Influence of hydrogen content and sputtering characteristics on the properties of ZnO:Al transparent conductive layers deposited on polymer substrate

ZnO:Al layers were deposited on a 100 μm thick PET (polyethylene terephthalate) film by magnetron sputtering of a ZnO/Al2O3 2 wt.% planar ceramic target in an Ar/H2 atmosphere at 0.3–0.6 Pa pressure. RF (13.56 MHz), DC and Pulsed DC power sources were used separately. The PET film was fixed on a rotating water-cooled drum. The investigation shows that the properties of ZnO layers depend mainly on the type of power source, the power level applied to the target and the concentration of hydrogen in the gas mixture. For all types of sources, the resistivity of the ZnO layers decreased when the sputtering power was reduced. Adding H2 to the Ar gas initially caused a relatively rapid decrease of the resistivity of the ZnO layers. The optimum concentrations of hydrogen in the gas mixture were approximately 1 vol.% and (10–12) vol.% for the RF- and DC-sources, respectively. Further increasing the hydrogen content in the gas mixture led to a slow increase of the resistivity. Unbalancing the magnetron allowed an additional reduction of the resistivity of the Al and H co-doped ZnO films. As a result, ZnO:Al layers with resistivities of 7.0 × 10− 4 Ω.cm (RF-source) and 1.2 × 10− 3 Ω.cm (DC-source) were obtained on the PET film. The best 350–400 nm thick layers had an optical transmittance of 82–83% for λ of 360–600 nm and a surface resistivity of 18–16 Ω/sq. The layers had a wurtzite structure, with the preferred c-axis orientation normal to the surface of the film. They were crack-free and had good adhesion to the substrate.