Ab initio and thermodynamic picture of Al adsorption of AlN(0001) surface – Role of bond creation and electron transition contributions

Abstract The role of bond creation and electron transition contribution were investigated in thermodynamic picture of adsorption obtained from intensive ab initio density functional theory (DFT) calculations. It was shown that electron transitions may reduce the overall energy gain from Al adsorption at AlN(000 1 ) surface by 50%. Accordingly properties of AlN polar N-terminated AlN(000 1 ) surface such as crystallographic structure, the type and energy of the surface states, the band profiles close to the surface were obtained. At the clean surface Al is adsorbed with the energy gain of 9.6 eV at H3 site creating bonds using two Al3p states by the overlap with the neighboring surface N atoms. The excess electrons are transferred to surface N broken bond states. At the critical coverage θ Al - c r = 1 4 M L all these states are occupied, therefore for higher Al coverage the electrons are transferred to Al3pz states. As their energy is higher by 2.5 eV, the adsorption energy gain is dropped to 4.089 eV, i.e. about 50%. For higher coverage Al adatoms are shifted to bridge and on-top configurations reducing the electron transfer and increasing energy gain by 2.5 eV for full coverage of Al adatoms located in the on-top positions. The temperature dependent pressure-coverage diagram was obtained showing the two stable regions. The first, up to 0.25 ML Al ( θ Al ≤ 1 4 M L ) with Al adatoms located in H3 positions, correspond to extremely low Al pressure, below 10−13 bar at 2500 K. The second region, at p Al > 2.18 × 10 - 5 b a r , is stable for high coverage, i.e. for θ Al > 0.999 M L (at 2500 K) with Al adatoms located in the on-top positions. For the pressures between, i.e. 1.80 × 10 - 12 b a r p Al 2.18 × 10 - 5 b a r , nonuniform coverage is stable, composed to these two phases. Finally at the coverage close to 1 ML, Langmuir entropic singularity in chemical potential was detected, corresponding to dramatic increase of the pressure over single Al layer which leads to transition to multiple Al layers and bulk Al.