Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H46: Focus Session: Wide Band Gap Semiconductors IV |
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Sponsoring Units: DMP Chair: Michael Stavola, Lehigh University Room: Baltimore Convention Center 349 |
Tuesday, March 14, 2006 11:15AM - 11:51AM |
H46.00001: Gallium Nitride-Based Nanowire Radial Heterostructures for Nanophotonics Invited Speaker: Semiconductor nanowires are attractive building blocks for the assembly of active photonics devices, providing a unique and flexible pathway for creating multicolor integrated nanophotonic systems beyond the limit of conventional planar structures. Realizing this potential will require novel electrically driven and interconnected nanowire building blocks with emission wavelengths that can be rationally tuned. To achieve this goal, we have exploited the controlled growth of well-defined GaN-based nanowire radial heterostructures, and their application as efficient and synthetically tunable multicolor light sources. Prepared by metal-organic chemical vapor deposition, these nanowire heterostructures consist of an n-GaN core and diverse multishells, including n-GaN/InGaN/p-GaN double heterostructures, n-GaN/InGaN/GaN/p-AlGaN/p-GaN single quantum well, and n-GaN/(InGaN/GaN)m/p-AlGaN/p-GaN multi-quantum well structures, where variation of indium mole fraction is used to tune emission wavelength. Transmission electron microscopy analysis shows that these nanowires are dislocation-free single crystals with triangular cross-sections and chemically distinct shells, while composition and thickness of individual shells are well controlled during synthesis. Under optical excitation, they exhibit strong photoluminescence consistent with bandgap emission of InGaN inner shell, and behave as freestanding Febry-P\'{e}rot optical cavities. Moreover, by contacting simultaneously n-GaN core and p-GaN shell, electroluminescence results demonstrate that in forward bias they function as high-brightness light-emitting diodes with tunable emission from 365 to 600 nm and high quantum efficiencies. The ability to synthesize rationally GaN-based nanowire radial heterostructures as electrically-driven, efficient and color-tunable light sources should open up significant potential for integrated photonics. [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:03PM |
H46.00002: GaN nanorods grown on Si(111) substrates by plasma-assisted molecular beam epitaxy Ching-Lien Hsiao, Li-Wei Tu, Tong-Wei Chi, Min Chen Various GaN nanorod structures grown on Si(111) substrate are realized by plasma-assisted molecular-beam epitaxy. Evolution of the nanorod structure from isolated regular nanorods, to isolated non-regular nanorods, and to dense nanorods is well controlled by the GaN buffer structure. Adding the parameter of beam-equivalent pressure of N/Ga ratio to the nanorod growth, the density of the regular nanorod becomes also a controllable item. There are several combinations of rod density and diameter in the nanorod growth. High density-small diameter and low density-large diameter can be grown directly on the surface without buffer layer. Low density-small diameter, low density-large diameter, and high density-large diameter can be achieved by inserting a GaN buffer. Nanorod of single crystal wurtzite structure without dislocation was characterized by high-resolution transmission electron microscopy. Only Ga and N signals were detected by energy-dispersion x-ray spectroscopy analysis. Single freestanding nanorod was prepared to perform micro-Raman spectroscopy. Wurtzite-type Raman modes at different scattering configurations have small line width and indicate the high crystalline quality of the nanorod. Fr\"{o}lich interaction and the surface vibrational modes are observed at high laser power densities which will be discussed in detail. [Preview Abstract] |
Tuesday, March 14, 2006 12:03PM - 12:15PM |
H46.00003: GaN Overgrowth on GaN Nanocolumns by Molecular Beam Epitaxy K. L. Averett, J. E. Van Nostrand, J. Boeckl, R. Cortez, J. D. Albrecht GaN nanocolumns grown by plasma-assisted molecular beam epitaxy are explored as host structures for overgrowth of bulk-like GaN films. We investigate the layers of vertical GaN nanocolumns $90 \pm 10$nm in width which were grown on sapphire and alternate substrates. We present photoluminescence, x-ray diffraction, and microscopy data that indicates that the columns are unstrained, low-defect wurtzite GaN nanostructures. The nanocolumns form discontinuous layers with areal densities controlled by grown conditions. The nanocolumn layers are subsequently overgrown with thick GaN films and compared with commercial GaN template materials used for device fabrication. The overgrowth of GaN is studied as a function Ga flux and growth temperature. We present photoluminescence and Hall effect characterization of the overgrown layers. The microstructure and morphology are probed by atomic force, scanning and transmission electron microscopy, as well as x-ray diffraction. [Preview Abstract] |
Tuesday, March 14, 2006 12:15PM - 12:27PM |
H46.00004: Growth of ZnO Nanotube without catalyst and templates Yoke Khin Yap, Samuel Mensah, Vijaya Kaystha Nanotubular structures of oxides materials have recently gained attention for their hydrophilic properties. These oxide nanotubes are attracting interest for biological applications including nanofuidic devices for single DNA molecule sensing, rapid disease diagnosis and DNA sequencing. Here we show that tubular cavities of ZnO can be directly grown on substrates without the use of catalyst and templates. This is obtained by the evaporation and the condensation of a mixture of ZnO and graphite powders in a double-tube vapor-phase transport system. Field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), X-ray powder diffraction, Raman spectroscopy and photoluminescence (PL) were employed to study and characterize the tubular structures. Results show that these tubular structures were single crystals of pure hexagonal Wurtzite structure. Our results show that rapid cooling rate and deficiency of oxygen during cooling contributed to the tubular ZnO nanostructures. A growth model and possible reasons for the growth of ZnO nanotubes will be discussed at the meeting. [Preview Abstract] |
Tuesday, March 14, 2006 12:27PM - 12:39PM |
H46.00005: Growth of ZnO Nanowires without the mixture of other ZnO nanostructures Samuel Mensah, Vijaya Kaystha, Yoke Khin Yap We describe a new procedure to grow pure ZnO nanowires without the mixture of other ZnO nanostructures like nanobelts and nanocombs. A variety of ZnO nanostructures have been grown on oxidized Si substrates by a vapor phase transport process at temperatures ranging from 900 --500 deg. C. A mixture of ZnO and graphite powders are used as raw materials and result in the growth of nanobelts, nanocombs, and nanorods at various temperature zones when Au catalysts are used. We found that pure ZnO nanowires can be grown on catalyst free substrates. This is obtained at the temperature zone 500 to 600 deg. C, a region beyond those for growing nanobelts and nanorods. The crucial step is the need of an Au-coated substrate adjacent to the plain substrates. Long nanowires with uniform diameter less than 50 nm were obtained. Field-emission scanning electron microscopy (FESEM) analysis shows that the gold vapor from the adjacent substrates creates the nucleation sites on the catalyst free substrates to initiate the nanowire growth. Characterization of these ZnO nanostructures was conducted by X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and photoluminescence (PL). Details of these will be discussed in the meeting. [Preview Abstract] |
Tuesday, March 14, 2006 12:39PM - 12:51PM |
H46.00006: Exceptionally Bright Visible-wavelength Luminescence from Sulfur-doped ZnO Nanowires John V. Foreman, Hongying Peng, Jianye Li, Soojeong Choi, Henry O. Everitt, Jie Liu Sulfur-doped ZnO nanowires have been grown by the vapor-liquid-solid technique using ZnS and carbon as starting materials. The broadband, visible-wavelength emission of unprecedented brightness from these nanowires is characterized by steady-state and time-resolved spectroscopy. Energy transfer is explored by simultaneously studying the fast ($<$50 ps) decay of band edge emission and the slow ($>$5 ns) decay of the visible-wavelength emission as a function of temperature and excitation intensity. The contributions of sulfur doping and nanostructuring to the generation of this intense visible-wavelength emission are clarified by characterizing ZnO nanowires and micropowders of different morphologies and dopant concentrations. The results can be understood in terms of a physically motivated rate equation model, for which several of the key parameters are experimentally constrained. [Preview Abstract] |
Tuesday, March 14, 2006 12:51PM - 1:03PM |
H46.00007: Optical properties of AlGaN/AlGaN microcavities containing GaN quantum wells Oleg Mitrofanov, Stefan Schmult, Michael Manfra, Michael Sergent, Richard Molnar The large oscillator strength and binding energy of excitons in GaN make GaN based microcavities attractive for studies of light-matter coupling. Lattice mismatch within the family of Nitride alloys, however, results in formation of many extended defects, which degrade optical quality of heterostructures and substantially increase inhomogeneous broadening of excitons. We will discuss optical properties of AlGaN/AlGaN microcavities for the spectral region around 350 nm grown by molecular beam epitaxy on thick GaN templates. The structural quality of the microcavities is maintained by compensating the compressive and tensile strains in the layers of the distributed Bragg reflectors (DBR). This approach results in the lowest elastic strain energy and allows the growth of thick coherently strained DBRs with reflectivity higher than 99{\%}. We will present photoluminescence studies on GaN quantum wells incorporated inside microcavities and discuss exciton-photon coupling. [Preview Abstract] |
Tuesday, March 14, 2006 1:03PM - 1:15PM |
H46.00008: Studies of Enhanced Internal Quantum Efficiency of Photoluminescence from Al$_{x}$Ga$_{1-x}$N Alloys Displaying Nanoscale Compositional Inhomogeneities Gregory A. Garrett, A. V. Sampath, C. J. Collins, W. L. Sarney, H. Shen, M. Wraback AlGaN epilayers, grown by plasma-assisted molecular beam epitaxy, show internal quantum efficiency (IQE) for photoluminescence (PL) much higher than expected for growth on sapphire where high defect densities ($> 10^{10}$ cm$^{-2}$) are generated. Results are similar to earlier successes seen in blue- green light emitting diodes (LEDs) incorporating InGaN active regions where carrier localization is seen due to indium segregation. With no predicted immiscibility gap for aluminum in GaN and no observed aluminum segregation, the IQE increase in our AlGaN layers is attributed to localization of carriers to regions of nanoscale compositional inhomogeneities (NCI) that inhibit movement of carriers to nonradiative sites. Sub- picosecond time-resolved PL, using gated downconversion in a nonlinear optical crystal, is used to measure carrier capture dynamics from the bulk epilayers into these NCI regions. Rate equation modeling is used to estimate the density of NCI regions and to help understand the observed changes in IQE for NCI AlGaN epilayers grown under different conditions. [Preview Abstract] |
Tuesday, March 14, 2006 1:15PM - 1:27PM |
H46.00009: Wide bandgap semiconductor nanoclusters: Magnetism and UV photoluminescence Jiji Antony, David E. McCready, Mark Engelhard, Chongmin Wang, Amit Sharma, Joseph Nutting, Daniel Meyer, You Qiang Transition metal-doped ZnO is of great interest to current research due to its wide variety of applications in spintronic materials. We prepared 5{\%} Ti, V, Co or Ni-doped ZnO nanoclusters using a third generation nanocluster source. TEM images show that the nanoclusters are monodispersive with a nanocrystalline size $<$ 10 nm. XRD patterns are identical to the bulk ZnO wurtzite structure. XPS detected the dopant elements in clusters and showed Ti in +4 oxidation state, V in +4 and +5, Co in +2, and Ni in +2 and +3. These analyses indicate that dopant elements do not exist as independent aggregates but are incorporated into the ZnO structure. All the doped ZnO nanoclusters are ferromagnetic above room temperature. Magnetic moments of Ni or V-doped ZnO are much larger than Ti or Co doped ZnO clusters at 300K. Double exchange interactions due to the mixed valance states can be the reason that Ni or V-doped ZnO clusters a better magnetic moment than the Ti or Co-doped clusters. UV-photoluminescence is observed at pure and low dopant concentration ZnO nanoclusters. [Preview Abstract] |
Tuesday, March 14, 2006 1:27PM - 1:39PM |
H46.00010: Microbeam High Resolution X-ray Diffraction and Reciprocal Space Mapping Characterization of Selective Area Grown InGaN/GaN Waveguides A.A. Sirenko, A. Kazimirov, S. Cornaby, D. H. Bilderback, A. Ougazzaden, B. Neubert, F. Scholz We present microbeam high-resolution x-ray diffraction (HRXRD) and Reciprocal Space Mapping (RSM) analysis of InGaN/GaN-based MQW ridge-waveguide arrays for monolithically integrated optoelectronic devices. InGaN/GaN waveguides were produced by the MOVPE technique in the Selective Area Growth (SAG) regime with the width of 6 microns and the oxide mask widths varied between 2 and 14 microns. Synchrotron Radiation-based triple- axis HRXRD measurements with the angular resolution of 13 arcsec were carried out at CHESS with the x-ray beamsize of 10 microns. Strain, thickness, and Indium composition variation in the active region of the ridge waveguides have been measured for different configurations of the SAG mask. Gas-phase diffusion coefficients have been determined for In and Ga precursors. Strain-induced relaxation effects in the active regions have been studied using RSM analysis. [Preview Abstract] |
Tuesday, March 14, 2006 1:39PM - 1:51PM |
H46.00011: Magnetotransport investigation of spin-orbit interactions in high-mobility GaN/AlGaN heterostructures. Stefan Schmult, Michael Manfra, Alex Punnoose, Michael Sergent, Richard Molnar Spin-splitting in the conduction band of a two-dimensional electron gas (2DEG) in GaN/AlGaN single interface heterostructures with mobilities of $\sim $80.000cm$^{2}$/Vs is investigated utilizing low field magnetoconductivity measurements. The observation of a distinct anti-localization feature near zero magnetic field signals the presence of significant spin-orbit coupling. A spin-orbit coupling value in the Rashba formalism of 6x10$^{-13}$eV{\AA} is extracted. For a typical 2DEG density of $\sim $10$^{12}$cm$^{-2}$ this value of the coupling strength corresponds to a spin-splitting energy of 0.3meV at the Fermi edge. The observed value is unexpectedly high for a wide-bandgap semiconductor. This talk summarizes our studies of the carrier density, mobility, and temperature dependence of the magnetoresistance data. [Preview Abstract] |
Tuesday, March 14, 2006 1:51PM - 2:03PM |
H46.00012: Low-temperature peculiarities of AlGaN/GaN heterostructure transport. Andriy Kurakin, Svetlana Vitusevich, Serhiy Danylyuk, Norbert Klein, Alexander Belyaev At present, AlGaN/GaN heterostructures are attracting a lot of scientific interest in semiconductor research and are extensively studied. Despite this there are a number of open questions concerning the low temperature properties of such systems with two-dimensional electron gas (2DEG). We report here our recent results on transmission-line model (TLM) patterns and high electron mobility transistor (HEMT) measurements in the temperature range of 0.3-10.0 K. A set of samples with different Al fractions of the barrier layer, wide bandgap spacer and different substrates were tested. It is important that accurate information on 2DEG systems becomes available with the systematic analysis of current-voltage characteristics of TLM patterns. The latter enables channel resistance to be extracted from total device resistance. It was found that channel conductance in low electric field shows metal-like temperature dependence. The peculiarities observed in low-temperature characteristics of AlGaN/GaN heterostructures are analyzed. [Preview Abstract] |
Tuesday, March 14, 2006 2:03PM - 2:15PM |
H46.00013: Electron Mobility in InGaN and InAlN Alloys Leon Hsu, Wladek Walukiewicz III-Nitride alloys containing In offer the possibility of engineering materials with bandgaps as small as 0.7 eV. We have calculated electron mobilities in InGaN and InAlN alloys taking into account the standard scattering mechanisms of acoustic and optical phonons, Coulomb scattering from charged impurities, and alloy disorder scattering. Effects of the non-parabolicity of the conduction band have also been included. We compare our calculations with experimental results on composition and concentration dependencies of the electron mobility in as-grown and in high-energy particle irradiated materials. This work was partially supported by the US DOE under Contract No. DE-AC03-76SF00098. [Preview Abstract] |
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