Luminescence of ZnS:Cu,Cl Phosphor Powder Excited by Photons, An Electric Field and Cathode rays. Y. T. Nien a and I. G. Chen b. Tainan 701, Taiwan

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1 / The Electrochemical Society Luminescence of ZnS:Cu,Cl Phosphor Powder Excited by Photons, An Electric Field and Cathode rays Y. T. Nien a and I. G. Chen b a Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan b Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan ZnS:Cu,Cl phosphor powder prepared by a solid state reaction at 900 o C showed a broad emission peak in nm, consisting of blue (440nm), self-activated (470nm) and green (510nm) bands. The emission color of the phosphor powder, originating from the relative intensity of these three bands, was found to depend on the excitation method, i.e., bluish green by photons, greenish blue under an electric field, and blue by cathode rays. This behavior was thought to result from the different penetration depth of excitation energy as well as from the spatial distribution of activators in the phosphor. I. INTRODUCTION Semiconductor zinc sulfide (ZnS), with bandgap energies of 3.7eV and 3.8eV in zincblende and wurtzite structures [1], respectively, has great potential for use in solar cells [2], infrared windows [3], and phosphor materials. These material properties are achieved by doping with transition or rare-earth metals [4,5] and are based on its large bandgap energy, direct recombination and high resistance to electric fields. Additionally, owing to the advantages of its simple manufacturing process, the convenience of being able to print a large area and its high power efficiency, ZnS phosphor powder could be excited by photons, cathode rays or an electric field for back lighting of liquid crystal panels or for flat panel displays. The most widely used form of this phosphor material is ZnS:Cu,Cl, in which Cu and Cl behave as acceptors and donors, respectively, and are both contributing to the emission. The luminescence or emission color of ZnS phosphor powder can be classified into five kinds, depending on the relative concentration of activators (Cu) and co-activators (Cl) [6]. Cu doping can cause phase transformation, Cu x S precipitation and tensile strain in ZnS by substituting Zn sites or interstitially entering the lattice, which were observed via Raman, transmission electron microscopy (TEM) and x-ray diffraction (XRD) analyses [7-9]. Luminescence can be categorized into photoluminescence (PL), cathodoluminescence (CL), and electroluminescence (EL) according to excitation sources, i.e., PL by photons, EL by an electric field, and CL by cathode rays. Luminescence has also been widely used to study the crystal quality of semiconductor materials due to the trapping of charge carriers in the impurity centers formed from lattice point or line defects, e.g., observation of ultraviolet CL from stacking faults in GaN[10] and green PL from oxygen vacancies in ZnO[11]. In this study, we will investigate the distribution of dopants, including substitutional Cu and interstitial Cu, via the excitation of ZnS:Cu,Cl phosphor powder using photons, an electric field, and cathode rays, which is based on their different penetration depth into the material. 1

2 II. EXPERIMENTAL For the ZnS:Cu,Cl phosphor powder synthesis, ZnS powders (99.99%, <10µm, Acros Organics, Geel, Belgium) were mixed with 1wt.% NaCl (99.5%, Showa Chemicals, Tokyo, Japan) and 800ppm CuS nanocrystallites in alcohol, where CuS nanocrystallites were prepared by the coprecipitation of CuCl 2 2H 2 O (99%, Showa Chemicals, Tokyo, Japan) and Na 2 S 9H 2 O (98%, Showa Chemicals, Tokyo, Japan) solutions. The mixed powders were dried in an oven at a temperature of 80 o C and then fired in a tubular furnace at 900 o C for 2h in the reducing atmosphere of 3%H 2 /Ar and sulfur. Finally, the ZnS:Cu,Cl powders were rinsed with NH 4 OH (29.7%, J.T. Baker, Phillipsburg, NJ) and de-ionized water before the luminescence measurements. The details of the synthesis of ZnS:Cu,Cl phosphors can be obtained from our previous report [7]. The PL measurement of ZnS:Cu,Cl powders was carried out using a xenon lamp with an excitation wavelength of 343nm. For the EL measurement, ZnS:Cu,Cl powders were mixed with castor oil and then injected into a cell of indium tin oxide (ITO)-coated glass and aluminum sheet with a space of 80µm. The weight ratio of ZnS:Cu,Cl powders to castor oil was 1/6. The EL cell, 15 15mm 2 in area, was excited with an alternating current bias voltage of V at a frequency of 1kHz. The PL and EL were examined using optic fibers and an optical detector with CCD arrays (USB2000, Ocean Optics, Dunedin, FL) at room temperature under ambient atmosphere. III. RESULTS AND DISCUSSION Figure 1 shows the luminescence spectra of ZnS:Cu,Cl with a Cu concentration of 800ppm under excitations of photons (PL) and an electric field (EL). This data reveals a non-gaussian distribution in the range of nm and consisting of blue (B), selfactivated (SA), and green (G) emission bands centered at around 440, 470 and 510nm, respectively. The G-band was caused by the activation and emission of the doped-cu substituting Zn in the host material (Cu Zn ). However, the appearance of the B-band was derived from the interstitial Cu impurities in the lattice (Cu i ). The above emission mechanisms have been explained by a model provided by Kawai[12] and Blicks[13]. The SA-band was thought to result from point defects, such as vacancies of Zn and S (V Zn, V S ) [14]. The inset in Fig. 1a shows the corresponding transition states mentioned above. In PL, it can be seen that the G-band dominated in the spectrum, showing a bluish green color. EL comprised a lower intensity G-band compared to that in PL, resulting in a greenish blue color. We ascribed the different spectral energy distribution or emission color of the same powders to the different excitation volume in the two excitation processes, as discussed below. The excitation volume by photons, an electric field and, later, cathode rays can be estimated approximately by their penetration depth into the phosphor powder. First, the optical absorption coefficient of ZnS at 350nm is 9.98cm -1 [15] and, therefore, the penetration depth (~α -1 ) is around 1mm according to the Beer-Lambert law, I=I o exp(-αx) [1] where I is the light intensity as the incident light (I o ) travels through the material with an absorption coefficient of α to a distance of x. As shown in the inset of Fig. 2, the average size of the phosphor powder was approximately 5µm, indicating that the whole powder 2

3 particle can be excited by photons with a wavelength of 343nm. However, according to the bipolar field emission model proposed by Fischer [16] and our previous studies [7-9], conductive Cu x S precipitates are necessary for luminescent ZnS particles to intensify the electric field and to allow tunneling electrons to recombine with radiation in luminescent centers. This is shown per the dependence of emission intensity on the electric field shown in Fig. 1b inset. Hence, for EL, the excitation volume is believed to localize in the area around Cu x S precipitates in ZnS, which agrees with the observation of the double EL comet lines in ZnS phosphor powder by Fischer [17]. Therefore, the two excitation processes actually resulted in different excitation volume as well as spatial carrier distribution in the powder and, further, inhomogeneous distribution of dopants or Cu x S impurity induced the different spectral energy distribution, as shown in Fig. 1. Figure 1. Luminescence spectra of ZnS:Cu,Cl (Cu:800ppm) powders excited by photons of 343nm (a) and an electric field of 3.75x10 6 V/m (b) at room temperature. B, SA and G correspond to the blue, self-activated and green resolved bands. The band structure of ZnS and its transitions are shown in inset (a). Inset (b) shows the electric field dependence of EL emission intensity. According to the diffusion laws, the concentration of Cu in ZnS:Cu,Cl after the solid state reaction can be seen as a gradient distribution from the powder surface, and decreasing to the powder center (see solid curve in Fig. 2). This activator distribution in the powder is often observed in the preparation of phosphors using solid state reactions.[18] Moreover, the solubility of Cu in ZnS was limited to a constant value, which was less than 400ppm (indicated by the dashed line in Fig. 2) [7-9]. Therefore, the Cu near the powder surface beyond the solubility limit was supposed to enter the lattice interstitially (Cu i ), and some will precipitate as Cu x S for the EL. Owing to the fact of reducing the system free energy and achieving the charge compensation, vacancies (V Zn or V S ) are expected to be introduced around Cu x S precipitates. The distribution of the above dopants and impurities is shown by the symbols in Fig.2. Therefore, in the case of 3

4 electric field excitation, the recombination of tunneling electrons and holes occurring in the region closer to Cu x S precipitates is believed to show mostly defect type emission (Cu i, V Zn or V S ). This agrees with the observed larger intensity B- and SA-bands in the EL spectrum of Fig. 1b compared to those in the PL spectrum of Fig. 1a. However, photons of 343nm excited the whole powder particle, and Cu Zn dominated in the spectrum as seen in Fig. 1a. Figure 2. Schematic Cu concentration (solid curve) in different depth of ZnS powder after the solid state reaction based on Fick s second law, describing the dependence of concentration (C) on time (t), position (x) and diffusion coefficient (D). The dashed line represents the solubility limit of Cu in ZnS and the symbols indicate the types of dopants and impurities, showing substitutional Cu ( ) under the limit and interstitial Cu ( ), Cu x S impurity (*) and vacancies ( ) beyond the limit near the powder surface. Inset: scanning electron micrograph of ZnS:Cu,Cl (Cu:800ppm). An optical spectrometer (MonoCL, Gatan) attached to a scanning electron microscope (SEM, JSM6330TF, JEOL) with a emission current of 63µA was employed to study the CL of the powders. According to the Monte-Carlo simulation [19,20], the penetration depth of electrons at the accelerating voltage of 5kV was around 200nm, as shown in the inset of Fig. 3. This indicates only the excitation of the surface layer of the powder. In Fig. 3, a narrower emission peak which consists of only B- and SA-bands (or lacks G-band) can be observed under the electron beam. As described in Fig. 1b and Fig. 2, interstitial Cu and vacancies were expected to be created near the powder surface due to more Cu beyond the solubility which resulted in B- and SA-bands in the EL. Therefore, it was reasonable that only B- and SA-bands were observed in the CL of Fig. 3 from exciting the powder surface by 5keV electrons. This spatial recombination center distribution inducing different spectral energy distribution was also observed in β-fesi 2 luminescent devices [21]. 4

5 Figure 3. Luminescence spectrum of ZnS:Cu,Cl (Cu:800ppm) powders excited by 5kV electron beams at room temperature. Inset: simulated electron beam distribution with residual 25% (dashed) and 5% (solid) energy after travelling in the material of ZnS. IV. Conclusions ZnS:Cu,Cl phosphor powder prepared by a solid state reaction at 900 o C in the reducing atmosphere showed a broad emission peak from 400nm to 600nm, consisting of B- (440nm), SA- (470nm) and G- (510nm) bands. The emission color of the phosphor powder was found to depend on the excitation method, i.e. bluish green by photons, greenish blue under an electric field and blue by cathode rays, and was thought to result from the different penetration depth of excitation energy as well as spatial distribution of activators or impurities in the phosphor. The distribution of activators and Cu x S impurities in the powder, therefore, was studied based on the changing spectral energy distribution. This analysis showed a decreasing concentration of Cu from the powder surface in the form of Cu Zn for G-band. However, some Cu beyond the solubility will enter ZnS lattice interstitially (Cu i ) and further precipitate as Cu x S near the powder surface to induce B- and SA-bands in the EL and CL spectra. References 1. S. Shionoya, in Phosphor Handbook, S. Shionoya and M. William, Editors, p.232, CRC Press, Boca Raton, FL (1990). 2. T. Nakada, K. Furumi and A. Kunioka, IEEE Trans. Electron Devices, 46,2093 (1999). 3. A. Fujii, H. Wada, K. I. Shibata, S. Nakayama, M. Hasegawa, in Window and Dome Technologies and Materials VII, R. W. Tustison, Editor, p. 206, SPIE, Bellingham, WA (2001). 4. D. S. McClure, J. Chem. Phys., 39, 2850 (1963). 5. R. E. Shrader, S. Larach, P. N. Yocom, J. Appl. Phys., 42, 4529 (1971). 6. W. van Gool, Philips Res. Rept. Suppl., 3, 1 (1961). 7. Y. T. Nien, I. G. Chen, C. S. Hwang, and S. Y. Chu, J. Electroceram., 17, 299 ( 2006). 8. Y. T. Nien and I. G. Chen, Appl. Phys. Lett., 89, (2006). 9. Y. T. Nien, I. G. Chen, C. S. Hwang and S. Y. Chu, J. Phys. Chem. Solids, 69, 366 (2008). 10. R. Liu, A. Bell, F. A. Ponce, C. Q. Chen, J. W. Yang and M. A. Khan, Appl. Phys. Lett., 86, (2005). 11. A. van Dijken, E. A. Meulenkamp, D. Vanmaekelbergh and A. Meijerink, J. Phys. Chem. B, 104, 1715 (2000). 5

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