Ab initiocalculations of SiC(110) and GaAs(110) surfaces: A comparative study and the role of ionicity

We report ab initio calculations of structural and electronic properties of β-SiC and of the nonpolar SiC(110) surface. The calculations are carried out self-consistently in the local-density approximation employing smooth norm-conserving pseudopotentials in separable form. Gaussian orbital basis sets are used for an efficient description of the wave functions. These are strongly localized at the carbon atoms, in particular. For the bulk crystal 40 Gaussians per unit cell with s, p, d, and ${\mathit{s}}^{\mathrm{*}}$ symmetry are found to be sufficient for good convergence. Our results for the SiC ground-state structural parameters and the bulk band structure are in excellent agreement with the results of previous plane-wave calculations and with experimental data. We scrutinize the character of the chemical bond in SiC by comparisons with diamond, Si, GaAs, and ZnS, which have been investigated on equal footing. The SiC(110) surface is described in a supercell geometry. The optimal surface relaxation is determined by eliminating the forces iteratively. We find a top-layer bond-length-contracting rotation relaxation in which the Si surface layer atoms move closer towards the substrate while the C surface-layer atoms relax only parallel to the ideal surface plane. SiC(110) exhibits an occupied and an empty dangling-bond band within the fundamental gap. The occupied band predominantly originates from the dangling bonds at the surface carbon atoms which behave like anions. The empty band mainly originates from the dangling bonds at the surface Si atoms which act as cations. We present and discuss our results for the SiC(110) surface in direct comparison with the GaAs(110) surface. Further comparisons with literature data on surfaces of more ionic II-VI compounds like ZnS or ZnO are given, as well. This allows us to address the physical origins of the surface relaxation behavior of these compound semiconductors and to identify characteristic differences and similarities in the relaxation which are related to the specific types of heteropolarity or ionicity of these systems.