JP2009535515A - Method for refining semiconductor materials using redox reactions - Google Patents

Abstract

  a) Oxidizing the solid semiconductor material to be purified with one or more ionic compounds at the anode 6 disposed in the anode electrolyte; b) disposed in the cathode electrolyte. Reducing, at the cathode 5, one or more compounds obtained in step a) to a purified solid semiconductor material, wherein one or more ionic compounds are also formed; A method for purifying a semiconductor material, wherein one or more ionic compounds formed in step b) are utilized in step a) and the anode and cathode are connected to each other for electron transfer. The formed ionic compound is purified externally. This method can be used, for example, for the purification of silicon.

Description

  The present invention relates to a method for purifying a semiconductor material by utilizing an oxidation-reduction reaction.

Silicon is one of the best known and most widely used semiconductor materials, and is often used in industries such as solar cells, diodes, transistors, integrated circuits (chips) and other areas of electronics. Yes. For such applications, for example high so-called SoG silicon (solar cell grade,> 99.9999% (> 6N) purity) or EG silicon (electronic grade,> 99.9999999% (> 9N) purity). Pure silicon is required. So-called MG silicon (metallurgical grade, 98-99% purity) is often used to obtain such pure silicon. Such silicon materials are generally obtained from SiO ₂ (eg, sand) by reduction with carbon (C) under the formation of carbon monoxide (carbothermal reduction).

  Examples of impurities that may be present in MG silicon are carbon, boron or phosphorus compounds and metal elements such as iron, aluminum, titanium and vanadium. Among other things, such impurities cause degradation of the semiconductor properties of the material, which can lead to a decrease in yield, for example for solar cells. Furthermore, such degraded semiconductor properties are also undesirable for other electronics applications.

  For example, certain impurities such as iron, aluminum, boron, phosphorus, titanium and vanadium are more harmful than others, and some impurities are more difficult to remove from silicon than others. The ease of removal is related, inter alia, to the difference in the separation factor or solubility of certain elements in fluid and solid silicon.

Currently, the standard method for silicon purification utilized consists of a number of steps, ie, in the first step, MG silicon is HCl gas with 90% SiHCl ₃ (trichlorosilane) and SiCl ₄ (tetrachloride). Silicon) is chlorinated to 10% and during this process hydrogen gas is formed.

  The newly formed gas phase silicon compound is then purified by fractional distillation until the amount of impurities reaches the ppb (parts per billion) and ppm (parts per million) levels.

  In the next step, the so-called “Siemens process”, the deposition of purified silicon takes place by the use of purified gaseous silicon compounds and hydrogen. This reaction is carried out on an electrically heated graphite electrode, for example at a temperature of 900-1100 ° C., forming 99.9999% (7N) pure polycrystalline silicon and HCl. A similar method for the purification of silicon is disclosed in US Pat. No. 4,213,937.

  The disadvantage of such a method is that, firstly, corrosive gases such as HCl and explosive gases such as hydrogen are required for the reaction, which places high demands on the equipment and safety requirements utilized. Is to impose. Second, this method consumes a great deal of energy due to the thermal process that converts the silicon compound to crystalline silicon.

Electrochemical deposition of silicon on a silicon cathode by using silicon tetrachloride (SiCl ₄ ) as a starting material is described in T.W. Matsuda et al., Chemistry Letters, 1996, pages 569-570.

The electrochemical synthesis of silane (SiH ₄ ) from MG silicon using hydrogen gas is described in T.W. Nohira et al., Electrochemistry, 2005, 73 (8), pages 692-699.

  JP 06-173064 discloses a method of electrolytic purification of titanium by electrolysis in a molten salt.

WO 02/099166 discloses a method for producing silicon by dissolving SiO _{2 in} an electrolyte composed of CaCl ₂ and CaO.

  An object of the present invention is to provide a method for purifying a semiconductor material with high energy efficiency.

  It is also an object of the present invention to provide such a method that does not require any corrosive or explosive gases.

  Furthermore, the object of the present invention is to provide such a method which can be carried out with simple equipment and without strict safety regulations.

One or more of the purposes are:
a) Oxidation step of an anode made of a semiconductor material containing one or more impurities disposed in an electrolyte, wherein one or more ionic compounds are present in the electrolyte, Reacting with a semiconductor material and one or more impurities, wherein A = semiconductor ion, X = halogen ion, H = hydrogen, y = valence of A, z = an integer greater than or equal to z I:
AX _z H _(yz) (I)
Forming one or more compounds and one or more related reaction products according to
b) At the cathode disposed in the electrolyte, one or more compounds according to formula I obtained in step (a) are added and reduced on the cathode to form a purified semiconductor material on the cathode. A step, wherein one or more ionic compounds formed in step b) are returned to step a). And achieved by a method according to the preamble, characterized in that the anode and the cathode are connected to each other for electron transfer.

  Although the present invention is further described based on silicon as an example of a semiconductor material, the present invention is also applicable to other semiconductor materials such as germanium.

  The present invention will be described with reference to the drawings.

  In this method, two half reactions are combined, an anodic reaction in which non-pure silicon is converted to a silicon halide compound and a cathodic reaction in which the purified silicon halide compound is converted to purified silicon.

  The present invention provides an energy efficient method by carrying out the first reaction (step a)) according to the method at the anode and the second reaction (step b) according to the method at the cathode. The electrons formed at the anode are used at the cathode, so that in principle no reversible (equilibrium) reaction is used, so no additional energy is required. Supplying additional energy by increasing the potential difference can significantly accelerate the reaction.

  A further advantage of the present invention is that HCl gas and hydrogen gas are no longer needed because ionic compounds such as chlorine ions or other ions can be utilized instead of gaseous compounds. These ionic compounds, when a silicon anode is utilized, react with the anode semiconductor material to form a compound according to Formula I, such as a silicon halide compound. In this way, one or more of the objects are achieved.

Furthermore, one or more impurities present in the semiconductor material to be purified react with one or more ionic compounds in one or more reactions to form the associated reaction product. When MG silicon is utilized as the anode, such related reaction products are, for example, FeCl ₃ , PCl ₃ , BCl ₃ , TiCl ₄ , AlCl ₃ and VCl ₄ .

  The electrolyte serves as a transport medium for the ionic reaction product from the cathode to the anode. Thus, returning these ionic reaction products is preferably done through the electrolyte.

  Adding one or more compounds according to Formula I to the cathode can be performed, for example, by bubbling such compounds as a gas at the cathode. However, other methods can also be used.

  FIG. 1 shows an example of a configuration 1 used by the inventor to carry out the method according to the invention. FIG. 1 displays the syringe pump 2 connected to the evaporator 3 (the evaporator 3 is connected to the electrolytic cell 4).

A cathode 5 and an anode 6 are present in the electrolytic cell 4 as shown in FIG. In FIG. 1, ion exchange is performed between the cathode 5 and the anode 6 as indicated by an arrow having Cl ⁻ next to it.

  According to step a) of the present invention, one or more ionic compounds are formed and collected at the anode 6 (see arrow at anode 6). These compounds are then purified externally. The purified desired silicon compound is then fed to the cathode by use of the syringe pump 2 to perform step b) of the method. Any unreacted silicon compound is discharged in the form of a gas at the top of the cathode 5 (see arrow on cathode 5).

  FIG. 2 is an embodiment of an electrolytic cell that can be used in the present invention. A preferred embodiment of the electrolytic cell 4 consists of a quartz tube 7 which is closed at one end and partially filled with an electrolyte 8. Two quartz tubes 9 having open bottoms are arranged in the electrolyte 8. The anode 6 and the cathode 5 are respectively disposed in these quartz tubes. In this additional manner of anode and cathode to the quartz tube 9, it is possible to separate the anode 6 and the cathode 5 from each other so that the vapors emitted at the cathode 5 and the anode 6 are separated separately. be able to. Also shown are a quartz tube with a reference electrode 10, a quartz tube with a thermocouple 11 for measuring temperature, and a compound supply tube in the form of a gas.

The compounds formed according to formula I are preferably purified according to the present invention. This is the two sub-steps c1) and c2) performed after step a) and before step b), ie
c1) extracting one or more compounds according to formula I and one or more related reaction products from the electrolyte;
c2) separating one or more compounds according to formula I from one or more related reaction products;
Can be carried out by a preferred embodiment of the method comprising an additional step c) comprising:

  Thus, in the case where MG silicon is used as the anode in this embodiment, both the formed silicon halide compound and related reaction products are extracted, preferably as a gas, from the electrolyte in which the anode is located. These can then be added to the cathode after purification, where reduction can be performed to form silicon and ions (eg, chlorine ions, etc.). These ions preferably migrate through the electrolyte to the anode where they react with MG silicon to form additional silicon halide compounds.

  Such an embodiment of the present invention ensures excellent purification and prevents electrolyte contamination. Impurities that may be present in the starting materials such as boron and phosphorus are removed before the purified silicon halide compound is produced in pure silicon by deposition on the cathode. Since boron and phosphorus are utilized as dopants in further processing of pure silicon, removal of these elements is particularly important. The amount of such dopants must be determined very accurately in order to obtain the desired properties, so it is very important that these elements are present as little as possible in the purified silicon. It is.

  Step c1) can be carried out, for example, by phase separation. Silicon halide compounds are volatile at the temperatures utilized and can therefore be collected as a gas from a fluid electrolyte. However, other extraction methods can be used.

  Step c2) can be carried out, for example, by fractional distillation. Silicon halide compounds and halogenated impurities have different boiling points and can therefore be separated by distillation. However, other separation methods can also be utilized.

  In step c2), the compounds according to formula I are 99.99% (4N), preferably 99.999% (5N), specifically 99.9999% (6N), more specifically 99. It is recommended to separate until it has a purity of 99999% (7N) or higher. The desired purity depends on the desired use, for example as already described above.

  Specifically, silicon is used as a semiconductor material in step a), and more specifically, MG silicon is used because it is easily available.

  The cathode material is preferably selected from the group consisting of silicon, carbon, silver, molybdenum, platinum, tungsten, and one or more combinations thereof. However, other conductive materials can also be used. Specifically, silicon as the cathode material is more preferable, and more specifically, silicon having a purity of at least a desired purity, that is, 99.99% (4N), preferably 99.9999% (6N) or higher.

  Such materials give sufficiently good properties with respect to electronic conductivity and are inert. These materials have a low resistance to electronic conduction at operating temperatures, provide minimal contamination to the silicon being purified, minimal contamination to the electrolyte, and they are simple crystals of silicon. For surface sites with low activation energy.

  The advantage of utilizing silicon as the cathode material, and more particularly silicon of at least the desired purity, is that removing the silicon formed from the cathode, for example by scraping the formed silicon, also rubs the cathode material. There is no formation of any impurities in the formed silicon by being dropped, which is simpler.

  The anode material preferably consists of a semiconductor material to be purified, and the semiconductor material to be purified forms the entire anode, i.e. the anode can consist of wires, rods or lumps of eg MG silicon. is there. The anode also consists of a core of inert material that serves as a carrier, which is preferably not a conductor for electrons, the inert material being sealed in a mantle of semiconductor material. It is also possible. Such a mantle can be applied to the core by deposition, sputtering or coating.

  Specifically, the semiconductor material to be purified is preferably porous or powdery. As a result, the material to be purified has a large surface area, which increases the reaction rate.

  In the method of the present invention, two electrolytes are used, more specifically, an electrolyte in which an anode is disposed and an electrolyte in which a cathode is disposed. These electrolytes are preferably the same. This makes the reactor design easier because the anode and cathode can be present in a single vessel. This also simplifies the return of ionic compounds from the cathode to the anode.

An electrolyte with a cathode disposed therein preferably has the following requirements:
-High ionic conductivity (low resistance) to minimize the energy loss caused by the resistance and achieve mass transfer of the formed ionic compounds;
A higher standard reduction potential (E ₀ ) than the compound according to formula I to prevent electrolysis of the electrolyte; a higher standard reduction potential allows it to apply a greater potential difference between the electrodes (see also below); This results in an increased reaction rate and is recommended;
-Low solubility of the semiconductor material formed on the cathode to prevent this loss in the electrolyte;
-Low solubility and diffusion rate of the semiconductor material formed on the cathode to prevent this contamination;
-High solubility in water to allow easy removal of the electrolyte while rinsing the semiconductor material formed on the cathode;
Meet.

The electrolyte in which the anode is arranged preferably has the following requirements:
-High ionic conductivity (low resistance) to achieve mass transfer of formed ionic compounds with minimal energy loss caused by resistance;
- higher standard reduction potential in order to prevent electrolysis of the electrolyte (E _0);
-Electrolyte contamination can lead to contamination of the semiconductor material formed on the cathode, so the low solubility of the anode material to prevent electrolyte contamination;
The low solubility of these compounds and products to simplify the extraction of the compounds according to formula I and related reaction products;
-Low solubility of related reaction products to prevent electrolyte contamination;
Meet.

Examples of the electrolyte include a molten salt. The electrolyte may be a salt of one or more halogens such as NaCl, KCl, LiCl and BaCl ₂ and one or more alkali (earth) metals, or one or more combinations thereof, and nitrates, carbonates And sulfates and combinations thereof, or fluids that are ionic at room temperature can be mentioned as electrolytes. Such an electrolyte can be used as an electrolyte in which an anode is disposed or an electrolyte in which a cathode is disposed.

  In one embodiment of the invention, a membrane is utilized that is permeable to the one or more ionic compounds formed in step b). The membrane exists between an electrolyte with an anode and an electrolyte with a cathode.

  Such membranes must be selectively permeable for the passage of ionic reducing compounds from the cathode to the anode, not to the oxidation products from the anode to the cathode. The membrane can prevent contamination of the electrolyte in which the cathode is located, such contamination consisting of positively charged ions (metals and others) that can be deposited on the cathode, and thus purified semiconductor material Can contaminate.

  The membrane is preferably made of a ceramic material such as La-M-O-Cl (M: an alkaline earth metal such as Ca / Sr). Such ceramic materials are examples of so-called “Solid Electrolyte” used in fuel cells.

  It is also possible to use a capture electrode connected to an external voltage source, which has an equivalent or somewhat higher voltage than the cathode, And in the electrolyte between the cathode.

  The capture electrode is not exposed to the compound according to Formula I applied to the cathode, the sole purpose of this capture electrode is to provide a site for the deposition and deposition of impurities (oxidation products) released at the anode. is there.

  In one embodiment of the invention, one or more compounds according to formula I are further added in step b) in order to compensate for any observable losses during the purification according to step c2). It is done. Such compounds should be of the desired purity, i.e. at least 99.99% (4N), more specifically at least 99.9999% (6N).

  The conditions for carrying out this reaction such as pressure, temperature and reduction potential are determined by the reaction rates of the anodic reaction and the cathodic reaction.

  From this, the reaction conditions are described for the case where silicon is used.

Cathodic reaction conditions It is recommended to obtain crystalline pure silicon. As is usual in semiconductors, the lowest temperature for deposition of silicon around the electrode caused by the conductive behavior of silicon increasing with increasing temperature is approximately 400 ° C. The transition from amorphous to crystalline silicon occurs at a temperature of 470 ° C., so the temperature at the cathode is preferably at least 400 ° C., more specifically at least 470 ° C. Also, in principle, amorphous silicon is formed at the cathode, which is then converted to crystalline silicon at a temperature above the cathode, for example, above 470 ° C. In some applications, amorphous silicon itself is also utilized.

  Also, the high resistance of silicon at lower temperatures can be used to form crystalline silicon at lower temperatures (<400 ° C.), which leads to local heating at the cathode resulting in the formation of crystalline silicon. . If the resistance increases, the temperature will also increase, which should result in an improved crystalline form of, for example, silicon. However, higher resistance should also lead to adverse energy usage increases. It is therefore important to find a good compromise.

  In order to obtain a high deposition rate of silicon and thus a high reaction rate, the concentration of the compound according to formula I at the cathode should be as high as possible. When silicon halide compounds are utilized, they are gaseous due to their boiling point at utilization temperatures of at least 400 ° C., and thus the reaction is carried out in three phases: a solid (cathode), It is carried out in a fluid state (electrolyte) and a gaseous state (silicon halide compound).

  The pressure should preferably be high to allow a high concentration of the compound according to formula I. If the silicon halide compound is added to the cathode as a gas (bubbling), the electrolyte should have as high a surface tension as possible and the bubbles should be as small as possible to obtain as high an internal pressure as possible. This is because the internal pressure in the gas bubble is determined by the external pressure of the fluid and the surface tension divided by the bubble radius.

  If membranes and / or capture electrodes are utilized, conditions such as temperature and pressure should be adjusted accordingly.

Anodic Reaction Conditions All products formed at the anode are desirably gaseous to limit electrolyte contamination to a minimum. Whether the product formed at the anode is gaseous depends on the temperature and pressure applied, in combination with the boiling point of the compound thus formed.

In order to obtain a gaseous product, the boiling point of the compound formed must be lower than the temperature at which the reaction takes place. For example, the desired SiCl ₄ has a boiling point of 57.6 ° C. and should be gaseous at the reaction temperature. Examples of contaminants often present in MG silicon that are gaseous at a reaction temperature of 400 ° C. are: BCl ₃ (12.5 ° C.), PCl ₃ (75.95 ° C.), TiCl ₄ (136.4 ° C.), PCl ₅ (160 ° C.), AlCl ₃ (182.7 ° C.), and FeCl ₃ (315 ° C.). At a reaction temperature of 400 ° C., such by-products are released as gases and therefore such by-products should not pose a problem during purification. But also, often resulting FeCl ₂ in the reaction has a boiling point of 1026 ° C., thus, not collected as a gas at a temperature of 400 ° C., and therefore should be removed by any method. For this purpose, for example, a membrane or a capture electrode is required.

  It is preferred to utilize an external voltage source (not limited to a certain type) to apply a potential difference selected between 0.01 V and the standard reduction potential of the electrolyte between the anode and cathode.

  Since a reversible reaction between the semiconductor and the compound according to formula I is used, very little energy is required to actually carry out this reaction. Theoretically, this corresponds to a potential difference of 0V. But in fact, there is no motive force for the reaction, so the reaction continues vaguely. In order to ensure that the reaction rate is high enough to be practically applied, a potential difference greater than 0V, for example greater than 0.01V should be applied. This potential difference must not exceed the standard reduction potential of the electrolyte used to prevent electrolysis. This potential difference is acceptable for the electrolyte used and is preferably as high as possible. Therefore, in order to enable a high reaction rate, it is preferable to use an electrolyte having a high standard reduction potential.

The reduction potential (E ₀ ) of SiCl ₄ is −1.451 V at 25 ° C. and −1.389 V at 850 ° C. The reduction potentials of a number of previously described electrolytes are: MgCl ₂ (−3.066 V at 25 ° C. and −2.42 V at 850 ° C.), CaCl ₂ (−3.887 V at 25 ° C. and -3.296V at 850 ° C), NaCl (-3.982V at 25 ° C) and KCl (-4.235V at 25 ° C). The availability of the electrolyte depends on the mobility of the ionic compound in the electrolyte.

  In one embodiment of the invention, the electrolyte is forced to flow from the cathode to the anode and vice versa. In such embodiments, the electrolyte is fed into the cathode-to-anode stream at a specific rate, specifically, the electrolyte-to-cathode flow is the cathode-to-anode electrolyte flow. And physically separated. This is possible, for example, by a closed loop in which the electrolyte is forced to flow, where the anode and cathode are arranged in a closed loop at individual sites. In particular, this is advantageous when a membrane or in particular a capture electrode is placed between the anode and the cathode.

  The advantage of such a structure is that the purification of the electrolyte is simplified when the flow rate is sufficient. Cations present in the electrolyte want to go to the cathode and are created at the anode. Because of the flow, they can only be moved by the route in which the membrane / capture electrode exists. Cations cannot move against the flow at a particular flow rate and are therefore always forced through the membrane or capture electrode. A capture electrode is preferred over a membrane because the capture electrode neutralizes cations and the membrane only captures cations.

  Such a circulation of the electrolyte can be realized, for example, by using a pump or using a density difference. The density difference can be realized, for example, by a temperature difference, or by the presence of gas bubbles (bubbling) at an electrode such as a cathode. However, other methods for realizing a circulating flow can also be utilized.

  An additional example of the application of circulation is that when chloride ions are present, they are not only transferred to the anode by diffusion, but also better mass transfer occurs at the electrode due to this flow, resulting in higher conversion rates. Is obtained.

  For this method, for example, MG silicon processing equipment (crushing, leaching), electrolytic cell (anode and cathode and electrolyte), separation equipment for purification of silicon halide compounds (distillation, absorption, adsorption, membrane separation) and pure This can be done by utilizing any suitable device or combination thereof, such as a clean silicon processing device (cleaning, crushing).

  The invention is further described below based on the following non-limiting examples.

A configuration as shown in FIG. 1 was used. As a syringe pump, a KD Scientific syringe pump having a 30 ml polypropylene syringe, model KDS200 was used. The syringe pump was filled with fluid purified SiCl ₄ and set to a syringe speed of 0.5 ml / min. For testing purposes, SiCl ₄ obtained from a Dutch VWR having a purity of> 99% was used. However, when performing the entire method, SiCl ₄ is formed at the anode and returned to the cathode after possible extraction and purification.

A syringe pump and a Teflon (registered trademark) tube are used to supply SiCl ₄ to the inlet of an evaporation device (made of borosilicate, which is provided with a cover at the inlet and outlet) consisting of a container having a capacity of 3 liters. Move in. The cover is hermetically sealed with an O-ring and then completely insulated with a glass fiber mat except for the bottom. The bottom of the container is placed on a hot plate having a constant temperature of 200 ° C.

If necessary, a valve was placed between the syringe pump and the evaporator so that the syringe could be replaced without affecting the evaporator pressure. The outlet of the evaporator is connected to the electrolytic cell by a borosilicate insulation tube. The purpose of the evaporator is to evaporate SiCl ₄ introduced into the reactor to prevent flash evaporation in the reactor.

  The electrolytic cell according to this example is the one shown in FIG. The outer quartz tube has a diameter of 60 mm, and this outer quartz tube has a total length of 400 mm. Add electrolyte to fill 1/3 of reactor.

  The cell was placed vertically in an electric oven that could be heated to 1200 ° C. (where the cell top was located outside the oven). The oven was an electric oven obtained from Westeneng Ovenbouw, The Netherlands and had a simple control device that allowed the temperature to be set before the reaction.

In this example, the anode was made from two strips of phosphorus-doped electronic purity n-type silicon having a resistivity between 1 and 30 ohm-cm. The purity of this material was higher than 99.9999%. The strip is approximately 1 cm wide and 15 cm long and 1 mm thick. Each strip has a thread at one end that is utilized to attach the strip to the end of a tungsten rod (2 mm diameter) by using a platinum wire. The other end of the tungsten rod was connected to a potentiostat as a counter electrode. Before submerging the anode in the electrolyte, the silicon portion of the anode was immersed in a 10% aqueous HF solution to remove any oxide from the surface. It is clear that only the silicon part of the anode is in contact with the electrolyte. The total anode area exposed to the electrolyte, was at least 42cm ^2.

The cathode is made of tungsten (length 400 m, diameter 2 mm) immersed in an electrolyte. SiCl ₄ serving as a starting material was supplied as a gas along the surface of the cathode. Tungsten electrode had a total reaction area of about 11cm ^2. The other end of the rod was connected to a potentiostat as a working electrode. The pseudo reference electrode was a 0.5 mm diameter platinum wire with an active reaction area of about 0.75 cm ² .

The electrolyte was a eutectic mixture of 50 mole percent KCl (purity 99.5%) and 50 mole percent NaCl (purity 99.5%) and had a melting point of 650 ° C. The total salt mass utilized in the electrolyte was 880 g. The salt mixture was placed in a quartz reaction tube and heated to a temperature of 810 ° C. to melt. After melting, the oven temperature was set to a temperature of 710 ° C. and held at this temperature for the entire duration of the experiment. Prior to submerging the electrode in the electrolyte, any dissolved oxide in the melt was removed by the addition of a small amount of NH ₄ Cl (99% purity). KCl, NaCl, and NH ₄ Cl were all obtained from VWR, The Netherlands.

The potentiostat with three electrodes was type PAR-273-A manufactured by EG & G Princeton Applied Research and was computer controlled with the LabVIEW program. This was utilized to perform a cyclic voltammetry test of the electrolytic cell. All experiments were performed in an argon gas atmosphere to condense any vapor released at the electrodes. Any uncondensed gas was passed through water to neutralize the SiCl ₄ still present therein.

The inventors have more specifically found that the use of NaCl and KCl as eutectic mixtures gives excellent results. The same as the previous experiment was also carried out by utilizing CaCl ₂ ; however, this was found to lead to quartz reactor erosion and corrosion due to the strong solubility of O ²⁻ ions. Therefore, the NaCl / KCl mixture was chosen because it has only limited solubility of O ²⁻ ions. NH ₄ Cl salt was added to remove any O ²⁻ ions present in the melt. The NH ₄ Cl salt decomposes at high temperature according to the following chemical equation 1. HCl formed by reaction 2, any both ^{O 2-} ions can ^{react, O 2-} ions are removed from the melt.
(1) NH ₄ Cl (solid) → NH ₃ (gas) + HCl (gas)
(2) 2HCl (gas) + O ²⁻ (liquid) → H ₂ 0 (gas) + 2Cl ⁻ (liquid)

Periodic voltammetry experiments were first performed without SiCl ₄ present to determine the experimental background. Measurements were then performed on the SiCl ₄ gas flow supplied by the syringe pump and the evaporator. The measurement results are shown in FIG.

  In all cases, the voltage was varied between 0 and -3 V with respect to the platinum pseudo reference electrode. In both cases, a scan rate of 5 mV / sec was utilized.

In the absence of any SiCl ₄ (black curve), the current rose very strongly when the potential difference was greater than −2.2 V for the Pt electrode. This peak appears to correspond to the electrolysis of the electrolyte with the formation of Cl ₂ (or SiCl ₄ on the anode) and the deposition of K and Na on the cathode. Since it has a lower reduction potential than potassium, sodium should precipitate first. The scan was repeated four times for each scan with similar results.

Next, SiCl ₄ gas was supplied to the system and the peak current was observed in the region between -1.2 and -1.7 V (gray curve). These peaks are thought to correspond to the reduction of SiCl ₄ to Si on the cathode, which is the desired reaction. Varying the potential between -1.2V and -1.7V while adding SiCl ₄ to the reactor to ensure that enough silicon is deposited so that sufficient sample or analysis can be obtained. The scan was repeatedly executed. After this was done for 1.5 hours, the entire system was switched off, the electrodes were removed, the electrolyte was cooled and solidified.

A thin film layer of material was observed on the tungsten electrode and the silicon anode was observed to contain a number of holes. Some of the unreacted SiCl ₄ released at the cathode was condensed. Only a small amount of condensation was observed from the anode, but not enough to allow the sample to be examined. It was also observed that black deposits were formed on the electrolyte deposited at the bottom of the quartz tube. In addition, fragments of tungsten electrodes and electrolyte fragments including black deposits were analyzed by TNO (Netherlands Institute of Applied Natural Sciences) using SEM-EDX. The EDX analysis of the tungsten cathode is displayed in FIG. The electrode was observed to contain a high level of both tungsten expected by the method and silicon deposited on the electrode. In addition, a small amount of impurities are present. From this analysis, it can be determined that the silicon layer is indeed deposited on the tungsten cathode.

  The portion of the electrolyte with black deposits was dissolved in water and washed to obtain a black powder which was analyzed. The EDX analysis of the black powder is displayed in FIG. This analysis shows that the powder is primarily pure silicon with very small amounts of sodium mainly from the electrolyte. This sodium may have precipitated during the electrolysis of the electrolyte to determine the background. The silicon powder is considered to have a porous structure having a particle size of about 50 μm.

From the above experiments, by utilizing this method, a significant amount of silicon was deposited on the cathode and as a porous powder by electroreduction of SiCl ₄ gas on the cathode placed in a molten electrolyte of chloride salt. Can be shown. It has also been observed that the silicon anode erodes during the experiment and provides a condensable vapor of silicon chloride.

FIG. 3 is a diagrammatic representation of the configuration used to carry out the method according to the invention. FIG. 2 is a diagrammatic representation of an electrolytic cell that can be used to carry out the method according to the invention. It is a figure expressing a cyclic voltammogram. It is a figure which shows the graph of SEM-EDX. It is a figure which shows the graph of SEM-EDX.

Claims (17)

a) oxidizing an anode disposed in an electrolyte, wherein the anode is made of a semiconductor material containing one or more impurities, and the electrolyte contains one or more ionic compounds present And the ionic compound reacts with the semiconductor material and one or more impurities, wherein A = semiconductor ion, X = halogen ion, H = hydrogen, y = A valence, z = z or more An integer, Formula I:
AX _z H _(yz) (I)
Forming one or more compounds and one or more related reaction products according to
b) adding one or more compounds according to formula I obtained in step (a) to the cathode disposed in the electrolyte, reducing at the cathode and forming a purified semiconductor material on the cathode Wherein the reduction also results in the formation of one or more ionic compounds, wherein the one or more ionic compounds formed in step b) are returned to step a), A method for purifying a semiconductor material by using an oxidation-reduction reaction, characterized in that an anode and a cathode are connected to each other for electron transfer. The method comprises an additional step c) performed after step a) and before step b), wherein step c) comprises two sub-steps c1) and c2), ie
c1) a sub-step of extracting one or more compounds according to formula I and one or more related reaction products from the electrolyte;
and c2) a sub-step of separating one or more compounds according to formula I from one or more related reaction products.   The process according to claim 2, wherein in step c2) one or more compounds according to formula I are separated to a purity of 99.99% (4N), preferably 99.9999% (6N) or higher.   4. A method according to any one or more of the preceding claims, wherein silicon is used as the semiconductor material in step a).   The method according to claim 4, wherein MG silicon (metallurgical purity) is used as the semiconductor material in step a).   6. A method according to any one or more of claims 1 to 5, wherein the cathode material is selected from the group consisting of silicon, carbon, silver, molybdenum, platinum, tungsten and one or more combinations thereof.   The method according to claim 6, wherein silicon having a purity of 99.99% (4N), preferably 99.9999% (6N) or higher is utilized as the cathode.   8. A method according to any one or more of the preceding claims, wherein a core of inert material sealed in a mantle of semiconductor material is utilized as the anode.   13. A method according to any one or more of the preceding claims, wherein the electrolyte in which the anode is arranged is the same as the electrolyte in which the cathode is arranged.   The method according to claim 9, wherein the electrolyte is a combination of NaCl and KCl, preferably a eutectic mixture of NaCl and KCl.   A membrane that is permeable to the one or more ionic compounds formed in step b) is used, which membrane is between the electrolyte in which the anode is arranged and the electrolyte in which the cathode is arranged. 11. A method according to any one or more of the preceding claims, wherein the method is present.   The method of claim 11, wherein a ceramic membrane is utilized as the membrane.   A capture electrode connected to an external voltage source is utilized, the capture electrode having a potential greater than or equal to the potential of the cathode, and the capture electrode is disposed in the electrolyte between the anode and the cathode. The method according to any one or more of 9 to 11.   14. A method according to any one or more of claims 2 to 13, wherein in step b) one or more compounds according to formula I are further added.   15. A process according to any one or more of the preceding claims, wherein the reduction is carried out at a temperature of at least 400 ° C, preferably at least 470 ° C.   16. An external voltage source is utilized to apply a potential difference between the anode and the cathode, the potential difference being selected between 0.01V and the standard reduction potential of the electrolyte. A method according to paragraph or clause.   17. A method according to any one or more of claims 9 to 16, wherein the electrolyte is forced to flow from the cathode to the anode and vice versa.

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