Structural and optical properties of GaN nanocolumns grown on (0 0 0 1) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy

GaN nanocolumns were grown with AlN buffer layers on (0 0 0 1) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy. The AlN buffer layers underneath the nanocolumns were used to nucleate the nanocrystals. The thickness of the AlN buffer layer affected the column configuration (size, shape), the density and the optical properties of the nanocolumns; when the thickness increased from 1.8 to 8.2 nm, the average column diameter gradually decreased from 150 to 52 nm with a small kink, but the column density peaked at a thickness of 3.2 nm at 5×109 cm−2 and finally decreased to 2×108 cm−2. Based on TEM observations, it is suggested that GaN nanocolumns were not grown just on AlN grain but on the edge of AlN grain. Further, the growth behavior of a nanocolumn as a function of AlN buffer layer thickness is suggested. The room-temperature photoluminescence intensity of the nanocolumns was maximized at a buffer thickness of 4.6 nm, where the intensity was 4 times stronger than that of high-quality bulk GaN crystals grown by HVPE with a threading dislocation density of ∼8×106 cm−2.