Sept. 30, 1958 J. A.

LELY
SUBLIMATION PROCESS FOR MANUFACTURING 2,854,364
SILICON CARBIDE CRYSTAS
Filed March 7, 1955

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JAN ANTHONY LELY

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 United States Patent Office 2,854,364
Patented Sept. 30, 1958.
2
of a size of, for example, from 4 to 10mms. are enabled to
g??W.
SUBLMATON PROCESS FOR MANUEFACTURING Due to the fact that in carrying out the process in
SLECON CARBEDE (CRYSTALS 5 accordance with the invention use is made of a space
bounded by silicon carbide the equilibrium vapour pres
Jan Anthony Lely, Emmasingel, Eindhoven, Netherlands, sure of silicon carbide and silicon will always obtain in
assignor, by mesne assignments, to North American this space so that the crystals which settle contain no free
Philips Company, ac., New York, N. Y., a corporation carbon. It is true that material of the outer layer of
of Delaware
0.
the silicon carbide bounding the sublimation space will
Application March 7, 1955, Serial No. 492,385 evaporate and dissociate and reach the furnace space but
Claims priority, application Netherlands March 19, 1954 this does not influence the sublimation.
Impermissible gas currents in the Sublimation space
6 Claims. (C. 148-1.5) can be counteracted by causing the protective gas to flow
through this space at a very low rate. Preferably, how
The invention relates to the production of silicon car 15 ever, use is made of a sublimation vessel which communi
bide crystals by sublimation and to semi-conductive de cates with the gas flow at one end only so that currents
vices comprising such crystals. ---- are substantially precluded. . ? -
The production of well-formed large crystals by means Suitable gases for the protective gas atmosphere are
of sublimation generally requires that the vapour pres 20 rare gases, hydrogen and carbon monoxide. Hydrocar
sure of the substance required to be sublimed is main bons will decompose at the prevailing temperature to
tained low, that the temperature gradient in the vessel form CH-4-H with the separation of carbon. Since
in which the sublimation is carried out is small and that this carbon may clog the installation, hydrocarbons
the occurrence of gas currents is avoided as far as possible. should preferably not be used.
Due to the fact that silicon carbide produces a suffi 25 When pure starting material (impurities less than
cient vapour pressure only at temperatures exceeding 0.002%) is utilized as the SiC lining, crystals substantial
2000 C. and that, at such high temperatures, of the usual ly clear as glass having a resistivity of 108 ohm-cm. and
heat resistance materials only carbon does not react with more can be obtained by a single sublimation. A start
silicon carbide so as to cause the absorption of foreign ing material of sufficient purity can be produced in known
impurities, the sublimation cannot be carried out in a 30 manner by heating silicic acid and carbon to a tempera
closed gastight vessel. Consequently, the sublimation ture of approximately 1800° C. according to the reaction
will have to be carried out at a total gas pressure equal to equation SiO2--3C--> SiC-+-2CO.
that in the furnace. The furnace itself can be closed so The sublimation produces a material purification of
as to be gastight so that the sublimation can be carried the original silicon carbide. It is true that impurities
out at any desirable gas pressure. 35 present in the silicon carbide such as, for example, N, P,
When silicon carbide evaporates it dissociates into As, B, Al and Fe will also evaporate so that on conden
gaseous silicon and solid carbon. The silicon pressure sation a certain amount of these impurities, which is de
is approximately 10 to 20% of the silicon carbide pressure. pendent on the partial pressure of their vapour in the gas
When a graphite vessel is used, not only silicon carbide atmosphere, will be incorporated in the crystal. How
vapour but also silicon vapour will diffuse through the wall 40 ever, this amount is very slight due to the fact that the
into the furnace space on heating. Consequently well distribution coefficient (that is to say the ratio of the
formed crystals having reasonably large dimensions and concentration of the impurity in the solid substance to
always satisfactory with respect to composition cannot be that in the vapour) is very small, for example approxi
obtained in this manner. mately 10-8 for aluminum in accordance with the partial
According to the invention these disadvantages are 45 pressure, and also 108 for nitrogen, with a concentration
obviated by carrying out the sublimation in a protective of 0.5% in the protective gas at atmospheric pressure.
gas in a space bounded by silicon carbide. The presence of impurities in the crystals produces
Silicon carbide is a very hard substance which cannot electrical conductivity, namely n-conductivity with ele
readily be machined so that a vessel entirely consisting. ments of a valency exceeding 4, such as for example
of silicon carbide can only be manufactured with diffi 50 N, P, As, which can act as donors, and p-conductivity
culty. According to a particular embodiment of the in with elements of a valency less than 4, such as for ex
vention the sublimation is carried out in a graphite vessel ample B, Al, and so on, which can act as acceptors.
which is internally lined with silicon carbide. This lin When donor impurities are present the colour of the sili
ing may, for example, be effected by arranging a core or con carbide becomes green to green-black and when
mandrel in a graphite cylinder open at one end and filling 55 acceptor impurities are present it becomes blue to blue
the space between the graphite wall and the core with black.
lumps of silicon carbide. After packing of the lamps, The amount of the donor and acceptor impurities can
the core can be removed carefully, leaving a central open be controlled by means of the vapour pressure of the
space, and the open upper end of the cylinder can be donor or acceptor in elementary form or in the form of a
closed with a lump of silicon carbide. As an alternative, 60 compound in the gas in which the sublimation is carried
the graphite vessel may be lined with powdered silicon out. During sublimation the partial pressure of the im
carbide to which water glass is added as a binding agent. purities in the sublimation space can be maintained
When sublimation is carried out at approximately constant. Consequently, in this event crystals are ob
atmospheric pressure, the Sublimation vessel is heated to tained having a composition which is constant through
approximately 2500 C. By the use of suitable heat 65 out the mass. V
insulation local temperature differences are restricted, so When nitrogen is used as a donor this is readily realised
that even at the points exhibiting the lowest temperatures, by adding this gas to the atmosphere. Phosphorus can
in a reasonably short period of time, for example from 4 be introduced in the gas in the form of PC or PH and
to 7 hours, at the obtaining low vapour pressure of, for ex arsenic and antimony in the form of ASCls and SbCls
ample, 5 to 50 mms. of mercury, which pressure is pro 70 respectively.
duced by the silicon carbide bounding the walls of the sub The amount of aluminum required to produce crystals
imatic in space which acts as the starting material, crystals having p-conductivity can be controlled by adding it in
 2,854,364
ex
al
vš 4
the form of AICl. However, in this event allowance carbon furnace 17. This carbon furnace consists of a
should be made for the fact that in hydrogen at a tem graphite cylinder open at both ends and having two saw
perature exceeding 1000 C. the reaction proceeds cuts formed in it for a large part of its length which ex
4AlCl3-+-6H2-1-3C-> AlC3--12HCl, and AICs at a tem tend so as to be parallel to its axis. At the sawed end
perature below 2000. C. does not have a sufficient vapour of the cylinder copper electrodes 18 comprising water
pressure. Consequently in the presence of AlCl3, which cooling portions 19 are arranged on each part between
up to 3000 C. itself does not react with C, the sublima the sawcuts. Finally the assembly is surrounded by an
tion can only be carried out in an atmosphere of inert insulating jacket 20 which is filled with soot 21.
rare gas or carbon monoxide. A similar complication At the lower end of the apparatus the protective gas
occurs when boron is added in the form of BCl and con 10 used in sublimation can be introduced. This gas flows
sequently in this event also the sublimation is carried out through a supply pipe 22, the duct 16 and the apertures
in an inert rare gas or in carbon monoxide. 8 and 6, between the vessel 1 and the cylinder 7, and
With a sufficient concentration of the impurities in the through apertures in and at the sides of the lump of silicon
initial material, i. e., the silicon carbide lining, it is also carbide 3 and communicates with the sublimation Space
possible to produce n-type and p-type crystals. The addi 15 23 without giving rise therein to inconvenient gas currents.
tion of controlled amounts of the donor and acceptor Thereupon the gas passes the grates 13 provided in
impurities can be effected in the starting material. In the circuit in order to condense the vapour carried along
order to produce this material use may, for example, be by the gas flow and flows out through apertures 24.
made of a mixture of SiO--C to which NaPO or Al-O The gas required to protect the carbon furnace 17 is
is added as initial material. The donor or acceptor is 20 separately introduced into the space between the graphite
then previously built in the SiC lattice. However, a mix cylinder 7 and the insulating jacket.
ture of SiC and the impurity in elementary form or in Example I
the form of a compound may also be used as initial ma
terial. In accordance with the distribution coefficient A cylindrical graphite vessel which is open at one end,
during Sublimation the separated crystals will have the has an inner diameter of 70 mms. and an outer diameter
desirable donor or acceptor concentration, of 80 mms. and is 130 mms. long, is lined internally by
Finally, crystals can be produced having adjacent zones piling lumps of pure silicon carbide (impurities less than
of different conductivity, more particularly of opposite 0.002%) so as to surround a central mandrel 25 mms.
conductivity type, by adding locally a controlled amount thick which is later removed. Consequently the silicon
of the donor and/or acceptor impurities to the crystals. 30 carbide layer, which acts as the starting material as well
This can be effected by varying the vapour pressure as the furnace lining is approximately 20 to 25 mms.
of an impurity during sublimation and, if the formation thick. A layer of approximately equal thickness is ar
of crystals having zones of opposite conductivity type is ranged on the bottom of the vessel also and the open
required, by causing the vapour pressure of donor and upper end of the vessel is closed by a lump of silicon
acceptor impurities respectively to predominate alter carbide. The total amount of silicon carbide which thus
nately. Surrounds the sublimation space 23, is approximately
As an alternative, the impurities can partly be added 500 grm.
to the initial material and partly to the gas atmosphere. The vessel is arranged in a device of the kind shown
In order to produce crystals comprising p-n junctions use in the drawing and the assembly is evacuated under a
may, for example, be made of SiC containing a controlled bell jar in order to remove the air to a maximum extent.
amount of impurity as initial material which is sublimed Thereupon pure hydrogen or argon is introduced until
in a protective gas together with an impurity which gives atmospheric pressure is attained and this gas is passed
rise to the formation of silicon carbide having the oppo through at a rate of approximately 1 litre per minute
site conductivity type. If in this case use is made of a measured at room temperature; this gas is also passed
protective gas containing a proportion of the last-men through the space comprising the furnace. Thereupon
tioned impurity, which proportion may, if required, be the temperature is raised in 1 hour to 2500-30 C. with
varied periodically, silicon carbide will separate out which aid of the carbon furnace and the vessel is maintained
has a conductivity corresponding to the first-mentioned at this temperature for 6 hours. Finally it is cooled
or last-mentioned impurity respectively according as the in approximately 5 hours.
vapour pressure of the impurity originating from the ini The loosely piled silicon carbide has become a coherent
tial material or the impurity which is added to the gas unit which proves to be substantially entirely recrystal
predominates in the sublimation atmosphere with the re lised. On the inner wall of the sublimation chamber
sult that crystals having p-n junctions are obtained. very pure crystals are arranged. These have been grown
The silicon carbide crystals produced in accordance by portions of the lining vaporizing, some of it decom
with the invention can be used in semi-conductive de- :5 5 posing, and depositing on other portions of the lining
vices, such as rectifiers, transistors and voltage-dependent in The
the form of very pure silicon carbide crystals.
crystals can be reached by carefully breaking up
resistors.
The process according to the invention can be carried the silicon carbide mass. The yield of useful crystals is
out with the use of an apparatus of the kind shown dia from 30 to 200. The remainder of the mass can be used
grammatically in the accompanying drawing, in the sole 60 again.
figure of which 1 designates a graphite cylinder open at The obtained transparent and substantially colourless
one end. On the bottom and along the walls lumps of crystals exhibit a very slight n-conductivity due to the
presence of traces of nitrogen (less than 104%) in the
silicon carbide 2 are piled and the aggregate is closed by gas flow and a resistivity of approximately 1000 ohm cm.
a lump 3 of the same substance. This vessel 1 is ar
ranged on a cylindrical member 4 which is filled with 65 Example II
soot 5 with a view to satisfactory heat insulation and com If silicon carbide is treated in the manner described in
prises small apertures 6. The assembly is surrounded by Example 1 in a gas flow to which nitrogen has been
a slightly larger graphite cylinder 7 comprising an aper added, products of high n-conductivity are obtained. In
ture 8 in the bottom 9. The upper end of the cylinder the following table the resistivity, the analytically deter
7 is closed by the graphite member 10 in which a cylin 70
drical member 11 made of graphite is arranged which mined nitrogen content, the number of incorporated nitro
similarly to the member 4 is filled with soot 12, and by gen atoms per cm. calculated therefrom and the num
a number of grates 13. The assembly is arranged to ber of current carriers per cm.8 determined from the Hall
gether with an insulating member 15 made of graphite coefficient at a temperature of from 600 to 800° C. are
and filled with soot 14 which comprises a duct 16 in a 75 listed of a number of crystals obtained in a gas atmos
 2,854,364
5 6
phere of varying nitrogen content. The agreement be protective gas, crystals are obtained having an Al-content
tween the electrically determined number of current car of 0.005% and p-conductivity.
riers and the number of analytically determined nitrogen Example V
atoms is striking. For the sake of completeness it should
be noted that the crystals were analysed by heating them If pure silicon carbide is subjected to a treatment of
to 2300° C. in vacuo, capturing the liberated gas and the kind described in Example III in an atmosphere of
determining the nitrogen content thereof. argon containing AlCl3, crystals of p-conductivity will be
produced at a concentration of 0.06%. After these
PerCellt Analyti- Calculated Calculated crystals have grown for 4 hours argon is introduced in
by wol- Resis- cally de- from from Hall 10 which the AlCl4 addition is replaced by 1% of nitrogen,
No. ume of tivity in termined analysis, coefficient,
N2 in ohm-cm. percent of atOIns
E2 at Ng in
electrons,
N1cm.3 CIn.3
after which the crystals grow for three hours during which
lat.Im. crystal time nitrogen-containing SiC settles. The occurrence of
rectification proves that a p-n junction is produced in the
0.5 0, 17 | 1.2X10-3 1. 65X1018 1.9X.018 crystals, which junction is also visible by the colour
2 ---------- 3.8X10-3 5.2X1018 -6X1018 difference.
13 ---------- 3X10-2 1.8x1010 4. 1?1019 What is claimed is:
75 0.006 4X10-2 || 5, 8X1019 7.7x1019
1. A process for producing silicon carbide crystals by
Example III Sublimation, comprising providing a heat-insulated carbon
vessel open at one end, completely lining the interior,
Addition of All enables the production of silicon car 20 including the open end, of the vessel with substantially
bide crystals of p-conductivity. An apparatus of the kind pure silicon carbide to define an empty center space, flow
shown in the drawing is filled with pure silicon carbide in ing a protective gas past the vessel so that it communicates
the manner described in Example 1. After evacuation with the lined open end thereof, and heating the vessel
under a bell jar, pure argon is introduced until atmos at an elevated temperature at which silicon carbide vapor
pheric pressure is attained and passed through at a rate 25 izes from portions of the lining into the empty space and
of one litre per minute measured at room temperature. substantial portions of which decompose into its consti
Argon is also passed as a protective gas through the space tuent atoms, which redeposit on other portions of the
comprising the furnace. Thereafter the furnace is heated lining in the form of very pure silicon carbide crystals.
and, when a temperature of from 1000 to 1500 C. is 30 2. A process as set forth in claim 1 wherein the silicon
reached, AlCl3 is added to the argon flow. The gas sup carbide lining contains a conductivity-determining im
ply pipe is maintained at a temperature of from 150 to purity selected from the group consisting of donor and
200° C. acceptor impurities, whereby vapor of the impurity is
The AlCl3 is added to the gas flow by passing the argon established in the center space during the heating step
loaded with a chlorine carbon compound, for example 35 tivity thereby to produce crystals of a predetermined conduc
CCl4, over aluminum at a temperature of from 400 to type.
500 C, The amount of CC present in the gas is thus 3. A process for producing silicon carbide crystals
quantitatively converted into AlCl3. The CCI concen by Sublimation, comprising providing a heat-insulated
tration in the gas can be readily controlled by means of carbon vessel open at one end, completely lining the in
the temperature imparted to the liquid CCI and of the 40 terior, including the open end, of the vessel with sub
rate of flow of the argon which is passed through this stantially pure silicon carbide to define an empty center
liquid. space, flowing a gas mixture constituted of a protective
The AlCls carried along by the gas flow reaches the gas and a gaseous impurity at a controlled vapor pressure
sublimation space by diffusion. and Selected from the group consisting of donor and
The sublimation is carried out at a temperature of ap acceptor impurities past the vessel so that it communi
proximately 2560° C. for example 6 hours after which 45 cates with the lined open end thereof, and heating the
cooling is effected for approximately 5 hours. When the vessel at an elevated temperature at which silicon carbide
temperature in the sublimation space has dropped to ap vaporizes from portions of the lining into the empty space
proximately 2000 C., the apparatus is washed further and substantial portions of which decompose into its
with argon having no ACis added to it. In the following 50 the constituent atoms, which redeposit on other portions of
table the analytically determined aluminum content, the lining in the form of very pure silicon carbide crystals
number of incorporated aluminum atoms per cm.8 calcu ductivity doped by impurities and exhibiting a predetermined con
lated therefrom and the number of current carriers per type.
cm.8 at 500 to 600° C. determined from the Hall coeffi 4. A process for producing silicon carbide crystals b
cient is given of a number of crystals produced by sublim Sublimation, comprising providing a heat-insulated re
ation in argon at atmospheric pressure with a varying 55 fractory the
vessel open only at one end, completely lining
interior, including the open end, of the vessel with
AlCls concentration. pure silicon carbide to define an empty center space,
flowing a protective gas at atmospheric pressure to the
Analyti- Calculated vessel so that it communicates with the lined open end
Percent of cally de- from From Hall
No. ACl3 in termined analysis, coefficient, 60 thereof, and heating the vessel at an elevated temperature
argon at percent of atoms of hcles/cIn.3 of approximately 2500 C. at which portions of the lining
l attin. Al in Al/??1.3
crystal vaporize into the empty space and redeposit on other
portions in the form of very pure silicon carbide crystals.
0.0093 ?! ? ? ? ? ? ? ? ? ? ? ? ? ?? - ? - ? - ? ? - 3X1017 5. A process as claimed in claim 4 wherein a gaseous
1019?????? 65 impurity selected from the group consisting of acceptor
0.056 0.003 2X1018 -...----------
0.53 0.014 1X1019
1.9 0.20 1. 4X10------------ and donor impurities is added to the protective gas so as
to incorporate impurity atoms into the grown crystal
Example IV whereby it exhibits a desired conductivity.
6. A process for producing silicon carbide crystals by
ir this example, the starting material was SiC contain 70 Sublimation, comprising providing a heat-insulated re
ing a proportion of 0.3% of aluminum. Such a product fractory vessel open only at one end, completely lining the
was obtained by heating a mixture of carbon, silicic acid interior, including the open end, of the vessel with pure
and aluminum oxide to a temperature of 2000 C. in hy silicon carbide to define an empty center space, flowing
drogen. if this product is subjected to a treatinent as de a gas mixture at atmospheric pressure and comprising a
scribed in Example with the use of carbon monoxide as 75 protective gas and an impurity gas selected from the
 2,854,864
7 8
group consisting of donor and acceptor impurities to the References Cited in the file of this patent
vessel so that it communicates with the lined open end UNITED STATES PATENTS
thereof, heating the vessel at an elevated temperature of 913,324 Tone ------------------ Feb. 23, 1909
approximately 2500° C. at which portions of the lining 992,698 Tone ------------------ May 16, 1911
vaporize into the empty space and redeposition other 5 2,005,956 Ridgway --------------- June 25, 1935
portions in the form of very pure silicon carbide crystals, 2,178,773 Benner et al. ------------ Nov. 7, 1939
and controlling the vapor pressure of the impurity gas to
produce crystals with a p-n junction therein. 2,701,216 Seiler --------------------- Feb. 1, 1955