A resistive gas sensor based on undoped p-type anatase

Observation of p-type gas sensitivity in undoped titanium oxide thin-films is reported. Layers of titanium oxide were sol-gel deposited on thermally oxidized Si wafers. The deposits were annealed at 500 °C for 60 min. XRD tests revealed the films to be mainly of anatase with an average grain size of ∼15 nm, and the SEM micrographs indicated an average particle size of 30 nm. The four-point-probe test, carried out at 320 °C, indicated the oxide semiconductor layer to be of p-type, and the Arrhenius diagram of electrical conduction resulted in an activation energy of 0.2 eV. The gas sensing properties of the layers were investigated on sample resistive gas sensors fabricated based on TiOx-SiO2-Si-SiO2 chips mounted on temperature-controlled micro-heaters. Operated at 320 °C, the resistance of the sample devices increased as ethanol vapour contamination was introduced into the surrounding atmosphere. The sensitivity to ethanol is high; ∼100% resistance increase was recorded for 50 ppm contamination. The sample sensors were capable of detecting ppm ethanol levels reversibly but concentrations above 3000 ppm affected the layers irreversibly. Increasing the partial pressure of oxygen had an inverse effect and reduced the resistance of the device as expected for a p-type oxide semiconductor.