Determination of thermal diffusivity of suspended porous silicon films by thermal lens technique

Abstract

In this paper, the thermal diffusivity of free-standing films of nanoporous silicon has been studied. The films were obtained by electrochemical etching of p-type silicon. Measures were performed under vacuum and in the temperature range 300–600 K, so that the internal sample surface was not contaminated by ambient pollutants and hydrogen desorption did not affect significantly the surface chemistry due to high temperature effects. An investigation technique, based on thermal lensing, was adopted since it seemed to be more suitable than other techniques generally used for the determination of thermal properties of porous layers on crystalline substrates. The comparison between theoretical previsions and experimental results obtained for thin layers of known properties shows that the developed technique is reliable and of easy application for solid samples of low thermal conductivity. The diffusivity of the investigated nanoporous silicon samples is reduced with respect to the one of crystalline silicon due to both the porosity of the material and to the reduction in diffusivity of each individual nanocrystal, resulting from the enhanced boundary scattering of lattice waves.