Growth diagram of AlN epilayers grown by plasma-assisted molecular beam epitaxy

We have investigated homoepitaxy of AlN films grown by molecular beam epitaxy (MBE) on AlN/sapphire templates. The MBE epitaxy of AlN at the low temperature range, which is suitable for AlGaN, encounters significant challenge in preventing Al droplet and pits, since the migration and desorption rate of Al atom are very low. In contrast, by elevating the growth temperature, such a difficulty can be effectively overcome, and we were able to grow AlN films with much improved surface morphology and obtained step flow growth mode without any Al droplets and pits. The cathodoluminescence spectroscopy indicate that the impurity incorporation and defect generation in the AlN epilayers was suppressed by elevating the growth temperature. A systematic investigation on the influence of Al beam flux and growth temperature in a very wide range on the AlN films has been conducted, and a comprehensive growth diagram of MBE AlN has been obtained.