A saturated solution of silicon and an element such as carbon having a segregation coefficient less than unity is formed by placing a solid piece of carbon in a body of molten silicon having a temperature differential decreasing toward the surface. A silicon carbide seed crystal is disposed on a holder beneath the surface of the molten silicon. As a rod or ribbon of silicon is slowly pulled from the melt, a supersaturated solution of carbon in silicon is formed in the vicinity of the seed crystal. Excess carbon is emitted from the solution in the form of silicon carbide which crystallizes on the seed crystal held in the cool region of the melt

Ciszek, T. F.

Schwuttke, G. H.

English

NASA Technical Reports Server

United States Patent [191 11 11 4,152,194 
Ciszek et al. [451 May 1,1979 
[54] GROWTH OF SILICON CARBIDE 
CRYSTALS ON A SEED WHILE PULLING 
SILICON CRYSTALS FROM A MELT 
[76] Inventors: Robert A. Frosch, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of Theodore F. Ciszek, Salt 
Point; Guenther H. Schwuttke, 
Poughkeepsie, both of N.Y. 
[21] Appl. No.: 820,499 
[22] Filed: Jul. 29, 1977 
[51] Int. ( 3 . 2  .............................................. BOlJ 17/20 
[52] U.S. Cl. ..................... 156/617 SP; 156DIG. 68; 
423/345 
[58] Field of Search ......... 156DIG. 68, 608, 617 SPY 
156/605, DIG. 88; 23/273 SP; 423/345 
1561 References Cited 
U.S. PATENT DOCUMENTS 
2,996,415 8/1961 Hergenrother .............. 156DIG. 68 
3,053,635 9/1962 Shockley ..................... 156DIG. 68 
OTHER PUBLICATIONS 
Swartz et al., J. of Electronic Materials, vol. 4, #2, 
Primary Examiner-Stephen J. Emery 
Attorney, Agent, or Firm-Wilfred Gritla; John R. 
Manning; Monte F. Mott 
1975, pp. 255-279. 
[571 ABSTRACT 
A saturated solution of silicon and an element such as 
carbon having a segregation coefficient less than unity 
is formed by placing a solid piece of carbon in a body of 
molten silicon having a temperature differential de- 
creasing toward the surface. A silicon carbide seed 
crystal is disposed on a holder beneath the surface of the 
molten silicon. As a rod or ribbon of silicon is slowly 
pulled from the melt, a supersaturated solution of car- 
bon in silicon is formed in the vicinity of the seed crys- 
tal. Excess carbon is emitted from the solution in the 
form of silicon carbide -which crystallizes on the seed 
crystal held in the cool >region of the melt. 
10 Claims, 4 Drawing Figures 
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4.152.194 
1 
GROWTH OF SILICON CARBIDE CRYSTALS ON 
A SEED WHILE PULLING SILICON CRYSTALS 
FROM A MELT 
ORIGIN O F  THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is 
subject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 
83-568 (72 Stat. 435; USC 2457). 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to growth of single 
crystals from molten solutions and, more particularly, 
to the production of silicon carbide crystals from a 
weak solution of carbon in silicon. 
2. Description of the Prior Art 
Silicon carbide is of considerable interest because, as 
a large bandgap semiconductor, electronic junction 
devices made from it can operate at much higher tem- 
peratures than those made from germanium or silicon. 
However, no successful technique has been devised to 
grow large, pure single crystals of silicon carbide. It 
cannot be solidified from the melt at reasonable pres- 
sures. It has been grown from molten metal and molten 
silicon solutions. However, the very low solubility in 
molten metals allowed only small crystals to be grown. 
The usual method of producing silicon carbide crystals 
is by decomposition of SiHCL3 and a hydrocarbon gas 
such as methane from the gas phase which deposits onto 
a substrate. This process is complex, difficult to control 
and limited to production of small grain, single crystals 
of 1 cm or less. 
SUMMARY OF THE INVENTION 
A novel method for producing crystals of silicon 
compound from weak molten solutions is provided in 
accordance with the invention. Large single crystals are 
produced and spurious crystals are avoided by dispos- 
ing a seed crystal in a cool region near the surface of the 
melt containing a supersaturated zone of the co-crystal- 
lizing element. The supersaturated zone is created and 
maintained by slowly pulling a silicon rod or ribbon 
from the surface of the melt. Since the segregation coef- 
ficient of the co-crystallizing element is less than unity, 
the element is rejected into the solution as the silicon 
solidifies and forms a zone supersaturated with the ele- 
ment. The excess element is emitted from the zone in 
the form of crystals of silicon compound which crystal- 
lize on the seed crystal disposed in the cool region of the 
melt just below the solidifying silicon member. 
2 
* R. I. %ace and G. A. Slack, The SGC and Ge-C Phase Diagrams. 
Proceedings of the Conference on Silicon Carbide, Boston, April 1959. 
Pergamon Press, New York (1960). 
** T. N. Tucker, Distribution Coefficient of Carbon in Silicon, paper 
presented at the October 1968 Meeting of the Electrochemical Society, 
Montreal, Canada. 
All starting materials should be of very high purity 
and all surfaces should be non-nucleating to silicon 
carbide, suitably quartz. The seed crystal should be 
positioned as close as possible to the end of the silicon 
crystal but not so close as to interface with the pulled 
lo crystal, suitably 0.5 to 2 cm, generally about 1 cm. The 
seed crystal must be moved downwardly to maintain 
the spacing to the pulled crystal as the molten silicon 
level drops due to silicon crystallization and removal 
from the melt onto the pulled crystal. 
The temperature in the cool supersaturated zone is at 
the temperature of phase transformation of silicon liquid 
to solid, i.e. about 1412"-5" C. The temperature differ- 
ential from the cool zone to the lower portion of the 
2o melt in which is disposed a source of the element is 
generally from 50" C. to 150" C., preferably less than 
loo" C. The process can be operated in an inert atmo- 
sphere such as under vacuum or an inert gas such as 
argon. Generally a positive overpressure of 2 psi is 
25 utilized to assure outgassing of impurities from the sys- 
tem. The pulling rate is adjusted to withdraw sufficient 
silicon so as to maintain a zone supersaturated with 
carbon. This, of course, depends on the rate carbon is 
removed from the supersaturated zone as silicon car- 
30 bide crystals. The pulling rate is generally from 0.1 
m d m i n  to 10 mm/min depending on the diameter of 
the rod, usually about 1 mm/min. 
These and many other objects and attendant advan- 
tages of the invention will become readily apparent as 
35 the invention becomes better understood by reference 
to the following detailed description when considered 
in conjuction with the accompanying drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a schematic view of a first embodiment of 
FIG. la is an enlarged view of the solid-liquid inter- 
FIG. 2 is a schematic view of a further embodiment 
FIG. 2u is an enlarged view of the S ic  seed crystal 
l5 
40 
the invention; 
face of rod and molten solution; 
45 of the invention; and 
holder. 
DESCRIPTION OF THE PREFERRED 
EMBODIMENTS 
Referring now to FIGS. 1 and la. the crystal-grow- 
ing apparatus of the invention generally includes a cru- 
cible 10 containing a body of molten silicon 12, a source 
14 of crystallizing element, differential heating means 
55 16, means for crystallizing 18 a silicon compound crys- 
50 
The silicon rod can be reused to form a new batch Of 
melt or may be utilized as a crystalline silicon substrate 
material for semi-conductors, Solar Cells or other pur- 
poses. 
of crystals of silicon combined with elements having 
very low solubility of less than IO00 parts per million in 
silicon and having a segregation coefficient of less than 
unity in silicon. The solubility limit of carbon in silicon 
is about 100 parts per million at the temperature of 65 pass-through 27 for silicon rod puller 20. 
molten silicon* and the segregation coefficient is about 
0.05**. The segregation coefficients of boron and cop- 
per in silicon are both less than unity. 
tal disposed in the cooler region of the melt 12 and 
silicon crystal pulling means 20, the upper die end 22 
being disposed adjacent the crystallizing means 18. The 
crucible 10 is disposed within an enclosure 24 of a suit- 
The invention is generally applicable to the formation 60 able furnace and an inert atmosphere is maintained 
within the furnace by means of vacuum or by overpress- 
uring the system with inert gas such as argon intro- 
duced into the enclosure 24 through inlet 26 and which 
leaves the system through any openings such as the 
Differential heating means 16 may take many diverse 
forms, readily available commercially which are capa- 
ble of maintaining the desired temperature differential 
3 
4,152,194 
4 
in which the lowest zone 42 of the melt is maintained at all components in the cool zone are formed of quartz, 
a temperature of about 1500" C. while the highest such as the crucible 102, the silicon carbide seed holder 
cooler zone 44 is maintained at the crystallization tem- 104 and attaching rod 106. The top 108 of the furnace 
perature of silicon, about 1415" C. Typically, the heat- shell 111 has a first pass-through 110 for the pulling rod 
ing means 16 is a picket fence graphite heater in which 5 112 and a second pass-through 114 for the movable seed 
high resistance graphite rods 38 are housed within slots holder piston 116. 
40 disposed within the wall 25 of the furnace enclosure. The furnace shell contains variable cross-section 
A temperature differential can be maintained by vary- graphite heating rods 118 connected to variable power 
ing the diameter of each rod 38 Or tapering the rods source 120 by means of conductors 122. The bottom 
along their length. The rods are heated by means of 10 wall 1% of the furnace shell has an j&t port 126 con- 
conductors 48 connected to power source and Control- nected to the Source of argon 1%. A piece of high pu- 
ler 50. rity graphite 130 is held in the bottom of the crucible by 
The silicon pulling means depicted Comprises a rib- quartz holders 132. A rotatable silicon rod holder 100 
bon puller in which the die 22 includes a capillary slot which can be formed of metal or graphite is connected 
52 into which the molten, carbon saturated silicon 15 to a hydraulically-actuated mechanism 1%. ne 
creeps upwardly by capillary action from the meniscus mechanism 1% includes a piston 1% and a hydraulic 
the crucible by means of holder 60 slowly dissolves to draulic oil lm. Speed adjustment is obtained by means 
emit carbon atoms 56 which enter the melt to form a of a needle valve 142 disposed in the oil flow line 144 
saturated silicon solution containing one hundred parts 20 between the hydraulic and the oil reservoir 
per million of carbon. As the silicon ribbon forms tank 146 The 
die puller 20, carbon is emitted back into the melt within holder may suitably be adjusted by a variable hydraulic the cooler zone 44 and forms a supersaturated sub-zone fluid 64. The excess carbon is emitted from solution in the 25 by turning on argon form of fl-silicon carbide crystals 68 which deposit on 128 such that argon flows through the system 
and out the pass-throughs 110, 114. A piece of high the end of the die 22. Silicon carbide crystals were grown in an apparatus purity graphite 130 is engaged within the clamps 132 similar to that of FIG. 1. A rectangular piece of high and high purity silicon is placed purity graphite was dipped into a molten silicon solu- 30 
54 ofthe melt- The high purity graphite rod l4 held in cylinder 138 connected to a Source of pressurized hy- 
is suitably pressurized by air within the 52 and is Pulled by hydraulic piston system 116 on the movable xed  crystal 
150. 
The system of FIG. is 
the 
tion having a cool zone at the top of the melt at a tern- 
perature of about 1415" C .  and a bottom hotter zone 
temperature of 1500" C. The graphite was dipped into 
solution for approximately one hour while solidifying a 
of the graphite piece by capillmy action. After removal 
The to heat the bottom bf 
the to a temperature Of about 1500" c. and the 
top ofthe melt to a temperature of 1415" c* After the 
molten silicon is saturated with carbon, the rod Pulhg  
UPW~db. As the level of sihon solution falls the hy- 
120 is 
continuous ribbon of silicon from the melt fed to the top 35 
of the graphite piece, well-developed &silicon carbide draulic mechanism is actuated to lower the 
crystals were observed on the top surface of the graph- rod lo6 such that the seed crystal 170 and the 
ite piece. me crystals ranged in she from 0.1 to 1 -. silicon carbide crystal growing thereon is mahtahed 
scraped from the graphite piece was made through It is to & understood that Only Preferred embodid 
X-ray diffraction analysis. A few crystals were attached menb of the invention have been described and that 
to a glass rod and Debye-Schemer diffraction patterns numerous substitutions, modifications and alterations 
were obtained. The d-spacings measured from the films are permissible without departing from the spirit and 
were checked against the A.S.T.M. card file. The crys- 45 S O P  of the invention as defmed in the following 
tals were identified as /?-silicon carbide. This zinc-blend Claims- 
type s t r u p r e  (cubic form) has a lattice constant of a what is Claimed is: 
=4.358 A. The measurements are summarized in Table 1. A method of growing silicon carbide crystals com- 
I. prising the steps of: 
TABLE 1 placing solid carbon in the lower portion of a body of 
silicon; 
heating the body of silicon to melt the silicon and HKL d 1/11 dissolve the carbon therein to form a weak solution d 10 
of carbon in silicon while maintaining the top por- 2.51 S 111 2.5 1 100 2.18 M 200 2.17 20 
1.54 M 220 1.54 63 55 tion of the solution near the crystallizing tempera- 
1.315 M 31 1 1.31 50 ture of silicon; 
1.27 W 222 1.26 5 pulling a silicon crystal from the surface of the solu- 
tion while rejecting carbon into the top portion of 400 1.09 6 1.00 w 33 1 1 .oo 18 
0.975 W 420 0.97 6 the solution to form a zone supersaturated with 
0.89 W 422 0.89 13 60 carbon; 
0.84 W 333,511 0.84 10 supporting a seed crystal of silicon carbide within the 
zone adjacent the interface of the pulled silicon 
Larger silicon carbide crystals can be grown and crystals; and 
spurious nucleation can be avoided by localizing silicon growing a crystal of silicon carbide on the seed crys- 
carbide crystal growth in a preselected area of the melt 65 tal. 
and by maintaining proper spacing between the grow- 2. A method according to claim 1 further including 
ing silicon carbide crystal and the interface with the the step of moving the seed crystal downwardly as said 
growing silicon rod or ribbon. In the system of FIG. 2, interface moves downwardly. 
is actuated and slOwly pulls the rod 
Identification of the crystal Structure of the crystals 40 about 1 cm below the surface of the silicon rod- 
50 
CRYSTAL REFLECTION A.S.T.M. CARD #1-119 &Sic 
4,152,194 
5 6 
3. A method according to claim 2 further including 
ture such that the lower portion is at a temperature no 
more than 150" C. higher than the temperature at the 
top portion of the solution. 
4. A method according to claim 3 in which the car- 
bon is in the f o m  of a solid piece of high purity graphite 
and further the piece 
within the lower portion of the solution. 
portion of the solution is at a temperature of about 1415" 
C. and the lower portion is at a temperature of about 
1500" C. 15 
6. A method according to claim 5 in which all sur- 
crystal are non-nucleating to crystals of silicon carbide. 
,. A method according to claim in which the non- 
nucleating surfaces are formed of quartz. 
8. A method according to claim 5 in which the lower 
portion is at a temperature no more than 100" C. higher 
than the temperature at the top portion of the solution. 
9. A method according to claim 8 in which the silicon 
10 carbide seed crystal is positioned from 0.5 to 2 cm from 
the end of the silicon crystal. 
con crystal is pulled from the solution at a rate of about 
0.1 mm/min. 
the step of heating the solution at a differential tempera- face in contact with the solution except for the seed 
the step of 
5* A method according to in which the 10. A method according to claim 9 in which the 
* * * * *  
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Growth of silicon carbide crystals on a seed while pulling silicon crystals from a melt

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