2, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Mexico City, Mexico City, Mexico
Cu2O is a p-type semiconductor that has promising properties for solar cells, lithium ion batteries, emitting diodes, photo-catalyst for hydrogen production, photo-catalyst for water and air decontamination, sensor for organic molecules, among others. In this work, the conversion of CuO thin films to Cu2O ones, in less than 30 s via an argon/dry-air microwave plasma, is studied. The process is carried out in a lab-made equipment built with low-cost components, such as a commercial microwave oven . Our simple, but reliable processing, is faster, cheaper and easier than other similar plasma treatments (watch video, https://goo.gl/yS7Y1h), this could allow for its application at mass scale production of Cu2O films.
CuO film targets are easily produced by dip-coating on glass substrates from a homogeneous copper acetate solution. CuO targets are annealed in groups at different temperatures, from 350 °C to 550 °C in increments of 50 °C, in open atmosphere. To obtain Cu2O, CuO thin films are treated for 15, 20, 25 or 30 s, under an argon/dry-air microwave plasma. The plasma processing is performed inside a quartz chamber at low vacuum (15 mbar), with small gas fluxes of argon (6 sccm) and dry-air (6 sscm), and with a microwave power of 1500 W.
Interestingly, pure Cu2O is only produced from a metastable form of metallic copper that is obtained after the plasma treatment. The conversion process can take from a couple of minutes, under a controlled flow of oxygen, to some hours, in open atmosphere. This partial oxidation of metallic copper is clearly driven by the oxygen availability right after the plasma treatment, when the sample is still hot. To our knowledge, this phenomenon has not been reported before.
To study the effect of oxygen, argon and dry-air fluxes are shut down immediately after the plasma treatment and a controlled flow of pure oxygen is supplied for 10 minutes. When no oxygen is used, the resulting film looks dark green; XRD patterns show metallic copper as the majority phase. Once pure oxygen is supplied after the treatment, the film acquires a lighter green color; for these films XRD patterns show a mixture of Cu and Cu2O. Pure Cu2O is achieved once a certain minimum amount of oxygen is supplied. The transformative role of oxygen has a profound effect not only in the crystalline phase of the film, but also in its optical and electrical properties. This phenomenon is being studied with the aim of tailoring some of the properties of pure Cu2O.
Depending on different process conditions, like time of plasma, the crystallite size of Cu2O can be increased and controlled. Nonetheless, there is also an optimal annealing temperature, around 400 °C, for which crystallite size can be maximized. By treating the CuO samples for a longer time, bandgap can be decreased from 2.35 to 2.17 eV. The advantages of our plasma processing lie in the simplicity, short time of treatment and, low cost of the built equipment.