Effects of small amounts of Al in GaN grown on sapphire (0001) by molecular beam epitaxy

A series of 0.7 mum thick GaN layers have been grown by solid-source rf-pla sma assisted molecular beam epitaxy on sapphire (0001) substrates with the addition of 0.10%-3.30% Al. The Al concentration was determined by secondar y ion-mass spectrometry and Auger-electron spectroscopy, while the layer qu ality was assessed by photoluminescence, Hall effect measurement, and high- resolution scanning electron microscopy. Microscopy revealed a surface roug hness varying with Al content. The smallest surface roughness was obtained at 0.10% and 3.30% Al. Low-temperature photoluminescence revealed dominatin g peaks attributed to the neutral donor-bound exciton. Its energy increased slightly with Al concentration, which established a correlation between th e Al concentration and the band gap. The surface morphology, the correspond ing optical and electrical properties, showed a clear improvement of the Ga N layer quality for the range of 0.10%-0.17% Al content. (C) 2001 American Institute of Physics.