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Study of congruent-melting composition of langasite and its effects on crystal quality
Journal of Crystal Growth 219 (2000) 263}268
Study of congruent-melting composition of langasite and its e!ects on crystal quality Shou-Qi Wang, Jiro Harada, Satoshi Uda* Electronics Device R&D Center, Mitsubishi Materials Co. Ltd., 2270-Yokoze, Chichibu, Saitama 368-8503, Japan Received 30 May 2000; accepted 8 June 2000 Communicated by M. Schieber
Abstract Congruent-melting composition of langasite (La Ga SiO ) was studied by a combination of melting-point measure ment, crystal growth and compositional characterizations. The melting points of sintered samples with various compositions within the solid solution range determined by X-ray di!raction (XRD) around the stoichiometric composition were measured by di!erential thermal analysis (DTA). Upon the growth of single crystals from the compositions with the highest melting point, the compositions of various parts of the crystals and residual melts were investigated by X-ray #uorescence spectrum (XFS). The congruent-melting composition was determined to be the composition from which a minimum deviation of composition of both the crystal and the residual melt was obtained. 2000 Elsevier Science B.V. All rights reserved. PACS: 47.20.Ma; 61.72.Ff; 81.10.Fq Keywords: Langasite; Congruent-melting composition; Di!erential thermal analysis; Stoichiometric composition
1. Introduction La Ga SiO (langasite, LGS) single crystal, with a zero temperature coe$cient for the resonant frequency at room temperature and an electromechanical coupling coe$cient which is three times larger than that of quartz, has been considered to be the most promising piezoelectric material for bulk acoustic wave (BAW) and surface acoustic wave (SAW) devices. Its property of slow
* Corresponding author. E-mail address: [email protected] (S. Uda).
acoustic velocity makes it possible to fabricate small-size intermediate frequency SAW "lter in a new mobile communication system, wide-bandcode division multiple access (W-CDMA) [1,2]. However, due to the lack of knowledge on congruent-melting composition, most of the crystals are grown from the stoichiometric composition. As a result, the composition of the crystal shifts not only along the growth direction but also the radial direction leading to the deviation of phase velocity of acoustic wave. Thus, the parameters of the devices, e.g. center frequency of SAW "lters deviates even if they are made of the same crystal and cannot meet the demand of W-CDMA. The most e!ective mass-production method of crystals with
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 6 2 0 - 5
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S.-Q. Wang et al. / Journal of Crystal Growth 219 (2000) 263}268
good homogeneity is to use congruent-melting composition. However, although a few reports are available concerning this problem [3,4], the congruent-melting composition of langasite has not been clari"ed and no industrial-class crystal has ever been grown successfully using the congruentmelting composition. In this paper, we studied the congruent-melting composition of langasite by investigating the correlation of melting-point temperatures with compositions and the compositional shift of crystals and residual melt during crystal growth. First, the solid solution range of langasite was studied by phase identi"cation of sintered samples with compositions around the stoichiometric one using X-ray di!raction (XRD). Then the melting-point temperatures of the samples with compositions within the solid solution range were measured by di!erential thermal analysis (DTA). Finally, based on the composition with the highest melting point, single crystals were grown and the compositional variation in the crystals as well as the residual melt was investigated by X-ray #uorescence spectroscopy (XFS). The congruent-melting composition was determined to be the composition with a minimum shift of both the crystal and the residual melt. The characterization of SAW "lters fabricated on such crystals showed an excellent uniformity of center frequency.
2. Experimental procedure 2.1. Solid solution range of langasite Around the stoichiometric composition of langasite, about 50 di!erent compositions were selected for the investigation of solid solution range. Mixtures of 99.99% pure La O , Ga O and SiO with the selected compositions were pressed at 10 kg f/cm to pellets and sintered at a temperature of 14503C for 5 h. Phases of the sintered samples were identi"ed by XRD and samples which showed coexisting phases were crushed and sintered again until the single phase of langasite was con"rmed or the coexisting phases were proved to be stable. The solid solution range at 14503C was determined by
plotting the compositions of the samples which showed single phase of langasite. 2.2. Measurement of melting points The melting point temperatures of the samples with compositions within the solid solution range were measured by DTA at a heating rate of 53C/min using Pt pans. In order to calibrate the thermocouple of the DTA equipment, Au (with a melting point of 10633C) and Si (with a melting point of 14143C) were used as standard samples. The melting points were plotted against the compositions and the composition with the highest melting point temperature provides the possible congruent-melting composition of langasite. 2.3. Crystal growth and characterizations Single crystals were grown by Czochralski method. Several compositions in the range near the maximum melting point determined by DTA measurement were selected as initial compositions. Pellets of mixed materials were sintered under the conditions described in Section 2.1 and were charged into an Ir crucible. The crystal growth was carried out in an atmosphere of Ar plus 3 vol% O with a radio frequency heating Czoch ralski equipment specially designed for langasite growth. Two types of crystals were grown for di!erent characterizations. One is small sized and the other is industrial-class sized. The small-size crystals with an average diameter of 21 mm and a length of 120 mm were grown with an index of melt solidi"ed of about 80% for composition analysis. Compositions of upper, middle and lower parts of the crystals and the residual melt were characterized by XFS to investigate the variation of the composition during the growth process. The industrial-class crystals with an average diameter of 85 mm and a length of 150 mm were grown and 3 size wafers were fabricated [5]. SAW "lters were made on these wafers and the distribution of the phase velocity of SAW was characterized by measuring the center frequencies of the SAW "lters.
S.-Q. Wang et al. / Journal of Crystal Growth 219 (2000) 263}268
265
3. Results and discussion 3.1. Relationship of melting points with compositions The compositions selected for the investigation of solid solution range and the resulted phases are shown in Fig. 1a. The solid solution range of langasite at the temperature of 14503C was found to be a narrow area around the stoichiometric composition. The variable span of La O component of the solid solution range is much narrower than that of Ga O and SiO . Coexisting phases were detected outside this area. In detail, LaGaO was detected in La O rich area, Ga O in Ga O rich area and La Si O in SiO rich area consistent with the phase relationships in the La O }Ga O }SiO system shown in Fig. 1b [3]. Melting points measured by DTA on the samples with the compositions within the solid solution range are shown in Fig. 2. The maximum value of melting point is about 1505$0.53C, while those of
Fig. 2. Melting points of langasite vs. composition measured by DTA.
the adjacent compositions among our synthesized samples are lower than this value within a range of 53C. By plotting the melting points versus the compositions, a surface with a peak of melting point can be obtained which is considered to approximately be the solidus surface of the quasi-ternary phase diagram. The peak of the melting points is located at a Ga O -rich composition compared with stoichiometric composition of La Ga SiO , in dicating the possible composition of congruent melting. 3.2. Congruent-melting composition
Fig. 1. Solid solution range of langasite. (a) Investigated compositions and the resulted phases. The area illustrated by dashed line indicates the solid solution range of langasite at a temperature of 14503C. (b) Phase relation diagram in the system La O }Ga O }SiO at 13003C after Ref. [3].
Considering the possible errors which occur during the synthesis of the samples and the DTA measurement, it is also necessary to determine the congruent-melting composition by detecting the compositional shift of crystals as well as residual melts during growth of single crystals starting from various compositions. Several compositions in the region of high melting points were selected from which single crystals were grown with an index of melt solidi"ed of about 80% (small-sized crystal in Fig. 3). The compositions of the initial materials, various parts of the crystals along the growth direction and the residual melt were analyzed using XFS and the results are shown in Fig. 4. As can be seen,
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Fig. 3. Photograph of langasite crystals. The small one is for the analysis of compositions and the large one is for the fabrication of 3 size wafers.
Fig. 4. Compositions of initial material, crystals and residual melt. The dashed line represents the compositions holding the formula of A BC D O while changing the net macroscopic distribution of Ga> and Si> in D site.
the compositions of the upper parts of the crystals were located within a shaded area surrounded by the initial compositions. The compositions of the bottoms of the crystals were located slightly outside the shaded area while the compositions of the residual melts were located considerably away from the shaded area. This is consistent with the crystallization process from the o!-congruent melting composition. Thus, it is believed that the congruentmelting composition is located inside the shaded area. In Fig. 4, the absolute values of the initial compositions were plotted directly while the compositions of crystals and residual melts were plotted relatively to the initial composition. The accuracy of the weighing process of each component was controlled to 0.1 g against a total weight of 2800 g at one time which guarantees an accuracy of the initial compositions not worse than $0.01 mol% for each component. Thus, the initial compositions are believed to be accurate enough to discuss the congruent-melting composition. On the other hand, the compositions of the crystals and the residual melts were evaluated by XFS with a resolution of $0.1 mol%. Consequently, it does not
seem to be appropriate to get more accurate composition of congruent melting from such values. However, the compositional shift of the crystal bottom, especially, the residual melt was large enough to draw the line connecting compositions of the initial material, crystal bottom and residual melt. The compositions of the crystal top, initial material, bottom part and residual melt should lie on these extrapolated lines in this order. These lines generally show the so-called `S-typea curve [6] since each of the three major melt constituents, La O , Ga O and SiO changes their concentra tion independently. However, the initial melt which is close enough to the congruent-melting composition should vary its melt constituents in the same manner during growth and the ratio among them becomes nearly constant leading to the straight connecting line. We see in Fig. 4 that the shorter line is straighter than the longer one, which experimentally supported the above discussion. The congruent-melting composition should be found on the extrapolated line from the composition of the crystal top. Thus, the congruent melt should lie in the region existing inside the shaded area where four lines would intersect. As a result, a composition of
S.-Q. Wang et al. / Journal of Crystal Growth 219 (2000) 263}268
La O "30.10$0.05 mol%, Ga O "50.60$ 0.05 mol% and SiO "19.3$0.05 mol% showed an ignorable shift in compositions of both the crystal and the residual melt thus is concluded to be the congruent-melting composition of langasite. In the crystal structure [7,8] of langasite which is described as A BC D O , A site is occupied by La>, B and C sites are occupied by Ga> while D site is randomly occupied by either Ga or Si with net macroscopic distribution of 50% Ga> and 50% Si> for the stoichiometric composition. The dashed line in Fig. 4 represents the compositions holding the formula of A BC D O while chang ing the net macroscopic distribution of Ga> and Si> in D site. It can be seen that this line crosses the shaded area suggesting that the di!erence between the congruent-melting composition and the stoichiometric one mainly derives from the net macroscopic distribution of Ga> and Si> in D site. 3.3. Ewect on crystal qualities At the last stage of the growth process, the composition of the melt could signi"cantly di!er from the initial one and this caused compositional shift along the growth direction in the crystal. It can be seen in Fig. 4 that the composition of the crystal bottom is considerably away from this area. This is di!erent from a theoretical equilibrium in which the composition of the crystal eventually equals the initial composition because the actual growth process is too fast to reach an equilibrium so the melt does not react with previously formed crystals by di!usion processes and cannot change their composition. Thus, the compositional shift becomes more serious when a higher yield is attempted. On the other hand, the large di!erence of the compositions between the melt and the grown crystal also makes the composition in the solid more variable to the growth condition such as temperature gradient near the interface, temperature distribution along the radial, growth rate and melt convection. As a result, growth layers are constructed parallel to the crystal front. Since the industrial-size crystals are grown along the Z-axis while wafers for SAW "lters are sliced in >-50 orientation, the wafers exhibit the discontinuous growth taking the form of
267
striations on wafer surface. Compositional variations due to both the discontinuous growth rate and the incongruency cause large deviation of SAW phase velocity. By using a congruent-melting composition, such problems can be e!ectively solved and this is especially important for the mass production. The compositions used for the small-crystal growth were also attempted for the growth of industrial-size crystal as shown in Fig. 3. Unlike small-size crystals, we faced di$culties in conic formation process using the stoichimetric and La O -rich compositions because polycrystalline phases often occurred when the diameter was over a certain value resulting in cracking. Grains of La Si O were detected in the boundary between the single crystal and the polycrystalline phases and they acted as seeds of the polycrystal. However, by using the composition determined above, such problem no longer occurred. Characterization of the SAW "lters fabricated on the wafers sliced from this crystal showed a deviation of SAW phase velocity of smaller than 100 ppm, much better than such value obtained from the crystals grown from the stoichiometric composition which is normally larger than 300 ppm. This proved that the homogeneity of the crystal is considerably improved by the application of congruent-melting composition.
4. Conclusion Congruent-melting composition of langasite was investigated by a combination of DTA measurement of melting points on sintered samples and XFS analysis of compositions of single crystals and residual melt. The solid solution range of langasite was studied by phase identi"cation of sintered samples with compositions around the stoichiometric one using XRD and the melting-point temperatures of samples with compositions within the solid solution range were measured by DTA. Based on the composition with the highest melting point, single crystals were grown and the composition variation in the crystals as well as the residual melt was investigated by XFS. The congruent-melting composition was determined to be the composition with a minimum shift of both the crystal and the
268
S.-Q. Wang et al. / Journal of Crystal Growth 219 (2000) 263}268
residual melt. The characterization of SAW "lters made of the crystals grown from the congruent-melting composition proved that the application of congruent composition has great e!ect on the improvement of homogeneity of langasite crystals.
References [1] A.N. Gotalskaya, D.I. Drezin, V.V. Bezdelkin, V.N. Stassevich, Proceedings of the IEEE Frequency Symposium, 1993, p. 339.
[2] O. A. Buzanov, A.V. Naumov, V.V. Nechaev, S.N. Knyazev, Proceedings of the IEEE Frequency Symposium, 1996, p. 131. [3] B. Chai, J.J. Lefaucheur, Y.Y. Ji, H. Qiu, Proceedings of the IEEE International Freqency Control Symposium, 1998, p. 748. [4] H. Takeda, K. Shimamura, V.I. Chani, T. Fukuda, J. Crystal Growth 197 (1999) 204. [5] Satoshi Uda, Akihiro Bungo, Chunyun Jian, Jpn. J. Appl. Phys. 38 (1999) 5516. [6] L.S. Darken, Trans. AIME 180 (1949) 430. [7] B.V. Mill, A.V. Butashin, G.G. Khodzhabagyan, E.L. Belokoneva, N.V. Belov, Dokl. Akad. Nauk SSSR 264 (1982) 1385. [8] J.A. Koshinski, International Meeting on Future Trends of Acoustic Wave Device, Tokyo, 1996.
Study of congruent-melting composition of langasite and its e!ects on crystal quality Shou-Qi Wang, Jiro Harada, Satoshi Uda* Electronics Device R&D Center, Mitsubishi Materials Co. Ltd., 2270-Yokoze, Chichibu, Saitama 368-8503, Japan Received 30 May 2000; accepted 8 June 2000 Communicated by M. Schieber
Abstract Congruent-melting composition of langasite (La Ga SiO ) was studied by a combination of melting-point measure ment, crystal growth and compositional characterizations. The melting points of sintered samples with various compositions within the solid solution range determined by X-ray di!raction (XRD) around the stoichiometric composition were measured by di!erential thermal analysis (DTA). Upon the growth of single crystals from the compositions with the highest melting point, the compositions of various parts of the crystals and residual melts were investigated by X-ray #uorescence spectrum (XFS). The congruent-melting composition was determined to be the composition from which a minimum deviation of composition of both the crystal and the residual melt was obtained. 2000 Elsevier Science B.V. All rights reserved. PACS: 47.20.Ma; 61.72.Ff; 81.10.Fq Keywords: Langasite; Congruent-melting composition; Di!erential thermal analysis; Stoichiometric composition
1. Introduction La Ga SiO (langasite, LGS) single crystal, with a zero temperature coe$cient for the resonant frequency at room temperature and an electromechanical coupling coe$cient which is three times larger than that of quartz, has been considered to be the most promising piezoelectric material for bulk acoustic wave (BAW) and surface acoustic wave (SAW) devices. Its property of slow
* Corresponding author. E-mail address: [email protected] (S. Uda).
acoustic velocity makes it possible to fabricate small-size intermediate frequency SAW "lter in a new mobile communication system, wide-bandcode division multiple access (W-CDMA) [1,2]. However, due to the lack of knowledge on congruent-melting composition, most of the crystals are grown from the stoichiometric composition. As a result, the composition of the crystal shifts not only along the growth direction but also the radial direction leading to the deviation of phase velocity of acoustic wave. Thus, the parameters of the devices, e.g. center frequency of SAW "lters deviates even if they are made of the same crystal and cannot meet the demand of W-CDMA. The most e!ective mass-production method of crystals with
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 6 2 0 - 5
264
S.-Q. Wang et al. / Journal of Crystal Growth 219 (2000) 263}268
good homogeneity is to use congruent-melting composition. However, although a few reports are available concerning this problem [3,4], the congruent-melting composition of langasite has not been clari"ed and no industrial-class crystal has ever been grown successfully using the congruentmelting composition. In this paper, we studied the congruent-melting composition of langasite by investigating the correlation of melting-point temperatures with compositions and the compositional shift of crystals and residual melt during crystal growth. First, the solid solution range of langasite was studied by phase identi"cation of sintered samples with compositions around the stoichiometric one using X-ray di!raction (XRD). Then the melting-point temperatures of the samples with compositions within the solid solution range were measured by di!erential thermal analysis (DTA). Finally, based on the composition with the highest melting point, single crystals were grown and the compositional variation in the crystals as well as the residual melt was investigated by X-ray #uorescence spectroscopy (XFS). The congruent-melting composition was determined to be the composition with a minimum shift of both the crystal and the residual melt. The characterization of SAW "lters fabricated on such crystals showed an excellent uniformity of center frequency.
2. Experimental procedure 2.1. Solid solution range of langasite Around the stoichiometric composition of langasite, about 50 di!erent compositions were selected for the investigation of solid solution range. Mixtures of 99.99% pure La O , Ga O and SiO with the selected compositions were pressed at 10 kg f/cm to pellets and sintered at a temperature of 14503C for 5 h. Phases of the sintered samples were identi"ed by XRD and samples which showed coexisting phases were crushed and sintered again until the single phase of langasite was con"rmed or the coexisting phases were proved to be stable. The solid solution range at 14503C was determined by
plotting the compositions of the samples which showed single phase of langasite. 2.2. Measurement of melting points The melting point temperatures of the samples with compositions within the solid solution range were measured by DTA at a heating rate of 53C/min using Pt pans. In order to calibrate the thermocouple of the DTA equipment, Au (with a melting point of 10633C) and Si (with a melting point of 14143C) were used as standard samples. The melting points were plotted against the compositions and the composition with the highest melting point temperature provides the possible congruent-melting composition of langasite. 2.3. Crystal growth and characterizations Single crystals were grown by Czochralski method. Several compositions in the range near the maximum melting point determined by DTA measurement were selected as initial compositions. Pellets of mixed materials were sintered under the conditions described in Section 2.1 and were charged into an Ir crucible. The crystal growth was carried out in an atmosphere of Ar plus 3 vol% O with a radio frequency heating Czoch ralski equipment specially designed for langasite growth. Two types of crystals were grown for di!erent characterizations. One is small sized and the other is industrial-class sized. The small-size crystals with an average diameter of 21 mm and a length of 120 mm were grown with an index of melt solidi"ed of about 80% for composition analysis. Compositions of upper, middle and lower parts of the crystals and the residual melt were characterized by XFS to investigate the variation of the composition during the growth process. The industrial-class crystals with an average diameter of 85 mm and a length of 150 mm were grown and 3 size wafers were fabricated [5]. SAW "lters were made on these wafers and the distribution of the phase velocity of SAW was characterized by measuring the center frequencies of the SAW "lters.
S.-Q. Wang et al. / Journal of Crystal Growth 219 (2000) 263}268
265
3. Results and discussion 3.1. Relationship of melting points with compositions The compositions selected for the investigation of solid solution range and the resulted phases are shown in Fig. 1a. The solid solution range of langasite at the temperature of 14503C was found to be a narrow area around the stoichiometric composition. The variable span of La O component of the solid solution range is much narrower than that of Ga O and SiO . Coexisting phases were detected outside this area. In detail, LaGaO was detected in La O rich area, Ga O in Ga O rich area and La Si O in SiO rich area consistent with the phase relationships in the La O }Ga O }SiO system shown in Fig. 1b [3]. Melting points measured by DTA on the samples with the compositions within the solid solution range are shown in Fig. 2. The maximum value of melting point is about 1505$0.53C, while those of
Fig. 2. Melting points of langasite vs. composition measured by DTA.
the adjacent compositions among our synthesized samples are lower than this value within a range of 53C. By plotting the melting points versus the compositions, a surface with a peak of melting point can be obtained which is considered to approximately be the solidus surface of the quasi-ternary phase diagram. The peak of the melting points is located at a Ga O -rich composition compared with stoichiometric composition of La Ga SiO , in dicating the possible composition of congruent melting. 3.2. Congruent-melting composition
Fig. 1. Solid solution range of langasite. (a) Investigated compositions and the resulted phases. The area illustrated by dashed line indicates the solid solution range of langasite at a temperature of 14503C. (b) Phase relation diagram in the system La O }Ga O }SiO at 13003C after Ref. [3].
Considering the possible errors which occur during the synthesis of the samples and the DTA measurement, it is also necessary to determine the congruent-melting composition by detecting the compositional shift of crystals as well as residual melts during growth of single crystals starting from various compositions. Several compositions in the region of high melting points were selected from which single crystals were grown with an index of melt solidi"ed of about 80% (small-sized crystal in Fig. 3). The compositions of the initial materials, various parts of the crystals along the growth direction and the residual melt were analyzed using XFS and the results are shown in Fig. 4. As can be seen,
266
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Fig. 3. Photograph of langasite crystals. The small one is for the analysis of compositions and the large one is for the fabrication of 3 size wafers.
Fig. 4. Compositions of initial material, crystals and residual melt. The dashed line represents the compositions holding the formula of A BC D O while changing the net macroscopic distribution of Ga> and Si> in D site.
the compositions of the upper parts of the crystals were located within a shaded area surrounded by the initial compositions. The compositions of the bottoms of the crystals were located slightly outside the shaded area while the compositions of the residual melts were located considerably away from the shaded area. This is consistent with the crystallization process from the o!-congruent melting composition. Thus, it is believed that the congruentmelting composition is located inside the shaded area. In Fig. 4, the absolute values of the initial compositions were plotted directly while the compositions of crystals and residual melts were plotted relatively to the initial composition. The accuracy of the weighing process of each component was controlled to 0.1 g against a total weight of 2800 g at one time which guarantees an accuracy of the initial compositions not worse than $0.01 mol% for each component. Thus, the initial compositions are believed to be accurate enough to discuss the congruent-melting composition. On the other hand, the compositions of the crystals and the residual melts were evaluated by XFS with a resolution of $0.1 mol%. Consequently, it does not
seem to be appropriate to get more accurate composition of congruent melting from such values. However, the compositional shift of the crystal bottom, especially, the residual melt was large enough to draw the line connecting compositions of the initial material, crystal bottom and residual melt. The compositions of the crystal top, initial material, bottom part and residual melt should lie on these extrapolated lines in this order. These lines generally show the so-called `S-typea curve [6] since each of the three major melt constituents, La O , Ga O and SiO changes their concentra tion independently. However, the initial melt which is close enough to the congruent-melting composition should vary its melt constituents in the same manner during growth and the ratio among them becomes nearly constant leading to the straight connecting line. We see in Fig. 4 that the shorter line is straighter than the longer one, which experimentally supported the above discussion. The congruent-melting composition should be found on the extrapolated line from the composition of the crystal top. Thus, the congruent melt should lie in the region existing inside the shaded area where four lines would intersect. As a result, a composition of
S.-Q. Wang et al. / Journal of Crystal Growth 219 (2000) 263}268
La O "30.10$0.05 mol%, Ga O "50.60$ 0.05 mol% and SiO "19.3$0.05 mol% showed an ignorable shift in compositions of both the crystal and the residual melt thus is concluded to be the congruent-melting composition of langasite. In the crystal structure [7,8] of langasite which is described as A BC D O , A site is occupied by La>, B and C sites are occupied by Ga> while D site is randomly occupied by either Ga or Si with net macroscopic distribution of 50% Ga> and 50% Si> for the stoichiometric composition. The dashed line in Fig. 4 represents the compositions holding the formula of A BC D O while chang ing the net macroscopic distribution of Ga> and Si> in D site. It can be seen that this line crosses the shaded area suggesting that the di!erence between the congruent-melting composition and the stoichiometric one mainly derives from the net macroscopic distribution of Ga> and Si> in D site. 3.3. Ewect on crystal qualities At the last stage of the growth process, the composition of the melt could signi"cantly di!er from the initial one and this caused compositional shift along the growth direction in the crystal. It can be seen in Fig. 4 that the composition of the crystal bottom is considerably away from this area. This is di!erent from a theoretical equilibrium in which the composition of the crystal eventually equals the initial composition because the actual growth process is too fast to reach an equilibrium so the melt does not react with previously formed crystals by di!usion processes and cannot change their composition. Thus, the compositional shift becomes more serious when a higher yield is attempted. On the other hand, the large di!erence of the compositions between the melt and the grown crystal also makes the composition in the solid more variable to the growth condition such as temperature gradient near the interface, temperature distribution along the radial, growth rate and melt convection. As a result, growth layers are constructed parallel to the crystal front. Since the industrial-size crystals are grown along the Z-axis while wafers for SAW "lters are sliced in >-50 orientation, the wafers exhibit the discontinuous growth taking the form of
267
striations on wafer surface. Compositional variations due to both the discontinuous growth rate and the incongruency cause large deviation of SAW phase velocity. By using a congruent-melting composition, such problems can be e!ectively solved and this is especially important for the mass production. The compositions used for the small-crystal growth were also attempted for the growth of industrial-size crystal as shown in Fig. 3. Unlike small-size crystals, we faced di$culties in conic formation process using the stoichimetric and La O -rich compositions because polycrystalline phases often occurred when the diameter was over a certain value resulting in cracking. Grains of La Si O were detected in the boundary between the single crystal and the polycrystalline phases and they acted as seeds of the polycrystal. However, by using the composition determined above, such problem no longer occurred. Characterization of the SAW "lters fabricated on the wafers sliced from this crystal showed a deviation of SAW phase velocity of smaller than 100 ppm, much better than such value obtained from the crystals grown from the stoichiometric composition which is normally larger than 300 ppm. This proved that the homogeneity of the crystal is considerably improved by the application of congruent-melting composition.
4. Conclusion Congruent-melting composition of langasite was investigated by a combination of DTA measurement of melting points on sintered samples and XFS analysis of compositions of single crystals and residual melt. The solid solution range of langasite was studied by phase identi"cation of sintered samples with compositions around the stoichiometric one using XRD and the melting-point temperatures of samples with compositions within the solid solution range were measured by DTA. Based on the composition with the highest melting point, single crystals were grown and the composition variation in the crystals as well as the residual melt was investigated by XFS. The congruent-melting composition was determined to be the composition with a minimum shift of both the crystal and the
268
S.-Q. Wang et al. / Journal of Crystal Growth 219 (2000) 263}268
residual melt. The characterization of SAW "lters made of the crystals grown from the congruent-melting composition proved that the application of congruent composition has great e!ect on the improvement of homogeneity of langasite crystals.
References [1] A.N. Gotalskaya, D.I. Drezin, V.V. Bezdelkin, V.N. Stassevich, Proceedings of the IEEE Frequency Symposium, 1993, p. 339.
[2] O. A. Buzanov, A.V. Naumov, V.V. Nechaev, S.N. Knyazev, Proceedings of the IEEE Frequency Symposium, 1996, p. 131. [3] B. Chai, J.J. Lefaucheur, Y.Y. Ji, H. Qiu, Proceedings of the IEEE International Freqency Control Symposium, 1998, p. 748. [4] H. Takeda, K. Shimamura, V.I. Chani, T. Fukuda, J. Crystal Growth 197 (1999) 204. [5] Satoshi Uda, Akihiro Bungo, Chunyun Jian, Jpn. J. Appl. Phys. 38 (1999) 5516. [6] L.S. Darken, Trans. AIME 180 (1949) 430. [7] B.V. Mill, A.V. Butashin, G.G. Khodzhabagyan, E.L. Belokoneva, N.V. Belov, Dokl. Akad. Nauk SSSR 264 (1982) 1385. [8] J.A. Koshinski, International Meeting on Future Trends of Acoustic Wave Device, Tokyo, 1996.