Dislocation mediated surface morphology of GaN

The surfaces of GaN films grown by metalorganic chemical vapor deposition ( MOCVD) and molecular beam epitaxy (MBE) were studied using atomic force mic roscopy (AFM). Due to the high dislocation densities in the films (10(8) cm (-2)), the typical surface morphologies of layers grown by both techniques were dominated by three dislocation mediated surface structures-pinned step s, spiral hillocks, and surface depressions. The characteristics of these s urface structures were found to depend on growth technique (MOCVD vs MBE) a nd the group-III to group-V ratio used in the growth of MBE GaN films. Pinn ed steps, created by the intersections of mixed character dislocations with the free surface, were found on all GaN films. The pinned steps were obser ved to be predominantly straight on the MOCVD GaN and curved into spiral hi llock formations on the MBE GaN. Spiral growth hillocks form when pinned st eps grow outward and around the dislocation under step-flow growth conditio ns. The tightness of the spiral hillocks on MBE GaN surfaces was found to i ncreases with III/V ratio. Surface depressions, caused by the high strain-e nergy 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 lo w III/V ratios. These observations are explained using theories developed b y Burton, Cabrera, and Frank [Philos. Trans. R. Soc. London, Ser. A 243, 29 9 (1951)] and Frank [Acta Crystallogr. 4, 497 (1951)]. (C) 1999 American In stitute of Physics. [S0021-8979(99)09409-8].