Ferroelectric thin films form striped polarization nanodomains in order to minimize the electrostatic energy. The boundaries of between domains have unique properties such as electromechanical clamping and electrical conductance and can also have crystallographic structures that are distinct from bulk materials. Experimental studies of the configuration and structure of the nanodomains of ultra-thin ferroelectric films in applied electric fields are challenging because the fabrication of top electrodes can destroy the domain pattern and the thinness of the ferroelectric layers leads to large leakage currents. Ferroelectric/dielectric superlattices, however, form nanodomains similar to ultra-thin ferroelectric films because the neighboring ferroelectric layers are weakly coupled due to the electrical isolation of the dielectric layers. The study of ferroelectric nanodomains in ferroelectric/dielectric superlattices, especially their spatial configuration and switching dynamics, provides insight into the structure and dynamics of a more general class of striped domains.
We have studied nanodomain structures in a PbTiO3/SrTiO3 superlattice using synchrotron coherent x-ray diffraction. The degree of transverse coherence of the x-ray probe is increased by illuminating the focusing optics through a narrow slit with a width matching the coherence length of the x-rays. The superlattice nanodomains produce speckle diffraction patterns which contain depend in detail on the arrangement and structure of the nanodomains. We have observed speckle scattering patterns using x-ray beams prepared with focusing optics based on Kirkpatrick-Baez mirrors and Fresnel zone plate. When the numerical aperture of the focusing optics is small, the spatial profile of the speckles is determined by the illumination function at far-field limit. However, when the numerical aperture is large, the spatial profile of the speckles is determined by the interference of the illumination functions and the angular width can be an order of magnitude smaller than the convergent angle of the focusing optics. The speckle visibility decreases with the increase of slit width and count time in both cases. The degree of correlation between speckle patterns decreases over time due to the drift of the experimental setup. A spatial map of the correlation coefficient indicates that there is no spatial correlation of the in-plane nanodomains, which agrees with the approximation of the coherence length calculated from the envelope of the speckle pattern.

We are investigating the coupling between ferroelectric domains of BaTiO₃ (BTO) single crystals and magnetic domains in permalloy (Py) thin films deposited on top of them. We reproducibly fabricate dense magnetic stripes near the transcritical state by controlling sputtering deposition parameters. The films display partial out-of-plane anisotropy which is clearly visible as dense stripe domains in magnetic force microscopy. The stripe domains are used as an indicator to investigate the local coupling behavior between Py thin films and BTO crystals. Piezoresponse force microscopy of the crystals shows clear presence and spatial modulation of a-c domains in BTO. When the Py/BTO crystals are annealed above the Curie temperature of BTO (130^#9675;C) and cooled down, stripe domains in Py develop local anisotropy modulation consistent with distribution of a-c domains. In particular, there is a clear and abrupt change in deduced in-plane magnetic anisotropy in Py at the a-c domain boundary of BTO. Magnetic stripe period also changes at the a-c domain boundary which is accounted for by magnetoelastic energy. The detailed coupling behavior achieved under different conditions is described by micromagnetic models.

The metallic features in materials, which provide low-resistance channels for electrical conduction, lead to effective screening of local electric dipole moments. Itinerant electrons disfavor both their formation and cooperative ordering. Consequently, most metals with a finite density of states and partial band occupation exhibit centric (inversion symmetric) crystal structures. Despite this contraindication, noncentrosymmetric metals (NCSM) lacking inversion were proposed more than fifty years ago, with some examples discovered serendipitously later. Here we describe a design framework to alleviate the property disparities and accelerate NCSM discovery: The primary ingredient relies on the removal of inversion symmetry through displacements of atoms whose electronic degrees of freedom are decoupled from the states at the Fermi level. Density functional theory calculations validate our crystal-chemistry strategy, and predict a polar perovskite ruthenate to be metallic and robust to spin-orbit interactions. Motivated by recent suggestions that degenerately doped ferroelectrics exhibit advantageous thermoelectric responses, we show that the thermopower in this polar metal exhibits large anisotropy for particular doping regimes along the polar axis. Although the high-density of states is unfavorable for a high power factor, our results indicate highly functional NCSM could be tailored for targeted thermoelectric applications.
This work was performed in collaboration with Dr. Danilo Puggioni and sponsored by the U.S. Army Research Office under grant no. W911NF-12-1-0133.

Fast, reversible redox reactions in solids at low temperatures without thermomechanical degradation are a promising strategy for the enhancement of the overall performance and lifetime of many energy and environmental technologies, including solid oxide fuel cells, batteries and catalytic converters. Despite many efforts to search for new materials and routes to overcome the technical challenges, the robust nature of the cation&’s oxidation state and the high thermodynamic barrier have hindered the realization of fast catalytic reactions and bulk diffusion at low temperatures. Here, we report a low-temperature topotactic phase transition in SrCoO_x grown directly by pulsed laser epitaxy as one of two distinct crystalline phases, either the perovskite SrCoO_3-δ or the brownmillerite SrCoO_2.5. Based on real-time temperature dependent characterizations with x-ray diffraction and optical spectroscopy, we found that the two topotactic phases can be reversibly switched at a remarkably reduced temperature (200~300 ^oC) in a considerably short time (< 1 min) without destroying the parent framework. This phase reversal accompanies distinct electronic (metal vs. insulator) and magnetic (ferromagnetic and antiferromagnetic) transitions. Therefore, our results on low temperature topotactic valence state reversal provide valuable insight not only in understanding the structure-physical property relationship in multivalent oxides, but also for identifying new opportunities for technological applications, such as low temperature catalysts.
The work was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division.

Thin films and heterostructures of the perovskite cobaltites are of great interest, not only from the point of view of fundamental physics and materials science, but also for technological applications such as solid oxide fuel cells, gas membranes and multiferroics. Their properties are, however, severely deteriorated from the bulk, being dominated by the presence of interfacial “dead layers”. Working with the prototypical SrTiO3(001)/La1-xSrxCoO3 (LSCO) system, we recently discovered that this degradation in the magnetic and electronic transport at the interface is caused by nanoscopic magneto-electronic phase separation. This was shown to occur primarily due to accumulation of oxygen vacancies near the interface, driven by the fascinating interplay between the strain state and oxygen vacancy ordering [1, 2]. In the present work we show how this understanding allows for engineering of the interfacial magnetic and electronic transport properties via manipulation of this oxygen vacancy superstructure [3, 4]. Using reciprocal space mapping, we demonstrate the ability to control, via the vacancy ordering, the critical strain relaxation thickness by changing the sign of the strain (from tensile on SrTiO3 to compressive on LaAlO3) and crystallographic orientation ((001) vs. (110)). Furthermore, SQUID magnetometry, polarized neutron reflectometry (PNR) and magneto-transport confirm the concomitant mitigation of the interfacial degradation for LSCO films grown on LaAlO3(001) and SrTiO3(110), as compared to films grown on SrTiO3 (001). Finally, we provide cross sectional electron energy loss spectroscopy (EELS) data showing the preservation of both oxygen and hole carrier concentration at the LaAlO3(001)/LSCO and SrTiO3(110)/LSCO interfaces, strikingly different to the severely depleted SrTiO3(001)/LSCO interface Our work thus opens up a new route to tailor interfacial electronic transport and magnetic properties, thereby engineering complex oxide device performance.
Work supported by NSF and DOE (neutron scattering). Research at ORNL supported by U.S. DOE-BES, MS&E Division, at UCM by ERC Starting Investigator Award.
[1] Torija et al., Adv. Mater. 23, 2711 (2011)
[2] Gazquez et al., Nano Lett. 11, 973 (2011)
[3] Gazquez, Bose et al., APL Materials 1, 012105 (2013)
[4] Bose et al. (in preparation)

We have investigated the I-V characteristics of 5-10 nm La_0.7Ca_0.3MnO_3 (LCMO) and La_0.5Sr_0.5MnO_3 (LSMO) thin films grown epitaxial on (001) SrTiO_3 (STO), (001) LaAlO_3 (LAO) and (110) NdGaO_3 substrates via off-axis radio frequency magnetron sputtering. These films show high crystallinity and have RMS roughness of 2-3 Å. The LSMO films with thickness above 6 nm show metal-insulator transitions (MIT) at ~275K and exhibit linear I-V characteristics in the whole temperature range investigated (10 K to 350 K). As the film thickness approaches the electrically dead layer thickness, LSMO becomes totally insulating and exhibits a strong nonlinear behavior in I-V. In the LCMO films, we observe a strong correlation between the I-V characteristics and the MIT. Linear I-V dependence has been observed in the paramagnetic insulating phase and nonlinear I-V starts to develop below the MIT transition temperature (~150 K). We discuss the origin of our observations within the phase separation model and explore how to engineer the I-V characteristics in manganite thin films via film thickness, substrate strain, and an electric field effect through a neighboring ferroelectric gate.
We acknowledge the support from NSF Grant CAREER No. DMR-1148783, the Center for NanoFerroic Devices (CNFD) and the Nanoelectronics Research Initiative (NRI).

Strongly correlated electron materials exhibit rich and unique properties including colossal magnetoresistance (CMR) in manganite system and so on. Since the discovery of the intrinsic inhomogenence, nanoscale electronic phase separation is thought to play an important role in correlated electronic materials, and the interplay between the nanoscale electric phases lead to exotic properties. In (La,Pr,Ca)MnO₃ /(LPCMO), the coexistence and competition between nano ferromagnetic metal phase and nano charge ordering insulator phase result in CMR effect [1]. By spatially confining materials to length scales smaller than the electronic nano phases, a huge effect, i.e., a steep MR change would emerge. Thus. it is possible to artificially design their behavior and expected to create drastic changes in magnetic and electric properties by capturing a single nano phase into nanostructure. For this purpose, we have developed 3D nano-template PLD technique [2, 3] to construct extremely small metal oxide nanostructures, and succeed to form LPCMO nanowall wire structures with 50 nm width. The LPCMO nanowall wire exhibited nano-confinement metal-insulator transition properties driven by both temperature and magnetic field. The drastic resistivity change of a single domain corresponds to the first-order insulator-metal transition of a single domain. For example, this nanowall wire sample showed a digital resistance change at 80 K while a film sample displays the gradual resistance change. Above 80 K, in addition to the insulator feature (negative temperature coefficient of resistance) the discrete drops in resistivity were observed. Very steep magnetoresistance change was also observed on LPCMO nanowall wire sample while smooth MR change appeared on thin film sample. We will also report a quantitative correlation between nano-spatial size and nano-confinement MIT properties and discuss the revealed a single phase dynamics in correlated oxide nano-structures.
[1] M. Uehara et al., Nature 399, 560 (1999)
[2] Y. Fujiwara, A. N. Hattori, K. Fujiwara and H. Tanaka, Jpn. J. Appl. Phys. 52 (2013)015001
[3] T. Kushizaki, K. Fujiwara, A. N. Hattori, T. Kanki and H. Tanaka, Nanotechnology 23 (2012) 485308

Electronic changes at polar interfaces between transition metal oxides offer the tantalizing possibility to stabilize novel ground states yet can also cause unintended reconstructions in thin films poised for next-generation devices. The very nature of these interfacial reconstructions should be qualitatively different for metallic and insulating films as the electrostatic boundary conditions and compensation mechanisms are distinct. Interfacial electronic charge transfers to equilibrate chemical potentials and balance interface charges are expected in a metal, and for insulators there is the possibility for either sustained internal electric fields or an interface dipole to cancel an otherwise growing electrostatic potential - the so-called ‘polar catastrophe&’ Here, we probe the charge distribution for manganite-titanate interfaces traversing the metal-to-insulator transition.
Using scanning transmission electron microscopy (STEM) in combination with electron energy loss spectroscopy (EELS), we measure the elemental concentration and valence of the cations at a series of La1-xSrxMnO3/SrTiO3 interfaces. We directly quantify both the charge transfer, manifest as valence changes on the interfacial manganese sites, as well as extrinsic defects such as cation interdiffusion and vacancies. We find an intrinsic interfacial electronic reconstruction in the insulating films (x le; 0.2), where the total charge measured quantitatively agrees with that needed to cancel the polar catastrophe. Surprisingly the width of the charge transferred region remains constant for the insulating films, despite differences in cation interdiffusion. As the manganite becomes metallic with increased hole-doping, the total charge build-up and its spatial range drop substantially consistent with screening lengths in metallic La0.7Sr0.3MnO3. Direct quantification of the intrinsic charge transfer and spatial width should lay the framework for devices harnessing these unique electronic phases.

The interlayer exchange coupling between magnetic layers separated by non-magnetic spacers can give rise to new spin structures that are distinct from the bulk constituents. In this work, we investigate the non-collinear spin structures that arise in superlattices containing paramagnetic LaNiO₃ and ferromagnetic La_0.7Sr_0.3MnO₃. Using ozone-assisted molecular beam epitaxy, we have fabricated a series of (LaNiO₃)_n/(La_0.7Sr_0.3MnO₃)₉ superlattices where n is varied from 1 to 9 unit cells on (001) SrTiO₃ and LSAT substrates. The total superlattice thickness is maintained at 60 nm by varying the number of superlattice repetitions. The magnetic structure of several superlattices was measured using polarized neutron reflectometry as a function of temperature and applied magnetic field. For low fields, we find the magnetization of neighboring La_0.7Sr_0.3MnO₃ layers to be non-collinear due to an antiferromagnetic interlayer exchange coupling, which persists to temperatures above 250 K. In the low-field regime we observe positive magnetoresistance using in-plane transport, which competes with the negative magnetoresistance of La_0.7Sr_0.3MnO₃ at high fields. We discuss underlying mechanisms for the observed effects and possible applications to oxide-based magnetoresistive devices.

The rare-earth perovskite nickelates (RNiO3) have a strong coupling between structure and electronic transport. The nickelates accommodate changes in the unit cell volume by elongating and shortening the in-plane Ni-O bond lengths and bending or flattening the in-plane Ni-O-Ni bond angles. Because transport in RNiO3 depends on these structural parameters, tuning the structural parameters affects electronic transport. The unit cell volume can be changed by applying pressure or by varying the radius of the rare earth ion. For instance, LaNiO3 is a paramagnetic metal in the bulk, while the other rare earth nickelates undergo a metal-insulator transition as the radius of the rare earth ion is changed. Structural parameters in the nickelates have been successfully tuned by layering in superlattices and with epitaxial strain. These are static effects; in order to dynamically tune electronic transport in RNiO3, we use the ferroelectric PbZr0.2TiO3 (PZT) to couple the polarization of the PZT to the electronic transport of RNiO3 films. Switching the polarization of the ferroelectric from up to down has a dramatic effect on electronic transport in the RNiO3, and can be understood within the context of charge and structure.

Heterostructures of complex oxides have attracted great attention for the interplay between structure, electronic, and magnetic properties, offering unique opportunities in device applications. Here we investigate the effects of epitaxial strain on the structural properties of LaNiO₃. We perform first-principles calculations based on density functional theory to investigate the NiO₆ octahedral tilts of strained LaNiO₃ layers in LaNiO₃/SrTiO₃ superlattices. Recent experimental results suggest that octahedral connectivity at LaNiO₃/SrTiO₃ interfaces determines the local structure, but the effect of epitaxial strain on the LaNiO₃ layers has remained unclear. We present quantitative results for the octahedral tilt angles as a function of both biaxial strain and distance from the substrate for LaNiO₃ grown on SrTiO₃ (001). Our results indicate that LaNiO₃ exhibits vanishing octahedral tilt angles under certain strain conditions, a finding that holds important consequences for its electronic properties.

Understanding interface in complex oxide heterostructure is scientifically and technologically important since interfaces and surfaces determine the physical properties through coupled competing order parameters (magnetic, charge, orbital, etc.). In this talk, we present two types of BiFeO₃ related superlattice (SL) systems, BiFeO₃/BiMnO₃ (BFO/BMO) and La_0.7Sr_0.3MnO₃/BiFeO₃ (LSMO/BFO). The structure of SL-BFO/BMO films is equivalent to 90° rotated nanoscale checkerboard (NCB) BFO/BMO. The film properties are in remarkable agreement with the theoretical predictions of a T_C = ~ 407 K for NCB-BFO/BMO. Our experimental results provide an explanation of the emergence of magnetism in complex oxide heterostructures and show how to create a room temperature multiferroic. In SL-LSMO/BFO, the BFO layer showed the magnetization of ~ 75 ± 25 kA/m at 10 K. By ab initio calculation based on the density functional theory, SL-LSMO/BFO has much larger interfacial ferromagnetism of > ~ 0.3 mu;_B/Fe than the canted moment 0.03 mu;_B/Fe in the bulk BiFeO₃. SL-BFO/BMO showed a high magnetic transition temperature (T_C ~ 400 K).

Oxide heterostructures play an important role in the design of next generation functional information storage and electronic devices. Given the complexities of these oxide heterointerfaces, studying their structure and chemistry at the atomic scale and providing vital information relating to their valence and electronic structure is pivotal to explain the observed properties, such as their electromechanical behavior and related magnetic ordering. A combination of scanning transmission electron microscopy and density functional theory is used to probe mixed phase bismuth ferrite and reveals systematic changes in electronic structure across a phase boundary in the film.
Techniques such as X-ray absorption spectroscopy (XAS) and near-edge X-ray fine absorption spectra (NEXAFS) typically detect minute spectral changes, but not well suited to investigate atomic scale interfacial phenomena. Conversely, scanning transmission electron microscopy (STEM) and STEM-based electron energy loss spectroscopy (EELS) is one method that is able to provide the spatial distribution of electronic phenomena at nanostructured oxide interfaces with single atom and vacancy sensitivity. The unique combination of STEM and density functional theory (DFT) computations further provides valuable insight into the electronic structure controlling functional responses of oxide heterostructures. In order to address the effect of strain-induced configurations at the interface and beyond, however, a progressive study traversing the boundary layer is still lacking and must be conducted.
Here we combine aberration-corrected STEM with sub-Ångstrom resolution EELS and DFT methods to reveal strain modulated electronic structure and bonding perturbations in mixed phase bismuth ferrite thin films. In contrast to previous works, we focus on the incremental spectral transitions in the electronic signatures across the boundary layer to explain the interfacial electronic structure and the concomitant role of strain induced structure on the observed properties. High angle annular dark field (HAADF) and EELS chemical mapping is utilized to observe the atomic structure. Non-linear least squares (NLSS) spectral peak fitting and DFT further confirm these observed changes are attributed to changes in bonding environment surrounding the central iron cation and result in a change in electronic structure. Furthermore, DFT analyses suggest the same changes are attributed to a breakage in the structural symmetry across the boundary due to the simultaneous presence of increasing epitaxial strain and off axial symmetry in the T phase within the square-pyramidal oxygen cages. We expect the results presented to have a significant impact on the fundamental approach to and understanding of the effect of epitaxial strain on the resultant ferroelectric, piezoelectric coefficients, and complex phase equilibria in multiferroics.

Complex perovskite oxides exhibit a rich spectrum of functional responses, including magnetism, ferroelectricity, highly correlated electron behavior, superconductivity, etc. The basic materials physics of such materials provide the ideal playground for interdisciplinary scientific exploration. Over the past decade we have been exploring the science of such materials (for example, colossal magnetoresistance, ferroelectricity, etc) in thin film form by creating epitaxial heterostructures and nanostructures. Among the large number of materials systems, there exists a small set of materials which exhibit multiple order parameters; these are known as multiferroics. Using our work in the field of ferroelectric(FE) and ferromagnetic oxides as the background, we are now exploring such materials, as epitaxial thin films as well as nanostructures. Specifically, we are studying the role of thin film growth, heteroepitaxy and processing on the basic properties as well as magnitude of the coupling between the order parameters. In our work we are exploring the switchability of the antiferromagnetic order using this coupling.
What is the importance of this work ? Antiferromagnets(AFM) are pervasive in the recording industry. They are used as exchange biasing layers in MTJ&’s etc. However, to date there has been no antiferomagnet that is electrically tunable. We believe that the multiferroic BiFeO3 is one compound where this can be observed at room temperature. The next step is to explore the coupling of a ferromagnet to this antiferromagnet through the exchange biasing concept. Ultimately, this will give us the opportunity to switch the magnetic state in a ferromagnet( and therefore the spin polarization direction) by simply applying an electric field to the underlying antiferromagnetic ferroelectric. In this talk, I will describe our progress to date on this exciting possibility.

In recent years, engineering and characterization of domain walls has developed with a view to future nanoscale device applications. One of the pivotal physical properties of domain walls is enhanced conduction, which has been extensively studied in different types of oxides in the last few years. In ferroelastic materials the domain walls are known to attract oxygen vacancies [E.K.H Salje et al., ChemPhysChem. (2010)]. It has been pointed out [S. Farokhipoor & B. Noheda PRL (2011)] that in BiFeO₃ the migration of oxygen vacancies to the walls can lower the electronic injection barrier at the interface with the metal and locally enhance conduction. Here we have investigated highly strained BiFeO₃ thin films grown on LaAlO₃ substrates [C. Beekman et al., Adv. Mater (2013)]. These films consist of a stripe-like coexistence of two polymorphs that are each different from BiFeO₃&’s bulk phase, leading to an interesting strain landscape across the film. By investigating the local conduction at different types of domain walls and phase boundaries by means of scanning probe techniques, we find a clear correlation between the strain state, the oxygen vacancy concentration and the local conductivity.

The domain structure of ferroelectric and multiferroic materials can have a significant effect on electron transport properties. Piezo Force Microscopy allows unique investigations of such nanoscale effects, and can further be implemented to monitor domain switching dynamics. BiFeO₃ thin films engineered with distinct strains and interfacial configurations are thereby shown to influence the ferroelectric domain distributions, with up to 50 mu;m of domain boundary per mu;m2. Coupling these results with conductive AFM, correlations between such domain configurations and through-film electron transport are comprehensively studied, identifying conditions for enhancing devices coupling ferroelectric polarization and conductivity. Characteristic switching processes are demonstrated for distinct specimen conditions as well, including nucleation times and growth velocities, domain wall densities at discrete steps during the switching process, and the distribution of 71°, 109°, and 180° initial and ultimate polarization reorientations. These results provide unique insight into the behavior of ferroelectric domains at the nanoscale for interfacial engineering of optimally performing electron transport devices.

Chemical substitution is widely used to induce phase transition in BiFeO₃ films. Our previous research discovered the phase transition from rhombohedral phase to orthorhombic phase through La doping in BiFeO₃ films. Here we report a thickness-dependent paraelectric-ferroelectric phase transition in La-doped BiFeO₃ films on Si substrates, opening up a new path to achieving phase transition. Epitaxial 20% La doping BiFeO₃ films (LBFO20) with thickness of 10 to 100 nm were grown on (001) SrTiO₃-buffered Si substrates with SRO as bottom electrodes (~20 nm) by pulsed laser deposition. X-ray Diffraction (XRD) results show the peak splitting in 60 and 100 nm LBFO20 films, but there are no peak splitting in 10 to 40 nm films, which demonstrates the paraelectric orthorhombic phase to ferroelectric rhombohedral phase transition. Piezoresponse force microscopy (PFM) and piezoelectric property measurement results are consistently with XRD data. Using PFM, we find that 100nm LBFO20 film cannot be switched with -15 V dc bias and 60nm film can be switched slightly, whereas LBFO20 films with thickness from 10 to 40 nm can be switched completely with -8 v dc bias, which further reveals the paraelectric-ferroelectric phase transition with decreasing thickness of films. Transmission electron microscopy (TEM) shows that lots of orthorhombic phase exists in 100 nm LBFO20 film, however, in 20 nm film, there is almost entirely made up of ferroelectric rhombohedral phase. Orthorhombic and rhombohedral phase boundary is also discovered in the films. In summary, our finding provides a new path to drive the paraelectric-ferroelectric phase transition and also offers a possible route to study morphotropic phase boundary (MPB).

Flexoelectric effect is the generation of an electric field by a strain gradient via electro-mechanical coupling. This effect was predicted theoretically by Kogan in 1964 and experimentally observed by Bursian and Zaikovskii in 1968. The phenomenon was given the name ‘flexoelectricity&’ by Indenbom et al. in 1981. However, there have been few studies on flexoelectric effects in solids. For bulk solids, it was believed that flexoelectric effects should be minimal. However, it was recently shown that the strain gradient in nano-structured materials and/or epitaxial oxide thin films could be 6 or 7 orders of magnitude larger than the corresponding bulk values. After this realization, there has been increased interest in this phenomenon. These electromechanical coupling effects have provided answers to many physical phenomena that could not be explained before.
In this presentation, we will address that fully strained BiFeO₃ films are self-poled, having a down ward polarization; this indicated that the interfacial effect is dominant. In contrast, the relaxed films have upward self-polarization, indicating that the flexoelectric effect is dominant. Moreover, quantification of the flexoelectric fields are estimated, indicating that the flexoelectric field in relaxed BFO is higher about one order of magnitude, compared with that of the uniaxially strained BFO film. Interestingly enough, the two kinds of films also exhibit different unidirectional current flows, referred to as the diode effect. By understanding the self-poling mechanisms in BiFeO₃ films, such as ferroelectric hysteresis and electronic transport characteristics, the configuration of the as-grown films can be optimized to allow full utilization of the ferroelectric functional device.

As CMOS transistors are scaled towards atomistic sizes, the research is underway to find beyond-CMOS logic devices which would complement CMOS in futures computational circuits. Majority of currently considered logic gates include spintronic devices, i.e. ones involving ferromagnets. With the currently available switching methods based on passing current through these devices - spin torque and spin Hall effects - these gates are expected to switch with longer time and larger energy than comparable electronic gates. Recently, switching of magnetization by application of voltage was demonstrated via magnetoelectric effects relying on interfaces of ferromagnets with oxides. We simulate a variety of spintronic gates switched with both spin torque and magnetoelectric effects. Various cases of switching are considered, values of magnetoelectric coefficients necessary for switching are determined. We show that only magnetoelectric switching results in time and energy comparable to those of CMOS devices.

Conduction mechanisms in thin ferroelectric Pb(Zr,Ti)O₃(PZT) films are controlled to a great extent by the film/electrode interfaces. In polycrystalline PZT films with heterophase grain boundaries coalescence of grain polarization charge and semiconducting grain boundaries can trigger such transport effects as intergrain photovoltaic effect [1] and clockwise current hysteresis [2].
We use direct current-voltage measurements and Scanning Spreading Resistance Microscopy to study transport characteristics of thin PZT films depending on its polarization. In direct current-voltage measurements a bias voltage is applied to the structure as a sequence of steps of the same amplitude and duration. The voltage rise rate is changed within the range of 1-0.001 V/s by variation of the step amplitude and duration. The current measurements are done in preliminary poled films.
The both experimental techniques, we use to study the polycrystalline (111) textured 100-nm-thick PZT film deposited on PbTiO₃/Ir/SiO₂/Si substrate and epitaxial (001) oriented 210-nm-thick PZT film deposited on SrRuO₃/SrTiO₃ substrate, revealed the same results at micro and nanoscale ranges. A current response from PZT film to applied bias contains a long relaxation component, which is shown to depend on voltage rise rate and the polarization direction. However the current dependence on the polarization is found to be completely different for the polycrystalline and epitaxial films. The current of polycrystalline film is much larger when the bias is directed against the polarization, which can be associated with screening of polarization charge by traps charge on grain boundaries with bias variation [2]. While the current of epitaxial films is larger if the bias direction coincides with the polarization, which cannot be explained by ferroelectric switching. We consider the measured current of the epitaxial film as the transient current caused by hole&’s capture at or electron&’s emission from traps level. The electron&’s emission probability is controlled by the impurity potential, which becomes asymmetrical in the presence of spontaneous polarization: the probability of electron's emission in the polarization direction is larger than that against the polarization. Since the electron moves again the external bias, the more favorable condition for quick electron emission and the current relaxation takes place when the bias is directed against the polarization. Therefore, the larger current value measured when the bias and polarization directions are coincides reflects the slower emission of electron and slower current relaxation. The clockwise current hysteresis is observed at any preliminary polarization of the both polycrystalline and epitaxial films. The model, which allows us to estimate characteristics of the transient current, is proposed.
[1] IEEE-TUFFC 58, pp. 2252-2258 (2011).
[2] MRS Proc. 1292, mrsf10-1292-k03-31(2011).

As a novel physical concept, Ferroelectric (FE) negative capacitance (NC) can be effectively utilized to reduce the sub-threshold swing (SS) of ultra-low power MOSFET applications below the fundamental physical limit of 60 mV/decade [1]. According to Salahuddin et al., the negative capacitance state of a ferroelectric capacitor is an unstable non-equilibrium state and can be effectively stabilized under static conditions if it is placed in series with dielectric capacitor. Recently, the experimental demonstrations of ferroelectric negative capacitance phenomenon have been reported by several research groups [2, 3]. However, the negative capacitance effect is only observed in a certain high temperature range, which closely depends on the Curie temperature of ferroelectric materials. In order to be compatible with MOSFET applications, it is necessary to look for proper ferroelectric materials, which can provide negative capacitance effect at the MOSFET operating temperature. In this talk, we will introduce the room temperature negative capacitance effect in LaAlO₃/Ba_0.8Sr_0.2TiO₃ (LAO/BSTO) superlattice heterostructure.
The artificial superlattices, [(LAO)_m/(BSTO)_n]11 (m=6, 12, and 18; n=18, 12, and 6) were grown on the SrRuO₃-covered (001)-SrTiO₃ (STO), (110)-GdScO₃ (GSO) and (110)-DyScO₃ (DSO) substrates by Laser-Molecular Beam Epitaxy technique. During the deposition, the substrate temperature was kept at 750 °C for the (LAO)_m/(BSTO)_n superlattice, 700 °C for the SrRuO₃ and the oxygen pressure at 50 mTorr. The capacitance of superlattices were measured by 4194A Impedance/Gain-Phase Analyzer with frequency up to 1 MHz at room temperature. These superlattices show an enhanced capacitance compared to single dielectric LAO film of the same thickness equal to the total thickness in the superlattice. This means that the capacitance of the superlattice has gone up, rather than going down, even after adding additional layers to LAO and thereby increasing the total thickness of the overall capacitance. This is the first observation of room temperature negative capacitance effect in ferroelectric/dielectric heterostructures at room temperature. The effect of lattice strain, FE thickness etc. on the negative capacitance will be discussed in this talk.

Recently, solid-state drives based on NAND flash memory have been a interest in replacement of hard-disk drives due to their high speed data transmission, low power, and high reliability. The reliability including endurance and retention characteristics of NAND flash memory cells is degraded as shrinking the gate pitch size of NAND flash memory cells. Sicne a few of trap sites located in tunnel oxide can affect to the reliability of NAND flash memory in the shrunk technology [1], it is a very critical challenge to improve the quality of tunnel oxide and its interface. Many studies have been reported, such as increasing the quality of tunnel oxide, inter-poly dielectric as well as inter-layer dielectrics [2,3]. Also, the effect of halogen ions on the tunnel oxide has been studied, which is ion implantion into the p-type gate [4]. In this paper, halogen ions are incorporated into tunnel oxide with the implantation process through inter-layer dielectric which is deposited on the formed cell structure for protecting cells against moisture, hydrogen, and defects. To diffuse the halogen ions upto tunnel oxide and cure the inter-layer dielectric, the samples are annealed. Interface-trap charge (ΔVit), direct-current current-voltage (DCIV), and charge-to-breakdown (QBD) methods are used to characterize electrical properties. It is found that the quality of tunnel oxide and interface is enhanced with the concentration of ions, which is due to the reduction of the number of trap sites caused by diffusion through the inter-layer dielectric. It is shown that the reliability characteristics after program/erase cycles and retention at high temperature are improved with the implant dose. In conclusion, the reliability of NAND flash memory can be improved with the decrease of the number of defects in dielectrics of its cells by halogen ions.
References
[1] J.D. Lee et al., Proc. IRPS , 497 (2003)
[2] S.Y. Wang et al., Solid-State Electron., 50, 1171 (2006).
[3] C.H. Liu et al., Proc. VLSI Tech., 35 (2009).
[4] C.C. Chen et al., Electrochem. Solid-State Lett., 3, 290 (2000)

In this work, we use the Charge-Optimized Many-Body potential (COMB), an empirical potential parameterized for Si/SiO2 [1,2] and Hf/HfO2 systems [3], with the aim of modeling novel semiconductor/oxide interfaces with realistic features of interface structure. The COMB potential is particularly well suited to model interfaces because it is a variable charge potential that allows charges to dynamically evolve during the simulation. The modified COMB potential has been implemented within Pysic [4], an ASE (Atomic Simulation Environment) calculator in an object-based Python environment.
Building on the framework of the COMB potential developed by Shan et al. [2] (used in LAMMPS [5]), we have modified the same in order to obtain more realistic results. The implemented potential has been tested on Si3Oy (y = 1-6) and HfOn (n = 2-6) clusters. Further, ab-initio results from the VASP code [6], and classical results from LAMMPS and Pysic codes for optimized silica and hafnia bulk phases have been compared.
After suitable testing, we model the Si/SiO2/HfO2 interface. Considering that a layer of silica is always formed between adjacent Si and HfO2 slabs, molecular dynamics simulations of the Si/SiO2 interface have been carried out, and the results have been compared with earlier ReaxFF studies [7]. Finally, we prepare a series of Si/SiO2/HfO2 interfaces and filter them to find characteristic representations of real interfaces. These key interfaces are further studied at the first principles level, looking at their electronic structure and the possible role of defects in electron transport across the interface.
[1] J. Yu et al, Physical Review B 75, 085311 (2007)
[2] Tzu-Ray Shan et al, Physical Review B 82, 235302 (2010)
[3] Tzu-Ray Shan et al, Physical Review B 81, 125328 (2010)
[4] http://butler.cc.tut.fi/~hynnine5/programs/pysic/doc/index.html
[5] S. Plimpton, J Comp Phys 117, 1-19 (1995)
[6] G. Kresse and J. Hafner, Physical Review B, 47-558 (1993)
[7] van Duin et al, The Journal of Physical Chemistry A 105, 9396-9409 (2001)

Oxide based resistive switching memory (RRAM) is one of the promising candidates for future non-volatile memory application due to its simple structure, low switching voltage (<3 V), fast switching speed (<10 ns), excellent scalability (<10 nm), and great compatibility with the CMOS process technology. The development of the RRAM technology involves key challenges such as the unclear physics of resistive switching in oxides, relatively poor uniformity, and large variability of the switching parameters. This talk gives an overview of efforts to address these issues through experiments and modeling.
The physical mechanism of resistive switching is generally attributed to the conductive filament (made up of oxygen vacancies) formation and rupture in the oxide due to field assisted oxygen ion migration. As a model system for device physics study, HfOx based RRAM devices were fabricated and characterized. To identify the electron conduction mechanism, various electrical characterization techniques such as I-V measurements at various temperatures, low-frequency noise measurements, and AC conductance measurements were employed. It was suggested that the trap-assisted-tunneling is the dominant conduction mechanism. In order to explore the oxygen ion migration dynamics, pulse switching measurements were performed. An exponential voltage-time relationship was found between the switching time and the applied voltage. To obtain a first-order understanding of the variability of resistive switching, a Kinetic Monte Carlo (KMC) numerical simulator was developed. The generation/recombination/migration probabilities of oxygen vacancies and oxygen ions were calculated, and the conductive filament configuration was updated stochastically according to those probabilities. The KMC simulation can reproduce many experimental observations in the DC I-V sweep, pulse switching, endurance cycling, and retention baking, etc. The tail bits in the resistance distribution are attributed to the oxygen vacancy left over in the gap region due to a competition between the oxygen vacancy generation and recombination. To enable circuit and system development using RRAM, a compact device model was developed. The compact model, implemented in MATLAB, HSPICE, and Verilog-A, which can be employed in many commonly available circuit simulators using the SPICE engine.

As the NAND flash memory cell is scaling down into sub-20 nm technologies, the reliability characteristics such as endurance and retention of NAND flash memory cells are dramatically degraded. Especially, in NAND flash memory cell, properties of floating gate (FG) as a charge storage node are the critical factors in reliability. Up to now, grain size of poly-silicon, carbon-doped FG and the dopant type of FG have been studied to enhance reliability of NAND cells [1,2]. In this paper, we will show rapid thermal oxidation (RTO) effect on poly-silicon as used FG for improving endurance characteristics. The varied RTO process temperature and time after the deposition of poly-silicon are investigated to modify a physical structure of FG and their electrical properties of poly-silicon. When the RTO process is skipped or the sample is annealed at low temperature (compared with reference samples), the shape of the upper FG becomes narrow. Furthermore, the height of FG is lower and the grain size of poly-silicon is smaller. We can confirm these results from TEM and AFM images. For the comparison of reliability between the samples, threshold voltage (Vth) distribution of cells before and after program/erase (P/E) cycles is measured. The sample annealed with lower RTO temperature exhibits better program disturbance characteristic and narrower width of Vth distribution. These results indicate that the small grain size of poly-silicon disperses the electric field between FG and inter-poly dielectrics, which reduces generation of interfacial and oxide trap sites during P/E cycles. Also, increased on-current through the strings and reduction of interference between cells lead narrow Vth distribution. In conclusion, high reliability of NAND Flash can be achieved with the proposed RTO technique in sub-20 nm technology.
References
[1] A. T. Voutsas et al., J. Electrochem. Soc., 140, 282 (1993).
[2] J. Pu et al., IEEE Electron Dev. Lett., 29, 688 (2008).

While ferroelectric materials are normally considered as insulators, semiconducting ferroelectrics have been known for a long time to exhibit bistable conduction characteristics.[1] With the advent of modern growth techniques for complex oxide heterostuctures, devices can now be fabricated which exhibit polarization controlled charge transport and switchable photovoltaic effects, as observed in semiconducting BiFeO3.[2] In addition, recent experimental and theoretical studies found that ferroelectric polar displacements persist in BaTiO3, a prototypical oxide ferroelectric, even up to quite high levels of electron doping.[3,4] The co-existence of the ferroelectric phase with conductivity opens the door to new functionalities which may provide a unique route for novel device applications. Using first-principles methods and electrostatic modeling we explore the effect that the switchable polarization of electron-doped BaTiO3 (n-BaTiO3) has on the electronic properties at the SrRuO3/n-BaTiO3 (001) interface. Ferroelectric polarization controls the accumulation and depletion of electron charge at the interface, and the associated bending of the n-BTO conduction band determines the transport regime across the interface. We find that the interface exhibits a wide Schottky tunnel barrier for one polarization orientation, whereas an Ohmic-like contact is present for the opposite polarization orientation, leading to a large change in interface resistance associated with polarization reversal. Calculations reveal a large (five orders of magnitude) change in the interface resistance as a result of polarization switching, making the polarization control of interface transport regimes a fascinating prospect for potential device applications.
[1] V. Fridkin, Ferroelectric semiconductors (Consultants Bureau: New York, 1980) 318pp.
[2] A. Q. Jiang, C. Wang, K. J. Jin, X. B. Liu, J. F. Scott, C. S. Hwang, T. A. Tang, H. B. Lu, and G. Z. Yang, Advanced Mater. 23, 1277 (2011).
[3] T. Kolodiazhnyi, M. Tachibana, H. Kawaji, J. Hwang, and E. Takayama-Muromachi, Phys. Rev. Lett. 104, 147602 (2010).
[4] Y. Wang, X. Liu, J. D. Burton, S. S. Jaswal, and E. Y. Tsymbal, Phys. Rev. Lett. 109, 247601 (2012).

There is growing interest in understanding unexpected properties arising at oxide interfaces, whether due to new physics arising from many body effects or simply chemical composition change-driven phenomena. In this context, a liquid-oxide interface is interesting to probe band bending effects (electrostatic) and chemical diffusion (such as by ionic species injection) driven resistance modulation. Electric double layer transistors (EDLTs) are well suited to study these competing effects. Vanadium oxide (VO2) is of contemporary interest as a model material to investigate correlated electron phenomena, metal insulator phase transition mechanisms, and as well as for potential applications in switching devices. Membranes of such oxides allow studies on electronic transport in confined structures, and enable measurements in chemical potential gradients. We demonstrate a robust lithographic patterning method to fabricate self-supported sub-50nm VO2 membranes that undergo a phase transition. Utilizing such self-supported membranes, we directly observed shift in metal-insulator transition temperature arising from stress relaxation and consistent opening of the hysteresis. Electric double layer transistors were then fabricated with the membranes and compared to thin film devices on nitride substrates. The ionic liquid allowed reversible modulation of channel resistance and distinguishing bulk process from the surface effects. From the shift in the metal-insulator transition temperature, the carrier density doped through electrolyte gating is estimated to be 1*10^20 cm^-3. Hydrogen annealing studies showed little difference in resistivity between the film and the membrane indicating rapid diffusion of hydrogen in the vanadium oxide rutile lattice, consistent with previous observations. We will present a detailed discussion on interpretation of transistor response that includes electrostatic, electrochemical and chemical effects, all of which can modulate the resistance of a phase change correlated oxide.

We demonstrate light induced modification of the metal-insulator transition in single crystalline VO2 ultrathin films (2-15nm) grown on (001) TiO2 substrates using molecular beam epitaxy. These films exhibit a metal insulator transition (MIT) from a metallic phase with a rutile structure to a monoclinic insulating phase at 278K. This temperature is well below the bulk transition temperature of 340K due to strain from the substrate lattice mismatch with the VO2 structure. Exposure to 257 nm wavelength UV light at 0.94W/cm2 power density suppresses the MIT causing the films to remain metallic to temperatures as low as 150K. Various thermal annealing experiments aimed at recovering the MIT transition will be presented. The ability to write, erase, and reconfigure metallic regions within an insulating VO2 film presents a flexible and powerful means to build electronic circuits using correlated oxides.

Strongly correlated electron systems are of much interest for the development of highly sensitive sensors and phase change memories that use the Mott transition driven by external forces. Especially vanadium dioxide (VO2) is a promising material to lead to realization of their devices because of drastic change of conductive properties with several orders of magnitude by temperature and an electric field. Recently, electric-double-layer transistor based on VO2 channel is attracting attention in the huge changes of resistance by the electric-filed [1,2].
In this research, we focus on a side-gate-type field-effect transistor (SG-FET) via air gap with VO2 nano-wire channel and tried to modulate channel resistance by an electric field under various kinds of gas atmospheres (air, N2 and etchellip;).
VO2 thin films were prepared by a pulsed laser deposition technique. In patterning of the SG-FET, we applied a nanoimprint lithography technique and successfully obtained over 60 devices at a time with high controllability of size, shape and position in a large area (4 mm x 4 mm).
When a gate voltage (Vg) was applied to a VO2 channel from both sides across air gap with 200 nm in width, the resistance drastically decreased at Vg=100 V and recovered at Vg=-100 V in air, exhibiting memory effect, while the resistance remained unchanged in N2 atmosphere. It is considered that the change of resistance is caused by a redox reaction. In this meeting, we will report the detail experimental results and their analysis.
This work was supported by JSPS KAKENHI Grant Numbers 21676001 and 25286058
[1] M. Nakano et al, Nature 487 (2012) 459. [2] J. Jeong et al, Science 339 (2013) 1402.

Magnetite (Fe₃O₄) is a prototype multiferroic material with both spontaneous magnetization and dielectric polarization. Below the well-known metal-insulator (Verwey) transition at 120 K, a reg-ular arrangement of the B-site Fe²⁺ and Fe³⁺ ions in an inverse spinel structure results in a charge-ordering pattern with an alternation of short and long Fe-Fe bonds. The coexistence of bond-centered and charge-centered charge ordering induces an electronic polarization along the monoclinic b-axis [1]. Recently, we have demonstrated pyroelectric detection of a spontaneous polarization in Fe₃O₄ films grown on Nb:SrTiO₃ substrates below the Verwey transition point [2]. Moreover, quasi-static pyroelectric measurements of Pd/Fe₃O₄/Nb:SrTiO₃ junctions revealed a very large effective pyroelec-tric coefficient of 735 nC/cm²K at 60 K. This value is comparable to the well-known large pyroelec-tric coefficient of Pb(Sc,Ta)O₃ films, 600 nC/cm²K, and much larger than the 20 nC/cm²K coefficient of PbTiO₃/MgO films. Such a large pyroelectric coefficient may be related to the interface between the Fe₃O₄ film and a semiconducting Nb:SrTiO₃ substrate that was used as a bottom electrode. We have therefore investigated the interfacial capacitance of Fe₃O₄/Nb:SrTiO₃ junctions below the Ver-wey transition point.
Fe₃O₄ films were grown on Nb:SrTiO₃(001) substrates by pulsed laser deposition [3]. Typical Fe₃O₄ film thicknesses were 13 to 546 nm. For the electric contacts, 100-nm-thick Pd top electrodes were deposited on the Fe₃O₄ film surface by e-beam evaporation. The polar state was characterized by measuring the pyroelectric response and by scanning nonlinear dielectric microscopy (SNDM). The capacitance value was measured by using an LCR meter as a function of temperature and frequency. The temperature dependence of capacitance for all samples presented strong dielectric dispersion be-tween 20 and 50 K. Above 60 K, the capacitance value was constant and not dependent on the Fe₃O₄ film thickness, even though the capacitance value should be inversely proportional to the film thick-ness. This indicates that the measurements are affected by interfacial capacitance due to the formation of a depletion layer between the Fe₃O₄ film and the Nb:SrTiO₃(001) substrate. A large interfacial ca-pacitance is expected to enhance the effective pyroelectric response of Pd/Fe₃O₄/Nb:SrTiO₃ junctions.
[1] J. Brink et al, J. Phys.: Condens. Mater. 20, 424217 (2008)
[2] R. Takahashi et al. Physical Review B, 86, 144105 (2012))
[3] R. Takahashi et al. Crystal Growth & Design 12, 2679-2683 (2012)

Complex oxide heterostructures are highly promising for technological applications as they offer novel device concepts and functionalities. One of the fundamental parameters that influence the characteristics of the metal/oxide heterostructure is the Schottky barrier formed at the interface. The Schottky barrier height (SBH) is strongly influenced by the atomic structure of the metal/oxide interface and is of fundamental interest as an intrinsic property of the system. The SrRuO₃/SrTiO₃(001) heterostructure is a prototypical system to study SBH at the oxide metal/dielectric interface. In recent years, the SrRuO₃ (SRO) has attracted a lot of attention as an electrode material for ultrathin ferroelectric films. Using ab-initio calculations, we have studied the p-type SBH and its dependence on the interface structure in SRO/STO heterostructure. In addition, we have estimated the p-SBH using semi-empirical Metal-Induced-Gap-States (MIGS) model. In particular we have considered three types of interfaces: RuO₂/SrO/TiO₂, RuO₂/BaO/TiO₂ and MnO₂/SrO/TiO₂. The ab-initio estimate of p-SBH comes out to be 1.27, 1.33 and 0.78 eV for RuO₂/SrO/TiO₂, RuO₂/BaO/TiO₂ and MnO₂/SrO/TiO₂ interfaces respectively. We find that semi-empirical MIGS model overestimate the p-SBH by ~2 eV. We have also studied the workfunctions of SRO surfaces with different terminations. The workfunction of SRO(001) surface with RuO₂ termination is larger by 2.42 eV than that with SrO termination. The calculated value of SRO(001) surface workfunction with RuO₂ termination is 5.0 eV and is in close agreement with the experimental value of 5.2 eV. We have also tested the validity of a recently proposed generic relation between the metal work function and the Young&’s modulus, in case of SRO.

We determined the atomic structures and energies of 109°, 180° and 71° domain walls in BiFeO₃, combining density functional theory +U calculations and aberration-corrected transmission electron microscopy images. Both the calculation and the measurement show that across the domain walls the Fe sublattice is relatively uniform and the Bi sublattice shows a substantial shift, implying that the change in polarization is due to the shift of the Bi sublattice. The oxygen octahedral rotation changes across the three types of walls correspond to the thicknesses obtained by analyzing the atomic displacement distributions from both the density functional theory +U calculations and HAADF images. The calculated wall energies (γ) follow the sequence γ₁₀₉ < γ₁₈₀ < γ₇₁ for the 109°, 180°, and 71° walls. We attribute the high 71° wall energy to an opposite tilting rotation of the oxygen octahedra and the low 109° wall energy to the opposite twisting rotation of the oxygen octahedra across the domain walls. [Phys. Rev. Lett. (accepted June 2013)].

The impact of the quality of TiN electrodes on the switching characteristics of HfOx-based resistive random access memory (ReRAM) devices is investigated in this work. It is shown that by changing the method of preparation of the top TiN electrode, the Ron/Roff ratio of the device is altered by up to three orders of magnitude.
Two types of TiN/HfOx/TiN devices have been fabricated where the top 200nm TiN electrode has been deposited by two different sputtering methods. For one device, the top TiN electrode was deposited by reactive sputtering, using a titanium target in a nitrogen environment. For the other device, the top TiN electrode was deposited by non-reactive sputtering, using a titanium nitride target. XRD, EDX and electrical measurements show the TiN layer, deposited in a nitrogen environment, is single-phase stoichiometric TiN and has a sheet resistance of 6Omega;/square. The other TiN layer, deposited by a titanium nitride target, contains oxygen and has a sheet resistance of 505Omega;/square. This indicates that this TiN layer readily reacts with oxygen in the HfOx layer beneath, forming a TiOxNy layer between the top electrode and the insulator.
The device with the stoichiometric titanium nitride layer results in bipolar switching, with a Ron/Roff ratio <5. The device with an oxygen titanium nitride layer results in unipolar switching, with a Ron/Roff ratio of 10^4. The larger Ron/Roff ratio in this device supports the theory of oxygen vacancies leading to the formation of conductive filaments. It is likely that the formation of the TiOxNy layer causes oxygen vacancies to be created in the HfOx layer, aiding the switching of the device, leading to an increase in the Ron/Roff ratio. In the other device, reaction between the stoichiometric TiN and the HfOx layer beneath is minimal, resulting in low concentration of oxygen vacancies in the HfOx layer and therefore leads to a smaller Ron/Roff ratio. These results show that an oxygen layer between the top electrode and insulator is advantageous for resistive memory applications.

Semiconductors with mobile dopants (SMDs), which are distinct from conventional semiconductors, exhibit a unique physical property; they demonstrate memristive hysteretic current-voltage curves. Memristive behaviour, theoretically proposed in 1971 and experimentally realised in 2008, refers to the dependence of a semiconductor&’s resistance on the history of the applied current flow and its internal state. The fundamental peculiarity of an SMD memristive hysteresis curve is that it exhibits two oppositely rotating directions, whose origin is still debatable. In this presentation, we introduce an SMD model to describe the electronic-barrier modulation due to the motion of mobile dopants in oxide semiconductor under an electric field. We will show that oxygen vacancy migration near the Schottky-interface region induces abnormal modulation of the electronic barrier; low (high) density of dopants near a metal-semiconductor interface lead to high (low) conductance, while the conventional ionic and electronic models predict the reverse behaviours. From this observation, we will show that the two directions of the hysteresis I-V curve originate from the spatial inhomogeneity of the mobile dopant distribution in the SMD.

The equilibrium charge carrier distribution at abrupt contacts is considered by not only allowing electrons but also cations or anions to be mobile, the control parameters being partial pressures, temperature and doping content [1]. This leads to the generalized ther-modynamic picture involving mixed conductors. A variety of interfacially dominated sys-tems (composites, nanoceramics, heterolayered systems) are treated and experimental evidence is given for the significance of ionic redistribution: ion transport along CaF2/BaF2 heterolayers [2], coupling of ion and electron redistribution at LiF/TiO2 con-tacts [3], generalized point defect chemistry in nanocrystalline SrTiO3 [4], the space charge storage in Li2O/Ru composites [5].
The significance of “nano-ionics” as basic discipline underlying these phenomena will be highlighted as far as emergent electron transport properties at complex oxide interfaces are concerned.
References
[1] J. Maier, Progress in Solid State Chemistry 23, 171-263 (1995); Nature Materials 4, 805-815 (2005).
[2] N. Sata, K. Eberman, K. Eberl, and J. Maier, Nature 408, 946-949 (2000); X. X. Guo and J.Maier, Ad-vanced Materials 21, 2619-2631 (2009).
[3] C. L. Li, L. Gu, X. X. Guo, D. Samuelis, K. Tang, and J. Maier, Nano Letters 12, 1241-1246 (2012).
[4] P. Lupetin, G. Gregori, and J. Maier, Angewandte Chemie International Edition 49, 10123-10126 (2010).
[5] J. Maier, Angewandte Chemie International Edition 52, 4998-5026 (2013).

The electrical control of magnetism using a field-effect transistor structure is currently an area of active research in spintronics. Recent progress in gating technology that uses high-k dielectrics, ferroelectric insulators, and electrolytes, has made it possible to change the magnetic properties of various ferromagnetic materials including 3d metals and oxides. The field effect using an ionic liquid has been considered very powerful because of the extremely high-density charge carrier accumulation [1-3]. Very recently, along with carrier doping, oxygen diffusion was found to be induced by the large electric field created at the interface between the oxide surface and ionic liquid [4]. This provides us an additional degree of freedom to control material&’s properties via electrochemical reactions. Here we report the electrochemical control of electrical and magnetic properties of spinel zinc ferrite (ZnxFe3minus;xO4) at room temperature.
Epitaxial thin films of Zn0.5Fe2.5O4 were grown on MgO(001) substrates by pulsed-laser deposition [5]. The film was patterned into a Hall bar geometry by photolithography and Ar ion milling. An ionic liquid, DEME-TFSI, was then dropped such that the Zn0.5Fe2.5O4 channel and Au gate electrode form an electric-double-layer capacitor. The device exhibited n-type behavior; the application of a positive gate bias VG increased the drain current ID (conductivity). A large hysteresis observed in the ID-VG curve is similar to those of VO2 devices for which oxygen diffusion by ionic-liquid gating was first reported. Further support to the presence of electrochemical reactions was obtained from the VG polarity dependence of the initial ID-VG curve. The application of a positive VG was found to be necessary to trigger the hysteresis and conductivity modulation. We speculate that the extraction of oxygen ions at the Zn0.5Fe2.5O4 surface occurs when negatively biased. The tendency of the change in conductivity against the variation in the oxygen nonstoichiometry was well reproduced by transport measurements using the films grown under various oxygen pressures. Utilizing this gate-induced bistable zero-bias states, we controlled the magnetoresistance of Zn0.5Fe2.5O4.
[1] M. Weisheit et al., Science 315, 349 (2007).
[2] Y. Yamada et al., Science 332, 1065 (2011).
[3] K. Shimamura et al., Appl. Phys. Lett. 100, 122402 (2012).
[4] J. Jeong et al., Science 339, 1402 (2013).
[5] T. Ichimura et al., Jpn. J. Appl. Phys. 52, 068002 (2013).

LiTi2O4 has attracted much attention as the only known oxide superconductor with a spinel structure. We are investigating the transport properties of expitaxially grown LiTi2O4 thin films on MgAl2O4 substrates using pulsed laser deposition. Since Li can easily evaporate during the deposition process, we use Li excess Li4Ti5O12 as a target and deposit films in vacuum [1]. LiTi2O4 films grown on (111) and (100) MgAl2O4 substrates showed superconducting transition temperatures at asymp;12 K, consistent with previous reports [1,2]. Transport properties including tunneling and the Hall effect have been measured. We will also discuss the thickness dependence of the transport properties and the electrostatic gating effect using an ionic liquid as the gate material. This work is supported by AFOSR. Ref. [1] Kumatani et al., Appl. Phys. Lett. 101, 123103 (2012). [2] Chopdekar et al., Physica C 469, 1885 (2009).

Oxide thin films and heterostructures have become one of the focal points of physical research in the last 20 years, with a large number of novel phenomena associated with oxide structures. It is well-established, that oxygen stoichiometry strongly affects oxide properties. In this context, oxygen-controlled properties of oxide surfaces are of special interest. In particular, surfaces are the key intermediate step during film growth that influences the structure and properties of interfaces in hetersotructures. High cationic and oxygen lability on surfaces can lead to multivariant adatom reconstructions. Furthermore, oxygen-related surface electrochemistry was shown to be directly influencing physical processes in bulk of thin films with effects on such phenomena as ferroelectric phase stability or metal-insulator phase transitions. In turn, oxygen evolution and reduction reactions at the surfaces are key processes in operation of energy conversion devices, catalysts, oxygen sensors and pumps, and other functional systems. In this work, we explored coupling between oxygen behavior and physical functionality on the surface of SrRuO₃ (SRO), one of the most studied oxide materials with perovskite structure, using few nanometer thick films of SRO epitaxially grown by pulsed laser deposition (PLD) on (001)SrTiO₃ substrates. We have for the first time succeeded in atomic-resolution scanning tunneling microscopy (STM) imaging of as-deposited SRO films grown at conditions, which are normally used for deposition of SRO electrode layers in complex oxide functional heterostructures. The STM images showed atomic patterns on the film surfaces, which were not reported before. Depending on growth/annealing conditions, zigzag and rectangular linear structures were observed and identified as formed by oxygen adatoms. Further, we performed a detailed in situ and ex situ experimental investigation of surfaces of SRO thin films using a combination of x-ray and ultraviolet photoelectron spectroscopy (PES), magnetometry, and magnetotransport measurements. Ab-initio DFT modeling revealed a link between the patterns of adsorbed oxygen atoms and the electronic and magnetic properties of the topmost atomic layer. DFT showed relatively large adsorption energies of oxygen in the observed surface structures indicating their stability at deposition temperature. The modeling predicted that oxygen adatoms possess frustrated local spin moments with possible spin-glass behavior of the surface covered by adsorbed oxygen due to random ferromagnetic/antiferromagnetic coupling between spins of oxygen adatoms and spins of the underlying SRO. In turn, the pristine SrO-terminated surface is half-metallic, which is of a strong interest for realization of spintronic devices. This research was sponsored by the DMSE, BES, U.S. DOE (AT, WS, SVK). Research at the ORNL's CNMS was sponsored by the Division of Scientific User Facilities, U.S. DOE.

Existence of two dimensional electron gas (2DEG) with high mobility at the atomically engineered interface between two wide band-gap perovskite insulators, SrTiO3 (STO) and LaAlO3 (LAO) has stimulated research interests to utilize these functional oxide interfaces in key electronic devices such as high mobility electron transistors (HMET). Although excellent interfacial transport properties were manifested, structuring these delicate heterointerfaces without damaging the substrate STO single crystal has remained elusive which is essential for their integration with other device components. Physical etching processes employing “top-down” patterning approach were unsuitable since they induce substrate conductivity through creation of oxygen vacancies.
We demonstrate development of a novel procedure for fabricating patterned functional interfaces based on an epitaxial-lift-off technique without utilizing any physical etching process. Devices incorporating precisely patterned interfaces of LAO-STO were fabricated and temperature dependent magneto transport properties were investigated. High-quality interface properties were found to be conserved in the patterned devices structures promoting future studies of low-dimensional confinement on high mobility interface conductivity as well as interfacial magnetism. In addition the pathway is shown to be capable of fabricating precise structures of other functional perovskites like ferroelectric PZT with dimensions down to nano-scale.

The rich array of conventional and exotic electronic properties that can be generated by oxide heterostructures are of great potential value for device applications. Particularly promising are the d-electron or f-electron-based mobile electron systems at interfaces in complex oxides. However, the actual physical implementation of functional complex oxide heterostructures is challenged by a higher defect density than is characteristic for semiconductor multilayers. Indeed, only single transistors bare of any circuit functionality have been realized from complex oxides. In the presentation we report that oxide electronic devices can be more defect-tolerant than semiconducting devices are.
We have fabricated robust oxide NMOS circuits and demonstrate LaAlO3-SrTiO3 chips with hundreds of thousands transistors. Providing the capability to process the signals of functional oxide devices such as sensors directly on oxide chips, these results illustrate the practicability and the potential of oxide electronics.

Here we investigate how charge transport properties scale down to the nanoscale regime, comparing the properties to standard semiconductor materials and providing a perspective on what it means to device manufacturing. Strontium titanate - the prototypical oxide material - has been widely studied for applications in thermoelectrics, nanoelectronics, catalysis, and other uses. We investigated how charge transport is effected at interfaces to stronitium titanate under a wide range of conditions - by varying contact size, interface shape, dopant concentration, surface structures and in various combinations and relate the the results to experiments utilizing standard semiconducting materials such as silicon and gallium arsenide. Also, the results of the analysis have wide ranging implications, especially for ferroelectric perovskite materials and serves as the basis for understanding and controlling switching effects - both polarization and oxygen migration based switching.

The two-dimensional electron gas (2DEG) at the interface between LaAlO₃ and SrTiO₃ (LAO/STO) displays a variety of electronic properties, including superconductivity, magnetism and Rashba spin-orbit interaction, and offers the possibility to control some of them using electric field effect. These features make it one of the most interesting multifunctional materials in oxide electronics.
Using a patterning technique based on electron beam lithography, we realized nanodevices with various geometries, which enable us to investigate different aspects of the physics of the LAO/STO system, both in the normal and in the superconducting regime.
Nanobridges with width down to 200 nm display superconducting properties that can be tuned by an electric field effect [1]. By optimizing the design of the nanostructure and the field effect geometry, we can achieve large tunability with low gate voltages, allowing the exploration of a wider range of dopings.
In these devices, we also studied the evolution in temperature and doping of the spin-orbit interaction, to map its relation with the superconducting state.
[1] D. Stornaiuolo et al., Applied Physics Letters, 101, 222601 (2012)

LaAlO₃/SrTiO₃ interfaces are known to display various intriguing properties such as metallic, insulating, superconducting, and magnetic properties at low temperatures depending on their growth conditions. Here we show that the transport properties of the interfaces are also affected by substrate surface-preparation methods. Using the deionized-water leaching method [1] and the commonly-used buffered hydrofluoric acid (BHF)-etching method, we have prepared atomically-flat, TiO₂-terminated SrTiO₃ (001) substrates. We have then grown epitaxial LaAlO₃ thin-films of various thicknesses using pulsed laser deposition. Electrical transport measurements show that the surface preparation has little effect on the transport properties of metallic samples with n_s > 10¹⁴ cm^-2. However, less metallic samples with n_s < 10¹³ cm^-2 show clear changes in their transport properties depending on the chemical etching methods. Our results suggest that the substrate preparation method is critical to the interfacial-transport properties.
[1] J. G. Connell, B. J. Isaac, G. B. Ekanayake, D. R. Strachan, and S. S. A. Seo, Appl. Phys. Lett., 101, 251607, (2012).

Following the discovery of the two-dimensional electron gas (2DEG) at the interface between polar LaAlO3 (LAO) and non-polar SrTiO3 (STO) grown in the [001] direction, many similar heterostructures with ever interesting physical properties have been proposed and investigated. Here using the first-principles theory, we explore the emergence of the 2DEG at the interface between two polar materials—a large material group—and thus broaden the field for designing highly conducing interfaces. We consider a model LAO-STO heterostructure stacked in the [111] direction. In the [111] direction both free standing LAO and STO are polar with alternatively charged planes-(LaO3)3- and (Al)3+ in the former and (SrO3)4 and (Ti)4+ in later with inevitable surface reconstruction. However, when LAO is deposited on STO, the reconstruction is partially relieved leading to a perfect Ti/LaO3 interface and n-type conductivity. We describe this phenomenon in terms of a modified polar catastrophe model predicting the critical thickness of the LAO layer of eight (LaO3-Al) bilayers. From the calculated electronic band structure we predict conductivity asymmetry between in-plane [1-10] and [11 -2] directions. Our findings are consistent with the experiment performed by S.S. Ryu, C.W. Bark, T. Hernandez, Hua Zhou, D.D. Fong, Y. Zhang, J. Podkaminer, X.Q. Pan, M.S. Rzchowski, and C.B. Eom.

For polar/nonpolar heterostructures, Maxwell&’s theory dictates that the electric potential in the polar components will increase divergently with the film thickness. For LaAlO3/SrTiO3, a conceptually intriguing route, termed charge reconstruction, has been proposed to avert such “polar catastrophe”. The existence of a polar potential in LaAlO3 is a prerequisite for the validity of the charge reconstruction picture, yet to date, its direct measurement remains a major challenge. Here we establish unambiguously the existence of the residual polar potential in ultrathin LaAlO3 films on SrTiO3, using a novel photovoltaic device design as an effective probe. The measured lower bound of the residual polar potential is 1.0 V. Such a direct observation of the giant residual polar potential within the unit-cell-scale LaAlO3 films amounts to a definitive experimental evidence for the charge reconstruction picture, and also points to new technological significance of oxide heterostructures in photovoltaic and sensing devices with atomic-scale control.

Transition-metal oxides display a variety of functionalities at their surfaces and interfaces, which have been induced by structural and electronic modifications. However, the origins of the functionalities are still in debate. To understand the origins of such phenomena and to further explore intriguing functionalities, it is important to elucidate, at the atomic level, their electronic structures and how those heterogeneous interfaces are formed.
We report direct visualizations of initial growth nature of sub-unit cell SrTiO₃ (STO) and LaAlO₃ (LAO) films formed on the top of atomically-ordered STO(001)-(radic;13×radic;13)-R33.7° substrate surfaces [1] using a scanning tunneling microscopy/spectroscopy (STM/STS) combined with pulsed laser deposition (PLD).[2] The STO(001)-(radic;13×radic;13)-R33.7° surface [3] is used as an excellent template in terms of atomic-scale structures and surface composition revealed by STM and x-ray photoelectron spectroscopy, respectively.
The STM/STS imaging of sub-UC LAO islands revealed that a TiO_x layer of the STO substrate has transferred to the topmost surface of the LAO layer, indicating that the TiO_x layer can be viewed as a graphene-like one unit-cell TiO_x sheet. [4] This structure can be regarded as a new oxide nanomaterial.
Further, we intentionally tune the atomic-scale surface structure of STO homoepitaxial thin film, and directly image surface electronic states of STO by using STS. We present the first-time, real-space observation of different symmetry constraints of STO surface electrons, controlled by both temperature and surface-defect concentrations.

These findings on the atomic-scale nature of perovskite surfaces lead to preparation of higher-quality thin films and surfaces/interfaces exhibiting novel electronic and magnetic properties.
[1] R. Shimizu, T. Hitosugi et al., Appl. Phys. Lett. 100, 263106 (2012).
[2] K. Iwaya, R. Shimizu, T. Hashizume, T. Hitosugi, Rev. Sci. Instrum. 82, 083702 (2011).
[2] R. Shimizu, T. Hitosugi et al., ACS Nano, 5, 7967 (2011).
[4] Ohsawa et al., submitted.

The two-dimensional electron gas (2DEG) confined at the interface between oxide band-insulators as LaAlO3 and SrTiO3 is of the highest interest. Until recently, the observation of the 2DEG has been restricted to the interface between TiO2-terminated SrTiO3(001) substrates and epitaxial LaAlO3 films. Surprisingly, a 2DEG has also been found when amorphous films of several oxides are deposited on TiO2-terminated SrTiO3(001) substrates. In all these cases, only the neutral TiO2 surface of SrTiO3(001) has been shown to be conductive. Here we show that this restriction can be surpassed. Properly treated (110) and (111) oriented SrTiO3 substrates were used to deposit LaAlO3 films by pulsed laser deposition assisted by reflection high energy electron diffraction (RHEED). In spite of the high energy of (110) and (111) surfaces of perovskites the films can be grown by a layer-by-layer mechanism, thus permitting flat interface and perfect control of the number of unit cells deposited. Remarkably, (110) and (111) LaAlO3/SrTiO3 interfaces are conducting. High-resolution transmission electron microscopy characterization of (110) samples shows atomically sharp interfaces with no observation of (001)-faceting. Moreover we show that amorphous interfaces do not form exclusively on the neutral TiO2-terminated SrTiO3(001) surface: conducting (110) interfaces with amorphous LaAlO3, SrTiO3 and YSZ can also be prepared. Remarkably for all these interfaces, whatever epitaxial or amorphous, a critical thickness is found below which the samples are insulating, although the number of LaAlO3 unit cells required to trigger the insulator-to-metal transition is dependent on the orientation. These findings open a new perspective both for materials research and for elucidating the ultimate microscopic mechanism of carrier doping.
Electron microscopy work at ORNL was supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division.

The formation of a highly conducting interface between the two nominal band gap insulators lanthanum aluminate (LaAlO₃ , LAO) and strontium titanate (SrTiO₃, STO) has been subject to many recent studies. Being a promising oxide heterostructure system to emulate and exceed the properties of semiconductor interface devices, substantial advances in the fabrication and understanding of oxide systems are likely to provide intriguing implications for physics as well as for technology.
The fundamental question, how the electrical conductivity can be induced at oxide interfaces, is still under controversial discussion. In order to understand whether the underlying LAO/STO interface reconstruction can be regarded as a universal effect at oxide interfaces between polar and non-polar perovskite materials, it is essential to find alternative perovskite systems next to LAO/STO which also form a conducting interface.
This contribution provides a detailed analysis of the structural and electrical properties of neodymium gallate/strontium titanate (NdGaO₃ /SrTiO₃ , NGO/STO) heterostructures exhibiting similar interface conductivity as the LAO/STO system.
As will be shown, the cation stoichiometry of NGO thin films can be tuned over a wide range by the variation of the PLD growth temperature. A detailed investigation of the electrical transport in NGO/STO heterostructures grown at various deposition temperatures reveals an interesting correlation between the NGO cation stoichiometry and the NGO/STO interface conductivity. Moreover, the NGO/STO system is characterized in terms of a defect chemistry model. An enhanced analysis of the defect structure of the NGO/STO interface is provided by in situ conductance measurements under high temperature oxygen equilibrium. At the same time, the high temperature experiment reveals a transition from metallic to insulating behavior at elevated temperatures. This thermal instability of the interface reconstruction is attributed to a preferential evaporation of gallium.
From the growth experiments as well as from the high temperature treatment, we were able to identify the importance of the B-site cation, namely gallium, for the existence of metallic conductivity at the NGO/STO interface. This provides insight into the conduction mechanism at the NGO/STO interface and rules out an Nd-ion implantation scenario as possible origin of the high electron concentration [1].
[1] F. Gunkel et al., Appl. Phys. Lett. 102, 071601 (2013).

Material systems dominated by interface effects provide promising chances to investigate new physical properties. A wide range of new physical effects can only be found in multilayered oxide thin film structures. The combination of a multiferroic to manipulate the transport properties of a conductive interface opens a path to an effective and ultrathin field effect device. We investigate the multiferroic material BiFeO3 in order to use it as a ferroelectric gate electrode for the quasi two dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 interface. The oxide thin films are prepared by Pulsed Laser Deposition in a controlled oxygen gas atmosphere. The ferroelectric properties of the BiFeO3 layers are characterized by Piezo-Force Microscopy (PFM). In addition PFM is utilized to switch the polarization of the BiFeO3 layer in out-of-plane direction by the use of the 2DEG as a back electrode. The transport properties of the 2DEG at the interface are strongly affected by an electric field in out-of-plane direction. This experiment was performed with gate electrodes on the backside of the substrate. The conductivity of the 2DEG is significantly modified by the existence of a multiferroic BiFeO3 top layer even in the unpoled state. As a first step to device fabrication and to make use of the PFM to manipulate the conductivity of the 2DEG the samples were microstructured by the use of an amorphous mask technique. Resistivity data of a microstructured line of the 2DEG is analyzed in correspondence to the area of the BiFeO3 polarized in the two possible out-of-plane polarizations of BiFeO3.

The fabrication of atomically-abrupt perovskite interfaces has usually depended on careful preparation prior to growth (i.e., in the preparation of substrates with unique surface termination) and extensive analysis afterwards (i.e., TEM-EELS and/or SXRD analysis of the chemical and crystallographic structure of the interface). We demonstrate a new in situ method to identify and control the surface termination and interface stacking during growth, based on careful use of reflection high-energy electron diffraction (RHEED), that significantly improves the atomic design of perovskite interfaces [1]. This technique requires independent supply of A- and B-site cations during deposition, achieved by shuttering the elemental sources in reactive molecular beam epitaxy (MBE) growth [2]. Rather than using oscillations of the specular spot to count the deposition of single ABO3 molecular layers, detailed information on surface termination and layer inversion can be extracted from diffracted beam (DB) oscillations and rocking curves [3]. The utility of this approach derives from the observation that the DB oscillations during shuttered deposition show a common phenomenology across many perovskite families for the full range of B-site valence between 3+ and 4+ and under different strain states; we will demonstrate this from data taken during the growth of titanate, manganite and ferrite films [4]. Conversely, RHEED rocking curves (RCs) are performed without deposition, and unique diffracted-beam RCs can be correlated with different surface terminations deduced from DB oscillations or other methods. Together, these advances in RHEED technique allow us to precisely prepare one perovskite surface before the deposition of an interface, resulting in the elimination of cation intermixing and anomalous lattice dilations that have previously been observed. This approach has also yielded magnetic tunnel junctions with record tunneling magnetoresistance [5].
[1] A. Yu. Petrov, X. Torrelles, A. Verna, H. Xu, A. Cossaro, M. Pedio, J. Garcia-Barriocanal, G. R. Castro and B. A. Davidson, Adv. Mater. (in press).
[2] J. H. Haeni, C. D. Theis and D. G. Schlom, J. Electroceram. 4, 385 (2000).
[3] T. Hikita, T. Hanada, M. Kudo and M. Kawai, Surf. Sci. 287-288, 377 (1993).
[4] A. Yu. Petrov, H. Xu and B. A. Davidson, in preparation.
[5] R. Werner, A. Yu. Petrov, L. Alvarez Miño, R. Kleiner, D. Koelle and B. A. Davidson, Appl. Phys. Lett. 98, 162505 (2011).

Spontaneous polarization in wurtzite crystal structure is one of dominant origins for the accumulation of two-dimensional electron gas (2DEG), which is applicable for the transparent field effect transistor. We have shown that MgZnO/ZnO interfaces generate high-mobility 2DEG due to the mismatch in electric polarization of constituent piezoelectric materials [1]. The electron mobility is now approaching to 1 million cm²V^-1s^-1 with using distilled pure ozone as oxygen source in molecular-beam epitaxy [2]. Compared with well-studied GaAs heterostrucutres, strong correlated 2D transport phenomena are pronounced in ZnO heterostruc