Evolution of defects and charge carrier transport mechanism in fluorine-doped tin oxide thin films upon thermal treatment

Fluorine-doped tin oxide (FTO) thin films were prepared by a radio frequency magnetron sputtering technique. The defects and charge carrier transport behavior in FTO thin films were evaluated during the transition process from amorphous to nanocrystalline structures. The stable lattice structure in FTO thin films was obtained as the annealing temperature reached 400  °C. Positron annihilation results indicated that defect evolution in the FTO thin films was shown in two stages, formation and reduction of vacancies/vacancy clusters. The carrier mobility of the FTO thin films annealed at 600  °C was enhanced twice the amount than that of the unannealed samples. The correlation between the results obtained from positron annihilation and the Hall effect revealed the importance of defect scattering in deciding the charge carrier mobility. A defect scattering mechanism was proposed to interpret the noticeable increment of carrier mobility in FTO thin films after thermal treatment.