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Electron traps in Tb3+-doped CaAl2O4
9 September 2002
Chemical Physics Letters 363 (2002) 241–244 www.elsevier.com/locate/cplett
Electron traps in Tb3þ-doped CaAl2O4 D. Jia a, Xiao-jun Wang a
b,*
, W.M. Yen
a
Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USA b Department of Physics, Georgia Southern University, Statesboro, GA 30460, USA Received 16 April 2002; in final form 25 June 2002
Abstract Tb3þ -doped CaAl2 O4 samples were prepared and studied. The 5 D3;4 to 7 FJ (J ¼ 6, 5, 4; . . .) afterglow emissions of Tb with a persistence time of 1 h are observed. Photoconductivity experiment determined that the ground state of Tb3þ is 0.4 eV above the top of the CaAl2 O4 valence band. Two thermoluminescence peaks ()3 and 114 °C) and two thermal stimulated current peaks ()5 and 119 °C) were also detected. The results indicated that the traps at )3 °C (1.08 eV) are close to Tb3þ emission centers, while the traps at 114 °C (0.66 eV) are far away from the centers. Ó 2002 Elsevier Science B.V. All rights reserved. 3þ
1. Introduction Defect-related traps and associated trapping dynamics are important issues for luminescent material research [1,2]. The charge-defects create traps that can trap excited electrons. The trapping rate were found in the same order of the radiative decay rate, therefore a great part of the excited electrons are going into the traps instead of giving radiative emission. As a result, the luminescence efficiency can be greatly reduced by these chargedefects related traps. There are many kinds of traps, such as F-center-like electron traps and V-center-like hole traps. In many cases, the traps are due to charge compensation of the charge-defects. A charge-defect is a point defect due to charge non-compensation so
*
Corresponding author. Fax: +1-912-681-0471. E-mail address: [email protected] (X.-j. Wang).
that the electrons or holes can be trapped at these charge defects by the Coulomb attraction. The traps due to the charge-defects have been found in many materials such as CaS:Eu2þ ,Cl (Al3þ , Y3þ ) [2,3]; Lu2 SiO5 :Ce3þ [4], and SrAl2 O4 :Eu2þ ,Dy3þ [5–7]. On the other hand, we may classify the traps into two categories based on their locations. One is the local trap that is close to the defect and the other is the distant trap that is away from the defect. The local traps can catch excited electrons through tunneling, and the distant traps can only capture the excited electrons that are ionized to the conduction band [8]. When a Tb3þ ion was doped into CaAl2 O4 it substituted a Ca2þ ion and became a charge-defect. In this work, we demonstrate a ceramic system, CaAl2 O4 :Tb3þ , that exhibits the properties of both local and distant defect-related traps. Long afterglow emission was observed in the system because of the Tb3þ -related traps. Photoconductivity, thermoluminescence and thermal stimulated cur-
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 1 7 0 - 3
242
D. Jia et al. / Chemical Physics Letters 363 (2002) 241–244
rent experiments were performed in order to study the traps and the trapping process.
2. Experiment Terbium-doped and undoped calcium monoaluminate ceramic samples were prepared by sintering. 0.01 mol CaCO3 and Al2 O3 powders (1.66 at.% excess CaCO3 ) were mixed with 1 at.% of Tb2 O3 as dopant and 5 at.% of B2 O3 as flux, respectively. The mixture was first sintered at 900 °C for 2 h and was ground again to achieve better mixing. The mixture was then sintered at 1350 °C for 5 h in reducing environment (5H2 % þ N2 gas flow), which prevented Tb3þ from being oxidized to more stable Tb4þ . The emission and excitation spectra of the sample were measured with a FluoroMaxII fluorometer. The afterglow emission was collected into an optic fiber cable and recorded with a cooled CCD. The fluorescence decay was measured with a Spex 500 M spectrometer equipped with photoncounting system. Photoconductivity experiments were performed to study the band position of Tb3þ relative to the band gap of CaAl2 O4 . The incident light for photoconductivity was obtained using a xenon lamp and an ISA Jobin Yvon monochromator. The sample was mounted in between a pair of Ni mesh electrodes and sapphire plates. Thermal stimulated current was also measured using the photoconductivity setup without incident light. Thermoluminescence spectra were recorded when the sample was placed inside a home-made cryostat and could be cooled down to liquid N2 temperature. A cartridge heater was used to heat the sample and an Omega thermal controller was employed to control the heating rate.
3. Results and discussions The emission and excitation spectra of CaAl2 O4 :Tb3þ are shown in Fig. 1. The 4f–4f emissions from the 5 D4 state to 7 FJ (J ¼ 6, 5, 4, 3) states of Tb3þ are found at 493, 543, 590, and 621 nm, respectively. The strongest 5 D4 to 7 F5 transition made the sample emission green. There
Fig. 1. Excitation (—) and emission (– – –) spectra of CaAl2 O4 :Tb3þ .
are two additional emissions shown in Fig. 1 at 415 and 438 nm, corresponding to the Tb3þ transitions from the 5 D3 state to the 7 F5 and 7 F4 states, respectively. The lifetime of the strongest 5 D4 to 7 F5 transition was measured to be 2.21 ms using a Nd:YAG pulsed laser (355 nm). The excitation spectrum was recorded when 543 nm emission of Tb3þ was monitored. The excitation includes 4f–4f transitions from 300 to 390 nm and two 4f–5d transitions at 245 and 285 nm. The lowest 4f–5d transition of Ce3þ in the same host is at 361nm [9]. Using the data and the Dorenbos’ equation [10], the lowest 4f–5d transition of Tb3þ is predicted at 282 nm, which is in good agreement with the observed value. Photoconductivity experiment is a very useful technique to study the relative energy positions for the electron states of impurity in the band gap of the host. When the electrons are photoexcited to the conduction band they produce photocurrent when a high voltage applied across the sample. The absorption spectrum of the undoped CaAl2 O4 and the photoconductivity spectrum of CaAl2 O4 :Tb3þ are shown in Figs. 2a and 2b, respectively. The absorption band edge is at about 215 nm, giving a value of the band gap of the CaAl2 O4 host to be 5.8 eV and being consistent with the sharp increase in the photocurrent. Several weak peaks of photocurrent at longer wavelengths were also observed and they all increase linearly with the incident light power, indicating
D. Jia et al. / Chemical Physics Letters 363 (2002) 241–244
Fig. 2. (a) Absorption spectrum of undoped CaAl2 O4 ceramic and (b) photoconductivity spectrum of CaAl2 O4 :Tb3þ excited by a 180 W Xe lamp.
that the currents were induced by one-photon processes. The photocurrent curve is shown in Fig. 3 in comparison with the excitation spectrum of CaAl2 O4 :Tb3þ , where the excitation spectrum is offset +0.4 eV to match the photocurrent spectrum. The photocurrents that appear at 4.8 eV (260 nm) and lower energies are believed due to the charge transfer from valence band to the excited states of Tb4þ ions, reducing the Tb4þ ions to Tb3þ ions in their excited states. The holes left in the valence band then gave the weak photocurrents. The reasons for the weakness are that the con-
Fig. 3. A comparison of photocurrent (– – –) and excitation (—) spectra.
243
Fig. 4. Afterglow decay curve for 543 nm emission and afterglow spectrum (inset).
centration of existing Tb4þ is low and the mobility of holes is also lower than that of electrons. This type of charge-transfer process was also observed in other systems, such as Ce3þ /Ce4þ in CaS, Ti3þ / Ti4þ in YAlO3 , and Cr3þ /Cr4þ in SrTiO3 [11–13]. The 0.4 eV offset between excitation and photocurrent indicate that the ground state of Tb3þ is 0.4 eV above the top of the host valence band. Similar results have been observed in YAG:Tb3þ [14]. Persistent afterglow of CaAl2 O4 :Tb3þ was observed. The sample was irradiated with a 50 W mercury lamp for 10 min before the afterglow spectrum was recorded. The 543 nm afterglow persisted for 1 h as shown in Fig. 4. To study the properties of the traps in CaAl2 O4 , thermoluminescence (TL) and thermal stimulated current (TSC) spectra of CaAl2 O4 :Tb3þ were obtained and are shown in Fig. 5 with solid line and dashed line, respectively. For both measurements, the sample was cooled below )50 °C and was irradiated with a xenon lamp for 10 min. The irradiated sample was heated at a rate of 0.04 °C/s to record the spectra. There are two TL peaks at )3 and 114 °C, corresponding to two different traps, Tp1 and Tp2, respectively. The depths of the traps were calculated to be 1.08 and 0.66 eV for Tp1 and for Tp2, respectively [15]. Two TSC peaks at )5 °C and 119 °C were also observed, which are consistent with the TL peaks. However, the total TL emission yield by Tp1 (the
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D. Jia et al. / Chemical Physics Letters 363 (2002) 241–244
state is 0.4 eV above the top of the host valence band. Thermoluminescence and thermal stimulated current experiments were performed. The results show that the electron traps with 1.08 eV depth is close to Tb3þ ions and the one with 0.66 eV is away from Tb3þ . Acknowledgements
Fig. 5. Thermoluminescence (—) and thermal stimulated current (– – –) spectra. Sample was cooled down to )50 °C, and irradiated with Xe lamp for 10 min. The heating rate is 0.04 °C/s.
area under Tp1 peak) is twice as strong as that of the Tp2, meaning a larger number of electrons are trapped at the Tp1 than that at Tp2, while the TSC is in the opposite. The total current at 119 °C (Tp2) is about thirty times higher than that of the one at )5 °C (Tp1). One explanation of the huge current difference is that the Tp1 is very close to the Tb3þ ions, and the Tp2 are much further away from Tb3þ ions. Thus the free electrons coming from the Tp1 are much easier to find Tb3þ and decay back to ground state and stay in conduction band for shorter time, making less contribution to photocurrent. On the other hand, the electrons from the Tp2 will stay in conduction band for longer time before finding Tb3þ ions, resulting in more contribution to the transient photocurrent.
4. Conclusions Ceramic CaAl2 O4 :Tb3þ samples were prepared and studied. Persistent afterglow emission of Tb3þ was observed. Two types of charge-defect related electron traps, local and distant traps to the emission center, were identified. The Tb3þ ground
This work was supported by grant DMR 9986693 from the National Science Foundation. D.J. would like to acknowledge the helpful discussions with Dr. R.S. Meltzer. X.J.W. wishes to thank the support of Cottrell College Science Awards from Research Corporation.
References [1] T. Kinoshita, M. Yamazaki, H. Kawazoe, H. Hosono, J. Appl. Phys. 86 (1999) 3729. [2] D. Jia, J. Zhu, B. Wu, J. Electrochem. Soc. 147 (2000) 3948. [3] D. Jia, J. Zhu, B. Wu, J. Electrochem. Soc. 147 (2000) 386. [4] P. Dorenbos, C.W.E. van Eijk, A.J.J. Bos, C.L. Melcher, J. Phys.: Condens. Mat. 6 (1994) 4167. [5] Y.H. Lin, Z.T. Zhang, Z.L. Tang, J.Y. Zhang, Z.S. Zheng, X. Lu, Mater. Chem. Phys. 70 (2001) 156. [6] J. Holsa, H. Jungner, M. Lastusaari, J. Niittykoski, J. Alloys Compd. 323 (2001) 326. [7] H. Yamamoto, T. Matsuzawa, J. Lumin. 72 (1997) 287. [8] D. Jia, X.J. Wang, E. van de Kolk, W.M. Yen, Opt. Commun. 204 (2002) 247. [9] D. Jia, R.S. Meltzer, W. Jia, W.M. Yen, X.J. Wang, Appl. Phys. Lett. 80 (2002) 1535. [10] P. Dorenbos, J. Lumin. 91 (2000) 91. [11] D. Jia, R.S. Meltzer, W.M. Yen, J. Lumin. 99 (2002) 1. [12] S. Basun, T. Danger, A.A. Kaplyanskii, D. McClure, K. Petermann, W.C. Wong, Phys. Rev. B 54 (1996) 6141. [13] S. Basun, U. Bianchi, V.E. Bursian, A.A. Kaplyanskii, W. Kleemann, L.S. Sochava, V.S. Vikhnin, J. Lumin. 66/67 (1996) 526. [14] C.W. Thiel, H. Cruguel, H. Wu, Y. Sun, G.J. Lapeyre, R.L. Cone, R.W. Equall, R.M. Macfarlane, Phys. Rev. B 64 (2001) 5107. [15] S.W.S. McKeever, in: Thermoluminescence of Solid, Cambridge University Press, Cambridge, UK, 1985, p. 85.
Chemical Physics Letters 363 (2002) 241–244 www.elsevier.com/locate/cplett
Electron traps in Tb3þ-doped CaAl2O4 D. Jia a, Xiao-jun Wang a
b,*
, W.M. Yen
a
Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USA b Department of Physics, Georgia Southern University, Statesboro, GA 30460, USA Received 16 April 2002; in final form 25 June 2002
Abstract Tb3þ -doped CaAl2 O4 samples were prepared and studied. The 5 D3;4 to 7 FJ (J ¼ 6, 5, 4; . . .) afterglow emissions of Tb with a persistence time of 1 h are observed. Photoconductivity experiment determined that the ground state of Tb3þ is 0.4 eV above the top of the CaAl2 O4 valence band. Two thermoluminescence peaks ()3 and 114 °C) and two thermal stimulated current peaks ()5 and 119 °C) were also detected. The results indicated that the traps at )3 °C (1.08 eV) are close to Tb3þ emission centers, while the traps at 114 °C (0.66 eV) are far away from the centers. Ó 2002 Elsevier Science B.V. All rights reserved. 3þ
1. Introduction Defect-related traps and associated trapping dynamics are important issues for luminescent material research [1,2]. The charge-defects create traps that can trap excited electrons. The trapping rate were found in the same order of the radiative decay rate, therefore a great part of the excited electrons are going into the traps instead of giving radiative emission. As a result, the luminescence efficiency can be greatly reduced by these chargedefects related traps. There are many kinds of traps, such as F-center-like electron traps and V-center-like hole traps. In many cases, the traps are due to charge compensation of the charge-defects. A charge-defect is a point defect due to charge non-compensation so
*
Corresponding author. Fax: +1-912-681-0471. E-mail address: [email protected] (X.-j. Wang).
that the electrons or holes can be trapped at these charge defects by the Coulomb attraction. The traps due to the charge-defects have been found in many materials such as CaS:Eu2þ ,Cl (Al3þ , Y3þ ) [2,3]; Lu2 SiO5 :Ce3þ [4], and SrAl2 O4 :Eu2þ ,Dy3þ [5–7]. On the other hand, we may classify the traps into two categories based on their locations. One is the local trap that is close to the defect and the other is the distant trap that is away from the defect. The local traps can catch excited electrons through tunneling, and the distant traps can only capture the excited electrons that are ionized to the conduction band [8]. When a Tb3þ ion was doped into CaAl2 O4 it substituted a Ca2þ ion and became a charge-defect. In this work, we demonstrate a ceramic system, CaAl2 O4 :Tb3þ , that exhibits the properties of both local and distant defect-related traps. Long afterglow emission was observed in the system because of the Tb3þ -related traps. Photoconductivity, thermoluminescence and thermal stimulated cur-
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 1 7 0 - 3
242
D. Jia et al. / Chemical Physics Letters 363 (2002) 241–244
rent experiments were performed in order to study the traps and the trapping process.
2. Experiment Terbium-doped and undoped calcium monoaluminate ceramic samples were prepared by sintering. 0.01 mol CaCO3 and Al2 O3 powders (1.66 at.% excess CaCO3 ) were mixed with 1 at.% of Tb2 O3 as dopant and 5 at.% of B2 O3 as flux, respectively. The mixture was first sintered at 900 °C for 2 h and was ground again to achieve better mixing. The mixture was then sintered at 1350 °C for 5 h in reducing environment (5H2 % þ N2 gas flow), which prevented Tb3þ from being oxidized to more stable Tb4þ . The emission and excitation spectra of the sample were measured with a FluoroMaxII fluorometer. The afterglow emission was collected into an optic fiber cable and recorded with a cooled CCD. The fluorescence decay was measured with a Spex 500 M spectrometer equipped with photoncounting system. Photoconductivity experiments were performed to study the band position of Tb3þ relative to the band gap of CaAl2 O4 . The incident light for photoconductivity was obtained using a xenon lamp and an ISA Jobin Yvon monochromator. The sample was mounted in between a pair of Ni mesh electrodes and sapphire plates. Thermal stimulated current was also measured using the photoconductivity setup without incident light. Thermoluminescence spectra were recorded when the sample was placed inside a home-made cryostat and could be cooled down to liquid N2 temperature. A cartridge heater was used to heat the sample and an Omega thermal controller was employed to control the heating rate.
3. Results and discussions The emission and excitation spectra of CaAl2 O4 :Tb3þ are shown in Fig. 1. The 4f–4f emissions from the 5 D4 state to 7 FJ (J ¼ 6, 5, 4, 3) states of Tb3þ are found at 493, 543, 590, and 621 nm, respectively. The strongest 5 D4 to 7 F5 transition made the sample emission green. There
Fig. 1. Excitation (—) and emission (– – –) spectra of CaAl2 O4 :Tb3þ .
are two additional emissions shown in Fig. 1 at 415 and 438 nm, corresponding to the Tb3þ transitions from the 5 D3 state to the 7 F5 and 7 F4 states, respectively. The lifetime of the strongest 5 D4 to 7 F5 transition was measured to be 2.21 ms using a Nd:YAG pulsed laser (355 nm). The excitation spectrum was recorded when 543 nm emission of Tb3þ was monitored. The excitation includes 4f–4f transitions from 300 to 390 nm and two 4f–5d transitions at 245 and 285 nm. The lowest 4f–5d transition of Ce3þ in the same host is at 361nm [9]. Using the data and the Dorenbos’ equation [10], the lowest 4f–5d transition of Tb3þ is predicted at 282 nm, which is in good agreement with the observed value. Photoconductivity experiment is a very useful technique to study the relative energy positions for the electron states of impurity in the band gap of the host. When the electrons are photoexcited to the conduction band they produce photocurrent when a high voltage applied across the sample. The absorption spectrum of the undoped CaAl2 O4 and the photoconductivity spectrum of CaAl2 O4 :Tb3þ are shown in Figs. 2a and 2b, respectively. The absorption band edge is at about 215 nm, giving a value of the band gap of the CaAl2 O4 host to be 5.8 eV and being consistent with the sharp increase in the photocurrent. Several weak peaks of photocurrent at longer wavelengths were also observed and they all increase linearly with the incident light power, indicating
D. Jia et al. / Chemical Physics Letters 363 (2002) 241–244
Fig. 2. (a) Absorption spectrum of undoped CaAl2 O4 ceramic and (b) photoconductivity spectrum of CaAl2 O4 :Tb3þ excited by a 180 W Xe lamp.
that the currents were induced by one-photon processes. The photocurrent curve is shown in Fig. 3 in comparison with the excitation spectrum of CaAl2 O4 :Tb3þ , where the excitation spectrum is offset +0.4 eV to match the photocurrent spectrum. The photocurrents that appear at 4.8 eV (260 nm) and lower energies are believed due to the charge transfer from valence band to the excited states of Tb4þ ions, reducing the Tb4þ ions to Tb3þ ions in their excited states. The holes left in the valence band then gave the weak photocurrents. The reasons for the weakness are that the con-
Fig. 3. A comparison of photocurrent (– – –) and excitation (—) spectra.
243
Fig. 4. Afterglow decay curve for 543 nm emission and afterglow spectrum (inset).
centration of existing Tb4þ is low and the mobility of holes is also lower than that of electrons. This type of charge-transfer process was also observed in other systems, such as Ce3þ /Ce4þ in CaS, Ti3þ / Ti4þ in YAlO3 , and Cr3þ /Cr4þ in SrTiO3 [11–13]. The 0.4 eV offset between excitation and photocurrent indicate that the ground state of Tb3þ is 0.4 eV above the top of the host valence band. Similar results have been observed in YAG:Tb3þ [14]. Persistent afterglow of CaAl2 O4 :Tb3þ was observed. The sample was irradiated with a 50 W mercury lamp for 10 min before the afterglow spectrum was recorded. The 543 nm afterglow persisted for 1 h as shown in Fig. 4. To study the properties of the traps in CaAl2 O4 , thermoluminescence (TL) and thermal stimulated current (TSC) spectra of CaAl2 O4 :Tb3þ were obtained and are shown in Fig. 5 with solid line and dashed line, respectively. For both measurements, the sample was cooled below )50 °C and was irradiated with a xenon lamp for 10 min. The irradiated sample was heated at a rate of 0.04 °C/s to record the spectra. There are two TL peaks at )3 and 114 °C, corresponding to two different traps, Tp1 and Tp2, respectively. The depths of the traps were calculated to be 1.08 and 0.66 eV for Tp1 and for Tp2, respectively [15]. Two TSC peaks at )5 °C and 119 °C were also observed, which are consistent with the TL peaks. However, the total TL emission yield by Tp1 (the
244
D. Jia et al. / Chemical Physics Letters 363 (2002) 241–244
state is 0.4 eV above the top of the host valence band. Thermoluminescence and thermal stimulated current experiments were performed. The results show that the electron traps with 1.08 eV depth is close to Tb3þ ions and the one with 0.66 eV is away from Tb3þ . Acknowledgements
Fig. 5. Thermoluminescence (—) and thermal stimulated current (– – –) spectra. Sample was cooled down to )50 °C, and irradiated with Xe lamp for 10 min. The heating rate is 0.04 °C/s.
area under Tp1 peak) is twice as strong as that of the Tp2, meaning a larger number of electrons are trapped at the Tp1 than that at Tp2, while the TSC is in the opposite. The total current at 119 °C (Tp2) is about thirty times higher than that of the one at )5 °C (Tp1). One explanation of the huge current difference is that the Tp1 is very close to the Tb3þ ions, and the Tp2 are much further away from Tb3þ ions. Thus the free electrons coming from the Tp1 are much easier to find Tb3þ and decay back to ground state and stay in conduction band for shorter time, making less contribution to photocurrent. On the other hand, the electrons from the Tp2 will stay in conduction band for longer time before finding Tb3þ ions, resulting in more contribution to the transient photocurrent.
4. Conclusions Ceramic CaAl2 O4 :Tb3þ samples were prepared and studied. Persistent afterglow emission of Tb3þ was observed. Two types of charge-defect related electron traps, local and distant traps to the emission center, were identified. The Tb3þ ground
This work was supported by grant DMR 9986693 from the National Science Foundation. D.J. would like to acknowledge the helpful discussions with Dr. R.S. Meltzer. X.J.W. wishes to thank the support of Cottrell College Science Awards from Research Corporation.
References [1] T. Kinoshita, M. Yamazaki, H. Kawazoe, H. Hosono, J. Appl. Phys. 86 (1999) 3729. [2] D. Jia, J. Zhu, B. Wu, J. Electrochem. Soc. 147 (2000) 3948. [3] D. Jia, J. Zhu, B. Wu, J. Electrochem. Soc. 147 (2000) 386. [4] P. Dorenbos, C.W.E. van Eijk, A.J.J. Bos, C.L. Melcher, J. Phys.: Condens. Mat. 6 (1994) 4167. [5] Y.H. Lin, Z.T. Zhang, Z.L. Tang, J.Y. Zhang, Z.S. Zheng, X. Lu, Mater. Chem. Phys. 70 (2001) 156. [6] J. Holsa, H. Jungner, M. Lastusaari, J. Niittykoski, J. Alloys Compd. 323 (2001) 326. [7] H. Yamamoto, T. Matsuzawa, J. Lumin. 72 (1997) 287. [8] D. Jia, X.J. Wang, E. van de Kolk, W.M. Yen, Opt. Commun. 204 (2002) 247. [9] D. Jia, R.S. Meltzer, W. Jia, W.M. Yen, X.J. Wang, Appl. Phys. Lett. 80 (2002) 1535. [10] P. Dorenbos, J. Lumin. 91 (2000) 91. [11] D. Jia, R.S. Meltzer, W.M. Yen, J. Lumin. 99 (2002) 1. [12] S. Basun, T. Danger, A.A. Kaplyanskii, D. McClure, K. Petermann, W.C. Wong, Phys. Rev. B 54 (1996) 6141. [13] S. Basun, U. Bianchi, V.E. Bursian, A.A. Kaplyanskii, W. Kleemann, L.S. Sochava, V.S. Vikhnin, J. Lumin. 66/67 (1996) 526. [14] C.W. Thiel, H. Cruguel, H. Wu, Y. Sun, G.J. Lapeyre, R.L. Cone, R.W. Equall, R.M. Macfarlane, Phys. Rev. B 64 (2001) 5107. [15] S.W.S. McKeever, in: Thermoluminescence of Solid, Cambridge University Press, Cambridge, UK, 1985, p. 85.