We prepare CuGaO 2 thin films on SiO 2 substrates by using the sol-gel spin-coating method with two combinations of Cu and a Ga source, Cu and Ga nitrate, or acetylacetonate. X-ray diffraction analysis reveals that the thin films prepared using nitrate sol that are annealed at a temperature of 850 °C - 950 °C show both c-axis-orientated peaks, (006) and a non- c-axis-oriented peak (012) with similar intensity; little dependence of signal intensity on annealing temperature is also shown. The films are opaque in appearance at these annealing temperatures. Scanning electron microscope observation reveals that the opaque appearance is due to the texture or cracks on the surface of the films. In contrast, the films prepared using acetylacetonate show a (006) peak with higher signal intensity than the (012) peaks. The films show more transparent appearance than that of the films by nitrate. The highest conductivity of the film is 5.7 × 10 -4Ω -1 ·cm -1, obtained in the films by nitrate annealed at 850 °C.
Transparent conductive oxide (TCO) thin films are widely used as a transparent electrode in opt-electric devices [
In the present work, CuGaO2 thin films were prepared by the sol-gel method using two combinations of a copper and gallium metal source that consist of the same counter anion or same complex ligand. One combination is copper nitrate and gallium nitrate that have the same anion, and the other is a combination of copper acetylacetonate and gallium acetylacetonate that have the same complex ligand.
The raw materials for metal nitrate, copper nitrate and gallium nitrate were of analytical grade and purchased from Wako chemicals. Gallium nitrate was supplied as gallium nitrate n-hydrate (n = 7 - 9), with 8 adopted as the n value for calculating molecular weight in the present work. Both copper nitrate trihydrate (2.42 g, 0.01 mol) and gallium nitrate n-hydrate (4.00 g, 0.01 mol) were separately dissolved into 2-methoxyethanol (25 mL) by stirring for 12 hours at room temperature in air. Then, the nitrate sol was obtained by mixing two solutions and stirring for 12 hours at room temperature in air. The reagents for metal acetylacetonate, copper acetylacetonate and gallium acetylacetonate were of analytical grade and purchased from Aldrich. Copper acetylacetonate (2.62 g, 0.01 mol) and gallium acetylacetonate (3.67 g, 0.01 mol) were dissolved separately into a mixture of 2-methoxyethanol (25 mL) and 2-aminoethanol (24.0 g) by stirring for 72 hours at room temperature in air. The acetylacetonate sol was obtained by mixing two solutions and stirring for 12 hours at room temperature in air.
The sols were spin-coated onto SiO2 substrate at a spinning speed of 3000 rpm. The coated films were first heated at 200˚C for 10 min, then, heated again at a higher temperature of 500˚C for 20 min using hot-plate-type heating devices. We used high temperature, 500˚C, in the post-coating heat treatment under strictly temperature control in order to form Cu-delafossite crystalline without Cu2+ [
The structural properties of the films were studied by X-ray diffraction (XRD; D8 Discover, Bruker) analysis in the θ - 2θ mode using CuKα radiation. Transmission spectra were measured using a UV/Vis spectrophotometer (U-3000, Hitachi). The surface morphologies of the films were observed using scanning electron microscopy (SEM; JSN76380LV, JEOL). Conductivity of the films was measured using evaporated Au interdigital electrode with a gap of 0.5 mm.
in the film prepared using nitrate sol shown in
In all the XRD patterns, signals of Ga2O3 were not observed and only a weak signal of CuO was observed as a shoulder of peaks in some of the films. The use of sols containing only the same anion or some complex ligand can be thought to have prevented the formation of by-products. As also observed by XRD measurements, the films with different raw materials had different structures. As a trend, structural properties of the films prepared using acetylacetonate had more significant annealing temperature dependence and more significant c-axis-oriented structure than the films prepared using nitrate sol.
Transmission spectra of CuGaO2 thin films prepared using nitrate sol followed by annealing at 800˚C, 900˚C and 1000˚C are shown in
900˚C was also observed in the films annealed at 850˚C and 950˚C, although the structural properties changed depending on the annealing temperature, as observed in XRD.
Figures 5(a)-(c) shows SEM images of CuGaO2 thin films prepared using metal nitrate annealed at 800˚C, 900˚C and 1000˚C, respectively. The film annealed at 800˚C shows formation of crystalline grains with approximately 0.5 μm diameters
on the surface. At an annealing temperature of 900˚C, textures with a scale of 5 - 10 μm were observed and crystalline grains were not observed. For the film annealed at 1000˚C, 1 - 2 μm square-structured grains were observed with cracks in the boundary of these grains. These results indicate that the surface morphology of the CuGaO2 films prepared using nitrate sol changes drastically depending on the annealing temperature. The crystalline grains observed at an annealing temperature of 800˚C can be thought to become larger with increase in annealing temperature. Then, the surface becomes one with the texture or larger crystalline grains.
Figures 5(d)-(e) shows SEM images of CuGaO2 thin films prepared using metal acetylacetonate annealed at 800˚C, 900˚C and 1000˚C, respectively. The films prepared using acetylacetonate show a relatively smooth surface compared with that in the films prepared by nitrate. At annealing temperatures of 800˚C - 900˚C, formation of crystalline grain is not well observed at the surface of the films. As shown in
In the films prepared using the acetylacetonate sol, the film showed conductivity of 3.8 × 10−6 Ω−1・cm−1 at an annealing temperature of 750˚C, which was higher than that in the film prepared by the nitrate sol at the same annealing temperature. The conductivity of the film increased with annealing temperature as well as in the films prepared by the nitrate sol; however, the highest conductivity obtained by acetylacetonate film was 1.6 × 10−5 Ω−1・cm−1 at the annealing temperature of 850˚C, which is a lower value than that in the film prepared using nitrate. The conductivity increased by less than one order from the annealing temperatures of 750˚C - 850˚C. In addition, decreasing conductivity at higher temperature was less than two orders from 850˚C to 950˚C.
CuGaO2 thin films were prepared by the sol-gel method using two kinds of combination of metal source materials, Cu and Al nitrate, and acetylacetonate. The films showed dependence of the structural, optical and electrical properties not only on the annealing temperature, but also on the metal source materials. In the case of the films prepared by the nitrate sol, XRD signal intensity did not show significant dependency on annealing temperature; however, surface morphology and conductivity showed significant dependency on annealing temperature. In contrast, in the case of the films prepared using the acetylacetonate sol, XRD signal intensity showed dependence on the annealing temperature. However, surface morphology and conductivity showed independency on the annealing temperature. The films prepared using acetylacetonate showed preferable transparency compared with the films prepared using nitrate. Transmittance of the films at wavelengths longer than 400 nm was more than 75% at annealing temperatures of 800˚C - 900˚C. The highest conductivity of 5.7 × 10−4 Ω−1・cm−1 was obtained by the film prepared by the nitrate sol followed by annealing at 850˚C. Because of the use of metal source materials with the same anion or same complex ligand and strict control of heat treatment, by-products were not observed at an annealing temperature higher than 850˚C.
This work has been financially supported by Kyoso Kenkyu Josei in 2017 of Ishinomaki Senshu University.
The author declares no conflicts of interest regarding the publication of this paper.
Ehara, T. (2018) Preparation of CuGaO2 Thin Film by a Sol-Gel Method Using Two Kinds of Metal Source Combination. Journal of Materials Science and Chemical Engineering, 6, 68-78. https://doi.org/10.4236/msce.2018.68006