Effect of porosity on the thermal properties of porous silicon as investigated by spatially-resolved Raman spectroscopy

Abstract

We performed spatially-resolved Raman spectroscopy on porous silicon films (pSi) grown on silicon substrates to map the laser-induced heating along the cross-section of the porous silicon/silicon material system. The pSi films were grown on N+ doped (100) Si wafers by electrochemical etching with hydrofluoric acid as the electrolyte. The pSi films were grown with etch currents of 10 mA and 20 mA to vary the thickness and the porosity of the pSi. Reflectance measurements showed that the 10 mA and 20 mA etched pSi films have a porosity of 85% and 90%, respectively. Spatially-resolved Raman spectroscopy was performed to map the Raman scattering from the Si substrate to the surface of the pSi film. The Raman maps show that the crystalline Si peak at 520 cm⁻¹ downshifts to 495 cm⁻¹ as the laser is moved from the Si substrate to the surface of the pSi due to laser-induced heating. The Raman spectra was shown to recover back to the 520 cm⁻¹ peak at locations close to the pSi surface. The spatially-resolved Raman maps further showed that the porosity and film thickness affect the thermal conductivity, and thermal distribution profile of pSi.