FTIR STUDIES OF H2O AND D2O DECOMPOSITION ON POROUS SILICON SURFACES

The decomposition of H2O and D2O on silicon surfaces was studied using transmission Fourier-transform infrared (FTIR) spectroscopy. These FTIR studies were performed in situ in an ultrahigh vacuum chamber using high surface area porous silicon samples. The FTIR spectra revealed that H2O (D2O) initially dissociates upon adsorption at 300 K to form SiH (SiH (SiD) and SiOH (SiOD) surface species, i.e., H2O --> SiH + SiOH. The decomposition of these surface species was then monitored using the Si-H (Si-D) stretch at 2090 cm-1 (1513 cm-1), SiO-H (SiO-D) stretch at 3680 cm-1 (2707 cm-1) and the Si-O-Si stretch at 900-1100 cm-1. As the silicon surface was annealed to 650 K, the FTIR spectra revealed that the SiOH surface species progressively decomposed to Si-O-Si species and additional SiH species, i.e., SiOH --> SiH + SiOSi. Above 650 K, the SiH surface species decreased concurrently with the desorption of H2 from the porous silicon surface. New blue-shifted infrared features in the Si-H stretching region were observed at 2119, 2176 and 2268 cm-1 after annealing above 600 K. Additional infrared studies of partially hydrogen-covered porous silicon surfaces exposed to O2 suggested that these blue-shifted Si-H stretching vibrations were associated with silicon surface atoms backbonded to one, two or three oxygen atoms, respectively.