxide thin film grown on the top of the buffer layer. NSMO is an ideal material for evaluating the strain-tuning given its strongly lattice-coupled charge/orbital ordering (COO) transition at 150 K. By varying the thickness of the Sr₃Al₂O₆ buffer layer, we succeeded in continuously tuning the NSMO in-plane lattice constant from fully strained to STO {100} to almost the bulk-like relaxed value. We observed a systematic change in the film properties including a COO transition, the latter previously never observed in {100} oriented thin films [4]. We believe that this technique can be a general way to tailor high-quality strain-tunable oxide thin films.
References:
[1] W. Si et al., Appl. Phys. Lett. 78, 240 (2001).
[2] K. Terai et al.., Appl. Phys. Lett. 80, 4437 (2002).
[3] A. D. Rata et al.., Phys. Rev. Lett. 100, 076401 (2008).
[4] M. Nakamura et al.., Appl. Phys. Lett. 86, 182504 (2005).

Multiferroics (i.e. a class of compounds simultaneously displaying ferroelectricity, ferroelasticity and magnetism) have generated a tremendous flurry of interest in recent years because of their fundamental complexity and potential for applications in nanoelectronics and energy conversion. Finding in nature single-phase compounds with those properties, however, has proved extremely difficult. Such a scarcity of bulk multiferroic materials has motivated researchers to investigate oxide-based materials in thin film and superlattices geometries where i) the properties of the ferroelectric can be tuned almost at will by choosing an appropriate substrate lattice parameter, (ii) the electrostatic coupling between different oxide layers can be exploited, and iii) interfaces, rather than the oxide itself, can show novel multi-functional properties which are absent in either of the bulk constituents. In the development of this field first-principles simulations have played a major role, leading to the present situation in which theory often leads the way to new devices.

Here, we present the results of a systematic first-principles study in which the effects of external mechanical and electrostatic perturbations on the equilibrium phase of technologically relevant PbTiO3 and BiFeO3 perovskites are analyzed. In particular, we have determined the phase diagram of each compound as a function of two macroscopic variables: the in-plain epitaxial strain (s), and the out-of-plane component of the electric displacement (D). In doing this we have accessed a wealth of new valuable insight into the physics of these materials, namely (i) the relations between strain, electrical and structural (e.g. octahedral O6 rotations) degrees of freedom, (ii) previously overlooked bulk phases which can be stabilized by effect of constraining parameters (s, D), and (iii) a survey of the magnetoelectric, piezoelectric and dielectric properties of each compound for optimization of related features in real applications. Importantly, we explain how to interpret the information contained in our diagrams for rational engineering of multiferroic superlattices based on these materials. We expect the results and methodology presented in this study to become a very useful and intuitive tool for theorists and also experimentalists working in the field of nanostructured oxide materials.

Multiferroics, materials where magnetic ordering and ferroelectricity coexist, have been highly sought after though often prove to be elusive due to the two phenomena frequently being mutually exclusive. LuFe₂O₄ has been reported to be simultaneously ferrimagnetic and ferroelectric below 250 K, the highest temperature of any known material, which has resulted in significant interest in the system. Unlike displacive ferroelectrics, LuFe₂O₄ is reported to develop a ferroelectric polarization through the ordering of Fe2+ and Fe3+ ions, though the multiferroic nature of LuFe₂O₄ has been disputed. The ability to deposit single-crystal thin films of LuFe₂O₄ is needed to both better understand and manipulate the properties of this material. Progress towards achieving LuFe₂O₄ in thin film form has been limited to polycrystalline films or films with and impurity phases present. We present an adsorption controlled growth technique allowing for the deposition of single-phase (0001)-oriented epitaxial thin films of LuFe₂O₄. Films were deposited on a variety of substrates including (111) MgAl₂O₄, (111) MgO, and (0001) 6H-SiC, while the composition was controlled by depositing in an iron-rich environment at pressures and temperatures where the excess iron desorbs from the film. Scanning transmission electron microscopy reveals reaction-free film-substrate interfaces. Consistent with the paramagnetic-to-ferrimagnetic phase transition seen in bulk LuFe₂O₄, the film magnetization increases rapidly below 240 K. Optical spectroscopy reveals a 3.4 eV direct band gap and 0.35 eV indirect band gap.

Interplay of charge, spin, orbital, and lattice degrees of freedom has become one the most important issues in the condensed matter physics in last two decades since intimate spin-charge-orbital-lattice coupling evokes various emerging phenomena in complex oxides including colossal magnetoresistance, half-metallicity, and multiferroics. The electric and magnetic properties are expected to be strongly affected by the lattice stain, which is easily realized with variation in the complex oxide epitaxial thin film. Thus the strain engineering can be considered as a key factor for manipulation of functionality of the oxide film. Here we present electronic aspects how the strain affects the electronic properties to vary the functionality in magnetism and ferroelectricity of the mltiferroic BiFeO₃, based on the various spectroscopic results. We also briefly introduce the strain effect on the spin reorientation transition in α-Fe₂O₃ thin films grown by the molecular beam epitaxy.

For any material to realize a multiferroic device there must be coupling between the ferromagnetism and ferroelectricity. This has been shown at low temperature by switching magnetically and electrically as far back as the 60s [1]. However, it was not until 2006 that any form of coupling was demonstrated at room temperature [2]: Zhao et al. switched antiferromagnetic domains in BiFeO3 using an electric field. Despite this progress, in switching antiferromagnetic domains, no one has directly observed switching, at room temperature, of ferromagnetic domains by using an electric field, or ferroelectric domains by using a magnetic field.
In this talk, we will show domain patterns that result from electrical switching in a co-planar geometry, imaged with a PFM. We then show how a magnetic field can induce ferroelectric switching to leave remanent ferroelectric domain patterns comparable to those achieved after the application of an electric-field; the authors believe this is the first demonstration of the equivalence in ferroelectric domain switching between electric and magnetic fields at room temperature. TEM is used to investigate the domain hierarchy and to look at preliminary evidence that the coupling involves switching of the finest domains, on a scale of order tens of nanometers. Finally, we calculate an order of magnitude estimate for the effective magnetoelectric coupling coefficient and compare it to other values in the literature. All our samples are single crystal slabs of ca. (PbZr0.53Ti0.47O3)0.6-(PbFe0.5Ta0.5O3)0.4 cut from single grains of bulk multiferroic ceramics [3] by using a Focused Ion Beam Microscope.
[1] E. Ascher et al., J. Appl. Phys. 37, 1404-1405, 1966
[2] T. Zhao et al., Nat. Mater. 5, 823 - 829, 2006
[3] D. Sanchez et al., AIP Adv. 1, 042169, 2011

Extensive research has been done on bulk and thin film properties of complex oxides. Though already bulk and thin film perovskites posses a wide variety of interesting properties, especially intriguing effects at the interface have become subject of extensive research. Recently, the research on interfaces has exposed an unexpected conducting state at the interface of lanthanum aluminate (LAO) and strontium titanate (STO), two bulk insulators. The origin of this quasi-two dimensional electron gas (2DEG) state stays not completely solved. The development of this system needs precise control of all parameters through the preparation and deposition process. This can be achieved by the use of a pulsed laser deposition (PLD) system. Laser ablation gives rise to the deposition of a wide range of oxide multilayer systems with thickness control at atomic level. Researching the ferroelectric and magnetic properties of multiferroics is a field of constantly emerging interest. Many interesting applications can be introduced. The resistivity of the 2DEG at the STO/LAO interface possesses a strong dependence on an out-of-plane electrical field. In combination with a ferroelectric layer on top of the STO/LAO interface system this can lead to a very thin memory unit. Here, results on the growth and characterization of the multiferroic BiFeO3 and on the LAO/STO interface system containing a 2DEG are shown. On the other hand, piezo force microscopy (PFM) is utilized as a tool to observe the ferroelectric domain structure of the BFO thin film with the 2DEG as a back electrode. Furthermore, nanostructured bridges of BFO are fabricated to utilize the PFM tip and the 2DEG to switch the remnant polarization of BFO using PFM lithography. Finally, the influence of the two achievable out-of-plane polarization states is researched with respect to device fabrication.

Bistability of ferroelectric polarization states serves as a basis for solid-state memory. It can also yield interesting photovoltaic effects in such a way that the directional photocarrier motion follows the inherent potential gradient imposed by the polarization vectors of ferroelectrics that are electrically switchable. Therein, the defining characteristic of such coherent switching between the polarization states is realized by single-domain switching. Here we provide visual evidence for such a coherent photovoltaic switching of lateral BiFeO₃ channels by a scanning photocurrent imaging that are spatially and spectrally resolved. Direct comparison of the sequential photovoltaic current images at different remanent polarization states of single-domain BiFeO₃ channels with those on multi-domains reveals that the switching characteristics is determined both by the internal polarization vector of the domain and oxygen vacancy migration at the domain walls.

Ferroelectric thin films and ferroelectric/dielectric superlattices have a series of atomic-scale and nanometer-scale structural features that have profound effects on the dynamical response to applied electric fields. We discuss recent results in three systems: a BaTiO₃/CaTiO₃ dielectric/ferroelectric superlattice (SL) in which the dielectric polarization of adjacent ferroelectric and dielectric layers are strongly coupled, a PbTiO₃/SrTiO₃ SL in which the polarization is more weakly coupled, and a uniform-composition BiFeO₃ thin film grown on a substrate providing a large compressive stress. The dynamics of these systems were probed in a series of time-resolved synchrotron x-ray microdiffraction experiments which probed the structure in electric fields applied using a thin-film capacitor geometry. In the BaTiO₃/CaTiO₃ system we show that a structural instability leads to x-ray reflections consistent with rotation of oxygen octahedra that has been previously predicted in density functional theory studies. We describe the time-evolution of these distortions in applied fields, and the connection of these dynamics to the relatively slow overall piezoelectric response of the superlattice. The PbTiO₃/SrTiO₃ system exhibits a weaker coupling of the polarization between layers, leading to the formation of a nanoscale striped domain pattern. Applied fields lead initially to changes in the x-ray domain diffuse scattering pattern that are consistent with a distortion of the domain patterns. At later times, after several tens of nanoseconds, the system transitions to a uniform polarization state accompanied by the disappearance of the domain diffuse scattering. The piezoelectric distortion of the field-induced uniform polarization state is consistent with an increased polarization of the SrTiO₃ layers. Finally, we describe the field- and time-dependence of the transition between structural symmetries of the compressively strained BiFeO₃ film. Taken together, these results show how emerging scattering techniques can provide experimental insight into phenomena predicted in field-dependent calculations, and point the way to the use of electric fields to modulate the properties of heteroepitaxial complex oxides.

The resistance-change-based memory (or RRAM) is a technology which has been the subject of an intense research in the past 10 years, and one of the most promising candidates for future high-density nonvolatile memory due to the fast writing speed, simple structure, and compatibility with current CMOS technology. Although many RRAMs with a good switching characteristics has been reported by many research groups, a fully acceptable RRAM showing reliable memory performance has not been yet presented because of lack of material architecture for reliable system and insufficient understanding of switching mechanism. More specifically, the understanding and functionalizing on each layer constituting the R-stack is essential to make a reliable high-density memory utilizing RRAM.
In this work, we proposed a material architecture for RRAM and a phenomenological model which is based on the movement of oxygen vacancies and the accompanying Schottky barrier modulation, and successfully manifested the memory performance of the optimized structure for single-level cell and multi-level cell. The material architecture is electrode/barrier layer (I)/base layer/oxygen exchange layer/barrier layer (II)/electrode. The key functions and requirements of each layer are well defined and the candidate materials are derived from the ab-initio calculation and the considerations of the material properties such as Gibb&’s free energy change and diffusion constant. Experimental result of the basic system with Pt electrode (Pt/TaOx/Ta2O5/Pt) show switching endurance over 1T cycles and switching time below 10ns, and the switching current below 100 uA. Another combination enables us to utilize non-Pt electrode such as W/AlO/TaOx/Ta2O5/IrOx and W/AlO/TaOx/ZrOx/Ru. The former shows almost the same performance to the basic system and the latter exhibits much higher On/Off ration up to 100M times without any degradation of other properties such as endurance and retention properties. The combination adopting full component reveals a successful MLC operation, in which the insertion of a thin barrier layer (II) at the Ta2O5/electrode interface make it possible to block the background leakage current so that the On/Off resistance ratio increases by decreasing the Off-state current.
In order to investigate the origin of resistance-changing we carried out several kinds of analytical measurements such as oxygen/tantalum mapping and depth profile using AES, in-situ HR-STEM observation and EELS analysis. Those measurement shows that the main element migrating during resistance-switching is oxygen. Additionally ab-initio calculation was performed to understand the analyzed data and the conducting path in TaOx lattice.

Recent discoveries of distinct functional behaviour, specific to domain walls in ferroelectric and multiferroic oxides, have prompted ideas of new kinds of electronic components, entirely based on the presence or absence of conducting domain wall channels. Realising working devices, however, requires control over the points at which domain walls are injected into a system as well as over the manner in which these walls subsequently behave. In ferromagnetic alloys, race-track memory and domain wall logic applications have forced this kind of control to be developed already, but a similar capability in ferroelectric and multiferroic oxides is currently non-existent. Here we demonstrate how site-specific domain wall injection and control over domain wall mobility can be realised in ferroelectric capacitors, by engineering non-uniformity in the electric field distribution. The creation of local field heterogeneity involved focused ion beam patterning of the interelectrode gap in simple mesoscale coplanar capacitors.

Multiferroics are compounds that show ferroelectricity and magnetism. BiFeO3, by far the most studied, has outstanding ferroelectric properties, a cycloidal magnetic order in the bulk, and many unexpected virtues such as conductive domain walls or a low bandgap of interest for photovoltaics. While this flurry of properties make BiFeO3 a paradigmatic multifunctional material, most are related to its ferroelectric character, and its other ferroic property - antiferromagnetism - has not been investigated extensively, especially in thin films. Here we bring insight into the rich spin physics of BiFeO3 in a detailed study of the static and dynamic magnetic response of strain-engineered films. Using Mössbauer and Raman spectroscopies combined with Landau-Ginzburg theory and effective Hamiltonian calculations, we show that the bulk-like cycloidal spin modulation that exists at low compressive strain is driven towards pseudo-collinear antiferromagnetism at high strain, both tensile and compressive. For moderate tensile strain we also predict and observe indications of a new cycloid. Accordingly, we find that the magnonic response is entirely modified, low energy magnon modes being suppressed as strain increases. Finally, we reveal that strain progressively drives the average spin angle from in-plane to out-of-plane, a property we use to control the exchange bias and giant-magnetoresistive response of spin valves.

In the recent past, the entire field of photo-ferroelectrics has been revitalized by the reports of photovoltaic (PV) effect in BiFeO₃ (BFO). Unlike traditional semiconductors the open circuit voltages in BFO are not limited by the width of band gap. Initial investigations assumed that the PV effect in BFO is primarily due to the presence of a potential step at the domain walls which enables a more efficient separation of charge carriers [1]. However, in a recent work [2] it was observed that the generation and recombination of photo-generated non-equilibrium carriers in a BFO single crystal are primarily affected by the presence of shallow energy levels and the domain walls might not be playing a major role in the corresponding PV effect, as initially proposed. Thus, till now there is no clear model available for the PV effect in BFO. In this work, in order to get a further insight into the actual mechanism for the generation of the PV effect in epitaxial BFO thin films, time and temperature resolved measurements have been performed. Time resolved measurements, namely photo induced transient spectroscopy (PITS), bring valuable data regarding generation and recombination of the photo-excited carriers. Providing that the ferroelectric domain configuration in BFO does not change at low temperature, variable temperature measurements of the PV effect can provide information on details regarding the electronic structure of BFO, namely the presence of shallow or deep levels in the band gap which may affect the conduction mechanism as reported in [2].
[1] Yang, S. Y.et al., Nature Nanotechnology, 2010, 5, 143-147
[2] Alexe, M., Nano Letters, 2012, 12, 2193-2198