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Semiconducting copper oxide (CuO) gas sensing layers show a remarkable conductance behavior if exposed to hydrogen sulfide (H2S) gas at low operating temperature (180 °C). At first conductance decreases as expected for a p-type semiconducting metal oxide offering reducing test gas. After a certain exposure time, however, a sudden steep increase in conductance can be observed. In a first approach this behavior is explained by the formation of metallic conducting copper sulfide (CuS, degenerate p-type semiconductor) clusters which eventually form conducting pathways across the sensing layers short-circuiting the remaining CuO phase. In the field of statistical physics such behavior can be described by the so-called percolation theory. Here we present a detailed experimental and theoretical analysis of the observed effect utilizing RF-magnetron-sputtered copper oxide films with different stoichiometry (CuO, Cu4O3 and Cu2O) as model systems. The layers are exposed to H2S for different time spans and analyzed with respect to their morphology (SEM, XRD) and chemical composition (XPS, ToF-SIMS). Analysis of the transient behavior of the conductance by means of a percolation model and comparison of the results to the experimental data allow the identification of different processes. For CuO samples first the formation of different non-CuS copper–sulfur–oxygen phases is observed followed by the percolation regime with the steep conductance increase. Afterwards diffusion processes superimposing the percolation leading to a slower conductance increase and eventually the process is dominated by diffusion of copper ions from the bulk. For oxides with other stoichiometry (Cu2O, Cu4O3) no percolation regime is observed which is attributed to higher diffusion rate of copper ions weakening the percolation effect in these samples. Based on these observations a model for the electronic conductance behavior of copper oxide gas sensors under exposure to hydrogen sulfide (H2S) at temperatures below 200 °C is proposed. A better understanding of these systems will enable the preparation of reliable sensors with inherent thresholds.

We deposited copper oxides by rf magnetron sputtering from a 4N Cu-target at room temperature, varying the oxygen flux and keeping the argon flow constant. Dependent on the oxygen flux Cu2O, Cu4O3or CuO were synthesized. The different compounds were characterized by XRD. The dielectric functions of the oxides were determined by spectroscopic ellipsometry and show significant differences between the compounds. The electrical properties, like the carrier concentration, of each compound can be tuned by adjusting the oxygen flux. We discuss the structural, optical and electrical properties of the copper oxides in terms of phase purity and stoichiometry deviations.

Secondary ion mass spectrometry (SIMS) is a technically matured analysis technique for the investigation of depth and lateral distributions in solids. The “raw data” of a SIMS measurement provides only qualitative information. For quantification so-called relative sensitivity factors (RSF) are mandatory. To our knowledge no RSFs have been determined for Cu2O and CuO so far. In this work the RSFs for 21 elements in Cu2O and 14 elements in CuO have been determined via ion implanted standards. For the RSF determination we present the plateau method (Section 3) for box-like implantation profiles. This method provides a lower uncertainty in RSF value compared to the obtained uncertainty using single implantation profiles. In addition, we have estimated the electron affinities of NO2, AsO2 and SbO to 0.55, 2.7 and 2.85 eV, respectively. Unexpectedly, we observe for Cu2O and CuO nearly identical RSF values. This behaviour cannot be attributed to a change in chemical composition caused by the SIMS sputter process. Furthermore, the calculated RSFs have been used to determine the impurity concentrations of our sputtered copper oxide thin films.

The oxygen conductor yttria-stabilized-zirconia (YSZ) is widely used in miniaturized solid oxide fuel cells (µSOFC) and may be suitable for solid state ion emitter applications e.g. as miniaturized ion engines for electric propulsion. Since the YSZ films are not completely free of stress during the growth, cracks in fabricated free-standing membranes are often observed.YSZ thin films were deposited on silicon substrates by radio frequency sputtering. Free-standing YSZ membranes were fabricated by partially removing the Si substrate by anisotropic wet-chemical etching using different masking patterns defined by electron beam lithography. We show how different sizes and etching conditions influence the strain in the fabricated membranes. To characterize these membranes we used optical microscopy and scanning electron microscopy.

The physics of cuprous oxide (Cu2O) is reviewed with respect to structural, optical, and electrical properties. New experimental findings are included especially in the context of theoretical band-structure calculations. Intrinsic defects related to nonstoichiometry are made responsible for the p-type conduction. First preliminary experiments on the alloying of Cu2O by sulfur are reported.

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.