The effect of Al in plasma-assisted MBE-grown GaN

We have grown GaN, with addition of a 0.10 to 0.33 % Al, on sapphire(0001) substrates by solid-source RF-plasma assisted MBE. The Al-concentration was determined by secondary ion-mass spectrometry and Auger-electron spectrosc opy, while the layer quality was assessed by photoluminescence and high-res olution scanning electron microscopy. Microscopy revealed a meandering patt ern and a surface roughness varying with Al-content. The smallest surface r oughness was obtained at 0.10 % Al. Photoluminescence revealed two main pea ks attributed to the neutral donor-bound exciton. Its energy increased slig htly with Al-concentration, which established a correlation between the Al- concentration and the band gap.