Low thermal conductivity porous Si at cryogenic temperatures for cooling applications

Porous Si thermal conductivity was determined in a wide temperature range from 20 to 350 K using the steady-state direct current method and a subsequent finite element method. The method was applied to a 40 µm thick porous Si layer of ∼60% porosity, formed on p-type Si of resistivity 1–10 Ω cm. It was found that in the temperature range 20–80 K the thermal conductivity of the studied porous Si layer was more than four orders of magnitude lower than that of bulk crystalline Si, due to phonon confinement in the nanostructured skeleton composing porous Si. The temperature distribution as a function of distance from a heater on the porous Si layer and as a function of depth from the porous Si/metal interface was also calculated for different values of the applied electric power and for two different temperatures, 20 and 290 K, using a combination of experimental results and simulations. The effectiveness of local thermal isolation from the Si substrate by a thick highly porous Si layer in a wide temperature range was demonstrated, showing that this material is challenging for use in Si micro-cooling devices. (paper)