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Czochralski growth of TeO2 single crystals under conditions of forced convection in the melt
j. . . . . . . . C R Y S T A L GROWTH
Journal of Crystal Growth 116 (1992) 364-368 North-Holland
Czochralski growth of TeO 2 single crystals under conditions of forced convection in the melt T a d e u s z L u k a s i e w i c z and A n d r z e j M a j c h r o w s k i Institute of Technical Physics, WAT, UI. Kaliskiego 2, 00-908 Warsaw, Poland Received 26 June 1991; manuscript received in final form 1 October 1991
Growth of paratellurite single crystals by the Czochralski method in an apparatus with resistance heating was investigated. It was found that forced convection in the melt was predominant besides the initial stage of the growth process. The dependence between the diameter of the crystal corresponding to the inversion of the flows in the melt and the rotation rate was investigated, and the importance of the vertical temperature gradient in the crucible was pointed out. It was compared with results obtained by other authors when RF heating was used.
I. Introduction Tetragonal paratellurite T e O 2 single crystals have been well known as an excellent acoustooptic material. Its properties were reported in a number of works [1-3]. T e O 2 single crystals completely free from inhomogeneities with very high structural perfection can be used in various highly efficient acousto-optic devices. The material is highly transparent in the range of 0.35-500 /~m and has a very high acousto-optic figure of merit M 2 = 793 × 10 -18 S 3 g - i for transverse acoustic wave propagating in [110] direction with extremely low velocity of 616 m s-]. There are two more acoustic modes which can be used in acousto-optic devices: longitudinal wave propagating in the [001] direction with M 2 = 34.5 × 10-18 s 3 g-~ and transverse wave propagating in the [001] plane making an angle of 35.9 o to the x-axis with M 2 = 200 x 10 -18 s 3 g-l. The latter mode shows stabilization of acoustic velocity with temperature [4]. T e O 2 is insoluble in water and has high refractive indices - the material is uniaxial positive with n o = 2.274 and n e = 2.430. The growth of paratellurite single crystals by the Czochralski method was first reported by Liebertz [5], who used the resistance heating.
Miyazawa and Kondo [6] studied the growth under conditions of RF heating.
2. Experimental method In our experiments, TeO 2 single crystals were grown from the melt by the Czochralski method. As a container, pure platinum crucibles were used. At first the raw material was obtained by raffination of commercially available powder of 99% T e O 2 in nitric acid. The chemical analyses showed that the content of impurities after the operation was at the same level as before it. Since the purity of the starting material is one of the most important parameters in crystal growth, in the next step we used 6N metallic tellurium which, after powdering, was dissolved in nitric acid. After oxidation, T e O 2 w a s precipitated from acid solution with 10% NH4OH, washed, dried and heated in 773 K for a few hours. Small quantities of T e O 3 were always present among the products and the last operation converted them into T e O 2. At the beginning of our experiments, the RF heating was used, but it gave too sharp temperature gradients at the interface; also, the temperature stability was poor, so it was changed into a
0022-0248/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
T. Lukasiewicz, A. Majchrowski / CZ growth of TeOe under forced convection in melt
resistance heating. We used a multi-zone resistance furnace with "Master-Slave" control and a regulation system containing Eurotherm High Stability Controllers. The temperature measured below the bottom of the crucible was kept with an accuracy of + 0.1 K. A schematic construction of the furnace is shown on the fig. 1. Small crystalline samples of TeOe were first obtained on platinum wire. Their growth direction was approximately [001]. In all of them the lineage structure was observed. By cutting samples in the [110] direction, the lineage structure
seed
365
was eliminated and seeds free from this kind of defect were obtained. Different values of temperature gradient near the interface from 40 to 100 K cm -~, pulling rate from 1 to 5 mm h-1 and seed rotation from 10 to 100 rpm were investigated. Using a crucible of 40 mm in diameter and 40 mm deep, the best results were obtained when a temperature gradient of 60 K cm-1 closely above the surface of the melt was used. Pulling rate was smaller than 2 mm h -~ (~ 1.5 m m h -1) and rotation rate was 80 rpm. We obtained single crystals of TeO2 in [110],
\ ndow
~g ;nts
i thermoc
ucible molten
2
thermocouples Fig. 1. Schematic diagram of the multi-zone furnace for the Czochralski growth of TeO~ single crystals.
366
T. Lukasiewicz, A. Majchrowski / CZ growth of TeO2 under forced convection in melt
Fig. 2. [001] TeO2 single crystal. Despite continuous decrease of the temperature of the melt, the cross-section dimensionswere changing in "jumps", whichcan be seen on the surface of the crystalas bright lines. Scale in centimeters. [001], [100] and [111] directions up to 50 mm in length and 15 x 15 mm 2 in cross-section. TeO 2 crystals pulled in [001] direction had square cross-sections and four [110] planes. The crosssections of the crystals pulled in [110] and [111] directions were rectangular and triangle, respectively. By decreasing the temperature gradient, [001] TeO 2 single crystals with octagonal crosssection limited by [110] and [100] faces were also obtained, but such crystals had many inclusions caused by constitutional supercooling. As was reported earlier, dissolved platinum from the walls of the crucible formed these inhomogeneities [7]. We investigated the growth conditions of bigger paratellurite single crystals using a crucible of 70 mm in diameter and 60 mm deep. The crystals obtained from this crucible were up to 60 mm in length and up to 25 x 25 mm 2 in cross section (fig. 2). The density of obtained TeO 2 single crystals is 6.04 g cm -3, which is compliant with the density calculated from measured lattice constants: a = 4.8088 ,4, and c = 7.6038 .~,. The crystals were colourless and free from inclusions, gas bubbles, cracks and distortions, which could be seen between crossed polarizers. The quality of the crystals was determined by chemical etching method,
as described by Grabmaier et al. [7]. The dislocation density was 6 x 10 3 cm -2. The crystals were applied in such acousto-optic devices as deflectors switching laser light from one guideline to another [8], modulators with shear [9] and longitudinal [10] acoustic waves and tunable optical filters working in solar spectrometers [11].
3. Discussion
The influence of crystal pulling rate on conditions of TeO 2 single crystal growth was investigaied. Higher pulling rates could be realized with increase of seed rotation; however, it was not possible to obtain good quality single crystals using pulling rates exceeding 3 mm h -1, despite high seed rotation and high temperature gradient near the interface. In practice, any pulling rate higher than 2 mm h-~ gave crystals with opaque zones including gas bubbles and inclusions of impurities. The strong tendency of TeO 2 single crystals to cracking, caused by high anisotropy of thermal expansion coefficients, requires the use of a low temperature gradient. Under such conditions, constitutional supercooling can occur and as a result cellular growth is possible. Using the
T. Lukasiewicz, A. Majchrowski / CZ growth of TeO 2 under forced convection in melt
d[mm]
367
c
14 13 12 1
1 9 8 7 6 5
ci
4 3 2 1
~)0
I
I
t
I
0,05
0,10
0,15
0.20
f
0,2 5C~'~ [ (rpmTmin)" tr41
Fig. 3. Relation between rotation rate and diameter of TeO 2 crystal corresponding to inversion of flows in the melt: (a) crucible diameter 40 mm, dependence calculated according to Miyazawa [13]; (b) crucible diameter 40 mm, depth of melt H = 40 ram, dependence calculated according to formula (3); (c) crucible diameter 70 ram, H = 60 ram, dependence as in (b).
method proposed by Brice [12], we calculated the thermal conditions which can cause cracking and cellular growth, especially in the case of bigger TeO 2 crystals. We used in our considerations the results of measurements obtained during the growth of small TeO2 single crystal. For TeO 2 crystals with cross-section larger than 20 × 20 mm 2, the constitutional supercooling begins to play the main role. The temperature conditions, which should be used to protect crystals against cracking, may cause cellular growth. Moreover, during the growth the level of the melt falls down and uncovered crucible walls act as an afterheater. It additionally decreases the temperature gradients and the quality of the crystal radically
decreases. The details of these calculations will be discussed separately in the next paper. To determine the relation between crystal rotation and character of the melt flow, TeO 2 crystals doped with 0.1 wt% TiO 2 were pulled. The shape of the striations formed by the dopant represented the shape of the profile of the interface. This informed us about the character of the dominating convection at every moment of the growth. Besides the initial stage of the growth process, forced convection was observed as being predominant. The inversion from natural to forced flow caused changes of diameter and shape of growing crystal. For example, in [001] TeO 2 crystals the circular cross-section becomes square.
368
T. Lukasiewicz, A. Majchrowski / CZ growth of TeO2 under forced concection in melt
We found that the diameter of the crystal corresponding to the inversion of the flows and the rotation rate are linked by the formula: d =mto -°'5.
(1)
lower ( ~ 2 K c m - 1 ) and the ATH H 3 unit had a distinct influence on the process of inversion of convection flows. Results of calculations, as shown in fig. 3, were compliant with experiments.
Miyazawa [13] gave
m = (4zc-2gB ATR3) °'25,
(2)
where g i_s the earth acceleration, B the thermal expansion coefficient of the melt, AT the radial temperature difference in the melt and R the radius of the crucible. The real conditions in our experiments are described by the following formula:
m=[aTr-2gB(ATR3+ATH H3)] 0'25
(3)
where ATH is the vertical difference of the temperature and H the depth of the melt. This equation was obtained by introducing the vertical difference of the temperature into Miyazawa's formula (2) (see ref. [13]). Miyazawa studied flows in crucibles heated by RF. Under such conditions, the radial temperature gradient is very high and the ATnH 3 unit can be neglected. In our experiments with resistance heating, the radial temperature gradient was much
References [1] G. Arlt, J. Liebertz and H. Schweppe, 8th Intern. Congr. on Acoustics, H-989, 1968. [2] N. Uchida and N. Ohmachi, J. Appl. Phys. 40 (1969) 4692. [3] N. Uchida, Phys. Rev. 4 (1971) 3736. [4] Y. Ohmachi and N. Uchida, Rev. Elec. Commun. Lab. 20 (1971) 529. [5] J. Liebertz, Kristall Tech. 4 (1969) 221. [6] S. Miyazawa and S. Kondo, Mater. Res. Bull. 8 (1973) 1215. [7] J.G. Grabmaier, R.D. Plattner and M. Schieber, J. Crystal Growth 20 (1973) 82. [8] Milewski, MSc Degree, WAT, Warsaw (1982). [9] I. Merta, W. Milewski and M. Szustakowski, Elektryczne i Akustyczne Metody Badafi Material6w i Struktur Biologicznych (Warsaw, 1984) p. 121. [10] I. Merta, private communication. [11} J. Koztowski, in: Proc. 4th Spring School on AcoustoOptics and Applications, Gdafisk, 1988, p. 327. [12] J.C. Brice, J. Crystal Growth 2 (1968) 395. [13] S. Miyazawa, J. Crystal Growth 49 (1980) 515.
Journal of Crystal Growth 116 (1992) 364-368 North-Holland
Czochralski growth of TeO 2 single crystals under conditions of forced convection in the melt T a d e u s z L u k a s i e w i c z and A n d r z e j M a j c h r o w s k i Institute of Technical Physics, WAT, UI. Kaliskiego 2, 00-908 Warsaw, Poland Received 26 June 1991; manuscript received in final form 1 October 1991
Growth of paratellurite single crystals by the Czochralski method in an apparatus with resistance heating was investigated. It was found that forced convection in the melt was predominant besides the initial stage of the growth process. The dependence between the diameter of the crystal corresponding to the inversion of the flows in the melt and the rotation rate was investigated, and the importance of the vertical temperature gradient in the crucible was pointed out. It was compared with results obtained by other authors when RF heating was used.
I. Introduction Tetragonal paratellurite T e O 2 single crystals have been well known as an excellent acoustooptic material. Its properties were reported in a number of works [1-3]. T e O 2 single crystals completely free from inhomogeneities with very high structural perfection can be used in various highly efficient acousto-optic devices. The material is highly transparent in the range of 0.35-500 /~m and has a very high acousto-optic figure of merit M 2 = 793 × 10 -18 S 3 g - i for transverse acoustic wave propagating in [110] direction with extremely low velocity of 616 m s-]. There are two more acoustic modes which can be used in acousto-optic devices: longitudinal wave propagating in the [001] direction with M 2 = 34.5 × 10-18 s 3 g-~ and transverse wave propagating in the [001] plane making an angle of 35.9 o to the x-axis with M 2 = 200 x 10 -18 s 3 g-l. The latter mode shows stabilization of acoustic velocity with temperature [4]. T e O 2 is insoluble in water and has high refractive indices - the material is uniaxial positive with n o = 2.274 and n e = 2.430. The growth of paratellurite single crystals by the Czochralski method was first reported by Liebertz [5], who used the resistance heating.
Miyazawa and Kondo [6] studied the growth under conditions of RF heating.
2. Experimental method In our experiments, TeO 2 single crystals were grown from the melt by the Czochralski method. As a container, pure platinum crucibles were used. At first the raw material was obtained by raffination of commercially available powder of 99% T e O 2 in nitric acid. The chemical analyses showed that the content of impurities after the operation was at the same level as before it. Since the purity of the starting material is one of the most important parameters in crystal growth, in the next step we used 6N metallic tellurium which, after powdering, was dissolved in nitric acid. After oxidation, T e O 2 w a s precipitated from acid solution with 10% NH4OH, washed, dried and heated in 773 K for a few hours. Small quantities of T e O 3 were always present among the products and the last operation converted them into T e O 2. At the beginning of our experiments, the RF heating was used, but it gave too sharp temperature gradients at the interface; also, the temperature stability was poor, so it was changed into a
0022-0248/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
T. Lukasiewicz, A. Majchrowski / CZ growth of TeOe under forced convection in melt
resistance heating. We used a multi-zone resistance furnace with "Master-Slave" control and a regulation system containing Eurotherm High Stability Controllers. The temperature measured below the bottom of the crucible was kept with an accuracy of + 0.1 K. A schematic construction of the furnace is shown on the fig. 1. Small crystalline samples of TeOe were first obtained on platinum wire. Their growth direction was approximately [001]. In all of them the lineage structure was observed. By cutting samples in the [110] direction, the lineage structure
seed
365
was eliminated and seeds free from this kind of defect were obtained. Different values of temperature gradient near the interface from 40 to 100 K cm -~, pulling rate from 1 to 5 mm h-1 and seed rotation from 10 to 100 rpm were investigated. Using a crucible of 40 mm in diameter and 40 mm deep, the best results were obtained when a temperature gradient of 60 K cm-1 closely above the surface of the melt was used. Pulling rate was smaller than 2 mm h -~ (~ 1.5 m m h -1) and rotation rate was 80 rpm. We obtained single crystals of TeO2 in [110],
\ ndow
~g ;nts
i thermoc
ucible molten
2
thermocouples Fig. 1. Schematic diagram of the multi-zone furnace for the Czochralski growth of TeO~ single crystals.
366
T. Lukasiewicz, A. Majchrowski / CZ growth of TeO2 under forced convection in melt
Fig. 2. [001] TeO2 single crystal. Despite continuous decrease of the temperature of the melt, the cross-section dimensionswere changing in "jumps", whichcan be seen on the surface of the crystalas bright lines. Scale in centimeters. [001], [100] and [111] directions up to 50 mm in length and 15 x 15 mm 2 in cross-section. TeO 2 crystals pulled in [001] direction had square cross-sections and four [110] planes. The crosssections of the crystals pulled in [110] and [111] directions were rectangular and triangle, respectively. By decreasing the temperature gradient, [001] TeO 2 single crystals with octagonal crosssection limited by [110] and [100] faces were also obtained, but such crystals had many inclusions caused by constitutional supercooling. As was reported earlier, dissolved platinum from the walls of the crucible formed these inhomogeneities [7]. We investigated the growth conditions of bigger paratellurite single crystals using a crucible of 70 mm in diameter and 60 mm deep. The crystals obtained from this crucible were up to 60 mm in length and up to 25 x 25 mm 2 in cross section (fig. 2). The density of obtained TeO 2 single crystals is 6.04 g cm -3, which is compliant with the density calculated from measured lattice constants: a = 4.8088 ,4, and c = 7.6038 .~,. The crystals were colourless and free from inclusions, gas bubbles, cracks and distortions, which could be seen between crossed polarizers. The quality of the crystals was determined by chemical etching method,
as described by Grabmaier et al. [7]. The dislocation density was 6 x 10 3 cm -2. The crystals were applied in such acousto-optic devices as deflectors switching laser light from one guideline to another [8], modulators with shear [9] and longitudinal [10] acoustic waves and tunable optical filters working in solar spectrometers [11].
3. Discussion
The influence of crystal pulling rate on conditions of TeO 2 single crystal growth was investigaied. Higher pulling rates could be realized with increase of seed rotation; however, it was not possible to obtain good quality single crystals using pulling rates exceeding 3 mm h -1, despite high seed rotation and high temperature gradient near the interface. In practice, any pulling rate higher than 2 mm h-~ gave crystals with opaque zones including gas bubbles and inclusions of impurities. The strong tendency of TeO 2 single crystals to cracking, caused by high anisotropy of thermal expansion coefficients, requires the use of a low temperature gradient. Under such conditions, constitutional supercooling can occur and as a result cellular growth is possible. Using the
T. Lukasiewicz, A. Majchrowski / CZ growth of TeO 2 under forced convection in melt
d[mm]
367
c
14 13 12 1
1 9 8 7 6 5
ci
4 3 2 1
~)0
I
I
t
I
0,05
0,10
0,15
0.20
f
0,2 5C~'~ [ (rpmTmin)" tr41
Fig. 3. Relation between rotation rate and diameter of TeO 2 crystal corresponding to inversion of flows in the melt: (a) crucible diameter 40 mm, dependence calculated according to Miyazawa [13]; (b) crucible diameter 40 mm, depth of melt H = 40 ram, dependence calculated according to formula (3); (c) crucible diameter 70 ram, H = 60 ram, dependence as in (b).
method proposed by Brice [12], we calculated the thermal conditions which can cause cracking and cellular growth, especially in the case of bigger TeO 2 crystals. We used in our considerations the results of measurements obtained during the growth of small TeO2 single crystal. For TeO 2 crystals with cross-section larger than 20 × 20 mm 2, the constitutional supercooling begins to play the main role. The temperature conditions, which should be used to protect crystals against cracking, may cause cellular growth. Moreover, during the growth the level of the melt falls down and uncovered crucible walls act as an afterheater. It additionally decreases the temperature gradients and the quality of the crystal radically
decreases. The details of these calculations will be discussed separately in the next paper. To determine the relation between crystal rotation and character of the melt flow, TeO 2 crystals doped with 0.1 wt% TiO 2 were pulled. The shape of the striations formed by the dopant represented the shape of the profile of the interface. This informed us about the character of the dominating convection at every moment of the growth. Besides the initial stage of the growth process, forced convection was observed as being predominant. The inversion from natural to forced flow caused changes of diameter and shape of growing crystal. For example, in [001] TeO 2 crystals the circular cross-section becomes square.
368
T. Lukasiewicz, A. Majchrowski / CZ growth of TeO2 under forced concection in melt
We found that the diameter of the crystal corresponding to the inversion of the flows and the rotation rate are linked by the formula: d =mto -°'5.
(1)
lower ( ~ 2 K c m - 1 ) and the ATH H 3 unit had a distinct influence on the process of inversion of convection flows. Results of calculations, as shown in fig. 3, were compliant with experiments.
Miyazawa [13] gave
m = (4zc-2gB ATR3) °'25,
(2)
where g i_s the earth acceleration, B the thermal expansion coefficient of the melt, AT the radial temperature difference in the melt and R the radius of the crucible. The real conditions in our experiments are described by the following formula:
m=[aTr-2gB(ATR3+ATH H3)] 0'25
(3)
where ATH is the vertical difference of the temperature and H the depth of the melt. This equation was obtained by introducing the vertical difference of the temperature into Miyazawa's formula (2) (see ref. [13]). Miyazawa studied flows in crucibles heated by RF. Under such conditions, the radial temperature gradient is very high and the ATnH 3 unit can be neglected. In our experiments with resistance heating, the radial temperature gradient was much
References [1] G. Arlt, J. Liebertz and H. Schweppe, 8th Intern. Congr. on Acoustics, H-989, 1968. [2] N. Uchida and N. Ohmachi, J. Appl. Phys. 40 (1969) 4692. [3] N. Uchida, Phys. Rev. 4 (1971) 3736. [4] Y. Ohmachi and N. Uchida, Rev. Elec. Commun. Lab. 20 (1971) 529. [5] J. Liebertz, Kristall Tech. 4 (1969) 221. [6] S. Miyazawa and S. Kondo, Mater. Res. Bull. 8 (1973) 1215. [7] J.G. Grabmaier, R.D. Plattner and M. Schieber, J. Crystal Growth 20 (1973) 82. [8] Milewski, MSc Degree, WAT, Warsaw (1982). [9] I. Merta, W. Milewski and M. Szustakowski, Elektryczne i Akustyczne Metody Badafi Material6w i Struktur Biologicznych (Warsaw, 1984) p. 121. [10] I. Merta, private communication. [11} J. Koztowski, in: Proc. 4th Spring School on AcoustoOptics and Applications, Gdafisk, 1988, p. 327. [12] J.C. Brice, J. Crystal Growth 2 (1968) 395. [13] S. Miyazawa, J. Crystal Growth 49 (1980) 515.