Thin films of nitrogen‐doped cuprous oxide (Cu2O:N) have been deposited by means of direct‐current and radio‐frequency sputtering using a metallic copper target and a mixture of argon, oxygen, and nitrogen for generating the plasma. The doping with nitrogen appears to significantly increase the electrical conductivity of the films. All samples exhibit a temperature‐activated transport behavior. It is shown that the activation energy decreases proportionally to the reciprocal distance between nitrogen atoms, which indicates that a constant fraction of nitrogen is most likely substitutionally incorporated on oxygen site in the Cu2O lattice and acts as an acceptor. Nevertheless, Raman measurements suggest that molecular nitrogen can also be found in the samples, bound at different sites inside the bulk and at the surface.
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.
The band alignment of p-Cu2O/n-AlxGa1–xN heterojunction with x up to 0.15 was studied by X-ray photoelectron spectroscopy. The conduction band offset between binary Cu2O and ternary AlxGa1–xN is found to decrease with increasing x. The data suggest that a flatband situation in the conduction band of p-Cu2O/n-AlxGa1–xN heterojunctions can be achieved for x about 0.4, which is an Al-content where n-type doping is still feasible. Thus, n-AlxGa1–xN with x between 0.4 and 0.6 may be a suitable window material for heterojunction solar cells with a p-Cu2O absorber layer. The current-voltage characteristics of the p-Cu2O/n-AlxGa1–xN heterojunctions under illumination confirm the anticipated improvement of the photovoltaic properties with increasing x.
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