Characterisation of Cu2O, Cu4O3, and CuO mixed phase thin films produced by microwave-activated reactive sputtering

Abstract

Copper readily forms three oxides, CuO, Cu4O3 and Cu2O, widely recognised as the most promising p-type oxides because of their desirable optical and electrical properties and potential use in solar cells, transparent electronics as well as other specialised applications such as electrodes for rechargeable lithium batteries, catalysis and memristors. For large-scale implementation of devices, magnetron sputtering is a practical method of producing metal oxides; however, sputtered copper oxides tend to form as a mixture of the oxides, with Cu2O being particularly difficult to produce reliably in pure form. Here, nanostructured thin films of copper oxides were prepared by a variation on reactive sputtering known as microwave-activated reactive sputtering under various rates of oxygen flow. Microwave-activated reactive sputtering was shown to be a suitable technique for the inexpensive production of large areas of copper oxide thin films at near room temperature, facilitating deposition on a wide variety of substrates including polymers. Furthermore, it was demonstrated that the sputtered films develop through CuO, Cu4O3 and Cu2O mixed phases as oxygen flow rate is increased. The preparation of a given stoichiometry for a particular application can be achieved by varying the flow rate of oxygen during the microwave-activated reactive sputtering process.