Insights into the origin of threading dislocations in GaN∕Al2O3 from atomic force microscopy

Threading dislocations in GaN∕Al2O3 are widely assumed to arise at the coalescence boundaries of misoriented islands. We have used atomic force microscopy to study a large number of coalescence boundaries and found no significant difference between the threading dislocation density in the boundary regions and the overall threading dislocation density for the large islands observed on partially coalesced samples. A secondary population of smaller islands is observed with a significantly higher dislocation density, and we suggest that when these small islands are overgrown by large islands, these threading dislocations are incorporated into the large islands, resulting in dislocation clusters.