Anisotropic etching versus interaction of atomic steps: Scanning tunneling microscopy observations on HF/NH4F-treated Si(111)

After ex situ etching with various solutions of hydrofluoric acid (HF) and ammonium fluoride (NH4F) Si(111) samples are transferred into ultrahigh vacuum with an ultrafast load-lock and characterized by scanning tunneling microscopy (STM): Concentrated HF selectively removes any surface oxide and, thus chemically prepares the initially burried, isotropically rough Si/SiO2 interface while highly buffered HF (i.e., NH4F) attacks bulk silicon anisotropically. After a rapid homogenization of the chemical surface termination (HF: various hydrides, fluorine, ...) towards a perfect, unreconstructed monohydride phase, Si(111)-(1×1):H, NH4F etching leads to a time-dependent transformation of isotropic roughness into a pattern of triangular etch defects with monohydride steps perpendicular to <2̄11≳ due to a preferential removal of lower-coordinated atomic defect sites. A predominant atomic step structure due to sample miscut (vicinal surfaces with azimuth ≠<2̄11≳) can oppose the anisotropic NH4F etching: At low step density (small polar angle of miscut) a meandering of atomic steps with straight monohydride portions is observed while at high step density strong step-step interaction counterbalances anisotropic removal and forces an etching by a homogeneous flow of (nonmonohydride) steps along the macroscopic misorientation. Local findings obtained with STM are compared to macroscopically averaged results from a simultaneous quantitative analysis of low-energy electron diffraction profiles.