Surface effects in GaN growth

By means of plane wave pseudopotential periodic supercell density functional theory calculations, we investigated the energetics of several GaN(0 0 0 1) surfaces containing atomic and molecular adsorbates. The surface structures were selected in order to get insight into GaN epitaxial growth, in situations where NH3 molecules are employed as the nitrogen precursor and H2 molecules as the carrier gas, typical of standard vapor phase growth techniques. Therefore, we considered NH2 complexes and H atoms as molecular and atomic adsorbates, respectively, assuming they derive from molecular dissociation from the gas flows in the growth chambers. We took into account different adsorption stoichiometries, varying the relative proportion of NH2 and H adsorbates, and different degrees of clusterization of the ad-atoms and ad-complexes. For each structure, we obtained the relaxed atomic configuration and the corresponding total energy. Hence, we describe here the relative structural stability in terms of a phase diagram that identifies the optimal growth conditions to attain at intermediate stages the most favorable equilibrium surfaces.