Surface nitridation induced AlN nano-columnar growth on c-sapphire

We probe the parametric dependence of the nitridation mechanism of Al2O3 surface by radio frequency nitrogen plasma in a molecular beam epitaxy system. Our quantitative analysis by XPS and RHEED shows that the chemical composition varies with exposure time and is independent of nitrogen plasma fluence and substrate temperature. Here we show that nitrogen incorporation into the host material is diffusion limited process and involves the conversion of Al2O3 into AlN, which has a higher rate initially and then saturates at 6 h of nitrogen plasma exposure. We deposit a thin AlN layer on this saturated nitrided sapphire substrate, which was found to consist of nanorods with a bimodal diameter distribution and apex morphology. By RHEED, XRD and TEM studies we attribute that the faceted nanocolumns are formed at misfit-induced dislocations with the dislocations propagating axially in the nanorods, while the oval apexed features are diffusion mediated. We find that nanorods of both the morphologies are c-oriented, single crystalline and strain relaxed, but possess different in-plane orientation.