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Growth structure observations in Czochralski-grown barium-lithium-niobate single crystals
Journal of Crystal Gro wili 26 (1974) 311-313 North-I-to/land Pub/ishinh Co.
LETTERS TO THE EDITORS GROWTH STRUCTURE OBSERVATiONS IN CZOCHRALSKI-GROWN BARIUMLITHIUM-NIOBATE SINGLE CRYSTALS H. MATTHES. A. MARSHALL. M. GAUNTLETT and J. HESSE A EG—Telefunken Forschungsinstitut, Frankfurt/M., Germany Received 4 June 1974; revised manuscript received 22 October 1974 Inhomogeneities in Czochralski grown single crystals of Ba
2 LiNb~O15are analysed and methods to over-
come them shown.
Single crystals of the composition Ba2LiNb5O15areby virtue of their favourable optical and non-linear optical properties of large importance for use as 1). a frequency Growth doubler of 1.06 jim YAG laser radiation details for obtaining large crack free single crystals of this material by the Czochralski method have been reported2) as have measurements of transmission, refractive indices and non-linear optical properties3). However, these optical measurements have revealed diffraction centres and inhomogeneities in the crystals which are detrimental in the practical applications. These difficulties are investigated here and solutions to overcome them discussed. The crystals, with growth parameters according to ref. 2 were orientated, cut on an annular diamond saw using a low rotation rate (100 rpm), polished with diamond paste to optical quality and light microscopically investigated. It was possible to detect different characteristic inhomogeneities according to the orientation of selected sample cuts with respect to the growth direction: By cuts parallel to the growth direction in both a and c axis grown crystals, a striation layer formation perpendicular to the growth axis was observed. An example of this inhomogeneity structure in an Unetched c grown sample in transmitted light is shown in fig. 1. The striations possess an equidistant spacing which became greater when the growth rate was increased or the rotation rate decreased during growth. The growth structures were also etchable on c faces
(HF (48°z~), ratio 2:1 at a temperature of 110°Cfor an etch time of 2 mm. By cuts perpendicular to the growth direction, Bya second characteristic structure type was observed.
with a mixture of the concentrated acids HNO3 (65°~),
perpendicular to the growth direction.
311
only polishing it was possible to detect in transmitted light a cell like pattern especially dense in the centre of the crystal. An example is shown in fig. 2 ofan unetched ________________________________
______________________________________________ ______________________________________________ -
___________________________________________
~ Fig. I.
.
.
.
.
Growth striations in transmitted light, Ba 2LiNb5O1~
312
H. MATTHES. A. MARSHALL,
M. GAU\I
~
LI TT AND J. HESSE
ir~.
;ij
~
~
~
.
4
t
Crysthi centre Fig. 2.
J~m~
¶
Cystal edge Cell clusters with different densities in the crystal cross-section.
disc in transmitted light in the direction of the growth axis. These observed inhomogeneities suggest that either the melt starting composition1) or/and the growth conditions2) were not optimally selected, The appearance of striation structure is well known in crystals grown from doped melts4 8) These structures are there due to the dependence of the distribution coefficient on the growth rate, that is, changes in the growth rate give rise to changes in the doping concentration. In the case of Ba 2LiNb5O15, an analogous mechanism should be assumed. By emission flame
(
4
spectroscopy and X-ray fluorescence analysis it was observed, that the composition of the grown crystals differed from that of the residual melts. Temperature changes at the solid liquid interface produce under such conditions changes in the composition of the growing crystal. Periodic temperature oscillations can be produced by the rotation of the crystal itself as a result of themisalignmentbetweenthethermalaxisofthegrowth chamber and the symmetry axis of the crystal9). This in practice is neatly always present. The observed cell structures can also be explained
~0
S 25pm Fig. 3. Suppression of striations by reduction of rotation rate.
GROWTH STRUCTURE OBSERVATIONS
from the difference in the composition of the grown crystal and the residual melt. An enrichment of a particular melt component at the interface causes constitutional supercooling. This gives rise to local growth acceleration observed in the crystals as pipe-like structures in the growth direction and cells perpendicularly. Because these discussed inhomogeneities seriously prejudice the use of this material as a frequency doubler, possibilities for obtaining better results by the growth process must be found. Therefore, on the basis of the interpretation given here two solutions are offered: (I) Adequate changes of the experimental conditions to decrease periodic temperature oscillations. (2) Exact investigation of the constitutional conditions to select a more favourable starting composition. By growth of doped single crystals, it was possible to overcome striations by reducing the rotation rate’ O)~ It has to be proved whether this change in the growth parameter could be applied successfully here. Fig. 3 shows the cross-section of a crystal cut parallel to the growth direction by which the rotation was reduced from 10 rev/mm to 0.5 rev/hour. The figure confirms that with the lower rotation rate no periodic inhomogeneities in the form of striations were formed. However, the cell structures were not affected by this alternation. To eleminate these also, it was necessary to select, according to suggestion (2) above, other melt compositions. For that the phase diagrams between the
313
compounds Li3NnO4—BaNb2O6 and LiNbO3—BaNb2O6 in the ternary system BaO—Li20—Nb2O5 were investigated’ 1)~ According to these constitutional studies a melt composition containing 80 mol °~ BaNb2O6 in the binary section Li3NbO4—BaNb2O6 is favourable for the growth of crystals without the here described periodic and non-periodic inhomogeneities. This paper is based on work financially supported by the Ministry of Education and Science of the Federal Republic of Germany as part of its technology programm. References I) H. Hirano, H. Takai and S. Koide. Japan. J. Appl. Phys. 9 (1970) 580. 2) H. Matthes, J. Crystal Growth 15 (1972) 157. 3) 0. Bernecker, H. Matthes and A. Marshall, Phys. Status Solidi (a) 17(1973)453. 4) Soc. K. Morizane, 113 (1966)A.51.F. Witt and H. C. Gatos, J. Electrochem. 5) A. F. Witt and H. C. Gatos. J. Electrochem. Soc. 113 (1966) 808. 6) B. K. Jindal, V. V. Karelin and W. H. Tiller, J. Electrochem. Soc. 120(1973) 101. 7) E.Jakeman and D. T. J. Hurle, Rev. Phys. Technol. 3(1972)3. 8) A. F. Witt and H. C. Gatos, in: Semiconductor Silicon (The Electrochemical Society, 1969) p. 146. 9) K. Morizane, A. F. Witt and H. C. Gatos, J. Electrochem. Soc. 114 (1967) 738. 10) C. Brehm, J.-Y. Boniort and P. Margotin, J. Crystal Growth II) 18(1973)191. V. Kramer, H. Matthes and A. Marshall. J. Mater. Sci., to be published.
LETTERS TO THE EDITORS GROWTH STRUCTURE OBSERVATiONS IN CZOCHRALSKI-GROWN BARIUMLITHIUM-NIOBATE SINGLE CRYSTALS H. MATTHES. A. MARSHALL. M. GAUNTLETT and J. HESSE A EG—Telefunken Forschungsinstitut, Frankfurt/M., Germany Received 4 June 1974; revised manuscript received 22 October 1974 Inhomogeneities in Czochralski grown single crystals of Ba
2 LiNb~O15are analysed and methods to over-
come them shown.
Single crystals of the composition Ba2LiNb5O15areby virtue of their favourable optical and non-linear optical properties of large importance for use as 1). a frequency Growth doubler of 1.06 jim YAG laser radiation details for obtaining large crack free single crystals of this material by the Czochralski method have been reported2) as have measurements of transmission, refractive indices and non-linear optical properties3). However, these optical measurements have revealed diffraction centres and inhomogeneities in the crystals which are detrimental in the practical applications. These difficulties are investigated here and solutions to overcome them discussed. The crystals, with growth parameters according to ref. 2 were orientated, cut on an annular diamond saw using a low rotation rate (100 rpm), polished with diamond paste to optical quality and light microscopically investigated. It was possible to detect different characteristic inhomogeneities according to the orientation of selected sample cuts with respect to the growth direction: By cuts parallel to the growth direction in both a and c axis grown crystals, a striation layer formation perpendicular to the growth axis was observed. An example of this inhomogeneity structure in an Unetched c grown sample in transmitted light is shown in fig. 1. The striations possess an equidistant spacing which became greater when the growth rate was increased or the rotation rate decreased during growth. The growth structures were also etchable on c faces
(HF (48°z~), ratio 2:1 at a temperature of 110°Cfor an etch time of 2 mm. By cuts perpendicular to the growth direction, Bya second characteristic structure type was observed.
with a mixture of the concentrated acids HNO3 (65°~),
perpendicular to the growth direction.
311
only polishing it was possible to detect in transmitted light a cell like pattern especially dense in the centre of the crystal. An example is shown in fig. 2 ofan unetched ________________________________
______________________________________________ ______________________________________________ -
___________________________________________
~ Fig. I.
.
.
.
.
Growth striations in transmitted light, Ba 2LiNb5O1~
312
H. MATTHES. A. MARSHALL,
M. GAU\I
~
LI TT AND J. HESSE
ir~.
;ij
~
~
~
.
4
t
Crysthi centre Fig. 2.
J~m~
¶
Cystal edge Cell clusters with different densities in the crystal cross-section.
disc in transmitted light in the direction of the growth axis. These observed inhomogeneities suggest that either the melt starting composition1) or/and the growth conditions2) were not optimally selected, The appearance of striation structure is well known in crystals grown from doped melts4 8) These structures are there due to the dependence of the distribution coefficient on the growth rate, that is, changes in the growth rate give rise to changes in the doping concentration. In the case of Ba 2LiNb5O15, an analogous mechanism should be assumed. By emission flame
(
4
spectroscopy and X-ray fluorescence analysis it was observed, that the composition of the grown crystals differed from that of the residual melts. Temperature changes at the solid liquid interface produce under such conditions changes in the composition of the growing crystal. Periodic temperature oscillations can be produced by the rotation of the crystal itself as a result of themisalignmentbetweenthethermalaxisofthegrowth chamber and the symmetry axis of the crystal9). This in practice is neatly always present. The observed cell structures can also be explained
~0
S 25pm Fig. 3. Suppression of striations by reduction of rotation rate.
GROWTH STRUCTURE OBSERVATIONS
from the difference in the composition of the grown crystal and the residual melt. An enrichment of a particular melt component at the interface causes constitutional supercooling. This gives rise to local growth acceleration observed in the crystals as pipe-like structures in the growth direction and cells perpendicularly. Because these discussed inhomogeneities seriously prejudice the use of this material as a frequency doubler, possibilities for obtaining better results by the growth process must be found. Therefore, on the basis of the interpretation given here two solutions are offered: (I) Adequate changes of the experimental conditions to decrease periodic temperature oscillations. (2) Exact investigation of the constitutional conditions to select a more favourable starting composition. By growth of doped single crystals, it was possible to overcome striations by reducing the rotation rate’ O)~ It has to be proved whether this change in the growth parameter could be applied successfully here. Fig. 3 shows the cross-section of a crystal cut parallel to the growth direction by which the rotation was reduced from 10 rev/mm to 0.5 rev/hour. The figure confirms that with the lower rotation rate no periodic inhomogeneities in the form of striations were formed. However, the cell structures were not affected by this alternation. To eleminate these also, it was necessary to select, according to suggestion (2) above, other melt compositions. For that the phase diagrams between the
313
compounds Li3NnO4—BaNb2O6 and LiNbO3—BaNb2O6 in the ternary system BaO—Li20—Nb2O5 were investigated’ 1)~ According to these constitutional studies a melt composition containing 80 mol °~ BaNb2O6 in the binary section Li3NbO4—BaNb2O6 is favourable for the growth of crystals without the here described periodic and non-periodic inhomogeneities. This paper is based on work financially supported by the Ministry of Education and Science of the Federal Republic of Germany as part of its technology programm. References I) H. Hirano, H. Takai and S. Koide. Japan. J. Appl. Phys. 9 (1970) 580. 2) H. Matthes, J. Crystal Growth 15 (1972) 157. 3) 0. Bernecker, H. Matthes and A. Marshall, Phys. Status Solidi (a) 17(1973)453. 4) Soc. K. Morizane, 113 (1966)A.51.F. Witt and H. C. Gatos, J. Electrochem. 5) A. F. Witt and H. C. Gatos. J. Electrochem. Soc. 113 (1966) 808. 6) B. K. Jindal, V. V. Karelin and W. H. Tiller, J. Electrochem. Soc. 120(1973) 101. 7) E.Jakeman and D. T. J. Hurle, Rev. Phys. Technol. 3(1972)3. 8) A. F. Witt and H. C. Gatos, in: Semiconductor Silicon (The Electrochemical Society, 1969) p. 146. 9) K. Morizane, A. F. Witt and H. C. Gatos, J. Electrochem. Soc. 114 (1967) 738. 10) C. Brehm, J.-Y. Boniort and P. Margotin, J. Crystal Growth II) 18(1973)191. V. Kramer, H. Matthes and A. Marshall. J. Mater. Sci., to be published.