Influence of oxidation temperature, film thickness and substrate on NO2 sensing of SnO2 ultra thin films

Thin SnO2 films of different thickness were fabricated by sputtering of tin followed by thermal oxidation. The oxidation was performed at 600, 700 and 800 °C and silicon and silicon dioxide were chosen as substrates for their compatibility to silicon CMOS technology. Work function measurements were carried out on these films upon exposure to NO2 in the concentration range 5–100 ppm at a particular temperature under dry and humid conditions using a commercial Kelvin probe measurement set-up. Kelvin probe measures contact potential difference (CPD) resulting from change in work function of a film material on exposure to a gas. The dependence of NO2 sensitivity of films on oxidation temperature, film thickness and substrate was investigated. The influence of temperature on NO2 response from 30 to 130 °C was investigated. It has been observed that humidity has negligible effect at 130 °C. It was further observed that response of SnO2 film on silicon to NO2 increases with oxidation temperature of the film whereas for the films grown on silicon dioxide the response was parabolic. In the later case, increase in signal was exhibited by the films oxidized up to 700 °C while it was found to decrease for films oxidized at 800 °C. The films of three different thickness (10, 35 and 55 nm) prepared at 800 °C on silicon showed negligible variation in sensitivity. However, sensitivity of the films grown on silicon dioxide is observed to be thickness dependent. The films were characterized using atomic force microscopy (AFM) and Auger electron spectroscopy (AES) to study the surface morphology and depth profile respectively. The observed results are discussed to understand the NO2 sensing behavior of ultra thin SnO2 films.