Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session A64: High Mobility, Wide Bandgap Oxides for ElectronicsFocus
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Sponsoring Units: DMP Chair: Turan Birol, University of Minnesota Room: Mile High Ballroom 4E |
Monday, March 2, 2020 8:00AM - 8:36AM |
A64.00001: Complex Oxide Heterointerfaces - New Materials, Terminations, and Orientations Invited Speaker: Mark Rzchowski In this talk I discuss our recent research activity in complex oxide heterostructures, addressing effects arising from spin-orbit coupling, novel spin structures, and interface modifications across a range of systems, highlighting similarities and differences. I will discuss two-dimensional charge liquids at heterointerfaces and their dependence on new materials, crystallographic orientation, and termination, magnetoelectric coupling in multiferrioc and composite oxide heterostructures, and materials and heterostructures exhibiting Berry curvature effects and interfacial coupling arising from crystalline and spin structure. |
Monday, March 2, 2020 8:36AM - 8:48AM |
A64.00002: Structural, Electronic, and Optical Properties of 2D Titanium Oxide Layers on MgO (001) Stephen Eltinge, Kidae Shin, Sangjae Lee, Juan Jiang, Charles H Ahn, Frederick J Walker, Sohrab Ismail-Beigi Two-dimensional transition metal oxides (2DTMOs) are a promising addition to the growing array of functional 2D materials, with potential applications related to their long-lived, strongly bound excitons. In addition, 2DTMOs are expected to be more stable than other 2D materials since they do not react with water or oxygen species. However, unlike some other chalcogenides, 2DTMOs do not naturally occur in stackable van der Waals-bonded layers, so they present challenges for structural prediction and characterization. We report on ab initio density functional theory simulations of 2D titanium oxide layers on the (001) surface of MgO. We consider both monolayer TiO2, including various surface reconstructions, and few-layer Mg2TiO4 based on a bulk inverse spinel structure. We examine the feasibility of various interfaces between the Mg2TiO4 overlayer and the MgO(001) substrate. We report on calculations of the optical absorption spectrum of low-energy structures in order to compare to experiment. Finally, we describe the improved treatment of the orbital energies of low-energy structures using a nonlocal hybrid functional. |
Monday, March 2, 2020 8:48AM - 9:00AM |
A64.00003: Ab initio calculation of ionic oxidation states in metal oxides Sohrab Ismail-Beigi The concept of the oxidation state of ions is intuitive and useful for determining or predicting the stability, metallicity, and/or magnetism of metal oxides (as well as other materials). Determining the oxidation state of an ion is almost always a straightforward excercise in the use of the periodic table together with atomic electronegativities. However, calculating the charge state of an atom in a crystal, i.e., assigning electrons to atoms, is a notoriously difficult and ill-defined problem. After a brief discussion of some of the most popular approaches used for assigning electrons to atoms and their relative merits, we discuss the relation of oxidation state to the band structure, band occupancy and band symmetry in metal oxides and what is required and what it means to compute oxidation states from the band structure. Examples of insulating and metallic bulk metal oxides as well as superlattices and interfacial systems will illustrate the ideas. |
Monday, March 2, 2020 9:00AM - 9:12AM |
A64.00004: Radical-based MBE growth, chemical doping, and electronic transport in SrSnO3 films Tristan Truttmann, Fengdeng Liu, Abhinav Prakash, Jin Yue, Thomas E Mates, Bharat Jalan In this talk, we present our recent study of radical-based molecular beam epitaxy (MBE) growth, and controlled doping in coherent, epitaxial n-doped SrSnO3/GdScO3 (001) films using La and Nd as dopants. By combining detailed growth, structural characterizations, secondary ion mass spectroscopy (SIMS) and temperature-dependent magnetotransport measurements, we show films with one-to-one correlation between dopant and activated carrier concentration. Carrier density exceeding 1 × 1020 cm−3 was achieved in doped SrSnO3 films, which is in excellent agreement with the dopant-solubility limit predicted by density functional theory calculations. A record-high room-temperature mobility of 70 cm2 V−1s−1 at 1 × 1020 cm−3 was obtained in a 12 nm La-doped SrSnO3 film, making this the thinnest perovskite oxide semiconductor with electron mobility exceeding 25 cm2 V−1s−1 at room temperature. We discuss the structure - dopant - scattering mechanisms - transport property relationships, providing essential knowledge for the design of electronic devices using these materials. |
Monday, March 2, 2020 9:12AM - 9:24AM |
A64.00005: Electronic and Magnetic Characterization of Doped Perovskite Stannate Epitaxial Thin Films Emily Lindgren, Hanjong Paik, Carolina Adamo, Alpha T. N'Diaye, Darrell Schlom, Yuri Suzuki La-doped BaSnO3 thin films have been identified as a promising high mobility semiconducting oxide, which could play an important role in the development of an all-oxide power electronics platform. In order to incorporate spin functionality into these materials, we explore magnetic doping of the conducting perovskite stannates. We have fabricated La and Ru doped BaSnO3 and SrSnO3 films on (001) SrTiO3 substrates grown by molecular beam epitaxy (MBE) and pulsed laser deposition (PLD). X-ray diffraction (XRD) was used to verify epitaxial growth and confirm high crystalline quality, with typical omega rocking curve FWHM of 0.05o deg. Films are optically transparent as verified by UV-vis spectrometry. They are also conductive, with room temperature mobilities up to 105 cm2/Vs, and standard carrier electronconcentrations of 1-2x1020 /cm3. Films show evidence of paramagnetism both in the field and temperature dependence of magnetization. |
Monday, March 2, 2020 9:24AM - 9:36AM |
A64.00006: Weak Electron-Electron Interaction effects in La-doped SrSnO3 Films Tianqi Wang, Jin Yue, Laxman R. Thoutam, Abhinav Prakash, Bharat Jalan Alkaline-earth stannates with perovskite structures have attracted significant research interest recently due to their wide bandgap, high optical transparency, and high room temperature mobility. These characteristics make them promising candidates for transparent electronics and power electronics applications. Significant progress has been made in the synthesis, characterization and device demonstration, but there are still many open fundamental questions regarding the electronic transport in stannates. |
Monday, March 2, 2020 9:36AM - 9:48AM |
A64.00007: Large Hysteretic Magnetoresistance in La-doped SrSnO3 Heterostructures: An Evidence of Magnetism? Laxman Raju Thoutam, Tristan Truttmann, Anil K Rajapitamahuni, Bharat Jalan In this talk, we present our recent observation of a robust, hysteretic magnetoresistance (MR) in low-doped Sr1-xLaxSnO3 films grown on GdScO3 (001)pc using radical-based MBE approach. At low temperatures, sample showed a transport behavior consistent with Efros-Shklovskii variable range hopping. The MR shows negative values for 6 K ≤ T ≤ 200 K whereas it crosses over to positive values at lower temperatures, 1.8 K ≤ T ≤ 5 K. A large anisotropy was observed at 1.8 K between the out-of-plane and the in-plane MR values with MR exceeding 100% for magnetic field in the plane of the sample. Significantly, a robust hysteretic behavior was observed at low temperatures, which disappears only at temperature above 5 K. We discuss the physical origin of the hysteresis, and the origin of large in-plane MR in low-doped SrSnO3 providing important and critical insights into the role of defects, strain and the choice of substrates on the electronic and magnetic properties. |
Monday, March 2, 2020 9:48AM - 10:00AM |
A64.00008: Strain Dependent Characterization of Flexible BaSnO3 Nanomembranes Prastuti Singh, Seung Sae Hong, Varun Harbola, Harold Hwang Doped BaSnO3 is a wide band-gap semiconductor that is known to exhibit high electron mobility at room temperature, showing great prospects as an alternative transparent conducting oxide to the industry standard indium-tin-oxide. Flexible transparent conducting oxides are of special interest for future photonics and optoelectronic devices but realizing high-quality films as flexible materials has been a challenge. In this work, we will discuss our efforts to fabricate nanometer thick, freestanding La-doped BaSnO3 film that are highly conducting and transparent using a water-soluble and lattice matched Ba3Al2O6 buffer layer1. Using this technique, we can transfer the nanomembranes onto a flexible device platform and characterize the physical and electronic properties of La-doped BaSnO3 as a function of strain. |
Monday, March 2, 2020 10:00AM - 10:12AM |
A64.00009: Structural characterization and optimization of PAMBE-grown Ga2O3 on STO (001) Tobias Hadamek, Eric Dombrowski, Agham Posadas, Martha R. McCartney, David John Smith, Alexander Demkov Wide band-gap semiconductor Ga2O3 is in the focus of research as a promising material for high-power device applications. Here, we report on the structural properties of Ga2O3 grown by plasma-assisted MBE on STO as a function of growth temperature. At elevated temperatures above 550 °C we observe two out-of-plane orientations of the beta-polymorph (1-2-2) and (100), each with four in-plane rotational domain orientations by XRD and RHEED. As we lower the temperature the high-energy surface (1-2-2) becomes suppressed and we obtain single-out-of-plane oriented (100) beta-Ga2O3. The possibility of thin interfacial layers of gamma-Ga2O3 is investigated by means of STEM. The relevance of the integration of Ga2O3 on STO is that STO can be epitaxially grown directly on Si (001) and hence, can function as a template layer for the integration of Ga2O3 onto the Si platform. A low temperature growth is therefore preferable. |
Monday, March 2, 2020 10:12AM - 10:24AM |
A64.00010: Temperature-dependent Magnetotransport Study of Si-doped β – Ga2O3 Films Anil Rajapitamahuni, Laxman Raju Thoutam, Praneeth Ranga, Sriram Krishnamoorthy, Bharat Jalan Monoclinic β – Ga2O3 has generated significant excitement due to the wide bandgap, high electrical breakdown and high electron mobility at room temperature. In this work, we will discuss temperature-dependent magnetotransport study of homoepitaxial Si-doped β – Ga2O3 (010) films grown via metal-organic vapor-phase epitaxy (MOVPE). Temperature dependent Hall measurements were carried out using Van der Pauw configuration. Hall measurement at room temperature yielded a linear slope and a carrier density (n) of ~ 4 x 1017 cm-3. A non-linear Hall resistance was however observed at 10 K < T < 150 K indicating the presence of multiple channel conduction. A transport model using two-channel conduction was used to fit the experimental magnetotransport data resulting in carrier densities, n1, n2 and the corresponding mobilities of µ1, and µ2 respectively. With decreasing temperature, n1 showed a freeze-out behavior with an activation energy of ~ 15 meV, whereas n2 remained nominally unchanged. By combining electrostatic gating using ion-gel as a gate-dielectric, detailed magnetotransport and transport modeling, we will discuss the origin of multiple carriers, defect-mobility relationship, and the relevant scattering mechanisms in MOVPE grown Si-doped β – Ga2O3. |
Monday, March 2, 2020 10:24AM - 10:36AM |
A64.00011: Stabilizing extraordinary disorder in single crystal high entropy oxides Wenrui Zhang, Alessandro Mazza, Elizabeth Skoropata, Thomas Zac Ward In functional oxides, compositional changes to the cation sublattice(s) enables tunability to electronic and/or magnetic phases. For correlated systems, even low levels of cation substitution can have a dominating influence on behavior due to the near degeneracy of the spin/charge/orbital energy scales. Further, the local distortion fields near doping sites often act as the catalysis for phase transitions. Controlling the type, number, and magnitude of these local distortions is highly desirable but often not experimentally accessible. We demonstrate the creation of exceptionally high disorder fields in single crystal oxides. Examples of epitaxial perovskite, spinel, and layered Ruddlesden-Popper phases possessing 5 or more elements randomly distributed on one or more of their cation sublattices. Synthesis of these materials relies on precise control of configuration entropy to dominate the free energy landscape, thus avoiding phase separation generally inherent in complex multinary systems. The impact of changing spin, charge, and lattice distortion field uniformity in these high entropy oxides will be discussed. |
Monday, March 2, 2020 10:36AM - 10:48AM |
A64.00012: Epitaxial Oxides on Glass: A Platform for Integrated Oxide Devices John E Ortmann, Martha R. McCartney, Agham Posadas, David John Smith, Alexander Demkov The fabrication of epitaxial, ultra-thin SrTiO3 (STO) on thick SiO2 without the need for complicated wafer-bonding processes has been demonstrated. The resulting transition metal oxide (TMO)-on-glass layer stack is analogous to traditional silicon-on-insulator (SOI) wafers, |
Monday, March 2, 2020 10:48AM - 11:00AM |
A64.00013: Growth and structure of Mg2TiO4 on MgO (001) Kidae Shin, Stephen Eltinge, Sangjae Lee, Hyungki Shin, Juan Jiang, Hawoong Hong, Bruce Davidson, Ke Zou, Sohrab Ismail-Beigi, Charles H Ahn, Frederick J Walker The unique properties of two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides, have been used for applications such as high mobility channels and as hosts for quantum optical phenomena. In transition metal oxides (TMOs), a similarly broad range of properties and devices may be possible to realize by leveraging the diverse properties and structures found in these materials and synthesizing them as ultrathin 2D films. In order to use 2D TMOs, understanding the limits of 2D oxide heteroepitaxy is crucial. Here, we report that growth of TiO2 on MgO using molecular beam epitaxy (MBE) results in formation of epitaxial Mg2TiO4. Using a combination of in situ crystal truncation rod (CTR) measurements and first-principles density functional theory (DFT) calculations, the layer-resolved structure of Mg2TiO4 that is a few unit cells thick is determined, and the thermodynamic driving force behind Mg2TiO4 formation is calculated. These measurements resolve distinct structural features at the Mg2TiO4 interface with MgO, which is due to the polarity of Mg2TiO4, pointing to new ways to synthesize distinct 2D electronic structures in thin films. |
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