Role of Ar/O2 mixture on structural, compositional and optical properties of thin copper oxide films deposited by DC magnetron sputtering
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In this study, the effect of oxygen content on a thin copper oxide layer deposited on BK7 and steel substrates by DC magnetron sputtering were investigated. Argon as working gas with impurity of 99.9% and various oxygen ratios were used to sputter a pure Cu cathode target in a cylindrical geometry. The produced samples were analyzed by X-ray diffraction (XRD), energy-dispersive X-ray (EDX), atomic force microscopy (AFM), and spectrophotometry techniques. The films thickness was measured by profilometer facility. The results show that by increasing oxygen content in the working gas the sputtering rate reduces. Moreover, the type of oxide phase (Cu2O or CuO) in the synthesized layer and consequently its optical properties dramatically depend on Ar/O2 ratio in the working gas.
KeywordsCopper oxide Thin films Magnetron sputtering Optical properties
Copper oxide as a thin film is an attractive material for various applications such as semiconductors, sensors, smart windows and photovoltaic devices [1, 2, 3]. Low-cost production abundant availability and nontoxic nature are some attractive features of this material. Thus, investigation of copper oxide films has both scientific and technological significance. It is important to know that the preparation technique can define the electrical and optical properties of the films . Various methods such as evaporation [5, 6, 7], chemical vapor deposition , sputtering [9, 10, 11, 12, 13], thermal oxidation , sol–gel process , were used to produce the copper oxide film. The advantages of sputtering method are its flexibility, scalability, complexity of possible films, reproducibility, ability to work at low temperature, capability to deposit precisely controlled hetero structures, and simplicity of controlling the film composition, structure and morphology. The DC magnetron sputtering technique in the reactive mode is one of the ways to synthesize copper oxide thin films due to the excellent uniformity and controllability.
In this paper, the role of oxygen content added to the argon as working gas (WG) on the structural, compositional and optical properties of copper oxide film during magnetron sputtering process are reported. The use of a premixed WG (Ar + O2), cylindrical geometry of the magnetron sputtering system, simultaneous applying of two substrates (i.e., steel and transparent Bk7 glass), and investigation of morphology and thickness alongside other properties of the deposited films are some of the new features of the current research.
The condition of sputtering process
Initial pressure (torr)
Working pressure (torr)
Deposition time (min)
Magnetic field (G)
1 × 10−5
1.5 × 10−2
The sputtering process for BK7 and steel substrates were simultaneously performed for each sputtering regime. Transparent substrate (BK7) was used to obtain optical properties (i.e. transmission). The films thickness was measured after deposition process with a Surface Profile Meter, Bruker Dektak XT profilometer. To measure the standard error, the thickness of the deposited layer obtained at seven different points of the samples. The structure of the films were characterized by X-ray diffraction using a STOE model STADI MP diffractometer which uses Cu kα (λ = 0.154 nm) radiation. The surface morphologies were observed by Atomic Force Microscopy, Park Scientific Instrument, Auto probe CP. The optical transmission and reflection measurements in the range of 175–2500 nm were performed using a UV–Vis–NIR Spectrophotometer, Varian, Cary 500. Energy-dispersive X-ray (EDX) spectroscopy was carried out with AMETEK EDAX analyzer.
Results and discussion
XRD and EDX results
EDX analysis of the films with various oxygen contents in the WG
Concentration (at. %) with 5% O2 in WG
Concentration (at. %) with 20% O2 in WG
Therefore, the controlling the phase composition in the deposited layer is possible by controlling the amount of O2 in the WG of the sputtering process.
Although it is a difficult task to find a relationship between structures of the surface and amount of O2 in the WG, but it can be connected to the creation of the new compositional phase (Cu, Cu2O or CuO). Comparing Fig. 5b with c and d, it can be suggested that the decrease in the grain size at higher oxygen ratios (10 and 20%) may be related to the smaller crystallites of CuO phase to Cu2O phase.
Therefore, the presence of O2 and its amount in the WG of the sputtering process not only change the composition of the sputtered film but also change the structure and morphology of the film.
Copper oxides (Cu2O and CuO) thin layers were produced by dc magnetron sputtering technique in a mixture of Ar and O2 gases and various oxygen contents. The increase in the amount of oxygen in the WG led to the decrease of the deposition rate and formation of the oxide phases from Cu2O to CuO. The results showed that by increasing the amount of oxygen in the WG, the optical band gap enhanced. The optical band gap energies of 2.10 and 2.26 eV obtained for 10 and 20% oxygen in the WG. Therefore, it can be concluded that by adjusting the amount of oxygen in the WG of the sputtering process the phase composition, optical behavior, and structure of the copper thin films can be controlled.
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