Buffer design for nitrogen polarity GaN on sapphire (0001) by RF-MBE and application to the nanostructure formation using KOH etching

GaN films were grown by radiofrequency plasma-assisted molecular beam epitaxy on a variety of sapphire (0001) substrates with different buffer layer and/or preparation processes, to show the controllability of lattice-polarity selection as well as their applicability to the nanostructure fabrication. Film polarities, which have been checked by reflection high-energy electron diffraction reconstructions and etched morphologies in KOH aqueous solution, were very sensitive to these preparation processes. Ga-polarity GaN could be obtained by an introduction of a high-temperature nitridation and an AlN interlayer deposition, and the N-polarity film with a flat surface and large grains could be grown on a low-temperature-nitrided sapphire in combination with a use of nitrogen-flux modulation sequence, while other pretreatment resulted in the mixed polarities. The resultant morphology of N-polarity GaN after KOH etching clearly reflects crystallographic orientation and the highly anisotropic-etching behavior show the usefulness of the low-temperature-nitrided sapphire for obtaining large-domain N-polarity GaN and the feasibility of nanostructure fabrication without the use of an expensive dry-etching process.