Selective area formation of GaN nanowires on GaN substrates by the use of amorphous AlxOy nucleation layer

Examples are presented that application of amorphous AlxOy nucleation layer is an efficient way of controlling spatial distribution of GaN nanowires grown by plasma-assisted molecular beam epitaxy. On GaN/sapphire substrates AlxOy stripes induce formation of GaN nanowires while a compact GaN layer is formed outside the stripes. We show that the ratio of nanowire length h to the thickness of the compact layer d can be tailored by adjusting impinging gallium and nitrogen fluxes. Calculations of the h/d aspect ratio were performed taking into account dependence of nanowire incubation time on the growth parameters. In agreement with calculations we found that the value of h/d ratio can be increased by increasing the N/Ga flux ratio in the way that the N-limited growth regime determines nanowire axial growth rate while growth of compact layer remains Ga-limited. This ensures the highest value of the h/d aspect ratio. Local modification of GaN growth kinetics caused by surface diffusion of Ga adatoms through the boundary separating the AlxOy stripe and the GaN/sapphire substrate is discussed. We show that during the nanowire incubation period gallium is transported out of the AlxOy stripe, which delays nanowire nucleation onset and leads to reduced length of GaN nanowires in the vicinity of the stripe edge. Simultaneously the growth on the GaN/sapphire substrate is locally enhanced, so the planar GaN layers adopts a typical edge shape of mesa structures grown by selective area growth. Ga diffusion length on a-AlxOy surface of ∼500 nm is inferred from our results.