Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1080~89 10924
The present invention relates to a method for
producing virtually perfect massive unicrystalline
gadolinium gallium garnet material. More particularly,
the present invention is directed to a method for pro-
ducing such material from an oxide melt containing a
predetermined amount of calcium, magnesium or strontium
ion.
Unicrystalline gadolinium gallium garnet
material in massive form is produced following the well
known Czochralski technique by pulling a seed rod from
a melt of Gd203 and Ga203 in a molar ratio of 3:5.
The resulting material is used in the form of wafers
~ as substrates in electronic applications such as for
- the epitaxial growth of iron garnet film. It is very
important that these substrates, and hence the crystal
from which they are formed, be free of imperfections,
e.g. linear edge dislocations. This is so since such
; dislocations will propagate into epitaxial layers formed
on crystalline substrates with well known detrimental
effects.
It has been found that the above noted imper-
fections occur with significant frequency unless the
crystal growing operation is very carefully monitored
. to detect signs of abnormal growth and the operational
thermal conditions appropriately adjusted by techniques
known to the art. Alternatively, it has been discovered
that virtually perfect unicrystalline material can be
produced without substantial abnormal growth by pre-
determined additions to the melt from which the crystal
is grown.
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~080589 - -
It is, therefore, an object of the present
invention to provide a method for producing massive
unicrystalline virtually perfect gadolinium gallium
garnet.
Other objects will be apparent from the
following description and claims taken in conjunction
with the drawing in which Figure 1 shows an apparatus
suitable for the practice of the present invention and
Figure 2 illlustrates a portion of a unicrystal-
line shape produced by the practice of the
present invention.
The method of the present invention for
producing virtually perfect unicrystalline gadolinium
gallium garnet involves the following steps:
(i) forming a melt by heating a mixture of
Gd2O3 and Ga2O3 in a molar ratio of 3:5
and adding a predetermined amount of at
least one metal ion selected from the
group consisting of calcium, magnesium
and strontium to provide from about ~S -~
to l00 ppm in the aggregate of selected
metal ion in the melt, the melt being
at a temperature in the range of 1700
to 1800C. . ~-
tii) inserting a seed rod of unicrystalline :
gadolinium gallium garnet into the melt,
tiii) maintaining an atmosphere ovèr the melt
which is substantially chemically inert
to the melt, and
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iO8~ 9
(iv) withdrawing the seed rod from the melt
such that gadolinium gallium garnet
material is solidified and crystallized
on the seed rod to form a virtually
perfect massive unicrystalline product
of increasing length and substantially
circular cross-section having a growth
axis common with the longitudinal axis of
the seed rod.
The improvement of the present invention with
respect to previous Czochralski growth techniques
resides in the addition to the oxide melt of at least
one of the metal ions of calcium, magnesium or strontium
to provide in the melt an appropriate amount of ion in
the range of about 15 ppm to 100 ppm whereby a virtually
perfect unicrystalline material free of imperfections,
e.g. edge dislocations is obtained. It has been found
that relatively fast crystal growth rates can be
achieved in the practice of the present invention with-
out detrimentally affecting the quality of the unicrystal-
line material which can range up to three inch diameters
and ten inches and more in lengths.
.~
~ In the practice of the present invention, the
. "
amount of selected metal ion provided in the melt
can be from about 15 parts per million (ppm) to about
100 ppm by weight, with the preferred amount being
from about 15 to 25 ppm. With lesser amounts of selected
metal ion, edge dislocations regularly occur in the
product crystal unless special precautions are taken
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108~)589
while at greater amounts undesirable discoloration of
the product crystal can occur.
The metal ion additions to the product crystal
are accomplished by melting additions of selected metal
oxides, carbonates or other ionizable compounds to
obtain the desired metal ion content in the melt from
which the product crystal is formed.
With reference to Figure 1, there is illustrated
a crystal growth chamber 1. A melt 9 of Gd203 and
Ga203 in a molar ratio of 3:5 having an addition of one
or more metal ions as described above is contained in
a crucible 8 which is preferably fabricated from iridium.
A washer 16, preferably iridium, having a central
aperture 17 rests on top the crucible 8 and acts as a
radiation shield to reduce heat loss from the melt 9.
The crucible 8 is bounded on its sides and bottom with
insulation lS. The insulation is preferably zirconia
and serves to: reduce the power required to sus~ain
the melt 9; reduce thermal gradients along the crucible;
and to dampen temperature fluctuations arising from line
voltage fluctuations, convective cooling effects from
the atmosphere, as well as other disturbances. Hollow
- tubing 11 forms an aperture through which the temperature
of the bottom of the crucible 8 can be determined by,
for example, a radiation pyrometer focused on the center
of the bottom of the crucible.
A ceramic washer 4, fabricated from alumina,
for example, is supported by tubing 5 preferably of
zirconia. The washer 4 serves as a secondary radiation
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108~)589
shield and to restrict the convective currents of the
atmosphere against entering the top of the crucible
and reaching the growing crystal 7. Thus, it serves
to reduce the vertical temperature gradients in the
vicinity of the growing crystal and to augment the
effects of the washer 16.
Sleeve 6, formed of silicon dioxide, for
example, serves to contain the insulation 15 and serves
as a part of the insulating assembly surrounding the
crucible 8. The tubing 5 which serves to support the
washer 4 also functions as a part of the insulating
system.
The crucible 8 and its surrounding insulating
assembly rests on a ceramic pedestal 12 composed of,
-; for example, zirconium oxide (ZrO2). The entire
assembly is enclosed in a bell jar 3 sealed to a base
~- plate 13. The base plate 13 is composed of any suit-
able material such as for example silicone-bonded fiber
- glass. The desired gas atmosphere for the inside of
the crucible 8, i.e. a gas atmosphere non-reactive
with the melt in the crucible, e.g. nitrogen with 2%
by volume oxygen, is introduced into sight tube 14
;`- which communicates with tubing 11. The gas exits
through the hole 18 in the bell jar 3 through which
the seed rod 2 is inserted. Seed rod 2 has an end
;; portion in the form of unicrystalline gadolinium gallium
-~ garnet material having its longitudinal axis 20 common
with the growth axis 30 of crystal 7 and the orientation
of the unicrystalline material of seed rod 2 is a pre-
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108~S~9
determined orientation depending on the ultimate
industrial use. Such a seed rod can be routinely
prepared and results in the production of a massive
unicrystalline material.
Using the above described apparatus, an
addition of a selected metal ion from the group of
calcium, magnesium and strontium is added to the
Gd203-Ga2O3 melt, for example, by including a compound
of the selected ion addition with the initial charge
prior to melting or by addition to the charge after
it has become molten. The temperature of the melt is
maintained in the range of 1700 to 1800C and a
unicrystalline mass is pulled from the melt following
procedures known to the art as exemplified by U.S.
patent 3,715,194. The resulting unicrystalline
material has a substantially circular uniform cross-
section and is virtually perfect and free of
imperfections. Figure 2 illustrates 2 portion of a ~ ;
shape of unicrystalline material 50 grown in accordance
; 20 with the present invention characterized by substantially
parabolic initial growth surfaces 60. When less than
about 15 ppm of selected metal ion are present, the
initial growth surfaces tend to follow the dotted line ~0
and unless rapidly corrected by adjustment of melt furnace
operation, results in crystal having imperfections 90. ~ -
- The following examples will further illustrate
the present invention.
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1080S89
EXAMPLE I
About 6000 grams of Gd203 and Ga203 in a molar
ratio of 3:5 were placed in an iridium crucible
having an inside diameter of 4.25 inches, a wall
thickness of .060 inch and a height of five inches.
The ~O3-Ga2 ~ material contained less than lO ppm
of calcium, magnesium and strontium. The crucible
was placed within a 10 turn induction heating coil
having an I.D. of 7.53 inches. The crucible stood on
a pedestal containing packed zirconia granules while
the space between the coil and the crucible was also
packed with zirconia granules. This entire apparatus
was enclosed in an aluminum bell jar having an aperture
at its top. A nitrogen atmosphere containing about
2% by volume oxygen was maintained inside the bell jar.
The induction heating coil was energized from a we~l -
known R.F. induction heating unit and the power was
increased until the induced current in the iridium
crucible heated it to a "white heat". Conductive heat
from the iridium crucible then melted the Gd203 and
~ O3to form a melt. A unicrystalline gadolinium
gallium garnet seed 0.25 inch diameter tclll>
orientation) was lowered through the aperture in the
bell jar until it contacted the surface Oc the melt.
The seed was then withdrawn from the melt at about
0.125 inch per hour for 9 hours. A highly distorted
boule of irregular cross-section was obtained which
contained numerous gross edge dislocations.
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10805~9
EXAMPtE II
About 6000 grams of Gd203 and G~03 in a molar
ratio of 3:5 without any addition were placed in an
iridium crucible having an inside diameter of 4.25
inches, a wall thickness o' 0.060 inch and a height of
5 inches. The Gd203-Ga203 material contalned less
than 10 ppm of calcium, magnesium, and strontium.
The crucible was placed within a 10 turn induction
heating coil having an I.D. of 7.5 inches. The
crucible stood on a pedestal containing packed ~ ;
zirconia granules while the space between the coil
and the crucible was also packed with zirconia granules.
This entire apparatus was enclosed in a glass bell jar
having an aperture at its top. A nitrogen atmosphere
containing about 2~ by volume oxygen was maintzined
inside the bell jar. The induction heating coil was
energized from a well known R.F. induction heating
unit and the power was increased until the induced
current in the iridium crucible heated it to a "white
heat". Conductive heat from the iridium crucible then ;
melted the Gd203-Ga203 material to form a melt. 1.5
grams of CaC03 were added to the Gd203-Ga203 melt in
the crucible to provide a calcium ion content of 100
`~ ppm in the melt. A unicrystalline gadolinium gallium
~; .
garnet seed 0.25 inch diameter ~<111>) was lowered
through the aperture in the bell jar until it contacted
the surface of the melt. The seed was then withdrawn
from the melt at about 0.125 inch per hour for 27 hours.
The final elongated boule was of uniform circular cross-
section having a final diameter measurement of 3.2 inches
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108~589
and a final length o~ 6.25 inches. This boule was
massive vi-tuall~J perfect unicrystalline gadolinium
gallium gæ~net free oî edge dislocations.
EXAM~L~ III
About 6000 grams o. Gd203 and Ga203 in a molar
ratio of 3:5 without any addition were placed in an
Lridium crucible having an inside tiameter of 4.25
inches, a wall thickness of 0.060 inch and a height of
5 inches. The Gd2o3-Ga2o3 material contained less
than lO ppm of calcium, magnesium, and strontium.
T~e crucible was placed within a 10 turn induction
heating coil having an I.D. of 7.5 inches. The
crucible stood on a pedestal containing packed
zirconia granules while the space between the coil
and the crucible was also packed with zirconia granules.
This entire appar2tus was enclosed in a glass bell jar
having an aperture at its top. A nitrogen atmosphere
containing about 2% by volume oxygen was maintained
i~.side the beil jar. The induction heating c9il was
energized from a well known R.F. induction heating
unit and the power was increased until the induced
current i~ the iridium crucible heated lt to a t'~hite
heat". Conductlve heat rrom the iridi~m cr~cible then
melted the Gd2O3-Ga203 mat_rlal to form a melt. a . s
grams of CaCO3 were added to the G~ 03-G~ ~ melt in
the crucible to provide a calcium ion content OL 3 3
ppm in the melt. A unicrystalline gadoliniu~ gallium
garnet seed 0.2S inch diameter ~<111>) was lowered
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10926
through the. zpe~ture in the bell jar until it contacted
the surface of the mel.. The seed was then withdrawn
from the melt at a~out 0.125 inch pe- hour for 30 hours.
The final elongated boule was of unifor~ circular cross-
section having z final dia~eter ~easure~ent of 3.2 inches
and a final length of 6.25 in-hes. This boule was
massive virtually pe-fect unicrystalline gadolinium -~
galliu~ garnet free of edge dislocations.
EXAMPLE IV
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Abo~t 5500 grams of 6d203 and Ga203in a ~olar
ratio of 3:5 without any addition were placed in an
iridium crucible having an inside diameter of 4.00
inches, a ~all thic~ness of O.lO inch ant a height of
4.5 inches. The Gd2O3- ~ O3material conla ned less
th&n lO ppm of calcium, magnesium, and strontium.
The crucible was placed within a lO turn induction
heating coil hav~ng an I.D. or 7.~ inches. The
crucible stocd on a pedestal contzining packed - -~
zirconia granules wh~le the space be~een the coil
and the crucible was also packed with zi~conia granules.
This enti-e appar2tus was er.closed in a glass bell jar
having an a?ertu~e at its top. A nitrogen at~ospr.er~
containir.g about 2% by volume o~ygen W2S maintaine~
inside the bell i r. The induct~on heating coil was
energi2ed f-om a well ~own R.F. i~duction heating
unit and the power was inc.eased until .~e induced
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108[)589
current i~ the i-idium crucible heated it .o a "white
heat". Conductive hea~ f~om the iridium crucible then
melted the Gd203-Ga203 materlal to form a melt. 0.66
grams of CaC0~ were adde~ to the Gd203^G~2~ melt in
- the crucible to provide a calcium ion content of 50
ppm in the melt. A unicrystalline gadolinium gzllium
garnet seed 0.2~ inch diam~ter t<lll~) W2S lowered
through the aperture in the bell jar until it contacted
the surface of the melt. The seed was then withdrawn
from the melt at about 0.18 inch per hour for 42 hours.
The final elongated boule wzs of uniform ci-cular cross-
; section having a final dlameter measurement or 2;25 inches
and a final length of 10.00 inches. This boule was
; massive virtuzlly perfect unicrys.alline gadolinium
gallium garnet free of edge dislocations.
..
EXAMPT.F V
.~ :
About ssao grams of Gd203 andGa203 in a molar
` ratio of 3:5 without any addition were placed in an
iridium crucible having an inside dl2meter of 4.0C
2a inches, a wall thickness of 0.100 inch and a height of
4.5 inches. The Gd2~-Ga2~ material con.air.ed less -
thar. lO ppm of calcium, magnesium, and strontlum.
The cruclble was 21aced witnir. a 10 turn induction
heating coil h~ving an I.D. of 7.5 inches. The
crucible stood on a pedestal con.aining packed
zirconia g~anules while the space between the coil
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1 O 8 ~ S 8 9 10924
and the crucible was also pa~ed with zi-conia granules.
This entire app2r2tus was enclosed in a glass bell jar
having an aperture at its top. A nitrogen atmosphere
containing about 2~ by volume oxygen was maintained
inside the bell jar. The intuction heatir.g coil was
energized from a well know~ R.F. induction heating
unit and the power was incre2sed until the induced
current in the iridium cr~cible heated it to a "white
heatn. Conductive heat from the iridium crucible then
melted the Gd203-Ga203 material to form a melt. ~.34
grams of CaC03 were added to the Gd2 ~ ~ ~ melt in
the crucible to provide a calcium ion content of 25
ppm in the melt. A unic-ystalline gadolinium gallium ~ -
garnet seed 0.2~ inch diameter ~ ) was lowered
through the aperture in the bell jar until it contacted
the surface of the melt. The seed was then withtrawn
from the melt at about 0.18 inch per hour for 34 hours.
The final elongated boule was of uniform circular cross-
section having a final diameter measurement of 2 2 inches
and a final length of 8.00 inches. This boule was
massive virtually perrect unicrystalline gadolinium ~;
gallium garnet free of edge dislocations.
As can be seen from the foregoing exzmples,
the gztolini~m g211i~m gzrnet boules of Examples II,
III, IV and ~ produced 'ro~ calcium ion containing
mel~s in accordance with the present invention were
free o. edge dislocations, whereaas the gadolinium
gallium garnet prQduct o~ Exzmple I which contzin2d
less than 10 ppm in .ne aggregate o. c21cium,
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10924
1080589
magnesium and strontium was grossly distorted and
contained numerous edge dislocations even though
; made by essentially the same procedure as Examples
II to V.
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