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 saturatio n of pairs of Ga and N dangling bonds rather than through exclusive oc cupation 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, compare d to 1.95 eV/cell for the bare surface. We present results for the N-H and Ga-H stretching and bending eigenfrequencies. Dissociative adsorp tion of NH3 via the formation of N-H and Ga-NH2 bonds is exothermic an d 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 nano pipe defects in GaN are examined.