Vacancy charging on Si(100)-(2×1): Consequences for surface diffusion and STM imaging

The present work investigates the structure and energetics of charged vacancies on Si(100)-(2×1) by calculations using density functional theory. The calculations predict multiple stable charge states for all vacancy structures investigated, although the neutral state is most stable for typical Si(100) surfaces. The multiplicity of possible states lends significant support to a hypothesized mechanism for nonthermal illumination influences on surface diffusion. The calculations also show that the +1 state of the upper dimer monovacancy is destabilized by structural relaxations, leading to negative-U properties. Implications of this work for possible artifacts in imaging by scanning tunneling microscopy are discussed.