Session B41: Oxide Thin Films, Surfaces, and Interfaces

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In high-power electronic applications, electric contacts between components have a strong impact on device efficiency. Predicting the electronic properties at these contacts could greatly facilitate device design and fabrication but such predictions have remained challenging. In this work, plausible interfacial structures between Ga2O3 and different metals and oxides are found using structure matching. The local density of state of these structures is then calculated in order to identify interfacial states in the band gap. From this analysis, the type of electrical contact (ohmic or schottky) at the interface and other important electronic properties can be predicted. This methodology provides a potential pathway to theory guided high-power electronic device engineering.   

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<div style="direction: ltr;">The ongoing search for earth-abundant transparent conducting oxides (TCOs) has spread across a wide variety of research fields, highlighting the correlated metal SrVO₃ as a potential candidate. SrVO₃ exhibits high conductivity and transparency in the visible. Moreover, SrVO₃ is an attractive candidate for future electronics. A major hurdle for the synthesis of high quality SrVO₃ is to mitigate the formation of defects throughout the thin film. These defects can cause electron scattering and result in high resistivity and obscure some of the interesting physics. Film growth was done with molecular beam epitaxy (MBE), providing a scalable and industry-compatible fabrication process. In this work we present low defect SrVO₃ films with residual resistivity ratios exceeding 15 and room temperature resistivities in the order of 30 µΩ cm. Careful analysis of the structural and electronic properties of the SrVO₃ films paves the way towards further improvement of their quality and their implementation as TCO in optoelectronics and renewable energy devices.</div>   

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The choice of electrostatic gating over the conventional chemical doping is attributed to the fact that the former can reversibly tune the carrier density without affecting the system’s level of disorder. However, this proposition seems to break down in field-effect transistors involving SrTiO₃ (STO) based two-dimensional electron gases. Such peculiar behavior is associated with the electron trapping under the external electric field. However, the microscopic nature of trapping centers remains an open question. Our work on the conducting interface between defect spinel oxide γ- Al₂O₃ and cubic perovskite STO reveals that the charge trapping under +ve back gate voltage (V_g) above the tetragonal to cubic structural transition temperature (T_c) of STO is contributed by the electric field-assisted thermal escape of electrons from the quantum well, and the clustering of oxygen vacancies (OVs). Interestingly, an additional source of trapping emerges below T_c, which arises from the trapping of free carriers at the ferroelastic twin walls of STO. Application of -ve V_g results in a charge detrapping from the twin wall, which also vanishes above T_c. The amount of trapped/detrapped charges at the twin wall is controlled by the net polarity of the wall and scales almost linearly with V_g.   

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LaVO₃ (LVO) has been proposed as a promising material for photovoltaics because its strongly correlated 3d electrons can facilitate the creation of multiple electron-hole pairs per incoming photon, which would lead to increased device efficiency. Our group grows thin films of LVO on SrTiO₃ substrates using pulsed laser deposition. We control La:V ratio of the films from ~60:40 to ~40:60 by adjusting laser fluences. We find that while V-rich films show behaviors that are similar to bulk LVO, films that are La-rich show remarkable differences in optical measurements, and more rich temperature-dependent transport behaviors, which indicates the presence of electronic phase separation. This study allows us to better understand the complex physical properties of strongly correlated insulators paving the way for their use as absorbers in high-performance photovoltaics.   

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Renewable energy catalysis technologies are governed by oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). An electrocatalyst is desirable to carry out OER and ORR efficiently for energy production on a large scale without expensive metals such as Pt or Ir. Ideally, the electrocatalyst should have high earth abundance, low cost and high stability. Recent studies have shown metal oxides are efficient catalysts for OER and ORR and are less expensive. We synthesized perovskites LaNiO₃, LaFeO₃, and LaFeO₃/LaNiO₃ thin films to study the catalytic properties of these materials. These oxide thin films were grown using molecular beam epitaxy. In-situ XPS measurements were performed to ensure film stoichiometry and measure electronic properties. Cyclic voltammetry and electrochemical impedance spectroscopy were performed to study OER catalysis with these materials. Scanning transmission electron microscopy images have shown high stability of LaFeO₃ during electrocatalysis. We further studied band alignment at the LaFeO₃/LaNiO₃ interface using XPS as well as using density functional theory computations to understand the interface and its impact on applicability of these materials as catalysts in future energy technologies.   

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Bismuth vanadate (BiVO₄) has many advantageous optoelectronic properties for water splitting that make it ideal for studying interfacial energetics. We present an integrated experimental and computational study aimed at an atomistic understanding of the interaction with water on the BiVO₄ (010) surface while varying the surface termination.

We performed first-principles calculations with Quantum Espresso (https://www.quantum-espresso.org/) and used STM, XPS, and resonant PES to elucidate the microscopic effects of varying the surface termination. Based on measured and simulated STM images, we demonstrate how surface termination may be varied to tune the photoelectrochemical performance [1]. Next, we explore how adsorbed water influences the formation of polarons using resPES. We identify structural moieties with adsorbed water that lead to the observed enhancement in the polaron signal. Finally, we compare the changes in surface energetics upon immersion in water of the BiVO₄ (010) surface with different surface terminations.

[1] D. Lee, W. Wang, C, Zhou, X. Tong, M. Liu, G. Galli, K.-S. Choi. “Modifying interfacial energetics in BiVO₄ photoanodes by surface termination.” Submitted.   

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The relationship between the sheet carrier concentration, ns, of LaCrO3(LCO)/SrTiO3(STO) heterostructures and their structure has been investigated. Under low oxygen partial pressure, the STO substrate is reduced during growth as evidenced by a high ns of 10¹⁶ cm⁻². By controlling the post-growth annealing conditions, heterostructures with ns of 10¹³–10¹⁶ cm⁻² are achieved. The atomic-scale structure of the samples is obtained using high-resolution synchrotron x-ray diffraction measurements. For heterostructures with ns at or below 3 × 10¹³ cm⁻², polar distortions are present within the LCO layers and increase in magnitude with a decrease in sheet carrier concentration. These distortions are absent for samples with ns on the order of 10¹⁵–10¹⁶ cm⁻² where interfacial carriers play a role in alleviating the polar discontinuity at the LCO/STO interface. These results suggest that interfacial charge carriers and polar distortions can act as complementary mechanisms to alleviate the polar discontinuity at polar/non-polar complex oxide interfaces.   

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Complex oxides have long been known as hosts for many fascinating phenomena such as superconductivity and colossal magnetoresistance. Recently, the successful synthesis of freestanding thin films of complex oxides opened up new ways of probing and controling these materials. In this work, by using ultrafast electron diffraction (UED), we reconstruct the real-space wrinkling dynamics due to a photoinduced coherent acoustic phonon in La2/3Ca1/3MnO3 freestanding thin films. From the wrinkling in the freestanding thin film, we further extract information on thin film morphology as well as strain properties with picosecond time resolution.   

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Recent investigations on the spinels CoMn₂O₄ and MnCo₂O₄ have shown their potential for applications in water splitting and fuel cell technologies as they exhibit strong catalytic behavior through oxygen reduction reactivity. To further understand these materials, cobalt-manganese spinels were synthesized as thin films using molecular beam epitaxy and characterized using experimental analyses. The Co_xMn_3-xO₄ samples vary in composition between Co₃O₄ and Mn₃O₄ to establish links between cation stoichiometry and material properties. Experiments that include in-situ x-ray photoelectron spectroscopy, x-ray diffraction, x-ray absorption spectroscopy, and spectroscopic ellipsometry reveal information on properties such as cation valence states, bond lengths and coordination, as well as electronic structure with references to density-functional theory models. The findings provide insights into the entire cobalt-manganese spinel system and related spinel oxides that are promising candidates for inexpensive oxygen reduction reaction catalysts.   

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The molecular beam epitaxy technique allows us to synthesise atomically smooth single-crystal thin films, the study of which triggers progress in condensed-matter physics and materials science. We present a method of growth of atomically perfect thin films of La_2-xSr_xCuO₄ (LSCO), the archetypical high-temperature copper oxide superconductor, using a custom-built molecular beam epitaxy system. We first describe the steps prior to the film growth, such as substrate preparation and calibration of fluxes from elemental sources. We then describe the atomic-layer-by-layer growth mode and the post-annealing procedures appropriate for different doping levels. We further illustrate and discuss the strategy to monitor the film growth and maintain the correct stoichiometry, based on real-time feedback from the Reflection High Energy Electron Diffraction (RHEED). The atomic force microscopy shows that the films are atomically smooth without any secondary phase precipitates. Magnetic measurements show that T_c is homogeneous to within 0.1 K over the entire film area.   

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The oxide perovskite BaTiO₃ is a prototypical displacive ferroelectric material with high-performance ferroelectric/piezoelectric properties. The structural simplicity of BaTiO₃ makes it possible to integrate it with other functional materials in a single crystalline form, such as with superconducting cuprates and semiconducting Si, MgO, and Ge, providing an opportunity to electrically control functional properties of adjacent layers. The synthesis of high-quality BaTiO₃ as thin films at low temperatures may allow the integration of ferroelectrics with other materials and devices that require a reduced thermal budget. We describe our synthesis of BaTiO₃ thin films by molecular beam epitaxy at various temperatures and find that ferroelectric epitaxial BaTiO₃ can be grown at temperatures as low as ~310 °C. Using reflection high energy diffraction, we demonstrate surface mobility of BaO and TiO₂ adatoms that are high enough to promote ferroelectric crystal growth at low temperatures.   

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The recent discovery of superconductivity in Sr-doped NdNiO₂ films grown on STO started a novel field within unconventional superconductivity. Our results revealed a strong lattice reconstruction in the case of NNO/STO and CCO/STO, resulting in a polar film. To avoid the polar catastrophe, the NiO₂/ CuO₂ surface to the vacuum reconstructs as well. Furthermore, we also observed a 2D electron gas at the interface between NNO/CCO and STO, caused by the polar discontinuity. For NNO/STO the two-dimensional electron gas extends over several layers, while for CCO/STO this electronic rearrangement is very localized at the CCO/STO interface. The electronic reconstruction found at the interface involves a strong occupation of the Ti 3d_xy state. In both cases, there is a significant electronic charge transfer from the surface Ni/Cu layers to the Ti interface layer. By introducing magnetism and electronic correlation, we observed that the d_3z2 orbital of Ni becomes itinerant while the same orbital for Cu remains doubly occupied, establishing a clear two- vs one-orbital active framework for the description of these systems.

[1] Y. Zhang, L. F. Lin, W.-J. Hu, A. Moreo, S. Dong and E. Dagotto (to appear in PRB).   

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LaNiO₃ films have triggered enormous interest owing to its paramagnetic and metallic behavior [1] as well as the recent discovery of superconductivity in RNiO₃-based heterostructures. The quality of the deposited film is known to depend strongly on the structure and composition of the single crystal substrate surface. Here, exploiting in situ synchrotron X-ray investigation during thin film growth by molecular beam epitaxy, we study and understand the growth behavior of LaNiO₃ films on (LaAlO₃)(Sr₂AlTaO₆) (001) substrates with different surface compositions, in combination with resonant anomalous diffraction measurements [2]. We find that the fabrication of atomically smooth, high quality and phase pure epitaxial LaNiO₃ films can be achieved on LSAT substrates, irrespective of initial substrate surface composition and layer-by-layer deposition sequence, illustrating that dynamic rearrangement of layers occurs during the growth of complex oxides on top of mixed-terminated substrates. This has important implications regarding the use of a wider variety of substrates for fundamental studies on complex oxide systems.   

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We find evidence for a surface to bulk core level shift in both the Co 2p and Fe 2p core level photoemission spectra of 5 nm CoFe₂O₄ (111) film grown on Al₂O₃ (0001). The Co 2p_3/2 and Fe 2p_3/2 surface components were distinguished from the bulk components by angle resolved X-ray photoemission spectroscopy (ARXPS). While many complex oxides show a strong preferential surface termination, the surface termination of CoFe₂O₄ (111) contains both Co and Fe, but the core level photoemission binding energy shifts tend to indicate that the surface is significantly different from the bulk, even for so thin a film. Furthermore, X-ray photoemission spectroscopy (XPS) measurement of 1.5 nm CoFe₂O₄ (111) film grown on Al₂O₃ (0001) shows a suboxide interlayer of cobalt. This has implications for cobalt ferrite as a magnetic storage media and could affect growth of this spinel in the degradation of steel in boiling water nuclear power applications.   

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The last two decades have seen enormous growth in the field of nanoengineering and nanomechanics using thin sheets owing to the variety of 2D materials available to us (2D Mater. 5, 032005 (2018)). With new advances in thin film growth techniques (Nat. Mater. 15, 1255–1260 (2016)), a new class of functional oxide thin films which are freestanding can be produced and readily incorporated in such nanomechanical implementations. However functional oxides are rigid and brittle in bulk, so it becomes imperative to verify the feasibility of nanomechanical deformations by investigating the breaking strength of these thin films. Here we report measurements of the ultimate strength of SrTiO₃ thin films using an atomic force microscope (AFM) by impinging upon a freestanding drumhead with an AFM tip. We demonstrate that in the sub-20 nm thickness regime of these thin films, SrTiO₃ can withstand an elastic extension of ~ 6% which is more than an order of magnitude higher than that for bulk. Furthermore, we also show that the fracture point of these films with respect to applied force is robust thus demonstrating their potential for use in nanomechanical platforms and devices.