Optimising porous silicon electrical properties for thermal sensing applications

Present day sensing elements used in thermo-resistive type uncooled detectors or bolometers either suffer from material incompatibility issues or have relatively poor performance in the long wave infrared region (LWIR). This work explores the potential of meso-porous silicon films as the base material for the sensing element in these bolometer applications. The performance of these highly sensitive bolometers depends on the electrical properties, specifically the electrical resistivity and temperature coefficient of resistance (TCR), of the sensing element. In this work, we explore the electrical properties of meso-porous silicon films and optimize these properties through fabrication conditions and surface treatment. We further compare the TCR values of single and multi-layer porous silicon films for use in multilayers to fabricate highly selective distributed Bragg reflectors (DBR) in LWIR detector applications. The experimental results show that a low value of resistance along with high value of TCR can be obtained by altering porosity and surface passivation in single and multi-layer porous silicon films. We measured the TCR within a three-layer porous silicon film (DBR stack) to be 6%/K, which is significantly higher than the TCR from contemporary sensing element materials. Our studies indicate that the use of meso-porous silicon films for the sensing element in bolometers can potentially overcome the performance and the material compatibility issues associated with existing technologies such as amorphous silicon and vanadium oxide in LWIR.