Carbon lines on the cubic SiC(001) surface

We investigated the structure and bonding of carbon atomic lines on the bet a-SiC(001) surface, using ab initio calculations. In agreement with experim ent, we find that reconstructions with sp-bonded chains are energetically f avored over those with sp(3)-like C bonds. However the energy difference be tween the two types of reconstructions is only a few meV/dimer, and for sma ll applied compressive stress geometries with sp(3)-bonded sites become fav ored, in addition, the formation energy of sp(3)-like defects on a sp-bonde d surface decreases rapidly with increasing defect concentration, pointing at stable sp(3) lines perpendicular to sp dimers, consistently with experim ent. A mechanism is proposed for the recently observed sp --> sp(3) transit ion on carbon terminated beta-SiC(001) surfaces.