Atomic layer engineering of perovskite oxides for chemically sharp heterointerfaces

Designing and controlling the properties of transition metal oxide quantum materials

This Perspective addresses the design, creation, characterization and control of synthetic quantum materials with strong electronic correlations. We show how emerging synergies between theoretical/computational approaches and materials design/experimental probes are driving recent advances in the discovery, understanding and control of new electronic behaviour in materials systems with interesting and potentially technologically important properties. The focus here is on transition metal oxides, where electronic correlations lead to a myriad of functional properties including superconductivity, magnetism, Mott transitions, multiferroicity and emergent behaviour at picoscale-designed interfaces. Current opportunities and challenges are also addressed, including possible new discoveries of non-equilibrium phenomena and optical control of correlated quantum phases of transition metal oxides.

Phase Selection and Structure of Low-Defect-Density γ-Al2O3 Created by Epitaxial Crystallization of Amorphous Al2O3

A multistep phase sequence following the crystallization of amorphous Al₂O₃ via solid-phase epitaxy (SPE) points to methods to create low-defect-density thin films of the metastable cubic γ-Al₂O₃ polymorph. An amorphous Al₂O₃ thin film on a (0001) α-Al₂O₃ sapphire substrate initially transforms upon heating to form epitaxial γ-Al₂O₃, followed by a transformation to monoclinic θ-Al₂O₃, and eventually to α-Al₂O₃. Epitaxial γ-Al₂O₃ layers with low mosaic widths in X-ray rocking curves can be formed via SPE by crystallizing the γ-Al₂O₃ phase from amorphous Al₂O₃ and avoiding the microstructural inhomogeneity arising from the spatially inhomogeneous transformation to θ-Al₂O₃. A complementary molecular dynamics (MD) simulation indicates that the amorphous layer and γ-Al₂O₃ have similar Al coordination geometry, suggesting that γ-Al₂O₃ forms in part because it involves the minimum rearrangement of the initially amorphous configuration. The lattice parameters of γ-Al₂O₃ are consistent with a structure in which the majority of the Al vacancies in the spinel structure occupy sites with tetrahedral coordination, consistent with the MD results. The formation of Al vacancies at tetrahedral spinel sites in epitaxial γ-Al₂O₃ can minimize the epitaxial elastic deformation of γ-Al₂O₃ during crystallization.

Interfacial stabilization for epitaxial CuCrO2 delafossites

ABO₂ delafossites are fascinating materials that exhibit a wide range of physical properties, including giant Rashba spin splitting and anomalous Hall effects, because of their characteristic layered structures composed of noble metal A and strongly correlated BO₂ sublayers. However, thin film synthesis is known to be extremely challenging owing to their low symmetry rhombohedral structures, which limit the selection of substrates for thin film epitaxy. Hexagonal lattices, such as those provided by Al₂O₃(0001) and (111) oriented cubic perovskites, are promising candidates for epitaxy of delafossites. However, the formation of twin domains and impurity phases is hard to suppress, and the nucleation and growth mechanisms thereon have not been studied for the growth of epitaxial delafossites. In this study, we report the epitaxial stabilization of a new interfacial phase formed during pulsed-laser epitaxy of (0001)-oriented CuCrO₂ epitaxial thin films on Al₂O₃ substrates. Through a combined study using scanning transmission electron microscopy/electron-energy loss spectroscopy and density functional theory calculations, we report that the nucleation of a thermodynamically stable, atomically thick CuCr_1-xAl_xO₂ interfacial layer is the critical element for the epitaxy of CuCrO₂ delafossites on Al₂O₃ substrates. This finding provides key insights into the thermodynamic mechanism for the nucleation of intermixing-induced buffer layers that can be used for the growth of other noble-metal-based delafossites, which are known to be challenging due to the difficulty in initial nucleation.

A spin-orbit coupling-induced two-dimensional electron gas in BiAlO3/SrTiO3 heterostructures

Both LaAlO₃ and BiAlO₃ are isostructural, isoelectronic and band insulators. Therefore, in analogy to the LaAlO₃/SrTiO₃ heterostructure, a quasi two dimensional electron gas (q-2DEG) could be anticipated in BiAlO₃/SrTiO₃ heterostructures. Our density functional theory based scalar relativistic calculations show that BiAlO₃/SrTiO₃ heterostructures remain insulating for a BiAlO₃ film thickness up to 5 unit cells. However, with spin orbit coupling included in the crystal Hamiltonian, we find a thickness dependent insulator to metal transition for BiAlO₃/SrTiO₃ heterostructures. However, unlike the Ti³⁺/Ti⁴⁺ electronic reconstruction in LaAlO₃/SrTiO₃, the conductivity in BiAlO₃/SrTiO₃ is found to originate from the subsurface Bi 6p states. The results suggest that the properties of q-2DEG in BiAlO₃/SrTiO₃ can be controlled using an external electric field, leading to a wide range of solid state applications.

Kinetically Controlled Fabrication of Single-Crystalline TiO2 Nanobrush Architectures with High Energy {001} Facets

This study demonstrates that precise control of nonequilibrium growth conditions during pulsed laser deposition (PLD) can be exploited to produce single-crystalline anatase TiO2 nanobrush architectures with large surface areas terminated with high energy {001} facets. The data indicate that the key to nanobrush formation is controlling the atomic surface transport processes to balance defect aggregation and surface-smoothing processes. High-resolution scanning transmission electron microscopy data reveal that defect-mediated aggregation is the key to TiO2 nanobrush formation. The large concentration of defects present at the intersection of domain boundaries promotes aggregation of PLD growth species, resulting in the growth of the single-crystalline nanobrush architecture. This study proposes a model for the relationship between defect creation and growth mode in nonequilibrium environments, which enables application of this growth method to novel nanostructure design in a broad range of materials.

Highly Oriented SrTiO3 Thin Film on Graphene Substrate

Growth of perovskite oxide thin films on Si in crystalline form has long been a critical obstacle for the integration of multifunctional oxides into Si-based technologies. In this study, we propose pulsed laser deposition of a crystalline SrTiO₃ thin film on a Si using graphene substrate. The SrTiO₃ thin film on graphene has a highly (00l)-oriented crystalline structure which results from the partial epitaxy. Moreover, graphene promotes a sharp interface by highly suppressing the chemical intermixing. The important role of graphene as a 2D substrate and diffusion barrier allows the expansion of device applications based on functional complex oxides.

Unraveling the enhanced Oxygen Vacancy Formation in Complex Oxides during Annealing and Growth

The reduction of oxides during annealing and growth in low pressure processes is a widely known problem. We hence investigate the influence of mere annealing and of growth in vacuum systems to shed light on the reasons behind the reduction of perovskites. When comparing the existing literature regarding the reduction of the perovskite model material SrTiO₃ it is conspicuous that one finds different oxygen pressures required to achieve reduction for vacuum annealing and for chemically controlled reducing atmospheres. The unraveling of this discrepancy is of high interest for low pressure physical vapor depositions of thin films heterostructures to gain further understanding of the reduction of the SrTiO₃. For thermal annealing, our results prove the attached measurement devices (mass spectrometer/ cold cathode gauge) to be primarily responsible for the reduction of SrTiO₃ in the deposition chamber by shifting the thermodynamic equilibrium to a more reducing atmosphere. We investigated the impact of our findings on the pulsed laser deposition growth at low pressure for LaAlO₃/SrTiO₃. During deposition the reduction triggered by the presence of the laser plume dominates and the impact of the measurement devices plays a minor role. During post annealing a complete reoxidization of samples is inhibited by an insufficient supply of oxygen.

Phase transitions via selective elemental vacancy engineering in complex oxide thin films

Defect engineering has brought about a unique level of control for Si-based semiconductors, leading to the optimization of various opto-electronic properties and devices. With regard to perovskite transition metal oxides, O vacancies have been a key ingredient in defect engineering, as they play a central role in determining the crystal field and consequent electronic structure, leading to important electronic and magnetic phase transitions. Therefore, experimental approaches toward understanding the role of defects in complex oxides have been largely limited to controlling O vacancies. In this study, we report on the selective formation of different types of elemental vacancies and their individual roles in determining the atomic and electronic structures of perovskite SrTiO3 (STO) homoepitaxial thin films fabricated by pulsed laser epitaxy. Structural and electronic transitions have been achieved via selective control of the Sr and O vacancy concentrations, respectively, indicating a decoupling between the two phase transitions. In particular, O vacancies were responsible for metal-insulator transitions, but did not influence the Sr vacancy induced cubic-to-tetragonal structural transition in epitaxial STO thin film. The independent control of multiple phase transitions in complex oxides by exploiting selective vacancy engineering opens up an unprecedented opportunity toward understanding and customizing complex oxide thin films.

Growth control of oxygen stoichiometry in homoepitaxial SrTiO3 films by pulsed laser epitaxy in high vacuum

In many transition metal oxides, oxygen stoichiometry is one of the most critical parameters that plays a key role in determining the structural, physical, optical, and electrochemical properties of the material. However, controlling the growth to obtain high quality single crystal films having the right oxygen stoichiometry, especially in a high vacuum environment, has been viewed as a challenge. In this work, we show that, through proper control of the plume kinetic energy, stoichiometric crystalline films can be synthesized without generating oxygen defects even in high vacuum. We use a model homoepitaxial system of SrTiO₃ (STO) thin films on single crystal STO substrates. Physical property measurements indicate that oxygen vacancy generation in high vacuum is strongly influenced by the energetics of the laser plume, and it can be controlled by proper laser beam delivery. Therefore, our finding not only provides essential insight into oxygen stoichiometry control in high vacuum for understanding the fundamental properties of STO-based thin films and heterostructures, but expands the utility of pulsed laser epitaxy of other materials as well.

Epitaxial stabilization and phase instability of VO2 polymorphs

The VO2 polymorphs, i.e., VO2(A), VO2(B), VO2(M1) and VO2(R), have a wide spectrum of functionalities useful for many potential applications in information and energy technologies. However, synthesis of phase pure materials, especially in thin film forms, has been a challenging task due to the fact that the VO2 polymorphs are closely related to each other in a thermodynamic framework. Here, we report epitaxial stabilization of the VO2 polymorphs to synthesize high quality single crystalline thin films and study the phase stability of these metastable materials. We selectively deposit all the phases on various perovskite substrates with different crystallographic orientations. By investigating the phase instability, phonon modes and transport behaviours, not only do we find distinctively contrasting physical properties of the VO2 polymorphs, but that the polymorphs can be on the verge of phase transitions when heated as low as ~400 °C. Our successful epitaxy of both VO2(A) and VO2(B) phases, which are rarely studied due to the lack of phase pure materials, will open the door to the fundamental studies of VO2 polymorphs for potential applications in advanced electronic and energy devices.

Surface Control of Epitaxial Manganite Films via Oxygen Pressure

The trend to reduce device dimensions demands increasing attention to atomic-scale details of structure of thin films as well as to pathways to control it. This is of special importance in the systems with multiple competing interactions. We have used in situ scanning tunneling microscopy to image surfaces of La5/8Ca3/8MnO3 films grown by pulsed laser deposition. The atomically resolved imaging was combined with in situ angle-resolved X-ray photoelectron spectroscopy. We find a strong effect of the background oxygen pressure during deposition on structural and chemical features of the film surface. Deposition at 50 mTorr of O2 leads to mixed-terminated film surfaces, with B-site (MnO2) termination being structurally imperfect at the atomic scale. A relatively small reduction of the oxygen pressure to 20 mTorr results in a dramatic change of the surface structure leading to a nearly perfectly ordered B-site terminated surface with only a small fraction of A-site (La,Ca)O termination. This is accompanied, however, by surface roughening at a mesoscopic length scale. The results suggest that oxygen has a strong link to the adatom mobility during growth. The effect of the oxygen pressure on dopant surface segregation is also pronounced: Ca surface segregation is decreased with oxygen pressure reduction.

Surface properties of atomically flat poly-crystalline SrTiO3

Comparison between single- and the poly-crystalline structures provides essential information on the role of long-range translational symmetry and grain boundaries. In particular, by comparing single- and poly-crystalline transition metal oxides (TMOs), one can study intriguing physical phenomena such as electronic and ionic conduction at the grain boundaries, phonon propagation, and various domain properties. In order to make an accurate comparison, however, both single- and poly-crystalline samples should have the same quality, e.g., stoichiometry, crystallinity, thickness, etc. Here, by studying the surface properties of atomically flat poly-crystalline SrTiO₃ (STO), we propose an approach to simultaneously fabricate both single- and poly-crystalline epitaxial TMO thin films on STO substrates. In order to grow TMOs epitaxially with atomic precision, an atomically flat, single-terminated surface of the substrate is a prerequisite. We first examined (100), (110), and (111) oriented single-crystalline STO surfaces, which required different annealing conditions to achieve atomically flat surfaces, depending on the surface energy. A poly-crystalline STO surface was then prepared at the optimum condition for which all the domains with different crystallographic orientations could be successfully flattened. Based on our atomically flat poly-crystalline STO substrates, we envision expansion of the studies regarding the TMO domains and grain boundaries.

Enhanced surface-and-interface coupling in Pd-nanoparticle-coated LaAlO3/SrTiO3 heterostructures: strong gas- and photo-induced conductance modulation

Pd nanoparticle (NP) coated LaAlO3/SrTiO3 (LAO/STO) heterointerface exhibits more notable conductance (G) change while varying the ambient gas (N2, H2/N2, and O2) and illuminating with UV light (wavelength: 365 nm) than a sample without the NPs. Simultaneous Kelvin probe force microscopy and transport measurements reveal close relationships between the surface work function (W) and G of the samples. Quantitative analyses suggest that a surface adsorption/desorption-mediated reaction and redox, resulting in a band-alignment modification and charge-transfer, could explain the gas- and photo-induced conductance modulation at the LAO/STO interface. Such surface-and-interface coupling enhanced by catalytic Pd NPs is a unique feature, quite distinct from conventional semiconductor hetero-junctions, which enables the significant conductance tunability at ultrathin oxide heterointerfaces by external stimuli.

Tunneling electroresistance induced by interfacial phase transitions in ultrathin oxide heterostructures

The ferroelectric (FE) control of electronic transport is one of the emerging technologies in oxide heterostructures. Many previous studies in FE tunnel junctions (FTJs) exploited 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 further enhanced by the electrostatic modification in the correlated electron oxide electrodes, and that FTJs with nanometer thin layers can effectively produce a considerably large electroresistance ratio at room temperature. To understand these surprising results, we employed an additional control parameter, which is related to the crossing of electronic and magnetic phase boundaries of the correlated electron oxide. The FE-induced phase modulation at the heterointerface ultimately results in an enhanced electroresistance effect. Our study highlights that the strong coupling between degrees of freedom across heterointerfaces could yield versatile and novel applications in oxide electronics.