GaN epitaxial growth on sapphire (0 0 0 1): the role of the substrate nitridation

Abstract

The nitridation of sapphire substrates was monitored in situ by reflection high-energy electron diffraction. The evolution of the lattice-mismatch evidences the formation of an AlN relaxed layer when exposing the sapphire surface heated at 850°C to an ammonia flow. GaN thin films were grown by gas-source molecular beam epitaxy on variously nitridated sapphire substrates. High-resolution transmission electron microscopy study reveals the existence of two different epitaxial relationships between GaN and sapphire (0 0 0 1). The well-known orientation with the c-axis of the GaN crystal perpendicular to the Al 2O 3 surface is observed when starting the growth on a nitridated substrate. On the other hand, growing GaN directly on bare Al 2O 3 surfaces results in a different crystallographic orientation where the c-axis is tiled by about 19° with respect to the sapphire basal plane. Photoluminescence measurements show that both the intensity of the yellow-band emission (∼ 2.2 eV) and the residual donor-acceptor pair recombinations are affected by the nitridation state of the starting surface. The dependence of the optical properties of GaN thin films versus the NH 3 exposure time is then used to optimize the nitridation step.