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-in- frared range fabricated using the autocloning method based on radio-frequency (RF) bias sputtering [1] - [3] , and it can additionally be used as an anti-reflection coating material for silicon solar cells [4] . However, Ta₂O₅ has recently attracted much attention as an active optical material, since broad red photoluminescence (PL) spectra at wavelengths from 600 to 650 nm were observed from thermal-oxidized amorphous Ta₂O₅ thin films [5] . In our previous work, we demonstrated blue PL from Ta₂O₅ thin films deposited by RF magnetron sputtering [6] .

In addition, many studies on rare-earth-doped 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 [7] . We have reported on various rare-earth (Er, Eu, Tm, and Y) doping into Ta₂O₅ thin films using simply co-sputtering of rare-earth oxide (Er₂O₃, Eu₂O₃, Tm₂O₃, and Y₂O₃) pellets and a Ta₂O₅ disc [8] -[12] . Moreover, in our recent study, we fabricated Er, Eu, and Ce co-doped Ta₂O₅ (Ta₂O₅: Er, Eu, Ce) thin films using the co-sputtering method, and observed yellow PL from the films.

In this study, in order to expand the useful wavelength range of our Ta₂O₅-based light-emitting sputtered films, we fabricated ytterbium-doped Ta₂O₅ (Ta₂O₅:Yb) thin films using the simple co-sputtering method for the first time.

Ta₂O₅:Yb thin films were deposited using an RF magnetron sputtering system (ULVAC, SH-350-SE). A Ta₂O₅ disc (Furuuchi Chemical Corporation, 99.99% purity, diameter 100 mm) was used as the sputtering target. We placed Yb₂O₃ pellets (Furuuchi Chemical Corporation, 99.9% purity, diameter 21 mm) on the Ta₂O₅ disc. The Ta₂O₅ disc and Yb₂O₃ pellets were co-sputtered by supplying RF power to the target. Figure 1 is a schematic diagram of the sputtering target, with three Yb₂O₃ pellets on the Ta₂O₅ disc. We prepared co-sputtered Ta₂O₅:Yb films with different Yb concentrations by placing two, three, four, or five Yb₂O₃ pellets on the Ta₂O₅ disc. The flow rate of argon gas introduced into the vacuum chamber was 10 sccm, and the pressure in the chamber during deposition was kept at ~1 Pa. The RF power supplied to the target was 200 W. Commercial fused silica plates (ATOCK Inc., 1 mm thick) were used as substrates. The substrates were not heated during sputtering. We subsequently annealed the samples in ambient air using an electric furnace (Denken, KDF S-70).

The PL spectra of the Ta₂O₅:Yb 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). An He-Cd laser (Kimmon, IK3251R-F, wavelength λ = 325 nm) was used to excite the films.

Figure 2 presents PL spectra of the Ta₂O₅:Yb films deposited using two, three, four, or five Yb₂O₃ pellets and annealed at 900˚C for 20 min. Intense PL peaks around a wavelength of 980 nm were observed from all of the samples. The 980-nm peaks seem to be the result of the ²F_5/2→²F_7/2 transition of Yb³⁺ [13] . The peak intensity once increased and subsequently decreased with increasing the number of Yb₂O₃ pellets (the Yb concentration) as [13] . Figure 2 indicates that the film deposited using three Yb₂O₃ pellets exhibited the strongest intensity of the 980-nm peak after annealing at 900˚C for 20 min.

Figure 3 presents PL spectra from Ta₂O₅:Yb films deposited using the standard three Yb₂O₃ pellets and annealed at 900˚C for 10, 20, 30, or 40 min. The 980-nm-peak intensities from the samples annealed for 10, 30, and 40 min were approximately the same, but the peak intensity from the sample annealed for 20 min was stronger than the others. We therefore considered that the proper annealing time is 20 min.

Figure 4 presents PL spectra from Ta₂O₅:Yb films deposited using the three Yb₂O₃ pellets and annealed at 700˚C, 800˚C, 900˚C, or 1000˚C for the standard 20 min. The 980-nm-peak intensity once increased and subsequently decreased with increasing the annealing temperature. The strongest peak intensity was observed from the sample annealed at 800˚C. Interestingly, the PL peak observed from the sample annealed at 700˚C was sharper than that of the other samples. From the X-ray diffraction (XRD) measurements, the sample annealed at 700˚C seemed to be amorphous phase, and the samples annealed at 800˚C, 900˚C, or 1000˚C seemed to be polycrystalline phase. We will continue to investigate the relationship between the width of the 980-nm peak and the crystallizability of our Ta₂O₅:Yb film.

Ta₂O₅:Yb thin films were prepared by our simple co-sputtering method for the first time, and PL spectra having sharp peaks at a wavelength of 980 nm were observed from the films after annealing from 700˚C to 1000˚C for 10 to 40 min. The reference conditions for fabricating our Ta₂O₅:Yb films (three Yb₂O₃ pellets; annealing temperature 800˚C; annealing time 20 min) determined to provide the strongest PL peak intensity. Such rare-earth-

doped Ta₂O₅ sputtered films can be used as high-refractive-index materials of Ta₂O₅/SiO₂ autocloned (multilayered) photonic crystals that can be applied to novel light-emitting devices [1] , and they will also be used as both anti-reflection [4] and down-conversion [13] [14] layers for realizing high-efficiency silicon solar cells.

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.