Kenta Miura^*, Takumi Osawa, Yuya Yokota, Zobaer Hossain, Osamu Hanaizumi
Graduate School of Science and Technology, Gunma University, Kiryu, Japan.
DOI: 10.4236/msce.2015.39006   PDF    HTML   XML   2,724 Downloads   3,457 Views   Citations

Abstract

We prepared CuO-Ta₂O₅ composite films using our simple co-sputtering method for the first time. Four specimens were prepared from an as-deposited CuO-Ta₂O₅ sample by cutting it using a diamond- wire saw, and the specimens were subsequently annealed at 600°C - 900°C. The X-ray diffraction and photoluminescence (PL) of the annealed specimens were evaluated. The CuO-Ta₂O₅ film annealed at 600°C seemed to be primarily amorphous phase, and a sharp PL peak at a wavelength of 450 nm, due to the existence of Cu²⁺, was observed from the film. In contrast, the CuO-Ta₂O₅ films annealed at 700°C, 800°C, and 900°C seemed to be tetragonal CuTa₂O₆ phases. We expect that good-quality CuTa₂O₆ films can be obtained using our very simple co-sputtering method and subsequent annealing above 900°C. Such CuTa₂O₆ films can be used in chemisorptions conductometric gas sensors.

Keywords

Ta₂O₅, CuO, Co-Sputtering, X-Ray Diffraction, Photoluminescence

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1. Introduction

Tantalum (V) oxide (Ta₂O₅) has a higher refractive index (n > 2) and lower phonon energy (100 - 450 cm⁻¹) than other popular oxides (e.g., silicon dioxide (SiO₂)). It is widely applicable to various passive/active optoelectronics elements such as anti-reflection coating films for silicon solar cells [1] , photonic crystals for the visible to near-infrared range fabricated using the autocloning method [2] [3] , and phosphors doped with rare earths [4] . We have so far prepared various rare-earth (Er, Eu, Yb, Tm, Y, and Ce) doped Ta₂O₅ thin films using simple co-sputtering of rare-earth oxide (Er₂O₃, Eu₂O₃, Yb₂O₃, Tm₂O₃, Y₂O₃, and CeO₂) pellets and a Ta₂O₅ disc [5] -[18] . By using our simple co-sputtering method, we can easily change the functional dopants in a Ta₂O₅ host by changing the constituent materials of pellets placed on the Ta₂O₅ disc.

Copper (Cu) is a transition metal used as a functional dopant in light-emitting materials such as ZnS:Cu [19] -[21] and ZnO:Cu [22] . We expect that new Ta₂O₅-based functional materials will be realized by doping with Cu instead of rare earths. In this short report, the first preparation of a Cu(II) oxide (CuO) and Ta₂O₅ composite (CuO-Ta₂O₅) film using our simple co-sputtering method will be presented.

2. Experiments

A CuO-Ta₂O₅ film was deposited using our radio-frequency (RF) magnetron sputtering system (ULVAC, SH-350-SE). A schematic diagram of the system was presented in our previous report [6] . A Ta₂O₅ disc (Furuuchi Chemical Corporation, 99.99% purity, diameter 100 mm) was used as a sputtering target in the system. We placed a CuO pellet (Furuuchi Chemical Corporation, 99.9% purity, diameter 20 mm) on the erosion area of the Ta₂O₅ disc as seen in Figure 1. It was co-sputtered by supplying RF power to the target. The flow rate of Ar gas introduced into the processing vacuum chamber was 15 sccm, and the pressure in the chamber during deposition was kept at ~5.4 × 10⁻⁴ Torr. RF power of 200 W was supplied to the target. A fused-silica plate (ATOCK Inc., 1 mm thick) was used as a substrate, and it was not heated during sputtering. We prepared four specimens from the as-deposited CuO-Ta₂O₅ sample by cutting it using a diamond-wire saw and subsequently annealed the four specimens in ambient air at 600˚C, 700˚C, 800˚C, or 900˚C for 20 min using an electric furnace (Denken, KDF S-70). We set the annealing time to 20 min because it is the standard condition for our rare-earth-doped Ta₂O₅ thin films [5] -[18] .

The X-ray diffraction (XRD) patterns of the specimens were recorded using an X-ray diffractometer (RIGAKU, RINT2200VF+/PCsystem). The PL spectra of the specimens were measured using a dual-grating monochromator (Roper Scientific, SpectraPro 2150i) and a CCD detector (Roper Scientific, Pixis: 100B, electrically cooled to −80˚C) under excitation using a He-Cd laser (Kimmon, IK3251R-F, wavelength λ = 325 nm).

3. Results and Discussion

Figure 2 presents XRD patterns of the four specimens annealed at 600˚C, 700˚C, 800˚C, and 900˚C. The CuO- Ta₂O₅ film annealed at 600˚C seemed to be primarily amorphous phase because no significant diffraction peak was observed. In contrast, three major peaks corresponding to tetragonal CuTa₂O₆ ((2 0 0), (2 1 1), and (3 1 0)) phases (JCPDS No. 00-024-0380) were observed from the specimens annealed at 700˚C, 800˚C, and 900˚C. CuTa₂O₆ can be used in chemisorptions conductometric gas sensors [23] . We found that the CuTa₂O₆ film can be easily obtained using our simple co-sputtering method and subsequent annealing above 700˚C.

Figure 3 presents PL spectra of the specimens annealed at 600˚C, 700˚C, 800˚C, and 900˚C. A sharp PL peak at a wavelength of ~450 nm was observed from the specimen annealed at 600˚C. The PL peak seems attributable to the transition from the conduction band of Ta₂O₅ to the t₂ energy level of Cu²⁺ in the band gap of Ta₂O₅ [19] [21] , and the peak seems to be obtained only when the CuO-Ta₂O₅ film is amorphous, as seen in Figure 2. In addition, weak and broad PL peaks ranging from 400 to 500 nm were observed from the specimens annealed at 700˚C and 800˚C. The peaks are similar to the ones that originate from oxygen-vacancy trap levels of Ta₂O₅ reported in [24] . Furthermore, no PL peak was observed from the specimen annealed at 900˚C. As mentioned above, we found that the CuO-Ta₂O₅ films annealed at 700˚C, 800˚C, and 900˚C were tetragonal CuTa₂O₆ phases, based on the results of the XRD measurements presented in Figure 2. Therefore, it seems that the CuO-Ta₂O₅ (CuTa₂O₆) films annealed at 700˚C and 800˚C had defects such as oxygen vacancies, but the one

annealed at 900˚C had almost no defects because the broad PL peaks seen from the films annealed at 700˚C and 800˚C were not observed. Thus it is expected that good-quality CuTa₂O₆ films applicable to the above-mentioned gas sensors [23] can be obtained using our very simple co-sputtering method and subsequent annealing above 900˚C.

4. Summary

We prepared CuO-Ta₂O₅ films using our simple co-sputtering method for the first time and subsequently annealed them at 600˚C - 900˚C. The XRD and PL properties of the annealed films were evaluated. The CuO-Ta₂O₅ film annealed at 600˚C seemed to be primarily amorphous phase, and a sharp PL peak at a wavelength of ~450 nm, due to the existence of Cu²⁺, was observed from the film. In contrast, the CuO-Ta₂O₅ films annealed at 700˚C, 800˚C, and 900˚C seemed to be tetragonal CuTa₂O₆ phases. It is expected that good-quality CuTa₂O₆ films without defects can be obtained using our very simple co-sputtering method and subsequent annealing above 900˚C.

Acknowledgements

Part of this work was supported by JSPS KAKENHI Grant Number 26390073; and the “Element Innovation” Project by Ministry of Education, Culture, Sports, Science and Technology in Japan. Part of this work was conducted at the Human Resources Cultivation Center (HRCC), Gunma University, Japan.

NOTES

^*Corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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