1. Introduction

Ce³⁺-doped materials are attractive for applications such as lasers, scintillators, and phosphors [1]. Recently, Ce³⁺-doped Lu₂SiO₅ (LSO:Ce) [2,3] was developed as scintillators having high conversion efficiency from high energy to visible light and a fast luminescence decay time. However, when the crystalline quality was degraded by defects created during the high-temperature crystal growth process, persistent afterglow from the crystals was observed [4]. Although such long afterglow is undesirable for scintillator applications, understanding of afterglow mechanisms can lead to improve the conversion efficiency and further points the way to finding novel persistent phosphors with visually sufficient phosphorescence intensities for the whole day.

The basic mechanism responsible for the afterglow is very similar to the donor-acceptor recombination process in semiconductors [5]. If donors and acceptors, being uniformly distributed in semiconductors, recombine radiatively through tunnelling, the decay curve of the recombination luminescence can be described by a t^-1 power function [5,6]. The decay times for the donor-acceptor recombination and the afterglow have distributions in the ranges of 10⁻⁴ to 1 s and 1 to 10⁴ s, respectively. The difference in time scale is due to energy depth of the trapped charges [6]. Oxides with such afterglow observed at room temperature are called persistent phosphors.

The first long persistent green phosphorescence was observed for Eu²⁺-doped alkaline-earth aluminates, SrAl₂O₄:Eu²⁺,Dy³⁺ (SAO:Eu:Dy) [7]. Consecutively, blue persistent phosphorescence was observed for Ce³⁺-doped melilite crystals, Ca₂Al₂SiO₇:Ce³⁺ (CASM:Ce) and CaYAl₃O₇:Ce³⁺(CYAM:Ce) [8,9]. The electron-spin resonance (ESR) study of CASM:Ce has deduced that electrons and holes produced by ultraviolet irradiation are trapped at oxygen vacancies and self-trapped at aluminium sites, respectively, and that the electron-hole recombination occurs at Ce³⁺ ions [10].

Figure 1 shows a unit cell of the LSO crystal with monoclinic structure of C_2h⁶ [11]. There are two lutetium sites denoted by Lu(1) and Lu(2), represented by complexes LuO₇ and LuO₆, respectively. Ce³⁺ ions enter the host lattice by substituting for the Lu³⁺ ions. ESR measurement has indicated that Ce³⁺ ions occupy the Lu(1) sites with an amount of 95% and Lu(2) sites with an amount of 5% [11]. There are five different oxygen sites in the LSO crystal. Deficits of oxygen and silicon ions in the crystal are strongly related to the degradation of the crystal quality.

 

Fig. 1 Schematic diagram showing a unit cell of the Lu₂SiO₅ crystal structure. Two lutetium sites are denoted by Lu(1) and Lu(2).

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In this paper, we report temperature dependence of decay curves of persistent phosphorescence observed in LSO:Ce and propose a mechanism responsible for the persistent phosphorescence, taking into account the LSO crystal structure. We discuss the role of these deficits in persistent phosphorescent process for the LSO:Ce crystal.

2. Experimental procedure

LSO:Ce crystals were grown using the high-temperature Czochralski technique. The cerium concentration was 0.1 mol % at the starting charge.

Optical absorption, luminescence and excitation spectra were measured in the temperature range between 10 and 300 K using the tunable source (100-600 nm) in the UVSOR facility at Institute for Molecular Science at Okazaki. Luminescence decay curves of Ce³⁺ ions were measured using the third harmonics (355 nm) output from a Spectra-Physics GCR100 pulsed Nd:YAG laser with a pulse width of 10 ns and a Hamamatsu Photonics R943-02 PMT detector, connected to a Yokogawa DL1740 digital oscilloscope and a personal computer (PC). Persistent phosphorescence was measured after the sample was excited for five minutes with the laser light of 355 nm. Optical signals after removal of the excitation light were sampled at 10 kHz, integrated over a period of 1 second, and passed through an A to D converter connected to a PC. The sample temperatures between 20 and 300 K or between 77 and 500 K were achieved using an Iwatani CA201 cryo-refrigerator or an Oxford OptistatDN-V, respectively.

3. Experimental results

Figure 2 shows the absorption, luminescence and excitation spectra of the LSO:Ce at 10 K. The absorption spectrum of LSO:Ce consists of several unresolved broad bands in the range of 100-400 nm. The blue luminescence excited at 355 nm has double peaks at 400 and 430 nm corresponding to the transitions from the lowest energy level of the ²E (5d) excited state to the low lying ²F_5/2 (4f) and ²F_7/2 (4f) ground states, respectively. The tail of the luminescence extended toward 600 nm may be due to the minor Ce³⁺ ions occupying the Lu(2) sites [11]. The excitation spectrum of the 450 nm luminescence is the same as the absorption spectrum except for the relative intensities. The bands denoted by arrows are assigned to four of the five optical transitions from the ²F_5/2 (4f) ground state to the ²E (5d) and ²T₂ (5d) excited states of Ce³⁺, and the broad bands below 200 nm may be due to the band-to-band transitions of the LSO host crystal.

 

Fig. 2 Absorption, luminescence, and excitation spectra observed in Ce³⁺-doped Lu₂SiO₅ at 10 K.

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Figure 3 shows the decay time of the 410 nm luminescence in LSO:Ce in the temperature range of 20-400 K. The decay time is 38 ns below 300 K, so that the optical transitions are parity- and spin-allowed. However, the decay times above 300 K drastically decrease due to thermal multiphonon-assisted nonradiative relaxation [12].

 

Fig. 3 Temperature dependence of the decay time of the Ce³⁺ luminescence in Lu₂SiO₅. Solid curve is calculated using the equation of 1/$\tau$ = 1/38 + 10⁶/3.6$\times$exp(−6360/T).

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After removal of the excitation light of 355 nm at room temperature, persistent phosphorescence was observed in spite of the fairly weak intensity compared with the Ce³⁺ luminescence. The line shape of the phosphorescence is in agreement with that of the Ce³⁺ luminescence presented in Fig. 2.

Figure 4 shows the decay curves of the persistent phosphorescence in the time range of 1-10³ s observed at 300 and 500 K. The initial intensity (t = 1 s) at 500 K are 10 times larger than that at 300 K. Both decay curves at 300 and 500 K do not fit to a single exponential function, but to t ^-0.2 and t ⁻¹ power functions, respectively.

 

Fig. 4 Decay curves of the persistent phosphorescence observed at 300 and 500 K with the 355 nm excitation for Ce³⁺-doped Lu₂SiO₅.

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Figure 5 shows the decay curves of the persistent phosphorescence at various measurement temperatures of the sample. The decay curves at 300 and 500K show the linearity in the log-log scales. The decay curves with the exception of 300 and 500 K fit to neither a single exponential function nor a power function, but fit better to the curves described by Eq. (1). This expression is derived from the recombination model of a nearest-neighbor pair consisting of self-trapped electron and self-trapped hole in PbBr₂ crystals [13]:

 

Fig. 5 Temperature dependence of the decay curve of the persistent phosphorescence observed with the 355 nm excitation for Ce³⁺-doped Lu₂SiO₅. The dash-dot curves are calculated using Eq. (1).

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Figure 6 shows the total intensity of the integrated decay curve in the time range from 1 to 10³ s as a function of temperature. The total intensity curve can be decomposed into three components 1, 2, and 3, corresponding to those in Fig. 5. The three components suggest, at least, three recombination processes, as a consequence of the existence of three different trapped electron or hole centers.

 

Fig. 6 Temperature dependence of the integrated intensity of the decay curve in the time range from 1 to 10³ s for Ce³⁺-doped Lu₂SiO₅. The components 1, 2 and 3 were estimated from the decomposed decay curve in Fig. 5.

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4. Mechanism of persistent phosphorescence

The line shape of the persistent phosphorescence in LSO:Ce is the same as that of the Ce³⁺ luminescence. This result shows that a hole localized at the activator Ce³⁺ forming Ce⁴⁺ recombines radiatively by tunneling with a nearby trapped electron, such as an electron trapped at a nearby oxygen vacancy. The tunneling recombination, with an assumption of an uniform distribution of trapped electrons and holes in the crystal, gives rise to the decay curve represented by the t⁻¹ power function [5,6,13]. This is consistent with the decay curve observed at 500 K as shown in Fig. 5. On the other hand, the slope of the decay curve observed at 300 K is much slower than that at 500 K and the curve fits to a t^-0.2 power function, resulting in fairly long persistent phosphorescence. The decay curves, except at 300, 335 and 500 K, can be described using Eq. (1) with two sets of the fitting parameters of A, $\tau$₁ and $\tau$₂ being strongly temperature dependent. This raised a serious question as to why these parameters associated with tunneling effect are influenced by temperatures.

Below we propose a model to account for this temperature dependence. It is possible that two UV photons create an electron-hole pair in the conduction and valence bands of the LSO crystal because the energy level of the ground state of Ce³⁺ in oxides lies in the middle of the band gap [10,14,15]. ESR studies on persistent phosphors, such as SAO:Eu:Dy [16] and CASM:Ce [10], have indicated that holes produced by UV excitation are self-trapped in the form of two AlO₄ complexes and a single AlO₄ complex accompanied by a Si-vacancy, respectively. Although self-trapped holes (STHs) are stable at low temperature, they have certain mobility when the temperature is elevated up to room temperature [17]. With an assumption that a STH hops to an adjacent AlO₄ tetrahedron, the hopping rate of the STHs is represented by the Arrhenius equation and the hopping of the STHs increases the number of Ce⁴⁺ ions [10,18]. This assumption can explain the marked increase of the phosphorescence intensity above 300 K in Fig. 6. However, the total intensity is almost constant between 330 and 430 K, where energy transfer occurs between the two decomposed components 1 and 2 as shown in Fig. 6. In addition, above 430 K, energy transfer occurs from the component 2 to the component 3 and dark centers, which account for the decrease of the overall intensity above 430 K.

Finally, we discuss the origin of the three components of the total intensity in Fig. 6. The three components are associated with three electron trapped centers because holes are localized at Ce³⁺ ions and in the form of Ce⁴⁺. There are five different oxygen sites in the LSO crystal denoted by O(1), O(2), O(3), O(4) and O(5) [11]. These oxygen ions are surrounded by Lu(1), Lu(2) and Si, and form complexes. The five complexes can be classified into three groups according to number of the ligand cations, that is, group 1: O(1)Lu(1)₂Si, O(2)Lu(1)Lu(2)Si, and O(3)Lu(2)₂Si; group 2: O(4)Lu(1)₂Lu(2)Si; and group 3: O(5)Lu(1)₂Lu(2)₂. The depths of the energy levels of electrons trapped at the five oxygen vacancies from the bottom of the conduction band vary with the above different surrounding. When the temperature is elevated from 300 to 500 K, electrons move to more stable, i.e. deeper electron-trapped centers. This is equivalent to the energy transfer processes from the component 1 to the component 2 followed by the component 3 and dark centers as described above. This implies that the three groups of electron-trapped centers correspond to the three components of the persistent phosphorescence in Fig. 6.

5. Conclusions

X-ray irradiation produces electron-hole pairs in scintillators. According to crystal structure, holes are self-trapped at cation lattices and electrons move freely in the conduction band. If STHs are unstable above room temperature, they move to Ce³⁺ ions and electron-hole recombination immediately occurs at Ce³⁺ ions. Such Ce³⁺ luminescence can be observed with the short decay times. However, if F⁺ color centers (an electron trapped at an oxygen vacancy) and STHs are stable near room temperature, the electron-hole recombination at Ce³⁺ sites produces afterglow luminescence with fairly long decay times of 1- 10⁴ s. Deficits in LSO, namely, oxygen and silicon vacancies are produced during the high-temperature crystal growth process and they play an important role in the formation of stable electron- and hole-trapped centers above room temperature.

The present results have shown the important role played by the intrinsic deficits in the host crystal. It is therefore implicit that crystal quality needs to be improved, before they can be considered for scintillation applications, whereas applications as passive optical sources can take advantage of the persistent phosphorescence. It is important to have a good understanding of the formation of these defects centers and the processes involved in order to be ultimately tailor-made for the desired optical response to special applications.