Peculiarities of surface doping with Cu in SnO2 thin film gas sensors

It is demonstrated that a small amount of Cu impurity (less than 1 at .%) significantly modifies the properties of tin oxide thin films used for the gas sensors. Different amount of C-ct (between 0.5 and 7 at .%) was sputtered on the top of the films. The structure, surface chemical composition, optical and electrical properties are studied for these films. It is shown that the surface doping with the sputtered Cu leads to nearly constant doping level in all the volume of the polycrystalline tin oxide thin film. It is shown experimentally that the selectivity of the resistance response to CO, H-2 and Cl-2 gases is improved for the tin oxide sensors by the small amount of the sputtered Cu. A correlation is revealed between the Cu effect and the electronic surface states that were detected by the XPS and the optical analysis.