Evaluation of the thermal conductivity of porous silicon layers by an optical pump-probe method

Measurements of the thermal conductivity for free-standing porous silicon layers were performed by means of an optical pump-probe experimental set-up. By transient heating due to laser pumping, a refractive index modulation was induced in the sample and, solving the heat propagation equation for inside the solid film, it is shown that the time decay of the nonlinear transmittance can be related to the thermal conductivity. The optical technique demonstrated here is contactless, quite simple and does not require much effort in data analysis, and is therefore very useful for thin-film characterization. The thermal conductivities of the porous silicon samples, whose porosities lay in the range 60-70%, were taken into account, and we found good agreement with results obtained with different techniques.