Investigation and Engineering of the Homogeneity and Current Injection of Molecular Beam Epitaxy Grown III-Nitride Nanowire Ultraviolet Light Emitting Diodes
Abstract
Self-assembled nanowires are attractive because of their innate ability to effectively strain relax without the creation of extended defects. This allows for interesting heteroepitaxial growths and extreme heterostructures. III-Nitride nanowires are of particular interest because of the wide range of direct bandgaps available in the material system, spanning form the infrared to the deep ultraviolet finding uses in sensors, photovoltaics, lasers and LEDs. The work presented here will be focused on nanowire LEDs with emission in the ultraviolet grown by molecular beam epitaxy. The first part of this work will discuss the possible inhomogeneities present in self-assembled nanowires and how these manifest themselves in ensemble devices. The effect of nonuniformities (specifically shorts) on the current spreading in devices where many individual diodes are wired in parallel is then addressed, and the use of a short-term-overload bias is shown as a way to reduce the presence of nonuniformities, increasing the efficiency of ensemble devices. Next, alternative substrates are investigated, with the growth of high-quality GaN nanowires being demonstrated on polycrystalline foils, the fabrication of the first UV LED grown directly on metal foil follows. The final portion of this work begins by addressing the grain-dependent uniformity issues present with growth on bulk polycrystalline foils through the use of thin nanocrystalline metal films and amorphous metals. Finally, a different nanowire LED structure is discussed in which the upper portion of the nanowires is coalesced to form a “thin-film” transparent conductive layer, enabling the substitution of the traditional fully conformal thin metal top contact with only a current spreading grid.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 2019
- Bibcode:
- 2019PhDT........62M
- Keywords:
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- Materials science;Electrical engineering