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 metal-organic chemical vapor deposition (MOCVD) of GaN on sapphire. The MOCVD 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 followed by high temperature (HT) growth at 1060-1080 degrees C. Throughout the different stages of growth, the microstructure was studied by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Two growth conditions were closely studied: brief pre-growth ammonia exposure of the sapphire ('Material A') and extensive pre-growth ammonia exposure of the sapphire ('Material B'). The as-grown Material B NL has a similar to 25 Angstrom hexagonal GaN wetting layer followed by predominantly (1 1 1) oriented cubic GaN. After HT exposure, Material B NL predominantly transforms to hexagonal GaN and has TDs. These TDs propagate 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, have rough morphologies and a high-degree-of-stacklng disorder (predominantly (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 between the HT hexagonal GaN and the NL islands is accommodated either by Shockley 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.