In-situ infrared vibrational study of the early stages of silicon oxidation at the interface with a non-aqueous electrolyte

As part of our investigation of the silicon/non-aqueous electrolyte interface, we present here the first in-situ vibrational data in a spectral range extended to wavenumbers below 2000 cm^-1. Our spectra were recorded using Fourier transform electrochemically modulated infrared spectroscopy. They cover the entire 1000-4200 cm^-1 range, giving access in particular to the Si-O-Si asymmetric stretching vibrations. The experiments were performed on the (111) surface of an n-type silicon electrode. Before immersion into the electrolyte, the HF-rinsed electrodes were covered with a passivating Si-H layer. Our results demonstrate that: (i) the residual water in the electrolyte (10 ppm) is molecularly adsorbed at the silicon surface; (ii) this water oxidizes the silicon surface slowly; and (iii) the silicon oxide grows by island formation. Quantitative estimation shows that after 1 week in the electrolyte the oxide islands are typically 0.6 nm thick and cover about 60% of the electrode surface; in these conditions, a typical island can be roughly described as a cluster of about 100 Si-O-Si vibrators.