Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V20: Focus Session: Superlattices and Oxides on Silicon |
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Sponsoring Units: DMP Chair: Kristopher Andersen, Naval Research Laboratory Room: Colorado Convention Center 105 |
Thursday, March 8, 2007 11:15AM - 11:27AM |
V20.00001: Tailoring the properties of artificially layered ferroelectric superlattices Matthew Dawber, Nicolas Stucki, Celine Lichtensteiger, Stefano Gariglio, Jean-Marc Triscone A key attraction of artificial ferroelectric superlattices is the potential to be able to tailor the properties of the material to a particular application. Here we demonstrate that the key ferroelectric parameters, polarization and critical temperature can be tuned over a very large range in PbTiO$_{3}$/SrTiO$_{3}$ superlattices by varying the ratio of the layer thicknesses. It is shown that the polarization can be tuned from 0-60 $\mu C/cm^{2}$ and the transition temperature from room temperature to 700$^o$C while maintaining a perfect crystal structure and low leakage currents in these heterostructures. We developed a simple model based on Landau theory that would guide straightforward production of samples with ferroelectric properties designed for particular applications. We also explore the phase transition behaviour with temperature in superlattices with very thin PbTiO$_{3}$ layers where we find not only unexpected evidence of ferroelectricity but also an unusual relationship between strain and polarization. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V20.00002: Electrostatics of superlattices by first principles Xifan Wu, Oswaldo Di\'eguez, Massimiliano Stengel, 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 calculated the layer polarizations $p_j$ as defined using the Wannier-based method of Wu, Di\'eguez, Rabe and Vanderbilt\footnote{X. Wu, O. Di\'eguez, K. Rabe and D. Vanderbilt, Phys.\ Rev.\ Lett.\ {\bf 97}, 107602 (2006).} for each neutral BaO, SrO, PbO, or TiO$_2$ layer, and modeled $p_j$ as a function of displacement field $D$ (which is uniform throughout the superlattice), the chemical identity of the layer itself, and the chemical identity of its near neighbors. We then test our expectation that the dependence on the identity of neighboring layers should decay rapidly with distance. If we apply a cut-off to the range of this interlayer interaction, we arrive at a model description that allows us to predict $p_j(D)$ for each layer, and thus the overall $P(D)$ (and trivially, also $P$ vs. electric field and related quantities) for a superlattice of arbitrary layer sequence. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V20.00003: Interfacial Intermixing in Ferroelectric Superlattices from First Principles Valentino R. Cooper, Karen Johnston, Karin M. Rabe Ferroelectric superlattices present a unique foundation for creating novel materials for modern devices. In ideal superlattices with perfectly flat, compositionally abrupt interfaces, first- principles studies have shown how factors such as strain due to lattice mismatches, charge compensation and bonding at the interface can be controlled to enhance the ferroelectric properties of the superlattice. In real superlattices, the presence of an additional factor, cation intermixing at the interface, is suggested by high-resolution COBRA studies\footnote{D.~D.~Fong et al. PRB \textbf{{71}}, 144112 (2005)}. As the period of a superlattice decreases, the effect of this intermixing would be expected to become increasingly important. Here, we present results of a first-principles study of the effect of interfacial intermixing on short-period $x_{\rm{PT}}$/$y_{\rm{ST}}$ superlattices. We find that the effect of intermixing on the superlattice polarization can indeed be substantial, and use first-principles information about atomic and electronic properties to interpret and model the effect. Implications for other superlattice combinations and experiments will be discussed. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V20.00004: Understanding, enhancing and fine-tuning polar properties in multicomponent perovskite superlattices Invited Speaker: Modern epitaxial thin-film techniques make it possible to synthesize artificial multicomponent perovskite-oxide superlattices (SLs) with interfaces that are atomically flat and compositionally abrupt. The behavior of such systems is dominated by strong interactions between individual SL layers, high levels of epitaxial strain and symmetry lowering relative to the bulk. All of these factors can be manipulated in order to enhance or custom-tailor the useful polar properties --- such as polarization and piezoelectric response --- for a wide variety of technological applications. First-principles computational techniques are a tool of choice to help us understand how the strain, symmetry and composition of these complex systems influence their polar properties. However, the prohibitive computational costs associated with such simulations, growing rapidly as the period of the SL increases, make it impossible to answer some broader, more interesting questions: in particular, how could the SL layer arrangement be optimized to obtain the best possible polar properties? Here we show how first-principles calculations combined with a simple model for SL polarization and a genetic-algorithm optimization allow us to find answers to the questions mentioned above and, among other things, to predictively identify the most polar perovskite-oxide SLs that can be grown on currently available substrates. This flexible modeling procedure can be applied to a wide variety of layered perovskite-oxide nanostructures, providing guidance for experimental development of nanoelectromechanical devices with substantially improved polar properties. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V20.00005: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V20.00006: Ferroelectricity and Polarization-Dependent Tunneling in BaTiO$_{3}$ Films Below 10 Layers Arthur P. Baddorf, Junsoo Shin, Sergei V. Kalinin, Von Braun Nascimento, E. Ward Plummer Predictions for the minimum critical film thickness for ferroelectricity have continuously decreased. For BaTiO$_{3}$, ferroelectricity has previously been observed experimentally down to 12 layers and predicted by first-principles calculations in 6 layer films. We have examined BaTiO$_{3}$ ultra-thin films grown on SrRuO$_{3}$/SrTiO$_{3}$ using laser-MBE in high oxygen pressures and report evidence of a ferroelectric state at room temperature by in-situ characterization of structure, using low energy electron diffraction (LEED I-V) and by scanning tunneling spectroscopy (STS). Films produce sharp (1x1) LEED patterns, indicating well-ordered tetragonal phase structure. Comparison of observed diffraction intensities for 4 and 10 layer films at 130 and 300 K with calculated intensities reveals a vertical displacement of the central Ti, corresponding to a polarization consistent with compressive strain. Reversible polarization switching was observed locally as a jump in the electron tunneling current at +/- 2.5 V using a scanning tunneling microscope. Research was sponsored by the Division of Materials Sciences and Engineering and the Center for Nanophase Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V20.00007: Surface morphology of PbTiO$_{3}$ films on SrTiO$_{3}$ (001) K. Latifi, Carol Thompson, D. D. Fong, G. B. Stephenson, P. H. Fuoss, J. A. Eastman, F. Jiang, S. K. Streiffer, R.-V. Wang Strain relaxation is a ubiquitous process in the synthesis of heteroepitaxial films. Films deposited onto a substrate with a small lattice parameter mismatch will often initially form as coherently strained (lattice matched) layers. As the film grows and exceeds a critical thickness, the stored elastic energy is released through the creation of crystal defects such as misfit dislocations. Even before dislocation introduction, epitaxial strain can lead to morphological instabilities of the growth interface, with the formation of mounds. We use atomic force microscopy to investigate surface morphology related to strain relaxation during the epitaxial growth of PbTiO$_{3}$ films ranging from 10 nm to 385 nm in thickness on SrTiO$_{3}$ (001) substrates. The ferroelectric phase transition temperature of coherently strained PbTiO$_{3}$ films is increased, but also depends on thickness. Therefore, for some range of typical growth temperatures of organo-metallic vapor phase epitaxy, it is possible for the film to undergo a paraelectric to ferroelectric phase transition during growth. This could lead to additional mechanisms of strain relaxation becoming active during growth. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V20.00008: Ferroelectricity in ultra-thin and thick ferroelectric films of~Pb$_{0.35}$Sr$_{0.65}$TiO$_{3}$ studied by second~harmonic generation Shiwei Liu, Jacques Chakhalian, Min Xiao, Chonglin Chen Second harmonic generation (SHG) measurements were performed in the reflection geometry using the femtosecond Ti:Sapphire pulse laser at the wavelength of 810 nm for a 16-nm-thick ultra-thin Pb$_{0.35}$Sr$_{0.65}$TiO$_{3}$ film and a 243-nm-thick Pb$_{0.35}$Sr$_{0.65}$TiO$_{3}$ film, which were epitaxially deposited on (001) MgO substrates by pulsed laser ablation (PLD). It is concluded that in the ultra-thin film the ferroelectric phase is still present and a remarkably sharp ferroelectric phase transition was observed. In contrast, the thick film exhibits a pronounced diffuse phase transition. Theoretical analysis based on the polarization diagrams show the compensated c-domain fraction is dominant in both films whereas the nonlinear susceptibility of the ultra-thin film has a different tensor property from the thick film. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V20.00009: Functional Interfaces of a Crystalline Oxide on Semiconductor Fred Walker, Curt Billman, Marco Buongiorno-Nardelli, Rodney McKee Interfaces play a pivotal role in the properties of complex oxides such as polarization of ferroelectrics, band offsets of gate dielectrics and the field effect in correlated electron materials. The concept of the interface phase has guided us in understanding these diverse roles of the interface. Moreover, this concept has been especially powerful in guiding the development of processes for the heteroepitaxial growth of oxides on semiconductors. Functional substitutions of elements at a crystalline-oxide and silicon interface have been accomplished using molecular beam epitaxy of alkaline earth oxides deposited on silicon. We present capacitance-voltage measurements to spectroscopically characterize the interface states induced by changing the composition of the interface from, for example, strontium to barium silicide. Research sponsored jointly by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy at Oak Ridge National Laboratory under contract DE-AC05-00OR22725 with UT-Battelle, LLC and at the University of Tennessee under contract DE-FG02-01ER45937. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V20.00010: Surprises in the Growth of SrTiO$_3$ on Silicon: a Charged Interface and Polar Film. C. Stephen Hellberg, Kristopher E. Andersen Growth of complex oxides on silicon has been of great interest for device applications. SrTiO$_3$ has served as the prototypical system, but initial optimism has faded as well ordered epitaxial films have been difficult to achieve. Recently there have been several developments that have dramatically improved our understanding of these systems. Growth of coherent lattice-matched films has finally been achieved, and the measured expansion of the out-of-plane lattice constant exceeds the prediction of the bulk elastic constants of SrTiO$_3$ by nearly 100\%. Simultaneously, growth by a different process in thermodynamic equilibrium yields islands of SrTiO$_3$. We will present first principles density functional calculations consistent with both experiments: The energetically favored interface is electrically charged, and the film grows ferroelectrically polarized, with an accompanying out-of-plane expansion. Additionally, the films are unstable to phase separation. Methods of substitutionally doping the interface to eliminate the charge are discussed. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V20.00011: Structural and electrical properties of crystalline LaAlO$_{3}$ on Silicon (100) James Reiner, Agham Posadas, Miaomiao Wang, T.P. Ma, Charles Ahn LaAlO$_{3}$ (LAO) is a promising gate insulator material for use in future generations of silicon technology because of its relatively large dielectric constant (24) and band gap (5.5 eV). The growth of crystalline LAO on silicon is impeded, however, by interface reactions. To overcome this difficulty, we have deposited epitaxial LAO on silicon (100) surfaces by using a 2 monolayer crystalline SrTiO$_{3}$ transition layer. The growth was performed by oxide molecular beam epitaxy. The crystallinity of the structure was confirmed by in situ reflection high energy electron diffraction (RHEED), and also x-ray diffraction and transmission electron microscopy (TEM). Cross-sectional TEM shows no SiO$_{2}$ at the complex oxide-silicon interface. Metal-oxide-semiconductor (MOS) capacitors have been fabricated and measured (I-V and C-V characteristics). Inelastic electron tunneling spectroscopy (IETS) measurements have been carried out on these MOS capacitors, which also indicate the absence of SiO$_{2}$ at the interface. [Preview Abstract] |
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