Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F28: Dopants and Defects in Semiconductors IV: NitridesFocus
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Sponsoring Units: DMP FIAP DCOMP Chair: Nicholas Harmon, University of Iowa Room: 291 |
Tuesday, March 14, 2017 11:15AM - 11:51AM |
F28.00001: Defect related electrical and optical properties of AlN bulk crystals grown by physical vapor transport Invited Speaker: Klaus Irmscher AlN crystallizes thermodynamically stable in the wurtzite structure and possesses a direct band gap of about 6 eV. It is the ideal substrate for the epitaxial growth of Al-rich Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N films that enable deep ultraviolet (UV) emitters. Appropriate AlN bulk crystals can be grown by physical vapor transport (PVT). Besides high structural perfection, such substrate crystals should be highly UV transparent and ideally, electrically conductive. It is well known that point defects like impurities and intrinsic defects may introduce electronic energy levels within the bandgap, which lead to additional optical absorption or electrical compensation. Among the impurities, which may be incorporated into the AlN crystals during PVT growth at well above 2000 $^{\circ}$ C, oxygen, carbon, and silicon play the major role. Based on our own experimental data as well as on experimental and theoretical results reported in literature, we discuss energy levels, charge states and possible negative-U behavior of these impurities and of vacancy-type defects. In particular, we develop a model that explains the absorption behavior of the crystals in dependence on the Fermi level that can be controlled by the growth conditions, including intentional doping. Further, we pay attention on spectroscopic investigations giving direct evidence for the chemical nature and atomic arrangement of the involved point defects. As examples local vibrational mode (LVM) spectroscopy of carbon related defects and recent reports of electron paramagnetic resonance (EPR) spectroscopy are discussed. [Preview Abstract] |
Tuesday, March 14, 2017 11:51AM - 12:03PM |
F28.00002: Effects of Sc-doping on the structure and physical properties of AlN: first-principles studies Chengxin Wang, Zhifan Wang, Yanning Zhang It was found in experiments that Sc doping can significantly improve the piezoelectric property of wurtzite AlN, making AlNSc compounds promising in the applications of piezoelectric acoustic device. However, the piezoelectric constant of Al$_{1-x}$NSc$_{x}$ drops quickly if the Sc doping content is larger than 43{\%}, probably due to the phase transition of Al$_{1-x}$NSc$_{x}$..$^{1,\, 2}$ In this work, we performed systematic first principles calculations on the strutural, mechanical and physical properties of Sc-doped AlN, as a dependence of Sc compositions, so as to understand how the Sc-doping affects the properties of AlN. The calculated lattice parameters and piezoelectric constant corresponds well with the experimental data, reaching to the peak at $x \quad =$ 43.5{\%}. Also we found that with the increasing Sc contents, the elastic constants of C$_{33}$, C$_{11}$ and C$_{44}$ decarese, whereas C$_{12}$ and C$_{13}$ increase. The total energy calculations show that Al$_{1-x}$NSc$_{x}$ with a wurtzite phase is more stable than the rocksalt phase as x \textless 31.25{\%}, and then the rocksalt Al$_{1-x}$NSc$_{x}$ that has few peizoelectric property is energetically preferred. Ab-initio molecular dynamic (AIMD) studies were further employed to analyze the phase transition of Al$_{0.5}$NSc$_{0.5}$. .1. A. Morito, K. Toshihiro, K. Kazuhiko, T. Akihiko, T. Yukihiro and K. Nobuaki, Advanced Materials \textbf{21}, 593-596 (2009). 2. O. Leon, A. MorĂ¡n and R. Gonzalez, Applied Physics Letters \textbf{95}, 162107-162107-162103 (2009). [Preview Abstract] |
Tuesday, March 14, 2017 12:03PM - 12:15PM |
F28.00003: Frequency-dependent EPR studies of strain localized around the Mg acceptor in free-standing GaN Ustun Sunay, Mary Zvanut, Jamiyanaa Dashdorj, Jacob Leach GaN is known for its applications in LEDs, but limited information exists about the local environment of Mg, the only p-dopant in GaN. We investigate GaN:Mg samples and report non-uniform strain around Mg acceptors and estimate a maximum hyperfine splitting using variable frequency electron paramagnetic resonance (EPR). We conducted EPR experiments at 4 K from 9.4 to 150 GHz to study strain variations local to Mg via analysis of the full width at half maximum (FWHM) of the Mg-acceptor signal. 1 mm-thick free-standing hydride vapor phase epitaxy grown GaN with a Mg concentration of 6x10$^{\mathrm{18}}$ cm$^{\mathrm{-3}}$ was studied so that the Mg could be probed in a minimally strained environment. The results are then compared with those from 4x10$^{\mathrm{19}}$ cm$^{\mathrm{-3\thinspace }}$Mg thin-film heteroepitaxial samples where the strain is thought to be significant. In the free-standing crystals, The FWHM of the Mg EPR signal increases monotonically as the microwave frequency increases, indicating variations in strain among Mg acceptors. The maximum value of the hyperfine splitting, extracted from the zero-frequency limit, is estimated to be 120 G. Measurements obtained at 9.4 GHz suggest that the average strain around the Mg in films is at least an order of magnitude greater than in the free-standing GaN. [Preview Abstract] |
Tuesday, March 14, 2017 12:15PM - 12:27PM |
F28.00004: Charge Transfer in Compensated GaN:Be Substrates Observed with Magnetic Resonance William Willoughby, Mary Ellen Zvanut, Jamiyanaa Dashdorj, Michal Bockowski GaN:Be layers are used for electrical isolation, and the broad Be-related yellow luminescence (YL) may be used for white-light production. To understand this behavior, we investigate charge transfer using photo-induced electron paramagnetic resonance (EPR). GaN substrates grown from Ga solution under high N pressure (HNPS) and doped with 10$^{\mathrm{19}}$ O/cm$^{\mathrm{3}}$ and 10$^{\mathrm{17}}$ to 10$^{\mathrm{19}}$ Be/cm$^{\mathrm{3}}$ were studied using time-dependent photo-EPR at 3.5 K. Excitation with $E_{ph}$ \textgreater 2.6 eV increased EPR amplitude, while subsequent illumination with $E_{ph}$ \textgreater 1 eV quenched EPR. A charge transfer model fit to the data included electron-excitation, capture onto ionized donors and neutral acceptors, and recombination of neutral donors and acceptors. The spectral dependence of the optical absorption cross-section of the negative charge state (A$^{\mathrm{-}})$ of a Be-related acceptor revealed an acceptor level $E_{V} \quad +$ 0.7 eV and structural relaxation of 0.5 eV for the A$^{\mathrm{-}} \quad \to $ A$^{\mathrm{0}} \quad +$ e$^{\mathrm{-}}_{\mathrm{CB}}$ transition. Preliminary analysis of quenching suggests an acceptor level at $E_{V} \quad +$ 1 eV and a relaxation of 1 eV for A$^{\mathrm{0}} \quad +$ e$^{\mathrm{-}}_{\mathrm{VB}} \quad \to $ A$^{\mathrm{-}}$. The deep acceptor level provides an explanation for the efficacy of Be in producing resistive substrates and for the YL mechanism used for light conversion. [Preview Abstract] |
Tuesday, March 14, 2017 12:27PM - 12:39PM |
F28.00005: Mg acceptors and the ultraviolet band in Mg-doped GaN. Ibrahima Diallo, Denis Demchenko, Michael Reshchikov The sharp ultraviolet luminescence (UVL) band with a maximum photoluminescence and zero-phonon line at approximately 3.25-3.30 eV is observed in magnesium (Mg) doped GaN. Using the hybrid density functional method, we calculate electronic and optical properties of Mg acceptors in GaN. We show that Mg substituting Ga is responsible for the experimentally observed sharp UVL band in Mg-doped GaN. We also analyze the dual nature of Mg acceptors in GaN. [Preview Abstract] |
Tuesday, March 14, 2017 12:39PM - 12:51PM |
F28.00006: Investigating doping effects in III-nitride materials by large-scale hybrid-functional calculations Ying-Chih Chen, Qing Shi, Vincent Michaud-Rioux, Hong Guo Electronic properties of semiconductors is strongly influenced by the presence of dopants. Large supercells having over a thousand atoms need to be calculated for dopant concentration below 1\% in standard density functional theory (DFT). To deal with the band gap issue of DFT with local/semilocal functionals, the hybrid-functional which mixes the exact exchange (EXX) with semilocal correlation, is often an effective approach. However, performing hybrid-functional simulation with large number of atoms is a challenge. To this end, we have implemented EXX in the framework of numerical atomic orbital (NAO) and pseudopotential which allows us to accurately calculate electronic structures of semiconductors with very small doping concentrations. We show that, (i) the calculated band gap and band structure of semiconductors are accurately comparable with plane-wave and projector augmented wave methods; and (ii) the band gap is nonlinearlly depended on the doping-concentration from 0.1\% to 3\% in gallium-nitride (GaN) crystals. The numerically very efficient NAO hybrid-functional approach is powerful for investigating semiconductor materials whenever the supercell contains more than several hundred atoms and beyond. [Preview Abstract] |
Tuesday, March 14, 2017 12:51PM - 1:03PM |
F28.00007: Schrodinger-Poisson Modeling of Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N/GaN Heterostructures Employing Tailored Depth-Dependent Aluminum Concentration for Polarization Grading Jeffrey Calame, Igor Chernyavskiy, Mario Ancona, David Meyer Polarization-gradient profiling of Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N/GaN heterostructures in the vertical (depth) direction, achieved by deliberate spatial tailoring of the aluminum concentration profile, can be used to control the spatial structure of the conducting electron gas in high electron mobility transistors. In particular, the typical two-dimensional electron gas of abrupt heterostructures can exhibit a more three-dimensional distribution in graded structures. This offers the possibility of improved device linearity through deliberate vertical heterostructure engineering, which can minimize or compensate for various scattering mechanisms that contribute to nonlinearity. Schrodinger-Poisson modeling (i.e., the Hartree approximation) is used to study the electron density profiles that result from such deliberate grading, and how those profiles evolve with the application of biasing vertical electric fields across the heterostructure. Implications of the results on device linearity will be discussed. Comparisons between the electron density profiles predicted by the Schrodinger-Poisson modeling and those obtained by density-gradient theory will be made in selected examples. [Preview Abstract] |
Tuesday, March 14, 2017 1:03PM - 1:15PM |
F28.00008: Optoelectronic Properties of Point Defects in Gallium Nitride: A Many-Body Perturbation Theory Perspective Kirk Lewis, Sahar Sharifzadeh Gallium nitride (GaN) and related alloys form a class of wide bandgap semiconductors that have broad applications in optoelectronics technology such as blue/ultraviolet optical devices and power electronics. However, these materials generally grow with high defect densities, which can substantially degrade their optical and electronic properties. An accurate and detailed knowledge of the influence of defects on the optoelectronic properties of these materials is central to the design of new high-performance materials. We employ first-principles many-body perturbation theory within the GW/BSE approximation to investigate the influence of defects on the optical and electronic properties of bulk GaN, taking the nitrogen vacancy as an example. GW calculations predict that introduction of the vacancy results in significant modification of the electronic properties of GaN due to the presence of shallow trap states, an effect that is partially captured by standard DFT. Additionally, we predict strong exciton binding energies associated with excitations near defects. Our analysis suggests that it is necessary to go beyond standard DFT to understand excited-states near shallow defects. [Preview Abstract] |
Tuesday, March 14, 2017 1:15PM - 1:27PM |
F28.00009: Structural and electronic properties of ZnGeN$_2$ Nicholas L. Adamski, Zhen Zhu, Darshana Wickramaratne, Chris G. Van de Walle ZnGeN$_2$ is a direct-band-gap earth-abundant semiconductor that is a candidate material for photovoltaic and light-emitting devices. Elucidating the potential use of ZnGeN$_2$ in such applications requires an accurate knowledge of its structural and electronic properties, as well as an understanding of the role of native point defects in the material at the microscopic level. Using density functional theory with a hybrid functional, we study the structural and electronic properties of ZnGeN$_2$. We investigate the role of native point defects, specifically antisites, vacancies and interstitials, and discuss their impact on electronic and optical properties. [Preview Abstract] |
Tuesday, March 14, 2017 1:27PM - 1:39PM |
F28.00010: Electronic structure and $p$-type doping of ZnSnN$_{\mathrm{2}}$ Tianshi Wang, Anderson Janotti, Chaoying Ni ZnSnN$_{\mathrm{2}}$ is a promising solar-cell absorber material composed of earth abundant elements. Little is known about doping, defects, and how the valence and conduction bands in this material align with the bands in other semiconductors. Using density functional theory with the the Heyd-Scuseria-Ernzerhof hybrid functional (HSE06), we investigate the electronic structure of ZnSnN$_{\mathrm{2}}$, its band alignment to other semiconductors, such as GaN and ZnO, the possibility of $p$-type doping, and the possible causes of the observed unintentional $n$-type conductivity. We find that the position of the valence-band maximum of ZnSnN$_{\mathrm{2}}$ is 0.55 eV higher than that of GaN, yet the conduction-band minimum is close to that in ZnO. As possible $p$-type dopants, we explore Li, Na, and K substituting on the Zn site. Finally, we discuss the cause of unintentional $n$-type conductivity by analyzing the position of the conduction-band minimum with respect to that of GaN and ZnO. [Preview Abstract] |
Tuesday, March 14, 2017 1:39PM - 1:51PM |
F28.00011: Nitrogen vacancy effects on the electronic structure of CrN Tomas Rojas, Sergio E. Ulloa Chromium nitride (CrN) is believed to be a small indirect gap semiconductor with interesting electronic and magnetic properties. It exhibits a phase transition at T~280K in which both the electronic and magnetic structures change from a paramagnetic cubic rock-salt to an antiferromagnetic orthorhombic structure. However, the transport properties of CrN thin films are not fully settled, exhibiting metallic and semiconducting behavior at low temperatures in different situations. In particular, the impact of nitrogen vacancies and other defects on the transport properties are yet to be analyzed in detail. We have performed ab initio calculations using the LSDA+U method to examine the effect of N vacancies in bulk CrN. By replacing or removing a nitrogen atom in an appropriately large supercell, we study the accompanying deformations of the lattice structure as well as the energetics and spatial distribution of the associated charge and spin distribution of the defect state. We also study and compare less likely defects such as Cr, N-N and Cr-N vacancies. Our results indicate that a high percentage of N vacancies results in a transition towards a metallic phase, which produces strong defects on the local magnetic arrangements and may even create a small absolute magnetization. [Preview Abstract] |
Tuesday, March 14, 2017 1:51PM - 2:03PM |
F28.00012: Prospects and limitations for $p$-type doping in boron nitride polymorphs Leigh Weston, Chris G. Van de Walle Using first-principles calculations, we examine the potential for $p$-type doping of BN polymorphs via substitutional impurities. Based on density functional theory with a hybrid functional, our calculations reveal that group-IV elements (C, Si) substituting at the N site result in acceptor levels that are more than 1 eV above the valence-band maximum in all of the BN polymorphs, and hence far too deep to allow for $p$-type doping. On the other hand, group-II elements (Be, Mg) substituting at the B site lead to shallower acceptor levels. However, for the ground-state hexagonal phase ($h$-BN), we show that $p$-type doping at the B site is inhibited by the formation of hole polarons. Our calculations reveal that hole localization is intrinsic to $sp^2$ bonded $h$-BN, and this places fundamental limits on hole conduction in this material. In contrast, the $sp^3$ bonded wurtzite ($w$-BN) and cubic ($c$-BN) polymorphs are capable of forming shallow acceptor levels. For Be dopants, the acceptor ionization energies are 0.31 eV and 0.24 eV for $w$-BN and $c$-BN, respectively; these values are only slightly larger than the ionization energy of the Mg acceptor in GaN. [Preview Abstract] |
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