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 ult rahigh vacuum with an ultrafast load-lock and characterized by scannin g tunneling microscopy (STM): Concentrated HF selectively removes any surface oxide and, thus chemically prepares the initially burried, iso tropically rough Si/SiO2 interface while highly buffered HF (i.e., NH4 F) 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 X 1):H, NH4F etching leads to a time-dependent transformation of isotro pic roughness into a pattern of triangular etch defects with monohydri de steps perpendicular to [211BAR] due to a preferential removal of lo wer-coordinated atomic defect sites. A predominant atomic step structu re due to sample miscut (vicinal surfaces with azimuth not-equal [211B AR]) can oppose the anisotropic NH4F etching: At low step density (sma ll polar angle of miscut) a meandering of atomic steps with straight m onohydride portions is observed while at high step density strong step -step interaction counterbalances anisotropic removal and forces an et ching by a homogeneous flow of (nonmonohydride) steps along the macros copic misorientation. Local findings obtained with STM are compared to macroscopically averaged results from a simultaneous quantitative ana lysis of low-energy electron diffraction profiles.