Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F44: Focus Session: Defects in Semiconductors: Lighting Materials |
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Sponsoring Units: DMP FIAP Chair: Anderson Janotti, University of California, Santa Barbara Room: Mile High Ballroom 4C |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F44.00001: Theoretical stability of Eu dopant in diamond Wenhao Hu, Michael E. Flatt\'e Due to their extremely long spin coherence times, rare earth ions are promising candidates for high resolution magnetic sensing. In addition, recent progress on high resolution magnetometry based on the diamond NV center suggests that the combination of diamond and rare earth ions might have perform well. In this article, we simulated europium complexes in diamond using density functional theory in the LSDA+U approximation. We used a 64-atom supercell for the diamond host, inserted Eu and removed 1-4 carbon atoms; atomic positions were allowed to relax with a force precision of 1.0 mRy/a.u. The formation energies of possible configurations of charged Eu in diamond were investigated. The first order Markov-Payne correction was used to remove the effects of the supercell size and fictitious charge background. The formation energy for substitutional Eu is very large, originating from the limited relaxation space for the nearest neighbor carbons, however the formation energy is much lower with 1-3 surrounding carbon vacancies. We find the most stable configuration is the +1 charged Eu with 1 neighboring vacancy. The work was supported by an AFOSR MURI. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F44.00002: Bistable lattice position of a single magnetic dopant in a semiconductor Jeffrey M. Moore, Victoria R. Kortan, C\"uneyt \c{S}ahin, Juanita Bocquel, Paul M. Koenraad, Michael E. Flatt\'e Electronic control of the lattice position of individual dopants has been demonstrated recently, including displacement of a single Si dopant in the surface layer of GaAs by a scanning tunneling microscope (STM)[1]. Fe dopants in GaAs have internal spin degrees of freedom associated with their core d states which can also be manipulated using a STM[2]. A reversible and hysteretic change in the topography measured near a single Fe dopant is observed when a negative bias voltage is applied. To determine if a lattice displacement is responsible, we have performed first-principles calculations to evaluate the formation energy of a single Fe atom embedded in GaAs as a function of displacement from the substitutional site. Our calculations support the existence of a second stable configuration, characterized by a displacement accompanied by a change in atomic configuration symmetry about the Fe from four-fold to six-fold symmetry. These results expand the range of demonstrated local configurational changes induced electronically for dopants, and thus may be of use for sensitive control of spin-spin interactions between dopants.\\[4pt] [1] J. K. Garleff et al., Physical Review B 84, 075459 (2011).\\[4pt] [2] J. Bocquel et al., Physical Review B 87, 075421 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F44.00003: The Influence of Donor-Acceptor Pairs on Excitation Efficiency in GaN:Eu Brandon Mitchell, Jonathan Poplawsky, Dong-Gun Lee, Yasufumi Fujiwara, Volkmar Dierolf The nature of Eu incorporation and resulting luminescence efficiency, in GaN, has been extensively investigated. By performing a comparative study on Eu:GaN samples grown under a variety of controlled conditions, and using a variety of experimental techniques, the configuration of the majority site has been concluded to contain a nitrogen vacancy (V$_{\mathrm{N}})$. The nitrogen vacancy can appear in two symmetries, which has a profound impact on the luminescence and magnetic properties of the sample. The structure of the minority site has also been identified, and we further propose, and give evidence to the idea that the excitation efficiency, of both sites, is the result of a donor acceptor pair in the vicinity of the Eu. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F44.00004: Electronic and Optical Properties of Luminescent Centers in Halides and Oxides Invited Speaker: Mao-Hua Du Luminescent materials, such as phosphors and scintillators, are widely used for fluorescent lighting, laser, medical imaging, nuclear material detection, etc. . The luminescence is usually activated by impurities (or activators), which act as luminescence centers. The activators are typically multi-valent ions that insert multiple electronic states in the band gap of the host material. In this talk, first-principles calculations of electronic structure and optical transitions are shown for a wide range of activators, including rare-earth ions (e.g., Ce$^{\mathrm{3+}}$, Eu$^{\mathrm{2+}})$, ns$^{\mathrm{2}}$ ions (the ions that have outer electronic configurations of ns$^{\mathrm{2}}$, such as Tl$^{\mathrm{+}}$, Pb$^{\mathrm{2+}}$, Bi$^{\mathrm{3+}})$, and transition-metal ions (e.g., Mn$^{\mathrm{4+}})$, in a large number of halides and oxides. The results reveal how the activator-ligand hybridization affects the emission energy and the luminescence mechanism. New phosphors and scintillators are proposed based on the chemical trends emerging from the calculations of a large number of materials. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F44.00005: Stability of oxygen dopants in group-III nitride alloys Ji-Sang Park, K.J. Chang Group-III nitride materials have attracted much attention for their potential applications in light-emitting devices such as light-emitting and laser diodes. Low resistivity p-type layers are demanding, however, the p-type doping efficiency is still low due to high Mg acceptor level and compensating donor defects such as interstitial hydrogen and nitrogen vacancy. Several donor-acceptor co-doping methods have been suggested to enhance the p-type doping efficiency in group-III nitrides; however, there is a lack of study on the stability and electronic properties of donor dopants in nitride alloys. In this study, we investigate site preference of oxygen dopants in group-III nitride alloys including ternary AlGaN and quaternary AlInGaN alloys through first-principles density functional calculations. We adjust the composition ratio of Al and In to make the band gap of AlInGaN same to that of AlGaN. In AlGaN, we find that the oxygen dopants tend to bond with Al atoms due to the high bond energy between Al and O. The same tendency is found in AlInGaN, whereas the dopants also become stable as they are bonded to In atoms due to small strain. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F44.00006: Phonon-Assisted Auger Recombination in Medium and Wide Band-Gap Materials from First Principles Daniel Steiauf, Emmanouil Kioupakis, Chris G. Van de Walle GaN and GaAs and their alloys are technologically important materials for solid-state optoelectronic devices such as LEDs and lasers. The internal quantum efficiency of these devices, defined as the fraction of electron-hole pairs converted to photons, is limited by nonradiative loss mechanisms. Auger recombination is such a mechanism which limits the efficiency at high carrier densities. The energy and momentum of an electron-hole pair is transferred to a third carrier instead of creating a photon. We present state-of-the-art results of first-principles calculations of the Auger recombination rate coefficients both for the simple direct purely Coulombic process and the indirect phonon-assisted process. We find the absolute values of these recombination rates as well as their relative importance. In GaAs, when energy and momentum of the recombining pair are transferred to an Auger electron, the phonon-assisted process is several orders of magnitude stronger than the direct process, while for recombinations that create an Auger hole, the direct and phonon-assisted processes contribute almost equally. For lower band gaps, the electron processes become equally strong, and also the direct process becomes comparable in magnitude. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F44.00007: Efficiency Droop in Nanostructured III-N LEDs: Multiscale Numerical Analysis and Design Optimization Rezaul Nishat, Vinay Chimalgi, Krishna Yalavarthi, Shaikh Ahmed Recently, optical emitters using InGaN nanostructures have attracted much attention for applications in lasers, solid-state lighting, near-field photolithography, free-space quantum cryptography, consumer displays, as well as diagnostic medicine and imaging. Nanostructures can accommodate a broader range of lattice mismatch thereby allowing full-solar-spectrum emission characteristic, and provide larger active surface area and higher temperature stability. Nevertheless, performance of these III-N LEDs is determined by an intricate interplay of complex, nonlinear, highly stochastic and dynamically-coupled structural fields, charge, and thermal transport processes at different length and time scales. In this work, we study the effects of these coupled processes on the electronic and optical emission properties in nanostructured III-N LEDs. The multiscale computational framework employs the atomistic valence force-field molecular mechanics, the 10-band \textit{sp}$^{3}s^{\ast }$\textit{-SO} tight-binding models, and a coupling to a TCAD toolkit to determine the terminal properties of the device. Finally, a series of numerical experiments are performed (by varying different nanoscale parameters such as size, geometry, crystal cut, composition, surface and contacts, and electrostatics) that mainly aim to improve the efficiency \textit{droop} and reliability of these LEDs. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F44.00008: Blue luminescence and the Zn acceptor in GaN: test case for the hybrid functional approach Denis Demchenko, Michael Reshchikov We present a comparison of exchange tuned hybrid density functional calculations with experimental data obtained for the Zn acceptor in GaN. Since this acceptor is one of the few reliably identified defects in GaN, we use Zn-doped GaN as a test case for the widely used HSE06 hybrid functional method of calculations of defect properties in semiconductors. Here, we present the experimental results of luminescence measurements in Zn-doped GaN from which we obtain Zn acceptor defect levels. They are compared with theoretically calculated defect thermodynamic and optical transition levels, as well as the zero phonon line associated with this acceptor. We also analyze the dependence of the results on the exchange tuning procedure used in HSE06 hybrid functional. Excellent agreement with experiment is obtained when the amount of exact exchange in HSE06 is tuned to reproduce the GaN experimental band gap. This favorable comparison with the experimental results for a well-established defect suggests that the exchange tuned HSE06 hybrid functional yields accurate defect properties in GaN and therefore has significant predictive power. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F44.00009: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F44.00010: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F44.00011: Multivalency of group-V elements in SnO$_2$ Haowei Peng, Stephan Lany Multivalence is an intrinsic property of elements being capable to change their valence state which usually companies with environmental perturbation such as lattice distortion. It commonly shows in transition metal compounds, but also in some some main-group elements, especially heavy group-IV and -V elements. Group-V elements were proposed as $n$-type dopants in SnO$_2$, and compared with the commercial FTO (F-doped SnO$_2$), the cation-site incorporation can facilitate various growth techniques. However, substituting for Sn$^{4+}$ ions, the group-V elements can possess either the desired $5+$ oxidation state that generates electron charge carriers, or a compensating $3+$ oxidation state. Hence, specific attention to this multivalence characteristics is indispensable. To this end, we accurately determine the defect transition energy level $\epsilon(1-/1+)$ with respect to the conduction band minimum, by combining the state-of-art quasi-particle GW and hybrid functional calculations. Group-V elements including P, As, Sb and Bi are considered, which have strong site-preference on Sn instead of on O sites. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F44.00012: Excited States of the divacancy in SiC Michel Bockstedte, Thomas Garratt, Viktor Ivady, Adam Gali The divacancy in SiC - a technologically mature material that fulfills the necessary requirements\footnote{J.~R.~Weber \emph{et al.}, PNAS \textbf{107}, 8513 (2010).} for hosting defect based quantum computing - is a good candidate for implementing a solid state quantum bit. Its ground state is isovalent to the NV center in diamond as demonstrated by density functional theory (DFT).\footnote{A.~Gali, phys. status solidi (b) \textbf{248}, 1337 (2011); J.~P. Gross \emph{et al.} \textbf{77}, 3041 (1996).} Furthermore, coherent manipulation of divacancy spins in SiC has been demonstrated.\footnote{F.~Koehl \emph{et al.}, Nature\textbf{479}, 84 (2011).} The similarities to NV might indicate that the same inter system crossing (ICS) from the high to the low spin state is responsible for its spin-dependent fluorescent signal. By DFT and a DFT-based multi-reference hamiltonian we analyze the excited state spectrum of the defects. In contrast to the current picture of the spin dynamics of the NV center, we predict that a static Jahn-Teller effect in the first excited triplet states governs an ICS both with the excited and ground state of the divacancy. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F44.00013: Cs-based gamma-radiation detector material Cs$_2$Hg$_6$S$_7$: First-principles study of extrinsic doping Jino Im, Shichao Wang, John A. Peters, Zhifu Liu, Bruce W. Wessels, Mercouri G. Kanatzidis, Arthur J. Freeman Semiconductor X-ray/$\gamma$-ray radiation detectors have broad applications, yet finding superior detector materials that work at room temperature is a challenge because of its contradictory requirements. In a previous study, the ternary compound Cs$_2$Hg$_6$S$_7$ was proposed as a possible candidate because of its high density, optimal band gap and high $\mu\tau$ values. However, the low resistivity originating from p-type carriers is a detrimental factor that limits its performance. As a strategy to increase the resistivity, we investigated compensation by extrinsic doping. Using first-principles density functional theory calculations we focused on finding a proper dopant which gives a shallow donor level that leads to a compensation of hole carriers. We tested a number of extrinsic dopants and, as a result, we found that indium is a promising dopant for the strategy to increase the resistivity of Cs$_2$Hg$_6$S$_7$. [Preview Abstract] |
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