Effects of substrate and annealing on GaN films grown by plasma-assisted molecular beam epitaxy

GaN films were deposited by plasma-assisted molecular beam epitaxy on different substrates, i.e. Si (111), Sapphire (0001) and 4H-SiC (0001) with 4° of miscutting orientation. Due to the lattice mismatch between GaN films and substrates, the substrates profoundly affect the growth mechanism of GaN films on them during the epitaxy. The smooth and complete GaN films can be obtained on 4H-SiC substrates based on the analyses of reflection high energy electron diffraction, scanning electron microscopy and atomic force microscopy. Higher quality of GaN films on 4H-SiC was also confirmed by high resolution X-ray diffraction. Photoluminescence measurements show that the compress stress existed in GaN films grown on sapphire, but tensile stress was observed in the films grown on 4H-SiC and Si. Meanwhile, in-situ annealing treatment of post expitxy can remove Ga droplets on GaN films and decrease the surface roughness and defect density of GaN films; however, thermal decomposition on the surface of GaN films was observed in the characterizations of X-ray photoelectron spectroscopy.