Novel architectured, dislocation-free, III-Nitride structures for the next generation optoelectronic devices
Abstract: III-Nitride (III-N) materials are promising building blocks of optoelectronic devices such as light emitting diode and laser diode due to the unique material properties. Furthermore, direct and tunable band gaps of III-N materials enable them to cover the entire visible-UV region of electromagnetic spectrum for device applications. Even though the possibility of the fabrication of many devices based on III-N materials have been already presented and some of them are also commercially available, these devices are still face formidable challenges mainly originated from the quality of the grown materials arising from lack of lattice-matched substrate and heteroepitaxy of difference layers to design devices. Generation of such structural defects especially dislocations during the growth degrade device efficiency, lagging III-N technologies far behind to be a mature one. 3D growth mode has been suggested as a solution for as mentioned challenges yet it poses another challenge which is inability to control the crystal facets defined the final structure. In this work, we have presented a novel growth technique based on 3D growth mode to address both blocking dislocations from underlying substrate and avoiding the generation of dislocations due to heteroepitaxy of high Al content AlGaN layer on GaN structure. More importantly, in this growth approach it is possible to control the crystal facets into the final structure. In the first step, NW geometry is used to block dislocations from substrate, then initial GaN nanowire is transformed into a GaN hexagonal flat-top prism by regrowth, in-situ annealing and controlled material redistribution in-between the different facets. In the first project, GaN prism has been used to fabricate a vertical cavity with a quality factor of over 500 by sandwiching the prism between a pair of distributed Bragg reflectors. For the second study, GaN prism has been used as micron-sized substrate to grow AlGaN layer with up to 90% Al. The characterization data indicate that GaN prism have a sufficiently small volume to accommodate the crystal strain built up by the AlGaN/GaN heterostructure elastically avoiding plastically relaxation by formation of dislocations between layers. We believe two studies will open up new possibilities for the design of highly efficient optoelectronic devices.
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