Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B13: Complex Oxide Heterostructures - Synthesis Techniques and Strain Effects |
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Sponsoring Units: DMP Chair: Divine Kumah, North Carolina State University Room: LACC 304A |
Monday, March 5, 2018 11:15AM - 11:27AM |
B13.00001: Extreme strain states in complex oxide membranes Seung Sae Hong, Di Lu, Arturas Vailionis, Yasuyuki Hikita, Harold Hwang Freestanding 2D layers provide an unprecedented degree of freedom to control physical properties. The recent advances in the thin film epitaxy of a water-soluble layer enable the synthesis of virtually all types of complex oxides as a macroscale freestanding membrane [1]. Based on this synthetic method, we have developed an on-chip platform of complex oxide membranes for which strain states can be continuously tuned by design. In the example of a colossal magnetoresistance material (La0.7Ca0.3MnO3), we successfully applied biaxial strain up to 5% and induced a dramatic shift in the metal-to-insulator transition in ultrathin manganite membranes. This versatile platform - compatible with magnetotransport, structural characterization, and optical probes - enables a new avenue to study oxide thin films and heterostructures beyond epitaxial strain. |
Monday, March 5, 2018 11:27AM - 11:39AM |
B13.00002: Coherent growth of oxide films on cleaved layered metal oxide substrate Prahald Siwakoti, Hangwen Guo, Zhen Wang, Zhiqiang Mao, Jiandi Zhang Understanding interface-induced effects requires controlled epitaxial growth of thin films on atomically smooth substrate surfaces. While conventional thin film growth on commercial substrates has proven to be a successful route, the appropriate substrate choices are limited and not all of them have well-defined surfaces for the epitaxy. Here we present an alternative method of growing oxide thin films on single-crystal substrates which will greatly increase the availability of substrates for future oxide films growth. La0.67Sr0.33MnO3 thin films were deposited on a cleaved Sr2RuO4 single crystal as a prototype system. Cleaving Sr2RuO4 in ultra-high vacuum along the [001] direction exposes clean atomically flat, SrO layer terminated surface that extends up to a few micrometers. The surface of the substrate has (√2×√2)R45° reconstruction, but the film maintains (1×1) structure, as determined by the low energy electron diffraction. Atomically sharp interface was probed by scanning tunneling electron microscopy. The surface chemical composition and the film stoichiometry were confirmed by angle-dependent X-ray photoelectron spectroscopy. These systems can serve as a tool to explore proximity effects between superconductors and ferromagnets. |
Monday, March 5, 2018 11:39AM - 11:51AM |
B13.00003: Emergent Phases at Oxide Interfaces in LaCoO3-based Heterostructures Sangjae Lee, Ankit Disa, Alexandru Georgescu, Gilberto Fabbris, Yichen Jia, Mark Dean, Sohrab Ismail-Beigi, Frederick Walker, Charles Ahn The interplay of orbital, spin, and charge states in transition metal oxides creates a rich phase space, displaying a wide range of novel phenomena, from magnetism to superconductivity. Incorporating TMOs into epitaxial heterostructures expands the space of new phenomena possible due to dimensional confinement and interfacial coupling. For cobaltates, orbital filling and spin configuration can display a number of different patterns based on an interplay between crystal field, exchange, and Hund’s energies. These energies can be tuned by combining the perovskite cobaltate LaCoO3 with other oxide perovskites, such as LaTiO3, in (LaCoO3)n/(LaTiO3)n heterostructures grown by molecular beam epitaxy. Interfacial charge transfer from Ti to Co in the heterostructure leads to significant modifications of the cobaltate unit cell structure, which allows the tuning of orbital polarization in the cobaltates. We investigate the varying degree of charge transfer and orbital polarization in these superlattices by controlling layer thickness, n, and by using x-ray absorption spectroscopy(XAS) and resonant inelastic x-ray scattering(RIXS) to characterize electronic properties. This study demonstrates an approach to systematically tune the orbital and charge states in LaCoO3-based heterostructures. |
Monday, March 5, 2018 11:51AM - 12:03PM |
B13.00004: Mobility Optimization in High-Pressure-Oxygen-Sputtered
Epitaxial Ba1-xLaxSnO3 Thin Films William Postiglione, Koustav Ganguly, Hwanhui Yun, Jong Seok Jeong, Andre Mkhoyan, Bharat Jalan, Chris Leighton Due to its high 300 K mobility and wide gap, perovskite BaSnO3 is of high interest for oxide electronics. Despite rapid progress, many questions remain regarding transport behavior, particularly in films, where defects limit mobility. While many publications investigating epitaxial BaSnO3 to date have focused on PLD or MBE grown films, here we focus on an industrially relevant, scalable technique: high pressure oxygen sputtering. In this work, single-phase, stoichiometric, smooth, epitaxial films of Ba0.98La0.02SnO3 have been grown, and the impact of thickness, growth rate, deposition temperature, and substrate on mobility systematically studied. 300 K mobilities up to 70 cm2V-1s-1 at 2 ×1020 cm-3 are obtained, even in 380-Å-thick unbuffered films. Important trends include a monotonic increase in mobility with deposition temperature, reduced mobility and carrier freeze-out at low thickness, optimal mobility around 1200 Å thickness, and insensitivity to the substrate mismatch. These are discussed in terms of accompanying structural characterization data, particularly transmission electron microscopy. |
Monday, March 5, 2018 12:03PM - 12:15PM |
B13.00005: Temperature and frequency dependent dielectric response in hybrid molecular beam epitaxy grown BaSnO3 films William Nunn, Abhinav Prakash, Ryan Haislmaier, Jin Yue, Bharat Jalan BaSnO3 has recently gained significant attention as a promising candidate for transparent conducting and power electronic applications due to its high room temperature mobility and large carrier density when doped n-type. However, to date, no reports of dielectric properties for thin film BaSnO3 have been produced. High quality, phase pure, epitaxial BaSnO3 films were grown on Nb-doped SrTiO3 (001) using a hybrid molecular beam epitaxy approach. A metal-insulator-metal capacitor structure was then fabricated using Pt top electrodes, and dielectric response was recorded using impedance spectroscopy. Dielectric constant and loss tangent were found to be 15 and 0.005, respectively, at 100 kHz and room temperature in 40-nm-thick films. Thickness was then varied to investigate the effect of dimensionality on dielectric properties as well as to determine the bulk-like response by isolating interface effects. We will discuss the dielectric constant and loss in BaSnO3 films as a function of temperature, frequency, film thickness, and stoichiometry, as well as the effect of introducing Ti to create the BaSnO3-BaTiO3 solid solution. |
Monday, March 5, 2018 12:15PM - 12:27PM |
B13.00006: Theoretical Investigation on Lanthanide Ad-atoms on Wurzite GaN Donghan Shin, Tobias Hadamek, Alexander Demkov Several oxides including rare earth oxide (REO) such as Gd2O3, have been used to passivate the GaN surface as well as to serve as the gate oxide in metal-oxide-semiconductor (MOS) applications. One of the problems in heteroepitaxy with REO on GaN is poor wetting related to the interface and film surface energy balance. The use of Zintl and Zintl-like intermetallic compounds can be a possible solution to overcome poor wetting. Using density functional theory, we investigate electronic structures of bare (0001)-oriented surface of wurzite GaN and lanthanide Eu and Gd ad-atoms on it. We also explore the possibility of forming a Zintl-like transition layer, EuGa2. Our results provide the microscopic understanding of the intermetallic layer formation and its role in the growth of REO on GaN. We validate our theoretical results through a comparison with the in-situ x-ray photoelectron spectroscopy data. |
Monday, March 5, 2018 12:27PM - 12:39PM |
B13.00007: Electrochemical Stabilities of Ni-Based Compounds from Bulk to Nanoscale Dimensions Liang-Feng Huang, James Rondinelli Ni-based compounds (e.g., oxides, hydroxides, oxyhydroxides) are important materials in various fields (e.g., energy, catalysis, corrosion, and oxidation), and exhibit spatial dimensions ranging from bulk down to the nanoscale. The electrochemical stability is a critical factor determining applications of Ni-based compounds in aqueous conditions, and can be described by phase diagrams consisting of electrode potential and solution pH, i.e., Pourbaix diagrams. Using density functional theory, we calculated the thermodynamic energies of bulk and nanoscale Ni-based compounds and examine the effects of water adsorption and substrate adhesion. The Pourbaix diagrams simulated using these thermodynamic energies are consistent with various direct electrochemical observations as well as recent electrochemical measurements by our experimental collaborators (the Scully Group at University of Virginia). This work both quantitatively reveals the roles of many factors in nanoscale electrochemistry and develops a first-principles approach for studying material electrochemistry at various conditions. |
Monday, March 5, 2018 12:39PM - 12:51PM |
B13.00008: Metal sub-oxide epitaxy on SrTiO3 via substrate interfacial oxygen scavenging Agham Posadas, Alexander Demkov SrTiO3 is a widely used substrate for the growth of other functional oxide thin films. However, it suffers from a relative ease in oxygen loss during thin film deposition making interfacial phenomena between SrTiO3 and other oxides more complicated to interpret. We utilize this easy reducibility of SrTiO3 to fabricate metal suboxides without the need to precisely control the oxygen partial pressure during growth, removing the need to worry about underoxidizing or overoxidizing the film. We have previously demonstrated this approach for the ferromagnetic semiconductor EuO. We extend this new, facile method of suboxide growth by growing epitaxial layers of oxides of the metal-insulator transition materials V2O3, VO2, and NbO2 in the absence of oxygen gas, with the oxidation state completely controlled only by temperature and metal flux. All of the oxygen needed to form the metal oxide is taken from the substrate resulting in an oxygen-deficient layer beneath the metal-oxide, forming a natural bottom electrode for the film for electrical measurements. We describe the growth process and structural characterization of the films in detail and show that this new approach is potentially useful for obtaining crystalline layers of intermediate oxidation states of metals with relative ease. |
Monday, March 5, 2018 12:51PM - 1:03PM |
B13.00009: Oxygen scavenging: Eu on SrTiO3 (001) Wei Guo, Agham Posadas, Alexander Demkov EuO is a promising material for spintronic devices due to its strong magnetism (7 µB per Eu 2+) and 100% spin polarization of the conduction band. However, growing stoichiometric, high quality EuO on substrates such as buffered -Si is difficult because of high stability of the competing Eu2O3 phase. We grow EuO/SrTiO3/Si structures by molecular beam deposition of metallic Eu in UHV on SrTiO3/Si without providing additional oxygen. We analyze the composition by x-ray photoelectron spectroscopy. In direct deposition of Eu on SrTiO3 (STO), oxygen is scavenged from the substrate to form EuO. This leaves behind a conductive oxygen deficient SrTiO3-δ layer. We investigate the growth mechanism by varying the STO thicknesses and temperature of Eu deposition. We find that single crystal EuO forms on thick STO leaving a SrTiO3-δ layer of ~4nm. If the STO layer is comparable to the scavenging depth at a given temperature, a solid state reaction occurs. On the other hand, at low temperature only a 1-2 nm-thick EuO layer forms, on top of which Eu metal is stable. |
Monday, March 5, 2018 1:03PM - 1:15PM |
B13.00010: Eu2O3 hexagonal and monoclinic phase on GaN (0001) by MBE Tobias Hadamek, Donghan Shin, Agham Posadas, Alexander Demkov, Sunah Kwon, Qingxiao Wang, Moon Kim One major issue preventing large scale manufacturing of GaN-based MOS devices is the lack of a high quality, thermally and chemically stable gate dielectric. Many different insulators have been grown for that purpose on GaN (0001). Here we report on the epitaxial growth of the “high-pressure” hexagonal phase of Eu2O3 on C-plane GaN (0001) by molecular beam epitaxy. A structural phase transition from the hexagonal to the monoclinic phase was observed with increasing film thickness by ex-situ X-ray diffraction 2theta-theta scans and reciprocal space maps. A conduction band offset of 0.8 eV between GaN and Eu2O3 was measured by XPS making Eu2O3 feasible as a gate dielectric for a high electron mobility n-type channel GaN-based field effect transistor. |
Monday, March 5, 2018 1:15PM - 1:27PM |
B13.00011: Interaction of Stem Cells with Metal Oxide Thin Films Wilfrid Prellier, Mariya Khokhlova Stem cells are extensively studied due to their unique properties and great potential in biomedical applications. Thanks to a variety of biological responses, metal-oxide coatings are widely used for interacting with stem cells. A very limited number of oxides have however been studied, and mechanism of the cell-material interaction is still not completely clear. |
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