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 $(\mathrm{Al},\mathrm{Ga})\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ 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 $\mathrm{Al}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ , 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 $(\mathrm{Al},\mathrm{Ga})\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ 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.