GaN and InGaN Nanowires on Si Substrates by Ga-Droplet Molecular Beam Epitaxy
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1080-O08-04
GaN and InGaN Nanowires on Si Substrates by Ga-Droplet Molecular Beam Epitaxy Kevin Goodman, Kejia Wang, Xiangning Luo, John Simon, Tom Kosel, and Debdeep Jena Electrical Engineering, University of Notre Dame, 275 Fitzpatrick Hall, Notre Dame, IN, 46556 ABSTRACT Molecular beam epitaxy growth of GaN and InGaN nanowires is accomplished on Si (111) substrates using Ga-droplet nucleation. Typical diameters range from 25-80 nm and lengths can be varied by increasing the growth time; the growth rate is ~0.25 microns/hour. The nanowires have been characterized structurally and optically. Photoluminescence spectra show band-edge emission of GaN nanowires centered at 362 nm at 290 K. Transmission electron microscopy images unveil that the nanowires are highly crystalline, and grow along the 0001 polar direction. Indium has also been successfully incorporated into GaN nanowires by modifying the growth conditions; the InGaN nanowires emit at ~520 nm, which provides a possible route to solving strain related problems of high In-composition InGaN based efficient green emitters. INTRODUCTION Today’s world is seeing an ever increasing cost of energy with no downturn in sight for the immediate future [1]. Therefore it is only logical to look for every possible method to conserve energy. One means to this end is to replace conventional lighting such as incandescent or fluorescent lamps with light emitting diodes which offer higher luminescent efficiencies and longer lifetimes. Light emitting diodes (LEDs) producing white light are possible by combining blue, red, and green wavelength light sources [2]. The III-V Nitride semiconductor family (InN, GaN, and AlN) spans this entire wavelength range. Therefore, by altering the compositions of ternary InGaN materials, all three wavelengths needed to construct white light sources can be produced from the single material family. Currently, the green region of the spectrum (specifically the 520 – 580 nm window) is not adequately covered by nitride LEDs, due to the low crystalline quality of high-Indium composition InGaN layers. A problem that surfaces in growing c-plane high In-composition InGaN 2D layers on GaN substrates is the considerable lattice mismatch between InN (c-lattice constant 0.353 nm), and the substrate (GaN, 0.319 nm). This difference results in high strain in the InGaN, resulting in the formation of dislocations and other defects in thick layers. These defects act to introduce recombination centers and electronic traps which lower the optical emission efficiency of light emitting devices. Recent research has shown [3] that one method to circumvent these problems is to grow non-2D structures, specifically pillars or 1D-nanowires of InGaN. Since the nanowires in principle are free to expand or contract to their desired inherent lattice constants in the growth planes, considerable (and perhaps complete) strain relaxation can be achieved in this fashion. This offers crystalline materials free of defects, which has the potential to increase the optical emission effi
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