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

Automated summary

Ab initio calculations were used to investigate the physical properties of AlN(0001) surface under Al coverage, revealing the adsorption behavior and energy dependence on coverage for Al adatoms. The adlayer undergoes structural changes from disordered and corrugated to ordered and atomically flat with increasing Al coverage, affecting the Al bonding energy. Thermodynamic analysis identified equilibrium regions with Al vapor, crucial for physical vapor transport AlN growth.