Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U18: Focus Session: Wide Band Gap Semiconductors V |
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Sponsoring Units: DMP Chair: Andrei Sirenko, NJIT Room: LACC 406A |
Thursday, March 24, 2005 8:00AM - 8:12AM |
U18.00001: Temperature dependence of the A, B, and C excitons in ZnO over 5-400 K: A modulated reflectivity study. S. Tsoi, M. Cardona, R. Lauck, H. Alawadhi, X. Lu, M. Grimsditch, A. K. Ramdas Optical properties of ZnO, a wide gap semiconductor with wurtzite structure, have generated renewed interest in the material in the context of opto-electronic phenomena and applications. The A, B, and C excitons of ZnO, arising from the combined effects of crystal field and spin-orbit splittings of the valence band, are investigated in the temperature range 5- 400 K, exploiting electro-, photo-, and wavelength-modulated reflectivity. The specimens studied have natural isotopic composition. The temperature dependence of the A, B, and C excitonic band gaps, fitted with a two harmonic oscillator model\footnote{M. Cardona, Phys. Status. Solidi b \textbf{220}, 5 (2000); R. P\"{a}ssler, J. Appl. Phys. \textbf{89}, 6235 (2001)} following Manj\'{o}n \emph{et al.}\footnote{F. J. Manj\'{o}n \emph{et al.}, Solid State Commun. \textbf{128}, 35 (2003)}, yields the magnitudes of the zero-point renormalizations 262 meV (A), 227 meV (B), and 249 meV (C), respectively. Isotopically controlled ZnO is currently being investigated to determine the isotopic mass dependence of the zero-point renormalizations. [Preview Abstract] |
Thursday, March 24, 2005 8:12AM - 8:24AM |
U18.00002: Nature of Room-temperature Photoluminescence in ZnO W. Shan, W. Walukiewicz, J.W. Ager III, K.M. Yu, H.B. Yuan, H.P. Xin, G. Cantwell, J.J. Song The temperature dependence of the photoluminescence (PL) transitions associated with various excitons and their phonon replicas in high-purity bulk ZnO has been studied at temperatures from 12 K to above room temperature (320 K). The evolution of the emission lines with temperature allows us to unambiguously study the PL process in ZnO and to elucidate its nature at room temperature. The room-temperature PL in ZnO is shown to be a free-exciton annihilation process. The process is dominated by the simultaneous emission of photons and longitudinal optical phonons with E$_{FX}$-1LO emission at the maximum due to the strong exciton-phonon coupling in the material. The results explain the discrepancy between the transition energy of free exciton determined by reflection measurement and the peak position obtained by PL measurement. [Preview Abstract] |
Thursday, March 24, 2005 8:24AM - 8:36AM |
U18.00003: Electronic Structure of BGaP Alloys Erik Alldredge, Kwiseon Kim We present the results of an {\em ab initio} investigation of zinc-blende BGaP alloys in the local density approximation. BP is a wide-band gap semiconductor with promising electronic and structural characteristics for potential optical and electronic applications. With an indirect gap of 2.0 eV, it has been considered as an optical window for silicon photoelectrochemical cells and has also been recently investigated as a thin buffer layer for the epitaxial growth of cubic GaN on silicon substrates. The use of boron, a first row element with deep atomic potentials and no $p$ core electrons in III-V compounds results in signficiant differences in electronic structure. For example, the $\Gamma$ conduction band minimum for BP is $p$- like ($\Gamma_{15c}$) in contrast to the $s$-like ($\Gamma_{1c}$) minimum typical of other III-V compounds such as GaP. We discuss the effect of this difference on the band structure of BGaP alloys and determine the bowing parameters for band gaps. [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 8:48AM |
U18.00004: Quantum-well depth of cubic “single stacking fault” inclusions in 4H-SiC p-i-n diodes determined by Ballistic Electron Emission Microscopy K.-B. Park, J. P. Pelz, M. Skowronski, J. Grim Current-induced single stacking-fault (SF) cubic inclusions formed in (1 1 --2 0) oriented 4H-SiC $p-i-n$ diodes were exposed in cross-section by polishing down to the intrinsic layer. Surprisingly non-leaky Schottky barrier (SB) Pt contacts were made on the polished surface, and were investigated by nm-resolution Ballistic Electron Emission Microscopy (BEEM) [1]. Enhanced BEEM current and a $\sim $0.25 eV lower SB height was observed over single SF inclusions, directly confirming they act as $\sim $0.5 nm wide quantum wells (QWs) and support propagating 2D electronic states. This indicates the QW conduction band minimum is $\sim $0.25 eV lower than the 4H-SiC host, consistent with calculations and much shallower than the $\sim $0.53 eV depth of double SF inclusions [1]. We also found that the BEEM amplitude (but not the SB height) is extremely sensitive to polishing scratches, likely due to hot-electron scattering from sub-surface defects. Work supported by ONR and NSF. [1] Yi Ding \textit{et al}., Phys. Rev. B \textbf{69}, 041305 (2004) [Preview Abstract] |
Thursday, March 24, 2005 8:48AM - 9:00AM |
U18.00005: Polarization Properties of III-Nitride Blue and UV Light-Emitting Diodes Kevin Knabe, Jagat Shakya, Kyoung Kim, Zhaoyang Fan, Jingyu Lin, Hongxing Jiang Polarization resolved electroluminescence (EL) studies of III-nitride blue and ultraviolet (UV) light emitting diodes (LEDs) were performed. The LEDs were fabricated on nitride materials grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates (0001). Transverse electric (TE) polarization dominates in the InGaN/GaN quantum well (QW) blue LEDs ($\lambda $ = 458 nm), whereas transverse magnetic (TM) polarization is dominant in the AlInGaN QW UV LEDs ($\lambda $ = 333 nm). For the case of edge emission in blue LEDs, a ratio ($r=I_\bot /I_{//} )$ of about 1.8:1 was observed between the EL intensities with polarization (TE mode) and (TM mode), which corresponds to a degree of polarization $\sim $ 0.29. The UV LEDs exhibit a ratio $r $of about 1:2.3, corresponding to a degree of polarization $\sim $ -0.4. This is due to the fact that the degree of polarization of the band edge emission of the Al$_{x}$In$_{y}$Ga$_{1-x-y}$N active layer changes with Al concentration. The low emission efficiency of nitride UV LEDs is partly related to this polarization property. Possible consequences and ways to enhance UV emitter performances related to this unique polarization property are discussed. Effects of photonic crystal incorporation on the polarization properties will also be discussed. [Preview Abstract] |
Thursday, March 24, 2005 9:00AM - 9:12AM |
U18.00006: Thermal Studies of Operating AlGaN/GaN/SiC Based High Electron Mobility Transistors Todd Holden, ZhiXun Ma, R. Sandhu, B. Heying, I. Smorchkova, M. Wojtowicz AlGaN/GaN structures grown on SiC substrates can be used to create high power, high electron mobility transistors by exploiting the peizo-electric effect. These materials are highly desirable for their high thermal conductivity, high bandgap, and slow degradation. We have performed Raman and AFM- based thermal measurements on operating and non-operating devices in order to study thermal defects. In addition to measuring temperatures at a sub-micron scale, we find that even structures with excellent electronic and optical properties display sub-micron sized thermal defects, resulting in lowered thermal conductivity for the structures. Finite element analysis was used to better understand the thermal flows and experimental results. [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:24AM |
U18.00007: Growth and Fabrication of III-Nitride Deep Ultraviolet Emitters T.M. Al Tahtamouni, M.L. Nakarmi, M. Khizar, Z.Y. Fan, J.Y. Lin, H.X. Jiang In recent years, there has been a great effort to develop AlGaN based compact deep ultraviolet (UV) light-emitting diodes (LEDs) ($\lambda <$ 300 nm) for applications such as bio-chemical agent detection and medical research/health care. To obtain deep UV emission with $\lambda <$ 300 nm, AlGaN quantum well (QW) based LED structures require an active layer with Al composition higher than 40{\%}. As a result, the alloy composition for p- and n-cladding layers should be more than that of the active layer. The high Al composition introduces dislocations and leads to poor p- and n-type conductivity in the cladding layers, which limits current injection. We report here on the epitaxial growth of deep UV LEDs with operating wavelengths ranging from 300 nm to 270 nm by metal-organic chemical vapor deposition (MOCVD). Our UV LED structure was deposited on AlN/sapphire templates. We have achieved deep UV LEDs with an output power of 1.4 mW at 350 mA dc driving at 280 nm. The use of AlN epilayers as templates to reduce the dislocation density and enhance the LED performance will be discussed. Different device architectures for enhanced LED performances will also be presented. [Preview Abstract] |
Thursday, March 24, 2005 9:24AM - 9:36AM |
U18.00008: Nitride Deep Ultraviolet Light-Emitting Diodes with Microlens Array Khizar Muhammad, Zhaoyang Fan, Kyoung Hoon Kim, Jingyu Lin, Hongxing Jiang We report on the fabrication of 285 nm AlGaN-based deep ultraviolet light-emitting diodes (UV LEDs) on sapphire substrates with integrated microlens array. Microlenses with a diameter of 12 microns were fabricated on the sapphire substrate by resist thermal reflow and plasma dry etching. LED devices were flip-chip bonded on high thermal conductive AlN ceramic submounts to improve the thermal dissipation, and the emitted UV light was extracted through the sapphire substrates. With the integrated microlense array, a 55{\%} enhancement in the output power at 20 mA DC driving was achieved compared with the same LED without microlens. The light extraction enhancement is the result of the reduced internal reflections of the light caused by the microlense surface profile. An output power of 0.22 mW at 20 mA was measured for a circular LED with a diameter of 275 microns. [Preview Abstract] |
Thursday, March 24, 2005 9:36AM - 9:48AM |
U18.00009: Direct Measurement of Curvature Dependent Ion Etching of GaN Bentau Cui, Philip Cohen, Amir Dabiran The formation of ion induced nanoscale patterns, such as ripple or dimples, can be described using a continuum equation including a surface roughening term (curvature-dependent sputtering or asymmetric attachment of mobile adatoms/defects), a surface smoothing term (thermal or/and ion induced diffusion), and noise term. By measuring the roughening coefficient of the continuum equation, we have found the Ehrlich-Schwoebel length is 5.2 nm and the step-edge barrier is 1.2 eV at 733K. A Kaufman ion source was used to supply sub-keV ions from glancing incidence. The surface morphology was examined using RHEED in situ and AFM afterwards. The samples were rotated at 1.2 rpm to preclude ripple generation. Dimple structures with diameters ranging from 30 nm to 800 nm, have been produced using both argon ions and nitrogen ions with energies ranging from 60 - 1200 eV at an ion flux of 3.6 ions s$^{-1}$nm$^{-2}$. Using both RHEED and AFM we measure a minimum in the local roughness near 320 C. From the evolution of the dimple dimensions we obtain the first direct measurement of the curvature driven roughening, and the roughening coefficient is measured to be 43.2 nm$^2$/s. The activation energy for surface relaxation has been measured to be 0.11~eV. \\[1ex] Partially supported by the AFOSR [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:00AM |
U18.00010: Spectroscopy and Modeling of Carrier Dynamics in Al$_x$Ga$_{1-x}$N Alloys with Compositional Inhomogeneity H. Shen, C.J. Collins, A.V. Sampath, G.A. Garrett, W.L. Sarney, M. Wraback AlGaN samples grown by plasma-assisted molecular beam-epitaxy on sapphire (0001) substrates, with 20-50{\%} Al content, show intense room-temperature photoluminescence (PL) that is significantly red-shifted from band edge. Low temperature PL shows two distinct peaks: a very bright red-shifted (RS) peak that decreases by a factor of $\sim $7 as the temperature is increased from 10 to 300 K, and a feature associated with band edge (BE) emission that rapidly decreases by greater than 3 orders of magnitude from 10 K to 300 K. Room-temperature monochromatic cathodoluminescence images at the RS peak reveal spatially nonuniform emission. Time resolved (TR) PL data for the BE emission peak show a prompt response at $t$~=~0 and an intensity dependent decay that saturates at higher pump intensity. TRPL data for the RS peak, however, show intensity dependent initial rise times with slow and nearly intensity independent decay times. From fitting to a phenomenological model, the density of the localized states associated with the RS feature, the carrier life times in the BE and the RS states, and the transfer time between the two states are determined. [Preview Abstract] |
Thursday, March 24, 2005 10:00AM - 10:12AM |
U18.00011: Deep UV time resolved optical studies of excitonic transitions in AlN epilayers N. Nepal, M.L. Nakarmi, K.B. Nam, J.Y. Lin, H.X. Jiang Optical properties of excitonic transitions in AlN epilayers have been studied. AlN epilayers were grown by metalorganic chemical-vapor deposition (MOCVD) and the optical properties were probed by deep ultraviolet (UV) time-resolved photoluminescence (PL) spectroscopy. Binding energies and lifetimes of the free-exciton (FX), neutral acceptor-bound exciton (I$_{1})$, and neutral donor-bound exciton (I$_{2})$ transitions have been measured. We found that the undoped AlN epilayer exhibits a strong band-edge emission line at 6.06 eV due to FX transition at 10 K. A PL emission line at 6.02 eV has been observed at 10 K in Mg-doped AlN, which is about 40 meV below the FX transition in undoped AlN epilayer. This transition has been assigned to the recombination of an exciton bound to neutral Mg acceptor (I$_{1})$ with a binding energy of, E$_{bx }$= 40 meV. The recombination lifetime of the I$_{1}$ transition in Mg doped AlN has been measured to be 130 ps. PL studies on Si-doped AlN have found that the I$_{2}$ transition (E$_{bx }$= 16 meV) with a recombination lifetime of 80 ps to be dominant transition at low temperatures. Our experimental study reveals a free-exciton binding energy of 80 meV in AlN, which is the largest exciton binding energy ever reported in semiconductors. [Preview Abstract] |
Thursday, March 24, 2005 10:12AM - 10:24AM |
U18.00012: Resonant shake-up satellites in photoemission at the Ga 3p photothreshold in GaN L. Plucinski, T. Learmonth, L. Colakerol, S. Bernardis, Y. Zhang, P.-A. Glans, K.E. Smith, A.A. Zakharov, R. Nyholm, I. Grzegory, T. Suski, S. Porowski, I. Friel, T. Moustakas Photoemission spectra recorded near the Ga 3$p$ photothreshold from both thin film and single crystal GaN have been found to contain shake-up satellites of the main Ga 3$d$ emission line. The intensity of these satellites resonates at this threshold, and the satellites are associated with a 3$d^{8}$ state. The correlation energies and binding energies for the satellite multiplet have been measured for the satellite and related Auger transitions. The satellite multiplet contains additional constant binding energy features not observed in previous studies of other Ga compounds. The present results are compared to published data for GaP and GaAs, as well as to our preliminary results for thin film In$_{x}$Ga$_{1-x}$N and Al$_{x}$Ga$_{1-x}$N. Our results will be discussed in the context of the degree of correlation and magnitude of the respective band gaps in these materials. \newline [Preview Abstract] |
Thursday, March 24, 2005 10:24AM - 10:36AM |
U18.00013: Resonance Raman Scattering in InGaN J.W. Ager III, W. Walukiewicz, W. Shan, K.M. Yu, S.X. Li, E.E. Haller, H. Lu, W.J. Schaff Resonance Raman studies of single phase In$_{1-x}$Ga$_{x}$N epitaxial films with 0 $<$ x $<$ 0.63 and free electron concentrations in the 10$^{18}$ cm$^{-3}$ range are presented. The A$_{1}$(LO) phonon scattering intensity is enhanced for excitation above the direct band gaps of the films. Examination of films with direct band gaps between 0.7 and 1.9 eV and laser energies from 1.96 to 2.71 eV show that the resonance is broad, extending to up to 2 eV above the direct gap. Multiphonon Raman scattering of the LO phonon up to {\em n} = 5 is also observed in alloy samples. Coupling of the electron plasmon to the LO phonon to form a longitudinal plasmon coupled (LOPC) mode, which is observed in the Raman spectra of n-type GaN, is not observed in In$_{1-x}$Ga$_{x}$N for x $<$ 0.15. These experimental results will be discussed in terms of the electron-phonon interaction in InGaN. [Preview Abstract] |
Thursday, March 24, 2005 10:36AM - 10:48AM |
U18.00014: Molten Salt Based Growth of GaN for Native Substrates Karen Waldrip, T. Kerley, A. West, F. Delnick, J. Tsao, D. Ingersoll The issue of material quality in III-nitrides is nearly ubiquitous in the field owing to the lack of a lattice-matched substrate on which to epitaxially grow thin films for LEDs. Conventional approaches to growth of bulk gallium nitride material require overpressures ranging from 4,000 to 45,000 atmospheres, and the kinetics of such processes is slow and difficult to scale. This work will describe and demonstrate proof of the underlying concepts for a process that may be capable of growing large-area bulk gallium nitride at fast growth rates, reasonable temperatures (450-950ûC), and 1 atm. This method circumvents the typical challenges associated with growth of bulk nitride material by precipitating from a molten salt, which provides the perfect host environment for the reactive nitride anion. Furthermore, molten salts can also function as an electrolyte, permitting electrochemical pre-growth purification processes in addition to several possible methods to electrochemically enhance the growth process, providing further control over the quality of the precipitate. We have recently demonstrated molten salt-based growth of wurtzite GaN crystals as large as 0.9 mm long by 0.6 mm wide in as little as two hours using these techniques. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 24, 2005 10:48AM - 11:00AM |
U18.00015: Electronic Structure Studies of Silicon Carbide Cationic Nanoclusters Prachi Pradhan, Asok K. Ray As a continuation of our studies on the high stabilities and associated electronic structure properties of Si$_{8}$C$_{2 }$to Si$_{14}$C$_{2}$ and Si$_{20}$C$_{n }$(n=3-6) clusters,$^{1}$ we report here detailed \textit{ab initio} electronic and geometric structure studies of small $Si_m C_n^+ $(1\underline {$<$ }m, n \underline {$<$} 4) cationic clusters. The theoretical formalism used is the local density approximation (LDA) to density functional theory (DFT) and the \textit{Gaussian03} suite of programs$^{2}$ with an all electron 6-311++G** basis set has been used. Complete geometry optimizations of different possible structures have been carried out. The stability of the clusters varies with the ratio of the number of silicon to carbon atoms in the cluster. In contrast to the neutral clusters,$^{3}$ cationic clusters appear to prefer more open structures. Results will be presented for binding energies, relative energies, fragmentation energies, vibrational frequencies, and adiabatic ionization potentials$^{3}$ for the optimized clusters. Detailed comparisons with published data in the literature will also be presented. * Work supported, in part, by the Welch Foundation, Houston, Texas (Grant No. Y-1525) $^{1}$M. N. Huda and A. K. Ray, Phys. Rev. A (R) \textbf{69}, 011201 (2004); Eur. Phys. J. D \textbf{31}, 63 (2004). $^{2}$ \textit{Gaussian03}, Revision A.1, M. J. Frisch \textit{et al.,} Gaussian Inc., Pittsburgh, PA , 2003. $^{3 }$P. Pradhan and A. K. Ray, J. Mol. Structure (Theochem), in press. [Preview Abstract] |
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U18.00016: Electronic Structure Studies of Silicon Carbide Anionic Nanoclusters Prachi Pradhan, Asok K. Ray As a continuation of our studies on the high stabilities and electronic structure properties of Si$_{8}$C$_{2 }$to Si$_{14}$C$_{2}$ clusters and Si$_{60}$C$_{n }$(n=3-6) clusters,$^{1}$ we report here \textit{ab initio} studies of small $Si_m C_n^- $(1\underline {$<$ }m, n \underline {$<$} 4) anionic clusters. The theoretical formalism used is the local density approximation (LDA) to density functional theory (DFT) and the \textit{Gaussian03} suite of programs$^{2}$ with an all electron 6-311++G** basis set has been used. Complete geometry optimizations of different possible structures have been carried out. Carbon-rich and silicon rich species show distinctly different patterns with respect to the vertical detachment energies. For carbon-rich aggregates, the VDE's show an even odd alternation, similar to that of the carbon anions. We present results on binding energies, relative energies, fragmentation energies, vertical detachment energies, vibrational frequencies, and adiabatic electron affinities$^{3}$ for the optimized clusters. Detailed comparisons with published data in the literature will also be presented. * Work supported, in part, by the Welch Foundation, Houston, Texas (Grant No. Y-1525). $^{1}$M. N. Huda and A. K. Ray, Phys. Rev. A (R) \textbf{69}, 011201 (2004); Eur. Phys. J. D \textbf{31}, 63 (2004). $^{2}$ \textit{Gaussian03}, Revision A.1, M. J. Frisch \textit{et al.,} Gaussian Inc., Pittsburgh, PA , 2003. $^{3 }$P. Pradhan and A. K. Ray, J. Mol. Structure (Theochem), in press. [Preview Abstract] |
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