Structural stability of scandium on nonpolar GaN (image) and (image) surfaces: A first-principles study

First-principles calculations based on density-functional theory have been implemented to study the scandium (Sc) adsorption and incorporation on nonpolar GaN (image) and (image) surfaces. It is found that Sc adatom prefers to reside at bridge positions, between the hollow and top sites, on both GaN nonpolar surfaces. In addition, calculating the relative surface energy of several Sc configurations, we constructed a phase diagram showing the energetically most stable surfaces as a function of the 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 ScN interlayers on nonpolar GaN (image) and (image) surfaces, which reduces the dislocation densities between GaN and ScN.