A theoretical comparative study of the surfactant effect of Sb and Bi on GaN growth

The role of Sb and Bi as surfactants in GaN growth is investigated using first-principles, periodic, self-consistent, density functional theory calculations. It is shown that N diffuses much slower than Ga on the Ga-rich GaN(0 0 0 1) surface. Surfactants such as Sb and Bi are considerably more mobile on this surface and they react with N to produce SbN and BiN intermediates. The diffusion of these intermediates on the Ga-rich GaN(0 0 0 1) surface is more facile than that of atomic N. Therefore, this intermediate-mediated transport mechanism would increase the effective diffusion length for N. As a result, Sb and Bi would improve step edge incorporation of N, leading to a reduction in the average surface roughness of the GaN samples. While the barrier for BiN diffusion on GaN(0 0 0 1) is only slightly lower than that of N, the calculated difference in the diffusion barriers of SbN and N on that surface is significant and this would cause the preferential sidewall facets to change from (11¯01) and (112¯2) to the vertical (112¯0) facets during lateral epitaxial overgrowth (LEO). Additional calculations show that Sb and Bi can act as surfactants on the GaN(112¯0) surface too. However, the adsorption of all the species on GaN(112¯0) is significantly weaker and the diffusion barriers of SbN and BiN are considerably higher compared to the GaN(0 0 0 1) surface. Consequently, the surfactant effect of Sb and Bi on the GaN(112¯0) surface should be less pronounced compared to that on the GaN(0 0 0 1) surface.