Optical, Thermal and Carrier Transport Properties of Porous Silicon Layer Determined Using Photoacoustic Spectroscopy

In this study, porous silicon layers were fabricated on p-type and n-type silicon substrates by electrochemical etching method at five different current densities (i.e. 16.98, 22.64, 28.29, 33.96 and 39.62) mA/cm2. The etching duration was fixed at 20 minutes. In the second series of preparation, the silicon substrates were anodized at six different etching durations (i.e. 20, 25, 30, 35, 40 and 45 minutes) with constant current density of 22.64 mA/cm2. Photoluminescence spectra, measured by fiber optics spectrophotometer show a peak that is blueshifted towards higher energy as the porosity is increased. Photoacoustic spectroscopic technique was used to characterize the optical, thermal and carrier transport properties of the samples. The measurements of the optical properties were carried out at three different modulation frequencies (15 Hz, 23 Hz and 33 Hz). The absorption peaks show a gradual blueshift towards higher energy as the porosity is increased. The band gap energies of the porous silicon layers were determined from the photoacoustic spectra measured at modulation frequencies of 23 Hz and 33 Hz, respectively. The band gap increases linearly with porosity from 1.80 eV to 2.00 eV for the porous silicon prepared on p-type silicon substrate, and from 1.70 eV to 1.86 eV for the porous silicon prepared on n-type silicon substrate. The band gap values of the p-type porous silicon are always higher than the n-type porous silicon samples. This indicates an active etching process has occurred in p-type silicon during anodization to produce larger micropores at higher porosity. The thermal diffusivity, carrier diffusion coefficient, surface recombination velocity and recombination lifetime of both p-type and n-type porous silicon layers were determined from the photoacoustic phase signal-frequency dependent using He-Ne laser as the excitation source. The thermal diffusivity (0.0493 - 0.0597) cm2/s and diffusion coefficient (3.33 - 4.07) cm2/s of porous silicon decrease consistently with the increasing of porosity. The thermal diffusivity (0.0493 - 0.0562) cm2/s and diffusion coefficient values (3.33 - 3.83) cm2/s of porous silicon prepared on p-type silicon substrates were obtained lower than the porous silicon prepared on n-type silicon substrates ((0.0541 - 0.0597) cm2/s and (3.69 - 4.07) cm2/s). This could be due to the enhancement of air and oxygen molecules impregnated inside the enlarged micropores with sponge-like spherical morphology that acted as the barriers for thermal diffusion. The surface recombination velocity of these samples ranges from 314.4 cm/s to 344.0 cm/s, while the recombination lifetime lies in the range from 71.6 cm/s to 97.6 cm/s. The surface recombination velocity of porous silicon increases as the increase of porosity. The surface recombination velocity of p-type porous silicon (324.1 - 344.0) cm/s is higher than the porous silicon layer prepared on n-type silicon substrates (314.4 - 330.3) cm/s. The XRD, EDX spectra and SEM photographs confirmed that the porous silicon layer remained crystalline and consists of a large network of tiny micropores impregnated with luminescent Si-oxygen bonds.

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