Growth defects in GaN films on sapphire: The probable origin of threading dislocations

Single crystal GaN films with a wurtzite structure were grown on the basal plane of sapphire. A high density of threading dislocations parallel to the c-axis crossed the film from the interface to the film surface. They were found to have a predominantly edge character with 1/3[11 (2) over bar 0] Burgers vector. In addition, dislocation half-loops, elongated along the c-axis of GaN, were also found on the prism planes. These dislocations had a mostly screw character with a [0001] Burgers vector. Substrate surface steps with a height of 1/6c(Al2O3) were found to be accommodated by localized elastic bending of GaN (0001)(GaN) planes in the vicinity of the film/substrate interface, Observations show that the region of the film, with a thickness of similar to 100 nm, adjacent to the interface is highly defective. This region is thought to correspond to the low-temperature GaN ''buffer'' layer which is initially grown on the sapphire substrate. Based on the experimental observations, a model for the formation of the majority threading dislocations in the film is proposed. The analysis of the results leads us to conclude that the film is under residual biaxial compression.