Characterization of optical properties of porous silicon using photoacoustic technique

Photoacoustic absorption spectra of the porous silicon samples ( P-Si) of different thickness and porosity percentage were measured at different modulation frequency. The absorption edge of the P-Si layer for all samples shows a blue shift from that of crystalline silicon ( C-Si) at 1.1 eV. At low modulation frequency the estimated energy gap ( 1.88 eV) is almost the same for all samples and the PA signal increases as the porosity percentage increases. At the higher modulation frequency, the spectra show an increase in the energy gap indicating the effect of quantum confinement as the porosity increasing. The Raman shifts of the P-Si samples are correlated with the particle size leading to an estimated average particle size. The quantum confinement interpretation of the PA results is in agreement with the Raman measurements that indicate the presence of such nanostructure in the P-Si layer.