Ga-metal inclusions in GaN grown on sapphire

We have shown that for GaN films grown directly on sapphire substrates using plasma-assisted molecular beam epitaxy, the application of a localised mechanical stress on the films can produce metallic droplets on the surface. By using X-rap photoelectron spectroscopy measurements we have demonstrated that the droplets consist of Ga, which can be removed by etching in HCl. This problem can be avoided using nitridation of the surface combined with a low-temperature buffer layer, or by using alternative substrates such as GaAs or Si. (C) 1999 Elsevier Science B.V. All rights reserved.