Al coverage of AlN(0001) surface and Al vapor pressure – Thermodynamic assessment based on ab initio calculations.

[Display omitted] Ab initio calculations were used to determine physical properties of AlN(0001) surface under Al coverage. It was shown that Al atoms are adsorbed in T4 sites for very low Al coverage, up to θ Al = 1 / 4 monolayers (ML). For higher Al coverage, θ Al = 1 / 4 M L up to θ Al = 1 M L the adlayer becomes disordered and corrugated vertically. In this coverage range ( 0 < θ Al ≤ 1 M L) the Al bonding energy is independent on the coverage and equal to E ads DFT ≅ 5.0 e V. For higher coverage, 1 M L ≤ θ Al ≤ 7 / 6 M L , the Al adlayer becomes ordered and atomically flat. The Al adsorption energy in this range is E ads DFT ≈ 6.0 e V , i.e. much higher. For higher coverage, θ Al > 1.25 M L , the Al adatoms are located in the second layer. The adsorption energy is reduced to E ads DFT = 3.97 e V at θ Al = 1.25 M L and linearly increasing to E ads DFT = 4.88 e V for the coverage increase up to θ Al = 2 M L. Full thermodynamic analysis identified two regions in equilibrium with Al vapor: first at very low pressures where θ Al ≤ 10 - 3 M L and the second for higher pressures which is 1 M L ≤ θ Al ≤ 7 / 6 M L. The second region is relevant for physical vapor transport (PVT) AlN growth. The nitrogen adsorption process, important for PVT AlN growth, takes place at single or double Al layer covered AlN(0001) surface. [ABSTRACT FROM AUTHOR]