Effect of step edges on the adsorption behavior on vicinal AlN(0001) surface during metal-organic vapor phase epitaxy: An ab initio study

We present systematic theoretical investigations for the adsorption behavior of Al and N adatoms on vicinal AlN(0001) surface under metal–organic vapor phase epitaxy (MOVPE) growth condition by means of ab initio electronic structure calculations. Prior to the investigation adatoms, we reveal the growth condition dependence of stable structure for bilayer step edges during the MOVPE. The vicinal surface with hydrogen terminated N atom and NH2 (N-H + Al-NH2) is found to be the most stable under N-rich limit whereas the surface with hydrogen terminated N atom (N-H + Al-H) with NH2 at the step edge is favorable over the wide range of Al chemical potential. This suggests that N-H + Al-NH2 and N-H + Al-H with NH2 at the step edge appear at high and low temperatures, respectively. For both surface models, we also elucidate that the adsorption energy of Al adatoms at the step edge is much lower than that in the terrace region, indicating that Al adatoms are easily incorporated into the step edge. The estimated values of Ehrlich-Schwobel barrier (ESB) of Al adatoms for N-H + Al-NH2 and N-H + Al-H with NH2 at the step edge are −1.4 and −1.9 eV, respectively. The relationship between the adsorption behavior and surface morphology during the MOVPE is discussed on the basis of calculated ESB as well as adsorption energies and migration barriers.