Electronic structures of polar and nonpolar GaN surfaces

GaN has a large ionicity. The polarity of its surface may have a substantial influence on its electronic structure, in particular its work function. Our first-principles calculations for several polar and nonpolar surfaces suggest that the polar surfaces are semimetallic, while the nonpolar surfaces are semiconducting. The polar surfaces have an array of dipole layers that result in a substantial increase or decrease of the work function depending on whether the surface is terminated by the N or Ga layer. The nonpolar surfaces have tilted surface NGa bonds, which result in an outward dipole layer that increases the work function.