On the photo-induced electrical conduction related to gas sensing of the Sb:SnO2/TiO2 heterostructure

In this work, changes on the electrical properties as function of the temperature and gas addition on the surface of the 4at%Sb:SnO2/TiO2 heterostructure are investigated. This heterojunction privileges the gas sensor application at temperatures nearer to the ambient temperature, when compared to existing devices, which is more efficient when sensitized with monochromatic light of specific wavelength. The results point that the conduction mechanism, under the thermally activated trapping influence, is occurring preferentially in the TiO2 layer, since the activation energy of the deepest level is 56 meV, comparable to donors from TiO2. Besides, an increase of the photo-current decaying rate was noticed when using the excitation of a He-Cd laser under O2 atmosphere and room temperature, compared to films with a sole TiO2 layer. Samples showed a high resistance variation (up to 40 times) with the addition of gas at temperatures higher than 330 K, that can be attributed to the electron trapping by the oxygen molecules adsorbed on the sample surface. A model that describes such trap-states and the conduction mechanism within the samples is proposed. This way, this work can be valuable to the better understanding of the effects and rules on the electrical behavior of this heterostructure, in order to apply this material as a gas sensor device.