Series of CuxO thin-films in the entire range of compositions 1≤x≤2 were obtained by varying the oxygen flux in an rf-sputter deposition process. Growth windows for three crystalline phases, i.e., the thermodynamically stable cuprous oxide Cu2O and cupric oxide CuO as well as the metastable paramelaconite Cu4O3, were observed. The crystalline phases persist non-stoichiometrically over a wide range of compositions. These flux-range windows are separated by ranges where highly disordered, almost amorphous material is obtained. All samples were analysed with respect to their thermoelectric properties, i.e., Seebeck coefficient, electrical, and thermal conductivity. Clear trends of these transport parameters were found and used to determine the thermoelectric figure of merit ZT. The ZT-values at room temperature are highest for the two thermodynamically stable crystalline phases CuO and Cu2O.

Abstract.Copper oxides, such as CuO and Cu 2 O, are promising materials for H 2 S detection because of the reversible reaction with H 2 S to copper sulfides (CuS, Cu 2 S). Along with the phase change, the electrical conductance increases by several orders of magnitude. On CuO x films the H 2 S reaction causes the formation of statistically distributed Cu x S islands. Continuous exposition to H 2 S 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 x S 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 H 2 S 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 H 2 S 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, Cu 2 O and Cu 4 O 3 ) 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.

Polycrystalline Cu2O thin films were prepared on c-sapphire substrates by reactive radio-frequency sputtering at various temperatures between 500 and 925 K employing a metallic target and utilizing an argon/hydrogen/oxygen gas mixture. It is demonstrated that the use of hydrogen in the sputter deposition process beneficially affects the transport properties of the Cu2O films obtained. Correlating the amount of hydrogen incorporated into the thin films, the film morphology and the transport and luminescence properties demonstrate that in this approach hydrogen is predominantly accumulated at the grain boundaries of the polycrystalline films, leading to a lower film resistivity due to the reduction of grain boundary scattering. It is demonstrated that a suitable employment of hydrogen in the growth process of Cu2O material for solar cell applications improves the material properties significantly.

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