Interface formation of scandium nitride on the GaN(0 0 0 1) surface: A first-principles study

Self-consistent periodic density functional theory calculations are used to investigate the role of scandium (Sc) impurity atoms during GaN growth. Adsorption and diffusion of Sc atoms on GaN(0 0 0 1)-2 × 2 surface is examined and it is shown that Sc atoms preferentially adsorb at the T4 sites at low and high coverages (from 1/4 up to 1 monolayer). It is also found that N atoms are relatively immobile compared to Ga as well as Sc atoms on the GaN(0 0 0 1) surface. In addition, calculating the relative surface energy of several configurations and various Sc concentrations, we constructed a phase diagram showing the energetically most stable surfaces as a function of the Sc and Ga chemical potentials. Based on these results, we have found that incorporation of Sc adatoms in the Ga-substitutional site is energetically more favorable compared with the adsorption on the top layers. This effect leads to the formation of an interfacial crystalline ScN layer on the GaN(0 0 0 1) surface, which can offer a good interfacial combination between Sc and GaN substrate. Our calculations show that the scandium incorporation is most favorable under a nitrogen environment, in agreement with experimental results.