Color-tunable electroluminescence from Eudoped TiO2/p+-Si heterostructured devices: engineering of energy transfer Chen Zhu,1 Chunyan Lv,1,2 Canxing Wang,1 Yiping Sha,1 Dongsheng Li,1 Xiangyang Ma,1,* and Deren Yang1 1
State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China 2 Department of Chemistry, Huzhou University, Huzhou 313000, Zhejiang, China * [email protected]
Abstract: We report on color-tunable electroluminescence (EL) from TiO2:Eu/p+-Si heterostructured devices using different TiO2:Eu films in terms of Eu content and annealing temperature. It is found that the Eurelated emissions are activated by the energy transferred from TiO2 host via oxygen vacancies, at the price of weakened oxygen-vacancy-related emissions. Both the higher Eu content and the higher annealing temperature for TiO2:Eu films facilitate the aforementioned energy transfer. In this context, the dominant EL from the TiO2:Eu/p+-Si heterostructured devices can be transformed from oxygen-vacancy-related emissions into Eu-related emissions with increasing Eu-content and annealing temperature for TiO2:Eu films, exhibiting different colors of emanated light. We believe that this work sheds light on developing silicon-based red emitters using the Eudoped oxide semiconductor films. © 2015 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (160.5690) Rare-earth-doped materials; (160.2100) Electro-optical materials.
References and links 1.
M. A. J. Klik, T. Gregorkiewicz, I. V. Bradley, and J. P. R. Wells, “Optically induced deexcitation of Rare-Earth ions in a semiconductor matrix,” Phys. Rev. Lett. 89(22), 227401 (2002). 2. E. F. Pecora, T. I. Murphy, and L. D. Negro, “Rare earth doped Si-rich ZnO for multiband near-infrared light emitting devices,” Appl. Phys. Lett. 101(19), 191115 (2012). 3. S. C. Qu, W. H. Zhou, F. Q. Liu, N. F. Chen, Z. G. Wang, H. Y. Pan, and D. P. Yu, “Photoluminescence properties of Eu3+-doped ZnS nanocrystals prepared in a water/methanol solution,” Appl. Phys. Lett. 80(19), 3605 (2002). 4. A. J. Neuhalfen and B. W. Wessels, “Thermal quenching of Er3+-related luminescence in In1−xGaxP,” Appl. Phys. Lett. 60(21), 2657 (1992). 5. Y. S. Liu, W. Q. Luo, R. F. Li, and X. Y. Chen, “Optical properties of Nd3+ ion-doped ZnO nanocrystals,” J. Nanosci. Nanotechnol. 10(3), 1871–1876 (2010). 6. A. Conde-Gallardo, M. García-Rocha, I. Hernández-Calderón, and R. Palomino-Merino, “Photoluminescence properties of the Eu3+ activator ion in the TiO2 host matrix,” Appl. Phys. Lett. 78(22), 3436 (2001). 7. C. Y. Fu, J. S. Liao, W. Q. Luo, R. F. Li, and X. Y. Chen, “Emission of 1.53 microm originating from the lattice site of Er3+ ions incorporated in TiO2 nanocrystals,” Opt. Lett. 33(9), 953–955 (2008). 8. S. Komuro, T. Katsumata, H. Kokai, T. Morikawa, and X. Zhao, “Change in photoluminescence from Er-doped TiO2 thin films induced by optically assisted reduction,” Appl. Phys. Lett. 81(25), 4733 (2002). 9. Y. Yang, L. Jin, X. Ma, and D. Yang, “Low-voltage driven visible and infrared electroluminescence from lightemitting device based on Er-doped TiO2/p+-Si heterostructure,” Appl. Phys. Lett. 100(3), 031103 (2012). 10. Y. Yang, C. Lv, C. Zhu, S. Li, X. Ma, and D. Yang, “Near-infrared electroluminescence from light-emitting devices based on Nd-doped TiO2/p +-Si heterostructures,” Appl. Phys. Lett. 104(20), 201109 (2014). 11. L. Q. Jing, B. F. Xin, F. L. Yuan, L. P. Xue, B. Q. Wang, and H. G. Fu, “Effects of surface oxygen vacancies on photophysical and photochemical processes of Zn-doped TiO2 nanoparticles and their relationships,” J. Phys. Chem. B 110(36), 17860–17865 (2006). 12. Z. Zhou, T. Komori, T. Ayukawa, H. Yukawa, M. Morinaga, A. Koizumi, and Y. Takeda, “Li- and Er-codoped ZnO with enhanced 1.54 μm photoemission,” Appl. Phys. Lett. 87(9), 091109 (2005).
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2819
13. M. Popa, L. Diamandescu, F. Vasiliu, C. M. Teodorescu, V. Cosoveanu, M. Baia, M. Feder, L. Baia, and V. Danciu, “Synthesis, structural characterization, and photocatalytic properties of iron-doped TiO2 aerogels,” J. Mater. Sci. 44(2), 358–364 (2009). 14. T. Ohsaka, “Temperature dependence of the Raman spectrum in anatase TiO2,” J. Phys. Soc. Jpn. 48(5), 1661– 1668 (1980). 15. Y. P. Du, Y. W. Zhang, L. D. Sun, and C. H. Yan, “Efficient energy transfer in monodisperse Eu-doped ZnO nanocrystals synthesized from metal acetylacetonates in high-boiling solvents,” J. Phys. Chem. C 112(32), 12234–12241 (2008). 16. Q. Zeng, Z. Zhang, Z. Ding, Y. Wang, and Y. Sheng, “Strong photoluminescence emission of Eu:TiO2 nanotubes,” Scr. Mater. 57(10), 897–900 (2007). 17. J. G. Li, X. H. Wang, K. Watanabe, and T. Ishigaki, “Phase structure and luminescence properties of Eu3+-doped TiO2 nanocrystals synthesized by Ar/O2 radio frequency thermal plasma oxidation of liquid precursor mists,” J. Phys. Chem. B 110(3), 1121–1127 (2006). 18. D. D. Wang, G. Z. Xing, M. Gao, L. L. Yang, J. H. Yang, and T. Wu, “Defects-mediated energy transfer in redlight-emitting Eu-doped ZnO nanowire arrays,” J. Phys. Chem. C 115(46), 22729–22735 (2011). 19. Y. Zhang, X. Ma, P. Chen, D. Li, and D. Yang, “Electroluminescence from TiO2/p+-Si heterostructure,” Appl. Phys. Lett. 94(6), 061115 (2009). 20. Y. Zhang, X. Ma, P. Chen, D. Li, X. Pi, D. Yang, and P. G. Coleman, “Enhancement of electroluminescence from TiO2/p+-Si heterostructure-based devices through engineering of oxygen vacancies in TiO2,” Appl. Phys. Lett. 95(25), 252102 (2009).
1. Introduction Rare-earth (RE)-doped semiconductor thin films have long received considerable attention due to their promising technological applications in light-emitting devices (LEDs) [1–4]. The well-known fact that the RE3+ ions alone are weakly luminescent due to the parity forbidden intra-4f transitions necessitates the use of host materials to excite the RE3+ ions efficiently in a wide spectral range [5]. As a wide band-gap and cost-effective oxide semiconductor, TiO2 has been proved to be a desirable host for the RE3+ ions in terms of photoluminescence (PL) [6– 8]. Of technological importance is that our group has recently realized low-voltage driven electroluminescence (EL) in the visible and near-infrared from the LEDs based on the TiO2:RE (Er and Nd)/p+-Si heterostructures [9,10]. Therein, the energy transfer from TiO2 host to RE3+ ions is responsible for the RE-related EL. Logically, red EL could be enabled from the devices based on TiO2:Eu/p+-Si heterostructures. This will, however, not come true if the details of energy transfer from TiO2 host to Eu3+ ions are not essentially clarified. Due to the different ionic radii, the RE3+ ions will exert effects to different degrees on the formation of point defects such as oxygen vacancies in TiO2 host and even on the crystal phases of TiO2 host, which will result in different scenarios of the aforementioned energy transfer in diverse TiO2:RE materials [11,12]. Actually, in our previous reports, it has been found that the stories of energy transfer for the EL actions in TiO2:Er/p+-Si and TiO2:Nd/p+-Si heterostructured devices are quite different [9,10]. Compared with Er and Nd, Eu has an ionic radius in between, thus probably exhibiting influence on TiO2 host in a different way. Therefore, the details of energy transfer responsible for the EL from TiO2:Eu/p+-Si heterostructured devices need to be essentially elucidated. In this work, we have investigated the EL actions of the TiO2:Eu/p+-Si heterostructured devices using different TiO2:Eu films in terms of their Eu contents and annealing temperatures. It is found that the weights of oxygen-vacancy-related emissions from TiO2 host and Eu-related emissions in the EL from the TiO2:Eu/p+-Si heterostructured device depend heavily on the Eu content and annealing temperature for the TiO2:Eu film used. Both the higher Eu content and the higher annealing temperature facilitate the energy transfer from TiO2 host to Eu3+ ions, favoring the red EL from the TiO2:Eu/p+-Si heterostructured devices. We believe that this work paves the way for developing Si-compatible red LEDs based on the Eu-doped oxide semiconductors. 2. Experimental DETAILS The TiO2:Eu films were deposited on silicon substrates by radio-frequency sputtering. Here, 1.5 × 1.5 cm2-sized, -oriented and heavily boron-doped silicon (p+-Si) slices with a
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2820
resistivity of ~0.001 Ω•cm were used as the substrates. Prior to being loaded into the sputter chamber, the silicon substrates were cleaned by the standard RCA process, followed with a dip in a dilute HF solution (HF:H2O = 1:10 in volume ratio) for 1 min. The TiO2 ceramic targets doped with 0.8%, 1.0% (molar ratio) Eu2O3 were used for sputtering, respectively. The sputter chamber was firstly evacuated to a base pressure of 5 × 10−3 Pa and was then inlet with Ar gas to a working pressure of ~1 Pa. The sputtering target was fed with a power of 120 W and the p+-Si substrates were maintained at 100 °C. With 90 min sputtering, ~100 nm TiO2:Eu films were deposited onto the p+-Si substrates. Here, the film thickness was measured with DEKTAK XT stylus profiler (Bruker, Mass, USA). Subsequent to the sputtering, the TiO2:Eu films on p+-Si substrates were annealed at 550 or 650 °C for 2h in O2 ambient to enable crystallization. In order to form the devices based on the TiO2:Eu/p+-Si heterostructures formed by the aforementioned processes, ~150 nm thick ITO films onto the TiO2:Eu films and ~100 nm thick Au films onto the backsides of p+-Si substrates were deposited by directcurrent (DC) magnetron sputtering, respectively. Herein, the ITO and Au films were all patterned to be circular with a diameter of ~1.0 cm, acting as the device electrodes. The actual chemical compositions of the TiO2:Eu films were characterized by Rutherford backscattering spectroscopy (NEC 5SDH-2, Tokyo, Japan), using 2.022MeV 4He ion beam with a scattering angle of 165°. X-ray (XRD) characterization was performed on a Rigaku D/max-gA X-ray diffractometer (Rigaku, Osaka, Japan) with graphite monochromatized Cu Kα radiation (λ = 1.54178 Å). Moreover, the TiO2:Eu films were characterized by microRaman spectroscopy (Bruker Senterra, Bilerica, USA) using 532 nm He-Cd laser beam. The photoluminescence (PL) spectra for the TiO2:Eu films were acquired by excitation using a 325 nm He-Cd laser and the photoluminescence excitation (PLE) spectra were recorded using a Xe lamp as the excitation source. To activate electroluminescence (EL) from each device, forward DC bias was applied on the device with the positive voltage connecting to the p+-Si substrate. No EL could be detected from the device under reverse bias. The EL spectra were recorded using Acton SpectraPro 2500i Spectrometers (Princeton Instruments, New Jersey, USA). All measurements as mentioned above were performed at room temperature. 3. Results and Discussion
Fig. 1. (a) XRD patterns and (b) Raman spectra of the TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films annealed at 550 and 650 °C, respectively, for 2h in O2 ambient.
According to the RBS characterization results, the molar ratios of Eu/Ti are 1.2% and 1.7% in the two TiO2:Eu films sputtered from the TiO2:Eu (0.8%) and TiO2:Eu (1.0%) targets, respectively. For the sake of simplicity, hereafter, the two TiO2:Eu films are denoted as TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films, correspondingly. Fig. 1(a) shows the XRD
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2821
patterns of the above-mentioned two TiO2:Eu films annealed at 550 and 650 °C, respectively, for 2h in O2 ambient. As can be seen, all peaks in the XRD pattern for each film can be indexed into anatase TiO2. It should be noted that no Eu-related phases are revealed in the XRD pattern of each TiO2:Eu film, indicating that the Eu3+ ions are substantially incorporated into the TiO2 host. Nevertheless, mind should be kept in that not all of the Eu3+ ions enter into the crystal grains of TiO2 host. Due to the polycrystalline nature of TiO2:Eu films, a part of Eu3+ ions are inevitably segregated into the grain boundaries. As will be mentioned later, only the Eu3+ ions resident in the TiO2 grains are possibly luminescent. Strikingly, with the same anneal the TiO2:Eu (1.2%) film exhibits larger XRD peak intensities than the TiO2:Eu (1.7%) film. Since such two films have almost the same thickness, it is believed that the TiO2:Eu (1.2%) film is of better crystallinity. Because the ionic radius of Eu3+ ion (0.095nm) is much larger than that of Ti4+ ion (0.061nm), more significant substitution of Eu3+ ions for the Ti lattices leads to degrading the crystallinity of TiO2 host. As mentioned above, the TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films annealed at 550 and 650 °C, respectively, were further characterized by micro-Raman spectroscopy, which is sensitive to the short-range order in materials. In general, anatase TiO2 has six Raman-active modes (A1g + 2B1g + 3Eg) according to group theory [13]. Ohsaka once reported the six allowed modes appeared at ~144 (Eg), 197 (Eg), 399 (B1g), 513 (A1g), 519 (B1g), and 639 cm−1 (Eg), respectively, for an anatase TiO2 single crystal [14]. Among them, the 144 cm−1 mode is generally strongest. For our TiO2:Eu films, only the band at ~144 cm−1 can be found in their Raman spectra, most likely due to the very small thickness of the films. Figure 1(b) shows the Raman bands of Eg mode near 144 cm−1 for the TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films annealed at 550 and 650 °C, respectively. As can be seen, both the higher Eu content and the higher annealing temperature for the TiO2:Eu film result in more significant red-shift of the Raman band near 144 cm−1, which suggests that more oversize Eu3+ ions are substituted for Ti4+ ions in TiO2 grains. Actually, the red-shift of Raman band has also been reported for Eudoped ZnO (ZnO:Eu) nanocrystals. Du et al. have found the red-shifts of the main two Raman peaks of E1 (LO) near 584 cm−1 and E2 (high) near 433 cm−1 with increasing Eu content in ZnO:Eu nanocrystals, which indicates that more significant substitution of Zn2+ ions with oversize Eu3+ ions occurs due to the higher doped Eu content [15].
Fig. 2. PL spectra for the TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films annealed at 550 and 650 °C, respectively. Direct comparison of PL intensities can be made between parts a and b.
Figure 2 shows the PL spectra for the TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films annealed at 550 and 650 °C, respectively. The pronounced sharp peak located at ~616 nm corresponds to the electrical dipole transition 5D0→7F2 of Eu3+ ions. This peak is accompanied with the
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2822
Stark shifted shoulder peak located at ~624 nm [16]. For either the TiO2:Eu (1.2%) or the TiO2:Eu (1.7%) film annealed at 650 °C, as shown in Fig. 2(b), there is a minor sharp peak at ~592 nm in PL spectrum. Besides the aforementioned Eu-related emission peaks, a broad emission band covering the range 380-550 nm, which is believed to be relevant to deep-level defects, appears in each PL spectrum as shown in Fig. 2. Actually, the deep-level defects in TiO2:Eu films are primarily the oxygen vacancies, which are substantially generated by the substitution of Ti4+ lattice sites with Eu3+ ions, as expressed by 2TiO2 Eu2 O3 ⎯⎯⎯ → 2 EuTi' + VO•• + 3Oo×
(1)
where EuTi' is a Eu3+ ion in a Ti site, VO•• is an oxygen vacancy, and Oo× is a neutral oxygen atom. The substitution of Eu3+ for Ti4+ creates oxygen vacancies and lattice distortions in the TiO2 host, making the site symmetry of Eu3+ deviate from the exact D2d symmetry. Therefore, the electrical dipole transition 5D0→7F2 dominates the emissions from Eu3+ ions [17]. As for the TiO2:Eu films annealed at 550 °C, it can be seen from Fig. 2(a) that the TiO2:Eu (1.7%) film possesses larger intensities of both the oxygen-vacancy-related emission band and the characteristic emission peaks of Eu3+ ions than the TiO2:Eu (1.2%) film. This is due to that the TiO2:Eu (1.7%) film contains more oxygen vacancies, as can be known from the above discussion and, moreover, is doped with more optically active Eu3+ ions. Concerning the TiO2:Eu films annealed at 650 °C, Fig. 2(b) shows the understandable phenomenon that the TiO2:Eu (1.7%) film exhibits much stronger Eu-related emissions than the TiO2:Eu (1.2%) film. Noteworthily, despite the aforementioned fact that the TiO2:Eu (1.7%) film contains a higher concentration of oxygen vacancies than the TiO2:Eu (1.2%) film, the former exhibits a little weaker oxygen-vacancy-related emission band. The reason for this seemingly abnormal phenomenon will be elucidated later.
Fig. 3. (a) PLE spectra monitoring the emissions at 616 and 420 nm, respectively, for the TiO2:Eu (1.7%) film annealed at 650 °C. (b) Schematic diagram of the proposed energy transfer from TiO2 host to Eu3+ ions in the case of PL.
The PLE spectra monitoring the emissions at ~420 and 616 nm, respectively, for the TiO2:Eu (1.7%) film annealed at 650 °C is shown in Fig. 3(a). When monitoring the Eurelated emission at 616 nm, the PLE spectrum features a pronounced broad band in the range of 240-380 nm with a peak at ~310 nm, accompanied with three minor bands centered at ~392, 465 and 535 nm, respectively, which correspond to the intra-4f transitions from the ground state 7F0 to the excited-states of 5L6 and 5D2 as well as to the transition from 7F1 to 5D1 of Eu3+ ions [18]. As for the PLE spectrum monitoring the oxygen-vacancy-related emission peaking at 420 nm from the TiO2 host itself, a broad band covering 240-380 nm also appears.
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2823
Therefore, it is known that the characteristic emissions of Eu3+ ions and the oxygen-vacancyrelated emission from the TiO2 host can be excited by the same UV light in the region of 240380 nm. Moreover, we have measured the decay traces of the 420 nm emission for the TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films annealed at 650 °C, respectively. Both decay traces (not shown herein) can be fitted with double exponential functions. It is found that the weighted mean lifetime of 420 nm emission decreases from 5.31 to 3.73 µs with the Eu content increasing from 1.2% to 1.7%. Accordingly, it is believed that the Eu-related emissions in the red should be triggered by the energy transferred from the TiO2 host via the deep level transitions related to oxygen vacancies. Returning to Fig. 2, it is seen that the oxygen-vacancy-related emissions from the TiO2 host covers the range of 380-550 nm. Moreover, Fig. 3(a) shows that the characteristic absorption bands centered at ~392, 465 and 535 nm, respectively, are relevant to the 616 nm emission from Eu3+ ions. Evidently, the oxygen-vacancy-related emission spectrum and the Eu-related absorption spectrum overlap within a specific wavelength range. In view of the this fact, the mechanistic understanding of Eu-related emissions resulted from the energy transfer from the TiO2 host to Eu3+ ions can be schematically illustrated in Fig. 3(b), although the positions of the oxygen-vacancy-related energy levels in the bandgap of TiO2 host relative to the 4f levels of Eu3+ ion are not readily defined. As shown in Fig. 3(b), the photo-excited electrons in the conduction band of TiO2 firstly drop down to different oxygen-vacancyrelated deep levels. A fraction of energy released from the recombination of electrons in the oxygen-vacancy-related levels with the photo-excited holes in the valence band of TiO2 resonantly transfers to the Eu3+ ions in close proximity to the oxygen vacancies. Such transferred energy excites electrons from the ground level 7F0 to the higher lying levels 5D2, 5 L6 and from 7F1 to 5D1 of Eu3+ ions. Successively, thus-excited electrons firstly drop down to 5 D0 level nonradiatively. Besides, electrons can be also excited from the ground level 7F0 to 5 D0 level directly. Finally, electrons at 5D0 level transit to the 7F1 and 7F2 levels, leading to the emissions peaking at ~592 and 616 nm, respectively. Then, returning to Fig. 2(b), it is believed that the energy transfer from TiO2 host to Eu3+ ions is more remarkable in the TiO2:Eu (1.7%) film than in the TiO2:Eu (1.2%) film because more Eu3+ ions are adjacent to the oxygen vacancies in the TiO2:Eu (1.7%) film. Therefore, the oxygen-vacancy-related emission band of TiO2:Eu (1.7%) film is relatively weaker despite that the TiO2:Eu (1.7%) film contains a higher concentration of oxygen vacancies. Furthermore, the comparison between Figs. 2(a) and (b) suggests that the energy transfer from TiO2 host to Eu3+ ions is more significant in the TiO2:Eu films annealed at 650 °C than in those annealed at 550 °C. By the way, it is worth mentioning that only the Eu3+ ions resident in the TiO2 grains are possibly luminescent in view of the aforementioned scenario of energy transfer between the TiO2 host and Eu3+ ions via the oxygen vacancies as the sensitizers. Regarding the Eu3+ ions segregated into the grain boundaries within TiO2 film, they are hardly luminescent due to the lack of sensitizers for the energy transfer.
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2824
Fig. 4. EL spectra acquired with different forward injection currents for the TiO2:Eu/p+-Si heterostructured devices using (a) 550 °C-annealed TiO2:Eu (1.2%) film, (b) 650 °C-annealed TiO2:Eu (1.2%) film, (c) 550 °C-annealed TiO2:Eu (1.7%) film and (d) 650 °C-annealed TiO2:Eu (1.7%) film. The upper-right insets show the digital camera images of the light emissions at 20 mA from the four devices. Direct comparison of EL intensities can be made among parts a-d.
Figure 4 shows the EL spectra acquired at different forward injection currents for the TiO2:Eu/p+-Si heterostructured devices using the TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films annealed at 550 and 650 °C, respectively. Overall, the devices using the 650 °C-annealed TiO2:Eu films exhibit more pronounced Eu-related emissions primarily peaking at ~592, 616 and 624 nm, respectively. For the two devices using the TiO2:Eu (1.2%) films, as shown in Figs. 4(a) and (b), each EL spectrum involves a broad visible emission band peaking at ~540 nm, which is believed to be related to the oxygen vacancies in anatase TiO2 [19,20]. It should be mentioned that such oxygen-vacancy-related EL bands are quite different from the oxygenvacancy-related PL bands shown in Fig. 2 in terms of the spectrum’s shape and peak wavelength. This point has not been essentially understood so far. Most likely, the carrier injection level in PL is much higher than that in EL, which results in the distribution of more electrons at the higher oxygen-vacancy-related levels in the case of PL, thus leading to shorter peak wavelength in the PL spectra than in the EL spectra. Importantly, at a given injection current the device using the 650 °C-annealed TiO2:Eu (1.2%) film exhibits much more pronounced Eu-related emissions while relatively weaker oxygen-vacancy-related emissions than that using the 550 °C-annealed TiO2:Eu(1.2%) film, as can be seen from the comparison between Figs. 4(a) and 4(b). As revealed by the micro-Raman characterization, for the TiO2:Eu films of identical Eu content the 650 °C annealing results in more significant substitution of Eu3+ ions for Ti lattice sites in TiO2 grains than the 550 °C annealing. In turn, there should be more oxygen vacancies in the 650 °C-annealed TiO2:Eu film, as can be derived from the aforementioned Equa.1. Therefore, the comparison between Figs. 4(a) and (b) suggests that the Eu-related EL is activated by the energy resonantly transferred from the non-radiative recombination of electrons and holes trapped by the oxygen vacancies in TiO2 host, which is at the price of weakened oxygen-vacancy-related emissions. It should be
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2825
mentioned that the scenario of energy transfer from TiO2 host to Eu3+ ions in the case of EL is essentially similar to that in the case of PL. Regarding the devices using the TiO2:Eu (1.7%) films, as shown in Figs. 4(c) and (d), the Eu-related emission peaks dominate the EL spectra. Especially, for the device using the 650 °C-annealed TiO2:Eu (1.7%) film, Fig. 4(d) shows that the EL spectra are primarily characteristic of Eu-related emission peaks. As mentioned earlier, the 650 °C-annealed TiO2:Eu (1.7%) film possesses the highest content of optically active Eu3+ ions and the highest concentration of oxygen vacancies among the four TiO2:Eu films. Therefore, Fig. 4(d) indicates that the energy transfer from TiO2 host to Eu3+ ions via the oxygen vacancies is most significant in the EL from the device using the 650 °C-annealed TiO2:Eu (1.7%) film . As can be seen from the photographs shown in the insets of Fig. 4 for the light emissions from the four devices, the color of emanated light changes from orange to red as the Eu content and the annealing temperature are increased for the TiO2:Eu film, vividly reflecting the enhanced energy transfer from TiO2 host to Eu3+ ions arising from the incorporation of more luminescent Eu3+ ions and oxygen vacancies into TiO2 host. 4. Conclusions In summary, we have demonstrated the color-tunable EL from the TiO2:Eu/p+-Si heterostructured devices by adjusting the Eu content and annealing temperature for the TiO2:Eu films. Concretely, the predominating constituent in the EL from the device is transformed from the oxygen-vacancy-related emissions into Eu-related emissions by increasing the Eu content from 1.2 to 1.7% and the annealing temperature from 550 to 650 °C for the TiO2:Eu film used in the device. Either the higher Eu content or the higher annealing temperature leads to incorporation of more luminescent Eu3+ ions into TiO2 grains of the films. Moreover, the higher Eu content results in generation of more oxygen vacancies in TiO2 grains. It is well proved that Eu-related emissions are activated by the energy resonantly transferred from the non-radiative recombination of electrons and holes via oxygen-vacancyrelated levels, which is at the price of weakened oxygen-vacancy-related emissions. Such energy transfer from TiO2 host to Eu3+ ions can be enhanced by increasing either the Eu content or the annealing temperature for TiO2:Eu film. In this context, the TiO2:Eu/p+-Si heterostructured device using the TiO2:Eu (1.7%) film annealed at 650 °C features predominantly the Eu-related red emission as a result of substantial energy transfer as mentioned above. Acknowledgments The authors would like to thank the financial supports from National Natural Science Foundation of China (No.51372219) and “973 Program” (Grant No. 2007CB613403) Zhejiang Provincial Natural Science Foundation (No. LY12E02002).
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2826
State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China 2 Department of Chemistry, Huzhou University, Huzhou 313000, Zhejiang, China * [email protected]
Abstract: We report on color-tunable electroluminescence (EL) from TiO2:Eu/p+-Si heterostructured devices using different TiO2:Eu films in terms of Eu content and annealing temperature. It is found that the Eurelated emissions are activated by the energy transferred from TiO2 host via oxygen vacancies, at the price of weakened oxygen-vacancy-related emissions. Both the higher Eu content and the higher annealing temperature for TiO2:Eu films facilitate the aforementioned energy transfer. In this context, the dominant EL from the TiO2:Eu/p+-Si heterostructured devices can be transformed from oxygen-vacancy-related emissions into Eu-related emissions with increasing Eu-content and annealing temperature for TiO2:Eu films, exhibiting different colors of emanated light. We believe that this work sheds light on developing silicon-based red emitters using the Eudoped oxide semiconductor films. © 2015 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (160.5690) Rare-earth-doped materials; (160.2100) Electro-optical materials.
References and links 1.
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2819
13. M. Popa, L. Diamandescu, F. Vasiliu, C. M. Teodorescu, V. Cosoveanu, M. Baia, M. Feder, L. Baia, and V. Danciu, “Synthesis, structural characterization, and photocatalytic properties of iron-doped TiO2 aerogels,” J. Mater. Sci. 44(2), 358–364 (2009). 14. T. Ohsaka, “Temperature dependence of the Raman spectrum in anatase TiO2,” J. Phys. Soc. Jpn. 48(5), 1661– 1668 (1980). 15. Y. P. Du, Y. W. Zhang, L. D. Sun, and C. H. Yan, “Efficient energy transfer in monodisperse Eu-doped ZnO nanocrystals synthesized from metal acetylacetonates in high-boiling solvents,” J. Phys. Chem. C 112(32), 12234–12241 (2008). 16. Q. Zeng, Z. Zhang, Z. Ding, Y. Wang, and Y. Sheng, “Strong photoluminescence emission of Eu:TiO2 nanotubes,” Scr. Mater. 57(10), 897–900 (2007). 17. J. G. Li, X. H. Wang, K. Watanabe, and T. Ishigaki, “Phase structure and luminescence properties of Eu3+-doped TiO2 nanocrystals synthesized by Ar/O2 radio frequency thermal plasma oxidation of liquid precursor mists,” J. Phys. Chem. B 110(3), 1121–1127 (2006). 18. D. D. Wang, G. Z. Xing, M. Gao, L. L. Yang, J. H. Yang, and T. Wu, “Defects-mediated energy transfer in redlight-emitting Eu-doped ZnO nanowire arrays,” J. Phys. Chem. C 115(46), 22729–22735 (2011). 19. Y. Zhang, X. Ma, P. Chen, D. Li, and D. Yang, “Electroluminescence from TiO2/p+-Si heterostructure,” Appl. Phys. Lett. 94(6), 061115 (2009). 20. Y. Zhang, X. Ma, P. Chen, D. Li, X. Pi, D. Yang, and P. G. Coleman, “Enhancement of electroluminescence from TiO2/p+-Si heterostructure-based devices through engineering of oxygen vacancies in TiO2,” Appl. Phys. Lett. 95(25), 252102 (2009).
1. Introduction Rare-earth (RE)-doped semiconductor thin films have long received considerable attention due to their promising technological applications in light-emitting devices (LEDs) [1–4]. The well-known fact that the RE3+ ions alone are weakly luminescent due to the parity forbidden intra-4f transitions necessitates the use of host materials to excite the RE3+ ions efficiently in a wide spectral range [5]. As a wide band-gap and cost-effective oxide semiconductor, TiO2 has been proved to be a desirable host for the RE3+ ions in terms of photoluminescence (PL) [6– 8]. Of technological importance is that our group has recently realized low-voltage driven electroluminescence (EL) in the visible and near-infrared from the LEDs based on the TiO2:RE (Er and Nd)/p+-Si heterostructures [9,10]. Therein, the energy transfer from TiO2 host to RE3+ ions is responsible for the RE-related EL. Logically, red EL could be enabled from the devices based on TiO2:Eu/p+-Si heterostructures. This will, however, not come true if the details of energy transfer from TiO2 host to Eu3+ ions are not essentially clarified. Due to the different ionic radii, the RE3+ ions will exert effects to different degrees on the formation of point defects such as oxygen vacancies in TiO2 host and even on the crystal phases of TiO2 host, which will result in different scenarios of the aforementioned energy transfer in diverse TiO2:RE materials [11,12]. Actually, in our previous reports, it has been found that the stories of energy transfer for the EL actions in TiO2:Er/p+-Si and TiO2:Nd/p+-Si heterostructured devices are quite different [9,10]. Compared with Er and Nd, Eu has an ionic radius in between, thus probably exhibiting influence on TiO2 host in a different way. Therefore, the details of energy transfer responsible for the EL from TiO2:Eu/p+-Si heterostructured devices need to be essentially elucidated. In this work, we have investigated the EL actions of the TiO2:Eu/p+-Si heterostructured devices using different TiO2:Eu films in terms of their Eu contents and annealing temperatures. It is found that the weights of oxygen-vacancy-related emissions from TiO2 host and Eu-related emissions in the EL from the TiO2:Eu/p+-Si heterostructured device depend heavily on the Eu content and annealing temperature for the TiO2:Eu film used. Both the higher Eu content and the higher annealing temperature facilitate the energy transfer from TiO2 host to Eu3+ ions, favoring the red EL from the TiO2:Eu/p+-Si heterostructured devices. We believe that this work paves the way for developing Si-compatible red LEDs based on the Eu-doped oxide semiconductors. 2. Experimental DETAILS The TiO2:Eu films were deposited on silicon substrates by radio-frequency sputtering. Here, 1.5 × 1.5 cm2-sized, -oriented and heavily boron-doped silicon (p+-Si) slices with a
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2820
resistivity of ~0.001 Ω•cm were used as the substrates. Prior to being loaded into the sputter chamber, the silicon substrates were cleaned by the standard RCA process, followed with a dip in a dilute HF solution (HF:H2O = 1:10 in volume ratio) for 1 min. The TiO2 ceramic targets doped with 0.8%, 1.0% (molar ratio) Eu2O3 were used for sputtering, respectively. The sputter chamber was firstly evacuated to a base pressure of 5 × 10−3 Pa and was then inlet with Ar gas to a working pressure of ~1 Pa. The sputtering target was fed with a power of 120 W and the p+-Si substrates were maintained at 100 °C. With 90 min sputtering, ~100 nm TiO2:Eu films were deposited onto the p+-Si substrates. Here, the film thickness was measured with DEKTAK XT stylus profiler (Bruker, Mass, USA). Subsequent to the sputtering, the TiO2:Eu films on p+-Si substrates were annealed at 550 or 650 °C for 2h in O2 ambient to enable crystallization. In order to form the devices based on the TiO2:Eu/p+-Si heterostructures formed by the aforementioned processes, ~150 nm thick ITO films onto the TiO2:Eu films and ~100 nm thick Au films onto the backsides of p+-Si substrates were deposited by directcurrent (DC) magnetron sputtering, respectively. Herein, the ITO and Au films were all patterned to be circular with a diameter of ~1.0 cm, acting as the device electrodes. The actual chemical compositions of the TiO2:Eu films were characterized by Rutherford backscattering spectroscopy (NEC 5SDH-2, Tokyo, Japan), using 2.022MeV 4He ion beam with a scattering angle of 165°. X-ray (XRD) characterization was performed on a Rigaku D/max-gA X-ray diffractometer (Rigaku, Osaka, Japan) with graphite monochromatized Cu Kα radiation (λ = 1.54178 Å). Moreover, the TiO2:Eu films were characterized by microRaman spectroscopy (Bruker Senterra, Bilerica, USA) using 532 nm He-Cd laser beam. The photoluminescence (PL) spectra for the TiO2:Eu films were acquired by excitation using a 325 nm He-Cd laser and the photoluminescence excitation (PLE) spectra were recorded using a Xe lamp as the excitation source. To activate electroluminescence (EL) from each device, forward DC bias was applied on the device with the positive voltage connecting to the p+-Si substrate. No EL could be detected from the device under reverse bias. The EL spectra were recorded using Acton SpectraPro 2500i Spectrometers (Princeton Instruments, New Jersey, USA). All measurements as mentioned above were performed at room temperature. 3. Results and Discussion
Fig. 1. (a) XRD patterns and (b) Raman spectra of the TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films annealed at 550 and 650 °C, respectively, for 2h in O2 ambient.
According to the RBS characterization results, the molar ratios of Eu/Ti are 1.2% and 1.7% in the two TiO2:Eu films sputtered from the TiO2:Eu (0.8%) and TiO2:Eu (1.0%) targets, respectively. For the sake of simplicity, hereafter, the two TiO2:Eu films are denoted as TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films, correspondingly. Fig. 1(a) shows the XRD
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2821
patterns of the above-mentioned two TiO2:Eu films annealed at 550 and 650 °C, respectively, for 2h in O2 ambient. As can be seen, all peaks in the XRD pattern for each film can be indexed into anatase TiO2. It should be noted that no Eu-related phases are revealed in the XRD pattern of each TiO2:Eu film, indicating that the Eu3+ ions are substantially incorporated into the TiO2 host. Nevertheless, mind should be kept in that not all of the Eu3+ ions enter into the crystal grains of TiO2 host. Due to the polycrystalline nature of TiO2:Eu films, a part of Eu3+ ions are inevitably segregated into the grain boundaries. As will be mentioned later, only the Eu3+ ions resident in the TiO2 grains are possibly luminescent. Strikingly, with the same anneal the TiO2:Eu (1.2%) film exhibits larger XRD peak intensities than the TiO2:Eu (1.7%) film. Since such two films have almost the same thickness, it is believed that the TiO2:Eu (1.2%) film is of better crystallinity. Because the ionic radius of Eu3+ ion (0.095nm) is much larger than that of Ti4+ ion (0.061nm), more significant substitution of Eu3+ ions for the Ti lattices leads to degrading the crystallinity of TiO2 host. As mentioned above, the TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films annealed at 550 and 650 °C, respectively, were further characterized by micro-Raman spectroscopy, which is sensitive to the short-range order in materials. In general, anatase TiO2 has six Raman-active modes (A1g + 2B1g + 3Eg) according to group theory [13]. Ohsaka once reported the six allowed modes appeared at ~144 (Eg), 197 (Eg), 399 (B1g), 513 (A1g), 519 (B1g), and 639 cm−1 (Eg), respectively, for an anatase TiO2 single crystal [14]. Among them, the 144 cm−1 mode is generally strongest. For our TiO2:Eu films, only the band at ~144 cm−1 can be found in their Raman spectra, most likely due to the very small thickness of the films. Figure 1(b) shows the Raman bands of Eg mode near 144 cm−1 for the TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films annealed at 550 and 650 °C, respectively. As can be seen, both the higher Eu content and the higher annealing temperature for the TiO2:Eu film result in more significant red-shift of the Raman band near 144 cm−1, which suggests that more oversize Eu3+ ions are substituted for Ti4+ ions in TiO2 grains. Actually, the red-shift of Raman band has also been reported for Eudoped ZnO (ZnO:Eu) nanocrystals. Du et al. have found the red-shifts of the main two Raman peaks of E1 (LO) near 584 cm−1 and E2 (high) near 433 cm−1 with increasing Eu content in ZnO:Eu nanocrystals, which indicates that more significant substitution of Zn2+ ions with oversize Eu3+ ions occurs due to the higher doped Eu content [15].
Fig. 2. PL spectra for the TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films annealed at 550 and 650 °C, respectively. Direct comparison of PL intensities can be made between parts a and b.
Figure 2 shows the PL spectra for the TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films annealed at 550 and 650 °C, respectively. The pronounced sharp peak located at ~616 nm corresponds to the electrical dipole transition 5D0→7F2 of Eu3+ ions. This peak is accompanied with the
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2822
Stark shifted shoulder peak located at ~624 nm [16]. For either the TiO2:Eu (1.2%) or the TiO2:Eu (1.7%) film annealed at 650 °C, as shown in Fig. 2(b), there is a minor sharp peak at ~592 nm in PL spectrum. Besides the aforementioned Eu-related emission peaks, a broad emission band covering the range 380-550 nm, which is believed to be relevant to deep-level defects, appears in each PL spectrum as shown in Fig. 2. Actually, the deep-level defects in TiO2:Eu films are primarily the oxygen vacancies, which are substantially generated by the substitution of Ti4+ lattice sites with Eu3+ ions, as expressed by 2TiO2 Eu2 O3 ⎯⎯⎯ → 2 EuTi' + VO•• + 3Oo×
(1)
where EuTi' is a Eu3+ ion in a Ti site, VO•• is an oxygen vacancy, and Oo× is a neutral oxygen atom. The substitution of Eu3+ for Ti4+ creates oxygen vacancies and lattice distortions in the TiO2 host, making the site symmetry of Eu3+ deviate from the exact D2d symmetry. Therefore, the electrical dipole transition 5D0→7F2 dominates the emissions from Eu3+ ions [17]. As for the TiO2:Eu films annealed at 550 °C, it can be seen from Fig. 2(a) that the TiO2:Eu (1.7%) film possesses larger intensities of both the oxygen-vacancy-related emission band and the characteristic emission peaks of Eu3+ ions than the TiO2:Eu (1.2%) film. This is due to that the TiO2:Eu (1.7%) film contains more oxygen vacancies, as can be known from the above discussion and, moreover, is doped with more optically active Eu3+ ions. Concerning the TiO2:Eu films annealed at 650 °C, Fig. 2(b) shows the understandable phenomenon that the TiO2:Eu (1.7%) film exhibits much stronger Eu-related emissions than the TiO2:Eu (1.2%) film. Noteworthily, despite the aforementioned fact that the TiO2:Eu (1.7%) film contains a higher concentration of oxygen vacancies than the TiO2:Eu (1.2%) film, the former exhibits a little weaker oxygen-vacancy-related emission band. The reason for this seemingly abnormal phenomenon will be elucidated later.
Fig. 3. (a) PLE spectra monitoring the emissions at 616 and 420 nm, respectively, for the TiO2:Eu (1.7%) film annealed at 650 °C. (b) Schematic diagram of the proposed energy transfer from TiO2 host to Eu3+ ions in the case of PL.
The PLE spectra monitoring the emissions at ~420 and 616 nm, respectively, for the TiO2:Eu (1.7%) film annealed at 650 °C is shown in Fig. 3(a). When monitoring the Eurelated emission at 616 nm, the PLE spectrum features a pronounced broad band in the range of 240-380 nm with a peak at ~310 nm, accompanied with three minor bands centered at ~392, 465 and 535 nm, respectively, which correspond to the intra-4f transitions from the ground state 7F0 to the excited-states of 5L6 and 5D2 as well as to the transition from 7F1 to 5D1 of Eu3+ ions [18]. As for the PLE spectrum monitoring the oxygen-vacancy-related emission peaking at 420 nm from the TiO2 host itself, a broad band covering 240-380 nm also appears.
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2823
Therefore, it is known that the characteristic emissions of Eu3+ ions and the oxygen-vacancyrelated emission from the TiO2 host can be excited by the same UV light in the region of 240380 nm. Moreover, we have measured the decay traces of the 420 nm emission for the TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films annealed at 650 °C, respectively. Both decay traces (not shown herein) can be fitted with double exponential functions. It is found that the weighted mean lifetime of 420 nm emission decreases from 5.31 to 3.73 µs with the Eu content increasing from 1.2% to 1.7%. Accordingly, it is believed that the Eu-related emissions in the red should be triggered by the energy transferred from the TiO2 host via the deep level transitions related to oxygen vacancies. Returning to Fig. 2, it is seen that the oxygen-vacancy-related emissions from the TiO2 host covers the range of 380-550 nm. Moreover, Fig. 3(a) shows that the characteristic absorption bands centered at ~392, 465 and 535 nm, respectively, are relevant to the 616 nm emission from Eu3+ ions. Evidently, the oxygen-vacancy-related emission spectrum and the Eu-related absorption spectrum overlap within a specific wavelength range. In view of the this fact, the mechanistic understanding of Eu-related emissions resulted from the energy transfer from the TiO2 host to Eu3+ ions can be schematically illustrated in Fig. 3(b), although the positions of the oxygen-vacancy-related energy levels in the bandgap of TiO2 host relative to the 4f levels of Eu3+ ion are not readily defined. As shown in Fig. 3(b), the photo-excited electrons in the conduction band of TiO2 firstly drop down to different oxygen-vacancyrelated deep levels. A fraction of energy released from the recombination of electrons in the oxygen-vacancy-related levels with the photo-excited holes in the valence band of TiO2 resonantly transfers to the Eu3+ ions in close proximity to the oxygen vacancies. Such transferred energy excites electrons from the ground level 7F0 to the higher lying levels 5D2, 5 L6 and from 7F1 to 5D1 of Eu3+ ions. Successively, thus-excited electrons firstly drop down to 5 D0 level nonradiatively. Besides, electrons can be also excited from the ground level 7F0 to 5 D0 level directly. Finally, electrons at 5D0 level transit to the 7F1 and 7F2 levels, leading to the emissions peaking at ~592 and 616 nm, respectively. Then, returning to Fig. 2(b), it is believed that the energy transfer from TiO2 host to Eu3+ ions is more remarkable in the TiO2:Eu (1.7%) film than in the TiO2:Eu (1.2%) film because more Eu3+ ions are adjacent to the oxygen vacancies in the TiO2:Eu (1.7%) film. Therefore, the oxygen-vacancy-related emission band of TiO2:Eu (1.7%) film is relatively weaker despite that the TiO2:Eu (1.7%) film contains a higher concentration of oxygen vacancies. Furthermore, the comparison between Figs. 2(a) and (b) suggests that the energy transfer from TiO2 host to Eu3+ ions is more significant in the TiO2:Eu films annealed at 650 °C than in those annealed at 550 °C. By the way, it is worth mentioning that only the Eu3+ ions resident in the TiO2 grains are possibly luminescent in view of the aforementioned scenario of energy transfer between the TiO2 host and Eu3+ ions via the oxygen vacancies as the sensitizers. Regarding the Eu3+ ions segregated into the grain boundaries within TiO2 film, they are hardly luminescent due to the lack of sensitizers for the energy transfer.
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2824
Fig. 4. EL spectra acquired with different forward injection currents for the TiO2:Eu/p+-Si heterostructured devices using (a) 550 °C-annealed TiO2:Eu (1.2%) film, (b) 650 °C-annealed TiO2:Eu (1.2%) film, (c) 550 °C-annealed TiO2:Eu (1.7%) film and (d) 650 °C-annealed TiO2:Eu (1.7%) film. The upper-right insets show the digital camera images of the light emissions at 20 mA from the four devices. Direct comparison of EL intensities can be made among parts a-d.
Figure 4 shows the EL spectra acquired at different forward injection currents for the TiO2:Eu/p+-Si heterostructured devices using the TiO2:Eu (1.2%) and TiO2:Eu (1.7%) films annealed at 550 and 650 °C, respectively. Overall, the devices using the 650 °C-annealed TiO2:Eu films exhibit more pronounced Eu-related emissions primarily peaking at ~592, 616 and 624 nm, respectively. For the two devices using the TiO2:Eu (1.2%) films, as shown in Figs. 4(a) and (b), each EL spectrum involves a broad visible emission band peaking at ~540 nm, which is believed to be related to the oxygen vacancies in anatase TiO2 [19,20]. It should be mentioned that such oxygen-vacancy-related EL bands are quite different from the oxygenvacancy-related PL bands shown in Fig. 2 in terms of the spectrum’s shape and peak wavelength. This point has not been essentially understood so far. Most likely, the carrier injection level in PL is much higher than that in EL, which results in the distribution of more electrons at the higher oxygen-vacancy-related levels in the case of PL, thus leading to shorter peak wavelength in the PL spectra than in the EL spectra. Importantly, at a given injection current the device using the 650 °C-annealed TiO2:Eu (1.2%) film exhibits much more pronounced Eu-related emissions while relatively weaker oxygen-vacancy-related emissions than that using the 550 °C-annealed TiO2:Eu(1.2%) film, as can be seen from the comparison between Figs. 4(a) and 4(b). As revealed by the micro-Raman characterization, for the TiO2:Eu films of identical Eu content the 650 °C annealing results in more significant substitution of Eu3+ ions for Ti lattice sites in TiO2 grains than the 550 °C annealing. In turn, there should be more oxygen vacancies in the 650 °C-annealed TiO2:Eu film, as can be derived from the aforementioned Equa.1. Therefore, the comparison between Figs. 4(a) and (b) suggests that the Eu-related EL is activated by the energy resonantly transferred from the non-radiative recombination of electrons and holes trapped by the oxygen vacancies in TiO2 host, which is at the price of weakened oxygen-vacancy-related emissions. It should be
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2825
mentioned that the scenario of energy transfer from TiO2 host to Eu3+ ions in the case of EL is essentially similar to that in the case of PL. Regarding the devices using the TiO2:Eu (1.7%) films, as shown in Figs. 4(c) and (d), the Eu-related emission peaks dominate the EL spectra. Especially, for the device using the 650 °C-annealed TiO2:Eu (1.7%) film, Fig. 4(d) shows that the EL spectra are primarily characteristic of Eu-related emission peaks. As mentioned earlier, the 650 °C-annealed TiO2:Eu (1.7%) film possesses the highest content of optically active Eu3+ ions and the highest concentration of oxygen vacancies among the four TiO2:Eu films. Therefore, Fig. 4(d) indicates that the energy transfer from TiO2 host to Eu3+ ions via the oxygen vacancies is most significant in the EL from the device using the 650 °C-annealed TiO2:Eu (1.7%) film . As can be seen from the photographs shown in the insets of Fig. 4 for the light emissions from the four devices, the color of emanated light changes from orange to red as the Eu content and the annealing temperature are increased for the TiO2:Eu film, vividly reflecting the enhanced energy transfer from TiO2 host to Eu3+ ions arising from the incorporation of more luminescent Eu3+ ions and oxygen vacancies into TiO2 host. 4. Conclusions In summary, we have demonstrated the color-tunable EL from the TiO2:Eu/p+-Si heterostructured devices by adjusting the Eu content and annealing temperature for the TiO2:Eu films. Concretely, the predominating constituent in the EL from the device is transformed from the oxygen-vacancy-related emissions into Eu-related emissions by increasing the Eu content from 1.2 to 1.7% and the annealing temperature from 550 to 650 °C for the TiO2:Eu film used in the device. Either the higher Eu content or the higher annealing temperature leads to incorporation of more luminescent Eu3+ ions into TiO2 grains of the films. Moreover, the higher Eu content results in generation of more oxygen vacancies in TiO2 grains. It is well proved that Eu-related emissions are activated by the energy resonantly transferred from the non-radiative recombination of electrons and holes via oxygen-vacancyrelated levels, which is at the price of weakened oxygen-vacancy-related emissions. Such energy transfer from TiO2 host to Eu3+ ions can be enhanced by increasing either the Eu content or the annealing temperature for TiO2:Eu film. In this context, the TiO2:Eu/p+-Si heterostructured device using the TiO2:Eu (1.7%) film annealed at 650 °C features predominantly the Eu-related red emission as a result of substantial energy transfer as mentioned above. Acknowledgments The authors would like to thank the financial supports from National Natural Science Foundation of China (No.51372219) and “973 Program” (Grant No. 2007CB613403) Zhejiang Provincial Natural Science Foundation (No. LY12E02002).
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Received 30 Oct 2014; revised 25 Dec 2014; accepted 26 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002819 | OPTICS EXPRESS 2826
