Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W20: Oxide Surfaces, Interfaces and Thin Films |
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Sponsoring Units: DCMP Chair: Arthur Smith, Ohio University Room: Morial Convention Center 212 |
Thursday, March 13, 2008 2:30PM - 2:42PM |
W20.00001: Electronic structure of epitaxial CrO$_{2}$(100) and CrO$_{2}$(110) films C. A. Ventrice Jr, H. Geisler, D. R. Borst, G. X. Miao, A. Gupta Half-metallic ferromagnets are conducting solids whose conduction electrons undergo magnetic ordering with a spin polarization of 100{\%} at 0 K. Although CrO$_{2}$ is predicted to be a half-metallic ferromagnet, previous attempts to make devices using CrO$_{2}$ have resulted in a degradation of performance instead of an enhancement. Using ultra-violet photoelectron spectroscopy at the CAMD synchrotron, we have measured the electronic properties of epitaxial CrO$_{2}$(110)/TiO$_{2}$(110) and CrO$_{2}$(100)/TiO$_{2}$(100) surfaces grown using a CrO$_{3}$ precursor. Clean, stoichiometric CrO$_{2}$ surfaces have been prepared either by exposure to white light, which desorbs OH groups from the surface, or by sputtering and annealing in O$_{2}$. The measurements of the CrO$_{2}$ surfaces show no emission at E$_{F}$ after sputtering and annealing the surfaces in oxygen. However, the white light prepared surfaces show a small density of states at E$_{F}$. Photon energy dependent photoemission experiments show no increase in the density of states at E$_{F}$ as the photon energy is lowered from 50 eV to 15 eV, which increases the bulk sensitivity of these measurements. These results indicate that CrO$_{2}$ behaves more like a semi-metal than a half-metal and that surface disorder can induce a semi-metal to semiconductor transition at its surfaces. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W20.00002: Variation of Growth Mode with Orientation for Epitaxial CrO$_{2}$ Films Krishna Chetry, Hunter Sims, William Butler, Arun Gupta Thin films of CrO$_{2}$ have been grown epitaxially on (110) and (100) oriented TiO$_{2}$ substrates. CrO$_{2 }$(100) films grow in a layer by layer mode, while an island growth mode is observed for CrO$_{2}$(110) films as confirmed from atomic force microscopy (AFM) studies. To better understand the differences in the growth mode, we have performed first principles-based calculations using density functional theory implemented within the VASP code to study the surface and interface energies of CrO$_{2}$ (100), (110), TiO$_{2}$ (100) and (110) systems. For these calculations a periodic repeating slab geometry is used with a sufficient vacuum width and thickness to converge the surface energy within 0.01J/m$^{-2}$ From our calculations we find that in case of (110) orientation $\sigma _{TiO_2} >\sigma _{CrO_2 } +\gamma $, where $\sigma $ is the surface energy and $\gamma $ is the interface energy between CrO$_{2}$ and TiO$_{2}$ system. This result is consistent with the island growth mode observed experimentally for (110) orientation. For the case of (110) orientation also we find that $\sigma _{TiO_2} <\sigma _{CrO_2 } +\gamma $ , which does not match with our experimental results. We speculate that formation of some oxygen deficient phase of chromium oxide in the very first monolayer, which then gets converted to CrO$_{2}$ by accepting oxygen from the second layer, favors the layer-by-layer growth mode in CrO$_{2}$ (100). [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W20.00003: Self-Assembled Epitaxial Multiferroic Nanocomposite Films Prepared by Polymer-Assisted Deposition Hongmei Luo, Hao Yang, Eve Bauer, T. Mark McCleskey, Anthony K. Burrell, Quanxi Jia Multiferroic materials, which show simultaneous electric and magnetic ordering, have attracted considerable interest recently due to their unusual physical properties and potential device applications. Here we demonstrate that a cost-effective chemical solution approach of polymer-assisted deposition (PAD) is a very promising technique to grow self-assembled epitaxial multiferroic nanocomposite thin films: such as BaTiO$_{3}$-NiFe$_{2}$O$_{4}$ films grown on (001)-oriented LaAlO$_{3}$ substrate. X-ray diffraction (XRD), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM) analyses show clear epitaxial relationship between the two phases and the substrate. The ferroelectric BaTiO$_{3}$ grains are embedded in the ferrimagnetic spinel NiFe$_{2}$O$_{4}$ matrix. The composite films exhibit both ferroelectric and ferrimagnetic properties. The structure and properties will be discussed and compared with the nanocomposite films prepared by pulsed-laser deposition (PLD) method. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W20.00004: Structural and electrical properties of self-assembled (BiFeO$_{3})_{0.5}$:(Sm$_{2}$O$_{3})_{0.5}$ nanocomposite films Hao Yang, H. Wang, J.L. MacManus-Driscoll, Q.X. Jia Self-assembled (BiFeO$_{3})_{0.5}$:(Sm$_{2}$O$_{3})_{0.5}$ nanocomposite films were deposited on (001) SrTiO$_{3}$ and Nb-doped SrTiO$_{3}$ substrates by pulsed laser deposition using a single uniformly mixed target. Analysis from both high-resolution X-ray diffraction and transmission electron microscopy revealed self-assembled epitaxial two-phase BiFeO$_{3}$ (BFO) and Sm$_{2}$O$_{3}$ (SmO) composites in nanoscale. The BFO and SmO domains have grown alternately and vertically aligned with average column size of 10 nm. The dielectric properties of BFO:SmO nanocomposite films were investigated and compared with those of pure BFO and SmO thin films. The dielectric constant of the nanocomposites can be well described by a parallel connection of two individual dielectrics of BFO and SmO. On the other hand, the dielectric loss of nanocomposite films is lower than the theoretical value from such a parallel connection model. This might originate from the reduction of leakage current density of BFO phase in nanocomposite film due to the much larger interfacial area and the strong out-of-plane strain of each BFO column. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W20.00005: Checkerboard pattern formation in spinel oxide films Toshihiro Asada, Yoichi Horibe, Soonyong Park, Nara Lee, Makoto Tanimura, Sang-Wook Cheong A few-nanometer-size columnar superlattice with a checkerboard (CB) structure has been fabricated by harnessing Jahn-Teller structural distortions$^{1,2}$. In this talk, we will discuss the growth mechanism of the CB structures in thin films, obtained from the results of our transmission electron microscopy. Ionic diffusion for the phase separation occurs along the direction parallel to the twin boundaries in the high-temperature tetragonal phase. This anisotropic phase separation suggests the importance of the strain associated with the twin structure in the high-temperature tetragonal phase. 1. S. Yeo \textit{et al.}, Appl. Phys. Lett. \textbf{89}, 233120 (2006). 2. C. L. Zhang \textit{et al}., Appl. Phys. Lett. \textbf{90}, 133123 (2007). [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W20.00006: Thermal Stability of Epitaxial SrTiO$_{3}$ Thin Films on Si (001) G. Yong, Rajeswari Kolagani, S. Adhikari, W. Vanderlinde, L. Salamanca-Riba, Y. Liang, S. Friedrich Epitaxial SrTiO$_{3}$ on Si(001) is important for application as a high $\kappa$ (Kappa) gate oxide and is also suitable as a buffer layer for the subsequent growth of other perovskite oxide thin films to enable integration of perovskite oxide functionality with Si. For the latter application, the thermal stability of the interface structure in epitaxial SrTiO$_{3}$ thin films grown by molecular-beam epitaxy on Si (001) becomes a key issue since most of the perovskite layers are grown at relatively high temperatures (750-850\r{ }C). SrTiO$_{3}$ and Si are intrinsically thermodynamically unstable in proximity to each other, with some reduction of the rTiO$_{3}$ and oxidation of the Si to be expected. We have used x-ray diffraction and micro-analytical techniques including optical microscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to study the thermal stability of epitaxial SrTiO$_{3}$ thin films grown by molecular beam epitaxy (MBE) on Si (001). Elemental composition of the amorphous interface layer (AIL) and of the epitaxial SrTiO$_{3}$ film will change during various thermal treatments and gas environments encountered in the subsequent growth of other oxide layers. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W20.00007: Design and characterization of Pb(Zr,Ti)O$_{3}$ /CoFe$_{2}$O$_{4}$ Multilayer epitaxial thin films Nora Ortega, Ashok Kumar, Ram Katiyar Mutiferroics are a novel class of next generation multifunctional materials. Intensive research is being pursued towards the development of materials with high magnetoelectric (ME) coupling. We have fabricated epitaxial Pb(Zr,Ti)O$_{3}$--CoFe$_{2}$O$_{4}$ (PZT-CFO) multilayer (ML) thin films using pulsed laser deposition on (001) oriented lattice matched SrRuO$_{3}$/SrTiO$_{3}$ (SRO/STO) substrates. X-ray diffraction and Raman analysis revealed that PZT and CFO were in the perovskite and spinel phases respectively, in the multilayer thin films having high crystalline quality. The TEM and STEM line scan of the multilayer thin films showed that the layered structure was maintained. Magnetic hysteresis loop showed the ferromagnetic behavior of ML structure, which is independent of the ML configuration. Ferroelectric properties and temperature dependence magnetization will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W20.00008: Epitaxial oxide heterostructures on silicon Agham Posadas, J.W. Reiner, F.J. Walker, C.H. Ahn Silicon-based MOSFETs will soon be limited by the large off- state leakage current due to tunneling through the ~1 nm thick silicon oxynitride gate dielectric layer. One solution is to replace the silicon oxynitride with a high dielectric constant material, such as LaAlO$_3$, which has a relatively large dielectric constant of \~24 and band gap of 5.6 eV. We have recently grown LaAlO$_3$ epitaxially onto silicon via a transition layer consisting of SrTiO$_3$. The thickness of this SrTiO$_3$ layer is kept between 2 and 5 unit cells because of considerations of epitaxial strain and the atomic-scale interactions between the perovskite structure and the silicon substrate. The oxide heterostructures show atomically abrupt interfaces and dielectric constants close to the bulk value of LaAlO$_3$. Frequency and voltage dependent measurements of the complex impedance of the as-grown oxide heterostructures show a pinned Fermi level and a high density of interface states. Annealing at low temperatures in wet oxygen shows that the Fermi level can be unpinned, with a greatly reduced density of interface states. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W20.00009: Pulsed Laser Deposition of SrTiO$_{3}$ Thin Films: Time-Resolved X-ray Studies Joel Brock, Gokhan Arikan, John Ferguson, Arthur Woll X-ray scattering and reflectivity have been used to measure the static atomic structure of surfaces and interfaces for several decades. Modern synchrotron x-ray facilities now deliver sufficient flux to make time-resolved measurements on a wide variety of evolving surface and interface systems feasible. Here, we use time-resolved x-ray diffuse scattering studies of Pulsed Laser Deposition (PLD) of SrTiO$_{3}$ films on the (001) surface of SrTiO$_{3}$ to obtain detailed insight into the fundamental, atomic-scale growth mechanism. The data demonstrate that during layer-by-layer growth, islands are nucleated during the 1$^{st}$ pulse. The size of the pulse determines the nucleation density. During and in between subsequent pulses, the islands coarsen and coalesce. Line shape analysis of the diffuse scattering reveals that adatom capture occurs at the same time as coarsening. The time-resolved x-ray data obtained as a function of miscut further demonstrate that kinetic growth models based on adatom diffusion on singular surfaces are missing essential physical mechanisms which are step-edge density dependent. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W20.00010: Nucleation of MB$_2$ thin films on Si by low temperature CVD: island statistics and growth kinetics A. Yanguas-Gil, N. Kumar, J.R. Abelson HfB$_2$ thin films are deposited onto H-terminated Si by CVD using the single source precursor Hf(BH$_4$)$_4$, which affords extremely conformal and smooth film growth. HfB$_2$ films are technologically interesting as hard coatings and as impurity diffusion barriers. We analyze the nucleation process by in-situ spectroscopic ellipsometry (SE) and ex-situ AFM. The objective is to derive mechanistic information on the growth kinetics from a statistical analysis of the early stages of growth. The experimental results reveal that HfB$_2$ nucleates on Si(001) forming 3D islands and that there is an agreement between the SE data and the AFM measurements. Quantitative information on island statistics is extracted from AFM data taken after various growth times. The island height distribution functions are consistent with the presence of a physisorbed state of the precursor molecule. This situation is similar to that of the capture zone 2D island growth models in the literature and is in agreement with previous results on the steady state growth of HfB$_2$. The island spatial distribution and the correlation between island height and capture zone area indicate that nucleation is random and keeps on taking place until coalescence is reached. We show that the nucleation rate can be enhanced by surface activation, leading to a reduction in the grain size and lateral correlation length of the films. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W20.00011: STM study of MOCVD-grown a-plane ZnO thin films: film thickness, growth temperature, and substrate miscut effects O. Dulub, U. Diebold, G. Saraf, Y. Lu ZnO films with a-plane orientation were grown on r-plane (011bar2) sapphire substrates using metal-organic chemical vapor deposition (MOCVD). The surface morphology of ZnO films with various thicknesses (20 -- 2000 nm), growth temperatures (300 - 580$^{o}$C), and substrate miscut orientations and angles (0.2 - 2$^{o})$ was characterized by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Our results show strong dependence of surface morphology on growth parameters and the substrate miscut. STM images reveal uniform surfaces with small, rectangular terraces during the initial growth stage (20 nm-thick film). Films with thicknesses between 100 and 450 nm have a characteristic wave-like surface morphology with needle-shaped domains running predominantly along the crystallographic c-direction. Films with a thickness of 2000 nm exhibit more flat surfaces, but with c-oriented facets. We observed an increasing surface quality of the films with increasing growth temperature. Films grown at 580$^{o}$C exhibit the flattest surface morphology. Film morphologies show strong dependency on the substrate miscut angle as well as on the miscut direction at all growth conditions. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W20.00012: Avrami kinetics of oxide film decomposition on Si(100) Ikuya Kinefuchi, Hiroki Yamaguchi, Yukinori Sakiyama, Shu Takagi, Yoichiro Matsumoto The thermal decomposition of ultrathin oxide layers on silicon surface was investigated with TPD technique. Oxide layers were formed on Si(100) at 400$^{\circ}$C by exposure to O$_{2}$ molecular beam. The desorption spectrum of SiO for the initial coverages between 1.7 and 2.6 monolayers exhibits a relatively low peak at the lower temperature and a high peak at the higher temperature. The former peak corresponds to the unstable binding state, where O atoms are presumably trapped at the dangling bonds. The latter corresponds to the more stable binding states of O atoms at the dimer bridge sites and the dimer backbond sites. The most of O atoms are at the stable binding states, from which the desorption rate is well described by Avrami kinetics. This result is consistent with the reaction model which takes account of the void formation and growth as observed in STM studies. The rate-determining step is the reaction at void perimeter even if the overlap between voids becomes quite large. The Avrami exponents determined from our experiment suggest that the increase in the initial coverage makes the oxide adlayer more stable and suppresses the rate of void nucleation at the potential nucleation sites. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W20.00013: Effect of Surface Roughness on Oxidation: Changes in Scale Thickness, Composition, and Residual Stress Serif Uran, Boyd Veal, Marcos Grimsditch, John Pearson, Andreas Berger The effect of surface roughness on the properties of the oxide scale formed on Fe--Cr--Al alloys during oxidation in air at high temperatures has been investigated. Large and systematic differences in scale thickness, in the composition of the oxides forming the scale, and in the residual stress levels are found. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W20.00014: UHV Studies of Oxygen's Role in Hydrogen Sensing with a Platinum-Gate Silicon Carbide Field-Effect Device Roger Tobin, Yung Ho Kahng, Reza Loloee, Ruby Ghosh Silicon carbide-based field-effect devices with catalytic metal gates are promising as robust high-temperature gas sensors in harsh environments. We report ultrahigh vacuum studies of the gate surface chemistry of prototype Pt-SiO$_{2}$-SiC sensors. Oxygen plays a crucial role in the device's hydrogen-sensing behavior. Adsorbed oxygen can remove hydrogen by reacting to form water, which rapidly desorbs. In an oxygen-rich environment this reaction competes with the diffusion of adsorbed hydrogen to the interface bonding sites that give rise to the sensor signal. This competition reduces the sensor signal by decreasing the occupation of interface sites. The same reaction, however, is essential to the reversibility of the sensor, as oxygen is needed to fully deplete the sensor of hydrogen. Exposure to H$_{2}$S suppresses the sensor's response to alternating hydrogen and oxygen pulses, apparently by interfering with oxygen adsorption. Continued oxygen exposure, however, restores functionality by effectively removing the sulfur. [Preview Abstract] |
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