Origins of threading dislocations in GaN epitaxial layers grown on sapphire by metalorganic chemical vapor deposition

The origins of threading dislocations (TDs) in GaN epitaxial layers grown on (0001) sapphire have been investigated by examining different stages of high-temperature (HT) GaN growth on low-temperature GaN nucleation layers (NLs) by transmission electron microscopy. Results indicate that after 20 s of HT growth, GaN islands were free of TDs. After 75 and 120 s of growth, most of the islands contained pure screw (c type) and pure edge (a type) TDs with an interspersion of mixed (c+a type) TDs. Most of the TDs originated from faulted regions located within NLs. TDs move toward the island top surface (c type) or curve toward island side facets (a, c+a type). Coalescence of HT GaN islands did not give rise to either a, c, or c+a type TDs. After 240 s of growth, most TDs were predominantly of a type and could result from climb and glide of basal plane (BP) dislocations that form by the dissociation of Shockley partials located within the faulted regions. BP dislocations are also observed attached to the side facets of islands away from the faulted regions and their possible origins are discussed. c and c+a type TDs form primarily by the coalescence of Frank partials near the GaN/sapphire interface.