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] -[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 are observed from thermal-oxidized amorphous Ta₂O₅ thin films [4] . We demonstrated blue PL from Ta₂O₅ thin films deposited by RF magnetron sputtering [5] . Furthermore, 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 to 450 cm⁻¹) compared with that of other oxide materials (e.g., SiO₂) [6] . We have reported on various rare-earth (Er, Eu, Y, and Ce) doping into Ta₂O₅ thin films using simply co-sputtering of rare-earth oxide (Er₂O₃, Eu₂O₃, Y₂O₃, and CeO₂) pellets and a Ta₂O₅ disc [7] -[11] . Such light-emitting Ta₂O₅-based sputtered films can be used as high-refractive-index materials of autocloned photonic crystals that can be applied to novel light-emission devices [1] , and they will also be used as anti- reflection [12] and down-conversion [13] [14] layers for realizing high-efficiency silicon solar cells.

Recently, we have also reported on the preparation of thulium-doped Ta₂O₅ (Ta₂O₅:Tm) thin films using the same co-sputtering method and their PL properties having sharp peaks around a wavelength of 800 nm due to Tm³⁺ [15] . In addition, the sensitization of PL from rare-earth ions by Ce³⁺ is well known [16] . We can obtain Ce³⁺ ions by sputtering of CeO₂ because a small amount of Ce³⁺ exists at the surface of CeO₂ [17] . It is therefore expected that strong PL will be obtained from Tm and Ce co-doped Ta₂O₅ (Ta₂O₅:Tm, Ce) thin film deposited by co-sputtering of Tm₂O₃ and CeO₂ pellets on a Ta₂O₅ disc. In this study, we prepared Ta₂O₅:Tm, Ce co-sput- tered thin films using RF magnetron sputtering for the first time, and the PL and X-ray diffraction (XRD) properties of the films after annealing at 700˚C, 800˚C, 900˚C, or 1000˚C were evaluated.

Ta₂O₅:Tm, Ce thin films were deposited using a RF magnetron sputtering system (ULVAC, SH-350-SE). A Ta₂O₅ disc (Furuuchi Chemical Corporation, 99.99% purity, 100 mm diameter) was used as the sputtering target. We placed one or two Tm₂O₃ and CeO₂ pellets (Furuuchi Chemical Corporation, 99.9% purity, 20 mm diameter) on the Ta₂O₅ disc. The Ta₂O₅ disc and the Tm₂O₃ and CeO₂ pellets were co-sputtered by supplying RF power to the target. The flow rate of Ar gas introduced into the vacuum chamber was 15 sccm, and the RF power supplied to the target was 300 W. Commercial fused-silica plates (ATOCK Inc., 1 mm thick) were used as substrates, and they were not heated during sputtering.

In this study, we deposited three samples (A, B, and C) (Table 1). We changed the Tm or Ce concentrations of the Ta₂O₅:Tm, Ce films by changing the numbers of Tm₂O₃ or CeO₂ pellets placed on the Ta₂O₅ disc [8] . We prepared four specimens from one as-deposited sample by cutting it using a diamond-wire saw, and we subsequently annealed the specimens in ambient air at 700˚C, 800˚C, 900˚C, or 1000˚C for 20 min using an electric furnace (Denken, KDF S-70).

The PL spectra of the Ta₂O₅:Tm, Ce 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. XRD patterns of the films were recorded using an X-ray diffractometer (RIGAKU, RINT2200VF+/PC system). Tm and Ce concentrations of the films after annealing were measured using an electron probe micro-analyzer (EPMA) (Shimadzu, EPMA- 1610).

Figure 1 presents PL spectra of specimens prepared from a Ta₂O₅:Tm, Ce film deposited using one Tm₂O₃ pellet and two CeO₂ pellets (sample A) and annealed at 700˚C, 800˚C, 900˚C, or 1000˚C for 20 min. PL peaks around a wavelength of 800 nm were observed for specimens annealed at 900˚C or 1000˚C. The 800-nm peak

seems to be the result of the ³H₄→³H₆ transition of Tm³⁺ [15] . No PL peak was observed for specimens annealed at 700˚C or 800˚C. Figure 2 presents PL spectra of specimens prepared from a Ta₂O₅:Tm, Ce film deposited using one Tm₂O₃ pellet and one CeO₂ pellet (sample B) and annealed at 700˚C, 800˚C, 900˚C, or 1000˚C for 20 min. Much stronger PL peaks at almost the same wavelength of 800 nm were observed for specimens annealed at 900˚C or 1000˚C than for those prepared from sample A and annealed at the same temperature. The PL peak intensity of the specimen annealed at 900˚C was 3.3 times stronger, and that of the specimen annealed at 1000˚C was 14.4 times stronger than that of the specimen prepared from the sample A and annealed at the same temperature. Figure 3 presents PL spectra of specimens prepared from a Ta₂O₅:Tm, Ce film deposited using two Tm₂O₃ pellets and one CeO₂ pellet (sample C) and annealed at 700˚C, 800˚C, 900˚C, or 1000˚C for 20 min. PL peaks of specimens annealed at 900˚C or 1000˚C were similar to those of samples A and B, but much weaker than those of sample B.

Figure 4 plots normalized PL peak intensities of the specimens annealed at 900˚C or 1000˚C and prepared from samples A, B, and C. In our experiments, sample B exhibited the strongest PL intensity. The Tm concentration of film prepared from sample B and annealed at 900˚C was measured to be ~1.0 mol%, and the Ce concentration was measured to be ~1.3 mol%. These concentrations were thus estimated to be the proper Tm and Ce concentrations of such a Ta₂O₅:Tm, Ce film to obtain strong PL intensity.

Figure 5 presents XRD patterns of the specimens prepared from sample B and annealed at 700˚C, 800˚C, 900˚C, or 1000˚C for 20 min. The specimens annealed at 900˚C or 1000˚C had almost the same major diffraction peaks corresponding to the (001); δ-Ta₂O₅ (orthorhombic), (200); δ-Ta₂O₅ (hexagonal); and (201) phases as our rare-earth doped Ta₂O₅ sputtered thin films [18] . The three phases seem to be very important for obtaining strong PL peaks from the present Ta₂O₅:Tm, Ce films, in addition to optimizing the Tm and Ce concentrations.

We prepared Ta₂O₅:Tm, Ce thin films using simply co-sputtering of one or two Tm₂O₃ and CeO₂ pellets on a Ta₂O₅ disc for the first time, and PL properties of the films annealed at 700˚C, 800˚C, 900˚C, or 1000˚C for 20 min were evaluated. PL peaks around a wavelength of 800 nm due to Tm³⁺ were observed for films annealed at 900˚C or 1000˚C. The peak intensities of films prepared using one Tm₂O₃ pellet and one CeO₂ pellet were much stronger than those of films prepared using one Tm₂O₃ pellet and two CeO₂ pellets, or films prepared using two Tm₂O₃ pellets and one CeO₂ pellet. The proper Tm concentration to obtain strong PL intensity was estimated to be ~1.0 mol%, and proper Ce concentration was estimated to be ~1.3 mol%. Based on XRD measurements, the (001); δ-Ta₂O₅ (orthorhombic), (200); δ-Ta₂O₅ (hexagonal); and (201) phases of the Ta₂O₅:Tm, Ce films seem to be very important for obtaining a strong PL peak. Such light-emitting Ta₂O₅-based sputtered films can be used as high-refractive-index materials of autocloned photonic crystals that can be applied to novel light-emission devices, and they will also be used as anti-reflection and down-conversion layers for realizing high-efficiency silicon solar cells.

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.