Low temperature sapphire nitridation: A clue to optimize GaN layers grown by molecular beam epitaxy

The sapphire nitridation temperature is investigated as a possible paramete r to improve the properties of GaN epilayers grown by molecular beam epitax y using a radio frequency plasma source. It is found out that lowering the nitridation temperature to values as low as 200 degrees C allows us to dras tically improve the GaN structural and optical properties. Careful examinat ion of the interface by transmission electron microscopy reveals that, in t his case, the interface between the nitridated sapphire and the AlN buffer consists of an ordered array of pure edge dislocations. In contrast, high n itridation temperatures result in a perturbed interface with the occurrence of cubic crystallites in the AlN buffer. These results, complemented by a thorough reflection high-energy electron diffraction analysis of the nitrid ation procedure and a secondary ion mass spectrometry investigation, are in terpreted in the framework of a model whereby a higher oxygen concentration is extracted from the substrate at high nitridation temperature, leading t o the formation of cubic grains with a smaller lattice parameter than the s urrounding matrix and to the concomitant occurrence of defects within the b uffer. (C) 1999 American Institute of Physics. [S0021-8979(99)05903- 4].