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

Pawel Strak, Konrad Sakowski, Jacek Piechota, Ashfaq Ahmad, Izabella Grzegory, Yoshihiro Kangawa, Stanislaw Krukowski

Research output: Contribution to journalArticlepeer-review

Abstract

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 θAl ≤ 1/3 ML and high Al coverageθAl ≅ 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 VN(s). This reaction chain energy gain is positive, ΔEDFTdet(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 ΔEDFTads−Al(N)=8.68eV. Molecular nitrogen dissociates with the energy gain, dependent on additional N coverage: ΔEDFTads−Al(N2)=7.65eV at low and ΔEDFTads−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.

Original languageEnglish
Article number121891
JournalSurface Science
Volume713
DOIs
Publication statusPublished - Nov 2021

All Science Journal Classification (ASJC) codes

  • Condensed Matter Physics
  • Surfaces and Interfaces
  • Surfaces, Coatings and Films
  • Materials Chemistry

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