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. 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 are observed from thermal-oxidized amorphous Ta₂O₅ thin films [4] . In our previous work, we demonstrated blue PL from Ta₂O₅ thin films deposited by RF magnetron sputtering [5] .

In addition, many studies on rare-earth-doped Ta₂O₅ have been conducted because Ta₂O₅ is a potential host material for new phosphors or efficient down-conversion luminescent materials due to its lower phonon energy from 100 to 450 cm⁻¹ than other popular oxides such as SiO₂ [6] . We have reported on various rare-earth (Er, Eu, Y, Yb, and Ce) doping into Ta₂O₅ thin films using simply co-sputtering of rare-earth oxide (Er₂O₃, Eu₂O₃, Y₂O₃, Yb₂O₃, and CeO₂) pellets and a Ta₂O₅ disc, and we have observed various PL from our rare-earth-doped Ta₂O₅ thin films [7] -[17] . We also fabricated thulium-doped Ta₂O₅ (Ta₂O₅:Tm) thin films using co-sputtering of three Tm₂O₃ pellets and a Ta₂O₅ disc, and we obtained a remarkable PL peak around a wavelength of 800 nm due to the ³H₄→³H₆ transition of Tm³⁺ from a Ta₂O₅:Tm co-sputtered film after annealing at 900˚C for 20 min [17] .

In this study, we prepared Ta₂O₅:Tm co-sputtered thin films using two Tm₂O₃ pellets, and we observed not only PL peaks around a wavelength of 800 nm but also violet PL peaks from the films after annealing for the first time.

A Ta₂O₅:Tm thin film was deposited using a (RF) magnetron sputtering system (ULVAC, SH- 350-SE). A schematic figure of the system was presented in our previous report [8] . A Ta₂O₅ disc (99.99% purity, diameter 100 mm) and two Tm₂O₃ pellets (99.9% purity, diameter 20 mm) were used as co-sputtering targets. The Tm₂O₃ pellets were placed on the Ta₂O₅ disc as shown in Figure 1. The flow rate of argon gas introduced into the vacuum chamber was 10 sccm, and the RF power supplied to the targets was 300 W. A fused-silica plate (1 mm thick) was used as a substrate, and it was not heated during co-sputtering.

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 600˚C, 700˚C, 800˚C, or 900˚C for 20 min using an electric furnace (Denken, KDF S-70). The annealing temperatures (600˚C - 900˚C) and the annealing time (20 min) are the same as those for our Ta₂O₅:Tm thin films reported in [17] .

The PL spectra of the annealed 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 with a He-Cd laser (Kimmon, IK3251R-F, wavelength λ = 325 nm).

Figure 2 presents PL spectra from the four specimens annealed at 600˚C, 700˚C, 800˚C, or 900˚C for 20 min. The 800-nm-peaks due to the ³H₄→³H₆ transition of Tm³⁺ were observed from all the specimens though the similar peak was so far observed only from our Ta₂O₅:Tm co-sputtered film prepared using three Tm₂O₃ pellets and annealed at 900˚C [17] . The 800-nm-peak intensities from the specimens annealed at 600˚C and 900˚C were much stronger than the specimens annealed at 700˚C and 800˚C, and the maximum intensity was obtained from

the specimen annealed at 900˚C. Broad PL spectra ranging from 400 to 750 nm were also observed from the specimens annealed at lower temperatures of 600˚C and 700˚C. The broad spectra seem to originate from the ¹G₄→³H₆ transition of Tm³⁺ [18] and/or oxygen-vacancy trap levels of Ta₂O₅ reported in [19] .

On the other hand, comparatively narrow PL peaks around a wavelength of 400 nm (violet) were observed from the specimens annealed at higher temperatures of 800˚C and 900˚C. We thus observed violet-light emission bands from our Ta₂O₅:Tm co-sputtered thin films for the first time. The peak intensity from the specimen annealed at 900˚C was approximately four-times stronger than that from the specimen annealed at 800˚C. The origin of the 400-nm peaks seems to be the same as our non-doped Ta₂O₅ thin films deposited using RF sputtering because we observed PL peaks around wavelengths of 400 - 430 nm from Ta₂O₅ films annealed at 500˚C or 600˚C [5] .

We previously reported that the δ-Ta₂O₅ (hexagonal) phase in a Ta₂O₅:Tm thin film is very important for obtaining a strong PL peak at the wavelength of 800 nm from the film [17] . Therefore, the crystallizabilities of our Ta₂O₅:Tm thin films seem to be very important to obtain such violet PL peaks. We will conduct X-ray diffraction (XRD) measurements of the films in order to determine the relationship between the violet PL intensity and the crystallizability, and we will try to make the origin of the violet PL clear.

Such a Ta₂O₅:Tm co-sputtered thin film seems to be used as a multi-functional coating film having both anti- reflection [3] and down-conversion [20] -[22] effects for realizing a high-efficiency silicon solar cell.

We prepared Ta₂O₅:Tm thin films using co-sputtering of two Tm₂O₃ pellets and a Ta₂O₅ disc, and we observed not only PL peaks around a wavelength of 800 nm due to the ³H₄→³H₆ transition of Tm³⁺ but also violet PL peaks from the films after annealing for the first time. Comparatively narrow PL peaks around a wavelength of 400 nm were observed from the films annealed at 800˚C and 900˚C for 20 min. The peak intensity from the film annealed at 900˚C was approximately four-times stronger than that from the film annealed at 800˚C. The origin of the 400-nm peaks seems to be the same as our non-doped Ta₂O₅ thin films deposited using RF sputtering because we observe PL peaks around wavelengths of 400 - 430 nm from the Ta₂O₅ films. We will conduct XRD measurements of the Ta₂O₅:Tm thin films in order to determine the relationship between the violet PL intensities and the crystallizabilities of the film, and we will try to make the origin of the violet PL clear. Such Ta₂O₅:Tm co-sputtered thin films seem to be used as multi-functional coating films having both anti-reflection and down- conversion effects for realizing high-efficiency silicon solar cells.

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) Observation of Violet-Light Emission Band for Thulium-Doped Tantalum-Oxide Films Produced by Co-Sputtering. Materials Sciences and Applications,06,656-660. doi: 10.4236/msa.2015.67067