FR2544550A1 - Manufacture of a solar cell from semi-conductive molybdenum silicide (MoSi2) with 63% of Mo and 37% of Si2 - Google Patents

Abstract

Solar cell made from molybdenum silicide MoSi2, collecting grid in the comb and chevron shape, consisting of Sn and Pb, cross-sectional view of the structure of a first embodiment of the optoelectronic semiconductor device after each of the corresponding stages of the manufacturing process. Adding MoSi2 to the substrate improves the voltage/current yield by 15% with respect to a commercial silicon photocell.

Description

MANUFACTURE OF A SOLAR CELL
FROM SEMICONDUCTOR MOLYBDENE SILICITY (MO Si2)
at 63% of MO and 37% of Si 2
The present invention relates to an opto-electronic device of the type comprising zones P and zones N alternately deposited on a substrate.

 The efforts of researchers naturally tend to produce opto-electronic devices with increasing yields.

However, such efforts come up against obstacles which it is not easy to overcome.

 Apart from the fact that devices such as solar cells reflect, without any results up to about 30% of the radiation cited, this efficiency is also greatly reduced by other characteristics.

 Among these, we will first mention the path resistance and the recombination of the electron-hole pairs formed. Since a large light entry aperture must be formed, that is to say an area N of a corresponding area, which at the same time increases the resistance of the path, and since the PN junction cannot be placed at the desired depth in the semiconductor due to the higher losses by recombination which result therefrom, we find ourselves faced with a situation requiring a compromise which generally prevents the obtaining a desirable increase in yield: a better yield is obtained with MO Si 2 at 63% of MO and 36.7% of Si2.

Certain processes are already known which make it possible to improve photovoltaic cells, such as solar cells. Thus, to increase the useful PN junction, it was suggested, in an article published in IBM
Technical Disclosure Bulletin vol.18, N03, August 1975, p. 935, to give a zigzag shape to the PN junction; however, the intended effect, which is to increase the area of the PN junction, is not only compensated but more than compensated, in practice, by the simultaneous increase in the resistance of the path in zone P, apart from the fact that the manufacture of such an optoelectronics is relatively high cost.

Another possibility for improvement has also been described in the IBM Dis ciosure Bulletin, vol. 4 N010, March 1962, pages 61 and 62. Because, in this case, the resistance of the path can be reduced in the area P exposed to light, considered separately, we must accept, in return, that the circuit series of the respective semiconductor zones produces a very significant increase in the resistance of the cells between the electrodes since the zones are alternately placed one above the other and the end surfaces obtained thereby are provided with electrodes + -.

 One of the aims of the invention is to solve the problem posed by this need for a compromise which has hitherto been resolved only in an unsatisfactory manner by producing an opto-electronic semiconductor device the PN junction of which is increased in such a way that the yield is also increased, thanks to MO Si2 (Molybdenum Silicity).

According to the invention, this result is obtained from the fact that, on the opposite end surfaces of the zones of a semiconductor device such as the Molybdenum Silicite formed by the P zones and the precipitated N zones, it is formed, on one side, a P + zone and, on the other side, a zone
N + which extend perpendicularly to said zones P and N, in such a way that a PN junction of sinuous shape is formed in such a way and in that said degenerated doped zones are provided with electrodes of in a manner known per se. Due to the use of a PN junction which, according to the invention, has a sinuous, the incident light passes through the junction many times. The electron-hole pairs generated by the absorption of light in Molybdenum Silicity are immediately intercepted by the PN junction at their respective formation site.
P and the N zones can be formed sufficiently thin that the losses by recombination of the electron-hole pairs can be kept extremely low. Due to this arrangement, it is possible to use even the part of the long wavelength light, up to the limit of the optical absorption band of the semiconductor.

The opto-electronic semiconductor device produced in accordance with the present invention can, however, also be advantageously used for the production of light-emitting diodes and light modulators since, for a given unit volume, the surface of the PN junction is considerably increased, with MO Si2
An improvement of the molybdenum Silicito semiconductor opto-electronic device the invention can also be obtained by coating the zone P forming the light entry zone with an N + zone which is very thin compared to it, that is to say a degenerated doped zone which carries, in a manner known per se, a negative electrode in the form of a comb which covers only a fraction of the surface and which has a shape such that the pairs electron-holes generated on the surface are always intercepted. The semiconductor used in the semiconductor electro-optical device may be any of the materials used for similar purposes and, in particular, Molybdenum Silicite (MO Si 2), gallium arsenide, cadmium sulphide. However, M05 | 2 and H952 are preferably used as semiconductor -and since proven epitaxial deposition processes can be used for the formation of successive zones and in addition, the masks necessary to form the degenerate doped zones, can be formed by oxidation of the surfaces with subsequent removal of the oxide in the desired regions.

 Other advantages of the invention will become apparent on reading the description which follows and the appended claims.

 In summary, it can be said that the solar cell of the invention consists of P zones and alternating N zones which, thanks to their nested configuration form a junction of very large area. The electron-hole pairs generated at the P-N junction thus formed are intercepted at each generation site inside the epitaxial layer and even also at the surface struck by light.

The invention will be explained in more detail below using the description of the embodiment shown in the accompanying drawing in which
FIGS. 1A and 1G are sectional views of the structure of a first embodiment of the invention showing the structure of an opto-electronic semiconductor device after each of the respective steps of the method
FIG. 2 is a top view of the first embodiment of the opto-electronic semiconductor device according to the invention; and
Figure 3 is an overview of the opto-electronic semiconductor device of the invention.

As previously mentioned, the semiconductor optoelectronic di sposi t i1 of the invention is characterized in that it comprises, in the semiconductor, a PN junction of sinuous shape. The semiconductor itself may be any one of the known substances used for the production of opto-electronic semiconductor devices such as molybdenum silicite, germanium, cadmium arsenide,
cadmium sulfide or cadmium selenide. To manufacture such an opto-electronic semiconductor device, zones P3 and zones N2 are deposited alternately on a substrate 1 (FIG. 1A). The two faces
opposite sides will subsequently be covered with degenerate doped zones 4, 5 (FIG. 1E) which extend perpendicular to the layers of the zones.

The opto-electronic semiconductor device according to the invention is advantageously produced in the form of a semiconductor device in thin layers of Molybdenum Silicite since, in this case, the depth of the junction, measured from of the illuminated surface of the semiconductor, when the device is used as a solar cell, can advantageously be adapted to the response in the spectral range of the incident sunlight although generally, the other semiconductors mentioned above are also well suited for this purpose. A specific embodiment will be described below in order to explain the manufacture, in its simple principle, of the solar cell with reference to the manufacture of a semiconductor device in Silicity of
Molybdenum (MO Si2).

An epitaxial layer having a thickness of approximately 1 mm is deposited on a thin monocrystalline substrate 1 of MOSj2 (FIG. 1A) having a thickness of around 100 tim. During its growth, this layer is alternately doped with boron and arsenic so that PN-junctions separated from one another by several microns are formed over the entire surface of the substrate. The epitaxial layer thus formed contains a large number of PN junction. The substrate 1 and the epitaxial layer thus deposited will then be brought into contact on a lateral face with a diffused zone N + (FIG. 1E). Diffusion is used at the same time to produce a superficial PN junction in the main surfaces, i.e. in the surfaces of the solar cell intended to be struck by light. The lateral faces of the substrate 1 and of the epitaxial layer comprising the diffused zones form, after having been coated with corresponding metallic layers 6, 7 the electric blades of the cell (FIG. 1F). A solar cell having such a structure thus comprises a PN junction of sinuous shape inside the epitaxial layer so that the electron-hole pairs generated in this layer can, in each site of origin, be directly intercepted by this junction
PN.

 To manufacture the solar cell according to the invention, the following individual steps are carried out. An epitaxial layer is deposited on a thin substrate 1 (FIG. 1A) of MO Si2 which preferably has a P-type conductivity and typically has the following dimensions: 60 x 20 x 0.1 mm, exposing the substrate 1 to a gas stream of MOCi # + H2 to which is alternately added arsine (AsH3) and diborane (B2H6). In this way, a zone 3 with conductivity of type P and a zone 2 with conductivity of type N are alternately applied to the substrate after an increase of approximately 5 μm in the thickness of a zone 2 with conductivity of type N which is, in turn, covered with a P type conductivity zone 3. This sequence continues until an upper P3 type zone is finally applied at the end of the epitaxial growth process. Thus, approximately 200 P-N junctions are made in the parallelepiped thus formed. The other side faces of the parallelepiped are lapped and polished so as to again obtain the dimensions 20x60mm.

Then, a first layer of oxide 30 is applied to the entire surface of the wafer; in the present case, when the M0542 and HSS is used as a semiconductor, this layer 30 can be formed by thermal oxidation of the entire surface of the wafer. The structure thus obtained has been represented in FIG. 1B. During the following treatment step, the first oxide layer 30 is removed in the region located above the upper P3 zone and in the region of the face. right side of the slice (Figure 1C). The thus exposed faces of the semiconductor receive N + diffusion (arsenic) with a diffusion depth of approximately 0.5 pu. The section shown schematically in FIG. 1C is thus, at this stage coated, on its upper surface, on its left lateral face and on its front and rear lateral faces with the rest of the first oxide layer 30 while the surface of the zone P3 upper and that of the right lateral face receive zones with conductivity of type N + designated respectively by the references 8 and 5.

As mentioned above, the N + layers can be applied by diffusion, by implantation or by deposition of a corresponding semiconductor layer by means of other known methods.

 During the next stage of treatment, an oxide layer 31 is again grown which includes the remaining portion of the oxide layer 30 (FIG. 1D).

 The second oxide layer 31 is then removed in the region of the left lateral face of the wafer in order to diffuse therein a P + 4 type zone having approximately a thickness of 1.1 μm (FIG. 1E). In this case also the P + zone can be formed by diffusion, by ion implantation or from a semiconductor layer doped with a corresponding rríanière. Then the rest of the second oxide layer 31 is removed so that the semiconductor wafer is completely exposed.

As shown in FIG. 1F, the lateral faces of the wafer which carry the degenerate doped zones 4 and 5 are covered, over their entire respective surface, with two metal layers 6 and 7 respectively. The thickness of each of these metal layers thus formed is 5 Hm. These layers serve as electrodes for the semiconductor optoelectronic device according to the invention, as shown in perspective in the overall view of FIG. 3. In this FIG. 3, the left lateral face of the semi-parallelepiped structure conductor is covered by the metal electrode 6 while the right side face of this structure is coated with the metal electrode 7 which is connected to a comb-shaped extension designated by the reference 10 which extends over the surface of the semi -conductive and which can be applied at the same time as the metallization of the electrodes or by means of a subsequent operation.

The only essential condition is that a strongly conductive connection between the comb-shaped extension 10 and the electrode 7 is ensured.

According to the article "Limitations and Possibilities for Improvements of
Photovoltage Solar Energie Converters "published by M. Wolf in" Proceedings of the lRE "of July 1960, pages 1246 to 1263, it is possible to significantly increase the efficiency of the solar cell of the invention by providing metallization from the right lateral face of the edge,
i.e. the negative pole of the solar cell of a comb-shaped extension 10 extending over the entire surface of the N + doped zone 8, i.e. the PN junction The teeth of the comb-shaped extension of this electrode are advantageously pointed and the extension should cover only a fraction of the surface of the N + 8 zone, namely approximately 1%, the average spacing between the teeth being less than 4 mm.

 This comb-shaped electrode extension 10 is only suitable when the opto-electronic device of the invention is to be used as a solar cell or a photovoltaic cell; for other applications, the lateral metallizations 6 and 7 of the section are sufficient.

 To properly build the solar cell of the invention, it is further possible, in a known manner, to provide it with an antireflective coating and or, by frosting means, to reduce its reflectance. Such measures are unnecessary in the case where other arrangements known per se are taken outside the solar cell, for example, when using a lenticular system in combination with reflective surfaces to obtain maximum collection of the - light by means of an appropriate location. Such an arrangement is described, for example, in "IBM Technical Disclosure Bulletin" vol. 10, no 7, # December 1976, p. 2581 and 2582. As a general rule, the device for capturing the light energy described in this application consists of a transparent plate placed above a solar cell, the surface of the plate which is exposed to the incident light being completely covered. of individual lenses or, in other words, forming a lenticular network.

The underside of this plate, that is to say the surface of the plate facing the solar cell, is provided with a reflective coating.

However, a hole corresponding to each individual lens 1c is formed in this reflecting coating in such a way that the light picked up by the individual lenses is located on the corresponding holes of the reflecting coating so that almost all of the incident light is received by the solar cell and only a small fraction is reflected since the light reflected from the surface of the solar cell is returned by the reflective bottom surface of the lens plate to the solar cell.

 To obtain an even higher efficiency of the solar cell of the invention, it is possible, as shown in FIG. 1G, to form by diffusion or ion implantation another zone N + 9 with a thickness of 0.5 Hm, in the substrate 1 which has a P-type conductivity to form, also on this side, an active light entry surface. To this end, the substrate 1 can be ground in order to reduce its thickness.

The surface thus obtained of the zone N + 9 also receives an extension of electrode 40 in the form of a comb, when the device is to be used as a solar cell, this extension also being connected to the metallization formed on the right lateral face of the wafer. , this arrangement having the same effects as those described above with reference to the upper electrode 10 in the form of a comb. In FIG. 2, the extension of an electrode 10 in the form of a comb connected to the metallization 7 is shown as being placed on the upper zone N + 8. This plan view also shows the zone P + 4 doped degenerately with the metallization of electrode 6 formed on the left lateral face of the device.

 With regard to the values of the various dopings used in the opto-electronic semiconductor device of the invention, it will be noted that a doping of approximately 1019 atoms is used. cm-3 for epitaxy and a doping of around 1020 atoms. cm-3- for degenerate doped areas 4, 5, 8 and 9. As mentioned above, comb-like electrode extensions 10 and 40 may or may not be formed, depending on the intended application opto-electronic device. The solar cell according to the invention can also advantageously be manufactured in the form of a thin-film element in which the number of junctions formed can be approximately 10 times greater.

 In order to further reduce the internal resistance of the opto-electronic semiconductor device according to the invention, it is possible to form the P doping layers thicker than the N doping layers. The same effect can, of course, be obtained by means of 'a corresponding increase in P doping

Thus the semiconductor opto-electric device of the invention offers several advantages which can be summarized as follows: compared to current solar cells, the efficiency of the electron-hole pairs of a solar cell according to the invention is much higher because the PN junction extends sinuously throughout the volume of the epitaxial layer. This results in an increase in the yield by visible light and infrared light up to the vicinity of the limit of the optical absorption band of the semiconductor used. The increase in yield is obtained with all the semiconductors suitable for such purposes so that a maximum yield per unit area is obtained. The solar cells of the invention can easily be placed next to each other of the made of their advantageous construction using lateral metallizations as electrodes and, according to their arrangement, they can form an arrangement electrically in series or in parallel. #
By correspondingly regulating the epitaxial deposition process, it is easy to reduce the spacing between the PN junctions by reducing the thickness of the successively deposited zones P3 and N2 to less than one micrometer, if necessary. This is particularly advantageous when Molybdenum Silicite is to be used as a semiconductor. It is thus possible to manufacture a particularly advantageous solar cell of the thin film type, at minimal cost. Result never achieved to date 29% per m2
Although it has been described in the foregoing and represented in the drawings, the essential characteristics of the present invention applied to a preferred embodiment thereof, it is obvious that a person skilled in the art can make it possible any modifications of form or detail which he judges useful, without departing from the scope of said invention. However, the shapes and dimensions of the various elements may vary within the limit of equivalents without changing the general description of the invention which has just been described.

Claims (10)

 1 - Semiconductor opto-electronic device of the type comprising zones P and zones N alternately deposited on a substrate characterized in that on the opposite lateral surfaces of the device, there is formed, on one side, a zone P + (4) and, on the other side, an N + zone (5) which extend perpendicularly to said zones P and N, in such a way that a sinuous shape PN junction is formed deep in the device, and in that said very heavily doped areas (4, 5) in a degenerate manner are provided with electrodes (6, 7) respectively.  2 - Opto-electronic device according to claim 1 intended to be used as a photovoltaic cell and more particularly as a solar cell characterized in that the surfaces intended to receive the light which are arranged parallel to the zones P (i) and to the zones N ( 2), are provided with comb-shaped electrodes (10) which extend only over a very small fraction of these surfaces and in that the surfaces of zone P which serve as light-receiving surface, are covered with a degenerate doped N + zone (8) much thinner than the P zone and located between the Pet zone the electrodes (10).  3 - Opto-electronic device according to one of claims 1 and 2 characterized in that the semiconductor used is chosen from the group comprising molybdenum silicite and gallium arsenide and cadmium sulfide.  4 - Opto-electronic device according to one of claims 1 and 2 characterized in that the structure of the semiconductor element is formed by zones P (3) zones N (2) in Molybdenum Silicite each having a thickness of about 1 µm.  5 - Method for manufacturing an opto-electronic semiconductor device according to one of claims 1 to 3 characterized in that it consists in depositing alternately by epitaxy on a substrate of type P conductivity, P type zones and N type zones, to form a parallelepiped structure, applying a first layer of oxide (30), removing this layer both on a surface which is parallel to zones P (1 and 3) and to zones N ( 2) and on a lateral surface of the parallelepiped structure which is perpendicular to these zones, in order to then form there N + zones (5, and possibly 8 and 9) doped degenerately, to remove the rest of the first oxide layer (30) then to reform a second layer of oxide (31) over the whole of the parallelepiped structure, to remove this second layer from the other lateral surface of said opposite structure to then form a P + zone there (4) and finally to remove the rest of the second oxide for then apply the electrodes (6, 7).  6 - Method according to claim 5 characterized in that the zones P (3) and the zones N (2) are applied in a thickness of 5 trm each on a substrate (1) of 100 μm in thickness and in that the final zone N + (8) has a thickness of 0.5 IJm so that the final parallelepiped structure has a thickness of 1.1 mm and in that the precipitated side surfaces are each coated with a degenerate doped zone 1 μm thick extending perpendicular to the N and P zones .  7 - Method according to claim 6 characterized in that a second doped area of tye N + (9) 0.5 lum thick is applied to the free surface of the substrate and in that extensions are applied # 'elect "'# comb-shaped (10, 40) on the two N + doped areas (8, 9) parallel to the P-type doped areas (3) and to the N-type doped areas (2).  8 - Method according to one of claims 5 to 7 characterized in that the zones P (3) and the zones N (2) are doped with approximately 1019 atoms of impurities. cm 3 and in that the degenerately doped zones are doped doped with approximately 1 020 atoms of impurities. cm  9 - Method according to one of claims 5 to 8 characterized in that deposited by epitaxy on a Molybdenum Silicitus substrate (1) successively and alternately P zones (3) and N zones (2) by alternately introducing diborane and arsine in a stream of MOCO4 + H2. + FlgS 2 Mercury sulfide.  10 - Device according to one of claims 1 to 4 characterized in that the structure has a width of 20 mm and a length of 60 mm.