In situ electrochemical-scanning tunneling microcopy (EC-STM) was employed to investigate the etching dynamics of the moderately doped n-Si(111) electrode during cyclic voltammetric perturbation and at the seven different potentials including the open circuit potential (OCP) in 40% NH4F solution at pH 10, which was prepared from 40% NH4F and concentrated NH4OH solution. The etching rate was significant at OCP and showed an exponential dependence on the potential applied to the silicon substrate electrode. Although some triangular pits were generated at the Si(111) surface, at the potentials more negative than OCP the site dependence in the removal of surface silicon atoms prevailed and led to the atomically flat Si(111):H surfaces with sharply defined steps of the step height 3.1 angstrom, where the interatomic distance of 3.8 angstrom was observed with a three-fold symmetry. At the potentials sufficiently more positive than OCP, macroporous hole was formed to limit further in situ EC-STM study. The results were compared with in situ EC-STM studies of the etching reaction of n-Si(111):H in the aqueous solution of dilute ammonium fluoride at pH 5 40% NH4F at pH 8, and 1 M NaOH reported in the literature. (C) 2008 Elsevier Ltd. All fights reserved.