Method for growing single crystals of thermally unstable ferroelectric materials

A method is disclosed for growing thermally unstable ferroelectric materials having the formula MH.sub.2 XO.sub.4, where M is potassium, rubidium, cesium or ammonium; H is hydrogen or deuterium and X is phosphorus or arsenic. The ferroelectric material is heated to melt temperature in a constant volume cylindrical chamber (10) which is moisture-free. Improved crystal formation is accomplished by axially cooling the melt from the bottom end (18) of the chamber by thermal conduction along the chamber longitudinal axis predominantly and only minimally by radial thermal conduction through the sides (16) of the chamber. The axial cooling produces a crystal interface which is flat and perpendicular to the chamber axis and which gradually progresses toward the chamber top to provide uniform growth of a single crystal of ferroelectric material.

BACKGROUND OF THE INVENTION 
The present invention relates generally to methods for growing single 
crystals of ferroelectric materials which are thermally unstable at 
temperatures below their melting points. More particularly, the present 
invention relates to growing single crystals from a melt of the 
ferroelectric material in a manner such that the material does not 
thermally decompose and so that a high quality crystal is produced. 
Ferroelectric materials which are generally classified as belonging to the 
KDP family include the following compounds: 
KH.sub.2 PO.sub.4 (or KDP), KD.sub.2 PO.sub.4 (or KD*P), RbH.sub.2 PO.sub.4 
(or RDP), RbD.sub.2 PO.sub.4 (or RD*P), CsH.sub.2 PO.sub.4, CsD.sub.2 
PO.sub.4, (NH.sub.4)H.sub.2 PO.sub.4 (or ADP), (NH.sub.4)D.sub.2 PO.sub.4 
(or AD*P), (NH.sub.3 D)H.sub.2 PO.sub.4, (NH.sub.3 D)D.sub.2 PO.sub.4, 
(NH.sub.2 D.sub.2)H.sub.2 PO.sub.4, (NH.sub.2 D.sub.2)D.sub.2 PO.sub.4, 
(NHD.sub.3)H.sub.2 PO.sub.4, (NHD.sub.3)D.sub.2 PO.sub.4, 
(ND.sub.4)H.sub.2 PO.sub.4, (ND.sub.4)D.sub.2 PO.sub.4, KH.sub.2 
AsO.sub.4, KD.sub.2 AsO.sub.4, RbH.sub.2 AsO.sub.4 (or RDA), RbD.sub.2 
AsO.sub.4, CsH.sub.2 AsO.sub.4 (or CDA), CsD.sub.2 AsO.sub.4 (or CD*A), 
(NH.sub.4)H.sub.2 AsO.sub.4 (or ADA), (NH.sub.4)D.sub.2 AsO.sub.4 (or 
AD*A), (NH.sub.3 D)H.sub.2 AsO.sub.4, (NH.sub.3 D)D.sub.2 AsO.sub.4, 
(NH.sub.2 D.sub.2)D.sub.2 AsO.sub.4, (NH.sub.2 D.sub.2)D.sub.2 AsO.sub.4, 
(NHD.sub.3)H.sub.2 AsO.sub.4, (NHD.sub.3)D.sub.2 AsO.sub.4, 
(ND.sub.4)H.sub.2 AsO.sub.4, (ND.sub.4)D.sub.2 AsO.sub.4 and mixtures of 
these. 
Compounds such as KHDPO.sub.4, (NH.sub.4)HDPO.sub.4, etc. or mixtures of 
deuterated and undeuterated counterparts are also considered part of the 
KDP family. Single crystals made from these materials are known to have 
good non-linear optical properties and are, therefore, used extensively in 
the electro-optical industries. 
The above-listed ferroelectric materials which belong to the KDP family are 
notoriously thermally unstable at temperatures below their melting points. 
Typically, the materials undergo partial decomposition as they are heated 
to temperatures near their melting point. Due to this thermal instability, 
conventional high temperature crystal growth methods, such as the 
Czochralski and Bridgman techniques where crystals are grown from their 
respective melts, have not been developed. Instead, the crystals are grown 
at much lower temperatures from aqueous solutions by solvent evaporation. 
The solvent evaporation method is the conventional procedure used in 
large-scale production of these crystals. This method is a slow process, 
with growth rates on the order of one millimeter per day being typical. 
The quality of the crystals grown by evaporation from solution suffers 
from microscopic solvent inclusions. Further, some optical and 
electro-optical properties of the crystals grown from solution will vary 
depending upon the amounts of potassium carbonate (K.sub.2 CO.sub.3) 
and/or phosphoric acid (H.sub.3 PO.sub.4) additives added to the growth 
solutions. These additives are conventionally used in different amounts by 
various commercial manufacturers to control solution pH and enhance 
crystal growth. 
It would be desirable to provide a relatively quick method for preparing 
quality crystals from materials belonging to the KDP family which does not 
involve the relatively slow growth of crystals from aqueous solutions. It 
would further be desirable to provide a method which produces crystals 
having uniform optic and electro-optic properties and which do not have 
microscopic solvent inclusions. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a method is provided which allows 
relatively rapid and simple growth of high quality crystals from 
ferroelectric materials of the KDP family. The method is based on the 
growth of temperature-sensitive KDP-type materials from a melt of the 
material in a manner which prevents thermal decomposition and produces a 
high quality crystal having uniform and reproducible optic and 
electro-optic properties. The method has application to growing single 
crystals of ferroelectric materials having the formula MH.sub.2 XO.sub.4, 
where M is potassium, rubidium, cesium or ammonium; H is hydrogen or 
deuterium and X is phosphorus or arsenic. 
The method involves providing a sealed vessel which defines a constant 
volume cylindrical chamber having a top end, side walls and a bottom end. 
Except for KH.sub.2 PO.sub.4, the chamber is substantially filled with the 
ferroelectric material to be processed into a single crystal and is also 
moisture-free. The chamber has an axis extending longitudinally from the 
chamber top end to the chamber bottom end. The ferroelectric material in 
the chamber is uniformly heated to a sufficient temperature to uniformly 
melt the material only and not decompose the material. The use of a 
constant volume sealed chamber limits the extent of decomposition of the 
material when melted. 
The melted material is then axially cooled to form a single solid crystal. 
The axial cooling is accomplished from the bottom end of the chamber by 
thermal conduction along the chamber axis, rather than by radial thermal 
conduction through the vessel side walls. This axial-only cooling produces 
a crystal interface between the solid crystal material and the melted 
material which is flat and perpendicular to the chamber axis. The crystal 
interface progresses from the chamber bottom end to the chamber top end 
due to the axial-only cooling and thereby provides uniform growth of the 
single crystal from the bottom end to the top end of the chamber. 
The method in accordance with the present invention provides a simple and 
relatively fast means for reproducibly and accurately growing high quality 
crystals from thermally unstable ferroelectric materials. The new method 
is an improvement over the slower solution type crystal growth methods 
presently in use which produce crystals having solvent inclusions and 
varying properties. 
The above discussed and many other features and attendant advantages of the 
present invention will become better understood by reference to the 
following detailed description when considered in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention relates to growing high quality single crystals from 
compounds in the KDP family. For the purposes of this specification, the 
KDP family will be defined as those ferroelectric materials having the 
formula MH.sub.2 XO.sub.4, where M is potassium, rubidium, cesium or 
ammonium; H is hydrogen or deuterium and X is phosphorus or arsenic. The 
ammonium may be partially or fully deuterated. 
The KDP family of compounds includes KH.sub.2 PO.sub.4 (KDP), 
(NH.sub.4)H.sub.2 PO.sub.4 (ADP), RbH.sub.2 PO.sub.4, CsH.sub.2 PO.sub.4, 
their corresponding arsenates and the fully and partially deuterated forms 
of the phosphates and arsenates as set forth in the Background of the 
Invention. 
The ferroelectric materials collectively known as the KDP family are 
thermally unstable at or near their melting temperatures. When heated to 
melting, these materials will decompose according to the chemical 
reaction: 
##STR1## 
where y is a positive number not greater than 1 and is a measure of the 
extent of thermal decomposition of the ferroelectric material. If the 
material is heated in open air, so that the water vapor, H.sub.2 O(g), is 
continuously removed from the reaction site, then the thermal 
decomposition will proceed to the point where only trace amounts of 
constitutioned water remain in the melt, i.e., y is almost equal to 1. 
It was found that if the same starting material was heated to melting in a 
sealed chamber, its thermal decomposition did not proceed to the same 
extent as it did in open air. The extent of decomposition decreased as the 
volume of the reaction chamber was diminished. If the vapor space in the 
reaction chamber was reduced to the point where its volume was equal to 
that of the molten mass, it was found that the extent of decomposition was 
negligible, with y being less than about 0.01. At such low values of y, it 
was discovered that the material behaved like a congruent melter, i.e., 
like a material that melted or crystallized reversibly at its melting 
point. 
In accordance with the present invention, high quality uniform crystals of 
KDP family materials except for KDP itself can be grown from their melts 
in a sealed or constant-volume vessel, if the vessel has the following 
features: (1) it must have minimal vapor space, or volume at most equal to 
the volume of the melt, (2) it must be provided with a nucleation, or 
seeding, end at which crystal growth may be initiated, and (3) it must be 
designed to allow for unidirectional cooling, starting from the nucleation 
end. In the case of KDP, the vapor space should be from about two-thirds 
to five-sixths of the constant-volume vessel. 
Two preferred exemplary vessels which may be used in the method of the 
present invention are shown in FIGS. 3 and 4. Both FIGS. 3 and 4 show the 
vessel at four stages (A, B, C, and D) during preparation of the sealed 
vessel prior to heating. Referring to FIG. 3, a first exemplary embodiment 
is shown in which a cylindrical vial 10 is made from vitreous silica or 
other suitable material. The vial 10 is preferably formed in the end of a 
cylindrical rod. The vial 10 defines a cylindrical chamber 12 which has a 
top end 14, side walls 16 and bottom end 18. The bottom end 18 preferably 
tapers inward to point 20 to provide a suitable location for initiation of 
crystal growth. 
Crystal growth may be initiated in either of two ways. The first is 
referred to as spontaneous nucleation. To achieve spontaneous nucleation, 
the crucible is designed in such a way that only one crystallite nucleus 
can form where crystal growth is to be initiated. Preferred designs of the 
crucible bottoms for spontaneous nucleation are shown in FIGS. 3 and 4 at 
18 and 44, respectively, and in FIG. 5 at 45. The conical bottoms 18 and 
44 have been used because they are simpler to fabricate. Since the bottom 
comes to a point, only one nucleus can form there. The design shown in 
FIG. 5 permits several nuclei to form in the bottom 47. However, only that 
nucleus which is favorably oriented (i.e., whose fastest growing direction 
is parallel to the vertical axis of the nucleating well 49) will emerge 
from the nucleating well 49, shutting off the other nuclei from the supply 
of nutrient material, viz., the melt. 
The second way to initiate growth is by seeding, in which a seed crystal is 
placed at the growth initiation site. The same designs of crucible bottom 
discussed may be used for seeded growth. Since the seed crystal has to be 
cut to a shape that will fit the crucible bottom, the design shown in FIG. 
5 is preferred. Many times it is desirable to grow crystals with a 
specific crystallographic orientation. Seeded growth provides this 
orientation control. Seeded growth has been utilized in crucibles having 
the bottom design of FIG. 5 to grow ADP crystals with controlled crystal 
orientation. 
Vials made from silica glass are not quite inert to molten KDP. They are 
attacked by the melt, and this chemical action alters the melt 
composition, making the crystallization of KDP from it difficult. There is 
also a problem in growing ADP in silica glass, i.e., the problem of 
adhesion. The ADP crystal contracts more than does the silica glass 
crucible as they are cooled down to room temperature. Since the ADP 
cyrstal adheres to the crucible wall, it is subjected to tension during 
this cooling, and fractures as a result. In order to solve the corrosion 
problem with KDP and the adhesion problem with ADP, it is preferred that 
the interior of the silica glass crucible be lined with a carbon film. 
This carbon film is conveniently deposited on the silica glass surface by 
baking the crucible in a furnace at about 800.degree. C. while a stream of 
acetone vapor and nitrogen is caused to flow in the crucible. This method 
of deposition of pyrolytic carbon on silica glass has been used in the 
past for crystal growth of cadmium telluride. Stainless steel crucibles 
that are electroplated with gold on the interior walls may also be used to 
grow crystals in accordance with the present invention. 
Prior to sealing, the vial 10 includes a neck portion 22 which defines 
opening 24. The ferroelectric material to be processed is introduced into 
chamber 12 through opening 24. The chamber 12 is substantially completely 
filled with the ferroelectric material as shown at 26 in FIG. 3B. The 
ferroelectric material which is introduced into chamber 12 may be in 
particulate form. The particles should be small enough so that they are 
easily handled and introduced through opening 24, while not being so small 
that they are entrained when the vacuum is applied to the vial 10 when it 
is sealed. 
Prior to sealing vial 10, it is preferred that a vacuum be applied to the 
chamber 12 as represented in FIG. 3B, in order to remove any moisture 
present in chamber 12. The amount of vacuum which is applied to chamber 12 
is not particularly critical so long as substantially all of the moisture 
present in chamber 12 is removed prior to sealing. A vacuum on the order 
of 5 Torr is acceptable. Also, the material may be slightly heated in 
order to ensure complete moisture removal. 
After substantially all of the moisture has been removed from chamber 12, 
the neck 22 is sealed as shown in FIG. 3C. Sealing of neck 22 is 
preferably accomplished by heating the neck to a sufficient temperature to 
melt the vitreous silica and provide a heat seal. Other possible methods 
for sealing the neck 22 can be used so long as the seal is sufficiently 
tight to maintain the vacuum and provide a constant volume cylindrical 
chamber. 
After sealing, the vial 10 is placed within an insulated housing 28 as 
shown in FIG. 3D. The insulated housing 28 must be made from a suitable 
insulating material and must also be of sufficient thickness to minimize 
radial thermal conduction in the direction represented by arrows 30. It is 
important that thermal conduction be predominantly axial in the direction 
represented by arrow 32. The insulation used for housing 28 may be any 
suitable insulating material such as asbestos or other low thermal 
conduction material. The diameter of the cylindrical chamber 12 can be 
varied depending upon the size of crystal to be grown. Typical crystal 
diameters are from about 0.5 inch to 1.5 inch. 
Referring now to FIG. 4, a second exemplary vial 40 for growing crystals in 
accordance with the present invention is shown. The vial 40 is also 
preferably formed in the end of a cylindrical vitreous silica rod and 
includes sidewalls 42, tapered bottom 44 and an open top 46. The interior 
walls of the vial 40 are also preferably carbon lined. The open top 46 
provides a much larger opening through which the ferroelectric material 
may be introduced into the chamber 48. In addition to a particulate charge 
of materials, charge material that is powder which is compressed into 
pellet form may be placed into chamber 48. Although the charge material 
may be in particulate form, it is preferred that the charge material be in 
the form of pressed tablets. The particulate, or powder, form has 
approximately half the density of the bulk solid form. Therefore, when it 
is melted down in the sealed crucible, at least half of the interior 
volume of the crucible is vapor space, the other half being occupied by 
the melt. This volume ratio is acceptable; however, it is preferred that 
the powder be pressed into tablets with density approximately that of the 
bulk solid and with diameter just less than the internal diameter of the 
crucible. The pellet may be inserted directly into the crucible or vial 
40. In practice, the conical bottom 44 of the crucible may be filled with 
powder, then the pressed tablets may be stacked over it. The value of y 
can be reduced to as low as 0.0001 with this technique. Also, by using 
compressed powder pellets, the possibility of entrainment of charge 
materials during removal of moisture by vacuum is eliminated. 
The chamber 40 is filled with the ferroelectric material 50 which is to be 
processed as shown in FIG. 4B. A plug 52 is inserted into the vial open 
top 46 and sealed to the vial 40 as also shown in FIG. 4B. The plug 52 
includes a vacuum outlet 54 which is adapted to be attached to a vacuum to 
provide removal of moisture from the ferroelectric material 50. After the 
chamber 48 has been evacuated to remove substantially all of the moisture 
from the ferroelectric material 50, the outlet 54 is sealed off as shown 
in FIG. 4C. Again, sealing is preferably accomplished by melting the 
vitreous silica material to provide an air-tight seal. The sealed vial 40 
is then placed in an insulating jacket 56 which is basically the same as 
the insulating jacket 28 previously described. 
The following description will be limited to a discussion of crystal growth 
in accordance with the present invention utilizing the vial shown in FIG. 
3. The description applies equally to the alternate vial configuration 
shown in FIG. 4. 
After the vial 10 is sealed and placed within the insulating housing 28, it 
is first necessary to heat the KDP-family material 26 to a sufficient 
temperature to form a uniform melt of material. A preferred exemplary 
furnace for heating the insulated vial is shown generally at 60 in FIG. 2. 
It is important that the furnace 60 be designed so that the KDP-family 
material 26 is uniformly heated. If there are hot spots in the furnace, 
local overheating of the ferroelectric material may result. The localized 
overheating can cause localized decomposition of the material. Such 
localized decomposition may result in bursting of the vial. Even if 
bursting of the vial does not occur, the melt composition would 
nevertheless undesirably deviate farther than necessary from the starting 
composition. Such deviations would require that crystal growth be slowed 
down. The furnace 60 includes an anodized cylindrical aluminum sleeve 
lining 62. The aluminum sleeve lining ensures that the temperature in the 
furnace is uniform. A conventional cylindrical heating element 64 is 
located around the aluminum sleeve 62 to provide sufficient heat to melt 
the ferroelectric material. Insulation sleeve 66 is provided to allow 
accurate control of temperature within the heating chamber 68 and to 
prevent undesirable conduction of heat to the surrounding atmosphere. The 
sleeve 62 may be made from metals other than aluminum, so long as the 
metal is a good conductor of heat. 
The steps of uniformly heating and axially cooling the KDP-family material 
in accordance with the present invention are shown in FIG. 1 and labeled 
as A, B and C. In step A, the material is shown after it has been heated 
in furnace 60 to a sufficient temperature to form a melt 70 of the 
ferroelectric material. The particular temperature to which the melt must 
be heated will vary depending upon the particular KDP-family material 
being processed. Preferably, the temperature of the melt will be about 
10.degree. C. to 15.degree. C. above the melting point of the material. 
Melting points for KDP-family materials typically range from about 
200.degree. C. to about 300.degree. C. For example, ADP has a melting 
point of about 200.degree. C., while KDP has a melting point of about 
270.degree. C. 
After the melt 70 has reached the desired uniform temperature, the vial 10 
and insulated housing 28 are slowly withdrawn from the bottom 69 of the 
furnace 60. The rate at which the vial 10 is withdrawn from the furnace 
will vary, depending upon the particular ferroelectric material being 
crystallized and the crosssectional diameter of the melt. In general, 
withdrawal rates of up to about 2 millimeters per hour have been applied 
successfully, and it is believed that growth rates up to at least about 4 
mm/hr are possible. However, slower rates are needed with KDP itself, from 
about 0.1 mm to about 0.7 mm per hour. 
As the vial 10 and insulated housing 28 are withdrawn from furnace 60, the 
melt cools by axial thermal conduction as represented by arrow 72. The 
tapered bottom of the chamber allows the formation of a small seed crystal 
at point 74 from which the single ferroelectric crystal is grown. The 
axial-only cooling provided by this particular configuration produces a 
crystal interface 76 between the ferroelectric crystal 78 and melt 80. The 
interface 76 is flat and perpendicular to the axis of the chamber 12. The 
rate of withdrawal or pulling of the vial from furnace 60 is controlled so 
that the axial-only cooling produces the desired flat interface only and 
does not produce a concave or convex interface in which undesirable nuclei 
may form. The crystal interface progresses from the bottom end of the 
chamber to the top end of the chamber as the vial is slowly pulled from 
the furnace 60. FIG. 1C shows the final crystallized melt 82 which is 
formed after complete axial cooling and crystallization of the melt. 
It is preferable that the insulated housing 28 fit tightly within the 
furnace chamber 68. The absence of an air space between the insulated 
housing 28 and aluminum sleeve 62 prevents air convection alongside the 
vial and provides the thermal stability which is essential to the crystal 
growth process. 
The crystal growth process in accordance with the present invention has 
been used to grow high quality crystals of ADP. ADP, KDP, CsH.sub.2 
AsO.sub.4, RbH.sub.2 AsO.sub.4 and (ND.sub.4)D.sub.2 PO.sub.4 are 
particularly preferred materials which are amenable to crystal growth 
utilizing the present invention. The crystal phase for KDP and ADP at room 
temperature belongs to the non-centro symmetrical point group 42 m. In 
addition the process may be used to grow crystals which have been doped 
with about 2% to about 5% of any other member of the KDP family. This 
effectively reduces the melting temperature of the basic member and 
facilitates growth of crystals. We have also found that the arsenates 
crystals are easier to process and that the process may be used to process 
binary mixture of the KDP family, e.g., two phosphate compounds, two 
arsenate compounds or a mixture of a phosphate and arsenate. 
The crystal growth method in accordance with the present invention provides 
an improved method for growing single crystals of thermally unstable 
ferroelectric material which is based on modifications of the Bridgman 
crystal growth technique. Due to the combined features of heating within a 
constant volume, moisture-free melt chamber, along with the provision for 
axial-only cooling during crystal formation, the present method provides a 
simple and efficient method for growing ferroelectric materials which is 
significantly faster than the prior solution growth techniques. Further, 
the quality of the crystals obtained are equal to or better than crystals 
formed by the solution growth techniques. 
Having thus described exemplary embodiments of the present invention, it 
should be noted by those skilled in the art that the within disclosures 
are exemplary only and that various other alternatives, adaptations and 
modifications may be made within the scope of the present invention. 
Accordingly, the present invention is not limited to the specific 
embodiments as illustrated herein, but is only limited by the following 
claims.