Synthesis and characterization of indium oxide thin films for NO₂ gas sensing application

Nitrogen dioxide is one of the most toxic gases whose major sources of emission are motor vehicles, industries, gas stoves, thermal power plants, and many more. It affects the respiratory and cardiovascular system of humans and also affects our ecosystem, and thereby, its detection at lower levels (below 10 ppm) is very important. Portable, efficient, stable, and affordable sensor devices are required for the detection of these toxic gases. Thin films of In₂O₃ having a thickness in the range of 50 – 250 nm are prepared on alumina and sapphire substrates by the thermal oxidation of In metal films deposited by thermal evaporation technique. The oxidation of In films was carried out in a horizontal furnace at different oxidation temperatures in the range of 500 - 600°C in ambient air. The grazing incidence x-ray diffraction studies confirmed the formation of In₂O₃ thin films, which exhibit a cubic structure. The preferred orientation was aligned along the (222) orientation. SEM analysis showed that the thin films possess uniform grain distribution and the surface is porous. The film resistance was found to increase with the increase in annealing temperature. The gas-sensing performance of In₂O₃ films prepared at various oxidation temperatures has demonstrated a better response toward low ppm NO₂ gas at room temperature with good stability. The response and recovery times were determined using a resistant–time graph. It is found that the sensing response for 5 ppm NO₂ is about 83 % with a response time of 10 s at room temperature. The present study reveals that NO₂ gas sensing response is influenced due to thickness variation and oxidation temperature