WO2006122877A1 - Dispersion containing silicon powder and coating process - Google Patents

Dispersion containing silicon powder and coating process Download PDF

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Publication number
WO2006122877A1
WO2006122877A1 PCT/EP2006/062040 EP2006062040W WO2006122877A1 WO 2006122877 A1 WO2006122877 A1 WO 2006122877A1 EP 2006062040 W EP2006062040 W EP 2006062040W WO 2006122877 A1 WO2006122877 A1 WO 2006122877A1
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Prior art keywords
silicon
aluminium
silicon powder
layer
dispersion
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PCT/EP2006/062040
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French (fr)
Inventor
Mario Gjukic
Robert Lechner
Martin Stutzmann
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Evonik Degussa Gmbh
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Publication of WO2006122877A1 publication Critical patent/WO2006122877A1/en

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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C10/00—Solid state diffusion of only metal elements or silicon into metallic material surfaces
    • C23C10/02—Pretreatment of the material to be coated
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/02—Silicon
    • C01B33/021—Preparation
    • C01B33/027—Preparation by decomposition or reduction of gaseous or vaporised silicon compounds other than silica or silica-containing material
    • C01B33/029—Preparation by decomposition or reduction of gaseous or vaporised silicon compounds other than silica or silica-containing material by decomposition of monosilane
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C10/00—Solid state diffusion of only metal elements or silicon into metallic material surfaces
    • C23C10/18—Solid state diffusion of only metal elements or silicon into metallic material surfaces using liquids, e.g. salt baths, liquid suspensions
    • C23C10/26—Solid state diffusion of only metal elements or silicon into metallic material surfaces using liquids, e.g. salt baths, liquid suspensions more than one element being diffused
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02367—Substrates
    • H01L21/0237—Materials
    • H01L21/02422—Non-crystalline insulating materials, e.g. glass, polymers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/02521—Materials
    • H01L21/02524—Group 14 semiconducting materials
    • H01L21/02532—Silicon, silicon germanium, germanium
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/02587—Structure
    • H01L21/0259—Microstructure
    • H01L21/02601—Nanoparticles
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02612—Formation types
    • H01L21/02617—Deposition types
    • H01L21/02623—Liquid deposition
    • H01L21/02628—Liquid deposition using solutions
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic System
    • H01L31/182—Special manufacturing methods for polycrystalline Si, e.g. Si ribbon, poly Si ingots, thin films of polycrystalline Si
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/546—Polycrystalline silicon PV cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the invention relates to a dispersion which contains nanosize silicon particles and to the production and use of this dispersion.
  • the invention further relates to a process for producing a polycrystalline silicon layer.
  • Processes in which polycrystalline layers are applied to a substrate are known in the literature. These are generally deposition processes. Plasma-aided chemical gas-phase deposition processes such as PECVD and ECR-CVD are most frequently used. In addition, the hot wire technique and ion-aided deposition processes (IAD processes) have attained some importance. A disadvantage of all these processes is that they are very complicated, costly and not very efficient.
  • the metal-induced layer exchange process starts out from a metal layer, generally aluminium, applied to the substrate.
  • an aluminium film is firstly vapour-deposited onto a substrate.
  • An amorphous silicon layer is subsequently applied by sputtering.
  • silicon atoms diffuse into the aluminium layer and form crystalline silicon nuclei. These nuclei grow to become grains and displace the aluminium. Finally, adjacent grains form a continuous polycrystalline silicon layer on the substrate.
  • the overall process is thus a layer exchange between aluminium and silicon. It is generally assumed that the driving force for the aluminium-induced layer exchange is the energy difference of the Gibbs potentials of amorphous silicon and crystalline silicon.
  • the process of aluminium-induced layer exchange has advantages in terms of simplicity, temperatures required and, associated therewith, useability of the substrates .
  • a disadvantage is that the silicon layer has to be applied by vapour deposition or sputtering. This step is complicated and expensive.
  • the technical object of the present invention is therefore to provide a means which allows a silicon layer to be applied inexpensively to a substrate.
  • a further technical object is to provide a process for producing a polycrystalline silicon layer, which is faster and cheaper compared to the prior art.
  • the invention provides a dispersion containing silicon powder and a liquid phase, in which the silicon powder is predominantly in crystalline form and the particles in the dispersion have a mean diameter D 50 of less than 500 nm and a BET surface area of more than 5 m 2 /g.
  • "predominantly crystalline” means that the powder particles can contain proportions of amorphous silicon.
  • This amorphous silicon can, for example, be detected by means of high-resolution transmission electron micrographs.
  • the region containing amorphous material is restricted to the regions close to the surface.
  • the proportion of crystalline material is greater than 95%, in general greater than 99%. Silicon powder without any amorphous content can also be present in the dispersion of the invention.
  • the mean particle diameter D 50 of the silicon powder present in the dispersion of the invention is less than 500 nm.
  • a silicon powder having a mean particle diameter D 50 of less than 500 nm is defined as nanosize for the purposes of the present invention.
  • the mean particle diameter D 50 can preferably be from 5 to 200 nm and particularly preferably from 50 to 150 nm.
  • Silicon powder containing crystallites having a mean particle diameter D 50 of less than 500 nm is defined as nanocrystalline silicon powder for the purposes of the present invention.
  • Polycrystalline silicon has a crystal size in the range from about 1 ⁇ m to
  • the BET surface area of the silicon powder present in the dispersion of the invention is at least 5 m 2 /g.
  • a silicon powder having a BET surface area of from 5 to 30 m 2 /g can be preferred.
  • Such a silicon powder can preferably be produced by heat treating a reaction mixture comprising at least one vaporized or gaseous silane and, if appropriate, at least one vaporized or gaseous dopant in the presence of an inert gas and hydrogen in a hot-wall reactor.
  • the proportion of silane is from 0.1 to 90% by weight, based on the sum of silane, dopant, hydrogen and inert gases.
  • the proportion of hydrogen, based on the sum of hydrogen, silane, inert gas and any dopant, is from 1 mol% to 96 mol%.
  • the residence time in the hot-wall reactor is from 0.1 s to
  • the maximum temperature in the hot-wall reactor should not exceed 1000 0 C.
  • SiH 4 Si 2 Hg, ClSiH 3 , Cl 2 SiH 2 , Cl 3 SiH and/or SiCl 4 , with SiH 4 being particularly preferred.
  • the present invention fully incorporates the contents of the German patent application number 10353996.4 filed on 19 November 2003 by reference.
  • a silicon powder having a BET surface area of from 50 to 700 m 2 /g can be advantageous.
  • Such a powder can preferably be produced by reaction of a vaporized or gaseous silane and, if appropriate, at least one vaporized or gaseous dopant by means of electromagnetic radiation in the microwave region at a pressure of from 10 to 1100 mbar in a plasma in the presence of an inert gas .
  • the proportion of silane is from 0.1 to 90% by weight, preferably from 1 to 10% by weight, based on the sum of silane, dopant and inert gases.
  • the reaction mixture is allowed to cool or is cooled and the reaction product is separated off in the form of a powder from gaseous substances.
  • the conversion of silane is at least 98%.
  • silanes preference is given to using SiH 4 , Si 2 H 6 , ClSiH 3 , Cl2SiH 2 ,
  • the power input is not limited. It is preferably selected so that the reflected microwave power which has not been absorbed is minimal and a stable plasma is formed.
  • the energy input is from 100 W to 100 KW, particularly preferably from 500 W to 6 KW.
  • the particle size distribution can be varied by means of the microwave power injected. Thus, at the same gas compositions and volume flows, higher microwave powers can lead to a smaller particle size and a narrower particle size distribution.
  • the microwave region is a region from 900 MHz to 2.5 GHz, with a frequency of 915 MHz being particularly preferred.
  • the proportion of hydrogen can be in a range from 1 to 96% by volume.
  • the process can be carried out with the reaction mixture obtained after the microwave treatment being thermally after-treated.
  • a wall-heated hot-wall reactor is particularly advantageous for this purpose.
  • the dimensions of the hot-wall reactor should be such that the residence time in it is from 0.1 s to 2 s.
  • the maximum temperature in the hot-wall reactor should not exceed 1000 0 C.
  • the silicon powder itself it is likewise possible for the silicon powder itself to be thermally after-treated.
  • the present invention fully incorporates the contents of the German patent application number 10353995.6 filed on 19 November 2003 by reference.
  • the silicon powder present in the dispersion of the invention can be present in unaggregated, predominantly aggregated or completely aggregated form.
  • "predominantly aggregated” means that some individual unaggregated particles are present in addition to the aggregates.
  • the silicon powder is preferably present in aggregated or predominantly aggregated form in the dispersion of the invention.
  • the silicon powder present in the dispersion of the invention can be doped. Furthermore, the silicon powder according to the invention can be doped. Preferred dopants are, particularly for use as semiconductor in electronic components, the elements phosphorus, arsenic, antimony, bismuth, boron, aluminium, gallium, indium, thallium, europium, erbium, cerium, praseodymium, neodymium, samarium, gadolinium, terbium, dysprosium, holmium, thulium, ytterbium, lutetium.
  • the proportion of these in the silicon powder according to the invention can be up to 1% by weight.
  • a silicon powder in which the doping component is present in the ppm or even ppb range will be desired. Preference is given to a range of from 10 13 to 10 15 atoms of doping component/cm 3 .
  • the particles of the silicon powder present in the dispersion of the invention can contain bonds of the Si-H, Si-O and/or Si-OH type on their surface. Such bonds can be detected, for example, by means of IR spectra.
  • a silicon powder which contains both hydrogen-terminated and oxygen- terminated silicon atoms is preferred for the dispersion of the invention.
  • the content of hydrogen on the surface of the silicon powder can preferably be up to 1 mol%.
  • bonds of the Si-H, Si-O and/or Si-OH type represent reactive sites for surface modification of silicon powders.
  • the Si-H bonds present in the silicon powder can react partly or completely with alkenes in a hydrosilylation reaction to form Si-C bonds.
  • alkenes it is possible to use aliphatic alkenes, unsaturated (di) carboxylic acids, unsaturated fatty acids or alkenes having aromatic radicals. Aliphatic alkenes having from 3 to 20 carbon atoms are particularly suitable.
  • alkenes examples include: 1-octadecene, 1-octene, 1-hexene, 1-butene, styrene, alpha-methylstyrene, acrylic acid, methacrylic acid, itaconic acid and oleic acid.
  • Alkynes can be used analogously to the alkenes for the hydrosilylation.
  • alkenyl-Si bonds are formed.
  • Ethynylbenzene can, for example, serve as alkyne.
  • a combination of alkenyl-Si bonds and surface-bonded functional groups can also be achieved by means of functionalized alkynes in a manner analogous to the introduction of functional groups in the case of alkenes .
  • the hydrosilylation reaction can also be used for introducing functional groups on the surface, for example sulphonate groups by means of vinylsulphonate or methallylsulphonate, and also OH groups by means of unsaturated alcohols, for example hexenol .
  • the Si-OH bonds present in the silicon powder can react with suitable substances to form a chemical bond.
  • the chemical bond can be a covalent, ionic or coordinate bond between the surface modifier and the particle, but hydrogen bonds are also possible.
  • a coordinate bond means complex formation.
  • an acid/base reaction of the Br ⁇ nsted or Lewis type, complex formation or an esterification can take place between the functional groups of the modifier and the particle .
  • Suitable substances can be, for example: a) saturated or unsaturated monocarboxylic and polycarboxylic acids having from 1 to 24 carbon atoms and also the corresponding acid anhydrides, chlorides, esters and amides and their salts, b) monoamines, polyamines, polyalkylenamines, for example methylamine, dimethylamine, trimethylamine, ethylamine, aniline, N-methylaniline, diphenylamine, triphenylamine, toluidine, ethylenediamine, diethylenetriamine, c) beta-dicarbonyl compounds such as acetylacetone, 2,4- hexanedione, 3, 5-heptanedione, acetoacetic acid, Ci-C 4 - alkyl acetoacetates and acetonylacetone, d) amino acids such as beta-alanine, glycine, valine, aminocaproic acid, leucine and isoleucine, e) silane
  • Silanes of the formula R a SiX 4 - a where the radicals R are identical or different and are nonhydrolysable groups, the radicals X are identical or different and are hydrolysable groups or hydroxy groups and a is 1, 2 or 3, can preferably serve as surface- modifying reagent.
  • the value a is preferably 1.
  • the hydrolysable groups X which can be identical or different, are, for example, hydrogen or halogen (F, Cl, Br or I) , alkoxy (preferably Ci-C ⁇ - alkoxy such as methoxy, ethoxy, n-propoxy, i-propoxy and butoxy) , aryloxy (preferably C ⁇ -Cio-aryloxy such as phenoxy) , acyloxy (preferably Ci-C 6 -acyloxy such as acetoxy or propionyloxy) , alkylcarbonyl (preferably C 2 - C7-alkylcarbonyl such as acetyl) , amino, monoalkylamino or dialkylamino preferably having from 1 to 12, in particular from 1 to 6, carbon atoms.
  • alkoxy preferably Ci-C ⁇ - alkoxy such as methoxy, ethoxy, n-propoxy, i-propoxy and butoxy
  • aryloxy preferably C ⁇ -Cio-aryloxy such as
  • Preferred hydrolysable radicals are halogen, alkoxy groups and acyloxy groups .
  • Particularly preferred hydrolysable radicals are Ci-C4-alkoxy groups, in particular methoxy and ethoxy.
  • the nonhydrolysable radicals R which can be identical or different, can be nonhydrolysable radicals R with or without a functional group.
  • the nonhydrolysable radical R without a functional group can be, for example, alkyl (preferably Ci-Cg-alkyl such as methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl and tert-butyl, pentyl, hexyl, octyl or cyclohexyl) , alkenyl (preferably C 2 -C 6 -alkenyl such as vinyl, 1-propenyl, 2-propenyl and butenyl) , alkynyl (preferably C 2 -C 6 -alkynyl such as acetylenyl and propargyl) , aryl (preferably C 6 -Cio-aryl such as phenyl and naphthyl) and corresponding alkaryls and aralkyls (e.g.
  • alkyl preferably Ci-Cg-alkyl such as methyl, e
  • alkyltrialkoxysilanes examples are : CH 3 SiCl 3 , CH 3 Si (OC 2 H 5 ) 3 , CH 3 Si (OCH 3 ) 3 , C 2 H 5 SiCl 3 , C 2 H 5 Si(OC 2 Hs) 3 , C 2 H 5 Si(OCH 3 ) 3 , C 3 H 7 Si(OC 2 Hs) 3 , (C 2 H 5 O) 3 SiC 3 H 6 Cl, (CHs) 2 SiCl 2 , (CH 3 ) 2 Si (OC 2 H 5 ) 2 , (CH 3 ) 2 Si(OH) 2 , C 6 H 5 Si(OCHs) 3 , C 6 H 5 Si(OC 2 Hs) 3 , C 6 H 5 CH 2 CH 2 Si(OCH 3 )S, (C 6
  • the nonhydrolysable radical R having a functional group can, for example, comprise an epoxide (e.g. glycidyl or glycidyloxy) , hydroxy, ether, amino, monoalkylamino, dialkylamino, substituted or unsubstituted anilino, amido, carboxy, acryl, acryloxy, methacryl, methacryloxy, mercapto, cyano, alkoxy, isocyanato, aldehyde, alkylcarbonyl, acid anhydride or phosphoric acid group as functional group.
  • an epoxide e.g. glycidyl or glycidyloxy
  • hydroxy, ether amino, monoalkylamino, dialkylamino, substituted or unsubstituted anilino, amido, carboxy, acryl, acryloxy, methacryl, methacryloxy, mercapto, cyano, alkoxy, isocyanato
  • These functional groups are bound to the silicon atom via alkylene, alkenylene or arylene bridging groups which may be interrupted by oxygen atoms or -NH groups .
  • the bridging groups preferably contain from 1 to 18, particularly preferably from 1 to 8 and in particular from 1 to 6, carbon atoms.
  • the abovementioned divalent bridging groups and any substituents present, as in the case of the alkylamino groups, are derived, for example, from the abovementioned monovalent alkyl, alkenyl, aryl, alkaryl or aralkyl radicals. It is naturally also possible for the radical R to have more than one functional group.
  • nonhydrolysable radicals R having functional groups are a glycidyl or glycidyloxy- (C1-C20) - alkylene radical, e.g. beta-glycidyloxyethyl, gamma- glycidyloxypropyl, delta-glycidyloxybutyl, epsilon- glycidyloxypentyl, omega-glycidyloxyhexyl and 2- (3, 4- epoxycyclohexyl) ethyl, a (meth) acryloxy- (Ci-C ⁇ ) -alkylene radical, e.g. (meth) acryloxymethyl, (meth) acryloxyethyl, (meth) acryloxypropyl or (meth) acryloxybutyl, and a 3-isocyanatopropyl radical.
  • silanes are gamma-glycidyloxy- propyltrimethoxysilane (GPTS) , gamma-glycidyloxypropyl- triethoxysilane (GPTES) , 3-isocyanatopropyltriethoxy- silane, 3-isocyanatopropyldimethylchlorosilane,
  • GPTS gamma-glycidyloxy- propyltrimethoxysilane
  • GPTES gamma-glycidyloxypropyl- triethoxysilane
  • 3-isocyanatopropyltriethoxy- silane 3-isocyanatopropyldimethylchlorosilane
  • APTS 3-aminopropyltrimethoxysilane
  • APTES 3-aminopropyltri- ethoxysilane
  • N- [N' - (2 ' -aminoethyl) -2-aminoethyl] -3- aminopropyltrimethoxysilane hydroxymethyltriethoxy- silane
  • the content of silicon powder in the dispersion of the invention is not restricted. It depends first and foremost on the intended further use of the dispersion. If thin silicon layers are to be produced by means of the dispersion of the invention, a dispersion having a low content of silicon powder is to be preferred. On the other hand, in the case of thick layers a dispersion having a high content of silicon powder would be preferred.
  • the dispersion of the invention has, as shown in Figure 3 for the example of two 15% strength dispersions containing different silicon powders, only a low tendency to reagglomerate .
  • the dispersion of the invention preferably has a content of silicon powder of from 1 to 60% by weight, particularly preferably from 10 to 50% by weight and very particularly preferably from 15 to 40% by weight.
  • the viscosity of the dispersion of the invention can be chosen over a wide range. For reasons of further processability, preference is given to a dispersion which has a low viscosity in the range below 100 mPas at 23 0 C even at a high content of silicon powder.
  • a dispersion according to the invention having a viscosity of less than 10 mPas can be particularly preferred, with the viscosities being determined at 23 0 C and a shear rate of 1000 s '1 .
  • the liquid phase of the dispersion of the invention can be water. Furthermore, the liquid phase can comprise one or more organic solvents or mixtures of the abovementioned organic solvents with water.
  • the solubility of the solid silicon powder phase in the liquid phase should be minimal and preferably be close to zero.
  • Preferred organic solvents are ones whose structural formulae encompass one or more hydroxyl-containing compounds.
  • alkyls and diols are preferred.
  • Particular preference is given to mixtures of two or more alcohols having differing volatilities. Dispersions according to the invention containing such mixtures are particularly suitable for producing silicon layers.
  • the invention further provides a process in which the silicon powder is added all at once, in portions or continuously to the liquid phase under dispersing conditions .
  • Suitable dispersing apparatuses can be, for example, rotor- stator machines, high-energy mills in which the particles are milled by collision with one another, planetary kneaders, stirred ball mills, ball mills operating as shaking apparatuses, shaking tables, ultrasonic apparatuses or combinations of the abovementioned apparatuses .
  • the invention further provides a process for coating a substrate with a nanosize silicon layer having a thickness of from 1 nm to 5 ⁇ m using the dispersion of the invention.
  • the coating can be applied, for example, by means of spin coating, dipping, painting, spraying or doctor blade coating.
  • the invention further provides a process for coating a substrate with a closed, polycrystalline silicon layer by means of aluminium-induced layer exchange, in which a dispersion containing nanosize silicon powder which is predominantly present in crystalline form is firstly applied to the surface of an aluminium-coated substrate to form a substrate/aluminium/nanosize silicon powder layer structure, with the layer thickness of the silicon powder being greater than or equal to the layer thickness of the aluminium, and, after drying of the nanosize silicon layer, - the substrate/aluminium/nanosize silicon powder layer structure is heat treated at a temperature below the eutectic temperature of the mixture of silicon and aluminium in an inert atmosphere to form a substrate/polycrystalline silicon/aluminium layer structure, possibly substrate/polycrystalline silicon/ (aluminium + nanosize silicon) if the layer thickness of the silicon before the heat treatment is greater than that of the aluminium.
  • the polycrystalline silicon obtainable by this process is doped with aluminium.
  • the concentration of aluminium corresponds to the equilibrium solubility of aluminium in silicon at the reaction temperature used. At a reaction temperature of about 55O 0 C, a concentration of from 10 18 to 10 19 atoms of aluminium/cm 3 is to be expected.
  • Coating can be carried out, for example, by printing-on of aluminium nanoparticles, electrolytic deposition or thermal vapour deposition.
  • aluminium surface it can be advantageous in terms of the quality of the silicon layer on the substrate for the aluminium surface to be oxidized prior to the heat treatment.
  • This oxidation can be carried out during application of the aluminium layer to the substrate in the presence of oxygen (passivation of the surface) .
  • Aluminium oxide layers of differing thickness can be formed as a function of time, oxygen concentration and reaction temperature. These aluminium oxide layers are, regardless of the reaction conditions, present only at the surface or in layers close to the surface.
  • the heat treatment can preferably be carried out at from 300 0 C to 55O 0 C.
  • the reaction time can be from some minutes to some hours .
  • the dispersion of the invention can preferably be used as dispersion.
  • Any substrate which does not react with aluminium or with silicon and is stable at the reaction temperatures can be used in the process of the invention.
  • Glass, silicon dioxide, polycrystalline silicon, ceramic or an organic polymer can preferably be used.
  • the thickness of the layer of the nanosize silicon prior to the heat treatment is preferably from 1 nm to 5 ⁇ m in the process of the invention. Particular preference is given to a layer thickness of from 100 nm to 1 ⁇ m.
  • aluminium layer present on the polycrystalline silicon layer after the heat treatment and, if appropriate, the aluminium oxide layer and the nanosize silicon layer can be removed by pickling techniques known to those skilled in the art .
  • the invention further provides for the use of the substrate coated by means of the process of the invention for producing solar cells, thin-film transistors, diodes, sensors or conductor tracks .
  • nanosize silicon powder can be used as starting material for aluminium-induced layer exchange is surprising.
  • This exchange has hitherto only been known for the use of amorphous silicon.
  • the driving force for the crystallization is considered to be the Gibbs free energy between amorphous and crystalline phases.
  • a mechanism of the aluminium-induced layer exchange on which the invention is based is not yet known.
  • the process of the invention makes it possible to dispense with the complicated and time-consuming deposition of amorphous silicon in vacuum units.
  • the layers obtained by means of the process of the invention have very good conductivity properties and in particular high charge carrier mobilities and are therefore of interest for use in electronic applications .
  • a tube having a length of 200 cm and a diameter of 6 cm is used as hot-wall reactor. It is composed of fused silica or Si/SiC having a fused silica inliner.
  • the tube is externally heated to 1000 0 C over a zone of 100 cm by means of resistance heating.
  • the pressure in the reactor is 1080 mbar.
  • the pulverulent product is separated off from gaseous substances .
  • the powder obtained has a BET surface area of 10.5 m 2 /g.
  • the hydrogen loading is about 1.3 mol%. According to XRD and transmission electron micrographs, it is free of amorphous constituents .
  • High-surface-area silicon powder P2 Construction of apparatus: a microwave generator (from
  • Muegge is used for generating the plasma.
  • the microwave radiation is focused in the reaction space by means of a tuner (3-rod tuner) .
  • a tuner (3-rod tuner)
  • the fine adjustment by means of the tuner and the precise positioning of the nozzle which functions as electrode a stable plasma is generated in the pressure range from 10 mbar to 1100 mbar and a microwave power of from 100 to 6000 W.
  • the microwave reactor comprises a fused silica tube which has a diameter (external) of 30 mm and a length of 120 mm and is used in the plasma applicator.
  • the microwave reactor can be followed by a hot-wall reactor. A longer fused silica tube having a length of 600 mm is used for this purpose.
  • the mixture leaving the microwave reactor is heated by means of an externally heated zone (length about 300 mm) .
  • a power of 600 W is introduced into the gas mixture by means of a microwave generator and a plasma is generated thereby.
  • the plasma flame leaving the reactor via a nozzle expands into a space whose volume is about 20 1 and therefore large compared to the reactor.
  • the pressure in this space and in the reactor is regulated to 200 mbar.
  • the pulverulent powder is separated off from gaseous substances .
  • the hydrogen loading is about 1.5 mol%. According to XRD and transmission electron micrographs, it is free of amorphous constituents.
  • Dispersion 1 10.6 ml of ethanol are added to 0.0421 g of silicon powder Pl and the mixture is admixed with 10.6 ml (1:1) of SAZ beads and dispersed by means of a disperser (SkandexDisperser DAS 200, from Lau GmbH) for 4 h. The temperature of the dispersion is maintained at 25-35 0 C by means of cooling. The dispersion has a silicon content of 0.5% by weight. It is applied to a glass substrate by means of spin coating at 2000 rpm. This results in a layer thickness of 90 nm.
  • the dispersions 2 to 16 are produced analogously. Here, the amount of ethanol remains the same. Silicon contents of the dispersions and the thicknesses of the layers produced therefrom by means of spin coating are indicated in table 1. The viscosity of the dispersions as a function of the silicon content is indicated in table 2.
  • Table 1 shows that the thickness of the silicon layer increases proportionally with the concentration of silicon in the dispersions according to the invention.
  • Table 2 shows that the dispersions according to the invention have a low viscosity even at high silicon contents .
  • Table 1 Layer thicknesses as a function of the Si content
  • Figure 1 and Figure 2 show the D 50 values of the silicon particles in the dispersions according to the invention having a silicon content of 15% by weight as a function of the dispersion time.
  • the dispersing apparatus used is a ball mill operating as shaking apparatus (Skandex Disperser DAS 200, from Lau GmbH) .
  • Liquid phases employed are ethanol ( ⁇ ) , a 1 : 1 ethanol/terpineol mixture (O) and methyl ethyl ketone (A) .
  • Figure 1 shows the results when using dispersions containing the high-surface-area silicon powder P2. Dispersions having particle sizes D 50 of less than 50 nm can be obtained as a function of the liquid phase.
  • Figure 2 shows the results when using dispersions containing the low-surface-area silicon powder Pl.
  • the D 50 values achieved are in the range from about 100 to 150 nm, with the 1:1 ethanol/terpineol mixture giving particularly- low values .
  • Figure 3 shows the reagglomeration behaviour of dispersions according to the invention having a silicon content of 15% by weight over a period of 3 weeks.
  • the D 50 of the silicon particles determined by means of PCS is plotted on the Y axis.
  • the upper curve denoted by A describes the reagglomeration behaviour of the low-surface-area silicon powder Pl in ethanol.
  • the D 50 is about 125 nm and is virtually constant over the period of 3 weeks, i.e. no appreciable reagglomeration is observed.
  • the lower curve denoted by ⁇ describes the reagglomeration behaviour of the high-surface-area silicon powder P2 in ethanol .
  • the curve shows that dispersions having very small particle sizes can be produced. Although the dispersion is not stable in respect of reagglomeration, it still has D 50 values below 100 nm after 3 weeks.
  • a Heraeus fused silica sheet HOQ 310 having dimensions of 25 x 25 mm 2 and a thickness of 1 mm serves as substrate.
  • This substrate is coated using aluminium slugs (Praxair) having a purity of 99.9995% by means of thermal vapour deposition from an electrically resistance-heated tungsten crucible in a vacuum unit at a background pressure of 10 ⁇ 6 mbar.
  • the vapour deposition rate is 0.5-3 nm/s.
  • the thickness of the vapour-deposited layer is 250 nm.
  • a dispersion of the high-surface-area silicon powder P2 in an ethanol/terpineol mixture is applied to the aluminium- coated substrate by means of spin coating and is dried at about 6O 0 C in a drying oven.
  • the thickness of the silicon layer is 1.2 ⁇ m.
  • Layer exchange takes place at a temperature of 55O 0 C in a nitrogen atmosphere in a furnace over a period of 40 hours.
  • a coherent layer of polycrystalline silicon is formed (characterization by means of reflection spectroscopy and Raman spectroscopy) .
  • the fact that lateral current transport transversely across the specimen can be measured indicates that a continuous network of contacting crystallites is present.
  • the residues of aluminium and silicon remaining on the surface are subsequently removed by means of suitable pickling solutions, so that only the polycrystalline silicon layer remains on the insulating substrate .
  • the layers obtained have very good conductivity properties and in particular high charge carrier mobilities and are therefore of interest for use in electronic applications .
  • a Schottky diode was produced as first component.
  • silver contacts were applied to the polycrystalline silicon layer after layer exchange and the current-voltage characteristics between two contacts having a spacing of about 1 mm were measured.
  • a series resistance of 50 k ⁇ and an ideality factor of about 1.5 were observed.

Abstract

Dispersion containing silicon powder and a liquid phase, in which the silicon powder is predominantly in crystalline form and the particles in the dispersion have a mean diameter D50 of less than 500 nm and a BET surface area of more than 5 m2/g. Process for coating a substrate with a nanosize silicon layer having a thickness of from 1 nm to 5 μm using the dispersion. Process for coating a substrate with a closed, polycrystalline silicon layer by means of aluminium-induced layer exchange, in which a dispersion containing nanosize silicon powder is firstly applied to the surface of an aluminium-coated substrate and, after drying of the nanosize silicon layer, the layer structure composed of substrate, aluminium and nanosize silicon powder is thermally treated in an inert atmosphere at a temperature below the eutectic temperature of the mixture of silicon and aluminium.

Description

Dispersion containing silicon powder and coating process
The invention relates to a dispersion which contains nanosize silicon particles and to the production and use of this dispersion. The invention further relates to a process for producing a polycrystalline silicon layer.
Processes in which polycrystalline layers are applied to a substrate are known in the literature. These are generally deposition processes. Plasma-aided chemical gas-phase deposition processes such as PECVD and ECR-CVD are most frequently used. In addition, the hot wire technique and ion-aided deposition processes (IAD processes) have attained some importance. A disadvantage of all these processes is that they are very complicated, costly and not very efficient.
While in the abovementioned processes the silicon is applied directly to the substrate, the metal-induced layer exchange process starts out from a metal layer, generally aluminium, applied to the substrate.
Here, an aluminium film is firstly vapour-deposited onto a substrate. An amorphous silicon layer is subsequently applied by sputtering. In the heat treatment of this structure, silicon atoms diffuse into the aluminium layer and form crystalline silicon nuclei. These nuclei grow to become grains and displace the aluminium. Finally, adjacent grains form a continuous polycrystalline silicon layer on the substrate. The overall process is thus a layer exchange between aluminium and silicon. It is generally assumed that the driving force for the aluminium-induced layer exchange is the energy difference of the Gibbs potentials of amorphous silicon and crystalline silicon. Compared to the abovementioned pure deposition processes for producing polycrystalline silicon layers, the process of aluminium-induced layer exchange has advantages in terms of simplicity, temperatures required and, associated therewith, useability of the substrates .
A disadvantage is that the silicon layer has to be applied by vapour deposition or sputtering. This step is complicated and expensive.
The technical object of the present invention is therefore to provide a means which allows a silicon layer to be applied inexpensively to a substrate.
A further technical object is to provide a process for producing a polycrystalline silicon layer, which is faster and cheaper compared to the prior art.
The invention provides a dispersion containing silicon powder and a liquid phase, in which the silicon powder is predominantly in crystalline form and the particles in the dispersion have a mean diameter D50 of less than 500 nm and a BET surface area of more than 5 m2/g.
For the purposes of the present invention, "predominantly crystalline" means that the powder particles can contain proportions of amorphous silicon. This amorphous silicon can, for example, be detected by means of high-resolution transmission electron micrographs. In general, the region containing amorphous material is restricted to the regions close to the surface. The proportion of crystalline material is greater than 95%, in general greater than 99%. Silicon powder without any amorphous content can also be present in the dispersion of the invention.
The mean particle diameter D50 of the silicon powder present in the dispersion of the invention is less than 500 nm. A silicon powder having a mean particle diameter D50 of less than 500 nm is defined as nanosize for the purposes of the present invention. The mean particle diameter D50 can preferably be from 5 to 200 nm and particularly preferably from 50 to 150 nm.
Silicon powder containing crystallites having a mean particle diameter D50 of less than 500 nm is defined as nanocrystalline silicon powder for the purposes of the present invention. Polycrystalline silicon, on the other hand, has a crystal size in the range from about 1 μm to
1 mm.
The BET surface area of the silicon powder present in the dispersion of the invention is at least 5 m2/g. A silicon powder having a BET surface area of from 5 to 30 m2/g can be preferred.
Such a silicon powder can preferably be produced by heat treating a reaction mixture comprising at least one vaporized or gaseous silane and, if appropriate, at least one vaporized or gaseous dopant in the presence of an inert gas and hydrogen in a hot-wall reactor. The proportion of silane is from 0.1 to 90% by weight, based on the sum of silane, dopant, hydrogen and inert gases. The proportion of hydrogen, based on the sum of hydrogen, silane, inert gas and any dopant, is from 1 mol% to 96 mol%. In general, the residence time in the hot-wall reactor is from 0.1 s to
2 s . The maximum temperature in the hot-wall reactor should not exceed 10000C.
As silanes, preference is given to using SiH4, Si2Hg, ClSiH3, Cl2SiH2, Cl3SiH and/or SiCl4, with SiH4 being particularly preferred. The present invention fully incorporates the contents of the German patent application number 10353996.4 filed on 19 November 2003 by reference.
Furthermore, a silicon powder having a BET surface area of from 50 to 700 m2/g can be advantageous. Such a powder can preferably be produced by reaction of a vaporized or gaseous silane and, if appropriate, at least one vaporized or gaseous dopant by means of electromagnetic radiation in the microwave region at a pressure of from 10 to 1100 mbar in a plasma in the presence of an inert gas . The proportion of silane is from 0.1 to 90% by weight, preferably from 1 to 10% by weight, based on the sum of silane, dopant and inert gases. The reaction mixture is allowed to cool or is cooled and the reaction product is separated off in the form of a powder from gaseous substances. In general, the conversion of silane is at least 98%. As silanes, preference is given to using SiH4, Si2H6, ClSiH3, Cl2SiH2,
Cl3SiH and/or SiCl4, with particular preference being given to SiH4.
The power input is not limited. It is preferably selected so that the reflected microwave power which has not been absorbed is minimal and a stable plasma is formed. In general, the energy input is from 100 W to 100 KW, particularly preferably from 500 W to 6 KW. The particle size distribution can be varied by means of the microwave power injected. Thus, at the same gas compositions and volume flows, higher microwave powers can lead to a smaller particle size and a narrower particle size distribution.
For the purposes of the invention, the microwave region is a region from 900 MHz to 2.5 GHz, with a frequency of 915 MHz being particularly preferred. Preference is given to hydrogen, if appropriate in admixture with an inert gas, being additionally introduced into the reaction. The proportion of hydrogen can be in a range from 1 to 96% by volume. Furthermore, the process can be carried out with the reaction mixture obtained after the microwave treatment being thermally after-treated. A wall-heated hot-wall reactor is particularly advantageous for this purpose. The dimensions of the hot-wall reactor should be such that the residence time in it is from 0.1 s to 2 s. The maximum temperature in the hot-wall reactor should not exceed 10000C. Apart from the thermal after-treatment of the reaction mixture, it is likewise possible for the silicon powder itself to be thermally after-treated. The present invention fully incorporates the contents of the German patent application number 10353995.6 filed on 19 November 2003 by reference.
The silicon powder present in the dispersion of the invention can be present in unaggregated, predominantly aggregated or completely aggregated form. For the present purposes, "predominantly aggregated" means that some individual unaggregated particles are present in addition to the aggregates. The silicon powder is preferably present in aggregated or predominantly aggregated form in the dispersion of the invention.
Furthermore, preference is given to silicon powders whose particles are nonporous in the dispersion of the invention. This means that the particles have no continuous pores.
The silicon powder present in the dispersion of the invention can be doped. Furthermore, the silicon powder according to the invention can be doped. Preferred dopants are, particularly for use as semiconductor in electronic components, the elements phosphorus, arsenic, antimony, bismuth, boron, aluminium, gallium, indium, thallium, europium, erbium, cerium, praseodymium, neodymium, samarium, gadolinium, terbium, dysprosium, holmium, thulium, ytterbium, lutetium. The proportion of these in the silicon powder according to the invention can be up to 1% by weight. In general, a silicon powder in which the doping component is present in the ppm or even ppb range will be desired. Preference is given to a range of from 1013 to 1015 atoms of doping component/cm3.
The particles of the silicon powder present in the dispersion of the invention can contain bonds of the Si-H, Si-O and/or Si-OH type on their surface. Such bonds can be detected, for example, by means of IR spectra. A silicon powder which contains both hydrogen-terminated and oxygen- terminated silicon atoms is preferred for the dispersion of the invention.
The content of hydrogen on the surface of the silicon powder can preferably be up to 1 mol%.
Furthermore, bonds of the Si-H, Si-O and/or Si-OH type represent reactive sites for surface modification of silicon powders. Thus, the Si-H bonds present in the silicon powder can react partly or completely with alkenes in a hydrosilylation reaction to form Si-C bonds. As alkenes, it is possible to use aliphatic alkenes, unsaturated (di) carboxylic acids, unsaturated fatty acids or alkenes having aromatic radicals. Aliphatic alkenes having from 3 to 20 carbon atoms are particularly suitable. Examples of suitable alkenes are: 1-octadecene, 1-octene, 1-hexene, 1-butene, styrene, alpha-methylstyrene, acrylic acid, methacrylic acid, itaconic acid and oleic acid.
Alkynes can be used analogously to the alkenes for the hydrosilylation. Here, alkenyl-Si bonds are formed. Ethynylbenzene can, for example, serve as alkyne. However, a combination of alkenyl-Si bonds and surface-bonded functional groups can also be achieved by means of functionalized alkynes in a manner analogous to the introduction of functional groups in the case of alkenes .
In general, the hydrosilylation reaction can also be used for introducing functional groups on the surface, for example sulphonate groups by means of vinylsulphonate or methallylsulphonate, and also OH groups by means of unsaturated alcohols, for example hexenol .
Furthermore, the Si-OH bonds present in the silicon powder can react with suitable substances to form a chemical bond. The chemical bond can be a covalent, ionic or coordinate bond between the surface modifier and the particle, but hydrogen bonds are also possible. For the present purposes, a coordinate bond means complex formation. Thus, for example, an acid/base reaction of the Brδnsted or Lewis type, complex formation or an esterification can take place between the functional groups of the modifier and the particle .
Suitable substances can be, for example: a) saturated or unsaturated monocarboxylic and polycarboxylic acids having from 1 to 24 carbon atoms and also the corresponding acid anhydrides, chlorides, esters and amides and their salts, b) monoamines, polyamines, polyalkylenamines, for example methylamine, dimethylamine, trimethylamine, ethylamine, aniline, N-methylaniline, diphenylamine, triphenylamine, toluidine, ethylenediamine, diethylenetriamine, c) beta-dicarbonyl compounds such as acetylacetone, 2,4- hexanedione, 3, 5-heptanedione, acetoacetic acid, Ci-C4- alkyl acetoacetates and acetonylacetone, d) amino acids such as beta-alanine, glycine, valine, aminocaproic acid, leucine and isoleucine, e) silanes which have at least one nonhydrolysable group or a hydroxy group, in particular hydrolysable organo- silanes which additionally have at least one nonhydrolysable radical. Silanes of the formula RaSiX4-a, where the radicals R are identical or different and are nonhydrolysable groups, the radicals X are identical or different and are hydrolysable groups or hydroxy groups and a is 1, 2 or 3, can preferably serve as surface- modifying reagent. The value a is preferably 1. In the general formula, the hydrolysable groups X, which can be identical or different, are, for example, hydrogen or halogen (F, Cl, Br or I) , alkoxy (preferably Ci-Cβ- alkoxy such as methoxy, ethoxy, n-propoxy, i-propoxy and butoxy) , aryloxy (preferably Cβ-Cio-aryloxy such as phenoxy) , acyloxy (preferably Ci-C6-acyloxy such as acetoxy or propionyloxy) , alkylcarbonyl (preferably C2- C7-alkylcarbonyl such as acetyl) , amino, monoalkylamino or dialkylamino preferably having from 1 to 12, in particular from 1 to 6, carbon atoms. Preferred hydrolysable radicals are halogen, alkoxy groups and acyloxy groups . Particularly preferred hydrolysable radicals are Ci-C4-alkoxy groups, in particular methoxy and ethoxy. The nonhydrolysable radicals R, which can be identical or different, can be nonhydrolysable radicals R with or without a functional group. The nonhydrolysable radical R without a functional group can be, for example, alkyl (preferably Ci-Cg-alkyl such as methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl and tert-butyl, pentyl, hexyl, octyl or cyclohexyl) , alkenyl (preferably C2-C6-alkenyl such as vinyl, 1-propenyl, 2-propenyl and butenyl) , alkynyl (preferably C2-C6-alkynyl such as acetylenyl and propargyl) , aryl (preferably C6-Cio-aryl such as phenyl and naphthyl) and corresponding alkaryls and aralkyls (e.g. tolyl, benzyl and phenethyl) . The radicals R and X may bear one or more customary substituents such as halogen or alkoxy. Preference is given to alkyltrialkoxysilanes . Examples are : CH3SiCl3, CH3Si (OC2H5) 3, CH3Si (OCH3) 3, C2H5SiCl3, C2H5Si(OC2Hs)3, C2H5Si(OCH3)3, C3H7Si(OC2Hs)3, (C2H5O)3SiC3H6Cl, (CHs)2SiCl2, (CH3) 2Si (OC2H5) 2, (CH3)2Si(OH)2, C6H5Si(OCHs)3, C6H5Si(OC2Hs)3, C6H5CH2CH2Si(OCH3)S, (C6Hs)2SiCl2, (C6H5) 2Si (OC2H5) 2, (i-C3H7)3SiOH, CH2=CHSi (0OCCH3) 3, CH2=CHSiCl3,
CH2=CH-Si(OC2H5)S, CH2=CHSi(OC2Hs)3, CH2=CH-Si(OC2H4OCHs)3, CH2=CH-CH2-Si (OC2H5) 3, CH2=CH-CH2-Si (OC2H5) 3, CH2=CH-CH2Si(OOOCHs)3, n-C6H13-CH2-CH2-Si (OC2H5) 3 and n-C8H17-CH2CH2-Si (OC2H5) 3. The nonhydrolysable radical R having a functional group can, for example, comprise an epoxide (e.g. glycidyl or glycidyloxy) , hydroxy, ether, amino, monoalkylamino, dialkylamino, substituted or unsubstituted anilino, amido, carboxy, acryl, acryloxy, methacryl, methacryloxy, mercapto, cyano, alkoxy, isocyanato, aldehyde, alkylcarbonyl, acid anhydride or phosphoric acid group as functional group. These functional groups are bound to the silicon atom via alkylene, alkenylene or arylene bridging groups which may be interrupted by oxygen atoms or -NH groups . The bridging groups preferably contain from 1 to 18, particularly preferably from 1 to 8 and in particular from 1 to 6, carbon atoms.
The abovementioned divalent bridging groups and any substituents present, as in the case of the alkylamino groups, are derived, for example, from the abovementioned monovalent alkyl, alkenyl, aryl, alkaryl or aralkyl radicals. It is naturally also possible for the radical R to have more than one functional group.
Preferred examples of nonhydrolysable radicals R having functional groups are a glycidyl or glycidyloxy- (C1-C20) - alkylene radical, e.g. beta-glycidyloxyethyl, gamma- glycidyloxypropyl, delta-glycidyloxybutyl, epsilon- glycidyloxypentyl, omega-glycidyloxyhexyl and 2- (3, 4- epoxycyclohexyl) ethyl, a (meth) acryloxy- (Ci-Cβ) -alkylene radical, e.g. (meth) acryloxymethyl, (meth) acryloxyethyl, (meth) acryloxypropyl or (meth) acryloxybutyl, and a 3-isocyanatopropyl radical.
Examples of corresponding silanes are gamma-glycidyloxy- propyltrimethoxysilane (GPTS) , gamma-glycidyloxypropyl- triethoxysilane (GPTES) , 3-isocyanatopropyltriethoxy- silane, 3-isocyanatopropyldimethylchlorosilane,
3-aminopropyltrimethoxysilane (APTS) , 3-aminopropyltri- ethoxysilane (APTES), N- (2-aminoethyl) -3-aminopropyltrimethoxysilane, N- [N' - (2 ' -aminoethyl) -2-aminoethyl] -3- aminopropyltrimethoxysilane, hydroxymethyltriethoxy- silane, 2- [methoxy (polyethylenoxy) propyl] trimethoxy- silane, bis (hydroxyethyl) -3-aminopropyltriethoxysilane, N-hydroxyethyl-N-methylaminopropyltriethoxysilane, 3- (meth) acryloxypropyltriethoxysilane and 3- (meth) acryloxypropyltrimethoxysilane .
The content of silicon powder in the dispersion of the invention is not restricted. It depends first and foremost on the intended further use of the dispersion. If thin silicon layers are to be produced by means of the dispersion of the invention, a dispersion having a low content of silicon powder is to be preferred. On the other hand, in the case of thick layers a dispersion having a high content of silicon powder would be preferred.
Furthermore, the stability of the dispersion of the invention against settling and reagglomeration has to be taken into account. The dispersion of the invention has, as shown in Figure 3 for the example of two 15% strength dispersions containing different silicon powders, only a low tendency to reagglomerate .
The dispersion of the invention preferably has a content of silicon powder of from 1 to 60% by weight, particularly preferably from 10 to 50% by weight and very particularly preferably from 15 to 40% by weight.
The viscosity of the dispersion of the invention can be chosen over a wide range. For reasons of further processability, preference is given to a dispersion which has a low viscosity in the range below 100 mPas at 230C even at a high content of silicon powder. A dispersion according to the invention having a viscosity of less than 10 mPas can be particularly preferred, with the viscosities being determined at 230C and a shear rate of 1000 s'1.
The liquid phase of the dispersion of the invention can be water. Furthermore, the liquid phase can comprise one or more organic solvents or mixtures of the abovementioned organic solvents with water. The solubility of the solid silicon powder phase in the liquid phase should be minimal and preferably be close to zero.
Preferred organic solvents are ones whose structural formulae encompass one or more hydroxyl-containing compounds. In particular, alkyls and diols are preferred. Particular preference is given to mixtures of two or more alcohols having differing volatilities. Dispersions according to the invention containing such mixtures are particularly suitable for producing silicon layers.
The invention further provides a process in which the silicon powder is added all at once, in portions or continuously to the liquid phase under dispersing conditions .
Suitable dispersing apparatuses can be, for example, rotor- stator machines, high-energy mills in which the particles are milled by collision with one another, planetary kneaders, stirred ball mills, ball mills operating as shaking apparatuses, shaking tables, ultrasonic apparatuses or combinations of the abovementioned apparatuses .
The invention further provides a process for coating a substrate with a nanosize silicon layer having a thickness of from 1 nm to 5 μm using the dispersion of the invention. The coating can be applied, for example, by means of spin coating, dipping, painting, spraying or doctor blade coating.
The invention further provides a process for coating a substrate with a closed, polycrystalline silicon layer by means of aluminium-induced layer exchange, in which a dispersion containing nanosize silicon powder which is predominantly present in crystalline form is firstly applied to the surface of an aluminium-coated substrate to form a substrate/aluminium/nanosize silicon powder layer structure, with the layer thickness of the silicon powder being greater than or equal to the layer thickness of the aluminium, and, after drying of the nanosize silicon layer, - the substrate/aluminium/nanosize silicon powder layer structure is heat treated at a temperature below the eutectic temperature of the mixture of silicon and aluminium in an inert atmosphere to form a substrate/polycrystalline silicon/aluminium layer structure, possibly substrate/polycrystalline silicon/ (aluminium + nanosize silicon) if the layer thickness of the silicon before the heat treatment is greater than that of the aluminium.
The polycrystalline silicon obtainable by this process is doped with aluminium. The concentration of aluminium corresponds to the equilibrium solubility of aluminium in silicon at the reaction temperature used. At a reaction temperature of about 55O0C, a concentration of from 1018 to 1019 atoms of aluminium/cm3 is to be expected.
Coating can be carried out, for example, by printing-on of aluminium nanoparticles, electrolytic deposition or thermal vapour deposition.
It has been found that it can be advantageous in terms of the quality of the silicon layer on the substrate for the aluminium surface to be oxidized prior to the heat treatment. This oxidation can be carried out during application of the aluminium layer to the substrate in the presence of oxygen (passivation of the surface) . Aluminium oxide layers of differing thickness can be formed as a function of time, oxygen concentration and reaction temperature. These aluminium oxide layers are, regardless of the reaction conditions, present only at the surface or in layers close to the surface. The heat treatment can preferably be carried out at from 3000C to 55O0C.
The reaction time can be from some minutes to some hours .
The dispersion of the invention can preferably be used as dispersion.
Any substrate which does not react with aluminium or with silicon and is stable at the reaction temperatures can be used in the process of the invention. Glass, silicon dioxide, polycrystalline silicon, ceramic or an organic polymer can preferably be used.
The thickness of the layer of the nanosize silicon prior to the heat treatment is preferably from 1 nm to 5 μm in the process of the invention. Particular preference is given to a layer thickness of from 100 nm to 1 μm.
The aluminium layer present on the polycrystalline silicon layer after the heat treatment and, if appropriate, the aluminium oxide layer and the nanosize silicon layer can be removed by pickling techniques known to those skilled in the art .
The invention further provides for the use of the substrate coated by means of the process of the invention for producing solar cells, thin-film transistors, diodes, sensors or conductor tracks .
The fact that nanosize silicon powder can be used as starting material for aluminium-induced layer exchange is surprising. This exchange has hitherto only been known for the use of amorphous silicon. The driving force for the crystallization is considered to be the Gibbs free energy between amorphous and crystalline phases. A mechanism of the aluminium-induced layer exchange on which the invention is based is not yet known. The process of the invention makes it possible to dispense with the complicated and time-consuming deposition of amorphous silicon in vacuum units.
The layers obtained by means of the process of the invention have very good conductivity properties and in particular high charge carrier mobilities and are therefore of interest for use in electronic applications .
Examples :
Powder :
Low-surface-area silicon powder Pl:
Construction of apparatus: a tube having a length of 200 cm and a diameter of 6 cm is used as hot-wall reactor. It is composed of fused silica or Si/SiC having a fused silica inliner. The tube is externally heated to 10000C over a zone of 100 cm by means of resistance heating.
An SiH4/argon mixture composed of 2000 seem of silane (standard cubic centimetres per minute; 1 seem = 1 cm3 of gas per minute, at O0C and atmospheric pressure) and 1000 seem of argon and also a stream of 5000 seem of argon are fed via a two-fluid nozzle into the hot-wall reactor from the top. The pressure in the reactor is 1080 mbar. In a downstream filter unit, the pulverulent product is separated off from gaseous substances .
The powder obtained has a BET surface area of 10.5 m2/g. The hydrogen loading is about 1.3 mol%. According to XRD and transmission electron micrographs, it is free of amorphous constituents .
High-surface-area silicon powder P2 : Construction of apparatus: a microwave generator (from
Muegge) is used for generating the plasma. The microwave radiation is focused in the reaction space by means of a tuner (3-rod tuner) . As a result of the design of the hollow waveguide, the fine adjustment by means of the tuner and the precise positioning of the nozzle which functions as electrode, a stable plasma is generated in the pressure range from 10 mbar to 1100 mbar and a microwave power of from 100 to 6000 W.
The microwave reactor comprises a fused silica tube which has a diameter (external) of 30 mm and a length of 120 mm and is used in the plasma applicator. The microwave reactor can be followed by a hot-wall reactor. A longer fused silica tube having a length of 600 mm is used for this purpose. The mixture leaving the microwave reactor is heated by means of an externally heated zone (length about 300 mm) . An SiH4/argon mixture composed of 100 seem (standard cubic centimetres per minute; 1 seem = 1 cm3 of gas per minute, at O0C and atmospheric pressure) and 900 seem of argon and also a mixture of argon and hydrogen, each 10 000 seem, are fed via a two-fluid nozzle into the microwave reactor. A power of 600 W is introduced into the gas mixture by means of a microwave generator and a plasma is generated thereby. The plasma flame leaving the reactor via a nozzle expands into a space whose volume is about 20 1 and therefore large compared to the reactor. The pressure in this space and in the reactor is regulated to 200 mbar. In a downstream filter unit, the pulverulent powder is separated off from gaseous substances .
The powder obtained has the following properties : BET surface area = 170 m2/g. The hydrogen loading is about 1.5 mol%. According to XRD and transmission electron micrographs, it is free of amorphous constituents.
Dispersions
Dispersion 1: 10.6 ml of ethanol are added to 0.0421 g of silicon powder Pl and the mixture is admixed with 10.6 ml (1:1) of SAZ beads and dispersed by means of a disperser (SkandexDisperser DAS 200, from Lau GmbH) for 4 h. The temperature of the dispersion is maintained at 25-350C by means of cooling. The dispersion has a silicon content of 0.5% by weight. It is applied to a glass substrate by means of spin coating at 2000 rpm. This results in a layer thickness of 90 nm.
The dispersions 2 to 16 are produced analogously. Here, the amount of ethanol remains the same. Silicon contents of the dispersions and the thicknesses of the layers produced therefrom by means of spin coating are indicated in table 1. The viscosity of the dispersions as a function of the silicon content is indicated in table 2.
Table 1 shows that the thickness of the silicon layer increases proportionally with the concentration of silicon in the dispersions according to the invention.
Table 2 shows that the dispersions according to the invention have a low viscosity even at high silicon contents .
Table 1 : Layer thicknesses as a function of the Si content
Dispersion Silicon content Layer thickness [% by weight] [nm]
1 0.5 90
2 1.75 115
3 2.78 170
4 4.17 230
5 5.56 300
6 9 450
7 12.5 620
8 16.7 800
9 25 1330
10 30 1660
11 35 2120 Table 2 : Viscosity8' as a function of the Si content
Figure imgf000018_0001
a) determined using a Haake RS 75 rheometer at 23 C
Figure 1 and Figure 2 show the D50 values of the silicon particles in the dispersions according to the invention having a silicon content of 15% by weight as a function of the dispersion time. The dispersing apparatus used is a ball mill operating as shaking apparatus (Skandex Disperser DAS 200, from Lau GmbH) .
Liquid phases employed are ethanol (■) , a 1 : 1 ethanol/terpineol mixture (O) and methyl ethyl ketone (A) .
Figure 1 shows the results when using dispersions containing the high-surface-area silicon powder P2. Dispersions having particle sizes D50 of less than 50 nm can be obtained as a function of the liquid phase.
Figure 2 shows the results when using dispersions containing the low-surface-area silicon powder Pl. The D50 values achieved are in the range from about 100 to 150 nm, with the 1:1 ethanol/terpineol mixture giving particularly- low values .
Figure 3 shows the reagglomeration behaviour of dispersions according to the invention having a silicon content of 15% by weight over a period of 3 weeks. The D50 of the silicon particles determined by means of PCS is plotted on the Y axis. The upper curve denoted by A describes the reagglomeration behaviour of the low-surface-area silicon powder Pl in ethanol. The D50 is about 125 nm and is virtually constant over the period of 3 weeks, i.e. no appreciable reagglomeration is observed. The lower curve denoted by ■ describes the reagglomeration behaviour of the high-surface-area silicon powder P2 in ethanol . The curve shows that dispersions having very small particle sizes can be produced. Although the dispersion is not stable in respect of reagglomeration, it still has D50 values below 100 nm after 3 weeks.
Aluminium-induced layer exchange:
A Heraeus fused silica sheet HOQ 310 having dimensions of 25 x 25 mm2 and a thickness of 1 mm serves as substrate. This substrate is coated using aluminium slugs (Praxair) having a purity of 99.9995% by means of thermal vapour deposition from an electrically resistance-heated tungsten crucible in a vacuum unit at a background pressure of 10~6 mbar. The vapour deposition rate is 0.5-3 nm/s. The thickness of the vapour-deposited layer is 250 nm. A dispersion of the high-surface-area silicon powder P2 in an ethanol/terpineol mixture is applied to the aluminium- coated substrate by means of spin coating and is dried at about 6O0C in a drying oven. The thickness of the silicon layer is 1.2 μm.
Layer exchange takes place at a temperature of 55O0C in a nitrogen atmosphere in a furnace over a period of 40 hours. A coherent layer of polycrystalline silicon is formed (characterization by means of reflection spectroscopy and Raman spectroscopy) . The fact that lateral current transport transversely across the specimen can be measured indicates that a continuous network of contacting crystallites is present. The residues of aluminium and silicon remaining on the surface are subsequently removed by means of suitable pickling solutions, so that only the polycrystalline silicon layer remains on the insulating substrate .
The layers obtained have very good conductivity properties and in particular high charge carrier mobilities and are therefore of interest for use in electronic applications . Hall measurements on the crystallized layers gave hole concentrations of p = 1.7 x 1018 cm"3, as would be expected from the solubility of aluminium in silicon at the process temperature. Hall mobilities of the charge carriers of 20- 45 cm2/Vs were observed.
A Schottky diode was produced as first component. For this purpose, silver contacts were applied to the polycrystalline silicon layer after layer exchange and the current-voltage characteristics between two contacts having a spacing of about 1 mm were measured. A series resistance of 50 kΩ and an ideality factor of about 1.5 were observed.

Claims

Claims :
1. Dispersion containing silicon powder and a liquid phase, characterized in that the silicon powder is predominantly in crystalline form and the particles in the dispersion have a mean diameter D50 of less than 500 nm and a BET surface area of more than 5 m2/g.
2. Dispersion according to Claim 1, characterized in that the silicon powder is completely in crystalline form.
3. Dispersion according to Claim 1 or 2, characterized in that the mean particle diameter D50 of the silicon powder is from 5 to 200 nm.
4. Dispersion according to any of Claims 1 to 3, characterized in that the BET surface area of the silicon powder is from 5 to 30 m2/g.
5. Dispersion according to any of Claims 1 to 3, characterized in that the BET surface area of the silicon powder is from 50 to 700 m2/g.
6. Dispersion according to any of Claims 1 to 5, characterized in that the silicon powder is in unaggregated, predominantly aggregated or completely aggregated form.
7. Dispersion according to any of Claims 1 to 6, characterized in that the silicon powder is nonporous .
8. Dispersion according to any of Claims 1 to 7, characterized in that the silicon powder is doped.
9. Dispersion according to any of Claims 1 to 8, characterized in that the suface of the silicon particles contains bonds of the Si-H, Si-O, Si-C and/or Si-OH type.
10. Dispersion according to any of Claims 1 to 9, characterized in that the content of silicon powder is from 1 to 60% by weight.
11. Dispersion according to any of Claims 1 to 10, characterized in that it has a viscosity at 230C of less than 100 mPas, particularly preferably less than 10 mPas, at 1000 s"1.
12. Dispersion according to any of Claims 1 to 11, characterized in that water and/or one or more organic solvents are constituent of the liquid phase.
13. Process for producing the dispersion according to any of Claims 1 to 12, characterized in that the silicon powder is added all at once, in portions or continuously to the liquid phase under dispersing conditions.
14. Process for coating a substrate with a nanosize silicon layer having a thickness of from 1 nm to 5 μm using the dispersion according to any of Claims 1 to 12.
15. Process for coating a substrate with a closed, polycrystalline silicon layer by means of aluminium- induced layer exchange, in which a dispersion containing nanosize silicon powder which is predominantly present in crystalline form is firstly applied to the surface of an aluminium-coated substrate to form a substrate/aluminium/nanosize silicon powder layer structure, with the layer thickness of the silicon powder being greater than or equal to the layer thickness of the aluminium, and, after drying of the nanosize silicon layer, the substrate/aluminium/nanosize silicon powder layer structure is heat treated at a temperature below the eutectic temperature of the mixture of silicon and aluminium in an inert atmosphere to form a substrate/polycrystalline silicon/aluminium layer structure, possibly substrate/polycrystalline silicon/ (aluminium + nanosize silicon) if the layer thickness of the silicon before the heat treatment is greater than that of the aluminium.
16. Process according to Claim 15, characterized in that the aluminium surface is oxidized prior to the heat treatment.
17. Process according to Claim 15 or 16, characterized in that the heat treatment is carried out at from 3000C to 55O0C.
18. Process according to any of Claims 15 to 17, characterized in that the dispersion according to any of Claims 1 to 16 is used.
19. Process according to any of Claims 15 to 18, characterized in that the substrate is glass, silicon dioxide, polycrystalline silicon, ceramic or an organic polymer.
20. Process according to any of Claims 15 to 19, characterized in that the thickness of the layer of the nanosize silicon prior to the heat treatment is from 1 nm to 5 μm.
21. Process according to any of Claims 15 to 20, characterized in that the aluminium layer present on the polycrystalline silicon layer after the heat treatment and, if appropriate, the aluminium oxide layer and the nanosize silicon layer are removed by pickling.
22. The use of the substrate coated according to any of Claims 15 to 21 for producing solar cells, thin-film transistors, diodes, sensors or conductor tracks.
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