Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W33: Insulators and Dielectrics I |
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Sponsoring Units: DCMP Chair: William Parker, Argonne National Laboratory Room: C143/149 |
Thursday, March 24, 2011 11:15AM - 11:27AM |
W33.00001: An \textit{Ab Initio }Study of PuO$_{2\pm 0.25}$, UO$_{2\pm 0.25}$, and U$_{0.5}$Pu$_{0.5}$O$_{2\pm 0.25}$ Li Ma, Asok Ray Hybrid density functional theory has been used to systematically study the electronic, geometric, and magnetic properties of strongly correlated materials PuO$_{2\pm x}$, UO$_{2\pm x}$, and U$_{0.5}$Pu$_{0.5}$O$_{2\pm x}$ with x=0.25. The calculations have been performed using the all-electron \underline {f}ull-\underline {p}otential \underline {l}inearized \underline {a}ugmented \underline {p}lane \underline {w}ave plus \underline {l}ocal \underline {o}rbitals basis (FP-L/APW+lo) method. Each compound has been studied at the ferromagnetic (FM) and anti-ferromagnetic (AFM) configurations with and without spin-orbit coupling (SOC) and \textit{full} geometry optimizations. The optimized lattice constants, bulk moduli, and band gaps will be reported. Total energy calculations indicate that the ground states are AFM for all compounds studied here and the band gaps are typically higher than 1.0 eV, and characteristic of semiconductors. The total energy is lowered significantly and the band gaps increase with the inclusion of SOC. The chemical bonds between the actinide metals and oxygen atoms are primarily ionic in character. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W33.00002: Characterization of point defects in UO2 by positron annihilation spectroscopy: a first-principles study Marc Torrent, Gerald Jomard abstract- Positron Annihilation Spectroscopy is a powerful experimental tool for probing defects in crystalline materials. The correct identification of defects with PAS requires the knowledge of accurate positron lifetimes for the various kinds of defects. That can be provided by numerical calculations in the framework of the Two-Component Density-Functional Theory. This method accurately treat on the same footing, the electrons and positron densities as well as the atomic structure. We have implemented this formalism within the Projector Augmented-Wave method in the ABINIT code, optimizing the electrons and positron densities self-consistently and calculating positron-induced forces accurately. That allows to properly determining the relaxed geometries of defects that trapped positron. We have applied the TC-DFT to various point defects in UO2. The use of the PAW method allows considering large super cells to simulate point defects, we have typically used cells that contain 32 UO2 unit formulas. We use the LDA+U framework in order to treat strong electronic correlations. This work is a first attempt to help for the interpretation of PAS experiments on UO2 but it seems really promising.- [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W33.00003: Effect of Dielectric Materials on the Topological Insulator Bi$_{2}$Se$_{3}$ Surface States Jiwon Chang, Leonard Register, Sanjay Banerjee, Bhagawan Sahu We study the effects of crystalline dielectric materials on the electronic surface states of a strong topological band insulator (TI) Bi$_{2}$Se$_{3}$ using a density functional based electronic structure method [1]. We will discuss the sensitivity of Dirac point degeneracy and linear band dispersion of the TI with respect to different dielectric surface terminations as well as different relative atom positions of the dielectric and the TI. Both passivated and non-passivated substrate surfaces will be considered. Two representative dielectrics SiO$_{2}$ and boron nitride will be chosen to understand the physics of interplay of interface potential, linear band dispersion and the chemical environments of the TI surface states. Our findings have implications in interpreting experiments and designing novel nanoelectronics device concepts based on TIs. \\[4pt] [1] ``Intrinsic and extrinsic perturbations on the surface states of topological insulator Bi$_{2}$Se$_{3}$,'' J. Chang, P. Jadaun, L. F. Register, S. K. Banerjee and B. Sahu (In preparation) [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W33.00004: First-Principles Investigation of Low Energy E' Center Precursors in Amorphous Silica Nathan Anderson, Ravi Vedula, Peter Schultz, Renee Van Ginhoven, Alejandro Strachan We show that oxygen vacancies are not necessary for the formation of E' centers in amorphous SiO2 and that a single O-deficiency can lead to two charge traps. Employing molecular dynamics with a reactive potential and density functional theory we generate an ensemble of stoichiometric and oxygen-deficient amorphous SiO2 atomic structures and identify low-energy network defects. Three-coordinated Si atoms appear in several low-energy defects both in stoichiometric and O-deficient samples where, in addition to the neutral oxygen vacancy, they appear as isolated defects. Various charge transition levels for each defect are also presented. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W33.00005: Structure and energetics of ferroelectric domain walls in LiNbO$_{3}$ from atomic level simulations Donghwa Lee, Haixuan Xu, Volkmar Dierolf, Venkatraman Gopalan, Simon Phillpot Due to its unique ferroelectric and nonlinear optical properties, LiNbO$_{3}$ has a wide range of applications in optoelectronics and nonlinear optics. These unique properties of LiNbO$_{3}$ are, however, quite sensitive to point defects and ferroelectric domain walls. Therefore, detailed characterization of the ferroelectric domain walls and their interaction with the defects at atomistic scale is highly important The local structure and energetics of the ferroelectric domain walls in LiNbO$_{3}$ are examined using density functional theory (DFT) and atomistic-level simulation methods. The energetics and electric dipoles associated with the pertinent point defects and domain walls in LiNbO$_{3}$ are investigated in detail. In particular, the variation of polarization due to 180\r{ } ferroelectric domain walls, the coupling of defect-domain wall interactions and their effects on domain wall motion are discussed [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W33.00006: Study of Defects That Trap Excitons in Yttrium Aluminum Garnets Doped With Rare-Earth Elements Farida Selim, Chris Varnery, Gary Collins, David Mckay, Sherif Reda Excitons play a fundamental role in transporting energy in photonic materials. Understanding and controlling excitons dynamics through their interactions with activating impurities and lattice defects is key to improving scintillation and optical properties. Singles crystals of yttrium aluminum garnet (YAG) crystals doped with rare-earths were studied by positron annihilation, thermolunuinescence and optical spectroscopy. Evidence of defect complexes was found in the YAG structure. Positron lifetime measurements were performed to characterize those defects. Effects of dopants on the optical properties and lattice defects were investigated. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W33.00007: Charge-Flow Regulation at Material Interfaces in Atomistic Models Steven Valone An important class of materials problems of great interest consists of composites of metals and metal oxides. At sharp metal/metal oxide interfaces, the oxygen concentration is varying radically over short distances. For this reason, at the atomistic level, variable-charge atomistic models are required that control charge flow at these interfaces. Charge flow is controlled through chemical potential equalization among its constituents. Existing models of chemical potential, such as the Iczkowski-Margrave (IM) model, are linear in the charge as is appropriate for metals. Here a new, ``fragment'' model Hamiltonian is constructed at the atomistic level commensurate with the IM model, as opposed to the one-electron model Hamiltonians that underlie tight-binding and related methods. Essential properties of the fragment model Hamiltonian include an alternative charge dependence compared to the IM model, charge-flow regulation, absent in the IM model, preservation of a sense of covalent-ionic balance, and capture of important theoretical limits. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W33.00008: Infrared laser ablation of polystyrene from microseconds to picoseconds Richard Haglund, Sergey Avanesyan, Kenneth Schriver, Malte Duering, Barry Luther-Davies, Hee Park, Singaravelu Senthilraja, Michael Klopf, Michael Kelley We describe experiments on both resonant and non-resonant infrared pulsed laser of polystyrene across time scales varying from microseconds to picoseconds for the purpose of determining the ways in which the rate of energy deposition changes the response of both the ablated material and the residual substrate. RIR-PLD has been shown to be a relatively low-temperature process leading to evaporation and deposition of intact molecules. We compare the characteristics of ablation craters and ablation plumes deposited by Nd:YAG and Er:YAG lasers, picosecond and nanosecond optical parametric oscillators, and two different infrared free-electron lasers with differing pulse profiles. The films were characterized by profilometry, digital optical microscopy, scanning electron microscopy, and Fourier-transform infrared spectroscopy. Based on the experiments and computational modeling, we discuss the constraints on laser parameters that produce non-destructive ablation by resonant infrared excitation. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W33.00009: The effect of substrate temperature on the structure and morphology of titanium nitride compounds grown by DC magnetron sputtering Mohammad R. Hantehzadeh, Reza Bavadi The TiN thin films were deposited on p-type silicon (100) substrates using reactive planar DC magnetron sputtering system. The target was 99.99{\%} pure Ti. The reactive sputter gas was a mixture of Ar (99.999{\%}) and N$_{2}$ (99.999{\%}) with the ratio Ar (97{\%}) and N$_{2}$(3{\%}) by volume. Structural characterization of the coating was done using X-ray diffraction (XRD). The surface roughness of the coating was determined using an Atomic Force Microscope (AFM). The reflectivity of thin films was investigated by a spectrophotometer system. The X-ray diffraction measurements showed that by increasing the substrate temperature during the growth, change in crystalline structure will occur. The crystallite size of the films determined by Scherer's equation, and the crystallite size measured by AFM also increased by increasing the substrate growth temperature. The surface reflectivity measurements indicate that by increasing the substrate growth temperature, the optical properties of the films changes. The change in optical properties and crystalline structure of the films indicate that substrate growth temperature plays an important role in structure and morphology of the grown layers. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W33.00010: Growth of Erbium doped Yttrium oxide thin films by atomic layer deposition Nicholas Becker, Thomas Proslier, J. Klug, J. Elam, T. Sanamyan, M. Dubinckii, J. Girolami, M. Pellin Er-doped Yttrium oxide (Er$^{3+}$:Y$_2$O$_3$) has gained recent attention for its possible use in optoelectronic devices. Here we report the use of atomic layer deposition (ALD) to synthesize thin films of Yttrium oxide with various doping levels of Erbium ions (Er$^{3+}$) using different chemistries. ALD uses self-limiting surface reactions to deposit highly conformal thin films over large areas and substrates of arbitrary shape. This allows for the uniform layered doping of Yttrium oxide with Er$^{3+}$. The spatial separation of the Erbium ions can be controlled using organometallic precursors with varying ligand sizes. The doping concentration (volume ratio of Er$^{3+}$ sites to Y$^{3+}$ sites) is controlled by the ratio of the precursor pulses. We comprehensively studied ALD-grown films of Er$^{3+}$:Y$_2$O$_3$ obtained from the Erbium precursors Er(MCp)$_3$ and Er(BA)$_3$ and the Yttrium precursors Y(MCp)$_3$ and Y(Cp)$_3$ using either water or hydrogen peroxide as an oxygen source. Detailed description of the studied optical and spectroscopic properties, stoichiometry, and physical characteristics of these films will be presented. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W33.00011: Atomic Layer Deposition of AlOx for Metal-AlOx-Metal Trilayers and Resonators A. Kozen, L. Henn-Lecordier, X. Chen, M. Schroeder, C. Musgrave, G. Rubloff The dielectric layer in conventional Josephson junction qubits is fabricated by thermal oxidation of aluminum. These dielectrics suffer from high loss tangents thought to be due to defect-related quantum two level systems. Our collaborators have identified the -OH rotor associated with hydroxyl species in the AlOx as a prime defect candidate. We demonstrate the fabrication of the AlOx dielectric in MIM structures using atomic layer deposition (ALD) from trimethylaluminum and both H2O and D2O precursors. ALD enables precise control of film growth at the atomic scale, while comparison of D2O vs H2O as the oxidation precursor should reveal the role of this defect in the loss tangent. We have developed the D2O based ALD process, observed similar kinetics and properties for D2O and H2O based ALD for AlOx, and characterized the materials by SIMS, XPS, and electrical measurements of MIM capacitor structures. Correlation between room temperature electrical measurements and low temperature resonator performance will be discussed. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W33.00012: Improvements of Defect Analysis by Space-Charge Wave Spectroscopy Kay-Michael Voit, Burkhard Hilling, Heinz-J\"urgen Schmidt, Mirco Imlau We report on our recent advancements in space-charge wave spectroscopy, which can be used to investigate defect structures in classical high-resistive semiconductors and insulators. It permits to estimate the effective trap concentrations as well as the effective donor density $N_{eff}$ and the product $\mu\tau$ of electron mobility and life-time in the conduction band. We present a novel method of space-charge wave excitation, using a superposition of a running and a static sinusoidal illumination pattern. Thus we acquire -- in contrast to the former oscillating pattern -- a distinct direction of movement. The proposed new technique can be regarded as an effective amelioration, as the theoretical analysis is no longer limited by numerous presumptions like low modulation depth or small oscillation amplitudes. It not only overcomes these limits of the experimental configuration improving accuracy of SCW spectroscopy, but it also provides additional information, such as the sign of the charge carriers. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W33.00013: First-principles studies of Ce and Eu doped inorganic scintillator gamma ray detectors Andrew Canning, Anurag Chaudhry, Rostyslav Boutchko, Stephen Derenzo We have performed DFT based band structure calculations for new Ce and Eu doped wide band gap inorganic materials to determine their potential as candidates for gamma ray scintillator detectors. These calculations are based on determining the 4f ground state level of the Ce and Eu relative to the valence band of the host as well as the position of the Ce and Eu 5d excited state relative to the conduction band of the host. Host hole and electron traps as well as STEs (self trapped excitons) can also limit the transfer of energy from the host to the Ce or Eu site and therefore limit the light output. We also present calculations for host hole traps and STEs to compare the energies to the Ce and Eu excited states. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W33.00014: Novel Approaches in Energy Conversion by Molecular Charge Transfer from Diamond Surfaces Franz A.M. Koeck, Jeff Sharp, Robert J. Nemanich Vacuum thermionic energy conversion is based on electron transfer from a hot emitter across a vacuum gap to the collector. Our approach for an efficient emitter utilizes nanostructured, negative electron affinity doped diamond films. With a low effective work function of less than 1.3 eV thermionic emission commences at 260 C and observes the law of Richardson -- Dushman with a significant emission current $>$ 5 mA at 500 C. Pairing this emitter with a similar collector results in a potential across the gap and introduction of an ohmic load establishes a current indicative of energy conversion. Utilizing ionization processes of gaseous species at the emitter surface can enhance inter-electrode charge transfer and increase output power. In the ionization process an electron is trapped in an occupied molecular orbital establishing a negative ion state. The electron affinity and negative ion binding energy determines stability of the transient negative species, and we present results for H2 and CH4. As these species are introduced in the inter-electrode gap an increase in output power is observed with a concurrent shift of maximum output power to lower load resistance. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W33.00015: Diamond Based Electron Emitters for Photon Enhanced Thermionic Energy Conversion Tianyin Sun, Franz A. Koeck, Robert J. Nemanich Energy conversion cells utilize either direct photon illumination or indirect thermal energy for electron excitation. Nitrogen-doped, hydrogen terminated nanocrystalline diamond films display a negative electron affinity and have shown low temperature thermionic emission which can be employed for energy conversion in a vacuum thermionic emission cell. However, the low work function of such films suggests that the current could be enhanced through visible light illumination to induce photoelectron emission. We present measurements of the spectrum of emitted electrons from N-doped diamond films for light illumination between 600 and 340nm, while the film is heated from ambient to 500C. Features due to thermionic and photo-emission are identified, and a complex interaction is observed between the two processes at various temperatures and illumination wavelengths. The results indicate the potential application of diamond emitters as combined thermal and photon energy converters, and we present a new approach to enhance the performance of diamond-film energy converters. [Preview Abstract] |
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