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Photo- and cathodoluminescence of Eu3+ or Tb3+ doped CaZrO3 films prepared by pulsed laser deposition
Optical Materials 73 (2017) 504e508
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Photo- and cathodoluminescence of Eu3þ or Tb3þ doped CaZrO3 films prepared by pulsed laser deposition Kazushige Ueda a, *, Yuhei Shimizu a, Hiroshi Takashima b, **, Florian Massuyeau c, phane Jobic c Ste a
Department of Materials Science, Graduate School of Engineering, Kyushu Institute of Technology, 1-1 Sensui, Tobata, Kitakyushu, 804-8550, Japan Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki, 3058568, Japan c Institut des Mat eriaux Jean Rouxel (IMN), Universit e de Nantes, CNRS, 2 rue de la Houssini ere, B.P. 32229, 44322, Nantes Cedex 3, France b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 June 2017 Received in revised form 21 August 2017 Accepted 4 September 2017
Luminescent films of Eu3þ or Tb3þ doped CaZrO3 were epitaxially grown on SrTiO3 or LaAlO3 (001) single crystal substrates by pulsed laser deposition (PLD). They showed a high transmittance in the visible region and a smooth surface. Uniform red photoluminescence (PL) was observed from Eu3þ 5% doped films, and blue and green PL was observed from Tb3þ 0.5% and 5% doped films, respectively. These PL spectra of the films turned out to be very similar to those of the powder samples. Practically, it has to be noticed the use of a LaAlO3 substrate is necessary for intense blue luminescence for Tb3þ 0.5% doped films to avoid optical absorption of emitted light by the SrTiO3 substrate. The cathodoluminescence (CL) spectra of the red and green luminescent films were almost the same as those PL spectra. In contrast, blue CL was significantly enhanced in CL compared to PL. This was attributed to the shorter lifetime of the 5D3-7FJ transitions and many excited electron-hole pairs, which were generated rapidly on high energy electron irradiation. © 2017 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Cathodoluminescence Perovskite CaZrO3 Thin film
1. Introduction Light emitting inorganic materials have been widely investigated for applications in lighting and displays. Nowadays, nitride, sulfide and oxide compounds are frequently used as practical luminescent materials for light emitting diodes (LEDs), electroluminescent (EL) devices and fluorescent lamps [1e4]. Nevertheless, among the large family of inorganic compounds, oxide materials have many advantages in practice because they are chemically and thermally more stable than their nitride and sulfide congeners, and easy to fabricate under ambient atmosphere. Accordingly, the fundamental properties of many luminescent oxides have been evaluated by photoluminescence (PL) and cathodoluminescence (CL). Pr3þ doped luminescent perovskite-type oxides such as CaTiO3:Pr3þ and SrTiO3:Pr3þ were examined previously from both
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Ueda), [email protected] (H. Takashima). http://dx.doi.org/10.1016/j.optmat.2017.09.007 0925-3467/© 2017 Elsevier B.V. All rights reserved.
scientific and practical viewpoints [5e9]. Although perovskite-type oxides show various electronic functions such as ferroelectricity, piezoelectricity, and semiconductivity, the luminescent function is not extensively investigated yet. The development of practical red luminescent Pr3þ and Al3þ codoped titanate for field emission displays led to further application of luminescent perovskite-type oxides to lighting components such as EL devices. Hence, Pr3þ doped (Ca,Sr)TiO3 thin films have been used as emitting layers in EL devices, which were operated at low voltage [10]. Highluminescence EL devices using Pr3þ doped (Ca,Sr)TiO3 films were also reported along with the ac and dc operation [11]. However, only red luminescence turns to be available in titanate phosphors. Recently, perovskite-type stannates doped with lanthanide (Ln) ions were examined to achieve several luminescent colors for EL devices [12,13]. Although some luminescent colors were found to be available in stannates, the important luminescent colors of red, green and blue (RGB) were not obtained from the only perovskitetype CaSnO3 host lattice unfortunately. In contrast to titanates and stannates, Ln doped zirconates, especially CaZrO3, were found to show RGB tricolor luminescence without changing the host lattice [14e17], which is anticipated to
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facilitate a process of luminescent device fabrications. Eu3þ doped CaZrO3 showed red luminescence, Tb3þ doped CaZrO3 showed green luminescence at medium Tb3þ doping concentrations, and blue luminescence was seen at a low Tb3þ doping concentration. In this study, to apply doped CaZrO3 with RGB luminescence, we first attempted the preparation of high quality thin films on single crystal substrates and examined their PL and CL properties. 2. Experiments Thin films of Ln doped CaZrO3 were grown on SrTiO3 (STO) (001) or LaAlO3 (LAO) (001) single crystal substrates at high temperatures using a pulsed laser deposition (PLD) technique with polycrystalline sintered disks as targets. The disks were prepared by a conventional solid state reaction and their chemical compositions were {(Ca0.97Mg0.03)0.95Eu0.05}ZrO3þd for red, {(Ca0.97Mg0.03)0.95Tb0.05} ZrO3þd for green, and {(Ca0.97Mg0.03)0.995Tb0.005}ZrO3þd for blue luminescence, as reported previously [14]. Mg codoping usually enhances the PL intensity of samples [12,14,18] and all Ln doped samples in this study were codoped with Mg ions even if not mentioned explicitly. The as-deposited films were annealed in air to obtain a higher crystallinity and intense luminescence. The PLD and annealing conditions are summarized in Table 1. Crystalline phases in the films were analyzed by X-ray diffraction (XRD) measurements (Rigaku, RINT 2500), and film surfaces were characterized by atomic force microscope (AFM) (Digital Instruments, Nanoscope III) and reflection high energy electron diffraction (RHEED) (KSA 400). Transmission spectra of the films were collected using an UV/Vis/ NIR spectrophotometer (Jasco, V-570). PL/PL excitation (PLE) spectra were measured using a conventional spectrofluorometer (Jasco, FP-6500), and fluorescence decay curves were obtained using a spectrofluorometer with a Xe flash lamp (HORIBA Jobin Yvon, Fluorolog-3). CL spectra of the films were obtained using a CNT field-emission electron gun with a gate electrode and a modular USB spectrometer with an optical fiber (Ocean Optics USB4000). The electron acceleration voltage for the electron gun was varied up to 2.0 kV in the CL measurements. 3. Results and discussion The XRD patterns of Tb3þ 5% doped CaZrO3 films deposited on STO substrates at different temperatures are shown in Fig. 1. The XRD patterns clearly show the formation of the main perovskitetype CaZrO3 phase with a small amount of a CaO-stabilized zirconia (CSZ) phase as an impurity for low temperature synthesis. As shown in the inset, films deposited at temperatures below approximately 700 C were amorphous in contrast with a film deposited at 740 C that was crystallized with distinguishable diffraction peaks of CaZrO3. After annealing for 1 h at 1000 C, the
Fig. 1. XRD patterns of Tb3þ 5% doped CaZrO3 thin films deposited at different substrate temperatures. Simulated XRD patterns of CaZrO3 (solid line) and CSZ (dashed line) are compared at the bottom. XRD patterns of the films in the as-deposited state (thin line) and after annealing (thick line) are shown in the inset.
amorphous films deposited below 700 C were crystallized accompanying a CSZ secondary phase. On the other hand, a highly crystalline phase pure CaZrO3 film was obtained from the film deposited at 740 C, revealing that the annealing only increased the crystallinity. Therefore, high substrate temperature was necessary during film deposition to obtain a high quality and pure perovskitetype films, and avoid the formation of the CSZ phase. Accordingly, the preparation of CaZrO3 films turned to be more challenging than CaTiO3 and CaSnO3 ones [19e23] due to the high stability of the CSZ phase. Referring to the crystallization behaviors in Tb3þ 5% doped CaZrO3 films, phase pure films for the other chemical compositions were obtained by depositing at 740 C and annealing at 1000 C. The XRD pattern of the pure CaZrO3 film in Fig. 1 indicates that the film grew up with strong preferential orientations on a STO single crystal substrate, and then the peak intensities of (00l) crystallographic planes, which are pseudo-cubic indexes, strongly increased. Further analyses of the film by RHEED measurements and XRD f scan measurements were carried out as shown in Fig. 2. A streaky pattern with some spots was observed in the RHEED measurements. Four-fold rotational symmetry of the (111) diffraction was observed in the f scan measurements. These results revealed the epitaxial growth of CaZrO3 on a STO single crystal substrate. Similar epitaxial growth of CaZrO3 was also observed on a LAO single crystal substrate due to the small differences in the lattice constants; a ¼ 3.91 Å for STO and a ¼ 3.79 Å for LAO [24]. The transmission spectra of the STO and LAO substrates and
Table 1 PLD and annealing conditions. PLD conditions
Targets
Substrates
Annealing conditions
Laser source Laser fluence Laser frequency Oxygen partial pressure Substrate temperature Atmosphere Temperature Duration
{(Ca0.97Mg0.03)0.95Tb0.05}ZrO3þd {(Ca0.97Mg0.03)0.995Tb0.005}ZrO3þd {(Ca0.97Mg0.03)0.95Eu0.05}ZrO3þd SrTiO3 (001) or LaAlO3 (001) single crystals ArF excimer (193 nm) ~1.2 Jcm2pulse1 16 Hz 0.5 Pa 640, 690, 740 C Air 1000 C 1h
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Fig. 2. (a) RHEED image of Tb3þ 5% doped CaZrO3 thin film and (b) XRD pattern of a 4 scan for (111) diffraction.
Tb3þ 5% doped CaZrO3 film sample are shown in Fig. 3 along with an AFM image of the film surface. Both the STO and LAO substrates show a transmission as high as 70% in the visible range. LAO is able to transmit UV light because of its large band gap (Eg ¼ 5.6 eV [25]) while this is not the case for STO (Eg ¼ 3.2 eV [26]). Therefore, LAO substrates are preferable for thin films of UV or violet light emitting materials. Although Ln doped CaZrO3 films were grown on both STO and LAO substrates, no obvious differences were detected in the epitaxial growth and film quality. Ln doped CaZrO3 films on the LAO substrates were transparent and showed a high transmission in the UV region as well as the visible region. The film thicknesses were estimated by the optical interference observed in the transmission spectra, and the typical film thickness was approximately 900 nm. The AFM image of the film surface revealed that the surface was smooth without cracks and composed of nanometer-sized texture. The PL and PLE spectra of a Tb3þ 5% doped film sample were compared with those of a powder sample as shown in Fig. 4. In the PLE spectra, Tb3þ doped CaZrO3 showed two excitation peaks in the UV region. One is attributed to the Tb3þ 4f-5d transition and the other is to the host lattice excitation. In CaZrO3:Tb3þ, the peaks at
Fig. 3. Transmission spectra of STO substrate (dashed line), LAO substrate (dot-dashed line), and Tb3þ 5% doped CaZrO3 thin film grown on STO substrate (solid line).
Fig. 4. PL and PLE spectra of Tb3þ 5% doped CaZrO3 powder (top) and thin film (bottom) along with photographs of the pressed powder and thin film under UV irradiation.
244 nm and 203 nm were assigned to the Tb3þ 4f-5d transition and host lattice excitation, respectively, as reported previously [14]. In contrast to the peak at 203 nm, the intensity of the peak at 244 nm decreased largely in the film sample compared to the powder sample. This is because the number of Tb3þ ions in the optical path for PLE spectra measurements is very small in the film even though the 4f-5d transition is a known allowed electronic dipole transition. In the PL spectra, two series of luminescence were observed in the blue and green regions related to the 5D3-7FJ and 5D4-7FJ (J ¼ 6e2) transitions, respectively. The 5D4-7FJ transitions are generally dominant at high Tb3þ concentrations because of a cross relaxation phenomenon (5D3-5D4 /7FJ -7FJ0 ). The major 5D4-7F5 transition peaking at 550 nm leads specifically to an intense green luminescence of CaZrO3:Tb3þ 5%. The shapes of the PL spectra for both the powder and film samples were very similar to each other. This is not only for the case of the Tb 5% doped sample but also for the other doped samples. PL spectra and photographs of all the film samples are shown in
Fig. 5. PL spectra and photographs of CaZrO3 thin films doped with Eu3þ 5% (bottom), Tb3þ 5% (middle), and Tb3þ 0.5% (top).
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Fig. 5. All the film samples showed uniform surface luminescence under UV excitation indicating that the dopant Ln ions were homogeneously distributed in the film. The PL spectra of the films deposited on the STO substrates were very similar to those of the powders in the green luminescent Tb3þ 5% doped sample and red luminescent Eu3þ 5% doped sample. However, the PL spectrum of the blue luminescent Tb3þ 0.5% doped film on the STO substrate was somewhat different from that of the powder; the intensity of the luminescent peak at 382 nm dropped considerably. This is obviously due to the absorption of the luminescence by the STO substrate as shown in Fig. 3. To avoid the absorption, the blue luminescent Tb3þ 0.5% doped film was deposited on a UV transparent LAO substrate. Consequently, a PL spectrum analogous to that of the powder sample, which indicates intense violet-blue luminescence, was obtained for the Tb3þ 0.5% doped film on the LAO substrate. Accordingly, uniform RGB luminescent films were obtained from the CaZrO3 host lattice by just changing the Ln dopant and its concentration. Fig. 6 shows decay curves of emission peaks for RGB luminescence. In Eu 5% doped sample, the decay of the peak at 616 nm, which was assigned to the 5D0-7F2 transition, was measured for red luminescence. In Tb 0.5 and 5% doped samples, the decay of the peaks at 382 and 549 nm, which were assigned to the 5D3-7F6 and 5 D4-7F5 transitions, were measured for blue and green luminescence, respectively. The red and green luminescence decayed in a very similar manner, while the blue luminescence decayed much faster. The lifetime t of each luminescence was roughly estimated by fitting a single exponential component and plotting as shown in the inset. The evaluated lifetimes t were 0.39 ms for blue luminescence derived from the 5D3-7F6 transition, 1.00 ms for green luminescence derived from the 5D4-7F5 transition, and 0.95 ms for red luminescence derived from the 5D0-7F2 transition. The lifetimes of 5D3-7FJ transitions in Tb3þ are usually much shorter than those for 5D4-7FJ transitions, and the values of t for the respective transitions in Tb3þ and Eu3þ were similar to reported values [27e33]. CL spectra and photographs are shown in Fig. 7 along with the PL spectra. CL was evaluated in an evacuated silica glass tube as shown in the inset. Clear CL was observed from each sample but the CL showed a slight lack of uniformity as shown in the photographs. Because the PL showed uniform luminescence as observed in Fig. 5,
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Fig. 7. CL spectra (thick lines) and photographs of CaZrO3 thin films doped with Eu3þ 5% (bottom), Tb3þ 5% (middle) and Tb3þ 0.5% (top) along with the PL spectra (thin lines).
the lack of CL uniformity was derived from electrons irregularly emitted from the electron gun. The CL intensities increased with an increase in the acceleration voltage and reached maximum at 2.0 kV. Blue CL intensity was the most intense among respective RGB CL intensities. The CL peaks of the 4f-4f transitions unique to Tb3þ and Eu3þ were observed, and the shapes of the CL spectra were basically the same as those of the PL spectra in all the samples. However, the results revealed that the blue luminescence was markedly enhanced in the CL. The energy of the accelerated electrons in the CL was approximately 2 keV, which was much larger than the energy of electrons excited by UV photons (several electronvolts) in the PL. In the CL processes, it is generally considered that the incident electrons, namely primary electrons, directly excite optical phonons, plasmons and core electrons rather than doped Ln activators, and the energy is converted into numbers of thermalized electronehole pairs rapidly [34,35]. Then, the energies of these pairs are transferred to the activators to give the luminescence. It was supposed, in this study, that the electronehole pairs, which were generated rapidly in the CaZrO3 host lattice, transferred their energy to Tb3þ ions, and the Tb3þ ions relaxed quickly through faster electronic transitions. Consequently, it was observed in CaZrO3 that the blue luminescence with the shorter lifetime was enhanced compared with green luminescence. In Tb3þ doped Y3Al5O12, the enhancement of the blue luminescence in CL has not been reported although the blue luminescence was observed in both PL and CL measurements [36e38]. On the other hand, intense blue CL was clearly observed in Tb3þ doped GaN, especially at higher incident electron energy [39,40]. Although the details of the enhancement are still unknown, it is considered that not only the lifetime but also site symmetry at Tb3þ sites influence the luminescence intensity and the cross relaxation in Tb3þ ions.
4. Conclusions
Fig. 6. Decay curves of luminescence of CaZrO3 doped with Eu3þ 5% (square), Tb3þ 5% (circle) and Tb3þ 0.5% (triangle). Peak intensities of red luminescence (lem ¼ 616 nm) for the 5D0-7F2 transition, green luminescence (lem ¼ 549 nm) for the 5D4-7F5 transition and blue luminescence (lem ¼ 382 nm) for the 5D3-7F6 transition were measured as a function of time.
Transparent Ln doped CaZrO3 phosphor films were epitaxially grown on STO or LAO single crystal substrates by a PLD technique. The preparation of the CaZrO3 films was not as simple as that of CaSnO3 or CaTiO3 films because of the presence of the stable CSZ phase, which appeared as a secondary phase under nonoptimized preparation conditions. It was found that phase pure CaZrO3 films
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were obtained by setting the substrate temperature to be high enough to suppress the formation of the CSZ phase and sequential annealing of the deposited films at 1000 C. The obtained films showed a high transparency in the visible region and exhibited uniform PL under UV excitation. The Tb3þ 0.5 and 5% doped samples showed blue and green luminescence, respectively, and the Eu3þ 5% doped samples showed red luminescence as observed in powder samples. To avoid the absorption of the luminescence by the STO single crystal substrates, LAO substrates were essential for the Tb3þ 0.5% doped blue luminescent samples. Decay curve measurements revealed that the 5D3-7F6 transitions for blue luminescence occurred faster than the 5D4-7F5 transitions for green luminescence and the 5D0-7F2 transitions for red luminescence. All the samples showed clear CL, as well as PL, and the shapes of the CL spectra of the Tb3þ 5% doped sample for green luminescence and the Eu3þ 5% doped sample for red luminescence were very similar to those of the PL spectra. Only the Tb3þ 0.5% doped sample for blue luminescence showed enhancement of the violet-blue luminescence, which was attributed to the short lifetime of the 5D3-7FJ transitions and numbers of electronehole pairs generated in the CL processes. References [1] S. Shionoya, W.M. Yen (Eds.), Phosphor Handbook, CRC Press LLC, Boca Raton, 1998. [2] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, 1994. [3] C. Ronda (Ed.), Luminescence, Wiley-VCH, Weinheim, 2008. [4] A. Kitai (Ed.), Luminescent Materials and Application, Wiley, Chichester, 2008. [5] A. Vecht, D.W. Smith, S.S. Chadha, C.S. Gibbons, J. Koh, D. Morton, New electron excited light emitting materials, J. Vac. Sci. Technol. B 12 (1994) 781e784. [6] P.T. Diallo, P. Boutinaud, R. Mahiou, J.C. Cousseins, Red luminescence in Pr3þdoped calcium titanates, Phys. Status Solidi A 160 (1997) 255e263. [7] S. Itoh, H. Toki, K. Tamura, F. Kataoka, A new red-emitting phosphor, SrTiO3: Pr3þ, for low-voltage electron excitation, Jpn. J. Appl. Phys. 38 (1999) 6387e6391. [8] H. Yamamoto, S. Okamoto, H. Kobayashi, Luminescence of rare-earth ions in perovskite-type oxides: from basic research to applications, J. Lumin. 100 (2002) 325e332. ^ men, R. Sakamoto, N. Sakamoto, S. Kunugi, M. Itoh, Photoluminescence [9] T. Kyo properties of Pr-doped (Ca,Sr,Ba)TiO3, Chem. Mater. 17 (2005) 3200e3204. [10] H. Takashima, K. Shimada, N. Miura, T. Katsumata, Y. Inaguma, K. Ueda, M. Itoh, Low-driving-voltage electroluminescence in perovskite films, Adv. Mater. 21 (2009) 3699e3702. ^men, M. Hanaya, H. Takashima, Electroluminescence near interfaces [11] T. Kyo between (Ca,Sr)TiO3:Pr phosphor and SnO2:Sb transparent conductor thin films prepared by solegel and spin-coating methods, J. Lumin. 149 (2014) 133e137. [12] K. Ueda, T. Yamashita, K. Nakayashiki, K. Goto, T. Maeda, K. Furui, K. Ozaki, Y. Nakachi, S. Nakamura, M. Fujisawa, T. Miyazaki, Green, orange, and magenta luminescence in strontium stannates with perovskite-related structures, Jpn. J. Appl. Phys. 45 (2006) 6981e6983. [13] K. Ueda, Y. Shimizu, Fabrication of TbeMg codoped CaSnO3 perovskite thin films and electroluminescence devices, Thin Solid Films 518 (2010) 3063e3066. [14] Y. Shimizu, S. Sakagami, K. Goto, Y. Nakachi, K. Ueda, Tricolor luminescence in rare earth doped CaZrO3 perovskite oxides, Mater. Sci. Eng. B 161 (2009) 100e103. [15] S.K. Gupta, P.S. Ghosh, N. Pathak, R. Tewari, Nature of defects in blue light emitting CaZrO3: spectroscopic and theoretical study, RSC Adv. 5 (2015) 56526e56533. [16] H. Zhang, X. Fu, S. Niu, Q. Xin, Blue luminescence of nanocrystalline CaZrO3: Tm phosphors synthesized by a modified Pechini solegel method, J. Lumin. 128 (2008) 1348e1352.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Photo- and cathodoluminescence of Eu3þ or Tb3þ doped CaZrO3 films prepared by pulsed laser deposition Kazushige Ueda a, *, Yuhei Shimizu a, Hiroshi Takashima b, **, Florian Massuyeau c, phane Jobic c Ste a
Department of Materials Science, Graduate School of Engineering, Kyushu Institute of Technology, 1-1 Sensui, Tobata, Kitakyushu, 804-8550, Japan Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki, 3058568, Japan c Institut des Mat eriaux Jean Rouxel (IMN), Universit e de Nantes, CNRS, 2 rue de la Houssini ere, B.P. 32229, 44322, Nantes Cedex 3, France b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 June 2017 Received in revised form 21 August 2017 Accepted 4 September 2017
Luminescent films of Eu3þ or Tb3þ doped CaZrO3 were epitaxially grown on SrTiO3 or LaAlO3 (001) single crystal substrates by pulsed laser deposition (PLD). They showed a high transmittance in the visible region and a smooth surface. Uniform red photoluminescence (PL) was observed from Eu3þ 5% doped films, and blue and green PL was observed from Tb3þ 0.5% and 5% doped films, respectively. These PL spectra of the films turned out to be very similar to those of the powder samples. Practically, it has to be noticed the use of a LaAlO3 substrate is necessary for intense blue luminescence for Tb3þ 0.5% doped films to avoid optical absorption of emitted light by the SrTiO3 substrate. The cathodoluminescence (CL) spectra of the red and green luminescent films were almost the same as those PL spectra. In contrast, blue CL was significantly enhanced in CL compared to PL. This was attributed to the shorter lifetime of the 5D3-7FJ transitions and many excited electron-hole pairs, which were generated rapidly on high energy electron irradiation. © 2017 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Cathodoluminescence Perovskite CaZrO3 Thin film
1. Introduction Light emitting inorganic materials have been widely investigated for applications in lighting and displays. Nowadays, nitride, sulfide and oxide compounds are frequently used as practical luminescent materials for light emitting diodes (LEDs), electroluminescent (EL) devices and fluorescent lamps [1e4]. Nevertheless, among the large family of inorganic compounds, oxide materials have many advantages in practice because they are chemically and thermally more stable than their nitride and sulfide congeners, and easy to fabricate under ambient atmosphere. Accordingly, the fundamental properties of many luminescent oxides have been evaluated by photoluminescence (PL) and cathodoluminescence (CL). Pr3þ doped luminescent perovskite-type oxides such as CaTiO3:Pr3þ and SrTiO3:Pr3þ were examined previously from both
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Ueda), [email protected] (H. Takashima). http://dx.doi.org/10.1016/j.optmat.2017.09.007 0925-3467/© 2017 Elsevier B.V. All rights reserved.
scientific and practical viewpoints [5e9]. Although perovskite-type oxides show various electronic functions such as ferroelectricity, piezoelectricity, and semiconductivity, the luminescent function is not extensively investigated yet. The development of practical red luminescent Pr3þ and Al3þ codoped titanate for field emission displays led to further application of luminescent perovskite-type oxides to lighting components such as EL devices. Hence, Pr3þ doped (Ca,Sr)TiO3 thin films have been used as emitting layers in EL devices, which were operated at low voltage [10]. Highluminescence EL devices using Pr3þ doped (Ca,Sr)TiO3 films were also reported along with the ac and dc operation [11]. However, only red luminescence turns to be available in titanate phosphors. Recently, perovskite-type stannates doped with lanthanide (Ln) ions were examined to achieve several luminescent colors for EL devices [12,13]. Although some luminescent colors were found to be available in stannates, the important luminescent colors of red, green and blue (RGB) were not obtained from the only perovskitetype CaSnO3 host lattice unfortunately. In contrast to titanates and stannates, Ln doped zirconates, especially CaZrO3, were found to show RGB tricolor luminescence without changing the host lattice [14e17], which is anticipated to
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facilitate a process of luminescent device fabrications. Eu3þ doped CaZrO3 showed red luminescence, Tb3þ doped CaZrO3 showed green luminescence at medium Tb3þ doping concentrations, and blue luminescence was seen at a low Tb3þ doping concentration. In this study, to apply doped CaZrO3 with RGB luminescence, we first attempted the preparation of high quality thin films on single crystal substrates and examined their PL and CL properties. 2. Experiments Thin films of Ln doped CaZrO3 were grown on SrTiO3 (STO) (001) or LaAlO3 (LAO) (001) single crystal substrates at high temperatures using a pulsed laser deposition (PLD) technique with polycrystalline sintered disks as targets. The disks were prepared by a conventional solid state reaction and their chemical compositions were {(Ca0.97Mg0.03)0.95Eu0.05}ZrO3þd for red, {(Ca0.97Mg0.03)0.95Tb0.05} ZrO3þd for green, and {(Ca0.97Mg0.03)0.995Tb0.005}ZrO3þd for blue luminescence, as reported previously [14]. Mg codoping usually enhances the PL intensity of samples [12,14,18] and all Ln doped samples in this study were codoped with Mg ions even if not mentioned explicitly. The as-deposited films were annealed in air to obtain a higher crystallinity and intense luminescence. The PLD and annealing conditions are summarized in Table 1. Crystalline phases in the films were analyzed by X-ray diffraction (XRD) measurements (Rigaku, RINT 2500), and film surfaces were characterized by atomic force microscope (AFM) (Digital Instruments, Nanoscope III) and reflection high energy electron diffraction (RHEED) (KSA 400). Transmission spectra of the films were collected using an UV/Vis/ NIR spectrophotometer (Jasco, V-570). PL/PL excitation (PLE) spectra were measured using a conventional spectrofluorometer (Jasco, FP-6500), and fluorescence decay curves were obtained using a spectrofluorometer with a Xe flash lamp (HORIBA Jobin Yvon, Fluorolog-3). CL spectra of the films were obtained using a CNT field-emission electron gun with a gate electrode and a modular USB spectrometer with an optical fiber (Ocean Optics USB4000). The electron acceleration voltage for the electron gun was varied up to 2.0 kV in the CL measurements. 3. Results and discussion The XRD patterns of Tb3þ 5% doped CaZrO3 films deposited on STO substrates at different temperatures are shown in Fig. 1. The XRD patterns clearly show the formation of the main perovskitetype CaZrO3 phase with a small amount of a CaO-stabilized zirconia (CSZ) phase as an impurity for low temperature synthesis. As shown in the inset, films deposited at temperatures below approximately 700 C were amorphous in contrast with a film deposited at 740 C that was crystallized with distinguishable diffraction peaks of CaZrO3. After annealing for 1 h at 1000 C, the
Fig. 1. XRD patterns of Tb3þ 5% doped CaZrO3 thin films deposited at different substrate temperatures. Simulated XRD patterns of CaZrO3 (solid line) and CSZ (dashed line) are compared at the bottom. XRD patterns of the films in the as-deposited state (thin line) and after annealing (thick line) are shown in the inset.
amorphous films deposited below 700 C were crystallized accompanying a CSZ secondary phase. On the other hand, a highly crystalline phase pure CaZrO3 film was obtained from the film deposited at 740 C, revealing that the annealing only increased the crystallinity. Therefore, high substrate temperature was necessary during film deposition to obtain a high quality and pure perovskitetype films, and avoid the formation of the CSZ phase. Accordingly, the preparation of CaZrO3 films turned to be more challenging than CaTiO3 and CaSnO3 ones [19e23] due to the high stability of the CSZ phase. Referring to the crystallization behaviors in Tb3þ 5% doped CaZrO3 films, phase pure films for the other chemical compositions were obtained by depositing at 740 C and annealing at 1000 C. The XRD pattern of the pure CaZrO3 film in Fig. 1 indicates that the film grew up with strong preferential orientations on a STO single crystal substrate, and then the peak intensities of (00l) crystallographic planes, which are pseudo-cubic indexes, strongly increased. Further analyses of the film by RHEED measurements and XRD f scan measurements were carried out as shown in Fig. 2. A streaky pattern with some spots was observed in the RHEED measurements. Four-fold rotational symmetry of the (111) diffraction was observed in the f scan measurements. These results revealed the epitaxial growth of CaZrO3 on a STO single crystal substrate. Similar epitaxial growth of CaZrO3 was also observed on a LAO single crystal substrate due to the small differences in the lattice constants; a ¼ 3.91 Å for STO and a ¼ 3.79 Å for LAO [24]. The transmission spectra of the STO and LAO substrates and
Table 1 PLD and annealing conditions. PLD conditions
Targets
Substrates
Annealing conditions
Laser source Laser fluence Laser frequency Oxygen partial pressure Substrate temperature Atmosphere Temperature Duration
{(Ca0.97Mg0.03)0.95Tb0.05}ZrO3þd {(Ca0.97Mg0.03)0.995Tb0.005}ZrO3þd {(Ca0.97Mg0.03)0.95Eu0.05}ZrO3þd SrTiO3 (001) or LaAlO3 (001) single crystals ArF excimer (193 nm) ~1.2 Jcm2pulse1 16 Hz 0.5 Pa 640, 690, 740 C Air 1000 C 1h
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Fig. 2. (a) RHEED image of Tb3þ 5% doped CaZrO3 thin film and (b) XRD pattern of a 4 scan for (111) diffraction.
Tb3þ 5% doped CaZrO3 film sample are shown in Fig. 3 along with an AFM image of the film surface. Both the STO and LAO substrates show a transmission as high as 70% in the visible range. LAO is able to transmit UV light because of its large band gap (Eg ¼ 5.6 eV [25]) while this is not the case for STO (Eg ¼ 3.2 eV [26]). Therefore, LAO substrates are preferable for thin films of UV or violet light emitting materials. Although Ln doped CaZrO3 films were grown on both STO and LAO substrates, no obvious differences were detected in the epitaxial growth and film quality. Ln doped CaZrO3 films on the LAO substrates were transparent and showed a high transmission in the UV region as well as the visible region. The film thicknesses were estimated by the optical interference observed in the transmission spectra, and the typical film thickness was approximately 900 nm. The AFM image of the film surface revealed that the surface was smooth without cracks and composed of nanometer-sized texture. The PL and PLE spectra of a Tb3þ 5% doped film sample were compared with those of a powder sample as shown in Fig. 4. In the PLE spectra, Tb3þ doped CaZrO3 showed two excitation peaks in the UV region. One is attributed to the Tb3þ 4f-5d transition and the other is to the host lattice excitation. In CaZrO3:Tb3þ, the peaks at
Fig. 3. Transmission spectra of STO substrate (dashed line), LAO substrate (dot-dashed line), and Tb3þ 5% doped CaZrO3 thin film grown on STO substrate (solid line).
Fig. 4. PL and PLE spectra of Tb3þ 5% doped CaZrO3 powder (top) and thin film (bottom) along with photographs of the pressed powder and thin film under UV irradiation.
244 nm and 203 nm were assigned to the Tb3þ 4f-5d transition and host lattice excitation, respectively, as reported previously [14]. In contrast to the peak at 203 nm, the intensity of the peak at 244 nm decreased largely in the film sample compared to the powder sample. This is because the number of Tb3þ ions in the optical path for PLE spectra measurements is very small in the film even though the 4f-5d transition is a known allowed electronic dipole transition. In the PL spectra, two series of luminescence were observed in the blue and green regions related to the 5D3-7FJ and 5D4-7FJ (J ¼ 6e2) transitions, respectively. The 5D4-7FJ transitions are generally dominant at high Tb3þ concentrations because of a cross relaxation phenomenon (5D3-5D4 /7FJ -7FJ0 ). The major 5D4-7F5 transition peaking at 550 nm leads specifically to an intense green luminescence of CaZrO3:Tb3þ 5%. The shapes of the PL spectra for both the powder and film samples were very similar to each other. This is not only for the case of the Tb 5% doped sample but also for the other doped samples. PL spectra and photographs of all the film samples are shown in
Fig. 5. PL spectra and photographs of CaZrO3 thin films doped with Eu3þ 5% (bottom), Tb3þ 5% (middle), and Tb3þ 0.5% (top).
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Fig. 5. All the film samples showed uniform surface luminescence under UV excitation indicating that the dopant Ln ions were homogeneously distributed in the film. The PL spectra of the films deposited on the STO substrates were very similar to those of the powders in the green luminescent Tb3þ 5% doped sample and red luminescent Eu3þ 5% doped sample. However, the PL spectrum of the blue luminescent Tb3þ 0.5% doped film on the STO substrate was somewhat different from that of the powder; the intensity of the luminescent peak at 382 nm dropped considerably. This is obviously due to the absorption of the luminescence by the STO substrate as shown in Fig. 3. To avoid the absorption, the blue luminescent Tb3þ 0.5% doped film was deposited on a UV transparent LAO substrate. Consequently, a PL spectrum analogous to that of the powder sample, which indicates intense violet-blue luminescence, was obtained for the Tb3þ 0.5% doped film on the LAO substrate. Accordingly, uniform RGB luminescent films were obtained from the CaZrO3 host lattice by just changing the Ln dopant and its concentration. Fig. 6 shows decay curves of emission peaks for RGB luminescence. In Eu 5% doped sample, the decay of the peak at 616 nm, which was assigned to the 5D0-7F2 transition, was measured for red luminescence. In Tb 0.5 and 5% doped samples, the decay of the peaks at 382 and 549 nm, which were assigned to the 5D3-7F6 and 5 D4-7F5 transitions, were measured for blue and green luminescence, respectively. The red and green luminescence decayed in a very similar manner, while the blue luminescence decayed much faster. The lifetime t of each luminescence was roughly estimated by fitting a single exponential component and plotting as shown in the inset. The evaluated lifetimes t were 0.39 ms for blue luminescence derived from the 5D3-7F6 transition, 1.00 ms for green luminescence derived from the 5D4-7F5 transition, and 0.95 ms for red luminescence derived from the 5D0-7F2 transition. The lifetimes of 5D3-7FJ transitions in Tb3þ are usually much shorter than those for 5D4-7FJ transitions, and the values of t for the respective transitions in Tb3þ and Eu3þ were similar to reported values [27e33]. CL spectra and photographs are shown in Fig. 7 along with the PL spectra. CL was evaluated in an evacuated silica glass tube as shown in the inset. Clear CL was observed from each sample but the CL showed a slight lack of uniformity as shown in the photographs. Because the PL showed uniform luminescence as observed in Fig. 5,
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Fig. 7. CL spectra (thick lines) and photographs of CaZrO3 thin films doped with Eu3þ 5% (bottom), Tb3þ 5% (middle) and Tb3þ 0.5% (top) along with the PL spectra (thin lines).
the lack of CL uniformity was derived from electrons irregularly emitted from the electron gun. The CL intensities increased with an increase in the acceleration voltage and reached maximum at 2.0 kV. Blue CL intensity was the most intense among respective RGB CL intensities. The CL peaks of the 4f-4f transitions unique to Tb3þ and Eu3þ were observed, and the shapes of the CL spectra were basically the same as those of the PL spectra in all the samples. However, the results revealed that the blue luminescence was markedly enhanced in the CL. The energy of the accelerated electrons in the CL was approximately 2 keV, which was much larger than the energy of electrons excited by UV photons (several electronvolts) in the PL. In the CL processes, it is generally considered that the incident electrons, namely primary electrons, directly excite optical phonons, plasmons and core electrons rather than doped Ln activators, and the energy is converted into numbers of thermalized electronehole pairs rapidly [34,35]. Then, the energies of these pairs are transferred to the activators to give the luminescence. It was supposed, in this study, that the electronehole pairs, which were generated rapidly in the CaZrO3 host lattice, transferred their energy to Tb3þ ions, and the Tb3þ ions relaxed quickly through faster electronic transitions. Consequently, it was observed in CaZrO3 that the blue luminescence with the shorter lifetime was enhanced compared with green luminescence. In Tb3þ doped Y3Al5O12, the enhancement of the blue luminescence in CL has not been reported although the blue luminescence was observed in both PL and CL measurements [36e38]. On the other hand, intense blue CL was clearly observed in Tb3þ doped GaN, especially at higher incident electron energy [39,40]. Although the details of the enhancement are still unknown, it is considered that not only the lifetime but also site symmetry at Tb3þ sites influence the luminescence intensity and the cross relaxation in Tb3þ ions.
4. Conclusions
Fig. 6. Decay curves of luminescence of CaZrO3 doped with Eu3þ 5% (square), Tb3þ 5% (circle) and Tb3þ 0.5% (triangle). Peak intensities of red luminescence (lem ¼ 616 nm) for the 5D0-7F2 transition, green luminescence (lem ¼ 549 nm) for the 5D4-7F5 transition and blue luminescence (lem ¼ 382 nm) for the 5D3-7F6 transition were measured as a function of time.
Transparent Ln doped CaZrO3 phosphor films were epitaxially grown on STO or LAO single crystal substrates by a PLD technique. The preparation of the CaZrO3 films was not as simple as that of CaSnO3 or CaTiO3 films because of the presence of the stable CSZ phase, which appeared as a secondary phase under nonoptimized preparation conditions. It was found that phase pure CaZrO3 films
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were obtained by setting the substrate temperature to be high enough to suppress the formation of the CSZ phase and sequential annealing of the deposited films at 1000 C. The obtained films showed a high transparency in the visible region and exhibited uniform PL under UV excitation. The Tb3þ 0.5 and 5% doped samples showed blue and green luminescence, respectively, and the Eu3þ 5% doped samples showed red luminescence as observed in powder samples. To avoid the absorption of the luminescence by the STO single crystal substrates, LAO substrates were essential for the Tb3þ 0.5% doped blue luminescent samples. Decay curve measurements revealed that the 5D3-7F6 transitions for blue luminescence occurred faster than the 5D4-7F5 transitions for green luminescence and the 5D0-7F2 transitions for red luminescence. All the samples showed clear CL, as well as PL, and the shapes of the CL spectra of the Tb3þ 5% doped sample for green luminescence and the Eu3þ 5% doped sample for red luminescence were very similar to those of the PL spectra. Only the Tb3þ 0.5% doped sample for blue luminescence showed enhancement of the violet-blue luminescence, which was attributed to the short lifetime of the 5D3-7FJ transitions and numbers of electronehole pairs generated in the CL processes. References [1] S. Shionoya, W.M. Yen (Eds.), Phosphor Handbook, CRC Press LLC, Boca Raton, 1998. [2] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, 1994. [3] C. Ronda (Ed.), Luminescence, Wiley-VCH, Weinheim, 2008. [4] A. Kitai (Ed.), Luminescent Materials and Application, Wiley, Chichester, 2008. [5] A. Vecht, D.W. Smith, S.S. Chadha, C.S. Gibbons, J. Koh, D. Morton, New electron excited light emitting materials, J. Vac. Sci. Technol. B 12 (1994) 781e784. [6] P.T. Diallo, P. Boutinaud, R. Mahiou, J.C. Cousseins, Red luminescence in Pr3þdoped calcium titanates, Phys. Status Solidi A 160 (1997) 255e263. [7] S. Itoh, H. Toki, K. Tamura, F. Kataoka, A new red-emitting phosphor, SrTiO3: Pr3þ, for low-voltage electron excitation, Jpn. J. Appl. Phys. 38 (1999) 6387e6391. [8] H. Yamamoto, S. Okamoto, H. Kobayashi, Luminescence of rare-earth ions in perovskite-type oxides: from basic research to applications, J. Lumin. 100 (2002) 325e332. ^ men, R. Sakamoto, N. Sakamoto, S. Kunugi, M. Itoh, Photoluminescence [9] T. Kyo properties of Pr-doped (Ca,Sr,Ba)TiO3, Chem. Mater. 17 (2005) 3200e3204. [10] H. Takashima, K. Shimada, N. Miura, T. Katsumata, Y. Inaguma, K. Ueda, M. Itoh, Low-driving-voltage electroluminescence in perovskite films, Adv. Mater. 21 (2009) 3699e3702. ^men, M. Hanaya, H. Takashima, Electroluminescence near interfaces [11] T. Kyo between (Ca,Sr)TiO3:Pr phosphor and SnO2:Sb transparent conductor thin films prepared by solegel and spin-coating methods, J. Lumin. 149 (2014) 133e137. [12] K. Ueda, T. Yamashita, K. Nakayashiki, K. Goto, T. Maeda, K. Furui, K. Ozaki, Y. Nakachi, S. Nakamura, M. Fujisawa, T. Miyazaki, Green, orange, and magenta luminescence in strontium stannates with perovskite-related structures, Jpn. J. Appl. Phys. 45 (2006) 6981e6983. [13] K. Ueda, Y. Shimizu, Fabrication of TbeMg codoped CaSnO3 perovskite thin films and electroluminescence devices, Thin Solid Films 518 (2010) 3063e3066. [14] Y. Shimizu, S. Sakagami, K. Goto, Y. Nakachi, K. Ueda, Tricolor luminescence in rare earth doped CaZrO3 perovskite oxides, Mater. Sci. Eng. B 161 (2009) 100e103. [15] S.K. Gupta, P.S. Ghosh, N. Pathak, R. Tewari, Nature of defects in blue light emitting CaZrO3: spectroscopic and theoretical study, RSC Adv. 5 (2015) 56526e56533. [16] H. Zhang, X. Fu, S. Niu, Q. Xin, Blue luminescence of nanocrystalline CaZrO3: Tm phosphors synthesized by a modified Pechini solegel method, J. Lumin. 128 (2008) 1348e1352.
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