IRON-RELATED LUMINESCENCE CENTERS IN

Radiation Effects & Defects in Solids, 2002, Vol. 157, pp. 1123–1126

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IRON-RELATED LUMINESCENCE CENTERS IN ZnWO4:Fe ¨ NSSONc, M. KIRMd, V. NAGIRNYIa,*, S. CHERNOVb, L. GRIGORJEVAb, L. JO a a b A. KOTLOV , A. LUSHCHIK , D. MILLERS , V. A. NEFEDOVe, V. PANKRATOVb and B. I. ZADNEPROVSKIe a Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia; Institute for Solid State Physics, University of Latvia, Kengaraga 8, LV-1066 Riga, Latvia; c Physics Department, Lund University, Professorsgatan 1, S-223 63 Lund, Sweden; d Institute of Experimental Physics, Hamburg University, Hamburg, Germany; e All-Russia Research Institute of Mineral Materials Synthesis, Institutskaya St. 1, Alexandrov, Vladimir Region, 601600 Russia

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A systematic spectroscopic study of single ZnWO4:Fe crystals with different iron concentrations has been performed under excitation by ultraviolet light, by synchrotron radiation or under photostimulation by near-infrared light. The luminescence of Fe3þ-related centres has been studied. It is shown that iron centres of different types efficiently promote the formation of crystal defects at low temperatures. Electrons and holes can be trapped near Fe2þ or Fe3þ ions, which is further revealed in phosphorescence, thermostimulated or photostimulated luminescence. At room temperature the main effect of iron impurity is to reduce the light yield of a ZnWO4 scintillator. Keywords: Tungstate crystals; ZnWO4:Fe; Impurity centres; Luminescence

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INTRODUCTION

Zinc tungstate, ZnWO4, is a typical representative of optically anisotropic systems with a complicated oxyanionic structure. It is a good candidate for application as scintillator. The main restricting factors for successful application of these crystals are the difficulties in producing pure and stoichiometric crystals. Because of a high segregation coefficient (1 in ZnWO4) iron is the most common unintended impurity in the crystal. Due to the presence of Fe2þ and Fe3þ ions a sequence of impurity-related colour centres may be created in the crystal, which can considerably affect the scintillation properties of the crystal. A number of publications report on ESR studies of local symmetries of various iron-related centres in ZnWO4 [1, 2]. The aims of the present study are to reveal the contribution of the iron impurity into the optical spectra of ZnWO4 and to clarify its possible effects on the processes influencing scintillation properties of the material. * Corresponding author. Fax: þ372-7-383 033; E-mail: [email protected]

ISSN 1042-0150 print; ISSN 1029-4953 online # 2002 Taylor & Francis Ltd DOI: 10.1080=1042015021000052845

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RESULTS AND DISCUSSION

A systematic spectroscopic study of single ZnWO4:Fe crystals with different iron concentrations, CFe, ranging from 106 to 104 mole=mole was performed under excitation by ultraviolet light, by synchrotron radiation or under photostimulation by near-infrared light. Iron concentrations in the samples were estimated from absorption spectra using the method described in [3]. Absorption spectra were measured with spectrophotometer Jasco V-570. Excitation spectra of various emissions in the energy region 3.5–35 eV were studied at 8 and 300 K at the SUPERLUMI station of HASYLAB (Hamburg, Germany). The emission and excitations spectra of phosphorescence and photostimulated recombination luminescence were investigated using synchrotron radiation from the storage ring MAX I (Lund, Sweden). Emission spectra in the energy region 1–4 eV were measured using a conventional spectroscopic setup at the Institute of Physics, Tartu. All the spectra were measured from the surface exposed to the incident exciting light to minimize the effect of reabsorption of the emission by impurity centres. It is shown that the emission spectra of the studied ZnWO4:Fe crystals consist mainly of two emission bands: the main band at 2.5 eV connected with the decay of oxyanionic molecular excitons and an impurity-related band at 1.7 eV (Fig. 1, curves 1 and 2). The emission band at 1.7 eV was first observed at nitrogen laser excitation [4]. Its excitation spectrum has been presented in [5]. The emission at 1.7 eV is most effectively excited in the region of the absorption band of Fe3þ centres. For that reason it has been preliminary ascribed to the luminescence centers associated with Fe3þ ions. Our study reveals the following features of the 1.7-eV emission. There is no linear dependence of the intensity of this emission on iron concentration in the crystal. In the available samples, the intensity was the largest in crystals containing 105 mole=mole of iron and decreased as CFe approached 104 mole=mole. The intensity of the 1.7 eV band strongly depends on the origin and the growth method of the crystal. It was maximal in a crystal grown by the Czochralski method with CFe ¼ 1.5 105 mole=mole, whereas after treatment of the sample in an oxidizing atmosphere the emission band at 1.7 eV practically disappeared and a new weaker emission band appeared at 2.1 eV. These data indicate that the centre responsible for the 1.7-eV emis-

FIGURE 1 Emission spectra of a pure ZnWO4 excited by 4.1-eV photons (1) and of ZnWO4:Fe excited by 3.8-eV photons (2) and X-rays (3) at 4.2 K. The phosphorescence spectrum of ZnWO4:Fe 30 minutes after X-irradiation at 4.2 K (4).


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sion has a more complicated structure, which in addition to the Fe3þ ion includes other lattice defects, partly because of the need to compensate for the extra positive charge of Fe3þ in a Zn2þ site. The above-described features resemble the behavior of Fe3þ-OH centres described in [3]. The emission at 1.7 eV is strongly anisotropic that provides additional evidence for low symmetry of the corresponding luminescence centres. Depending on crystal growth conditions and crystal treatment in oxidizing or reducing atmosphere, several types of Fe3þ-related centres are created. It is clear that complicated impurity-related centres formed on the basis of Fe3þ ions may effectively trap electrons and holes created by ionising radiation, thus substantially hindering scintillating performance of ZnWO4. To clarify what role Fe3þ-related centers play in the processes of hole and electron trapping, we studied ZnWO4:Fe exposed to the X-ray and synchrotron radiation. Figure 1 presents the X-ray excited emission spectrum (curve 3), measured at 4.2 K for the sample with CFe ¼ 6 105 mole=mole. The contribution of the red emission to the total steadystate emission at X-ray excitation is relatively small. It is determined mainly by the phosphorescence fraction of the emission, which has relatively low initial amplitude, but its half-decay time at 4.2 K is nearly 1 hour. The phosphorescence spectrum measured 30 minutes after the X-ray irradiation is shown in Figure 1, curve 4. The contribution of the red emission into the phosphorescence is comparable to that of the main emission at 2.5 eV. This means that a substantial part of charge carriers created by the ionizing radiation at low temperatures is efficiently trapped in a proximate vicinity of the Fe2þ or Fe3þ ions. A subsequent recombination of impurity-trapped holes or electrons due to tunneling or thermal transitions in close pairs occurs through the excited state of Fe3þ-related luminescent centers, giving rise to the red emission of ZnWO4:Fe. At temperatures higher than 80 K no red phosphorescence recombination component was observed in the emission of ZnWO4:Fe under pulsed electron-beam excitation, indicating that at these temperatures the process described above is inefficient, i.e. no trapping of charge carriers occurs near the iron impurity. A considerable number of separated localized charge carriers that do not contribute to the phosphorescence is created at 4.2 K. We found that a part of these colour centers can be completely bleached by visible light in the energy region 2.5–1.4 eV selected by a double-quartz monochromator. The half-decay time at such a bleaching process was about 40 seconds. It was accompanied by recombination luminescence peaking at 2.5 eV. We used this feature

FIGURE 2 Creation spectrum of non-correlated electron-hole pairs in ZnWO4:Fe at 8 K and excitation spectrum of the main 2.5 eV emission of a pure ZnWO4 at 4.2 K (2).


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to study the creation spectrum of non-correlated electron-hole (e–h) pairs in ZnWO4:Fe. At each energy the sample was irradiated by the same number of photons, after which it was stimulated by the light of 1.8 eV and the time dependence of recombination photostimulated luminescence (PSL), selected by a green optical filter SZS-22, was measured. The intensity (SPSL) integrated over the first 25 s was taken as a measure of the number of recombining centers. The creation spectrum measured for a ZnWO4:Fe crystal at 8 K is shown in Figure 2, curve 1. For comparison an excitation spectrum for the main 2.5 eV emission is shown (curve 2). The main rise in the creation spectrum starts at energies higher than 6 eV. This result is consistent with our earlier results on phosphorescence excitation spectra of ZnWO4 [5]. It confirms that non-correlated e–h pairs are mainly created in ZnWO4 at energies nearly 2 eV higher than the fundamental absorption edge. The efficiency of e–h pair creation reaches a plateau at energies 9–13 eV. The further growth starting from 13 eV is due to the creation of two and more e–h pairs by one photon. It is also clear that in ZnWO4:Fe separated e–h pairs can be created at excitation in the impurity related bands below 6 eV and thus affect the processes of energy transport in the ZnWO4 scintillator (Fig. 2, curve 1).

3.

CONCLUSION

Considering that no Fe3þ emission can be excited in ZnWO4:Fe by high-energy photons or electrons at room temperature, the main effect of Fe impurity in ZnWO4 may be the reduction of the light yield of the main emission. By increasing the iron concentration the intensity of the main emission decreases. The intensity of the main emission excited at 4.2 K by photons of 4.1 eV decreases by a factor of 10 at CFe changing from 2 105 to 104 mole=mole. Beside the direct reabsorption of the main emission by Fe2þ centers the reason of such an intensity reduction is probably a non-radiative recombination of charge carriers created in the vicinity of complicated iron-related centers in the crystal. Acknowledgements This work was partly supported by the Estonian Science Foundation (grant 5029), the IHPContracts HPRI-CT-1999-00040 and HPRI-CT-1999-00058 of the European Commission. References [1] Nilsen, W. G. and Kurtz, S. K. (1964). Phys. Rev., 136, A262. [2] Watterich, A., Wo¨ hlecke, M., Mu¨ ller, H., Raksanyi, K., Breitkopf, A. and Zelei, B. (1992). J. Phys. Chem. Solids, 53, 889. [3] Fo¨ ldvari, I., Capelletti, R., Peter, A., Cravero, I. and Watterich, A. (1986). Solid State Commun., 59, 855. [4] Grigorjeva, L., Millers, D., Chernov, S., Pankratov, V. and Watterich, A. (2001). Radiat. Meas., 33, 645. [5] Nagirnyi, V., Feldbach, E., Jo¨ nsson, L., Kirm, M., Kotlov, A., Lushchik, A., Nefedov, V. A. and Zadneprovski, B. I. (2002). Nucl. Instr. and Methods in Phys. Res. A, 486, 395.