Nitridation mechanism of sapphire and its influence on the growth and properties of GaN overlayers

GaN epilayers were grown on sapphire and were nitridated for various time l engths at different temperatures (0-60min, 750-850 degreesC) using the N-2 helicon wave plasma assisted evaporation deposition process. The relationsh ip between the structure of the nitridated layers and the epitaxial growth of GaN was investigated performing a synchrotron X-ray analysis. It was fou nd that when the nitridated layer had the highest quality crystallinity, th e GaN overlayer grown on the nitridated layer showed greatly improved cryst alline and luminescent properties. Although the amount of compressive strai n in the optimally nitridated layer was about 1%, this layer showed low in- plane mosaicity and the largest domain size. However, excessive nitridation causes strain relaxation by reduction of the lateral domain size and the i ncrease of lateral mosaicity, resulting in the degradation of GaN overlayer s. Therefore, the adequately nitridated layer with compressive strain is a better provider of nuclei of subsequent nitride overlayers than the fully r elaxed layer. (C) 2001 Elsevier Science B.V. All rights reserved.