New Method for Production of Solar

Materials Transactions, Vol. 50, No. 12 (2009) pp. 2873 to 2878
#2009 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
New Method for Production of Solar-Grade Silicon by Subhalide Reduction
Kouji Yasuda1; *1 , Kunio Saegusa2 and Toru H. Okabe1; *2
1
2
Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan
Tsukuba Research Laboratory, Sumitomo Chemical Co., Ltd., Tsukuba 300-3294, Japan
In this study, a new method for producing Si, called ‘‘halidothermic reduction’’, was investigated with the purpose of producing solar-grade
silicon (SOG-Si); by this method, SiCl4 was reduced by gaseous subhalide used as the reductant. Si was produced by reacting SiCl4 with Al
subhalides, which were produced by reacting AlCl3 with metallic Al. Fibrous Si with diameters ranging from submicrons to several tens of
micrometers was deposited as a result of halidothermic reduction of SiCl4 by an Al subchloride (AlClx ) reductant at 1273 K. The size of Si
deposits and the reaction rate were increased by simultaneously supplying AlCl3 and SiCl4 vapors to a reaction tube holding Al metal. The
impurity level of the obtained Si was found to be lower than the detection limit of X-ray fluorescence. Halidothermic reduction is suitable for
producing high-purity Si since all reactants and byproducts exist in the vapor phase. Further, this process has high productivity since the overall
reaction is a highly scalable metallothermic reduction reaction. [doi:10.2320/matertrans.M2009260]
(Received July 31, 2009; Accepted September 24, 2009; Published November 25, 2009)
Keywords: high-purity silicon, solar-grade silicon, polycrystalline silicon, smelting, reduction process, metallothermic reduction, subhalide
1.
Introduction
In the last few decades, the Siemens process1) has been
most commonly used for manufacturing high-purity silicon
(Si); the Komatsu process2) has also been used for this
purpose, albeit to a lesser extent. High-purity Si used in solar
cells (SOG-Si; solar-grade Si, 6N purity) has been supplied
only from the off-grade portion of semiconductor-grade Si
(SEG-Si, 12N purity) manufactured by the Siemens process.
The use of solar cells has increased significantly in the 21st
century. The global production volume of photovoltaic (PV)
cells has been increasing at the rate of 40% per year, reaching
a value of 6,941 MW in 2008.3,4) Moreover, the global
production volume of polycrystalline Si reached 57,400 tons
in 2008; this is expected to double in the present decade.4,5)
This increased production of PV cells and polycrystalline Si
has resulted in serious shortages in SOG-Si supply, and this
has severely retarded the growth of the PV industry. The
main factor responsible for the shortage in SOG-Si supply is
the low productivity of the conventional processes used for
manufacturing high-purity Si. This is because these processes
are batch-type processes, involve slow reactions, and have
low energy efficiency. The reactions involved in conventional Si production processes are thermal decomposition
and/or hydrogen (H2 ) reduction of silane gases such as
trichlorosilane (SiHCl3 ) and silane (SiH4 ). Hence, in order
to meet the growing demand for SOG-Si, it is necessary to
develop a new SOG-Si production process that has high
productivity.
Various methods for SOG-Si production/Si purification
have been developed and improved by researchers in order
to improve the current process. These methods are as
follows:6,7) (1) thermal decomposition and/or H2 reduction
of silane gases by improving the existing Si production
methods that are based on the Siemens process;8–11) (2)
metallothermic reduction of Si halide compounds by zinc
(Zn), aluminum (Al), and sodium (Na);12–15) (3) upgrading
metallurgical-grade Si (MG-Si, 98–99% purity) via metallurgical purification methods.16–20) Among the pyrometallurgical processes used for SOG-Si production, metallothermic
reduction has very high productivity when high-purity Si
compounds and reductants are available. The reductant
used for the production of SOG-Si via metallothermic
reduction must be highly pure and should not form
intermetallic compounds with Si; further, the byproducts
formed in this reaction and the unreacted reductant should
be easily separable from the reaction mixture. Practically,
Zn13,15) and Al14,15) are the only reductants that satisfy
the aforementioned conditions and are suitable for use in
the production of high-purity Si. Besides metallothermic
reduction, thermal decomposition1,2,8–11,21,22) and disproportionation23,24) can also be used to produce SOG-Si from Si
compounds.
Herein, we report a new method denoted as ‘‘halidothermic
reduction’’, which is based on subhalide reduction and uses a
gaseous subhalide as the reductant, for SOG-Si production.
A subhalide is the halide of a low-valence metal having
multiple valence states and is stable under a reducing
atmosphere. For example, the highest valence state of
titanium (Ti) is +4, and titanium tetrachloride (TiCl4 ) is
stable in the Ti–Cl system at a certain temperature and
chlorine potential. In addition to TiCl4 , titanium dichloride
(TiCl2 ) and titanium trichloride (TiCl3 ) exist as stable
subchlorides under a reducing atmosphere.
The general reaction involved in the production of a metal
M from its halide (MX) by metallothermic reduction using
a reductant R is
MX þ R ! M þ RXn ;
where X is a halogen, and n is the highest valence number of
M. When a subhalide RXm (m < n) is used as the reductant
instead of R, the following halidothermic reduction occurs:
MX þ RXm ! M þ RXn ðm < nÞ
*1Present
address: New Process Development Department, Osaka Titanium Technologies Co., Ltd., Amagasaki 660-8533, Japan
*2Corresponding author, E-mail: okabe@iis.u-tokyo.ac.jp
ð1Þ
ð2Þ
Several patents concerning metal production using subhalides as reductants have been published previously;25–27)
K. Yasuda, K. Saegusa and T. H. Okabe
Carbon
Carbothermal reduction
CO, CO2
Metallurgical-grade Si (2N purity)
Cl2
Chlorination
Electrolysis
of AlCl3
Crude SiCl4 (+ FeCl3, AlCl3)
Purification of SiCl4
1.0
(a)
Silica ore
Partial pressure of i, pi / atm
2874
FeCl3, AlCl3
AlCl3 (g )
0.8
Ptotal = 1 atm
aAl = 1 (Al metal existence)
0.6
0.4
AlCl (g )
0.2
AlCl2 (g )
Al
AlCl3
Reduction of SiCl4 by AlClx
This study
Polycrystalline Si (6N purity)
Fig. 1 Flowchart of the new method for polycrystalline Si production using
Al subchloride as a reductant (halidothermic reduction). Polycrystalline Si
is produced by the reduction of SiCl4 using Al subchlorides. The
byproduct (AlCl3 ) is recovered, and a part of it is electrolyzed to obtain Al
metal and Cl2 gas. Al metal is used for the production of Al subchlorides,
and Cl2 gas is used for the chlorination of metallurgical-grade Si.
however, experimental verification of the availability of
subhalides has not yet been reported. In 1998, Uda et al.
experimentally demonstrated that TiCl2 can be reduced to Ti
by halidothermic reduction using a liquid subhalide, DyCl2
dissolved in a NaCl–KCl melt, as the reductant.28) However,
to the best of the authors’ knowledge, there is no report on
the use of a gaseous subhalide for halidothermic reduction.
On the basis of our technical discussions,29–31) we
attempted to develop a new method for producing Si via
halidothermic reduction using a gaseous subhalide. We
selected SiCl4 as the Si compound since it is the easiest
to handle among all Si halides. We selected aluminum
subchloride (AlClx , x ¼ 1 or 2) as the reductant because Al
has low affinity for Si; hence, no intermetallic compound is
formed in the Si–Al system.32) The reaction involved in Si
production by halidothermic reduction is
SiCl4ðgÞ þ AlClxðgÞ ! SiðsÞ þ AlClyðgÞ ðx < y 3Þ: ð3Þ
Figure 1 shows the flowchart of the proposed process,
which involves four major steps: chlorination of Si, reduction
of SiCl4 , electrolysis of the byproduct AlCl3 , and production
of AlClx . Metallic Si is produced by the reduction of SiCl4
by AlClx . A part of the byproduct AlCl3 is subjected to
electrolysis in order to reproduce Al metal and Cl2 gas. Al
metal is used for producing AlClx , and Cl2 gas is used for
producing SiCl4 by chlorination of MG-Si.
In this paper, the halidothermic reduction of SiCl4 to Si
by AlClx reductant was experimentally verified, and its
feasibility to SOG-Si was demonstrated.
2.
1000
1100
1200
1300
Temperature, T / K
1400
1500
(b)
SiCl4
Thermodynamic Analysis
In the mid 20th century, extensive investigations were
carried out on the nature of the Al–Cl system in order to
Standard Gibbs energy of formation, ∆G f0
( = RT lnpCl2 ) / kJ (mol·Cl2)-1
AlClx
Al2Cl6 (g )
0
900
Production
of AlClx
-150
-200
Si + Cl2 = SiCl
2
2/3 Si + Cl2 = 2/3 SiCl3
-250
1/2 Si +
2 Al
-300
-350
l
/3 Al 2C 6
Cl 2 = 1
2/3 Al +
-400
900
1000
1100
+ Cl
2
Al + Cl2 = AlCl2
Cl 2 = 1/2
SiCl 4
=2A
lCl
l
2/3 Al + Cl2 = 2/3 AlC 3
1200
1300
1400
1500
Temperature, T / K
Fig. 2 Thermodynamic properties of Al subchlorides. (a) Equilibrium
vapor pressure of Al chlorides.44) The vapor pressure is calculated under
the conditions that the gases equilibrate with Al metal and the total
pressure of the system is 1 atm. (b) Ellingham diagram for Si chlorides and
Al chlorides.44)
develop a new method for refining Al metal using a subhalide
(subhalide refining or the Gross process33,34)). In subhalide
refining, AlClx is produced from AlCl3 and low-purity Al,
and pure Al is then produced by disproportionation of AlClx .
According to previous reports on subhalide refining, the
disproportionation reaction is carried out between 1013 K
and 1573 K.33–43) Figure 2(a) shows the partial gas pressure
in the Al–Cl system calculated from the thermochemical
data44) under the conditions that the total pressure of the
system is 1 atm and metallic Al exists in the system. Below
1100 K, AlCl3 and its dimer Al2 Cl6 are stable in the gas
phase. The stability of AlClx (x ¼ 1; 2) increases with
temperature, particularly above 1150 K. At temperatures
above 1150 K, as shown in the Ellingham diagram for
chlorides (Fig. 2(b)), AlClx (x ¼ 1; 2) reduces SiCl4 to Si
according to reactions (4) and (5).
SiCl4ðgÞ þ 2AlClðgÞ ¼ SiðsÞ þ 2AlCl3ðgÞ
ð4Þ
G ¼ 229 kJ at 1300 K44Þ
SiCl4ðgÞ þ 4AlCl2ðgÞ ¼ SiðsÞ þ 4AlCl3ðgÞ
G ¼ 292 kJ at 1300 K44Þ
ð5Þ
From thermodynamic considerations, Si subchlorides (SiCl2
and SiCl3 ) are expected to be produced along with metallic Si
during halidothermic reduction. However, these Si chlorides
are also reduced to Si by AlClx if a sufficient amount of
New Method for Production of Solar-Grade Silicon by Subhalide Reduction
metallic Al exists in the system. As indicated in reactions (4)
and (5), Si formation by halidothermic reduction proceeds
with a large chemical driving force, whereas the conventional
Siemens process involves the following thermodynamically
unfavorable reactions:
SiHCl3ðgÞ þ H2ðgÞ ¼ SiðsÞ þ 3HClðgÞ
ð6Þ
45Þ
G ¼ +36.1 kJ at 1400 K
4SiHCl3ðgÞ ¼ SiðsÞ þ 3SiCl4ðgÞ þ 2H2ðgÞ
G ¼ 56:3 kJ at 1400 K45Þ
3.
ð7Þ
Methods
The production of AlClx and the reduction of SiCl4 by
AlClx at 1273 K are experimentally demonstrated for
verification of the feasibility of Si production by halidothermic reduction. Figure 3 shows the schematic illustration of
the experimental setup consisting of an AlClx production
part and a SiCl4 reduction part. In the AlClx production
part, AlClx is produced by the reaction between AlCl3
vapors supplied from tube (A) and liquid Al in crucible (1)
placed at the bottom of the quartz reaction tube. In the
reduction part, SiCl4 vapors are reduced by gaseous AlClx
to form solid Si and AlCl3 gas, as shown in reactions (4)
and (5). Both these reactions are performed at 1273 K for
2 h. For recovery of Si, crucible (2) is placed beneath the
outlet of the (Ar + SiCl4 ) gas supply tube (B).
Vapor of AlCl3 was supplied to the SiO2 reaction tube
(45 mm 42 mm 450 mm length) via an Ar gas stream
(flow rate: 50 sccm) successively through the AlCl3 -containing tube and the reaction tube represented as tube (A) in
Fig. 3. Vapor of AlCl3 was generated by heating AlCl3
powder (1.367 g, 5:07 102 mol, 99.99% purity) in a glass
tube maintained at 433 K by a ribbon heater. Silicon
tetrachloride was supplied via an Ar gas stream (flow rate:
50 sccm) into the SiCl4 -containing bottle and then to the
reaction tube represented as tube (B) in Fig. 3. Vapor of
SiCl4 was generated from liquid SiCl4 (3.22 g, 1:90 102 mol, 99.998% purity) in a glass bottle maintained at
275 K in a thermostat bath. A mixture of Ar and AlCl3 vapors
AlClx
SiCl4
reduction production
part
part
Quartz tube
Heater
Quartz tube
Al metal
Carbon crucible (2) Carbon crucible(1)
Tube (A)
Ar + AlCl3 (+ SiCl4)
Tube (B)
Ar + SiCl4
Outlet
Fig. 3 Schematic illustration of the experimental setup for polycrystalline
Si production by halidothermic reduction. Al subchlorides (AlClx ) are
produced by the reaction between AlCl3 and Al metal at 1273 K in the
AlClx production part of the set up. Si is produced by reducing SiCl4 gas
with AlClx gas in the SiCl4 reduction part of the set up. During in situ
subhalide production/reduction, SiCl4 is supplied in the form of a mixture
of Ar, AlCl3 , and SiCl4 directly through tube (A) to the AlClx production
part. In this experiment, no gas is flown through tube (B).
2875
was flown to the bottom of the reaction tube, and a mixture
of Ar and SiCl4 vapors was flown into the part of the tube
that was 50 mm away from the bottom. Aluminum grains
(6.65 g, 4:98 102 mol, diameter: 2–5 mm, 99.99% purity)
were taken in a graphite crucible (32 mm 28 mm 10 mm depth/15 mm height). The Al grains were placed at
the bottom of the reaction tube. At the beginning of the
experiment, the atmosphere was maintained by flowing Ar
gas through the AlCl3 supply tube at a rate of 50 sccm; the
AlCl3 powder was maintained at room temperature during
this stage. The reaction tube was inserted into an electric
furnace; the furnace was then heated from room temperature
for 3 h, and the temperature of the Al grains was maintained
at 1273 K. Then, AlCl3 vapors were generated by heating
solid AlCl3 to 433 K. Simultaneously, SiCl4 was supplied
from the SiCl4 -containing bottle via an Ar gas flow. During
in situ subhalide production/reduction, a mixture of Ar,
AlCl3 , and SiCl4 vapors was supplied through tube (A), and
no gas was flown through tube (B). After 2 h, the temperature
of AlCl3 was decreased, and Ar gas supply to the SiCl4 containing bottle was stopped. At the end of the reaction,
all the liquid SiCl4 evaporated, while a small amount of
AlCl3 powder remained in the tube. The obtained samples
were analyzed by XRD (Rigaku Corp., Cu-K line, RINT
2500), SEM (JEOL Co., Ltd., 15 kV, JSM-5600LV), and
XRF (JEOL Co., Ltd., JSX-3210).
4.
Results and Discussions
Figure 4(a) shows the reaction tube and the crucibles after
the 2-hour reaction. The inner wall of the quartz reaction tube
in the AlClx production part was initially transparent;
however, the color of the wall turned brown after the
experiment. White AlCl3 deposits were seen on the inner
walls of the quartz tube near its open end, i.e., AlCl3 , a
byproduct of the halidothermic reduction reaction, was
deposited on the low-temperature part of the reaction tube
(sublimation temperature of AlCl3 is 454.1 K44)). Brown
deposits were found adhered onto the surfaces of crucibles
(1) and (2). Crucible (1) was used in the AlClx production
part and was located downstream of the gas flow (Fig. 4(c)),
while crucible (2) was used in the reduction part (Fig. 4(b))
and was located upstream of the gas flow. Even in the case of
crucible (1), the brown deposit was found apart from the Al
grains, which indicates that the deposit was produced in a
gas-phase reaction. Both of these deposits were revealed as
crystalline Si by X-ray diffraction (XRD) (Fig. 5(a)). X-ray
fluorescence (XRF) analysis revealed the presence of only Si
in the deposit, thereby confirming the formation of highpurity metallic Si without Al contamination. Scanning
electron microscope (SEM) images revealed the fibrous
nature of the obtained Si (Figs. 5(b) and 5(c)); the fibers had
lengths of several tens of micrometers and diameters of 100–
300 nm. These deposits were not produced when AlCl3 was
not supplied to the reaction tube. Si deposition in the AlClx
production part was probably due to the backflow of SiCl4
into it. Thermodynamic analysis revealed that SiCl4 was the
major gaseous component in the Si–Cl system and that
Si subchlorides hardly existed under the experimental
conditions employed in this study (SiCl2 : 0.02 atm, SiCl3 :
2876
K. Yasuda, K. Saegusa and T. H. Okabe
(a)
AlClx
Reduction
production
part
part
(b)
(c)
Crucible (2)
Reduction part
Crucible (1)
AlClx production part
Fig. 4 Photographs of the reaction tube and crucibles after the reaction. (a) The reaction tube. (b) Crucible (2) used in the SiCl4 reduction
part. (c) Crucible (1) used in the AlClx production part. The reaction is carried out at 1273 K for 2 h. Ar/SiCl4 , and Ar/AlCl3 mixtures are
supplied via two separate tubes.
(a)
Intensity / a.u.
× Si
20
: #27-1402
×
×
×
40
60
2 theta / degree (Cu-K α)
(b)
×
×
80
(c)
5 µm
5 µm
Fig. 5 Characterization of Si deposits obtained by halidothermic reduction. (a) XRD pattern of the obtained Si deposits. (b) and (c) SEM
images of the obtained Si deposits. The reaction is carried out at 1273 K for 2 h. Ar/SiCl4 , and Ar/AlCl3 mixtures are supplied via two
separate tubes.
New Method for Production of Solar-Grade Silicon by Subhalide Reduction
(a)
2877
(b)
SiO2 reaction tube
Inlet gas tube (A)
Ar + AlCl3 + SiCl4
mixed gas
Graphite crucible
Si deposit
Remaining Al
(c)
Top part of deposit
10 µm
(d)
Boundary part with Al metal
30 µm
Fig. 6 Characterization of Si deposits obtained by in situ subhalide production/reduction. (a) Photograph and (b) schematic illustration of
the perpendicular section of crucible (1) after the experiment. (c) SEM image of the Si grains in the topmost layer of the deposits.
Columnar Si grains are produced by halidothermic reduction. (d) SEM image of Si deposited at the boundary with Al metal. Spherical Si
grains are produced by an aluminothermic reduction. The reaction is carried out at 1273 K for 2 h. A mixture of Ar, SiCl4 , and AlCl3 is
supplied via a single tube.
0.005 atm).44) Therefore, the amount of Si produced by
disproportionation of SiCl2 and SiCl3 was considerably
smaller than that produced by halidothermic reduction,
although these disproportionation reactions were likely to
proceed as side reactions. The experimental results shown
here confirm that the deposited Si is produced from the
reduction of SiCl4 by AlClx , thereby confirming the
feasibility of Si production by halidothermic reduction.
We also demonstrated the feasibility of ‘‘in-situ subhalide
production/reduction’’, in which AlCl3 and SiCl4 vapors are
simultaneously supplied by an Ar gas stream to the Al metal
via tube (A). In this reaction, SiCl4 is reduced in the gas phase
by gaseous AlClx , which is produced in the vicinity of the
area where the reaction between Al and AlCl3 occurs in
crucible (1) (Fig. 3). As shown in Figs. 6(a) and 6(b), Si is
deposited just above the Al metal in crucible (1). The topmost
layer of the Si deposits is at a higher level than the layer of the
original Al grains; this indicates that Si is formed as a result
of a gas-phase reaction and not because of the physical
contact between Al metal and SiCl4 gas. As can be seen in
Fig. 6(c), the topmost layer of the Si deposits comprises
columnar Si grains (diameter: 10–20 mm; length: 100–
300 mm). These grains are larger in size than those produced
when AlCl3 and SiCl4 vapors are separately supplied. The
morphology of the Si grains produced herein is characteristic
of the product formed by halidothermic reduction. The Al
content in the Si deposits was less than the detection limit of
XRF. The sample recovered from the vicinity of Al in the
tube is identified by XRD and XRF to be a mixture of
metallic Si and metallic Al (composition: 73.3 at% Si;
26.7 at% Al). In this part, SiCl4 is directly reduced to Si by
Al metal. The Si deposits obtained in this case comprise
spherical grains (Fig. 6(d)). The morphology of these Si
grains is identical to that of the Si grains produced when
SiCl4 is reacted with Al metal in the absence of AlCl3 vapors.
The spherical morphology of the Si grains probably results
from solidification of the liquid Si–Al alloy formed at high
temperatures. These experimental results clarified that in-situ
subhalide production/reduction is advantageous due to the
accelerated reaction rate and increased Si size. In this regard,
the Al remained as a solid solution should be removed by
refining treatments such as directional solidification.
Halidothermic reduction is advantageous for SOG-Si
production because in this reaction, the reactants (SiCl4 and
AlClx ) and byproducts (AlCl3 , SiCl2 , and SiCl3 ) are in the
vapor phase; hence, the product (Si) can be easily separated
from the byproducts and the unreacted reductant. Further,
contamination of the produced Si by the byproducts and
unreacted reductant can be avoided. Moreover, pure Si is
produced in large productivity in the halidothermic reduction
under the Al metal existence because the overall reaction is
an efficient metallothermic reduction process. On the basis of
2878
K. Yasuda, K. Saegusa and T. H. Okabe
the new Si production method investigated in this study, a
high-speed and continuous reduction process can be designed
by utilizing the powdery morphology of the Si deposits. In
the near future, high-purity SiCl4 is expected to become an
important raw material for SOG-Si production because it is
produced in large quantities as a byproduct in the Siemens
process, which will continue to be an essential process for the
production of ultra-high-purity SEG-Si.
5.
Summary
The recent increase in the production of solar cells has
resulted in a shortage of solar-grade Si, which is widely
used in these cells. High-purity Si is produced by thermal
decomposition/hydrogen reduction reactions such as the
Siemens process. However, since such processes have low
productivity, alternative processes for producing solar-grade
Si have been investigated. Here, we present a new method for
producing Si, called ‘‘halidothermic reduction’’, in which
subhalide reduction is carried out using gaseous subhalide as
the reductant. Si is produced by reacting SiCl4 with gaseous
Al subhalides, which are produced by reacting AlCl3 with
metallic Al. Through fundamental studies at 1273 K, Si fibers
with diameters ranging from submicrons to several tens of
micrometers are obtained by halidothermic reduction. The
impurity level of the obtained Si is lower than the detection
limit of X-ray fluorescence (XRF). This process affords highpurity Si since all the reactants and byproducts exist in the
vapor phase. Further, this process has high productivity since
the overall reaction is a highly scalable metallothermic
reduction reaction. This method is expected to be an effective
alternative route for the production of solar-grade Si.
Acknowledgements
The authors are grateful to Professor Kazuki Morita,
Institute of Industrial Science of the University of Tokyo, and
Dr. Naoyuki Goto, Sumitomo Chemical Co. Ltd., for fruitful
discussions. The authors gratefully acknowledge Sumitomo
Chemical Co. Ltd. for their financial support.
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