Photo Sensitivity Activation of SnO_2 Thin Film Gas Sensors at Room Temperature

摘要

The action of light is known to influence adsorption/desorption processes of molecular species on a semiconductor, by changing carrier concentration at the surface. These effects might provide a way of increasing the yield and selectivity in semiconducting gas sensors working at room temperature. We have studied the photo-response of thin films of n-type SnO_2, varying the gaseous atmospheres in the test chamber at different temperatures, showing that the response of the oxide film to these variations can be effectively enhanced by light. In addition we have found evidence of photodesorption of oxygen previously chemisorbed at the surface of the films. Our results indicate also that for long exposure times, other overwhelming processes can take place, they have a partial explanation in the balance between photodesorption and chemisorption of oxygen caused by irradiation. Specifically the inversion layer field pulls the photogenerated holes towards the surface, where they neutralise the chemisorbed oxygen species and so simultaneously reduce the surface field. In steady-state illumination photo-assisted desorption is balanced by new chemisorption processes.