Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B21: Focus Session: Dopants and Defects in Semiconductors I |
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Sponsoring Units: DMP Chair: Eugene Haller, University of California, Berkeley Room: 323 |
Monday, March 16, 2009 11:15AM - 11:51AM |
B21.00001: Hydrogen multicenter bond in oxide and nitride semiconductors Invited Speaker: Hydrogen is a very reactive atom, occurring in virtually all organic and in many inorganic compounds. It can form a purely covalent bond, in which two hydrogen atoms share a pair of electrons in a two-electron two-center bond, as well as polar covalent bonds, such as in an H$_{2}$O molecule. In solids, hydrogen is usually considered as an interstitial impurity. In elemental semiconductors, such as silicon, hydrogen forms a three-center bond when located at the bond center. In compound semiconductors, hydrogen bonds to the anionic species in p-type material, and to the cationic species in n-type. Thus far, hydrogen in solids has been found to form chemical bonds with one, two, or at most three other atoms. Higher coordination numbers are exceedingly rare and have been reported only for clusters. In this talk we will show that hydrogen is capable of forming multicenter bonds in solids, occupying substitutional sites. As examples, we discuss substitutional hydrogen impurities in oxides (ZnO, MgO, SnO$_{2}$, TiO$_{2})$ [1,2] and nitrides (InN, AlN, GaN) [3]. Based on first-principles calculations we show that hydrogen replaces oxygen (nitrogen) and forms genuine chemical bonds with multiple metal atoms, in truly multicoordinated configurations. These multicenter bonds are surprisingly strong despite the large hydrogen-metal distances when compared to typical values in hydrogen two-center bonds. Hydrogen in the multicenter bond configuration is a shallow donor in a number of materials. In conducting oxides, it provides a consistent explanation for the observed dependence of electrical conductivity on oxygen partial pressure, thus resolving a long-standing controversy on the role of point defects in unintentional n-type conductivity [1,2]. \\[4pt] [1] A. Janotti and C. G Van de Walle, Nature Materials \textbf{6}, 44 (2007). \\[0pt] [2] A. K. Singh, A. Janotti, M. Scheffler, and C. G. Van de Walle, Phys. Rev. Lett. \textbf{101}, 055502 (2008). \\[0pt] [3] A. Janotti and C. G. Van de Walle, Appl. Phys. Lett. \textbf{92}, 032104 (2008). [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B21.00002: Hydrogen in anion vacancies of semiconductors Mao-Hua Du, David Singh Hydrogen typically terminates the dangling bonds around vacancies in semiconductors, thereby, partially or completely passivating the vacancies. However, it has been shown recently that hydrogen in anion vacancies of many semiconductors, such as ZnO, MgO, InN, SnO$_{2}$, and GaN, takes multi-coordinated structures and acts as shallow donors, providing $n$-type conductivity to the materials. We study the hydrogen in the anion vacancies of a series of II-VI and III-V semiconductors using density functional calculations. The results on these materials show that, in the anion vacancies of polar II-VI semiconductors, the hydrogen is usually anionic and is coordinated with more than one cation atoms as a result of the relatively high ionicity of the host materials. The hydrogen coordination number depends on the host anion size. On the other hand, in more covalent semiconductors such as some III-V semiconductors, the single cation-H bonding configuration may become most stable. In the anion vacancies of ZnX and CdX where X represents anions, hydrogen is typically amphoteric except for oxides, in which the small anion size prohibits the formation of the cation-cation bond that is required for the acceptor configuration. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B21.00003: Defect Creation and Annihilation in GaN and ZnO Chris Van de Walle, Anderson Janotti ZnO is an extremely attractive material for a number of optoelectronic and electronic applications. Among its advantages is its radiation hardness, which is even greater than that of GaN. Based on our comprehensive investigations of intrinsic point defects [1,2], we have developed a model for defect creation and annihilation during and after irradiation. The calculations, based on pseudopotential-density-functional theory combined with LDA+U [2] produce formation energies, stability of charge states as a function of Fermi level, and migration barriers for each of the point defects. Migration barriers allow us to determine annealing temperatures at which we predict various defects to be mobile. In ZnO, the key factors responsible for radiation hardness are (1) the low migration barriers of point defects and (2) the charge-state matching of dominant defect pairs. Quantitative arguments for both ZnO and GaN will be presented, and the results compared with experimental observations. The insights provided by our modeling can be fruitfully applied to understand irradiation effects in semiconductors and insulators in general. [1] S. Limpijumnong and C. G. Van de Walle, Phys. Rev. B 69, 035207 (2004). [2] A. Janotti and C. G. Van de Walle, Phys. Rev. B 76, 165202 (2007). [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B21.00004: Asymmetric hole localization and multiple hole binding of acceptors in ZnO Stephan Lany, Alex Zunger Holes bound at cation-site acceptors in oxides, such as Li$_{Zn}$ or the Zn vacancy in ZnO tend to be localized on a single oxygen neighbor rather than to be delocalized over symmetrically equivalent sites. As a consequence of this localization, the acceptor level lies deep in the gap, typically $\sim $1 eV above the VBM. In contrast, conventional local density calculations do not show this symmetry breaking, and predict the acceptor level much too shallow. This failure of approximate functionals has been attributed to the residual self-interaction, which underestimates the energy splitting between occupied and unoccupied states. We identify a criterion for the cancellation of the self-interaction in terms of a generalized Koopmans theorem, and use this criterion to define a self-interaction correction (SIC) potential that does not rely on empirical parameters. After the SIC, the unoccupied hole states are correctly placed in energy with respect to the spectrum of the occupied host states. We use this method to predict the acceptor levels of cation-site acceptors and the Zn vacancy in ZnO, and of acceptors in In$_{2}$O$_{3}$ and SnO$_{2}$. We find that these acceptors have too deep levels to cause $p$-type conductivity, and we further predict that nominal single acceptors can generally bind multiple holes (up to 3). [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B21.00005: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 12:39PM - 12:51PM |
B21.00006: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 12:51PM - 1:03PM |
B21.00007: Unusual uniaxial stress results on the stretch mode of OH related defects in ZnO Kevin Martin, W. Beall Fowler Some uniaxial stress studies of the frequency dependence of O-H-related defects in ZnO have produced surprising results$^{1,2}$. For example, the Li:OH defect in ZnO (H-I*) is oriented along the c-axis, yet the OH stretch mode decreases in frequency when stress is applied along the c direction and increases when stress is perpendicular to the c-direction. Another example is the Cu:OH defect, in which the OH is aligned along one of the three non-c tetrahedral directions. Stress along the c-direction produces a strongly non-linear increase in frequency. These examples and others indicate something unusual is happening in these systems. One possibility is that the piezoelectric effect in ZnO is responsible for the ``backward'' behavior of the frequency shift of these defects. The piezoelectric effect in ZnO is caused by the lack of cancellation between the ``clamped-ion'' term (i.e., electronic contribution) and the term related to the change in the u parameter (``internal strain''), with the latter dominating$^{3}$. When c stress is applied, the value of u increases, thus the two interpenetrating hexagonal lattices (one for Zn, the other for O) increase their overlap. We will attempt to explain the experimental results within this framework. $^{1}$Lavrov and Weber, Phys. Rev. B, \textbf{73}, 035208 (2006), and $^{2}$ Phys.Stat. Sol. (b) \textbf{243}, 2657 (2006) $^{3}$Corso, et al Phys. Rev B. \textbf{50}, 10715 (1994) [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B21.00008: Theoretical study of Si in ZnO John Lyons, Anderson Janotti, Chris Van de Walle Recently, the presence of silicon in relatively high concentrations has been detected in samples of ZnO [1]. The properties of this impurity have not yet been investigated. Here we present a first-principles study of the electronic and structural properties of Si in zinc-blende ZnO using density functional calculations with LDA, GGA, and hybrid functionals. Our calculations show that substitutional Si on a Zn site is lower in energy than either Si on an oxygen site or a Si interstitial. The calculations consistently predict Si to be a shallow donor in ZnO, with the 2+ charge state being most stable across the band gap. The formation energy of substitutional Si is relatively low, supporting experimental evidence which shows a concentration of 10$^{17}$ cm$^{-3}$ Si in ZnO samples. The properties of Ge in ZnO are also studied for comparison and show behavior similar to that of Si. [1] M.D. McCluskey and S.J. Jokela, Physica B \textbf{401-402}, 355 (2007). [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B21.00009: Photoinduced EPR study of electron traps in TiO$_{2}$ crystals: Oxygen vacancies and Ti$^{3+}$ ions Shan Yang, Adam Brant, Larry Halliburton Electron paramagnetic resonance (EPR) provides a sensitive method to monitor native defects in wide-band-gap semiconductors. In-situ illumination with laser light at low temperature (photoinduced EPR) forms paramagnetic defects in fully oxidized bulk TiO$_{2}$ crystals. Illumination with 442 nm laser light at 30 K and below produces four electronlike centers and one holelike center. Three of the electronlike centers have S = 1/2 and are assigned, respectively, to a substitutional Ti$^{3+}$ ion in the otherwise perfect lattice, a substitutional Ti$^{3+}$ ion adjacent to a Si$^{4+}$ ion, and a substitutional Ti$^{3+}$ ion adjacent to an oxygen vacancy. The fourth electronlike center has S = 1 and is assigned to two Ti$^{3+}$ ions adjacent to one oxygen vacancy. The holelike center has S = 1/2 and consists of a hole shared equally by two adjacent oxygen ions in the otherwise perfect lattice. Spin-Hamiltonian parameters, obtained from complete sets of angular dependence data, are presented for each of the centers. This work was supported by NSF Grant No. DMR-0804352. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B21.00010: The nature of Group-V acceptor impurities in SnO$_{2}$ Joel Varley, Anderson Janotti, Chris Van de Walle Group-V elements have long been considered leading candidates for achieving p-type doping in semiconducting oxides. Using first-principles calculations, we investigate the feasibility of achieving ambipolar doping in SnO$_{2}$ using the Group-V elements N, P, and As. We address the electronic structure of these impurities by performing systematic density functional calculations using hybrid functionals. This approach overcomes the band-gap problems inherent in calculations using the local density approximation or generalized gradient approximation, thus allowing us to accurately determine energies of defect levels. We discuss the stability of the isolated impurities both as substitutional and interstitial defects, based on calculated formation and migration energies. We also investigate their possible passivation by hydrogen and examine binding energies and activation energies of hydrogen-acceptor complexes. We conclude that the Group-V elements are deep acceptors that will not enable p-type doping of SnO$_{2}$. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B21.00011: Manipulation of Single Oxygen Vacancies on TiO$_{2}$(110) Danda Acharya, Peter Sutter Oxygen vacancies are among the primary chemically active defects on the surface of reducible transition metal oxides, playing a key role in surface chemistry, catalysis, and photocatalysis. We report the controlled manipulation of individual O-vacancies on reduced TiO$_{2}$(110)-1x1 using a low temperature scanning tunneling microscope. Localized voltage pulses trigger the hopping of single vacancies along a bridging oxygen (O$_{br})$ row. We discuss the microscopic manipulation mechanism and demonstrate atomic-scale control by constructing linear and more complex arrangements of vacancies. Single defect manipulation is used to probe the interaction of closely spaced vacancies, and to establish the possibility of forming highly reactive double and a triple O-vacancy clusters. Detailed experimental and theoretical analysis reveals that bridge-bonded O-vacancy pairs are stable and have lower energy than pairs of vacancies separated by two or more lattice spacings. The existence of stable vacancy pairs with exposed low-coordinated Ti atoms has implications on the reactivity of TiO$_{2}$(110) and of similar metal oxide surfaces. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B21.00012: Coordination Defects and Nanoclusters of TiO$_{2}$ Ken Park, Vincent Meunier, Minghu Pan, Nan-Hsin Yu, Ward Plummer Titanium oxide is one of the most investigated photocatalytic systems. It is capable of converting toxic organic and inorganic materials to benign products, as well as turning solar energy into a chemical one. Many believe that the catalytic activation involves charge transfer localized at surface defects with lower stoichiometry and/or coordination. In this study, scanning tunneling microscopy (STM) and density functional theory (DFT) are used to gain insight into such defects on TiO$_{2}$(110). STM reveals defects ranging from a few {\AA}ngstroms to a few nanometers in size, but all of a uniform height of 3 {\AA}. These topographically distinct defects are determined as fully stoichiometric nanoclusters by DFT. Despite the full stoichiometry, they possess undercoordinated atomic sites including 3- and 4-coordinated Ti and 1-coordinated O atoms. Their electronic and chemical properties will be discussed. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B21.00013: Kinetic Monte Carlo study for the thermal stability of hydrogen in ZnO Junhyeok Bang, Kee Joo Chang Zinc oxide (ZnO) has attracted much attention due to a variety of applications to transparent optoelectronic devices. It is known that undoped ZnO exhibits n-type conductivity. Hydrogen, which is unintentionally incorporated, is considered as a promising candidate for shallow donors in ZnO. However, it is still difficult to explain n-type conductivity in annealed ZnO due to the low thermal stability of H. Here we study the diffusion of H in ZnO using first-principles calculations and then perform kinetic Monte Carlo (kMC) simulations for the thermal stability of H. The migration energy of a substitutional H is much higher than that for an interstitial H. Using as input the energy barriers for H diffusion, kMC simulations show that interstitial and substitutional H atoms diffuse out at different annealing temperatures around 125 and 475 $^{o}$C, respectively, in good agreement with experiments. When H atoms are injected from air into ZnO, we find that they are likely to be trapped at O-vacancy sites, leading to the n-type conductivity in annealed samples. [Preview Abstract] |
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