Hall effect measurements in gas sensors based on nanosized Os-doped sol-gel derived SnO2 thin films

The influence of dopants on the electrical properties of gas sensitive layers used in semiconductor gas sensors has to be carefully understood for getting a deeper insight in the relationship between the sensor performance and its chemical composition. In this work, undoped and Os-doped SnO₂ thin films have been prepared by the sol-gel process with an Os-Sn atomic ratio of 5%. The films have been characterized by resistivity and Hall effect measurements in a temperature range from 100 K to 500 K, both in air and in vacuum. The results have been investigated according to grain boundary scattering mechanism. We found that in air, the ambient oxygen species adsorbed on the film increase the height of the grain boundary barriers and the activation energy for the electrical conductivity increases in the doped film. In vacuum, the results showed that the height of the intergranular barrier is lower than the corresponding value in air. Both in air and in vacuum, the conductivity of the Os-doped sample is higher than the value in the undoped SnO₂ sample. The same occurs for the Hall mobility and the carrier concentration. The experimental results have been used to explain the better methane sensitivity, at low temperature, of the Os-doped films as compared with the undoped ones.