Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S37a: Complex Oxide Interfaces and Heterostructures - Defects at Oxide InterfacesFocus
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Sponsoring Units: DMP DCMP Chair: James Rondinelli, Northwestern University Room: 383 |
Thursday, March 16, 2017 11:15AM - 11:27AM |
S37a.00001: Surface core level shifts of germanium in the presence of alkaline-earth metals Ali Hamze, Shen Hu, Edward Lin, Agham Posadas, HsinWei Wu, David Smith, John Ekerdt, Alex Demkov The integration of perovskites on semiconductors for use as gate oxides in field-effect transistors has been of interest due to their high dielectric constants. Since it was found that .5 monolayer (ML) of strontium on silicon allows for the epitaxial growth of perovskites on silicon, many studies of the growth of perovskites have been carried out on silicon and other semiconductors, such as germanium and gallium arsenide. In this work, we investigate the surface core level shifts (SCLSs) of the germanium 3d core level in the presence of .5 ML of the alkaline-earth metals (AEMs) barium and strontium using density functional theory. Results found using the local density approximation and generalized gradient approximation to the exchange-correlation energy, different valence electron configurations, and for various slab surface areas are compared to each other and to x-ray photoemission spectroscopy (XPS) data. We find the germanium surface dimers flatten in the presence of .5 ML of AEMs, forming a Zintl layer, in agreement with experiment. We also find the surface core levels shift towards lower binding energy, and that the shift is the same for all the surface atoms, in qualitative agreement with the XPS data. [Preview Abstract] |
Thursday, March 16, 2017 11:27AM - 11:39AM |
S37a.00002: Structures, Properties and Defects of SrTiO$_3$/GaAs Hetero-interfaces Liang Hong, Kunal Bhatnagar, Ravi Droopad, Serdar \"{O}\u{g}\"{u}t, Robert Klie SrTiO$_3$ thin film can be epitaxially grown on GaAs substrate and used as a platform for growing other oxides to create functional metal-oxide-semiconductor devices, where a high-quality SrTiO$_3$/GaAs interface is essential. We studied the structural and electronic properties of SrTiO$_3$/GaAs hetero-interfaces at atomic level using scanning transmission electron microscopy and first-principles calculations. Our results suggest the preferred termination of GaAs (001) is significantly dependent on the oxygen concentration in the first oxide layer. The favorable interface structure is characterized as oxygen-deficient SrO in contact with arsenic and is observed in both experiment and simulation. The electronic properties are calculated and found to be tunable by interfacial defects such as oxygen, gallium and arsenic vacancies. [Preview Abstract] |
Thursday, March 16, 2017 11:39AM - 11:51AM |
S37a.00003: Designing antiphase domain boundary by atomic controlled heterointerfaces Hangwen Guo, Zhen Wang, Mohammad Saghayezhian, Jing Tao, Antonio Vecchione, Yimei Zhu, Jiandi Zhang, E. Ward Plummer Domain boundaries are one of the most commonly observed phenomena in crystal and thin film growth. They often show random formation by nature but can have large impact on physical properties. So far, there are very limited examples to exhibit designable domain boundary arrangement. In this work, we employed a methodology to control the nucleation and growth of antiphase boundary (APB), by growing thin film oxides on top of freshly cleaved layered compound. At the cleaved surface, the step of two adjunct terraces serves a natural seeding bed to nucleate APB. Utilizing high resolution scanning transmission electron microscopy (STEM), we directly visualized that APBs can merge into pyramid-like shape when two steps are close. Our observation opens up a new route to design and control domain boundaries in thin film transition metal oxides. [Preview Abstract] |
Thursday, March 16, 2017 11:51AM - 12:03PM |
S37a.00004: Thermodynamic and segregation analysis of oxygen vacancies in the Σ5 [001] CSL SrTiO3 twist grain boundary from first-principles Maziar Behtash, Kesong Yang We studied the sigma-5 (001) SrTiO3 (STO) twist grain boundary (GB) using first-principles DFT calculations. We modeled three types of GB terminations, SrO-SrO (SS), SrO-TiO2 (ST), & TiO2-TiO2 (TT), and studied their thermodynamic stability. We found that the SS and ST GB terminations are more energetically favorable than the TT GB termination. Oxygen vacancy formation in the GB structures was found to be more favorable than in bulk STO. Oxygen vacancy segregation energies were also calculated in the SS and ST GBs, to evaluate the preferred vacancy positions in these systems. In the SS system, oxygen vacancies exhibited a strong tendency to segregate to the SrO layers at the GB, rather than the grain interior. The reverse behavior was found in the ST system, where oxygen vacancies exhibited a strong tendency to remain within the TiO2 layers of the grain rather than segregate to the GB. To compare oxygen vacancy formation energies for each system on the same scale, we also calculated absolute oxygen vacancy formation energies for various vacancy positions in the SS and ST GB structures, along with that of bulk STO. Our results revealed that oxygen vacancy formation in both the SS and ST twist GB structures is substantially easier than in bulk STO. [Preview Abstract] |
Thursday, March 16, 2017 12:03PM - 12:15PM |
S37a.00005: Solid-gate control of insulator to 2D metal transition at SrTiO3 surface. Alejandro Schulman, Pablo Stoliar, Ai Kitoh, Marcelo Rozenberg, Isao H. Inoue As miniaturization of the semiconductor transistor approaches its limit, semiconductor industries are facing a major challenge to extend information processing beyond what can be attainable by conventional Si-based transistors. Innovative combinations of new materials and new processing platforms are desired. Recent discovery of the 2D electron gas (2DEG) at the surface of SrTiO3 (STO) and its electrostatic control, have carried it to the top of promising materials to be utilized in innovative devices. We report an electrostatic control of the carrier density of the 2DEG formed at the channel of bilayer-gated STO field-effect devices. By applying a gate electric field at room temperature, its highly insulating channel exhibits a transition to metallic one. This transition is accompanied by non-monotonic voltage-gain transfer characteristic with both negative and positive slope regions and unexpected enhancement of the sheet carrier density. We will introduce a numerical model to rationalize the observed features in terms of the established physics of field-effect transistors and the physics of percolation. Furthermore, we have found a clear signature of a Kondo effect that arises due to the interaction between the dilute 2DEG and localized Ti 3d orbitals originated by oxygen vacancies near the channel. [Preview Abstract] |
Thursday, March 16, 2017 12:15PM - 12:51PM |
S37a.00006: TBD - Complex Oxide Interfaces and Heterostructures Invited Speaker: Ulrike Diebold |
Thursday, March 16, 2017 12:51PM - 1:03PM |
S37a.00007: Stability and electronic structure of GdTiO$_{\mathrm{3}}$ surfaces Karthik Krishnaswamy, Anderson Janotti, Lars Bjaalie, Chris G. Van de Walle GdTiO$_{\mathrm{3}}$ (GTO) is a Mott insulator with the Ti atoms in a $+$3 oxidation state. Similar to the extensively studied LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ (LAO/STO), the GTO/STO (001) interface also exhibits a polar discontinuity that results in the formation of a high-density two-dimensional electron gas (2DEG). However, in contrast to the LAO/STO system, where the LAO surface strongly influences the 2DEG properties and causes an LAO-thickness dependence of the 2DEG density; the surface termination of GTO does not affect the 2DEG properties at the GTO/STO interface. Using first-principles calculations, we examine the surface properties of LAO and GTO, and determine their impact on the 2DEG properties. We find that the energetic positions of surface states in LAO and GTO explain the differences in their impact on the 2DEG. In addition, we find that the polar TiO$_{\mathrm{2}}$ surface termination of GTO is very stable, with a low surface energy, even compared to a hydrogenated surface. The TiO$_{\mathrm{2}}^{\mathrm{\thinspace }}$termination is insulating, while the GdO termination is found to be metallic. Our results shed light on the peculiar behavior of the polar surfaces of Mott insulators in general. [Preview Abstract] |
Thursday, March 16, 2017 1:03PM - 1:15PM |
S37a.00008: Scavenging of oxygen from SrTiO$_3$ by metals and its implications for oxide thin film deposition Agham Posadas, Kristy Kormondy, Wei Guo, Patrick Ponath, Jacqueline Kremer, Tobias Hadamek, Alexander Demkov SrTiO$_3$ is a widely used substrate for the growth of other functional oxide thin films. However, SrTiO$_3$ loses oxygen very easily during oxide thin film deposition even under relatively high oxygen pressures. In some cases, there will be an interfacial layer of oxygen-deficient SrTiO$_3$ formed at the interface with the deposited oxide film, depending on the metals present in the film. By depositing a variety of metals layer by layer and measuring the evolution of the core level spectra of both the deposited metal and SrTiO$_3$ using x-ray photoelectron spectroscopy, we show that there are three distinct types of behavior that occur for thin metal films on SrTiO$_3$. We discuss the implications of these types of behavior for the growth of complex oxide thin films on SrTiO$_3$, and which oxide thin films are expected to produce an interfacial oxygen-deficient layer depending on their elemental constituents. [Preview Abstract] |
Thursday, March 16, 2017 1:15PM - 1:27PM |
S37a.00009: Phase inversion and frequency doubling of reflection high-energy electron diffraction intensity oscillations in the layer-by-layer growth of complex oxides Zhangwen Mao, Wei Guo, Dianxiang Ji, Tianwei Zhang, Chenyi Gu, Chao Tang, Zhengbin Gu, Yuefeng Nie*, Xiaoqing Pan \textit{In situ} reflection high-energy electron diffraction (RHEED) and its intensity oscillations are extremely important for the growth of epitaxial thin films with atomic precision. The RHEED intensity oscillations of complex oxides are, however, rather complicated and a general model is still lacking. Here, we report the unusual phase inversion and frequency doubling of RHEED intensity oscillations observed in the layer-by-layer growth of SrTiO$_{\mathrm{3}}$ using oxide molecular beam epitaxy. In contacts to the common understanding that the maximum(minimum) intensity occurs at SrO(TiO$_{\mathrm{2}})$ termination, respectively, we found that both maximum or minimum intensities can occur at SrO, TiO$_{\mathrm{2}}$, or even incomplete terminations depending on the incident angle of the electron beam, which raises a fundamental question if one can rely on the RHEED intensity oscillations to precisely control the growth of thin films. A general model including surface roughness and termination dependent mean inner potential qualitatively explains the observed phenomena, and provides the answer to the question how to prepare atomically and chemically precise surface/interfaces using RHEED oscillations for complex oxides. [Preview Abstract] |
Thursday, March 16, 2017 1:27PM - 1:39PM |
S37a.00010: Atomic-scale characterization of oxygen vacancy ordering in La$_{0.5}$Sr$_{0.5}$CoO$_{3-\delta \, }$thin films on SrTiO$_{3}$ via in-situ cooling experiments Xue Rui, Jeff Walter, Chris Leighton, Robert Klie La$_{1-x}$Sr$_{x}$CoO$_{3-\delta }$ thin films have been studied extensively due to their rich magnetic phase behavior and interesting interplay between strain, defects, and magnetism. At x $=$ 0, ferromagnetic order is observed under tensile strain (in sharp contrast to bulk), whereas at higher x strain can stabilize oxygen vacancy ordering, with profound effects on transport and magnetism. For La$_{0.5}$Sr$_{0.5}$CoO$_{3-\delta }$ films grown on SrTiO$_{3\, }$substrates, the phase transition of SrTiO$_{3}$ from cubic to tetragonal at around 105 K can further modify the magnetic/transport properties of the films due to the coupling of the out-of-phase tilt of TiO$_{6}$ octahedral with the CoO$_{6\, }$octahedral network. In this contribution, we utilize atomic-resolution imaging and spectroscopies in a scanning transmission electron microscope to study the coupling between the TiO$_{6}$ octahedral tilt in the SrTiO$_{3}$ substrate and the oxygen vacancy ordering in the La$_{0.5}$Sr$_{0.5}$CoO$_{3-\delta }$ thin films using in-situ cooling experiments. The structural analysis will be compared to films grown on LaAlO$_{3}$, and the formation of magnetic domains will be studied using angular resolved electron energy-loss spectroscopy. [Preview Abstract] |
Thursday, March 16, 2017 1:39PM - 1:51PM |
S37a.00011: Growth and Electronic Structure Characterization of (SrCoO$_{\mathrm{x}})_{\mathrm{n}}$:(SrTiO$_{\mathrm{3}})_{\mathrm{1}}$ Superlattices Say Young Cook, Tassie Andersen, Richard Rosenberg, Hawoong Hong, Laurence Marks, Dillon Fong We report on the synthesis of a (SrCoO$_{\mathrm{x}})_{\mathrm{1}}$:(SrTiO$_{\mathrm{3}})_{\mathrm{1}}$ superlattice by oxide molecular beam epitaxy and the characterization of its electronic structure by soft x-ray spectroscopy. X-ray photoelectron and absorption spectroscopy reveal that Ti remains octahedrally coordinated with a 4$+$ oxidation state, while the Co oxidation state is intermediate of 3$+$ and 4$+$. Despite having the same half an oxygen vacancy per Co atom found in brownmillerite SrCoO$_{\mathrm{2.5}}$, which consists of alternating tetrahedral and octahedral layers of Co, the confinement of oxygen vacancies to isolated single atomic layers of SrCoO$_{\mathrm{x}}$ stabilizes square pyramidal coordination of Co, as observed by the linear dichroism in the Co 2p-3d x-ray absorption. The corresponding stabilization of Co$^{\mathrm{4+}}$ along with Co$^{\mathrm{3+\thinspace }}$within the square pyramidal SrCoO$_{\mathrm{2.5}}$ layers gives rise to a Fermi-edge step observed at strong Co 2p-3d resonance in the resonant photoemission spectroscopy of the valence band, and reveals a band gap of 1.7 eV. Comparisons with a Sr(Co,Ti)O$_{\mathrm{x}}$ alloy and a (SrCoO$_{\mathrm{x}})_{\mathrm{2}}$:(SrTiO$_{\mathrm{3}})_{\mathrm{1}}$ superlattice also will also be presented. The obtained results demonstrate artificial superlattices as effective means to defect engineer complex oxides by harnessing the confinement of oxygen vacancies to control the oxygen coordination environment of the transition metal. [Preview Abstract] |
Thursday, March 16, 2017 1:51PM - 2:03PM |
S37a.00012: Evolution of Oxygen Vacancy Ordered Phases in SrCoO$_{x}$ Under Varying Molecular Beam Epitaxy Growth Conditions Tassie Andersen, Say Young Cook, Hawoong Hong, Dillon Fong, Laurence Marks Strontium cobaltite, SrCoO$_{x}$ (SCO), is interesting for applications as a functional oxide, as its oxygen vacancy concentration tunes properties from an antiferromagnetic insulator (x=2.5) to a ferromagnetic metal (x=3.0). This work focuses on growth and control of vacancy content in brownmillerite (BM-SCO) and SrCoO$_{2.5+z}$ phases via molecular beam epitaxy (MBE) – necessary for device and heterostructure fabrication where vacancy-induced functionalities must be controlled. In-situ x-ray oxide MBE at Sector 33ID-E of the Advanced Photon Source was used for synthesis. Films of SrCoO$_{2.5+z}$ were grown epitaxially on SrTiO$_{3}$ (001) via shuttered deposition of SrO/CoO$_{1-2}$ unit-cells with different schemes. During and after growth scattered intensity in the out-of-plane direction (00L) was measured with 8 keV x-rays. X-ray Absorption Near-Edge Spectroscopy spectra at the Co K-edge (7.71 keV) were measured. Films exhibited BM-SCO and a second vacancy ordered phase, similar to the SrCoO$_{2.875}$ phase, dependent on the shutter scheme and overall deposition time. Direct growth of this phase rather than oxidation of BM-SCO to obtain SCO$_{2.5+z}$ phases suggests that this is a metastable phase that could allow for further customization of cobaltite-containing materials. [Preview Abstract] |
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