U.S. patent application number 09/198263 was filed with the patent office on 2001-12-13 for method of growing silicon crystal in liquid phase and method of producing solar cell.
Invention is credited to IWANE, MASAAKI, NAKAGAWA, KATSUMI, NISHIDA, SHOJI, UKIYO, NORITAKA.
Application Number | 20010051387 09/198263 |
Document ID | / |
Family ID | 18207436 |
Filed Date | 2001-12-13 |
United States Patent
Application |
20010051387 |
Kind Code |
A1 |
NAKAGAWA, KATSUMI ; et
al. |
December 13, 2001 |
METHOD OF GROWING SILICON CRYSTAL IN LIQUID PHASE AND METHOD OF
PRODUCING SOLAR CELL
Abstract
The method of the present invention of growing single crystal
silicon in a liquid phase comprises preparing a melt by dissolving
a solid of silicon containing boron, aluminum, phosphorus or
arsenic at a predetermined concentration into indium melted in a
carbon boat or a quartz crucible, supersaturating the melt, and
submerging a substrate into the melt, thereby growing a silicon
crystal containing a dopant element. This method can provide a
method of growing a thin film of crystalline silicon having a high
crystallinity and a dopant concentration favorably controlled,
thereby serving for mass production of inexpensive solar cells
which have high performance as well as image displays which have
high contrast and are free from color ununiformity.
Inventors: |
NAKAGAWA, KATSUMI;
(ATSUGI-SHI, JP) ; NISHIDA, SHOJI; (HIRATSUKA-SHI,
JP) ; UKIYO, NORITAKA; (ATSUGI-SHI, JP) ;
IWANE, MASAAKI; (ATSUGI-SHI, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Family ID: |
18207436 |
Appl. No.: |
09/198263 |
Filed: |
November 24, 1998 |
Current U.S.
Class: |
438/57 ;
257/E21.115; 257/E21.414; 438/89 |
Current CPC
Class: |
Y02E 10/547 20130101;
H01L 21/02513 20130101; H01L 21/02625 20130101; C30B 29/06
20130101; H01L 21/02532 20130101; H01L 29/66765 20130101; H01L
31/1804 20130101; Y02P 70/50 20151101; H01L 21/02579 20130101; H01L
31/1892 20130101; H01L 21/02628 20130101; H01L 21/02576 20130101;
H01L 21/02381 20130101; H01L 27/1296 20130101; H01L 31/068
20130101; Y02P 70/521 20151101; C30B 19/02 20130101 |
Class at
Publication: |
438/57 ;
438/89 |
International
Class: |
H01L 021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 28, 1997 |
JP |
9-328186 |
Claims
What is claimed is:
1. A method of growing a silicon crystal in a liquid phase, which
comprises using a melt prepared by dissolving a solid of silicon
containing a dopant at a predetermined concentration into liquid
indium.
2. A method of growing a silicon crystal in a liquid phase
according to claim 1, wherein the dopant is boron or aluminum.
3. A method of growing a silicon crystal in a liquid phase
according to claim 1, wherein the dopant is phosphorus or
arsenic.
4. A method of growing a silicon crystal in a liquid phase, which
comprises using a melt prepared by dissolving a solid of indium
containing a dopant at a predetermined concentration into liquid
indium.
5. A method of growing a silicon crystal in a liquid phase
according to claim 4, further comprising using a melt prepared by
further dissolving silicon into the melt in which the dopant is
dissolved.
6. A method of growing a silicon crystal in a liquid phase
according to claim 4, wherein the dopant is boron or aluminum.
7. A method of growing a silicon crystal in a liquid phase
according to claim 5, wherein the dopant is boron or aluminum.
8. A method of growing a silicon crystal in a liquid phase
according to claim 4, wherein the dopant is phosphorus or
arsenic.
9. A method of growing a silicon crystal in a liquid phase
according to claim 5, wherein the dopant is phosphorus or
arsenic.
10. A method of producing a solar cell, which comprises the steps
of: preparing a melt by dissolving a solid of silicon containing a
dopant at a predetermined concentration into liquid indium; forming
a first silicon layer of a first conductivity type on a substrate
by bringing the substrate into contact with the melt; and forming a
second silicon layer of a second conductivity type on the first
silicon layer of the first conductivity type.
11. A method of producing a solar cell according to claim 10,
wherein the substrate is a silicon wafer which has a porous layer
formed on a surface thereof by anodization.
12. A method of producing a solar cell according to claim 11,
further comprising a step of separating the silicon wafer from the
first silicon layer of the first conductivity type in the porous
layer after forming the second silicon layer of the second
conductivity type.
13. A method of producing a solar cell according to claim 12,
wherein the separating step is carried out by using an adhesive
tape.
14. A method of producing a solar cell, which comprises the steps
of: preparing a melt by dissolving a solid of indium containing a
dopant at a predetermined concentration into liquid indium and then
further dissolving silicon into the liquid indium; forming a first
silicon layer of a first conductivity type on a substrate by
bringing the substrate into contact with the melt; and forming a
second silicon layer of a second conductivity type on the first
silicon layer of the first conductivity type.
15. A method of producing a solar cell according to claim 14,
wherein the substrate is a silicon wafer which has a porous layer
formed on a surface thereof by anodization.
16. A method of producing a solar cell according to claim 15,
further comprising a step of separating the silicon wafer from the
first silicon layer of the first conductivity type in the porous
layer after forming the second silicon layer of the second
conductivity type.
17. A method of producing a solar cell according to claim 16,
wherein the separating step is carried out by using an adhesive
tape.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of growing a
silicon crystal in a liquid phase. A silicon crystal produced by
the method of the present invention can be used in silicon devices
having a large area such as solar cells and picture element driving
circuits for liquid crystal display devices.
[0003] 2. Related Background Art
[0004] Solar cells are prevailing as electric power sources which
are systematically linked with driving power sources for various
kinds of appliances and commercial line power. It is desirable that
solar cells can be manufactured at low costs. For example, it is
desired to produce solar cells on inexpensive substrates at a low
cost. Silicon is generally used as a semiconductor for composing
solar cells. Single crystalline silicon is extremely excellent from
a viewpoint of efficiency of converting a light energy into
electric power, that is, photoelectric conversion efficiency. From
a viewpoints of enlargement of areas and reduction of manufacturing
costs, on the other hand, amorphous silicon is advantageous. In the
recent years, it has come to use polycrystalline silicon for the
purpose of obtaining a cost as low as that of amorphous silicon and
a photoelectric conversion efficiency as high as that of the single
crystalline silicon.
[0005] However, it cannot be said that the expensive crystalline
materials are sufficiently utilized by a method which is
conventionally adopted to manufacture silicon devices using single
crystalline silicon or polycrystalline silicon since the method is
configured to slice a lump crystal to form plate-like substrates
and hardly capable of preparing substrates which have thickness of
0.3 mm or smaller, thereby allowing the substrates to have
thicknesses larger than a thickness (20 .mu.m to 50 .mu.m)
generally required to absorb incident rays. Furthermore, there has
recently been proposed the spin method of forming a silicon sheet
by flowing drops of melted silicon into a template. However, a
silicon sheet formed by this method has a quality insufficient for
use as a semiconductor and cannot provide a photoelectric
conversion efficiency which is so high as that in the case of using
a general crystalline silicon.
[0006] There has been proposed and actually applied to trial
production of a solar cell under the circumstances described above,
an idea of growing on an inexpensive substrate a silicon crystal of
a good quality until it has a required and sufficient thickness,
and form an active region (for example, a photoelectric conversion
region) thereon. Moreover, there has been proposed an idea of
growing a silicon crystal epitaxially on a substrate of a good
quality and then peel off the silicon crystal and reuse the
substrate.
[0007] On a premise that large area devices such as solar cells are
to be produced in mass, however, it is not so easy to grow a
silicon crystal until it has a thickness required for absorbing
incident rays. A silicon crystal of a good quality is generally
grown by the thermal CVD method of thermally decomposing a raw
material gas such as silane chloride. In order to grow a single
crystal at a high rate on the order of 1 .mu.m/minute in
particular, it is general to use the so-called epitaxial growing
furnace. However, such a growing furnace is not only unsuited to
mass production since it can treat 10 wafers at most at one batch
but also requires a high raw material cost since it utilizes a raw
material gas at a low efficiency. Though it is possible to treat
100 or more wafers at one batch by utilizing the so-called low
pressure CVD furnace, this furnace also provides a crystal
insufficient in a quality thereof and allows the crystal to grow at
a rate only on the order of 0.01 .mu.m/minute, thereby being low in
productivity.
[0008] As another method of growing a silicon crystal, there is
known a liquid phase growing method of supersaturating a liquid
metal solution in which silicon is dissolved and allowing a crystal
to deposit from the solution onto a substrate. This liquid phase
growing method is capable of growing a crystal of a high quality at
a high rate on the order of 1 .mu.m/minute and treating 100 or more
wafers at one batch, thereby being suited to mass production.
However, the liquid phase growing method is not generally used for
growing silicon and has some technical problems to be solved though
it widely prevails as a method of growing compound
semiconductors.
[0009] One of important problems lies in selection of a metal which
is to be used as a solvent. It is desirable that a metal to be used
for this purpose has a solubility for silicon which is as high as
possible and can hardly be incorporated into deposited silicon.
Furthermore, a metal having a lower melting point and a lower vapor
pressure can be handled easier. Tin is used most generally as a
solvent for silicon. Tin can be handled relatively easily since it
has allow melting point and a relatively high solubility for
silicon. It has been considered that tin is a preferable solvent
since tin and silicon belong to the same IV group of the Periodic
Table, and tin is inactive as a dopant even when it is incorporated
into deposited silicon.
[0010] However, the inventors have recently found that tin is
incorporated into silicon in a prettily large amount when growth
conditions (in particular, a growth temperature) are inadequate,
thereby deforming a lattice of a silicon crystal and adversely
affecting electric characteristics of a semiconductor probably due
to the atomic size of tin very different from that of silicon
though they are atoms belonging to the IV group. From this
viewpoint, there is posed a doubt in aptitude of tin as a solvent
which is used to grow a crystal for a solar cell with high
efficiency.
[0011] In addition to tin, elements such as gallium, indium and
aluminum which belong to the III group can be mentioned as metals
which are usable as solvents. Gallium and indium, in particular,
having a low melting point can be handled easily. Since gallium is
extremely expensive, indium is hopeful for use as a practical melt.
However, indium posed a problem which is described later in
controlling by introducing dopant a conductivity type of a silicon
crystal which is grown using an indium melt. There are known
examples wherein gallium is used as p-type dopant in combination
with an indium melt (G. F. Zheng et al.: Solar Energy Materials and
Solar Cells. 40 (1996) 231-238). Though gallium is usable at
relatively low concentrations, it cannot be used for doping at high
concentrations since a solid of gallium can be dissolved into
silicon at concentrations within a relatively low solubility and is
extremely expensive. On the other hand, examples which use n-type
dopants in combination with indium melts are disclosed by Japanese
Patent Application Laid-Open Nos. 9-183695 and 9-183696.
[0012] Boron and aluminum are generally used as p-type dopants,
whereas phosphorus and arsenic are often used as n-type dopants. It
is therefore conceivable to use these dopants for growing silicon
crystals in liquid phase with the indium melt. In practice,
however, problems were posed in conductivity types or
reproducibility of conductivities of grown silicon crystals in
certain cases. Furthermore, it is feared that a metal of the III
group such as indium which is originally active by itself as a
dopant may control a crystal to a strong p-type when incorporated
into silicon and may be incapable of controlling it to p.sup.--type
or n-type.
[0013] The problems described above makes it still impossible to
judge whether or not the liquid phase method has a true aptitude
for growth of silicon crystals on scales of mass production and
whether or not solar cells utilizing thin films of silicon crystals
have practical utility.
[0014] Thin films of silicon crystals are also used as devices for
driving picture elements of liquid crystal displays and so on.
Progresses made in the mass communication media have produced
increasing demands for a display having a larger screen and capable
of more minutely driving at a higher speed. Though the TFTs (thin
film transistors) of amorphous silicon have hitherto been utilized
as a driving circuit for picture elements to cope with the demands
for a display having a larger screen, the amorphous silicon cannot
meet any longer the demands for a display which can be more
minutely driven at a higher speed, and it is becoming to use TFTs
of polycrystalline silicon. In addition, there has been increasing
demands for polycrystalline silicon which has higher carrier
mobility and other characteristics.
[0015] The liquid phase growing method is also suited for growing
such crystalline silicon of a high quality on a large substrate
such as a glass plate. Though use of a glass plate or the like
makes it unallowable to heat a solution to a high temperature, it
is possible to grow a crystal of a good quality by using indium as
a solvent. Though it is impossible to grow a thick crystal at a low
growth temperature which lowers a solubility of silicon into
indium, there is no problem in formation of a crystal to be used as
a TFT having a thickness of the order of 0.1 to 0.5 .mu.m which is
far smaller than that of a solar cell. When indium is used as a
solvent for production of a TFT, a problem related to
reproducibility may be posed. Therefore, a concentration of a
dopant must be precisely controlled in order to enhance
reproducibility of characteristics of the TFT. In formation of a
film having a large area, an ununiform distribution of a dopant
concentration is not preferable which produces an ununiform
distribution of characteristics of TFT, thereby producing
variations in image density on a display device. In certain cases
where indium was used as a dopant, it was impossible to
sufficiently prevent the dopant from being distributed ununiformly
on surfaces.
SUMMARY OF THE INVENTION
[0016] The present invention has been achieved in view of the
current circumstances described above, and an object of the present
invention is to provide a method of precisely controlling a dopant
to be incorporated into crystalline silicon which is grown in a
liquid phase using indium as a solvent, thereby enabling mass
production of solar cells having a high efficiency and a light
weight as well as driving circuits for a high precision and high
speed display having a large area.
[0017] The present invention therefore provides a method of growing
a silicon crystal, which comprises using a melt prepared by
dissolving a solid of silicon containing a dopant at a
predetermined concentration into liquid indium. Furthermore, the
present invention provides a method of growing a silicon crystal,
which comprises using a melt prepared by dissolving a solid of
indium containing a dopant at a predetermined concentration into
liquid indium.
[0018] Moreover, the present invention provides a method of
producing a solar cell, which comprises the steps: preparing a melt
by dissolving a solid of silicon containing a dopant at a
predetermined concentration into liquid indium; forming a first
silicon layer of a first conductivity type on a substrate by
bringing the substrate into contact with the melt; and forming a
second silicon layer of a second conductivity type on the first
silicon layer of the first conductivity type.
[0019] In addition, the present invention provides a method of
producing a solar cell, which comprises the steps of: preparing a
melt by dissolving a solid of indium containing a dopant at a
predetermined concentration into liquid indium and further
dissolving silicon into the melt; forming a first silicon layer of
a first conductivity type on a substrate by bringing the substrate
into contact with the melt; and forming a second silicon layer of a
second conductivity type on the first silicon layer of the first
conductivity type.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a sectional view for showing one example of a
solar cell according to the present invention;
[0021] FIGS. 2A, 2B and 2C are sectional views for showing one
example of production steps of a solar cell according to the
present invention;
[0022] FIG. 3 is a sectional view for showing one example of an
apparatus used for carrying out a method of producing a silicon
crystal according to the present invention;
[0023] FIG. 4 is a sectional view for showing one example of an
apparatus used for carrying out the method of producing a silicon
crystal according to the present invention;
[0024] FIG. 5 is a sectional view for showing one example of an
apparatus used for carrying out the method of producing a silicon
crystal according to the present invention; and
[0025] FIGS. 6A, 6B, 6C, 6D, 6E and 6F are sectional views for
showing one example of production steps of a thin film transistor
(TFT) of polycrystalline silicon to which the method of the present
invention is applied.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] The present invention has been achieved on the basis of
knowledge obtained by experiments which are described below.
[0027] First, commercially available indium pellets were put into a
carbon crucible, heated and melted at 1000.degree. C. in a hydrogen
gas flow to obtain liquid indium. A melt was prepared by bringing
non-doped polycrystalline silicon into contact with the liquid
indium and dissolving silicon into the liquid indium until it was
saturated. Then, the melt was gradually cooled until it was
supersaturated. When the melt was cooled to 980.degree. C., a
substrate of non-doped polycrystalline silicon was brought into
contact with the melt, whereby a silicon crystal having a thickness
of 10 .mu.m was epitaxially grown on the substrate. A measurement
of specific resistance of the silicon crystal by the four-probe
method indicated approximately 0.2 .OMEGA.cm. Specific resistance
was varied within a range from 0.1 to 0.5 .OMEGA.cm in similar
experiments which were carried out using three different lots of
commercially available indium as the melt.
[0028] A similar experiment was carried out using indium pellets
refined to a high purity (6N), whereby a grown silicon layer on the
substrate has an extremely high resistance (a difference in
resistance between the grown silicon and the substrate could not be
evaluated). By secondary ion mass spectrometry (SIMS) analysis of
impurities contained in the grown silicon layer, no indium itself
was unanticipatedly detected in any sample (below measurable
limit). However, it was found that various kinds of impure elements
such as gallium and aluminum of the III group in particular other
than indium were contained in samples which were grown using
commercially available indium. From this result, it is presumed
that indium itself can hardly be incorporated into a silicon
crystal grown in a liquid phase, but the elements of the III group
other than indium which were contained in the commercially
available indium pellets were easily incorporated into the silicon
crystals, thereby lowering resistance. In other words, it is
necessary for precise control of conductivity of silicon to
precisely control impurities, in particular, elements of the III
group which are contained in indium.
[0029] Then, silicon was grown using a melt which was prepared by
dissolving silicon into highly pure indium until it was saturated
and then dissolving pellets of boron and aluminum. However,
specific resistance of silicon grown as described above was low in
reproducibility. The result of SIMS analysis indicated variations
in concentrations of boron and aluminum in the silicon.
Furthermore, silicon was grown using a melt which was prepared by
dissolving silicon into highly pure indium until it was saturated,
and then dissolving powders of phosphorus and arsenic. The silicon
grown as described above was certainly of the n-type but its
specific resistance was low in reproducibility. The result of SIMS
analysis indicated variations in concentrations of phosphorus and
arsenic in the silicon.
[0030] The inventors considered a cause for these variations as
described below. Since boron (density=2.23), aluminum
(density=2.70), phosphorus (density=2.69) and arsenic (density=3.9)
which are used as the dopants are prettily lighter than indium
(density=7.28), a solution of the dopant tends to be concentrated
on a surface of the indium melt, whereby the indium solution can
hardly be uniform as a whole. Furthermore, solubilities of
impurities which are subsequently dissolved into indium are
influenced at a high possibility by a concentration of silicon
which has already been dissolved in indium. In particular, when
indium is nearly saturated with silicon, a slight variation in the
saturation remarkably produces influences on the solubilities of
the impurities, whereby the elements which are put into the melt as
impurities are not always dissolved actually and concentrations of
the elements of the impurities may be unstable in the melt.
[0031] It is possible to dissolve the elements of the impurities
before dissolving silicon into indium by reversing the dissolving
order. In this case, since pellets or powders of the impurities are
to be put in trace amounts as compared with that of silicon, it is
difficult to uniformly distribute the elements of the impurities in
the melt as a whole.
[0032] For the reason described above, it is considered that indium
makes it hard to obtain a high reproducibility of doping though
indium itself has an excellent property as a melt that it can
hardly be incorporated into silicon crystals and facilitates to
obtain silicon crystals of high qualities. The inventors therefore
considered to dilute and then dissolve impurities into liquid
indium. However, a diluent to be used for this purpose must be a
substance which can hardly be incorporated into grown silicon
crystals or produces no adverse influence even when it is
incorporated into the silicon crystals.
[0033] Indium can be used as a first example of adequate diluent.
An alloy prepared by dissolving impurities into indium at a
predetermined concentration makes it possible to more accurately
control concentrations of the impurities and prevent adverse
influences from being produced by a diluent incorporated into
silicon. Further, such an alloy is advantageous also from a
viewpoint of having a slight different density from that of a
solvent, that is, liquid indium. When the diluted impurities are
dissolved before dissolving silicon, it is possible to prevent the
influence due to a concentration of silicon and use pellets or
powders in large amounts, thereby uniformly dissolving the elements
of the impurities into the melt.
[0034] Silicon can be used as a second example of adequate diluent.
When the elements of the impurities are preliminarily diluted with
silicon, the elements of the impurities are always and
simultaneously dissolved into indium with silicon, thereby
facilitating to maintain concentrations of the elements constant
relative to that of silicon.
[0035] The present invention which has been achieved on the idea
described above will be described in details below with reference
to effects and preferred embodiments thereof. However, the present
invention is not limited to the following examples.
EXAMPLE 1
[0036] In Example 1, a solar cell having a structure shown in FIG.
1 was produced using a metal grade silicon substrate which had a
low purity and was inexpensive due to the low purity.
[0037] Meant by the metal grade silicon is silicon which has a
purity on the order of 99% and is obtained by metallurgically
reducing silica. A substrate 101 of a metal grade polycrystalline
silicon which was 0.1 mm thick and 4 inches in diameter was
produced by dissolving a metal grade silicon nugget and gradually
cooling it in a carbon die coated with silicon nitride. The
substrate 101 contained boron at a high concentration and was of a
strong p-type. Using a liquid phase growing apparatus which had a
configuration shown in FIG. 3, a layer 102 of a p-type
polycrystalline silicon was grown on the substrate 101.
[0038] In the apparatus shown in FIG. 3, a crucible 301 made of
quartz glass is filled with a dissolved indium melt 302. The
apparatus is accommodated as a whole in a quartz bell-jar 303 and
heated to a desired temperature from outside with electric furnaces
304. Hydrogen gas is always introduced into the quartz bell-jar 303
to maintain a reducing atmosphere in the bell-jar 303. Further, a
reference numeral 305 represents a substrate susceptor made of
quartz glass which holds ends of a substrate 300 of a highly pure
polycrystalline silicon or the substrate 101 of the metal grade
polycrystalline silicon 101 having a diameter of 4 inches. The
substrate 100 or 300 of the polycrystalline silicon is held
obliquely so as to go and come smoothly into and out of the melt
302. A reference numeral 306 designates a load lock chamber which
can be partitioned from the quartz bell-jar 303 with a gate valve
307. When setting silicon in the susceptor 305 or replacing silicon
with another, the susceptor 305 is hoisted up with a hoist
mechanism 308 and the gate valve 307 is closed to prevent an
interior of the quartz bell-jar 303 from being exposed to
atmosphere. A reference numeral 309 represents a dopant introducer
which is configured also as a load lock mechanism and allows
pellets 310 containing a dopant to be put into the indium melt 302
in a condition where the gate valve 307 is opened and the susceptor
305 is hoisted up.
[0039] Now, the method of growing the layer 102 of the p-type
polycrystalline silicon will be described concretely. First, the
indium melt 302 was heated to 1000.degree. C. and pellets 310 of
highly pure indium containing 1% by weight of aluminum were put
into the indium melt 302. Since the indium pellets had a density
which was nearly the same as that of indium, it was considered that
the indium pellets were to uniformly disperse in the melt. Then,
the substrate 300 of highly pure polycrystalline silicon was
submerged as shown in FIG. 3. The substrate 300 was maintained in
this condition for 30 minutes to dissolve silicon into the indium
melt 302 until it is saturated.
[0040] Then, the gate valve 307 was closed, the substrate 300 of
the highly pure polycrystalline silicon was removed from the
susceptor 305, and a substrate 101 of metal grade polycrystalline
silicon having the diameter of 4 inches was placed in the
susceptor. After replacing an internal gas of the load lock chamber
306 first with nitrogen and then with hydrogen, the gate valve 307
was opened and the susceptor 305 was hoisted down to a preheating
position (not shown in the drawings) over the melt 302 to wait for
temperature rise of the substrate 101. Thereafter, cooling of the
interior of the quartz bell-jar was started at a rate of 1.degree.
C./minute. When temperature reached 990.degree. C., the substrate
101 was submerged into the melt 302. Thirty minutes later, the
susceptor 305 was hoisted up and the load lock chamber 306 was
closed with the gate valve 307. After replacing an internal gas of
the load lock chamber 306 with nitrogen, the substrate 101 was
taken outside. A p-type polycrystalline silicone layer 102 having a
thickness of 30 .mu.m had been grown on the substrate 101.
[0041] A PSG layer (phosphor silicate glass layer) having a
thickness of 200 .ANG. was deposited on the surface of the p-type
polycrystalline silicon layer 102 at a temperature of 560.degree.
C. using a CVD apparatus (not shown in the drawings). An
n.sup.+-type silicon layer 103 was formed on the surface side by
annealing the PSG layer in a nitrogen gas flow at a temperature of
1050.degree. C. for 30 minutes and diffusing phosphorus (P). The
remaining PSG was eliminated by etching with an aqueous solution of
hydrofluoric acid. Furthermore, aluminum was deposited to a
thickness of 2 .mu.m on the surface of the n.sup.+-type silicon
layer 103 by sputtering and comb-teeth like grid electrodes 104
were formed by photolithography. Successively, a titanium oxide
film having a thickness of 600 .ANG. was deposited by sputtering as
an antireflection film 105. At this stage, pads of the grid
electrodes 104 were masked to prevent titanium oxide from being
deposited thereon. A solar cell produced as described above will
hereinafter referred to as a solar cell 1.
[0042] The characteristic of the solar cell 1 was evaluated with an
AM-1.5 solar simulator to obtain a photoelectric conversion
efficiency of 13%. Furthermore, 21 subcells each having an area of
1 cm.sup.2 were formed on the substrate 101 and checked for a
distribution of the photoelectric conversion efficiency. The result
indicated a distribution within .+-.2% which was a favorable
result. Moreover, a silicon crystal was grown successively five
times while replenishing aluminum and silicon in the same
procedures as those for the first growth in the amount of aluminum
and silicon lost in each growth due to the deposition from the
melt. This experiment indicated the variation of the photoelectric
conversion efficiency within .+-.3% at one and the same location of
each substrate, which was a favorable result.
[0043] As a comparative example, a solar cell 2 was produced in the
same procedures as those for the solar cell 1, except that pellets
of pure aluminum were used as the pellets 310 containing the
dopant. In this case, aluminum could hardly be incorporated into a
p-type polycrystalline silicon layer 102 even when a dopant was
replenished in a theoretically adequate amount. When the dopant was
replenished in an amount exceeding the adequate amount, however,
irregular spots were generated on the surface of the substrate 101
and the p-type polycrystalline silicon layer 102, which were
considered to be formed by reaction between silicon and aluminum.
It is presumed that a layer of melted aluminum was formed on a
surface of the melt and reacted with the substrate 101 or the
silicon layer 102. The solar cell 2 had remarkably ununiform
photoelectric conversion efficiencies which were and certain
subcells exhibited no photoelectric conversion efficiencies at all.
Thus, the effects of the present invention was clarified by this
comparison.
EXAMPLE 2
[0044] Example 2 shows a principle of a method of producing a
light-weight and highly efficient solar cell at a low cost by
repeatedly using an expensive silicon wafer in steps shown in FIGS.
2A to 2C. First, a porous layer 202 which was 5 .mu.m thick was
formed on a surface of a p.sup.+-type (100) single crystalline
silicon wafer 201 having a diameter of 2 inches by the so-called
anodization which applies a positive voltage in hydrofluoric acid.
The porous layer is composed of a large number of micropores having
a diameter of 100 .ANG. which are formed by ununiformly dissolving
silicon due to electrochemical action of hydrofluoric acid and
extend in a direction of a film thickness while complicatedly
tangling with one another. It is possible to epitaxially grow a
single crystalline silicon on this layer since a portion remaining
as a skeleton maintains a property of a single crystal. Methods of
forming a porous layer and application of the porous layer to solar
cells are detailed by Japanese Patent Application Laid-Open Nos.
5-283722 and 7-302889.
[0045] FIG. 4 shows an apparatus for growing single crystalline
silicon which was used in Example 2. Reference numerals 401 and 402
represent members which compose a carbon boat. The member 401 is
provided with a cavity for dropping substrates 403, 403a and 403b
for dissolving and a cavity for dropping a substrate 404 for
growing. The member 402 is provided with a hole in which an indium
melt 405 is to be accommodated. The members 401 and 402 are
configured to slide relative to each other.
[0046] A polycrystalline silicon substrate 403 for dissolving
silicon into a melt and a single crystalline silicon substrate 404
having a porous layer 202 formed on the surface for growing a
crystal were arranged in the member 401. The member 402 was laid on
the member 401 and a predetermined amount of highly pure indium
pellets were placed in the hole of the member 402. When the indium
pellets were heated in a hydrogen flow, they were melted into a
melt 405 as shown in FIG. 4. After maintaining the growing
apparatus at 1050.degree. C. for five minutes, the temperature was
adjusted to 1000.degree. C. and the melt 405 was brought into
contact with the substrate 403 for dissolving by sliding the member
402. As the substrate 403 for dissolving, a p-type polycrystalline
silicon substrate doped with boron having specific resistance of
0.01 .OMEGA.cm was used. After keeping this state for one hour,
cooling of the apparatus as a whole was started at a rate of
1.degree. C./minute. When temperature reached 980.degree. C., the
member 402 was slid to bring the melt 405 into contact with a
surface of the porous layer 202 and cooled for one minute to form a
p.sup.+-type silicon layer 203 having a thickness of approximately
1 .mu.m. Thereafter, the melt was returned to its initial position
by sliding the member 402 once again and left standing for
cooling.
[0047] When the apparatus was cooled to room temperature, the
hardened melt and the substrate 403 for dissolving were removed,
whereafter highly pure indium pellets and the substrate 403a for
dissolving made of the p-type polycrystalline silicon doped with
boron and having specific resistance of 1 .OMEGA.cm were newly
arranged and heated in a manner similar to that at the preceding
stage. After bringing the melt 405 into contact with the substrate
403a for dissolving at a temperature of 1000.degree. C., keeping it
in this condition for one hour, cooling of the apparatus as a whole
was started at a rate of 1.degree. C./minute. When temperature was
lowered to 980.degree. C., the melt 405 was brought into contact
with the surface of the p.sup.+-type silicon layer 203 by sliding
the member 402 once again and cooled for thirty minutes, thereby
forming a p-type silicon layer 204 which was approximately 30 .mu.m
thick. Then, the melt was returned to its initial position by
sliding the member 402 once again and left standing for
cooling.
[0048] When the melt was cooled to room temperature, the hardened
melt and the substrate 403a for dissolving were removed, whereafter
highly pure indium pellets, and a substrate 403a for dissolving
made of n-type polycrystalline silicon doped with phosphorus and
having specific resistance of 0.01 .OMEGA.cm were newly disposed
and heated in a manner similar to that at the preceding stage.
After bringing the melt 405 into contact with the substrate 403b
for dissolving at a temperature of 1000.degree. C. and keeping it
in this condition for one hour, cooling of the apparatus as a whole
was started at a rate of 1.degree. C./minute. When temperature was
lowered to 980.degree. C., the melt 405 was brought into contact
with the surface of the p-type silicon layer 204 by sliding the
member 402 and cooled for thirty seconds, thereby forming an
n.sup.+-type silicon layer 205 which was approximately 0.5 .mu.m
thick. Thereafter, the melt was returned to its initial position by
sliding the member 402 once again and left standing for
cooling.
[0049] Furthermore, aluminum was deposited to form 2 .mu.m thick
layer on the n.sup.+-type silicon layer 205 by sputtering while
masking the layer 205, thereby forming grid electrodes 206. A
titanium dioxide film 207 having a thickness of 600 .ANG. and a
magnesium fluoride film 208 having a thickness of 1000 .ANG. were
stacked as antireflection layers 207 and 208 by sputtering. In
sputtering of the antireflection layers, grid tabs were masked so
that the antireflection layers were not deposited thereon.
[0050] A transparent adhesive tape 209 was bonded to a surface of
the antireflection layer thus formed. After a stacked body from the
p.sup.+-type silicon layer 203 to the antireflection layer 208 was
peeled from the silicon wafer 201 by destroying the porous layer
202 by applying forces in directions indicated by arrows in FIG.
2B, an aluminum sheet 210 was bonded to a back surface of the
p.sup.+-type silicon layer 203 with an electroconductive adhesive,
thereby forming a solar cell 3.
[0051] The characteristic of the solar cell 3 was evaluated with an
AM-1.5 solar simulator to obtain a photoelectric conversion
efficiency of 18%. Furthermore, 26 subcells each having an area of
0.25 cm.sup.2 were formed on a substrate 210 of an aluminum sheet
and checked for a distribution of photoelectric conversion
efficiencies. This result indicated a distribution within .+-.3%,
which was a favorable result.
[0052] As a comparative example, a solar cell 4 was produced in the
same procedures as in the case of the solar cell 3, except that a
melt was prepared by arranging powders of boron and phosphorus as
dopants in the hole of the member 402 together with indium pellets
and that a non-doped polycrystalline silicon was used as the
silicon for dissolving. Possibly due to a fact that boron and
phosphorus were not uniformly distributed in the melt in the liquid
phase growth, photoelectric conversion efficiencies of subcells
were 10% at most and distributed within a broad range, and certain
subcells exhibit no photoelectric conversion efficiency at all.
EXAMPLE 3
[0053] Example 3 shows steps for mass production of solar cells
having a structure which is basically the same as that of the solar
cell produced in Example 2 and proves that the method of the
present invention is preferably applicable to mass production.
[0054] A porous layer 202 having a thickness of 2 .mu.m were formed
on each 6-inch silicon wafer 201. In this case, the porous layers
202 could be formed on each of the wafers at a time and a working
efficiency could be remarkably enhanced by connecting ten silicon
wafers 210 in series in a solution of hydrofluoric acid and
supplying a current to the wafers.
[0055] An apparatus for growing silicon crystal according to the
present invention was based on the same principle as that of the
apparatus adopted for Example 1 shown in FIG. 3, provided that a
substrate susceptor 505 was used which is made of quartz glass and
configured to be capable of accommodating ten substrates. Quartz
glass crucibles 501 and quartz bell-jars (not shown in the
drawings) are deepened correspondingly. The apparatus can be
configured so as to accommodate a larger number of substrates to
enhance a production efficiency. Three quartz bell-jars having
similar internal structures are connected to a common load lock
chamber by way of gate valves so that substrates can move from one
bell-jar into another without being exposed to atmosphere.
[0056] First, a melt was prepared by placing highly pure indium
pellets in a crucible 501 of a first quartz bell-jar, heating and
melting the pellets at 1000.degree. C. Highly pure indium pellets
containing 1% by weight of aluminum were put into the melt, and
then a polycrystalline silicon substrate for dissolving was
submerged into the melt and kept in this condition for 30 minutes
to dissolve silicon into the indium melt until it was saturated,
thereby preparing a melt for growing a p.sup.+-type silicon
layer.
[0057] Then, a melt was prepared by placing highly pure indium
pellets in a crucible of a second quartz bell-jar, heating and
melting the pellets at 1000.degree. C. Then, ten substrates of
polycrystalline silicon doped with boron and having a specific
resistance of 0.05 .OMEGA.cm were attached to a susceptor 505,
submerged into the indium melt, kept in this condition for 30
minutes to dissolve silicon until the indium melt was saturated,
thereby preparing a melt for growing a p-type silicon layer.
[0058] Further, a melt was prepared by placing highly pure indium
pellets in a crucible of a third quartz ball-jar, heating and
melting the pellets at 1000.degree. C. Highly pure indium pellets
containing 1% by weight of arsenic were put into the melt, and a
polycrystalline silicon substrate for dissolving was submerged into
the melt and kept in this condition for 30 minutes to dissolve
silicon into the indium melt until it was saturated, thereby
preparing a melt for growing an n.sup.+-type silicon layer.
[0059] With the gate valves kept closed, the polycrystalline
silicone substrate for dissolving was removed from the susceptor
505 and a silicon wafer 201 (hereinafter simply referred to
"substrate") having a diameter of six inches and a porous layer 202
formed on a surface thereof was set in the susceptor. After
replacing an internal gas of the load lock chamber first with
nitrogen and then with hydrogen, the gate valve of the first quartz
bell-jar was opened, the susceptor 505 was hoisted down to its
preheating position, an interior of the quartz bell-jar was
maintained at 1050.degree. C. for ten minutes and then cooled to
1000.degree. C., and gradual cooling of the interior of the quartz
bell-jar was started at a rate of 0.2.degree. C./minute. When
temperature reached 995.degree. C., the substrate was submerged
into the melt 502 as shown in FIG. 5. After keeping this condition
for ten minutes, the susceptor 505 was hoisted up. A p.sup.+-type
silicon layer 203 having a thickness of approximately 2 .mu.m was
grown on the porous layer 202. Since this apparatus treated a large
number of substrates and required a time for pulling the susceptor
into and out of the melt, a crystalline silicon growing rate was
set at a low level in order not to vary the thickness of the
p.sup.+-type silicon layer 203 of each substrate.
[0060] After completely hoisting up the susceptor, the first quartz
bell-jar was closed to maintain a hydrogen atmosphere in the load
lock chamber, the gate valve of the second bell-jar was opened, the
susceptor 505 was hoisted down to its preheating position and an
interior of the bell-jar was maintained at 1000.degree. C. for ten
minutes. Then, gradual cooling of the interior of the quartz
bell-jar was started at a rate of 1.degree. C./minute. When
temperature reached 980.degree. C., the substrate was submerged
into the melt 502 as shown in FIG. 5. After keeping this condition
for 30 minutes, the susceptor 505 was hoisted up and the load lock
chamber was closed. A p-type silicon layer 204 having a thickness
of approximately 30 .mu.m was grown on the p.sup.+-type silicon
layer 203.
[0061] While keeping the hydrogen atmosphere in the load lock
chamber, the gate valve of the third quartz bell-jar was opened,
the susceptor 505 was hoisted down to its preheating position, an
interior of the quartz bell-jar was maintained at 1000.degree. C.
for ten minutes and then gradual cooling was started at a rate of
0.2.degree. C./minute. When temperature reached 995.degree. C., the
substrate was submerged into the melt 502. After maintaining this
condition for two minutes, the susceptor 505 was hoisted up and the
load lock chamber was closed. An n.sup.+-type silicon layer 205
having a thickness of approximately 0.4 .mu.m was grown on the
p-type silicon layer 204.
[0062] Thereafter, comb-teeth like grid electrodes 206 were formed
on the surface of the n.sup.+-type silicon layer 205 by printing a
copper paste by the screen printing method and calcining the paste.
Successively, a titanium dioxide film 207 having a thickness of 600
.ANG. was formed by coating a metal alkoxide solution by the
sol-gel method and calcining the solution, and a film (208) of
silicon oxide 800 .ANG. thick was formed in the similar procedures
as two antireflection layers 207 and 208. Ten or more substrates
can easily be treated at a time by the screen printing method and
the sol-gel method which are capable of treating a large number of
substrates. These methods are preferable. Successively, an adhesive
tape 209 was bonded to a surface of the antireflection layer, the
layers of the p.sup.+-type silicon layer 203 from the upper layers
were peeled from the substrate 201 by applying a force to the
substrate 201 so as to destroy the porous layer 202, and the tape
209 was peeled off with an organic solvent. Thereafter, a back
surface of the p.sup.+-type silicon layer 203 was coated with an
electroconductive ink, bonded to an aluminum support plate 210 and
calcined for setting, thereby producing solar cells 5.
[0063] Ten solar cells 5 were evaluated with an AM-1.5 solar
simulator to obtain photoelectric conversion efficiencies of
17.+-.0.3%, which were favorable and uniform. Furthermore, a solar
cell module 1 was produced by connecting the ten solar cells in
series and bonding them to a heat-resistant glass plate having a
thickness of 3 mm with a PVC resin. This solar cell module 1 had an
output of approximately 30 W.
[0064] Successively to the module 1, a module 2 was produced in
similar procedures. During the producing, the melts were not cooled
but kept in melted conditions. However, the polycrystalline silicon
substrate for dissolving was submerged again into the melt in each
of the quartz bell-jars to dissolve silicon until the melt was
saturated since silicon concentration was lowered by deposition of
a silicon crystal on the substrate. Boron was supplied together
with silicon into the melt in the second quartz bell-jar. Since
dopant concentrations were lowered in the melts in the first and
third quartz bell-jars, pellets containing a predetermined amount
of aluminum or arsenic were replenished into the melts in the first
and third quartz bell-jars before replenishing silicon. The method
of the present invention is capable of uniformly supplying a dopant
with a high repeatability even when using a large crucible for mass
production, whereby the module 2 also exhibited a characteristic
equalled to that of the module 1.
[0065] As a comparative example, ten solar cells were produced at a
batch by replenishing the melts with pellets or powders each
containing a single element of aluminum, boron or phosphorus. These
solar cells exhibited remarkably variable characteristics, and
therefore a solar cell module 3 composed of these solar cells in
series had an output characteristic of 5 W, clarifying that the
method of the present invention is extremely excellent in mass
production of modules connecting in series.
EXAMPLE 4
[0066] Example 4 shows an example that the method of the present
invention was applied to the production of a thin film transistor
(TFT) of polycrystalline silicon formed on a glass plate which was
to be used in a driving circuit for a liquid crystal display
device. FIGS. 6A to 6F schematically show production steps. A
stacked films of aluminum/chromium having a thickness of 2000 .ANG.
were deposited on a glass substrate 601 having a size of 4-inch
square by sputtering. A pattern was formed as a gate electrode 602
on these films by photolithography (see FIG. 6A). Using disilane
and ammonia as raw material gases, a silicon nitride film having a
thickness of 3000 .ANG. was deposited as a gate insulating film 603
on the gate electrode 602 by the CVD method (see FIG. 6B).
[0067] Used in Example 4 was a growing apparatus having a structure
which was similar to that of the apparatus shown in FIG. 3 except
that two quartz bell-jars were connected to a common load lock
chamber by way of gate valves. First, an n-type polycrystalline
silicon substrate doped with arsenic having a specific resistance
of 0.5 .OMEGA.cm and a size of 4-inch square was submerged into an
indium melt 302 in a crucible of a first quartz bell-jar as shown
in FIG. 3 and maintained in this condition for thirty minutes to
dissolve silicon into the melt 302 until it was saturated, thereby
preparing a melt for an n-type silicon layer for dissolving. After
indium pellets containing 2% by weight of boron were dropped in a
predetermined amount into a highly pure indium melt in a second
quartz bell-jar, a highly pure polycrystalline silicon substrate
having a size of 4-inch square was dissolved thereto, thereby
preparing a melt for a p.sup.+-type silicon layer for
dissolving.
[0068] Then, the gate valve was closed, the polycrystalline silicon
substrate was dismounted from a susceptor, and the glass substrate
601 on which the gate insulating film 603 had been formed was set
in the susceptor. After an internal gas of the load lock chamber
was replaced with nitrogen and then with hydrogen, the gate valve
of the first quartz bell-jar was opened, and the susceptor was
hoisted down to its preheating position and held at 600.degree. C.
for ten minutes to wait for temperature rise of the substrate 601.
Thereafter, gradual cooling of an interior of the quartz bell-jar
was started at a rate of 0.2.degree. C./minute. When temperature
reached 595.degree. C., the substrate 601 was submerged into the
melt. The substrate was maintained in this condition for 30 minutes
until an n-type polycrystalline silicon layer 604 having a
thickness of 3000 .ANG. was grown on the gate insulating film 603.
Then, the susceptor was hoisted up, the gate valve was closed, the
gate valve of the second quartz bell-jar was opened while
maintaining the hydrogen atmosphere, and the susceptor was hoisted
down to its preheating position and kept at 600.degree. C. for ten
minutes, whereafter gradual cooling of an interior of the quartz
bell-jar was started at a rate of 0.2.degree. C./minute. When
temperature reached 595.degree. C., the substrate 601 was submerged
in the melt and maintained in this condition for five minutes,
whereby a p.sup.+-type silicon layer 605 having a thickness of 500
.ANG. was grown on the n-type silicon layer 604 (see FIG. 6C).
Though silicon was grown at a very low rate in Example 4 due to the
use of the glass substrate which did not allow the melt to be
heated to a high temperature, the growth could be completed in a
time within a range similar to that for the other examples since a
necessary layer thickness was small.
[0069] After depositing stacked films of chromium/aluminum by
sputtering, a source electrode 606 and a drain electrode 607 were
patterned by photolithography (see FIG. 6D). Using the electrodes
606 and 607 as masks, unnecessary portions of the p.sup.+-type
layer at a channel portion 608 were removed by dry etching (see
FIG. 6E). Furthermore, a silicon oxide layer 609 was deposited on
the surface by sputtering for surface protection (see FIG. 6F).
[0070] In order to check the TFT for its basic characteristic, -5 V
and 0 V were applied to the gate electrode while applying 5 V
across the source and drain electrodes. This result indicated an
on/off ratio of 10.sup.6. Moreover, a distribution of on/off ratios
of b 10.sup.4 TFTs formed in the substrate was within an extremely
narrow range of 120%. Accordingly, it is possible to obtain display
devices having a high contrast and free from ununiformity in colors
by producing a driving circuit of TFTs according to the method of
the present invention.
[0071] As a comparative example, a dopant was supplied as pellets
or powders each singly composed of a dopant element. In this
comparative example, on/off ratios of TFTs were distributed within
a wide range of 10.sup.2, whereby the TFTs could not be expected to
be usable for driving display devices.
[0072] As understood from the foregoing description, the method of
the present invention is capable of growing silicon crystals of a
high quality having a dopant concentration favorably controlled,
thereby making it possible to produce high performance solar cells,
driving circuits for liquid crystal display devices and so on at a
low cost and with a high reproducibility.
* * * * *