Etching of n-Si(111) in 40% NH₄F Solution Investigated by OCP, In Situ EC-STM, and ATR-FTIR Spectroscopic Methods

The etching dynamics of a moderately doped n-Si(111):H electrode misoriented in the [112̄] direction in 40% NH 4 F aqueous solution was investigated by open circuit potential (OCP) measurement, in situ electrochemical-scanning tunneling microcopy (EC-STM), and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. It was demonstrated that OCP could be used as an in situ probe in judging that the well-defined Si(111):H surface was prepared successfully in the 40% NH 4 F solution. The OCP initially decreased when an oxide-covered silicon wafer was immersed into the solution, increased, and finally reached a plateau value, where the EC-STM image was observed to be composed of the atomically flat terraces with a threefold symmetry of interatomic distance 3.8 Å separated by a step of 3.1 Å. ATR-FTIR spectra showed that a strong and sharp peak with a full width at half-maximum of 0.94 cm -1 was observed from p-mode and a simultaneous background-like signal was observed from s-mode, suggesting that the surface chemical bond of the silicon wafer was mainly H−Si(111) normal to the surface with negligible contribution from other hydrogens bonded to surface silicon atoms. Time-dependent EC-STM images at eight different potentials including OCP revealed that the site dependence in the removal of surface silicon atoms was apparent at potentials up to the OCP and led to lateral etching with straight-step shaped terraces maintained. The first set of etching rate data was reported for the interfacial reaction at the n-Si(111):H under different potentials in the saturated fluoride solution. The data were rationalized within the framework of the recent report of the extended kinetic Monte Carlo simulation (Zhou, H.; Fu, J.; Silver, R. M. J. Phys. Chem. C 2007, 111, 3566).