Delafossite structured p-type wide bandgap semiconductor, CuYO2 thin films were prepared on SiO2 substrate by sol-gel method using copper (II) acetate and yttrium (III) acetate as source materials. The films preparation process was studied by varying annealing temperature after the preparation of gel films by spin coating, followed by thermal annealing at higher temperature. In the present work, one step annealing directly from Cu-Y-gel under nitrogen flow was used. X-ray diffraction (XRD) revealed that the film annealed at 800。C is significantly c-axis oriented, shows only (002) and (004) peaks at 15.6。 and 31.5。, respectively. The optical bandgap of 3.7 - 3.8 eV is estimated by (αhν)2 plot which is higher than previous works. In addition, the films with highly c-axis orientation showed photoluminescence (PL) at room temperature with very broad peak at 2.3 eV. The films annealed at different temperature showed different structural properties.
Transparent conductive oxides (TCOs) are widely used as materials in a variety of optoelectronic applications. Although some TCOs have already been produced for applications, most of them have n-type conductivity such as indium tin oxide  and Al-doped ZnO . Thus, p-type TCOs are required to develop a variety of optoelectronic applications employing p-n junctions. Cu-delafossite structure wide ban gap semiconductor materials have recently attracted much attention because the discovery of p-type conductivity of CuAlO2 .
CuYO2 is one of p-type TCO having delafossite structure, which has been prepared by some methods. For example, solid phase reaction of CuO and Y2O3 though Cu2Y2O5  and so-gel method using copper nitrate and yttrium nitrate as metal sources, employing air and nitrogen-flow two steps annealing  have been reported. The CuYO2 films have displayed green PL properties that have a peak at 540 nm (2.3 eV). The result suggests that the CuYO2 have a possibility to be a material for transparent light emitting devices.
In the present work, we describe the preparation of delafossite type CuYO2 thin films by the sol-gel method, in which copper acetate and yttrium acetate were used as metal sources. The gel films were annealed by one step annealing under nitrogen flow, not by two steps as described above. The dependence of the crystal structure, especially the c-axis orientation on annealing temperature is discussed.
CuYO2 thin films were prepared on SiO2 substrates by the sol-gel method. Prior to deposition of the thin films, the substrates were degreased by ultrasonication in EtOH. Copper (II) acetate monohydrate (Wako Chemicals) was dissolved in a mixture of 2-methoxyethanol and 2-aminoethanol by stirring for 12 h at room temperature. The molar ratio of 2-aminoethanol, chelating agent, to copper acetate was maintained at 4:1 and the color of the solution was dark blue. Yttrium acetate tetrahydrate (Wako Chemicals) was dissolved in a mixture of 2-me- thoxyethanol and 2-aminoethanol by stirring for 12 h at room temperature. The molar ratio of 2-aminoethanol to aluminum acetate basic was maintained at 2:1. After stirring, a colorless homogeneous solution was obtained. The two solutions were mixed with a Cu/Y ratio of 1:1 and stirred at room temperature for 12 h to form a sol. The sol was with total metal ion concentrations of 0.40 M. The sol was spin-coated onto a SiO2 substrate with spinning speed of 3000 rpm for 5 s. In the case of the samples prepared for transmission spectroscopy measurements, the sol adsorbed on the back side of the substrate was carefully removed after spin-coating. The coated films were first heated at 200˚C for 10 min, and then heated again at a higher temperature of 500˚C for 20 min using hot-plate-type heating devices. The spin-coating and subsequent heat treatment procedures were repeated for 6 times to obtain the desired film thickness of 0.4 μm. The prepared gel films were finally annealed at temperatures in the range of 750˚C - 900˚C for 10 h under nitrogen flow. The temperature was increased from room temperature to the specific temperature over a period of 3 h, held at the specific temperature for 10 h, and then cooled to room temperature over 6 h.
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). PL spectrum was measured using He-Cd laser (325 nm, 3.8 eV) for excitation.
3. Results and Discussion
Figure 1 shows XRD patterns for the films prepared using spin-coated gel films annealed at temperatures in the range of 750˚C - 900˚C. The broad signal observed at around 22˚ is due to the amorphous SiO2 substrate. The
Figure 1.XRD patterns for thin films annealed at temperatures in the range of 750˚C - 900˚C.
XRD patterns changed depending on the annealing temperature significantly at every increasing of 50 K. For the film annealed at 750˚C, XRD pattern has three kinds of peaks, CuYO2, Y2O3 and CuO. The peaks of CuYO2 are observed at 15.6˚ (002) and 31.4˚ (004). Both of the peaks are c-axis oriented, and no other CuYO2 peaks are observed. Apart from the peaks of CuYO2, peaks attributed to Y2O3 at 29.3˚ (222). 34.0 (440), and CuO at 35.7˚ (002)(-111), are observed. The film consists of ternary metal oxide CuYO2 that is target material in the present work and binary metal oxide Y2O3 and CuO. Due to the XRD peak intensity, it is considered that the fraction volume of Y2O3 is larger than that of CuYO2 at 750˚C. In the sample annealed at 800˚C, only the two peaks of (002) and (004) of CuYO2 are observed with higher intensity than the films annealed at other temperature. This result means that the direct synthesis of pure CuYO2 from Cu-Y-gel without making Cu2Y2O5 is successfully achieved in the present work. It is considered that the direct synthesis becomes possible because the metal ions and 2-aminoethanol complexes are used as metal sources which oxidized by different mechanism from non- chelating metal ions. In the films annealed at 850˚C, the film displays peaks of CuYO2, not only the c-axis oriented peaks, but also a peak of (101) at 29.9˚. The intensity of the (101) peak increased with annealing temperature at 900˚C with decreasing of c-axis oriented peaks, (002) and (004). The c-axis peak is finally disappeared at annealing temperature of 900˚C. In addition, the XRD signal intensity is decreased at higher temperature again. This result indicates that the material CuYO2 is not stable at higher temperature than 850˚C, thus the decomposition reaction of CuYO2 occurs at higher temperature than 850˚C. The results indicate that the CuYO2 crystalline fraction volume ration once increased with annealing temperature and becomes the highest at 800˚C, then decreased again at higher temperature. Along with the decreasing of CuYO2 peaks, the peaks of CuO are observed as well as in the case of films annealed at lower temperatures.
Optical transmission spectra of the films annealed at 800˚C and 900˚C are shown in Figure 2. The film annealed at 800˚C, which has the highest XRD peak intensity of c-axis orientated peaks, has a transparency of more than 60% at the wavelength longer than 500 nm region. In contrast, the film annealed at 900˚C shows lower transparency than the film annealed at 800˚C at longer wavelength than 400 nm, however, shows higher transparency at shorter wavelength. In addition, the absorption edge is blue shifted significantly compared with the film annealed 800˚C. The result indicates that the band structure of the films changed drastically between the annealing temperature 800˚C and 900˚C. Figure 3(a) and Figure 3(b) shows a plot of (αhv)2 against the photon energy of the film annealed at 800˚C, and 900˚C, respectively. In the film annealed at 800˚C, the optical bandgap determined from the plot is 3.78 eV, which is higher than that of previously reported CuYO2  and the value is sufficiently high for use as a TCO material. In contrast, the optical bandgap of the films annealed at
Figure 2. Absorption spectra of thin films annealed at 800˚C and 900˚C.
Figure 3. A plot of (αhv)2 against the photon energy of the film annealed at (a) 800˚C shown above; and (b) 900˚C shown below.
900˚C is 5.27 eV, which is significantly higher than the film annealed at 800˚C. In addition, this result is inconsistent with the assignment by XRD that the film is (101) oriented CuYO2 crystalline. It is considered that the film is consist of CuYO2 crystalline fraction and amorphous Y2O3 fraction that has bandgap of 5.6 eV . As mentioned previously, the fraction volume ratio of CuYO2 crystalline is thought to be low, thus, the optical bandgap of the film is thought to be determined by properties of the amorphous Y2O3 fraction that have significantly higher fraction volume ratio than CuYO2 crystalline in the film. The structural property dependency on the annealing temperature observed in the present work is not consistent with previously reported sol-gel preparation of CuYO2 films . It is because the synthesis of CuYO2 is directly from Cu-Y-gel, not from the Cu2Y2O5 that is promptly formed by the annealing of gel films in air before the annealing under nitrogen flow.
In the present work, the film becomes crystalline CuYO2 and Y2O3 at temperature of 750˚C, then becomes highly c-axis oriented CuYO2 at 800˚C, then finally, becomes mixture of non-c-axis oriented CuYO2, CuO and amorphous Y2O3 at higher temperature than 850˚C. It is considered this complicated dependence of structure on annealing temperature is caused by existence of many synthesis and decomposition reactions occur in the films simultaneously. For example, oxidization of Cu and Y gel, solid phase reaction of CuO and Y2O3 to CuYO2, decomposition reaction of CuYO2 to amorphous Y2O3 and CuO, and crystalline orientation changing of CuYO2 are thought to occur in the temperature region studied in the present work.
Figure 4 displays the PL spectrum of the film annealed at 800˚C which CuYO2 crystalline in the film is highly c-axis oriented. The film exhibited a broad emission band with a broad peak at photon energy of 2.3 eV. The spectrum is very similar to previously reported CuYO2 films . Although the preparation details are different, the CuYO2 film prepared in the present work has similar optical properties with previous works. The origin of the PL is assigned to be due to the Cu+ interconfiguration transition from 3d94s1 to 3d10 with Stokes shift .
Delafossite material, CuYO2 thin films are prepared by sol-gel method using Cu acetate and Y acetate as metal source materials. Spin-coated gel films were annealed under nitrogen flow without carrying out promptly annealing in air. As a result, CuYO2 is successfully synthesized without synthesize Cu2Y2O5. XRD patterns reveal that the film annealed at lower temperature shows the patterns of Y2O3 crystalline rather than CuYO2. At annealing temperature of 800˚C, the film shows highly c-axis oriented CuYO2 crystalline. The PL spectrum is similar to the previous works. Due to the result of XRD and transmission spectra, it is suggested that the films
Figure 4. PL spectrum of thin films annealed at 800˚C, measured at room temperature.
becomes mixture of (101) oriented CuYO2 and amorphous Y2O3 at annealing temperature of 850˚C and 900˚C. These complicated dependency of structural properties of the films on the annealing temperature is caused by the existence of many reactions occur in the films simultaneously.
This work was partially supported by Ishinomaki Senshu University Kenkyu-Jyosei.