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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Gain comparison in polar and nonpolar/ semipolar gallium-nitride-based laser diodes

Experimental gain spectra at continuous-wave (CW) operation of laser diodes (LDs) fabricated from c-plane, nonpolar and semipolar GaN-based materials emitting in violet, blue and green spectral regions are presented. Gain spectra were obtained using the Hakki–Paoli method at high resolution. The ability of the setup to resolve the sharp Fabry–Perot longitudinal peaks of the lasers at all injection currents allowed us to accurately measure the current density to reach transparency on the devices characterized in this work, and determine both total losses and differential modal gain curves up to threshold. We present a comparison of transparency current density and modal gain for nonpolar and c-plane LDs in violet and blue regions. The main parameters for the analysis are the internal electric fields (for c-plane lasers), hole effective masses and valence band splittings. In a preliminary analysis, we investigated the gain spectrum of semipolar green LDs and observed a significantly lower total linewidth (homogeneous plus inhomogeneous) than reported elsewhere, which seems to indicate that inhomogeneous broadening is not the main issue governing the evolution toward high performance green LDs.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Connecting thermoelectric performance and topological-insulator behavior: Bi 2 Te 3 and Bi 2 Te 2 Se from first principles

Thermoelectric performance is of interest for numerous applications such as waste-heat recovery and solid-state energy conversion and will be seen to be closely connected to topological-insulator behavior. In this paper, we here report first-principles transport and defect calculations for Bi 2 Te 2 Se in relation to Bi 2 Te 3 . The two compounds are found to contain remarkably different electronic structures in spite of being isostructural and isoelectronic. We also discuss these results in terms of the topological-insulator characteristics of these compounds.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2014

High-performance broadband optical coatings on InGaN/GaN solar cells for multijunction device integration

We demonstrate InGaN/GaN multiple quantum well solar cells grown by metalorganic chemical vapor deposition on a bulk (0001) substrate with high-performance broadband optical coatings to improve light absorption. A front-side anti-reflective coating and a back-side dichroic mirror were designed to minimize front surface reflections across a broad spectral range and maximize rear surface reflections only in the spectral range absorbed by the InGaN, making the cells suitable for multijunction solar cell integration. Application of optical coatings increased the peak external quantum efficiency by 56% (relative) and conversion efficiency by 37.5% (relative) under 1 sun AM0 equivalent illumination.

14 SOLAR ENERGY↗
DOE OSTI2011

Thermal conductivity as a metric for the crystalline quality of SrTiO{sub 3} epitaxial layers

Measurements of thermal conductivity {Lambda} by time-domain thermoreflectance in the temperature range 100<T<300 K are used to characterize the crystalline quality of epitaxial layers of a prototypical oxide, SrTiO{sub 3}. Twenty samples from five institutions using two growth techniques, molecular beam epitaxy and pulsed laser deposition (PLD), were analyzed. Optimized growth conditions produce layers with {Lambda} comparable to bulk single crystals. Many PLD layers, particularly those that use ceramics as the target material, show surprisingly low {Lambda}. For homoepitaxial layers, the decrease in {Lambda} created by point defects correlates well with the expansion of the lattice parameter in the direction normal to the surface.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Optical performance of top-down fabricated InGaN/GaN nanorod light emitting diode arrays

Vertically aligned InGaN/GaN nanorod light emitting diode (LED) arrays were created from planar LED structures using a new top-down fabrication technique consisting of a plasma etch followed by an anisotropic wet etch. The wet etch results in straight, smooth, well-faceted nanorods with controllable diameters and removes the plasma etch damage. 94% of the nanorod LEDs are dislocation-free and a reduced quantum confined Stark effect is observed due to reduced piezoelectric fields. Despite these advantages, the IQE of the nanorod LEDs measured by photoluminescence is comparable to the planar LED, perhaps due to inefficient thermal transport and enhanced nonradiative surface recombination.

42 ENGINEERING↗