Bismuth oxysulfide thin films for light and humidity sensing

Abstract

Bismuth oxysulfide (BOS) films formed on dielectric glass substrate by the chemical bath deposition have a high sensitivity to moisture content demonstrating decrease of the electrical resistance up to three orders of magnitude when the relative humidity (RH) increases from 5 to 85%. High sensitivity of resistive structure representing 0.65 μm thick film between two Ag contacts is associated with peculiarities of its microstructure. BOS films are composed of randomly oriented thin plate crystals. The presence of a great number of intergrain boundaries restricts transport of charge carriers (both in the dark and under illumination), which is desirable for resistive humidity sensors. As BOS films possess a high surface-to-volume ratio, this makes them ultra-sensitive to adsorption of various molecules and, in particular, water. It is supposed that ionic conductivity in a thin film of adsorbed water plays a crucial role in fast response of sensor (1–3 s) to adsorption-desorption cycles.