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

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Abstract
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 4F 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.