Effects of the Al composition in AlGaN buffer layer on a-plane GaN films grown on r-plane sapphire substrate by MOCVD

In this work, we have investigated the effect of Al composition in AlGaN low-temperature buffer layer (BL) on the crystal quality of a-pane GaN thin films grown on r-plane sapphire substrates. GaN films grown using AlGaN BL with 15% Al exhibit smoothest surface morphology, lowest value of full width at half maximum of X-ray rocking curve and least impurity incorporation, as compared to those films grown using GaN and AlN BLs. AlN BLs result in the worst crystal quality of GaN films due to the large lattice match between GaN and AlN BL, while GaN BLs lead to intermediary quality of GaN films due to large lattice match between GaN BL and r-sapphire substrate. Adding 15% AlN in to GaN BLs can significantly reduce the lattice mismatch between BL and r-sapphire substrate, while still keep the lattice mismatch between BL and GaN films relatively small, and thus optimizes the crystal quality of a-GaN films grown on r-sapphire substrates.