Fabrication and characterisation of NB:TIO2 thin film for hydrogen gas sensing application.

Abstract

A gas sensor is a selective device used in monitoring the presence or concentration level of a particular gas in the ambient atmosphere. Gas sensors operate on the principle which is anchored on any of the following three classifications, that is, spectroscopic, optical, and solid-state gas sensing methods. In spectroscopic techniques, the gas sensor is based on basic gas properties such as molecular mass or vibration spectrum, while for optical gas sensors; measurements of the absorption spectra are involved. Solid-state gas sensors apply the fact that there is a change in the electrical properties of a sensing material whenever there is exposure to gas. Data collected on hydrogen sensors indicate that all the sensors have a low response time and are less sensitive to respond to even very low leakages of hydrogen. This work was prompted by the fact that there is continued research and study of new gas sensing materials, and therefore a likelihood in improvement in terms of response to the gas sensing properties as well as widen the choice and variety of hydrogen gas sensors fabricated using different types of materials. Thin films of Nb:TiO2 for gas sensing applications have been deposited using radio frequency (RF) magnetron sputtering. The samples were deposited at different partial pressures and sputtering power. The objectives were to analyse the optical, electrical and gas sensing properties of the thin films. The general results on optical and electrical properties of pure TiO2 and doped 2%wt Nb: TiO2 and 4%wt Nb: TiO2 have shown the different amount of thin-film transmittance depending on deposition conditions. The increase in partial pressure has been observed to cause a decrease in transmittance in doped TiO2, which has been attributed to competition for oxygen molecules between TiO2 and NbO phase. The deposition power has also been observed to give similar results in terms of transmittance, this is because at lower power a thinner film forms while at a higher deposition power a thicker film is formed thus resulting in to decrease in transmittance. The amount of doping influences the number of free electrons and thus influencing the optical and electrical properties of thin films. The band gaps for the three types of thin films were observed to vary depending on the deposition conditions. The drop in bandgap after the post-deposition annealing was observed and is fact attributed to improved crystallinity due to an increase in electrically activated charge carriers. Finally, the sensing capability of the thin film device has been observed to improve with annealing, a factor that has been attributed to the crystallinity and charge carriers