In Situ Fourier-Transform Electromodulated Infrared Study of Porous Silicon Formation: Evidence for Solvent Effects on the Vibrational Linewidths

We report on an experimental investigation of porous silicon layer (PSL) formation and silicon/hydrofluoric acid (HF) interfaces, using internal-reflection Fourier-transform infrared spectroscopy and Fourier-transform electrochemically modulated infrared spectroscopy. Low-doping Si samples (p ≈ 3 × 10¹⁵cm⁻³, resistivity ≈4 Ωcm) have been used for PSL growing. The in situ electromodulated spectra during the formation of PSL and the transmission spectra of the PSL in contact with the electrolyte contain only a broad band at around 2100 cm⁻¹, whereas the transmission spectra of the dried PSL give rise to three sharp peaks at around 2085, 2115, and 2140 cm⁻¹. All these vibrational peaks are ascribed to surface- Sill chemical species. The broadening of the Sill spectrum in the former cases has been found to be due to interaction of Sill species with the solvent. Under cathodic conditions—either for n-Si in the dark or for p-Si with illumination—the spectrum of the Si/HF interface exhibits three sharp Sill peaks, characteristic of a surface Sill in contact with a gas phase. This is attributed to the presence of hydrogen gas bubbles on the Si surface. It is observed that the Si surface is covered with Si—H bonds in HF solution at all the applied potentials and current densities. An attempt has been made to detect intermediate species such as SiHF₃ in the process of anodic dissolution of Si in HF, and it is concluded that if these species exist at all, their lifetime is shorter than 0.3 ms.