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Floating zone growth of monochromator grade crystals of YB66
Journal of Crystal Growth 128 (1993)429-434 North-Holland
j........ C R Y S T A L GROW
Floating zone growth of monochromator grade crystals of Yutaka Kamimura and Joe Wong c
a,
Takaho Tanaka
a
T H
YB66
Shigeki O t a n i a, Y o s h i o Ishizawa a, Z o f i a U. R e k b
a National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305, Japan b Stanford Synchrotron Radiation Laboratory, Stanford, California 94309, USA c Lawrence Livermore National Laboratory, Livermore, California 94551, USA
Monochromator grade crystals of YB66 have been grown by the indirect heating floating zone (IHFZ) method. For growth at the congruent composition of [B]/[Y] = 62, some high quality crystals have been obtained by keeping a convex growth interface through the crystal using a minimized heating power and downwards drive. Reproducible growth of monochromator grade crystals has been achieved by growing crystals under an incongruent condition where the compositions of the feed rod and the zone are about [B]/[Y] = 56 and 40, respectively.
1. Introduction
2. Experimental procedure
YB66 is known as a peculiar boron compound because of its complicated crystal structure [1] and amorphous like behavior in phonon related properties [2]. Because of its large d-value (d = 0.586 nm for (400) reflection) and the absence of intrinsic absorption, YB66 has recently been selected as a unique candidate for a soft X-ray monochromator in the 1-2 keV region for synchrotron radiation applications [3]. Our investigation on the growth of monochromator grade crystals of YB66 has continued, using the indirect heating floating zone (IHFZ) method [4]. In a previous paper we reported deterioration in quality due to a concave growth interface, although high quality can be obtained near the seed end by seeding, double zone passes, and upwards drive. We also pointed out that a high quality crystal can be obtained by controlling the interface shape to be convex. In this paper we report recent advances in the crystal growth of YB66 , including growth with a convex growth interface at the congruent composition and growth under incongruent conditions for the purpose of reproducibile growth of high quality YB66 single crystals.
The raw powder of YB66 was synthesized from amorphous boron and yttrium oxide by a borothermal reduction method. In previous work [4], the YB66 crystals included impurities of C, Fe and A1. The C impurity came both from raw boron and from a carbon susceptor in the reaction furnace. The Fe impurity also came from the raw boron. In order to reduce the C impurity, the carbon susceptor was changed to a composite susceptor consisting of TiB 2 and BN, and the boron source was changed to Callery Inc. (USA). The latter was also effective in diminishing the Fe impurity. The AI contamination was introduced during the pulverization process with the use of an alumina mortar, which was now substituted with a stainless steel ball mill. The stainless steel contamination was leached out by dilute HC1 solution and decanted. A 180 mm long feed rod with a diameter of 12 mm was prepared by sintering after a cold isostatic press treatment. The crystals of YB66 have been grown by the I H F Z method, shown in fig. 1, where the molten zone is heated by radiation from an inductively heated tungsten ring situated between a RF work coil and the molten zone. The details of the
0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
430
Y. Kamimura et al. / FZ growth of monochromator grade crystals
BN
Molten z o n e Fig. 1. Schematic diagram of the indirect heating floating zone method. I H F Z system used for these experiments have been reported previously [4]. The zone has been doubly passed; the first pass is to densify the feed rod which has a relatively low density of about 60% and the second pass is to achieve high quality single crystals. For the first pass, the feed rod and the seed crystal were set at the upper and lower sides of the tungsten ring, respectively, and were driven downwards. For growth at the congruent composition, the molten zone was formed by melting the bottom end of the feed rod which has a congruent composition. On the other hand, for growth under incongruent conditions, a tip of optional composition was inserted between the feed rod and the seed rod. It can coexist in equilibrium with the feed rod and the growing crystal after it has been melted to form the molten zone. At the end of the first pass, the zone pass was finished by different ways depending on the zone pass modes of the second pass. For the congruent growth, the remaining part of the feed rod was detached from the first pass crystal by pulling it up before the molten zone freezes. In the case of upwards drive, another seed crystal was substituted with the feed rod at the upper side of the tungsten ring leaving the first pass crystal as it was. The molten zone for the second pass was formed by melting the bottom end of the seed crystal. After forming the molten zone, the seed crystal and the first pass crystal, which was now used as the feed rod, were driven upwards. For downwards drive, the first pass crystal was once
of YB66
removed from the furnace. Then it was set at the upper side of the tungsten ring with the zone end at the bottom. Another seed crystal was set at the lower side of the tungsten ring. The molten zone was formed by melting the bottom end of the first pass crystal. The first pass crystal and the seed crystal were diiven downwards. For the incongruent growth, the molten zone for the second pass should have the same composition as that of the molten zone used in the first pass. At the end of the first pass, the zone was frozen without detaching the remaining part of the feed rod from the crystal. After the whole rod was removed from the furnace, the feed rod was cut off leaving the frozen zone with the first pass crystal. Then the first pass crystal was set at the upper side of the tungsten ring with the frozen zone at the bottom. The molten zone for the second pass was formed by melting again the frozen zone, whose composition was evidently in equilibrium with that of the first pass crystal. Then, the first pass crystal and the seed crystal were drived downwards, the zone leveling condition for self-flux being kept from the initial stage of the second pass. The crystal quality was roughly evaluated by X-ray rocking curve analysis using an ordinary powder (Cu K a ) X-ray diffractomater, by fixing the counter at 15.13 ° (20) and rotating the crystal only. The disk crystal for measurement was cut perpendicular to the [100] growth axis using the Laue method. Four rocking curves were obtained for each sample, one for every 45 ° azimuthal rotation.
3. Results and d i s c u s s i o n
3.1. Growth at the congruent composition In the previous work, the crystals of YB66 w e r e grown at the congruent composition of [B]/[Y] = 62 by driving the crystal and the feed rod upwards after seeding and necking. Several crystals achieved high enough quality for monochromator use at the seed end of the crystals, but such high quality deteriorated progressively towards the zone end of the crystals. This was found to be
Y. Karnirnura et al. / FZ growth of monochromator grade crystals of YB66
due to a concave growth interface. Chemical etching and Laue topography on a vertical section of the crystal showed that sub-grain boundaries grow nearly perpendicular to the growth interface. Thus the concave growth interface introduces successively new sub-grain boundaries caused by turbulence-induced temperature fluctuations occurring in the surface region which become incorporated into the growing crystal. A convex growth interface which projects the subgrain boundaries out from the growing crystal is necessary to obtain sub-grain free crystals. Many attempts to change the growth interface from concave to convex, such as minimizing the heating power and lowering the growth rate to decrease a latent heat exhaust, failed as the feed rod and the seed crystal were driven upwards. According to Kobayashi's calculation [5], the shape of the growth interface is dependent on the Biot number H = eo'T3m/As, where e, o~, Tm and
431
A~ are emissivity, Stefan-Boltzmann constant, melting temperature and thermal conductivity of solid, respectively. The Biot number is the ratio of radiation heat loss from the molten zone and heat loss by thermal conduction through the growing crystal. For small Biot numbers H ~ 0.01, an increase of heating power causes an increase of the zone length without change of the convex shape of the growth interface. On the other hand, for large Biot numbers H ~ 1.0, the increase of heating power causes a shape change of the growth interface from convex to concave without any change of zone length. The convex growth interface can be kept only for the minimum heating power. It looks as if heat accumulates at the center of the zone due to a small rate of thermal conduction loss through the crystal. This is the case of YB66 which shows amorphous-like thermal properties and has quite low thermal conductivity [2].
Fig. 2. (a) Concave growth interface (upper side) and convex melting interface (lower side) by upwards drive with a pull rate at 20 mm/h. (b) Convex growth interface (lower side) by downwards drive with a pull rate at 10 mm/h.
432
Y. Kamimura et a L / FZ growth of monochromator grade crystals of YB66
Moreover, strong convection of ambient He gas heats the crystal located at the upper side of the molten zone and decreases the temperature gradient along the growth direction, resulting in a decrease of heat flow through the crystal. This enhances the tendency of the growth interface to become concave and makes the convex growth interface almost impossible. Fig. 2a shows an example of the concave growth interface for upwards drive at a growth rate of 20 m m / h . The figure also shows that the melting interface is highly convex. There is essentially no change for both the concavity of the growth interface and the convexity of the melting interface by decreasing the growth rate to 5 m m / h . The asymmetry between the growth interface and the melting interface shapes is a reflection of asymmetry between the temperature gradients of the crystals located at the upper and lower sides of the molten zone, respectively. The convection of He gas plays an opposite role at the lower side of the molten zone. It cools the crystal and increases the temperature gradient, resulting in an increase of thermal conduction loss through the crystal. In fact, the convex growth interface was kept easily under considerably wide growth conditions for downwards drive. One example of such a convex growth interface is shown in fig. 2b. Fig. 3 shows a vertical section of the YB66 crystal grown by downwards drive, where many sub-grains that
appeared just after seeding are gradually grown out by the convex growth interface. However, the use of downwards drive caused a new difficulty for the seeding and necking processes. It was quite difficult to form a droplet of molten YB66 at the bottom of the feed rod for seeding by a thin seed crystal, because the bottom of the feed rod was strongly cooled by the convection of He gas with the addition that little radiation came to heat the bottom of the feed rod from the tungsten ring. Thus, it was necessary for the seed crystal to have the same diameter as that of the growing crystal at steady state. Of course, the number of sub-grains decreased considerably after a long pass of the molten zone, as shown in fig. 3. In several crystals qualities high enough for monochromator use have been achieved. However, the reproducibility of obtaining a crystal free of subgrains was quite low.
3.2. Growth under incongruent conditions The crystallographic quality of flux grown crystals is often higher than that of crystals grown by the melt growth because of the lower growth temperature. A great advantage of the F Z method is its capability to grow crystals even under incongruent conditions, where the composition of the molten zone is different from and also in equilibrium with those of the growing crystal and the
Fig. 3. Vertical section of the crystal grown by the downwards drive. The crystal diameter is about 11 mm. The number of sub-grains is gradually decreasing from the seed end (left side) to the zone end (right side).
Y Kamimura et a L / FZ growth ofmonochromator grade crystals of }1866 40 I
433
50 60 70 90 BIY i
i
|
! I
I
I
congruent
~, [B]/[Y]----62, Tm=2100°C el
O.
zone
&
q
i
i
98 atomic*/, B
i
Fig. 5. Photographs of the crystal of YB62 (a) and YBs6 (b) grown at the same growth rate of 10 mm/h. There is no deposition of the evaporated substance on the YB56 crystal, indicating the lower growth temperature.
I
99
Fig. 4. Phase relation around YB66 under the crystal growth conditions. The ordinate is not scaled because of the difficulty of temperature measurement with the IHFZ method. The melting temperature at the congruent composition is 2100°C [6]. The compositions on the liquidus line and on the solidus line, designated by a solid triangle and a solid circle, correspond to the compositions of the zone and both the crystal and the feed rod, respectively. The corresponding arrangement of the floating zone is shown on the right-hand side.
feed rod. YB66 can be grown under incongruent conditions by using self-flux. From the phase relation obtained in our previous work [4], the compositions of the feed rod and the molten zone were estimated to be [B]/[Y] = 58 and [B]/[Y] = 50, respectively. Later these values have been corrected to about 56 and 40, respectively, by chemical analyses of the end part of the crystal and the frozen zone. The corrected phase relation is shown in fig. 4 with the arrangement of the floating zone. We confirmed the melting point of 2100°C at the congruent composition reported previously [6]. On the other hand, for incongruent growth, the growth temperature decreased nearly to the eutectic temperature. The corresponding decrease of the heating power was about 2%. However, we could not measure the growth temperature in the present I H F Z configuration. The surface of the molten zone reflected so much the radiation from the tungsten ring that the value observed by pyro-
scope showed much higher temperature than real ones. The crystals grown at the congruent composition and under incongruent conditions at the same growth rate of 10 m m / h are compared in figs. 5a and 5b, respectively. As opposed to the crystal grown from the congruent composition, there is no deposition of evaporated substances on the crystal grown under incongruent conditions. This also shows a decrease of the growth temperature.
-12 -11
8
J J
15 r4 t2 ^jo
~3rain
{~ rl
| [ Length.~rom seeding
,I
J
[/polnt~mm)
L. ,53.o4
30 60
I0 1 ~31 . 0 1 5 30 45 60 75
Angle(min )
Fig. 6. Variation of the hard X-ray rocking curve. The abscissa and the ordinate show the distance from the seeding position and the relative intensity of the reflected X-rays, respectively.
434
Y..Kamimura et al. / FZ growth of monochromator grade crystals" of YB66
T h e result of the hard X-ray rocking curve was m u c h more striking. Fig. 6 shows the variation of the rocking curve along the growth axis of the crystal. M a n y peaks were observed just after the seeding. However, the n u m b e r of peaks rapidly decreased as the distance from the seeding position increased, and the rocking curve at only 11 m m already shows a single, sharp p e a k whose F W H M has achieved 3 arc min. This behavior of the crystals grown u n d e r incongruent conditions is quite different from that of the crystals of the congruent composition shown in fig. 3, where the single domain could only be achieved by a zone pass of more than 40 mm. Moreover, the behavior u n d e r incongruent conditions seems to be certainly reproducible, judging from several growth experiments carried out in our laboratory.
M o r e systematic experiments are now underway in order to clarify the best melt composition for incongruent growth.
References [1] S.M. Richards and J.S. Kasper, Acta Cryst. B 25 (1969) 237. [2] P.A. Medwick, D.G. Cahill, A.K. Raychaudhuri, R.O. Pohl, F. Gompf, N. Nucker and T. Tanaka, AlP Conf. Proc. 231 (1990) 363. [3] J. Wong, G. Shimkabeg, W. Goldstein, M. Eckart, T. Tanaka, Z.U. Rek and H. Tompkins, Nucl. Instr. Methods A 291 (1990) 243. [4] T. Tanaka, S. Otani and Y. Ishizawa, J. Crystal Growth 99 (1990) 994. [5] N. Kobayashi, J. Crystal Growth 43 (1978) 417. [6] D.W. Oliver and G.D. Brower, J. Crystal Growth 11 (1971) 185.
j........ C R Y S T A L GROW
Floating zone growth of monochromator grade crystals of Yutaka Kamimura and Joe Wong c
a,
Takaho Tanaka
a
T H
YB66
Shigeki O t a n i a, Y o s h i o Ishizawa a, Z o f i a U. R e k b
a National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305, Japan b Stanford Synchrotron Radiation Laboratory, Stanford, California 94309, USA c Lawrence Livermore National Laboratory, Livermore, California 94551, USA
Monochromator grade crystals of YB66 have been grown by the indirect heating floating zone (IHFZ) method. For growth at the congruent composition of [B]/[Y] = 62, some high quality crystals have been obtained by keeping a convex growth interface through the crystal using a minimized heating power and downwards drive. Reproducible growth of monochromator grade crystals has been achieved by growing crystals under an incongruent condition where the compositions of the feed rod and the zone are about [B]/[Y] = 56 and 40, respectively.
1. Introduction
2. Experimental procedure
YB66 is known as a peculiar boron compound because of its complicated crystal structure [1] and amorphous like behavior in phonon related properties [2]. Because of its large d-value (d = 0.586 nm for (400) reflection) and the absence of intrinsic absorption, YB66 has recently been selected as a unique candidate for a soft X-ray monochromator in the 1-2 keV region for synchrotron radiation applications [3]. Our investigation on the growth of monochromator grade crystals of YB66 has continued, using the indirect heating floating zone (IHFZ) method [4]. In a previous paper we reported deterioration in quality due to a concave growth interface, although high quality can be obtained near the seed end by seeding, double zone passes, and upwards drive. We also pointed out that a high quality crystal can be obtained by controlling the interface shape to be convex. In this paper we report recent advances in the crystal growth of YB66 , including growth with a convex growth interface at the congruent composition and growth under incongruent conditions for the purpose of reproducibile growth of high quality YB66 single crystals.
The raw powder of YB66 was synthesized from amorphous boron and yttrium oxide by a borothermal reduction method. In previous work [4], the YB66 crystals included impurities of C, Fe and A1. The C impurity came both from raw boron and from a carbon susceptor in the reaction furnace. The Fe impurity also came from the raw boron. In order to reduce the C impurity, the carbon susceptor was changed to a composite susceptor consisting of TiB 2 and BN, and the boron source was changed to Callery Inc. (USA). The latter was also effective in diminishing the Fe impurity. The AI contamination was introduced during the pulverization process with the use of an alumina mortar, which was now substituted with a stainless steel ball mill. The stainless steel contamination was leached out by dilute HC1 solution and decanted. A 180 mm long feed rod with a diameter of 12 mm was prepared by sintering after a cold isostatic press treatment. The crystals of YB66 have been grown by the I H F Z method, shown in fig. 1, where the molten zone is heated by radiation from an inductively heated tungsten ring situated between a RF work coil and the molten zone. The details of the
0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
430
Y. Kamimura et al. / FZ growth of monochromator grade crystals
BN
Molten z o n e Fig. 1. Schematic diagram of the indirect heating floating zone method. I H F Z system used for these experiments have been reported previously [4]. The zone has been doubly passed; the first pass is to densify the feed rod which has a relatively low density of about 60% and the second pass is to achieve high quality single crystals. For the first pass, the feed rod and the seed crystal were set at the upper and lower sides of the tungsten ring, respectively, and were driven downwards. For growth at the congruent composition, the molten zone was formed by melting the bottom end of the feed rod which has a congruent composition. On the other hand, for growth under incongruent conditions, a tip of optional composition was inserted between the feed rod and the seed rod. It can coexist in equilibrium with the feed rod and the growing crystal after it has been melted to form the molten zone. At the end of the first pass, the zone pass was finished by different ways depending on the zone pass modes of the second pass. For the congruent growth, the remaining part of the feed rod was detached from the first pass crystal by pulling it up before the molten zone freezes. In the case of upwards drive, another seed crystal was substituted with the feed rod at the upper side of the tungsten ring leaving the first pass crystal as it was. The molten zone for the second pass was formed by melting the bottom end of the seed crystal. After forming the molten zone, the seed crystal and the first pass crystal, which was now used as the feed rod, were driven upwards. For downwards drive, the first pass crystal was once
of YB66
removed from the furnace. Then it was set at the upper side of the tungsten ring with the zone end at the bottom. Another seed crystal was set at the lower side of the tungsten ring. The molten zone was formed by melting the bottom end of the first pass crystal. The first pass crystal and the seed crystal were diiven downwards. For the incongruent growth, the molten zone for the second pass should have the same composition as that of the molten zone used in the first pass. At the end of the first pass, the zone was frozen without detaching the remaining part of the feed rod from the crystal. After the whole rod was removed from the furnace, the feed rod was cut off leaving the frozen zone with the first pass crystal. Then the first pass crystal was set at the upper side of the tungsten ring with the frozen zone at the bottom. The molten zone for the second pass was formed by melting again the frozen zone, whose composition was evidently in equilibrium with that of the first pass crystal. Then, the first pass crystal and the seed crystal were drived downwards, the zone leveling condition for self-flux being kept from the initial stage of the second pass. The crystal quality was roughly evaluated by X-ray rocking curve analysis using an ordinary powder (Cu K a ) X-ray diffractomater, by fixing the counter at 15.13 ° (20) and rotating the crystal only. The disk crystal for measurement was cut perpendicular to the [100] growth axis using the Laue method. Four rocking curves were obtained for each sample, one for every 45 ° azimuthal rotation.
3. Results and d i s c u s s i o n
3.1. Growth at the congruent composition In the previous work, the crystals of YB66 w e r e grown at the congruent composition of [B]/[Y] = 62 by driving the crystal and the feed rod upwards after seeding and necking. Several crystals achieved high enough quality for monochromator use at the seed end of the crystals, but such high quality deteriorated progressively towards the zone end of the crystals. This was found to be
Y. Karnirnura et al. / FZ growth of monochromator grade crystals of YB66
due to a concave growth interface. Chemical etching and Laue topography on a vertical section of the crystal showed that sub-grain boundaries grow nearly perpendicular to the growth interface. Thus the concave growth interface introduces successively new sub-grain boundaries caused by turbulence-induced temperature fluctuations occurring in the surface region which become incorporated into the growing crystal. A convex growth interface which projects the subgrain boundaries out from the growing crystal is necessary to obtain sub-grain free crystals. Many attempts to change the growth interface from concave to convex, such as minimizing the heating power and lowering the growth rate to decrease a latent heat exhaust, failed as the feed rod and the seed crystal were driven upwards. According to Kobayashi's calculation [5], the shape of the growth interface is dependent on the Biot number H = eo'T3m/As, where e, o~, Tm and
431
A~ are emissivity, Stefan-Boltzmann constant, melting temperature and thermal conductivity of solid, respectively. The Biot number is the ratio of radiation heat loss from the molten zone and heat loss by thermal conduction through the growing crystal. For small Biot numbers H ~ 0.01, an increase of heating power causes an increase of the zone length without change of the convex shape of the growth interface. On the other hand, for large Biot numbers H ~ 1.0, the increase of heating power causes a shape change of the growth interface from convex to concave without any change of zone length. The convex growth interface can be kept only for the minimum heating power. It looks as if heat accumulates at the center of the zone due to a small rate of thermal conduction loss through the crystal. This is the case of YB66 which shows amorphous-like thermal properties and has quite low thermal conductivity [2].
Fig. 2. (a) Concave growth interface (upper side) and convex melting interface (lower side) by upwards drive with a pull rate at 20 mm/h. (b) Convex growth interface (lower side) by downwards drive with a pull rate at 10 mm/h.
432
Y. Kamimura et a L / FZ growth of monochromator grade crystals of YB66
Moreover, strong convection of ambient He gas heats the crystal located at the upper side of the molten zone and decreases the temperature gradient along the growth direction, resulting in a decrease of heat flow through the crystal. This enhances the tendency of the growth interface to become concave and makes the convex growth interface almost impossible. Fig. 2a shows an example of the concave growth interface for upwards drive at a growth rate of 20 m m / h . The figure also shows that the melting interface is highly convex. There is essentially no change for both the concavity of the growth interface and the convexity of the melting interface by decreasing the growth rate to 5 m m / h . The asymmetry between the growth interface and the melting interface shapes is a reflection of asymmetry between the temperature gradients of the crystals located at the upper and lower sides of the molten zone, respectively. The convection of He gas plays an opposite role at the lower side of the molten zone. It cools the crystal and increases the temperature gradient, resulting in an increase of thermal conduction loss through the crystal. In fact, the convex growth interface was kept easily under considerably wide growth conditions for downwards drive. One example of such a convex growth interface is shown in fig. 2b. Fig. 3 shows a vertical section of the YB66 crystal grown by downwards drive, where many sub-grains that
appeared just after seeding are gradually grown out by the convex growth interface. However, the use of downwards drive caused a new difficulty for the seeding and necking processes. It was quite difficult to form a droplet of molten YB66 at the bottom of the feed rod for seeding by a thin seed crystal, because the bottom of the feed rod was strongly cooled by the convection of He gas with the addition that little radiation came to heat the bottom of the feed rod from the tungsten ring. Thus, it was necessary for the seed crystal to have the same diameter as that of the growing crystal at steady state. Of course, the number of sub-grains decreased considerably after a long pass of the molten zone, as shown in fig. 3. In several crystals qualities high enough for monochromator use have been achieved. However, the reproducibility of obtaining a crystal free of subgrains was quite low.
3.2. Growth under incongruent conditions The crystallographic quality of flux grown crystals is often higher than that of crystals grown by the melt growth because of the lower growth temperature. A great advantage of the F Z method is its capability to grow crystals even under incongruent conditions, where the composition of the molten zone is different from and also in equilibrium with those of the growing crystal and the
Fig. 3. Vertical section of the crystal grown by the downwards drive. The crystal diameter is about 11 mm. The number of sub-grains is gradually decreasing from the seed end (left side) to the zone end (right side).
Y Kamimura et a L / FZ growth ofmonochromator grade crystals of }1866 40 I
433
50 60 70 90 BIY i
i
|
! I
I
I
congruent
~, [B]/[Y]----62, Tm=2100°C el
O.
zone
&
q
i
i
98 atomic*/, B
i
Fig. 5. Photographs of the crystal of YB62 (a) and YBs6 (b) grown at the same growth rate of 10 mm/h. There is no deposition of the evaporated substance on the YB56 crystal, indicating the lower growth temperature.
I
99
Fig. 4. Phase relation around YB66 under the crystal growth conditions. The ordinate is not scaled because of the difficulty of temperature measurement with the IHFZ method. The melting temperature at the congruent composition is 2100°C [6]. The compositions on the liquidus line and on the solidus line, designated by a solid triangle and a solid circle, correspond to the compositions of the zone and both the crystal and the feed rod, respectively. The corresponding arrangement of the floating zone is shown on the right-hand side.
feed rod. YB66 can be grown under incongruent conditions by using self-flux. From the phase relation obtained in our previous work [4], the compositions of the feed rod and the molten zone were estimated to be [B]/[Y] = 58 and [B]/[Y] = 50, respectively. Later these values have been corrected to about 56 and 40, respectively, by chemical analyses of the end part of the crystal and the frozen zone. The corrected phase relation is shown in fig. 4 with the arrangement of the floating zone. We confirmed the melting point of 2100°C at the congruent composition reported previously [6]. On the other hand, for incongruent growth, the growth temperature decreased nearly to the eutectic temperature. The corresponding decrease of the heating power was about 2%. However, we could not measure the growth temperature in the present I H F Z configuration. The surface of the molten zone reflected so much the radiation from the tungsten ring that the value observed by pyro-
scope showed much higher temperature than real ones. The crystals grown at the congruent composition and under incongruent conditions at the same growth rate of 10 m m / h are compared in figs. 5a and 5b, respectively. As opposed to the crystal grown from the congruent composition, there is no deposition of evaporated substances on the crystal grown under incongruent conditions. This also shows a decrease of the growth temperature.
-12 -11
8
J J
15 r4 t2 ^jo
~3rain
{~ rl
| [ Length.~rom seeding
,I
J
[/polnt~mm)
L. ,53.o4
30 60
I0 1 ~31 . 0 1 5 30 45 60 75
Angle(min )
Fig. 6. Variation of the hard X-ray rocking curve. The abscissa and the ordinate show the distance from the seeding position and the relative intensity of the reflected X-rays, respectively.
434
Y..Kamimura et al. / FZ growth of monochromator grade crystals" of YB66
T h e result of the hard X-ray rocking curve was m u c h more striking. Fig. 6 shows the variation of the rocking curve along the growth axis of the crystal. M a n y peaks were observed just after the seeding. However, the n u m b e r of peaks rapidly decreased as the distance from the seeding position increased, and the rocking curve at only 11 m m already shows a single, sharp p e a k whose F W H M has achieved 3 arc min. This behavior of the crystals grown u n d e r incongruent conditions is quite different from that of the crystals of the congruent composition shown in fig. 3, where the single domain could only be achieved by a zone pass of more than 40 mm. Moreover, the behavior u n d e r incongruent conditions seems to be certainly reproducible, judging from several growth experiments carried out in our laboratory.
M o r e systematic experiments are now underway in order to clarify the best melt composition for incongruent growth.
References [1] S.M. Richards and J.S. Kasper, Acta Cryst. B 25 (1969) 237. [2] P.A. Medwick, D.G. Cahill, A.K. Raychaudhuri, R.O. Pohl, F. Gompf, N. Nucker and T. Tanaka, AlP Conf. Proc. 231 (1990) 363. [3] J. Wong, G. Shimkabeg, W. Goldstein, M. Eckart, T. Tanaka, Z.U. Rek and H. Tompkins, Nucl. Instr. Methods A 291 (1990) 243. [4] T. Tanaka, S. Otani and Y. Ishizawa, J. Crystal Growth 99 (1990) 994. [5] N. Kobayashi, J. Crystal Growth 43 (1978) 417. [6] D.W. Oliver and G.D. Brower, J. Crystal Growth 11 (1971) 185.