Many studies on rare-earth-doped tantalum (V) oxide (Ta₂O₅) have been conducted because Ta₂O₅ is a potential host material for new phosphors due to its low phonon energy (100 - 450 cm⁻¹) compared with other oxide materials such as SiO₂ [1] . Visible photoluminescence (PL) from erbium-doped Ta₂O₅ (Ta₂O₅:Er) produced by the sol-gel method [2] [3] and ion implantation [4] has been reported. Their PL spectra have main peaks at a wavelength of 550 nm due to the ⁴S_3/2→⁴I_15/2 transition of Er³⁺, and at a wavelength of 670 nm due to the ⁴F_9/2→⁴I_15/2 transition of Er³⁺. We previously demonstrated that Ta₂O₅:Er thin films deposited using a simple co-sputtering method exhibited such PL peaks at wavelengths of 550 and 670 nm after annealing at 600˚C to 1100˚C [5] [6] . Recently, we also fabricated ytterbium-doped Ta₂O₅ (Ta₂O₅:Yb) thin films using the same co-sputtering method in order to expand the useful wavelength range of our rare-earth-doped and light-emitting Ta₂O₅ co-sputtered films [7] . We observed PL spectra having sharp peaks at a wavelength of 980 nm from the Ta₂O₅:Yb thin films after annealing at 700˚C to 1000˚C [7] . The 980-nm peaks seemed to be the result of the ²F_5/2→²F_7/2 transition of Yb³⁺ [7] .

Furthermore, in our recent works, we demonstrated Tm and Ce co-doped Ta₂O₅ (Ta₂O₅:Tm, Ce) [8] , Er and Ce co-doped Ta₂O₅ (Ta₂O₅:Er, Ce) [9] , and Er, Eu, and Ce co-doped Ta₂O₅ (Ta₂O₅:Er, Eu, Ce) [10] thin films prepared using the co-sputtering method. In this work, we fabricated an Er and Yb co-doped Ta₂O₅ (Ta₂O₅:Er, Yb) thin film using radio-frequency (RF) magnetron co-sputtering of Ta₂O₅, Er₂O₃, and Yb₂O₃ for the first time, and evaluated its PL property.

A Ta₂O₅:Er, Yb thin film was prepared using our co-sputtering method reported in [5] -[13] . A Ta₂O₅ disc (99.99% purity, diameter 100 mm), two Er₂O₃ pellets (99.9% purity, diameter 21 mm), and two Yb₂O₃ pellets (99.9% purity, diameter 21 mm) were used as co-sputtering targets. The Er₂O₃ and Yb₂O₃ pellets were placed on the Ta₂O₅ disc as shown in Figure 1. The film was deposited using a RF magnetron sputtering system (ULVAC, SH-350-SE). The flow rate of Ar gas introduced into the vacuum chamber was 10 sccm, and the RF power supplied to the targets was 200 W. A fused-silica plate (1 mm thick) was used as a substrate, and it was not heated during co-sputtering. We subsequently annealed the film in ambient air at 900˚C for 20 min using an electric furnace (Denken, KDF S-70). The PL spectrum of the Ta₂O₅:Er, Yb film was 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 with a He-Cd laser (Kimmon, IK3251R-F, wavelength λ = 325 nm).

Figure 2 presents PL spectra of Ta₂O₅:Er, Yb (red line) and Ta₂O₅:Er (without Yb, black line) [5] [6] co-sput- tered thin films. We observed typical PL peaks around wavelengths of 550, 670, 850, and 980 nm from both the films. The 550-, 670-, and 850-nm peaks seem to be the results of the ⁴S_3/2→⁴I_15/2, ⁴F_9/2→⁴I_15/2, and ⁴I_9/2→⁴I_15/2 transitions of Er³⁺, respectively [5] [14] [15] . The 980-nm peaks seem to be the results of the ⁴I_11/2→⁴I_15/2 transition of Er³⁺ or the ²F_5/2→²F_7/2 transition of Yb³⁺ [7] [14] -[16] . From Figure 2, we can find that the 550-, 670-, and 850-nm peaks from the Ta₂O₅:Er film decreased by Yb doping. In contrast, the intensity of the 980-nm peak from the Ta₂O₅:Er, Yb film was stronger than that from the Ta₂O₅:Er film. This seems to be because of overlapping between the above-mentioned ⁴I_11/2→⁴I_15/2 transition of Er³⁺ and the ²F_5/2→²F_7/2 transition of Yb³⁺, and energy transfers from Er³⁺ to Yb³⁺ reported in [14] . Figure 3 illustrates energy level diagrams of Er³⁺ and Yb³⁺ [14] [15] . The energies of ⁴S_3/2 (the origin of the 550-nm peak), ⁴F_9/2 (the origin of the 670-nm peak), and ⁴I_9/2 (the origin of the 850-nm peak) states of Er³⁺ seem to transfer through the ⁴I_11/2 state of Er³⁺ to the ²F_5/2 state of Yb³⁺ as presented in Figure 3.

Figure 4 presents PL spectra of the same Ta₂O₅:Er, Yb film (red line) and a Ta₂O₅:Yb film (without Er, green line) reported in [7] . The 980-nm peak from the Ta₂O₅:Yb film is much stronger than that from the Ta₂O₅:Er, Yb film. This seems to be because the opposite energy transfer from Yb³⁺ to Er³⁺ occurred in the Ta₂O₅:Er, Yb film. The energy of the ²F_5/2 state of Yb³⁺ partially transfer to the ⁴I_11/2 state of Er³⁺ at first, and subsequently relax to the ⁴I_13/2 state as presented in Figure 3. Finally light emission around a wavelength of 1550 nm due to the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ seems to occur [14] . The 1550-nm emission may cause the decrease of the 980- nm-peak intensity. Unfortunately, our detector did not detect the light emission in the wavelength range. We will try to evaluate the light-emission properties of our Ta₂O₅:Er, Yb films in the near-infrared range in order to make the mechanism of the energy transfer between Er³⁺ and Yb³⁺ clearer.

Such a Ta₂O₅:Er, Yb co-sputtered thin film can be used as a high-refractive-index and light-emitting material of a multilayered photonic crystal that can be applied to a novel light-emitting device [17] , and it will also be used as a multi-functional coating film having both anti-reflection [18] and down-conversion [14] -[16] effects for realizing a high-efficiency silicon solar cell.

A Ta₂O₅:Er, Yb thin film was fabricated using our simple co-sputtering method for the first time, and its PL spectrum was evaluated. Energy transfers between Er³⁺ and Yb³⁺ in our Ta₂O₅:Er, Yb co-sputtered film were discussed by comparing between PL spectra of the film and our Ta₂O₅:Er or Ta₂O₅:Yb films. Such a Ta₂O₅:Er, Yb co-sputtered thin film can be used as a high-refractive-index and light-emitting material of a multilayered photonic crystal that can be applied to a novel light-emitting device, and it will also be used as a multi-func- tional coating film having both anti-reflection and down-conversion effects for realizing a high-efficiency silicon solar cell.

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