1. Introduction

Luminescent materials with broad emission band have become the key to the success of many applications such as solid-state lighting and broad band amplifiers for optical fiber communication. White LEDs provide an efficient way for general lighting [1]. However, the most commercialised white LEDs, obtained by combining blue LEDs with YAG:Ce³⁺ yellow phosphor suffer from a poor color rendering (color rendering index 70~74) [2] because of a narrow band-width of the blue emission and the lack of red emission from the phosphor. Solid-state lighting with broad emission band, especially in the red color region is the key to the wide use of this energy-efficient lighting technology. Broadband luminescent materials are also actively studied in the telecommunication bands (near infrared region) as novel candidates for optical amplifier, in order to meet the rapidly increasing demand for data transmission by increasing the number of channels in the dense wavelength-division multiplexing (DWDM) system [3–5].

Broadband emission can be normally achieved by using transition metal ions, rare-earth ions with 4f-5d configuration or charge transfer transition, ions with s² configuration and inter-band transition [6]. To obtain still broader emission bands, three ways are generally used: Co-doping two or more broadband active centers in one host lattice [7, 8], combination of several phosphors with different emission wavelengths [9, 10] and piling up different luminescent layers [11].

It is well known that the energy levels of transition metal ions or certain rare-earth ions such as Ce³⁺ and Eu²⁺ depend strongly on their chemical environment in the host lattice [6]. This property makes possible emission tuning by changing the chemical environment surrounding these ions. Based on this concept, we design a thin film structure with composition gradient and doped with Eu²⁺ ions. This composition gradient provides continuously changing chemical environment for the active ions so that ultra-broad emission bands could be achieved.

In this work, we will demonstrate the technical feasibility of this gradient thin film structure. Our preliminary attempt is devoted to the preparation of thin films with continuously adjusted composition and tunable emission wavelength by using cathodic co-sputtering.

2. Experimental methods

The selected system is CaS-ZnS:Eu²⁺/SrS-ZnS:Eu²⁺. It has been reported that the solid solution, Ca_1-xSr_xS:Eu powder, can be prepared by high temperature solid-state reaction, and these phosphors have continuously shifted emission wavelength [12]. Therefore the CaS/SrS:Eu system is suitable for the gradient thin film structure. Two φ50-mm ceramic disks (CaS)_66.7-(ZnS)_33.3: 0.3 mol% Eu²⁺ and (SrS)_66.7-(ZnS)_33.3: 0.3 mol% Eu²⁺ used as the targets for sputtering were prepared by hot-pressing from the raw materials CaS (3N), SrS (3N), ZnS (3N) and EuCl₃ (3N). There are two reasons for using ZnS. Firstly, the melting point of ZnS is lower than that of CaS and SrS, leading to lower sintering temperature. Secondly, because of the big ionic radius difference between Eu²⁺ and Zn²⁺, Eu²⁺ ions tend to replace Ca²⁺ ions and Sr²⁺ ions but not Zn²⁺ [13], so that the addition of ZnS couldn't affect the environment of Eu²⁺ ions.

The co-sputtering was performed at room temperature with a cathodic magnetron sputtering system Plassys MP600s. Before sputtering, the pressure in the deposition chamber was reduced down to 10⁻⁷ mbar. Then 0.033 mbar Ar was introduced into the chamber for the build-up of plasma. Five thin films were fabricated with different ratio of power applied on the two targets. The detailed information on the applied power is listed in Table 1.BK7 glass and silica glass were used as substrates. The latter was used for the thin films Sr4Ca7 and Sr0Ca7 in order to withstand an annealing at 900°C.

Table 1. Ratio of power applied on the targets P_SrS/P_CaS and the Ca/(Ca + Sr) ratio of the annealed thin films analyzed by EDS.

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To improve the luminescence efficiency, 2-hour heat-treatment under vacuum was carried out after deposition. For the thin films Sr6Ca0, Sr6Ca6 and Sr4Ca6, the annealing temperature was 500°C while for the thin films Sr4Ca7 and Sr0Ca7 it was 900°C.

The composition of the as-treated thin films was analyzed with an energy-dispersive X-Ray spectroscopy (EDS), using a field emission scanning electron microscope JSM7100F (JEOL) equipped with an EDS system (Oxford-INCA). X-ray diffraction patterns were recorded using a Philips PW3710 diffractometer operating with Cu Kα radiation (λ = 1.5418 Å). Luminescence measurements were performed with an Edinburgh Instruments FLS920 fluorescence spectrometer at room temperature. A Xe lamp Xe900 (450W) was used as light source. The emission was detected by a Hamamatsu R928 photomultiplier detector. All the spectra obtained were corrected by using the excitation and emission correction curves provided by the manufacturer to remove the influence from the instrument. The external quantum efficiency of the luminescence was determined with the technique described in Reference [14] by using a barium sulfate coated integrating sphere attached to the FLS920.

3. Results and discussion

The EDS results in Table 1 demonstrate the composition tunability of the thin films: With the decrease of P_SrS/P_CaS, the ratio of power applied on the two targets, the Ca/(Ca + Sr) ratio increases gradually.

The as-deposited thin films are amorphous according to the x-ray diffraction (XRD) patterns, as shown in Fig. 1(a).The crystallinity can be improved by heat treatment. One example is presented in Fig. 1(a): for the sample Sr6Ca0, an annealing at 300°C for 2 hours leads to the crystallization of the cubic SrS phases. After an annealing at 500°C, the diffraction peaks of ZnS can also be identified.

 

Fig. 1 (a) XRD patterns of the as-deposited and heat treated Sr6Ca0 thin films. The annealing temperature ranges from 220°C to 500°C and the duration is 2 hours. (b) XRD patterns of the series of thin films after annealing at 500°C or 900°C.

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Figure 1(b) shows the XRD patterns of the thin films after heat treatment. By examining the position of the two most intense diffraction peaks (200) and (220), it can be seen that these peaks of the co-deposited thin films are located between the corresponding peaks of SrS (JCPDS 8-489) and CaS (JCPDS 75-261). Moreover, when the content of Ca increases, these peaks shift to the corresponding peaks of CaS. These phenomena indicates that the crystals in the thin films belong to the solid solution phase Ca_1-xSr_xS and the content of Ca in the crystals increases when the power ratio P_SrS/P_CaS decreases, as expected and designed.

The crystallinity of these thin films is essential for the photoluminescence efficiency. This importance is demonstrated in Fig. 2(a), by taking Sr6Ca0 thin film as an example. It can be clearly seen that the as-deposited thin film shows negligible luminescence under excitation at 457 nm. After an annealing at 300°C, the temperature at which crystallization starts (shown in Fig. 1(a)), much stronger luminescence then can be observed. A higher annealing temperature leads to a stronger luminescence. The luminescence of Eu²⁺ ions involves the d-f electronic transition. Because of the strong interaction between the d-orbital electrons and the lattice vibration, the luminescence efficiency of Eu²⁺ is intimately related to its environment. Good crystallinity provides ordered structure and reduces structural defects so that efficient luminescence of Eu²⁺ can be achieved. The importance of crystallinity for the luminescence is also observed for another ion with d-f transition, represented by Ce³⁺ in Y₃Al₅O₁₂ (YAG) thin films [15].

 

Fig. 2 Emission spectra of the thin films excited by 467-nm blue light. (a) Luminescence of the as-deposited and heat treated Sr6Ca0 thin films. (b) Normalized emission bands of the series of thin films after annealing, showing a continuous shift. The inserted photos shows the color change of the photoluminescence of the thin films. A 495-nm high-pass filter was used during the record of the photos in order to remove the incident blue light. The inserted curve presents the variation of the FWHM of the emission bands of the thin films and of the powders reported in [12].

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Considering the importance of crystallinity for the luminescence, the thin films Sr6Ca0, Sr6Ca6, Sr4Ca6 were annealed at 500°C and the thin films Sr4Ca7, Sr0Ca7 were annealed at 900°C to obtain efficient photoluminescence. Figure 2(b) presents the emission spectra of the annealed thin films, under 467-nm light excitation. With the normalized spectra, it can be clearly seen that the emission bands shift to longer wavelength when the content of Ca in the films increases. The color of the luminescence changes from yellow to orange and finally to dark red, as shown in the inserted photos in Fig. 2(b).

The luminescence of Eu²⁺ arises from the d-f transition of 4f⁶5d¹(T_2g) to 4f⁷(⁸S_7/2). In cubic crystals like CaS and SrS, the 4f⁶5d¹ configuration split into two terms T_2g and E_g. The degree of splitting depends strongly on the ligand field strength. When the content of Ca increases in Ca_1-xSr_xS, the inter-cation-anion distance decreases so that the ligand field strength increases. This leads to a larger splitting of 4f⁶5d¹ and to lower the gap between 4f⁶5d¹(T_2g) and 4f⁷(⁸S_7/2) [16]. As a consequence, the emission band of Eu²⁺ shifts to longer wavelength.

Interestingly, by checking the variation of the full width at half maximum (FWHM, insert curve of Fig. 2(b)), it can be seen that the emission band of Eu²⁺ in the co-deposited thin films is significantly wider than that of the thin films with only Sr or Ca. This can be attributed to the diversity of the environment of Eu²⁺ in the co-deposited thin films, e.g. the fluctuation of the local Ca/(Ca + Sr) ratio around the Eu²⁺ ions. Compared with the Ca_1-xSr_xS powders prepared by flux assisted solid-state reaction [12], which is also presented in the insert, the broadening of emission band of Eu²⁺ in the thin films is greatly enhanced. This can be explained by the fact that due to a much higher quenching rate, a more non-equilibrium state is obtained by sputtering technique, leading to more chemically distributed sites even the thin films are thermally annealed after the deposition. This result indicates that thin films can provide more diversified chemical environment which is the key to broaden the luminescence associated with d-f electronic transition.

The tunable emission band of the Ca_1-xSr_xS: Eu thin film demonstrates the feasibility of fabricating gradient film having ultra-broad emission band. By gradually decreasing P_SrS and increasing P_CaS during the sputtering, the emission band can shift to long wavelength from the bottom layer to the top layer, resulting in a ultra-broad entire emission, which is schemed in Fig. 3(a).This gradient structure has several advantages. Firstly, by controlling the composition gradient, it is possible to get rid of any spectral discontinuity in the emission band, which is usually difficult to achieve by combining several phosphors or luminescent layers. Secondly, the continuous change of composition can minimize the lattice mismatch between the layers, which can improve the mechanical stability of the thin films. Thirdly, compared with the powder phosphors which should be homogenously mixed with epoxies for application, the thin film structure is more compatible with the miniaturization and integration requirement of the devices [17]. In addition, sputtering technique is a low-cost mass production technique widely used for continuous fabrication of rewritable DVDs, protective coatings, photovoltaic solar cells...

 

Fig. 3 (a) Scheme of a thin film with composition gradient having a ultra-broad emission band. (b) Emission spectrum of the tri-layer thin film Sr0Ca7/Sr4Ca6/Sr6Ca0 excited by 467-nm blue light. The FWHM reaches 92 nm.

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As a preliminary attempt, we prepared a tri-layer thin film by successively depositing the thin films Sr6Ca0, Sr4Ca6 and Sr0Ca7 on a silica glass substrate. The thickness of the layers is sequentially 430 nm, 142 nm and 305 nm. The thin film was then annealed at 900°C for two hours. Its emission spectrum under 467-nm light excitation is presented in Fig. 3(b). Still broader emission is observed for this thin film in comparison with the single-layer thin films. The FWHM of the emission peak increases to 92 nm. By using the fluorescence spectrometer equipped with an integrating sphere, the external quantum yield of the luminescence was measured and turned out to be 45%. According to the transmission spectrum, 13% of the incident light at 467 nm is absorbed by the tri-layer thin film, even though its thickness is less than 1 μm. It can be estimated that with a thickness of 4.5 μm, the absorption at 467 nm can reach 50%. This strong absorption arises from the intensive d-f transition of Eu²⁺, which is allowed by the selection rule.

Considering the strong absorption and the effective broad emission in the red region, the co-deposited Ca_1-xSr_xS thin films are promising to make up the red spectral deficiency of the YAG:Ce-coated blue LEDs which are the most used way for general lighting. Here we have calculated the emission spectra of two possible light sources by combining the emission bands of the GaN LED [18], the commercial YAG:Ce powder (Hongda Co. Ltd.) and the tri-layer thin film with different mixture ratio between the three bands (Fig. 4(a)). According to the chromaticity coordinates calculated from the emission spectra, the two light sources provide white light as shown in Fig. 4(b). Their correlated color temperatures (CCT) are 5455K and 4004K respectively. The color rendering indices (CRI) of these two possible light sources are 93 and 95, which are greatly improved in comparison with the commercial YAG:Ce-coated blue LEDs (CRI typically around 70~74 [2]).

 

Fig. 4 (a) Calculated combination of the emission spectra of the GaN LED [18], the commercial YAG:Ce powder (Hongda Co. Ltd.) and the tri-layer thin film, with two difference mixture ratios between the three bands. (b) CIE 1931 chromaticity coordinates of two possible light sources having the emission spectra shown in Fig. 4(a).

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4. Conclusion

In conclusion, we have demonstrated the possibility to fabricate thin films of Ca_1-xSr_xS:Eu solid solution by cathodic co-sputtering. The experiments show that by controlling the ratio of power applied on the two targets CaS-ZnS:Eu and SrS-ZnS:Eu, the Ca/(Ca + Sr) ratio in the thin films can be continuously adjusted. Meanwhile, the luminescence of the thin films under blue light excitation shifts from yellow to dark red. A broadening of the emission band is also observed in the co-deposited thin film, which could be attributed to the composition fluctuation. Based on these results, we propose that the Ca_1-xSr_xS:Eu thin film can be combined with YAG:Ce phosphors and blue LEDs to fabricate novel white light LEDs with high color rendering index. More importantly, the technique developed in the work allows to control the composition gradient of the thin films by simply programming the power applied on the targets during the co-sputtering. Using the controllable composition gradient, it is expected that ultra-broad emission bands can be achieved and the profile of the emission band can be controlled.