Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B23: Focus Session: Dopants and Defects in Semiconductors II |
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Sponsoring Units: FIAP Chair: Gerd J. Duscher, University of Tennessee at Knoxville Room: 325 |
Monday, March 18, 2013 11:15AM - 11:27AM |
B23.00001: A modification of Eu incorporation sites by the dissociation of hydrogen defect complexes in Mg co-doped Eu doped gallium nitride Brandon Mitchell, Jonathan Poplawsky, Volkmar Dierolf Europium doped gallium nitride (Eu:GaN) is a promising candidate as a material for red LEDs that can monolithically be integrated with existing nitride based lighting technology. Photoluminescence (PL) and cathodoluminescence (CL) studies have revealed, however, that the majority incorporation environment (site) for the Eu is not efficiently excited by electron hole pairs. To improve this efficiency, Mg was co-doped into Eu:GaN during metal organic chemical vapor deposition and multiple new incorporation environments were discovered. These new sites show a high efficiency at room temperature and have been attributed to the coupling of a Mg-H complex to the majority Eu site. However, we also observe that sustained electron beam irradiation produced a semi-permanent change in the CL spectra of the sample. It was demonstrated that this change occurs in two distinct steps which exhibit a pronounced temperature dependence. Our observations point toward a dynamic system in which the Mg-H bond is broken and the hydrogen moves within the epi-layer. Details of this behavior will be discussed. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B23.00002: The origin of the high hole density in In$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N:Mg William Willoughby, Mary Ellen Zvanut InGaN is the nitride of choice for applications requiring high hole density and emission tunability. The increased hole density with In incorporation may be explained by several different mechanisms; however, our electron paramagnetic resonance (EPR) studies reveal a surprising feature: the number of Mg-related acceptors decreases with increasing hole density. In$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N films, with x between 0.02 and 0.11 and thickness between 0.25 and 0.44 $\mu$m, were grown p-type by doping with Mg to a concentration of 2-3 $\times$ 10$^{19}$ cm$^{-3}$. Hall measurements reveal the expected hole density increase from 5-30x10$^{17}$ cm$^{-3}$ with increasing In mole fraction. However, unlike GaN:Mg where the EPR Mg signal tracks the hole density, the EPR intensity of the Mg-related signal in InGaN is found to decrease as the hole density increases. Together, compensating defects and a lowering of the acceptor level may explain the decrease in EPR intensity and the increase in hole density observed as the In mole fraction is increased. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B23.00003: Stability and electronic structure of Mg dopants in InGaN alloys Ji-Sang Park, K.J. Chang Nitride semiconductors have attracted much attention due to their applications for light emitting and laser diodes. High conductivity p-type nitride layers are demanding for various optoelectronic devices, however, hole concentrations are generally low because of the deep acceptor level of Mg and the compensation of hole carriers by donor defects. In this work, we investigate the stability and electronic properties of Mg dopants in InGaN alloys through first-principles density functional calculations. We generate the alloy structure with the In content of 10\% by using the special quasi-random structure approach. Considering various Mg sites surrounded with different numbers of the In atoms in the second nearest neighborhood, we find that Mg dopants prefer to be located near the Ga atoms rather than the In atoms due to the local bonding effect. Incorporation of the In atoms not only reduces the band gap but also decreases the ionization energy of Mg in Ga-rich regions. However, the ionization energy tends to increase as the number of the In atoms in the second nearest neighborhood increases, although this configuration is energetically unfavorable. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B23.00004: How localized acceptors limit $p$-type conductivity in GaN Invited Speaker: John L. Lyons Despite the impressive development of GaN as an optoelectronic material, $p$-type conductivity is still limited. Only a single acceptor impurity, magnesium, is known to lead to $p$-type GaN. But Mg is far from a well-behaved acceptor. Hydrogen is known to passivate Mg, necessitating a post-growth anneal for acceptor activation. In addition, the ionization energy is quite large ($\sim$ 200 meV in GaN), meaning only a few percent of Mg acceptors are ionized at room temperature. Thus, hole conductivity is limited, and high concentrations of Mg are required to achieve moderately $p$-type GaN. Other acceptor impurities have not proven to be effective $p$-type dopants, for reasons that are still unresolved. Using advanced first-principles calculations based on a hybrid functional, we investigate the electrical and optical properties of the isolated Mg acceptor and its complexes with hydrogen in GaN, InN, and AlN.\footnote{J. L. Lyons, A. Janotti, and C. G. Van de Walle, Phys. Rev. Lett. \textbf{108}, 156403 (2012).} We employ a technique that overcomes the band-gap-problem of traditional density functional theory, and allows for quantitative predictions of acceptor ionization energies and optical transition energies. Our results allow us to explain the deep or shallow nature of the Mg acceptor and its relation to the optical signals observed in Mg-doped GaN. We also revisit the properties of other group-II acceptors in GaN. We find that all cation-site acceptors show behavior similar to Mg$_{\mathrm{Ga}}$, and lead to highly localized holes. The Zn$_{\mathrm{Ga}}$ and Be$_{\mathrm{Ga}}$ acceptors have ionization energies that are even larger than that of Mg, making them ineffective dopants. All acceptors cause large lattice distortions in their neutral charge state, in turn leading to deep, broad luminescence signals that can serve as a means of experimentally verifying the deep nature of these acceptors. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B23.00005: Role of self-trapping in luminescence and $p$-type conductivity of wide-band-gap oxides Joel Varley, Anderson Janotti, Cesare Franchini, Chris Van de Walle Using hybrid functional calculations, we investigate the behavior of holes in the valence band of a range of wide-band-gap oxides including ZnO, MgO, In$_2$O$_3$, Ga$_2$O$_3$, Al$_2$O$_3$, SnO$_2$, SiO$_2$, and TiO$_2$. We find that, due to the orbital composition of the valence band, holes tend to form localized small polarons with characteristic lattice distortions, even in the absence of defects or impurities. These self-trapped holes (STHs) are energetically more favorable than delocalized, free holes in the valence band in all materials but ZnO and SiO$_2$. Based on calculated optical absorption and emission energies we show that STHs provide an explanation for the luminescence peaks that have been observed in many of these oxides. Additionally, we demonstrate that polaron formation prohibits $p$-type conductivity in this class of materials. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B23.00006: Carbon Defect Complex as a Source of Yellow Luminescence in GaN Denis Demchenko, Mikhail Reshchikov Using hybrid functional theory compared with experimental measurements, we demonstrate that yellow luminescence often observed in both carbon-doped and pristine GaN is the result of electronic transitions via C$_N$-O$_N$ complex. In contrast to the common isolated defects, C$_N$-O$_N$ complex is energetically favorable, and its calculated optical properties as well as the thermodynamic transition level show excellent agreement with the measured luminescence data. Calculated transitions via the localized defect states of this complex are (experimental values are given in brackets): thermodynamic transition level of 0.75 eV (0.85 eV), absorption energy 3.30 eV (3.32 eV), emission energy 2.25 eV (2.20 eV), and zero phonon transition 2.70 eV (2.60 eV). This complex has not been proposed as a source of the yellow band in GaN, while all other defects previously suggested to be sources of this band exhibit high formation energies and would produce red or infrared photoluminescence. Thus, combining hybrid density functional theory and experimental measurements we propose a solution to a long-standing problem of the GaN yellow luminescence. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B23.00007: Identification of the defect responsible for current collapse in GaN/AlGaN HEMTs Yevgeniy Puzyrev, Xiao Shen, Sokrates Pantelides Recent experiments show that GaN/AlGaN high-electron-mobility transistors (HEMTs) suffer significant current collapse during stress conditions characterized by the presence of charge trap level $\sim$ 0.50 eV below conduction band. This phenomenon has been attributed to thermally activated defect diffusion without specifying responsible defects. Here we report first-principles density-functional calculations of the hydrogenated substitutional oxygen complexes and show that the electric-field-enhanced formation of this defect complex provides an explanation for observed phenomenon. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B23.00008: Fe charge state kinetics in semi-insulating Fe-doped GaN Ustun Sunay, Jamiyanaa Dashdorj, Mary Ellen, Kevin Udwary, Jacob Leach GaN is a wide bandgap semiconductor with applications in LEDs and high-power devices. One of the problems plaguing this material is a high concentration of residual donors. This issue can be resolved by doping GaN with deep acceptors such as Fe, which compensates donors and creates semi-insulating material. Recently, a photo-induced electron paramagnetic resonance (EPR) spectroscopy study of Fe-doped GaN showed significantly long relaxation times [1]. The study proposed a charge transfer mechanism between Fe$^{3+}$ and Fe$^{4+}$ as an explanation for the phenomenon. However, absorption data from the same samples showed the existence of both Fe$^{2+}$ and Fe$^{3+}$ which suggests that the proposed model involving Fe$^{4+}$ is incorrect and a theory involving an intermediate center is more likely. 3.5 K 10 GHz EPR was performed on HVPE grown free-standing Fe/Si co-doped GaN. Data show an unexpected situation where both donor and Fe$^{3+}$ acceptor signals exist simultaneously. Together with the photo-EPR results, these data reinforce the necessity of invoking a multi-step mechanism for compensation. A model for compensation based on charge transfer between Fe3$+$ and a donor will be described based on EPR and additional material characterization measurements. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B23.00009: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 1:27PM - 1:39PM |
B23.00010: Nonradiative carrier capture rates at defects from first-principles calculations Qimin Yan, Audrius Alkauskas, Chris G. Van de Walle We develop a computational methodology to determine nonradiative carrier capture rates at defects in wide-band-gap semiconductors. In our theoretical framework, we consider carrier capture via multiphonon emission as the dominant nonradiative mechanism for deep defects in wide-band-gap materials at low and moderate carrier densities. Our methodology is based on the static approximation for the electron-phonon coupling. We employ a state-of-the-art hybrid density functional approach to describe the electronic structure. For charged defect systems, the screening effect by excess carriers is taken into account. As test cases, we investigate deep centers including C$_{\rm N}$ and $V_{\rm Ga}$ in GaN and Li$_{\rm Zn}$ in ZnO. Calculated carrier capture rates are in good agreement with available experimental data. This work was supported by DOE, NSF, Swiss NSF, and by the UCSB SSLEC. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B23.00011: Optical properties of Ga$_{1-x}$Mn$_x$As from large scale ab initio calculations J. Jackson, R. Cardenas, G. Bester The properties of Mn impurities in GaAs are revisited employing a new methodology based on atomic effective potentials (AEPs [1]) which yields LDA accuracy at considerably reduced computational expense. We consider the case of very low Mn concentrations that cannot be considered using conventional ab initio methods and discuss the metal/insulator transition in terms of the Mn-d band localization and its interpretation as a shallow acceptor. We discuss practical methods to improve upon the LDA bandgap in GaAs together with the excessive delocalization of the Mn states. Using a configuration-interaction technique we calculate the optical spectra of Ga$_{1-x}$Mn$_x$As including the fine-structure (FSS) splitting which is of importance to the development of quantum computing devices based upon magnetic impurities in semiconductors [2].\\[4pt] [1] J. R. C\'{a}rdenas and G. Bester, Phys. Rev. B {\bf 86}, 115332 (2012)\\[0pt] [2] D. E. Reiter, T. Kuhn, V.M. Axt, Phys. Rev. B {\bf 83}, 155322 (2011) [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B23.00012: Impact of gamma-irradiation on the properties of n-type AlGaN/GaN heterostructures Elena Flitsiyan, Leonid Chernyak Gamma-photon irradiation of AlGaN/GaN HEMTs with the modest dose of 700 Gy$^{\ast}$ resulted in significant deterioration of their DC characteristics. To understand the nature of the observed effect, we carried out a series of variable temperature EBIC measurements in the vicinity of HEMT's gate in-situ in Scanning Electron Microscope. The measurements were performed on 3 different devices, which were exposed to various gamma-irradiation doses. Temperature dependent EBIC measurements allowed obtaining activation energies for levels in the material's forbidden gap, which are responsible for carrier recombination. While the diffusion length decreases significantly with increasing irradiation dose, the activation energy, associated with carrier recombination, gets larger. This fact indicates generation of new deep levels caused by gamma-photon irradiation. These levels act as traps for electrons in AlGaN/GaN HEMT channel, thus reducing the drain current and leading to degradation of other device characteristics. The investigated effects of gamma irradiation are likely related to, epitaxial layer quality and composition. Therefore, the study of materials with variations in these properties is necessary to fully understand the irradiation-induced mechanisms. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B23.00013: Electronic band structure, doping, and defects in the semiconducting Half Heusler compound CoTiSb Jason Kawasaki, Linda Johansson, Martin Hjort, Rainer Timm, Brian Schultz, Thiagarajan Balasubramanian, Anders Mikkelsen, Chris Palmstrom We report transport and electronic band structure measurements on epitaxial films of the Half Heusler compound CoTiSb. CoTiSb belongs to the family of Half Heuslers with 18 valence electrons per formula unit that are predicted to be semiconducting despite being composed of all metallic components. Here the CoTiSb films were grown by molecular beam epitaxy on a lattice matched InAlAs buffer. The films are epitaxial and single crystalline, as measured by reflection high-energy electron diffraction and X-ray diffraction. Scanning tunnelling spectroscopy and temperature-dependent transport measurements reveal that the films are semiconducting, with unintentionally doped carrier concentrations comparable to that of highly doped conventional compound semiconductors. These carrier concentrations can be modulated by doping with Sn. The band structure of the films was measured by angle resolved photoemission spectroscopy at the MAX-Lab Synchrotron facility. The bulk bands are in general agreement with density functional theory calculations, with a valence band maximum at $\Gamma $ and surface states within the bulk band gap. The effects of defects are explored in order to explain the ARPES results. [Preview Abstract] |
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