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 nucleatio n layers deposited at 540-degrees-C on (0001)-oriented sapphire substr ates have a measurable crystalline component, although the x-ray data and the lack of absorbance features near the direct band gap of GaN su ggest that the crystallite size is very small. Upon annealing to highe r temperatures, the crystallite size increases and the crystal perfect ion improves markedly, until at temperatures near those empirically de termined to be optimum for growth of an epitaxial overlayer, it approa ches that of good quality single-crystal material. Most of the recryst allization of the nucleation layer occurs during the ramp from its dep osition temperature to the growth temperature of the GaN overlayer, an d there appears to be no advantage to prolonged annealing at high temp eratures prior to epitaxial growth. In fact, x-ray diffractometer resu lts suggest that the nucleation layer deteriorates after 20 min at tem peratures above 1015-degrees-C, under the conditions used in this stud y.