Physicochemistry of point defects in fluorine doped zinc tin oxide thin films

Zinc tin oxide (ZTO) and F-doped zinc tin oxide (FZTO) films with Zn concentration up to 35 at.% were prepared by chemical spray pyrolysis technique. The X-ray diffraction results showed an expansion in the lattice of tin oxide by either doping with fluorine or adding Zn due to the incorporation of fluorine into oxygen vacancies or the replacement of the host Sn atoms by Zn, respectively. The X-ray photoelectron spectroscopy results of the FZTO films yield oxygen vacancy concentration [VO] in the range 1021-1022 cm− 3 and substitutional fluorine concentration [FO] in the range (1.71-9.66) × 1020 cm− 3. For relatively low Zn concentration the electron concentration measured using Hall effect is close to [FO] but lower than [VO] by two orders of magnitude. The results suggested neutral oxygen vacancies. The overall results showed that tin is in tetravalent oxidation state in the whole range of studied Zn concentrations. All films under investigation show high transparency in the visible range (T ≥ 82%). In addition, the optical transmittance shows a tail in the near IR region due to free carrier absorption. The optical energy gap of the FZTO films falls in the range 3.86 eV-4.45 eV and exhibits a UV shift with the increase in free carrier concentration due to the Burstein-Moss effect.