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 o n (0001) sapphire have been investigated by examining different stages of h igh-temperature (HT) GaN growth on low-temperature GaN nucleation layers (N Ls) by transmission electron microscopy. Results indicate that after 20 s o f HT growth, GaN islands were free of TDs. After 75 and 120 s of growth, mo st 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 f rom faulted regions located within NLs. TDs move toward the island top surf ace (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 fro m climb and glide of basal plane (BP) dislocations that form by the dissoci ation of Shockley partials located within the faulted regions. BP dislocati ons 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 TD s form primarily by the coalescence of Frank partials near the GaN/sapphire interface. (C) 2001 American Institute of Physics.