We present a new defect model for the reconstructed Si(001) surface. Ab initio molecular-dynamics simulations at finite temperature show the existence of interstitial dimers which are recessed from the surface. These dimers and adjacent on-surface dimers are both symmetric. The possibility that missing dimer vacancies observed by scanning tunneling microscopy contain these new surface defects is pointed out.

• Received 14 July 1989

DOI:https://doi.org/10.1103/PhysRevLett.65.1909