Highly Conductive Zinc Oxide Based Transparent Conductive Oxide Films Prepared Using RF Plasma Sputtering Under Reducing Atmosphere

Abstract

The spectral properties and colour functions of a radio frequency (RF)-based sputtering plasma source was monitored during consecutive sputter deposition of zinc doped indium oxide (IZO) thin films under argon and argon/hydrogen mix. The effect of target exposure to the hydrogen gas on charge density/mobility and spectral transmittance of the deposited films was investigated. We demonstrate that consecutive exposure to the hydrogen gas during the deposition process progressively affects the properties of thin films with a certain degree of continuous improvement in electrical conductivity while demonstrating that reverting to only argon from argon/ hydrogen mix follows a complex pathway, which has not been reported previously in such detail to our knowledge. We then demonstrate that this effect can be used to prepare highly conductive zinc oxide thin films without indium presence and as such eliminating the need for the expensive indium addition. We shall demonstrate that complexity observed in emission spectra can be simply identified by monitoring the colour of the plasma through its colour functions, making this technique a simple real-time monitoring method for the deposition process.

Hypothesis on the Influence of the Magnetic Behaviour of Hydrogen Doped Zinc Oxide during Its Plasma Sputtering Process

“…As can be seen from the results presented in Figure 4, the electrical properties of the deposited films significantly varied for ZnO, depending on the position for the substrate, while for IZO the variation was significantly less elaborated (comparing the associated edge and centre samples). In a previous study, we demonstrated the enhanced electrical properties of the IZO when deposited under the argon-hydrogen mix [29]. These results combined with the observations presented in Figure 3 suggest that one way to explain this is to assume that more hydrogen doping is occurring at the edge of the substrate compared to the centre, unless the thickness of the coating was causing this observation.…”

“…However, such reports only exist at the research state, and no commercial TCO based on pure ZnO doped with hydrogen is present in the market to the best of our knowledge. We recently reported on enhancing the electrical properties of IZO and ZnO systems by repeated exposure of the sputtering target to an argon/hydrogen mixture as the working gas [29]. We have now identified interesting complexities associated with reactive hydrogen/argon sputtering of un-doped ZnO targets, which we believe are associated with the reported magnetic properties of hydrogenated ZnO particles, and this report outlines our observations.…”

“…The ZnO coated substrates were tested for their electrical properties via a four-point probe and Hall effect measurement equipment. In this report, no study was carried out on the crystal structure of the deposited thin films; however, in a previous report where we examined similar samples of zinc oxide thin films deposited in the presence of hydrogen, the ZnO films were fully crystalline, showing the hexagonal ZnO zincite phase (PDF 01-075-1533) with a (101) preferential orientation [29].…”

Langmuir Probe Technique for Plasma Characterization during Pulsed Laser Deposition Process

“…In the recent years, through the work of Giuffredi et al [30,31] and Fominski et al [32], it was shown that plasma monitoring with ion probes can be a successful approach for in-situ analysis during formation of complex films. The real-time monitoring during the deposition process was extended to other techniques like magnetron sputtering [33,34], PECVD [35] or plasma spray [36,37].…”