**4. Conclusions**

Undoped and fluorine-doped SnO2 thin films deposited by APCVD at two different temperatures, 590 ◦C and 610 ◦C, were studied by structural and magnetotransport probes. GIXRD revealed the polycrystalline nature of the thin films with the average crystallite size of 10–25 nm, while SEM indicated the presence of an additional inhomogeneity on a bigger scale in form of grains with the average size 80–200 nm. Samples deposited at 590 ◦C were found to have somewhat bigger grains, while samples deposited at 610 ◦C showed more (200) preferred orientation of crystallites. Hall effect measurements showed that the carrier density for both undoped and fluorine-doped samples was in the range 1–3 × 10<sup>20</sup> cm<sup>−</sup><sup>3</sup> (assuming a substantial level of natural defects and unintentional doping) and has a negligible temperature dependence from room temperature down to −269 ◦C, indicating that the Fermi level lies either in the conduction band or in the region where the conduction band mixes with the impurity levels. Carrier mobility extracted from the Hall effect and DC resistivity turned out to have values 10–20 cm<sup>2</sup>/Vs and to be temperature independent down to −269 ◦C, showing that the usual scattering of phonons and ionized impurities play a minor role in these samples. Such temperature-independent transport properties are ascribed to the dominant grain boundary scattering, where the interior of the grains is degenerately doped, i.e., the Fermi level is positioned above the conduction band minimum, and due to high dopant concentration, the electrostatic barriers at the grain boundaries are negligible. However, for our samples, there was no obvious correlation between the grain size and the carrier mobility, as the sample with the biggest carrier mobility had medium size grains, while the sample with the smallest carrier mobility had the biggest grains. The highest charge carrier mobility was found in the sample with the largest (200) texture coefficient of crystallites, which is consistent with the results published in Ref. [27]. More systematic studies are needed to separate the influence of grain boundary scattering and scattering on the preferred orientation crystallites and to optimize the transport properties of SnO2 thin films prepared by APCVD.

**Author Contributions:** K.J.: sample preparation, SEM, GIXRD experiments and data analysis, writing, original draft preparation, visualization; D.G.: conceptualization, samples preparation; J.R.P. and A.H.: GIXRD experiment; Z.S.: TOF-ERDA experiment and data analysis; M.C. and Ž.R.: magnetotransport probe experiment, analysis and ˇ discussion, draft writing; L.P.: impedance spectroscopy experiment and data analysis; M.B.: SEM experiments and data analysis. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by European Regional Development Fund (ERDF) under the (IRI) project "Improvement of solar cells and modules through research and development" (grant number KK.01.2.1.01.0115) and Croatian Science Foundation Project No. IP-2018-01-5246 and No. IP-2013-11-1011 (magnetotransport experiment).

**Acknowledgments:** We acknowledge Elettra Sincrotrone Trieste for providing access to its synchrotron radiation facilities for GIXRD experiment (MCX beamline). M. Culo and Ž. Rapljenovi´ ˇ c thank Ð. Drobac for cutting the samples for Hall bar geometry and B. Radatovi´c for providing a sample holder and helping with high temperature measurements used to verify temperature trend of resistivity above the room temperature.

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