An atomic force microscopy study of a temperature dependent morphology transition of GaN grown on sapphire by MOCVD

The dependence of the surface morphology of GaN epilayers grown by metal-organic chemical vapour deposition on growth temperature at two different pressures has been studied using atomic force microscopy. First, the theoretical morphology for GaN is derived from the crystal structure. Connected nets were found for the {0 0 0 2}, {1 (1) over bar 0 1} the {1 (1) over bar 0 0} faces. Experimental results show that on Ga terminated GaN films both the (0 0 0 1) and the {1 (1) over bar 0 1} faces are present for a considerable temperature range. are present for a considerable temperature range. At low deposition temperatures the {1 (1) over bar 0 1} faces dominate, at higher growth temperatures the morphology is determined by growth in the [0 0 0 1] direction. It is shown a growth rate parameter, alpha(GaN),proportional to the relative growth rates of the (0 0 0 1) and the {1 (1) over bar 0 1} faces, can be to describe the morphology of the films. The (0 0 0 1) surfaces show growth spirals emerging from screw dislocations. single or double spirals are composed of monoatomic steps. The double steps tend to split at the spiral centres, which be explained by entropic repulsion and a high surface diffusion. Most spiral centres are accompanied by hollow cores, size of which is larger than predicted using Frank's theory. A possible explanation for the large diameter of the hollow is given by the precipitation of vacancies along the dislocations. (C) 1999 Elsevier Science B.V. All rights reserved.