GaN nanocolumns on sapphire by ammonia-MBE: From self-organized to site-controlled growth

Catalyst-free GaN nanocolumns have been grown by ammonia-assisted molecular-beam epitaxy on (0 0 0 1) sapphire substrates. Two types of samples have been studied: self-organized nanocolumns grown directly onto the substrate and site-controlled nanocolumns grown through an array of apertures in a SiN mask. The nanocolumns were 100-300 nm in diameter, 200-1000 nm in height and c-oriented. The micro-photoluminescence spectra were dominated by the donor-bound exciton D0X peak and its energy position is consistent with that of a completely relaxed material. The nanocolumns show a residual n-type doping much more pronounced in the site-controlled compared to the self-organized growth case.