Dislocation mediated surface morphology of GaN

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Abstract
The surfaces of GaN films grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were studied using atomic force microscopy (AFM). Due to the high dislocation densities in the films (108 cm−2), the typical surface morphologies of layers grown by both techniques were dominated by three dislocation mediated surface structures—pinned steps, spiral hillocks, and surface depressions. The characteristics of these surface structures were found to depend on growth technique (MOCVD vs MBE) and the group-III to group-V ratio used in the growth of MBE GaN films. Pinned steps, created by the intersections of mixed character dislocations with the free surface, were found on all GaN films. The pinned steps were observed to be predominantly straight on the MOCVD GaN and curved into spiral hillock formations on the MBE GaN. Spiral growth hillocks form when pinned steps grow outward and around the dislocation under step-flow growth conditions. The tightness of the spiral hillocks on MBE GaN surfaces was found to increases with III/V ratio. Surface depressions, caused by the high strain-energy density near dislocations, were also observed on the surfaces of the GaN films. Two characteristic depression sizes were found on all MOCVD GaN films whereas depressions were observed only on MBE GaN films grown with low III/V ratios. These observations are explained using theories developed by Burton, Cabrera, and Frank [Philos. Trans. R. Soc. London, Ser. A 243, 299 (1951)] and Frank [Acta Crystallogr. 4, 497 (1951)].