Preferential etching of Si(111) and Si(001) in dilute NH₄F solutions: as probed by in situ STM

In situ scanning tunneling microscopy (STM) was used to examine the etching process of n-Si(111) and Si(001) electrodes in dilute NH₄F under cathodic potential control. For Si(111), time-dependent STM images have revealed the pronounced effect of the microscopic structure of surface Si atoms on their dissolution rates. The multiple hydrogen bonded Si atoms at the kink sites and dihydride steps eroded faster than the monohydride terminated Si. Presumably, the higher polarity at these defect sites is responsible for the difference. Steric consideration further favors the higher activity at the more open kink sites. The monohydride terminated Si(111) surface represents the most stable surface structure, which guides the dissolution of the Si(001) surface to the formation of {111} facets. The initial stage {111} facet formation on a Si(001) surface was revealed by in situ STM.