Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session R41: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides IV |
Hide Abstracts |
Sponsoring Units: DMP Chair: Maitri Warusawithana, Penn State University Room: Baltimore Convention Center 344 |
Wednesday, March 15, 2006 2:30PM - 2:42PM |
R41.00001: High-quality Thin Single-Crystal Y2O3 films Grown on Si (111) C.-W. Nieh, W.-C. Lee, Z.-K. Yang, Y.-J. Lee, Pen Cheng, T.-D. Lin, M. Hong, J. Kwo, C.-H. Hsu High-quality nano thick ($\sim $3nm) single-crystal Y2O3 films have been MBE-grown epitaxially on Si (111) despite a 2{\%} lattice mismatch. Studies carried out by x-ray diffraction, x-ray reflectivity, and cross-sectional HRTEM show the excellent structural properties. In-situ RHEED was used to monitor the growth quality, showing a sharp pattern of a (4x4) reconstruction. The strong oscillations of x-ray reflectivity and Pendellosung fringes surrounding the Y2O3 diffraction peaks point out that the film surfaces and the oxide/Si interfaces are atomically sharp and uniform. A record small FWHM of 0.01 degree in rocking scans was observed at Y2O3 (444), indicating the excellent quality of the nano films. The films are well aligned with the substrate with an orientation relationship of Si (111) // Y2O3 (111), and an in-plane expitaxy of Si [-110]// Y2O3 [-101]. In the thickness range of microelectronics applications, the nano Y2O3 layers have achieved a degree of perfection rarely observed in hetero-structures of oxide/semiconductor. [Preview Abstract] |
Wednesday, March 15, 2006 2:42PM - 2:54PM |
R41.00002: Comparison of the Surface Structures of SrTiO3 (100) and BaTiO3 (100). D. Li, M. Nikiforov, Dawn Bonnell While much research has been focused towards understanding the atomic structures on SrTiO3 surfaces, controversy still remains over the origin of the reconstructions. This talk will provide a rational description of the driving forces for previously observed and newly observed reconstructions on the (100) surface and compare these to analogous structures that occur on BaTiO3 (100). The degree to which the potential for ferroelectric ordering influences surface reconstruction will be discussed. [Preview Abstract] |
Wednesday, March 15, 2006 2:54PM - 3:06PM |
R41.00003: Coverage and bonding of thin, buried epitaxial SrTiO$_{3}$ on Si(100) Lena Fitting, Venugopalan Vaithyanathan, Melanie Jones, Darrell G. Schlom, David A. Muller SrTiO$_{3}$ can be used as a buffer layer for the growth of perovskite oxide heterostructures, opening up possibilities for the incorporation of novel materials into existing Si-based technology. While these layers have been studied during growth, a serious question for such thin layers is if their structure remains unaltered after they have been overgrown. Here, we present results of thin SrTiO$_{3}$ films grown on Si(100). Scanning transmission electron microscopy (STEM) of the buried structures shows the formation of islands and a non-uniform coverage of the first few monolayers after capping with a-Si. The island size increases if the SrTiO$_{3}$ film is grown on a SrSi$_{x}$ seed. An important question that arises is the existence of oxygen vacancies at the interface between the oxide and the substrate. Oxygen deficiency leads to a doping of the empty Ti 3d band in bulk SrTiO$_{3}$ with electrons, thereby inducing a metallic phase as the Ti formal valence changes from 4+ to 3+. Using electron energy loss spectroscopy in a STEM the Ti valence across the interface is probed on the atomic scale. [Preview Abstract] |
Wednesday, March 15, 2006 3:06PM - 3:18PM |
R41.00004: Structure of SrTiO$_3$ thin films grown on Si C. Stephen Hellberg, Kristopher Andersen Although SrTiO$_3$ films on Si have been extensively studied, the structure of the interface is not yet known. We present first-principles density-functional calculations of the energetics of the numerous proposed structures for the interface. We find that the energetically favored structure is one that has not been previously considered. The structure is consistent with x-ray and TEM probes of the interface. We discuss the implications of the interface for device applications. [Preview Abstract] |
Wednesday, March 15, 2006 3:18PM - 3:30PM |
R41.00005: Band offsets of SrTiO$_3$ thin films grown on Si Kristopher Andersen, C. Stephen Hellberg SrTiO$_3$ is a promising material to replace SiO$_2$ as the gate dielectric in metal-oxide-semiconductor field-effect transistors (MOS-FETs). The advantage of SrTiO$_3$ is its large dielectric constant: an order of magnitude greater than other promising oxides. However, the negligible conduction band offset found experimentally for heterostructures of SrTiO$_3$ (001) grown on Si (001) limits the usefulness of SrTiO$_3$ in devices because of leakage current. Here we present first-principles calculations on Si/SrTiO$_3$ heterostructures and investigate ways to modify the band offsets. Experimentally motivated structures are considered, and the effects of dopants and defects on the electronic structure are quantified. [Preview Abstract] |
Wednesday, March 15, 2006 3:30PM - 3:42PM |
R41.00006: COBRA Studies of Ultrathin Ferroelectric Oxide Films Codrin Cionca, D.P. Kumah, C-B Eom, D-M Kim, Y. Yacoby, D.A. Walko, R. Clarke We used Coherent Bragg Rod Analysis (COBRA) and x-ray synchrotron radiation to investigate the structure of epitaxial thin films of PbTiO$_{3}$ deposited by sputtering on SrTiO$_{3}$ substrates. The measurements were performed under different electric field conditions, including the presence of conducting electrodes. The data analysis reveals details of the atomic displacements in different layers and shows subtle variations as a function of distance from the interface. The effects of symmetry breaking at the substrate-film interface are discussed. [Preview Abstract] |
Wednesday, March 15, 2006 3:42PM - 4:18PM |
R41.00007: Ferroelectricity in Ultrathin Perovskite Films Invited Speaker: The ferroelectric phase transition in ultrathin perovskite films is exquisitely sensitive to strain and to compensation of depolarizing fields arising from divergences in polarization at interfaces. However, in most experiments there is significant uncertainty in the degree of compensation by internal or external space charge (even for electroded thin films), or by the formation of domain structures that minimize depolarization energy. This in turn affects interpretation of size effects in thin films, as depolarization contributions are film thickness dependent. Here we will describe our work utilizing in-situ synchrotron x-ray scattering to study the phase transition and equilibrium 180\r{ } domain structure in fully coherent, epitaxial PbTiO$_{3}$ thin films deposited onto SrTiO$_{3}$ and SrRuO$_{3}$/SrTiO$_{3}$ substrates by metalorganic chemical vapor deposition. We will show that we can control the degree of top interface compensation by controlling the gas environment in our deposition and analysis system, and monitor this via details of the 180\r{ } stripe domain pattern that forms for PbTiO$_{3}$ on insulating SrTiO$_{3}$. Furthermore, we will demonstrate that compensation can be accomplished at the ferroelectric film surface by adsorption of molecular or ionic species, in contrast to electronic mechanisms that occur in the presence of a conducting electrode. Exploitation of this for novel devices or templating techniques may be possible, both through chemical control of polarity and through polarization control of ionic adsorption. [Preview Abstract] |
Wednesday, March 15, 2006 4:18PM - 4:30PM |
R41.00008: Interface effect to the nanoscale ferroelectricity Chun-gang Duan, Renat F. Sabirianov, Wai-Ning Mei, Sitaram S. Jaswal, Evgeny Y. Tsymbal Recent experimental results demonstrate that in thin films ferroelectricity persists down to film thickness of a few unit cells. This finding opens an avenue for novel electronic devices based on ultrathin ferroelectrics, but also raises questions about factors controlling ferroelectricity and the nature of the ferroelectric state at the nanoscale. Here we report a first-principles study of KNbO$_3$ ferroelectric thin films placed between two metal electrodes, either SrRuO$_3$ or Pt. We show that the bonding at the ferroelectric-metal interface imposes severe constraints on the displacement of atoms, destroying the bulk tetragonal soft mode in thin ferroelectric films. This does not, however, quench local polarization. The critical thickness for the net polarization of the KNbO$_3$ film we found are about 1 nm for Pt and 1.8 nm for SrRuO$_3$ electrodes. [Preview Abstract] |
Wednesday, March 15, 2006 4:30PM - 4:42PM |
R41.00009: Ferroelectric properties of nm-scale barium and lead titanate films Y. -H. Tang, M. -H. Tsai The first-principles calculations of the total energies of nm-scale barium titanate (BaTiO$_{3})$ and lead titanate (PbTiO$_{3})$ films with respect to the off-center Ti ion displacement, d$_{Ti}$, have been carried out to understand ferroelectric properties of these films. Since the driving force of ferroelectricity is the attractive total dipole-dipole interactions associated with the collective off-center displacements of the transition metal ions in the octahedrons [1], the ferroelectric properties of nm-scale films is expected to be different from those of thick films because of the drastic reduction of the number of available dipoles. Thus, it is interesting to know whether the nm-scale films retain ferroelectricity. The calculated total energies as a function of d$_{Ti}$ along the (110) direction show a potential well at about 0.17 and 0.15{\AA} for BaTiO$_{3}$ and PbTiO$_{3}$, respectively. By symmetry, the potential well is the same for -- d$_{Ti}$. The existence of double potential wells is a characteristic of ferroelectricity. Thus, the total energy result shows that nm-scale BaTiO$_{3}$ and PbTiO$_{3}$ films retain ferroelectricity and resultant dipoles are parallel with the films. \newline \newline [1] M. --H. Tsai\textbf{, Y. --H. Tang}, and S. K. Dey, J. Phys.: Condens. Matter \textbf{15}, 7901 (2003). [Preview Abstract] |
Wednesday, March 15, 2006 4:42PM - 4:54PM |
R41.00010: Effects of surface layer composition in stabilizing thin film ferroelectrics Alexie Kolpak, Andrew Rappe Using first-principles density functional theory calculations, we explore the ramifications of chemically altering the properties of the metal/ferroelectric interfaces in thin film ferroelectric capacitors. We show that a 3 unit-cell-thick BaTiO$_3$ film with SrRuO$_3$ electrodes has a stable, bulk-like polarization when the surface BaO layers are replaced with PbO, in contrast to the unmodified SrRuO$_3$/BaTiO$_3$ film which only begins to support a small polarization at 8-unit cell thickness. At the other extreme, BaO surface layers completely suppress polarization in PbTiO$_3$ films even with Pt electrodes, which have been shown to stabilize a large bulk-like polarization in unaltered one unit-cell-thick PbTiO$_3$ films. Our results show that in addition to screening by the metal electrodes, the atomic structure and the work functions of the ferroelectric surface layers play key roles in determining stability. Furthermore, our work suggests that particular properties can be achieved in ferroelectric thin films through a combination of chemical surface modification and judicious choice of ferroelectric and electrode materials. [Preview Abstract] |
Wednesday, March 15, 2006 4:54PM - 5:06PM |
R41.00011: Visable-Deep UV Dielectric Functions and Electronic Band Structure of Lead Zirconate Titanate Thin Films Hosun Lee, T.D. Kang, Y.S. Kang, S.-J. Cho, H. Morkoc, J. Li, S.-H. Wei, P.G. Snyder We measure pseudodielectric functions in the visible-deep ultraviolet spectral range of Pb(Zr$_{x}$Ti$_{1-x})$O$_{3}$ (x=0.2, 0.56, 0.82) (PZT), Pb$_{0.98}$Nb$_{0.04}$(Zr$_{0.2}$Ti$_{0.8})_{0.96}$O$_{3}$,$_{ }$Pb$_{0.91}$La$_{0.09}$(Zr$_{0.65}$Ti$_{0.35})_{0.98}$O$_{3}$, and$_{ }$Pb$_{0.85}$La$_{0.15}$Ti$_{0.96}$O$_{3}$ films. Using a parametric optical constant model, we estimate the dielectric functions ($\varepsilon )$ of the perovskite oxide thin films. Taking the second derivative of the fitted layer dielectric functions and using the standard critical point model, we determine the parameters of the critical points. In the second derivative spectra, the lowest band gap energy peak near 4 eV is fitted as a double peak for annealed PZTs due to perovskite phase. As-grown PZTs have mainly pyrochlore phase and the lowest band gap peak is fitted as a single peak. We also examine the effect of dopants La and Nb, which substitute at Pb and Zr (Ti) sites, respectively. We found three band gaps E$_{a}$ ($\sim $3.9eV), E$_{b}$ ($\sim $4.5 eV)and E$_{c}$ ($\sim $6.5$_{ }$eV) in the order$_{ }$of increasing energy. The E$_{a}$ and E$_{b}$ band gap energies were not sensitive to Zr composition. We discuss the change of critical point (CP) parameters for PZTs in comparison to the band structure calculations based on local-density-approximation. The near-constancy of the lowest band gap energy independent of Zr composition is consistent with the band calculations. [Preview Abstract] |
Wednesday, March 15, 2006 5:06PM - 5:18PM |
R41.00012: Electrostatics of superlattices by first principles Xifan Wu, Oswaldo Di\'eguez, Karin Rabe, David Vanderbilt A complete theory of epitaxial perovskite superlattices requires an understanding of both epitaxial strain effects and of electrostatic boundary conditions. Here, focusing on the latter issue, we have carried out first-principles calculations of the nonlinear dielectric properties of short-period BaTiO$_3$/SrTiO$_3$ and PbTiO$_3$/SrTiO$_3$ superlattices having the in-plane lattice constant of SrTiO$_3$. In particular, we have mapped the energy $E$ vs.~polarization $P$ for these superlattices, and for the parent bulk materials, by extending the $E(P)$ mapping algorithm of Di\'eguez and Vanderbilt to handle strain relaxation in the $c$-direction. We then test whether the superlattice results can be understood quantitatively in terms of the $E(P)$ information for the bulk materials and similar information on the interfaces. We reformulate the theory in terms of the electric displacement field $D$ (since this is uniform throughout the superlattice) and the corresponding internal energy $U(D)$, and investigate how to model $U(D)$ in terms of ab-initio derived bulk and interface information. We stress that once the $U(D)$ is obtained for a given superlattice, all the electrostatic properties, both linear and non-linear, can be predicted. [Preview Abstract] |
Wednesday, March 15, 2006 5:18PM - 5:30PM |
R41.00013: Epitaxial strain effects from first principles Claude Ederer, Nicola Spaldin Epitaxial strain can substantially enhance the spontaneous polarizations and Curie temperatures of ferroelectric thin films compared to the corresponding bulk materials. In this work we use first principles calculations to calculate the effect of epitaxial strain on the spontaneous polarization of the ferroelectrics BaTiO$_3$, PbTiO$_3$, and LiNbO$_3$, and the multiferroic material BiFeO$_3$. We show that the epitaxial strain dependence of the polarization varies considerably for the different systems, and in some cases is in fact very small. We discuss possible reasons for this different behavior, and show that the effect of epitaxial strain can easily be understood in terms of the piezoelectric and elastic constants of the unstrained materials. Our results provide a computational tool for the quantitative prediction of strain behavior in ferroelectric thin films. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700