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

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=1/4ML up to θ_Al=1ML the adlayer becomes disordered and corrugated vertically. In this coverage range (0<θ_Al≤1ML) the Al bonding energy is independent on the coverage and equal to E_ads^DFT≅5.0eV. For higher coverage, 1ML≤θ_Al≤7/6ML, the Al adlayer becomes ordered and atomically flat. The Al adsorption energy in this range is E_ads^DFT≈6.0eV, i.e. much higher. For higher coverage, θ_Al>1.25ML, the Al adatoms are located in the second layer. The adsorption energy is reduced to E_ads^DFT=3.97eV at θ_Al=1.25ML and linearly increasing to E_ads^DFT=4.88eV for the coverage increase up to θ_Al=2ML. Full thermodynamic analysis identified two regions in equilibrium with Al vapor: first at very low pressures where θ_Al≤10^-3ML and the second for higher pressures which is 1ML≤θ_Al≤7/6ML. 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.