In situ characterization of the n-Si/acetonitrile interface by electromodulated infrared internal-reflection spectroscopy

We present a systematic experimental investigation on the infrared (IR) vibrational absorption spectroscopy of the n-Si/acetonitrile interface utilizing the attenuated total internal-reflection geometry in the near-infrared (1.1-5 μm) spectral region. The IR absorption of the interface has been isolated selectively by electromodulation, and studied as a function of modulation potential. The electrochemical behavior of the interface has been checked by current/voltage and impedance measurements. The IR spectra are composed of a broad background and various sharp vibrational peaks. The background can be analyzed as the sum of two contributions: (i) absorption by surface states at shorter wavelengths (<2 μm), (ii) free-carrier absorption at longer wavelengths. The free-carrier contribution is itself composed of a Drude-like component (proportional to λ^3/2) and an interband component. The vibrational peaks can be ascribed to the C≡N, C-H, Si-H, and (Si-)O-H chemical bonds. The shapes and magnitudes of the C≡N and C-H peaks can be quantitatively understood in terms of displaced ions and acetonitrile molecules near the surface upon the electrode potential modulation. The shape of the C≡N peak also gives an indication of a weak interaction of the acetonitrile molecules with the electrode surface. The Si-H and (Si-)O-H peaks can be interpreted in terms of Stark effect modulation of the infrared absorption of these species. The shape of the O-H peak indicates the presence of nonequivalent sites at the interface. Upon the electrode aging and oxidation the magnitude of the Si-H peak decreases and the (Si-)O-H peak increases and correspondingly the surface-state density increases which provides a direct in situ physicochemical information regarding the slow oxidation of the electrode surface.