Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S20: Focus Session: Ferroelectric Thin Films |
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Sponsoring Units: FIAP DMP Chair: Fred Walker, ORNL Room: LACC 407 |
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S20.00001: Giant Enhancement of Ferroelectricity in Strained BaTiO3 Thin Films Invited Speaker: Epitaxial thin films often have quite different properties than bulk single crystals due to epitaxial and thermal strains arising from substrate constraints. This offers the opportunity to modify ferroelectric properties by heteroepitaxy and strain engineering. Biaxial compressive strain has been used to dramatically enhance the ferroelectric properties of BaTiO$_{3}$ thin films on (110) GdScO$_{3}$ and (110) DyScO$_{3}$ substrates [1]. This strain, imposed by commensurate epitaxy, can result in a ferroelectric transition temperature ($T_{c})$ nearly 500~\r{ }C higher and a remanent polarization ($P_{r})$ at least 250{\%} higher than bulk BaTiO$_{3}$ single crystals. This is the largest increase in $T_{c}$ ever reported for a ferroelectric, and it is consistent with thermodynamic prediction. This work demonstrates a route to a lead-free ferroelectric for non-volatile memories and electro-optic devices, and is a general means for achieving extraordinary physical properties in thin films through strain engineering. [1] K. J. Choi et al. \textit{SCIENCE}, \textbf{306}, 1005 (2004). This work has been done in collaboration with K. J. Choi, M. Biegalski, Y. L. Li, A. Sharan, J. Schubert, R. Uecker, P. Reiche, Y. B. Chen, X. Q. Pan, V. Gopalan, L.-Q. Chen, D. G. Schlom. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S20.00002: Polarization Scaling in Ultra-thin Epitaxial Ferroelectric Heterostructures: Experimental Results R. Ramesh, V. Nagarajan, J.Q. He, C. Jia, H. Kohlstedt, R. Waser, S. Prasertchoung, T. Zhao, K. Lee, Y.K. Kim, S. Baik Scaling of the structural order parameter and the polarization was investigated in ultra-thin epitaxial PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ /SrRuO$_{3}$/SrTiO$_{3}$ model ferroelectric heterostructures. High Resolution Electron Microscopy and Synchrotron X-Ray studies show that a high tetragonality (c/a$\sim $1.06) is maintained down to 40 {\AA} thick films, suggesting indirectly that ferroelectricity is fully preserved at such ultrathin thicknesses. However, measurement of the switchable polarization ($\Delta P)$ using a novel pulsed probe setup revealed a systematic drop from $\sim 140\mu C/cm^2$ for a 150 {\AA} thick film to $11\mu C/cm^2$ for a 40 {\AA} thick film. This contradiction between the structural measurements and the measured switchable polarization is explained by an increasing presence of a strong depolarization field, which creates a pinned 180$^{o}$ polydomain state for the thinnest films. This work was supported by MRSEC Grant {\#} 00-8008, DOE Grant DE-FG02-01ER45937 and NSF-DFG Grant 02-44288. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S20.00003: Polarization scaling in ultrathin epitaxial ferroelectric heterostructures: First-principles results Javier Junquera, Philippe Ghosez, Karin M. Rabe To resolve the apparent inconsistency between the high c/a and the low measured switchable polarization of ultrathin Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_3$ (PZT) films \footnote{V. Nagarajan {\it et al.}, previous abstract}, we have carried out first-principles effective hamiltonian simulations. The epitaxial strain constraints and the thickness dependent residual depolarization field $\mathcal{E}$$_d$, arising from an incomplete screening of the dipole surface density by real metallic electrodes, \footnote{ J. Junquera and Ph. Ghosez, Nature {\bf 422}, 506 (2003)} are properly included. As the thickness decreases below 150\AA, the increase of $\mathcal{E}$$_d$ in the uniformly polarized state drives a phase transition to a state with 180$^o$ stripe domains, similar to that observed for PbTiO$_3$ films on insulating substrates \footnote{D. D. Fong {\it et al.}, Science, {\bf 304} 1650 (2004)}. Although the net polarization is zero, each domain exhibits the bulk strained polarization and tetragonality, 1.25 \% larger than in the unstrained sample, yielding a consistent interpretation of the experimental data. Work supported by DOE Grant DE-FG02-01ER45937 [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S20.00004: Kinetics of polarization switching in epitaxial BaTiO3 David Towner, Bruce Wessels The kinetics of polarization switching in BaTiO$_{3}$ ferroelectric epitaxial thin films were studied using optical second harmonic generation (SHG). Epitaxial films were polydomain having both a and c variants. Upon application of a poling field the SHG signal increased according to the expression 1- exp-At$^{n}$ . The rise is attributed to a time dependent increase in the fraction of aligned domains. The kinetic exponent n was of the order of 0.2 indicating fractal dimensionality. The observed kinetics are consistent with a model developed to describe the electro-optic and dielectric relaxation response of polydomain thin films with a continuous distribution of domain sizes (Hoerman PRB 65 2002). [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S20.00005: Ferroelectric properties of epitaxial BiFeO3 thin films grown by MOCVD S. Y. Yang, F. Zavaliche, Y.H. Chu, Y.J. Lee, L. Mohaddes-Ardabili, T. Zhao, Q. Zhan, R. Ramesh Recently, perovskite BiFeO$_{3}$ (BFO) has attracted great attention due to the coexistence of ferroelectric and magnetic properties. Particularly, ferroelectric properties in thin films make the BFO an outstanding candidate as a substitute for Pb-based ferro/piezoelectric material. Epitaxial BFO thin films were grown by Metallorganic chemical vapor deposition (MOCVD) equipped with liquid delivery system on SrRuO$_{3}$/SrTiO$_{3}$ (001) using Bi(thd)$_{3}$ and Fe(thd)$_{3}$ as precursors. A systematic study on deposition process control such as stoichiometric composition and growth temperature was carried out. To investigate the effect of thickness on structure and ferroelectric properties, BFO films with thicknesses in the range from 20 to 200 nm were grown. In addition, results will be discussed by comparison with size effect obtained from Pb(Zr$_{0.2}$Ti$_{0.8})$O$_{3}$ thin films. This work has been supported by the ONR under a MURI program and partly under the UMD-MRSEC program. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S20.00006: Finite Temperature Properties of KTaO$_3$ Thin Films from First-Principles Alireza Akbarzadeh, Laurent Bellaiche, Kevin Leung, Jorge \'I\~niguez, David Vanderbilt Thin films made of the incipient ferroelectric KTaO$_3$ are studied using a parameterized effective Hamiltonian, $H_{\rm eff}$. Quantum effects are turned off and on by performing classical Monte Carlo and, path integral quantum Monte Carlo simulations respectively. The films are simulated to be grown along the [001] pseudo-cubic direction. Different electrical and mechanical boundary conditions are investigated. Particular striking predictions are (1) that, unlike in the bulk, quantum effects are unable to suppress ferroelectricity in KTaO$_3$ thin films and, (2) the formation of complex ferroelectric nanodomains, depending on the boundary conditions.\\ This work is supported by ONR grants N 00014-01-1-0365, N 00014-04-1-0413 and N 00014-01-1-0600 and NSF grants DMR-9983678 and DMR-0404335 [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S20.00007: Local Electro-Optic Response of Strained SrTiO$_3$ Films Grown on DyScO$_3$ Hongzhou Ma, Jeremy Levy, Mike D. Biegalski, Darrell G. Schlom, Susan Trolier-McKinstry, R. Uecker, P. Reiche The electro-optical response of SrTiO$_3$ thin films grown on DyScO$_3$ substrates was studied with confocal scanning optical microscopy (CSOM) and apertureless near-field scanning optical microscopy (ANSOM). The polarization dependent electro-optical coefficients reveal that the c axis of the strained SrTiO$_3$ is oriented in-plane along the (110) and (-110)directions. The hysteretic electro-optic response at room temperature results from uniform strain in the SrTiO$_3$ film. Time-resolved ANSOM is used to study the domain dynamics at microwave frequencies. This work was supported by the National Science Foundation through grants DMR-0103354 and DMR-0333192, and the U.S. Department of Energy through contract W-31-109-ENG-38. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S20.00008: Local Piezoresponse of Strained BaTiO$_3$/Si Heterostructures Ajay K. Kochhar, Jeremy Levy, Venu Vaithyanathan, Darrell G. Schlom Ferroelectric-silicon heterostructures can provide new functionality with applications for classical and quantum computing architectures. For quantum computing, it is important to have reversible polarization oriented parallel to the growth direction. Strained BaTiO$_3$/relaxed (Ba,Sr) TiO$_3$/Si heterostructures were grown by oxide-MBE. X-ray diffraction measurements indicate an out-of-plane orientation for the BaTiO$_3$ films. Thin (1.5 nm) Au electrodes are deposited on top of the films, and piezoresponse measurements are performed using an atomic-force microscope in contact mode. Local hysteresis curves are imaged by an interleave method that ensures proper registry of successive scans taken at various applied bias voltages. The results suggest that such thin film structures exhibit local hysteresis with varying coercive fields over sub-micrometer regions. This work was supported by DARPA QuIST DAAD-19-01-1-0650. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S20.00009: Ferroelectric domain structure in BiFeO3 films F. Zavaliche, Y. H. Chu, J. Wang, S.Y. Yang, E. Reilly, T. Zhao, Q. Zhan, L. Mohaddes-Ardabili, R. Ramesh In bulk, BiFeO$_{3}$ (BFO) possesses a rhombohedrally distorted perovskite structure; in thin films, the structure is sensitive to heteroepitaxial constraints, and a large polarization was measured. To investigate the ferroelectric domain structure, films of various thicknesses were grown by both pulsed laser deposition, and metal-organic chemical vapor deposition on (001), (110) and (111) oriented SrTiO$_{3}$ (STO) substrates. The ferroelectric domain structure was studied by piezoelectric force microscopy. Contrary to the case of films grown on STO(111), we found that as-grown BFO films on (100) and (110) oriented STO show a strong in-plane polarization component for thicknesses above $\approx $30 nm. This finding is in agreement with the atomistic model of ferroelectricity in distorted rhombohedral BFO. The stability of switched domains is also investigated. This work has been supported in part by the U. of Maryland NSF-MRSEC under grant {\#}DMR 00-80008, and by the ONR under a MURI program. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S20.00010: Size Effect in Ferroelectric BiFeO$_3$ Films T. Zhao, F. Zavaliche, L. Mohaddes-Ardabili, S.Y. Yang, Y.H. Chu, Q. Zhan, H. Zheng, E. Reilly, D.G. Schlom, R. Ramesh BiFeO$_{3}$ (BFO) has recently attracted attention because it is considered as a promising candidate for the lead-free ferroelectric memory cells and/or piezoelectric sensors and actuators. However the understanding of ferroelectricity of BFO is still limited, especially when the vertical and lateral dimensions decrease in thin films. We are studying the ferroelectric and piezoelectric responses of epitaxial BFO films grown by pulsed laser deposition on single crystal SrTiO$_{3}$(STO) (100,110, 111 orientations) as well as STO/Si. The epitaxy of the films were confirmed by TEM and XRD. The electric properties were characterized by polarization hysteresis, pulsed polarization and piezoelectric force microscopy (PFM) measurements. The polarization in BFO is along the (111) direction of its rhombohedrally distorted perovskite structure which is different from the (001)-polarization in tetragonal PZT. We observe a systematic decrease in piezoresponse as the thickness is decreased from 400nm down to 30nm. We will present the results of this systematic study in this paper. This work is supported by an ONR-MURI. [Preview Abstract] |
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S20.00011: Nanoscale smoothing and the analysis of interfacial charge and dipolar densities Javier Junquera, Morrel H. Cohen, Karin M. Rabe, Philippe Ghosez The transfer of charge that occurs in a metal-dielectric interface is a difficult issue, whose deep understanding requires first-principles calculations. The problem then arises about how to extract from the inmense detail provided by the first-principles results the physical quantities of interest. The difficulty comes from the fact that the interface-related dipole densities are overwhelmed by the much larger variations of the total microscopic charge density. Accordingly, nano-smoothing procedures have been developed \footnote{L. Colombo {\it et al.}, Phys. Rev. B {\bf 44}, 5572 (1991)} in order to localize the physically relevant charge densities to the interface. Here we discuss the criteria for validity that the smoothing procedure should meet to leave the physical quantities unaffected. We have applied the model to compute directly the polarization charge density of a realistic ferroelectric capacitor \footnote{J. Junquera and Ph. Ghosez, Nature {\bf 422}, 506 (2003)} and find it to be much smaller than the bulk ferroelectric polarization. We attribute this reduction to the penetration of the metallic wave functions into the ferroelectric, which leads to screening of the polarization charge within the ferroelectric itself. Work supported by DOE Grant DE-FG02-01ER45937 [Preview Abstract] |
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