The diffusion of Ga and N adatoms on (00•¯1) and (00•1) surfaces was studied by using density-functional theory. The calculations revealed very different diffusivities of Ga and N adatoms on the equilibrium surfaces. That is, whereas Ga was very mobile at typical growth temperatures, the diffusion of N was slower by some orders of magnitude. It was noted that the desorption rate in excess N ambients could become smaller than the adsorption rate. Therefore, extensive regions could be formed in which the surface was covered mainly by N atoms. These adatoms were likely to influence the migration path and the diffusion barriers to Ga adatoms. The diffusion of Ga adatoms on the present, N-terminated, surfaces could thus be considered to be extreme cases of N coverage. The energetically favored binding sites were located at the face-centered cubic and hexagonal close-packed positions, and the transition site was the bridge position. For both surface orientations, the diffusion barrier to Ga adatoms was strongly affected. On (00•1), the migration barrier increased from 0.4 to 1.8eV while, on (00•¯1), it increased from 0.2 to 1.0eV. It was concluded that excess N at the surface, which could occur under N-rich conditions, significantly reduced the mobility of Ga adatoms. The reason for this was the formation of strong Ga-N bonds, which had to be broken during adatom migration.

T.Zywietz, J.Neugebauer, M.Scheffler: Applied Physics Letters, 1998, 73[4], 487-9