STRUCTURES AND ENERGETICS FOR POLAR AND NONPOLAR SIC SURFACE RELAXATIONS

We have investigated the nature of relaxations and resultant stability gains for polar (both Si- and C-terminated) and nonpolar (equal concentration of Si and C) surfaces of zinc-blende (cubic) and wurtzite (hexagonal) surfaces of SiC, using the tight-binding atom-superposition and electron-delocalization band technique. The ideally truncated C surfaces of (111) and (100) -SiC are predicted to exhibit larger inward displacements and stabilizations than the Si surfaces, which is in agreement with the semiempirical total-energy band calculations of Lee and Joannopoulos but not with the empirical potential-function prediction of Takai et al. The inward displacements on these surfaces have associated with them increased surface-atom to substrate bonding stabilization and the destabilization of the dangling surface-state band on the (111) surfaces and of both dangling surface-state bands on the (100) surfaces. The Si- and C-terminated (100) surfaces are calculated to undergo dimerizations similar to those observed on the Si(100) and C(100) surfaces with bond formation and some stabilizing overlap of the remaining surface dangling orbitals. The atoms on the nonpolar surfaces each have a single dangling surface orbital, empty for Si and doubly occupied for C. The relaxations at the -SiC(110) surface show a combination of buckling and downward displacement. The zig-zag chains in the surface plane acquire some character due to bonding overlaps between the dangling sp-hybrid surface orbitals on surface Si and C atoms along the chains. The relaxed -SiC(101 0) surface displays buckled and asymmetric SiC dimers on the surface with contracted bond distance, again due to in-phase overlaps between the sp-hybridized dangling surface orbitals on dimer atoms. The relaxation pattern and ensuing stability for the -SiC(112 0) surface are intermediate between those for the -SiC(110) and -SiC(101 0) surfaces and is explained on the basis of the geometric structure and the directions of the dangling surface-state orbitals on neighboring surface atoms, which lead to intermediate stabilization. © 1990 The American Physical Society.