Etch-pit initiation by dissolved oxygen on terraces of H-Si(111)

Abstract

Dissolved oxygen in 40% aqueous ammonium fluoride solution initiates the formation of etch pits in the terraces of the otherwise ideal H-Si(111) surface. The etch pits are observed byex situ scanning tunneling microscopy in an argon atmosphere following emersion from the aqueous fluoride solution. Removal of O2from the fluoride solution by sparging with argon substantially reduces the initiation of etch pits. We propose the following mechanism of etch-pit initiation. Oxygen molecules are reduced to superoxide anion radicals at the negative open-circuit potential of the silicon surface. A small fraction (less than 0.4%) of these superoxide anions abstract hydrogen atoms from the H-Si(111) terraces to form silicon radicals (dangling bonds), which are then susceptible to etching in neutral to basic aqueous solutions. Hydrogen atom abstraction by aqueous superoxide anion radical also explains the known enhancement by water of oxide growth on hydrogen-terminated silicon surfaces.