- Email: [email protected]
Growth and property of Zn-doped near-stoichiometric LiTaO3 crystal
ARTICLE IN PRESS Journal of Crystal Growth 312 (2010) 1879–1882
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth and property of Zn-doped near-stoichiometric LiTaO3 crystal Wei Zheng a,n, Dongpeng Wang b, Yuheng Xu b a b
College of Material Science and Engineering, 317 Linyuan Street 4, Harbin University of Science and Technology, Xiangfang District, Harbin 150040, China School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e in fo
abstract
Article history: Received 9 February 2009 Received in revised form 29 December 2009 Accepted 1 March 2010 Communicated by R.S. Feigelson Available online 6 March 2010
Near-stoichiometric LiTaO3 (SLT) and Zn-doped near-stoichiometric LiTaO3 (Zn:SLT) crystals with 10–15 mm in diameter and 10 mm in length were grown by using TSSG technique with K2O as the flux. The effect of adding amount of K2O was discussed in the growing process. The crystals were characterized by inductively coupled plasma-optical emission (ICP-OES), X-ray diffraction (XRD) and differential thermal analysis (DTA). The lattice constants of Zn:SLT were smaller than those of SLT and Curie temperature was higher than that of SLT. It was found that Zn doping is an efficient way to improve the optical damage resistance ability of SLT crystal. Compared with SLT crystal, Zn:SLT exhibited a much higher optical damage threshold, more than 500 MW/cm2, which was attributed to Zn self-compensated effect that formed the charge compensated complexes, (ZnTa)3 –3(ZnLi) + in SLT crystal. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Zn doping A1. Optical damage A2. TSSG technique B1. Stoichiometric LiTaO3
1. Introduction The congruent LiTaO3 (CLT) and LiNbO3 (CLN) crystals have been well known as one of the most excellent and useful ferroelectric materials for piezoelectric, nonlinear optical and linear electro-optic applications. They are shown to suffer retrievable optical damage in high-power optical conversion application involving visible wavelengths caused by photorefractive effect [1]. The optical damage (photorefractive damage) has been found to have a large negative effect on the properties of these crystals and then limited their optical applications. It is believed that this damage in CLT crystal was attributed to the existence of anti-site Ta (Nb) defects, Ta4Li+ (Nb4Li+ ), due to Li deficiency in these congruent crystals. It was proved that nearstoichiometric LiTaO3 (SLT) crystals grown from lithium-rich solutions appeared to be less sensitive to optical damage [2–4]. Besides increasing molar ratio of Li to Ta, another efficient way is the doping in a proper amount of some elements, such as Mg, Zn, In, Sc into the crystal to improve the ability of optical damage resistance. Recently, SLT crystal doped with Mg (1.2 mol%) [5] and Sc (0.44 mol%) [6] were reported to make an evident advance in optical damage resistance. Although SLT crystals have a great potential for many device applications, their growth research has been done less compared with near-stoichiometric LiNbO3 (SLN) crystals, which is maybe because of a higher melting point (SLT about 1550 1C and SLN
n
Corresponding author. Tel.: + 86 451 86392500; fax: +86 451 86392577. E-mail address: [email protected] (W. Zheng).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.03.003
about 1150 1C). The complicated double crucible system has been employed to supply self-flux Li2O to the melt continuously when SLT is growing from lithium-rich solutions [2]. Here we report on the growth of SLT and Zn-doped SLT crystals using top seed solution growth (TSSG) technology with K2O flux in a conventional CZ furnace. It is the first time that Zn-doped SLT crystal was grown. ZnO dopant played an active role in improving the optical damage resistance ability by compensating intrinsic defects left in SLN crystal [7]. In this paper the crystal growth procedure was described and discussed in detail and some characteristics of the as-grown crystals were presented. The ability of optical damage resistance was weighed up by facular distortion method quasiquantitatively.
2. Crystal growth and polarization The starting materials were Li2CO3, Ta2O5, K2CO3 and ZnO with the purity of 99.99%, 99.999%, 99.95% and 99.99%, respectively. The nominal melt composition for SLN crystal was characterized by a 48.7/51.3 molar ratio of Li2O/Ta2O5. The adding amount of K2CO3 was 10.9, 14.0 and 17.5 mol%, respectively. 0.5 mol% ZnO was doped into the melt and the nominal melt composition of all crystals was listed in Table 1. The mixed powder was heated up to 1250 1C and synthesized for 10 h. The crystals are grown along Z-axis. The pulling speed was 0.2 mm/h and rotating rates were about 15–20 rpm. The as-grown crystal was annealed from 1300 1C to room temperature. All crystals were colorless and transparent with the size of about 15 mm in diameter and 10 mm in length.
ARTICLE IN PRESS 1880
W. Zheng et al. / Journal of Crystal Growth 312 (2010) 1879–1882
Table 1 The raw material composition of SLT crystal series.
Press body mA
Crystal no.
Molar ratio of Li to Ta
K2CO3 amount in melt (mol%)
ZnO amount in melt (mol%)
SLT1 SLT2 SLT3 Zn:SLN
0.949 0.949 0.949 0.949
10.9 14.0 17.5 14.0
0 0 0 0.5
Rectifier
LT powder Ceramic plate
Pt electrode
~ 220V
Crystal R
Fig. 2. Schematic diagram of crystal polarizing set-up.
6
7
1
8 9
2
10
3
4 Fig.3. Metallographic photo of domain structure of CLT crystal ( 500).
5
1 Refractory material; 2 Al2O3 insulation; 3 RF coil; 4 Support; 5 Thermo-couple; 6 Water-cooled chamber; 7 Al2O3 cover; 8 Crystal; 9 Melt; 10 Al2O3 tubes Fig. 1. Schematic diagram of Czochralski RF furnace.
A CZ furnace was employed in growth process and the coil was connected with 25 kW, 20 kHz radio frequency generator shown schematically in Fig. 1. In the crystal growing process it was unexpected in the SLT1 growing that Pt crucible volatilized seriously at 1565 1C because the Pt melting point is about 1770 1C.So a Al2O3 ceramic cover was employed on the Pt crucible to limit its volatilization. A thin Pt layer was deposited on the inner surface of ceramic lid and seed crystal bar in the SLT1 growing process. With the increase of flux fraction in the melt, the SLT growing temperature decreased and Pt crucible volatizing loss became less. There was no visible volatilization in the growing of SLT3 and Zn:SLT crystals. The annealed crystal was cut the two ends off to a column shape connected with Pt electrodes and then embedded in the same composition polycrystalline powder in a furnace for polarizing as shown in Fig. 2. It was heated to 700 1C for 10 h and polarized in a DC electric field of 4 mA/cm2 for 10 min and
Fig. 4. Metallographic photo of domain structure of Zn:SLT crystal ( 500).
then was cooled down to room temperature at the rate of 50 1C. It was found that SLT crystals after polarization had a hexagonal domain structure, which was significantly different from CLT crystal irregular domain by a mixed HF and HNO3 acid etching. In Fig. 3 CLT domain exhibits several shapes: triangles, lines and hexagons. However, there is only regular hexagons for Zn:SLN due to its single domain structure as shown in Fig. 4.
ARTICLE IN PRESS W. Zheng et al. / Journal of Crystal Growth 312 (2010) 1879–1882
c
1
2
5
3
1881
Table 2 The chemical composition of the SLT crystals.
6
Crystal
K2O (mol%)
ZnO (mol%)
Li2O/Ta2O5 (molar ratio)
SLT1 SLT2 SLT3 Zn:SLT
1 10 4 1 10 4 4 10 3 1 10 4
– – – 0.42
0.9912 70.0001 0.9924 70.0001 0.9935 70.0001 0.9932 70.0001
7 4 1 Ar+laser, 2 Adjustable light attenuator, 3 Light shed; 4 Detector; 5 Convex lens; 6 Crystal sample; 7 Observation screen
Sample
Fig. 5. Set-up for measurement of optical damage threshold.
All crystals were cut into sheets with size of 10 1.5 10 mm3 (a b c). The (0 1 0) face of all crystals was polished to optical quality for optical measurement.
3. Characterization An inductively coupled plasma-optical emission spectrometer (ICP-OES) (IRIS Interpid HR Duo) was used to investigate the chemical composition of the crystals. The samples were prepared by microwave digestion with HNO3 (240 1C, 20 min). By X-ray powder diffraction analysis the lattice parameters were calculated with a SHMADZU XRD-6000 diffractometer and by differential thermal analysis the Curie temperature was refined from DTA curve data with a ZRY-2P DTA analyzer. The samples were taken from the left crystal fragments after cutting and then ground in an agate mortar. The powder specimen was heated in a furnace at 700 1C for 2 h to eliminate residual stress in shatter process. The ability of optical damage resistance was weighed up by determining optical damage threshold in the setup shown in Fig. 5. An Ar + laser with wavelength of 488.0 nm was employed as light source. After passing the diaphragm and convex lens, the laser beam was focused on crystal placed on focal point of lens. The transmittance light spot projected on the screen deformed in c-direction of crystals if the crystal occurs optical damage. The variation of light spot shape was observed by adjusting the output power of Ar + laser. The optical damage threshold, K, is defined as the lowest optical density on the crystal when the light spot on the observation screen begins to deform in 30 s irradiation accumulation [8], which is expressed as K¼
W 4W ¼ S pD2
ð1Þ
where W is the intensity of beam through the light attenuator, S is the area of light spot irradiation on the crystal and D is diameter of the spot. When the crystal is placed on the focal plane of the lens, the beam diameter is calculated by using D ¼ 1:22 2
fl d
Table 3 Lattice constants, Curie temperature and optical damage threshold of crystals.
ð2Þ
where f is focal length, l is laser wavelength and d is beam diameter before going through the lens.
4. Discussion The chemical composition of the crystals is listed in Table 2. The lattice constants, Curie temperature and photo damage threshold are listed in Table 3 together with quoted data for comparison. The K2O concentration in SLT3 crystal increases significantly, one magnitude higher than those in SLT2 and SLT1
CLT [9,10] SLT (from Li2O 58% melt) [11] SLT1 SLT2 SLT3 Zn:SLT
Curie temperature (1C)
Lattice constants (nm) A0
C0
605 675
0.5154 0.5152
1.3781 1.3774
679 678 674 688
0.5152 0.5151 0.5153 0.5151
1.3775 1.3776 1.3776 1.3771
Optical damage threshold K (MW/cm2)
11.5 –
455 480 460 4500
(4 10 3 versus 1 10 4 mol%). However, Pt crucible volatilizes seriously in the growing process of SLT1 because of its higher melting temperature compared with SLT2 and SLT3. Therefore, combined with the results in crystal growth, the proper amount of K2O flux seems to be from 14.0 to 17.0 mol% in congruent melt. All SLT crystals are near-stoichiometric from whatever flux amount melt and those Curie temperatures are higher than that of the congruent. The efficient segregation coefficient of Zn element in SLT is less than 1, about 0.8. There still exists a small amount of Li vacancies, (VLi) and anti-site Ta, (TaLi)4 + , forming (TaLi)4 + –4(VLi) in near-stoichiometric LiTaO3 crystal. Based on the data in Tables 2 and 3, it is deduced that there is a self-compensated effect in Zn:SLT similar to Zn:SLN [12]. In Zn:SLT crystal Zn ions in Ta sites are compensated with those in Li sites and form the charge compensated complexes (ZnTa)3 –3(ZnLi) + , which leads to a lower concentration of Li vacancy in Zn:SLT than in SLT. As a result of it, the unit cell contracts in crystals. Therefore, lattice constants of Zn:SLT represents smaller than those of SLT and Curie temperature higher than that of SLT. Usually, (TaLi)4 + /Ta5 + functions as a photorefractive center to improve photorefractive effect in CLT. Doping of Zn ions weakens photorefractive effect by making intrinsic defects fewer in Zn:SLT. Although limited to the maximum of lasers power output, we failed to detect optical damage threshold of Zn:SLT crystal exactly, Zn:SLT crystal shows a much higher optical damage resistance ability than SLT. The facular on the screen did not deform after Zn:SLT crystal had been irradiated for about 10 min by 500 MW/cm2.incident light.
5. Conclusion Zn-doped near-stoichiometric LiTaO3 crystal series with size of 15 mm in diameter and 10 mm in length were grown successfully by TSSG method with K2O flux. The proper K2O amount of was from 14.0 to 17.0 mol% in congruent melt. The grown crystals were colorless and transparent without macro defects. Doping of Zn ions in near-stoichiometric LiTaO3 crystal can increase the resistance ability of optical damage considerately. Optical damage threshold of Zn (0.5 mol%):SLT crystal is more than 500 MW/cm2.
ARTICLE IN PRESS 1882
W. Zheng et al. / Journal of Crystal Growth 312 (2010) 1879–1882
Acknowledgement The subject was supported by Harbin Youth Fund Project (Grant 2005AFQ XJ 033). References [1] A. Ashkin, G.D. Boyd, J.M. Dziedzic, R.G. Smith, A.A. Ballman, J.J. Levinstein K. Nassau, Appl. Phys. Lett. 9 (1966) 72. [2] K. Kitamura, Y. Furukawa, K. Niva, V. Gopalan, T.E. Mitchell, Appl. Phys. Lett. 73 (1998) 3073–3076. [3] D.P. Dimitar, J. Rottenberg, S. Samuelson, J. Cryst. Growth 287 (2006) 296–299.
[4] I.G. Kim, S. Tekakawa, Y. Furukawa, M. Lee, K. Kitamura, J. Cryst. Growth 229 (2001) 243–247. [5] W.T. Hsu, Z.B. Chen, C.A. You, M.C. Chou, Y.Y. Lin, Y.C. Huang, D.K. Rai C.W. Lan, J. Cryst. Growth 311 (2009) 66–71. [6] M. Nakamura, S. Takekawa, Y. Liu, K. Kitamura, J. Cryst. Growth 311 (2009) 272–277. [7] T. Volk, V. Pryalkin, N. Rubinina, Opt. Lett. 15 (1990) 996–1001. [8] W. Zheng, B. Liu, J. Bi, Y. Xu, Opt. Commun. 246 (2005) 297–301. [9] S. Fang, B. Wang, T. Zhang, F. Ling, R. Wang, Opt. Mater. 28 (2006) 207–211. [10] D. Wang, X. Zhang, J. Hou, Ningxia Eng. Technol. 2 (2003) 230–233. [11] Y. Furukawa, K. Kitamura, E. Suzuki, K. Niwa, J. Cryst. Growth 197 (1999) 889–895. [12] W. Zheng, N. Zhang, L. Zhao, Y. Xu, Opt. Commun. 227 (2003) 259–263.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth and property of Zn-doped near-stoichiometric LiTaO3 crystal Wei Zheng a,n, Dongpeng Wang b, Yuheng Xu b a b
College of Material Science and Engineering, 317 Linyuan Street 4, Harbin University of Science and Technology, Xiangfang District, Harbin 150040, China School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e in fo
abstract
Article history: Received 9 February 2009 Received in revised form 29 December 2009 Accepted 1 March 2010 Communicated by R.S. Feigelson Available online 6 March 2010
Near-stoichiometric LiTaO3 (SLT) and Zn-doped near-stoichiometric LiTaO3 (Zn:SLT) crystals with 10–15 mm in diameter and 10 mm in length were grown by using TSSG technique with K2O as the flux. The effect of adding amount of K2O was discussed in the growing process. The crystals were characterized by inductively coupled plasma-optical emission (ICP-OES), X-ray diffraction (XRD) and differential thermal analysis (DTA). The lattice constants of Zn:SLT were smaller than those of SLT and Curie temperature was higher than that of SLT. It was found that Zn doping is an efficient way to improve the optical damage resistance ability of SLT crystal. Compared with SLT crystal, Zn:SLT exhibited a much higher optical damage threshold, more than 500 MW/cm2, which was attributed to Zn self-compensated effect that formed the charge compensated complexes, (ZnTa)3 –3(ZnLi) + in SLT crystal. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Zn doping A1. Optical damage A2. TSSG technique B1. Stoichiometric LiTaO3
1. Introduction The congruent LiTaO3 (CLT) and LiNbO3 (CLN) crystals have been well known as one of the most excellent and useful ferroelectric materials for piezoelectric, nonlinear optical and linear electro-optic applications. They are shown to suffer retrievable optical damage in high-power optical conversion application involving visible wavelengths caused by photorefractive effect [1]. The optical damage (photorefractive damage) has been found to have a large negative effect on the properties of these crystals and then limited their optical applications. It is believed that this damage in CLT crystal was attributed to the existence of anti-site Ta (Nb) defects, Ta4Li+ (Nb4Li+ ), due to Li deficiency in these congruent crystals. It was proved that nearstoichiometric LiTaO3 (SLT) crystals grown from lithium-rich solutions appeared to be less sensitive to optical damage [2–4]. Besides increasing molar ratio of Li to Ta, another efficient way is the doping in a proper amount of some elements, such as Mg, Zn, In, Sc into the crystal to improve the ability of optical damage resistance. Recently, SLT crystal doped with Mg (1.2 mol%) [5] and Sc (0.44 mol%) [6] were reported to make an evident advance in optical damage resistance. Although SLT crystals have a great potential for many device applications, their growth research has been done less compared with near-stoichiometric LiNbO3 (SLN) crystals, which is maybe because of a higher melting point (SLT about 1550 1C and SLN
n
Corresponding author. Tel.: + 86 451 86392500; fax: +86 451 86392577. E-mail address: [email protected] (W. Zheng).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.03.003
about 1150 1C). The complicated double crucible system has been employed to supply self-flux Li2O to the melt continuously when SLT is growing from lithium-rich solutions [2]. Here we report on the growth of SLT and Zn-doped SLT crystals using top seed solution growth (TSSG) technology with K2O flux in a conventional CZ furnace. It is the first time that Zn-doped SLT crystal was grown. ZnO dopant played an active role in improving the optical damage resistance ability by compensating intrinsic defects left in SLN crystal [7]. In this paper the crystal growth procedure was described and discussed in detail and some characteristics of the as-grown crystals were presented. The ability of optical damage resistance was weighed up by facular distortion method quasiquantitatively.
2. Crystal growth and polarization The starting materials were Li2CO3, Ta2O5, K2CO3 and ZnO with the purity of 99.99%, 99.999%, 99.95% and 99.99%, respectively. The nominal melt composition for SLN crystal was characterized by a 48.7/51.3 molar ratio of Li2O/Ta2O5. The adding amount of K2CO3 was 10.9, 14.0 and 17.5 mol%, respectively. 0.5 mol% ZnO was doped into the melt and the nominal melt composition of all crystals was listed in Table 1. The mixed powder was heated up to 1250 1C and synthesized for 10 h. The crystals are grown along Z-axis. The pulling speed was 0.2 mm/h and rotating rates were about 15–20 rpm. The as-grown crystal was annealed from 1300 1C to room temperature. All crystals were colorless and transparent with the size of about 15 mm in diameter and 10 mm in length.
ARTICLE IN PRESS 1880
W. Zheng et al. / Journal of Crystal Growth 312 (2010) 1879–1882
Table 1 The raw material composition of SLT crystal series.
Press body mA
Crystal no.
Molar ratio of Li to Ta
K2CO3 amount in melt (mol%)
ZnO amount in melt (mol%)
SLT1 SLT2 SLT3 Zn:SLN
0.949 0.949 0.949 0.949
10.9 14.0 17.5 14.0
0 0 0 0.5
Rectifier
LT powder Ceramic plate
Pt electrode
~ 220V
Crystal R
Fig. 2. Schematic diagram of crystal polarizing set-up.
6
7
1
8 9
2
10
3
4 Fig.3. Metallographic photo of domain structure of CLT crystal ( 500).
5
1 Refractory material; 2 Al2O3 insulation; 3 RF coil; 4 Support; 5 Thermo-couple; 6 Water-cooled chamber; 7 Al2O3 cover; 8 Crystal; 9 Melt; 10 Al2O3 tubes Fig. 1. Schematic diagram of Czochralski RF furnace.
A CZ furnace was employed in growth process and the coil was connected with 25 kW, 20 kHz radio frequency generator shown schematically in Fig. 1. In the crystal growing process it was unexpected in the SLT1 growing that Pt crucible volatilized seriously at 1565 1C because the Pt melting point is about 1770 1C.So a Al2O3 ceramic cover was employed on the Pt crucible to limit its volatilization. A thin Pt layer was deposited on the inner surface of ceramic lid and seed crystal bar in the SLT1 growing process. With the increase of flux fraction in the melt, the SLT growing temperature decreased and Pt crucible volatizing loss became less. There was no visible volatilization in the growing of SLT3 and Zn:SLT crystals. The annealed crystal was cut the two ends off to a column shape connected with Pt electrodes and then embedded in the same composition polycrystalline powder in a furnace for polarizing as shown in Fig. 2. It was heated to 700 1C for 10 h and polarized in a DC electric field of 4 mA/cm2 for 10 min and
Fig. 4. Metallographic photo of domain structure of Zn:SLT crystal ( 500).
then was cooled down to room temperature at the rate of 50 1C. It was found that SLT crystals after polarization had a hexagonal domain structure, which was significantly different from CLT crystal irregular domain by a mixed HF and HNO3 acid etching. In Fig. 3 CLT domain exhibits several shapes: triangles, lines and hexagons. However, there is only regular hexagons for Zn:SLN due to its single domain structure as shown in Fig. 4.
ARTICLE IN PRESS W. Zheng et al. / Journal of Crystal Growth 312 (2010) 1879–1882
c
1
2
5
3
1881
Table 2 The chemical composition of the SLT crystals.
6
Crystal
K2O (mol%)
ZnO (mol%)
Li2O/Ta2O5 (molar ratio)
SLT1 SLT2 SLT3 Zn:SLT
1 10 4 1 10 4 4 10 3 1 10 4
– – – 0.42
0.9912 70.0001 0.9924 70.0001 0.9935 70.0001 0.9932 70.0001
7 4 1 Ar+laser, 2 Adjustable light attenuator, 3 Light shed; 4 Detector; 5 Convex lens; 6 Crystal sample; 7 Observation screen
Sample
Fig. 5. Set-up for measurement of optical damage threshold.
All crystals were cut into sheets with size of 10 1.5 10 mm3 (a b c). The (0 1 0) face of all crystals was polished to optical quality for optical measurement.
3. Characterization An inductively coupled plasma-optical emission spectrometer (ICP-OES) (IRIS Interpid HR Duo) was used to investigate the chemical composition of the crystals. The samples were prepared by microwave digestion with HNO3 (240 1C, 20 min). By X-ray powder diffraction analysis the lattice parameters were calculated with a SHMADZU XRD-6000 diffractometer and by differential thermal analysis the Curie temperature was refined from DTA curve data with a ZRY-2P DTA analyzer. The samples were taken from the left crystal fragments after cutting and then ground in an agate mortar. The powder specimen was heated in a furnace at 700 1C for 2 h to eliminate residual stress in shatter process. The ability of optical damage resistance was weighed up by determining optical damage threshold in the setup shown in Fig. 5. An Ar + laser with wavelength of 488.0 nm was employed as light source. After passing the diaphragm and convex lens, the laser beam was focused on crystal placed on focal point of lens. The transmittance light spot projected on the screen deformed in c-direction of crystals if the crystal occurs optical damage. The variation of light spot shape was observed by adjusting the output power of Ar + laser. The optical damage threshold, K, is defined as the lowest optical density on the crystal when the light spot on the observation screen begins to deform in 30 s irradiation accumulation [8], which is expressed as K¼
W 4W ¼ S pD2
ð1Þ
where W is the intensity of beam through the light attenuator, S is the area of light spot irradiation on the crystal and D is diameter of the spot. When the crystal is placed on the focal plane of the lens, the beam diameter is calculated by using D ¼ 1:22 2
fl d
Table 3 Lattice constants, Curie temperature and optical damage threshold of crystals.
ð2Þ
where f is focal length, l is laser wavelength and d is beam diameter before going through the lens.
4. Discussion The chemical composition of the crystals is listed in Table 2. The lattice constants, Curie temperature and photo damage threshold are listed in Table 3 together with quoted data for comparison. The K2O concentration in SLT3 crystal increases significantly, one magnitude higher than those in SLT2 and SLT1
CLT [9,10] SLT (from Li2O 58% melt) [11] SLT1 SLT2 SLT3 Zn:SLT
Curie temperature (1C)
Lattice constants (nm) A0
C0
605 675
0.5154 0.5152
1.3781 1.3774
679 678 674 688
0.5152 0.5151 0.5153 0.5151
1.3775 1.3776 1.3776 1.3771
Optical damage threshold K (MW/cm2)
11.5 –
455 480 460 4500
(4 10 3 versus 1 10 4 mol%). However, Pt crucible volatilizes seriously in the growing process of SLT1 because of its higher melting temperature compared with SLT2 and SLT3. Therefore, combined with the results in crystal growth, the proper amount of K2O flux seems to be from 14.0 to 17.0 mol% in congruent melt. All SLT crystals are near-stoichiometric from whatever flux amount melt and those Curie temperatures are higher than that of the congruent. The efficient segregation coefficient of Zn element in SLT is less than 1, about 0.8. There still exists a small amount of Li vacancies, (VLi) and anti-site Ta, (TaLi)4 + , forming (TaLi)4 + –4(VLi) in near-stoichiometric LiTaO3 crystal. Based on the data in Tables 2 and 3, it is deduced that there is a self-compensated effect in Zn:SLT similar to Zn:SLN [12]. In Zn:SLT crystal Zn ions in Ta sites are compensated with those in Li sites and form the charge compensated complexes (ZnTa)3 –3(ZnLi) + , which leads to a lower concentration of Li vacancy in Zn:SLT than in SLT. As a result of it, the unit cell contracts in crystals. Therefore, lattice constants of Zn:SLT represents smaller than those of SLT and Curie temperature higher than that of SLT. Usually, (TaLi)4 + /Ta5 + functions as a photorefractive center to improve photorefractive effect in CLT. Doping of Zn ions weakens photorefractive effect by making intrinsic defects fewer in Zn:SLT. Although limited to the maximum of lasers power output, we failed to detect optical damage threshold of Zn:SLT crystal exactly, Zn:SLT crystal shows a much higher optical damage resistance ability than SLT. The facular on the screen did not deform after Zn:SLT crystal had been irradiated for about 10 min by 500 MW/cm2.incident light.
5. Conclusion Zn-doped near-stoichiometric LiTaO3 crystal series with size of 15 mm in diameter and 10 mm in length were grown successfully by TSSG method with K2O flux. The proper K2O amount of was from 14.0 to 17.0 mol% in congruent melt. The grown crystals were colorless and transparent without macro defects. Doping of Zn ions in near-stoichiometric LiTaO3 crystal can increase the resistance ability of optical damage considerately. Optical damage threshold of Zn (0.5 mol%):SLT crystal is more than 500 MW/cm2.
ARTICLE IN PRESS 1882
W. Zheng et al. / Journal of Crystal Growth 312 (2010) 1879–1882
Acknowledgement The subject was supported by Harbin Youth Fund Project (Grant 2005AFQ XJ 033). References [1] A. Ashkin, G.D. Boyd, J.M. Dziedzic, R.G. Smith, A.A. Ballman, J.J. Levinstein K. Nassau, Appl. Phys. Lett. 9 (1966) 72. [2] K. Kitamura, Y. Furukawa, K. Niva, V. Gopalan, T.E. Mitchell, Appl. Phys. Lett. 73 (1998) 3073–3076. [3] D.P. Dimitar, J. Rottenberg, S. Samuelson, J. Cryst. Growth 287 (2006) 296–299.
[4] I.G. Kim, S. Tekakawa, Y. Furukawa, M. Lee, K. Kitamura, J. Cryst. Growth 229 (2001) 243–247. [5] W.T. Hsu, Z.B. Chen, C.A. You, M.C. Chou, Y.Y. Lin, Y.C. Huang, D.K. Rai C.W. Lan, J. Cryst. Growth 311 (2009) 66–71. [6] M. Nakamura, S. Takekawa, Y. Liu, K. Kitamura, J. Cryst. Growth 311 (2009) 272–277. [7] T. Volk, V. Pryalkin, N. Rubinina, Opt. Lett. 15 (1990) 996–1001. [8] W. Zheng, B. Liu, J. Bi, Y. Xu, Opt. Commun. 246 (2005) 297–301. [9] S. Fang, B. Wang, T. Zhang, F. Ling, R. Wang, Opt. Mater. 28 (2006) 207–211. [10] D. Wang, X. Zhang, J. Hou, Ningxia Eng. Technol. 2 (2003) 230–233. [11] Y. Furukawa, K. Kitamura, E. Suzuki, K. Niwa, J. Cryst. Growth 197 (1999) 889–895. [12] W. Zheng, N. Zhang, L. Zhao, Y. Xu, Opt. Commun. 227 (2003) 259–263.