Substrate pre-treatment and initial growth: Strategies towards high-quality III-nitride growth on sapphire by molecular beam epitaxy

Sapphire substrates annealed at 1200 °C in N2 : O2 (3 : 1) developed terrace-and-step morphology, ideal for III-nitride growth by molecular beam epitaxy. In situ treatment of sapphire substrates, using Ga deposition and desorption prior to growth, is shown to be negative for GaN growth. Nitridation transforms the sapphire substrate surface to a lateral structure similar to AlN (mismatch < 2.5%). The surface lattice parameters after nitridation did not depend on the substrate temperature. AlN nucleation layer growth conditions have been optimized for growth of GaN. Ideal Al / N flux ratio was found to be 0.6 for a 9 nm thick nucleation layer.