Abstract:
The present disclosure relates to a method for manufacturing a multi-junction solar cell device comprising the steps of: providing a first engineered substrate; providing a second substrate; forming at least one first solar cell layer on the first engineered substrate to obtain a first wafer structure; forming at least one second solar cell layer on the second substrate to obtain a second wafer structure; bonding the first wafer structure to the second wafer structure; detaching the first engineered substrate; removing the second substrate; and bonding a third substrate to the at least one first solar cell layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national phase entry under 35 U.S.C. §371 of International Patent Application PCT/EP2013/055144, filed Mar. 13, 2013, designating the United States of America and published in English as International Patent Publication WO 2013/143854 A1 on Oct. 3, 2013, which claims the benefit under Article 8 of the Patent Cooperation Treaty and under 35 U.S.C. §119(e) to European Patent Application Serial No. 12290107.7, filed Mar. 28, 2012, the disclosure of each of which is hereby incorporated herein in its entirety by this reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to the manufacture of multi junction solar cell substrates, in particular, to the manufacture of multi junction solar cell substrates comprising layer transfer processes and the manufacture of solar cell devices for terrestrial and space-related applications. 
       BACKGROUND 
       [0003]    Photovoltaic or solar cells are designed for converting the solar radiation to electrical current. In concentrator solar photovoltaic applications, the incoming sunlight is optically concentrated before it is directed to solar cells. For example, the incoming sunlight is received by a primary mirror that reflects the received radiation toward a secondary mirror that, in turn, reflects the radiation toward a solar cell, which converts the concentrated radiation to electrical current by the generation of electron-hole pairs in III-V semiconductor or single crystal silicon, for example. Alternatively, the sunlight could be concentrated onto solar cells by using transmittive optics like Fresnel lenses. 
         [0004]    Since different semiconductor material compositions show optimal absorption for different wavelengths of the incoming solar light, multi junction solar cells have been proposed that comprise, for example, three cells showing optimal absorption in different wavelength ranges. A triple cell structure may comprise a GaInP top cell layer with a gap value of 1.8 eV, a GaAs intermediate cell layer with a gap value of 1.4 eV and a Ge bottom cell layer with a gap value of 0.7 eV, for instance. In principle, III-V or group IV semiconductors can be used as active cells layers of multi junction solar cell devices manufactured by layer transfer/bonding. Multi junction solar cells are usually manufactured by monolithic epitaxial growth. The monolithic growth process requires, in general, that any formed layers be substantially lattice matched to previously formed layers or the underlying substrate. However, the epitaxial growth of the solar cell layers on growth substrates still provides a demanding problem in view of lattice mismatches. For example, it is not suitable to epitaxially grow an InP solar cell layer on a Ge substrate, since the crystalline and optical characteristics of the InP solar cell layer would be heavily deteriorated due to crystal mismatch. In addition, in conventionally used layer transfer processes, intermediate substrates are lost after the transfer of epitaxially grown layers. 
         [0005]    Thus, despite the recent engineering progress, there is still a need for an improved manufacturing process for multi junction solar cell devices wherein solar cell layers with a low defect rate are achieved and an intermediate substrate can be recycled. 
       DISCLOSURE 
       [0006]    The present invention addresses the above-mentioned need and, accordingly, provides a method for manufacturing a multi junction solar cell device, comprising the steps of:
       providing a first engineered substrate;   providing a second substrate;   forming at least one first solar cell layer on the first engineered substrate to obtain a first wafer structure;   forming at least one second solar cell layer on the second substrate to obtain a second wafer structure;   bonding the first wafer structure to the second wafer structure;   detaching the first engineered substrate;   removing the second substrate; and   bonding a third substrate to the at least one first solar cell layer.       
 
         [0015]    Alternatively, a first substrate and a second engineered substrate may be provided. 
         [0016]    The term “engineered substrate” comprises a substrate that is different from a mere pure bulk substrate but rather includes a layer or interface that is formed in the substrate in order to facilitate its removal from the structure resulting of the bonding of the first and second wafer structures. In particular, the “engineered substrate” may comprise a zipper layer between a seed layer and a base substrate. In particular, the engineered substrate may comprise a base substrate that is detached from the seed layer in the step of removal of the engineered substrate. 
         [0017]    Detachment by means of the zipper layer allows for recycling the detached substrate. 
         [0018]    In this document, the expression “detachment of the engineered substrate” should be interpreted as the detachment of the base substrate. This detachment step may be followed by the removal of the possible residue of the zipper layer (if any), and of the removal of the seed layer from the remaining structure. 
         [0019]    The zipper layer may be a weakened layer formed, e.g., by an appropriate treatment, for instance, a hydrogen and/or helium implantation in a substrate, that delimits an upper seed layer and a lower base substrate. 
         [0020]    The zipper layer may be formed by a buried porous layer by anodic etching at a surface of the base substrate. Then, epitaxial growth of the seed layer can be performed on top of the porous layer. 
         [0021]    The zipper layer may be provided in the form of an absorbing layer for laser lift-off, chemical lift-off or mechanical splitting in an intermediate strained layer during an epitaxy sequence: SiGe in Si matrix, in particular, an intermediate strained layer of SiGe at 20% in a Si substrate. In this alternative, the zipper layer may be formed by the seed layer itself, for instance, the seed layer can be selectively and chemically etched away to detach the engineered substrate. 
         [0022]    The zipper layer may also be formed of a SiN absorbing layer for laser lift-off inserted between a seed layer and a transparent base substrate, as known, for example, from WO2010/015878. 
         [0023]    Another possibility for an engineered substrate reads as follows: A removable (presenting a low bonding energy of less than 1.5 J/m 2 , and preferentially less than 1 J/m 2 ) bonding interface is formed between facing surfaces of a seed layer and a base support. In that possibility, the zipper layer is formed by the removable bonding interface. A first solar cell layer may be grown by epitaxy on the seed layer while preserving the removable character of the bonding interface, with the engineered substrate being heated to an epitaxial growth temperature. The low energy bonding is obtained by performing a treatment for augmenting the roughness of the facing surface of one of the seed layer or the substrate in particular carried out by chemical attack or etching, by effecting a treatment for decreasing hydrophilicity of the facing surface of one of the seed layer or the substrate (or the bonding layer in SiO 2  or Si 3 N 4  on each of them). Moreover, a different material for the bonding layer can be chosen such that weak intrinsic mutual chemical affinity of the interface materials is achieved. The detachment of the base substrate may be performed by application of a thermal treatment or mechanical stresses applied from a jet of fluid or a blade, for example. This kind of detachment is disclosed, for instance, in WO03/063214. 
         [0024]    As already mentioned, the engineered substrate includes a seed layer formed at the top of the zipper layer or removable bonding interface. The seed layer is transferred from the seed substrate to a base substrate by layer transfer from wafer to wafer, for example, by the S MART C UT ® process. The seed layer may or may not contain an epitaxial layer that has been formed originally by epitaxy on the seed substrate. Alternatively, the seed layer has been transferred or detached from a bulk seed substrate. In a preferred embodiment of the present invention, the seed layer is not used as a solar cell layer but rather a first solar cell layer is grown on the seed layer. 
         [0025]    According to a particular example in the above-described method, the first engineered substrate comprises a zipper layer and a first seed layer and the method comprises, in particular, the subsequently performed steps of:
       a) detaching a base substrate of the first engineered substrate at the zipper layer;   b) removing the first seed layer;   c) bonding the third substrate to the at least one first solar cell layer, in particular, by means of an electrically conductive bonding contact; and   d) removing the second substrate.       
 
         [0030]    The seed layer is not necessarily used as part of a solar cell layer because its band gap may not be adapted for an optimal efficiency in the sequence of cell layers that will eventually form the multi-junction solar cell. Therefore, in an embodiment of the present invention, the seed layer is only used for the growth of the first solar cell layer, thereby resulting in a multi-junction solar cell device comprising a first solar cell layer (as a bottom cell), satisfying both crystal and electrical characteristics. In fact, according to the above example, one might remove the seed layer later in the process, e.g., by chemical treatment such as etching. 
         [0031]    In the above-described examples, the solar cell layers can be grown with high crystal and electrical quality. For example, a dislocation density of less than 10 6 /cm 2  can be achieved. 
         [0032]    The detachment at the zipper layer may be performed by laser lift-off. A laser beam is radiated to the engineered substrate, for example, comprising a sapphire substrate, and provokes the decomposition of the material in the zipper layer, thereby allowing for detachment at the zipper layer. 
         [0033]    Furthermore, in the above-described example, the second substrate provides mechanical stability to the stacked solar cell layers during and after detachment of the first engineered substrate and up to the bonding of the third substrate to the same. The third substrate is a final base substrate of the solar cell device. The stack of solar cell layers formed on the final base substrate can be subject to finish processing in order to achieve the final solar cell. Finish processing or patterning may include the formation of mesas (formed over the complete surface of the wafer) from the stacked solar cell layers by etching, deposition and patterning of anti-reflective coatings and electrical contacting of the same. The final base substrate may be made of a thermally and electrically conductive material, for example, aluminum or copper or molybdenum or tungsten or doped semiconductors like silicon for Ohmic contact. 
         [0034]    A final base substrate made of tungsten or molybdenum or doped semiconductors like Ge, GaAs or InP may be particularly suitable for receiving the stack of solar cell layers provided on a second substrate made of GaAs or GaAsOS (see below), for instance. In particular, the difference in CTE of the final base substrate to the CTE of the second substrate should be less than 30% in order to avoid problems related to the bonding to the final substrate. 
         [0035]    According to an alternative embodiment of the inventive method for the manufacture of a multi junction solar cell, the first engineered substrate comprises a bulk substrate with a layer formed therein by implanted ionic species (and defining the seed layer between the implanted surface of the bulk substrate and the layer of implanted species) and the method comprises, in particular, the subsequently performed steps of:
       a) detaching part of the bulk substrate of the first engineered substrate at the layer formed by the implanted ionic species;   b) removing the first seed layer;   c) bonding the third substrate to the at least one first solar cell layer, in particular, by means of an electrically conductive bonding; and   d) removing the second substrate.       
 
         [0040]    In this embodiment, the need for the fabrication of an engineered substrate comprising a seed layer above a zipper layer is simplifying the overall processing. 
         [0041]    According to another example, the inventive method comprises the steps of:
       providing a first engineered substrate;   providing a second substrate;   forming at least one first solar cell layer on the first engineered substrate to obtain a first wafer structure;   forming at least one second solar cell layer on the second substrate to obtain a second wafer structure;   bonding the first wafer structure to the second wafer structure;   detaching the first engineered substrate;   removing the second substrate; and   bonding a third substrate to the at least one first solar cell layer;   and particularly comprises the successively performed steps of:
           a) removing the second substrate;   b) patterning the at least one first solar cell layer and the at least one second solar cell layer;   c) bonding a temporary (fourth) substrate to the patterned at least one second solar cell layer;   d) detaching the first engineered substrate; and   e) bonding the third substrate to the at least one patterned first solar cell layer.   
               
 
         [0056]    According to this example, it is the first engineered base substrate that provides mechanical stability to the stacked solar cell layers during the patterning process (like mesa and cap layer etching of the stacked solar cell layers, and/or anti-reflective coating realization and/or front side metallization) and during the bonding to the temporary substrate. The structure including the temporary substrate after detachment (and residues removal) of the first base substrate is bonded to the final base substrate and after removal of the temporary substrate the already finished or semi-finished (at least with respect to the processing of the solar cell layers) multi-junction solar cell is achieved. The temporary substrate may be in glass wafer, silicon and the bonding may be performed by glue, wax, etc., such that to be easily reversible by solvent or heat treatment or other methods well known by one skilled in the art. The detachment of the temporary substrate is obtained by dissolution of the glue with a solvent or heat treatment that does not damage the subcell layers. 
         [0057]    In all of the above-described embodiments, the final bonding to the final base substrate can be performed by means of an electrically conductive (metallic, or with conductor oxides, transparent conductor oxide TCO, silicide, etc.) bonding layer formed between the first solar cell layer and the final base substrate. The electrically conductive bonding can also be performed by direct bonding, i.e., without any intermediate bonding layers and by directly contacting the surfaces of the first solar cell and the final base substrate. An electrically conductive bonding also particularly helps to provide for thermal conductivity to the final base substrate that may function as a heat sink or may be connected to a heat sink and preferably is itself thermally and electrically conductive. The conductive bonding may be obtained by forming a eutectic contact such as Au/Sn or by a thermo-compression technique such as Au/Au thermo-compression. 
         [0058]    The step of bonding the first wafer structure to the second wafer structure can include a high-temperature treatment performed at a temperature of about 400° C. to about 600° C., and more preferably between 450° C. and 550° C., thereby allowing a reliable bond between the semiconductor materials of the solar cell layers. 
         [0059]    The advantage of the process is that the bonding of the solar cell layer is performed while the final base substrate is not yet present in the structure. Thus, risk of metallic contaminations is avoided during bonding and thermal treatment, such that high temperature may be used. Moreover, the expansion of the final base substrate that would be different from the expansion of the first and second substrates at such a temperature, due to different coefficient of thermal expansion (CTE), would damage the structure, in particular, it would weaken the bonding interface between the semiconductor materials. 
         [0060]    A final base substrate made of tungsten or molybdenum or doped semiconductors like Ge, GaAs or InP may be particularly suitable for receiving the stack of solar cell layers provided on a second substrate made of GaAs or GaAsOS (see below), for instance. In particular, the difference in CTE of the final base substrate to the CTE of the second substrate should be less 
         [0061]    In case of direct bonding of the first wafer structure to the second wafer structure, which forms the preferred embodiment according to the invention, the polishing of the surface of the solar cell layers to be bonded may be performed in order to smooth the surface, better than to 0.5 nm RMS over a field of 5×5 micrometers, for example, to obtain an enhanced bonding strength between the solar cell layers and improved efficiency and reliability of the subsequent solar cell. 
         [0062]    Alternatively, an electrically conductive, optically transparent material can be used as a bonding layer and facilitates the adhesion of the two structures. 
         [0063]    Furthermore, in the above examples, the second substrate may also be provided in the form of an engineered substrate comprising a zipper layer and a second seed layer, whereupon the at least one second solar cell layer is formed. Particularly, the second substrate may comprise a sapphire substrate and the second seed layer may comprise or consist of InP, Ge or GaAs. In that instance, the removal of the second substrate can be performed by detachment of the sapphire substrate, for example, from the seed layer. 
         [0064]    The first engineered substrate, on the other hand, may comprise a sapphire substrate and the first seed layer may comprise or consist of InP, GaSb, InAs or GaAs in all of the above examples for the inventive method. 
         [0065]    According to a particular variant in the above-described examples, the at least one first solar cell layer comprises two layers, a first layer and a second layer, that have been grown by epitaxy on the first layer, and the at least one second solar cell layer also comprises two layers, namely a fourth layer and a third layer, that have been grown by epitaxy on the fourth layer in order to provide inverted grown layers. In other terms, in the second solar cell, the layer having the smaller band gap is grown on top of the layer having the higher band gap. According to a particular example, the first layer (bottom cell) comprises or consists of GaInAs and/or the second layer comprises or consists of GaInAsP and/or the fourth layer (top cell in the finished multi-junction solar cell device) comprises or consists of GaInP and/or the third layer comprises or consists of GaAs. Thus, a four-cell multi junction solar cell device is achieved wherein the material of each cell is optimized for a particular wavelength of the incoming solar light. 
         [0066]    By the term “layer,” it is understood to be a functional photovoltaic layer. Such functional layer may comprise several individual layers having a different crystal nature or composition, and includes the necessary junctions, tunnel diode, barrier layers, etc. 
         [0067]    Furthermore, an embodiment is provided wherein no inversion of solar cell layers during the manufacturing process is needed. According to this embodiment, a method for manufacturing a multi-junction solar cell device comprises the steps of:
       providing a first engineered substrate;   providing a second substrate, in particular, a second engineered substrate;   forming at least one first solar cell layer on the first engineered substrate to obtain a first wafer structure;   forming at least one second solar cell layer on the second substrate to obtain a second wafer structure;   bonding a handling substrate to the second wafer structure at the at least one second solar cell layer;   removing, in particular, detaching, the second substrate to obtain a third wafer structure;   bonding the third wafer structure to the first wafer structure at the at least one first and the at least one second solar cell layer;   detaching the first engineered substrate to obtain a fourth wafer structure;   bonding the fourth wafer structure at the at least one first solar cell layer to a final substrate; and   detaching the handling substrate.       
 
         [0078]    In this embodiment, particularly the first engineered substrate, comprises a sapphire base substrate and an InP seed layer and a first and a second layer are formed on the InP layer. The second substrate may be a bulk GaAs substrate or an engineered substrate comprising a sapphire base substrate and a GaAs seed layer, with a third and a fourth layer being formed successively and with no inverted growth on the GaAs layer (i.e., the layer having a larger band gap is grown on top of the layer that has a smaller band gap). In this case, the second layer and the third layer are bonded to each other without inversion after attachment of the handling substrate and detachment of the second substrate (see also detailed description below). 
         [0079]    The present invention also provides an intermediate semiconductor device, comprising:
       a first substrate comprising GaAs or Ge, in particular, a first engineered substrate comprising GaAs or Ge;   a fourth layer, a third layer, a second layer and a first layer in this sequence on the first substrate (i.e., the fourth layer being in direct contact with the first engineered substrate), wherein the first layer comprises or consists of GaInAs and/or the second layer comprises or consists of GaInAsP and/or the fourth layer comprises or consists of GaInP and/or the third layer comprises or consists of GaAs. Note, that the individual layers are enumerated such that comparison with the layers provided according to the above-described examples of the method for the manufacture of a multi-junction solar cell device according to the present invention is facilitated.       
 
         [0082]    The intermediate semiconductor device may further comprise a final base substrate, in particular, a final base substrate made of Cu, Mo or Al or W or Si, bonded to the first solar cell layer, in particular, by a conductive bonding contact. 
         [0083]    A final base substrate made of tungsten or molybdenum or doped semiconductors like Ge, GaAs or InP may be particularly suitable for receiving the stack of solar cell layers provided on a second substrate made of GaAs or GaAsOS (see below), for instance. In particular, the difference in CTE of the final base substrate to the CTE of the second substrate should be less than 30% in order to avoid problems related to the bonding to the final substrate. 
         [0084]    Another intermediate semiconductor device provided herein comprises:
       a first substrate comprising InP, in particular, a first engineered substrate comprising InP;   a first layer, a second layer, a third layer and a fourth layer formed in this sequence (i.e., the first layer being in direct contact with the first engineered substrate) on the first substrate wherein the first layer comprises or consists of GaInAs and/or the second layer comprises or consists of GaInAsP and/or the third layer comprises or consists of GaAs and/or the fourth layer comprises or consists of GaInP.       
 
         [0087]    The intermediate semiconductor device may further comprise a contact formed on the fourth layer. A handling substrate, in particular, a handling substrate comprising or made of a glass or silicon material, is positioned on the fourth layer and contact. 
         [0088]    Corresponding to the “non-inverted” method described above, it is also provided an intermediate semiconductor device comprising:
       a first substrate comprising InP, in particular, a first engineered substrate comprising InP;   a first layer, a second layer, a third layer and a fourth layer formed in this sequence on the first substrate, wherein the first layer comprises or consists of GaInAs and/or the second layer comprises or consists of GaInAsP and/or the third layer comprises or consists of GaAs and/or the fourth layer comprises or consists of GaInP; and   a handling substrate, in particular, a handling substrate comprising or made of a glass material, formed on the fourth layer.       
 
         [0092]    In addition, a multi junction solar cell device is provided that is obtainable by one of the above-described examples of the method for the manufacture of a multi-junction solar cell device according to the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0093]    Additional features and advantages of the present invention will be described with reference to the drawings. In the description, reference is made to the accompanying figures that are meant to illustrate preferred embodiments of the invention. It is understood that such embodiments do not represent the full scope of the invention. 
           [0094]      FIG. 1  illustrates an example for the inventive method for the manufacturing of a multi junction solar cell comprising two wafer bonding steps. 
           [0095]      FIG. 2  illustrates another example for the inventive method for the manufacturing of a multi junction solar cell comprising three wafer bonding steps. 
           [0096]      FIG. 3  illustrates another example for the inventive method for the manufacturing of a multi-junction solar cell where no inversion of the formed solar cell layers is included. 
       
    
    
     DETAILED DESCRIPTION 
       [0097]    An example for the inventive method for the manufacturing of a multi junction solar cell comprising four solar cell layers is shown in  FIG. 1 . A first engineered base substrate comprising a zipper layer  2  for later detachment of the first base substrate  1  is provided. The term “engineered” generally refers to a substrate that is treated in a way to introduce some layer or interface within the substrate that facilitates detachment of the substrate after wafer bonding in a later processing step. For example, the first base substrate  1  is a sapphire substrate. Sapphire may preferably be chosen in view of its coefficient of thermal expansion, which is of importance for the temperature change (up and down) during epitaxy and for the further processing, in particular, bonding step (see below). Moreover, sapphire is transparent to laser light and can, thus, allow for laser lift-off in a later processing step (see below). A seed layer  3 , for example, an InP, Ge, GaSb, InAs or GaAs seed layer, is formed on the first base substrate  1 . For example, the seed layer may be grown on a seed substrate and then be transferred to the base substrate. Alternatively, the seed layer is taken off from a bulk seed substrate. The zipper layer  2  may comprise a layer of SiN interposed between the base substrate and the seed layer. In a variant, the zipper layer is the seed layer itself, InP, for example. Since the seed layer  3  will not function as a solar cell layer but only as a seed for the epitaxial growth of a first solar cell layer  4 , this seed layer can be used as a sacrificial layer to facilitate detachment of the base substrate, for example, being the media where decomposition will occur or, as another example, enabling separation by lateral selective etching of the seed layer. 
         [0098]    The thickness of the InP seed layer may be in the range of 50 nm to 1 μm. On the first solar cell layer  4 , a second solar cell layer  5  is grown. 
         [0099]    Furthermore, a second substrate  6  is provided, whereupon a fourth solar cell layer  7  and a third solar cell layer  8  are epitaxially grown. The second substrate  6  may be a bulk GaAs or Ge substrate. Alternatively, the second substrate  6  may be an engineered GaAsOS or GeOS (“OS” indicating “on sapphire”) substrate comprising a zipper layer similar to the one provided for the first base substrate  1  to allow the recycling of a portion of the substrates  1  and/or  6  that allowed the epitaxy of the subcell layers. In case of an engineered substrate, the portion that can be recycled is the base substrate. It is particularly advantageous when the base substrate is expensive, such as sapphire. The GaAs and Ge layer bonded via the zipper layer to the base substrate is a seed layer that is substantially lattice-matched with material of the third and the fourth solar cells  7 ,  8  so that epitaxy of the solar cells leads to sufficient crystalline quality (dislocation density less than 10 6 /cm 2 ) for subsequent operation in the multi junction solar cell devices. 
         [0100]    The four solar cell layers  4 ,  5 ,  7 ,  8  show absorption maxima for incident solar light for different wavelengths. It should be noted that the first solar cell layer  4  becomes the bottom cell and the fourth solar cell layer  7  becomes the top cell in the finished multi-junction solar cell device (see description below). According to the present example, all of the four monocrystalline solar cell layers  4 ,  5 ,  7 ,  8  are formed by epitaxial growth. In principle, the material of the solar cell layers can be selected from III-V semiconductors of the group consisting of InGaAs, GaAs, AlGaAs, InGaP, InP, GaSb, AlGaAsSb and InGaAsP. For example, the first solar cell layer  4  may be comprised of InGaAs, the second solar cell layer  5  may be comprised of InGaAsP, the third solar cell layer  7  may be comprised of GaAs, and the fourth solar cell layer  8  may be comprised of GaInP or InGaP. Each solar cell layer may include functional layers such as barrier for minority carrier, tunnel diode, etc. Appropriate tunnel junction layers may also be provided between particular ones of the solar cell layers by deposition or growth on a respective solar cell layer. 
         [0101]    As it is illustrated in  FIG. 1 , the resulting wafer structures comprising the first base substrate  1  and the first two solar cell layers  4  and  5  and the second substrate  6  and second two solar cell layers  7  and  8 , respectively, are bonded together. In the most preferred embodiment, the bonding is performed by direct bonding of the structures, or in other words, molecular adhesion of the two faces in contact, without any bonding layers. Molecular adhesion is performed preferably under partial vacuum. Contacting and bonding can be performed at relatively high temperatures of about 400° C. to 600° C. and, more preferably, between 450° C. and 550° C. Preferably, the contacting step is performed at room temperature followed by an annealing step reaching max temperature between 400° and 600° C., although it is not excluded to perform the contacting step at a higher temperature. This bonding step is crucial for the quality of the resulting multi junction solar cell and it is favorable to perform high-temperature bonding in order to achieve a bonding interface (between solar cell layers  5  and  8 ) without significant defects. Furthermore, multiple wafer structures comprising the first base substrate  1  may be bonded adjacently to a larger wafer structure comprising the second substrate  6 . For example, GaAs can be provided at a size of 6″ and InP at a size of 2 or 3″. 
         [0102]    Subsequently, the first base substrate  1  is detached from the seed layer  3  by means of the zipper layer  2 , for example, the first engineered substrate is cleaved at the zipper layer  2  allowing the detachment of first base substrate  1  from the seed layer  3 . For instance, the zipper layer  2  may be provided in the form of a weakened layer formed by the implantation of ionic species, for example, hydrogen or helium ions. In this case, the first substrate  1  can be provided as a bulk InP substrate, the implantation through the main top surface of the bulk substrate forms the weakened layer  2  that creates the seed layer  3  and the first base substrate  1 , respectively, in the top portion and the bottom portion of the bulk substrate. Then, detachment of the first base substrate  1  can be mediated by the application of mechanical forces to delaminate at the weakened layer formed by hydrogen or helium ions. 
         [0103]    Moreover, residue of the base substrate and/or bonding layer and the seed layer  3  may be removed by etching and/or polishing, for example, in particular, if the seed layer  3  shows poor electrical and/or thermal properties and could not be used in the subsequent device. The thus obtained structure is then bonded to a final base substrate  10 . This bonding process may be performed by direct conductive bonding the first solar cell layer  4  to the final base substrate  10  or by means of a metallic bonding layer  9  that is formed on the exposed surface of the first solar cell layer  4  or the final base substrate  10  before the bonding process. It is also possible to form a metallic bonding layer on the exposed surface of the first solar cell layer  4  and the final base substrate  10 , respectively. 
         [0104]    It is noted that the detached first base substrate  1  can be recycled to form a new engineered substrate for the re-usage as a growth substrate for epitaxial layers. 
         [0105]    In the next step illustrated in  FIG. 1 , the second substrate  6  is removed, e.g., by grinding and/or thinning or etching. If the second substrate  6  is provided in the form of an engineered substrate as mentioned above, detachment is mediated by the zipper layer of the engineered substrate. The resulting structure is then subject to some finish processing comprising the formation of a plurality of mesas comprising etched solar cell layers. The formation of the mesas can be achieved by lithographic processing after the formation of an appropriately patterned photoresist and optionally formed anti-reflective coating. Note that the first solar cell layer  4  grown on seed layer  3  is the bottom cell in the resulting multi-junction solar cell and that solar cell layers  7  and  8  have to be formed in reverse order with respect to the final orientation in the resulting multi-junction solar cell. 
         [0106]    It is noted that the final base substrate  10  may function as a thermally and electrically conductive substrate. Particularly, the final base substrate  10  may be made of Cu or Al, in particular, made of an aluminium sheet stiff enough to provide support of the stacked solar cell layers. Other envisaged candidates for the final base substrate  10  comprise Si, such as doped silicon, Mo and W. 
         [0107]    A final base substrate made of tungsten or molybdenum or doped semiconductors like Ge, GaAs or InP may be particularly suitable for receiving the stack of solar cell layers provided on a second substrate made of, for instance, GaAs or GaAsOS (see below). In particular, the difference in CTE of the final base substrate to the CTE of the second substrate should be less than 30% in order to avoid problems related to the bonding to the final substrate. 
         [0108]    It is furthermore noted that according to a variant of the example illustrated in  FIG. 1 , a bulk substrate, in particular, an InP bulk substrate, may be provided instead of the first engineered substrate. 
         [0109]      FIG. 2  illustrates an alternative example for the inventive method for the manufacturing of a multi junction solar cell comprising three wafer bonding steps. A first engineered substrate is provided in the form of an InPOS substrate (InP layer bonded on sapphire substrate) comprising a first zipper layer  12 , a first base substrate  11  in sapphire and an InP seed layer  13 . On the InP seed layer  13 , a first solar cell layer  14  and second solar cell layer  15  are epitaxially grown. A second engineered substrate is further provided in the form of a GaAsOS (GaAs on sapphire) substrate comprising a second zipper layer  17 , a sapphire second base substrate  16  and a GaAs seed layer  18 . On the GaAs seed layer  18 , a fourth solar cell layer  19  and a third solar cell layer  20  are epitaxially grown. Wafer bonding at the second and third solar cell layers  15  and  20  is performed as in the example shown in  FIG. 1 . As both base substrates  11  and  16  are in sapphire, the base substrates have the same CTE, such that the same material expansion occurs in the bottom and in the top of the bonded structure and bonding temperature may be as high as 600° C. without decreasing the strength of the bonding. However, in the next step of this example, the second substrate  16  is detached. Detachment is facilitated by the second zipper layer  17 . Thereafter, the GaAs seed layer  18  is removed (e.g., by polishing, thinning and/or etching). 
         [0110]    The advantage of both first and second base substrates  11 ,  16  in sapphire is that both are perfectly matched in CTE with the whole stack of solar cell layers, which makes it possible to apply bonding temperatures as high as 600° C. without degrading the solar cell. A similar reasoning applies to materials as, for instance, Ge, GaAs or InP, which may be used as first and second base substrates  11 ,  16 . The used first base substrate  11  may not be conductive, as, for instance, sapphire, which makes the bonding to a final base substrate that is conductive necessary. Preferentially, the materials of the first and second base substrate  11 ,  16  may be different, but the difference in their CTE should be less than 30% in order to allow high bonding temperatures. The use of Si for the first or second base substrate, however, is not suitable due to the difference in CTE between the seed layer used for growth of the solar cell layers and the base substrate with regard to the high temperatures used for epitaxial growth of the solar cell layers. 
         [0111]    Different from the example shown in  FIG. 1 , finish processing including the formation of a mesa from the stack of solar cell layers is performed on the InPOS first engineered substrate rather than the final base substrate, which may be advantageous in terms of the efficiency of the workflow and ease of the processing. However, if it is preferred, the finish processing may be performed after transfer to a final base substrate. A contact  21  may be formed atop of the solar cell layer  19  (top cell). The resulting structure has to be transferred to a final base substrate. For this, it is bonded by means of glue material  22 , for example, to another substrate  23  that may be provided in form of glass and provided a mechanical support of the solar cell layer structure. 
         [0112]    Subsequently, the first base substrate  11  can be detached. Detachment is facilitated by the first zipper layer  12 . A final base substrate  25  is then bonded to the surface exposed by detachment by means of the zipper layer. The bonding of the final base substrate  25  can be achieved by means of a metallic bonding layer  24 , for example. Subsequently, both the glue material  22  and the other substrate  23  are removed. It is noted that whereas in  FIG. 2  the glue material  22  and the other substrate  23  are illustrated as separate elements, in principle, they can be formed uniformly. 
         [0113]    Whereas the second substrate is provided in the form of an engineered substrate, it is also possible to provide it in the form of a GaAs bulk substrate, whereupon the third and fourth solar cell layers are epitaxially grown. In this case, the GaAs second substrate is removed by grinding or etching. The final base substrate  25 , as well as the solar cell layers, may be made of the same materials as described with reference to  FIG. 1 . 
         [0114]    The engineered substrates of the above-described examples can be fabricated by means of layer transfer of the respective seed layers to the base substrates. For example, an InP epitaxial layer or, as another example, a strained or relaxed InGaAs layer, may be the surface layer of an initial bulk InP seed substrate, which is subsequently transferred to a sapphire substrate by the well-known S MART C UT ® process. The remainder of the initial bulk seed substrate can be recycled after the transfer. The base substrates can also be recycled after the respective detachment steps. For forming an InP seed layer, it is preferable to apply the well-known S MART C UT ® process onto a bulk InP seed substrate. Applying the same process onto an epitaxial stack composed of InP may be chosen, however, offering more options regarding, for instance, the choice of doping levels, crystalline quality and exact process to manufacture the seed layer. As an alternative to the S MART C UT ® technology, any layer transfer process can be used, such as the bond and thin back technique. 
         [0115]      FIG. 3  shows another example for manufacturing a multi-junction solar cell. Two engineered substrates are provided: a first sapphire base substrate  30  including a zipper layer  31  and a second sapphire base substrate  35  including another zipper layer  36 . On the first base substrate  30 , an InP layer  32  is formed and on the second base substrate  35 , a GaAs layer  37  is formed. Note, that according to an alternative, a bulk GaAs substrate may be provided instead of the second engineered substrate. 
         [0116]    A first and a second solar cell layer  33  and  34  are formed on the InP layer and a third and a fourth solar cell layer  38  and  39  are formed on the GaAs layer  37 . Materials for the solar cell layers can be chosen as described in the above examples. Then, for lift-off of the second substrate and removal of the GaAs layer  37 , a handling substrate  41  is attached by means of an adhesive layer  40  to the fourth solar cell layer  39 . The handling substrate  41  can be a glass substrate and the adhesive layer can be a glue layer. After lift-off of the second base substrate  35  and removal of the GaAs layer  37 , bonding at the second and third solar cell layers  34  and  38  is performed, resulting in the structure shown on the right-hand-side of the upper row in  FIG. 3 . 
         [0117]    After lift-off of the first base substrate  30  and removal of the InP layer  32 , the solar cell layer stack is attached to the final base substrate  42  by a metallic bonding layer  43 . The adhesive layer  40  and the handling substrate  41  are removed and, subsequently, the solar cell layer stack is patterned and a contact  44  is formed atop of the solar cell layer  39  (top cell). An advantage of this example can be seen in that no inversion of the third and fourth solar cell layers  38  and  39  is necessary. On the other hand, another step of attachment to a handling substrate  41  is needed and the bonding at the second and third solar cell layers  34  and  38  poses some problem due to the limited temperature at which the bonding can take place. 
         [0118]    All previously discussed embodiments are not intended as limitations but serve as examples illustrating features and advantages of the invention. It has to be understood that some or all of the above-described features can also be combined in different ways. In particular, it is possible according to the invention to form multi junction solar cells not only composed of four junctions (as generally disclosed in the previous embodiments), but also two, three, five or more.