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Growth and optical properties of Yb doped new scintillator crystals
Optical Materials 24 (2003) 275–279 www.elsevier.com/locate/optmat
Growth and optical properties of Yb doped new scintillator crystals A. Yoshikawa a,*, H. Ogino a, J.H. Lee a, M. Nikl b, N. Solovieva b, N. Garnier c, C. Dujardin c, K. Lebbou c, C. Pedrini c, T. Fukuda a a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b Institute of Physics AS CR, Cukrovarnicka 10, 162 53 Prague, Czech Republic c Physical Chemistry of Luminescent Materials, Claude Bernard/Lyon1 University, CNRS UMR 5620, 69622 Villeurbanne Cedex, France Received 8 December 2002; accepted 24 February 2003
Abstract Single crystal of heavily Yb3þ -doped Lu3 Al5 O12 (Yb:LuAG) were grown by the micro-pulling-down method. Radioand photoluminescence spectra of the Yb:LuAG were studied. Two bands peaking round 340–350 and 480–500 nm could be ascribed to charge transfer (CT) luminescence of Yb3þ , as their position nearly coincide with the Yb3þ -related CT emission in YAG matrix. The highest emission intensity was observed for Yb concentration of 5% with respect to the Lu site. Temperature dependence of photoluminescence spectra of the Yb: LuAG (5%) was measured and points to remarkable thermal quenching above liquid nitrogen temperature. Ó 2003 Elsevier B.V. All rights reserved. PACS: 81.10.Fq; 82.30.Fi; 98.38.Am; 77.84.Bw; 61.50.)f Keywords: Yb3þ ; Charge transfer luminescence; Garnet; Single crystal growth from the melt
1. Introduction Yb3þ ions are well known luminescent centers, which show efficient 2 F5=2 ! 2 F7=2 infrared luminescence transitions between the only two spin– orbit 4f13 configuration states. This transition around 1 lm is of strong interest for the diode* Corresponding author. Tel.: +81-22-217-5167; fax: +81-22217-5102. E-mail address: [email protected] (A. Yoshikawa).
pumped Yb3þ lasers [1,2]. For this purpose, heavily Yb3þ -doped 3 Al5 O12 (Yb:YAG) crystals are very promising for microchip lasers [3]. Moreover, Yb3þ can exhibit also other kinds of luminescence. The 5d excited states are situated at too high energy to give rise to any radiative 5d ! 4f transitions [4,5]. However, Yb3þ -doped crystals are known to exhibit often fast charge transfer (CT) luminescence in the visible and UV regions [6–13]. This process involves allowed transitions between the p level of ligands (oxygen, halides) forming the top of valence band of
0925-3467/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-3467(03)00134-4
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A. Yoshikawa et al. / Optical Materials 24 (2003) 275–279
the crystal, and the mentioned two 4f states of Yb3þ . The interest in CT luminescence of Yb3þ is due to the following: 1. Fast decay of several tens of ns decay time at low temperatures can compete with the Ce3þ emission decay: Ce3þ center is widely used in scintillator materials. 2. Fast and efficient scintillator loaded with Yb is highly desirable detector for neutrino physics [8–14]. 3. Scintillators emitting in the green–red spectral region with nanosecond radiative lifetime are strongly required if the detection by avalanche photodiodes––new small, compact, monolithic device of high quantum efficiency and feasible internal gain, is considered [14,15]. In the present article, we report the single crystal growth of heavily Yb3þ -doped Lu3 Al5 O12 (Yb:LuAG) using the micro-pulling-down (l-PD) method. X-ray- and photo-excited CT luminescence of Yb3þ is obtained and its temperature dependence is reported as well.
preferable size. The heating was by induction at a frequency of 15 kHz. The temperature at the liquid–solid interface was measured by means of an optical pyrometer. The atmosphere was Ar in order to avoid oxidation of the Ir crucible. The initial seed crystal with h1 1 1i orientation was taken from a Cz-grown YAG crystal. Further crystal growth experiments used LuAG seed cut from crystals obtained in the initial experiments. The growth rate was 0.05–1.0 mm/min. The chemical composition was analysed by electron microprobe analysis (EPMA) using the JEOL JXA-8621MX. The distribution of Yb3þ cation in the crystal was measured with an electron probe 10 lm in diameter. Emission spectra were measured from liquid nitrogen temperature (LNT) up to RT using Spectrofluorometer 199S equipped by X-ray tube (25 kV, 15 mA) and hydrogen flash lamp as the excitation sources. Luminescence was detected by photomultiplier XP2233 in photon counting regime. Emission spectra were corrected for the experimental distortions.
3. Result and discussion 2. Experimental
3.1. Material elaboration
Starting materials were prepared from the stoichiometric mixture of 99.99% pure a-Al2 O3 , Lu2 O3 and Yb2 O3 powders produced by High Purity Chemicals Co. Mixed powders were cold pressed under 3000 kg/cm2 into disk pellets of 16 mm in diameter and sintered in a platinum crucible in air during 24 h at 1400 °C. In order to identify the obtained phase, powder X-ray diffraction analysis was carried out in the 2h range 20–80° using the RINT Ultima (RIGAKU) diffractometer. The atmosphere was air and the Xray source was CuKa, the accelerating voltage was 40 kV, and the current was 40 mA. All the X-ray experiments were carried out at room temperature (RT). Single crystals were grown by the l-PD method. Using crucible with a die makes it possible to control the diameter of the grown crystals with a
Evolution of the powder X-ray diffraction as a function of the Yb concentration were shown in Fig. 1. No impurity phase were detected and we concluded that they formed complete solid solution. As the Yb concentration increase, diffraction peaks shift to the left, constantly. This tendency is reasonable. As the ionic radii of Yb3þ is larger than that of Lu3þ , the lattice constant tends to increase linearly as the concentration of Yb3þ increases. This change is in good agreement with VegardÕs low. The rod shaped single crystals with 3 mm in diameter and 15–50 mm in length could be grown by the l-PD method (Fig. 2(a)–(c)). In order to control the diameter constant, the melt has to spread homogeneously at the entire part of the die. The Yb:LuAG melt does not wet Ir well. Therefore, we applied slightly higher temperature than
A. Yoshikawa et al. / Optical Materials 24 (2003) 275–279
277
x = 1.00
Intensity / a.u.
x = 0.75 x = 0.50 x = 0.30 x = 0.15 x = 0.05 x = 0.00 10
20
30
40
50
60
70
80
2θ/ degree (Cu Kα) Fig. 1. Evolution of the powder X-ray diffraction as a function of the Yb concentration in (Lu1x Ybx )3 Al5 O12 .
Fig. 2. (Lu1x Ybx )3 Al5 O12 single crystals grown by the l-PD method. Yb concentrations are (a) 5%, (b) 15% and (c) 30% with respect to the Lu site, (d) polished plate prepared for the optical characterization.
melting temperature, so as to decrease the melt viscosity and could spread the melt homogeneously at the die. The crystals were transparent without visible cracks and inclusions. The polished plates of 1 mm thickness, 3 mm width and 8 mm length were prepared for optical measurements (Fig. 2(d)). The Yb distribution was investigated using the EPMA. Yb was homogeneously distributed in LuAG host from top to the end (Fig. 3).
3.2. Luminescence characteristics Radioluminescence spectra of the samples described above are given in Fig. 4 together with a reference sample of BGO scintillator (plate of 7 7 1 mm). Two bands peaking round 340–350 and 480–500 nm can be ascribed to the CT transition of Yb3þ as their position nearly coincide with the Yb3þ -related CT emission in YAG matrix [10,12]. The highest emission intensity was
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A. Yoshikawa et al. / Optical Materials 24 (2003) 275–279
5 may be due to the presence of the other emission centers. Namely, Yb2þ center is very probably present and emitting round 360 nm as its presence seems to be evidenced by the presence of slow ms decay components in this spectral region [11]. Presence of Yb2þ is definitely harmful, if the application of such a material as a fast ns scintillator is considered and postgrowth annealing must be employed and optimized to suppress such a center, which is the goal for the future investigations.
35
Yb concentration / %
x=0.30 30 25 20 15
x=0.15
10 5
x=0.05
0 0
20
40
60
80
100
Solidification fraction / % Fig. 3. The Yb distribution in the (Lu1x Ybx )3 Al5 O12 single crystals investigated with the EPMA.
obtained for the Yb concentration of 5% with respect to the Lu site. In this case the total emission intensity is of about 10% of the BGO reference sample. Temperature dependence of photoluminescence spectra of the Yb:LuAG (5%) is shown in Fig. 5. Above LNT thermal quenching is remarkable. The small difference in the spectra shape between radio- and photoluminescence in Figs. 4 and
4. Summary As a candidate for new scintillator material, Yb:LuAG is studied. Transparent and crack-free Yb:LuAG single crystals could be grown by the l-PD method with h1 1 1i orientation. Under Xray and UV excitations the CT transition of Yb3þ has been observed in the prepared Yb:LuAG crystals. The highest intensity was observed for the Yb concentration of 5% with respect to the Lu site. In this case the emission intensity at RT is of about 10% of BGO reference sample. Above LNT the thermal quenching of CT emission in Yb:LuAG is remarkable and the photoluminescence intensity is decreased by more than one order at RT.
Fig. 4. Radioluminescence spectra of the (Lu1x Ybx )3 Al5 O12 single crystals and BGO reference sample of 7 7 1 mm. Excitation by X-ray at room temperature.
A. Yoshikawa et al. / Optical Materials 24 (2003) 275–279
279
LNT 110K
Intensity / a. u.
140K 170K 200K 230K 260K RT
280
320
360
400
440
480
520
560
600
Wavelength / nm Fig. 5. Temperature dependence for photoluminescence spectra of LuAG:Yb5% (excited at 225 nm).
Acknowledgements The authors would like to thank Mr. Murakami, of the Laboratory for Developmental Research of Advanced Materials in IMR, for his assistance with the EPMA analysis.
[6] [7] [8] [9] [10] [11]
References [12] [1] P. Lacovara, H.K. Choi, C.A. Wang, R.L. Aggarwal, T.Y. Fan, Opt. Lett. 16 (1991) 1089. [2] T.Y. Fan, IEEE J. Quantum Electron. 29 (1993) 1457. [3] P. Yang, P. Deng, J. Xu, Z. Yin, J. Cryst. Growth 216 (2000) 348. [4] D.S. McClure, in: P. Day (Ed.), Electronic States of Inorganic Compounds: New Experimental Techniques, Reidel, Dordrecht, 1975, p. 113. [5] P. Dorenbos, J. Lumin. 91 (2000) 91.
[13] [14] [15]
E. Nakazawa, Chem. Phys. Lett. 56 (1978) 161. E. Nakazawa, J. Lumin. 18–19 (1979) 272. R.S. Raghavan, Phys. Rev. Lett. 78 (1997) 3618. L. van Pieterson, M. Heeroma, E. de Heer, A. Meijerinkm, J. Lumin. 91 (2000) 177. N. Guerassimova, N. Garnier, C. Dujardin, A.G. Petrosyan, C. Pedrini, Chem. Phys. Lett. 339 (2001) 197. A. Yoshikawa, T. Akagi, M. Nikl, N. Solovieva, K. Lebbou, C. Dujardin, C. Pedrini, T. Fukuda, Nucl. Instr. and Meth. A 486 (2002) 79. N. Guerassimova, C. Dujardin, N. Garnier, C. Pedrini, A.G. Petrosyan, I.A. Kamenskikh, V.V. Mikhailin, I.N. Shpinkov, D.A. Spassky, K.L. Ovanesyan, G.O. Shirinyan, R. Chipaux, M. Cribier, J. Mallet, J.-P. Meyer, Nucl. Instr. and Meth. A 486 (2002) 278. P. Antonini et al., Nucl. Instr. and Meth. A 460 (2001) 467. M. Moszynski, M. Szawlowski, M. Kapusta, M. Balcerzyk, Nucl. Instr. and Meth. A 485 (2002) 504. P.A. Rodnyi, E.I. Gorohova, S.B. Mikhrin, A.N. Mishin, A.S. Potapov, Nucl. Instr. and Meth. A 486 (2002) 244.
Growth and optical properties of Yb doped new scintillator crystals A. Yoshikawa a,*, H. Ogino a, J.H. Lee a, M. Nikl b, N. Solovieva b, N. Garnier c, C. Dujardin c, K. Lebbou c, C. Pedrini c, T. Fukuda a a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b Institute of Physics AS CR, Cukrovarnicka 10, 162 53 Prague, Czech Republic c Physical Chemistry of Luminescent Materials, Claude Bernard/Lyon1 University, CNRS UMR 5620, 69622 Villeurbanne Cedex, France Received 8 December 2002; accepted 24 February 2003
Abstract Single crystal of heavily Yb3þ -doped Lu3 Al5 O12 (Yb:LuAG) were grown by the micro-pulling-down method. Radioand photoluminescence spectra of the Yb:LuAG were studied. Two bands peaking round 340–350 and 480–500 nm could be ascribed to charge transfer (CT) luminescence of Yb3þ , as their position nearly coincide with the Yb3þ -related CT emission in YAG matrix. The highest emission intensity was observed for Yb concentration of 5% with respect to the Lu site. Temperature dependence of photoluminescence spectra of the Yb: LuAG (5%) was measured and points to remarkable thermal quenching above liquid nitrogen temperature. Ó 2003 Elsevier B.V. All rights reserved. PACS: 81.10.Fq; 82.30.Fi; 98.38.Am; 77.84.Bw; 61.50.)f Keywords: Yb3þ ; Charge transfer luminescence; Garnet; Single crystal growth from the melt
1. Introduction Yb3þ ions are well known luminescent centers, which show efficient 2 F5=2 ! 2 F7=2 infrared luminescence transitions between the only two spin– orbit 4f13 configuration states. This transition around 1 lm is of strong interest for the diode* Corresponding author. Tel.: +81-22-217-5167; fax: +81-22217-5102. E-mail address: [email protected] (A. Yoshikawa).
pumped Yb3þ lasers [1,2]. For this purpose, heavily Yb3þ -doped 3 Al5 O12 (Yb:YAG) crystals are very promising for microchip lasers [3]. Moreover, Yb3þ can exhibit also other kinds of luminescence. The 5d excited states are situated at too high energy to give rise to any radiative 5d ! 4f transitions [4,5]. However, Yb3þ -doped crystals are known to exhibit often fast charge transfer (CT) luminescence in the visible and UV regions [6–13]. This process involves allowed transitions between the p level of ligands (oxygen, halides) forming the top of valence band of
0925-3467/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-3467(03)00134-4
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A. Yoshikawa et al. / Optical Materials 24 (2003) 275–279
the crystal, and the mentioned two 4f states of Yb3þ . The interest in CT luminescence of Yb3þ is due to the following: 1. Fast decay of several tens of ns decay time at low temperatures can compete with the Ce3þ emission decay: Ce3þ center is widely used in scintillator materials. 2. Fast and efficient scintillator loaded with Yb is highly desirable detector for neutrino physics [8–14]. 3. Scintillators emitting in the green–red spectral region with nanosecond radiative lifetime are strongly required if the detection by avalanche photodiodes––new small, compact, monolithic device of high quantum efficiency and feasible internal gain, is considered [14,15]. In the present article, we report the single crystal growth of heavily Yb3þ -doped Lu3 Al5 O12 (Yb:LuAG) using the micro-pulling-down (l-PD) method. X-ray- and photo-excited CT luminescence of Yb3þ is obtained and its temperature dependence is reported as well.
preferable size. The heating was by induction at a frequency of 15 kHz. The temperature at the liquid–solid interface was measured by means of an optical pyrometer. The atmosphere was Ar in order to avoid oxidation of the Ir crucible. The initial seed crystal with h1 1 1i orientation was taken from a Cz-grown YAG crystal. Further crystal growth experiments used LuAG seed cut from crystals obtained in the initial experiments. The growth rate was 0.05–1.0 mm/min. The chemical composition was analysed by electron microprobe analysis (EPMA) using the JEOL JXA-8621MX. The distribution of Yb3þ cation in the crystal was measured with an electron probe 10 lm in diameter. Emission spectra were measured from liquid nitrogen temperature (LNT) up to RT using Spectrofluorometer 199S equipped by X-ray tube (25 kV, 15 mA) and hydrogen flash lamp as the excitation sources. Luminescence was detected by photomultiplier XP2233 in photon counting regime. Emission spectra were corrected for the experimental distortions.
3. Result and discussion 2. Experimental
3.1. Material elaboration
Starting materials were prepared from the stoichiometric mixture of 99.99% pure a-Al2 O3 , Lu2 O3 and Yb2 O3 powders produced by High Purity Chemicals Co. Mixed powders were cold pressed under 3000 kg/cm2 into disk pellets of 16 mm in diameter and sintered in a platinum crucible in air during 24 h at 1400 °C. In order to identify the obtained phase, powder X-ray diffraction analysis was carried out in the 2h range 20–80° using the RINT Ultima (RIGAKU) diffractometer. The atmosphere was air and the Xray source was CuKa, the accelerating voltage was 40 kV, and the current was 40 mA. All the X-ray experiments were carried out at room temperature (RT). Single crystals were grown by the l-PD method. Using crucible with a die makes it possible to control the diameter of the grown crystals with a
Evolution of the powder X-ray diffraction as a function of the Yb concentration were shown in Fig. 1. No impurity phase were detected and we concluded that they formed complete solid solution. As the Yb concentration increase, diffraction peaks shift to the left, constantly. This tendency is reasonable. As the ionic radii of Yb3þ is larger than that of Lu3þ , the lattice constant tends to increase linearly as the concentration of Yb3þ increases. This change is in good agreement with VegardÕs low. The rod shaped single crystals with 3 mm in diameter and 15–50 mm in length could be grown by the l-PD method (Fig. 2(a)–(c)). In order to control the diameter constant, the melt has to spread homogeneously at the entire part of the die. The Yb:LuAG melt does not wet Ir well. Therefore, we applied slightly higher temperature than
A. Yoshikawa et al. / Optical Materials 24 (2003) 275–279
277
x = 1.00
Intensity / a.u.
x = 0.75 x = 0.50 x = 0.30 x = 0.15 x = 0.05 x = 0.00 10
20
30
40
50
60
70
80
2θ/ degree (Cu Kα) Fig. 1. Evolution of the powder X-ray diffraction as a function of the Yb concentration in (Lu1x Ybx )3 Al5 O12 .
Fig. 2. (Lu1x Ybx )3 Al5 O12 single crystals grown by the l-PD method. Yb concentrations are (a) 5%, (b) 15% and (c) 30% with respect to the Lu site, (d) polished plate prepared for the optical characterization.
melting temperature, so as to decrease the melt viscosity and could spread the melt homogeneously at the die. The crystals were transparent without visible cracks and inclusions. The polished plates of 1 mm thickness, 3 mm width and 8 mm length were prepared for optical measurements (Fig. 2(d)). The Yb distribution was investigated using the EPMA. Yb was homogeneously distributed in LuAG host from top to the end (Fig. 3).
3.2. Luminescence characteristics Radioluminescence spectra of the samples described above are given in Fig. 4 together with a reference sample of BGO scintillator (plate of 7 7 1 mm). Two bands peaking round 340–350 and 480–500 nm can be ascribed to the CT transition of Yb3þ as their position nearly coincide with the Yb3þ -related CT emission in YAG matrix [10,12]. The highest emission intensity was
278
A. Yoshikawa et al. / Optical Materials 24 (2003) 275–279
5 may be due to the presence of the other emission centers. Namely, Yb2þ center is very probably present and emitting round 360 nm as its presence seems to be evidenced by the presence of slow ms decay components in this spectral region [11]. Presence of Yb2þ is definitely harmful, if the application of such a material as a fast ns scintillator is considered and postgrowth annealing must be employed and optimized to suppress such a center, which is the goal for the future investigations.
35
Yb concentration / %
x=0.30 30 25 20 15
x=0.15
10 5
x=0.05
0 0
20
40
60
80
100
Solidification fraction / % Fig. 3. The Yb distribution in the (Lu1x Ybx )3 Al5 O12 single crystals investigated with the EPMA.
obtained for the Yb concentration of 5% with respect to the Lu site. In this case the total emission intensity is of about 10% of the BGO reference sample. Temperature dependence of photoluminescence spectra of the Yb:LuAG (5%) is shown in Fig. 5. Above LNT thermal quenching is remarkable. The small difference in the spectra shape between radio- and photoluminescence in Figs. 4 and
4. Summary As a candidate for new scintillator material, Yb:LuAG is studied. Transparent and crack-free Yb:LuAG single crystals could be grown by the l-PD method with h1 1 1i orientation. Under Xray and UV excitations the CT transition of Yb3þ has been observed in the prepared Yb:LuAG crystals. The highest intensity was observed for the Yb concentration of 5% with respect to the Lu site. In this case the emission intensity at RT is of about 10% of BGO reference sample. Above LNT the thermal quenching of CT emission in Yb:LuAG is remarkable and the photoluminescence intensity is decreased by more than one order at RT.
Fig. 4. Radioluminescence spectra of the (Lu1x Ybx )3 Al5 O12 single crystals and BGO reference sample of 7 7 1 mm. Excitation by X-ray at room temperature.
A. Yoshikawa et al. / Optical Materials 24 (2003) 275–279
279
LNT 110K
Intensity / a. u.
140K 170K 200K 230K 260K RT
280
320
360
400
440
480
520
560
600
Wavelength / nm Fig. 5. Temperature dependence for photoluminescence spectra of LuAG:Yb5% (excited at 225 nm).
Acknowledgements The authors would like to thank Mr. Murakami, of the Laboratory for Developmental Research of Advanced Materials in IMR, for his assistance with the EPMA analysis.
[6] [7] [8] [9] [10] [11]
References [12] [1] P. Lacovara, H.K. Choi, C.A. Wang, R.L. Aggarwal, T.Y. Fan, Opt. Lett. 16 (1991) 1089. [2] T.Y. Fan, IEEE J. Quantum Electron. 29 (1993) 1457. [3] P. Yang, P. Deng, J. Xu, Z. Yin, J. Cryst. Growth 216 (2000) 348. [4] D.S. McClure, in: P. Day (Ed.), Electronic States of Inorganic Compounds: New Experimental Techniques, Reidel, Dordrecht, 1975, p. 113. [5] P. Dorenbos, J. Lumin. 91 (2000) 91.
[13] [14] [15]
E. Nakazawa, Chem. Phys. Lett. 56 (1978) 161. E. Nakazawa, J. Lumin. 18–19 (1979) 272. R.S. Raghavan, Phys. Rev. Lett. 78 (1997) 3618. L. van Pieterson, M. Heeroma, E. de Heer, A. Meijerinkm, J. Lumin. 91 (2000) 177. N. Guerassimova, N. Garnier, C. Dujardin, A.G. Petrosyan, C. Pedrini, Chem. Phys. Lett. 339 (2001) 197. A. Yoshikawa, T. Akagi, M. Nikl, N. Solovieva, K. Lebbou, C. Dujardin, C. Pedrini, T. Fukuda, Nucl. Instr. and Meth. A 486 (2002) 79. N. Guerassimova, C. Dujardin, N. Garnier, C. Pedrini, A.G. Petrosyan, I.A. Kamenskikh, V.V. Mikhailin, I.N. Shpinkov, D.A. Spassky, K.L. Ovanesyan, G.O. Shirinyan, R. Chipaux, M. Cribier, J. Mallet, J.-P. Meyer, Nucl. Instr. and Meth. A 486 (2002) 278. P. Antonini et al., Nucl. Instr. and Meth. A 460 (2001) 467. M. Moszynski, M. Szawlowski, M. Kapusta, M. Balcerzyk, Nucl. Instr. and Meth. A 485 (2002) 504. P.A. Rodnyi, E.I. Gorohova, S.B. Mikhrin, A.N. Mishin, A.S. Potapov, Nucl. Instr. and Meth. A 486 (2002) 244.