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The temperature gradient technique (TGT) growth and optical properties of Yb-doped YAlO3 single crystal
ARTICLE IN PRESS
Journal of Crystal Growth 280 (2005) 483–489 www.elsevier.com/locate/jcrysgro
The temperature gradient technique (TGT) growth and optical properties of Yb-doped YAlO3 single crystal Guangjun Zhao, Hongjun Li, Jiang Zhu, Mingyin Jie, Xiaoming He, Jun Xu Shanghai Institute of Optics and Fine Mechanics, CAS, Shanghai 201800, PR China Received 6 January 2005; accepted 29 March 2005 Available online 23 May 2005 Communicated by M. Schieber
Abstract The dark-brown colored 5 at% Yb-doped YAlO3 (Yb:YAP) single crystal was grown successfully by temperature gradient technique (TGT) for the first time. The TGT-grown Yb:YAP crystal with the perovskite structure and excellent crystallization perfection were confirmed by the X-ray diffractions techniques. The dark-brown color of TGTYb:YAP crystal turned into the colorless after annealing in the air at 1200 1C for 10 h. The absorption spectra, LDexcited infrared emission and X-ray excited luminescence spectra of the air-annealed Yb:YAP single crystal were investigated at the room temperature. The results indicate that the TGT-Yb:YAP single crystals can be used for the laser and scintillation applications. r 2005 Elsevier B.V. All rights reserved. PACS: 61.10.Yh; 42.55.Rz; 78.20.e; 87.64.Ni Keywords: A2. TGT growth; B1. Yb:YAP; B1.Yb3+ doping
1. Introduction With the rapid development of diode pumped solid-state laser (DPSSL) in the latest decades, Yb3+-doped single crystals, which possess lower thermal load and higher laser energy capacity, have Corresponding author. Tel.: 86 21 69918551; fax: 86 21 69918485. E-mail addresses: [email protected], [email protected] (G. Zhao).
attracted much attention in DPSSL system [1,2]. Very recently, following the proposition of using 176 Yb as the target for the low-energy solar neutrino spectroscopy (LENS) proposed by Raghavan [3], the heavily Yb-doped single crystals with the fast UV charge transfer luminescence (CTL) have increasingly been regarded as the promising scintillators not only for neutrino physics but also for the general radiation detection [4]. Among a large amount of Yb-doped single crystals, Yb-doped Y3Al5O12 (YAG) single crystal
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.03.090
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with a cubic structure and with excellent thermalmechanical properties is a well-established LAAPD laser gain medium [1,2]. As a promising scintillator, the Yb:YAG crystals have also been extensively investigated and many exciting properties were found in this materials [4–8]. Yb-doped YAlO3 single crystal (brief in Yb:YAP) with the orthorhombic D16 2h space group is a biaxial crystal, which belongs to Y2O3–Al2O3 system and has the similar thermal-mechanical properties to YAG crystal. Its nature of birefringence makes Yb:YAP crystal can produce linearly polarized laser and can be free of detrimental thermal-lens effect occurred in Yb:YAG crystal [9]. On the other hand, the recent results revealed that 5at% doped Yb:YAP crystals manifest maximum light output of 30007150 e h/MeV by means of LAAPD at T ¼ 105 2 K [8,10], which suggested that Yb:YAP crystal be an attractive scintillator not only for neutrino physics, but also for nuclear imaging (PET) applications [5,10]. As a scintillator, Yb:YAP crystal has higher density (5.5 g/cm3) than that of Yb:YAG (4.5 g/cm3), and its CTL did not seriously quenched even the concentration of Yb ions in YAP host reached at 40 at% [10], while the Yb:YAG crystal has the seriously concentration the CTL quench effect [8]. This means that the highly Yb-doped YAP crystal is a very promising scintillation crystal for the general radiation detection and PET application [5,10,11]. In this paper, we have grown the 5 at% Ybdoped YAP single crystal with the diameter of 3 inches by the temperature gradient technique (TGT) for the first time. The structure and crystallization perfection of the TGT-grown Yb:YAP crystal were determined by the X-ray diffraction (XRD) techniques. The optical absorption, 940 nm-LD pumped and X-ray excited luminescence (XEL) properties of TGT-Yb:YAP crystal were also investigated in this paper.
which was similar to that described elsewhere [12]. The tapered molybdenum crucible with a lower seed end was used in our growth experiment. The typical dimension of the molybdenum crucible used in growth experiments is OD ¼ 80 mm, ID ¼ 76 mm with the cylindrical part height of 90 mm. After the highly pure Y2O3 (X99.999%), Yb2O3 (X99.999%) and Al2O3 (X99.95%) powders were weighed out in 0.995:0.005:1.000 mole ratios, the powders were totally mixed and extruded into the cylindrical blocks with the diameter of 75 mm and height of 30 mm at the 500 kg/cm2 pressure. These blocks (total weight of 1300 g) were sintered at 1350 1C for 24 h in air, and were then loaded into the Mo crucible with the cylindrical [0 1 0]-orientated YAP seed in the crucible. After the materials totally melted in Mo container and kept the molten for several hours, the crystallization of Yb:YAP was start and droven by the slow cooling (0.5–1 1C/h) with a high-precision temperature controller. The whole crystallization process was completed automatically in the highly pure Ar gas atmosphere in the furnace. When the crystallization finished, the Yb:YAP crystal was annealed in situ and then cooled down to room temperature by the optimal cooling rate (30–50 1C/h). The 3 inches 5 at% doped Yb:YAP crystal grown by TGT was cracked into two parts when it was removed from the Mo crucible due to the slight mechanical impact on the crucible. The cracked TGT-grown Yb:YAP crystal with darkbrown color was showed in the Fig. 1(a). The polished slices before and after air-annealing at 1200 1C for 10 h can be seen in the Fig. 1(b), and the dark-brown colored Yb:YAP crystal turned into almost colorless after the air-annealing. The dark-brown coloration of as-grown TGT-Yb:YAP crystal resulted from the contamination from graphite heater in the TGT furnace, and the detailed analysis will be seen in the Section 3.2.
2. Experimental procedures
2.2. Characterization of TGT-Yb:YAP crystal
2.1. The growth of Yb:YAP crystal by TGT
The structure and crystallization perfection of TGT-Yb:YAP crystal were examined by the XRD techniques. The experimental diffraction data of the 5 at% doped Yb:YAP crystal were obtained
The 5 at% doped Yb:YAP single crystal was grown in the TGT furnace with the graphite heater
ARTICLE IN PRESS G. Zhao et al. / Journal of Crystal Growth 280 (2005) 483–489
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using the MXP18AHF powder diffractometer with a rotating anode operating at 40 KV and 100 mA (Cu Ka-radiation, y/2y-scanning mode), and the precise lattice parameters were refined with powdered sample of crystals using the full profile Rietveld analysis. To precisely characterize crystallization perfection of the TGT-grown 5 at% doped Yb:YAP crystal, the rocking curve of TGT-Yb:YAP crystal was carried out on the four-crystal high-resolution X-ray diffractometer with 2y/o scanning mode. The Cu Ka1 X-ray was monochromatized by four pieces of standard Ge (2 2 0) crystal and the resolution of the instrument is 800 . The (0 1 0)-oriented slices with the size of 10 10 4.5 mm3 were cut from before and after air-annealed TGT-Yb:YAP crystals, and then mechanically polished into the thickness of 4 mm for the optical properties measurements. The absorption spectrum of the slice was measured by the UV/VIS Spectrophotometer (Mode V-570, JASCO) at room temperature. The emission spectra of the crystal slice excited by 940-nm-LD were obtained by JY-TRIAX550 spectrophotometer at room temperature. The XEL spectrum was measured on a XEL spectrometer, FluorMain, where an F-30 movable clinical X-ray tube (W anticathode target) was used as the X-ray source operated under the condition of 80 kV and 4 mA at room temperature. Luminescence spectra were obtained by homemade 44 W plate grating monochromator and
Relative Intensity
Fig. 1. (a) The photo of 3 inches 5 at% doped Yb:YAP single crystal grown by TGT method (cracking into two parts and dark-colored before air-annealing); (b) Photos of TGT-Yb:YAP slices before and after annealing in the air at 1200 1C for 10 h.
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Fig. 2. The X-ray diffraction pattern of 5 at% doped TGTgrown Yb:YAP crystal.
Hamamatsu R928-28 photomutiplier with the data acquired by computer.
3. Results and discussions 3.1. The structure and perfection of TGT-Yb:YAP crystal X-ray powder diffraction patters of the pure YAP and 5 at% doped TGT-Yb:YAP crystals are showed in the Fig. 2. It can be clearly seen that the indexed peaks of TGT-Yb:YAP crystal are identified with those of pure YAP crystal in terms of
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relative intensity and position of the diffraction peaks. This indicated that 5 at% doped TGT-Yb: YAP crystal has the perovskite-like structure with the Pnma space group. The lattice parameters of 5 at% doped TGT-Yb:YAP crystal refined by ( b ¼ 7:3702 A, ( Rietveld method are a ¼ 5:3296 A; ( c ¼ 5:1781 A, respectively. Compared with lattice ( parameters of pure YAP crystal a ¼ 5:3291 A, ( ( b ¼ 7:3709 A, c ¼ 5:1796 A [11], it was found that there is little variation of the unit cell of perovskite-type YAP structure after 5 at% Yb ions added into the pure YAP host. In general, the refraction peaks (0 2 0), (0 4 0), and (0 6 0) appear in the XRD rocking curves of the 5 at% doped TGT-Yb:YAP with (0 1 0) face. In our experiment, the strongest refraction peak indexed by (0 4 0) centered at 23.61041 with its fullwidth at half-maximum (FWHM) of 65.329200 was carefully recorded in the Fig. 3. According to the Darwinian theory [13], the theoretical FWHM value of the rocking curve of the perfect (0 1 0)oriented YAP crystal have been calculated (FWHM ¼ 16.6800 ) by the authors [14]. The experimental rocking curve FWHM of TGTYb:YAP crystal (65.329200 ) is at the same magnitude with that of the perfect YAP crystal (16.6800 ), which means that 5 at% doped Yb:YAP crystal grown by TGT method has a good crystallization perfection.
3.2. The absorption and LD-excited IR emission spectra The absorption spectra in the range of 190–1200 nm of the 5 at% TGT-Yb:YAP crystals before and after air-annealing are shown in Fig. 4. It is clearly seen in Fig. 4 that there exist two distinct absorption bands in both Yb:YAP crystal samples before and after air-annealing: one is strong absorption centered at 220 nm which is corresponding to the transition from the ground state 2F7/2 to the charge transfer state of the Yb3+ ions in the YAP host [15], and the other absorption band with four strong absorption peaks centered at 932, 960, 980 and 998 nm, which is obviously ascribed to the typical transitions from the ground state 2F7/2 to the sublevels of 2F5/ 3+ in YAP crystal. These strong absorption 2 of Yb 3+ of Yb in YAP host around 940–980 nm suggests that the TGT-Yb:YAP crystal is very suitable for LD pumping for the DPSSL applications. In addition, there is a very weak absorption region from 270 to 400 nm in Fig. 4, this absorption region is very similar to those previously observed in the TGT-grown 5 at% doped Yb:YAG crystal and Cz-grown 20 at% doped Yb:YAP crystals by authors [11,16], and the unknown color centers related oxide or cation vacancies should be responsible for this weak absorption.
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Fig. 3. The rocking curve of the 5 at% TGT-Yb:YAP crystal ((0 4 0) refraction peak on (0 1 0) face).
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Fig. 4. The absorption spectra of 5 at% TGT-Yb:YAP crystal before and after air-anealing.
ARTICLE IN PRESS G. Zhao et al. / Journal of Crystal Growth 280 (2005) 483–489
It is very essential that Yb:YAP crystal should have the high transmission in the useful luminescence wavelength bands, i.e. the CTL bands and IR laser emission bands, to allow its own emitted light pass through the crystal with very little energy loss. Therefore, it is very necessary for its further scintillation and laser applications to take the annealing treatment in air-atmosphere with high temperature and long time which can greatly enhance the transmission of Yb:YAP crystal grown by TGT. Fig. 6 is the IR emission spectrum (indicated by red dot line) of the air-annealed 5 at% doped TGT-Yb:YAP crystal excited under the LD source with the wavelength of 940 nm at room temperature. The emission peaks are located at 999, 1013, and 1036 nm corresponding to the transitions from the sublevels of 2F5/2 level to the components of the 2F7/2 ground state, and these emission peaks can be served as the possible laser outputs wavelength. Due to having a strong reabsorption at 998 nm, showed in the Fig. 6, it is very suitable that the Yb:YAP crystal lasing at 1031 and 1036 nm when LD pumped at 960 and 980 nm. 3.3. X-ray excited luminescence XEL spectrum of air-annealed 5 at% TGTYb:YAP crystal showed in Fig. 7 has demonstrated clearly the CTL of Yb3+ ions in YAP host with double peaks centered at about 366 and
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From the Fig. 4, it also can be found that the dark-brown colored TGT-Yb:YAP crystal before air-annealing has a much higher background absorption in the overall range of 120–1500 nm than that of the colorless air-annealed TGTYb:YAP crystal. This implies that the air-annealed TGT-Yb:YAP crystal has higher transmittance than that of the sample before air-annealed. Fig. 5 showed that the transmittance of the 5 at% doped TGT-Yb:YAP crystals with the same thickness of 4 mm before and after air-annealing. From the Fig. 5, it can be seen that the transmittance of the air-annealed TGT-Yb:YAP crystal is twice as high as that of the as-grown TGT-Yb:YAP crystal in the overall range of 120–1500 nm. This means that there exist much more uniform light scattering centers in the as grown Yb:YAP crystals, and these light scattering centers also should be responsible for the dark-brown coloration of the as-grown TGT-Yb:YAP crystal. Because the cylindrical graphite heater was employed in our TGT furnace, the Yb:YAP crystal with the contamination from the graphite heater could not be avoided during the TGT growth process, and the carbon scattering centers will be easily formed in as-grown crystal. After annealed in the air at 1200 1C for 10 h, the carbon scattering centers in TGTYb:YAP crystal will be escaped in the form of carbon dioxide gas, which makes the dark-brown colored Yb:YAP crystal turn into the colorless as showed in the Fig. 1(b).
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Fig. 6. IR absorption and emission spectra of 5 at% TGTYb:YAP crystal after annealed in the air.
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Fig. 7. X-ray excited luminescence of 5 at% TGT-Yb:YAP crystal after annealed in the air.
540 nm, which are corresponding to the transitions of the charge transfer state (CTS) to 2F7/2 and 2F5/2 states of Yb3+ respectively. Compared with the double CTL peaks centered at around 350 and 520 nm with the excitation by the UV photons around 230–250 nm in the previous reports [10,16,17], the X-ray excited double CTL peaks centered 366 and 540 nm are red-shift resulting from the higher photon energy of X-ray deposited on the Yb:YAP crystal. The very fast decay time (from sub-nanoseconds to several tens of nanoseconds) of the CTL in Yb:YAP crystal has been determined by the authors [10,11]. This suggests that it can be used for a fast scintillator. In addition, the very weak X-ray excited luminescence in the red spectral region of 590–700 nm can also be seen in the Fig. 7, which is similar that observed in the Cz-grown 20 at% doped Yb:YAP crystal [11]. The reasons for this red spectral emission probably can be ascribed to the intrinsic defects in pure YAP host [18,19] or some unintentional trivalent rare earth impurities such as Eu3+, Ho3+, and Tb3+ etc. presented in the TGT-grown Yb:YAP crystal.
4. Conclusion The dark-brown 5 at% doped Yb:YAP crystal with diameter of 3 inches has been grown by temperature gradient technique (TGT). The lattice
( b ¼ 7:3702 A, ( c¼ parameters of a ¼ 5:3296 A, ( of 5 at% doped TGT-Yb:YAP crystal 5:1781 A was determined by the precise X-ray diffraction (XRD) measurement. The rocking curve with FWHM of 65.329200 shows that the crystallization of the Yb:YAP crystal grown by TGT is perfect. The carbon light scattering centers in the as-grown Yb:YAP crystal, resulting from the contamination from graphite heater during TGT growth process, are as explained for dark-brown coloration of the crystal. The air-annealed high TGT-Yb:YAP crystal is colorless and high transparent. The absorption and LD excited emission spectra of air-annealed 5 at% Yb:YAP crystal showed that it can severed as the good laser media operated at around 1.0 mm when 960 nm or 980 nm LD pumped at room temperature. The fast charge transfer luminescence (CTL) peaking at around 366 and 540 nm was clearly determined in TGTYb:YAP crystal, which suggests that it can be served as the promising fast scintillation crystal for radiation detection. References [1] W.F. Krupke, IEEE J. Sel. Top. Quantum Electron. 6 (2000) 1287. [2] L.D. Deloach, S.A. Payne, et al., IEEE J. Quantum Electron. 29 (1993) 1179. [3] R.S. Raghavan, Phy. Rev. Lett. 78 (1997) 3681. [4] R. Chipaux, et al., Nucl. Instrum. Methods A 486 (2002) 228. [5] S. Belogurov, G. Bressi, et al., Nucl. Instrum. Methods A 496 (2003) 385. [6] G. Bressi, et al., Nucl. Instrum. Methods A 461 (2001) 467. [7] P. Antonini, et al., Nucl. Instrum. Methods A 486 (2002) 220. [8] S. Belogurov, et al., Nucl. Instrum. Methods A 516 (2004) 58. [9] G.A. Massey, Appl. Phys. Lett. 17 (1970) 213. [10] M. Nikl, N. Solovieva, et al., Appl. Phys. Lett. 84 (2004) 882. [11] Guangjun Zhao, Xionghui Zeng, et al., J. Crystal Growth 267 (2004) 522. [12] Xu Jianwei, Zhou Yongzong, et al., J. Crystal Growth 193 (1998) 123. [13] A.R. Lang, V.F. Miuskov, Appl. Phys. Lett. 7 (1965) 214. [14] Hongjun Li, Guangjun Zhao, et al., Mater. Lett. 58 (2004) 3253. [15] L. Van Pieterson, M. Heeroma, et al., J. Lumin. 91 (2000) 177.
ARTICLE IN PRESS G. Zhao et al. / Journal of Crystal Growth 280 (2005) 483–489 [16] Guangjun Zhao, Jiliang Si, et al., J. Crystal Growth 252 (2003) 355. [17] J.B. Shim, A. Yoshikawa, et al., J. Appl. Phys. 95 (2004) 3063.
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[18] I.A. Kamenskikh, N. Guerassimova, et al., Opt. Mater. 24 (2003) 267. [19] V.G. Baryshevski, et al., J. Phys.: Condens. Matter 5 (1993) 7893.
Journal of Crystal Growth 280 (2005) 483–489 www.elsevier.com/locate/jcrysgro
The temperature gradient technique (TGT) growth and optical properties of Yb-doped YAlO3 single crystal Guangjun Zhao, Hongjun Li, Jiang Zhu, Mingyin Jie, Xiaoming He, Jun Xu Shanghai Institute of Optics and Fine Mechanics, CAS, Shanghai 201800, PR China Received 6 January 2005; accepted 29 March 2005 Available online 23 May 2005 Communicated by M. Schieber
Abstract The dark-brown colored 5 at% Yb-doped YAlO3 (Yb:YAP) single crystal was grown successfully by temperature gradient technique (TGT) for the first time. The TGT-grown Yb:YAP crystal with the perovskite structure and excellent crystallization perfection were confirmed by the X-ray diffractions techniques. The dark-brown color of TGTYb:YAP crystal turned into the colorless after annealing in the air at 1200 1C for 10 h. The absorption spectra, LDexcited infrared emission and X-ray excited luminescence spectra of the air-annealed Yb:YAP single crystal were investigated at the room temperature. The results indicate that the TGT-Yb:YAP single crystals can be used for the laser and scintillation applications. r 2005 Elsevier B.V. All rights reserved. PACS: 61.10.Yh; 42.55.Rz; 78.20.e; 87.64.Ni Keywords: A2. TGT growth; B1. Yb:YAP; B1.Yb3+ doping
1. Introduction With the rapid development of diode pumped solid-state laser (DPSSL) in the latest decades, Yb3+-doped single crystals, which possess lower thermal load and higher laser energy capacity, have Corresponding author. Tel.: 86 21 69918551; fax: 86 21 69918485. E-mail addresses: [email protected], [email protected] (G. Zhao).
attracted much attention in DPSSL system [1,2]. Very recently, following the proposition of using 176 Yb as the target for the low-energy solar neutrino spectroscopy (LENS) proposed by Raghavan [3], the heavily Yb-doped single crystals with the fast UV charge transfer luminescence (CTL) have increasingly been regarded as the promising scintillators not only for neutrino physics but also for the general radiation detection [4]. Among a large amount of Yb-doped single crystals, Yb-doped Y3Al5O12 (YAG) single crystal
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.03.090
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G. Zhao et al. / Journal of Crystal Growth 280 (2005) 483–489
with a cubic structure and with excellent thermalmechanical properties is a well-established LAAPD laser gain medium [1,2]. As a promising scintillator, the Yb:YAG crystals have also been extensively investigated and many exciting properties were found in this materials [4–8]. Yb-doped YAlO3 single crystal (brief in Yb:YAP) with the orthorhombic D16 2h space group is a biaxial crystal, which belongs to Y2O3–Al2O3 system and has the similar thermal-mechanical properties to YAG crystal. Its nature of birefringence makes Yb:YAP crystal can produce linearly polarized laser and can be free of detrimental thermal-lens effect occurred in Yb:YAG crystal [9]. On the other hand, the recent results revealed that 5at% doped Yb:YAP crystals manifest maximum light output of 30007150 e h/MeV by means of LAAPD at T ¼ 105 2 K [8,10], which suggested that Yb:YAP crystal be an attractive scintillator not only for neutrino physics, but also for nuclear imaging (PET) applications [5,10]. As a scintillator, Yb:YAP crystal has higher density (5.5 g/cm3) than that of Yb:YAG (4.5 g/cm3), and its CTL did not seriously quenched even the concentration of Yb ions in YAP host reached at 40 at% [10], while the Yb:YAG crystal has the seriously concentration the CTL quench effect [8]. This means that the highly Yb-doped YAP crystal is a very promising scintillation crystal for the general radiation detection and PET application [5,10,11]. In this paper, we have grown the 5 at% Ybdoped YAP single crystal with the diameter of 3 inches by the temperature gradient technique (TGT) for the first time. The structure and crystallization perfection of the TGT-grown Yb:YAP crystal were determined by the X-ray diffraction (XRD) techniques. The optical absorption, 940 nm-LD pumped and X-ray excited luminescence (XEL) properties of TGT-Yb:YAP crystal were also investigated in this paper.
which was similar to that described elsewhere [12]. The tapered molybdenum crucible with a lower seed end was used in our growth experiment. The typical dimension of the molybdenum crucible used in growth experiments is OD ¼ 80 mm, ID ¼ 76 mm with the cylindrical part height of 90 mm. After the highly pure Y2O3 (X99.999%), Yb2O3 (X99.999%) and Al2O3 (X99.95%) powders were weighed out in 0.995:0.005:1.000 mole ratios, the powders were totally mixed and extruded into the cylindrical blocks with the diameter of 75 mm and height of 30 mm at the 500 kg/cm2 pressure. These blocks (total weight of 1300 g) were sintered at 1350 1C for 24 h in air, and were then loaded into the Mo crucible with the cylindrical [0 1 0]-orientated YAP seed in the crucible. After the materials totally melted in Mo container and kept the molten for several hours, the crystallization of Yb:YAP was start and droven by the slow cooling (0.5–1 1C/h) with a high-precision temperature controller. The whole crystallization process was completed automatically in the highly pure Ar gas atmosphere in the furnace. When the crystallization finished, the Yb:YAP crystal was annealed in situ and then cooled down to room temperature by the optimal cooling rate (30–50 1C/h). The 3 inches 5 at% doped Yb:YAP crystal grown by TGT was cracked into two parts when it was removed from the Mo crucible due to the slight mechanical impact on the crucible. The cracked TGT-grown Yb:YAP crystal with darkbrown color was showed in the Fig. 1(a). The polished slices before and after air-annealing at 1200 1C for 10 h can be seen in the Fig. 1(b), and the dark-brown colored Yb:YAP crystal turned into almost colorless after the air-annealing. The dark-brown coloration of as-grown TGT-Yb:YAP crystal resulted from the contamination from graphite heater in the TGT furnace, and the detailed analysis will be seen in the Section 3.2.
2. Experimental procedures
2.2. Characterization of TGT-Yb:YAP crystal
2.1. The growth of Yb:YAP crystal by TGT
The structure and crystallization perfection of TGT-Yb:YAP crystal were examined by the XRD techniques. The experimental diffraction data of the 5 at% doped Yb:YAP crystal were obtained
The 5 at% doped Yb:YAP single crystal was grown in the TGT furnace with the graphite heater
ARTICLE IN PRESS G. Zhao et al. / Journal of Crystal Growth 280 (2005) 483–489
485
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using the MXP18AHF powder diffractometer with a rotating anode operating at 40 KV and 100 mA (Cu Ka-radiation, y/2y-scanning mode), and the precise lattice parameters were refined with powdered sample of crystals using the full profile Rietveld analysis. To precisely characterize crystallization perfection of the TGT-grown 5 at% doped Yb:YAP crystal, the rocking curve of TGT-Yb:YAP crystal was carried out on the four-crystal high-resolution X-ray diffractometer with 2y/o scanning mode. The Cu Ka1 X-ray was monochromatized by four pieces of standard Ge (2 2 0) crystal and the resolution of the instrument is 800 . The (0 1 0)-oriented slices with the size of 10 10 4.5 mm3 were cut from before and after air-annealed TGT-Yb:YAP crystals, and then mechanically polished into the thickness of 4 mm for the optical properties measurements. The absorption spectrum of the slice was measured by the UV/VIS Spectrophotometer (Mode V-570, JASCO) at room temperature. The emission spectra of the crystal slice excited by 940-nm-LD were obtained by JY-TRIAX550 spectrophotometer at room temperature. The XEL spectrum was measured on a XEL spectrometer, FluorMain, where an F-30 movable clinical X-ray tube (W anticathode target) was used as the X-ray source operated under the condition of 80 kV and 4 mA at room temperature. Luminescence spectra were obtained by homemade 44 W plate grating monochromator and
Relative Intensity
Fig. 1. (a) The photo of 3 inches 5 at% doped Yb:YAP single crystal grown by TGT method (cracking into two parts and dark-colored before air-annealing); (b) Photos of TGT-Yb:YAP slices before and after annealing in the air at 1200 1C for 10 h.
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Fig. 2. The X-ray diffraction pattern of 5 at% doped TGTgrown Yb:YAP crystal.
Hamamatsu R928-28 photomutiplier with the data acquired by computer.
3. Results and discussions 3.1. The structure and perfection of TGT-Yb:YAP crystal X-ray powder diffraction patters of the pure YAP and 5 at% doped TGT-Yb:YAP crystals are showed in the Fig. 2. It can be clearly seen that the indexed peaks of TGT-Yb:YAP crystal are identified with those of pure YAP crystal in terms of
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relative intensity and position of the diffraction peaks. This indicated that 5 at% doped TGT-Yb: YAP crystal has the perovskite-like structure with the Pnma space group. The lattice parameters of 5 at% doped TGT-Yb:YAP crystal refined by ( b ¼ 7:3702 A, ( Rietveld method are a ¼ 5:3296 A; ( c ¼ 5:1781 A, respectively. Compared with lattice ( parameters of pure YAP crystal a ¼ 5:3291 A, ( ( b ¼ 7:3709 A, c ¼ 5:1796 A [11], it was found that there is little variation of the unit cell of perovskite-type YAP structure after 5 at% Yb ions added into the pure YAP host. In general, the refraction peaks (0 2 0), (0 4 0), and (0 6 0) appear in the XRD rocking curves of the 5 at% doped TGT-Yb:YAP with (0 1 0) face. In our experiment, the strongest refraction peak indexed by (0 4 0) centered at 23.61041 with its fullwidth at half-maximum (FWHM) of 65.329200 was carefully recorded in the Fig. 3. According to the Darwinian theory [13], the theoretical FWHM value of the rocking curve of the perfect (0 1 0)oriented YAP crystal have been calculated (FWHM ¼ 16.6800 ) by the authors [14]. The experimental rocking curve FWHM of TGTYb:YAP crystal (65.329200 ) is at the same magnitude with that of the perfect YAP crystal (16.6800 ), which means that 5 at% doped Yb:YAP crystal grown by TGT method has a good crystallization perfection.
3.2. The absorption and LD-excited IR emission spectra The absorption spectra in the range of 190–1200 nm of the 5 at% TGT-Yb:YAP crystals before and after air-annealing are shown in Fig. 4. It is clearly seen in Fig. 4 that there exist two distinct absorption bands in both Yb:YAP crystal samples before and after air-annealing: one is strong absorption centered at 220 nm which is corresponding to the transition from the ground state 2F7/2 to the charge transfer state of the Yb3+ ions in the YAP host [15], and the other absorption band with four strong absorption peaks centered at 932, 960, 980 and 998 nm, which is obviously ascribed to the typical transitions from the ground state 2F7/2 to the sublevels of 2F5/ 3+ in YAP crystal. These strong absorption 2 of Yb 3+ of Yb in YAP host around 940–980 nm suggests that the TGT-Yb:YAP crystal is very suitable for LD pumping for the DPSSL applications. In addition, there is a very weak absorption region from 270 to 400 nm in Fig. 4, this absorption region is very similar to those previously observed in the TGT-grown 5 at% doped Yb:YAG crystal and Cz-grown 20 at% doped Yb:YAP crystals by authors [11,16], and the unknown color centers related oxide or cation vacancies should be responsible for this weak absorption.
5.5 5.0
200000
23.6104
4.5 Optical Density
Intensity (a.u.)
175000 150000 125000
FWHM=65.3292"
100000 75000 50000
4.0
After air-annealing Before air-annealing
3.5 3.0 2.5 Yb3+
2.0 1.5 1.0
25000
λ1= 932nm λ2= 960nm λ3= 980nm λ4= 998nm
290nm
0.5
0 23.4
220nm
0.0 23.5
23.6 23.7 Degree (°)
23.8
23.9
Fig. 3. The rocking curve of the 5 at% TGT-Yb:YAP crystal ((0 4 0) refraction peak on (0 1 0) face).
200
400
600 800 1000 1200 Wavelength (nm)
1400
Fig. 4. The absorption spectra of 5 at% TGT-Yb:YAP crystal before and after air-anealing.
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It is very essential that Yb:YAP crystal should have the high transmission in the useful luminescence wavelength bands, i.e. the CTL bands and IR laser emission bands, to allow its own emitted light pass through the crystal with very little energy loss. Therefore, it is very necessary for its further scintillation and laser applications to take the annealing treatment in air-atmosphere with high temperature and long time which can greatly enhance the transmission of Yb:YAP crystal grown by TGT. Fig. 6 is the IR emission spectrum (indicated by red dot line) of the air-annealed 5 at% doped TGT-Yb:YAP crystal excited under the LD source with the wavelength of 940 nm at room temperature. The emission peaks are located at 999, 1013, and 1036 nm corresponding to the transitions from the sublevels of 2F5/2 level to the components of the 2F7/2 ground state, and these emission peaks can be served as the possible laser outputs wavelength. Due to having a strong reabsorption at 998 nm, showed in the Fig. 6, it is very suitable that the Yb:YAP crystal lasing at 1031 and 1036 nm when LD pumped at 960 and 980 nm. 3.3. X-ray excited luminescence XEL spectrum of air-annealed 5 at% TGTYb:YAP crystal showed in Fig. 7 has demonstrated clearly the CTL of Yb3+ ions in YAP host with double peaks centered at about 366 and
90
1.4 1.2
60 after air-annealing
50 40 30 20
before air-annealing
0 200
400
600 800 1000 Wavelength (nm)
1.0 998nm 1036nm
0.8 940nm
0.6 absorption
0.4 0.2
10 1200
Fig. 5. The transmission spectra of 5 at% TGT-Yb:YAP crystal before and after air-annealing.
1013nm
980nm
70 Optical Denstiy
Transmittance (%)
999nm 960nm
80
0.0 850
900
emission
2.5 2.3 2.0 1.8 1.5 1.3 1.0 0.8 0.5 0.3 0.0
Fluorescence Intensity (a.u.)
From the Fig. 4, it also can be found that the dark-brown colored TGT-Yb:YAP crystal before air-annealing has a much higher background absorption in the overall range of 120–1500 nm than that of the colorless air-annealed TGTYb:YAP crystal. This implies that the air-annealed TGT-Yb:YAP crystal has higher transmittance than that of the sample before air-annealed. Fig. 5 showed that the transmittance of the 5 at% doped TGT-Yb:YAP crystals with the same thickness of 4 mm before and after air-annealing. From the Fig. 5, it can be seen that the transmittance of the air-annealed TGT-Yb:YAP crystal is twice as high as that of the as-grown TGT-Yb:YAP crystal in the overall range of 120–1500 nm. This means that there exist much more uniform light scattering centers in the as grown Yb:YAP crystals, and these light scattering centers also should be responsible for the dark-brown coloration of the as-grown TGT-Yb:YAP crystal. Because the cylindrical graphite heater was employed in our TGT furnace, the Yb:YAP crystal with the contamination from the graphite heater could not be avoided during the TGT growth process, and the carbon scattering centers will be easily formed in as-grown crystal. After annealed in the air at 1200 1C for 10 h, the carbon scattering centers in TGTYb:YAP crystal will be escaped in the form of carbon dioxide gas, which makes the dark-brown colored Yb:YAP crystal turn into the colorless as showed in the Fig. 1(b).
487
950 1000 1050 1100 1150 Wavelength (nm)
Fig. 6. IR absorption and emission spectra of 5 at% TGTYb:YAP crystal after annealed in the air.
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60
366nm CTS
2F 7/2
Intensity (a.u.)
50 40 2F CTS 5/2 540nm
30
690nm
20 10 300
400
500 600 Wavelength (nm)
700
800
Fig. 7. X-ray excited luminescence of 5 at% TGT-Yb:YAP crystal after annealed in the air.
540 nm, which are corresponding to the transitions of the charge transfer state (CTS) to 2F7/2 and 2F5/2 states of Yb3+ respectively. Compared with the double CTL peaks centered at around 350 and 520 nm with the excitation by the UV photons around 230–250 nm in the previous reports [10,16,17], the X-ray excited double CTL peaks centered 366 and 540 nm are red-shift resulting from the higher photon energy of X-ray deposited on the Yb:YAP crystal. The very fast decay time (from sub-nanoseconds to several tens of nanoseconds) of the CTL in Yb:YAP crystal has been determined by the authors [10,11]. This suggests that it can be used for a fast scintillator. In addition, the very weak X-ray excited luminescence in the red spectral region of 590–700 nm can also be seen in the Fig. 7, which is similar that observed in the Cz-grown 20 at% doped Yb:YAP crystal [11]. The reasons for this red spectral emission probably can be ascribed to the intrinsic defects in pure YAP host [18,19] or some unintentional trivalent rare earth impurities such as Eu3+, Ho3+, and Tb3+ etc. presented in the TGT-grown Yb:YAP crystal.
4. Conclusion The dark-brown 5 at% doped Yb:YAP crystal with diameter of 3 inches has been grown by temperature gradient technique (TGT). The lattice
( b ¼ 7:3702 A, ( c¼ parameters of a ¼ 5:3296 A, ( of 5 at% doped TGT-Yb:YAP crystal 5:1781 A was determined by the precise X-ray diffraction (XRD) measurement. The rocking curve with FWHM of 65.329200 shows that the crystallization of the Yb:YAP crystal grown by TGT is perfect. The carbon light scattering centers in the as-grown Yb:YAP crystal, resulting from the contamination from graphite heater during TGT growth process, are as explained for dark-brown coloration of the crystal. The air-annealed high TGT-Yb:YAP crystal is colorless and high transparent. The absorption and LD excited emission spectra of air-annealed 5 at% Yb:YAP crystal showed that it can severed as the good laser media operated at around 1.0 mm when 960 nm or 980 nm LD pumped at room temperature. The fast charge transfer luminescence (CTL) peaking at around 366 and 540 nm was clearly determined in TGTYb:YAP crystal, which suggests that it can be served as the promising fast scintillation crystal for radiation detection. References [1] W.F. Krupke, IEEE J. Sel. Top. Quantum Electron. 6 (2000) 1287. [2] L.D. Deloach, S.A. Payne, et al., IEEE J. Quantum Electron. 29 (1993) 1179. [3] R.S. Raghavan, Phy. Rev. Lett. 78 (1997) 3681. [4] R. Chipaux, et al., Nucl. Instrum. Methods A 486 (2002) 228. [5] S. Belogurov, G. Bressi, et al., Nucl. Instrum. Methods A 496 (2003) 385. [6] G. Bressi, et al., Nucl. Instrum. Methods A 461 (2001) 467. [7] P. Antonini, et al., Nucl. Instrum. Methods A 486 (2002) 220. [8] S. Belogurov, et al., Nucl. Instrum. Methods A 516 (2004) 58. [9] G.A. Massey, Appl. Phys. Lett. 17 (1970) 213. [10] M. Nikl, N. Solovieva, et al., Appl. Phys. Lett. 84 (2004) 882. [11] Guangjun Zhao, Xionghui Zeng, et al., J. Crystal Growth 267 (2004) 522. [12] Xu Jianwei, Zhou Yongzong, et al., J. Crystal Growth 193 (1998) 123. [13] A.R. Lang, V.F. Miuskov, Appl. Phys. Lett. 7 (1965) 214. [14] Hongjun Li, Guangjun Zhao, et al., Mater. Lett. 58 (2004) 3253. [15] L. Van Pieterson, M. Heeroma, et al., J. Lumin. 91 (2000) 177.
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[18] I.A. Kamenskikh, N. Guerassimova, et al., Opt. Mater. 24 (2003) 267. [19] V.G. Baryshevski, et al., J. Phys.: Condens. Matter 5 (1993) 7893.