Origin of the anisotropy in the anodic dissolution of silicon

We report on an in situ, real-time, electrochemical, polarized, attenuated total reflection Fourier transform infrared spectroscopic investigation of Si(100), (111), and (110) surfaces in diluted HF solution (cF =0.05 M, pH 3). Prior to the IR measurements, cyclic voltammetry was carried out on the samples in a standard three-electrode arrangement, then IR spectra were recorded as a function of potential using 50-mV potential increments. The results give evidence for potential-induced absorption changes of the chemical surface species (Si-O, Si-H, Si-OH), which depend somewhat on substrate orientation. The potential-induced changes regard the shape and magnitude of all the above vibration bands and include a shift for some vibrations. In addition, the SiOH contribution appears about twice as large on (100) as on (110) and (111). The SiOH species, present just above the onset of anodic current flowing, at potentials around 0 V, appear as a key factor in the anisotropic dissolution of silicon. These observations are accounted for semiquantitatively in the framework of a kinetic model. The main factor responsible for the anisotropic effects appears to be the ability of surface silanols to couple and form siloxane bridges, a reaction governed by the surface concentration of available sites and the geometry of their neighborhood.