Epitaxy of an Al-Droplet-Free AlN Layer with Step-Flow Features by Molecular Beam Epitaxy

We investigate epitaxy of AlN layers on sapphire substrates by molecular beam epitaxy. It is found that an atomically flat surface can be obtained under Al-rich conditions at growth temperature of 780 °C. However, the growth window to obtain an Al−droplet-free surface is too narrow to be well-controlled. However, the growth window can be greatly broadened by increasing the growth temperature up to 950 °C, where an Al-droplet-free surface with a step-flow feature is obtained due to the enhanced re-evaporization rate and migration ability of Al adatoms. The samples grown at the higher temperature also show a higher crystalline quality than those grown at lower temperatures.