Roger E. Welser

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Electron radiation induced hydrocarbon contamination can be either a problem or a useful tool in electron beam analyses and lithographies. We have used atomic force microscopy to study electron beam written contamination structures. Contamination is shown not only to arise from the primary electron beam but also from the energy scattered outside of the(More)
Electro-optical/infrared (EO/IR) sensors and photovoltaic power sources are being developed for a variety of defense and commercial applications. One of the critical technologies that will enhance both EO/IR sensor and photovoltaic module performance is the development of high quality nanostructure-based antireflection coatings. In this paper, we review our(More)
-We report on investigations of the initial drift in DC current gain, or beta ( ), during the early stages of reliability testing of MOCVD-grown carbon doped InGaP/GaAs HBTs. The drift is compared for different HBT structures with varying burn-in percentages. Competing mechanisms are observed in which can either increase or decrease during the initial(More)
We have demonstrated the DC characteristics and CDMA performance of an NpN InGaP/InGaAsN/GaAs double heterojunction bipolar transistor (DHBT) and compared the results to conventional InGaP/GaAs HBTs. A 100 mV reduction in base-emitter turn-on voltage and 35 mV reduction in offset voltage is established relative to InGaP/GaAs HBTs using a low energy-gap(More)
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Oblique-angle deposition of indium tin oxide (ITO) is used to fabricate optical thin-film coatings with a porous, columnar nanostructure. Indium tin oxide is a material that is widely used in industrial applications because it is both optically transparent and electrically conductive. The ITO coatings are fabricated, using electron-beam evaporation, with a(More)
We demonstrate that GaAs-based HBTs with very low base currents at both low and high injection levels can be achieved using either Al0.35Ga0.65As or InGaP in the emitter with the proper optimization of structure and growth. We observe an order of magnitude reduction in space charge recombination current as the Al composition, and hence the energy-gap, of(More)
The performance of a long-wave infrared type-II InAs/GaSb superlattice photodetector with a 50% cutoff wavelength of approximately 8.7 μm is presented. The ability to lower dark current densities over traditional P-type-Intrinsic-N-type diodes is offered by way of hetero-structure engineering of a pBiBn structure utilizing superlattice p-type (p) and n-type(More)
We report time resolved measurements of spin and carrier relaxation in InAs films with carrier densities of 1.3 1016 and 1.6 1016 cm−3 grown on 001 and 111 GaAs, respectively. We used standard pump-probe and magneto-optical Kerr effect spectroscopy at different excitation wavelengths, power densities, and temperatures. We observed sensitivity of carrier and(More)
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