Surface and crystal structure of nitridated sapphire substrates and their effect on polar InN layers

Comprehensive analysis of the surface and crystal properties has been performed at clean c-plane sapphire substrates, sapphire layers after nitridation, and subsequently grown InN layers deposited by metal-organic vapor phase epitaxy. The (1 × 1) surface of clean sapphire reconstructs into a (31 × 31)R ± 9° structure after annealing at 1050 °C, which was performed prior to the nitridation process. The formation of crystalline AlN was observed for nitridation above 800 °C. X-ray photoelectron spectroscopy performed on the nitridated layers shows that NAl chemical bonds dominate this structure, while the number of NO bonds is negligibly small. Amorphous AlNxOy layers form during nitridation below 800 °C, where NO bonds dominate. All layers formed by nitridation show defects associated with N bonds. The morphology of the nitridated layers affects the surface and crystal quality of the subsequently grown polar InN layers. N-polar InN layers with a smooth surface and single crystalline structure were grown on the AlN nitridated layers, while In-polar InN layers with a rough surface and a polycrystalline structure were grown on the amorphous nitridated layers.