Microscopic Investigation of Al0.43Ga0.57N on Sapphire

The microscopic structure of Al0.43Ga0.57N films grown on four types of structures on sapphire substrates was investigated by transmission electron microscopy. Al0.43Ga0.57N grown on a low-temperature-deposited AlN buffer layer is predominantly composed of small grains of about 50 to 250 nm in diameter. The crystalline quality of Al0.43Ga0.57N grown on a high-temperature grown GaN layer is much improved. However, it generates a crack network due to the difference of the lattice constant of Al0.43Ga0.57N and that of GaN. Critical thickness for crack generation is as thin as 50 nm or less. The low-temperature-deposited AlN interlayer prevents crack generation in the thick Al0.43Ga0.57N layer, while simultaneously improving the crystalline quality significantly, acting as a dislocation filter, especially for dislocations having screw components.