Photoluminescence and Crystalline Properties of CuO-Ta₂O₅ Composite Films Prepared Using Co-Sputtering

We prepared CuO-Ta ₂ O ₅ composite films by our simple co-sputtering method using three CuO pellets and a Ta ₂ O ₅ disc as a co-sputtering target, and subsequently annealed the films in ambient air at 900℃, 1000℃, and 1100℃ for 20 min. We evaluated photoluminescence (PL) and X-ray diffraction properties of the annealed films, and discussed the relationship between sharp PL peaks ( λ ~ 450 nm) observed from all the films and their crystallizabilities. We considered that the 450-nm peaks originated from Cu _2.1 (Ta ₄ O ₁₂ ) crystal phases in the films.

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

1. Introduction

Tantalum (V) oxide (Ta₂O₅) is a higher refractive index (n > 2) and lower phonon energy (100 - 450 cm⁻¹) material than other popular oxides (e.g., SiO₂). It can be widely applicable to various passive or active optoelectronic elements such as anti-reflection coatings for silicon solar cells [1] , photonic crystals prepared using the autocloning method [2] [3] , and novel 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 radio-frequency (RF) magnetron co-sput- tering of rare-earth oxide (Er₂O₃, Eu₂O₃, Yb₂O₃, Tm₂O₃, Y₂O₃, and CeO₂) pellets and a Ta₂O₅ disc [5] - [18] , and we have obtained various photoluminescence (PL) properties from the films.

Copper (Cu) is one of transition metals, and it is used as a functional dopant in light-emitting materials such as ZnS:Cu [19] - [21] and ZnO:Cu [22] . It is expected that novel Ta₂O₅-based functional materials will be realized by doping with Cu instead of rare-earths into Ta₂O₅. We have prepared Cu(II) oxide (CuO) and Ta₂O₅ co- sputtered (CuO-Ta₂O₅) composite films using a CuO pellet and a Ta₂O₅ disc as a co-sputtering target, and we have evaluated X-ray diffraction (XRD) and PL properties of the films after annealing at 600˚C - 900˚C [23] . In this short report, we will present the preparation of CuO-Ta₂O₅ composite films using more CuO pellets and a Ta₂O₅ disc as a co-sputtering target, and the evaluations of PL and XRD properties of the films annealed at higher temperatures of 900˚C - 1100˚C than those in our previous report [23] . Subsequently, we will discuss the relationship between the PL and XRD properties.

2. Experiments

A CuO-Ta₂O₅ film was deposited using our RF magnetron sputtering system (ULVAC, SH-350-SE). A schematic figure of the system was presented in our previous report [6] . A Ta₂O₅ disc (Furuuchi Chemical Corporation, 99.99% purity, diameter 100 mm) was installed as a sputtering target in the system. We placed three CuO pellets (Furuuchi Chemical Corporation, 99.9% purity, diameter 20 mm) on the erosion area of the Ta₂O₅ disc as presented in Figure 1. 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. The CuO pellets and the Ta₂O₅ disc were co-sputtered by supplying RF power to a cathode under the Ta₂O₅ disc. The RF power was set to 200 W. A fused-silica plate was used as a substrate, and it was not heated during deposition. We prepared four specimens from the as-deposited CuO-Ta₂O₅ sample by cutting it using a diamond-wire saw, and we subsequently annealed three of the four specimens in ambient air at 900˚C, 1000˚C, or 1100˚C for 20 min using an electric furnace (Denken, KDF S-70).

The PL spectra of the three 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). The XRD patterns of the specimens were recorded using an X-ray diffractometer (RIGAKU, RINT2200VF+/PC system).

3. Results and Discussion

Figure 2 presents PL spectra of the three specimens annealed at 900˚C, 1000˚C, and 1100˚C. Sharp PL peaks at λ ~ 450 nm were observed from all the specimens. The relative intensities of the PL peaks from the specimens annealed at 900˚C, 1000˚C, and 1100˚C were 1, 1.86, and 2.14, respectively. Figure 3 presents XRD patterns of the same specimens. Three significant diffraction peaks were observed from all the specimens. These peaks correspond to orthorhombic Cu_2.1(Ta₄O₁₂) ((0 0 2), (2 0 0), and (0 2 2)) phases (JCPDS No.01-076-7904). Therefore, the CuO-Ta₂O₅ films annealed at 900˚C - 1100˚C seems to have Cu_2.1(Ta₄O₁₂) crystal phases.

A similar PL peak at λ ~ 450 nm have already been observed only from an amorphous CuO-Ta₂O₅ film annealed at 600˚C in our previous work [23] . The peak seemed to be attributed to the transition from the conduction band of Ta₂O₅ to the t₂ energy level of Cu²⁺ in the bang gap of Ta₂O₅ [19] [21] [23] . However, as mentioned above, the presented CuO-Ta₂O₅ films annealed at 900˚C - 1100˚C seemed to be not amorphous but partially Cu_2.1(Ta₄O₁₂) crystal phases. The origin of the sharp PL peaks presented in Figure 2 may be different from that reported in [23] . In addition, we have reported that CuO-Ta₂O₅ films prepared using a CuO pellet become tetragonal CuTa₂O₆ phases after annealing at 700˚C - 900˚C, and no sharp PL peak was observed from the films

Figure 1

Schematic top view of the sputtering target for co- sputtering of three CuO pellets and a Ta 2 O 5 disc

Figure 2

PL spectra of CuO-Ta 2 O 5 composite films annealed at 900˚C, 1000˚C, and 1100˚C

Figure 3

XRD patterns of CuO-Ta 2 O 5 films annealed at 900˚C, 1000˚C, and 1100˚C

[23] . Therefore, the sharp 450-nm peaks observed from the CuO-Ta₂O₅ films annealed at 900˚C - 1100˚C seem to originate from the Cu_2.1(Ta₄O₁₂) phases in the films. We will continue to further investigate the origin of the 450-nm peaks by characterizing morphologies of the films using a scanning electron microscope.

4. Summary

We prepared CuO-Ta₂O₅ composite films by our simple co-sputtering method using three CuO pellets and a Ta₂O₅ disc as a co-sputtering target, and subsequently annealed the films in ambient air at 900˚C, 1000˚C, and 1100˚C for 20 min. We evaluated PL and XRD properties of the annealed films, and discussed the relationship between sharp PL peaks (λ ~ 450 nm) observed from all the films and their crystallizabilities. From the results presented in this short report, we considered that the 450-nm peaks originated from Cu_2.1(Ta₄O₁₂) crystal phases in the films. Further investigations are going to be conducted in order to make the origin of the 450-nm peaks clearer.

Acknowledgements

Part of this work was supported by JSPS KAKENHI Grant Number 26390073. Part of this work was conducted at the Human Resources Cultivation Center (HRCC), Gunma University, Japan.

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