Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session L37: Semiconductors II: Structure and Phase Diagrams |
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Sponsoring Units: DCMP Chair: Alex Zunger, National Renewable Energy Laboratory Room: Morial Convention Center 229 |
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L37.00001: First-Principles epitaxial phase-diagram, short-range order, microstructure and electronic properties of (In,Ga)N zincblende alloys on GaN Zhe Liu, Paulo Piquini, Alex Zunger A first-principle total energy cluster expansion method is developed to study thermodynamic properties of epitaxial semiconductor alloys coherent to substrate (i.e., $<$ critical thickness $h_c$), in which coherent strain energy as a function of atomic configuration is explicitly described. The search for epitaxial ground state structures of (In, Ga)N alloy grown on GaN (001) substrate concludes that epitaxial strain suppresses phase separation, which is normally observed for bulk (In,Ga)N alloy and relaxed films. Two (102) superlattices: (InN)$_2$/(GaN)$_2$ and (InN)$_4$/(GaN)$_1$ are determined to be the epitaxial ground state structures. Composition-temperature phase diagram calculated by Monte Carlo method shows that homogeneous solid solution phase is thermodynamic stable at typical growth temperature of blue and green LED by MBE and MOCVD ($x$(In)$\sim 0.20-0.30$ and $h_c \sim 10-30$nm). Such calculated phase diagram can be used to understand the controversy regarding atomic microstructures in (In,Ga)N quantum well devices. Short-range-ordering of the solid solution phase and its influence on the electronic properties are also discussed. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L37.00002: First principles and valence force field study of III-V quaternary alloys Koushik Biswas, Alberto Franceschetti, Stephan Lany We report on the elastic properties and formation energies of Ga$_{x}$In$_{1-x}$P$_{y}$N$_{1-y}$ quaternary alloys using first principles and valence force field (VFF) calculations. The elastic constants of the binary compounds (GaP, InP, GaN, and InN) were calculated using the local density approximation (LDA). The resulting VFF parameters, \textit{$\alpha $} (bond stretching) and \textit{$\beta $} (bond angle bending) were used within the Keating model to calculate the formation energies of GaInP, GaInN, InPN, and GaPN ordered structures. We found that the VFF formation energies of phosphide-nitride alloys (e.g. GaPN) were not in very good agreement with the LDA formation energies. Conventionally, the bond bending parameter \textit{$\beta $} for a ternary alloy is chosen as the arithmetic mean of the binary constituents. To improve the accuracy of the VFF model, we lifted such restriction on the \textit{$\beta $}-parameter and we also introduced the parameter \textit{$\sigma $} (bond length-bond angle interaction). The VFF parameters \textit{$\alpha $}, \textit{$\beta $}, and \textit{$\sigma $} were fitted to the LDA-calculated formation energies of a large number of ternary ordered structures and were used to calculate the formation energy of the Ga$_{x}$In$_{1-x}$P$_{y}$N$_{1-y}$ quaternary alloy. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L37.00003: First Principles Phase Diagram Calculation For Al$_{x}$Ga$_{1-x}$N Jeremy Nicklas, John Wilkins First principles phase diagram calculations were performed for the wurtzite and zincblende structures of the quasibinary system AlN-GaN. The cluster expansion method using the code ATAT was performed without and with excess vibrational contributions to the free energy, $F_{vib}$. The ab initio calculations were performed with VASP using the PAW pseudopotentials with PBE for the exchange and correlation energies. Preliminary results show miscibility gaps for both structures with a decrease in the consolute points, ($X_{C}$,$T_ {C}$), when including $F_{vib}$. The wurtzite structure is predicted to be approximately symmetric while the zincblende is predicted to be quite assymetric. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L37.00004: Simulation studies of GST phase change alloys Glenn Martyna In order to help drive post-Moore's Law technology development, switching processes involving novel materials, in particular, GeSbTe (GST) alloys are being investigated for use in memory and eFuse applications. An anneal/quench thermal process crystallizes/amorphosizes a GST alloy which then has a low/high resistance and thereby forms a readable/writeable bit; for example, a ``one'' might be the low resistance, conducting crystalline state and a ``zero'' might be the high resistance, glassy state. There are many open questions about the precise nature of the structural transitions and the coupling to electronic structure changes. Computational and experimental studies of the effect of pressure on the GST materials were initiated in order to probe the physics behind the thermal switching process. A new pathway to reversible phase change involving pressure-induced structural metal insulator transitions was discovered. In a binary GS system, a room-temperature, direct, pressure-induced transformation from the high resistance amorphous phase to the low resistance crystalline phase was observed experimentally while the reverse process under tensile load was demonstrated via ab initio MD simulations performed on IBM's Blue Gene/L enabled by massively parallel software. Pressure induced transformations of the ternary material GST-225 (Ge2Sb2Te5) were, also, examined In the talk, the behavior of the two systems will be compared and insight into the nature of the phase change given. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L37.00005: Local structure and Phase transition of (GeTe)$_n$(Sb$_2$Te$_3$)$_m$ pseudo-binary system for the phase-change memory Jino Im, Jae-Hyeon Eom, Jisoon Ihm A theoretical investigation on the local structure and phase transition between the crystalline and the amorphous phase of (GeTe)$_n$(Sb$_2$Te$_3$)$_m$ pseudo-binary system(GST) for the phase-change memory is presented. Based on the study of the coordination number for the amorphous phase of GST, the local structure of the amorphous phase of the GST is shown to be composed of the stibnite-like building block for the Sb$_2$Te$_3$ and chain-like building block for the GeTe. The phase transition between the crystalline and the amorphous phase of GST is explained by relative repositioning of these building blocks. Density functional total energy minimization calculations show that the crystallization energy and the volume change in transition also agree with experimental data. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L37.00006: Raman scattering studies of Ge-Sb-Te nanoparticles Christine Kim, Hae-Young Shin, Ah Reum Jeong, William Jo, Seokhyun Yoon We have measured Raman scattering spectra of Ge-Sb-Te (GST) nanoparticles which are synthesized by a pulsed laser ablation method. The nanoparticles were grown under different growth conditions such as temperatures and/or pressures. Our measurements could provide information towards the optimal growth conditions for better crystalline quality of the GST nanoparticles. We have also measured nitrogen-doped GST nanoparticles. Comparision between Raman responses of nitrogen- doped- and undoped-GST nanoparticles will be presented. Our results suggest that Raman scattering spectroscopy can be used to study phases and phase changes through local structural information in the GST nanoparticles, which are being developed for low-power non-volatile memory applications. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L37.00007: Theory of unsaturated silicon lattices Feng Zhang, David Stucke, Dragan Stojkovic, Vincent Crespi Several molecules are known to contain stable silicon double or triple bonds that are sterically protected by bulky side groups. Through first-principles computation, we demonstrate that well-defined $\pi$ bonds can also be formed in two prototypical {\it crystalline} Si structures: Schwarzite Si-168 and dilated diamond. The sp$^2$-bonded Si-168 is thermodynamically preferred over diamond silicon at a modest negative pressure of -2.5 GPa. Ab-initio molecular dynamics simulations of Si-168 at 1000 K reveal significant thermal stability. Si-168 is metallic in density functional theory, but with distinct $\pi$-like and $\pi^*$-like valence and conduction band complexes just above and below the Fermi energy. A bandgap buried in the valence band but close to the Fermi level can be accessed via hole doping in semiconducting Si$_{144}$B$_{24}$. A less-stable crystalline system with a silicon-silicon triple bond is also examined: a rare-gas intercalated open framework on a dilated diamond lattice. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L37.00008: Phase diagram of silicon using a DFT-based neural network potential Oliviero Andreussi, Joerg Behler, Michele Parrinello The phase diagram of silicon is computed by means of Classical Molecular Dynamics. A recently developed [Behler and Parrinello, Phys. Rev. Lett. 98 146401 (2007)] neural-network potential based on Density Functional Theory calculations in the Local Density Approximation is used. This potential was shown to be several orders of magnitude faster than corresponding LDA-DFT calculations, while the accuracy is essentially maintained. Results on the liquid-solid coexistence curve are in good agreement with ab-initio calculations and demonstrate the quality of the neural-network potential. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L37.00009: Fine Structures of Ge Nanoclusters on Si(111): STM Observations and First-Principles Theory A.S. Rao, H.F. Ma, M.C. Xu, D.X. Shi, H.J. Gao, R. Gudipati, H.L. Dang, Sanwu Wang Germanium-based nanoclusters grown on silicon substrates have potential applications in optoelectronics and nanotechnology. A variety of Ge nanostructures formed on intact Si(111) have been observed. However, the fine structures of the atomic arrangements in the Ge nanoclusters have remained elusive. We performed scanning tunneling microscopy observations and first-principle calculations for investigating the fine structures of the Ge nanoclusters on the Si(111)-7$\times $7 surface. We obtained atomic structures of the nanoclusters formed with a certain process involving deposition at temperature of 423 K and annealing at 550 K. We found that Ge nanocluster, located predominately in the faulted half unit cells of the Si(111)-7$\times $7 surface, contained approximately six Ge atoms with three bonded center Si adatoms. We also observed that the obtained nanostructures were stable up to 600 K. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L37.00010: Damping of high amplitude phonons in bismuth: classical and quantum mechanical simulations Eamonn Murray, Aaron Hurley, David Prendergast, Tadashi Ogitsu, David Fritz, David Reis, Stephen Fahy Using both classical and quantum mechanical simulations together with first-principles results, we investigate the damping mechanism of high amplitude excitations of the $A_{1g}$ phonon mode in bismuth. Pump-probe experiments using ultrafast lasers can generate and measure large amplitude coherent oscillations of the $A_{1g}$ phonon mode in bismuth. A substantial reduction in the lifetime of the phonon is observed when higher amplitude oscillations are produced. With third-order couplings obtained from first-principles we calculate the rate of energy loss from the $A_{1g}$ mode over several picoseconds. We find that as the highly excited $A_{1g}$ mode decays, it produces highly excited modes in relatively small regions of the Brillouin Zone, leading to an increase in the decay rate into these modes. We show how this can greatly affect the observed lifetime of the high amplitude excitation of the $A_{1g}$ mode. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L37.00011: Structure of Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ supermodulation from ab initio calculations Y. He, S. Graser, P.J. Hirschfeld, H.-P. Cheng We present results of density functional theory (DFT) calculation of the structural supermodulation in Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ structure, and show that the supermodulation is indeed a spontaneous symmetry breaking of the nominal crystal symmetry, rather than a phenomenon driven by interstitial O dopants. The structure obtained is in excellent quantitative agreement with recent x-ray studies, and reproduces several qualitative aspects of scanning tunnelling microscopy (STM) experiments as well. The primary structural modulation affecting the CuO2 plane is found to be a buckling wave of tilted CuO$_5$ half-octahedra, with maximum tilt angle near the phase of the supermodulation where recent STM experiments have discovered an enhancement of the superconducting gap. We argue that the tilting of the half-octahedra and concommitant planar buckling are directly modulating the superconducting pair interaction. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L37.00012: Discovery of Dynamics of Jahn-Teller Effect Dan Liu Taking S=1/2 NaSiTi$_{2}$O$_{6}$ as example, we discovered the dynamics of the Jahn-Teller effect of solids by extending the molecular frontier orbital theory from chemistry to solid state physics. At the orbital and spin levels, the dynamics of the Jahn-Teller effect is exposed to involve spin flipping and spontaneous orbital transition that is driven by the spin-orbital coupling to keep the angle momentum reservation. The orbital transition leads to a dramatic structural change, i.e., the JT distortion. In analogue to the singlet-triplet intersystem crossing of photochemistry, the JT effect is a non-adiabatic process, associated with the first-order phase transition. The JT effect agrees well with the low-dimensional, S=1 Haldane gap on the basis of the antimagnetic Heisenberg model, if emphasizing that the S=1 parameter of Haldane gap indicates the parallel spins of neighboring orbitals, rather than the on-site parallel spins. We provide insight to understanding of the unusual structural, magnetic properties of S=1/2 NaTiSi$_{2}$O$_{6}$, as well as the S=1LiVGe$_{2}$O$_{6}$ and S=3/2 LiCrGe$_{2}$O$_{6}$. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L37.00013: Dynamics of A-B transition of the DNA double helices Hao Wang, Thomas Cheatham, Peter Gannett, James Lewis The conformational transitions of DNA and the sensitivity of DNA structure to the surrounding environment are very relevant to its chemical and biological function and potential applications in nano-technology. Different conformations of DNA, even with the same sequence, exhibit different electronic structures, resulting in different conduction properties. We present theoretical work on the dynamical features of electronic states in the A-B transition of a model DNA duplex of d(CGCGCGCGCG)$_2$ (10 base-pairs, 628 atoms) as the molecule undergoes conformational changes and thermal fluctuations at room temperature. We couple state-of-the art empirical force field molecular dynamics (MD) simulations with an {\it ab-initio} electronic structure method based on density-functional theory, called F{\scriptsize IREBALL}. For the A-B transition, we calculated the effects of conformational change on the electronic structure for each snapshot obtained from nanosecond MD simulations. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L37.00014: Mechanism of GeSbTe phase change materials: an ab initio molecular dynamics study Jean-Yves Raty, C\'eline Otjacques, Jean-Pierre Gaspard, Christophe Bichara Among phase change materials, Ge2Sb2Te5 (225) is one of the most successfully used in applications. Accepted models are based on EXAFS spectra and suppose a complete reorganization of bonds during amorphization, with Ge changing from sixfold to tetrahedral coordination. We perform ab initio MD simulations of the (225), (124) and (415) liquid alloys. We show that the crystalline, liquid and amorphous structure of these systems are similar, with very little sp3 hybridization around Ge atoms and a majority of p-sigma bonds. Using a set of quenched liquid configurations we reproduce the EXAFS measurements on the (225) composition and explain how the static Debye Waller factor due to the vacancies in the crystal phase leads to a cancellation of individual neighbors contribution to the EXAFS signal while in the amorphous, a larger coherence occurs, enhancing the EXAFS signal. The computed electrical conductivities of the three phases (cubic solid, liquid and amorphous) prove to be very different, accordingly with the experiment. [Preview Abstract] |
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