Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X10: Insulators and Dielectrics: Defects, Structure, and Mechanical and Dynamical Properties |
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Sponsoring Units: DCMP Chair: Tim Fister, Argonne National Laboratory Room: 304 |
Thursday, March 19, 2009 2:30PM - 2:42PM |
X10.00001: Dynamics of implant damage as a precursor to nanocrystal nucleation Matthew J. Beck, Sokrates T. Pantelides Ion implantation into \emph{a}-SiO$_{2}$ leads to the self-assembly of metal or semiconductor nanocrystal arrays having applications in optical and non-volatile memory devices. The production of uniform arrays of similarly-sized nanocrystals within the \emph{a}-SiO$_{2}$ matrix has been shown to depend strongly on nucleation conditions. Here we report results of quantum mechanical calculations probing the atomic-scale dynamics in the time immediately following ion-induced low-energy recoils. We show that individual low-energy recoils (with KE$\sim$100 eV) do not produce individual, isolated defects in the \emph{a}-SiO$_{2}$ structure, but rather produce nanoscale defect pockets. These defect pockets are sources for oxygen out-diffusion, and subsequently represent seed regions for nanocrystal nucleation. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X10.00002: Optical Emission from F$_{H}$(CN$^{-}$ ) centers in CsCl Joseph West, Richard Dallinger, Ryan Lidster Strong, previously unreported, emission from at least two different excited electronic states of F$_{H}$(CN$^{-})$ centers in CsCl has been measured following excitation at 532 nm, 514 nm and 568 nm. The temperature dependence of emission at 532 nm was obtained over the temperature range of 16K through 160K. No emission under excitation at 633 nm was detected. The presence of emission following excitation at the higher energies, but absent under 633 nm excitation, may suggest that the well-known energy transfer process from the electronic excited F-center states to the vibrational CN$^{-}$ energy levels in this system occurs from a single common relaxed excited electronic state associated with the absorption band at 633 nm. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X10.00003: Ab-Initio investigation of defects in GaTe Cedric Rocha Leao, Vincenzo Lordi Materials that are good candidates for room-temperature radiation detectors should ideally possess several characteristics that are sometimes contradictory. A high carrier mobility-lifetime product is required to maximize the collection of radiation generated charge, but a relatively large bandgap is desired to minimize thermal noise. High resistivity is usually also desired, to reduce background current which degrades the detection resolution. Furthermore, a high average atomic number increases absorption of high energy radiation. GaTe is attracting recent attention for the potential to satisfy many of these criteria, but its properties are still poorly understood. Like other III-VI compounds, GaTe is a layered material, but its unusual anisotropy in the atomic planes results in rather unique mechanical and electronic properties. In this talk, we discuss ab-initio calculations of the transport properties of GaTe with respect to its application to radiation detectors. Guided by very recent experimental results, we analyze the occurrence of native defects in this materials and possible compensating extrinsic defects, and their effects on the transport properties. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X10.00004: Electronic properties of oxygen vacancy in HfO$_{2}$ within GW calculations Eun-Ae Choi, Kee Joo Chang Hafnia (HfO$_{2})$ has attracted much attention as a high-k dielectric material, which substitutes for silicon gate oxide in nanoscale metal-oxide-semiconductor (MOS) devices. However, there remain several problems to be resolved in hafnia-based devices, such as flat band shift and threshold voltage instability. Oxygen vacancy, as the most common intrinsic defect, is regarded as a major cause of these problems. As previous calculations mostly rely on the local-density-functional approximation (LDA), the defect levels of oxygen vacancy are not accurately determined because of the LDA band gap problem. Here we perform GW calculations for the defect levels of oxygen vacancy in monoclinic HfO$_{2}$. Our calculations show that the Fermi level pinning of p+ poly Si gate electrode is due to the charge transfer from oxygen vacancy to the electrode. In addition, the charge trap of oxygen vacancy can lead to the threshold voltage instability in both nMOS and pMOS devices. Finally, we suggest that oxygen vacancy may be a cause of the gate leakage current by the Poole-Frenkel conduction. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X10.00005: DFT Energetics of Noble Gas Impurities and Schottky Defects in UO2 Alexander Thompson, Chris Wolverton There is a strong need to better understand the mechanisms of nuclear fuel swelling in uranium dioxide (UO2) due to formation of fission product gases. Using density functional theory (DFT+U) calculations, we have explored the energetics of noble gases in UO2, Schottky defects (SD) in UO2, and the interaction between these defects. We find: (i) The noble gas atoms show a strong size dependence of the incorporation energy. (ii) The energetics of the SD in three different geometries shows that the preferred geometry is not simply driven by electrostatic effects. (iii) The energetics of each of the noble gas atoms incorporated in a SD show a strong favorable binding, due to strain relief. (iv) For Ar, Kr, and Xe, the binding energy of a noble gas impurity with the SD is larger than the energy required to form a SD, thereby providing an energetic pathway for the ``spontaneous'' formation of these defects. (v) From our calculations, we have constructed a simple model of the critical number of noble gas atoms required to form a bubble. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X10.00006: Polarization Patterns In GeTe From Bulk To Ferroelectric Nanoclusters Engin Durgun, Riad Shaltaf, Xavier Gonze, Philippe Ghosez, Jean-Yves Raty In this study, we investigated the ferroelectric and structural properties of GeTe crystal and nanoclusters, an alternative type of ferroelectric material, up to 1500 atoms from first-principles calculations based on density functional theory (DFT). Firstly, the dynamical, dielectric and elastic properties of GeTe in ferroelectric phase have been investigated [1]. Next, we demonstrate, for the first time at the DFT level, the existence in the interior of sufficiently large dots of polarization vortices giving rise to a net and reversible toroidal moment of polarization (G) [2]. The amplitude of G decreases with the size of the system and is totally suppressed below a critical diameter of 2.7 nm. The pattern of atomic distortions and the size evolution of the properties are discussed in relationship with the existence of a surface region within which the atoms behave differently. The validity of the results is checked for thicker structures with more layers and also for nanowires which can be considered as an infinite limit. (*)engin.durgun@ulg.ac.be [1] R. Shaltaf, E. Durgun, J.-Y. Raty, Ph. Ghosez, and X. Gonze Phys. Rev. B 78, 205203 (2008) [2] E. Durgun, R. Shaltaf, J.-Y. Raty, X. Gonze, and Ph. Ghosez Nano. Lett. (submitted) [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X10.00007: First-principles study of point defects in $\kappa $-Al$_{2}$O$_{3}$ Justin R. Weber, Anderson Janotti, Chris G. Van de Walle The development of gate-stack structures for novel CMOS applications has stimulated interest in point defects that may occur in oxide dielectrics. We consider Al$_{2}$O$_{3}$ as a possible gate-stack material, and study defects in the low density $\kappa $ phase. The choice of the $\kappa $-Al$_{2}$O$_{3}$ phase is based on the similarity of its density to that of amorphous Al$_{2}$O$_{3}$. We analyze native point defects such as vacancies, self-interstitials, and antisites, as well as various relevant impurities. Our first-principles calculations are based on density functional theory (DFT). Hybrid functionals were utilized as a means of overcoming the band-gap problem. This approach allows us to accurately assess the positions of defect levels. We use calculated band offsets to make predictions about the location of these defect levels with respect to the band gap of relevant semiconductors used as channel materials. We will discuss which defects may impede the optimal performance of devices. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X10.00008: First-principles study on the electromigration of oxygen vacancy in metal oxides Sang Ho Jeon, Won-joon Son, Bae Ho Park, Seungwu Han The oxygen vacancy, which is a fundamental defect in oxides, plays a critical role in defining many electrical properties of oxides ranging from ionic conductivities to leakage behaviors. As such, to control the density and spatial distribution of the oxygen vacancy has often been an important goal in many researches on electronic devices, particularly for high-density devices, such as resistance-change random access memories(ReRAM). Despite its importance, the electromigration of oxygen vacancy has not been studied much from the microscopic point of view. In this presentation, we studied on the migration of the oxygen vacancy in metal oxides, such as MgO and TiO2. First, using the nudged elastic band(NEB) method, we estimated the migration barrier of charged oxygen vacancy under an external field. Then, we calculate the zone-center phonon modes of the bulk system to obtain the attempt frequency of the vacancy diffusion. Based on these results, we estimated the migration time of oxygen vacancy in metal oxide by using harmonic transition state theory, and it was in good agreement with the result of molecular dynamics(MD) calculation. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X10.00009: First-principles calculations of Ce activation in RE$_{2}$M$_{2}$O$_{7}$ (RE = La, Y; M = Ti, Zr, Hf). Anurag Chaudhry, Andrew Canning, Rostyslav Boutchko, Stephen Derenzo, Niels Gronbech-Jensen First-principles electronic structure calculations of Ce-doped La and Y compounds with composition RE2M2O7 (RE = La, Y; M = Ti, Zr, Hf) are performed using the pseudopotential method based on the local density approximation in density functional theory. The positions of the 4f states relative to the valence band maximum and the position of the lowest 5d excited state relative to the conduction band minimum of the host material are determined. The prediction of Ce activation is based on the following criteria: (1) The energy difference between the occupied Ce 5d excited state (Ce3+)* and the host material conduction band minimum (CBM) and (2) the degree of localization of the (Ce3+)* excited state on the Ce atom. Our theoretical investigations indicate that Ce activation is not possible in these host materials. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X10.00010: Electric Field Induced Sub-Microsecond Resistive Switching Nilanjan Das, Stephen Tsui, Ya-Qi Wang, Yuyi Xue, Ching-Wu (Paul) Chu Electric field induced resistive switching in metal-oxide interfaces has attracted extensive recent interest. While many agree that lattice defects play a key role, details of the physical processes are far from clear. There is debate, for example, regarding whether the electromigration of pre-existing point defects or the field-created larger lattice-defects dominates the switch. We investigate several Ag-Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (PCMO) samples exhibiting either sub-microsecond fast switching or slow quasistatic DC switching. It is found that the carrier trapping potentials are very different for the pre-existing point defects associated with doping (and/or electromigration) and the defects responsible for the sub-$\mu $s fast-switching. Creation/removal of the defects with more severe lattice-distortions and spatial spreading (trapping potential $\ge $ 0.35 eV), therefore, should be the dominating mechanism during sub-$\mu $s switching. On the other hand, the shallow defects (trapping potential $<<$ 0.2eV) associated with doping/annealing are most likely responsible for the resistance hysteresis (slow switch) during quasistatic voltage sweep. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X10.00011: Temperature Dependence of Gigahertz-Range Ultra-High Frequency Micromechanical Resonators Josef-Stefan Wenzler, Tyler Dunn, Diego Guerra, Pritiraj Mohanty We report measurements of bulk mode resonators in the ultrahigh frequency range up to 4 GHz. The devices are fabricated with a stack of materials and actuated using piezeoelectric technique. Typical dimensions of these resonators are 100 $\mu $m in length and width and 10 $\mu $m in thickness. The temperature dependence of mode frequencies and quality factor Q are investigated for temperatures ranging from 0.3 K - 400 K. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X10.00012: Temperature dependence of mechanical stiffness and dissipation in ultrananocrystalline diamond resonators Vivekananda Adiga, Anirudha Sumant, Sampath Suresh, Chris Gudeman, Orlando Auciello, John Carlisle, Robert Carpick We have studied the mechanical softening and dissipation of ultrananocrystalline diamond (UNCD) resonators with temperature. Resonant excitation and ring down measurements were conducted under ultra high vacuum (UHV) conditions in a decoupled UHV atomic force microscope (AFM) to determine the Young's Modulus and quality factor (Q) in UNCD cantilever structures. The temperature dependence of Young's modulus revealed the characteristic Wachtman's empirical relation. From this measurement the Debye temperature was estimated to be $\sim $ 1460\r{ }K, significantly lower than Debye temperature of 1860\r{ }K for single crystal diamond. The quality factors of different resonators increased as the cantilevers were cooled from 300\r{ }K to 30\r{ }K and with the hydrogen termination of the cantilever surface. The results indicate that surface and bulk defects significantly contribute to the observed dissipation as well as the mechanical softening in UNCD resonators. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X10.00013: Evidence of universality in the dynamical response of micromechanical diamond resonators at millikelvin temperatures Matthias Imboden, Pritiraj Mohanty We report kelvin to millikelvin-temperature measurements of dissipation and frequency shift in megahertz-range resonators fabricated from ultra-nanocrystalline diamond. Frequency shift $\delta $f/f$_{0}$ and dissipation Q$^{-1}$ demonstrate temperature dependence in the millikelvin range similar to that predicted by the glass model of tunneling two level systems. The logarithmic temperature dependence $\delta $f/f$_{0}$ is in good agreement with such models, which include phonon relaxation and phonon resonant absorption. Dissipation shows a weak power law, Q$^{-1} \quad \sim $ T$^{1/3}$, followed by saturation at low temperature. A comparison of both the scaled frequency shift and dissipation in equivalent micromechanical structures made of single-crystal silicon and gallium arsenide indicates universality in the dynamical response. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X10.00014: Small to medium atomic size-mismatch leads to alloy phase-separation yet huge mismatch can lead to ordering Xiuwen Zhang, Giancarlo Trimarchi, Mayeul d'Avezac, Alex Zunger Most alkali halide alloys AX-BX (where A and B are alkali elements and X=F, Cl, Br, or I) are expected to have miscibility gaps (phase separation) which increases with the lattice mismatch. Even though LiX-RbX and LiX-CsX with lattice mismatches of $20-33\%$ and $19-40\%$ respectively might be expected to have pronounced miscibility gaps, they were experimentally found to have ordered structures. Here, we investigate the possible stabilization of ordered compounds with respect to random configurations. In the family of LiNaBr$_{2}$, LiKBr$_{2}$, LiRbBr$_{2}$, and LiCsBr$_{2}$, we find that as the lattice mismatch increases, the formation enthalpy of the random structure increases, (mainly due to the volume deformation), but the formation enthalpy of the ordered structure decreases becoming negative for the latter two. The ordered structures consist of distorted LiX4 tetrahedral arranged in layers, with Rb (or Cs) sitting between layers at the center of the resulting triangular prism. We analyze the origin of ordering from the large local distortion induced by the huge lattice mismatch. [Preview Abstract] |
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