Heteroepitaxy mechanisms of AlN on nitridated c- and a-plane sapphire substrates

Abstract

We investigate the metalorganic vapor phase epitaxy of c-oriented AlN on c- and a-plane sapphire substrates, focusing on the effect of sapphire nitridation on the AlN structure. Prior to AlN growth, the sapphire surface is subjected to nitridation via an in-situ NH3 treatment. We demonstrate that nitridation without H2 thermal etching treatment realizes high quality AlN on both c- and a-plane sapphires, indicating that a reaction between NH3 and oxygen on the sapphire surface is a critical factor in the material growth. It is proposed that nitridation initially creates nanometer-scale inversion domains in the AlN epilayer, but as growth proceeds, the N-polar domains are annihilated, leaving voids. Such growth behaviors can be regarded as spontaneous selective area growth with strain-adsorbing void formation, and lead to crack-free, ∼5 μm thick AlN layers, which produce x-ray line widths as narrow as 180 and 483 arc sec for the (0002) and (101¯2) reflections, respectively, on c-plane sapphire, and 237 and 433 arc sec for these reflections on a-plane sapphire.