Dislocation mediated surface morphology of GaN

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)].

Effects of hydrogen during molecular beam epitaxy of GaN

“…The depth of depressions varies from 2 to 6 nm. Studies of MOCVD-grown GaN films have found similar surface depressions and attribute them to the dislocations intersecting the surface [1,9]. For the sample here, it is likely that those surface depression valleys are filled with metallic Ga before annealing, as observed by Hsu et al [14].…”

“…Subsequent annealing of this step-bunched surface produces small surface depressions, similar to those commonly reported for the MOCVD GaN and associated with dislocations intersecting the surface [1,9].…”

“…metal organic chemical vapor deposition (MOCVD) [1] and reactive molecular beam epitaxy (RMBE) using ammonia [2]) and those without intentionally introduced hydrogen (e.g. plasma-assisted MBE, PAMBE [3]).…”

Threading dislocations in epitaxial ferroelectric PbZr0.2Ti0.8O3films and their effect on polarization backswitching

“…Although the possible presence of TDs in (multi-)functional ternary-oxide thin films and heterostructures has been reported [6][7][8][9][10], details on their origin, role, and particularly avoidance, are not sufficiently understood.…”

Ga Adlayer Coverage and Surface Morphology Evolution during GaN Growth by Plasma-assisted Molecular Beam Epitaxy

“…When the flux further increases to a coverage of 2.44ML [ Fig. 2(e)], corresponding to the Ga bilayer structure, due to the existence of mixed-type dislocations, the stepped terraces show typical MBE-growth of spiral hillocks [14]. Finally, when the coverage exceeds the bilayer coverage, since small nano-islands consisted of Ga clusters are formed, Ga-droplet conditions are reached [ Fig.…”