Microscopic Investigation of Al_0.43Ga_0.57N on Sapphire

Abstract

The microscopic structure of Al 0.43 Ga 0.57 N films grown on four types of structures on sapphire substrates was investigated by transmission electron microscopy. Al 0.43 Ga 0.57 N 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 Al 0.43 Ga 0.57 N 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 Al 0.43 Ga 0.57 N 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 Al 0.43 Ga 0.57 N layer, while simultaneously improving the crystalline quality significantly, acting as a dislocation filter, especially for dislocations having screw components.