Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session W21: Dopants and Defects in Semiconductors IV |
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Sponsoring Units: DMP Chair: Anderson Janotti, University of California Santa Barbara Room: 323 |
Thursday, March 19, 2009 11:15AM - 11:27AM |
W21.00001: Temperature Dependence of the Minority Carrier Lifetime in n$^{-}$ Epilayers of 4H-SiC Paul Klein, Amitesh Shrivastava, Tangali Sudarshan Controlling the lifetime of minority carriers in n$^{-}$ epilayers of 4H-SiC is of great current interest, as short lifetimes lead to a high forward voltage drop in high-voltage, bipolar switching devices. As such devices operate at elevated temperatures, the temperature dependence of the carrier lifetime is of particular interest. For materials where the lifetime is controlled by trapping at deep defects (e.g. Z1/Z2), increasing the temperature results in the thermal emission of trapped carriers, leading to an increase in the carrier lifetime. In this work the temperature dependence of the carrier lifetime, measured by time-resolved photoluminescence decay at low injection, has been studied in a range of epitaxial layers. In addition to the classic temperature dependence, some samples exhibit a lifetime that decreases rapidly with temperature in a thermally activated manner. This behavior is believed to result from the fact that the deep defect concentration is too low in these samples to limit the lifetime, and that other processes have become dominant. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W21.00002: Issues with Deep Defect Spectra in Electron Irradiated 4H SiC F. Yan, R.P. Devaty, W.J. Choyke, T. Kimoto, T. Ohshima, G. Pensl Recently, Steeds \textit{et al.} [1] discussed deep levels induced by high fluence electron irradiation in 4H SiC. We have also observed the particular triplet assigned to di-carbon antisites using both ion implantation and electron irradiation. Here we specifically address data obtained by 170 keV electron irradiation at a fluence of 5x10$^{16 }$cm$^{-2}$. We shall discuss details of the no phonon lines of the triplet as well as two sets of vibrational modes well beyond the highest energy of the SiC lattice spectrum. Theory suggests that one should observe four no phonon lines and groups of four lines for each observed localized mode. Our high resolution spectra reveal differences in the LVM spectra with respect to those reported by Steeds \textit{et al.} We obtain strong spectra at a fluence of 5x10$^{16 }$cm$^{-2}$ whereas Steeds \textit{et al.} report that they do not see the triplet in the irradiated area using greater than 10$^{19}$ cm$^{-2}$, but do see it beyond the periphery of the TEM beam. We can explain this in terms of the transverse straggling of the electrons in his TEM beam. Finally, we report the reappearance of this triplet due to an anneal at 1100\r{ }C after it had already been annealed out at 1400\r{ }C. [1] J. W. Steeds\textit{ et al.}, Phys. Rev. B \textbf{77}, 195203 (2008). [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W21.00003: Charge transfer kinetics of carbon vacancy defect in 4H-SiC J. Dashdorj, M.E. Zvanut, J.G. Harrison There has been much detailed work aimed at understanding carbon vacancy related defects and their complexes in SiC, but there are no reports of charge transfer kinetics between the carbon vacancy and other defect centers. In this study, optical cross sections of the positively charged carbon vacancy, V$_{c}^{+}$, in high purity semi-insulating 4H-SiC were measured by time-dependent photo-electron paramagnetic resonance, EPR. The measurements were performed by a X-band EPR spectroscopy at 80 K. Selected photon energies used in this study were between 0.8 and 3.13 eV. A single defect model considering only capture and emission of electrons from V$_{c}^{+}$ was shown to fit well the measured data. The photon energy-dependence of the cross sections exhibit threshold value of 1.6 eV and peak value of 2.15 eV for the capture, and threshold value of 1.9 eV and peak value of 2.45 eV for the emission processes, respectively. In this talk, we will discuss the above results in terms of charge transfer mechanisms including the effects of the electronic density of states and participation of phonons. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W21.00004: EPR Study of SiC Defects Related to N$_{2}$ and O$_{2}$ Annealing. Sarah Thomas, Mary Ellen Zvanut SiC is a promising replacement for Si in future high power, high temperature electronic devices. It is well known that the Si/SiO$_{2}$ interface in MOSFETs has electronically active defects, and recent work has shown the same is true for SiC. Our research focuses on identifying the cause and location of defects in thermally treated SiC substrates using EPR at 9.8 GHz. Samples underwent isochronal anneals from 400 to 1000 $^{o}$C in high purity dry ($<$0.9 ppm H$_{2}$O) N$_{2}$ or O$_{2}$. Room temperature EPR spectra showed two defects, defect A and defect B, with line-widths of 4G and 10G, respectively. The temperature dependence was similar for the N$_{2}$ and O$_{2}$ anneals until 800 $^{o}$C, when the concentration of defect A, which stayed constant in N$_{2}$, decreased in O$_{2}$. In both ambients defect B was eliminated, and it was determined that this defect was due to cutting. That the amount of defect A decreased during the O$_{2}$ anneals, but not during the N$_{2}$, suggests that oxidation, perhaps through etching, removes the signal. During the talk we will compare the results of oxidation and reactive ion etching studies, as these will give a better understanding of the location of defect A. [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W21.00005: Pulsed ENDOR at 240 GHz of nitrogen centers in 4H-SiC: towards a detailed description of the wavefunction. Johan van Tol, Mary-Ellen Zvanut SiC is a very suitable semiconductor material for high power and high temperature applications. The electronic properties of many different defects and dopants in various polytypes have been studied by electron paramagnetic resonance (EPR), including various common nitrogen substitutional defects. In particular, \textit{high frequency} EPR has proven very powerful in separating the EPR signals of different sites, while the nuclear transitions of hyperfine coupled $^{29}$Si and $^{13}$C that are observed by electron nuclear double resonance (ENDOR) can be well separated. Here we report on new data on the hexagonal nitrogen center in 4H-SiC, for which the hyperfine interaction with the surrounding silicon and carbon shells was measured by pulsed ENDOR at 240 GHz. While these measurements give precise values for the electron spin density on the surrounding atoms, the assignment of these densities to particular atomic sites has proven challenging. The multivalley structure of the conduction band in this indirect semiconductor complicates the analysis. We will discuss the observed results and propose a tentative assignment on the basis of the Kohn-Luttinger theory. Supported by the NSF though grants DMR-0654118 and NSF DMR-0520481. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W21.00006: X-ray photoelectron spectroscopy of Ni doped boron carbides Nina Hong, M.A. Langell, S. Adenwalla Ni acts as an n-type dopant for semiconducting boron carbide (BC). A series of samples with increased Ni doping were grown on Si substrates using plasma enhanced chemical vapor deposition (PECVD) and characterized using IV measurements and X-ray photoelectron spectroscopy (XPS). Increased Ni doping leads to a linear increase in Ni concentration as evidenced by the intensity of the Ni 2p photoemission peak relative to that of the B 1s peak; concomitantly, the IV curves indicate that the BC becomes increasingly n doped. B1s peak shapes shows B-C and B-B bonding structure, and the C1s peak shows B$_{11}$C icosahedra bonds and C-B-C chain bonds in all samples. The overall binding energies for B and C agree with the results from sputter deposited stoichiometric B$_{4}$C [1]; in these PECVD grown samples, however, the graphite peak commonly seen in the sputter deposited B$_{4}$C is absent. [1] I. Jimenez, L. J. Terminello, \textit{et al.} J. Elect. Spec. Relat. Phenom., 101-103, 611-615 (1999). [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W21.00007: First-principles study of Oxygen vacancies in Mg$_{x}$Zn$_{1-x}$O alloys Adisak Boonchun, Walter Lambrecht A first principles study of oxygen vacancies in Mg$_{x}$Zn$_{1-x}$O alloys has been carried out within the LDA$+U_s+U_d$ FP-LMTO approach. Different types of oxygen vacancies are distinguished by their number of Mg and Zn nearest neighbors. We find that the energy of formation is lowest for oxygen vacancies surrounded by four Zn nearest neighbors. Because of the Boltzmann factor this implies that the probability of finding oxygen vacancies with one or more Mg as nearest neighbors is strongly suppressed. Unlike in pure ZnO and MgO, we do not find negative U behavior but this may in part be because of the small size of the supercell. The 2+/+ and +/0 transitions level gradually move to higher energy as the number of nearest neighbor Mg atoms increases. Defect levels in rocksalt MgO are also presented. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W21.00008: Cation dopant distributions in Mn-doped ZnO nanostructures and thin films: experiment and Monte Carlo simulations T.C. Droubay, D.J. Keavney, S.M. Heald, T.C. Kaspar, B.P. Kaspar, C.M. Wang, C.A. Johnson, K.M. Whitaker, D.R. Gamelin, S.A. Chambers Anion or cation doping at relatively high concentrations of several atomic percent is frequently suggested to realize synthetic materials with qualitatively new functionality. While the statistical probability of obtaining singles, dimers, and trimers has been determined for bulk lattices, these distributions are significantly altered in nanostructures and thin films due to the presence of under-coordinated surface sites. The dopant distributions in nanostructures and thin films of doped wurtzite ZnO have been determined from Monte Carlo simulations. Using empirical expressions derived from the MC simulations that accurately predict dopant bonding configurations as a function of surface-to-volume ratio and concentration, experimental results for epitaxial films of Mn-doped ZnO will be discussed. X-ray absorption and x-ray magnetic circular dichroism revealed that Mn(II) substituted for Zn in the Mn:ZnO films, which were deposited by PLD using targets created from Mn:ZnO nanoparticles. However, while substitutional, the Mn distribution is not stochastic but rather tends to segregate, yielding higher local concentrations than anticipated. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W21.00009: Weak localization effects in Al-doped ZnO films Priya.V Chinta*, Q.Y. Chen*, O. Lozano*, P.V. Wadekar*, W.K. Chu, S.W. Yeh, N.J. Ho, L.W. Tu, Y.S. Chang, W.Y. Pang, I. Lo, H.W. Seo Metal-semiconductor transitions (MST) at low temperatures were studied for (0001)-oriented Zn$_{1-x}$Al$_{x}$O thin films deposited by simultaneous RF magnetron sputtering of ZnO and Al onto (11-20)-oriented Al$_{2}$O$_{3}$ substrates. The MST occurs at 190K, 102K and 260K for x=2{\%}, 3{\%} and 10{\%} of Al-doping, respectively. The samples display negative magnetoresistance at low temperatures with zero-field electrical resistivity being as low as 3.3 $\times$ 10$^{-4} \quad \Omega $-cm for x=3{\%}. The charge scattering mechanisms below the MST will be discussed in light of weak localization and coulomb interactions due to disorder in the system. *Also with Dept of Physics, NSYSU, Taiwan. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W21.00010: First-Principles Theoretical Analysis of Dopant Adsorption and Diffusion on Surfaces of II-VI Compound Semiconductor Nanocrystals Tejinder Singh, T. J. Mountziaris, Dimitrios Maroudas We present a first-principles theoretical analysis of dopant adsorption and diffusion on facets of II-VI semiconductor nanocrystal surfaces and discuss its implications for dopant incorporation into growing nanocrystals. We focus on ZnSe nanocrystals with diameters d$\sim $5 nm that have polyhedral shapes with well-defined facets. Using density functional theory calculations, we find that ZnSe(001)-(2$\times $1) is the energetically favorable surface facet for dopant binding, with multiple adsorption sites. We find that the binding energy for Mn adsorption onto various sites of the ZnSe(001)-(2$\times $1) surface increases with increasing dopant surface concentration. This low binding energy at low dopant surface concentration provides an explanation for doping difficulties during nanocrystal growth. In addition, we have analyzed several dopant migration pathways for Mn diffusion on the ZnSe(001)-(2$\times $1) surface and calculated the corresponding activation barriers as a function of dopant surface concentration. We find that Mn atoms can migrate fast along the Se dimer rows. However, Mn migration to a trough site is governed by a high-barrier process that may lead to dopant incorporation into the ZnSe nanocrystal. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W21.00011: Effect of Oxygen Vacancies in In$_2$O$_3$ Kalum Palandage, Adil-Gerai Kussow, Alkim Akyurtlu, Gayanath Fernando In order to assess the effect of oxygen vacancies on its electronic structure, we have calculated the band structure of In$_2$O$_3$ (in the Ia$_3$ structure) with and without oxygen vacancies using density functional theory within the local density approximation. A $4\times4\times4$ Monkhorst-Pack grid of k- points were used to sample the Brillouin zone while permitting full structural relaxation and self-consistency. A noticeable change that is observed is in the nature of the direct band gap of In$_2$O$_3$ at the zone center, which becomes indirect with the addition of a single oxygen vacancy to an ideal 40-atom unit cell. There is also a clear tendency toward metallic behavior with the inclusion of a single oxygen vacancy, which appears to be independent of the location of the vacancy. In addition, spin-polarized calculations reveal negligible magnetization due to the introduction of these vacancies. The threshold vacancy concentration necessary for metallic behavior, our results from a symmetry analysis of the relevant valence and conduction band states, which are crucial for optical transitions, as well as effects due to Cr-doping will be presented. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W21.00012: Transport properties of transparent conducting oxide thin film, Nb:In2O3 O. Lozano*, Q.Y. Chen*, P.V. Chinta*, P.V. Wadekar*, L.H. Chu, D. Wijesundera, Wei-Kan Chu, H.W. Seo, L.W. Tu, Y.S. Chang, W.Y. Pang, I.K. Lo, S.W. Yeh, N.J. Ho Thin films of Nb-doped In$_{2}$O$_{3}$ were deposited on YSZ(001) by magnetron co-sputtering. The well-oriented thin films were studied as a function of Nb doping by x-ray diffraction, optical absorption spectroscopy and magneto-transport measurement. The optical transparency in the visible and infrared spectral ranges is 97-99{\%} while the electrical resistivity is about 0.4 m$\Omega $-cm. The variation of these properties with respect to doping will be discussed in the context of scattering and optical transition mechanisms. *Also with Department of Physics and Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung, Taiwan, Republic of China. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W21.00013: A First-Principle Density-Functional Theory of BxGa1-xBiyAs1-y Quaternary Alloys Zuozi Chen, Lei Liu, Peter Y. Yu, Zhi Xun Ma, S.S. Mao Both B and Bi are isovalent impurities in GaAs when they substitute for Ga and As, respectively, at low concentrations. At higher concentrations they can form alloys with GaAs. They have opposite effects on the host GaAs crystal in terms of the lattice constant and band gap. B is smaller than Ga and will increase the band gap of GsAs in addition to converting it from a direct band gap semiconductor into an indirect one. On the other hand, Bi is larger than As and will decrease the band gap of GaAs, turning it into a semi-metal at high concentration. In principle, by incorporating B and Bi into GaAs in appropriate concentration one can tune the band gap of the alloy over a large range of values from the far infra-red to the near uv. We have performed a first-principle density-functional calculation of the total energy, lattice parameters and the band gap of the cubic BxGa1-xBiyAs1-y alloy system. A generalized quasi-chemical approach is adopted to handle the disorder effects induced by alloying. The constant band gap energy surface Eg(x,y) of the quaternary alloy was found to display a two-dimensional bowing in the x-y plane. The range of compositions for which the alloy is lattice-matched to GaAs is also obtained. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W21.00014: Diffusion of point defects in CdTe John Jaffe, Charles Henager We have investigated the mobility of isolated native point defects in CdTe by first-principles calculations. Cd vacancies and interstitials, Te interstitials and Te-on-Cd antisites were considered. Diffusion barriers were found by the NEB (nudged- elastic-band) technique within the PAW-LDA method as implemented in the VASP code. Diffusion constants are estimated, and some implications for the growth of radiation detector material are suggested, especially in regard to the formation of Te precipitates. Comparisons to experimental and earlier theoretical studies are also provided. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W21.00015: Engineering Oxygen Vacancy Distribution by Exteranl Strain Da-Jun Shu, Shu-Ting Ge, Mu Wang, Nai-Ben Ming The most common defects on surfaces of transition metal oxides are oxygen vacancies, which play critical roles in applications such as heterogeneous catalysis, photoelectrolysis, biocompatibility, etc.. If the nature and distribution of the oxygen vacancies can be controlled, the surface properties will then be modified for different applications. For this purpose, one needs to understand both the influence of oxygen vacancies on the surface properties and the responses of oxygen vacancies to different external fields. We have conducted comprehensive first principles calculations on the surface energy of strained rutile TiO2(110) with oxygen vacancies. The formation energy of each type of oxygen vacancy is calculated as a function of external strain. We find that the type of the most easily formed oxygen vacancy can be tuned by the strain and therefore suggest that the distribution of oxygen vacancies can be engineered by external strain, which helps to improve the applications of TiO2 surface where oxygen vacancies play important roles. The dependence of surface elastic properties on the type of oxygen vacancy is found to be responsible for the interplay between external strain and oxygen vacancies. [Preview Abstract] |
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