Influence of Al/N flux ratio during nucleation layer growth on the structural properties of AlN grown on sapphire by molecular beam epitaxy

AlN nucleation and buffer layers have been grown with different Al/N flux ratio by molecular beam epitaxy on sapphire. Thick AlN on top of a nucleation layer grown with Al/N flux ratio=1.0 exhibited deep hexagonal holes (diameter 100–200 nm, density ∼10⁹ cm^-2). Investigation of the nucleation layer surface revealed that the holes were formed already during the nucleation. The formation of holes in AlN buffer layers could be avoided by using a N-rich (Al/N=0.5) nucleation layer. It is demonstrated that the holes in AlN buffer layers can be effectively avoided choosing a N-rich nucleation layer growth and then switch to Al/N flux ratio =1.0 for buffer layer growth. The two-step AlN growth gave high quality AlN, with excellent crystalline quality and smooth surface.