Relaxation mechanisms in metal-organic vapor phase epitaxy grown Al-rich $(Al,Ga)N∕GaN$ heterostructures

The relaxation mechanisms in metal-organic vapor phase epitaxy grown $(Al,Ga)N∕GaN$ heterostructures are studied. The first stage of the relaxation process is a two-dimensional–three-dimensional growth transition with the formation of mesalike islands separated by V-shaped trenches. The tensile stress relief is obtained by an elastic relaxation of the islands edges. In the case of $AlN∕GaN$⁠, the apexes of the V trenches reach the heterointerface and misfit dislocations are nucleated at the islands coalescence region. These dislocations are $a$ type and glide in the basal plane to promote further relaxation. For $(Al,Ga)N∕GaN$ with an Al concentration below 70%, the apexes of the V trenches do not reach the heterointerface, prohibiting the nucleation of misfit dislocations. For thicker layers, the next stage of the relaxation is the cracking of the films.