Bulletin of the American Physical Society
76th Annual Meeting of the Southeastern Section of APS
Volume 54, Number 16
Wednesday–Saturday, November 11–14, 2009; Atlanta, Georgia
Session EC: Semiconductors |
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Chair: Vadym Apalkov, Georgia State University Room: Paris |
Thursday, November 12, 2009 3:45PM - 3:57PM |
EC.00001: Growth of InN and In rich InGaN by ``High Pressure Chemical Vapor Deposition'' (HPCVD) Max Buegler, Mustafa Alevli, Ramazan Atalay, Goksel Durkaya, Jielei Wang, Indika Senevirathna, Muhammad Jamil, Ian Ferguson, Nikolaus Dietz We present the growth of III-nitrides under elevated nitrogen pressure to stabilize the growth surface at elevated temperatures. The achievable growth temperatures are significantly higher than in conventional low pressure MOCVD systems. With this the integration of In-rich materials into GaN and GaAlN becomes possible. In addition we present structural and optical properties of thin films grown by HPCVD at a reactor pressure of 15bar and temperatures of 870$^{o}$C. The Samples have been analyzed by Raman, optical absorption, IR reflectance and photoluminescence spectroscopy and by XRD. The growth of single phase layers of high crystalline quality has been proven by XRD with InN (0002) Bragg reflex FWHM's of 200arcsec (2$\Theta -\Omega $-scan) and 1600arcsec (rocking curves). These is backed up by Raman spectra with InN E$_{2}$(high) peak FWHM's of below 10cm$^{-1}$. Free carrier concentrations in the mid 10$^{18}$ cm$^{-3}$ to low 10$^{19}$cm$^{-3}$ have been calculated from IR reflection spectra. Photoluminescence spectroscopy showed luminescence at 0.77eV. [Preview Abstract] |
Thursday, November 12, 2009 3:57PM - 4:09PM |
EC.00002: Optical properties of InGaN layers Jielei Wang, Mustafa Alevli, Ramazan Atalay, Goksel Durkaya, Max Buegler, Indika Senevirathna, Nikolaus Dietz Optical absorption spectroscopy provides important properties of a material including fundamental absorption edge and defect structures. We use it to investigate the band gap of In$_{x}$Ga$_{(1-x)}$N with different Gallium contents grown by HPCVD. The results have shown that a bowing parameter of b = 2.5 eV allows one to reconcile our results and the literature data for the band gap of In$_{x}$Ga$_{(1-x)}$N alloys over the whole composition range. In addition, we calculate the thicknesses of the In$_{x}$Ga$_{(1-x)}$N thin films through simulation of the transmission using a multilayer stack model. The free electron concentration in the In$_{x}$Ga$_{(1-x)}$N thin film determined here is also found to be in good agreement with that obtained from simulations of infrared reflection measurements we carried out. [Preview Abstract] |
Thursday, November 12, 2009 4:09PM - 4:21PM |
EC.00003: Characterization of ``High Pressure Chemical Vapour Deposition'' grown InGaN layers by IR reflectance spectroscopy Indika Senevirathna, Max Buegler, Ramazan Atalay, Goksel Durkaya, Jielei Wang, Nikolaus Dietz We present the experimental setup, experimental results and a model to simulate the gathered IR reflection spectra in order to gain access to the materials properties. In Detail we will present results on optical Properties of hexagonal InGaN epitaxial films. The films have been grown on GaN/Sapphire substrates by ``High Pressure Chemical Vapor Deposition.'' The Dielectric function, the free carrier concentration, the thickness, the optical phonons and the carrier mobility in the InGaN layers have been determined by simulating the experimental IR reflectance spectra. For the simulation we used a multi layer stack model and in each layer a combination of classical harmonic Lorentz oscillators and the Drude model. We simulated the spectra all over the transparent, the restrahlen and the free carrier absorption region. The free carrier absorption, Phonon related features as well as interference fringes can be observed in the experimental data and can be matched well with our model. The free carrier concentrations in the analyzed InGaN layers are in the mid 10$^{18}$ cm$^{-3}$ ranges. [Preview Abstract] |
Thursday, November 12, 2009 4:21PM - 4:33PM |
EC.00004: Nonthermal Laser Assisted Ge Quantum Dot Formation on Si(100)-2x1 by Pulsed Laser Deposition Ali Er, Hani Elsayed-Ali The effect of laser-induced electronic excitations on the self-assembly of Ge quantum dots (QDs) on Si(100)-2x1 grown by pulsed laser deposition is studied. The samples were cleaned by using modified Shiraki method and then transferred into the deposition chamber. The vacuum system was then pumped down, baked for at least 24 hours, and the sample was then flashed to 1200 \r{ }C in order for the 2x1 reconstruction to form. The experiment was conducted under a pressure $\sim $1x10$^{-10}$~Torr. A Q-switched Nd:YAG laser was used to ablate a Ge target. In-situ RHEED and STM and ex-situ AFM were used to study the morphology of the grown QD. The dependence of the QD morphology on substrate temperature and ablation and excitation laser energy density was studied. Electronic excitation is shown to affect the surface morphology. Laser irradiation of the Si substrate is shown to decrease the roughness of films grown at a substrate temperature of $\sim $400 $^{o}$C. Electronic excitation also affected the surface coverage, cluster density, uniformity and decreased the temperature required to form 3-dimensional QDs to $\sim $250 \r{ }C at which no crystalline film formation is possible without excitation laser. Possible mechanisms such as two hole localization following the phonon kick will be discussed. [Preview Abstract] |
Thursday, November 12, 2009 4:33PM - 4:45PM |
EC.00005: ABSTRACT WITHDRAWN |
Thursday, November 12, 2009 4:45PM - 4:57PM |
EC.00006: Anomalous capacitance effects in GaN/Al$_{0.026}$Ga$_{0.974}$N structures G. Rothmeier, L. Byrum, N. Dietz, A.G.U. Perera, S. Matsik, I. Ferguson, A. Bezinger, H.C. Liu The effects of interface defect states on the capacitance characteristics of an $n^{+}${\-}GaN/Al$_{0.026}$Ga$_{0.974}$N/$i${\-}GaN/$n^{+}${\-}GaN structure are reported. An anomalous high-frequency capacitance peak was observed in the capacitance-frequency (C-f) profiles. Using IR spectroscopy, the defect related absorption centers with activation energies of 125~$\pm $~1 and 139~$\pm $~2~meV were attributed to C-donor/N-vacancy and Si-donor states pinned to the $n^{+}${\-}GaN layer, respectively. Si defect states at the $i${\-}GaN/$n^{+}${\-}GaN interface were found to produce the high-frequency capacitance peak. The peak can result from resonance scattering due to the hybridization of localized Si-donor states in the band gap (with electronic levels above the conduction band minimum) and continuous conduction band states at the $i${\-}GaN/$n^{+}${\-}GaN interface. [Preview Abstract] |
Thursday, November 12, 2009 4:57PM - 5:09PM |
EC.00007: Capacitance hysteresis due to interface defect states in $n^{+}$-GaN/Al$_{0.1}$Ga$_{0.9}$N heterostructures L. Byrum, G. Ariyawansa, R. Jayasinghe, N. Dietz, A.G.U. Perera, S. Matsik, I. Ferguson, A. Bezinger, H.C. Liu Capacitance-voltage-frequency measurements of $n^{+}${\-}GaN/Al$_{0.1}$Ga$_{0.9}$N heterostructures are reported. A distinct capacitance{\-}step and hysteresis were observed, and attributed to the abrupt change in electron occupation in C{\-}donor/N{\-}vacancy defect states located just above the Fermi level (200~meV) at the heterointerface, with an activation energy of 149~$\pm $~1~meV. With a forward scan direction, the defect states are initially empty. As the bias increases, the defect states will be pulled below the Fermi level, causing an abrupt change in trap state occupation and, thus, capacitance. As these defect states become occupied an accumulation region will form. When the scan direction is reversed, the defect states are now initially filled. The charge in the accumulation region leads to lower effective fields at the interface and, hence, a higher required voltage for the capacitance-step, which occurs as the defect states are pulled above the Fermi level and are abruptly emptied. The difference in initial defect trap state occupation between a forward and reverse scan direction results in the observed hysteresis. [Preview Abstract] |
Thursday, November 12, 2009 5:09PM - 5:21PM |
EC.00008: GaN-Based Heterojunction Structures for Simultaneous Detection of Ultraviolet/Infrared Radiation R. Jayasinghe, G. Ariyawansa, N. Dietz, A.G.U. Perera, S. Matsik, I. Ferguson, S. Laframboise, A. Bezinger, M. Buchanan, H.C. Liu Ultraviolet and infrared (UV/IR) dual-band photodetectors have potential applications in various areas, such as fire/flame detection, solar astronomy, military sensing, situational awareness applications, and combustion process monitoring. Since GaN-based UV/IR detectors do not respond to solar or artificial visible lighting, the false detection rate is strongly reduced. Use of a single detector for detecting UV/IR can also eliminate the difficulties of operating several individual detectors with separate electronics and cooling mechanisms. Here, we report a dual-band detector which simultaneously detects UV (250 -- 360 nm) and IR (5 -- 14 $\mu $m) regions, showing near zero spectral crosstalk. This allows the detection of both UV and IR incident radiation, separately identifying the relative strength of each photocurrent component. Further improvements for the detector are also discussed. [Preview Abstract] |
Thursday, November 12, 2009 5:21PM - 5:33PM |
EC.00009: Towards absorption enhancement and design optimization of Split-off band infrared photodetectors Manmohan Shishodia, A. G. Unil Perera Room temperature photodetectors operating in infrared (IR) region are important for astronomy, biomedical, defence and security related applications. Recently developed short wavelength infrared (2-5$\mu $m) detectors utilizing light absorption through split-off band transitions in mature GaAs/AlGaAs material system may offer an efficient alternative to the intrinsically slow present day microbolometer detectors. The total quantum efficiency of these detectors, defined as the product of absorption efficiency, internal quantum efficiency, and collection efficiency, usually limited by low absorption, can be improved through IR antenna induced surface plasmon enhanced absorption. The antenna induced absorption besides free carrier and split-off absorption should improve the total quantum efficiency ($\eta )$ and hence the responsivity (R), two being related by R=q$\eta \lambda $/hc, of these detectors. The optimized detector designs capable of reinforcing absorption due to free carriers and the antenna in the split-off region, and the theoretical results on absorption enhancement and performance improvement will be presented. [Preview Abstract] |
Thursday, November 12, 2009 5:33PM - 5:45PM |
EC.00010: Quantum Dot and Quantum Well Photodetectors for Multispectral Imaging G. Ariyawansa, V. Apalkov, A.G.U. Perera, G. Huang, P. Bhattacharya, M. Buchanan, Z.R. Wasilewski, H.C. Liu Multi-color infrared detection has become an important tool for identification of targets in complex backgrounds, aiding to effective target recognition. While detectors capable of sensing all the spatial, spectral, and polarimetric information from a scene are identified as the third generation infrared detectors, here, focus is on the development of multi-color detectors with wavelength selection capability based on the applied bias voltage. A superlattice quantum dot infrared photodetector (SL-QDIP) structure consisting of two quantum dot superlattices (QDs embedded in a GaAs/AlGaAs superlattice) separated by a graded barrier will be presented. This device structure enables photocurrent generation only in one superlattice depending on the applied voltage polarity. Two response peaks at 4.9 and 7.3 $\mu $m were observed under -1 V and 0.5 V, respectively. The specific detectivity values at 80 K are 4$\times $10$^{10}$ and 8$\times $10$^{9}$ Jones for the 4.9 $\mu $m (at -1 V) and 7.3 $\mu $m (at 0.5 V) peaks, respectively. Moreover, several quantum well designs will also be discussed as potential alternatives. [Preview Abstract] |
Thursday, November 12, 2009 5:45PM - 5:57PM |
EC.00011: Analysis of dark current mechanisms for split-off band infrared detectors at high temperature region Y.F. Lao, D. Kurkcuoglu, P.V.V. Jayaweera, S.G. Matsik, A.G.U. Perera An analysis of dark current mechanisms has been performed on high-operating-temperature (140-330K) split-off (SO) band infrared detectors based on p-GaAs/AlGaAs heterojunction structures. In contrast to tunneling and thermionic emission at low temperatures, carrier spreading effects due to drift-diffusion transportation dominate the main source of dark current for SO detectors working at high temperatures. The barrier height of heterojunction plays a critical role in determining a transition temperature for the alternation of dark current channels and operating temperatures of SO detectors. Current spreading effects induce non-uniformity of R0A as measured on devices with different mesa sizes. A theoretical model is used to explain experimental current-voltage curves and optimize device uniformity such as using high doping of p-GaAs region, high barrier height etc. [Preview Abstract] |
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