Effect of Step Edges on Adsorption Behavior for GaN(0001) Surfaces during Metalorganic Vapor Phase Epitaxy: An Ab Initio Study

The adsorption behavior of adatoms on stepped GaN(0001) surfaces during metalorganic vapor phase epitaxy (MOVPE) is theoretically investigated on the basis of ab initio calculations. The calculations using vicinal surfaces consisting of single layer step edges along the [1 (1) over bar 00] direction reveal that the structure of step edges depends on the growth condition. The vicinal surface with H-terminated N atoms (N-ad-H+Ga-H) and that with both H-terminated N atoms and NH2 (N-ad-H+Ga-NH2) are found to be stabilized under the MOVPE growth condition. Furthermore, different adsorption sites and energies of Ga and N adatoms are obtained depending on the atomic configurations of step edges and terraces. The most stable adsorption site of the Ga adatom is located at the step edge irrespective of the reconstructions, but the adsorption energy for the surface with N-ad-H+Ga-NH2 (-3.54 eV) is much lower than that with N-ad-H+Ga-H (-2.68 eV). One of the striking results of the adsorption behavior of the Ga adatom is the presence or absence of the Ehrlich-Schwoebel barrier, depending on the structure of step edges. On the basis of the calculated adsorption energies and energy barriers, the adsorption behavior at the step edges on GaN(0001) surfaces depending on the growth condition of MOVPE is successfully explained.