Infrared photo-induced absorption spectroscopy of porous silicon

The quantum confinement model, which assigns some of the luminescence features in porous silicon to size quantization in Si nano-crystallites, also predicts quantization of both the conduction and valence bands into sub-levels. In order to resolve this effect we have used a new experimental technique called "photo-induced infrared absorption spectroscopy". Here, a pump, visible laser, optically induces carriers in the conduction/valence band. Optical transitions between the quantized sub-levels are resolved by a probe, infrared beam in the energy range 50-300 meV. A broad photo-induced absorption signal has been observed in the 60-250 meV spectral range, in agreement with the prediction of the quantum confinement model. However, the photo-induced absorption signal decreases with the decreasing temperature, resolving activation energy of about 10 meV. This behavior can be understood if the allowed optical transitions are from the exciton singlet state only. Also, we found additional features in the photo-induced absorption spectrum that are correlated with the Si=O vibrational modes. Our results indicate a strong coupling between bulk excitonic states and surface states in small Si nano-crystallites.