Relaxations at GaN (1010) and (110) Surfaces

We present an ab-initio calculation of GaN wurtzite (1010) and zinc-blende (110) surface structures and formation energies. Our method employs ultrasoft pseudopotentials and plane-wave basis. These features enable us to obtain accurate results using small energy cut-off and large supercells. The (110) surface shows a Ga-N surface dimer rotation of ∼ 14°, i.e. about one half that of the ordinary III–V non-nitride compounds, and a 5% contraction of the surface bond-length (more than the double that occurring in GaAs). For the (1010) surface, a layer rotation angle of about 11° and a bond-length contraction of 6% has been found. Zinc-blende GaAs (110) and wurtzite ZnO (1010) surfaces have been studied as well, for the sake of comparison. GaAs results are in good agreement with the experimental findings. For ZnO a large bond contraction and a rotation angle of around 11% result. Thus, our findings place GaN closer in behaviour to the highly ionic II–VI compounds than to the non-nitride III–V semiconductors.