Adsorption of nitrogen at AlN(000-1) surface – Decisive role of structural and electronic factors

Adsorption of atomic and molecular nitrogen at AlN(000–1) surface was investigated by ab initio calculations and thermodynamic analysis. According to earlier works{Kempisty et al. Appl. Surf. Sci. 2020, 532, 147,719} in equilibrium with Al vapor, the AlN(000–1) surface is thermodynamically stable in two states: low Al coverage θ_Al ≤ 1/3 ML and high Al coverageθ_Al ≅ 1 ML. In these two cases nitrogen adsorption is completely different. At low Al-covered surface the nitrogen atom is strongly bounded to N surface atom, creates the N₂ admolecule that is finally detached leaving surface vacancy V_N(s). This reaction chain energy gain is positive, ΔE_DFT^det(N−N₂)=3.50eV. Therefore, the atomic nitrogen present in plasma assisted molecular beam epitaxy (PA-MBE) fluxes induces the surface decay. N₂ is adsorbed molecularly at the bare surface with the coverage independent energy gain about 1 eV. At the fully Al-covered surface atomic nitrogen is adsorbed in T4 sites with no barrier and large energy gain ΔE_DFT^ads−Al(N)=8.68eV. Molecular nitrogen dissociates with the energy gain, dependent on additional N coverage: ΔE_DFT^ads−Al(N₂)=7.65eV at low and ΔE_DFT^ads−Al(N₂)=2.77eV at high, respectively. This change is related to the reduction of electron transfer contribution, caused by Fermi level shift down due to electron transfer from Al to N surface states. The thermodynamic analysis shows incomplete N coverage above the adsorbed Al layer due to the above adsorption energy reduction effect. The resulting incomplete N coverage is responsible for creation of nitrogen vacancies during AlN physical vapor transport (PVT) growth and their coalescence into Al-rich inclusions.