1. Introduction

Wurtzite zinc oxide, a direct band semiconductor with wide band-gap (~3.3eV at room temperature) has attracted much attention as a luminescent material for both fundamental research and applications. When fired in a reducing atmosphere, ZnO with oxygen vacancies (usually denoted as ZnO:Zn) can give off efficient green emission that is of interest for the applications on flat panel displays [1-3]. Trivalent rare earth ions (RE³⁺) doped ZnO phosphors have been studied extensively during the last two decades [4-9], however, the results are quite disappointing, in that the desired characteristic emissions from RE³⁺ are so weak under the indirect UV excitation of the ZnO host. To achieve the efficient luminescence of ZnO:RE³⁺ phosphor, enhancing energy transfer (ET) from the semiconductor host to the RE³⁺ ions is required. The verification of the mechanism behind the ET process is also an important issue.

The relatively large radius of RE³⁺ and charge mismatching between the RE³⁺ and divalent Zn²⁺ ions are blamed to the unsuccessful incorporation of RE³⁺ into ZnO matrix, hence the inefficient ET is often observed. Lithium and RE³⁺-codoped phosphors exhibit more intense RE³⁺ emissions than singly doped RE³⁺ samples, resulted from the enhancement of solubility for RE³⁺ inserting the host [10] or other mechanisms [11-13]. S. Bachir et al. reported that the ET between ZnO host and RE³⁺ occurred only in the case of Tm³⁺-doped ZnO and explained the ET by a two-step mechanism involving electron-hole recombination via Tm²⁺ [4]. W. Jia et al. sintered the sample ZnO:1% EuF₃, Li in N₂ and found efficient ET from ZnO to Eu³⁺ evidenced by the enhanced absorption intensity ratio of UV band around 380 nm to f-f transitions of RE³⁺ in the excitation spectrum [7].

In comparison with the two methods frequently used to obtain the white light: one is the combination of GaN-based blue light-emitting diode (LED) chip with YAG:Ce³⁺ yellow phosphor, the other is the mixing of three red-green-blue (RGB) individual LEDs to generate white light, the method involved in In_xGa_1-xN-based blue and near-UV LED systems incorporating down-converting luminescent phosphors can provide relatively satisfied white light with both high color rendering and luminous efficacy. To achieve the high-performance white LEDs as illuminating sources, many endeavors have been made to improve the color-rendering-index, external quantum efficiency and luminous efficiency. Alternative phosphors to conventional materials for white light generation with strong absorption of n-UV light, high color purity and stability, hence the high luminescence efficiency is expected to be developed.

In this contribution, we report the successful synthesis of Eu³⁺, Li⁺-codoped ZnO:Zn phosphor that is suitable candidate for white LED pumped by near-UV (380-410 nm) light. The prominent ET from ZnO host to RE³⁺ ions is observed. The possible mechanisms are discussed based on steady state and time-resolved spectra and diffuse reflectance data.

2. Experimental

Un-doped and Eu³⁺, Li⁺-codoped ZnO:Zn samples are fabricated through solid-state reaction technique at 1100°C under reducing atmospheres for 3 hours. Mixed powders of analytical grade (99.9%) raw materials of ZnO, Li₂CO₃, and Eu₂O₃ (Beijing Chemicals Co. Ltd.) are carefully ground for 30 minutes, keeping the molar ratios of Eu³⁺ and Li⁺ equally in each sample, and then sintered in muffle using an alumina crucible as a container. While in reduced condition, a bigger crucible is filled with carbon sticks surrounding a smaller crucible containing the sample. X-ray diffraction (XRD) patterns are recorded by a Rigaku D/Max 2500V PC diffractometer operated at 18 kW with Cu Kα radiation (λ = 1.5406Å). The Li contents of the samples are determined using the inductively coupled plasma-optical emission spectrometer (ICP-OES) (Thermo Scientific iCAP 6300). Photoluminescence (PL), photoluminescence excitation (PLE), and diffuse reflectance spectra are measured using a Spectra-fluorometer (Hitachi F-4500). For time resolved PL measurements, the third (355 nm) harmonic of a Nd-YAG laser (Spectra-Physics, GCR 130) is used as the excitation source. Signal is detected by a Tektronix digital oscilloscope (TDS 3052).

3. Results and discussion

Figure 1 shows the X-ray diffraction patterns of undoped ZnO:Zn (c) and Eu³⁺, Li⁺-codoped samples with various doping concentrations: (a) 5at %, (b) 1at %, respectively. All the diffraction peaks of the undoped ZnO:Zn and codoped sample of 1% are well indexed as the hexagonal wurtzite ZnO (JCPDS card No. 36-1451). The impurity-related diffraction peaks (asterisk-indicated in curve (a)) indicates that new phases occur when the doping concentration increases as high as 5%. The slightly shift of diffraction peaks toward smaller angles with the doping concentration growing comes from the increase of the interplain distance, which is mainly due to the larger radius of Eu³⁺ (0.095 nm) than that of Zn²⁺ (0.074 nm). We argue that small amount of rare earth ions are most likely occupying the sites of Zn ions. However, the diffraction peak shift is not the sufficient evidence for this deduction.

 

Fig. 1. X-ray diffraction patterns of (a): 5% Eu³⁺, 5% Li⁺-codoped ZnO:Zn, (b): 1% Eu³⁺, 1% Li⁺-codoped ZnO:Zn and (c): undoped ZnO:Zn

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The PL and PLE spectra of Eu³⁺, Li⁺-codoped ZnO:Zn are shown in Fig. 2. The relative spectra of pure Eu₂O₃ powders are plotted together for comparison in the lower part of the profile. A broad green emission band centered at 520 nm (curve a), known as self-activated (SA) emission related to defects is observed in the PL spectra of codoped sample when excited under 390, 410 and 270 nm, respectively. The mechanism behind this green emission is not well established yet [1]. Among the mechanisms proposed [3,14-17], oxygen vacancies-caused emission is commonly accepted [3,16,17]. In addition to the broad green emission band, strong emissions from the Eu³⁺ dominated by the ⁵D₀-⁷F₂ transitions are also observed under the various excitation wavelengths, indicating that the Eu³⁺ ions are located at the lower symmetric sites. It is worth noting that the peak positions of ⁵D₀-⁷F₂ transitions varies with different excitations: the peak of the transitions is at 615 nm upon indirect near-UV pumping at 390 or 410 nm but changes to 609 nm when directly exciting ⁵D₂ level by 466 nm (see curve a, b and d in Fig. 2). This suggests that Eu³⁺ ions are located at different sites in which the transitions of ⁵D₀ level to the preferred splitting levels of ⁷F₂ state are different due to the crystal-fields around Eu³⁺. One can assume that there are two kinds of Eu³⁺ in the Eu³⁺, Li⁺-codoped ZnO:Zn sample: one is inside the lattices of ZnO host and the other is at interstitial sites at the boundary. These different sites for Eu³⁺ are further confirmed by the PLE spectra when monitored at 609 nm and 615 nm, respectively. In the former case, the PLE spectrum (curve χ in Fig. 2) features the broad absorption band centered at 270 nm, which is attributed to the O^2- → Eu³⁺ charge transfer (CT) transition, and has about 10 nm blue-shift compared to the PLE spectrum of Eu₂O₃ monitored at the same wavelength (curve δ in Fig. 2). In addition to this CT band, an intense absorption peak of Eu^{3+ 5}D₂ level at 466 nm and a broad band centered at around 390 nm are also observed. In the latter, monitored at 615 nm, the CT band is absent in curve β, showing the difference from the PLE spectrum monitored at 609 nm. The curve β also presents a strong excitation peak around 390 nm with full width at half maximum (FWHM) of ~40 nm but a relatively weak absorption peak for ⁵D₂ level of doped Eu³⁺. The reason behind this difference is that the efficient absorption of direct-band ZnO host dominates over the direct excitation of RE³⁺ ions inside the host lattices. In view of the band gap absorption of ZnO (~3.3eV) at room temperature and the spectra monitored at 520 and 615 nm (curves α and β in Fig. 2), we are further convinced that there exists a prominent ET process between the ZnO host to the RE³⁺ activators which are believed to occupy two kinds of sites respectively in the Eu³⁺, Li⁺-codoped sample.

 

Fig. 2. PL (right) and PLE (left) spectra of 1% Eu³⁺, 1% Li⁺ -codoped ZnO:Zn and pure Eu₂O₃ sintered in a reducing atmosphere at 1100°C. For Eu³⁺, Li⁺-codoped ZnO:Zn, a: λ_ex=390 nm, b:λ_ex=410 nm, c: λ_ex=270 nm, d: λ_ex=466 nm, α: λ_em=520 nm, β: λ_em=615 nm, χ: λ_em=609 nm; for Eu₂O₃, e: λ_ex=466 nm, δ: λ_em=609 nm; ↓: excitation peaks occurring in Eu₂O₃ but fading in the case of codoped sample; the broken positions in the PLE spectra are half wavelengths of monitored emissions.

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Moreover, the absorption peaks marked with downward arrows manifested in curve δ are almost absent in the PLE spectra (curves β and χ in Fig. 2) of Eu³⁺, Li⁺-codoped ZnO:Zn samples. Considering the 10 nm shifts between the abovementioned CT bands in curves χ and δ, one can deduce that Eu³⁺ ions located at boundary sites in co-doped sample are subjected to quite different environment from those in pure Eu₂O₃ powder. Eu³⁺ ions possess numerous energy levels in the 370-420 nm range with several discrete peaks as shown in the PLE spectrum of Eu₂O₃ (curve δ in Fig. 2). However, in the corresponding range for Eu³⁺, Li⁺-codoped ZnO:Zn, only a broad band around 390 nm appears when monitored ⁵D₀-⁷F₂ transitions (curves β and χ). Such excitation peaks at ~390 nm must be ascribed to the absorption of ZnO host instead of rare earth ions.

It is noteworthy that the broadening of the excitation peaks centered at ~390 nm makes the application possible for the phosphors in near-UV excited white LED. The left pattern of Fig. 3 presents the diffuse reflectance spectra of ZnO, ZnO:Zn, Eu³⁺,Li⁺-codoped ZnO and Eu³⁺,Li⁺-codoped ZnO:Zn samples, respectively. The broadening effects is also observed when Eu³⁺ and Li⁺ ions are introduced into the semiconductors, and the codoped sample sintered in the reducing condition shows stronger broadening effects compared to that in ambient. It has been reported that enhancement of luminescence can be achieved by Li⁺ -doping based on some reasonable mechanisms [10-13], and the lithium prefers to occupy the interstitial sites acting as shallow donors in ZnO phosphors [18-20]. By means of the ICP-OES method, Li contents in the samples codoped with 5at % and 1at % Eu³⁺, Li⁺ can be determined to be 0.90% and 0.12%, respectively, showing the same decreasing tendency after sintered at higher temperature [21]. Therefore, the enhanced red luminescence from RE³⁺ intraconfigurational transitions and the excitation broadening from the ZnO host can be explained by the Eu³⁺, Li⁺ incorporation and the formation of a series of shallow donor levels due to the reducing condition. The reduced atmosphere increases the quantity of oxygen vacancies and the incorporation of activators, hence followed by the increase of the lattice perturbation. Also, Li⁺ ions are most likely to be reduced to Li atoms, leading to the preferable occupation of interstitial sites. All these factors result in a series of energy levels near below the conduction band as temporary excitation energy storage. The right pattern in Fig. 3 shows another evidence of our arguments aforementioned, in which the diffuse reflectance spectra of Eu³⁺,Li⁺-codoped ZnO:Zn and Li⁺-free ZnO:Zn,Eu³⁺ samples are presented. The red shift of the reflectance edge in the lower curve indicates that the introduction of Li⁺ can enhance the absorption of near-UV light, supporting our interpretation based on the formation of shallow donor levels in the semiconductor host.

 

Fig. 3. Left: Diffuse reflectance spectra of ZnO, un-doped ZnO:Zn, ZnO:Eu³⁺, Li⁺ sintered in air and Eu³⁺, Li⁺-codoped ZnO:Zn samples; Right: Diffuse reflectance spectra of Li⁺-free ZnO:Zn, 1% Eu³⁺, 1% Li⁺-codoped ZnO:Zn, 1% Eu³⁺ samples.

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In our current work, ET is observed from the semiconductor host to the RE³⁺ no mater where they are located (inside the lattices or at the interstitial sites of grain boundary). Thus, the shallow donor levels originated from reduction condition play a crucial role as temporary energy storage which transfers the excitation energy to the luminescence centers yielding the green broad emission and characteristic sharp lines for Eu³⁺. Figure 4 depicts intensity decays of undoped ZnO:Zn and Eu³⁺,Li⁺-codoped ZnO:Zn samples. The lifetimes of broad green emission (λ_em=500 nm) and Eu³⁺ red emission (λ_em=615 nm) are obtained by fitting the decay curves with two exponential components, and their values are listed in Table 1. It is important to mention that after codoping RE³⁺ and Li⁺ as activators and compensators, the lifetimes of green emission become longer (from τ₁= 0.2 μs, τ₂= 1.8 μs to τ₁= 0.6 μs, τ₂= 4.3 μs), indicating that the electrons can also be trapped by the shallow donor levels then subsequently released with radiation through recombination with the holes trapped by defects like oxygen vacancies. In addition, these arguments can well explain the red shift of green emissions from 500 nm in ZnO:Zn (data not shown here but well-documented elsewhere) to 517 nm in Eu³⁺,Li⁺-codoped sample (see curve a in Fig. 2). The green emission of ZnO:Zn arises from the recombination of intrinsic shallower donors or electrons in conduction band, whereas in the case of Eu³⁺,Li⁺-codoped ZnO:Zn sample the green emission is from the recombination of the shallow donors created by extrinsic impurities like interstitial Li with the holes trapped in deep centers like oxygen vacancies.

 

Fig. 4. Decay curves of undoped ZnO:Zn and Eu³⁺, Li⁺-codoped ZnO:Zn samples sintered in reducing atmosphere monitored at 500 nm and 615 nm, respectively.

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Table 1. Lifetimes of undoped ZnO:Zn (τ_u) and Eu³⁺,Li⁺-codoped ZnO:Zn (τ_c) samples fitted by biexponential decay function: A ₁ exp(t/τ₁)+A ₂ exp(t/τ₂).

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In summary, we report the synthesis of Eu³⁺, Li⁺-codoped ZnO:Zn phosphor as a promising candidate for potential application in near-UV white LEDs. Both broad green band and intense sharp emissions from 4f-4f transitions are observed under the indirect excitation of near-UV light. Energy transfer occurs between the ZnO host to RE³⁺ located at different sites of semiconductor matrix. The ET process is enhanced due to the formation of shallow donor levels that act as temporary energy storage for excited electrons.