Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session B71: Engineering and Synthesis of Complex Oxide HeterostructuresFocus Recordings Available
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Sponsoring Units: DMP Chair: Yeongjae Shin, Yale University Room: Hyatt Regency Hotel -Jackson Park C |
Monday, March 14, 2022 11:30AM - 11:42AM |
B71.00001: Mapping hidden space-charge distributions across crystalline metal oxide/group IV semiconductor interfaces Scott Chambers, Matthew Chrysler, Joseph Ngai, Tien-Lin Lee, Judith Gabel, Steven Spurgeon, Bethany Matthews, Mark Bowden, Zihua Zhu, Peter Sushko Dopant profiles in semiconducting heterojunctions can have major effects on band energies which in turn drive transport properties. Here we use core-level photoelectron line shapes excited with hard x-rays to extract information about electric fields resulting from internal charge transfer in epitaxial La-doped Sr(Ti,Zr)O3/Ge(001) heterostructures. Experiments were carried out using both n- and p-type Ge substrates. These heterojunctions were not amenable to electronic characterization of all regions by transport measurements because the doped substrates act as electrical shunts, precluding probing the more resistive films and masking interface conductivity. However, the core-level line shapes were found to be a rich source of information on built-in potentials that exist throughout the heterostructure and yielded valuable insight into the impact of band bending on band alignment at the buried interfaces. The electronic effects expected for Ge with uniform n- and p-type doping are eclipsed by those of unintended oxygen dopants in the Ge near the interface. This study illustrates the power of hard x-ray photoemission spectroscopy and related modeling to determine electronic structure in material systems for which insight from traditional transport measurements is limited. |
Monday, March 14, 2022 11:42AM - 11:54AM |
B71.00002: Tuning band alignment at a semiconductor-crystalline oxide heterojunctionvia electrostatic modulation of the interfacial dipole Matthew Chrysler, Judith Gabel, Tien-Lin Lee, Aubrey Penn, Bethany Matthews, Demie Kepaptsoglou, Quentin Ramasse, Jay Paudel, Raj Sah, Joseph Grassi, Zihua Zhu, Alex Gray, James M LeBeau, Steven Spurgeon, Scott Chambers, Peter Sushko, Joseph Ngai Charge transfer across semiconductor heterojunctions and the electric fields that arise therefrom underpin the functionality of virtually all device technologies. In conventional semiconducting heterojunctions, band alignments are rigid under doping and charge transfer. In contrast, here we demonstrate that band alignment can be altered through charge transfer at hybrid heterojunctions comprised of crystalline oxides and semiconductors. The interfacial dipole associated with bonding across the SrTiO3/Si heterojunction can be tuned through space charge, thereby enabling band offsets and ultimately band alignment to be altered via doping. Oxygen impurities in Si act as donors that create space charge by transferring electrons across the interface into SrTiO3. The space charge induces an electric field that modifies the interfacial dipole, thereby tuning the band alignment from type II to III. The transferred charge, accompanying built-in electric fields, and change in band alignment are manifested in electrical transport and hard x-ray photoelectron spectroscopy measurements. Ab initio models reveal the interplay between polarization and band offsets. We find that band offsets can be tuned by modulating the density of space charge across the interface. Modulating the interface dipole to enable electrostatic altering of band alignment opens additional pathways to realize functional behavior in semiconducting hybrid heterojunctions. |
Monday, March 14, 2022 11:54AM - 12:06PM |
B71.00003: Hybrid Molecular Beam Epitaxy and Electronic Transport of Alkaline Earth Stannates Tristan K Truttmann, Jin-Jian Zhou, I-Te Lu, Anil Kumar Rajapitamahuni, Fengdeng Liu, Richard D James, Marco Bernardi, Bharat Jalan Perovskite oxides have been the subject of intense ongoing study due to the diverse assortment of available properties and phenomena that they and their derivatives exhibit. However, until recently, there has been an unfilled gap for perovskite oxides that exhibit a high carrier mobility at room temperature. This gap was filled with the discovery of BaSnO3 single crystals exhibiting room-temperature electron mobilities as high as 320 cm2V-1s-1. Here we present the growth of epitaxial thin-films of BaSnO3 and its noncubic analogs through the A-site substitution of Ba with smaller Sr and Ca. This includes a systematic doping study of SrSnO3 over a wide range of carrier concentrations with record room-temperature electron mobilities exceeding 70 cm2V-1s-1, and the first ever growth of CaSnO3 by molecular beam epitaxy, whose ultrawide bandgap make it an encouraging ultra-wide bandgap material for applications in high-power devices and novel heterostructure-based transistors combining other functional perovskite oxides. |
Monday, March 14, 2022 12:06PM - 12:18PM |
B71.00004: Hybrid Molecular Beam Epitaxy of Germanium-Containing Oxides Fengdeng Liu, Tristan K Truttmann, DOOYONG LEE, Bethany Matthews, Iflah Laraib, Anderson Janotti, Steven Spurgeon, Scott Chambers, Bharat Jalan Wide bandgap and high carrier mobility are two important materials characteristics for transparent conducting oxides and power device applications. Germanium-containing oxides such as SrGeO3 and rutile GeO2 meet both these requirements according to ab initio calculations. Predicted room-temperature mobilities exceed those of the well-known alkaline earth stannates such as BaSnO3 and SrSnO3. In this talk, we present the first demonstration of hybrid molecular beam epitaxy (MBE) growth of Ge-based oxides including perovskite SrGe1-xSnxO3 and rutile Ge1-xSnxO2. By combining high-resolution X-ray diffraction, scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and first-principles calculations, we demonstrate the successful growth of phase-pure, epitaxial, and coherent SrSn1-xGexO3 films on GdScO3 (110) substrates up to x = 0.16 and of coherent Ge1-xSnxO2 films on TiO2 (001) with x = 0.54. These findings confirm the viability of the hybrid MBE method for the growth of germanium-containing oxides and open the door to further research on high-quality germanate films. |
Monday, March 14, 2022 12:18PM - 12:30PM |
B71.00005: Remote epitaxy of SrTiO3 films on wet-transferred graphene using hybrid molecular beam epitaxy Hyojin Yoon, Tristan K Truttmann, Fengdeng Liu, Qun Su, Vivek Saraswat, Sebastian Manzo, Steven J Koester, Michael Arnold, Jason Kawasaki, Bharat Jalan Lattice mismatch between a crystalline film and substrate is often a key limitation in engineering heterostructures. If the mismatch is sufficiently large, defects such as dislocations can form when lattice mismatch-induced strain relaxes, resulting in often-undesirable consequences for the physical properties. The ability to accommodate strain relaxation without forming such defects can therefore have a significant impact on the synthesis science of epitaxial films. In this talk, we will present our results using a “remote” epitaxy approach to achieve defect-free relaxation by inserting a monolayer/bilayer of wet-transfer graphene between the SrTiO3 (STO) film and the substrates. By using hybrid molecular beam epitaxy that employs a metal-organic precursor, titanium isopropoxide, to supply both Ti and oxygen (without the need for additional oxygen), oxidation of the graphene layer can be avoided. Phase-pure, epitaxial STO films were grown on substrates, and were then successfully exfoliated and transferred onto other substrates. We confirmed that the transferred STO membrane is single-crystalline and that the graphene layer remained on the substrate after exfoliation. We will discuss how pinholes or wrinkles in graphene can influence the growth and surface morphology of the STO film in addition to discussing strain relaxation mechanisms. |
Monday, March 14, 2022 12:30PM - 12:42PM |
B71.00006: In situ X-ray studies of epitaxial growth of LaTiO3 (001) on graphene by molecular beam epitaxy Xi Yan, Hui Cao, YAN LI, Hawoong Hong, Nathan Guisinger, Hua Zhou, Dillon D Fong Remote epitaxy is a novel synthesis technique that allows the fabrication of ultrathin, flexible, and highly unique heterostructures that cannot be created by other methods. The technique was recently applied to construct freestanding, complex oxide membranes [1]. By inserting graphene between the film and substrate during thin film growth, the van der Waals interactions between graphene and other materials are strong enough to enable epitaxial growth but weak enough to allow the rapid release the thin film. The technique may eventually be exploited to create novel, single crystal heterostructures, as each film can be “stacked” atop another, regardless of degradation due to misfit strain or incompatibilities due to high temperature reactions. However, details regarding how remote epitaxy actually occurs remains unknown as this requires an in situ investigation of the remote epitaxy process. Here we describe the results of in situ synchrotron X-ray measurements conducted during the epitaxial growth of LaTiO3 on SrTiO3 (001) both with and without bilayer graphene as an intermediate layer. Crystal Truncation Rods (CTRs) were measured during the growth, and COherent Bragg Rod Analysis (COBRA) was used to reconstruct the electron density. Intensity oscillations at 001/2 (HKL) of each layer were observed during deposition, demonstrating the layer-by-layer growth mode. Raman spectroscopy was used to confirm the presence of the graphene after the deposition. The results will be compared to those from SrTiO3 / graphene / SrTiO3 (001). |
Monday, March 14, 2022 12:42PM - 12:54PM |
B71.00007: Graphene buffer layer impacts enhanced metal-insulator-transition (MIT) in remote epitaxial VO2 thin film HUI CAO, Xi Yan, YAN LI, Liliana Stan, Nathan Guisinger, Wei Chen, Hua Zhou, Dillon D Fong VO2 is an archetypal correlated material, which exhibits a dramatic MIT near room temperature. The MIT characteristics and reconfigurability of VO2-based heterostructures and associated devices via electric field control have attracted significant attention and effort from both fundamental understanding and technologically important applications. However, the precise synthesis of VO2 epitaxial thin film is a daunting challenge, as it is difficult to obtain films with high structural quality and good stoichiometry. Oxygen vacancies in the as-grown VO2-δ thin film without post-treatment often deteriorates the electrical features of the MIT process, such as a broader switching temperature window and lower on/off ratios. In this talk, we demonstrate that much enhanced MIT properties can be attained in an as-grown VO2 thin film if few monolayers of graphene are used as a buffer layer placed on top of a Al2O3 substrate before VO2 growth. Electrical transport measurements show clearly that the on/off ratio is enhanced 10 fold and the switching temperature window is significantly narrowed for VO2 thin films with a graphene buffer layer as compared to VO2 grown directly on Al2O3. We utilize a suite of structural, chemical and spectroscopic characterization methods to reveal the mechanisms behind the improvement in MIT behavior due to the presence of graphene during the growth. |
Monday, March 14, 2022 12:54PM - 1:06PM |
B71.00008: Strain effect on the ground-state crystal structure of Sr2SnO4 Ruddlesden-Popper oxides Hwanhui Yun, Dominique M Gautreau, K. Andre Mkhoyan, Turan Birol Ruddlesden-Popper (RP) oxides (An+1BnO3n+1), which consist of perovskite ABO3 slabs with AO rock-salt layers inserted between them, can exhibit diverse crystal structures. Here, we explore the lattice dynamics of Sr2SnO4, n=1 RP strontium stannates and investigate the strain effects on the ground-state crystal structure by employing first-principles calculations and group theoretical symmetry analyses. Dominant irrep modes are shown to be X point modes, X3+, X2+, X4+, which correspond to octahedral rotations about different crystallographic axes and associated Sr displacements. The behavior of each unstable structural mode with irreps X3+, X2+, X4+ and relevant order parameters with respect to imposed biaxial strain is analyzed. The predicted crystal structures are compared with earlier experimental structures of Sr2SnO4. |
Monday, March 14, 2022 1:06PM - 1:18PM |
B71.00009: Solid-source metal-organic molecular beam epitaxy for epitaxial SrRuO3 films Anusha Kamath Manjeshwar, Anil Kumar Rajapitamahuni, Javier Garcia Barriocanal, DOOYONG LEE, Tristan K Truttmann, Jin Yue, William T Nunn, M. Zzaman, R. Shahid, Kenta Amemiya, Vijay Raj Singh, Richard D James, Bharat Jalan SrRuO3 is the metal electrode of choice in complex oxide electronics due to its high room-temperature conductivity, thermal and chemical stability, and close lattice match with several oxide substrates for epitaxial growth. Challenges in the molecular beam epitaxy (MBE) growth of SrRuO3 thin films arises from the “stubbornness” of Ru, which is difficult to evaporate and to stabilize in the +4 oxidation state. Though electron-beam and ozone-assisted MBE has produced films with low defect densities, this approach raises concerns over flux instabilities. We present a novel solid-source metal-organic MBE approach to supply pre-oxidized Ru by subliming its solid metal-organic precursor in an effusion cell. We demonstrate the growth of phase pure, epitaxial SrRuO3 on SrTiO3 (001) and DyScO3 (110) substrates using high-resolution X-ray diffraction. Combining temperature-dependent electrical transport and magnetometry, we show epitaxial SrRuO3 on SrTiO3 (001) with robust ferromagnetism accompanied by an anomalous Hall effect below the Curie temperature of ~150 K. We discuss the effect of cation stoichiometry and epitaxial strain on the crystal structure, surface morphology, electronic and magnetic properties using X-ray spectroscopic techniques. |
Monday, March 14, 2022 1:18PM - 1:30PM |
B71.00010: Solid Source Metal-Organic MBE of Atomically Smooth IrO2 Films Sreejith Nair, DOOYONG LEE, Bharat Jalan Experimental realization of novel quantum phenomena in Iridium-based complex oxides has been challenging due to the difficulty in synthesizing atomically precise films owing to low vapor pressure, and difficulty in oxidation of Ir. Using a novel solid-source metal-organic MBE approach employing Ir(acac)3, a solid metal-organic precursor for Ir and a rf-plasma source for oxygen, we demonstrate atomically controlled growth of single crystalline, phase-pure, epitaxial IrO2 films on TiO2 and r-Al2O3 substrates. Atomically smooth films with root mean square (rms) roughness of ~ 3-4 Å in Atomic Force Microscopy were obtained regardless of substrate’s orientation. All films exhibit streaky Reflection High-Energy Electron Diffraction (RHEED) patterns consistent with smooth film morphology. Thickness dependent strain relaxation accompanied by a broadening of the X-Ray Diffraction film rocking curve was observed in the films. Temperature-dependent electrical transport measurements revealed metallic behaviour down to 1.8 K with room temperature resistivity of ~ 70 μΩ-cm in 12 nm IrO2 film/TiO2 (110) comparable to bulk single crystals. Further, we also discuss correlation between growth conditions, strain relaxation, dimensionality and magneto-transport properties in IrO2 films. |
Monday, March 14, 2022 1:30PM - 2:06PM |
B71.00011: Moiré engineering and novel electronic transport at oxide interfaces Invited Speaker: Changgan Zeng Moiré engineering has recently emerged as an effective approach to control quantum phenomena in condensed matter systems. Here we demonstrate electronic moiré patterns in films of a prototypical magnetoresistive oxide, La0.67Sr0.33MnO3, epitaxially grown on LaAlO3 substrates. Microscopic moiré profiles of both the electronic conductivity and ferromagnetism are observed, which can be attributed to the coexistence and interaction of two distinct incommensurate patterns of strain modulation. Our work provides a potential route to achieving spatially patterned electronic textures on demand in strained epitaxial materials. |
Monday, March 14, 2022 2:06PM - 2:18PM |
B71.00012: Thermal Laser Epitaxy for Oxide Heterostructures Hans Boschker, Dongyeong Kim, Thomas J Smart, Sander Smink, Lena N Majer, Rezgar Osman, Jochen Mannhart, Wolfgang Braun For the scientific development of quantum-matter heterostructures and for a range of potential applications, the growth of high-purity heterostructures is required. We have developed a new thin-film deposition technique that is especially suited to the growth of an extremely wide range of heterostructures with atomic precision. Thermal laser epitaxy (TLE) uses chemical elements as sources which are evaporated with continuous-wave lasers [1]. The lasers’ virtually arbitrary power density allows for the evaporation of almost all elements of the periodic table in the same setup[2]. |
Monday, March 14, 2022 2:18PM - 2:30PM |
B71.00013: Thermal Laser Epitaxy of elements from across the periodic table Thomas J Smart, Hans Boschker, Jochen Mannhart, Wolfgang Braun, Fabian Felden Thermal Laser Epitaxy (TLE) is a particularly promising new technique for the growth of complex oxides and heterostructures. [1] At its heart, TLE uses lasers to generate vapors from individual elemental sources. The use of lasers for epitaxial growth provides near arbitrary power densities, a lack of source contamination and increased efficiency due to the source being directly illuminated by the laser. |
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