Effect of process parameters on the growth and properties of impurity-doped zinc oxide transparent conducting thin films by RF magnetron sputtering

Zinc oxide transparent conducting thin films co-doped with aluminum and ruthenium were grown on polyethylene terephthalate substrates at room temperature using RF magnetron sputtering. The crystal growth and physical properties of the films were investigated with respect to the variation of discharge power density from 1.5 to 6.1 W/cm² and sputtering pressure from 0.13 to 2.0 Pa. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) showed that the films grown with 3.6 W/cm² power density and sputtering pressure of 0.4 Pa had the best crystallinity and larger pyramid-like grains. The optimized electrical resistivity had a lowest measured value of about 9 × 10⁻⁴ Ω cm. The low carrier mobilities of the films (3-8.9 cm² V⁻¹ s⁻¹) have been discussed in terms of what is believed to be the dominant effect of ionized impurity scattering, but in addition chemisorption of oxygen on the film surface and effect of grain boundaries are also thought to be significant. The transmittances of the films in the visible range are greater than 80%, while the optical band gaps are in the order of 3.337-3.382 eV.