Adatom Interactions on GaN(0001) Surface I: Coverage-Dependent Adsorption

The adsorption of an adatom on the surface of a crystal is affected by the surface coverage due to adatom–adatom and adatom–substrate interactions. We study the dependence of adsorption energy of Ga and N adatoms on wurtzite GaN(0001) as a function of decreasing surface coverage from 0.25 to 0.04 monolayers through first-principles calculations. The adsorption energies of Ga and N adatoms on the flat, clean GaN(0001) substrate do not converge with decreasing coverage. Further, it appears that the Ga and N adatoms significantly distort the substrate lattice from the flat configuration. We found that these distortions increase with increase in system size (or, equivalently, with reducing coverage). This observation is counterintuitive since it is expected that lattice distortions should decrease with decreasing surface coverage. We separate the different contributions to the adsorption energy and identify the part of adsorption energy that arises due to lattice distortions. This contribution appears to be m...