Copper‐oxide compound semiconductors provide a unique possibility to tune the optical and electronic properties from insulating to metallic conduction, from bandgap energies of 2.1 eV to the infrared at 1.40 eV, i.e., right into the middle of the efficiency maximum for solar‐cell applications. Three distinctly different phases, Cu2O, Cu4O3, and CuO, of this binary semiconductor can be prepared by thin‐film deposition techniques, which differ in the oxidation state of copper. Their material properties as far as they are known by experiment or predicted by theory are reviewed. They are supplemented by new experimental results from thin‐film growth and characterization, both will be critically discussed and summarized. With respect to devices the focus is on solar‐cell performances based on Cu2O. It is demonstrated by photoelectron spectroscopy (XPS) that the heterojunction system p‐Cu2O/n‐AlGaN is much more promising for the application as efficient solar cells than that of p‐Cu2O/n‐ZnO heterojunction devices that have been favored up to now.

Using photoelectron spectroscopy, we investigate the band alignments of the Cu2O/ZnO heterointerface and compare the findings with the corresponding values for Cu2O/GaN. While for Cu2O/ZnO, we find a valence band offset (VBO) of 2.17 eV and a conduction band offset (CBO) of 0.97 eV, both values are considerably reduced for Cu2O/GaN where the numbers are 1.47 eV (VBO) and 0.24 eV (CBO), respectively. The large CBO between ZnO and Cu2O will very likely result in low photovoltaic power conversion efficiencies as is the current status of Cu2O/ZnO solar cells.

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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.

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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 Cu 2 O and CuO so far. In this work the RSFs for 21 elements in Cu 2 O 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 NO 2 , AsO 2 and SbO to 0.55, 2.7 and 2.85 eV, respectively. Unexpectedly, we observe for Cu 2 O 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.

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.