Chopping frequency dependence of photoacoustic spectrum in porous silicon

The dependence of photoacoustic (PA) spectra on the chopping frequency in the range of 10 to 180 Hz was studied in porous silicon (PS) samples of varying thicknesses the PS layer. The enhancement of PA signals over those of bulk Si, reported in our previous work, is ascribed to the combined effects of interstitial gas 'pressure and intrinsic change of absorption. Although the enhancement seen in the long wavelengths, above 600 nm in the thick PS samples, is mainly caused by the "pressure" effect, the enhancement in the short wavelengths, below 600 nm, appears to be mainly due to the intrinsic effects related to the quantum confinement effect because it increases as the porosity increases and seems to remain at higher chopping frequencies (f = 180 Hz). We could also estimate the thermal diffusivity of the PS samples from the crossover frequency of the change of slope in the log PA signal vs the log chopping frequency plot. The dependencies of the PA spectral edges on porosity and chopping frequency are shown and discussed in reference to the double layered structure model proposed in our previous work.