EP1618612A1 - Sintered semiconductor material - Google Patents

Sintered semiconductor material

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Publication number
EP1618612A1
EP1618612A1 EP04742838A EP04742838A EP1618612A1 EP 1618612 A1 EP1618612 A1 EP 1618612A1 EP 04742838 A EP04742838 A EP 04742838A EP 04742838 A EP04742838 A EP 04742838A EP 1618612 A1 EP1618612 A1 EP 1618612A1
Authority
EP
European Patent Office
Prior art keywords
powders
heat treatment
silicon
semiconductor
present
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP04742838A
Other languages
German (de)
French (fr)
Inventor
Alain Straboni
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Centre National de la Recherche Scientifique CNRS
Universite de Poitiers
Original Assignee
Centre National de la Recherche Scientifique CNRS
Universite de Poitiers
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Centre National de la Recherche Scientifique CNRS, Universite de Poitiers filed Critical Centre National de la Recherche Scientifique CNRS
Publication of EP1618612A1 publication Critical patent/EP1618612A1/en
Withdrawn legal-status Critical Current

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Classifications

    • 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
    • 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
    • H01L31/02—Details
    • H01L31/0236—Special surface textures
    • H01L31/02363—Special surface textures of the semiconductor body itself, e.g. textured active layers
    • 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
    • Y02E10/547—Monocrystalline 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

  • FIG. 1 represents a conventional photovoltaic cell 1.
  • the photovoltaic cell 1 comprises a plane semiconductor material 3.
  • the material 3 generally made of polycrystalline silicon, comprises three different doping zones.
  • a thick central zone 3a is lightly doped with type P.
  • An upper zone 3b is doped with type N, and possibly overdoped on the surface.
  • a lower zone 3c is heavily doped with the P (P + ) type.
  • An aluminum layer 6 covers the underside of the cell.
  • the comb 5 and the layer 6 are both intended for transmitting the photovoltaic current and are connected to the terminals + and - not shown of the cell.
  • the material 3 conventionally comes from a polycrystalline silicon bar obtained from a silicon bath molten. The bar is sawn to obtain wafers which are then doped to obtain the material 3. This manufacturing process, close to the process for manufacturing monocrystalline silicon wafers, is expensive and limits the possible dimensions of the wafers.
  • the dopants- migrating through the porosity channels and spreading throughout the material are required at least a thousand times higher before the material can be used in a solar cell.
  • the surface of the materials obtained is uncontrolled and rough. Such a surface condition prevents the prediction of surface junctions, necessarily bad, in particular because of large leakage currents.
  • An object of the present invention is to produce a semiconductor material or a component by sintering semiconductor powders usable in the electronic field, in particular in the photovoltaic field.
  • Another object of the present invention is to produce a semiconductor material by sintering semiconductor powders having low roughness and / or a surface condition with controlled texturing.
  • the present invention provides a method of forming a semiconductor material from powders comprising at least one constituent belonging to the group consisting of the elements of column IV of the Mendeleev table and their alloys.
  • the method comprises a step of compressing said powders and a step of heat treatment such that at least part of the powders is melted or made viscous.
  • the compression and heat treatment steps are simultaneous.
  • the heat treatment is such that only powders belonging to a particular zone of the material are melted or made viscous.
  • the powders comprise silicon powders and powders of at least one other constituent, the heat treatment being such that the silicon is not molten and that at least one of the other constituents is melted or made viscous.
  • the powders comprise doped semiconductor powders and undoped semiconductor powders, the heat treatment being such that only the doped powders are melted.
  • the compression step is preceded by a step consisting in placing powders on a tray, the powders being different in their nature, their particle size and / or their doping according to their location. on the tray.
  • the present invention provides a semiconductor material obtained at least partially by compression and heat treatment of powders comprising at least two distinct zones formed of distinct constituents belonging to the group consisting of the elements of column IV of the Mendeleev table and their alloys.
  • said zones are superimposed.
  • the present invention also provides a structure or a component formed of or comprising at least one semiconductor material comprising grains and / or aggregates having prohibited bands of different value.
  • FIG. 1 represents a cell conventional photovoltaics
  • FIG. 2 illustrates an embodiment of the method according to the present invention
  • Figure 3 shows a material according to the present invention
  • Figure 4 shows a structure according to the present invention
  • Figures 5a and 5b illustrate other embodiments of the method according to the present invention
  • Figures 6, 7A to 7C illustrate ways of doping a material according to the present invention
  • Figures 8, 9 and 10 show materials according to the present invention.
  • FIG. 2 illustrates an embodiment of the method according to the present invention.
  • An upper plate 20 covers the powders 15.
  • the assembly is placed in a treatment enclosure and the layer of semiconductor powders 15 is compacted by application of a pressure P.
  • the compaction can be carried out by cold compression, that is to say that is to say at ambient temperature, or by hot compression, at a temperature T, for example between 950 and 1300 ° C.
  • the sintering is carried out at least partially in the liquid phase, that is to say that, during or after compression, a heat treatment is applied such that at least part of the powders is melted.
  • a heat treatment is applied such that at least part of the powders is melted.
  • F the letter F in Figure 2.
  • the terms “liquid phase” and “fusion” should be understood in a broad sense. As will be seen hereinafter, the expression “liquid phase” can also designate a viscous phase corresponding to an supercooled liquid, the term “fusion” then designating “supercooling”.
  • Partial melting can be carried out selectively, for example depending on the area of the material, the nature of the powders, or according to the heating means used.
  • the porosity is substantially zero (in practice, less than 0.2%). Also, the merger results in an increase in the size of the grains, which is desirable, the obstacle to the movement of the carriers created by the grain boundaries then being reduced. Although this is possible, it is not necessary for the entire material to be sintered in the liquid phase. In fact, during his research, the inventor realized that the characteristics of a material intended to form a solar cell did not need to be homogeneous throughout the material.
  • the so-called "absorber” part that is to say the zone intended to transform the photons received into electron-hole pairs, must have a very high quality microstructure, namely a porosity s '' getting as close to zero as possible and the largest possible grain size.
  • the part forming the junction is to say the zone intended to transform the photons received into electron-hole pairs.
  • zones are, for example, the heavily doped N-type or P-type conductive zones which act as contact with the N and P zones of the junction. It suffices that these zones have sufficient conductivity, and a porosity as large as 40 or 50% may be sufficient.
  • the heat treatment can be carried out so as to selectively cause a fusion only in the areas where a quality microstructure is desired.
  • the present invention it is possible to easily control the morphology of the surface of the material obtained. Indeed, especially when the partial melting step takes place during compression, the surface of the material faithfully reproduces the surface of the plates 10 and 20. With flat and smooth plates, the surface, analyzed by electron microscopy, appears like a plain plane with very low roughness. It will also be noted that an advantage of carrying out a hot compression of the powders rather than a cold compression makes it possible to obtain a material having a low overall porosity in a relatively short time, hence saving time, of energy and cost. It will also be noted that the liquid phase in which the material passes at least partially may be of very short duration, for example less than one minute.
  • powders of a size of 20 nanometers, sintered for half an hour by hot compression under a pressure of 120 bar (12 MPa) at a temperature 'of 1325 ° C provide a material with a porosity close to 4%.
  • a heat treatment by laser beam causing a melting on the surface of the material will allow the porosity of the surface layer of the material to be reduced to practically zero.
  • partial melting step is not necessarily distinct from the sintering step proper.
  • the partial melting step can be carried out simultaneously with compression.
  • the lower and upper plates are mechanical plates sufficiently robust to allow compression. They must be compatible with the nature of the semiconductor powders used so as not to introduce impurities. For example, they may be graphite or silicon carbide plates.
  • the powders of layer 15 are for example powders of pure silicon or of silicon enriched in elements of column IV of the table of Mendeleev, such as carbon, germanium, tin, or their alloys. It is also possible to use powders of other semiconductors, and to produce, by sintering, materials of germanium, of gallium arsenide AsGa, etc.
  • the powders used can be of nanometric, micrometric or even millimeter size. Preferably, the size of the powders is less than the thickness of the material which it is desired to obtain. However, it can also be slightly higher, the powders can be crushed during sintering.
  • the powders used can come from sawing residues of mono or polycrystalline semiconductor ingots. It is also possible to use very fine powders resulting from by-products of the decomposition reactors of the silicon compounds, such as the silane or trichlorosilane gases. These powders, typically of the order of 20 nanometers, currently have no industrial use. They are very inexpensive and their use makes the process according to the present invention even more economical.
  • the powder bed 15 There are various ways of making the powder bed 15. For example, one or more heaps of powders can be placed in various places on the tray 10 and equalized with the desired thickness using a scraper.
  • the powder bed 15 can also be produced by aerosol. In this case, a gas containing suspended solid particles is sent to the treatment enclosure. The particles are deposited on the plate 10 and form the powder bed 15. Also, it is possible to use masks to place the powders at particular places in the layer 15.
  • liquid phase if necessary, the viscous phase
  • viscous phase One way to obtain the liquid phase (if necessary, the viscous phase) is to use a mixture of powders such that part of the constituents melts (if necessary, be made viscous) during the heat treatment which takes place, remember, either during the compression step or after.
  • Germanium melts (melting temperature 937 ° C), but not silicon (melting temperature 1410 ° C).
  • melting germanium facilitates the transport of silicon atoms from one silicon grain to another, during their agglomeration.
  • germanium spreads in pores and mouths, hence the desired reduction in porosity. The same result can be obtained with a mixture of powders of silicon and tin.
  • Liquid phase sintering can also be obtained by mixing powders of various materials, such as glass powders or ceramic materials, with the silicon powders.
  • silica powders become soft and pasty from around 1100 ° C and can also be used as a fluxing agent for sintering silicon powders. It should be noted that, in this case, it is not strictly speaking a liquid phase, and that this term should rather be understood to mean a viscous phase, resulting from the passage of a constituent in the state of supercooled liquid.
  • the liquid phase can be partially or partially removed during or after sintering, for example by annealing at high temperature, as above 1200 ° C. in the case of germanium. It is also possible to promote the evacuation of the liquid phase by pumping at a pressure lower than the partial pressure of the constituent considered.
  • the mixture of silicon powders and melting agent need not be homogeneous.
  • the molten part of the powders need only relate to the surface part of the mixture. This can be achieved by surface heating with a laser beam.
  • the material obtained is a material comprising a surface area having a high quality structure.
  • the liquid phase can also be obtained by selectively melting powders having a particular type of doping.
  • doped powders can be selectively melted by induction, because their conductivity is higher than that of silicon.
  • the pressure and / or the temperature can vary during the implementation of the process according to the present invention.
  • the pressure can be applied for a shorter duration than the heat treatment.
  • pressure can be applied from intermittently during heat treatment.
  • the heat treatment can comprise several stages of which only one or more causes the fusion.
  • FIG. 3 represents a material 25 obtained by the method of FIG. 2.
  • the material 25 is in the form of a thin wafer, of thickness typically between 100 and 1000 ⁇ m. If necessary, we can have greater thicknesses, 2000 ⁇ m for example, or smaller, such as 50 ⁇ m.
  • the material 25 is mechanically robust, of suitable porosity and its surface condition is optimal. The dimensions of the material 25 can be quite large.
  • FIG. 4 represents a structure 26 according to the present invention.
  • the structure 26 comprises a mechanical support 27, such as an insulating or conductive ceramic, graphite, glass, a metal or an alloy, on which a semiconductor material 28 is fixed.
  • the structure 26 is very robust and can be obtained in several ways . For example, we can first make the material 25 of Figure 3 and fix it in any way, for example by gluing, on the support 27.
  • Such a plate is for example composed of silicon carbide SiC, silicon nitride Si N j , silica glasses whether or not enriched with boron, phosphorus, nitrogen, etc.
  • the structure 26 is thus obtained directly by the method of FIG. 2.
  • the thickness of the structure 26 can be arbitrary.
  • the support 27 can have a fairly small thickness, for example from one to a few millimeters, or fairly large, for example from one to a few centimeters.
  • the structure 26 will be preferred for example in the case of semiconductor materials 28 of low thickness, for example 50 micrometers, or when it is desired to produce very large semiconductor plates.
  • the material 25 and the structure 26, which are very inexpensive, can serve as a base for producing photovoltaic cells, by application of conventional doping, metallization, etc. methods.
  • the photovoltaic field is not the only possible application of the material 25 or of the structure 26.
  • the material 25 or the material 28 of the structure 26 can serve as a support for the semiconductor layers deposited subsequently, which are then the active layers, the materials 25 or 28 serving only as a support.
  • This application is particularly advantageous.
  • the materials 25 and 28 are compatible with the deposited layers, and in particular have the same coefficient of expansion.
  • the active layers are deposited, for example in the vapor phase, the high temperature then poses no problem of difference in expansion between the deposited layers and the plate.
  • the material 25 or the structure 26 can constitute plates used for components for CCD cameras or flat screens, these components being able to comprise thin film transistors.
  • FIG. 5a illustrates a method according to the present invention in which a layer of semiconductor powders 30 is placed between a lower plate 32 of planar surface and an upper plate 34 whose lower surface has indentations 35.
  • the indentations 35 can have a size of around a fifth of the thickness of layer 30.
  • the lower surface of the plate 34 prints the design of the indentations 35 in the layer 30.
  • the material obtained by sintering the layer 30 retains so faithful to its surface, the pattern transmitted by the plate 34.
  • the texture of the surface of the material is thus perfectly controlled and it can for example be adapted to better absorption of light.
  • FIG. 5b illustrates another example of texture that can be obtained on the surface of a material according to the present invention.
  • a lower plate 40 has parallel parallelepiped ribs 42.
  • a bed of semiconductor powders 44 is placed on the plate 40 and surmounted by an upper plate 46 of planar surface.
  • the material obtained has on its surface parallel depressions corresponding to the ribs of the plate 40. As will be seen below, these depressions can be filled with another material.
  • the doping obtained can be homogeneous, when powders of a particular type of doping, N or P, are distributed uniformly between the compression plates. It is also possible, by appropriately distributing more or less doped N or P type powders, to form, within the material, distinct zones having doping of different type and concentration. As has been seen, in the case of a mixture of pure silicon powders and doped silicon, the liquid phase can be obtained by melting only the doped powders. Note that this also provides the advantage of reducing the porosity of the doped areas to practically zero. We can also plan to melt only some of the doped zones.
  • a doped material can also be obtained by sintering a bed of undoped semiconductor powders to which are mixed dopants or impurities in the form of powders, such as boron, phosphorus, antimony, arsenic, gallium, l aluminum, etc. It will be noted that these constituents easily melt and that, by melting, they optimize the microstructure of the zone where they are present.
  • Homogeneous doping of the material can also be obtained using undoped powders and by circulating a gas carrying doping elements during the implementation of the method according to the present invention.
  • the porosity of the powder bed is very high, for example of the order of 50%.
  • the porosity is said to be open, that is to say that there exist within the bed , powders or material in formation of interconnected circulation channels and opening onto the outside. If a doping gas then circulates, the doping gas spreads throughout the material and dopes it uniformly.
  • the partial melting step which clogs the porosity channels, must only take place after doping or in areas not of interest.
  • FIG. 6 illustrates another way to dope the material during its development.
  • a lower plate 60 comprises a conduit 62 opening out to the outside.
  • the conduit 62 further includes openings 64 located on the upper surface of the tray 60.
  • a bed of powders 65 is placed on the tray 60 to form the semiconductor material.
  • a tray 66 having conduits 68 and 70 leading to the outside and to the lower surface of the tray 66.
  • the conduits 68 each connect the outside of the tray to a particular opening in the bottom surface of the tray 66
  • the duct 70 connects the outside of the tray 66 to several openings located on the lower surface of the tray 66.
  • a doping gas for example of the P type, is sent into the conduit 62.
  • This gas due to the large number of open porosities existing at the start of the formation of the material, causes, with regard to the openings 64, the doping of areas 74 delimited in dotted lines.
  • the different doped zones 74 can join.
  • the heat treatment step must be adapted to the desired result. In fact, the open porosities close during the heat treatment step. Depending on the moment of action of the gas during the process, it is possible to carry out localized doping.
  • Doping gases are also sent into conduits 68 and 70 to respectively form doped zones 76 and 78.
  • FIG. 7A schematically represents a view partially in section and in perspective of a P-type material 80 obtained by sintering powders according to the method of the present invention.
  • the material 80 has depressions 82 and 84 which have been obtained using a plate having projecting elements of corresponding shape, of a type similar to those of the plate 40 of FIG. 5b.
  • the width of depressions 82 and 84 can be as small as 1 ⁇ m.
  • the edges of depressions 82 and 84 are well defined.
  • the depression 82 is in the form of a meander and the depression 84 is rectilinear.
  • the depressions 82 and 84 are then each filled with semiconductor powders having doping of the desired type and concentration.
  • the material 80 has heavily doped N-type areas (N + ) and a heavily doped P-type area 88 (P + ). These zones were obtained by filling the depression 82 with N-type powders, and the depression 84 with P-type powders, then by sintering these powders. To do this, the material can simply be subjected to a heat treatment step.
  • FIG. 7C represents a top view of a semiconductor material 90 according to the present invention, in which heavily doped N-type areas 92 and heavily doped P-type areas 94 were obtained according to the method described in relation to the figures 7A and 7B. Zones 92 and 94 are intersected.
  • the face which comprises the zones 92 and 94 is intended to be the face not exposed to light. This makes it unnecessary to make a collecting comb like the comb 5 in FIG. 1 and correspondingly increases the illuminated surface of the photocell.
  • the materials comprising PN junctions described above are components very close to the finished product that a photocell represents.
  • the process according to the present invention makes it possible to get even closer to the finished product.
  • the PN junction is in the thickness of the material, it is possible to place a bed of aluminum powders at the base of the bed of semiconductor powders during the manufacture of the material.
  • the material obtained after sintering thus comprises the lower conductive layer, which no longer needs to be deposited thereafter.
  • a heavily doped P-type zone like zone 3c in FIG. 1, is naturally produced in contact between the P-type material and aluminum.
  • the upper collecting comb can also be produced during the preparation of the material, by placing suitable powders, such as aluminum, in the appropriate places. It is also possible, for current transmission, to place transparent conductive ceramic powders over the entire surface of the material exposed to light.
  • FIG. 8 schematically represents a top view of a material 100 according to the present invention.
  • the material 100 was obtained, for example by applying the method according to the present invention, to a bed of powders comprising powders of tin Sn, germanium Ge, silicon Si and carbon C.
  • a zone 102 formed of tin along the edge 104 of the material 100.
  • the zone 102 results from the sintering of tin powders placed along the lateral edge 104.
  • the irregular contour of the zone 102 is explained in particular by the fact that the tin melts at the temperatures used in the process and tends to spread in the open pores of the material.
  • the material 100 also includes islands 106 of germanium Ge, resulting from the sintering of germanium powders.
  • the silicon powders give rise to islands 108 of silicon and the carbon powders, which, in the example shown have been deposited rather towards the edge 112 of the material, give rise to islands of carbon C.
  • the material 100 includes islands 114 of SiGe alloy, islands 116 of Si x Ge, islands 118 of SiyC.
  • the material can also include islands of Ge x C and Si x GeyC.
  • These alloys are born in contact with grains of different nature during the heat treatment, the various grains agglomerating by sintering. If desired, the formation of these alloys can be limited by placing powders of a different nature so that they do not mix too much. It is also possible to have powders of various alloys in the bed of powders to be sintered, in order to increase the proportion of the alloys.
  • the powders used or the materials obtained can be doped as described above.
  • the material 100 is particularly advantageous in photovoltaic applications.
  • the wavelength of the radiation absorbed by a semiconductor element depends on the value of the band gap of this element.
  • silicon whose band gap is 1.1 eV
  • Infrared radiation is practically not absorbed by silicon.
  • Ultraviolet radiation is absorbed quickly by silicon, but the excess energy represented by the difference between the energy of the radiation and the value of the band gap is lost.
  • Germanium whose band gap is 0.7 eV, is particularly well suited for absorbing infrared light.
  • An alloy of Si x Ge type has a band gap between the band band of silicon and that of germanium.
  • An alloy of Si x C type has a forbidden band much greater than that of silicon.
  • An alloy of this type responds particularly well to blue and ultraviolet radiation.
  • the material 100 has a locally variable band gap. This is an extremely important advantage, since radiation can be used to best advantage in a photovoltaic application. For example, the material 100 can practically respond to the entire solar spectrum, which is not the case for a conventional silicon photocell.
  • FIG. 9 schematically represents a bed of powders 120 intended for the preparation of a material according to the present invention.
  • the powder bed 120 comprises a lower layer 122 of tin powders, followed by a layer 124 of germanium powders, followed by a layer 126 of silicon powders, the whole being surmounted by a layer 128 of powders.
  • 'an alloy Si x C of carbon and silicon The powder layers 122, 124, 126 and 128 are arranged in increasing order of the prohibited band.
  • the semiconductor material obtained thus comprises several superimposed layers of materials with different prohibited bands.
  • the face of the material which has the largest band gap layer, Si x C is exposed to light.
  • the Si x C alloy layer absorbs and around ultraviolet radiation and allows visible and infrared radiation to pass through.
  • the silicon layer absorbs visible light and is practically transparent to infrared radiation, which is absorbed by the germanium layer.
  • Various alloys created during sintering aid in the absorption of radiation.
  • the layer of tin, buried, is mainly used to collect the carriers born from the photovoltaic effect. As before, a PN junction can be achieved by appropriate doping.
  • the material obtained by the powder bed of FIG. 9 is advantageous in that the radiation successively passes through layers of decreasing forbidden band. This allows more complete absorption of the radiation.
  • the plates used to compress the bed of powders are not necessarily planar and can be of any shape.
  • FIG. 10 thus represents a semiconductor material 130 in the form of a tile which can be integrated into the structure of a roof.
  • the material 130 hereinafter called the tile, has a non-planar end 131 making it possible to cover the next tile 130 'and to connect to it.
  • the tile 130 is obtained by sintering a bed of semiconductor powders using trays of corresponding shape. The powder bed was produced so as to successively create a thin layer 132 heavily doped with type N (N + ), a layer 134 doped with type N, followed by a layer 136 doped with type P.
  • N + heavily doped with type N
  • P layer 136 doped with type P
  • P + highly doped P-type
  • the tile 130 is connected to the tile 130 'by any conductive fixing means 140, such as a solder or a flexible wire, connecting the N + layer of a tile to the zone P + of the next tile.
  • the solar cells represented by the tiles 130 and 130 ′ are thus connected in series.
  • any suitable means may be used, such as resistive ovens, lamp furnaces, solar furnaces, etc., the energy being transferred by conduction, convection, radiation, etc.
  • any structure or component comprising or formed from one or more materials according to the present invention is part of the field of the present invention.
  • the materials according to the present invention are not limited to the materials obtained by the method according to the present invention.
  • any semiconductor material comprising grains and / or aggregates having different forbidden bands is part of the field of the present invention, whatever its mode of production.

Abstract

The invention relates to a method for forming a semiconductor material obtained by sintering powders and to a semiconductor material. The method comprises a compression and heat treatment stage such that one part of the powder is melted or becomes viscous. The material can be used in the photovoltaic field.

Description

MATERIAU SEMICONDUCTEUR OBTENU PAR FRITTAGE SEMICONDUCTOR MATERIAL OBTAINED BY SINTERING
La présente invention concerne le domaine des semiconducteurs, et en particulier, mais non exclusivement, les matériaux semiconducteurs utilisables pour réaliser des cellules photovoltaïques, nommées aussi photopiles. La figure 1 représente une cellule photovoltaïque classique 1. La cellule photovoltaïque 1 comprend un matériau semiconducteur plan 3. Le matériau 3 , en général en silicium polycristallin, comprend trois zones de dopage différent. Une zone centrale épaisse 3a est faiblement dopée de type P. Une zone supérieure 3b est dopée de type N, et éventuellement surdopée en surface. Une zone inférieure 3c est fortement dopée de type P (P+) . Au-dessus de la zone 3b, destinée à être exposée à la lumière, se trouve un peigne conducteur 5. Une couche d'aluminium 6 recouvre la face inférieure de la cellule. Le peigne 5 et la couche 6 sont tous deux destinés à transmettre le courant photovoltaïque et sont reliés aux bornes + et - non représentées de la cellule. Sur la zone 3b et le peigne 5, se trouve de préférence une couche anti-reflet non représentée, pour limiter la réflexion des rayons lumineux à la surface de la photopile.The present invention relates to the field of semiconductors, and in particular, but not exclusively, semiconductor materials which can be used to produce photovoltaic cells, also called photocells. FIG. 1 represents a conventional photovoltaic cell 1. The photovoltaic cell 1 comprises a plane semiconductor material 3. The material 3, generally made of polycrystalline silicon, comprises three different doping zones. A thick central zone 3a is lightly doped with type P. An upper zone 3b is doped with type N, and possibly overdoped on the surface. A lower zone 3c is heavily doped with the P (P + ) type. Above zone 3b, intended to be exposed to light, is a conductive comb 5. An aluminum layer 6 covers the underside of the cell. The comb 5 and the layer 6 are both intended for transmitting the photovoltaic current and are connected to the terminals + and - not shown of the cell. On the zone 3b and the comb 5, there is preferably an anti-reflection layer, not shown, to limit the reflection of the light rays on the surface of the solar cell.
Le matériau 3 provient de façon classique d'un barreau de silicium polycristallin obtenu à partir d'un bain de silicium fondu. Le barreau est scié pour obtenir des plaquettes qui sont ensuite dopées pour obtenir le matériau 3. Ce procédé de fabrication, proche du procédé de fabrication de plaquettes de silicium monocristallin, est coûteux et limite les dimensions possibles des plaquettes .The material 3 conventionally comes from a polycrystalline silicon bar obtained from a silicon bath molten. The bar is sawn to obtain wafers which are then doped to obtain the material 3. This manufacturing process, close to the process for manufacturing monocrystalline silicon wafers, is expensive and limits the possible dimensions of the wafers.
L'inventeur a présenté lors d'une conférence à Munich (17 h European Photovoltaïc Solar Energy Conférence and Exhibition, Munich 21-26 Octobre 2001) un procédé de fabrication de plaquettes de silicium polycristallin par frittage de poudres de silicium. Dans ce procédé, des poudres de silicium de 5 μ ou de 20 μm sont placées entre les plateaux d'une presse. L'ensemble est comprimé avec une pression P comprise entre 70 MPa (700 bars) et 900 MPa (9000 bars) . Ensuite, la couche compactée est introduite dans un four de frittage, où elle est chauffée à une température T comprise entre 950°C et 1050°C. Le frittage, qui permet la croissance de ponts entre les grains et une rigidification du matériau, a été réalisé aux températures indiquées pendant une durée de deux à huit heures, sous une basse pression d'argon (100 Pa) . Les matériaux obtenus sont assez solides mécaniquement pour pouvoir être manipulés. Cependant, leur porosité est élevée, supérieure à 15%. En outre, la taille des grains est faible, ceux-ci n'ayant pas augmenté sensiblement de taille au cours du traitement . Le produit mobilité-durée de vie des porteurs minoritaires est faible, de l'ordre de 10~' c_v7-V~^- (lQ-llm2- -l dns χe système international) . Les matériaux obtenus sont inutilisables dans le domaine photovoltaïque. Par exemple, du fait de la porosité élevée, il est impossible de doper une zone particulière du matériau, les dopants- migrant par les canaux de porosité et se répandant partout au sein du matériau. Quant au produit mobilité-durée de vie des porteurs minoritaires, il faut des valeurs au moins mille fois supérieures pour que le matériau puisse être utilisé dans une photopile. En outre, la surface des matériaux obtenus est non contrôlée et rugueuse. Un tel état de surface empêche la prévision de jonctions de surface, nécessairement mauvaises, en particulier à cause de courants de fuite importants .The inventor presented at a conference in Munich (5 p.m. European Photovoltaïc Solar Energy Conference and Exhibition, Munich October 21-26, 2001) a process for manufacturing polycrystalline silicon wafers by sintering silicon powders. In this process, 5 μm or 20 μm silicon powders are placed between the plates of a press. The assembly is compressed with a pressure P of between 70 MPa (700 bars) and 900 MPa (9000 bars). Then, the compacted layer is introduced into a sintering oven, where it is heated to a temperature T of between 950 ° C. and 1050 ° C. Sintering, which allows the growth of bridges between the grains and a stiffening of the material, was carried out at the temperatures indicated for a period of two to eight hours, under a low argon pressure (100 Pa). The materials obtained are mechanically strong enough to be able to be handled. However, their porosity is high, greater than 15%. In addition, the size of the grains is small, the latter not having increased appreciably in size during the treatment. The mobility-life product of minority carriers is low, of the order of 10 ~ 'c_v7-V ~ ^ - (lQ-ll m 2- -l dns χ e international system). The materials obtained cannot be used in the photovoltaic field. For example, due to the high porosity, it is impossible to dop a particular zone of the material, the dopants- migrating through the porosity channels and spreading throughout the material. As for the mobility-life product of minority carriers, values are required at least a thousand times higher before the material can be used in a solar cell. In addition, the surface of the materials obtained is uncontrolled and rough. Such a surface condition prevents the prediction of surface junctions, necessarily bad, in particular because of large leakage currents.
Un objet de la présente invention est de réaliser un matériau semiconducteur ou un composant par frittage de poudres semiconductrices utilisable dans le domaine électronique, notamment dans le domaine photovoltaïque.An object of the present invention is to produce a semiconductor material or a component by sintering semiconductor powders usable in the electronic field, in particular in the photovoltaic field.
Un autre objet de la présente invention est de réaliser un matériau semiconducteur par frittage de poudres semiconductrices présentant une faible rugosité et/ou un état de surface à texturation contrôlée.Another object of the present invention is to produce a semiconductor material by sintering semiconductor powders having low roughness and / or a surface condition with controlled texturing.
Pour atteindre ces objets, la présente invention prévoit un procédé de formation d'un matériau semiconducteur à partir de poudres comprenant au moins un constituant appartenant au groupe constitué par les éléments de la colonne IV du tableau de Mendeleïev et leurs alliages. Le procédé comprend une étape de compression desdites poudres et une étape de traitement thermique telle qu'une partie au moins des poudres est fondue ou rendue visqueuse.To achieve these objects, the present invention provides a method of forming a semiconductor material from powders comprising at least one constituent belonging to the group consisting of the elements of column IV of the Mendeleev table and their alloys. The method comprises a step of compressing said powders and a step of heat treatment such that at least part of the powders is melted or made viscous.
Selon un mode de réalisation de la présente invention, les étapes de compression et de traitement thermique sont simultanées .According to an embodiment of the present invention, the compression and heat treatment steps are simultaneous.
Selon un mode de réalisation de la présente invention, le traitement thermique est tel que seules des poudres appartenant à une zone particulière du matériau sont fondues ou rendues visqueuses.According to an embodiment of the present invention, the heat treatment is such that only powders belonging to a particular zone of the material are melted or made viscous.
Selon un mode de réalisation de la présente invention, les poudres comprennent des poudres de silicium et des poudres d'au moins un autre constituant, le traitement thermique étant tel que le silicium n'est pas fondu et qu'au moins un des autres constituants est fondu ou rendu visqueux.According to an embodiment of the present invention, the powders comprise silicon powders and powders of at least one other constituent, the heat treatment being such that the silicon is not molten and that at least one of the other constituents is melted or made viscous.
Selon un mode de réalisation de la présente invention, les poudres comprennent des poudres semiconductrices dopées et des poudres semiconductrices non dopées, le traitement thermique étant tel que seules les poudres dopées sont fondues . Selon un mode de réalisation de la présente invention, l'étape de compression est précédée d'une étape consistant à placer des poudres sur un plateau, les poudres étant différentes quant à leur nature, leur' granulométrie et/ou leur dopage selon leur emplacement sur le plateau.According to an embodiment of the present invention, the powders comprise doped semiconductor powders and undoped semiconductor powders, the heat treatment being such that only the doped powders are melted. According to an embodiment of the present invention, the compression step is preceded by a step consisting in placing powders on a tray, the powders being different in their nature, their particle size and / or their doping according to their location. on the tray.
Selon un mode de réalisation de la présente invention, lors de l'étape de compression, lesdites poudres sont pressées entre des plateaux dont la surface est propre à texturer la surface du matériau. La présente invention prévoit aussi un matériau semiconducteur obtenu au moins partiellement par compression et traitement thermique de poudres comportant au moins deux zones distinctes formées de constituants distincts appartenant au groupe constitué par les éléments de la colonne IV du tableau de Mendeleïev et leurs alliages.According to an embodiment of the present invention, during the compression step, said powders are pressed between plates whose surface is suitable for texturing the surface of the material. The present invention also provides a semiconductor material obtained at least partially by compression and heat treatment of powders comprising at least two distinct zones formed of distinct constituents belonging to the group consisting of the elements of column IV of the Mendeleev table and their alloys.
Selon un mode de réalisation de la présente invention, lesdites zones sont superposées .According to an embodiment of the present invention, said zones are superimposed.
La présente invention prévoit aussi une structure ou un composant formé d'un ou comprenant au moins un matériau semi- conducteur comportant des grains et/ou des agrégats présentant des bandes interdites de valeur différente.The present invention also provides a structure or a component formed of or comprising at least one semiconductor material comprising grains and / or aggregates having prohibited bands of different value.
Ces objets, caractéristiques et avantages, ainsi que d' autres de la présente invention seront exposés en détail dans la description suivante de modes de réalisation particuliers faite à titre non-limitatif en relation avec les figures jointes parmi lesquelles : la figure 1 représente une cellule photovoltaïque classique ; la figure 2 illustre un mode de mise en oeuvre du procédé selon la présente invention ; la figure 3 représente un matériau selon la présente invention ; la figure 4 représente une structure selon la présente invention ; les figures 5a et 5b illustrent d'autres modes de mise en oeuvre du procédé selon la présente invention ; les figures 6, 7A à 7C illustrent des façons de doper un matériau selon la présente invention ; et les figures 8, 9 et 10 représentent des matériaux selon la présente invention.These objects, characteristics and advantages, as well as others of the present invention will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures, among which: FIG. 1 represents a cell conventional photovoltaics; FIG. 2 illustrates an embodiment of the method according to the present invention; Figure 3 shows a material according to the present invention; Figure 4 shows a structure according to the present invention; Figures 5a and 5b illustrate other embodiments of the method according to the present invention; Figures 6, 7A to 7C illustrate ways of doping a material according to the present invention; and Figures 8, 9 and 10 show materials according to the present invention.
La figure 2 illustre un mode de mise en oeuvre du procédé selon la présente invention.FIG. 2 illustrates an embodiment of the method according to the present invention.
Sur un plateau inférieur 10 est placé un lit de poudres semiconductrices 15, par exemple des poudres de silicium. Un plateau supérieur 20 recouvre les poudres 15. L'ensemble est placé dans une enceinte de traitement et la couche de poudres semiconductrices 15 est compactée par application d'une pression P. La compaction peut être effectuée par compression à froid, c'est-à-dire à température ambiante, ou par compression à chaud, à une température T, par exemple comprise entre 950 et 1300°C.On a lower plate 10 is placed a bed of semiconductor powders 15, for example silicon powders. An upper plate 20 covers the powders 15. The assembly is placed in a treatment enclosure and the layer of semiconductor powders 15 is compacted by application of a pressure P. The compaction can be carried out by cold compression, that is to say that is to say at ambient temperature, or by hot compression, at a temperature T, for example between 950 and 1300 ° C.
Selon la présente invention, le frittage est effectué au moins partiellement en phase liquide, c'est-à-dire que, pendant ou après la compression, il est appliqué un traitement thermique tel qu'une partie au moins des poudres est fondue. Cela est symbolisé par la lettre F en figure 2. Dans la présente invention, les termes "phase liquide" et "fusion" doivent être entendus dans un sens large. Comme on le verra par la suite, l'expression "phase liquide" peut désigner aussi une phase visqueuse correspondant à un liquide surfondu, le terme "fusion" désignant alors "surfusion".According to the present invention, the sintering is carried out at least partially in the liquid phase, that is to say that, during or after compression, a heat treatment is applied such that at least part of the powders is melted. This is symbolized by the letter F in Figure 2. In the present invention, the terms "liquid phase" and "fusion" should be understood in a broad sense. As will be seen hereinafter, the expression “liquid phase” can also designate a viscous phase corresponding to an supercooled liquid, the term “fusion” then designating “supercooling”.
La fusion partielle peut être réalisée de manière sélective, par exemple en fonction de la zone du matériau, de la nature des poudres, ou selon le moyen de chauffage utilisé.Partial melting can be carried out selectively, for example depending on the area of the material, the nature of the powders, or according to the heating means used.
Dans les zones frittées en phase liquide, la porosité est sensiblement nulle (en pratique, inférieure à 0,2%). Aussi, la fusion entraîne une augmentation de la taille des grains, ce qui est souhaitable, l'obstacle au déplacement des porteurs créé par les frontières de grains étant alors diminué. Bien que cela soit possible, il n'est pas nécessaire que l'ensemble du matériau soit fritte en phase liquide. En effet, au cours de ses recherches, l'inventeur s'est aperçu que les caractéristiques d'un matériau destiné à former une photopile n'avaient pas besoin d'être homogènes dans l'ensemble du matériau.In the liquid phase sintered areas, the porosity is substantially zero (in practice, less than 0.2%). Also, the merger results in an increase in the size of the grains, which is desirable, the obstacle to the movement of the carriers created by the grain boundaries then being reduced. Although this is possible, it is not necessary for the entire material to be sintered in the liquid phase. In fact, during his research, the inventor realized that the characteristics of a material intended to form a solar cell did not need to be homogeneous throughout the material.
Par exemple, dans une cellule photovoltaïque, la partie dite "absorbeur", c'est-à-dire la zone destinée à transformer les photons reçus en paires électrons-trous, doit posséder une microstructure de très haute qualité, à savoir une porosité s 'approchant le plus possible de zéro et une taille de grains la plus grande possible. La partie formant la jonctionFor example, in a photovoltaic cell, the so-called "absorber" part, that is to say the zone intended to transform the photons received into electron-hole pairs, must have a very high quality microstructure, namely a porosity s '' getting as close to zero as possible and the largest possible grain size. The part forming the junction
(ou zone de collection) , destinée à récupérer les porteurs, doit présenter également ce type de caractéristiques . Par contre, d'autres zones du matériau n'ont pas besoin d'une microstructure de haute qualité et peuvent présenter sans inconvénient une porosité médiocre. De telles zones sont par exemple les zones conductrices fortement dopées de type N ou de type P faisant office de contact avec les zones N et P de la jonction. Il suffit que ces zones présentent une conductivité suffisante, et une porosité aussi grande que 40 ou 50% peut suffire.(or collection area), intended to recover the carriers, must also have this type of characteristics. On the other hand, other areas of the material do not need a high quality microstructure and can have poor porosity without disadvantage. Such zones are, for example, the heavily doped N-type or P-type conductive zones which act as contact with the N and P zones of the junction. It suffices that these zones have sufficient conductivity, and a porosity as large as 40 or 50% may be sufficient.
En conséquence, selon la présente invention, le traitement thermique peut être mené de façon à provoquer de manière sélective une fusion seulement dans les zones où une microstructure de qualité est souhaitée.Consequently, according to the present invention, the heat treatment can be carried out so as to selectively cause a fusion only in the areas where a quality microstructure is desired.
On va ci-après donner quelques exemples de matériaux obtenus .We will give below some examples of materials obtained.
Un certain nombre d'essais ont été menés avec des pressions comprises entre 10 MPa et 30 MPa (100 et 300 bars) . Les températures ont été comprises entre 950°C et 1350°C. Les poudres utilisées ont été soit des poudres de silicium pur, soit des poudres de silicium mêlées à des poudres d'autres éléments de la colonne IV du tableau de Mendelieïev, comme le germanium, soit des poudres de silicium mêlées à des poudres de matériaux non semiconducteurs, comme la silice Si02. La granulométrie des poudres utilisées a été comprise entre 20 nanomètres et 700 micromètres. Les résultats obtenus sont spectaculaires. Les objets de la présente invention ont été atteints et des matériaux utilisables dans le domaine photovoltaïque ont été obtenus .A number of tests have been carried out with pressures between 10 MPa and 30 MPa (100 and 300 bars). The temperatures were between 950 ° C and 1350 ° C. The powders used were either pure silicon powders or silicon powders mixed with powders of other elements of column IV of the Mendelieev table, such as germanium, or silicon powders mixed with powders of materials non-semiconductor, such as silica Si02. The particle size of the powders used was between 20 nanometers and 700 micrometers. The results obtained are spectacular. The objects of the present invention have been achieved and materials usable in the photovoltaic field have been obtained.
On notera que, selon la présente invention, il est possible de contrôler aisément la morphologie de la surface du matériau obtenu. En effet, notamment lorsque l'étape de fusion partielle a lieu au cours de la compression, la surface du matériau reproduit fidèlement la surface des plateaux 10 et 20. Avec des plateaux plans et lisses, la surface, analysée par microscopie électronique, se présente comme un plan uni à très faible rugosité. On notera aussi qu'un avantage d'effectuer une compression à chaud des poudres plutôt qu'une compression à froid permet d'obtenir un matériau présentant une faible porosité d'ensemble en un temps relativement faible, d'où une économie de temps, d'énergie et de coût. On notera aussi que la phase liquide dans laquelle passe au moins partiellement le matériau peut être de très courte durée, par exemple inférieure à une minute.It will be noted that, according to the present invention, it is possible to easily control the morphology of the surface of the material obtained. Indeed, especially when the partial melting step takes place during compression, the surface of the material faithfully reproduces the surface of the plates 10 and 20. With flat and smooth plates, the surface, analyzed by electron microscopy, appears like a plain plane with very low roughness. It will also be noted that an advantage of carrying out a hot compression of the powders rather than a cold compression makes it possible to obtain a material having a low overall porosity in a relatively short time, hence saving time, of energy and cost. It will also be noted that the liquid phase in which the material passes at least partially may be of very short duration, for example less than one minute.
Par exemple, dans un exemple pratique, des poudres d'une taille de 20 nanomètres, frittées pendant une demi-heure par compression à chaud sous une pression de 120 bars (12 MPa) à une température' de 1325°C, fournissent un matériau de porosité voisine de 4%. Un traitement thermique par rayon laser provoquant une fusion en surface du matériau permettra d'abaisser la porosité de la couche superficielle du matériau à pratiquement zéro.For example, in a practical example, powders of a size of 20 nanometers, sintered for half an hour by hot compression under a pressure of 120 bar (12 MPa) at a temperature 'of 1325 ° C, provide a material with a porosity close to 4%. A heat treatment by laser beam causing a melting on the surface of the material will allow the porosity of the surface layer of the material to be reduced to practically zero.
On notera que l'étape de fusion partielle n'est pas nécessairement distincte de l'étape de frittage proprement dite. L'étape de fusion partielle peut être menée de manière simultanée à la compression. On va maintenant donner des exemples de mise en oeuvre du procédé selon la présente invention.Note that the partial melting step is not necessarily distinct from the sintering step proper. The partial melting step can be carried out simultaneously with compression. We will now give examples of implementation of the method according to the present invention.
Les plateaux inférieur et supérieur sont des plateaux mécaniques suffisamment robustes pour permettre la compression. Ils doivent être compatibles avec la nature des poudres semi- conductrices utilisées pour ne pas y introduire des impuretés. Par exemple, il peut s'agir de plateaux en graphite ou en carbure de silicium.The lower and upper plates are mechanical plates sufficiently robust to allow compression. They must be compatible with the nature of the semiconductor powders used so as not to introduce impurities. For example, they may be graphite or silicon carbide plates.
Les poudres de la couche 15 sont par exemple des poudres de silicium pur ou de silicium enrichi en éléments de la colonne IV du tableau de Mendeleïev, comme le carbone, le germanium, l'étain, ou leurs alliages. On peut aussi utiliser des poudres d'autres semiconducteurs, et réaliser par frittage des matériaux en germanium, en arséniure de gallium AsGa, etc. Les poudres utilisées peuvent être de taille nano- métrique, micrométrique, voire millimétrique. De préférence, la taille des poudres est inférieure à l'épaisseur du matériau que l'on souhaite obtenir. Cependant, elle peut être aussi légèrement supérieure, les poudres pouvant être écrasées au cours du frittage. On peut aussi faire un mélange de poudres de diverses granulométries pour réaliser le lit de poudres 15, afin notamment de contrôler de manière commode et efficace la porosité d'ensemble ou de zones du matériau obtenu.The powders of layer 15 are for example powders of pure silicon or of silicon enriched in elements of column IV of the table of Mendeleev, such as carbon, germanium, tin, or their alloys. It is also possible to use powders of other semiconductors, and to produce, by sintering, materials of germanium, of gallium arsenide AsGa, etc. The powders used can be of nanometric, micrometric or even millimeter size. Preferably, the size of the powders is less than the thickness of the material which it is desired to obtain. However, it can also be slightly higher, the powders can be crushed during sintering. One can also make a mixture of powders of various particle sizes to make the powder bed 15, in particular in order to conveniently and effectively control the porosity of the whole or of areas of the material obtained.
Les poudres utilisées peuvent être issues de résidus de sciage de lingots semiconducteurs mono ou polycristallins. On peut aussi utiliser des poudres très fines résultant de sous- produits des réacteurs de décomposition des composés du silicium, comme les gaz silane ou trichlorosilane . Ces poudres, typiquement de l'ordre de 20 nanomètres, n'ont actuellement aucune utilisation industrielle. Elles sont très bon marché et leur utilisation rend le procédé selon la présente invention encore plus économique.The powders used can come from sawing residues of mono or polycrystalline semiconductor ingots. It is also possible to use very fine powders resulting from by-products of the decomposition reactors of the silicon compounds, such as the silane or trichlorosilane gases. These powders, typically of the order of 20 nanometers, currently have no industrial use. They are very inexpensive and their use makes the process according to the present invention even more economical.
On peut procéder de diverses manières pour réaliser le lit de poudres 15. Par exemple, on peut placer un ou plusieurs tas de poudres en divers endroits du plateau 10 et égaliser à l'épaisseur voulue à l'aide d'un racleur. Le lit de poudres 15 peut aussi être réalisé par aérosol. Dans ce cas, un gaz contenant des particules solides en suspension est envoyé dans l'enceinte de traitement. Les particules se déposent sur le plateau 10 et forment le lit de poudres 15. Aussi, il est possible d'utiliser des masques pour placer les poudres à des endroits particuliers de la couche 15.There are various ways of making the powder bed 15. For example, one or more heaps of powders can be placed in various places on the tray 10 and equalized with the desired thickness using a scraper. The powder bed 15 can also be produced by aerosol. In this case, a gas containing suspended solid particles is sent to the treatment enclosure. The particles are deposited on the plate 10 and form the powder bed 15. Also, it is possible to use masks to place the powders at particular places in the layer 15.
On notera que les conditions de mise en oeuvre du procédé (pression, traitement thermique, nature et granulométrie des poudres, durée de traitement) permettent de contrôler les caractéristiques des matériaux obtenus et de les ajuster de manière souhaitée.It will be noted that the conditions for implementing the process (pressure, heat treatment, nature and particle size of the powders, duration of treatment) make it possible to control the characteristics of the materials obtained and to adjust them as desired.
Une façon d'obtenir la phase liquide (le cas échéant, la phase visqueuse) est d'utiliser un mélange de poudres tel qu'une partie des constituants fonde (le cas échéant, soit rendu visqueux) pendant le traitement thermique qui a lieu, rappelons- le, soit pendant l'étape de compression, soit après.One way to obtain the liquid phase (if necessary, the viscous phase) is to use a mixture of powders such that part of the constituents melts (if necessary, be made viscous) during the heat treatment which takes place, remember, either during the compression step or after.
Par exemple, on peut réaliser un mélange homogène de germanium et de silicium et le porter à une température comprise entre 937 et 1410°C. Le germanium fond (température de fusion 937°C), mais pas le silicium (température de fusion 1410°C) . En fondant, le germanium facilite le transport d'atomes de silicium d'un grain de silicium à un autre, lors de leur agglomération. En outre, le germanium se répand dans les pores et les bouche, d'où la réduction souhaitée de la porosité. Le même résultat peut être obtenu avec un mélange de poudres de silicium et d'étain.For example, a homogeneous mixture of germanium and silicon can be produced and brought to a temperature between 937 and 1410 ° C. Germanium melts (melting temperature 937 ° C), but not silicon (melting temperature 1410 ° C). By melting, germanium facilitates the transport of silicon atoms from one silicon grain to another, during their agglomeration. In addition, germanium spreads in pores and mouths, hence the desired reduction in porosity. The same result can be obtained with a mixture of powders of silicon and tin.
On peut obtenir aussi un frittage en phase liquide en mélangeant aux poudres de silicium des poudres de matériaux divers, comme des poudres de verre ou de matériaux céramiques. Par exemple, les poudres de silice deviennent molles et pâteuses à partir d'environ 1100°C et peuvent aussi être utilisées comme agent fondant pour fritter les poudres de silicium. On notera que, dans ce cas, il ne s'agit pas à proprement parler d'une phase liquide, et qu'il faut plutôt entendre par ce terme une phase visqueuse, résultant du passage d'un constituant à l'état de liquide surfondu.Liquid phase sintering can also be obtained by mixing powders of various materials, such as glass powders or ceramic materials, with the silicon powders. For example, silica powders become soft and pasty from around 1100 ° C and can also be used as a fluxing agent for sintering silicon powders. It should be noted that, in this case, it is not strictly speaking a liquid phase, and that this term should rather be understood to mean a viscous phase, resulting from the passage of a constituent in the state of supercooled liquid.
De manière générale, la phase liquide peut être évacuée partiellement ou en partie pendant ou après le frittage par exemple par un recuit à haute température, comme supérieure à 1200°C dans le cas du germanium. On peut aussi favoriser l'évacuation de la phase liquide en effectuant un pompage à une pression inférieure à la pression partielle du constituant considéré. Selon la présente invention, le mélange de poudres de silicium et d'agent fondant n'a pas besoin d'être homogène. Par exemple, dans une photopile où absorbeur et jonction sont sur une même face, la partie fondue des poudres n'a besoin que de concerner la partie superficielle du mélange. On peut obtenir cela en faisant un chauffage superficiel par rayon laser. On peut aussi obtenir cela réalisant une couche 15 en deux sous- couches, une sous-couche inférieure avec des poudres de silicium et une sous-couche supérieure avec un mélange de poudres de silicium et d'agent fondant, germanium par exemple, seul l'agent fondant fondant au cours du frittage. Le matériau obtenu est un matériau comportant une zone superficielle présentant une structure de haute qualité.Generally, the liquid phase can be partially or partially removed during or after sintering, for example by annealing at high temperature, as above 1200 ° C. in the case of germanium. It is also possible to promote the evacuation of the liquid phase by pumping at a pressure lower than the partial pressure of the constituent considered. According to the present invention, the mixture of silicon powders and melting agent need not be homogeneous. For example, in a solar cell where the absorber and junction are on the same face, the molten part of the powders need only relate to the surface part of the mixture. This can be achieved by surface heating with a laser beam. This can also be achieved by making a layer 15 in two sublayers, a lower sublayer with silicon powders and an upper sublayer with a mixture of silicon powders and fluxing agent, germanium for example, only 1 'fondant fondant during sintering. The material obtained is a material comprising a surface area having a high quality structure.
La phase liquide peut aussi être obtenue en fondant sélectivement des poudres présentant un type de dopage particulier. Ainsi, par exemple, dans un mélange de poudres de silicium dopé et de silicium pur, on peut fondre sélectivement par induction les poudres dopées, car leur conductivité est plus élevée que celle du silicium.The liquid phase can also be obtained by selectively melting powders having a particular type of doping. Thus, for example, in a mixture of doped silicon powders and pure silicon, doped powders can be selectively melted by induction, because their conductivity is higher than that of silicon.
Bien entendu, dans le procédé selon la présente invention, plusieurs étapes de compression et/ou plusieurs étapes de traitement thermique peuvent avoir lieu. La pression et/ou la température peuvent varier au cours de la mise en oeuvre du procédé selon la présente invention. Par exemple, la pression peut être exercée pendant une durée plus courte que le traitement thermique. Aussi, la pression peut être appliquée de manière intermittente au cours du traitement thermique. Aussi, le traitement thermique peut comporter plusieurs étapes dont seules une ou plusieurs occasionne la fusion.Of course, in the method according to the present invention, several stages of compression and / or several stages of heat treatment can take place. The pressure and / or the temperature can vary during the implementation of the process according to the present invention. For example, the pressure can be applied for a shorter duration than the heat treatment. Also, pressure can be applied from intermittently during heat treatment. Also, the heat treatment can comprise several stages of which only one or more causes the fusion.
On notera aussi qu'on peut réaliser un empilement de plusieurs plateaux mécaniques emprisonnant plusieurs lits de poudres semiconductrices, afin de fabriquer un grand nombre de matériaux en même temps .It will also be noted that it is possible to produce a stack of several mechanical plates trapping several beds of semiconductor powders, in order to manufacture a large number of materials at the same time.
La figure 3 représente un matériau 25 obtenu par le procédé de la figure 2. Le matériau 25 se présente sous la forme d'une plaquette fine, d'épaisseur typiquement comprise entre 100 et 1000 μm. Si besoin est, on peut avoir des épaisseurs plus importantes, 2000 μm par exemple, ou plus faibles, comme 50 μm. Le matériau 25 est robuste mécaniquement, de porosité adaptée et son état de surface est optimal. Les dimensions du matériau 25 peuvent être assez grandes.FIG. 3 represents a material 25 obtained by the method of FIG. 2. The material 25 is in the form of a thin wafer, of thickness typically between 100 and 1000 μm. If necessary, we can have greater thicknesses, 2000 μm for example, or smaller, such as 50 μm. The material 25 is mechanically robust, of suitable porosity and its surface condition is optimal. The dimensions of the material 25 can be quite large.
La figure 4 représente une structure 26 selon la présente invention. La structure 26 comprend un support mécanique 27, comme une céramique isolante ou conductrice, du graphite, du verre, un métal ou un alliage, sur lequel est fixé un matériau semiconducteur 28. La structure 26 est très robuste et peut être obtenue de plusieurs manières. Par exemple, on peut d'abord réaliser le matériau 25 de la figure 3 et le fixer d'une manière quelconque, par exemple par collage, sur le support 27. On peut aussi de manière avantageuse, pour former le support 27, utiliser un des deux plateaux 10 ou 20 de nature telle que le matériau semiconducteur y adhère lors du frittage des poudres de la couche 15. Un tel plateau est par exemple composé de carbure de silicium SiC, de nitrure de silicium Si Nj, de verres de silice enrichis ou non en bore, phosphore, azote, etc. On obtient ainsi la structure 26 directement par le procédé de la figure 2. L'épaisseur de la structure 26 peut être quelconque. Le support 27 peut avoir une épaisseur assez faible, par exemple de un à quelques millimètres, ou assez importante, par exemple de un à quelques centimètres. La structure 26 sera préférée par exemple dans le cas de matériaux semiconducteurs 28 de faible épaisseur, par exemple 50 micromètres, ou lorsque l'on souhaite réaliser des plaques semiconductrices de très grande dimension.FIG. 4 represents a structure 26 according to the present invention. The structure 26 comprises a mechanical support 27, such as an insulating or conductive ceramic, graphite, glass, a metal or an alloy, on which a semiconductor material 28 is fixed. The structure 26 is very robust and can be obtained in several ways . For example, we can first make the material 25 of Figure 3 and fix it in any way, for example by gluing, on the support 27. We can also advantageously, to form the support 27, use a of two plates 10 or 20 of a nature such that the semiconductor material adheres thereto during the sintering of the powders of the layer 15. Such a plate is for example composed of silicon carbide SiC, silicon nitride Si N j , silica glasses whether or not enriched with boron, phosphorus, nitrogen, etc. The structure 26 is thus obtained directly by the method of FIG. 2. The thickness of the structure 26 can be arbitrary. The support 27 can have a fairly small thickness, for example from one to a few millimeters, or fairly large, for example from one to a few centimeters. The structure 26 will be preferred for example in the case of semiconductor materials 28 of low thickness, for example 50 micrometers, or when it is desired to produce very large semiconductor plates.
Le matériau 25 et la structure 26, très bon marché, peuvent servir de base pour réaliser des cellules photo- voltaïques, par application de procédés classiques de dopage, métallisation, etc. Cependant, le domaine photovoltaïque n'est pas la seule application possible du matériau 25 ou de la structure 26.The material 25 and the structure 26, which are very inexpensive, can serve as a base for producing photovoltaic cells, by application of conventional doping, metallization, etc. methods. However, the photovoltaic field is not the only possible application of the material 25 or of the structure 26.
Par exemple, le matériau 25 ou le matériau 28 de la structure 26 peuvent servir de support à des couches semi- conductrices déposées par la suite, qui sont alors les couches actives, les matériaux 25 ou 28 ne servant que de support. Cette application est particulièrement avantageuse. En effet, les matériaux 25 et 28 sont compatibles avec les couches déposées, et possèdent notamment le même coefficient de dilatation. Lors du dépôt des couches actives, par exemple en phase vapeur, la température élevée ne pose alors aucun problème de différence de dilatation entre couches déposées et plateau.For example, the material 25 or the material 28 of the structure 26 can serve as a support for the semiconductor layers deposited subsequently, which are then the active layers, the materials 25 or 28 serving only as a support. This application is particularly advantageous. Indeed, the materials 25 and 28 are compatible with the deposited layers, and in particular have the same coefficient of expansion. When the active layers are deposited, for example in the vapor phase, the high temperature then poses no problem of difference in expansion between the deposited layers and the plate.
Par exemple, le matériau 25 ou la structure 26 peuvent constituer des plaquettes servant à des composants pour caméras CCD ou écrans plats, ces composants pouvant comporter des transistors en couches minces .For example, the material 25 or the structure 26 can constitute plates used for components for CCD cameras or flat screens, these components being able to comprise thin film transistors.
On va maintenant décrire quelques possibilités offertes par le procédé selon la présente invention, concernant la textu- • ration des matériaux, leur dopage et la réalisation de matériaux semiconducteurs "composites".We will now describe some possibilities offered by the method according to the present invention, on textu- • ration of materials, their doping and manufacturing semiconductor "composite" materials.
La figure 5a illustre un procédé selon la présente invention dans lequel une couche de poudres semiconductrices 30 est placée entre un plateau inférieur 32 de surface plane et un plateau supérieur 34 dont la surface inférieure présente des indentations 35. Les indentations 35 peuvent avoir une taille de l'ordre du cinquième de l'épaisseur de la couche 30. Lors de la ou des étapes de compression, la surface inférieure du plateau 34 imprime le dessin des indentations 35 dans la couche 30. Le matériau obtenu par frittage de la couche 30 conserve de manière fidèle, à sa surface, le motif transmis par le plateau 34. La texture de la surface du matériau est ainsi parfaitement contrôlée et l'on peut par exemple l'adapter à une meilleure absorption de la lumière. Il va de soi qu'il est préférable de réaliser dans ce cas le traitement thermique conduisant à la fusion partielle pendant l'étape de compression, afin de conserver de manière optimale le motif transféré par le plateau. Bien entendu, la fusion partielle pourra aussi intervenir après, si la modification du motif du fait de la fusion n'affecte pas de manière inopportune les caractéristiques souhaitées.FIG. 5a illustrates a method according to the present invention in which a layer of semiconductor powders 30 is placed between a lower plate 32 of planar surface and an upper plate 34 whose lower surface has indentations 35. The indentations 35 can have a size of around a fifth of the thickness of layer 30. During the compression step (s), the lower surface of the plate 34 prints the design of the indentations 35 in the layer 30. The material obtained by sintering the layer 30 retains so faithful to its surface, the pattern transmitted by the plate 34. The texture of the surface of the material is thus perfectly controlled and it can for example be adapted to better absorption of light. It goes without saying that it is preferable to carry out in this case the heat treatment leading to partial melting during the compression step, in order to optimally preserve the pattern transferred by the plate. Of course, the partial merger may also take place afterwards, if the modification of the reason due to the merger does not inappropriately affect the desired characteristics.
La figure 5b illustre un autre exemple de texture pouvant être obtenue à la surface d'un matériau selon la présente invention. Un plateau inférieur 40 présente des nervures parallélépipédiques parallèles 42. Un lit de poudres semi- conductrices 44 est placé sur le plateau 40 et surmonté d'un plateau supérieur 46 de surface plane. Après mise en oeuvre du procédé selon la présente invention, le matériau obtenu présente à sa surface des dépressions parallèles correspondant aux nervures du plateau 40. Comme on le verra ci-après, ces dépres- sions peuvent être remplies par un autre matériau.FIG. 5b illustrates another example of texture that can be obtained on the surface of a material according to the present invention. A lower plate 40 has parallel parallelepiped ribs 42. A bed of semiconductor powders 44 is placed on the plate 40 and surmounted by an upper plate 46 of planar surface. After implementing the method according to the present invention, the material obtained has on its surface parallel depressions corresponding to the ribs of the plate 40. As will be seen below, these depressions can be filled with another material.
On va maintenant décrire, à travers quelques exemples, et en relation avec les figures 6, 7A à 7C, diverses manières de doper le matériau selon la présente invention.We will now describe, through a few examples, and in relation to FIGS. 6, 7A to 7C, various ways of doping the material according to the present invention.
Tout d'abord, dans le procédé de la présente inven- tion, il est possible d'utiliser des poudres de matériaux semiconducteurs préalablement dopées . Le frittage de ces poudres fournit un matériau dopé directement.First of all, in the process of the present invention, it is possible to use powders of previously doped semiconductor materials. The sintering of these powders provides a directly doped material.
Le dopage obtenu peut être homogène, lorsque des poudres d'un type de dopage particulier, N ou P, sont réparties de manière uniforme entre les plateaux de compression. On peut aussi, en répartissant de manière adéquate des poudres de type N ou P plus ou moins dopées, former, au sein du matériau, des zones distinctes présentant un dopage de type et de concentration différentes . Comme cela a été vu, dans le cas d'un mélange de poudres de silicium pur et de silicium dopé, la phase liquide peut être obtenue en fondant uniquement les poudres dopées. On notera que cela procure en outre l'avantage de réduire à pratiquement zéro la porosité des zones dopées. On peut aussi prévoir de ne faire fondre que certaines des zones dopées .The doping obtained can be homogeneous, when powders of a particular type of doping, N or P, are distributed uniformly between the compression plates. It is also possible, by appropriately distributing more or less doped N or P type powders, to form, within the material, distinct zones having doping of different type and concentration. As has been seen, in the case of a mixture of pure silicon powders and doped silicon, the liquid phase can be obtained by melting only the doped powders. Note that this also provides the advantage of reducing the porosity of the doped areas to practically zero. We can also plan to melt only some of the doped zones.
On peut aussi obtenir un matériau dopé en frittant un lit de poudres semiconductrices non dopées auxquelles sont mélangés des dopants ou impuretés sous forme de poudres, comme du bore, du phosphore, de l'antimoine, de l'arsenic, du gallium, de l'aluminium, etc. On notera que ces constituants fondent facilement et que, en fondant, ils optimisent la microstructure de la zone où ils sont présents.A doped material can also be obtained by sintering a bed of undoped semiconductor powders to which are mixed dopants or impurities in the form of powders, such as boron, phosphorus, antimony, arsenic, gallium, l aluminum, etc. It will be noted that these constituents easily melt and that, by melting, they optimize the microstructure of the zone where they are present.
Un dopage homogène du matériau peut aussi être obtenu à l'aide de poudres non dopées et en faisant circuler un gaz porteur d'éléments dopants lors de la mise en oeuvre du procédé selon la présente invention. En effet, au début du traitement, la porosité du lit de poudres est très importante, par exemple de l'ordre de 50%. La porosité est dite ouverte, c'est-à-dire qu'il existe au sein du lit, de poudres ou du matériau en formation des canaux de circulation interconnectés et débouchant sur l'extérieur. Si un gaz dopant circule alors, le gaz dopant se répand dans l'ensemble du matériau et le dope de façon uniforme. L'étape de fusion partielle, qui bouche les canaux de porosité, ne doit intervenir qu'après le dopage ou dans des zones n'intéressant pas celui-ci.Homogeneous doping of the material can also be obtained using undoped powders and by circulating a gas carrying doping elements during the implementation of the method according to the present invention. Indeed, at the start of the treatment, the porosity of the powder bed is very high, for example of the order of 50%. The porosity is said to be open, that is to say that there exist within the bed , powders or material in formation of interconnected circulation channels and opening onto the outside. If a doping gas then circulates, the doping gas spreads throughout the material and dopes it uniformly. The partial melting step, which clogs the porosity channels, must only take place after doping or in areas not of interest.
Pour réaliser une jonction PN, on peut par exemple, réaliser par frittage de poudres un matériau de type N. On le fond localement, par exemple en surface. On le dope ensuite avec un dopage de type P par l'intermédiaire des porosités, par exemple par un gaz . Les parties n' ayant pas fondu se trouvent dopées de type P, alors que les parties ayant fondu, sans porosité, conservent le dopage de type N. Une jonction PN de grande taille peut être ainsi réalisée. La figure 6 illustre une autre façon de doper le matériau au cours de son élaboration. Un plateau inférieur 60 comprend un conduit 62 débouchant sur 1 ' extérieur. Le conduit 62 comporte en outre des ouvertures 64 situées à la surface supérieure du plateau 60. Un lit de poudres 65 est placé sur le plateau 60 pour former le matériau semiconducteur. Au-dessus, est placé un plateau 66 comportant des conduits 68 et 70 débouchant sur 1 ' extérieur et à la surface inférieure du plateau 66. Les conduits 68 relient chacun l'extérieur du plateau à une ouverture particulière de la surface inférieure du plateau 66. Le conduit 70 relie l'extérieur du plateau 66 à plusieurs ouvertures situées sur la surface inférieure du plateau 66.To make a PN junction, it is possible, for example, to produce an N type material by powder sintering. It is locally melted, for example at the surface. It is then doped with P-type doping via the porosities, for example by a gas. The parts which have not melted are doped with type P, while the parts which have melted, without porosity, retain the doping with type N. A large PN junction can thus be produced. Figure 6 illustrates another way to dope the material during its development. A lower plate 60 comprises a conduit 62 opening out to the outside. The conduit 62 further includes openings 64 located on the upper surface of the tray 60. A bed of powders 65 is placed on the tray 60 to form the semiconductor material. Above, is placed a tray 66 having conduits 68 and 70 leading to the outside and to the lower surface of the tray 66. The conduits 68 each connect the outside of the tray to a particular opening in the bottom surface of the tray 66 The duct 70 connects the outside of the tray 66 to several openings located on the lower surface of the tray 66.
Lors de l'étape de compression, un gaz dopant, par exemple de type P, est envoyé dans le conduit 62. Ce gaz, du fait du grand nombre de porosités ouvertes existant au début de la formation du matériau, provoque, au regard des ouvertures 64, le dopage de zones 74 délimitées en pointillés. Selon les conditions de l'envoi du gaz, les différentes zones dopées 74 peuvent se rejoindre. L'étape de traitement thermique devra être adaptée au résultat souhaité. En effet, les porosités ouvertes se ferment au cours de l'étape de traitement thermique. Selon le moment d'action du gaz au cours du procédé, il est possible de réaliser des dopages localisés. Des gaz dopants sont aussi envoyés dans les conduits 68 et 70 pour former respectivement des zones dopées 76 et 78. Comme il est possible de modifier de façon séparée les conditions d'envoi des gaz dans chacun des conduits 68 et le conduit 70, on peut obtenir une taille, un type et une concentration de dopage différents pour chacune des zones 76 et 78. Par conditions d'envoi des gaz, on entend notam- ment leur nature, leur débit ou leur pression, leur temps d'action, le moment où ils agissent, etc.During the compression step, a doping gas, for example of the P type, is sent into the conduit 62. This gas, due to the large number of open porosities existing at the start of the formation of the material, causes, with regard to the openings 64, the doping of areas 74 delimited in dotted lines. Depending on the conditions for sending the gas, the different doped zones 74 can join. The heat treatment step must be adapted to the desired result. In fact, the open porosities close during the heat treatment step. Depending on the moment of action of the gas during the process, it is possible to carry out localized doping. Doping gases are also sent into conduits 68 and 70 to respectively form doped zones 76 and 78. As it is possible to modify the conditions for sending gases separately in each of conduits 68 and conduit 70, it is possible to obtain a different size, type and concentration of doping for each of the zones 76 and 78. By conditions of gas delivery, we mean in particular their nature, their flow or pressure, their action time, the moment where they act, etc.
On va maintenant décrire, en relation avec les figures 7A à 7C, une autre façon de doper le matériau obtenu selon la présente invention. La figure 7A représente schématiquement une vue partiellement en coupe et en perspective d'un matériau 80 de type P obtenu par frittage de poudres selon le procédé de la présente invention. Le matériau 80 présente des dépressions 82 et 84 qui ont été obtenues à l'aide d'un plateau présentant des éléments en saillie de forme correspondante, d'un type similaire à ceux du plateau 40 de la figure 5b. La largeur des dépressions 82 et 84 peut être aussi faible que 1 μm. Les bords des dépressions 82 et 84 sont bien délimités. La dépression 82 est en forme de méandre et la dépression 84 est rectiligne. Les dépressions 82 et 84 sont ensuite remplies chacune de poudres semiconductrices présentant un dopage de type et de concentration souhaités .We will now describe, in relation to FIGS. 7A to 7C, another way of doping the material obtained according to the present invention. FIG. 7A schematically represents a view partially in section and in perspective of a P-type material 80 obtained by sintering powders according to the method of the present invention. The material 80 has depressions 82 and 84 which have been obtained using a plate having projecting elements of corresponding shape, of a type similar to those of the plate 40 of FIG. 5b. The width of depressions 82 and 84 can be as small as 1 μm. The edges of depressions 82 and 84 are well defined. The depression 82 is in the form of a meander and the depression 84 is rectilinear. The depressions 82 and 84 are then each filled with semiconductor powders having doping of the desired type and concentration.
En figure 7B, le matériau 80 présente des zones 86 fortement dopées de type N (N+) et une zone 88 fortement dopée de type P (P+) . Ces zones ont été obtenues en remplissant la dépression 82 de poudres de type N, et la dépression 84 de poudres de type P, puis en frittant ces poudres. Pour ce faire, on peut simplement soumettre le matériau à une étape de traitement thermique.In FIG. 7B, the material 80 has heavily doped N-type areas (N + ) and a heavily doped P-type area 88 (P + ). These zones were obtained by filling the depression 82 with N-type powders, and the depression 84 with P-type powders, then by sintering these powders. To do this, the material can simply be subjected to a heat treatment step.
La figure 7C représente une vue de dessus d'un matériau semiconducteur 90 selon la présente invention, dans lequel des zones 92 fortement dopées de type N et des zones 94 fortement dopées de type P ont été obtenues selon le procédé décrit en relation avec les figures 7A et 7B. Les zones 92 et 94 sont inter-digitées . La face qui comporte les zones 92 et 94 est destinée à être la face non exposée à la lumière. Cela rend inutile la réalisation d'un peigne collecteur comme la peigne 5 de la figure 1 et augmente de manière correspondante la surface éclairée de la photopile.FIG. 7C represents a top view of a semiconductor material 90 according to the present invention, in which heavily doped N-type areas 92 and heavily doped P-type areas 94 were obtained according to the method described in relation to the figures 7A and 7B. Zones 92 and 94 are intersected. The face which comprises the zones 92 and 94 is intended to be the face not exposed to light. This makes it unnecessary to make a collecting comb like the comb 5 in FIG. 1 and correspondingly increases the illuminated surface of the photocell.
On notera que les matériaux comportant des jonctions PN décrits ci-dessus sont des composants très proches du produit fini que représente une photopile. Le procédé selon la présente invention permet de se rapprocher encore plus du produit fini. D'une part, lorsque la jonction PN est dans l'épaisseur du matériau, il est possible de placer un lit de poudres d'aluminium à la base du lit de poudres semiconductrices lors de la' fabrication du matériau. Le matériau obtenu après frittage comprend ainsi la couche conductrice inférieure, qui n'a plus besoin d'être déposée par la suite. Une zone fortement dopée de type P, comme la zone 3c de la figure 1, se trouve produite naturellement au contact entre le matériau de type P et l'aluminium. On peut aussi placer une fine couche de poudres fortement dopées de type P, par exemple de un à quelques micromètres, sur la couche de poudres d'aluminium lors de la fabrication du matériau. Le peigne collecteur supérieur peut être aussi réalisé lors de l'élaboration du matériau, en plaçant des poudres adéquates, comme d'aluminium, aux endroits appropriés. On peut aussi, pour la transmission du courant, placer des poudres de céramique conductrice transparente sur toute la surface du matériau exposée à la lumière.It will be noted that the materials comprising PN junctions described above are components very close to the finished product that a photocell represents. The process according to the present invention makes it possible to get even closer to the finished product. On the one hand, when the PN junction is in the thickness of the material, it is possible to place a bed of aluminum powders at the base of the bed of semiconductor powders during the manufacture of the material. The material obtained after sintering thus comprises the lower conductive layer, which no longer needs to be deposited thereafter. A heavily doped P-type zone, like zone 3c in FIG. 1, is naturally produced in contact between the P-type material and aluminum. One can also place a thin layer of heavily doped p-type powders, for example from one to a few micrometers, on the layer of aluminum powders during the manufacture of the material. The upper collecting comb can also be produced during the preparation of the material, by placing suitable powders, such as aluminum, in the appropriate places. It is also possible, for current transmission, to place transparent conductive ceramic powders over the entire surface of the material exposed to light.
D'autre part, lorsque la jonction PN est en surface comme en figure 7C, on peut déposer des poudres conductrices sur les poudres destinées à former les zones dopées (surface non éclairée du matériau) avant leur traitement thermique. Le matériau obtenu comporte ainsi deux zones conductrices inter- digitées, qui forment des collecteurs particulièrement efficaces des porteurs créés par effet photoélectrique. On va maintenant décrire un matériau selon la présente invention obtenu par frittage de poudres semiconductrices de nature différente. Les poudres utilisées peuvent appartenir à tout élément de la colonne IV du tableau de Mendeleïev, et/ou à leurs alliages . La figure 8 représente schématiquement une vue de dessus d'un matériau 100 selon la présente invention. Le matériau 100 a été obtenu, par exemple par application du procédé selon la présente invention, à un lit de poudres comprenant des poudres d'étain Sn, de germanium Ge, de silicium Si et de carbone C. Une zone 102 formée d'étain longe le bord 104 du matériau 100. La zone 102 résulte du frittage de poudres d'étain placées le long du bord latéral 104. Le contour irrégulier de la zone 102 s'explique notamment par le fait que l'étain fond aux températures utilisées dans le ' procédé et a tendance à se répandre dans les porosités ouvertes du matériau. Le matériau 100 comporte aussi des îlots 106 de germanium Ge, résultant du frittage de poudres de germanium. De même, les poudres de silicium donnent naissance à des îlots 108 de silicium et les poudres de carbone, qui, dans l'exemple représenté ont été déposés plutôt vers le bord 112 du matériau, donnent naissance à des îlots de carbone C.On the other hand, when the PN junction is on the surface as in FIG. 7C, it is possible to deposit conductive powders on the powders intended to form the doped zones (unlit surface of the material) before their heat treatment. The material obtained thus comprises two intersecting conductive zones, which form particularly efficient collectors of the carriers created by the photoelectric effect. We will now describe a material according to the present invention obtained by sintering semiconductor powders of different nature. The powders used can belong to any element in column IV of the Mendeleev table, and / or to their alloys. FIG. 8 schematically represents a top view of a material 100 according to the present invention. The material 100 was obtained, for example by applying the method according to the present invention, to a bed of powders comprising powders of tin Sn, germanium Ge, silicon Si and carbon C. A zone 102 formed of tin along the edge 104 of the material 100. The zone 102 results from the sintering of tin powders placed along the lateral edge 104. The irregular contour of the zone 102 is explained in particular by the fact that the tin melts at the temperatures used in the process and tends to spread in the open pores of the material. The material 100 also includes islands 106 of germanium Ge, resulting from the sintering of germanium powders. Similarly, the silicon powders give rise to islands 108 of silicon and the carbon powders, which, in the example shown have been deposited rather towards the edge 112 of the material, give rise to islands of carbon C.
En outre, le matériau 100 comporte des îlots 114 d'alliage SiGe, des îlots 116 de SixGe, des îlots 118 de SiyC. Le matériau peut comporter aussi des îlots de GexC et de SixGeyC. Ces alliages naissent au contact des grains de différente nature lors du traitement thermique, les divers grains s ' agglomérant par frittage. Si cela est souhaité, on peut limiter la formation de ces alliages en plaçant les poudres de nature différente de façon à ce qu'elles ne se mélangent pas trop. On peut aussi disposer des poudres d'alliages divers dans le lit de poudres à fritter, pour augmenter la proportion des alliages . En outre, les poudres utilisées ou les matériaux obtenus peuvent être dopés comme cela est décrit ci-dessus.In addition, the material 100 includes islands 114 of SiGe alloy, islands 116 of Si x Ge, islands 118 of SiyC. The material can also include islands of Ge x C and Si x GeyC. These alloys are born in contact with grains of different nature during the heat treatment, the various grains agglomerating by sintering. If desired, the formation of these alloys can be limited by placing powders of a different nature so that they do not mix too much. It is also possible to have powders of various alloys in the bed of powders to be sintered, in order to increase the proportion of the alloys. In addition, the powders used or the materials obtained can be doped as described above.
On notera qu'avec les procédés classiques de fabrication de matériaux semiconducteurs, comme les procédés utilisant des bains fondus, seuls des alliages homogènes peuvent être obtenus et un matériau "composite" comme le matériau 100 ne peut être obtenu.It will be noted that with conventional methods of manufacturing semiconductor materials, such as methods using molten baths, only homogeneous alloys can be obtained and a "composite" material such as material 100 cannot be obtained.
Le matériau 100 est particulièrement avantageux dans des applications photovoltaïques .The material 100 is particularly advantageous in photovoltaic applications.
En effet, la longueur d'onde des radiations absorbées par un élément semiconducteur dépend de la valeur de la bande interdite de cet élément. Ainsi, le silicium, dont la bande interdite vaut 1,1 eV, est naturellement optimisé pour la lumière visible. Les radiations infrarouges ne sont pratiquement pas absorbées par le silicium. Les radiations ultraviolettes, elles, sont absorbées rapidement par le silicium, mais l'excès d'énergie représenté par la différence entre l'énergie du rayonnement et la valeur de la bande interdite est perdu. Le germa- nium, dont la bande interdite vaut 0,7 eV, est particulièrement bien adapté pour absorber la lumière infrarouge. Un alliage de type SixGe a une bande interdite comprise entre la bande interdite du silicium et celle du germanium. Un alliage de type SixC a une bande interdite très supérieure à celle du silicium. Un alliage de ce type répond particulièrement bien aux radiations bleues et ultraviolettes .Indeed, the wavelength of the radiation absorbed by a semiconductor element depends on the value of the band gap of this element. Thus, silicon, whose band gap is 1.1 eV, is naturally optimized for visible light. Infrared radiation is practically not absorbed by silicon. Ultraviolet radiation is absorbed quickly by silicon, but the excess energy represented by the difference between the energy of the radiation and the value of the band gap is lost. Germanium, whose band gap is 0.7 eV, is particularly well suited for absorbing infrared light. An alloy of Si x Ge type has a band gap between the band band of silicon and that of germanium. An alloy of Si x C type has a forbidden band much greater than that of silicon. An alloy of this type responds particularly well to blue and ultraviolet radiation.
Il en résulte que le matériau 100 est à bande interdite localement variable. Cela constitue un avantage extrêmement important, car on peut utiliser au mieux les radiations dans une application photovoltaïque. Par exemple, le matériau 100 peut pratiquement répondre à l'intégralité du spectre solaire, ce qui n'est pas le cas pour une photopile classique en silicium.As a result, the material 100 has a locally variable band gap. This is an extremely important advantage, since radiation can be used to best advantage in a photovoltaic application. For example, the material 100 can practically respond to the entire solar spectrum, which is not the case for a conventional silicon photocell.
La figure 9 représente schématiquement un lit de poudres 120 destiné à l'élaboration d'un matériau selon la présente invention. Le lit de poudres 120 comprend une couche inférieure 122 de poudres d'étain, suivie d'une couche 124 de poudres de germanium, suivie d'une couche 126 de poudres de silicium, le tout étant surmonté d'une couche 128 de poudres d'un alliage SixC de carbone et de silicium. Les couches de poudres 122, 124, 126 et 128 sont disposées par ordre croissant de bande interdite.FIG. 9 schematically represents a bed of powders 120 intended for the preparation of a material according to the present invention. The powder bed 120 comprises a lower layer 122 of tin powders, followed by a layer 124 of germanium powders, followed by a layer 126 of silicon powders, the whole being surmounted by a layer 128 of powders. 'an alloy Si x C of carbon and silicon. The powder layers 122, 124, 126 and 128 are arranged in increasing order of the prohibited band.
Après frittage, le matériau semiconducteur obtenu comporte ainsi plusieurs couches superposées de matériaux de bandes interdites différentes. Dans une application photovoltaïque, la face du matériau qui comporte la couche de bande interdite la plus grande, SixC, est exposée à la lumière. La couche d'alliage SixC absorbe le rayonnement ultraviolet et alentour et laisse passer les radiations visibles et infra- rouges. La couche de silicium absorbe la lumière visible et est pratiquement transparente aux radiations infrarouges, qui sont absorbées par la couche de germanium. Divers alliages créés au cours du frittage aident à l'absorption du rayonnement. La couche d'étain, enterrée, sert principalement à collecter les porteurs nés de l'effet photovoltaïque. Comme précédemment, une jonction PN peut être réalisée par un dopage approprié .After sintering, the semiconductor material obtained thus comprises several superimposed layers of materials with different prohibited bands. In a photovoltaic application, the face of the material which has the largest band gap layer, Si x C, is exposed to light. The Si x C alloy layer absorbs and around ultraviolet radiation and allows visible and infrared radiation to pass through. The silicon layer absorbs visible light and is practically transparent to infrared radiation, which is absorbed by the germanium layer. Various alloys created during sintering aid in the absorption of radiation. The layer of tin, buried, is mainly used to collect the carriers born from the photovoltaic effect. As before, a PN junction can be achieved by appropriate doping.
Par rapport au matériau de la figure 8, le matériau obtenu par le lit de poudres de la figure 9 est avantageux en ce que les radiations traversent successivement des couches de bande interdite décroissante. Cela permet une absorption plus complète du rayonnement.Compared with the material of FIG. 8, the material obtained by the powder bed of FIG. 9 is advantageous in that the radiation successively passes through layers of decreasing forbidden band. This allows more complete absorption of the radiation.
Bien entendu, la présente invention n'est pas limitée aux exemples décrits et toute variante, modification ou équivalent à la portée de l'homme de l'art fait partie du domaine de la présente invention.Of course, the present invention is not limited to the examples described and any variant, modification or equivalent within the reach of ordinary skill in the art is part of the field of the present invention.
En particulier, les plateaux utilisés pour comprimer le lit de poudres ne sont pas nécessairement plans et peuvent être de forme quelconque.In particular, the plates used to compress the bed of powders are not necessarily planar and can be of any shape.
La figure 10 représente ainsi un matériau semi- conducteur 130 en forme de tuile pouvant s'intégrer à la structure d'un toit. Le matériau 130, ci-après appelé tuile, comporte une extrémité 131 non plane permettant de recouvrir la tuile suivante 130' et de s'y connecter. La tuile 130 est obtenue par frittage d'un lit de poudres semiconductrices à l'aide de plateaux de forme correspondante. Le lit de poudres a été réalisé de façon à créer successivement une fine couche 132 fortement dopée de type N (N+) , une couche 134 dopée de type N, suivie d'une couche 136 dopée de type P. A l'extrémité opposée à l'extrémité 131 se trouve une zone peu étendue 138 fortement dopée de type P (P+) . La tuile 130 est connectée à la tuile 130' par un moyen de fixation conducteur quelconque 140, comme une soudure ou un fil souple, reliant la couche N+ d'une tuile à la zone P+ de la tuile suivante. Les photopiles représentées par les tuiles 130 et 130' sont ainsi connectées en série. Divers autres groupements d'un ensemble de tuiles, en série et/ou en parallèle, permettent d'obtenir les caractéristiques souhaitées d'une installation.FIG. 10 thus represents a semiconductor material 130 in the form of a tile which can be integrated into the structure of a roof. The material 130, hereinafter called the tile, has a non-planar end 131 making it possible to cover the next tile 130 'and to connect to it. The tile 130 is obtained by sintering a bed of semiconductor powders using trays of corresponding shape. The powder bed was produced so as to successively create a thin layer 132 heavily doped with type N (N + ), a layer 134 doped with type N, followed by a layer 136 doped with type P. At the opposite end at the end 131 is a small area 138 highly doped P-type (P +). The tile 130 is connected to the tile 130 'by any conductive fixing means 140, such as a solder or a flexible wire, connecting the N + layer of a tile to the zone P + of the next tile. The solar cells represented by the tiles 130 and 130 ′ are thus connected in series. Various other groupings of a set of tiles, in series and / or in parallel, make it possible to obtain the desired characteristics of an installation.
On notera que, pour l'étape de fusion, on pourra utiliser tout moyen approprié, comme des fours résistifs, four à lampes, four solaire etc., l'énergie étant transférée par conduction, convection, radiation, etc.It will be noted that, for the melting step, any suitable means may be used, such as resistive ovens, lamp furnaces, solar furnaces, etc., the energy being transferred by conduction, convection, radiation, etc.
On notera aussi que toute structure ou composant comprenant ou formé d'un ou de plusieurs matériaux selon la présente invention fait partie du domaine de la présente invention. On notera aussi que les matériaux selon la présente invention ne sont pas limités aux matériaux obtenus par le procédé selon la présente invention. Par exemple, tout matériau semiconducteur comportant des grains et/ou des agrégats présentant des bandes interdites différentes fait partie du domaine de la présente invention, quel que soit son mode d'obtention. It will also be noted that any structure or component comprising or formed from one or more materials according to the present invention is part of the field of the present invention. It will also be noted that the materials according to the present invention are not limited to the materials obtained by the method according to the present invention. For example, any semiconductor material comprising grains and / or aggregates having different forbidden bands is part of the field of the present invention, whatever its mode of production.

Claims

REEM)ICZ-.TIOMS REEM) ICZ-.TIOMS
1. Procédé de formation d'un matériau semiconducteur (25, 90, 100, 130) à partir de poudres comprenant au moins un constituant appartenant au groupe constitué par les éléments de la colonne IV du tableau de Mendeleïev et leurs alliages, caractérisé en ce qu'il comprend une étape de compression desdites poudres et une étape de traitement thermique telle qu'une partie au moins des poudres est fondue ou rendue visqueuse.1. Method for forming a semiconductor material (25, 90, 100, 130) from powders comprising at least one constituent belonging to the group consisting of the elements of column IV of the Mendeleev table and their alloys, characterized in that that it comprises a step of compressing said powders and a step of heat treatment such that at least part of the powders is melted or made viscous.
2. Procédé selon la revendication 1, caractérisé en ce que les étapes de compression et de traitement thermique sont simultanées .2. Method according to claim 1, characterized in that the stages of compression and heat treatment are simultaneous.
3. Procédé selon la revendication 1 ou 2, caractérisé en ce que le traitement thermique est tel que seules des poudres appartenant à une zone particulière du matériau sont fondues ou rendues visqueuses.3. Method according to claim 1 or 2, characterized in that the heat treatment is such that only powders belonging to a particular zone of the material are melted or made viscous.
4. Procédé selon l'une quelconque des revendications 1 à 3, caractérisé en ce que les poudres comprennent des poudres de silicium et des poudres d'au moins un autre constituant, le traitement thermique étant tel que le silicium n'est pas fondu et qu'au moins un des autres constituants est fondu ou rendu visqueux.4. Method according to any one of claims 1 to 3, characterized in that the powders comprise silicon powders and powders of at least one other constituent, the heat treatment being such that the silicon is not molten and that at least one of the other constituents is melted or made viscous.
5. Procédé selon l'une quelconque des revendications 1 à 3, caractérisé en ce que les poudres comprennent des poudres semiconductrices dopées et des poudres semiconductrices non dopées, le traitement thermique étant tel que seules les poudres dopées sont fondues .5. Method according to any one of claims 1 to 3, characterized in that the powders comprise doped semiconductor powders and undoped semiconductor powders, the heat treatment being such that only the doped powders are melted.
6. Procédé selon l'une quelconque des revendications 1 à 5, caractérisé en ce que l'étape de compression est précédée d'une étape consistant à placer des poudres sur un plateau (10), les poudres étant différentes quant à leur nature, leur granulométrie et/ou leur dopage selon leur emplacement sur le plateau.6. Method according to any one of claims 1 to 5, characterized in that the compression step is preceded by a step consisting in placing powders on a tray (10), the powders being different as to their nature, their particle size and / or their doping according to their location on the plate.
7. Procédé selon l'une quelconque des revendications 1 à 6, caractérisé en ce que, lors de l'étape de compression, lesdites poudres sont pressées entre des plateaux (10, 20) dont la surface est propre à texturer la surface du matériau.7. Method according to any one of claims 1 to 6, characterized in that, during the compression step, said powders are pressed between plates (10, 20) whose surface is suitable for texturing the surface of the material.
8. Matériau semiconducteur (25, 90, 100, 130) obtenu au moins partiellement par compression et traitement thermique de poudres, caractérisé en ce qu'il comporte au moins deux zones distinctes (102, 106, 108, 110, 114, 116, 118) formées de constituants distincts appartenant au groupe constitué par les éléments de la colonne IV du tableau de Mendeleïev et leurs alliages . 8. Semiconductor material (25, 90, 100, 130) obtained at least partially by compression and heat treatment of powders, characterized in that it comprises at least two distinct zones (102, 106, 108, 110, 114, 116, 118) formed of separate constituents belonging to the group consisting of the elements of column IV of Mendeleev's table and their alloys.
9. Matériau selon la revendication 8, caractérisé en ce que lesdites zones sont superposées .9. Material according to claim 8, characterized in that said zones are superimposed.
10. Structure ou composant formé d'un ou comprenant au moins un matériau semi-conducteur comportant des grains et/ou des agrégats présentant des bandes interdites de valeur différente. 10. Structure or component formed from or comprising at least one semiconductor material comprising grains and / or aggregates having prohibited bands of different value.
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JP4869061B2 (en) 2012-02-01
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US20070178675A1 (en) 2007-08-02

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