Elsevier

Vacuum

Volume 144, October 2017, Pages 217-228
Vacuum

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

https://doi.org/10.1016/j.vacuum.2017.08.005Get rights and content

Highlights

  • Microwave-activated reactive sputtering found to be inexpensive method to produce copper oxide thin films.

  • Detailed characterisation of the sputtered films is possible with the aid of XPS, XRD, optical and Raman measurements.

  • Controlled deposition of mixtures of various phases of Cu2O, Cu4O3, CuO can be determined by oxygen flow rate.

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.

Introduction

The facile and inexpensive production of large areas of copper oxide thin films is desirable because of the very many potential applications. For example, it has been noted that the development of new applications of transparent electronics such as displays and solar cells with improved efficiency and reduced costs requires inexpensive materials for both n-type and p-type semiconductors along with lower energy production methods [1], [2]. One of the main factors for the optimal use of copper oxides in particular applications is the control of the deposition process [3].

Cuprous oxide (Cu2O) and cupric oxide (CuO), are attractive material choices for transparent electronics because copper is an inexpensive and abundant metal, and the oxides are natural p-type semiconductors with a direct band gap [4], [5], [6]. Both oxides may be useful for solar cells, although CuO has higher absorbance over a wider wavelength range than Cu2O. On the other hand, for transparent electronics Cu2O has the advantage of greater transparency in the visible spectrum. The two oxides have distinctly different properties, with Cu2O being a yellow/red colour and CuO being a much darker brown/black colour in thin film form due to differences in the band gap and dispersion of the extinction coefficient over the visible/near infrared region of the spectrum [7]. Bandgaps in the range of 2.0–2.6 eV have been reported for Cu2O films [8], while nominally CuO films are typically in the lower range of 1.2–1.6 eV [9]. The rather wide ranges reported for material and electrical properties reflect the fact that films produced by many of the proposed fabrication methods invariably contain varying mixtures of copper and copper oxide phases and are rarely pure forms of the oxides. This was demonstrated by Drobny & Pulfrey [10], who found that reactive sputtering of copper oxide at higher oxygen partial pressures deposited more oxygen rich phases: Cu2O + Cu at lower partial pressures, then Cu2O only, then Cu2O + CuO, and then finally CuO only at higher partial pressures.

However, since the early 2000s, it has been recognised that sputtering can in fact result in three oxides, with Cu4O3 being readily formed in addition to CuO and Cu2O and many earlier papers have failed to recognise this fact [11], [12]. Cu4O3, known as paramelaconite in mineral form, is itself an interesting material, with potential use in catalysis [13], [14], [15].

Ogwu et al. [16], found that the sheet resistance of copper oxide thin films prepared by reactive magnetron sputtering increased with increased O2 flow rates during production. Therefore, it can be assumed that the presence of the more oxygen rich phases of copper oxide result in greater resistivity. Another very promising application area, for Cu2O thin films, is in memristors [17], the fourth passive circuit element (after resistors, capacitors and inductors) [18].

For the case of solar cells, it is usual to combine copper oxides with an n-type semiconductor film such as ZnO to fabricate p-n heterojunctions [19], [20]. Solar cells produced from copper oxide thin film have a theoretical efficiency of approximately 20% [21], [22]; however, the best achieved efficiency so far is 6% [23].

There are numerous methods to produce copper oxide thin films, such as thermal oxidation [24], electrodeposition [9], chemical brightening [25], spraying [8], chemical vapour deposition [26], plasma evaporation [8], vacuum evaporation [27], molecular beam epitaxy [28], reactive sputtering [10], [9], [25]. All noted methods produce a mixture of phases of Cu, CuO, and Cu2O [29]. Balamurugan and Mehta [8] used the activated reactive evaporation technique and varied the nanocrystalline structure by varying the deposition parameters. The crystallinity was then analysed by x-ray diffractometer (XRD). The results showed that a single phase of Cu2O could be deposited at relatively low substrate temperatures using this technique.

Papadimitropoulos et al. [27] grew copper oxide layers by vacuum evaporation of copper onto silicon substrates in a nitrogen-oxygen atmosphere at temperatures between 185C and 450C. The Tauc-Lorentz model was successfully used to extract the refractive indices of the films. Li et al. (2011) used a variation of sputtering known as HITUS and showed that single phase Cu2O could only be prepared within a very narrow range of deposition parameters.

The consensus of previous work is that copper oxides are potentially very useful materials but are particularly difficult to grow as a pure phase, free of contamination from other phases. While oxygen flow rate is the most important parameter, it appears that other process parameters also need careful control. This suggests that commercial applications requiring pure phases to be deposited reproducibly on large areas may be difficult to achieve, especially when cost and substrate factors require room temperature deposition without further processing steps such as annealing.

This paper focuses on a scalable deposition method with proven commercial success, although not yet in widespread use, known as microwave-activated reactive sputtering (MARS). This method uses a pulsed DC power supply for sputtering plus a separate microwave source in a rotating drum configuration to achieve large area, room temperature deposition of a wide variety of metal oxides, nitrides and oxynitrides [30], [31]. Excellent control of deposition parameters allows deposition of high performance optical filters [32], including rugate-type filters, which require thick films to be grown with continuously variable refractive index [33], [34]. Other advantages of this type of sputtering are that materials with very high melting points can be sputtered easily [35], whereas techniques such as evaporation can be difficult or impossible. Sputtered films usually have better substrate adhesion than evaporated films. Sputtering can also be undertaken at low temperatures in order to avoid damage to the substrate or other layers. Given that a target can be composed of a large amount of material and is maintenance free, the technique is well-suited for ultrahigh vacuum applications.

In this paper copper oxide films have been grown by MARS. With these films of 500 nm thickness it has not been possible to obtain quantitative compositional information with the techniques available but qualitative results from optical spectroscopy, XRD, Raman and XPS are presented in some detail. It is demonstrated that the sputtered films develop through CuO, Cu4O3 and Cu2O mixed phases as oxygen flow rate is increased and can be successfully and reproducibly grown at room temperature. A pure form of Cu2O is only produced in a very narrow range of oxygen flow.

Section snippets

Experimental procedures

In this work, a MARS system (MicroDyn® 40,000) equipped with a 127  mm × 380  mm high-purity copper target (>99.99%), 10 kW DC power supply (Advanced Energy MDX 10 with Spark-le V arc controller) and a 3 kW plasma source was used. This is a turbo-pumped system with an added Polycold 330 water trap and is capable of reaching an operating pressure of ∼10−6 torr in 15 min. The rotating drum in this model rotates about a horizontal axis at 60 revs/min and is capable of holding over 3600 cm2 of

Characterisation

Samples were imaged at various magnifications using a Hitachi S4100 field emission scanning electron microscope (SEM). This system has magnification of 40,000 times, a resolution of 1.5 nm, and acceleration voltage for primary electrons of up to 30 kV. The crystalline structure of the thin films was determined by X-ray diffractometry (XRD) (Siemens D5000) with CuK α radiation (40 kV, 30 mA). The diffraction angle was set between 30° and 50° with 1 scan (count) per second at 0.2 increments.

X-ray

SEM analysis

Fig. 1 shows the SEM images of the thin layers deposited under oxygen flows of 10, 14, and 17sccm revealing their thicknesses and smooth surface topography.

XRD analysis

The XRD results of the samples can be seen in Fig. 2.

The location of the spikes in intensity on the XRD plot at the various 2θ angles of incidence can be related to the material composition and crystalline orientation. The 2θ angle values for each copper composition were taken from JADE5 PDF tables or Blobaum et al. [11] and can be seen in

Conclusions

It is demonstrated that the sputtered films develop through CuO, Cu4O3 and Cu2O mixed phases as oxygen flow rate is increased and can be successfully and reproducibly grown at room temperature. A pure form of Cu2O is only produced under an oxygen flow rate of 10sccm; however, this may be contaminated by Cu metal.

The results indicate that Cu4O3 can be reliably produced by MARS at oxygen flow rates between 11sccm and 16sccm, and possibly at higher oxygen flow rates as well.

For the production of

Acknowledgments

XPS data were acquired at the National EPSRC XPS User's Service at Newcastle University, an EPSRC Mid-Range Facility. This work incorporates data from the Victorian node of the Australian National Fabrication Facility (ANFF), a company established under the National Collaborative Research Infrastructure Strategy to provide nano and microfabrication facilities for researchers in Australia, through the La Trobe University Centre for Materials and Surface Science. Data are reproduced under a

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