THE EFFECT OF THERMAL ANNEALING ON GAN NUCLEATION LAYERS DEPOSITED ON (0001) SAPPHIRE BY METALORGANIC CHEMICAL-VAPOR-DEPOSITION

It has been shown by optical and x-ray measurements that GaN nucleation layers deposited at 540-degrees-C on (0001)-oriented sapphire substrates have a measurable crystalline component, although the x-ray data and the lack of absorbance features near the direct band gap of GaN suggest that the crystallite size is very small. Upon annealing to higher temperatures, the crystallite size increases and the crystal perfection improves markedly, until at temperatures near those empirically determined to be optimum for growth of an epitaxial overlayer, it approaches that of good quality single-crystal material. Most of the recrystallization of the nucleation layer occurs during the ramp from its deposition temperature to the growth temperature of the GaN overlayer, and there appears to be no advantage to prolonged annealing at high temperatures prior to epitaxial growth. In fact, x-ray diffractometer results suggest that the nucleation layer deteriorates after 20 min at temperatures above 1015-degrees-C, under the conditions used in this study.