Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B46: Focus Session: Wide Band Gap Semiconductors II |
Hide Abstracts |
Sponsoring Units: DMP Chair: Paul Klein, Naval Research Laboratory Room: Baltimore Convention Center 349 |
Monday, March 13, 2006 11:15AM - 11:51AM |
B46.00001: Interaction of the N vacancy with H and Mg acceptors in $p$-type GaN Invited Speaker: Results from recent experimental studies suggest that the N vacancy (V$_{N})$ may compensate Mg acceptors in GaN in addition to the compensation arising from H introduced during growth. To investigate this possibility further, density-functional-theory calculations were performed to determine the interactions of V$_{N}$ with H, Mg, and the MgH center in GaN, and modeling was performed to determine the state populations at elevated temperatures. The results indicate that V$_{N}$H and MgV$_{N}$H complexes with H inside the vacancy are highly stable in $p$-type GaN and act to compensate or passivate Mg acceptors. Furthermore, barriers for formation of these complexes were investigated and the results indicate that they can readily form at temperatures $>$ 400\r{ } C, which is well below temperatures typically used for GaN growth. Overall, the results indicate that the V$_{N}$ compensation behavior suggested by experiments arises not from isolated V$_{N}$, but rather from V$_{N}$H and MgV$_{N}$H complexes with H located inside the vacancy. [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:03PM |
B46.00002: First Principles Green's Function Calculations of the Electronic and Optical Properties of Point Defects in GaN Sohrab Ismail-Beigi Despite the successful use of GaN in light-emitting and laser diodes, its materials properties are not well understood or controlled: typical samples have high densities of point and extended defects. A wealth of experimental information exists on these defects, but disentangling the contributions of individual defects from aggregated measured signals requires theoretical input. To date, first principles studies of these defects have used density functional theory (DFT) to study their formation energies, charge states, and structures. However, DFT provides poor predictions of electronic excitations and thus optical and luminescence properties, whereas many experiments probe these very properties. We apply state-of-the-art first principles Green's functions methods (based on the $GW$ approximation and the Bethe-Salpeter Equation) to GaN point defects. These methods have proven accurate enough to provide direct comparisons to experiments for a variety of materials. We report mainly on the properties of the gallium and nitrogen vacancies, $V_{Ga}$ and $V_N$, native defects with favorable formation energies in typical GaN samples. [Preview Abstract] |
Monday, March 13, 2006 12:03PM - 12:15PM |
B46.00003: Electron paramagnetic resonance of a donor in aluminum nitride crystals. Sean Evans, Nancy Giles, Larry Halliburton, Glen Slack, Sandra Schujman, Leo Schowalter Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) have been used to characterize a dominant donor in single crystals of aluminum nitride (AlN). A broad EPR signal, with g (parallel) = 2.002 and g (perpendicular) = 2.006, is observed in the as-grown crystals. Exposure to x-rays (i.e., ionizing radiation) increases the concentration of this center by a factor of five to ten (depending on sample), thus indicating that most of these centers are initially present in the crystals in a nonparamagnetic charge state. ENDOR identifies a strong hyperfine interaction with one aluminum neighbor along the c axis (described by A (parallel) = 111.30 MHz, A (perpendicular) = 54.19 MHz, and P = 0.289 MHz) and weaker equivalent hyperfine interactions with three additional aluminum neighbors in the basal plane. These aluminum interactions indicate that the responsible center is a deep donor at a nitrogen site. The observed paramagnetic defect is either a neutral oxygen substituting for nitrogen or a neutral nitrogen vacancy. This work was supported at West Virginia University by the National Science Foundation (Grant DMR-0508140). One of the authors (SME) received support from the WV EPSCoR STEM fellowship program. [Preview Abstract] |
Monday, March 13, 2006 12:15PM - 12:27PM |
B46.00004: Comparison of point defects in 4H SI SiC grown by halide chemical vapor deposition and physical vapor transport M.E. Zvanut, H.J. Chung, Alexander Polyakov Semi-insulating (SI) SiC may be produced by chemical vapor deposition using halides (HCVD) or physical vapor transport (PVT). Our electron paramagnetic resonance (EPR) studies address the point defects detected in PVT and HCVD 4H SI SiC substrates. EPR measurements are made `in equilibrium' prior to illumination and after exposure to light. All samples reported here have resistivity of 10$^{10}$ ohm-cm and activation energy greater than 0.8 eV. PVT samples exhibit the positively charged carbon vacancy (V$_{c}^{+})$ before illumination, which may be quenched by 1.5 eV light and revived at energies greater than 1.9 eV. The `equilibrium' EPR measurement of the HCVD samples reveals an intrinsic defect which can be quenched by bandgap illumination. The amount of this unidentified defect is estimated to be an order of magnitude greater than that of V$_{c}^{+}$ typically observed in PVT substrates. [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 12:39PM |
B46.00005: Observation of a three-site defect in SI 4H-SiC N.Y. Garces, W.E. Carlos, E.R. Glaser, M.A. Fanton High temperature anneals were used to study the evolution of native defects in semi-insulating 4H-SiC grown by PVT or HTCVD methods. The samples were annealed in an argon atmosphere for 30 min at temperatures from 1400$^{o}$C to 2100$^{o}$C. Using electron paramagnetic resonance (EPR), we observe a defect that is tentatively identified as V$_{C}$-C$_{Si}$-V$_{C}$. This spin S=1 defect is characterized by g$_{\vert \vert }$=2.0029; g$_{\bot }$=2.0038, a fine structure splitting D$\sim $96G, and several hyperfine interactions with the low abundant Si (A$\sim $5.5G) and C (A$\sim $30G) neighboring nuclei. This center is diamagnetic in the ground state but can be excited into a paramagnetic triplet state by sub-bandgap light. The EPR intensity of this center increases significantly with annealing. This defect could result during annealing by the movement of an adjoining C atom onto the V$_{Si}$ site of the divacancy (V$_{C}$--V$_{Si})$ whose intensity decreases. This may be the simplest of a family of more complex defects that play a role in the SI character. A search for optical signatures associated with this defect from 0.7-3.2 eV PL studies will also be discussed. [Preview Abstract] |
Monday, March 13, 2006 12:39PM - 12:51PM |
B46.00006: Defects Controlling the Minority Carrier Lifetime in n$^{-}$ 4H-SiC epitaxial layers . P.B. Klein, B.V. Shanabrook, S.W. Huh, A.Y. Polyakov, M. Skowronski, J.J. Sumakeris, M.J. O'Loughlin The relationship between the minority carrier lifetime (MCL) and the concentration of deep trapping centers in n$^{-}$ 4H-SiC was investigated by low-injection time-resolved photoluminescence (TRPL) and by deep level transient spectroscopy (DLTS) and minority carrier transient spectroscopy (MCTS). Layers of varying layer thicknesses were studied in order to enable the separation of bulk lifetimes from surface recombination effects. A linear dependence of the inverse bulk MCL on the concentration of the Z1/Z2 defect, and the lack of significant correlation of the MCL on the concentration of any other traps, suggests that the Z1/Z2 defect controls the MCL. The results of DLTS measurements in 4H-SiC p-i-n diodes under forward bias reinforce the idea that Z1/Z2 alone is the lifetime killer in this material. [Preview Abstract] |
Monday, March 13, 2006 12:51PM - 1:03PM |
B46.00007: Transition metal doped SiC: defect levels and magnetism M.S. Miao, Walter R.L. Lambrecht The properties of transition metal substitutions and interstitials in 3C and 4H SiC are studied by first principles supercell calculations. The defect levels change slightly for different polytypes or for different layers in 4H SiC. The calculated defect levels are generally in good agreements with DLTS results. Ti in 4H SiC has two acceptor levels close to the conduction band minimum, corresponding to cubic and hexagonal layers.V, Cr and Mn are amphoteric and have both donor and acceptor levels in the gap. We found Mn has deep trap levels which may be useful to achieve semi-insulating SiC, as is also well-known to be the case for V. The Cr and Mn acceptor levels are deep in the gap. They are unlikely to induce holes at the valence band maximum. Therefore the well-known hole-mediated ferromagnetic mechanism does not apply to Cr and Mn doped SiC. However, our calculations showed ferromagnetic coupling for Cr or Mn doped at neighboring sites. The ferromagnetic coupling is very strong but localized for Cr:SiC. However, it is relatively weak but long range for Mn:SiC. Such features are determined by the nature of the impurity bands. The highest occupied defect state of Cr:SiC is antibonding $e$ which is localized whereas the state of Mn:SiC is $t2$ which is delocalized and strongly couples with the surrounding C dangling bonds. [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:15PM |
B46.00008: Heat Capacity of ZnO: Isotope Effects Manuel Cardona, Jorge Serrano, Reinhard K. Kremer, Gisela Siegle, Aldo H. Romero, Rudolf Lauck We have measured the heat capacity of zinc oxide for several single crystals with different isotopic composition in the 5 -- 350 K temperature range. In order to analyze the dependence of the heat capacity on the isotope mass of the oxygen and zinc atoms, we have performed first-principles calculations within the harmonic approximation. The Zn mass affects mainly the acoustic phonons, thus leading to isotopic effects on the heat capacity mostly at low temperatures, whereas the O mass affects mainly the optic phonons, which become thermally active at higher temperatures. This behavior is reproduced quantitatively by the calculations, and is also in agreement with theoretical predictions reported for wurtzite GaN, an isostructural semiconductor. The possibility of using these data to obtain the density of phonon states projected on the corresponding atoms will be discussed. [Preview Abstract] |
Monday, March 13, 2006 1:15PM - 1:27PM |
B46.00009: Unambiguous identification of an OH-Li center in ZnO: Experiment G.A. Shi, M. Stavola, W.B. Fowler Theory has found that isolated H is always a donor in ZnO [1] and has led to a number of studies of the properties of H in this promising wide bandgap semiconductor. Of interest here is an OH vibrational line at 3577.3 cm$^{-1}$ that is dominant in ZnO grown by the hydrothermal method [2,3]. We show that the two naturally abundant isotopes of Li [$^{6}$Li (7.5{\%}) and $^{7}$Li (92.5{\%})] cause the D-stretching counterpart of the 3577.3 cm$^{-1}$ IR line to be split into two components that can be separately resolved, even though the 3577.3 cm$^{-1}$ line itself shows no Li-related splitting. This unexpected result establishes unambiguously that the 3577.3 cm$^{-1}$ IR line is due to an OH-Li complex. Overtone and oxygen isotope data for the OH-Li center provide an unusually complete picture of the vibrational properties of this defect. Because isotope splittings for elements heavier than Li will be more difficult to detect, this OH-Li center may be considered a model system for H trapped by impurities in ZnO. \newline \newline [1] C. G. Van de Walle, Phys. Rev. Lett. \textbf{85}, 1012 (2000). \newline [2] L. E. Halliburton \textit{et al.}, J. Appl. Phys. \textbf{96}, 7168 (2004). \newline [3] E. V. Lavrov \textit{et al.}, Phys. Rev. B \textbf{71}, 035205 (2005). [Preview Abstract] |
Monday, March 13, 2006 1:27PM - 1:39PM |
B46.00010: Unambiguous identification of an OH-Li center in ZnO: Theory W.B. Fowler, G.A. Shi, M. Stavola Three structures[1] have been suggested for the OH-Li complex in ZnO whose Li isotope shift has been observed by Shi, Stavola, and Fowler: (a), an O-H-second neighbor Li; (b), Li-O-H; and (c), O-H-Li (a bond-centered hydrogen), with all three atoms along the c-axis. Wardle\textit{ et al.} [2] have theoretically investigated the energies of two of these structures. They found (c) to be the ground state and (b) to be 0.5 eV higher in energy. We have used[3] CRYSTAL2003 to evaluate the structural and vibrational properties of this defect. We also find the ground state to be bond centered, with structure (b) higher in energy by 0.5 eV and structure (a) higher by 0.7 eV. We have then moved the O, H, and Li atoms by hand along the c-axis to obtain harmonic force constants and anharmonic constants and have used the harmonic force constants to predict the Li-related isotope shifts. Li-related isotope shifts for OD are predicted to be only 0.3 {\%} and 25{\%} of the experimental value, respectively, for structures (b) and (a). Structure (c), the bond-centered structure, is favored both by its stability and by a predicted Li-related isotope shift for OD of 76{\%} of the experimental value. 1. L. E. Halliburton \textit{et al.}, J. Appl. Phys. \textbf{96}, 7168 (2004). 2. M. G. Wardle \textit{et al.,} Phys. Rev. B \textbf{71}, 155205 (2005). 3. V. R. Saunders \textit{et al.,} \textit{Crystal2003 User's Manual}, University of Torino, Torino, 2003. [Preview Abstract] |
Monday, March 13, 2006 1:39PM - 1:51PM |
B46.00011: Vibrational properties of a Li -- OH complex in ZnO K. R. Martin, P. M. Blaney, G. A. Shi, M. Stavola, W. B. Fowler Considerable interest has developed on the potential use of II-VI oxides as electronic and optical materials. In several cases alkali atoms have been suggested as dopants. We report on the theoretical and experimental investigation of infrared and vibrational properties of a Li-OH complex in ZnO. Earlier infrared experiments[1] revealed a broad OH-like band centered at 3577.3 cm$^{-1}$ (12K), with full width at half maximum ranging from 0.4 cm$^{-1}$ (12K) to 41.3 cm$^{-1}$ (300K) and a corresponding shift in peak position of - 29 cm$^{-1}$, suggesting a significant coupling of the OH stretch with other modes. We have performed similar experiments on the OD version of this defect and have theoretically investigated[2,3] the coupling of these stretch modes to other modes. The lack of a significant isotope dependence of the resulting parameters suggests that the defect couples to the host rather than the large-amplitude motion of the H or D itself. 1. L. E. Halliburton \textit{et al}., J. Appl. Phys. \textbf{96}, 7168 (2004). 2. B. N. J. Persson and R. Ryberg, Phys. Rev. B \textbf{32}, 3586 (1985). [3] M. Budde \textit{et al}., Phys. Rev. B \textbf{63}, 195203 (2001). [Preview Abstract] |
Monday, March 13, 2006 1:51PM - 2:03PM |
B46.00012: Oxygen vacancies and titanium interstitials in rutile and anatase John Jaffe, Michael Henderson Native point defects in reduced TiO$_{2}$, namely the oxygen vacancy and Ti interstitial, were investigated computationally for both the rutile and anatase structures. The generalized gradient approximation to density functional theory was used along with a plane-wave expansion and ultrasoft pseudopotentials. Defect formation energies were calculated after geometry relaxation from O vacancies created in the bulk rutile and anatase lattices, from the experimental Ti interstitial position in rutile, and from several trial initial geometries for the Ti interstitial in anatase. Contrary to traditional assumptions but consistent with much recent evidence, the Ti interstitial was found to be predominant over the O vacancy in rutile under most conditions. Donor ionization energies in rutile were consistent with experiment. Surprisingly, the calculations also indicate a dominant role for the Ti interstitial in anatase (lower formation energy than the O vacancy, though not by as much as in rutile, and much shallower ionization levels.) We evaluate these findings against experimental data on pure n-type TiO$_{2}$, and discuss possible implications for transition metal cation doping as well as anion doping of TiO$_{2}$. [Preview Abstract] |
Monday, March 13, 2006 2:03PM - 2:15PM |
B46.00013: Electron Distributions in Pure TiO$_{2}$ in Anatase Phase and with Hydrogen Impurities* S. Byahut, Sudha Srinivas, Lee Chow, R.H. Scheicher, Junho Jeong, R.H. Pink, T.P. Das** The electronic structures of pure TiO$_{2}$ in anatase phase and with hydrogen impurity have been investigated by the Hartree-Fock Cluster Procedure. They are used to study the associated $^{47}$Ti and $^{17}$O nuclear quadrupole coupling constants e$^{2}$qQ and asymmetry parameters $\eta $ . For pure TiO$_{2}$ in the anatase phase, comparison will be made with theoretical and experimental e$^{2}$qQ in the rutile phase, the value of $\eta $ for $^{47}$Ti vanishing in the anatase phase due to local axial symmetry. *Supported by US-Nepal Research Program, UGC Nepal, and Senior Fulbright Post-Doctoral Fellowship (S.B.) **Also at UCF, Orlando [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700