Experimental and theoretical investigation on the structural properties of GaN grown on sapphire

We have investigated the growth mechanisms of group-III nitrides on c-plane sapphire substrates with experiments by pulsed-laser deposition and first-principles calculations. It has been experimentally revealed that the in-plane alignment between the nitrides and sapphire is [10-10]nitride//[11-20]sapphire and the nitride films have the N polarity for the most cases. We have found that the insertion of an Al-rich AlN buffer layer effectively turns over the GaN crystals from the N polarity to the Ga polarity, although the Ga-rich GaN buffer layer does not cause change in the polarity. The theoretical energy calculations of a sapphire slab with an adatom explain the experimental results, such as the in-plane alignment and the polarity change, quite well.