EP1618612A1

EP1618612A1 - Sintered semiconductor material

Info

Publication number: EP1618612A1
Application number: EP04742838A
Authority: EP
Inventors: Alain Straboni
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite de Poitiers
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite de Poitiers
Priority date: 2003-04-14
Filing date: 2004-04-09
Publication date: 2006-01-25
Also published as: JP4869061B2; JP2006523021A; WO2004093202A1; US20070178675A1; US8105923B2

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

SEMICONDUCTOR MATERIAL OBTAINED BY SINTERING

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.

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 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.

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.

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.

According to an embodiment of the present invention, the compression and heat treatment steps are simultaneous.

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.

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.

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.

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.

According to an embodiment of the present invention, 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.

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.

FIG. 2 illustrates an embodiment of the method according to the present invention.

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.

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”.

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.

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.

For 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

(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.

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.

We will give below some examples of materials obtained.

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.

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.

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.

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.

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. 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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. 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

The material 100 is particularly advantageous in photovoltaic applications.

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.

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.

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.

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.

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.

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.

In particular, 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. 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.

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.

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

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. Method according to claim 1, characterized in that the stages of compression and heat treatment are simultaneous.

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. 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. 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. 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. 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. 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. Material according to claim 8, characterized in that said zones are superimposed.

10. Structure or component formed from or comprising at least one semiconductor material comprising grains and / or aggregates having prohibited bands of different value.