Study of nanoscale metal oxide coatings produced by magnetron sputtering of copper followed by heat treatment

The composition and microstructure of nanoscale metal oxide coatings deposited by magnetron RF sputtering copper on the quartz glass with followed heat treatment were investigated. The dependence of the film thickness and the content of oxides CuO and Cu2O in it depending on the temperature and time of heat treatment conditions were obtained. Studies have shown that, depending on the mode of receipt of copper oxide films may be multiphasic and include various oxide phases, predominantly, CuO and Cu2O with different bandgap and p-type conductivity.


Introduction
Copper oxides are widely used in solid state electronics and optoelectronics [1]. One important advantage of using the copper oxide lies in non-toxicity and widely availability [2]. Various methods for the obtaining of copper oxide, such as thermal oxidation, a reactive high-frequency magnetron sputtering, hydrothermal synthesis, electrodeposition, thermal evaporation, and others are known [3]. It is also known that copper oxide has two phases with different stoichiometry, Cu 2 O and CuO [3]. Interest in the study of copper oxide films of different stoichiometries connected with the possibility of their use in solar cells [4,5]. Application of this type of photovoltaic constrained insufficient knowledge of the physical processes at the interface of oxide structures with developed surface morphology.
Theoretical efficiency conversion of solar energy into electrical energy for copper oxide is 9-12 %, but still the experimental oxide thin film heterostructures CuO x /ZnO, obtained by different research groups [4][5][6][7][8], have low efficiency. In addition to the n-ZnO/p-CuO heterojunctions, the heterostructures of copper oxide with PZT are also of interest for photovoltaic applications [9] due to the ferroelectric photovoltaic effect and significant electric fields at the interface of the PZT and different semiconductors [9][10][11][12][13]. Thus, copper oxide can be considered as a promising photosensitive layer in combination with ZnO or PZT. The main method of increasing the efficiency of solar cells based on copper oxide is to improve the quality of the oxide layers, which requires a detailed examination and developing of processes for obtaining oxide layers. This is a continuation study for the works [3] about development and investigation of thin-film layers of copper oxides for photovoltaic applications. As a convenient method of depositing layers, we use magnetron sputtering.

Methods of obtaining and studying samples
On a purified quartz and glass substrates have deposited at room temperature copper thin film, 100 nm thick by magnetron sputtering on installation Plasmalab System 100 (at a pressure of 5 mbar and a charge power of 100 mW). Substrates were cut mechanically into rectangular samples of size 10x20 mm. Then, the samples were placed in electric furnace PT-1,2-70 and have suffered thermal oxidation in air at temperatures ranging from 220 to 500 C and the time (t) from 5 to 60 minutes.
Evaluation of the film thickness were performed at the facility AlphaStep D-120. The values of film thickness by temperature and annealing time of copper film are shown in table 1. As seen from the profilometry data, metal film 100 nm thick is fully oxidized at 500 C for 30 minutes. The prepared samples were investigated by Raman spectroscopy, X-ray analysis and atomic force microscopy. Optical and electrical properties have been studied. Survey of the phase composition of the films by Raman spectroscopy performed with the setup LamRam HR800 using green laser with wavelength 532 nm, 2 mW beam power, accumulation time 30 seconds and the number of scans 10. The surface morphology was studying by the atomic force microscope "Integra Thermo" with a resolution of 10 nm in the horizontal plane, and the vertical better than 1 nm. The optical properties of the films were studied by Varian Cary® 50 UV-Vis Spectrophotometer. Measurements of the changing electrophysical parameters were carried out on the installation ECOPIA HMS-5000.

Results and discussion
Studying the phase composition of the films formed by oxidation at various temperatures of nanoscale copper layers on the air, carried out independently in two waysby Raman spectroscopy and X-ray analysis (figure 1). Analyzing of Raman spectra identified lines Cu 2 O (98 cm -1 , 145 cm -1 , 217 cm -1 ) and CuO (296 cm -1 , 342 cm -1 , 627 cm -1 ). Results showed that structure of the copper oxide film varies depending on the temperature and time of annealing in air. Therefore, Cu 2 O phase formed by isochronous annealing for 30 minutes at temperatures of 220-250 °C. With further increasing the temperature to 350 °C is obtained the film as a mixture phases Cu 2 O and CuO, but at a temperature above 500 °C formed a single phase CuO.
X-ray analysis allowed to draw similar conclusions about the change in the phase composition of the copper oxide films with increasing annealing temperature. In a manner, by varying the annealing temperature in the range of 220 -500°C we are able to manage phase composition and to obtain copper oxide as single-phase CuO or Cu 2 O and multiphasic Cu 2 O + CuO films.
Morphology of obtained surface layers were examined by atomic force microscopy (AFM) in tapping mode on a Nano-lab NTEGRA-Therma. Analysis of the data showed particles ranging in size from 100 to 300 nm are evenly distributed on the sample surface. The surface of obtained layers rough, while the scope of heights at the characteristic surface topography length of 10 microns. For samples subjected to thermo processing at higher temperatures, this scope is larger. Investigation  According to the theory, in the process of annealing molecular oxygen at atmospheric pressure depending on the processing temperature may oxidize copper fully, partially or in two stages: Cu + 0,5O 2 ↔ CuO; 2Cu + 0,5O 2 ↔ Cu 2 O; Cu 2 O + 0,5O 2 ↔ 2CuO. Due to the polycrystalline structure of copper, according to [9], the rate of oxidation of copper with the bulk diffusion of deposits in the lattice and grain boundary diffusion can be expressed as where h m and h 0maximal and the initial film thickness of the copper oxide, t -time, D -the effective diffusion coefficient.
Study optical properties of the films obtained at temperatures of 220-250 °C showed that samples have high transmittance (50%). Whereas the films obtained at temperatures of 350-500 °C have a high absorption coefficient in the visible spectrum and substantially transparent in the infrared spectrum (figure 2). By transmission and absorption spectra were calculated band gap of 1.2 and 2.28 eV for CuO and Cu 2 O respectively. Based on the differences in transmission for samples more promising for use in photovoltaics is Cu 2 O. Investigation of electrophysical parameters showed that the samples have p-type conductivity and high surface resistance.

Conclusions
As a result of experimental studies were obtained dependences of the oxide film thickness and the percentage content of CuO and Cu 2 O oxides upon the annealing temperature-time conditions. Thus, at isochronous annealing for 30 minutes at temperatures of 220-250 °C formed Cu 2 O, by further increasing the temperature to 350 °C obtained mixture phase Cu 2 O and CuO and at a temperature above 500 °C formed a single phase CuO.