Journal of Materials Science and Chemical Engineering
Vol.03 No.08(2015), Article ID:58656,4 pages
10.4236/msce.2015.38005

Photoluminescence Properties of Europium and Cerium Co-Doped Tantalum-Oxide Thin Films Prepared Using Co-Sputtering Method

Kenta Miura*, Tetsuhito Suzuki, Osamu Hanaizumi

Graduate School of Science and Technology, Gunma University, Kiryu, Japan

Email: *mkenta@gunma-u.ac.jp

Copyright © 2015 by authors and Scientific Research Publishing Inc.

This work is licensed under the Creative Commons Attribution-NonCommercial International License (CC BY-NC).

http://creativecommons.org/licenses/by-nc/4.0/

Received 3 July 2015; accepted 4 August 2015; published 7 August 2015

ABSTRACT

We fabricated europium and cerium co-doped tantalum (V) oxide (Ta2O5: Eu, Ce) thin films using our co-sputtering method for the first time, and evaluated photoluminescence (PL) properties of the films after annealing at 600˚C - 1100˚C for 20 min. Four remarkable PL peaks at wavelengths of 600, 620, 700, and 705 nm were observed from the film annealed at 900˚C. The intensities of the 700- and 705-nm peaks due to the 5D07F4 transition of Eu3+ were much stronger than those of the 600-nm (5D07F1) and 620-nm (5D07F2) peaks of the film annealed at 900˚C. It seems that energy transfer from Ce3+ to Eu3+ occurs in the film, and much energy is selectively used for the 5D07F4 and 5D07F1 transitions. Such a Ta2O5: Eu, Ce 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.

Keywords:

Tantalum Oxide, Europium, Cerium, Co-Sputtering, Photoluminescence

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 (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] [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 were observed from thermal-oxidized amorphous Ta2O5 thin films [4] . Many studies on rare-earth-doped Ta2O5 have also been conducted because Ta2O5 is a potential host material for new phosphors due to its lower phonon energy (100 - 450 cm−1) than other popular oxide materials (e.g. silicon dioxide (SiO2)) [5] . We have fabricated various rare-earth doped Ta2O5 thin films using simply co-sputtering of rare-earth oxide pellets and a Ta2O5 disc, and we obtained various PL properties from these rare-earth-doped Ta2O5 thin films [6] - [11] . We reported on red or orange PL from europium (Eu)-doped Ta2O5 (Ta2O5: Eu) thin films deposited using the same co-sputtering method [9] . In our recent study, we fabricated erbium (Er), Eu, and cerium (Ce) co-doped Ta2O5 (Ta2O5: Er, Eu, Ce) thin films using co-sputtering of Er2O3, Eu2O3, CeO2 and Ta2O5, and observed yellow PL from a film annealed at 900˚C [10] . The yellow light emission seemed to be obtained from the result of enhancement of the 550-nm (green) PL peak due to Er3+ by Ce3+ doping [12] . We also prepared Er and Ce co-doped Ta2O5 (Ta2O5: Er, Ce) thin films using co-sputtering of Er2O3, CeO2 and Ta2O5. An enhanced green PL peak that seems to be sensitized by Ce3+ was observed from a film annealed at 900˚C [11] . We can obtain Ce3+ from CeO2 (cerium (IV) oxide) pellets because a small amount of Ce3+ exists at the surface of CeO2 [13] .

In this study, we fabricated Eu and Ce co-doped Ta2O5 (Ta2O5: Eu, Ce) thin films using our co-sputtering method for the first time, and we evaluated PL properties of the films.

2. Experimental

Ta2O5: Eu, Ce thin films were deposited using our RF magnetron sputtering system (ULVAC, SH-350-SE). A schematic figure of the system was presented in our previous report [7] . A Ta2O5 sintered-compact disc (Furuuchi Chemical Corporation, 99.99% purity, diameter 100 mm) was used as a sputtering target in the system. We placed an Eu2O3 sintered-compact pellet (Furuuchi Chemical Corporation, 99.9% purity, diameter 20 mm) and two CeO2 sintered-compact pellets (Furuuchi Chemical Corporation, 99.9% purity, diameter 20 mm) on the Ta2O5 disc as presented in Figure 1. They were co-sputtered by supplying RF power to the target. The flow rate of Ar gas introduced into the processing vacuum chamber was 15 sccm, and the pressure in the chamber during deposition was kept at ~5.4 × 10−4 Torr. The RF power supplied to the target was 200 W. Fused-silica plates (ATOCK Inc., 1 mm thick) were used as substrates, and they were not heated during sputtering. The thicknesses of the films were set to be ~1.5 μm by adjusting the sputtering times of the films.

We subsequently annealed the Ta2O5: Eu, Ce co-sputtered thin films in ambient air at 600˚C, 700˚C, 800˚C, 900˚C, 1000˚C, or 1100˚C for 20 min using an electric furnace (Denken, KDF S-70). We set the annealing time to 20 min because it was the proper condition for our Er-doped Ta2O5 (Ta2O5: Er) films to obtain strong PL intensities [6] [8] . The PL spectra of the 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. The Eu and Ce concentrations of the films after annealing were measured using an electron probe micro-analyzer (EPMA) (Shimadzu, EPMA-1610). The X-ray diffraction (XRD) patterns of the films were recorded using an X-ray diffractometer (RIGAKU, RINT2200VF+/PC system).

Figure 1. Schematic top view of the sputtering target for co-sputtering of an Eu2O3 pellet, two CeO2 pellets, and a Ta2O5 disc.

3. Results and Discussion

Figure 2 presents PL spectra of Ta2O5: Eu, Ce films annealed at 600˚C, 700˚C, 800˚C, 900˚C, 1000˚C, or 1100˚C for 20 min. Four remarkable PL peaks at wavelengths of 600, 620, 700, and 705 nm were observed only from the film annealed at 900˚C. No PL peak was observed from the films annealed at 600˚C, 700˚C, 800˚C, 1000˚C, or 1100˚C. The peaks at the wavelengths of 600 and 620 nm seem to be the results of the 5D07F1 and 5D07F2 transitions of Eu3+, respectively [9] , and the peaks at the wavelength of 700 and 705 nm seem to be due to the 5D07F4 transition of Eu3+ [9] . We could not observe a remarkable peak around a wavelength of 650 nm due to the 5D07F3 transition of Eu3+ that was observed in [9] . On the other hand, we found an additional 705-nm peak from the film annealed at 900˚C.

The Eu and Ce concentrations of the film annealed at 800˚C were measured to be around 1.5 and 2.8 mol%, respectively. The concentrations may be almost the same as those of the films annealed at the other temperatures.

Figure 3 presents XRD patterns of the films annealed at 600˚C, 700˚C, or 800˚C (Figure 3(a)) and those of the films annealed at 900˚C, 1000˚C, or 1100˚C (Figure 3(b)). The films annealed at 600˚C, 700˚C, and 800˚C seemed to be almost amorphous phases because no significant diffraction peak was observed from them as seen in Figure 3(a). On the other hand, three major peaks corresponding to the (0 0 1); β-Ta2O5 (orthorhombic), (2 0 0); δ-Ta2O5 (hexagonal), and (2 0 1) Ta2O5 phases were observed from the films annealed at 900˚C [7] . These crystalline phases of Ta2O5 seem to be very important for obtaining significant PL peaks from our Ta2O5: Eu, Ce films annealed at 900˚C. Furthermore, other diffraction peaks due to CeTa7O19 and EuTa7O19 crystals were observed from the films annealed at 1000˚C and 1100˚C. In particular, four peaks corresponding to the hexagonal CeTa7O19 phases ((1 0 0), (0 0 6), (1 1 1), and (1 1 5)) were remarkably observed as presented in Figure 3(b). Therefore, it seems that the hexagonal CeTa7O19 phases should be avoided in order to obtain PL from our Ta2O5: Eu, Ce films.

Figure 4 illustrates energy level diagrams of Eu3+ and Ce3+ [14] [15] . As presented in Figure 4, electrons are excited to the 2D5/2 state by the He-Cd laser irradiation (λ = 325 nm), and they relax to the 2D3/2 state of Ce3+. Subsequently, energy transfer from the 2D3/2 state of Ce3+ to the 5D1 state of Eu3+ occurs, and the electrons relax

Figure 2. PL spectra of Ta2O5: Eu, Ce co-sputtered thin films annealed at 600˚C, 700˚C, 800˚C, 900˚C, 1000˚C, or 1100˚C.

(a) (b)

Figure 3. XRD patterns of Ta2O5: Eu, Ce films annealed at (a) 600˚C, 700˚C, or 800˚C and (b) 900˚C, 1000˚C, or 1100˚C.

Figure 4. Energy level diagrams of Eu3+ and Ce3+.

to the 5D0 state of Eu3+. Finally, the transitions of 5D07F1, 5D07F2, and 5D07F4 occur, and light emission at wavelengths of 600, 620, 700, and 705 nm can be observed. In our previous study, the 620-nm peaks observed from our Ta2O5: Eu co-sputtered thin films were much stronger than the other peaks around the wavelengths of 600 and 700 nm [9] . However, as seen in Figure 2, the intensities of the 600-nm (5D07F1) and 620-nm (5D07F2) peaks were almost the same, and the intensity of the 700- and 705-nm (5D07F4) was much stronger than that of the 620-nm peak of the Ta2O5: Eu, Ce co-sputtered thin film annealed at 900˚C. It seems that energy transfer from Ce3+ to Eu3+ occurs in the film, and much energy is selectively used for the 5D07F4 and 5D07F1 transitions.

Such Ta2O5-based thin films seem to be used as high-refractive-index and light-emitting materials of “autocloning” photonic crystals that can be applied to novel light-emission devices [1] , and they also seem to be used as multi-functional coating films having both anti-reflection [3] and down-conversion [16] -[18] effects for realizing high-efficiency silicon solar cells.

4. Conclusion

We fabricated Ta2O5: Eu, Ce thin films using our co-sputtering method for the first time, and evaluated the PL properties of the films after annealing at 600˚C - 1100˚C for 20 min. Four remarkable PL peaks at wavelengths of 600, 620, 700, and 705 nm were observed from the film annealed at 900˚C. The intensities of the 700- and 705-nm peaks due to the 5D07F4 transition of Eu3+ were much stronger than those of the 600-nm (5D07F1) and 620-nm (5D07F2) peaks of the film annealed at 900˚C. It seems that energy transfer from Ce3+ to Eu3+ occurs in the film, and much energy is selectively used for the 5D07F4 and 5D07F1 transitions.

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.

Cite this paper

KentaMiura,TetsuhitoSuzuki,OsamuHanaizumi, (2015) Photoluminescence Properties of Europium and Cerium Co-Doped Tantalum-Oxide Thin Films Prepared Using Co-Sputtering Method. Journal of Materials Science and Chemical Engineering,03,30-34. doi: 10.4236/msce.2015.38005

References

  1. 1. Hanaizumi, O., Miura, K., Saito, M., Sato, T., Kawakami, S., Kuramochi, E. and Oku, S. (2000) Frontiers Related with Automatic Shaping of Photonic Crystals. IEICE Transactions on Electronics, E83-C, 912-919.

  2. 2. Sato, T., Miura, K., Ishino, N., Ohtera, Y., Tamamura, T. and Kawakami, S. (2002) Photonic Crystals for the Visible Range Fabricated by Autocloning Technique and Their Application. Optical and Quantum Electronics, 34, 63-70.
    http://dx.doi.org/10.1023/A:1013382711983

  3. 3. Cid, M., Stem, N., Brunetti, C., Beloto, A.F. and Ramos, C.A.S. (1998) Improvements in Anti-Reflection Coatings for High-Efficiency Silicon Solar Cells. Surface and Coatings Technology, 106, 117-120.
    http://dx.doi.org/10.1016/S0257-8972(98)00499-X

  4. 4. Zhu, M., Zhang, Z. and Miao, W. (2006) Intense Photoluminescence from Amorphous Tantalum Oxide Films. Applied Physics Letters, 89, Article ID: 021915.
    http://dx.doi.org/10.1063/1.2219991

  5. 5. Sanada, T., Wakai, Y., Nakashita, H., Matsumoto, T., Yogi, C., Ikeda, S., Wada, N. and Kojima, K. (2010) Preparation of Eu3+-Doped Ta2O5 Phosphor Particles by Sol-Gel Method. Optical Materials, 33, 164-169.
    http://dx.doi.org/10.1016/j.optmat.2010.08.018

  6. 6. Singh, M.K., Fusegi, G., Kano, K., Bange, J.P., Miura, K. and Hanaizumi, O. (2009) Intense Photoluminescence from Erbium-Doped Tantalum Oxide Thin Films Deposited by Sputtering. IEICE Electronics Express, 6, 1676-1682.
    http://dx.doi.org/10.1587/elex.6.1676

  7. 7. Bange, J.P., Singh, M.K., Kano, K., Miura, K. and Hanaizumi, O. (2011) Structural Analysis of RF Sputtered Er Doped Ta2O5 Films. Key Engineering Materials, 459, 32-37.
    http://dx.doi.org/10.4028/www.scientific.net/KEM.459.32

  8. 8. Singh, M.K., Miura, K., Fusegi, G., Kano, K. and Hanaizumi, O. (2013) Visible-Light Emission Properties of Erbium-Doped Tantalum-Oxide Films Produced by Co-Sputtering. Key Engineering Materials, 534, 154-157.
    http://dx.doi.org/10.4028/www.scientific.net/KEM.534.154

  9. 9. Miura, K., Arai, Y., Osawa, T. and Hanaizumi, O. (2012) Light-Emission Properties of Europium-Doped Tantalum- Oxide Thin Films Deposited by Radio-Frequency Magnetron Sputtering. Journal of Light & Visual Environment, 36, 64-67.
    http://dx.doi.org/10.2150/jlve.36.64

  10. 10. Miura, K., Osawa, T., Suzuki, T., Yokota, Y. and Hanaizumi, O. (2015) Yellow Light Emission from Ta2O5:Er, Eu, Ce Thin Films Deposited Using a Simple Co-Sputtering Method. Results in Physics, 5, 26-27.
    http://dx.doi.org/10.1016/j.rinp.2014.11.003

  11. 11. Miura, K., Osawa, T., Suzuki, T., Yokota, Y. and Hanaizumi, O. (2015) Fabrication and Evaluation of Green-Light Emitting Ta2O5:Er, Ce Co-Sputtered Thin Films. Results in Physics, 5, 78-79.
    http://dx.doi.org/10.1016/j.rinp.2015.02.002

  12. 12. Du, Q., Zhou, G., Zhou, J., Zhou, H. and Zhan, J. (2012) Enhanced Photoluminescence of CaZrO3:Er3+ by Efficient Energy Transfer from Ce3+. Materials Research Bulletin, 47, 3774-3779.
    http://dx.doi.org/10.1016/j.materresbull.2012.06.022

  13. 13. Roh, J., Hwang, S.H. and Jang, J. (2014) Dual-Functional CeO2:Eu3+ Nanocrystals for Performance-Enhanced Dye- Sensitized Solar Cells. ACS Applied Materials Interfaces, 6, 19825-19832.
    http://dx.doi.org/10.1021/am505194k

  14. 14. Kim, G.C., Mho, S.I. and Park, H.L. (1995) Observation of Energy Transfer between Ce3+ and Eu3+ in YAlO3:Ce,Eu. Journal of Materials Science Letters. 14, 805-806.
    http://dx.doi.org/10.1007/BF00278135

  15. 15. Wang, Y.F., Wang, S., Wu, Z.L., Li, W.R. and Ruan, Y.F. (2013) Photoluminescence Properties of Ce and Eu Co- Doped YVO4 Crystals. Journal of Alloys and Compounds, 551, 262-266.
    http://dx.doi.org/10.1016/j.jallcom.2012.10.042

  16. 16. Rodriguez, V.D., Tikhomirov, V.K., Mendez-Ramos, J., Yanes, A.C. and Moshchalkov, V.V. (2010) Towards Broad Range and Highly Efficient Down-Conversion of Solar Spectrum by Er3+-Yb3+ Co-Doped Nano-Structured Glass-Ce- ramics. Solar Energy Materials and Solar Cells, 94, 1612-1617.
    http://dx.doi.org/10.1016/j.solmat.2010.04.081

  17. 17. Aarts, L., van der Ende, B.M. and Meijerink, A. (2009) Downconversion for Solar Cells in NaYF4:Er, Yb. Journal of Applied Physics, 106, Article ID: 023522.
    http://dx.doi.org/10.1063/1.3177257

  18. 18. Ueda, J. and Tanabe, S. (2011) Broadband near Ultra Violet Sensitization of 1 μm Luminescence in Yb3+-Doped CeO2 Crystal. Journal of Applied Physics, 110, Article ID: 073104.
    http://dx.doi.org/10.1063/1.3642984

NOTES

*Corresponding author.