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Microscopic voids in FZ-grown NdGG garnet; Occurrence and morphology
656
Journal of Crystal Growth 67 (1984) 656—659 North-Holland, Amsterdam
LETTER TO THE EDITORS MICROSCOPIC VOIDS IN FZ-GROWN NdGG GARNET; OCCURRENCE AND MORPHOLOGY K. KITAMURA, M. TSUTSUMI and S. KIMLJRA Notional Institute for Research in Inorganic Materials, I - I Nansil.i, .S’akura - mura, Niiharigun, Ibaraki 305, Japan
and H. KOMATSU Research Insitute for Iron, Steel and Other Metals, Tohoku Universiti. Katahira, Sendai 980, Japan Received 14 March 1984: manuscript received in final form 30 May 1984
Microscopic voids with a diameter of a few gm were found in NdGG single crystals grown by the FZ method. The occurrence of the voids strongly depended on the growth rate. They were concentrated in the (211) facet regions under certain growth conditions. Each void exhibited a lot of small flat faces, which were less than I gm in diameter, on the concave surface. The (211) and (100) faces were superior to the (110} faces in size and distinctness.
It is common procedure to use an after-heater when single crystals of rare-earth garnets are grown by the floating zone method (FZ method) with a radiation convergence type heater; thereby the thermal gradient is decreased to avoid crack development in the grown crystal during cooling [1,2]. However, through studies on the growth of neodinium gallium garnet (NdGG). it has been recognized that such usage of after-heater tends to cause characteristic inclusions in the grown crystals. These inclusions were confirmed to be microscopic voids of several ~tm in diameter by means of a SEM (Scanning Electronic Microscope). Their occurrence and morphology are described and the implications thereof are discussed in this letter. Mode of occurrence of the voids: A NdGG garnet was grown at a growth rate of 2 mm/h and a seed crystal rotation rate of 40 rpm. The grown garnet was cut perpendicularly to the [Ill] growth direction and polished to serve as a section. Fig. I shows the distribution of the voids in the crystal boule observed using the dark-field tllumination method and the presence of voids can be visualized by the scattered light. The figure dem-
onstrates the characteristic distribution of the voids, which are concentrated in the {211 } facet regions. Fig. 2 shows the features of the voids under a polarizing microscope with parallel Nicols. The
2
m
I 1g. I. l~i~irihuiionof soids rese,iled hs Oie d,irk—ficld 1— lurnination method. In the crystal of NdGG grown by the FZ method at a growth rate of 2 mm/h, the voids are concenirated in the (211) facet regions.
0022-0248/84/$03.00 © Elsevier Science Publishers By. (North-Holland Physics Publishing Division)
K. Kitamura et al.
/
Microscopic voids in FZ-grown NCIGG garnet
off-facet
657
the garnet grown at a growth rate of I mm/h, the voids were almost eliminated. At a growth rate of 2 mm/h, the voids were concentrated in the {211}
Fig. 2. Voids observed under a polarizing microscope with parallel Nicols exhibiting the spherical shape as black dots.
photograph indicates that the voids are spherical and that their size is limited in range from I to several ~tm. It is remarked that stress-hirefringence, which is often observed in association with crystal defects such as inclusions [3], was not recognized around the voids. Some characteristics in the occurrence of such voids can be summarized as follows: (1) The occurrence density of the voids is increased by using an after-heater which is applied to decrease the thermal gradient. (2) the occurrence of voids significantly varies depending on the growth rate. In
I
~.,
1pm ~ —
/ /
11.2 ~ ,~( ‘~2~
194
~
~i;;~~:”
213l\~/—’ ~_‘I~_i111
011
~
121
2~(~,61~oJ
/
~
/
~ ~1-i ~
2
(Ti
B
211
—
121
Fig. 4. Cut surface of voids observed by means of SEM. (A) The sample was fractured perpendicularly to the [III] growth
Fig. 3. Distribution of voids in a sample grown at rate of 6 mm/h. The voids are concentrated both in the core and in the intermediate regions, the latter appearing as “a ring” in a
direction and the surface was observed along a direction somewhat oblique to that direction. (B) (hkl of each face recognized in (A), which was determined from the stereographic projec-
section perpendicular to the growth direction [4[.
tion.
658
K. Kitarnura et al.
/
Microscopic voids in FZ-grown NdGG garnet
facet regions of grown crystal as shown figs. 1 and 2. At a growth rate above 4 mm/h. they were formed also in the off-facet region as shown in fig. 3. In this case, the voids were incorporated preferentially both in the core region and in the intermediate region. In the core region, the crystal was grown at the largest growth rate since the growing interface was convex toward the melt, and in the intermediate region the thick stagnant layer was located along the growing interface [4]. (3) The voids are more remarkable in NdGG crystals grown in an 02 atmosphere than in those grown in aN 2 atmosphere.
—
shown in fig. 4A, which demonstrates small faces of low index planes of garnet. The (hkl) index of each face was determined by the stereographic projection as shown in fig. 4B. The faces in fig. 4A were determined as (211) (100), (321), (210) and (110) planes. Among such faces, the (211) and { 100} faces are seen clearly being larger than others in fig. 4A. On the other hand, the (110) faces are small and obscure. Fig. 5A also shows the surface of a void in the
111
crystal fractured perpendicularly to the [211] direc-
~
~
_____________________________________
-4 I
~
________________________
Morphology of the voids: Although the voids apparently exhibit the shape of a nearly perfect sphere under an optical microscope, it has been revealed by means of SEM that numerous small faces (less than I jzm in diameter) were formed as flat planes on the surface of the voids. The crystal was intentionally fractured perpendicularly to the [111] growth direction (fig. 4A) and the [211] direction (fig. 5A). The fractured surface was ohserved in the SEM after carbon coating. Each void bisected by the fracture appeared as a hemispherical hole. An example of the surface of the void is
_________
tion. The same tendency of the development of the faces was recognized as the (211) and {100} faces were superior to the (110) faces in size and clarity. Especially in fig. 5A, the (110) faces are almost undetectable. A crystal of NdGG was heated to 1530°C (the melting temperature of NdGG is about 1560°C) for a week and then cooled slowly, but the tend-
~
/~
f2~o
2O1.~j”~ 211
ency of the development of the faces remained the same as before the thermal treatment.
“...~
\
Discussion: Fiom the mode of occurrence of the
11 2
voids, it can be deduced that the voids originated from bubbles in the melt which was mechanically
001
stirred, for the melt was under the influence of rotation of a seed crystal and a charge rod in the counterwise direction in the FZ method. In this case, the bubbles were spherical due to the surface tension and the size became homogeneous. Their size and density may depend on the partial pres-
. -.
iio~ 142 129
121 -
010. “—,
912:132 Si
~
-
() / 11 2/
/ /
B Fig. 5. Cut surface of a void observed by means of SEM. (A) The sample was fractured perpendicularly to the [211] direction and the observation by SEM was carried out along that direction. (B) (hkl) of each face determined from the stereographic projection.
sure of oxygen in the atmosphere [5]. Such bubbles drifting in the melt tend to be unavoidably ineluded in the growing crystal when the growth rate is so large or the melt is so stagnant that they cannot escape from the interface. It is noteworthy that the voids were concentrated in the (211 } facet regions under appropriate growth conditions, in
K. Kitamura ci al.
/
Microscopic voids in FZ - grown NdGG garnet
spite of the fact that they were found in the off-facet regions in the crystal grown only at a large growth rate. It possibly suggests that the growth rate on the (211) facet region is larger than that on the off-facet regions under certain conditions. Concerning the morphology of voids, numerous studies have previously been performed. In most cases, the voids included in the crystals from vanous causes were believed to reflect either the equilibrium form or the growth form [6]. If they exhibited the growth form, the thermal treatment should have some effect in changing the form into the equilibrium one, as typically observed in the case of Ni-oxide [7]. In the present study, the thermal treatment indicated no sign of change. Besides, if the morphology revealed in this study were reflecting such a growth or an equilibrium form of garnet, the sequence of the size, (211 }, (100)>> (110) would become contradictory to the sequence of morphological importance of such faces reported previously. Thus the morphology of the presently found voids may be explained on the basis of neither the equilibrium nor the growth form. Van Erk et al. [8] showed the development of faces in the order of decreasing a-factor, as (llo}, (211), (321), (100), from the growth experiments. They suggested that at the growth temperature (900—1000°C in the flux growth), the a-factor of (321) and (100) must be close to the critical value for surface roughening. Tolksdorf and Bartels [9] supported such an order from the LPE growth of YIG on the spherical surface of GGG crystal. Although they recognized the (110), (211) and (321) facets, the (100) facets did not appear. In the study carried out by Bennema et al. [10]using a PBC analysis, the morphological importance of such faces has been concluded as (110) (211} >> (100). The flat faces presently observed on the surface of the voids seem to be some traces of surface
659
melting or remelting which takes place at a ternperature very close to the melting temperature. However, the surface structure of each face could not be observed clearly since the size was small and the surface actually observed was concave. Therefore, it is difficult to discuss in detail the formation mechanism of such faces. However, because such morphology of the voids must reflect an anisotropy of the surface condition, it can be deduced from the observation that the surface condition of the (211) faces is different from that of the (110) faces, but rather similar to that of the (100} faces, the a-factor of which must be already over the critical value for surface roughening near the melting temperature. As described above, the characteristic occurrence and morphology of the voids observed in FZ-grown NdGG garnet are quite suggestive when one discusses the surface condition and formation mechanism of the (211) facet. A problem concerning the growth rate on the (211) facet region will be discussed elsewhere [11].
References [1] K. Kitamura, S. Kimura and K. Watanabe. J. Crystal Growth 67(1982)475. [2] M. Higuch, K. Kitamura and S. Kimura, to be prepared. [3] BK. Tanner, and D.J. Fathers, Phil. Mag 29 (1974) 1081. [4] K. Kitamura, N. Ii, 1. Shindo and S. Kimura, J. Crystal Growth 46(1979) 277. [5] N. Ii and I. Shindo, J. Crystal Growth 46 (1979) 569. [6] R.S. Nelson, D.J. Maret and R.S. Barns, Phil. Mag. 10 (1965) 91. [7] G. Colomer, M. Dechamps, G. Dhalenne and A. Revcolevschi, J. Crystal Growth 56 (1981) 93. [8] W. van Erk, H.J.G.J. van Hoek-Martens and G. Bartels, J. Crystal Growth 48 (1980) 621. [9] W.Tolksdorf and I. Bartels, J. Crystal Growth 54 (1981) .
.
[10] P. Bennema, E.A. Giess and J.E. Weidenborner, J. Crystal Growth 62 (1983) 41. [11] K. Kitamura, S. Kimura, T. Oshikiri and H. Komatsu, to be prepared.
Journal of Crystal Growth 67 (1984) 656—659 North-Holland, Amsterdam
LETTER TO THE EDITORS MICROSCOPIC VOIDS IN FZ-GROWN NdGG GARNET; OCCURRENCE AND MORPHOLOGY K. KITAMURA, M. TSUTSUMI and S. KIMLJRA Notional Institute for Research in Inorganic Materials, I - I Nansil.i, .S’akura - mura, Niiharigun, Ibaraki 305, Japan
and H. KOMATSU Research Insitute for Iron, Steel and Other Metals, Tohoku Universiti. Katahira, Sendai 980, Japan Received 14 March 1984: manuscript received in final form 30 May 1984
Microscopic voids with a diameter of a few gm were found in NdGG single crystals grown by the FZ method. The occurrence of the voids strongly depended on the growth rate. They were concentrated in the (211) facet regions under certain growth conditions. Each void exhibited a lot of small flat faces, which were less than I gm in diameter, on the concave surface. The (211) and (100) faces were superior to the (110} faces in size and distinctness.
It is common procedure to use an after-heater when single crystals of rare-earth garnets are grown by the floating zone method (FZ method) with a radiation convergence type heater; thereby the thermal gradient is decreased to avoid crack development in the grown crystal during cooling [1,2]. However, through studies on the growth of neodinium gallium garnet (NdGG). it has been recognized that such usage of after-heater tends to cause characteristic inclusions in the grown crystals. These inclusions were confirmed to be microscopic voids of several ~tm in diameter by means of a SEM (Scanning Electronic Microscope). Their occurrence and morphology are described and the implications thereof are discussed in this letter. Mode of occurrence of the voids: A NdGG garnet was grown at a growth rate of 2 mm/h and a seed crystal rotation rate of 40 rpm. The grown garnet was cut perpendicularly to the [Ill] growth direction and polished to serve as a section. Fig. I shows the distribution of the voids in the crystal boule observed using the dark-field tllumination method and the presence of voids can be visualized by the scattered light. The figure dem-
onstrates the characteristic distribution of the voids, which are concentrated in the {211 } facet regions. Fig. 2 shows the features of the voids under a polarizing microscope with parallel Nicols. The
2
m
I 1g. I. l~i~irihuiionof soids rese,iled hs Oie d,irk—ficld 1— lurnination method. In the crystal of NdGG grown by the FZ method at a growth rate of 2 mm/h, the voids are concenirated in the (211) facet regions.
0022-0248/84/$03.00 © Elsevier Science Publishers By. (North-Holland Physics Publishing Division)
K. Kitamura et al.
/
Microscopic voids in FZ-grown NCIGG garnet
off-facet
657
the garnet grown at a growth rate of I mm/h, the voids were almost eliminated. At a growth rate of 2 mm/h, the voids were concentrated in the {211}
Fig. 2. Voids observed under a polarizing microscope with parallel Nicols exhibiting the spherical shape as black dots.
photograph indicates that the voids are spherical and that their size is limited in range from I to several ~tm. It is remarked that stress-hirefringence, which is often observed in association with crystal defects such as inclusions [3], was not recognized around the voids. Some characteristics in the occurrence of such voids can be summarized as follows: (1) The occurrence density of the voids is increased by using an after-heater which is applied to decrease the thermal gradient. (2) the occurrence of voids significantly varies depending on the growth rate. In
I
~.,
1pm ~ —
/ /
11.2 ~ ,~( ‘~2~
194
~
~i;;~~:”
213l\~/—’ ~_‘I~_i111
011
~
121
2~(~,61~oJ
/
~
/
~ ~1-i ~
2
(Ti
B
211
—
121
Fig. 4. Cut surface of voids observed by means of SEM. (A) The sample was fractured perpendicularly to the [III] growth
Fig. 3. Distribution of voids in a sample grown at rate of 6 mm/h. The voids are concentrated both in the core and in the intermediate regions, the latter appearing as “a ring” in a
direction and the surface was observed along a direction somewhat oblique to that direction. (B) (hkl of each face recognized in (A), which was determined from the stereographic projec-
section perpendicular to the growth direction [4[.
tion.
658
K. Kitarnura et al.
/
Microscopic voids in FZ-grown NdGG garnet
facet regions of grown crystal as shown figs. 1 and 2. At a growth rate above 4 mm/h. they were formed also in the off-facet region as shown in fig. 3. In this case, the voids were incorporated preferentially both in the core region and in the intermediate region. In the core region, the crystal was grown at the largest growth rate since the growing interface was convex toward the melt, and in the intermediate region the thick stagnant layer was located along the growing interface [4]. (3) The voids are more remarkable in NdGG crystals grown in an 02 atmosphere than in those grown in aN 2 atmosphere.
—
shown in fig. 4A, which demonstrates small faces of low index planes of garnet. The (hkl) index of each face was determined by the stereographic projection as shown in fig. 4B. The faces in fig. 4A were determined as (211) (100), (321), (210) and (110) planes. Among such faces, the (211) and { 100} faces are seen clearly being larger than others in fig. 4A. On the other hand, the (110) faces are small and obscure. Fig. 5A also shows the surface of a void in the
111
crystal fractured perpendicularly to the [211] direc-
~
~
_____________________________________
-4 I
~
________________________
Morphology of the voids: Although the voids apparently exhibit the shape of a nearly perfect sphere under an optical microscope, it has been revealed by means of SEM that numerous small faces (less than I jzm in diameter) were formed as flat planes on the surface of the voids. The crystal was intentionally fractured perpendicularly to the [111] growth direction (fig. 4A) and the [211] direction (fig. 5A). The fractured surface was ohserved in the SEM after carbon coating. Each void bisected by the fracture appeared as a hemispherical hole. An example of the surface of the void is
_________
tion. The same tendency of the development of the faces was recognized as the (211) and {100} faces were superior to the (110) faces in size and clarity. Especially in fig. 5A, the (110) faces are almost undetectable. A crystal of NdGG was heated to 1530°C (the melting temperature of NdGG is about 1560°C) for a week and then cooled slowly, but the tend-
~
/~
f2~o
2O1.~j”~ 211
ency of the development of the faces remained the same as before the thermal treatment.
“...~
\
Discussion: Fiom the mode of occurrence of the
11 2
voids, it can be deduced that the voids originated from bubbles in the melt which was mechanically
001
stirred, for the melt was under the influence of rotation of a seed crystal and a charge rod in the counterwise direction in the FZ method. In this case, the bubbles were spherical due to the surface tension and the size became homogeneous. Their size and density may depend on the partial pres-
. -.
iio~ 142 129
121 -
010. “—,
912:132 Si
~
-
() / 11 2/
/ /
B Fig. 5. Cut surface of a void observed by means of SEM. (A) The sample was fractured perpendicularly to the [211] direction and the observation by SEM was carried out along that direction. (B) (hkl) of each face determined from the stereographic projection.
sure of oxygen in the atmosphere [5]. Such bubbles drifting in the melt tend to be unavoidably ineluded in the growing crystal when the growth rate is so large or the melt is so stagnant that they cannot escape from the interface. It is noteworthy that the voids were concentrated in the (211 } facet regions under appropriate growth conditions, in
K. Kitamura ci al.
/
Microscopic voids in FZ - grown NdGG garnet
spite of the fact that they were found in the off-facet regions in the crystal grown only at a large growth rate. It possibly suggests that the growth rate on the (211) facet region is larger than that on the off-facet regions under certain conditions. Concerning the morphology of voids, numerous studies have previously been performed. In most cases, the voids included in the crystals from vanous causes were believed to reflect either the equilibrium form or the growth form [6]. If they exhibited the growth form, the thermal treatment should have some effect in changing the form into the equilibrium one, as typically observed in the case of Ni-oxide [7]. In the present study, the thermal treatment indicated no sign of change. Besides, if the morphology revealed in this study were reflecting such a growth or an equilibrium form of garnet, the sequence of the size, (211 }, (100)>> (110) would become contradictory to the sequence of morphological importance of such faces reported previously. Thus the morphology of the presently found voids may be explained on the basis of neither the equilibrium nor the growth form. Van Erk et al. [8] showed the development of faces in the order of decreasing a-factor, as (llo}, (211), (321), (100), from the growth experiments. They suggested that at the growth temperature (900—1000°C in the flux growth), the a-factor of (321) and (100) must be close to the critical value for surface roughening. Tolksdorf and Bartels [9] supported such an order from the LPE growth of YIG on the spherical surface of GGG crystal. Although they recognized the (110), (211) and (321) facets, the (100) facets did not appear. In the study carried out by Bennema et al. [10]using a PBC analysis, the morphological importance of such faces has been concluded as (110) (211} >> (100). The flat faces presently observed on the surface of the voids seem to be some traces of surface
659
melting or remelting which takes place at a ternperature very close to the melting temperature. However, the surface structure of each face could not be observed clearly since the size was small and the surface actually observed was concave. Therefore, it is difficult to discuss in detail the formation mechanism of such faces. However, because such morphology of the voids must reflect an anisotropy of the surface condition, it can be deduced from the observation that the surface condition of the (211) faces is different from that of the (110) faces, but rather similar to that of the (100} faces, the a-factor of which must be already over the critical value for surface roughening near the melting temperature. As described above, the characteristic occurrence and morphology of the voids observed in FZ-grown NdGG garnet are quite suggestive when one discusses the surface condition and formation mechanism of the (211) facet. A problem concerning the growth rate on the (211) facet region will be discussed elsewhere [11].
References [1] K. Kitamura, S. Kimura and K. Watanabe. J. Crystal Growth 67(1982)475. [2] M. Higuch, K. Kitamura and S. Kimura, to be prepared. [3] BK. Tanner, and D.J. Fathers, Phil. Mag 29 (1974) 1081. [4] K. Kitamura, N. Ii, 1. Shindo and S. Kimura, J. Crystal Growth 46(1979) 277. [5] N. Ii and I. Shindo, J. Crystal Growth 46 (1979) 569. [6] R.S. Nelson, D.J. Maret and R.S. Barns, Phil. Mag. 10 (1965) 91. [7] G. Colomer, M. Dechamps, G. Dhalenne and A. Revcolevschi, J. Crystal Growth 56 (1981) 93. [8] W. van Erk, H.J.G.J. van Hoek-Martens and G. Bartels, J. Crystal Growth 48 (1980) 621. [9] W.Tolksdorf and I. Bartels, J. Crystal Growth 54 (1981) .
.
[10] P. Bennema, E.A. Giess and J.E. Weidenborner, J. Crystal Growth 62 (1983) 41. [11] K. Kitamura, S. Kimura, T. Oshikiri and H. Komatsu, to be prepared.