TY - JOUR
T1 - Adsorption of nitrogen at AlN(000-1) surface – Decisive role of structural and electronic factors
AU - Strak, Pawel
AU - Sakowski, Konrad
AU - Piechota, Jacek
AU - Ahmad, Ashfaq
AU - Grzegory, Izabella
AU - Kangawa, Yoshihiro
AU - Krukowski, Stanislaw
N1 - Funding Information:
The research was partially supported by Poland National Science center [grant number DEC-2016/23/B/ST5/02278 and DEC- 2017/27/B/ST3/01899] and partially by Japan JST CREST [grant number JPMJCR16N2] and by JSPS KAKENHI [grant number JP16H06418]. This research was carried out with the support of the Interdisciplinary center for Mathematical and Computational Modelling at the University of Warsaw (ICM UW) under grants no GB77–29, GB84–23 and GB76–25.
Publisher Copyright:
© 2021 Elsevier B.V.
PY - 2021/11
Y1 - 2021/11
N2 - 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.
AB - 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.
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U2 - 10.1016/j.susc.2021.121891
DO - 10.1016/j.susc.2021.121891
M3 - Article
AN - SCOPUS:85112482612
VL - 713
JO - Surface Science
JF - Surface Science
SN - 0039-6028
M1 - 121891
ER -