Slow etching of triangular pits on atomically flat monohydride terminated Si(1 1 1) surface in 40% NH4F solution

The etching reactions of monohydride (MH) terminated Si(1 1 1) surface were observed in oxygen-free 40% NH4F solution by an in situ electrochemical scanning tunneling microscope. The results showed that the MH silicon steps in triangular pits played an important role in producing the ideally flat MH terminated Si(1 1 1) surface. The etching rate of MH silicon steps in the triangular pits is about 8.3 nm/min at open circuit potential which is significantly lower than the value of about 28 nm/min for terrace MH steps. The etching rate of MH terminated steps in triangular pits is 3.3–4.5 times lower than the values for MH terminated steps of the terrace edges and its etching ratio of steps over pits was maximized at −0.4 V; consequently, the staircase structure with a constant step width was produced on the Si(1 1 1) surface.