Solid-solution LiTaxNb1−xO3 single crystal growth by Czochralski and edge-defined film-fed growth technique

Journal of Crystal Growth 35 (1976) 127—132 © North-Holland Publishing Company

SOLID-SOLUTION LiTa~Nbi x03 SINGLE CRYSTAL GROWTH BY CZOCHRALSKI AND EDGE-DEFINED FILM-FED GROWTH TECHNIQUE T. FUKUDA and H. HIRANO Toshiba Research and Derelopmcnt Center, Tokyo Shibaura Electric Co., Ltd., 1 Komukai-Toshibacho, Kawasaki, Japan Received 22 January 1976; revised manuscript received 19 April 1976

LiTaxNbl_x03 (LTN) solid-solution single crystals were grown by the Czochralski (CZ) technique and the edge-defined film-fed growth (EFG) technique. For the CZ technique, the crystal quality was easily impaired by marked cellular growth due to constitutional supercooling as x increased in the melt, except in the vicinity of LiTaO 3. The problem was minimized by high thermal gradient (> 300°C/cm) and low pulling rate (<1 mm/h), but striation occurred. For the EFG technique, pronounced inhomogeneity due to growth cell was not observed but microvoids existed. The crystal composition of the EFG grown crystal was almost constant, irrespective of weight fraction solidified, g, whereas that of CZ grown crystal varied largely withg. Growth conditions for homogeneous LTN crystal are discussed.

1. Introduction

2. Experimental

LiNbO3 and LiTaO3 single crystals have attracted much attention for their widespread utilization in nonlinear optic, piezoelectric and acoustic surface device applications and have been intensively studied. Recently, a Ta-doped LiNbO3 single crystal was found to be particularly attractive as an acoustic surface filter [1] and much interest has developed in the solid-solution LiTa~Nbi~~O3 (designated as LTN) single crystal. However, no paper on LTN single crystal growth has been published, LiNbO3—LiTaO3 [2,3] system phase equilibrium shows that a complete solid-solution exists in this system, and that variations of crystal composition and constitutional supercooling occur easily during growth, due to the wide separation between liquidus and solidus lines which make the growth of homogeneous crystals difficult. This paper is concerned with the growth and some characteristics of LTN single crystals. The crystals are grown by the Czochralski (CZ) and edge-defined, film-fed growth (EFG) techniques. The growth conditions of homogeneous single crystal are discussed.

Single LTN crystals were grown by the conventional CZ and EFG techniques. Starting materials were prepared by mixing high purity Li2CO3, Nb205 and Ta205 (99.999%) from ~~hns~n Matthey Co., in a congruent melt ratio. Though congruent melt compositions of LiNbO3 and LiTaO3 are reported to be 48.5 [4] and 48.6 [5] mole% Li20 and 48.75 [6] and 49.0 [71 rnole% Li20, respectively, that of LTN is not known. So the 48.6 mole% Li20 is used as an approximate value in the present LTN study. In the CZ technique, the mixture was placed in a 50 mm in diameter and 50 mm in height Pt or Ir crucible and melted by rf heating. LTN single crystals were initially grown on a LiTaO3 crystal. Oriented crystals cut from the initial crystals were used as seeds for later crystal growth runs. Single crystals were pulled along X ( a) axis, Y-axis and Z ( c) axis. The melt composition, pulling rate, crystal rotation rate and thermal gradient above the melt surface are listed in table 1. The thermal gradient was adjusted by the position of the Pt-reflector and alumina heat shield. In the EFG technique, LTN plate single crys127

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T. Eukiida, 11. Ilirano

/ Solid-solution LiTaxNbi

03 single crystalgro wth

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Table 1 LTN single crystal growth conditions CZ technique Melt composition,

~:~05~

0.1, 0.2,

EFG technique 0.05, 0.2

Thermal gradient (°C/cin)

30, 50, 100, 20t), 300

60

Pulling rate (mm/h)

0.5,1,2,5,9

42

Seed rotation (rpm)

20, 40, 50, 100, 150

Pulling axis

x. y, z,

__________________________________________________ ___

_________

z

tals were grown with melt compositions x = 0.05 and x = 0.20 by the apparatus as used in the growth of LiNbO 3 plate crystal [8]. The pulling axis and speed were Z-axis and 0.7 mm/mm, respectively. The thermal gradient above the melt film surface was 60°C/cni. The crystal composition was determined by X-ray fluorescent analysis. The density was measured pycnometrically. X-ray diffraction measurements were performed by means of a diffmactometer, using CuKa madiation and Si as an internal standard. Lattice constants were calculated using reflection in the 20 = 20°to 60° range.

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3. Results and discussion 3.]. Czochralskigrowth

lig. 1. lypical as-grown LTN single crystals. Melt composition x = 0.05 and thermal gradients: 30°C/cm(a), 100°C/cm

LTN single crystals ranging in size to 10 20 mm in diameter and 50 mm in length could be grown at the melt compositions used. The crystal quality was very sensitive to growth conditions, especially to a thermal gradient above the melt surface and pulling rate. Typical as-grown LTN crystals are shown in fig. 1. The growth conditions are as follows: melt composition, x = 0.05; pulling axis, x (= a) axis; pulling rate, 5 mm/h; rotation rate, 20 rpm; thermal gradient, 30, 100, 300°C/cm. Non-circular crystal shapes and pronounced microsegregation, as seen in fig. la, where observed in the crystal under a low thermal gradient, under which crystals of high quality could be grown in the case of end components, LiNbO3 and LiTaO3. These problems

(b), 300 C/cm (c).

were minimiLed as the thermal gradient was increased (see figs. lb abd ic, or as pulling rate was decreased. The effect of rotation rate on these problems was not distinct in the present experiment. Fig. 2 shows typical growth cells observed in the bottom surfaces of c-axis pulled and a-axis pulled crystals. The shape of growth cell of c-axis pulled crystal is a trigonal hillocks, correspond with the {102} plane, which is nearly the same as that of Rh-doped LiNbO3 [9]. The cell shape observed in the a-axis pulled crystal is a V-shape groove elongated to the direction inclined about 43°to the c-axis, which is explained by the trace of f 102/ plane on the a-plane. For a crystal showing a growth cell in

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[10], solid-solution which is known to be a typical tantalate~niobate crystal. Table 2 shows the relation between the growth conditions of the temperature gradient, and evidence

_______________________________________________________

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of a growth cell observed in the growth of an LTN crystal with melt composition x = 0.05. The LTN crystals (“° 15 mm in diameter) were grown with pulhng speed 1 --~2mm/h. From table I, it is obvious that high thermal gradients are required to avoid growth cell, which are easily supposed to be attributed to cellular growth due to constitutional supercooling. A higher thermal gradient ((4 allowed a faster growth rate (F), but, in practice, increased

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of c—axis pulled cryst.ils tat and a-axis pulled c rs ~t.ik i b I. Table 2

the bottom surface, an inhomogeneous birefringence pattern corresponding to the cell pattern was distinctly revealed, when the crystal was viewed under erossed nicols (see fig. 3). The inhomogeneous pattern was eliminated with the growth cell by increasing the thermal gradient or decreasing the pulling rate in the growth. However, striation appeared in place of it. The striation appeared like that observed often in LiNbO 3 grown from a stoichiometric melt, and differed from the lamellar structure observed in KTN

The relation betsveen growth condition and growth cell ——-—-

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Thermal gradient (°C/cm) Pulling rate (mm/h) Growrh cell -

50

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Yes Yes Yes No

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T. Fukuda, II. Iuirano /Solid-solution LiTaxNb]x03 single crystal gro wth

furnace were to grow under rather high thermal gradient, about 200°C/cm and with slower pulling rate, less than 1 mm/h. As the LiTaO3 concentration x increased in melt, a higher G/V value was required to avoid the cellular growth. A higher thermal gradient (>400°C/cm) and lower pulling speed (<0.5 mm/h) minimized the cellular growth, but striation and cracking during cooling were induced. However, when the melt composition x was in the vicinity of LiTaO3, an LTN crystal of good quality was easily obtained, 3.2. EFG growth LTN plate single crystals 15 X 40 X 1 mm in size were grown from both melt compositions x = 0.05

and x = 0.20. The crystal quality of an LTN single crystal (x = 0.05) was comparable with that of a LiNbO 3 plate single crystal, which had been described elsewhere [8]. Fig. 4 shows the plate plane and a cross-section of the LTN (x = 0.05) plate crystal viewed in transmission with polarized light. The LTN plate crystal contained some imperfections such as bubble voids, microcracks and a sub-grain boundary, but no pronounced growth cells causing optical inhomogeneity and microsegragation, as was seen in the CZ grown LTN crystals, were found. For the plate crystal grown from the composition x = 0.20, the number of such imperfections increased a little under the same condition as that of LiNbO3. Bubble voids may be brought about by the cellular growth, the same as in the case of A1203 [11]. No attempt was made to achieve an optimum condition to avoid the voids. 3.3. Crystal composition and lattice constants Table 3 gives crystal composition of CZ grown LTN single crystals under the following growth conditions: melt composition x = 0.05; pulling axis, a axis; pulling rate, 1 mm/h; and rotation rate is 50 rpm. In table 3, g menas the weight fraction solidified versus initial melt. Crystal composition C~shows the values obtained from X-ray fluorescence analysis and c’~ shows the extrapolated values from the LiNbO3-LiTaO3 density line based on the measured LTN crystal density. The C~is in good agreement withCv, as seen in table 3. Table 3 mdi-

~ ________

________ ________ ______

~

_______________

__________

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Fig. 4. MicrosLopic observation of LTN plate crystal gro\sn by the FFG technique: plane perpendi ular to growth axis

(a) and plane p~tralleIto growth axis (b).

Table 3

LTN single crystal compositional variation (melt composition x = 0.5) Sample No. Weight

g

Crystal composition

LTN-30-5

0.0052 0.034 0.042 0.069 0.076 0.079 0 116 020

0.15* 0.132* 0.132* 0.12* 0.12* 0.12* 0 105* 0055*

LTN-30-6 LTN-30-7

2.76 18.04 22.37 36.63 40.13 41.81 61 0 ~

0.1428 0.1332 0.1221

T. Eukuda, H. Hirano / Solid-solution LiTaxiVbj....xOj single crystal growth

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Fig. 6. Lattice constants versus crystal composition of I,TN single crystal.

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3

eates that the composition of the LTN crystal, grown with melt composition x = 0.05, is 0.15 at an initial part (g 0.005) and varies markedly down to 0.055 at g = 0.20. The variation of composition in the typical crystal (20 X 40 mm in size, 40 g) over the length was 0.03, which was equivalent to about 18°Cvariation of the Curie temperature. In fig. 5, the compositions of EFG and CZ grown crystals from melt composition x = 0.05 are plotted against g. It was found that the composition of an EFG grown crystal was almost constant, irrespective of g, and nearly the same as a melt composition, whereas that of a CZ grown crystal varied withg and also varied depending on growth condition, such as pulling rate. For the conservative crystal growth, the crystal composition C~can be given by the following equations [131: —

I

MoIe% LiTaO

Fig. 5. Compositions of the EFG and CZ grown crystals from melt composition x = 0.05 plotted against g.

x

I

(2)

0

where C0 = initial liquid composition, ke = effective distribution coefficient, k0 = equilibrium distribution eoeffieient,D = liquid diffusion coefficient, 6 = dif-

fusion boundary thickness at rotating crystal interface, V = growth rate. These equations indicate that C~varies withg, depending on ke, and that it is necessary to make ke approach unity in order to avoid the compositional variation. For the CZ technique, it may be possible to satisfy the requirement by growing the crystal with very high pulling rate, as is known from eq. (2). But, in practice the crystal quality would be exceedingly impaired. Therefore, compositional variation is inevitable for the conservative CZ grown LTN crysal. As a growth condition for minimizing the compositional variation within the crystal, consideration was paid to growing to a crystal makingg as small as possible, with the use of a large crucible. For the EFG technique, ke was experimentally shown to be nearly unity, as is seen in fig. 6. The result is in good agreement with that proved theoretically by Swartz et al. [14]. Therefore, the problem of compositional variation for the LTN would be eltminated by ustng the FFG technique. Fig. 6 shows the lattice constants versus crystal composition of LTN single crystal. The lattice constant c decreases and a increases slightly with composition, respectively. These data are in good agreement with Vegard’s law. The lattice constants of LTN studied in

132

T. Fukuda, II. Hirano / Solid-solution LiTaxNbj_x0

sintered powder have been reported by Shapiro et al. [2], respectively. The present data are in agreement with those studied with sintered powder by Peterson et al. and Shapiro et a]., though little differences between them, apparently be due to the difference in sample preparation, existed.

3 single crystalgrowth

References [1]

K. Yamanouchi and K. Shibayama, Report of the 1964 Autumn Meeting, the Acoustic Society of Japan, p. 371.

121 G.E. Bridenbough and P. Green, J. Chern. Phys. Peterson, 46 (1967)P.M. 4009. 131 G.E. Peterson, JR. Carruthers and A. Carnevale, J. Chem. Phys. 53 (1974) 2436.

141 P. Lerner, C. Legras and J.P. Dumas, J. Crystal Growth 4. Summary

3~4(1968) 231.

151 JR. Carruthers, G.E. Peterson and M. Grasso, J. Appl. Phys. 42(1971)1846.

LTN single crystals were grown by the CZ and EFG techniques, and the growth conditions for homogeneous crystals were discussed. The main results are as follows: (1) For the CZ technique, the quality of LTN was likely to be impaired by marked cellular growth due to constitutional supercooling. The problem was mmimized by a high thermal gradient above the melt surface and a slow pulling rate. (2) For the EFG3technique, an be LTN plate Bubble crystal in size could grown. 15 X 40 X 1 mm voids, which might be caused by cellular growth, were observed, but a pronounced growth cell, such as observed in CZ crystals, was not observed. (3) The crystal composition of the EFG grown LTN crystal was almost constant irrespective of g, while that of CZ grown LTN varied with increasing g, depending on growth conditions.

Acknowledgement The authors would like to thank Mr. H. Hirao for the X-ray fluorescent analysis and Messrs. T. Ito and S. Matsumura for their help in sample preparations.

[6] S. Miyazawa and H. Iwasaki, J. Crystal Growth 10 (1971) 276. 171 R.L. Barns and J. Carruthers, J. Appl. Cryst. 3(1970) 395. 18] T. Fukuda and H. Hirano, Mater. Res. Bull. 10 (1975) 801. [9] S. Kondo, S. Miyazawa and 1-I. lwasaki, J. Crystal Growth 26 (1974) 323.

1101 S.A. Lavy and R. Gashler, Mater. Res. Bull., 3 (1968) 417. [11] B. Chalmers, HE. Labelle, Jr. and AL. Mlavsky, Mater. Res. Bull. 6 (1971) 681. 1121 Rigerman, Z.I. Shapio,Izv. S.A.Akad. Fcdulov, and(1965) L.G. Nauk Yu.N. SSSR.Venevstev Ser. Fiz. 29 1047. [13] M. Zief and W.R. Wilcox, Fractional Solidification, (Dekker, New York, 1967) p.133. 1141 J.C. Swartz, T. Surek Mater. 4 (1975) 255. and B. Chalmers, J. Electron.