Band gap of porous silicon estimated by photoacoustic spectra

The conditions under which photoacoustic (PA) measurements of porous Si samples can yield reliable results on the widening of their band gaps are studied. The enhancement of the PA signals observed from porous Si samples is stronger than those from the powdered ones. Although this enhancement makes the observation of both the magnitude and phase of the PA signals easy, it also causes an apparent shift in the observed gap of the samples with a thin porous Si layer on the Si substrate. A simplified two layer model is examined, including the effect of scattering to separate this effect. The results of the samples with a thicker porous Si layer reveal the real shift of the band gap.