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Патент USA US3053648

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Sept. 11, 1962
w. SHOCKLEY
3,053,635
METHOD OF GROWING SILICON CARBIDE CRYSTALS
Filed Sept. 26, 1960
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WILLIAM
SHOCKLEY
INVENTOR
sYiaaww/m
ATTORNEY
United States Patent
1
ice
3,053,635
Patented Sept. 11, 1962
2
Referring to the drawing, a graphite crucible 11 is
3,053,635
METHGD 0F GROWING SHJCON
CARBEDE CRYSTALS
.
William Shockley, Los Altos, Cali?, assignor to Cievite
Corporation, Cleveland, Ohio, a corporation of Ohio
Filed Sept. 26, 1960, Ser. No. 58,308
17 Claims. (Cl. 23-208)
placed in an oven 12 and heated by an induction heater
13. The molten pool ‘14 may comprise an iron and Sili
con solution which is in contact with the graphite cru~
cible whereby the solution becomes saturated with car
bon. The solubility of silicon in a carbon saturated mol
ten iron solution increases from an atomic fraction of
0.355 at 14210° C. to v0.376 at 1700° C. (Table 8, page
448 of Journal Acta Metallurgica, -vol. 2, May 1954,
This invention relates generally to a method of grOW
ing silicon carbide crystals, and more particularly to a 10 Chiprnan et al.). \As a consequence of this increase in
the solubility of silicon with increased temperature, it
method of growing silicon carbide crystals ‘from a molten
is evident that at higher temperatures more silicon may
metal or alloy containing silicon and carbon in solution
therewith.
\It is a general object of the present invention to provide
be added without causing supersaturation. However, if
a seed to is lowered into the molten solution, a localized
15 region will be cooled. This region of the solution is
a method for ‘growing silicon carbide crystals.
super-saturated with silicon carbide. vIf the seed 16 is
It is another object of the present invention to pro~
silicon carbide, silicon carbide will crystallize onto the
vide a method of growing silicon carbide crystals having
seed rather than silicon since the silicon carbon bond has
relatively high purity.
higher energy than the silicon silicon bond. The crystal
‘It is a further object of the present invention to pro
may ‘be continuously withdrawn as indicated by the ar
vide a method for growing silicon carbide crystals at a
row 17. To maintain the concentration of silicon in
temperature which is substantially below the melting tem
the remainder of the molten solution, silicon may be
perature of silicon carbide.
added by Well known techniques as indicated by the ar
It is still a further object of the present invention to
row 18. Thus, it is seen that iron is a metal which has
provide a method of growing silicon carbide crystals
from a molten metal or alloy containing silicon and car 25 the characteristics set ‘forth above.
‘In order to avoid the precipitation of graphite crystals,
bon in solution therewith.
it is advantageous to approach the condition of super
‘It is still a further object of the present invention to
saturation from silicon rich solutions on the phase dia
provide a method for growing silicon carbide crystals
gram. For this reason, it may be advantageous to use
having predetermined doping impurities therein to give
material having predetermined carrier concentrations.
These and other objects of the invention will become
more clearly apparent from the following description
taken in conjunction with the accompanying drawing.
The drawing schematically shows apparatus suitable
30 not a graphite crucible but a silicon carbide crucible, or
equal to the energy of the carbon carbon bond. These
energies are considerably higher than that of the silicon
silicon bond. The method of growing silicon carbide
crystals according to this invention relies upon these
differences in bonding energy.
cent at 1420° C. to nearly 2.2 atomic percent at 1700“
C. (Table 7, same authors). Thus, it is seen that the
even an inert crucible and to add extra silicon to the
solution.
Alternately, the changing saturation concentration of
carbon with temperature may be employed to effect the
35 growth of a silicon carbide crystal. The solubility of
for carrying out the invention.
graphite in an alloy ‘containing approximately 0.35 atomic
'As is well known, the energy of the silicon carbon
fraction of silicon increases from about 1.05 atomic per-.
bond is very great, being quoted as being approximately
solubility of the carbon increases with increasing tem
perature. :When a crystal is lowered into the solution,
silicon carbide is precipitated onto the crystal. The crys
tal may then be withdrawn and carbon added in the
According to the invention, a molten metal or alloy
manner previously described.
has in solution therewith carbon and silicon to form a
‘In general, it is seen that the solubility of silicon car
liquid metal or alloy solvent. The concentration of car 45
bide increases in the iron with increasing temperature.
bon and silicon being such that at a predetermined tem
By maintaining a ?xed concentration at a higher tempera
perature the concentration of silicon carbide is at satura
ture of either silicon or carbon and producing a localized,
tion. A small localized region of the solution is then
cold spot in the solution, a supersaturated region is formed.
cooled. This region is then supersaturated ‘with silicon
carbide and the silicon carbide can be precipitated in 50 Silicon carbide crystal may be precipitated from this
region of the solution.
. .
crystalline form from the solution. The temperature in
The silicon carbide crystal is grown at temperatures,
a small region may be lowered by inserting a silicon
14220-1700° C., substantially below the melting point of
carbide seed into the solution. The localized region ad
silicon carbide. The lstrength of the carbon silicon bond
jacent the seed is supersaturated and silicon carbide de
posits on the seed in crystalline form. The seed and 55 inthe silicon carbide will cause the. crystals to reject
crystal forming thereon may be continuously withdrawn
vigorously atoms which are not of the right, size to ?t
from the solution ‘as silicon and carbon are continuously
into the crystal lattice of the silicon carbide crystal.
added to maintain the proper concentration of silicon
This effect is known to be pronounced for silicon and
carbide in the solution.
germanium crystals which are characterized by segrega:
The metal or alloy solvent is selected so that it forms 60 tion factors when grown from a melt of the order: of
a molten (liquid) bath which dissolves an appreciable
170-4 to 10'? (The segregation factors measure the rel:
quantity of silicon carbide ‘without any vaporization of the
ative solubility of an impurity in theliquid as compared
metal or alloy at said temperature. The solubility of
to the crystal.) These segregation factors hold for silicon
silicon carbide in the molten ‘bath should increase with
and germanium at the melting point. Therejectionrwould
temperature; preferably there should be a large solubility 65 presumably be even stronger if the crystal were grown
dependence on temperature. The solution process should
from a solution which allowed it to be grown at tempera
be reversible whereby a lowering of the temperature
tures below the melting temperature.
'
'
‘
supersaturates the solution with silicon carbide. Pref
Conventional methods of doping the'silicon carbide
eralbly, a large temperature difference should exist be
crystals in the melt can be employed by utilizing atoms
tween saturation and spontaneous nucleation of silicon 70 of suitable size factors. For example, nitrogen and
carbide in order to permit growth of ‘a single crystal from
phosphorous atoms as donors and boron and aluminum
a cooled zone.
atoms as acceptors will be more suitable in silicon carbide
3,058,635
A.
3
solubility of silicon carbide with increasing temperature
with the solubility process being reversible, maintaining
than atoms coming farther along in the periodic table.
These elements may be used to make n-type and p-type
said bath at a molten temperature, adjusting the concen
silicon carbide. Since the radius of the carbon and silicon
tration of the carbon and silicon at said temperature until
atoms are quite di?erent, substitutional impurities from
the third and ?fth colums of the periodic table will have 6 the bath is at a saturation point for silicon carbide, in
setting a silicon carbide crystal seed into said molten bath,
preferential substitutional positions on one or the other of
and withdrawing the seed from the bath, said seed serving
the carbon and silicon sites depending upon the atomic
to cool a surrounding portion of the bath to form a
radii and electronegativity values. Their roles as donors
portion which is supersaturated with silicon carbide
and acceptors are, however, independent of site at which
whereby silicon carbide precipitates on said seed as it
they ?nd themselves even though the ionization energies
is withdrawn from the solution.
differ somewhat.
2. A method as claimed in claim 1 in which said molten
It is evident that a variety of molten metals and alloys
bath is formed in a silicon carbide crucible.
may be used as solvents. Several atomic percent of car
3. The method of growing silicon carbide crystals
bon is soluble in copper at 1800° C. and silicon is com
which comprises melting a solvent which dissolves an ap
pletely miscible. Over 10 atomic percent of carbon is
preciable quantity of silicon carbide at a temperature be
soluble in nickel at 1600° C., and again silicon is com~
low the solvent’s vaporization temperature and which has
pletely miscible. Aluminum, aluminum-zinc alloy, man
an increasing solubility of silicon carbide with increasing
ganese, cobalt, bismuth and tin are a few others which
temperature with the solution process being reversible in
have the desired characteristics. It is a general con
sequence of the thermodynamics of these systems that the 20 a graphite crucible to form a molten bath of the solvent
saturated with carbon, maintaining said bath at a molten
solubility of silicon carbide will increase with increasing
temperature, adding silicon to said molten bath to saturate
temperature. The solution process is reversible. The rel
said bath with silicon carbide at said molten temperature,
ative strength of the silicon silicon, carbon carbon and
inserting a silicon carbide crystal seed into said molten
silicon carbon bonds lead to the previously described
bath,
and withdrawing the seed from the bath, said seed
25
behavior in these metal solutions.
serving to cool a surrounding portion of the bath to form
For example, copper may be used as the solvent metal
a portion which is supersaturated with silicon carbide
for the purpose of growing silicon carbide crystals. The
whereby
silicon carbide precipitates on said seed as it is
solubility of carbon and coper varies from the order of
withdrawn from the bath.
one atomic percent at 1600° C. to three atomic percent
4. The method of growing silicon carbide crystals
at 1900° C., Table 1. Silicon is highly soluble in copper
which
comprises forming a molten bath of carbon and
and in fact lowers its melting point. Thus, it is relatively
silicon dissolved in a molten solvent which dissolves an
easy to produce a solution saturated with silicon carbide
appreciable quantity of silicon carbide at a temperature
from which the silicon carbide crystal may then be grown
below the solvent’s vaporization temperature and has an
as previously described.
increasing solubility of silicon carbide with increasing
As previously described, various of the metals may
temperature with the solution process being reversible,
be exploited for the purpose of growing silicon carbide.
maintaining said bath at a molten temperature, adjusting
The advantage of one compared to another being asso
the concentration of the carbon and silicon at said tem
ciated with such factors as their suitability for adding
perature until the bath is at a saturation point for silicon
doping elements and to the degree that the atoms of the
40 carbide, inserting a silicon carbide crystal seed into said
metal solvents themselves contribute bene?cially or ad
bath to cool a surrounding portion of said bath to form a
versely to the crystal as grown.
portion which is supersaturated with silicon carbide
A tube 19 may be provided for directing a cooling jet
whereby silicon carbide precipitates on said seed, with
of inert gas towards a portion of the melt to produce a
drawing said seed tfrom said bath together with the silicon
cold region whereby the solution is supersaturated. The
jet may also serve to stir the solution in the vicinity of as er
the crystal so that the crystal may be withdrawn without
disturbance due to latent heat.
In a continuous process graphite, silicon rods or silicon
carbide rods may be continually fed into the melt to main
tain the proper concentration. An electric current may
be passed through the rod as it is inserted into the melt
whereby the temperature is maintained near that of the
melt. The rod then has negligible effect upon the tempera
ture of the melt. Further, surface tension of the molten
solution will not prevent mixing of the added material.
It is apparent that di?erent apparatus may be employed
for carrying out the invention. The apparatus shown is
illustrative only.
carbide depositing thereon, and continuously adding sili
con and carbon to said molten bath to compensate for
that removed as silicon carbide crystal.
5. The method as in claim 4 in which said molten sol
vent is additionally characterized in that it has a large
temperature difference between saturation and spontane
ous nucleation of silicon carbide to permit growth of sili
con carbide.
6. The method of growing silicon carbide crystals
which comprises forming a molten bath of carbon and
silicon dissolved in iron, maintaining said bath at a molten
temperature, adjusting the concentration of the carbon
and silicon at said temperature until the bath is at a satur
ation point for silicon carbide, inserting a silicon carbide
crystal seed into said molten bath, and withdrawing the
Thus, it is seen that a method of growing silicon carbide 60 seed from the bath, said seed serving to cool a surround
crystals has been described. The crystals are grown from
ing portion of the bath to form a portion which is super
a metal or alloy solution at temperatures substantially be
saturated with silicon carbide whereby silicon carbide pre
low the melting point of silicon carbide. The crystals
have a high purity. Suitable doping elements are easily
added.
This application is a continuation-in-part of the copend
cipitates on said seed as it is withdrawn from the solution.
7. A method as in claim 6 in which said molten bath
65 is formed in a silicon canbide crucible.
ing application, Serial No. 648,889, ?led March 27, 1957,
entitled “Method of Growing Silicon Carbide Crystals,”
now abandoned.
I claim:
1. The method of growing silicon carbide crystals which
comprises forming a bath of carbon and silicon dissolved
in a molten solvent which dissolves an appreciable quan
tity of silicon carbide at a temperature below the solvent’s
vaporization temperature and which has an increasing 75
8. The method of growing silicon carbide crystals
which comprises melting iron in a graphite crucible to
form a molten bath of iron saturated with carbon, main~
taining said bath at a molten temperature, adding silicon
to said molten bath to saturate said bath with silicon car
bide at said molten temperature, inserting a silicon car
bide crystal seed into said molten bath, and withdrawing
the seed ‘from the bath, said seed serving to cool a sur
rounding portion of the bath to form a portion which is
supersaturated with silicon carbide whereby silicon car
3,053,635
.
5
bide precipitates on said seed as it is withdrawn from the
bath.
9. The method of growing silicon carbide crystals
which comprises forming a molten bath of carbon and
silicon dissolved in iron, maintaining said bath at a molten
temperature, adjusting the concentration of the carbon
and silicon at said temperature until the bath is at a satur
14. The method of growing silicon carbide crystals
which comprises forming a molten bath of carbon and
silicon dissolved in a molten solvent selected from the
group consisting of iron, copper, nickel, aluminum, alu
minum-zinc alloy, manganese, cobalt, bismuth and tin,
maintaining said bath at a molten temperature, adjusting
the concentration of the carbon and silicon at said tem
perature until the bath is at a saturation point for silicon
carbide, inserting a silicon carbide crystal seed into said
ation point for silicon carbide, inserting a silicon carbide
crystal seed into said bath to cool a surrounding portion
of said bath to form a portion which is supersaturated 10 molten bath, and withdrawing the seed from the bath,
with silicon carbide whereby silicon carbide precipitates
said seed serving to cool a surrounding portion of the
on said seed, withdrawing said seed vfrom said bath to
bath (to form a portion which is supersaturated with sili
con carbide whereby silicon carbide precipitates on said
seed as it is withdrawn from the solution.
bath to compensate for that removed as silicon carbide 15
15. A method as in claim 14 in which said molten bath
crystal.
is formed in a silicon carbide crucible.
gether with the silicon carbide depositing thereon, and
continuously adding silicon and carbon to said molten
10. The method of growing silicon carbide crystals
16. The method of growing silicon carbide crystals
which comprises forming a molten bath of carbon and
which comprises melting a solvent selected from the
silicon dissolved in copper, maintaining said bath at a
group consisting of iron, copper, nickel, ‘aluminum, alu
molten temperature, adjusting the concentration of the 20 minum-zinc alloy, manganese, cobalt, bismuth and
carbon and silicon at said temperature until the bath is at
a saturation point 'for silicon carbide, inserting a silicon
tin in a graphite crucible to form a bath of said sol
vent saturated with carbon, maintaining said bath at a
carbide crystal seed into said molten bath, and withdraw
molten temperature, adding silicon to said molten bath
ing the seed from the bath, said seed serving to cool a
to saturate said bath with silicon carbide at said molten
surrounding portion of the bath to form a portion which 25 temperature, inserting a silicon carbide crystal seed into
is supersaturated with silicon carbide whereby silicon car
said molten bath, and withdrawing the seed from the
bide precipitates on said seed as it is withdrawn from the
bath, said seed serving to cool a surrounding portion of
solution.
the bath to ‘form a portion which is supersaturated with
11. A method as in claim 10 in which said molten bath
silicon carbide whereby silicon carbide precipitates on
is :formed in a silicon carbide crucible.
12. The method of growing silicon carbide crystals
which comprises melting copper in a graphite crucible to
30 said seed as it is withdrawn from the bath.
17. The method of growng silicon carbide crystals
which comprises forming a molten bath of carbon and
silicon dissolved in a molten solvent selected from the
form a molten bath of copper saturated with carbon,
maintaining said bath at a molten temperature, adding
group consisting of iron, copper, nickel, aluminum, alu
silicon to said molten bath to saturate said bath with 35 minum-zinc alloy, manganese, cobalt, bismuth and tin,
silicon carbide at said molten temperature, inserting a
maintaining said bath at a molten temperature, adjusting
silicon carbide crystal seed into said molten bath, and
the concentration of the carbon and silicon at said tem
withdrawing the seed from the bath, said seed serving
perature until the bath is at ‘a saturation point for silicon
to cool a surrounding portion of the bath to form a
carbide, inserting a silicon carbide crystal seed into said
portion which is supersaturated with silicon carbide 40 bath to cool a surrounding portion of said bath to ‘form
whereby silicon carbide precipitates on said seed as it
a portion which is supersaturated with silicon carbide
is withdrawn from the bath.
whereby silicon carbide precipitates on said seed, with
13. The method of growing silicon carbide crystals
drawing said seed from said bath together with the silicon
which comprises forming a molten bath of carbon and
carbide depositing thereon, and continuously adding sili
silicon dissolved in copper, maintaining said bath at a 45 con and carbon to said molten bath to compensate for
molten temperature, adjusting the concentration of the
that removed as silicon carbide crystal.
carbon and silicon at said temperature until the bath is
at a saturation point for silicon carbide, inserting a silicon
References Cited in the ?le of this patent
carbide crystal seed into said bath to cool a surrounding
portion of said bath to ‘form a portion which is super 50
UNITED STATES PATENTS
saturated with silicon carbide whereby silicon carbide
2,113,354
McKenna ___________ _._ Apr. 5, 1938
precipitates on said seed, withdrawing said seed from said
2,729,542
bath together with the silicon carbide depositing thereon,
2,851,342
and continuously adding silicon and carbon to said molten
bath to compensate ‘for that removed as silicon carbide 55 2,854,364
Van Der Pyl ___________ __ Jan. 3, 1956
Bradshaw et al. _______ ._ Sept. 9, 1958
2,908,553
Frank et a1, .._.._>___._,,._,__, Oct. 13, 1959
crystal,
Lely __, ______________ __ Sept. 30, 1958
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