Adsorption and desorption of S on and off Si(001) studied by ab initio density functional theory

We present detailed ab initio density functional calculations of equilibrium atomic geometry, electronic states, and chemical bonding for the adsorption of elemental S on Si(001). Following recently reported room temperature low-energy electron diffraction, Auger electron spectroscopy, thermal desorption spectroscopy, and work function measurements by Papageorgopoulos et al. [Phys. Rev. B 55, 4435 (1997)], three different adsorption models have been studied: hemisulfide (2×1) structure, monosulfide (1×1) structure, and disulfide (1×1) structure. For hemisulfide and monosulfide structures, the calculated location of S above the Si(001) surface is in excellent agreement with the experiment. An analysis of surface free energy suggests that, in the allowed range of S chemical potential, the monosulfide structure is more stable than the hemisulfide and disulfide structures. A signature of desorption of the SiS unit is obtained from the study of the disulfide structure.