Characterisation of Cu 2 O, Cu 4 O 3 , and CuO mixed phase thin films produced by microwave-activated reactive sputtering

Abstract

Copper readily forms three oxides, CuO, Cu 4 O 3 and Cu 2 O, 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 largescale 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 Cu 2 O 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. Microwaveactivated 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.

The Phase Evolution and Physical Properties of Binary Copper Oxide Thin Films Prepared by Reactive Magnetron Sputtering

“…The calculated optical E g values can be obtained as 2.51 ± 0.02 eV, 1.65 ± 0.1 eV, and 1.42 ± 0.01 eV for Cu 2 O, Cu 4 O 3 , and CuO, respectively. These are consistent with the previous reported results [2,4,7,12,15,28]. Furthermore, the measured results of the band gap indicate that, although the morphologies of the films under various O 2 partial pressures are different, the band gap value of each type of single-phase copper oxide remains almost constant.…”

“…Raman spectra were also introduced to confirm the film structure and detect the trace impurity. As shown in Figure 1b, all the Raman peaks marked using vertical bars agree well with experiments [15] and with previous calculations [27]. The XRD and Raman results indicate that the three types of Cu 2 O, Cu 4 O 3 , and CuO films can be prepared through magnetron sputtering by only tuning the O 2 flowing rate.…”

“…Although Cu 2 O has the advantage of good transparency in the visible light range, its low carrier concentration or large resistivity leads to poor performances [3,10]. The second oxide phase, Cu 4 O 3 , discovered during the late 1870s [13], is a metastable mixed-valence intermediate compound between Cu 2 O and CuO [1,4,9,14,15]. To date, research about the electronic structure of Cu 4 O 3 has been limited.…”

“…Among those methods, magnetron sputtering at room temperature is desirable for the growth of thin films with good physical properties. Moreover, one can easily deposit the three types of binary copper oxides or their mixed phases by merely tuning the oxygen partial pressure during depositions [9,14,15]. …”

“…Consequentially, the critical parameters for the synthesis of the Cu 4 O 3 metastable phase need insightful exploration. On the other hand, the physical properties of thin films (such as preferred orientation, optical band gaps, mobilities, and carrier concentrations) can also be tuned by changing the oxygen partial pressure during deposition [9,14,15]. The effects during depositions of oxygen chemical potential on the films’ physical properties still need to be investigated further.…”