DISLOCATION GENERATION IN GAN HETEROEPITAXY

In this work, we study the microstructural evolution, with particular emphasis on threading dislocation (TD) generation, in the two-step met al-organic chemical vapor deposition (MOCVD) of GaN on sapphire. The M OCVD growths were carried out at atmospheric pressure in a horizontal two-flow reactor. Nominally, 200 Angstrom thick nucleation layers (NL) were deposited at temperatures in the range 525-600 degrees C followe d by high temperature (HT) growth at 1060-1080 degrees C. Throughout t he different stages of growth, the microstructure was studied by trans mission electron microscopy (TEM) and atomic force microscopy (AFM). T wo growth conditions were closely studied: brief pre-growth ammonia ex posure of the sapphire ('Material A') and extensive pre-growth ammonia exposure of the sapphire ('Material B'). The as-grown Material B NL h as a similar to 25 Angstrom hexagonal GaN wetting layer followed by pr edominantly (1 1 1) oriented cubic GaN. After HT exposure, Material B NL predominantly transforms to hexagonal GaN and has TDs. These TDs pr opagate into the HT GaN and lead to a TD density of 2 x 10(-10) after 1 mu m of HT growth. Material A NLs, before and after HT exposure, hav e rough morphologies and a high-degree-of-stacklng disorder (predomina ntly (1 I 1) oriented cubic GaN). On Material A NLs, The HT GaN grows by a coarse island mechanism in which the GaN laterally overgrows the NL without generating TDs. Stacking disorder and misorientation betwee n the HT hexagonal GaN and the NL islands is accommodated either by Sh ockley or Frank partial dislocations or local strain. The majority of TDs are subsequently generated at the coalescence of the HT islands. ( C) 1998 Elsevier Science B,V. All rights reserved.