Investigation of AlF3 doped ZnO thin films prepared by RF magnetron sputtering

Transparent conducting ZnO thin films co-doped with aluminium and fluorine (AZO:F) were produced on glass substrates by RF magnetron sputtering at room temperature. The chemical AlF3 was selected as a dopant, which allowed Al and F elements to be simultaneously substituted into sub-lattices sites of Zn and O, respectively. The microstructure, electrical and optical properties of ZnO thin films as a function of AlF3 concentration were evaluated and compared to the films doped with Al2O3. From XRD analysis, it was revealed that the favoured orientation of ZnO films shifted from (0 0 2) to (1 0 3) as the amount of AlF3 increased from 1 to 7 wt%. The film doped with 2 wt% AlF3 showed improved crystallinity and a microstructure with larger, pyramid-like grains that were 120 nm long and 50 nm wide. As a result, the electrical resistivity of the AZO:F films had a minimum of 5.2 × 10−4 Ω cm. The improvement in the electrical resistivity of AZO:F films was due to the increase in carrier concentration from 7.0 × 1020 to 1.25 × 1021 cm−3 and the mobility from 4.7 to 9.3 cm2 V−1 s−1. Meanwhile, the electrical resistivity of the ZnO film doped with AlF3 was found to be lower than that of the Al2O3-doped film. This was mainly attributed to the increase in carrier concentration by substituting Al and F atoms into the Zn and O sub-lattice sites, respectively, despite the slight decrease in the mobility. This increase in carrier concentration was also found to affect the optical property of the films due to the Moss–Burstein shift.