Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S13: Focus Session: Oxide-Semiconductor Interfaces, Oxygen Vacancy Defects, Doped SrTiO3 |
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Sponsoring Units: DMP Chair: Serge Nakhmanson, University of Connecticut Room: 007D |
Thursday, March 5, 2015 8:00AM - 8:12AM |
S13.00001: Electrical characterization of n-SrTiO$_{3-\delta}$ / p-Si(100)epitaxial diode structures Ryan Cottier, Daniel Currie, Nikoleta Theodoropoulou Semiconducting, single crystalline n-p (n-STO/p-Si) junctions were grown by MBE (Molecular Beam Epitaxy) using a technique that suppresses the formation of a SiO$_{2}$ interfacial layer. The STO layer thickness varied from 3.6 to 30 nm, and oxygen vacancies were induced during growth by controlling the background O$_{2}$ pressure ($\delta $ up to 0.4{\%}). The STO films are compressively strained by 1.7{\%} due to the lattice mismatch with Si, and strain induced ferroelectric behavior was observed for 4 nm thick films at 300K. The epitaxial quality was verified through x-ray diffraction, atomic force microscopy, scanning electron microscopy and transmission electron microscopy. I-V measurements (5-300K) showed characteristics of an n-p junction down to T$=$10 K. C-V measurements at 300 K for a frequency range of 1 kHz to 1 MHz show typical depletion layer capacitance behavior for reverse bias. Under forward biased, both V$_{\mathrm{bi}}$ and the dielectric constant decrease with increasing thickness. A sharp peak in the capacitance is observed for voltages higher than V$_{\mathrm{bi}}$ for frequencies below 10 kHz. The origin of the observed behavior is considered in terms of strain, interface states, charged defects, ferroelectricity, an interfacial dipole, band bending, and an inversion layer at the interface. [Preview Abstract] |
Thursday, March 5, 2015 8:12AM - 8:24AM |
S13.00002: Transport properties of SrTiO$_{3-\delta}$ thin films grown by Molecular Beam Epitaxy on p-Si(001) substrates Nikoleta Theodoropoulou, Daniel Currie, Ryan Cottier, Arturo Ponce-Pedraza, Jesus Cantu, Oscar Villarreal SrTiO$_{3}$ (STO) films were grown on p-Si(001) and STO(001) bulk substrates using molecular beam epitaxy (MBE). Oxygen vacancies were introduced by controlling the Oxygen pressure during growth (P(O$_{2})$: 4 $\times$ 10$^{-8}$ - 8 $\times$ 10$^{-7})$ resulting in SrTiO$_{3-\delta}$ with $\delta $ $\sim$ 0.02{\%} for the lowest P(O$_{2})$. The single-phase STO/Si films were of high crystalline quality as verified by x-ray diffraction, transmission electron microscopy, and atomically flat. Transport measurements were performed on the STO/Si structures in a Van der Pauw configuration. The P(O$_{2})$ during growth determines the conduction behavior which changes from strongly localized transport that fits a Variable Range Hopping (VRH) model (low P(O$_{2})$-high disorder) to thermally activated transport (high P(O$_{2})$-low disorder). The resistivity of the strongly disordered STO/Si films decreased from 1 Ohm$\cdot$cm to 3x10$^{-2}$ Ohm$\cdot$cm as the film thickness increased (3nm-60nm). The perpendicular magnetoresistance (MR) is positive at 300K and becomes negative at T$=$3-20K. We consider competing effects on the STO/Si heterostructure such as 1.7{\%} compressive strain induced by lattice mismatch to Si, defects due to oxygen vacancies, the bulk STO antiferrodistortive phase transition at 105K, and structural dislocations. [Preview Abstract] |
Thursday, March 5, 2015 8:24AM - 8:36AM |
S13.00003: Control of a Two-Dimensional Electron Gas on SrTiO$_3$ (111) by Atomic Oxygen Siobhan McKeown Walker, A. de la Torre, F.Y. Bruno, A. Tamai, T.K. Kim, M. Hoesch, M. Shi, M.S. Bahramy, P.D.C. King, F. Baumberger We report on the formation of a two-dimensional electron gas (2DEG) at the bare surface of (111) oriented SrTiO$_{3}$. Angle resolved photoemission experiments reveal highly itinerant carriers with a 6-fold symmetric Fermi surface and strongly anisotropic effective masses [1]. The electronic structure of the 2DEG is in good agreement with self-consistent tight-binding supercell calculations that incorporate a confinement potential due to surface band bending. Our measurements provide insight into the nontrivial consequences of quantum confinement along the [111] direction which is directly relevant to an understanding of electronic structure at (111) orientated interfaces. We further demonstrate that alternate exposure of the surface to ultraviolet light and atomic oxygen allows tuning of the carrier density and the complete suppression of the 2DEG. [1] S. McKeown Walker et al., Phys. Rev. Lett. 113, 177601 (2014) [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 9:12AM |
S13.00004: Depth- and momentum- resolved electronic structure at buried oxide interfaces from standing-wave angle-resolved photoemission Invited Speaker: Charles Fadley It is clear that interfaces in complex oxide heterostructures often represent emergent materials that possess surprising properties not associated with the parent oxides, such as two-dimensional electron gases (2DEGs), superconductivity, and magnetism. A detailed knowledge of the composition, atomic structure, and electronic structure through such interfaces is thus critical. Photomission (PES) and angle-resolved photoemission (ARPES) represent techniques of choice for such studies, but have certain limitations in being too surface sensitive and in not being able to focus specifically on buried interfaces or heterostructure layers. In this talk, I will discuss combining two newer elements of PES/ARPES to deal with this challenge: - the use of soft x-rays in the ca. few hundred-to-2000 eV regime [1], or even into the true hard x-ray regime [2], to probe more deeply into the structure, and - tailoring of the x-ray intensity profile into a strong standing wave (SW) through reflection from a multilayer heterostructure to provide much enhanced depth resolution [3]. The relative advantages of soft/hard x-ray PES and ARPES and their complementarity to conventional VUV ARPES in the ca. 5-150 eV regime will be considered [1]. As illustrative examples, by combining SW-PES and SW-ARPES, it has been possible to measure for the first time the detailed concentration profiles and momentum-resolved electronic structure at the SrTiO3/La0.67Sr0.33MnO3 interface [3] and to directly measure the depth profile of the 2DEG at SrTiO3/GdTiO3 interfaces [4]. Future directions for such measurements will also be discussed.\\[4pt] [1] C.S. Fadley, Synchrotron Radiation News \underline {25}, 26 (2012);\\[0pt] [2] A.X. Gray et al., Nature Materials \underline {11}, 957 (2012);\\[0pt] [3] A.X. Gray et al., Phys. Rev. B \underline {82}, 205116 (2010) and Europhys. Letters \underline {104}, 17004 (2013);\\[0pt] [4] S. Nemsak et al., to be published. [Preview Abstract] |
Thursday, March 5, 2015 9:12AM - 9:24AM |
S13.00005: Continuous strain modulation of strontium titanate (SrTiO$_{3})$ on semiconductor interface by thermal strains Lei Zhang, Yakun Yuan, Shiming Lei, Bernd Kabius, Venkatraman Gopalan, Roman Engel-Herbert Strain engineering is a general strategy for tuning the desired material properties, such as enhancing carrier mobility and increasing the spontaneous polarization and Curie temperature in ferroelectric films. Control over the strain state in thin film is provided by the substrate with lattice mismatch. Although growth of strained perovskite oxides was demonstrated, the limited number of suitable substrates of high quality imposed difficulty towards utilizing the strain. We'll discuss a novel route towards wafer-scale strain engineering of ferroelectric oxide based on large thermal mismatch between film and substrate, enabling to continuously modulate the strain state. SrTiO$_{3}$ films were grown on Si~(001) between 400 $^{\circ}$C and 900 $^{\circ}$C above the critical film thickness. The films relaxed at growth temperature and accumulated a tensile strain ranging from 0.2{\%} to 0.7{\%} during cool-down, which was proportional to the temperature difference. X-ray reciprocal space maps and geometric phase analysis obtained from cross section transmission electron microscopy have been used to relate the film's strain state to the ferroelectric properties, probed by second harmonic generation and piezo force microscopy. [Preview Abstract] |
Thursday, March 5, 2015 9:24AM - 9:36AM |
S13.00006: Switchable two-dimensional electron gas at the interface of a ferroelectric and nonpolar insulator: BaTiO3/SrTiO3 Kurt Fredrickson, Alex Demkov We theoretically investigate the interface between a ferroelectric BaTiO3 film and a nonpolar insulating SrTiO3 substrate. We find that thin BaTiO3, under 5 nm, can stabilize two polarization states. While the nonpolarized state is insulating, for the polarized heterostructure, we discovered the existence of two-dimensional gases. In this case, the heterostructure undergoes an electronic reconstruction in order to prevent the polar catastrophe. The two-dimensional gases, formed as a result, screen the polarization, leading to a substantially reduced potential drop across the ferroelectric film. We emphasize that the two-dimensional electron and hole gases are created by the polarization of the sample, and is not due to the polar nature of either material or doping. [Preview Abstract] |
Thursday, March 5, 2015 9:36AM - 9:48AM |
S13.00007: Electronic carrier transport at epitaxial oxide-semiconductor interfaces Lior Kornblum, Eric Jin, Charles Ahn, Fred Walker The epitaxial growth of transition metal perovskite oxides on conventional semiconductors is a promising approach for integrating the wealth of electronic phenomena found in these oxides with existing devices and technologies. Some oxide functionalities require charge transport to and from the semiconductor, making the semiconductor-oxide interface an important focal point in the utilization of epitaxial oxides in electronic devices. We present our findings on electronic carrier transport in the conduction band of titanate perovskites ($R$TiO$_{3})$ epitaxially grown on silicon and on germanium. Metal oxide semiconductor devices were fabricated by evaporation of metal contacts on top of epitaxially-grown oxides on semiconductors. Transport measurements show diode-like transport across the interface of some of the structures, whereas only leakage currents are observed in others. These results are discussed in light of the physical and electronic structure at the oxide-semiconductor interface. [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:00AM |
S13.00008: Growth of Dielectric SrTiO3(111) Film with Atomically Well Defined Surface Jiandong Guo, Yan Liang, Wentao Li, Shuyuan Zhang Oxide heterointerfaces exhibit novel properties that can be controlled by external fields. The (111) surfaces of perovskite oxides are particularly interesting since the six-fold symmetry is compatible with other quantum materials, e.g., topological insulators. We grow high quality SrTiO3(111) film on Nb-doped substrate by oxide molecular beam epitaxy. By adjusting the flux rates of Sr and Ti, we keep the reconstruction of film surface unchanged as that on substrate all through the growth. Thus the cation stoichiometry is achieved since the surface reconstruction is determined by Sr-Ti concentration ratio and can be monitored in real-time. Moreover reconstruction allows the stable layer-by-layer growth of the polar film. The in situ scanning tunneling microscope shows the atomically well defined film surface with broadened terraces. And the C-V measurements indicate that the tunable range of the carrier density is 2E13 per cm2 for a 50 nm film. This insulator/metal homoepitaxial system provides a template for the growth of novel low-dimensional structures with flexible tunability by gate voltage. [Preview Abstract] |
Thursday, March 5, 2015 10:00AM - 10:12AM |
S13.00009: Growth of epitaxial poly-crystalline transition metal oxide thin films Sungmin Woo, Hoidong Jeong, Sang A Lee, Hosung Seo, Morgane Lacotte, Adrian David, Hyun You Kim, Wilfrid Prellier, Yunseok Kim, Woo Seok Choi By comparing single- and poly-crystalline transition metal oxides (TMOs), one can study intriguing physical phenomena such as electronic and ionic conduction at the grain boundaries, phonon propagation, and various domain properties. In this work, we propose an approach to simultaneously fabricate single- and poly-crystalline epitaxial TMO thin films using substrate epitaxy of poly-crystalline SrTiO$_{\mathrm{3}}$ (STO). In order to grow TMO thin films epitaxially with atomic precision, an atomically flat surface of the substrate is required. We first examined (100), (110), and (111) oriented single-crystalline STO surfaces, which required different annealing conditions to achieve an atomically flat surface. A poly-crystalline STO surface was then prepared at the optimum condition for which all the domains with different crystallographic orientations could be successfully flattened. Studying the surface properties (surface potential, topography, and orientation) of poly-crystalline STO helped us to understand the formation of the atomically flat surface. Based on our research, we envision expansion of the studies regarding the epitaxial poly-crystalline TMO thin films and heterostructures.. [Preview Abstract] |
Thursday, March 5, 2015 10:12AM - 10:24AM |
S13.00010: The surface of SrTiO$_{\mathbf{3}}$ (111): effect of annealing in vacuum and in oxygen Mohammad Saghayezhian, Lina Chen, Gaomin Wang, Hangwen Guo, Jiandi Zhang, Earl W. Plummer The surface of SrTiO$_{3}$ (111) have created a new playground for new physics, exhibiting novel properties such as 2DEG and topological phases such as quantum spin Hall effect. Due to the polar nature of the surface, it is very susceptible to different kinds of reconstructions which results in various terminations. There has been a fair amount of investigations on SrTiO$_{3}$ (111) as a function of sputtering and annealing, while less attention has been paid to its reconstruction when the surface comes in contact with oxygen or the mere effect of annealing in vacuum. We have focused on the surface reconstruction and chemical composition of SrTiO$_{3}$ (111) as a function of annealing temperature and oxygen pressure using LEED and ARXPS. We observed that annealing in oxygen brings more Ti to the surface in comparison with annealing in vacuum. Our data show that the SrTiO$_{3}$ (111) surface is highly reactive and easily absorbs carbon. Furthermore, we show that in contrast to SrTiO$_{3}$ (001), where carbon tends to be physisorbed and can easily be removed by low temperature annealing, on SrTiO$_{3}$ (111), carbon only leaves the surface after annealing to very high temperature. Also, our data show that the presence of oxygen can facilitate de-contamination of the surface and makes the surface more ordered. [Preview Abstract] |
Thursday, March 5, 2015 10:24AM - 10:36AM |
S13.00011: Influence of Strain on the Thermoelectric Properties of electron-doped $SrTiO_{3}$ Thin Films Alexandros Sarantopoulos, Elias Ferreiro-Vila, Cesar Magen, Myriam H. Aguirre, Victor Pardo, Francisco Rivadulla The discovery of a two dimensional electron gas with high mobility at the interface between insulating $LaAlO_{3}/SrTiO_{3}$ (LAO/STO) opened the possibility of fabricating functional devices based on this interfacial effect. Therefore, it is important to study the influence of the growth parameters on the properties of the constituent materials. Here, we demonstrate that the thermoelectric properties of epitaxial thin films of Nb:STO can be finely tuned by adjusting the growth conditions in a PLD system. By growing the sample on different substrates, we demonstrate that the amount of vacancies depends on the degree of epitaxial compressive stress. The vacancies produced lead to impurity scattering at low temperatures. We show that the magnetoresistance response, and non-linear behavior of the Hall effect, characteristic of LAO/STO interfaces, can be reproduced in thin films of Nb:STO with a controlled number of vacancies. Moreover, we show that the Seebeck coefficient is a valid tool to obtain information about the degeneracy of the electronic band structure. [Preview Abstract] |
Thursday, March 5, 2015 10:36AM - 10:48AM |
S13.00012: Spectroscopic ellipsometry study on doped SrTiO$_{3}$ superlattice films Yunsang Lee, Y.K. Seo, E. Choi, J.W. Seo, J. Lee We report on the spectroscopic ellipsometry study on the low-dimensional confinement of chemical doping in SrTiO$_{3}$. We fabricated superlattice films composed of the stacking of insulating SrTiO$_{3}$ (STO) and metallic La-doped SrTiO$_{3}$ (SLTO) layers. As the dimensionality is varied from three to two dimensions by changing the thickness of the SrTiO$_{3}$ layers, phase transition from metal to insulator occurred through interplay of charge, spin, orbital, and lattice degrees of freedom. The optical conductivity spectra obtained from the spectrosocopic ellipsometry show a significant change below the charge transfer gap near 3 eV through the insulator-metal transition. We detail our spectroscopic finding on the STO/SLTO superlattice, and compare them with the transport and structural properties of the films. [Preview Abstract] |
Thursday, March 5, 2015 10:48AM - 11:00AM |
S13.00013: Two-dimensional electron gas at surfaces of (001), (110), and (111) oriented SrTiO$_{3}$ induced by Ar$^{+}$-irradiation Ludi Miao, Renzhong Du, Yuewei Yin, Qi Li Two-dimensional electron gases (2DEGs) at transition metal oxide surfaces and interfaces have attracted much attention due to their fascinating exotic properties such as superconductivity, large magneto-resistance (MR), and ferromagnetism. We have created 2DEGs at the surfaces of (001), (110), and (111) oriented SrTiO$_{3}$ (STO) by Ar$^+$-irradiation and measured their transport properties. The 2DEGs exhibit a fully metallic behavior with the 2D charge carrier density around $2\times 10^{14}$ cm$^{-2}$ and the mobility as large as 5500 cm$^{2}$V$^{-1}$s$^{-1}$ at low temperatures, which is tunable by electric fields applied through STO back gates. We have measured MR anisotropy of the 2DEGs at surfaces of STO with all orientations. We observed combinations of two types of components in their anisotropic MR at low temperatures. While the first type is an STO-orientation-independent two-fold component results from the Lorentz force effect, the second type shows stark differences between these 2DEGs as a consequence of distinct Fermi surface symmetries with different STO orientations. Indeed, it is four-fold for STO (001), two-fold for STO (110) and six-fold for STO (111), respectively. [Preview Abstract] |
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