Photoacoustic measurements of optical energy gap of porous silicon as a two layer opaque material

Abstract

Photoacoustic (PA) technique was applied to study the optical absorption and measure the optical energy gap of porous silicon (PSi) as a suitable example of the two layer samples (porous layer and Si substrate). The PSi layers were prepared by the electrochemical anodization etching process with different current density of p-type silicon wafers. The formation of the porous layer has been identified by photoluminescence (PL) and AFM measurements. Thermal diffusivity of the prepared samples was also measured to confirm thermal isolation properties of the formed PSi. The optical absorption, energy gap, and thermal diffusivity of PSi were investigated by analyzing the experimental data from PA measurements. The values of energy gap have been found to be porosity-dependent. In contrast to the conventional techniques, PA measures the energy band gap of both layers simultaneously with high accuracy. In the range of porosity (27–52%) of the studies, our results show that the optical band-gap energy of p-type PSi (1.7–1.92 eV) was higher than that obtained for silicon substrate (1.11 eV). These energies were used to obtain the diameter of the Si quantum dots in the porous layer. These results introduce PA technique as a suitable candidate for optical depth profile analysis of multilayer solids. Moreover, PA technique can be used for particles size estimation of low dimensional materials.