Tantalum (V) oxide (Ta₂O₅) is a high-refractive-index material used in passive optical elements such as Ta₂O₅/ SiO₂ multilayered wavelength filters for dense wavelength-division multiplexing (DWDM). It has also been used as a high-index material of Ta₂O₅/SiO₂ multilayered photonic-crystal elements for the visible to near- infrared range fabricated using the “autocloning” method based on radio-frequency (RF) bias sputtering [1] [2] , and it can additionally be used as an anti-reflection coating material for silicon solar cells [3] .

However, Ta₂O₅ has recently attracted much attention as an active optical material, since broad red photoluminescence (PL) spectra at wavelengths of 600 to 650 nm were observed from thermal-oxidized amorphous Ta₂O₅ thin films [4] . Many studies on rare-earth-doped Ta₂O₅ have also been conducted because Ta₂O₅ is a potential host material for new phosphors due to its lower phonon energy (100 - 450 cm⁻¹) than other popular oxide materials (e.g. silicon dioxide (SiO₂)) [5] . We have fabricated various rare-earth doped Ta₂O₅ thin films using simply co-sputtering of rare-earth oxide pellets and a Ta₂O₅ disc, and we obtained various PL properties from these rare-earth-doped Ta₂O₅ thin films [6] - [11] . We reported on red or orange PL from europium (Eu)-doped Ta₂O₅ (Ta₂O₅: Eu) thin films deposited using the same co-sputtering method [9] . In our recent study, we fabricated erbium (Er), Eu, and cerium (Ce) co-doped Ta₂O₅ (Ta₂O₅: Er, Eu, Ce) thin films using co-sputtering of Er₂O₃, Eu₂O₃, CeO₂ and Ta₂O₅, and observed yellow PL from a film annealed at 900˚C [10] . The yellow light emission seemed to be obtained from the result of enhancement of the 550-nm (green) PL peak due to Er³⁺ by Ce³⁺ doping [12] . We also prepared Er and Ce co-doped Ta₂O₅ (Ta₂O₅: Er, Ce) thin films using co-sputtering of Er₂O₃, CeO₂ and Ta₂O₅. An enhanced green PL peak that seems to be sensitized by Ce³⁺ was observed from a film annealed at 900˚C [11] . We can obtain Ce³⁺ from CeO₂ (cerium (IV) oxide) pellets because a small amount of Ce³⁺ exists at the surface of CeO₂ [13] .

In this study, we fabricated Eu and Ce co-doped Ta₂O₅ (Ta₂O₅: Eu, Ce) thin films using our co-sputtering method for the first time, and we evaluated PL properties of the films.

Ta₂O₅: Eu, Ce thin films were deposited using our RF magnetron sputtering system (ULVAC, SH-350-SE). A schematic figure of the system was presented in our previous report [7] . A Ta₂O₅ sintered-compact disc (Furuuchi Chemical Corporation, 99.99% purity, diameter 100 mm) was used as a sputtering target in the system. We placed an Eu₂O₃ sintered-compact pellet (Furuuchi Chemical Corporation, 99.9% purity, diameter 20 mm) and two CeO₂ sintered-compact pellets (Furuuchi Chemical Corporation, 99.9% purity, diameter 20 mm) on the Ta₂O₅ disc as presented in Figure 1. They were 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. The RF power supplied to the target was 200 W. Fused-silica plates (ATOCK Inc., 1 mm thick) were used as substrates, and they were not heated during sputtering. The thicknesses of the films were set to be ~1.5 μm by adjusting the sputtering times of the films.

We subsequently annealed the Ta₂O₅: Eu, Ce co-sputtered thin films in ambient air at 600˚C, 700˚C, 800˚C, 900˚C, 1000˚C, or 1100˚C for 20 min using an electric furnace (Denken, KDF S-70). We set the annealing time to 20 min because it was the proper condition for our Er-doped Ta₂O₅ (Ta₂O₅: Er) films to obtain strong PL intensities [6] [8] . The PL spectra of the films were measured using a dual-grating monochromator (Roper Scientific, SpectraPro 2150i) and a CCD detector (Roper Scientific, Pixis: 100B, electrically cooled to −80˚C). A He-Cd laser (Kimmon, IK3251R-F, wavelength λ = 325 nm) was used to excite the films. The Eu and Ce concentrations of the films after annealing were measured using an electron probe micro-analyzer (EPMA) (Shimadzu, EPMA-1610). The X-ray diffraction (XRD) patterns of the films were recorded using an X-ray diffractometer (RIGAKU, RINT2200VF+/PC system).

Figure 2 presents PL spectra of Ta₂O₅: Eu, Ce films annealed at 600˚C, 700˚C, 800˚C, 900˚C, 1000˚C, or 1100˚C for 20 min. Four remarkable PL peaks at wavelengths of 600, 620, 700, and 705 nm were observed only from the film annealed at 900˚C. No PL peak was observed from the films annealed at 600˚C, 700˚C, 800˚C, 1000˚C, or 1100˚C. The peaks at the wavelengths of 600 and 620 nm seem to be the results of the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of Eu³⁺, respectively [9] , and the peaks at the wavelength of 700 and 705 nm seem to be due to the ⁵D₀ → ⁷F₄ transition of Eu³⁺ [9] . We could not observe a remarkable peak around a wavelength of 650 nm due to the ⁵D₀ → ⁷F₃ transition of Eu³⁺ that was observed in [9] . On the other hand, we found an additional 705-nm peak from the film annealed at 900˚C.

The Eu and Ce concentrations of the film annealed at 800˚C were measured to be around 1.5 and 2.8 mol%, respectively. The concentrations may be almost the same as those of the films annealed at the other temperatures.

Figure 3 presents XRD patterns of the films annealed at 600˚C, 700˚C, or 800˚C (Figure 3(a)) and those of the films annealed at 900˚C, 1000˚C, or 1100˚C (Figure 3(b)). The films annealed at 600˚C, 700˚C, and 800˚C seemed to be almost amorphous phases because no significant diffraction peak was observed from them as seen in Figure 3(a). On the other hand, three major peaks corresponding to the (0 0 1); β-Ta₂O₅ (orthorhombic), (2 0 0); δ-Ta₂O₅ (hexagonal), and (2 0 1) Ta₂O₅ phases were observed from the films annealed at 900˚C [7] . These crystalline phases of Ta₂O₅ seem to be very important for obtaining significant PL peaks from our Ta₂O₅: Eu, Ce films annealed at 900˚C. Furthermore, other diffraction peaks due to CeTa₇O₁₉ and EuTa₇O₁₉ crystals were observed from the films annealed at 1000˚C and 1100˚C. In particular, four peaks corresponding to the hexagonal CeTa₇O₁₉ phases ((1 0 0), (0 0 6), (1 1 1), and (1 1 5)) were remarkably observed as presented in Figure 3(b). Therefore, it seems that the hexagonal CeTa₇O₁₉ phases should be avoided in order to obtain PL from our Ta₂O₅: Eu, Ce films.

Figure 4 illustrates energy level diagrams of Eu³⁺ and Ce³⁺ [14] [15] . As presented in Figure 4, electrons are excited to the ²D_5/2 state by the He-Cd laser irradiation (λ = 325 nm), and they relax to the ²D_3/2 state of Ce³⁺. Subsequently, energy transfer from the ²D_3/2 state of Ce³⁺ to the ⁵D₁ state of Eu³⁺ occurs, and the electrons relax

to the ⁵D₀ state of Eu³⁺. Finally, the transitions of ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, and ⁵D₀ → ⁷F₄ occur, and light emission at wavelengths of 600, 620, 700, and 705 nm can be observed. In our previous study, the 620-nm peaks observed from our Ta₂O₅: Eu co-sputtered thin films were much stronger than the other peaks around the wavelengths of 600 and 700 nm [9] . However, as seen in Figure 2, the intensities of the 600-nm (⁵D₀ → ⁷F₁) and 620-nm (⁵D₀ → ⁷F₂) peaks were almost the same, and the intensity of the 700- and 705-nm (⁵D₀ → ⁷F₄) was much stronger than that of the 620-nm peak of the Ta₂O₅: Eu, Ce co-sputtered thin film annealed at 900˚C. It seems that energy transfer from Ce³⁺ to Eu³⁺ occurs in the film, and much energy is selectively used for the ⁵D₀ → ⁷F₄ and ⁵D₀ → ⁷F₁ transitions.

Such Ta₂O₅-based thin films seem to be used as high-refractive-index and light-emitting materials of “autocloning” photonic crystals that can be applied to novel light-emission devices [1] , and they also seem to be used as multi-functional coating films having both anti-reflection [3] and down-conversion [16] -[18] effects for realizing high-efficiency silicon solar cells.

We fabricated Ta₂O₅: Eu, Ce thin films using our co-sputtering method for the first time, and evaluated the PL properties of the films after annealing at 600˚C - 1100˚C for 20 min. Four remarkable PL peaks at wavelengths of 600, 620, 700, and 705 nm were observed from the film annealed at 900˚C. The intensities of the 700- and 705-nm peaks due to the ⁵D₀ → ⁷F₄ transition of Eu³⁺ were much stronger than those of the 600-nm (⁵D₀ → ⁷F₁) and 620-nm (⁵D₀ → ⁷F₂) peaks of the film annealed at 900˚C. It seems that energy transfer from Ce³⁺ to Eu³⁺ occurs in the film, and much energy is selectively used for the ⁵D₀ → ⁷F₄ and ⁵D₀ → ⁷F₁ transitions.

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.

KentaMiura,TetsuhitoSuzuki,OsamuHanaizumi, (2015) Photoluminescence Properties of Europium and Cerium Co-Doped Tantalum-Oxide Thin Films Prepared Using Co-Sputtering Method. Journal of Materials Science and Chemical Engineering,03,30-34. doi: 10.4236/msce.2015.38005