Theoretical adlayer surface morphology of wurtzite 2 × 2 reconstructions of the GaN(0001) surface

In the first-principles calculations presented here we employ a density functional formalism using a pseudopotential plane-wave basis set in order to obtain the minimum energy configurations of various GaN(0001) 2 × 2 surfaces involving N atoms. The calculated formation energies of the 2 × 2 ideal model are compared with a previously proposed laterally contracted Ga bilayer model. We show how the order and stability of the different reconstructions are influenced by the choice of the standard 2 × 2 ideal or contracted bilayer model. On the basis of these results, we have characterized the effect on the adlayer surface of N segregation on the top of the surface, and the stability dependence on the number of substitutions for the different models employed. Our results predict that not all the adlayer structures containing nitrogen are unstable relative to the commonly considered N (H3) adatom configuration.