Difference of the responses between SnO2 and ZnO to reducing gases at 300°C and below via optical and electrical approaches

The responses of SnO2 and ZnO were investigated to several atmospheres via optical and electric approaches. Optical absorption spectroscopy was adopted to detect the formation of the defect levels in the energy-band-gaps by the reducing treatments, while the changes in the electric properties of the materials were evaluated by Hall effect measurements. The electric measurements revealed that the response of the SnO2 films to 0.5% H2/N2 consisted of two components. One was steep rise of the electric conductance as the films were exposed to the gas, and the second was gradual increase after the conductance jump, which lasted for several hours until reaching a plateau. The SnO2 films after reaching the plateau exhibited the enhancement of optical absorption between 340 and 800 nm and it was attributed to the formation of oxygen vacancies by the reducing treatment. The Hall effect measurements revealed that the increase of the electric conductivity of the SnO2 films was a consequence of increase of both the carrier concentration and the mobility. On the other hand, a change of the optical absorption was not observed for ZnO and Al-doped ZnO films against 0.5% H2/N2 up to the treatment temperature of 400°C. The Al–ZnO films showed the carrier concentration dependence in the responses. When Al was doped in ZnO to enhance the carrier concentration at a relatively low level (~1019 cm−3), the change of the electric conductivity according to the atmosphere did follow the change of the mobility. As the carrier concentration increased to ~1021 cm−3 with higher Al doping, the response became dominantly influenced by the change of the carrier-concentration according to the atmosphere. Even though SnO2 and ZnO are both typical semiconductive oxides, the interaction with the atmospheres is different from each other.