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

[摘要] 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 theta(Al) <= 1/3ML and high Al coverage theta(Al)congruent to 1ML. 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 N-2 admolecule that is finally detached leaving surface vacancyV(N)(s). This reaction chain energy gain is positive, Delta E-DFT(det) (N - N-2) = 3.50 eV. Therefore, the atomic nitrogen present in plasma assisted molecular beam epitaxy (PA-MBE) fluxes induces the surface decay. N-2 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 Delta E-DFT(ads-Al) (N) = 8.68 eV. Molecular nitrogen dissociates with the energy gain, dependent on additional N coverage: Delta E-DFT(ads-Al)(N-2) = 7.65 eV at low and Delta E-DFT(ads-Al)(N-2) = 2.77 eV 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.

[发布日期] 2021-11-01

[关键词] Adsorption;Aluminum nitride;Surface;Density functional theory