Energetics of AlN thin films and the implications for epitaxial growth on SiC

We present an ab initio study of the energetics of thin films of AlN on the Si-terminated SiC(0001) surface. We demonstrate the existence of a vacancy-stabilized NAl wetting layer that can be obtained in both the $\sqrt{3}\times \sqrt{3}$ and 2×2 reconstructions through an N-rich deposition of Al and N. We show that the latter reconstruction is compatible with a nonabrupt neutral interface which promotes the formation of thick overlayers. Our study of the competition between two-dimensional and three-dimensional growth reveals that only large islands ($r_c>~60$ Å) are stable with respect to the initial two-dimensional phase.