Reduction of the defect density in GaN films using ultra-thin AlN buffer layers on 6H-SiC

A low dislocation density of ∼ 10^7-8 cm^-2 in GaN thin films on 6H-SiC(0001) substrates grown by metalorganic chemical vapor deposition was achieved. By considering possible origins of dislocations in the GaN/AlN/SiC structure, two major dislocation reduction routes are proposed; ultra-thin AlN buffer layers and smooth AlN surfaces in an atomic scale. Experimentally, the effects of the surface roughness and structural perfection of the AlN buffer layer on GaN film quality were extensively investigated as a function of AlN film thickness. The reduced dislocation density was realized by using ultra-thin AlN buffer layers having a thickness of ∼ 1.5 nm, which is below the critical value for misfit dislocation generation. The smoother surface morphology and enhanced structural quality of ultra-thin AlN buffer layers were found to be the main parameters in reducing the defect density in the GaN film.