Strain relaxation in (Al,Ga)N/GaN heterostructures

Strain relaxation mechanisms in metal-organic vapour phase epitaxy grown (Al,Ga)N/GaN heterostructures are presented. Relaxation first occurs through a 2D–3D transition. For pure AlN, misfit a-type dislocations are introduced at the coalescence front of growth islands. For (Al,Ga)N (Al concentration≤70%), the second relaxation step is cracking. When cracked, relaxation of the films occurs by the introduction of long and straight a+c-type dislocations and small bowed a-type dislocation half-loops bordering the cracks. These two relaxing features lead for Al0.2Ga0.8N films above 2μm thick to full relaxation.