Thermal conductivity reduction in highly doped mesoporous silicon: The effect of nano-crystal formation

Abstract

The study of heat transfer properties in mesoporous silicon, fabricated from highly doped p-type and n-type silicon wafers, is presented. Measurements were performed by a laser induced transient thermal grating technique, which allowed us to determine the effective (in-plane) thermal conductivity. It is shown that the thermal conductivity undergoes a significant decrease with respect to bulk values mainly due to a reduction of the phonon mean free path of the solid matrix. This reduction can be ascribed to the formation of nano-crystalline domains, which are a consequence of the wet etching fabrication method. Additionally, the in-plane thermal conductivity was analyzed by employing a modified effective medium approach, which includes the phonon mean free path reduction due to the presence of both the nanometric pores and the nano-crystalline domains. The theoretical analysis shows good agreement with our measurements, indicating that the inclusion of phonon mean free path reduction to an effective medium approach is a well-suited method for studying the thermal conductivity of porous silicon.