Electronic bond rupture of Si atoms onSi(111)−(2×1)induced by valence excitation

Abstract

Scanning tunneling microscopy study reveals the electronic local bond rupture of threefold-coordinated Si atoms at intrinsic sites on $\mathrm{Si}(111)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ under nanosecond-laser excitation at $1064\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The rate of bond rupture leading to monovacancy formation on the surface depends superlinearly on the excitation intensity. This primary step of surface-structural change is followed by efficient formation of vacancy clusters with two distinctive morphologies: vacancy strings aligned along the Si-atom chain in one dimension and vacancy islands developed across the chains in two dimensions. Quantitative analysis of the vacancy clustering process shows that the bond-rupture rate at sites nearest to pre-existing surface defects is enhanced more than a factor of 1000 relative to perfect sites. We also studied effects of different excitation wavelength and of Fermi-level positions on the bond-rupture process. The laser-induced surface bond-rupture mechanism is discussed in terms of two-hole localization of optically generated nonequilibrated valence holes on the surface sites.