Effect of doping concentration, solvent proportions and solution aging on the figure of merit of chemically sprayed ZnO:F thin films

Undoped zinc oxide (ZnO) and fluorine doped zinc oxide (ZnO:F) thin films were deposited on sodalime glass substrates by the ultrasonic chemical spray technique. The effect of the relevant deposition variables, namely, doping concentration, solvent proportions and solution aging on the Haacke´s Figure of Merit, φ_M, of ZnO:F thin films, is reported for the first time. φ_M was evaluated from the optical and electrical characteristics of the films. A wide range of F/Zn doping from 5 to 90 at%, in the starting solution, was used due to the high volatility of F during the deposition process and the concomitant poor incorporation into the ZnO lattice. In order to have F-doped solutions with high chemical stability, a mix of acetic acid:water:methanol, at different volume proportions, were tested as solvent; nevertheless the φ_M magnitudes remain unchanged, irrespective of solvent proportions. Regarding solution aging, despite it is a deposition variable that no plays any role in similar wide band gap semiconductor oxides, in the case of ZnO:F films, φ_M was significantly influenced through the electrical transport properties. Complementary information on structural and morphological characteristics of deposited ZnO:F thin films, for films with high φ_M, is also reported. From the results, optimization of deposition variables was achieved, as highly conductive and transparent ZnO:F thin films with a maximum, φ_M = 7.57 × 10⁻³ (Ω/□)⁻¹, were deposited with a doping concentration of 30 at%, solvent proportion of 50:50:900, and an aging of 37 days. The obtained results show that ZnO:F thin films are potential candidates for transparent conductive oxide applications.