Energetics of H and NH2 on GaN(10(1)over-bar-0) and implications for the origin of nanopipe defects

We present first-principles calculations of the formation energy for H-terminated GaN(<10(1)over bar 0>) surfaces. The calculations indicate that H adsorption on GaN(<10(1)over bar 0>) will proceed by saturation of pairs of Ga and N dangling bonds rather than through exclusive occupation of only one type of bonding site. At T=0, the surface energy of the fully H-terminated surface is found to be 0.02 eV/cell, compared to 1.95 eV/cell for the bare surface. We present results for the N-H and Ga-H stretching and bending eigenfrequencies. Dissociative adsorption of NH3 via the formation of N-H and Ga-NH2 bonds is exothermic and reduces the surface formation energy to a value which is less than 0.1 eV at T=0. The implications of these results for the origin of nanopipe defects in GaN are examined.