In-situ infrared study of the silicon surface in HF electrolyte

As part of an in-situ study of the behavior of the silicon surface in hydrofluoric acid, we have investigated porous silicon layer formation using in-situ Fourier-transform electrochemically modulated infrared Spectroscopy. The porous silicon layers were grown on p-Si wafers ((100)-oriented, doping p -3 × 10¹⁵ cm^-3). A prominent vSiH band at around 2100 cm^-1 is found under growth conditions as well as for cathodic potentials. The same band is observed in standard infrared absorption Spectroscopy, and is found to split into three narrow lines at 2085, 2115, and 2140 cm^-1 when the silicon surface is removed from the electrolyte. This shows that the width of the initial band is due to interaction of the SiH vibrators with the electrolyte. Intermediate species in the dissolution process, such as SiHF₃, have been looked for. Failure to detect such species provides an upper bound for their lifetime of 0.3 milliseconds.