U.S. patent application number 10/120357 was filed with the patent office on 2002-08-22 for liquid phase growth method of silicon crystal, method of producing solar, cell, and liquid phase growth apparatus.
Invention is credited to Iwane, Masaaki, Nakagawa, Katsumi, Nishida, Shoji, Ukiyo, Noritaka.
Application Number | 20020112660 10/120357 |
Document ID | / |
Family ID | 18355886 |
Filed Date | 2002-08-22 |
United States Patent
Application |
20020112660 |
Kind Code |
A1 |
Nishida, Shoji ; et
al. |
August 22, 2002 |
Liquid phase growth method of silicon crystal, method of producing
solar, cell, and liquid phase growth apparatus
Abstract
Provided are a liquid phase growth method of silicon crystal
comprising a step of injecting a source gas containing at least
silicon atoms into a solvent to decompose the source gas and,
simultaneously therewith, dissolving the silicon atoms into the
solvent, thereby supplying the silicon atoms into the solvent, and
a step of dipping or contacting a substrate into or with the
solvent, thereby growing a silicon crystal on the substrate; and a
method of producing a solar cell utilizing the aforementioned
method. Also provided is a liquid phase growth apparatus of a
silicon crystal comprising means for holding a solvent in which
silicon atoms are dissolved, and means for dipping or contacting a
substrate into or with the solvent, the apparatus further
comprising means for injecting a source gas containing at least
silicon atoms into the solvent. These provide a liquid phase growth
method of a silicon crystal and a production method of a solar cell
each having high volume productivity and permitting continuous
growth.
Inventors: |
Nishida, Shoji;
(Kanagawa-ken, JP) ; Nakagawa, Katsumi;
(Kanagawa-ken, JP) ; Ukiyo, Noritaka;
(Kanagawa-ken, JP) ; Iwane, Masaaki;
(Kanagawa-ken, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Family ID: |
18355886 |
Appl. No.: |
10/120357 |
Filed: |
April 12, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10120357 |
Apr 12, 2002 |
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09208377 |
Dec 10, 1998 |
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6391108 |
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Current U.S.
Class: |
117/54 |
Current CPC
Class: |
C30B 19/02 20130101;
C30B 19/06 20130101; C30B 29/06 20130101 |
Class at
Publication: |
117/54 |
International
Class: |
C30B 019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 12, 1997 |
JP |
9-342709 |
Claims
What is claimed is:
1. A liquid phase growth method of a silicon crystal comprising a
step of injecting a source gas comprising at least silicon atoms
into a solvent to decompose the source gas and, simultaneously
therewith, dissolving the silicon atoms into the solvent, thereby
supplying the silicon atoms into the solvent, and a step of dipping
or contacting a substrate into or with the solvent, thereby growing
a silicon crystal on the substrate.
2. The method according to claim 1, comprising a step of agitating
the solvent and the silicon atoms by at least one of the source
gas, a gas evolved by the decomposition of the source gas, and a
gas injected into the solvent together with the source gas.
3. The method according to claim 1, comprising a step of agitating
the solvent and the silicon atoms by use of a mechanical means.
4. The method according to claim 1, wherein the solvent is a
solvent comprised of a metal.
5. The method according to claim 4, wherein the metal is at least
one selected from In, Sn, Bi, Ga, and Sb.
6. The method according to claim 1, wherein the source gas
comprises SiH.sub.4.
7. The method according to claim 1, wherein the source gas
comprises Si.sub.nH.sub.2n+2 (n is an integer of 2 or more).
8. The method according to claim 1, wherein the source gas
comprises of a silane halide.
9. The method according to claim 1, wherein the source gas contains
a dopant.
10. A method of producing a solar cell comprising at least a step
of forming a silicon layer by liquid phase growth, the method
comprising a step of injecting a source gas comprising at least
silicon atoms into a solvent to decompose the source gas and,
simultaneously therewith, dissolving the silicon atoms into the
solvent, thereby supplying the silicon atoms into the solvent, and
a step of dipping or contacting a substrate into or with the
solvent, thereby growing a silicon crystal on the substrate to form
the silicon layer.
11. The method according to claim 10, comprising a step of
agitating the solvent and the silicon atoms by at least one of the
source gas, a gas evolved by the decomposition of the source gas,
and a gas injected into the solvent together with the source
gas.
12. The method according to claim 10, comprising a step of
agitating the solvent and the silicon atoms by use of a mechanical
means.
13. The method according to claim 10, wherein the solvent is a
solvent comprised of a metal.
14. The method according to claim 13, wherein the metal is at least
one selected from In, Sn, Bi, Ga, and Sb.
15. The method according to claim 10, wherein the source gas
comprises SiH.sub.4.
16. The method according to claim 10, wherein the source gas
comprises Si.sub.nH.sub.2n+2 (n is an integer of 2 or more).
17. The method according to claim 10, wherein the source gas
comprises a silane halide.
18. The method according to claim 10, wherein the source gas
contains a dopant.
19. The method according to claim 10, further comprising a step of
forming an n-type layer, after the step of forming the silicon
layer by liquid phase growth.
20. The method according to claim 19, wherein the n-type layer is
formed by diffusing a dopant into a part of the silicon layer.
21. A liquid phase growth apparatus of a silicon crystal comprising
means for holding a solvent in which silicon atoms are dissolved,
and means for dipping or contacting a substrate into or with the
solvent, the apparatus further comprising means for injecting a
source gas comprising at least silicon atoms into the solvent.
22. A liquid phase growth apparatus of a silicon crystal comprising
a solvent reservoir for holding a solvent in which silicon atoms
are dissolved, a source gas inlet pipe having an opening portion in
the solvent held in the solvent reservoir, a wafer cassette for
holding a substrate, the wafer cassette being arranged to be freely
taken into or out of the solvent held in the solvent reservoir, and
a heater.
23. The apparatus according to claim 22, further comprising a
mechanical agitating means arranged to be freely taken into or out
of the solvent.
24. A liquid phase growth apparatus of a silicon crystal comprising
a solvent reservoir and a growth vessel each for holding a solvent
in which silicon atoms are dissolved, a pipe for circulating the
solvent between the solvent reservoir and the growth vessel, a
source gas inlet pipe having an opening portion in the solvent held
in the solvent reservoir, a wafer cassette for holding a substrate,
the wafer cassette being arranged to be freely taken into or out of
the solvent held in the growth vessel, and a heater.
25. The apparatus according to claim 24, further comprising means
for making a difference between the temperature of the solvent in
the solvent reservoir and the temperature of the solvent in the
growth vessel.
26. The apparatus according to claim 25, wherein the means for
making the difference between the temperature of the solvent in the
solvent reservoir and the temperature of the solvent in the growth
vessel comprises a heater block surrounding the solvent
reservoir.
27. The apparatus according to claim 24, wherein at least a part of
the pipe functions as a heat exchanger.
28. A liquid phase growth apparatus of a silicon crystal comprising
a solvent reservoir for holding a solvent in which silicon atoms
are dissolved, a pipe both ends of which are connected to the
solvent reservoir and which has an aperture portion except for the
both ends, the pipe being provided for circulating the solvent, a
source gas inlet pipe having an opening portion in the solvent held
in the solvent reservoir, a holding member for holding a substrate
so that the substrate is in contact with the solvent at the
aperture portion, and a heater.
29. The apparatus according to claim 28, further comprising means
for making a difference between the temperature of the solvent in
the solvent reservoir and the temperature of the solvent in the
vicinity of the aperture portion.
30. The apparatus according to claim 29, wherein the means for
making the difference between the temperature of the solvent in the
solvent reservoir and the temperature of the solvent in the
vicinity of the aperture portion comprises a heater block
surrounding the solvent reservoir.
31. The apparatus according to claim 28, wherein at least a part of
the pipe functions as a heat exchanger.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a liquid phase growth
method of a silicon crystal, a method for producing a solar cell,
and a liquid phase growth apparatus and, more particularly, to a
liquid phase growth method that permits continuous growth and
volume production.
[0003] 2. Related Background Art
[0004] Liquid phase growth methods have the advantage of the
capability of obtaining crystals with high quality close to
stoichiometric compositions because of crystal growth from the
quasi-equilibrium state and are used in production of LEDs
(light-emitting diodes), laser diodes, and so on, as techniques
already established in such compound semiconductors as GaAs.
Recently, attempt has been made to utilize the liquid phase growth
of Si in order to obtain a thick film (for example, Japanese Patent
Application Laid-Open No. 58-89874) and application to solar cells
is also under research.
[0005] In the conventional liquid phase growth methods, in general,
a solution containing a substance for growth as a solute is cooled
into a supersaturated state to deposit the excess solute (the
substance for growth) on a substrate. On that occasion, it is
necessary to preliminarily dissolve the solute into a solvent until
saturated, prior to depositing (or growing) the solute on the
substrate. Ordinary methods for dissolving the solute into the
solvent include one for preliminarily mixing the solute in an
amount enough to saturate at a temperature during the dissolution
into the solvent and heating the solvent, and one for heating a
large amount of a base material of the solute (over a saturation
amount) in contact with the solvent and keeping it at the
dissolving temperature to saturate. In the former case, a newly
weighed amount of the solute is charged into the solvent or the old
solvent is replaced by another solvent in which the solute was
preliminarily dissolved, after every completion of growth. In the
latter case, the base material of the solute is taken into and out
of the solvent before or after the growth and the base material
will be used up at last to cause some harm in taking it into or out
of the solvent or result in an insufficient dissolved amount.
Therefore, the old base material needs to be replaced by a new base
material. In either case, time loss occurs, because the apparatus
is stopped for supplying the raw material when used up or because
the growth is suspended. Therefore, the methods according to the
conventional techniques had the problem in terms of volume
productivity.
[0006] The present invention has been accomplished as a consequence
of intensive and extensive research by the inventors in order to
solve the problem in the conventional techniques as discussed above
and an object of the present invention is, therefore, to provide a
liquid phase growth method that is simple and easy and that has
high volume productivity.
SUMMARY OF THE INVENTION
[0007] Therefore, the present invention provides a liquid phase
growth method of a silicon crystal comprising a step of injecting a
source gas comprising at least silicon atoms into a solvent to
decompose the source gas and, simultaneously therewith, dissolving
the silicon atoms into the solvent, thereby supplying the silicon
atoms into the solvent, and a step of dipping or contacting a
substrate into or with the solvent, thereby growing a silicon
crystal on the substrate.
[0008] Further, the present invention provides a method of
producing a solar cell comprising at least a step of forming a
silicon layer by liquid phase growth, the method comprising a step
of injecting a source gas comprising at least silicon atoms into a
solvent to decompose the source gas and, simultaneously therewith,
dissolving the silicon atoms into the solvent, thereby supplying
the silicon atoms into the solvent, and a step of dipping or
contacting a substrate into or with the solvent, thereby growing a
silicon crystal on the substrate to form said silicon layer.
[0009] Moreover, the present invention provides a liquid phase
growth apparatus of a silicon crystal comprising means for holding
a solvent in which silicon atoms are dissolved, and means for
dipping or contacting a substrate into or with the solvent, the
apparatus further comprising means for injecting a source gas
comprising at least silicon atoms into the solvent.
[0010] Further, the present invention provides a liquid phase
growth apparatus of a silicon crystal comprising a solvent
reservoir for holding a solvent in which silicon atoms are
dissolved, a source gas inlet pipe having an opening portion in the
solvent held in the solvent reservoir, a wafer cassette for holding
a substrate, the wafer cassette being arranged to be freely taken
into or out of the solvent held in the solvent reservoir, and a
heater.
[0011] Moreover, the present invention provides a liquid phase
growth apparatus of a silicon crystal comprising a solvent
reservoir and a growth vessel each for holding a solvent in which
silicon atoms are dissolved, a pipe for circulating the solvent
between the solvent reservoir and the growth vessel, a source gas
inlet pipe having an opening portion in the solvent held in the
solvent reservoir, a wafer cassette for holding a substrate, the
wafer cassette being arranged to be freely taken into or out of the
solvent held in the growth vessel, and a heater.
[0012] In addition, the present invention provides a liquid phase
growth apparatus of a silicon crystal comprising a solvent
reservoir for holding a solvent in which silicon atoms are
dissolved, a pipe both ends of which are connected to the solvent
reservoir and which has an aperture portion except for the both
ends, the pipe being provided for circulating the solvent, a source
gas inlet pipe having an opening portion in the solvent held in the
solvent reservoir, a holding member for holding a substrate so that
the substrate is in contact with the solvent at the aperture
portion, and a heater.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic sectional view showing an example of
the liquid phase growth apparatus according to the present
invention;
[0014] FIG. 2 is a schematic sectional view showing an apparatus
that permits the dissolution of silicon and liquid phase growth to
be carried out simultaneously, as an example of the liquid phase
growth apparatus according to the present invention;
[0015] FIG. 3 is a schematic sectional view showing an apparatus
having a mechanical agitating means, as an example of the liquid
phase growth apparatus according to the present invention; and
[0016] FIG. 4 is a schematic sectional view showing an apparatus in
which a substrate is in contact with a solvent at an aperture
portion, as an example of the liquid phase growth apparatus
according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] An example of the liquid phase growth apparatus, which is
used in the liquid phase growth method of the present invention, is
illustrated in FIG. 1. In FIG. 1 reference numeral 101 designates a
wafer cassette, 102 substrates (wafers), 103 a solvent reservoir
(crucible), 104 a solvent (melt), 105 a reaction product gas, 106 a
source gas inlet pipe, 107 a reactor tube, and 108 an electric
furnace (heater).
[0018] Further, numeral 109 denotes gas outlet holes provided in
the source gas inlet pipe, 110 a gate valve, and 111 an exhaust
port.
[0019] The liquid phase growth method and liquid phase growth
apparatus of the present invention will be described referring to
FIG. 1. As illustrated in FIG. 1, the solvent reservoir (crucible)
103 made of carbon is filled with the solvent comprised of a metal
(hereinafter referred to as metal solvent) 104 and the supply pipe
106 for introduction of the source gas is set along the side wall
and bottom surface of the crucible 103. The wafer cassette 101
carrying the wafers 102 is located above the crucible 103 and is
moved vertically to dip the wafers 102 in the metal solvent 104 or
lift the wafers 102 up out of the metal solvent 104, thereby
performing growth start operation/growth end operation. The wafer
cassette 101 is equipped with a rotating mechanism and the wafer
cassette 101 is rotated thereby during growth to uniform
thicknesses of a grown film in each wafer surface and thicknesses
of grown films among the wafers. The crucible 103, source gas inlet
pipe 106, and wafer cassette 101 are housed in the reactor tube 107
and are heated by the electric furnace 108 located outside the
reactor 107.
[0020] Specific procedures of the liquid phase growth method of the
present invention will be described. First, the unsaturated metal
solvent, or the metal solvent 104 after an end of growth is heated
to a predetermined temperature (a little higher than the growth
temperature) and kept thereat before stabilized. Then the source
gas, for example SiH.sub.4, as a supply source of Si is allowed to
flow in the source gas inlet pipe 106, so that the source gas
(SiH.sub.4) is injected into the metal solvent through the gas
outlet holes 109 opening in the surface of the inlet pipe placed at
the bottom surface of the crucible, whereupon the source gas
(SiH.sub.4) comes into contact with the metal solvent. When
SiH.sub.4 is used as the source gas, the SiH.sub.4 coming into
contact with the metal solvent soon reacts to be decomposed into Si
atoms and H.sub.2 molecules. The Si atoms are dissolved into the
metal solvent. At this time, the H.sub.2 molecules thus evolved
agitate the metal solvent to promote the dissolution of the Si
atoms into the solvent. It can also be contemplated that the
solvent is positively agitated with an agitating mechanism (not
illustrated) provided separately. After the SiH.sub.4 gas is
injected into the metal solvent for a certain time, the flow of the
SiH.sub.4 gas is stopped and the metal solvent is slowly cooled by
controlling the electric furnace 108. When the temperature of the
metal solvent reaches the growth start temperature, the wafer
cassette 101 is moved down to dip the wafers 102 into the metal
solvent 104. Preferably, the wafer cassette 101 is rotated at the
rate of several rpm during the growth so as to uniform the
thicknesses of grown films. After a lapse of a predetermined growth
time, the wafer cassette 101 is moved up out of the metal solvent
104, thereby terminating the growth. Since in the present
embodiment the wafers 102 are mounted at a fixed inclination in the
wafer cassette 101, there remains little metal solvent 104 on the
wafer surfaces when they are drawn up out of the metal solvent 104.
There are, however, some cases where a small amount of the metal
solvent remains at contact portions (support portions) between the
wafers 102 and the wafer cassette 101. In such cases, the wafer
cassette is rotated at the rotational speed of several ten or
higher rpm, whereby the remaining metal solvent can be thrown off.
Subsequently, the wafer cassette is lifted up into a preliminary
chamber (not illustrated) separated from the reactor, in which the
wafers are exchanged. Then the above steps are repeated, thereby
continuously performing liquid phase growth operations.
[0021] The feature of the present invention is that the source
material can be continuously supplied into the solvent with the
source gas kept in contact with the metal solvent, which eliminates
the time loss due to the exchange of the base material as the
solute and the like in the conventional methods, thereby enhancing
the volume productivity.
[0022] In the present invention, as the material for the solvent
reservoir for storing the metal solvent and as the material for the
wafer cassette for supporting the wafers, there is preferably used
high-purity carbon or high-purity quartz or the like. Similarly,
high-purity carbon or high-purity quartz or the like is also used
as a preferred material for the source gas inlet pipe used in the
present invention, and high-purity quartz is used as a preferred
material for the reactor tube. As the source gas used herein, there
are preferably included silanes such as SiH.sub.4, Si.sub.2H.sub.6,
. . . , Si.sub.nH.sub.2n+2 (n: natural number) and silane halides
such as SiH.sub.2Cl.sub.2, SiHCl.sub.3, SiCl.sub.4,
SiH.sub.2F.sub.2, and Si.sub.2F.sub.6.
[0023] Further, it is preferable to add hydrogen (H.sub.2) to the
source gas, as a carrier gas or for the purpose of obtaining a
reducing atmosphere to promote the crystal growth. The ratio of
amounts of the source gas and hydrogen is properly determined
depending upon the forming method, type of the source gas, and
forming conditions, and the ratio is preferably not less than
1:1000 nor more than 100:1 (based on the ratio of flow rates of
introduced gases) and more preferably not less than 1:100 nor more
than 10:1.
[0024] As the solvent used in the present invention, a solvent
comprised of a metal such as, In, Sn, Bi, Ga, Sb, or the like is
preferred. Epitaxial growth is effected by contacting the source
gas into the solvent to dissolve the Si atoms thereinto and
thereafter slowly cooling the solvent or by providing a temperature
difference in the solvent while supplying the Si atoms from the
source gas into the solvent.
[0025] The temperature and pressure in the liquid phase growth
method employed in the present invention differ depending upon the
forming method, the type of the source material (gas) used, etc.,
but the temperature of the solvent is desirably controlled in the
range of not less than 600.degree. C. nor more than 1050.degree. C.
when silicon is grown using the solvent of Sn or In, for example.
The appropriate pressure is generally in the range of 10.sup.-2
Torr to 760 Torr and more preferably in the range of 10.sup.-1 Torr
to 760 Torr.
[0026] When the conductivity type (the p-type/n-type) of Si needs
to be controlled, a gas containing a dopant such as P, B, etc. may
be introduced into the solvent as the occasion demands. Further, a
solar cell element can be formed by growing a silicon crystal by
use of indium as a solvent without addition of a particular dopant
to form a silicon layer and thereafter forming an n-type layer, for
example, by a method for diffusing the dopant into a part of the
silicon layer by such a method as thermal diffusion, ion
implantation, or the like.
[0027] The growth of a desired crystal by the method of the present
invention will be described in more detail using examples, but it
should be noted that the present invention is by no means intended
to be limited to these examples.
EXAMPLE 1
[0028] In the present example an epitaxial layer of Si was grown
using the liquid phase growth apparatus of the structure
illustrated in FIG. 1. The solvent was In and the source gas was
SiH.sub.4. While the wafer cassette 101 carrying five 5" Si wafers
102 was kept on standby in a preliminary chamber (not illustrated),
the solvent reservoir 103 storing the In solvent 104 was heated by
the heater 108 to keep the temperature of the solvent at the
constant temperature of 960.degree. C. Here, 5" means that the
diameter of the wafers is 5 inches. Then the wafer cassette 101
kept on standby in the preliminary chamber was guided into the
reactor 107 while opening the gate valve 110 and it was held
immediately above the solvent reservoir 103. The gate valve was
kept open thereafter. The source gas SiH.sub.4, together with
H.sub.2 gas (in the ratio of gas flow rates:
SiH.sub.4/H.sub.2=1:1), was injected through the source gas inlet
pipe 106 into the In solvent 104 and these gases were kept flowing
for 30 minutes. After the flow of the gases was stopped, the heater
108 was controlled so as to start slowly cooling the solvent in the
reactor tube 107 at a rate of -1.degree. C./min. When the
temperature of the In solvent 104 reached 950.degree. C., the wafer
cassette 101 was moved down into the In solvent 104 while being
rotated at the rotational speed of 10 rpm. When the wafer cassette
101 was completely dipped in the In solvent 104, the down movement
was stopped and the wafer cassette was held at that position. Then
the liquid phase growth was carried on for 60 minutes while
rotating the wafer cassette. After that, the wafer cassette 101 was
drawn up out of the In solvent 104 and was temporarily stopped
immediately above the solvent reservoir 103. Then the rotational
speed was increased up to 120 rpm to throw the partly remaining In
off the wafer cassette, and the liquid phase growth was
completed.
[0029] Cross sections of the wafers thus obtained were observed
with a scanning electron microscope and a transmission electron
microscope and it was verified that the epitaxial silicon layers
thus grown had a thickness of about 15 .mu.m and also had good
crystallinity.
EXAMPLE 2
[0030] In the present example an epitaxial layer of Si was grown by
using the liquid phase growth apparatus of the structure
illustrated in FIG. 3 and using a mechanical agitating means
(agitating mechanism) in combination to dissolve the solute in the
solvent. The solvent was In and the source gas was Si.sub.2H.sub.6.
While the agitating mechanism 312 was kept on standby in a
preliminary chamber (not illustrated), the solvent reservoir 303
storing the In solvent 304 was heated by a heater 308 to keep the
temperature of the solvent at the constant temperature of
960.degree. C. Then the agitating mechanism 312 kept on standby in
the preliminary chamber was guided into the reactor tube 307 while
opening the gate valve 310 and was held immediately above the
solvent reservoir 303. The gate valve was kept opening thereafter.
The source gas Si.sub.2H.sub.6, together with H.sub.2 gas (in the
ratio of gas flow rates: Si.sub.2H.sub.6/H.sub.2=1:1), was injected
through the source gas inlet pipe 306 into the In solvent 304 and
the agitating mechanism 312 was moved down into the In solvent 304
while being rotated at a rotational speed of 20 rpm. When the
blades 313 of the agitating mechanism were adequately dipped in the
In solvent, the down motion was stopped and the agitating mechanism
was held at that position. Then the gases were allowed to flow for
30 minutes while agitating the solvent. After the end of the flow
of the gases, the agitating mechanism 312 was drawn up to the
preliminary chamber and then the wafer cassette 301 carrying five
5" Si wafers 302 this time was guided from a preliminary chamber
(not illustrated) into the reactor tube 307 to be held immediately
above the solvent reservoir 303 for 10 minutes. Then the heater 308
was controlled to start slowly cooling the solvent in the reactor
tube 307 at a rate of -1.5.degree. C./min. When the temperature of
the In solvent 304 reached 950.degree. C., the wafer cassette 301
was moved down into the In solvent 304 while being rotated at a
rotational speed of 10 rpm. When the wafer cassette 301 was
completely dipped in the In solvent 304, the down motion was
stopped and the wafer cassette 301 was held at that position. Then
the liquid phase growth was carried on for 45 minutes while
rotating the wafer cassette. After that, the wafer cassette 301 was
lifted up out of the In solvent 304 and was temporarily stopped
immediately above the solvent reservoir 303. Then the rotational
speed was increased up to 120 rpm, thereby throwing the partly
remaining In off the wafer cassette, and the liquid phase growth
operation was ended. In FIG. 3 numeral 305 represents the reaction
product gas, 309 the gas outlet holes, and 311 the exhaust
port.
[0031] Cross sections of the wafers thus obtained were observed
with a scanning electron microscope and a transmission electron
microscope and it was verified that the epitaxial silicon layers
thus grown had a thickness of about 15 .mu.m and also had good
crystallinity.
EXAMPLE 3
[0032] In the present example an epitaxial layer of Si was grown
using the apparatus illustrated in FIG. 2.
[0033] The solvent was Sn and the source gas was SiH.sub.2Cl.sub.2.
The apparatus illustrated in FIG. 2 has a solvent reservoir 214
made of quartz, a growth vessel 203 in which a wafer cassette 201
carrying substrates (wafers) 202 are dipped, and quartz pipes 209a,
209b, 210 routed out of one side surface of the solvent reservoir
214, through the growth cell 203, and back to another side surface
of the solvent reservoir 214, inside an electric furnace 207. The
pipes 209a, 209b serve as heat exchangers. The solvent reservoir
214 and heat exchanger 209b are further surrounded by a heater
block 208 so as to be able to control the temperature
independently. Numeral 211 designates a rotor for circulation, 212
a gate valve, 206 a source gas inlet pipe, and 213 an exhaust port.
Further, numeral 204 is the solvent and 205 the reaction product
gas.
[0034] The solvent 204 of Sn sufficiently purified in a hydrogen
atmosphere was charged into the solvent reservoir 214, growth
vessel 203, and quartz pipes 209a, 209b, 210 and the temperature
inside the electric furnace 207 was kept at the constant
temperature of 950.degree. C. The temperature of the solvent
reservoir 214 was set 10.degree. C. higher by the heater block 208
than the temperature inside the electric furnace 207 and outside
the heater block 208 and the solvent 204 was circulated by the
rotor 211.
[0035] After a lapse of a sufficient time, the wafer cassette 201
carrying five 5" p.sup.+ (100) Si wafers 202 (wafers doped with a
relatively large amount of a p-type dopant and having the principal
plane of the crystal plane orientation of (100)) was guided from a
preliminary chamber (not illustrated) into the growth vessel 203
while opening the gate valve 212 to be held immediately above the
Sn solvent 204. The source gas SiH.sub.2Cl.sub.2, together with
H.sub.2 gas (in the ratio of gas flow rates:
SiH.sub.2Cl.sub.2/H.sub.2=1:5), was injected through the source gas
inlet pipe 206 into the Sn solvent 204 in the solvent reservoir 214
and the gases were kept flowing. After a lapse of 30 minutes, the
wafer cassette 201 was moved down into the Sn solvent 204 in the
growth vessel 203 while being rotated at a rotational speed of 10
rpm. When the wafer cassette 201 was completely dipped in the Sn
solvent 204, the down movement was stopped and the wafer cassette
was held at that position. Then the liquid phase growth was carried
on for 60 minutes while rotating the wafer cassette. Then the wafer
cassette 201 was drawn up out of the Sn solvent 204 and was
temporarily stopped immediately above the Sn solvent 204. The
rotational speed was increased up to 150 rpm to throw the partly
remaining Sn off the wafer cassette 201, and the liquid phase
growth operation was terminated.
[0036] Cross sections of the wafers thus obtained were observed
with a scanning electron microscope and a transmission electron
microscope and it was verified that the epitaxial silicon layers
thus grown had a thickness of about 20 .mu.m and also had good
crystallinity.
EXAMPLE 4
[0037] In the present example an Si layer was grown on
polycrystalline Si substrates by use of the apparatus illustrated
in FIG. 4. The solvent was In+Ga (Ga content: 0.1 atomic %) and the
source gas was SiH.sub.4. The substrates were each obtained by
processing polycrystalline Si formed by the casting method into the
width 40 mm, the length 250 mm, and the thickness 0.6 mm, polishing
the surface thereof, and thereafter cleaning it.
[0038] The apparatus illustrated in FIG. 4 has a solvent reservoir
414 of carbon, and flat pipes 409a, 409b, 410 made of carbon in an
electric furnace 407, the pipes 409a, 409b, 410 being routed so as
to leave one side surface of the solvent reservoir 414, contact a
slider 402 on which a plurality of substrates 401 are placed, at an
aperture portion 403, and then return to another side surface of
the solvent reservoir 414. The pipes 409a, 409b serve as heat
exchangers. The solvent reservoir 414 and heat exchanger 409b are
further surrounded by heater block 408, so that the temperature can
be controlled independently. Numeral 411 denotes the rotor for
circulation, 406 the source gas inlet pipe, and 413 the exhaust
port. Further, numeral 404 represents the solvent and 405 the
reaction product gas.
[0039] The solvent 404 of In+Ga sufficiently purified in a hydrogen
atmosphere was charged into the solvent reservoir 414 and flat
pipes 409a, 409b, 410, and the position of the slider 402 was
preliminarily adjusted so that the Si substrates 401 were not in
contact with the solvent 404 at the aperture portion 403 of the
flat pipe. In that state, the temperature inside the electric
furnace 407 was kept at the constant temperature of 950.degree. C.
and, at the same time, the temperature of the solvent reservoir 414
was set 10.degree. C. higher by the heater block 408 than the
temperature inside the electric furnace 407 and outside the heater
block 408. The solvent 404 was circulated by the rotor 411. At this
time the length of the aperture portion 403 was 100 mm and the
circulation rate of the solvent 404 was 40 mm/min. In the present
example three Si substrates were placed on the slider.
[0040] Then the source gas SiH.sub.4, together with the H.sub.2 gas
(in the ratio of gas flow rates: SiH.sub.4/H.sub.2=1:1), was
injected through the source gas inlet pipe 406 into the In+Ga
solvent 404 and the gases were kept flowing. After a lapse of 30
minutes, the slider 402 was conveyed at a conveyance speed of 20
mm/min and the liquid phase growth was effected at the aperture
portion 403 with the polycrystalline Si substrate 401 being kept in
contact with the In+Ga solvent 404. After all the polycrystalline
Si substrates 402 have passed the aperture portion 403, the
conveyance of the slider 401 was stopped and the liquid phase
growth was ended.
[0041] Cross sections of the wafers were observed with a scanning
electron microscope and a transmission electron microscope, with
the result that the Si layers thus grown had a thickness of about
20 .mu.m. The orientations of the Si layers thus grown were
inspected by the ECP (Electron Channeling Pattern) method and it
was found that they inherited the crystal orientations of the
respective grains of the base polycrystalline Si substrates. The
present example verified that the crystalline Si layer was able to
be grown continuously while conveying the substrates as described
above.
[0042] Example 4 described above showed the example using the
substrates placed on the slider, but it is also possible, for
instance, to bring a web-like substrate having an Si layer attached
on a surface thereof into contact with a solvent and convey the
substrate in one direction by the roll-to-roll method, thus
continuously growing the Si layer.
EXAMPLE 5
[0043] In the present example n.sup.+/p-type thin-film
single-crystal solar cells were made using the liquid phase growth
method of the present invention. First, by using the apparatus
illustrated in FIG. 1, an epitaxial Si layer was grown on a 500
.mu.m-thick p.sup.+ Si wafer (.rho.=0.01 .OMEGA..multidot.cm) in
the similar fashion to Example 1. The epitaxial growth was carried
out in the same manner as in Example 1 except that the wafer was
different and that the slow cooling rate of the In solvent 104 was
-2.degree. C./min.
[0044] The thickness of the Si layer thus grown was evaluated by a
step gage or the like to be about 30 .mu.m. Then thermal diffusion
of P was effected at a temperature of 900.degree. C. on the surface
of the Si layer thus grown with a diffusion source of POCl.sub.3,
thereby forming the n.sup.+ layer. The junction depth obtained was
about 0.5 .mu.m. The dead layer in the surface of the n.sup.+ layer
thus formed was wet-oxidized and thereafter removed by etching,
thereby obtaining the junction depth of about 0.2 .mu.m with a
moderate surface concentration.
[0045] In the last place, by EB (Electron Beam) evaporation, a
collector electrode (Ti/Pd/Ag (40 nm/20 nm/1 .mu.m)) and an ITO
transparent conductive film (82 nm) were deposited on the n.sup.+
layer and a back surface electrode (Al (1 .mu.m)) was deposited on
the back surface of the substrate, thereby forming the solar
cell.
[0046] The I-V characteristics of the thin-film single-crystal Si
solar cells thus obtained were measured under irradiation with
light of AM 1.5 (100 mW/cm.sup.2). In the cell area of 6 cm.sup.2,
typically an open-circuit voltage 0.6 V, a short-circuit current 33
mA/cm.sup.2, a fill factor 0.77, and an energy conversion
efficiency 15.2% were obtained.
[0047] The present invention has enabled to continuously perform
the crystal growth without interruption for supply of a source
material in the liquid phase growth method of a silicon crystal.
The present invention is suitably applicable to volume production
methods of devices required to have some thickness, particularly,
to those of solar cells.
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