Observation of Violet-Light Emission Band for Thulium-Doped Tantalum-Oxide Films Produced by Co-Sputtering

Abstract

We prepared thulium-doped tantalum (V) oxide (Ta2O5:Tm) thin films using co-sputtering of two Tm2O3 pellets and a Ta2O5 disc, and we observed photoluminescence (PL) peaks not only around a wavelength of 800 nm due to the 3H43H6 transition of Tm3+ but also around a wavelength of 400 nm (violet) from the films after annealing for the first time. Comparatively narrow PL peaks around the 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 radio-frequency sputtering because we observe PL peaks around 400 - 430 nm from the Ta2O5 films. Such a Ta2O5:Tm co-sputtered thin film seems to be used as a multi-functional coating film having both anti-reflection and down-conversion effects for realizing a high-efficiency silicon solar cell.

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Miura, K. , Suzuki, T. and Hanaizumi, O. (2015) Observation of Violet-Light Emission Band for Thulium-Doped Tantalum-Oxide Films Produced by Co-Sputtering. Materials Sciences and Applications, 6, 656-660. doi: 10.4236/msa.2015.67067.

1. Introduction

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 3H43H6 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.

2. Experimental

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).

3. Results and Discussion

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 3H43H6 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

Figure 1. Schematic diagram of the sputtering target for co- sputtering of two Tm2O3 pellets and a Ta2O5 disc.

Figure 2. PL spectra of Ta2O5:Tm co-sputtered thin films annealed at 600˚C, 700˚C, 800˚C, or 900˚C for 20 min.

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 1G43H6 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.

4. Summary

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 3H43H6 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.

Acknowledgements

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.

NOTES

*Corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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