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X-ray topographic analysis of dislocations and growth bands in a melt grown gadolinium gallium garnet crystal
Mat. Res. Bull. Vol. 8, pp. 43-52, 1973. the United States.
Pergamon Press, Inc.
Printed in
X-RAY TOPOGRAPHIC ANALYSIS OF DISLOCATIONS AND GROWTH BANDS IN A MELT GROWN GADOLINIUMGALLIUM GARNET CRYSTAL
H. L. Glass Research and Technology Division North American Rockwell Electronics Group, Anaheim, CA 92803
(Received October 30, 1972; Communicated by R. A. Huggins) ABSTRACT A melt grown crystal of gadolinium gallium sarn~t containing dislocations with a density of about 3 x lO~/cm was analyzed by X-ray diffraction topography. Most of the dislocations appear to have Burgers vectors in the direction parallel to the growth axis. These dislocations make relativelysmall angles with the growth axis so they are predominantly screw in character. Some evidence for the existence of decorated edge dislocations was found. In addition, anisotropy of growth band contrast was observed. This effect is interpreted in terms of elastic stresses between bands of different lattice parameter. Introduction Single crystals of gadolinium gallium garnet (chemical formula Gd3Gas012; shorthand notation GdGaG) are of interest as substrates for epitaxial magnetic bubble domain garnet films (i).
Since substrate defects
are readily replicated by the epitaxial films (2, 3) and since the replicated defects generally have adverse effects on bubble properties (i), the characterization and control of substrate defects are of considerable importance. Dislocations constitute one class of substrate defects.
Several
investigators have observed dislocations in garnets using chemical etching (4) optical microscopy (5), x-ray diffraction topography (6), or a combination of these techniques (7, 8).
For the most part, these observations were limited
to general surveys of dislocation densities and distributions.
43
However,
44
GADOLINIUM GALLIUM GARNET
Vol. 8, No. 1
Burgers vector directions were determined in flux grown YGaG (Y3Ga5012) using a photoelastic method (5) and by x-ray topography in melt grown YAG (Y3AI5012)
(6) and in flux grown YIG (Y3Feb012) and YGalG (Y3[Ga,Fe]5012)
(7).
This paper reports a Burgers vector analysis of dislocations in a melt grown GdGaG crystal using x-ray diffraction topography.
The previously
reported melt grown garnet (YAG) contained only a small number of dislocations which were confined to the outer regions of the boule and which appeared to be associated with inclusions.
The GdGaG crystal used in the present study,
on the other hand, contained a fairly high density of dislocations distributed throughout the boule. In addition to the Burgers vector analysis, this paper also describes some observations of strain anisotropy associated with grown bands. Experimental The material examined was a boule of GdGaG grown from the melt by the Czochralski method.
The boule was grown under conditions which produced
a slightly concave interface to repress facet development.
Most of the
boule was cut into thick wafers (about 0.75mm) for use as substrates for epitaxial film deposition.
These wafers were cut perpendicular to the
boule axis and were mechanically polished.
Except for a few isolated
scratches, the depth of residual surface damage was estimated to be no more than about 0.i ~m. Two of the substrate wafers were examined prior to film deposition using x-ray double crystal reflection topography,
These results, as well as topo-
graphs of wafers with films, have been reported elsewhere (2, 3).
For the
reader's convenience, a magnified topograph of one of the wafers without an epitaxial film is reproduced in Figure i.
The dislocations, which are viewed
nearly end-on, appear as high contrast point-like features distributed over the entire area of the wafer; but with a higher density near the center of the boule.
The dislocation density is about 3 x 103/cm 2 near the center.
Im addition to the perpendicularly cut wafers, one wafer cut parallel and close to the boule axis was prepared hy similar techniques but to a thickness of about 75 ~m.
This thin, longitudinal wafer was approximately rectangular
with dimensions of 12 x 20 mm. (ll~planes;
The wafer surface was nearly parallel to the
the long dimension being parallel to the [~i0] direction while
the short dimension was parallel to [ii[], the boule axis.
These orientation
relationships are shown in the stereographic projection of Figure 2 which also
Vol. 8, No. i
GADOLINIUM GALLIUM GARNET
45
FIG. i FeK~ (842) double crystal reflection topograph of (iii) GdGaG substrate wafer.
(042)
[1"10] (220) FIG. 2
Cubic (112) stereographic projection showing diffracting planes used in topography. Boule axis is [ii[].
46
GADOLINIUM GALLIUM GARNET
Vol. 8, No. 1
indicates the various (hkl) reflections for which topographs were recorded . (In some cases the (El) reflection was used instead of or in addition to the (hkl) reflectlo~ shown.)
The (842} form reflections near the center
of the stereographic projection were used for double crystal reflection topographs (2) wit~ FeK~ radiation.
All of the other indicated reflections
were used for Lang transmission topographs (9) with A g K ~
radiation.
Result..___.~s Figure 3 shows a (40~) Lang topograph of the central portion of the thin, longitudi~l wafer. (a)
Several types of defects are visible including:
The long, parallel, slightly curved bands which run approximately perpendicular to the boule axis and which cover the entire area of the wafer.
These have been identified previously (2) as growth bands
which delineate successive interfaces during boule growth. (b)
The shorter, dark lines which lie at angles of about 15 or 20 degrees to the boule axis.
Some of these short lines link up to form shallow
V-shaped figures. (c)
The long, wavy dark lines which lie nearly parallel to the boule axis and which exhibit contrast similar to the short lines described in (b).
(d)
The straight, dark lines which are nearly parallel to the boule axis. Some of these li~es are quite long and very intense. In addition to these four types of defects, other features are also
visible; mainly surface scratches. Topographs recorded using (04~), (24~), (~24), and (44~) reflections are similar in appearance to the (40~) topograph shown in Figure 3.
A (44~)
topograph was shown in reference (2). Topographs made with (220), (452), (4~4), and (~62) reflections, while similar to each other, are considerably different in appearance from Figure 3. Figure 4, a (~62) topograph. with very strorLg contrast. contrast.
This difference may be seen in
In this figure the lines of feature (d) appear Some surface scratches also exhibit strong
Features (a), (b), and (c), however, have nearly vanished.
Although the topographs fall naturally into the two categories exemplified by Figures 3 and 4, within each Category there are noticeable variations in defect contrast, particularly for the defects of type (d). Figure 5 shows a (248) asymmetric reflection double crystal topograph of the face of the wafer which had served as entrance surface for the Lang topographs.
Features (a) and (d) show strong contrast, while (b) and (c)
Vol. 8, No. 1
GADOLINIUM GALLIUM GARNET
FIG. 3 AgK~ I (40~) transmission topograph of (112) GdGaG longitudinal wafer. Oriented as in Figure 2 with boule axis horizontal.
FIG. 4 AgK~ 1 (~62) transmission topograph of (112) GdGaG longitudinal wafer shown in Figure 3.
47
48
GADOLINIUM GALLIUM GARNET
Vol. 8, No. 1
FIG. 5 FeK~ (248) double crystal reflection topograph of entrance surface of (112) GdGaG wafer shown in Figure 3. Print reversed for comparison with transmission topographs.
show weak contrast.
The lines of feature (d) are particularly interesting
since they exhibit both white and black contrast and because each of these lines is distinct at one end but becomes diffuse at the other end. Discussion The growth bands, feature (a), have been described elsewhere (2).
The
band contrast arises from slight lattice parameter differences between bands. These lattice parameter fluctuations have been ascribed to compositional variations resulting from thermal instability during boule growth.
Thus
the crystal may be thought of as a stack of lamellae which lle normal to the growth axis.
Neighboring lamellae differ slightly in composition.
Comperison of Figures 3 and 4 shows a considerable difference in lamellar contrast.
This indicates that the strain associated with the
lamellae is anisotroplc.
The strain anisotropy may be explained as follows.
Vol. 8, No. 1
GADOLINIUM GALLIUM GARNET
49
Each lamella has an unstrained, equilibrium lattice parameter determined by its composition.
But the lamellae are not free bodies.
On the contrary,
there is a stress exerted on each lamella by its neighbors which have slightly different compositions.
A lamella with slightly smaller lattice
parameter than its neighbor will experience a tensile stress tending to increase its lattice parameter in directions parallel to the lamella. Conversely, the lamella with larger lattice parameter will experience a compressive stress.
Thus, the inter-lamellar stresses tend to reduce the
lattice parameter differences in directions parallel to the lamellae.
By
a Poisson ratio effect, the lattice parameter differences in directions normal to the lamellae (parallel to the growth axis) will be enhanced.
This
explains the high lamellar contrast for diffracting planes normal to (or making large angles with) the growth axis (Figure 3) and the low contrast in topographs made with diffracting planes parallel to the growth axis (Figure 4).
The double crystal reflection topograph of Figure 5 was recorded
using planes nearly parallel to the growth axis.
The visibility of the
growth bands in this topograph may be ascribed to the high sensitivity of the double crystal method and possibly, to relaxation of the interlamellar stresses at the specimen surface. Similar observations of band strain anisotropy have been reported for oxygen bands in silicon (i0) and for striations in faceted regions of YAG (6). The features (b) and (c) are line defects.
Both of these groups exhibit
greatly reduced contrast for diffracting planes which are normal to the [ii~] growth axis, as in Figure 4.
Thus, it appears that these defects are
dislocations having Burgers vectors parallel to [Ii~] (9), probably ~[iii](5). Since these dislocation lines make relatively small angles with the [ii~] direction, they are predominantly screw in character.
This result is not
unexpected, since screw dislocations parallel to the boule axis should be easily propagated during crystal growth from the melt by the Czochralski method. The dislocations axis [ii~].
(b) tend to lie at angles of 15-20 ° from the growth
Assuming that these dislocation lines run through the wafer
from entrance surface to exit surface, the angle between the dislocation lines and the [112] surface normal can be calculated from the lengths of the lines and the specimen thickness.
This angle is about 78 ° .
Thus it
appears that these dislocations tend to lie along [21~] and [12~] directions.
50
GADOLINIUM GALLIUM GARNET
Vol. 8, No. 1
These are directions normal to planes on which faceting tends to occur (6,11). Although they show contrast effects similar to dislocations (b), the longer dislocation lines (c) are more nearly parallel to the [Ii[] growth direction. The intensely dark lines, feature (d), are also line defects.
This is
most clearly revealed by the reflection topograph, Figure 5, in which each of these lines appears distinct at one end and then tails off.
This is
characteristic of line defects which run from the front surface (distinct end) into the interior of the sample (12).
In the present case, these lines
presumably continue through to the back surface. The black-whlte contrast across these lines in Figure 5 suggests that these defects are dislocations which are predominantly edge in character; probably with Burgers vectors lying in or near the plane of the specimen (i12)°
The only simple Burgers vector which lles in this plane and would
correspond to an edge dislocation is a [[i0] (5).
However, this is a rather
large Burgers vector and would be associated with a high strain energy. Furthermore, these dislocations do not all exhibit the same variation in contrast with diffraction vector (reflecting plane).
Thus it appears that
these dislocations do not all have the same Burgers vector.
While a few
may have a [[i0] Burgers vectors, it is likely that most of these dislocations have Burgers vectors of ¢[i[i] ore[ill]. of the f o r m ~ l l l >
These Burgers vectors
correspond to lowest strain energy (5).
The Burgers vectors of these dislocations (d) could not be definitively determined since no reflections were found for which these dislocations vanished.
In fact, some of these dislocations show rather strong contrast
in all topographs although variatlons in contrast are distinguishable. The observed contrast variations are consistent with the interpretation given in the preceding paragraph.
The failure of these dislocations to go
out of contrast could conceivably be due to impurltyprecipitatlon along the dislocations.
Such decoration can modify the strain field (13) and nullify
the usual conditions for image contrast (I0).
Possible decorating species
are iridium (from the crucible) or components of the Gd203-Ga203 system. The failure to see evidence of decoration of dislocations of types (b) and (c) is not surprising, since decoration is more likely to occur on edge dislocations than on screw (14). The origin of edge dislocations aligned parallel to the boule axis in a Czochralski grown crystal is not clear. of impurities may be significant factors.
Thermal gradients and the presence The earlier study of melt grown
Vol. 8, No. 1
GADOLINIUM GALLIUM GARNET
51
YAG(6) showed dislocations associated with inclusions in the outer regions of the boule.
In the present study no inclusions were observed; however, the
edge dislocations occurred with noticeably higher density near the periphery of the boule. The results reported in this paper are for one particular boule of GdGaG.
This boule was somewhat unusual, since most boules of GdGaG have
very low dislocation densities.
This difference appears to be due to the
particular growth conditions employed which produced a slightly concave solid/liquid interface.
In spite of the apparent uniqueness of this boule,
most of the observed dislocation characteristics do not seem unusual and are probably representative of melt grown GdGaG with growth axis. Conclusions Under certain growth conditions GdGaG crystals grown by the Czochralski me~hod may contain a fairly high density of dislocations.
X-ray topography
reveals that most of these dislocations are of mixed character but with large screw components.
The Burgers vectors were found to be parallel to
the growth axis, in this case.
A few line defects were found which
lie nearly parallel to the growth axis and which appear to be dislocations having large edge components. decorated.
There is some evidence that these are
The topographs also show an anisotropy in growth band contrast.
This effect is ascribed to interband stresses which result from the lattice parameter differences between bands. Acknowledgements The author thanks W. R. Wilcox and S. B. Austerman for helpful discussions and D. Medellin and G. W. Johnson for specimen polishing. References i.
L. J. Varnerin, IEEE Trans. on Mag. MAG-7, 404 (1971).
2.
H. L. Glass, Mat. Res. Bull. ~, 385 (1972).
3.
H. L. Glass and T. N. Hamilton, Mat. Res. Bull. ~, 761 (1972).
4.
B. Cockayne and D. B. Gasson, J. Matls. Sci. ~, 112 (1966).
5.
M. J. Prescott and J. Basterfield, J. Matls. Sci. ~, 583 (1967).
6.
J. Basterfield, M. J. Prescott and B. Cockayne, J. !~tls. Sci. ~, 33 (1968).
7.
R. F. Belt, J. Appl. Phys. 40, 1644 (1969).
8.
P. E. Elkins, M. F. Ehman and H. L. Glass, to be published.
9.
A. R. Lang, J. Appl. Phys. 30, 1748 (1959).
52
GADOLINIUM GALLIUM GARNET
Vol. 8, No. 1
i0.
G. H. Schwuttke in Direct Observation of Imperfections in Crystals, edited by J. B. Newkirk and T. H. Wernlck, p. 497. Interscienee Publishers, New York (1962).
ii.
H. L. Glass, Mat. Res. Bull. 7, No. i0 (1972).
12.
U. Bonse in Ref. i0 p. 431.
13.
V. L. Indenbom~ V. I. Nikitenko and L. S. Milevskii, Soviet Phys. Doklady 6, 1034 (1962).
14.
W. C. Dash, J. Appl. Phys. 27, 1193 (1956).
Pergamon Press, Inc.
Printed in
X-RAY TOPOGRAPHIC ANALYSIS OF DISLOCATIONS AND GROWTH BANDS IN A MELT GROWN GADOLINIUMGALLIUM GARNET CRYSTAL
H. L. Glass Research and Technology Division North American Rockwell Electronics Group, Anaheim, CA 92803
(Received October 30, 1972; Communicated by R. A. Huggins) ABSTRACT A melt grown crystal of gadolinium gallium sarn~t containing dislocations with a density of about 3 x lO~/cm was analyzed by X-ray diffraction topography. Most of the dislocations appear to have Burgers vectors in the
Since substrate defects
are readily replicated by the epitaxial films (2, 3) and since the replicated defects generally have adverse effects on bubble properties (i), the characterization and control of substrate defects are of considerable importance. Dislocations constitute one class of substrate defects.
Several
investigators have observed dislocations in garnets using chemical etching (4) optical microscopy (5), x-ray diffraction topography (6), or a combination of these techniques (7, 8).
For the most part, these observations were limited
to general surveys of dislocation densities and distributions.
43
However,
44
GADOLINIUM GALLIUM GARNET
Vol. 8, No. 1
Burgers vector directions were determined in flux grown YGaG (Y3Ga5012) using a photoelastic method (5) and by x-ray topography in melt grown YAG (Y3AI5012)
(6) and in flux grown YIG (Y3Feb012) and YGalG (Y3[Ga,Fe]5012)
(7).
This paper reports a Burgers vector analysis of dislocations in a melt grown GdGaG crystal using x-ray diffraction topography.
The previously
reported melt grown garnet (YAG) contained only a small number of dislocations which were confined to the outer regions of the boule and which appeared to be associated with inclusions.
The GdGaG crystal used in the present study,
on the other hand, contained a fairly high density of dislocations distributed throughout the boule. In addition to the Burgers vector analysis, this paper also describes some observations of strain anisotropy associated with grown bands. Experimental The material examined was a boule of GdGaG grown from the melt by the Czochralski method.
The boule was grown under conditions which produced
a slightly concave interface to repress facet development.
Most of the
boule was cut into thick wafers (about 0.75mm) for use as substrates for epitaxial film deposition.
These wafers were cut perpendicular to the
boule axis
Except for a few isolated
scratches, the depth of residual surface damage was estimated to be no more than about 0.i ~m. Two of the substrate wafers were examined prior to film deposition using x-ray double crystal reflection topography,
These results, as well as topo-
graphs of wafers with films, have been reported elsewhere (2, 3).
For the
reader's convenience, a magnified topograph of one of the wafers without an epitaxial film is reproduced in Figure i.
The dislocations, which are viewed
nearly end-on, appear as high contrast point-like features distributed over the entire area of the wafer; but with a higher density near the center of the boule.
The dislocation density is about 3 x 103/cm 2 near the center.
Im addition to the perpendicularly cut wafers, one wafer cut parallel and close to the boule axis was prepared hy similar techniques but to a thickness of about 75 ~m.
This thin, longitudinal wafer was approximately rectangular
with dimensions of 12 x 20 mm. (ll~planes;
The wafer surface was nearly parallel to the
the long dimension being parallel to the [~i0] direction while
the short dimension was parallel to [ii[], the boule axis.
These orientation
relationships are shown in the stereographic projection of Figure 2 which also
Vol. 8, No. i
GADOLINIUM GALLIUM GARNET
45
FIG. i FeK~ (842) double crystal reflection topograph of (iii) GdGaG substrate wafer.
(042)
[1"10] (220) FIG. 2
Cubic (112) stereographic projection showing diffracting planes used in topography. Boule axis is [ii[].
46
GADOLINIUM GALLIUM GARNET
Vol. 8, No. 1
indicates the various (hkl) reflections for which topographs were recorded . (In some cases the (El) reflection was used instead of or in addition to the (hkl) reflectlo~ shown.)
The (842} form reflections near the center
of the stereographic projection were used for double crystal reflection topographs (2) wit~ FeK~ radiation.
All of the other indicated reflections
were used for Lang transmission topographs (9) with A g K ~
radiation.
Result..___.~s Figure 3 shows a (40~) Lang topograph of the central portion of the thin, longitudi~l wafer. (a)
Several types of defects are visible including:
The long, parallel, slightly curved bands which run approximately perpendicular to the boule axis and which cover the entire area of the wafer.
These have been identified previously (2) as growth bands
which delineate successive interfaces during boule growth. (b)
The shorter, dark lines which lie at angles of about 15 or 20 degrees to the boule axis.
Some of these short lines link up to form shallow
V-shaped figures. (c)
The long, wavy dark lines which lie nearly parallel to the boule axis and which exhibit contrast similar to the short lines described in (b).
(d)
The straight, dark lines which are nearly parallel to the boule axis. Some of these li~es are quite long and very intense. In addition to these four types of defects, other features are also
visible; mainly surface scratches. Topographs recorded using (04~), (24~), (~24), and (44~) reflections are similar in appearance to the (40~) topograph shown in Figure 3.
A (44~)
topograph was shown in reference (2). Topographs made with (220), (452), (4~4), and (~62) reflections, while similar to each other, are considerably different in appearance from Figure 3. Figure 4, a (~62) topograph. with very strorLg contrast. contrast.
This difference may be seen in
In this figure the lines of feature (d) appear Some surface scratches also exhibit strong
Features (a), (b), and (c), however, have nearly vanished.
Although the topographs fall naturally into the two categories exemplified by Figures 3 and 4, within each Category there are noticeable variations in defect contrast, particularly for the defects of type (d). Figure 5 shows a (248) asymmetric reflection double crystal topograph of the face of the wafer which had served as entrance surface for the Lang topographs.
Features (a) and (d) show strong contrast, while (b) and (c)
Vol. 8, No. 1
GADOLINIUM GALLIUM GARNET
FIG. 3 AgK~ I (40~) transmission topograph of (112) GdGaG longitudinal wafer. Oriented as in Figure 2 with boule axis horizontal.
FIG. 4 AgK~ 1 (~62) transmission topograph of (112) GdGaG longitudinal wafer shown in Figure 3.
47
48
GADOLINIUM GALLIUM GARNET
Vol. 8, No. 1
FIG. 5 FeK~ (248) double crystal reflection topograph of entrance surface of (112) GdGaG wafer shown in Figure 3. Print reversed for comparison with transmission topographs.
show weak contrast.
The lines of feature (d) are particularly interesting
since they exhibit both white and black contrast and because each of these lines is distinct at one end but becomes diffuse at the other end. Discussion The growth bands, feature (a), have been described elsewhere (2).
The
band contrast arises from slight lattice parameter differences between bands. These lattice parameter fluctuations have been ascribed to compositional variations resulting from thermal instability during boule growth.
Thus
the crystal may be thought of as a stack of lamellae which lle normal to the growth axis.
Neighboring lamellae differ slightly in composition.
Comperison of Figures 3 and 4 shows a considerable difference in lamellar contrast.
This indicates that the strain associated with the
lamellae is anisotroplc.
The strain anisotropy may be explained as follows.
Vol. 8, No. 1
GADOLINIUM GALLIUM GARNET
49
Each lamella has an unstrained, equilibrium lattice parameter determined by its composition.
But the lamellae are not free bodies.
On the contrary,
there is a stress exerted on each lamella by its neighbors which have slightly different compositions.
A lamella with slightly smaller lattice
parameter than its neighbor will experience a tensile stress tending to increase its lattice parameter in directions parallel to the lamella. Conversely, the lamella with larger lattice parameter will experience a compressive stress.
Thus, the inter-lamellar stresses tend to reduce the
lattice parameter differences in directions parallel to the lamellae.
By
a Poisson ratio effect, the lattice parameter differences in directions normal to the lamellae (parallel to the growth axis) will be enhanced.
This
explains the high lamellar contrast for diffracting planes normal to (or making large angles with) the growth axis (Figure 3) and the low contrast in topographs made with diffracting planes parallel to the growth axis (Figure 4).
The double crystal reflection topograph of Figure 5 was recorded
using planes nearly parallel to the growth axis.
The visibility of the
growth bands in this topograph may be ascribed to the high sensitivity of the double crystal method and possibly, to relaxation of the interlamellar stresses at the specimen surface. Similar observations of band strain anisotropy have been reported for oxygen bands in silicon (i0) and for striations in faceted regions of YAG (6). The features (b) and (c) are line defects.
Both of these groups exhibit
greatly reduced contrast for diffracting planes which are normal to the [ii~] growth axis, as in Figure 4.
Thus, it appears that these defects are
dislocations having Burgers vectors parallel to [Ii~] (9), probably ~[iii](5). Since these dislocation lines make relatively small angles with the [ii~] direction, they are predominantly screw in character.
This result is not
unexpected, since screw dislocations parallel to the boule axis should be easily propagated during crystal growth from the melt by the Czochralski method. The dislocations axis [ii~].
(b) tend to lie at angles of 15-20 ° from the growth
Assuming that these dislocation lines run through the wafer
from entrance surface to exit surface, the angle between the dislocation lines and the [112] surface normal can be calculated from the lengths of the lines and the specimen thickness.
This angle is about 78 ° .
Thus it
appears that these dislocations tend to lie along [21~] and [12~] directions.
50
GADOLINIUM GALLIUM GARNET
Vol. 8, No. 1
These are directions normal to planes on which faceting tends to occur (6,11). Although they show contrast effects similar to dislocations (b), the longer dislocation lines (c) are more nearly parallel to the [Ii[] growth direction. The intensely dark lines, feature (d), are also line defects.
This is
most clearly revealed by the reflection topograph, Figure 5, in which each of these lines appears distinct at one end and then tails off.
This is
characteristic of line defects which run from the front surface (distinct end) into the interior of the sample (12).
In the present case, these lines
presumably continue through to the back surface. The black-whlte contrast across these lines in Figure 5 suggests that these defects are dislocations which are predominantly edge in character; probably with Burgers vectors lying in or near the plane of the specimen (i12)°
The only simple Burgers vector which lles in this plane and would
correspond to an edge dislocation is a [[i0] (5).
However, this is a rather
large Burgers vector and would be associated with a high strain energy. Furthermore, these dislocations do not all exhibit the same variation in contrast with diffraction vector (reflecting plane).
Thus it appears that
these dislocations do not all have the same Burgers vector.
While a few
may have a [[i0] Burgers vectors, it is likely that most of these dislocations have Burgers vectors of ¢[i[i] ore[ill]. of the f o r m ~ l l l >
These Burgers vectors
correspond to lowest strain energy (5).
The Burgers vectors of these dislocations (d) could not be definitively determined since no reflections were found for which these dislocations vanished.
In fact, some of these dislocations show rather strong contrast
in all topographs although variatlons in contrast are distinguishable. The observed contrast variations are consistent with the interpretation given in the preceding paragraph.
The failure of these dislocations to go
out of contrast could conceivably be due to impurltyprecipitatlon along the dislocations.
Such decoration can modify the strain field (13) and nullify
the usual conditions for image contrast (I0).
Possible decorating species
are iridium (from the crucible) or components of the Gd203-Ga203 system. The failure to see evidence of decoration of dislocations of types (b) and (c) is not surprising, since decoration is more likely to occur on edge dislocations than on screw (14). The origin of edge dislocations aligned parallel to the boule axis in a Czochralski grown crystal is not clear. of impurities may be significant factors.
Thermal gradients and the presence The earlier study of melt grown
Vol. 8, No. 1
GADOLINIUM GALLIUM GARNET
51
YAG(6) showed dislocations associated with inclusions in the outer regions of the boule.
In the present study no inclusions were observed; however, the
edge dislocations occurred with noticeably higher density near the periphery of the boule. The results reported in this paper are for one particular boule of GdGaG.
This boule was somewhat unusual, since most boules of GdGaG have
very low dislocation densities.
This difference appears to be due to the
particular growth conditions employed which produced a slightly concave solid/liquid interface.
In spite of the apparent uniqueness of this boule,
most of the observed dislocation characteristics do not seem unusual and are probably representative of melt grown GdGaG with
X-ray topography
reveals that most of these dislocations are of mixed character but with large screw components.
The Burgers vectors were found to be parallel to
the growth axis,
A few line defects were found which
lie nearly parallel to the growth axis and which appear to be dislocations having large edge components. decorated.
There is some evidence that these are
The topographs also show an anisotropy in growth band contrast.
This effect is ascribed to interband stresses which result from the lattice parameter differences between bands. Acknowledgements The author thanks W. R. Wilcox and S. B. Austerman for helpful discussions and D. Medellin and G. W. Johnson for specimen polishing. References i.
L. J. Varnerin, IEEE Trans. on Mag. MAG-7, 404 (1971).
2.
H. L. Glass, Mat. Res. Bull. ~, 385 (1972).
3.
H. L. Glass and T. N. Hamilton, Mat. Res. Bull. ~, 761 (1972).
4.
B. Cockayne and D. B. Gasson, J. Matls. Sci. ~, 112 (1966).
5.
M. J. Prescott and J. Basterfield, J. Matls. Sci. ~, 583 (1967).
6.
J. Basterfield, M. J. Prescott and B. Cockayne, J. !~tls. Sci. ~, 33 (1968).
7.
R. F. Belt, J. Appl. Phys. 40, 1644 (1969).
8.
P. E. Elkins, M. F. Ehman and H. L. Glass, to be published.
9.
A. R. Lang, J. Appl. Phys. 30, 1748 (1959).
52
GADOLINIUM GALLIUM GARNET
Vol. 8, No. 1
i0.
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