Bulletin of the American Physical Society
75th Annual Meeting of the Southeastern Section of APS
Volume 53, Number 13
Thursday–Saturday, October 30–November 1 2008; Raleigh, North Carolina
Session JC: Semiconductors and some Atmospheric Physics |
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Chair: James Dickerson, Vanderbilt University Room: Holiday Inn Brownstone Lincoln |
Friday, October 31, 2008 1:30PM - 1:42PM |
JC.00001: The Wavefunction of Nitrogen Shallow Donors in 4H-SiC Probed by 240 GHz Pulsed EPR/ENDOR Johan van Tol, Mary-Ellen Zvanut SiC is a very suitable semiconductor material for high power and high temperature applications. New fabrication techniques are also overcoming its drawback that it can crystallize in any of over 200 known polytypes. In view of the growing interest in this material for various applications, the electronic properties of many different defects and dopants have been studied by EPR.\footnote{Greulich-Weber, Phys. Stat. Sol.A 162, 95 (1997).} In particular, high frequency EPR has proven very powerful in separating the EPR signals of different sites, and the ENDOR signals of different nuclei.\footnote{van Duijn-Arnold et al., Phys. Rev. B. 64, 085206 (2001). } However, the multivalley structure of the conduction band makes it difficult to assign measured spin densities to specific nuclei. Here we present $^{29}$Si and $^{13}$C pulsed ENDOR measurements at 240 GHz on the N$_{h}$ center in 4H-SiC and a N$_{h}$-N$_{c}$ pair,\footnote{Zvanut and van Tol, Physica B. 401-402, 76 (2007).} and discuss the results in terms of the electron wavefunction (spin-distribution) of these shallow donors. Supported by NSF grants DMR-0084173 and NSF DMR-0520481. [Preview Abstract] |
Friday, October 31, 2008 1:42PM - 1:54PM |
JC.00002: Substrate Temperature effect on the transition characteristics of Vanadium (IV) oxide Tsung-Han Yang, Wei Wei, Chunming Jin, Jay Narayan One of the semiconductor to metal transition material (SMT) is Vanadium Oxide (VO2) which has a very sharp transition temperature close to 340 K as the crystal structure changes from monoclinic phase (semiconductor) into tetragonal phase (metal phase). We have grown high-quality epitaxial vanadium oxide (VO2) films on sapphire (0001) substrates by pulsed laser deposition for oxygen pressure 10-2torr and obtained interesting results without further annealing treatments. The epitaxial growth via domain matching epitaxy, where integral multiples of planes matched across the film-substrate interface. We were able to control the transition characteristics such as the sharpness (T), amplitude (A) of SMT transition and the width of thermal hysteresis (H) by altering the substrate temperature from 300~$^{\circ}$C, 400~$^{\circ}$C, 500~$^{\circ}$C, and 600~$^{\circ}$C. We use the XRD to identify the microstructure of film and measure the optical properties of film. Finally the transition characteristics is observed by the resistance with the increase of temperature by Van Der Pauw method from 25 to 100~$^{\circ}$C to measure the electrical resistivity hystersis loop during the transition temperature. [Preview Abstract] |
Friday, October 31, 2008 1:54PM - 2:06PM |
JC.00003: Bilayered Oxide thin films for transparent electrode application Titas Dutta, Jagdish Narayan Ga doped ZnO films with electrical and optical properties comparable to indium tin oxide (ITO) is a promising candidate for transparent conducting oxides (TCOs) because of its superior stability in hydrogen environment, benign nature and relatively inexpensive supply. However, ZnO based TCO films suffer from low work function, which is a critical parameter for device applications. We report here the growth of a novel bilayered structure consisting of very thin (few monolayers) ITO, MoO$_{x }$layer on Zn$_{0.95}$Ga$_{0.05}$O film for transparent electrode applications by using pulsed laser deposition technique at different temperatures and oxygen partial pressure. The characteristics of the ITO film and the heterostructure have been investigated in detail using XRD, TEM, XPS, and electrical and optical property measurements. It is envisaged that the overall transmittance and the resistivity are dictated by the thicker layer of ZnGa$_{0.05}$O beneath the ITO layer. Hence, this study is aimed to improve the surface characteristics without affecting the overall transmittance and sheet resistance. This will enhance the transport of the carriers across the heterojunction in the device, thus, resulting in the increase in device efficiency. [Preview Abstract] |
Friday, October 31, 2008 2:06PM - 2:18PM |
JC.00004: Virtual Sources in OMCVD Growth of ZnO: The importance of real-time diagnostics for process development E.J. Adles, X. Liu, D.E. Aspnes Growth of zinc oxide (ZnO) by organometallic chemical vapor deposition (OMCVD) is of high current interest because ZnO is nominally a plentiful, low-cost replacement for nitride devices, which rely on Ga. and In, and because CVD is a scalable process. However the extreme reactivity of the common zinc precursor, diethylzinc (DEZ), and the high volatility of ZnO itself make growth via CVD a challenging balance between deposition and sublimation. Using an OMCVD reactor with an integrated spectroscopic polarimeter, we have investigated growth of ZnO on sapphire in real time and have identified 3 factors that must be managed for successful growth. First, the immediate reaction of DEZ with the oxidizer species forms particles of ZnO and/or ZnO adducts, which we term the virtual source. These particles are large enough to scatter light, although the scatter vanishes at a well-defined distance above the growth surface as a result of particle size being reduced below the scattering threshold by sublimation. Second, a seed layer is necessary for growth to begin. Third, the volatility of ZnO at growth temperatures results in large exchange currents between the virtual source and the deposited material such that deposition is essentially reversible. A sequence of real-time spectra illustrates the formation of a seed layer, subsequent growth, and removal of deposited material by sublimation. [Preview Abstract] |
Friday, October 31, 2008 2:18PM - 2:30PM |
JC.00005: Correlations Between XAS and Spectroscopic Ellipsometry Studies and Ab-Initio Quantum Calculations on RPE-MOCVD Deposited Titanium Silicate Alloys Nicholas Stoute, Gerald Lucovsky, David Aspnes We report thin film titanium silicate alloys, with a range of compositions between 0 and 100{\%} TiO$_{2}$, deposited on Si(100) substrates through Remote Plasma Enhanced Metal Organic Chemical Vapor Deposition (RPE-MOCVD).~ Samples were measured in both their as-deposited condition and after a range of annealing temperatures.~ The conduction-band electronic structure of these alloys were analyzed though O K$_{1}$ and Ti L$_{2,3}$ X-Ray Absorption Spectroscopy (XAS) measurements as well as Spectroscopic Ellipsometry measurements preformed in the 1.5 to 6 and 4.5 to 8.5 eV energy ranges.~ Results were correlated with previous theoretical and experimental studies as well as new Ab-Initio quantum calculations. Emphasis is placed on correlating spectroscopic data with calculations on 4-fold coordinated tetrahedral as well as 6-fold coordinated rutile and anatase structures to obtain spectroscopic signatures of phase changes and crystallization. Investigations into the effect of bond distortions on the electronic structure will also be presented. [Preview Abstract] |
Friday, October 31, 2008 2:30PM - 2:42PM |
JC.00006: Tunable THz plasmon resonances in a InGaAs/InP heterostructure Himanshu Saxena, Robert Peale, Walter Buchwald Gate-bias tuned plasmon resonances excited by THz radiation in a two dimensional electron gas are reported. A commercial InGaAs/InP HEMT wafer is patterned with source, drain, and 500 nm period grating gate contacts.~ The grating couples THz radiation to the plasmons, defines their wavevector, and tunes the sheet charge density with applied bias.~ Fourier spectroscopy over the range 10 -- 200 cm$^{-1}$ at 4 K reveals absorption at the fundamental plasmon frequency along with several higher harmonics. These resonances shift to lower frequency with sheet-charge depletion as expected from theory and the device electrical properties.~ The device has potential as a tunable narrow-band detector for spectrometer on a chip and space situational awareness applications. [Preview Abstract] |
Friday, October 31, 2008 2:42PM - 2:54PM |
JC.00007: Epitaxial growth of zinc oxide thin films on silicon (100) substrates with zirconia buffer layer Ravi Aggarwal, Chunming Jin, Wei Wei, Jagdish Narayan, Roger J. Narayan As an II-VI semiconductor, with wide bandgap and high exciton binding energy, zinc oxide has been favored for the new opto-electronic devices. One of the key issues for such applications is the integration of the zinc oxide onto silicon substrates. In this paper, we report a new integration methodology for depositing high quality zinc oxide thin films on silicon substrates. We have developed a novel epitaxial system for this purpose. An yttria stabilized zirconia (YSZ) buffer layer was used for depositing high quality, single crystalline zinc oxide films on Si (100) substrates. The heterostructure was developed with a pulsed laser deposition system. The results show that ZnO films grow epitaxially on YSZ buffered Si (100) substrates, with c-axis perpendicular to the substrate surface. High resolution image demonstrated that the interface between YSZ and ZnO is atomically smooth without any evidence of reaction. These zinc oxide films on Si (100), with YSZ buffer, showed excellent photoluminescence, evidenced with an extremely high exciton emission centered at 377 nm, at room temperature. [Preview Abstract] |
Friday, October 31, 2008 2:54PM - 3:06PM |
JC.00008: Cavity Ring Down Spectroscopy for Atmospheric Research Israel Begashaw, Solomon Bililign, Anthony Cochran, Christopher Jessamy The study of the atmosphere requires full appreciation for several important chemical processes that occur at very small scales. Cavity ring down spectroscopy is one of the most sensitive absorption techniques available. The technique involves measuring the rate of absorption of light by a sample placed in a high-finesse optical cavity. Our lab uses an Nd-Yag pumped dye laser. The laser light is passed through an isolator and telescope set up and is coupled into an optical cavity containing highly reflective mirrors. As the light reflects back and forth between the two mirrors, a small amount leaks out of the cavity the decay time of the leaked light is measured. This decay time constant is dependent on the cavity design and sample placed in the cavity. We are using this sensitive technique to study the spectra of water vapor around 715-740nm. Work is also underway to study NO3 + Isoprene with the same technique. Preliminary results will be reported. [Preview Abstract] |
Friday, October 31, 2008 3:06PM - 3:18PM |
JC.00009: Wavelet-based adaptive mesh refinement algorithm for atmospheric chemical transport modeling Yevgenii Rastigejev Numerical modeling of chemical transport in the Earth atmosphere is essential for addressing problems and issues related to atmospheric air quality, greenhouse gases budget and climate forcing. Chemical transport models (CTM) combine chemical reactions with advection by a meteorologically predicted flow velocity. The resulting system of equations is extremely stiff, nonlinear and involves a large number of chemically interacting species. The difficulty of solving these equations imposes severe limitations on the spatial resolution of the CTMs. Wavelet-based Adaptive Mesh Refinement (WAMR) algorithm has been developed to address these difficulties. WAMR allows a fine grid in the regions where sharp transitions and cruder grid in the regions of smooth solution behavior. Thus WAMR results in much more accurate solutions than conventional finite difference methods implemented on a uniform grids. Numerical experiments showed the algorithm ability to achieve much higher accuracy than traditional numerical methods with the same number of grid points. [Preview Abstract] |
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