Ductile relaxation in cracked metal-organic chemical-vapor-deposition-grown AlGaN films on GaN

A study of the strain relaxation of cracked (Al,Ga)N∕GaN films grown by metal-organic chemical-vapor deposition on (0001) sapphire is presented. The strain measured by x-ray diffraction is shown to depend both on the film thickness and on the high-temperature exposure time (including both growth duration and annealing). These macroscopic observations are linked to the film microstructure by using transmission electron microscopy and cathodoluminescence observations. Relaxation may occur either by the introduction of long, straight, a+c misfit dislocations or by the introduction of additional cracks bordered by bowed a-type misfit dislocations. The nucleation mechanism and the efficiency in relaxing the misfit strain of these two features are discussed.