Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M49: Focus Session: Oxide Thin Films: Growth and Properties |
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Sponsoring Units: DMP Chair: Stefano Gariglio, University of Geneva Room: Mile High Ballroom 1C |
Wednesday, March 5, 2014 11:15AM - 11:27AM |
M49.00001: Atomic layer-by-layer growth of oxide thin films by laser MBE Qingyu Lei, Guozhen Liu, Maryam Golalikhani, Ke Chen, Suilin Shi, Fuqiang Huang, Andrew Farrar, Dmitri Tenne, Rakesh Singh, Xiaoxing Xi We have studied an atomic layer-by-layer thin film growth technique for complex oxide thin films and heterostructures. By using a laser-MBE system and monitoring the reflection high-energy electron diffraction (RHEED) intensity to control the flux for each atomic layer in-situ, we actively control the structure and stoichiometry down to the atomic layer level. In the growth of SrTiO$_{3}$ from the separate SrO and TiO$_{2}$ targets, or from metal Sr and oxide TiO$_{2}$ target, we studied the phases of the specular and diffraction spot intensities as well as that of the Kikuchi lines. UV Raman spectroscopy was used to probe the symmetry breaking due to the cation off-stoichiometry. Similar stoichiometry control as shown by reactive MBE has been demonstrated. We also studied the target preparation of various oxides, including the highly reactive La$_{2}$O$_{3}$ and BaO. We have successfully applied this atomic layer-by-layer growth method to the deposition of LaAlO$_{3}$ and LaNiO$_{3}$ thin films and superlattices. [Preview Abstract] |
Wednesday, March 5, 2014 11:27AM - 11:39AM |
M49.00002: Optimization of atomically smooth and metallic surface of SrTiO$_{3}$ for the growth of ultra-thin manganite films In Hae Kwak, Sima Varnoosfaderan, Arthur Hebard, Amlan Biswas Atomically smooth, TiO$_{2}$ terminated SrTiO$_{3}$ substrates can be prepared using a combination of chemical and thermal annealing treatments. Such substrates have been widely used to grow sharp oxide interfaces between SrTiO$_{3}$ and materials such as LaAlO$_{3}$. Insulating SrTiO$_{3}$ can also be made metallic by inducing oxygen vacancies or by doping with metals such as niobium. However, such treatments usually generate a rough surface. Thus, further growth of epitaxial thin films or study of the surface itself has been limited. Here, we report the optimal conditions to fabricate atomically smooth and metallic SrTiO$_{3}$ surfaces which show steps of one unit cell height. We directly confirmed the metallic characteristic of SrTiO$_{3}$ using sheet resistance vs. temperature ($R(T))$ measurements. The $R(T)$ data provides information on the physical origin of metallic behavior in SrTiO$_{3}$, which might also be relevant to the current research interest in 2DEG SrTiO$_{3}$ and oxide interfaces. We will also discuss the thin film growth of strain-induced insulating manganites on top of atomically smooth and metallic SrTiO$_{3}$ using pulsed laser deposition. [Preview Abstract] |
Wednesday, March 5, 2014 11:39AM - 11:51AM |
M49.00003: Spectroscopic ellipsometry study on metal-insulator transition for ultrathin La-doped SrTiO$_{3}$ films Yunsang Lee, Yunkyung Seo, Dongjae Lee, E. Choi, J.W. Seo, J. Lee We investigated the metal-insulator transition for ultrathin La-doped SrTiO$_{3}$ (LSTO) films by using spectroscopic ellipsometric technique. As the film thickness decreased below 10 unit cells, phase transition from metal to insulator occurred through interplay of charge, spin, orbital, and lattice degrees of freedom. The optical spectra below the charge transfer gap near 3 eV changed significantly through the insulator-metal transition, exhibiting the coherent-incoherent crossover behavior. We detail our spectroscopic results on the LSTO ultrathin films, and compare them with the transport and structural properties of the films. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M49.00004: Bimodal island size distribution in heteroepitaxial growth Randall Headrick, Priya Chinta A bimodal size distribution of two dimensional islands is inferred during interface formation in heteroepitaxial growth of Bismuth Ferrite on (001) oriented SrTiO$_3$ by sputter deposition. Features observed by in-situ x-ray scattering are explained by a model where coalescence of islands determines the growth kinetics with negligible surface diffusion on SrTiO$_3$. Small clusters maintain a compact shape as they coalesce, while clusters beyond a critical size impinge to form large irregular connected islands and a population of smaller clusters forms in the spaces between the larger ones. [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M49.00005: Epitaxial strain induced phase transitions in La-doped BiFeO$_{3}$ thin films on Si substrates Deyang Chen, Christopher T. Nelson, Xiaohong Zhu, Claudy R. Serrao, Ya Gao, Di Yi, Jian Liu, Ramamoorthy Ramesh, Dechang Zeng, Darrel G. Schlom Epitaxial strain is a powerful pathway to trigger phase transitions with emergent phenomena in oxide thin films, e.g., strain induced ferroelectric to ferroelectric (PE-PE) phase transition from tetragonal-like to rhombohedral-like phase in Pb(Zr$_{\mathrm{x}}$Ti$_{\mathrm{1-x}})$O$_{3}$ and BiFeO$_{3}$ films. In this study, we report a strain driven antiferroelectric to ferroelectric (AFE-FE) phase transition from orthorhombic (O) to rhombohedral (R) phase in La$_{\mathrm{x}}$Bi$_{\mathrm{1-x}}$FeO$_{3}$ (LBFO) thin film on Si substrates. The ground state of La$_{\mathrm{x}}$Bi$_{\mathrm{1-x}}$FeO$_{3}$ bulk is antiferroelectric PbZrO$_{3}$ type orthorhombic phase. We show that epitaxial strain from Si substrates can stabilize a rhombohedral structure of LBFO in 20 nm films and intermediate strains position LBFO into a nanoscale mixture of rhombohedral and orthorhombic phases in 30-100 nm films and then strain relaxation in 125nm films leads to the orthorhombic phase. Transmission electron microscopy (TEM) shows atomically sharp O/R morphotropic phase boundary (MPB) with O phase domains larger than 10 nm in width. In summary, our findings open a new path to drive AFE-FE phase transition in LBFO and provide a route to study O/R MPB. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M49.00006: Bimodal island size distribution in heteroepitaxial film growth: BiFeO$_{3}$ on SrTiO$_{3}$ Priya Chinta, Ishviene Cour, Randy Headrick, Matthew Dawber Growth and control of complex oxide thin films on an atomic level is highly critical in understanding the behavior of both interfaces and complex oxide system. Real time X-ray scattering measurements during heteroepitaxial film deposition provide details of initial nucleation and growth giving insight into atomic scale processes and growth mechanisms. In this work we present experimental data for growth of multiferroic epitaxial BiFeO$_{3}$ (001) thin films on SrTiO$_{3}$ substrates using \textit{in-situ} diffuse x-ray scattering. A bimodal size distribution of two dimensional islands where monodispersed set of large clusters and a broad distribution of smaller islands are observed during coalescence evident from two different components of diffuse scattering. Features observed by \textit{in-situ} x-ray scattering are explained by a model where coalescence of islands determines the growth kinetics with negligible surface diffusion on SrTiO$_{3}$. Small clusters maintain a compact shape as they coalesce, while clusters beyond a critical size impinge to form large irregular connected islands and a population of smaller clusters forms in the spaces between the larger ones. \textit{Ex-situ} atomic force microscopy (AFM) was used to measure the final surface morphology of the films at each stage. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M49.00007: Epitaxial growth of polar KTaO$_{3}$ thin-films on polar perovskite substrates J. Thompson, J. Nichols, J. Hwang, S.S.A. Seo The atomic polarity plays an important role in a wide range of physical phenomena at heterointerfaces. For example, the polar/non-polar nature of a LaAlO$_{3}$/SrTiO$_{3}$ system induces partial conducting electrons at the heterointerfaces to avoid diverging electrostatic potential, the so-called ``polar catastrophe,'' which results in intriguing two-dimensional transport and magnetic properties. In this presentation, we discuss another system in which the role of the polar interface is important: the KTaO$_{3}$/GdScO$_{3}$ (KTO/GSO) polar/polar system. At the KTO/GSO interface, there is a ``polar conflict'' heterointerface along the [001] direction, where the AO and BO$_{2}$ layers have reversed net charges so that there is a conflict between the chemical bonding and the electrostatic charges, i.e. K$^{1+}$O$^{2-}$(1-)/Sc$^{3+}$O$_{2}^{4-}$(1-) or Ta$^{5+}$O$_{2}^{4-}$(1$+)$/Gd$^{3+}$O$^{2-}$(1$+)$, which is unstable in the electrostatic point of view. We ask a fundamental question: ``How is the polar conflict resolved in the atomically flat heterointerfaces of such polar/polar systems?'' We have synthesized epitaxial KTO thin films on GSO substrates using pulsed laser deposition. From X-ray diffraction and high-resolution transmission electron microscopy, we have observed that the polar conflict is quite effectively avoided by forming only two non-polar mono-layers at the heterointerface, resulting in high-quality epitaxial thin films on top of the layers. Our result suggests a new way to create two-dimensional confined layers using the polar conflict of the heterointerfaces of two polar materials. [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M49.00008: Epitaxy of polar semiconductor $Co_{3}O_{4}$ (110): growth, structure, and characterization Kristy Kormondy, Agham Posadas, Alexander Slepko, Ajit Dhamdhere, David Smith, Khadijih Mitchell, Stefan Zollner, Luke Marshall, Jianshi Zhou, Alexander Demkov The (110) plane of catalytic $Co_{3}O_{4}$ exhibits significantly higher rates of carbon monoxide conversion due to the presence of active Co$^{3+}$ species at the surface. However, experimental studies of $Co_{3}O_{4}$ (110) surfaces and interfaces have been limited due to the difficulties in growing high-quality films. In this paper, we present thin (1- 25nm) $Co_{3}O_{4}$ films grown by molecular beam epitaxy in the polar (110) direction on $MgAl _{2}O_{4}$ substrates. Reflection high-energy electron diffraction, atomic force microscopy, x-ray diffraction, and transmission electron microscopy measurements attest to the high quality of the as-grown films. We note that the film surface roughens at intermediate thickness, but slowly smoothens as growth continues, returning to an RMS surface roughness less than 1 {\AA}. Furthermore, we investigate the electronic structure and optical properties of this material by core level and valence band x-ray photoelectron spectroscopy, first-principles density functional theory calculations, and ellipsometry. A valence band offset of 3.5 eV is measured for the $Co_{3}O_{4}/MgAl _{2}O_{4}$ heterostructure. Magnetic measurements show the signature of antiferromagnetic ordering at 46 K. [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:03PM |
M49.00009: Effect of Particle Size Distribution on the Magnetostrictive Properties of Cobalt Ferrite Cajetan Nlebedim, David Jiles Magnetostrictive materials are technologically useful for developing stress sensors and actuators. Oxide based magnetostrictive materials such cobalt ferrite are more appropriate especially in situations where it is desirable to avoid losses due to eddy current. A very important factor that strongly affects the magnetostrictive properties of this class of materials is the microstructure. This study investigates the relationship between the pre-sintering particle size distribution and the magnetostrictive properties of cobalt ferrite. This is important because final microstructure, hence the magnetostrictive properties of ferrites prepared via the solid-state reaction technique will depend strongly on the pre-sintering particle size distribution. Samples derived by combining powders with the smallest and largest particle size distributions gave the highest magnetostriction amplitude and strain sensitivity for measurement in the parallel direction. Samples from the largest particle size distribution gave the least in the parallel direction but highest in the perpendicular direction. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:15PM |
M49.00010: Room-Temperature Ferroelectricity in Hexagonal TbMnO$_{3}$ Thin Films Tula R. Paudel, Dong Jik Kim, Haidong Lu, J.D. Burton, John G. Connell, Evgeny Y. Tsymbal, S.S. Ambrose Seo, Alexei Gruverman Magnetoelectric multiferroics exhibit coupling between the ferroelectric and magnetic order parameters, allowing control of electric polarization by a magnetic field or magnetization by an electric field. This property is appealing for novel device applications but they require room-temperature functionality. Among a limited group of single-phase multiferroic materials, rare-earth manganites, such as TbMnO$_{3}$, are promising due to their strong magnetoelectric coupling. However, the ferroelectric transition temperature of TbMnO$_{3}$ in the bulk orthorhombic phase is very low. Here, we report room-temperature ferroelectricity of epitaxially-stabilized hexagonal TbMnO$_{3}$ thin films which is accompanied by significant polarization-dependent resistive switching. The first principle calculation and group theoretical analysis reveals that the ferroelectric polarization of hexagonal TbMnO$_{3}$ is associated with the lattice instability of prototypical paraelectric phase at the zone boundary and is also an improper ferroelectric similar to other manganites such as YMnO$_{3}$. Our results demonstrate a possibility to engineer new single-phase multiferroics by epitaxial growth, which broadens the range of functional materials desirable for novel electronic devices. [Preview Abstract] |
Wednesday, March 5, 2014 1:15PM - 1:27PM |
M49.00011: The Control of Anisotropic Transport in Manganites by Stripy Domains Changcheng Ju, Xiaomei Lu, Yinghao Chu Epitaxial thin film acts as a significant tool to investigate novel phenomena of complex oxide systems. Extrinsic constraint1 of uniform or certain designed buffer layer strain could be easily implanted to these materials. However, the strain distribution might be quite complicated by involving micro- or nano-lattice distortions which could partially relax the strain and determine the complex phase diagrams of thin film, meanwhile introducing structural and physical inhomogeneities. In this work , we report 71$^{\circ}$ striped ferroelectric domains created in BFO can also epitaxially lock the perovskite manganites leading to the emerge of ordered structural domain. LSMO/BFO hetero-epitaxial samples are deposited by PLD. The 71$^{\circ}$ periodic striped domains and coherent growth are demonstrated by PFM and X-ray analysis. Plan-view TEM and X-ray RSM have been used to confirm the epitaxial relationships of the functional layers and IP lattice constant. Both the simulation and structural analysis demonstrate we can create a periodic ordered stripe structural domain in LSMO. And this will leave an anisotropic distribution of structural domain walls which makes it possible to capture the anisotropic tunneling for strong electron--lattice coupling in manganites. Temperature-dependent resistivity measurements reveal a substantial anisotropic resistivities and a remarkable shift of the MI transition between the perpendicular and parallel to the stripe domain directions. [Preview Abstract] |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M49.00012: Structural Characterization of Strain Relaxed (100)-Oriented Nd$_{0.5}$Sr$_{0.5}$MnO$_3$ Thin Films Di Lu, Yasuyuki Hikita, Arturas Vailionis, Hiroki Sato, Bongju Kim, Takeaki Yajima, Christopher Bell, Harold Hwang Half-doped manganites exhibit intriguing charge ordering (CO) properties. Pseudocubic (110)-oriented thin films can preserve bulk properties and show a charge ordering-ferromagnetic (CO-FM) transition. However, for (100) oriented films grown on traditional perovskite substrates, no CO-FM transition has been reported so far. Here we successfully realized the CO-FM transition in (100)-oriented $\mathrm{Nd_{0.5}Sr_{0.5}MnO_3}$ (NSMO) thin films grown on $\mathrm{SrTiO_3}$ substrates by inserting a perovskite-like flexible buffer layer $\mathrm{Sr_3Al_2O_6}$. From temperature-dependent X-ray elastic scattering, we observed changes in the NSMO lattice constants exactly at the CO-FM transition temperature determined from transport and magnetization measurements. Moreover, we observed CO peaks suggesting a different ordering pattern compared to the bulk or (110)-oriented thin films. These results provide new opportunities to create and study novel electronic ground states unexplored in films grown on the rigid substrates used up to now. [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M49.00013: Atomic manipulation with Scanning Tunneling Microscopy on the surface of a manganite thin film Rama Vasudevan, Alexander Tselev, Arthur Baddorf, Sergei Kalinin Manganites have attracted significant attention in the past two decades, due to an extraordinarily rich spectrum of phenomena stemming from inherent complexity linking spin, charge, lattice and orbital degrees of freedom that result in properties including half-metallicity and giant magnetoresistance. Here, we report atomic manipulation with STM on the surfaces of 25 unit-cell thick La5/8Ca3/8MnO3 (LCMO) SrTiO3 (STO) substrates. We demonstrate that by applying triangular first-order reversal curve (FORC) waveforms of increasing amplitude to STM tips in-situ, it is possible from both A and B terminations to individually extract single units, form vacancies, remove units from layers below, rearrange atoms in the surrounding lattice, and therefore cause reactions to occur at the atomic level. These experiments point to the possibility of STM to manipulate atoms on the surfaces of manganites, opening up further avenues of research into fundamental physical properties defined at atomic scales. This research was sponsored by the Division of Materials Sciences and Engineering (RKV, AT, SVK) and by the Scientific User Facilities Division (APB) of BES, DOE. Research was conducted at the CNMS, which is sponsored at ORNL by the Scientific User Facilities Division, BES, DOE. [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M49.00014: Critical thickness for ferromagnetism in insulating LaMnO$_{3}$ films X. Renshaw Wang, N. Poccia, D.P. Leusink, Tura R. Paudel, E.Y. Tsymbal, C.J. Li, W.M. Lv, T. Venkatesan, Ariando Ariando, H. Hilgenkamp The interplay between exchange interactions, interfacial charges, and confinement effects controls the electronic, magnetic, and transport properties of complex oxide thin films. Here we report the emergence of ferromagnetism in insulating LaMnO$_{3}$ thin films grown on SrTiO$_{3}$ substrates beyond a critical thickness. LaMnO$_{3}$ (001) films are deposited by a pulsed laser deposition technique with thicknesses varying from 1 unit cell to 24 unit cells. The position dependent local magnetization is then mapped with micrometer resolution using scanning superconducting quantum interference device microscopy. We find that the magnetic ground state switches from non-ferromagnetic to ferromagnetic within a change of one unit cell above the critical thickness of 5 unit cells with characteristic domain size of about 20 $\mu $m. Further increase of film thickness up to 24 unit cells leads to reduction of the domain size to about 10 $\mu $m. The critical thickness is qualitatively explained in terms of the charge transfer in polar LaMnO$_{3}$ (001) thin films based on results of additional experimental data, density-functional calculations, and the electrostatic modeling. [Preview Abstract] |
Wednesday, March 5, 2014 2:03PM - 2:15PM |
M49.00015: Atomically-Resolved In-Situ Studies of Surface Structure Evolution of PLD-Grown La$_{5/8}$Ca$_{3/8}$MnO$_{3}$ Thin Films Alexander Tselev, Rama Vasudevan, Liang Qiao, Michael Biegalski, Arthur Baddorf, Sergei Kalinin Here, we report atomically resolved in-situ Scanning Tunneling Microscopy (STM) studies of La$_{5/8}$Ca$_{3/8}$MnO$_{3}$ (LCMO) thin films grown by RHEED-assisted PLD. Films were grown on TiO$_{2}$-terminated (001) SrTiO$_{3}$ substrates at a substrate temperature of 750 $^{\circ}$C and O$_{2}$ pressure of 50 mTorr. \textit{In-situ} UHV STM was performed at room temperature. LCMO is known to grow in layer-by layer (LBL) mode. We find that the initial growth does not follow the best physically possible LBL growth (with only three u.c. layers exposed). RHEED oscillations decay during deposition of the first 10-15$^{\rm th}$ unit cells. Subsequently, the RHEED intensity oscillations grow and remain persistent. STM images of 16 u.c.-thick films revealed surfaces with up to five u.c. layers being exposed in a stepped island-like morphology with 1/2 u.c. step heights. Such morphology allowed studies of atomic surface structure of both terminations. 25 u.c.-thick samples were found to be almost single-terminated. The minor termination is ordered and exhibits (1x1) reconstructions; RHEED suggests that this termination is the La/Ca-O termination. A 250 u.c.-thick film was found to be single-terminated with only three u.c. layers exposed. [Preview Abstract] |
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