Plasma assisted molecular beam epitaxy growth mechanism of AlGaN epilayers and strain relaxation on AlN templates

Abstract

Abstract Several series of high Al composition AlGaN epilayers were grown on AlN/sapphire templates by plasma assisted molecular beam epitaxy. Three growth regions, Ga droplets, intermediate Ga-rich, and N-rich, have been identified based on in situ and various ex situ characterizations, and AlGaN growth diagrams have been summarized. The Al composition is basically determined by Al supply when it is less than the effective N radical beam supply. X-ray diffraction reciprocal space maps have revealed residual strain status of AlGaN on AlN, demonstrating lattice relaxation as the layer thickness increases. The experimental critical thicknesses are observed increasing as Al composition increases, but they deviate from the theoretical values, which is attributed to the combined influence of nonequilibrium growth conditions, kinetics, phase separation and dislocation generation. Energy bandgaps were determined by optical absorption edges, suggesting a bowing parameter of 1.1 eV.