The surfaces of films which had been grown by metalorganic chemical vapor deposition and molecular beam epitaxy were studied by using atomic force microscopy. Due to the high dislocation densities in the films (108/cm2), the typical surface morphologies of the layers which had been grown using either technique were dominated by 3 dislocation-mediated surface structures. These were pinned steps, spiral hillocks and surface depressions. The characteristics of these surface structures were found to depend upon the growth technique used and upon the group-III to group-V ratio which was used during molecular beam epitaxial growth. Pinned steps, which were created by the intersection of mixed-character dislocations with the free surface, were found on all of the films. The pinned steps were observed to be predominantly straight on the samples prepared using metalorganic chemical vapor deposition, and were curved into spiral hillock formations on molecular beam epitaxial samples. Spiral growth hillocks formed when pinned steps grew outwards, and around the dislocation, under step-flow conditions. The tightness of the spiral hillocks on molecular beam epitaxial samples increased with the III/V ratio. Surface depressions, which were caused by the high strain-energy densities near to dislocations, were also observed on the surfaces. Two characteristic depression sizes were found on all metalorganic chemical vapor deposited films, whereas depressions were observed only on molecular beam epitaxial films which were grown using low III/V ratios. The observations were explained in terms of the Burton-Cabrera-Frank theory.

Dislocation-mediated surface morphology of GaN B.Heying, E.J.Tarsa, C.R.Elsass, P.Fini, S.P.DenBaars, J.S.Speck: Journal of Applied Physics, 1999, 85[9], 6470-6