Trap levels in Y-aluminum garnet scintillating crystals

Radiation Measurements 38 (2004) 673 – 676 www.elsevier.com/locate/radmeas

Trap levels in Y-aluminum garnet scintillating crystals A. Veddaa;∗ , D. Di Martinoa , M. Martinia , J. Maresb , E. Mihokovab , M. Niklb , N. Solovievab , K. Blazekc , K. Nejezchlebc a INFM

and Dipartimento di Scienza dei Materiali dell’ Universita di Milano “Bicocca”, Via Cozzi 53, Milano 20125, Italy b Institute of Physics, AS CR, Cukrovarnick, a 10, Prague 162 53, Czech Republic c Crytur Ltd., Palackeho 175, Turnov 51101, Czech Republic Received 24 November 2003; received in revised form 24 November 2003; accepted 16 February 2004

Abstract Point defects acting as trap levels were investigated on undoped, Ce- and (Ce, Si)-doped Y3 Al5 O12 (YAG) crystals by TSL measurements performed over a wide temperature range (10–800 K). Below room temperature, a composite glow curve was observed, whose intensity strongly increased after Ce doping. Moreover, Ce doping introduced new trap levels giving rise to glow peaks in the 100–200 K range. On the other hand, Si co-doping did not in=uence the low T glow curve in a signi>cant way. The spectral emission of the TSL was found to be governed by the Ce3+ 5d–4f radiative transition, while defect related higher energy emission bands were detected only in the undoped crystal. Above RT, the glow curve was found to be much more in=uenced by Si co-doping since a strong increase of a glow peak at about 250◦ C was noticed. Scintillation time decays of Ce- and Ce,Si-doped samples are also reported and compared with TSL data. The signi>cance of the results and the potential impact of defect states on the scintillation properties are discussed. c 2004 Elsevier Ltd. All rights reserved.  Keywords: Aluminum garnets; Scintillators; Cerium; Thermoluminescence

1. Introduction In the search for new fast scintillators for medical and industrial applications, cerium-doped aluminum garnets are presently being investigated. Speci>cally, Y- and Lu-aluminum garnets (Y3 Al5 O12 –YAG, and Lu3 Al5 O12 – LuAG) have been the subject of several studies (Moszynski et al., 1994; Kaczmarek et al., 1999; Wisniewski et al., 1999; Lempicki et al., 1995; Nikl et al., 2000; Zorenko et al., 2000). In both cases, the emission related to Ce3+ 5d– 4f allowed transition is fast with a time decay lower than 100 ns; moreover, it peaks in the 500–600 nm interval so that it is very suitable for photodiode-based detection as well. Trap levels were investigated both in YAG and LuAG crystals by thermally stimulated luminescence (TSL) measurements below and above room temperature. Concerning ∗ Corresponding author. Tel.: +39-02-644-851-62; fax: +39-02-644-854-00. E-mail address: [email protected] (A. Vedda).

c 2004 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter
doi:10.1016/j.radmeas.2004.02.012

LuAG:Ce, a relation between a strong TSL peak at 285 K and slow components observed in the scintillation decay was proposed (Nikl et al., 2000). Moreover, Zr co-doping was found to aGect the LuAG:Ce TSL properties above RT, by promoting the increase of glow peaks at 80◦ C and 280◦ C (Vedda et al., 2002, 2003). For what regards cerium-doped YAG, several studies dealt with detailed investigations on luminescence time decay properties under both selective and ionizing excitation. A slow component in the cerium emission decay with a time constant of several hundred nanoseconds was found, and related to energy transfer between a defect emission at about 300 nm and a Ce absorption band (Robbins et al., 1979a, b; Ludziejewski et al., 1997). Moreover, a lengthening of the regular Ce3+ radiative decay (from about 70 up to 100 ns) was also observed in the temperature interval 100–400 K, and attributed to the presence of defects acting as carrier traps and giving rise to delayed recombination of carriers at Ce3+ sites. In fact, the presence of trap levels was monitored by TSL measurements that displayed several glow peaks in the same temperature interval where decay lengthening was observed (Zych et al., 2000).

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Due to the in=uence of trap levels on time decay properties, their nature is worth to be better investigated, together with possible thermal or chemical crystal treatments that might be able to reduce their concentration. In this work, we studied the defect properties of Ce3+ -doped Y-aluminum garnet crystals by wavelength-resolved TSL measurements performed in a wide temperature range (10–800 K). The work is grounded on extended experimental data obtained on undoped and Ce3+ -doped samples grown in highly controlled conditions; moreover, due to the positive role that aliovalent dopants may have in reducing the concentration of intrinsic lattice defects (Nikl, 2000), the eGect of co-doping was considered. Recently, a reduction of the TSL signal above RT was observed in YAG:Ce, Mg samples (Wisniewski et al., 1999). The eGect of co-doping with tetravalent ions is here investigated: taking into account the negative results obtained by Zr co-doping on LuAG:Ce crystals (Vedda et al., 2002, 2003)—Zr 4+ being supposed to enter LuAG lattice at Lu3+ substitutional sites—in this case co-doping with Si4+ ions was pursued, which for their ionic radius should occupy Al3+ sites. 2. Experimental conditions Single crystals of Ce-doped and (Ce, Si)-doped YAG were grown by the Czochralski method from a Molybdenum crucible 1 using 6N Y2 O3 and 5N Al2 O3 raw materials; samples with CeO2 concentrations in the melt of 0.23 and 0:32 wt% were prepared, while about 100 ppm of SiO2 was added in the melt to grow other 0:25 wt% Ce; 100 ppm Si co-doped crystals. An undoped sample was also considered as a reference. Low temperature wavelength-resolved TSL glow curves were obtained in the 10–310 K interval after X-ray irradiation at 10 K by a Philips 2274 operated at 20 kV; the TSL signal was detected by a CCD (Jobin Yvon Spectrum 3000) working in the wavelength region 280–715 nm and the heating rate was 0:1 K=s. TSL measurements above RT were performed after X-irradiation at RT by a Machlett OEG 50 X-ray tube also operated at 20 kV. In this case, the glow curves were detected by a photomultiplier (EMI 9635QB) from RT up to 450◦ C. A heating rate of 1◦ C=s was adopted. RT scintillation decays were obtained in single photon counting mode by a spectro=uorometer (Edinburgh Instruments 199S) equipped with a 22 Na radioisotope (511 keV photons) excitation source with a repetition rate of 20 kHz. 3. Experimental results Wavelength-resolved TSL measurements after X-ray irradiation at 10 K were performed on undoped, Ce- and 1

Crystals grown at CRYTUR Ltd., Czech Republic.

Fig. 1. Contour plot of the wavelength-resolved TSL measurement of YAG: 0:32 wt% Ce after X-ray irradiation at 10 K.

Ce,Si-doped samples; in Fig. 1 the contourplot relative to the measurement performed on YAG: 0:32 wt% Ce is reported as a representative case. A composite glow curve extending from 10 K up to approximately 220 K is observed, with several overlapping peaks. The spectral composition of all peaks is governed by the typical emission related to the radiative 5d–4f emission of Ce3+ ions. The glow curves of diGerently doped samples are compared in Fig. 2, where the data are obtained after integration of the wavelength-resolved measurements over the 280–715 nm interval. We remark that the TSL spectral emission of YAG: 0:25 wt% Ce; 100 ppm Si was again dominated by 5d–4f Ce3+ radiative transition. At variance, a diGerent TSL spectrum was observed in the case of the undoped sample: the emission typical of Ce3+ was still observed due to the presence of trace amounts of this dopant; however, and particularly at temperatures below 100 K, a broad and composite high energy structure in the 300–400 nm interval was also observed. Concerning the data presented in Fig. 2, several remarks can be made: >rst of all, it clearly appears that Ce doping strongly in=uences the shape of the glow curve, by promoting the disappearance of the sharp peak at 50 K detected in the undoped crystal, and by introducing several new peaks in the 100– 250 K interval. Moreover, the intensities of the glow peaks are markedly increased after Ce doping. On the other hand, the eGect of Si co-doping on the shape and intensity of glow curves is by far more limited, the major diGerences between Ce- and Ce,Si-doped crystals being observed in the 170– 200 K region. To further investigate the eGect of Ce doping, RT scintillation time decay measurements were performed and the

A. Vedda et al. / Radiation Measurements 38 (2004) 673 – 676 7

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2.5×10

YAG:Ce,Si 7

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1×10

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YAG:Ce,Si YAG:Ce (x10)

YAG undoped (x10)

0 50

100 150 200 Temperature (K)

250

300

Fig. 2. TSL glow curves of undoped YAG, YAG: 0:32 wt% Ce and YAG: 0:25 wt% Ce; 100 ppm Si after X-ray irradiation at 10 K. The glow curves were obtained from integration of wavelength-resolved measurements in the 280–715 nm interval. Heating rate = 0:1 K=s. The glow curves of YAG:Ce and YAG:Ce,Si are shifted on the ordinate scale for better clarity.

1 Ce-doped YAG (Ce,Si)-doped YAG

Scintillation Intensity (arb.units)

TSL Intensity (arb. units)

TSL Intensity (arb. units)

3×10

675

0.1

50 100 150 200 250 300 350 400 450 Temperature (°C) Fig. 4. TSL glow curves of undoped YAG, YAG: 0:32 wt% Ce and YAG: 0:25 wt% Ce; 100 ppm Si after X-ray irradiation at RT. Heating rate = 1◦ C=s. The glow curves of YAG:Ce and YAG:Ce, Si are shifted on the ordinate scale for better clarity.

characterized by a coeQcient alpha (for details see Nikl et al., 2000; Nikl, 2000) of about 5.0–5.1%. The TSL glow curves after X-ray irradiation at RT of diGerent samples were then compared and the data are presented in Fig. 4. In this case, as explained in the Experimental section, the total emitted light was collected by a photomultiplier. In the investigated temperature region from RT up to 450◦ C, a markedly diGerent in=uence of doping on glow curves with respect to the pattern observed at low T is obtained. Namely, the most striking eGect is the very strong increase of the principal 230◦ C peak by Si doping (note that glow curves of undoped and Ce-doped samples were multiplied by a factor 10). Moreover, diGerences on the temperature position and intensity of other minor peaks both below 200◦ C and above 300◦ C are also observed. 4. Discussion and conclusions

0

100

200

300 400 Time (ns)

500

600

Fig. 3. RT normalized scintillation decay (excitation by 511 keV photons of 22 Na radioisotope) of 0:23 wt% Ce and 0:25 wt% Ce; 100 ppm Si-doped YAG samples.

results concerning 0:23 wt% Ce and 0:25 wt% Ce; 100 ppm Si-doped YAG samples are reported in Fig. 3. In both cases, the leading decay component shows the decay time of about 57–58 ns and the contribution of very slow components is

The TSL measurements performed both below and above RT prove the existence of several localized trap levels in YAG crystals. Due to the very good purity of the raw materials used to grow the samples, an intrinsic nature of the defects responsible for such localized levels can be suggested. In cerium-doped crystals, the fact that all TSL peaks are characterized by emission at Ce3+ sites implies that Ce3+ ions are the main recombination centres, and that carriers thermally freed from traps undergo delocalization to the conduction band before their radiative recombination; concerning the diGerent, high-energy emission additionally observed in the undoped sample, we remark that emission bands in a similar energy region were observed by photoluminescence (PL) and cathodoluminescence (CL) measurements (Robbins et al., 1979b; Wong et al., 1984) and attributed to

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lattice defects. In CL experiments, a decrease of such emission by increasing rare-earth activator concentration and temperature was noticed and interpreted by a competition between defects and activators in the recombination pathways of free carriers, also governed by thermally activated energy transfer between defects and activators (Robbins et al., 1979b). A similar situation could occur also in the TSL process, where defect emission is observable mainly at T lower than 100 K and only when the activator concentration is very low, like in the nominally undoped crystal. Coming back to Ce-doped samples, from low T measurements it can be noticed that Ce3+ dopant not only provides the TSL recombination centre, but it also favours the creation of new trapping states mainly in the 100–200 K region. These might possibly be due to intrinsic defects related to lattice distortion introduced by doping. Apart from the strong modi>cations of the shape of the glow curve, the noticeable increase of the overall TSL intensity after cerium doping (note that the glow curve of the undoped sample in Fig. 2 was multiplied by a factor 5) can be ascribed to the increase of the concentration of luminescent sites. In principle, it is well known that defect states giving rise to glow peaks below RT could compete with prompt radiative decay giving rise to a lengthening of the recombination process. As already mentioned in the Introduction, such a process was even veri>ed in the case of YAG crystals (Zych et al., 2000). In this case, Si co-doping has no strong eGect on the composite glow curve and so, a modi>cation of scintillation time decay features following Si co-doping is not expected. Indeed, closely similar scintillation decays were obtained for Ce- and (Ce,Si)-doped YAG samples as displayed in Fig. 3. A diGerent, although not more favourable, situation was revealed above RT, where Si doping caused the signi>cant increase of the glow peak in the 250◦ C region and of a higher temperature shoulder. Since a similar peak exists also in Si-free samples although with a much lower intensity, it may be supposed that the responsible defect is of intrinsic nature but that Si co-doping creates a local situation in the lattice in which the formation of such a defect is favoured. Such a deep trap is surely harmful for the intended application of the crystal as scintillator, since it can trap carriers in a stable way so competing with prompt radiative recombination pathways. Such deep traps are mostly the reason for the radiation damage phenomena and coloring of the scintillators causing the long term instabilities of their optical parameters (Nikl, 2000). Hence, in the considered concentration of the order of 100 ppm in the melt, Si co-doping was not useful to reduce lattice defects neither at low nor at high temperatures, and in the latter case even opposite eGects were noticed. The approach of defect reduction by doping with higher valence ions, which gave very good results, for example, in the case of the intrinsically Pb-poor PbWO4 lattice (where doping by trivalent La, Lu or Gd strongly suppressed TSL peaks and favoured scintillation speed and radiation hardness (Nikl, 2000), is not eGective

in the case of YAG garnet crystals). As already mentioned in the Introduction, the same result was obtained by considering Zr co-doping in LuAG:Ce crystals (Vedda et al., 2002, 2003), proving the diGerent nature and dynamics of point defects in garnet crystals. The eGect of diGerent chemical and thermal treatments should be further veri>ed in the future, in order to shed light on the nature of such intrinsic point defects and to limit their concentration in optimized crystals. Acknowledgements Financial support of NATO SfP 973510 project and FF-P/125 project of the Ministry of Industry and Trade, Czech Republic, is gratefully acknowledged. References Kaczmarek, S.M., et al., 1999. Changes in optical properties of Ce:YAG crystals under annealing and irradiation processing. Cryst. Res. Technol. 34, 719–728. Lempicki, A., et al., 1995. LuAlO3 :Ce and other aluminate scintillators. IEEE Trans. Nucl. Sci. 42, 280–284. Ludziejewski, T., et al., 1997. Investigation of some scintillation properties of YAG:Ce crystals. Nucl. Instrum. Meth. A 398, 287–294. Moszynski, M., et al., 1994. Properties of the YAG scintillator. Nucl. Instrum. Meth. A 345, 461–467. Nikl, M., 2000. Wide band gap scintillation materials: progress in the technology and material understanding. Phys. Stat. Sol. (a) 178, 595–620. Nikl, M., et al., 2000. Traps and timing characteristics of LuAG : Ce3+ scintillator. Phys. Stat. Sol. (b) 181, R10–R12. Robbins, D.J., et al., 1979a. The temperature dependence of rare-earth activated garnet phosphors. I. Intensity and lifetime measurements on undoped and Ce-doped Y3 Al5 O12 . J. Electrochem. Soc. 126, 1213–1220. Robbins, D.J., et al., 1979b. Investigation of competitive recombination processes in rare-earth activated garnet phosphors. Phys. Rev. B 19, 1254–1269. Vedda, A., et al., 2002. Defect states in Lu3 Al5 O12 :Ce crystals. Rad. EGects Defects Solids 157, 1003–1007. Vedda, A., et al., 2003. Thermoluminescence of Zr-codoped Lu3 Al5 O12 :Ce crystals. Phys. Stat. Sol. (a) 195, R1–R3. Wisniewski, K., et al., 1999. Excited state absorption and thermoluminescence in Ce- and Mg-doped yttrium aluminum garnet. Acta Phys. Pol. A 95, 403–413. Wong, C.M., Rotman, S.R., Warde, C., 1984. Optical studies of cerium doped yttrium aluminum garnet single crystals. Appl. Phys. Lett. 44, 1038–1040. Zorenko, Y., et al., 2000. Application of scintillators based on single-crystalline Lu3 Al5 O12 : Ce3+ >lms for radiation monitoring in biology and medicine. Semicond. Phys. Quantum Electron. Optoelectron. 3, 213–218. Zych, E., Brecher, C., Glodo, J., 2000. Kinetics of cerium emission in a YAG:Ce single crystal: the role of traps. J. Phys.: Condens. Matter 12, 1947–1958.