The role of the fluorine concentration and substrate temperature on the electrical, optical, morphological and structural properties of chemically sprayed ZnO:F thin films

The effect of both the fluorine/zinc ratio, [F]/[Zn], in the starting solution and the substrate temperature (TS) on the transport, structural and optical properties of chemically sprayed fluorine-doped zinc oxide (ZnO:F) thin films is reported. An important decrease in the resistivity (on the order of 1.2×10−2 Ωcm) is observed in films deposited at 500 °C, from a starting solution with a [F]/[Zn] ratio as low as 10 at.%. In order to obtain such decrease, an adequate combination of, an excess in acetic acid, the addition of hydrofluoric acid and an aging process of 9 days of the solution were required. A minimum resistivity was obtained in films deposited with a [F]/[Zn] of 20 at.%, beyond this value the films obtained show higher resistivity values. As the [F]/[Zn] ratio in the solution increases, the grain size decreases, as observed by scanning electron microscopy. A (002) preferential growth is observed in films deposited with [F]/[Zn] ratios in solution lower than 20 at.%, whereas a switching from (002) to (101) is observed in the films deposited with the highest [F]/[Zn] ratio. A typical transmittance higher than 85% and a band gap (Eg) of 3.3 eV were obtained in the films deposited at the optimum conditions.