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Magnetopolaron Effect On Shallow Donors In Gan, A. Wysmolek, R. Stepniewski, M. Potemski, B. Chwalisz-Pietka, K. Pakula, J. M. Baranowski, David C. Look, S. S. Park, S. K. Lee

Physics Faculty Publications

Resonant interaction between longitudinal-optic (LO) phonons and electrons bound on shallow donors in GaN is studied using magnetoluminescence of neutral-donor bound excitons (D⁰X). The experiments were performed on high-quality freestanding GaN material and heteroepitaxial GaN layers grown on sapphire. In addition to the principal recombination channel of D⁰X, in which donors are left in their ground states, two-electron satellites (TES) involving different excited donor excitations, as well as replicas of the principal D⁰X transition due to LO phonons, were observed for the oxygen and silicon donors. In order to separate transitions involving ground and …

Morphology Of The Near-Terminator Martian Ionosphere: A Comparison Of Models And Data, Jane L. Fox, Katherine E. Yeager

Physics Faculty Publications

We have constructed low and high solar activity models of the Martian thermosphere/ionosphere for solar zenith angles from 60 to 90° in 5 degree increments. The solar fluxes that we have adopted are those from the Solar 2000 v2.22 models of Tobiska (2004), without enhancements of the soft X-ray fluxes. The background neutral density and temperature profiles are similar to those that we have recently presented (Fox, 2004). We compute the density profiles for 14 ions and nine neutral species. For all the models, we present altitude profiles of the photoionization rates, electron impact ionization rates, total ion production rates, …

Radiative Recombination And Ultralong Exciton Photoluminescence Lifetime In Gan Freestanding Film Via Two-Photon Excitation, Yongchun Zhong, Kam Sing Wong, Weili Zhang, David C. Look

Physics Faculty Publications

We have measured the photoluminescence (PL) lifetime of a freestanding GaN film using one-photon and two-photon excitations to demonstrate the dramatic difference in exciton recombination dynamics at the surface and in the bulk. An ultralong exciton PL lifetime of 17.2 ns at 295 K is observed from a GaN freestanding film using two-photon excitation, whereas less than 100 ps lifetime is observed for one-photon excitation, suggesting that nonradiative processes from surface defects account for the short PL lifetime measured. A monotonic increase in two-photon excited PL lifetime with increasing temperature and the linear dependence of the exciton lifetime with emission …

The Features Of Drosophila Core Promoters Revealed By Statistical Analysis, Naum I. Gershenzon, Edward N. Trifonov, Ilya P. Ioshikhes

Physics Faculty Publications

Background: Experimental investigation of transcription is still a very labor- and time-consuming process. Only a few transcription initiation scenarios have been studied in detail. The mechanism of interaction between basal machinery and promoter, in particular core promoter elements, is not known for the majority of identified promoters. In this study, we reveal various transcription initiation mechanisms by statistical analysis of 3393 nonredundant Drosophila promoters.

Results: Using Drosophila-specific position-weight matrices, we identified promoters containing TATA box, Initiator, Downstream Promoter Element (DPE), and Motif Ten Element (MTE), as well as core elements discovered in Human (TFIIB Recognition Element (BRE) and Downstream …

Direct Evidence Of Impurity Decoration Of Ga Vacancies In Gan From Positron Annihilation Spectroscopy, S. Hautakangas, I. Makkonen, V. Ranki, J. Puska, K. Saarinen, X. Xu, David C. Look

Physics Faculty Publications

Positron annihilation spectroscopy, supported by ab initio theory, has been applied to verify the decoration of Ga vacancies in GaN by oxygen and hydrogen. Our results indicate that the Doppler broadening measurement of electron momentum distribution is sensitive enough to distinguish even between N and O atoms neighboring the Ga vacancy. We identify isolated V_Ga in electron irradiated GaN and V_Ga-O_N complexes in highly O-doped high-purity GaN. Evidence of H decoration of Ga vacancies is obtained in epitaxial GaN grown by metalorganic chemical-vapor deposition.

Development Of A Fourier Transform Far Infrared (Ftfir) Spectrometer To Characterize Broadband Transmission Properties Of Common Materials In The Terahertz Region, William Paul Ford

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With sub-millimeter wave or terahertz devices becoming more readily available, there is interest in developing sensors in this region of the spectra. To support this interest, we have developed a Fourier Transform Far InfraRed (FTFIR) spectrometer to characterize broadband transmission and reflectance coefficients of materials. The spectrometer utilizes a broadband blackbody source, a Michelson interferometer, and silicon bolometer. The path difference in the Michelson is obtained using a linear stage and data acquisition and stage control were both implemented in a Labview programming environment. The details of the experimental setup and experimental results are presented in this thesis. The instrument …

Electronic Properties Of Silicon-Based Nanostructures, Gian Giacomo Guzman-Verri

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We have developed a new unifying tight-binding theory that can account for the electronic properties of recently proposed Si-based nanostructures, namely, Si graphene-like sheets and Si nanotubes. We considered the sp²s* and sp³ models up to first- and second-nearest neighbors, respectively. Our results show that the corresponding Si nanotubes follow the so-called Hamada's rule. Comparison to a recent ab initio calculation is made.

Models Of Disordered Media And Predictions Of Associated Hydraulic Conductivity, L. Aaron Blank Jr.

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In the late 20th century there was a spill of Technetium in eastern Washington State at the US Department of Energy Hanford site. Resulting contamination of water supplies would raise serious health issues for local residents. Therefore, the ability to predict how these contaminants move through the soil is of great interest. The main contribution to contaminant transport arises from being carried along by flowing water. An important control on the movement of the water through the medium is the hydraulic conductivity, K, which defines the ease of water flow for a given pressure difference (analogous to the electrical conductivity). …