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

Abstract

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