**4. Conclusions**

This study has examined the electrical and optical properties of Ti:SnO2 thin films deposited on glass substrates and then annealed at temperatures ranging from 200–500 ◦C. The experimental results have shown that the thickness of the Ti:SnO2 films is insensitive to the annealing temperature. However, as the annealing temperature increases, strong peaks in the XRD patterns emerge corresponding to (110), (101), and (211) phases. Hence, it is inferred that the films increase a well-crystalline structure at higher annealing temperatures. The Ti:SnO2 film annealed at the lowest temperature of 300 ◦C shows both the minimum resistivity of 5.65 <sup>×</sup> 10−<sup>3</sup> <sup>Ω</sup>·cm. The energy gap and optical transmittance both increase with increasing annealing temperature and have values of 3.28 eV and 74.2% at an annealing temperature of 500 ◦C. The AFM results show that for the samples annealed at temperatures of more than 300 ◦C, the mean surface roughness reduces with an increase in annealing temperature. The SEM observations suggest that the lower surface roughness is the result of larger grain size. In particular, the grain size decreases from 14.89 nm in the film annealed at 300 ◦C to 11.56 nm in the film annealed at 500 ◦C. The Ti:SnO2 film annealed at a temperature of 500 ◦C shows the highest ΦTC of 3.99 <sup>×</sup> <sup>10</sup>−<sup>4</sup> <sup>Ω</sup>−1. The characterization results have suggested that the optimal performance of this film is due to optical transmittance.

**Author Contributions:** Data curation—formal analysis, C.-F.L. and Y.-S.H.; Writing—Original draft preparation methodology—investigation T.-H.C.; Review and editing—investigation C.-H.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Ministry of Science and Technology, Taipei, Taiwan, under Grant No. 106-2628-E-992 -302 -MY3.

**Acknowledgments:** The authors gratefully acknowledge the financial support provided to this study by the Ministry of Science and Technology.

**Conflicts of Interest:** The authors declare no conflict of interest.
