Porous silicon ZnO/SnO2 structures for CO2 detection

Abstract

In this work, we report the CO2 sensing properties of tin oxide (SnO2) and zinc oxide (ZnO) thin films deposited onto the macro porous silicon (PS) substrates. PS was synthesized by electrochemical etching of p-type monocrystalline silicon along with the metal oxide (MO) thin film deposition using precipitation method. X-ray diffraction (XRD) confirms the presence of tetragonal and wurtzite phases of SnO2 and ZnO respectively. Scanning electron microscopy (SEM) reveals the formation of substrate porosity dependent porous metal oxide nanostructures. CO2 sensing properties of SnO2/PS and ZnO/PS were studied as a function of deposition time, gas concentration and operation temperature (up to 300 °C). Sensing response of approximately 19 was obtained with SnO2/PS, whereas, ZnO/PS reveals a relatively faster response time (∼65s). Porous silicon templates improve the gas-sensing properties of both SnO2 and ZnO films by a factor of 2 with respect to the films over cSi substrates due to increase in specific surface area. Irrespective of substrate type, the SnO2 films showed higher sensing response than ZnO films due to the association and dissociation reactions of SnO2 with adsorbed atmospheric oxygen. An increased conductivity of ZnO films synthesized with zinc carbonate precursor and subsequently their sensing response (∼4) at lower temperatures (100 °C), has been attributed to an additional interface and new nano passages created due to Zn2SiO4 formation. As compared to the c-Si substrate, the faster response time was achieved for all films using PS substrates.