H₂S dosimeter with controllable percolation threshold based on semi-conducting copper oxide thin films

Abstract. Copper oxides, such as CuO and Cu₂O, are promising materials for H₂S detection because of the reversible reaction with H₂S to copper sulfides (CuS, Cu₂S). Along with the phase change, the electrical conductance increases by several orders of magnitude. On CuO_x films the H₂S reaction causes the formation of statistically distributed Cu_xS islands. Continuous exposition to H₂S leads to island growth and eventually to the formation of an electrical highly conductive path traversing the entire system: the so-called percolation path. The associated CuO_x ∕ Cu_xS conversion ratio is referred to as the percolation threshold. This pronounced threshold causes a gas concentration dependent switch-like behaviour of the film conductance. However, to utilize this effect for the preparation of CuO-based H₂S sensors, a profound understanding of the operational and morphological parameters influencing the CuS path evolution is needed.

Thus, this article is focused on basic features of H₂S detection by copper oxide films and the influence of structural parameters on the percolation threshold and switching behaviour. In particular, two important factors, namely the stoichiometry of copper oxides (CuO, Cu₂O and Cu₄O₃) and surface morphology, are investigated in detail. CuO_x thin films were synthesized by a radio frequency magnetron sputtering process which allows modification of these parameters. It could be shown that, for instance, the impact on the switching behaviour is dominated by morphology rather than stoichiometry of copper oxide.