Improvement of the properties of commercial SnO2 by Cd treatment

Commercial SnO2 thin films have been annealed after deposition of a thin upper layer of cadmium. When annealed for 30 min at 850 K under argon flow, XPS depth profiles show that the cadmium diffuses into the tin oxide film. When air is used during annealing instead of argon, there is not homogeneous cadmium diffusion in the underlayer, but CdO formation at the surface. The conductivity of the films annealed under argon is two-fold higher than that of commercial SnO2. It appears that the main part of this improvement is related to an increase in carrier mobility and not to an increase in carrier density. Measurement of the variation in conductivity with temperature shows that, before and after treatment, there is a small decrease in conductivity when the temperature increases, which can be attributed to carrier scattering by ionized impurity and a vibration scattering effect. Moreover, even if the films are degenerated, the effect of grain boundaries is not totally negligible. Therefore, it can be concluded from all the results described above that cadmium diffusion into the films during annealing essentially improves the grain boundary properties, but does not strongly modify the electrical properties of the crystallites.