Highly aligned, template-free growth and characterization of vertical GaN nanowires on sapphire by metal-organic chemical vapour deposition

We report the growth of exceptionally well aligned and vertically oriented GaN nanowires on (1 (1) over bar 02) r-plane sapphire wafers via metal-organic chemical vapour deposition. The nanowires were grown without the use of either a template or patterning. Transmission electron microscopy indicates the nanowires are single crystalline, free of threading dislocations, and have triangular cross-sections. The high degree of vertical alignment is explained by the crystallographic match between the [11 (2) over bar0] oriented nanowires and the r-plane sapphire surface. We find that the degree of alignment and size uniformity of the nanowires are highly dependent on the nickel nitrate catalyst concentration used, with the highest degree of uniformity and alignment occurring at concentrations much more dilute than typically employed for vapour-liquid-solid-based nanowire growth. Additionally, we report here a strong dependence of the optical and electrical properties of the nanowires on the growth temperature, which we hypothesize is due to increased carbon incorporation at lower growth temperatures.