Modification of dislocation behavior in GaN overgrown on engineered AlN film-on-bulk Si substrate

The changes that the AlN buffer and Si substrate undergo at each stage of our substrate engineering process, previously shown to lead to a simultaneous and substantial reduction in film crack density and dislocation density in overgrown GaN, are presented. Evidence of ion-implantation assisted grain reorientation for AlN islands coupled with physical isolation from the bulk Si substrate prove to be the dominating driving forces. This is further emphasized with x-ray diffraction analysis that demonstrates a reduction in the in-plane lattice constant of AlN from 3.148 Å to 3.113 Å and a relative change in rotation of AlN islands by 0.135° with regard to the Si substrate after substrate engineering. Misfit dislocations at the AlN-Si interface and disorder that is normally associated with formation of amorphous SiN_x at this interface are considered to be two of the major contributors to dislocation nucleation within overgrown GaN. Following our technique, the disappearance of disorder at the AlN-Si interface is observed. Extensive ellipsometry and transmission electron microscopy suggests that larger AlN islands with a smoother surface morphology could further reduce the dislocation density below that previously reported. A 1.2 μm GaN layer deposited on an AlN buffer with larger islands and smoother morphology exhibits a 14× reduction in surface pit density after undergoing the ion-implantation assisted substrate modification technique.