Transmission electron microscopy was used to study AlN nucleation layers, grown by using metalorganic vapor phase epitaxy, in order to link the resistivity of unintentionally doped GaN films to the nucleation layer microstructure. It was observed that high-resistivity GaN films grew on large-grained nucleation layers which, in turn, grew on smooth sapphire surfaces. These high-resistivity films contained a higher density of edge dislocations than screw or mixed dislocations. Low-resistivity films were found to grow on fine-grained nucleation layers that grew on rougher sapphire surfaces. These low-resistivity films contained a density of edge dislocations which was close to the combined density of screw and mixed dislocations.

Correlation between Nucleation Layer Structure, Dislocation Density and Electrical Resistivity for GaN Films Grown on a-Plane Sapphire by Metalorganic Vapor Phase Epitaxy. M.E.Twigg, D.D.Koleske, A.E.Wickenden, R.L.Henry, S.C.Binari: Applied Physics Letters, 2001, 79[26], 4322-4