Abstract:
A method of manufacturing a solar cell by providing a first semiconductor substrate and depositing a first sequence of layers of semiconductor material to form a first solar subcell, including a first bond layer disposed on the top of the first sequence of layers. A second semiconductor substrate is provided, and on the top surface of the second substrate a second sequence of layers of semiconductor material is deposited forming at least a second solar subcell. A second bond layer is disposed on the top of said second sequence of layers. The first solar subcell is mounted on top of the second solar subcell by joining the first bond layer to the second bond layer in an ultra high vacuum chamber, and the first semiconductor substrate is removed.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to co-pending U.S. patent application Ser. No. 12/258,190 filed Oct. 24, 2008. 
         [0002]    This application is related to co-pending U.S. patent application Ser. No. 12/253,051 filed Oct. 16, 2008. 
         [0003]    This application is related to co-pending U.S. patent application Ser. No. 12/190,449 filed Aug. 12, 2008. 
         [0004]    This application is related to co-pending U.S. patent application Ser. No. 12/187,477 filed Aug. 7, 2008. 
         [0005]    This application is related to co-pending U.S. patent application Ser. No. 12/218,582 filed Jul. 18, 2008. 
         [0006]    This application is related to co-pending U.S. patent application Ser. No. 12/218,558 filed Jul. 17, 2008. 
         [0007]    This application is related to co-pending U.S. patent application Ser. No. 12/123,864 filed May 20, 2008. 
         [0008]    This application is related to co-pending U.S. patent application Ser. No. 12/102,550 filed Apr. 14, 2008. 
         [0009]    This application is related to co-pending U.S. patent application Ser. No. 12/047,842, and U.S. Ser. No. 12/047,944, filed Mar. 13, 2008. 
         [0010]    This application is related to co-pending U.S. patent application Ser. No. 12/023,772, filed Jan. 31, 2008. 
         [0011]    This application is related to co-pending U.S. patent application Ser. No. 11/956,069, filed Dec. 13, 2007. 
         [0012]    This application is also related to co-pending U.S. patent application Ser. Nos. 11/860,142 and 11/860,183 filed Sep. 24, 2007. 
         [0013]    This application is also related to co-pending U.S. patent application Ser. No. 11/836,402 filed Aug. 8, 2007. 
         [0014]    This application is also related to co-pending U.S. patent application Ser. No. 11/616,596 filed Dec. 27, 2006. 
         [0015]    This application is also related to co-pending U.S. patent application Ser. No. 11/614,332 filed Dec. 21, 2006. 
         [0016]    This application is also related to co-pending U.S. patent application Ser. No. 11/445,793 filed Jun. 2, 2006. 
         [0017]    This application is also related to co-pending U.S. patent application Ser. No. 11/500,053 filed Aug. 7, 2006. 
     
    
     GOVERNMENT RIGHTS STATEMENT 
       [0018]    This invention was made with government support under Contract No. FA9453-06-C-0345 awarded by the U.S. Air Force. The Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0019]    1. Field of the Invention 
         [0020]    The present invention relates to the field of semiconductor devices, and to fabrication processes and devices such as multijunction solar cells based on III-V semiconductor compounds including a metamorphic layer. Such devices are also known as inverted metamorphic multijunction solar cells. 
         [0021]    2. Description of the Related Art 
         [0022]    Solar power from photovoltaic cells, also called solar cells, has been predominantly provided by silicon semiconductor technology. In the past several years, however, high-volume manufacturing of III-V compound semiconductor multijunction solar cells for space applications has accelerated the development of such technology not only for use in space but also for terrestrial solar power applications. Compared to silicon, III-V compound semiconductor multijunction devices have greater energy conversion efficiencies and generally more radiation resistance, although they tend to be more complex to manufacture. Typical commercial III-V compound semiconductor multijunction solar cells have energy efficiencies that exceed 27% under one sun, air mass 0 (AM0), illumination, whereas even the most efficient silicon technologies generally reach only about 18% efficiency under comparable conditions. Under high solar concentration (e.g., 500×), commercially available III-V compound semiconductor multijunction solar cells in terrestrial applications (at AM1.5D) have energy efficiencies that exceed 37%. The higher conversion efficiency of III-V compound semiconductor solar cells compared to silicon solar cells is in part based on the ability to achieve spectral splitting of the incident radiation through the use of a plurality of photovoltaic regions with different band gap energies, and accumulating the current from each of the regions. 
         [0023]    In satellite and other space related applications, the size, mass and cost of a satellite power system are dependent on the power and energy conversion efficiency of the solar cells used. Putting it another way, the size of the payload and the availability of on-board services are proportional to the amount of power provided. Thus, as payloads become more sophisticated, the power-to-weight ratio of a solar cell becomes increasingly more important. 
         [0024]    Typical III-V compound semiconductor solar cells are fabricated on a semiconductor wafer in vertical, multijunction structures. The individual solars cells or wafers are then disposed in horizontal arrays, with the individual solar cells connected together in an electrical series circuit. The shape and structure of an array, as well as the number of cells it contains, are determined in part by the desired output voltage and current. 
         [0025]    Inverted metamorphic solar cell structures based on III-V compound semiconductor layers, such as described in M. W. Wanlass et al., Lattice Mismatched Approaches for High Performance, III-V Photovoltaic Energy Converters (Conference Proceedings of the 31 st  IEEE Photovoltaic Specialists Conference, Jan. 3-7, 2005, IEEE Press, 2005), present an important conceptual starting point for the development of future commercial high efficiency solar cells. However, the materials and structures for a number of different layers of the cell proposed and described in such reference present a number of practical difficulties relating to the appropriate choice of materials and fabrication steps. 
         [0026]    Prior to the present invention, the materials and fabrication steps disclosed in the prior art have not been adequate to produce a commercially viable and energy efficient solar cell using commercially established fabrication processes for producing an inverted metamorphic multijunction cell structure. 
       SUMMARY OF THE INVENTION 
       [0027]    Briefly, and in general terms, the present invention provides a method of manufacturing a solar cell comprising providing a first semiconductor substrate; depositing on the first substrate a first sequence of layers of semiconductor material to form a first solar subcell, including a first bond layer disposed on the top of said first sequence of layers; providing a second semiconductor substrate; depositing on the top surface of the second substrate a second sequence of layers of semiconductor material forming at least a second solar subcell, including a second bond layer disposed on the top of said second sequence of layers; mounting the first solar subcell on top of the second solar subcell by joining the first bond layer to the second bond layer; and removing the first semiconductor substrate. 
         [0028]    In another aspect, the present invention provides a method of manufacturing a solar cell by providing a first semiconductor substrate; depositing on a first substrate a first sequence of layers of semiconductor material forming at least one solar cell; providing a second semiconductor substrate; depositing on the second substrate a second sequence of layers of semiconductor material forming at least one solar cell; bonding the first sequence of layers to the second sequence of layers; mounting a surrogate second substrate on top to the second substrate; and removing the first substrate. 
         [0029]    In another aspect, the present invention provides a method of manufacturing a solar cell including forming a first semiconductor structure including providing a first semiconductor substrate for the epitaxial growth of semiconductor material; forming a first subcell on said substrate comprising a first semiconductor material with a first band gap and a first lattice constant; forming a second subcell comprising a second semiconductor material with a second band gap and a second lattice constant, wherein the second band gap is less than the first band gap and the second lattice constant is greater than the first lattice constant; forming a lattice constant transition material positioned between the first subcell and the second subcell, said lattice constant transition material having a lattice constant that changes gradually from the first lattice constant to the second lattice constant; forming a second semiconductor structure including providing a second semiconductor substrate; depositing on the top surface of the second substrate a sequence of layers of semiconductor material forming at least a third solar subcell; mounting the first semiconductor structure to the second semiconductor structure; and removing the first semiconductor substrate. 
         [0030]    In still another aspect, the present invention provides a multijunction solar cell comprising a first solar subcell having a first band gap; a second solar subcell disposed over the first solar subcell having a second band gap smaller than the first band gap; a graded interlayer disposed over the second subcell having a third band gap greater than the second band gap; a third solar subcell disposed over the graded interlayer having a fourth band gap smaller than the second band gap and lattice mismatched with respect to the second subcell; and a fourth solar subcell disposed over said third subcell having a fifth band gap smaller than said fourth band gap and lattice mismatched with respect to said third subcell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    The invention will be better and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein: 
           [0032]      FIG. 1  is a graph representing the bandgap of certain binary materials and their lattice constants; 
           [0033]      FIG. 2  is a cross-sectional view of the solar cell of the invention after the deposition of semiconductor layers on the first growth substrate; 
           [0034]      FIG. 3A  is a cross-sectional view of a portion of the solar cell of the invention after the deposition of a first semiconductor layer on the second growth substrate; 
           [0035]      FIG. 3B  is a cross-sectional view of the solar cell of  FIG. 3A  after the next process step; 
           [0036]      FIG. 3C  is a cross-sectional view of the solar cell of  FIG. 3B  after next process step; 
           [0037]      FIG. 3D  is a cross-sectional view of the solar cell of  FIG. 3D  after next process step; 
           [0038]      FIG. 3E  is a cross-sectional view of the solar cell of  FIG. 3D  after next process step; 
           [0039]      FIG. 4  is a cross-sectional view of the solar cell of  FIG. 3E  with an orientation depicting the second growth substrate on the top of the Figure; 
           [0040]      FIG. 5  is a cross-sectional view of the solar cell of a first embodiment of the present invention depicting the substrate of  FIG. 2  bonded with the substrate of  FIG. 4 ; 
           [0041]      FIG. 6A  is a cross-sectional view of first a portion of the solar cell in a second embodiment of the present invention after the deposition of semiconductor layers on the first growth substrate; 
           [0042]      FIG. 6B  is a cross-sectional view of the solar cell of  FIG. 6A  after the next process step; 
           [0043]      FIG. 6C  is a cross-sectional view of the solar cell of  FIG. 6B  after next process step; 
           [0044]      FIG. 6D  is a cross-sectional view of the solar cell of  FIG. 6C  after next process step; 
           [0045]      FIG. 7A  is a cross-sectional view of a second portion of the solar cell in a second embodiment of the present invention after the deposition of a first semiconductor layer on the second growth substrate; 
           [0046]      FIG. 7B  is a cross-sectional view of the substrate of  FIG. 7A  after the next process step; 
           [0047]      FIG. 7C  is a cross-sectional view of the substrate of  FIG. 7B  with an orientation depicting the second growth substrate on the top of the Figure; 
           [0048]      FIG. 8  is a cross-sectional view of the solar cell of the second embodiment of the present invention depicting the substrate of  FIG. 6D  bonded with the substrate of  FIG. 7C ; 
           [0049]      FIG. 9  is a highly simplified cross-sectional view of the solar cell of  FIG. 5  or  FIG. 8 ; 
           [0050]      FIG. 10  is a cross-sectional view of the solar cell of  FIG. 9  after the next process step in which a backside metal contact layer is deposited; 
           [0051]      FIG. 11A  is a cross-sectional view of the solar cell of  FIG. 10  after next process step in which a surrogate substrate is adhered to the backside metal contact layer; 
           [0052]      FIG. 11B  is a cross-sectional view of the solar cell of  FIG. 11A  after the next process step in which the first growth substrate is removed; 
           [0053]      FIG. 11C  is a cross-sectional view of the solar cell of  FIG. 11B  after the next process step in which the etch stop layer is removed. 
           [0054]      FIG. 11D  is another cross-sectional view of the solar cell of  FIG. 11B  with an orientation depicting the surrogate substrate on the bottom of the Figure; 
           [0055]      FIG. 12  is a highly simplified cross-sectional view of the solar cell of  FIG. 11D ; 
           [0056]      FIG. 13  is a cross-sectional view of the solar cell of  FIG. 12  after the next process step in which the grid lines are formed over the contact layer; 
           [0057]      FIG. 14  is a cross-sectional view of the solar cell of  FIG. 13  after the next process step; 
           [0058]      FIG. 15  is a top plan view of a wafer in which the solar cells are fabricated; 
           [0059]      FIG. 16  is a cross-sectional view of the solar cell of  FIG. 14  after the next process step; 
           [0060]      FIG. 17A  is a cross-sectional view of the solar cell of  FIG. 16  after the next process step; 
           [0061]      FIG. 17B  is a cross-sectional view of the solar cell of  FIG. 16  after the next process step in which the surrogate substrate is removed; 
           [0062]      FIG. 18  is a top plan view of the wafer of  FIG. 17  depicting the surface view of the trench etched around each of the solar cells; 
           [0063]      FIG. 19  is a cross-sectional view of the solar cell of  FIG. 17B  after the next process step in which the individual solar cells are scribed or diced from the wafer; 
           [0064]      FIG. 20  is a cross-sectional view of the solar cell of  FIG. 19  after the next process step in which a cover glass is attached to each cell; 
           [0065]      FIG. 21  is a bottom plan view of four cells cut from the wafer in which the solar cells are fabricated after the process step of  FIG. 20 ; and 
           [0066]      FIG. 22  is a graph of the doping profile in a base and adjoining emitter layer in the metamorphic solar cell according to the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0067]    Details of the present invention will now be described including exemplary aspects and embodiments thereof. Referring to the drawings and the following description, like reference numbers are used to identify like or functionally similar elements, and are intended to illustrate major features of exemplary embodiments in a highly simplified diagrammatic manner. Moreover, the drawings are not intended to depict every feature of the actual embodiment nor the relative dimensions of the depicted elements, and are not drawn to scale. 
         [0068]    The basic concept of fabricating an inverted metamorphic multijunction (IMM) solar cell is to grow the subcells of the solar cell on a substrate in a “reverse” sequence. That is, the high band gap subcells (i.e. subcells with band gaps in the range of 1.8 to 2.1 eV), which would normally be the “top” subcells facing the solar radiation, are grown epitaxially on a semiconductor growth substrate, such as for example GaAs or Ge, and such subcells are therefore lattice-matched to such substrate. One or more lower band gap middle subcells (i.e. with band gaps in the range of 1.2 to 1.8 eV) can then be grown on the high band gap subcells. 
         [0069]    At least one lower subcell is formed over the middle subcell such that the at least one lower subcell is substantially lattice-mismatched with respect to the growth substrate and such that the at least one lower subcell has a third lower band gap (i.e. a band gap in the range of 0.7 to 1.2 eV). A surrogate substrate or support structure is provided over the “bottom” or substantially lattice-mismatched lower subcell, and the growth semiconductor substrate is subsequently removed. (The growth substrate may then subsequently be re-used for the growth of a second and subsequent solar cells). 
         [0070]    A variety of different features of inverted metamorphic multijunction solar cells are disclosed in the related applications noted above. The present invention is directed to the tandem construction or combination by a mechanical bonding process of a first semiconductor structure, including an IMM solar cell, with a second semiconductor structure, including one or more additional subcells, wherein the subcell in the second structure that is most closely adjacent to the first structure has a similar lattice constant to the bonding layer of the first structure. In the present invention, the resulting construction includes four subcells, with band gaps of approximately 1.91 eV, 1.42 eV, 1.02 eV, and 0.67 eV respectively, although other band gaps (such as, e.g. 1.98 eV for the top cell) are also contemplated to be within the scope of the design according to the present invention. 
         [0071]      FIG. 1  is a graph representing the band gap of certain binary materials and their lattice constants. The band gap and lattice constants of ternary materials are located on the lines drawn between typical associated binary materials (such as the ternary material GaAlAs being located between the GaAs and AlAs points on the graph, with the band gap of the ternary material lying between 1.42 eV for GaAs and 2.16 eV for AlAs depending upon the relative amount of the individual constituents). Thus, depending upon the desired band gap, the material constituents of ternary materials can be appropriately selected for growth. 
         [0072]    The lattice constants and electrical properties of the layers in the semiconductor structure are preferably controlled by specification of appropriate reactor growth temperatures and times, and by use of appropriate chemical composition and dopants. The use of a vapor deposition method, such as Organo Metallic Vapor Phase Epitaxy (OMVPE), Metal Organic Chemical Vapor Deposition (MOCVD), Molecular Beam Epitaxy (MBE), or other vapor deposition methods for the reverse growth may enable the layers in the monolithic semiconductor structure forming the cell to be grown with the required thickness, elemental composition, dopant concentration and grading and conductivity type. 
         [0073]      FIG. 2  depicts a portion of the multijunction solar cell to be fabricated according to the present invention after the sequential formation of the three subcells A, B and C on a GaAs growth substrate. More particularly, there is shown a substrate  101 , which is preferably gallium arsenide (GaAs), but may also be germanium (Ge) or other suitable material. For GaAs, the substrate is preferably a 15° off-cut substrate, that is to say, its surface is orientated 15° off the (100) plane towards the (111)A plane, as more fully described in U.S. patent application Ser. No. 12/047,944, filed Mar. 13, 2008. 
         [0074]    In the case of a Ge substrate, a nucleation layer (not shown) is deposited directly on the substrate  101 . On the substrate, or over the nucleation layer (in the case of a Ge substrate), a buffer layer  102  and an etch stop layer  103  are further deposited. In the case of GaAs substrate, the buffer layer  102  is preferably GaAs. In the case of Ge substrate, the buffer layer  102  is preferably InGaAs. A contact layer  104  of GaAs is then deposited on layer  103 , and a window layer  105  of AlInP is deposited on the contact layer. The subcell A, consisting of an n+ emitter layer  106  and a p-type base layer  107 , is then epitaxially deposited on the window layer  105 . The subcell A is generally latticed matched to the growth substrate  101 . 
         [0075]    It should be noted that the multijunction solar cell structure could be formed by any suitable combination of group III to V elements listed in the periodic table subject to lattice constant and bandgap requirements, wherein the group III includes boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (T). The group IV includes carbon (C), silicon (Si), germanium (Ge), and tin (Sn). The group V includes nitrogen (N), phosphorous (P), arsenic (As), antimony (Sb), and bismuth (Bi). 
         [0076]    In the preferred embodiment, the emitter layer  106  is composed of InGa(Al)P and the base layer  107  is composed of InGa(Al)P. The aluminum or Al term in parenthesis in the preceding formula means that Al is an optional constituent, and in this instance may be used in an amount ranging from 0% to 30%. The doping profile of the emitter and base layers  106  and  107  according to the present invention will be discussed in conjunction with  FIG. 23 . 
         [0077]    Subcell A will ultimately become the “top” subcell of the inverted metamorphic structure after completion of the process steps according to the present invention to be described hereinafter. 
         [0078]    On top of the base layer  107  a back surface field (“BSF”) layer  108  is deposited and used to reduce recombination loss, preferably p+ AlGaInP. 
         [0079]    The BSF layer  108  drives minority carriers from the region near the base/BSF interface surface to minimize the effect of recombination loss. In other words, a BSF layer  18  reduces recombination loss at the backside of the solar subcell A and thereby reduces the recombination in the base. 
         [0080]    On top of the BSF layer  108  is deposited a sequence of heavily doped p-type and n-type layers  109  which forms a tunnel diode which is an ohmic circuit element to connect subcell A to subcell B. These layers are preferably composed of p++ AlGaAs, and n++ InGaP. On top of the tunnel diode layers  109  a window layer  110  is deposited, preferably n+GaInP 2 . The window layer  110  used in the subcell B operates to reduce the interface recombination loss. It should be apparent to one skilled in the art, that additional layer(s) may be added or deleted in the cell structure without departing from the scope of the present invention. 
         [0081]    On top of the window layer  110  the layers of subcell B are deposited: the n-type emitter layer  111  and the p-type base layer  112 . These layers are preferably composed of InGaP and In 0.015 GaAs respectively (for a Ge substrate or growth template), or InGaP and GaAs respectively (for a GaAs substrate), although any other suitable materials consistent with lattice constant and bandgap requirements may be used as well. Thus, subcell B may be composed of a GaAs, GaInP, GaInAs, GaAsSb, or GaInAsN emitter region and a GaAs, GaInAs, GaAsSb, or GaInAsN base region. The doping profile of layers  111  and  112  according to the present invention will be discussed in conjunction with  FIG. 23 . 
         [0082]    In the preferred embodiment of the present invention, the middle subcell emitter has a band gap equal to the top subcell emitter, and the bottom subcell emitter has a band gap greater than the band gap of the base of the middle subcell. Therefore, after fabrication of the solar cell, and implementation and operation, neither the middle subcell B nor the bottom subcell C emitters will be exposed to absorbable radiation. Substantially radiation will be absorbed in the bases of cells B and C, which have narrower band gaps then the emitters. Therefore, the advantages of using heterojunction subcells are: (i) the short wavelength response for both subcells will improve, and (ii) the bulk of the radiation is more effectively absorbed and collected in the narrower band gap base. The effect will be to increase J sc . 
         [0083]    On top of the cell B is deposited a BSF layer  113  which performs the same function as the BSF layer  109 . A p++/n++ tunnel diode  114  is deposited over the BSF layer  113  similar to the layers  109 , again forming an ohmic circuit element to connect subcell B to subcell C. These layers  114  are preferably compound of p++ AlGaAs and n++ InGaP. 
         [0084]    A barrier layer  115 , preferably composed of n-type InGa(Al)P, is deposited over the tunnel diode  114 , to a thickness of about 1.0 micron. Such barrier layer is intended to prevent threading dislocations from propagating, either opposite to the direction of growth into the middle and top subcells B and C, or in the direction of growth into the bottom subcell A, and is more particularly described in copending U.S. patent application Ser. No. 11/860,183, filed Sep. 24, 2007. 
         [0085]    A metamorphic layer (or graded interlayer)  116  is deposited over the barrier layer  115  using a surfactant. Layer  116  is preferably a compositionally step-graded series of InGaAlAs layers, preferably with monotonically changing lattice constant, so as to achieve a gradual transition in lattice constant in the semiconductor structure from subcell B to subcell C while minimizing threading dislocations from occurring. The bandgap of layer  116  is constant throughout its thickness preferably approximately 1.5 eV or otherwise consistent with a value slightly greater than the bandgap of the middle subcell B. The preferred embodiment of the graded interlayer may also be expressed as being composed of (In x Ga 1-x ) y  Al 1-y As, with x and y selected such that the band gap of the interlayer remains constant at approximately 1.50 eV. 
         [0086]    In the surfactant assisted growth of the metamorphic layer  116 , a suitable chemical element is introduced into the reactor during the growth of layer  116  to improve the surface characteristics of the layer. In the preferred embodiment, such element may be a dopant or donor atom such as selenium (Se) or tellurium (Te). Small amounts of Se or Te are therefore incorporated in the metamorphic layer  116 , and remain in the finished solar cell. Although Se or Te are the preferred n-type dopant atoms, other non-isoelectronic surfactants may be used as well. 
         [0087]    Surfactant assisted growth results in a much smoother or planarized surface. Since the surface topography affects the bulk properties of the semiconductor material as it grows and the layer becomes thicker, the use of the surfactants minimizes threading dislocations in the active regions, and therefore improves overall solar cell efficiency. 
         [0088]    As an alternative to the use of non-isoelectronic one may use an isoelectronic surfactant. The term “isoelectronic” refers to surfactants such as antimony (Sb) or bismuth (Bi), since such elements have the same number of valence electrons as the P of InGaP, or as in InGaAlAs, in the metamorphic buffer layer. Such Sb or Bi surfactants will not typically be incorporated into the metamorphic layer  116 . 
         [0089]    In an alternative embodiment where the solar cell has only two subcells, and the “middle” cell B is the uppermost or top subcell in the final solar cell, wherein the “top” subcell B would typically have a bandgap of 1.8 to 1.9 eV, then the band gap of the interlayer would remain constant at 1.9 eV. 
         [0090]    In the inverted metamorphic structure described in the Wanlass et al. paper cited above, the metamorphic layer consists of nine compositionally graded InGaP steps, with each step layer having a thickness of 0.25 micron. As a result, each layer of Wanlass et al. has a different bandgap. In the preferred embodiment of the present invention, the layer  116  is composed of a plurality of layers of InGaAlAs, with monotonically changing lattice constant, each layer having the same bandgap, approximately 1.5 eV. 
         [0091]    The advantage of utilizing a constant bandgap material such as InGaAlAs is that arsenide-based semiconductor material is much easier to process in standard commercial MOCVD reactors, while the small amount of aluminum assures radiation transparency of the metamorphic layers. 
         [0092]    Although the preferred embodiment of the present invention utilizes a plurality of layers of InGaAlAs for the metamorphic layer  116  for reasons of manufacturability and radiation transparency, other embodiments of the present invention may utilize different material systems to achieve a change in lattice constant from subcell B to subcell C. Thus, the system of Wanlass using compositionally graded InGaP is a further embodiment of the present invention. Other embodiments of the present invention may utilize continuously graded, as opposed to step graded, materials. More generally, the graded interlayer may be composed of any of the As, P, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter greater or equal to that of the second solar cell and less than or equal to that of the third solar cell, and having a bandgap energy greater than that of the second solar cell. 
         [0093]    In another embodiment of the present invention, an optional second barrier layer  117  may be deposited over the InGaAlAs metamorphic layer  116 . The second barrier layer  117  will typically have a different composition than that of barrier layer  115 , and performs essentially the same function of preventing threading dislocations from propagating. In the preferred embodiment, barrier layer  117  is n+ type GaInP. 
         [0094]    A window layer  118  preferably composed of n+ type GaInP is then deposited over the barrier layer  117  (or directly over layer  116 , in the absence of a second barrier layer). This window layer operates to reduce the recombination loss in subcell “C”. It should be apparent to one skilled in the art that additional layers may be added or deleted in the cell structure without departing from the scope of the present invention. 
         [0095]    On top of the window layer  118 , the layers of cell C are deposited: the n+ emitter layer  119 , and the p-type base layer  120 . These layers are preferably composed of n type InGaAs and p type InGaAs respectively, or n type InGaP and p type InGaAs for a heterojunction subcell, although another suitable materials consistent with lattice constant and bandgap requirements may be used as well. The doping profile of layers  119  and  120  will be discussed in connection with  FIG. 23 . 
         [0096]    A BSF layer  121 , preferably composed of p+ InGaAlAs, is then deposited on top of the cell C, the BSF layer performing the same function as the BSF layers  108  and  113 . 
         [0097]    Finally, a bond layer  122 , preferably composed of a p+ lattice matched InGaAs layer (preferably In 0.30 GaAs) is deposited on the BSF layer  121 . 
         [0098]      FIG. 3A  is a cross-sectional view of a portion of the solar cell of the invention after the deposition of a nucleation layer  203  on a second growth substrate  200  which is preferably p-type germanium. 
         [0099]      FIG. 3B  is a cross-sectional view of the substrate of  FIG. 3A  after the next process step in which a heavily doped n-type gallium arsenide layer, such as InGaAs, is deposited. 
         [0100]    The emitter region  202  is formed in the Ge substrate by diffusion of dopants from one or more upper layers into the Ge substrate, thereby changing upper portion of the p-type germanium substrate  200  to an n-type region  201  which forms the emitter region of the solar cell. Typically, a heavily doped n-type gallium arsenide layer  204 , such as n+ InGaAs, is deposited over the nucleation layer  203 , and is a source of arsenic dopants into the emitter region  201 . The remainder or lower portion of the substrate  200  remains p-type, and forms the p-type base region  201  of the solar subcell, which will be subsequently referred to as subcell D. 
         [0101]      FIG. 3C  is a cross-sectional view of the substrate of  FIG. 3B  after the next process step in which a n++ InGaAs layer  205  is deposited over layer  204 . The layer  206  forms the first portion of a tunnel diode. 
         [0102]      FIG. 3D  is a cross-sectional view of the substrate of  FIG. 3D  after next process step in which a p++ AlGaAs layer  206  is deposited over layer  205 . The layer  206  forms a second portion of a tunnel diode. 
         [0103]      FIG. 3E  is a cross-sectional view of the substrate of  FIG. 3D  after the next process step in which a p+ InGaAs layer  207  is deposited over layer  206 . The layer  207  will subsequently serve as the bonding layer when the semiconductor structure of  FIG. 3  is attached to or bonded with the semiconductor structure of  FIG. 2 . 
         [0104]      FIG. 4  is a cross-sectional view of the substrate of  FIG. 3E  with an orientation depicting the second growth substrate on the top of the Figure. Subsequent Figures of this embodiment following in this application will assume such orientation. 
         [0105]    It should be apparent to one skilled in the art, that additional layer(s) may be added or deleted in the cell structure of  FIG. 4  without departing from the scope of the present invention. 
         [0106]      FIG. 5  is a cross-sectional view of the solar cell of a first embodiment of the present invention depicting the semiconductor structure of  FIG. 2  bonded with the semiconductor structure of  FIG. 4 . 
         [0107]    The method of bonding the wafers of the structures of  FIG. 2  and  FIG. 4  according to the present invention is as follows. The wafers are placed in an ultra high vacuum chamber. The wafers are then exposed to H+ ions at 400° C. to remove all oxides and carbonaceous materials on the bonding surfaces of each wafer. They are then aligned, and are mechanically brought together or compressed to form a bond at near zero pressure. Subsequent processing of the bonded structure will be described below, beginning with the description of  FIG. 9 . 
         [0108]    Turning to a second embodiment of the present invention, in which the tunnel diode is formed on the second substrate rather than the first substrate,  FIG. 6A  depicts a cross-sectional view of first a portion of the solar cell in a second embodiment of the present invention after the deposition of semiconductor layers on the first growth substrate up to the B SF layer  121 , similar to that of the first embodiment depicted in  FIG. 2 . 
         [0109]      FIG. 6B  is a cross-sectional view of the structure of  FIG. 6  after the next process step, in which a p++ lattice matched InGaAs tunnel diode layer  123  is deposited over BSF layer  121 . 
         [0110]      FIG. 6C  is a cross-sectional view of the structure of  FIG. 6B  after next process step in which an n++ lattice matched InGaAs tunnel diode layer  124  is deposited over layer  123 . 
         [0111]      FIG. 6D  is a cross-sectional view of the structure of  FIG. 6C  after the next process step in which a n+ lattice matched InGaAs bond layer  125  is deposited over layer  124 . 
         [0112]      FIG. 7A  is a cross-sectional view of a second portion of the solar cell in a second embodiment of the present invention after the deposition of a nucleation layer  203  on the second growth substrate  200 , similar to that of the first embodiment depicted in  FIG. 3A . 
         [0113]      FIG. 7B  is a cross-sectional view of the structure of  FIG. 7A  after the next process step in which a bond layer, preferably a n+ lattice matched InGaAs layer, is deposited over the nucleation layer  203 . 
         [0114]      FIG. 7C  is a cross-sectional view of the structure of  FIG. 7B  with an orientation depicting the second growth substrate on the top of the Figure. 
         [0115]      FIG. 8  is a cross-sectional view of the solar cell of the second embodiment of the present invention depicting the substrate of  FIG. 6  bonded with the substrate of  FIG. 7C . 
         [0116]      FIG. 9  is a highly simplified cross-sectional view of the solar cell semiconductor of  FIG. 5  or  FIG. 8  depicting just a few of the layers ( 201 ,  202 , and  203 ) at the top of the semiconductor structure and of few of the layers ( 105 ,  104 ,  103 ,  102 ) at the bottom of the structure over the GaAs substrate  101 . 
         [0117]      FIG. 10  is a cross-sectional view of the solar cell of  FIG. 9  (i.e., a simplified version of  FIG. 5  or  FIG. 8 ) after the next process step in which a metal contact layer  301  is deposited over the exposed surface of the germanium substrate  201 . The metal is preferably the sequence of metal layers Ti/Au/Ag/Au, although other metal sequences can be used as well. 
         [0118]      FIG. 11A  is a cross-sectional view of the structure of  FIG. 10  after the next process step in which an adhesive layer  302  is deposited over the metal layer  301 . The adhesive is preferably Wafer Bond (manufactured by Brewer Science, Inc. of Rolla, Mo.). A surrogate substrate  303 , preferably sapphire, is attached to the structure of  FIG. 10  by the adhesive. Alternative, the surrogate substrate may be GaAs, Ge or Si, or other suitable material. The surrogate substrate is preferably about 40 mils in thickness, and is perforated with holes about 1 mm in diameter, spaced 4 mm apart, to aid in subsequent removal of the adhesive  302  and the substrate  303 . 
         [0119]      FIG. 11B  is a cross-sectional view of the solar cell of  FIG. 11A  after the next process step in which the GaAs substrate  101  is removed by a sequence of lapping and/or etching steps in which the substrate  101 , and the buffer layer  102  are removed. The choice of a particular etchant is growth substrate dependent. 
         [0120]      FIG. 11C  is a cross-sectional view of the solar cell of  FIG. 11B  after the next process step in which the etch stop layer  103  is removed preferably by a HCl/H 2 O solution. 
         [0121]      FIG. 11D  is another cross-sectional view of the solar cell of  FIG. 11C  with an orientation depicting the surrogate on the bottom of the Figure. Subsequent Figures of this embodiment following in this application will assume such orientation. 
         [0122]      FIG. 12  is a more highly simplified cross-sectional view of the solar cell of  FIG. 11D . 
         [0123]      FIG. 13  is a cross-sectional view of the solar cell of  FIG. 12  after the next sequence of process steps in which a photoresist mask (not shown) is placed over the contact layer  104  to form the grid lines  501 . As will be described in greater detail below, the grid lines  501  are deposited via evaporation and lithographically patterned and deposited over the contact layer  104 . The mask is subsequently lifted off to form the finished metal grid lines  501  as depicted in the Figures. 
         [0124]    As more particularly described in U.S. patent application Ser. No. 12/218,582, filed Jul. 18, 2008, the composition of the metal grid lines  501  is preferably the sequence of layers Pd/Ge/Ti/Pd/Au, although other suitable materials and sequence of layers may be used as well. 
         [0125]      FIG. 14  is a cross-sectional view of the solar cell of  FIG. 13  after the next process step in which the grid lines are used as a mask to etch down the surface to the window layer  105  using a citric acid/peroxide etching mixture. 
         [0126]      FIG. 15  is a top plan view of a wafer in which four solar cells are implemented. The depiction of four cells is for illustration for purposes only, and the present invention is not limited to any specific number of cells per wafer. 
         [0127]    In each cell there are grid lines  501  (more particularly shown in cross-section in  FIG. 14 ), an interconnecting bus line  502 , and a contact pad  503 . The geometry and number of grid and bus lines, is merely illustrative and the present invention is not limited to the illustrated embodiment. 
         [0128]      FIG. 16  is a cross-sectional view of the solar cell of  FIG. 14  after the next process step in which an antireflective (ARC) dielectric coating layer  130  is applied over the entire surface of the “bottom” side of the wafer with the grid lines  501 . 
         [0129]      FIG. 17A  is a cross-sectional view of the solar cell of  FIG. 16  after the next process step according to the present invention in which two channels  510  and  511 , or portions of the semiconductor structure are etched down into the Ge substrate  201 , preferably using phosphide and arsenide etchants, thereby defining a peripheral boundary and leaving a mesa structure which constitutes the solar cell. The cross-section depicted in  FIG. 17A  is that as seen from the A-A plane shown in  FIG. 18 . 
         [0130]      FIG. 17B  is a cross-sectional view of the solar cell of  FIG. 16  after the next process step in which the surrogate substrate  303  and adhesive  302  are removed, leaving only the metal contact layer  301  which forms the backside contact of each of the solar cells. The surrogate substrate is preferably removed by the use of the etchant EKC  922 . The surrogate substrate includes perforations over its surface, preferably with a diameter is 0.033 inches and separated by 0.152 inches, that allow the flow of etchant through the surrogate substrate  303  to permit its lift off. After lift off, the surrogate substrate may be reused in subsequent wafer processing operations. 
         [0131]      FIG. 18  is a top plan view of the wafer of  FIG. 15  depicting the channels  510  and  511  etched around the periphery of each cell using phosphide and arsenide etchants. 
         [0132]      FIG. 19  is a cross-sectional view of the solar cell of  FIG. 17B  after the individual solar cells (cell  1 , cell  2 , etc. shown in  FIG. 18 ) are cut or scribed from the wafer through the channel  511 , leaving a vertical edge  512  extending through the Ge substrate  512  and the metal contact layer  301 . 
         [0133]      FIG. 20  is a cross-sectional view of the solar cell of  FIG. 19  after the next process step in an embodiment of the present invention in which a cover glass  514  is secured to the top of each of the individual cells by an adhesive  513 . 
         [0134]      FIG. 21  is a bottom plan view of the four individual solar cells shown in  FIG. 18 , now separated from the wafer (shown as an imaginary dotted line), showing the exposed backside metal contact layer  301 . 
         [0135]      FIG. 22  is a graph of a doping profile in the emitter and base layers in one or more subcells of the inverted metamorphic multijunction solar cell of the present invention. The various doping profiles within the scope of the present invention, and the advantages of such doping profiles are more particularly described in copending U.S. patent application Ser. No. 11/956,069 filed Dec. 13, 2007, herein incorporated by reference. The doping profiles depicted herein are merely illustrative, and other more complex profiles may be utilized as would be apparent to those skilled in the art without departing from the scope of the present invention. 
         [0136]    It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of constructions differing from the types of constructions described above. 
         [0137]    Although the preferred embodiment of the present invention utilizes two semiconductor structures, one with a vertical stack of three subcells, and the other structure a single subcell based on a germanium substrate, the present invention can apply to stacks with fewer or greater number of subcells and substrates i.e. two junction cells, four junction cells, five junction cells, etc. as well as having more than two bonded semiconductor structures. In the case of four or more junction cells, the use of more than one metamorphic grading interlayer may also be utilized. The composition and individual band gaps of the subcells depicted in the present invention are merely illustrative, and those skilled in the art may be able to configure five, six or more subcells with different band gap ranges using the principles of the present invention. 
         [0138]    In addition, although the present embodiment is configured with top and bottom electrical contacts, the subcells may alternatively be contacted by means of metal contacts to laterally conductive semiconductor layers between the subcells. Such arrangements may be used to form 3-terminal, 4-terminal, and in general, n-terminal devices. The subcells can be interconnected in circuits using these additional terminals such that most of the available photogenerated current density in each subcell can be used effectively, leading to high efficiency for the multijunction cell, notwithstanding that the photogenerated current densities are typically different in the various subcells. 
         [0139]    As noted above, the present invention may utilize an arrangement of one or more, or all, homojunction cells or subcells, i.e., a cell or subcell in which the p-n junction is formed between a p-type semiconductor and an n-type semiconductor both of which have the same chemical composition and the same band gap, differing only in the dopant species and types, and one or more heterojunction cells or subcells. Subcell A, with p-type and n-type InGaP is one example of a homojunction subcell. Alternatively, as more particularly described in U.S. patent application Ser. No. 12/023,772 filed Jan. 31, 2008, the present invention may utilize one or more, or all, heterojunction cells or subcells, i.e., a cell or subcell in which the p-n junction is formed between a p-type semiconductor and an n-type semiconductor having different chemical compositions of the semiconductor material in the n-type regions, and/or different band gap energies in the p-type regions, in addition to utilizing different dopant species and type in the p-type and n-type regions that form the p-n junction. 
         [0140]    In some cells, a thin so-called “intrinsic layer” may be placed between the emitter layer and base layer, with the same or different composition from either the emitter base layer. The intrinsic layer functions to suppress minority-carrier recombination in the space-charge region. Similarly, either the base layer or, the emitter layer may also be intrinsic or not-intentionally-doped (“ND”) over part or all of its thickness. 
         [0141]    The composition of the window or BSF layers may utilize other semiconductor compounds, subject to lattice constant and band gap requirements, and may include AlInP, AlAs, AlP, AlGaInP, AlGaAsP, AlGaInAs, AlGaInPAs, GaInP, GaInAs, GaInPAs, AlGaAs, AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb, AlGaInSb, AIN, GaN, InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials, and still fall within the spirit of the present invention. 
         [0142]    While the invention has been illustrated and described as embodied in an inverted metamorphic multijunction solar cell, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
         [0143]    Thus, while the description of this invention has focused primarily on solar cells or photovoltaic devices, persons skilled in the art know that other electronic and optoelectronic devices, such as, transistors, thermophotovoltaic (TPV) cells, photodetectors and light-emitting diodes (LEDS) are very similar in structure, physics, and materials to photovoltaic devices with some minor variations in doping and the minority carrier lifetime. For example, photodetectors can be the same materials and structures as the photovoltaic devices described above, but perhaps more lightly-doped for sensitivity rather than power production. On the other hand LEDs can also be made with similar structures and materials, but perhaps more heavily-doped to shorten recombination time, thus radiative lifetime to produce light instead of power. Therefore, this invention also applies to photodetectors and LEDs with structures, compositions of matter, articles of manufacture, and improvements as described above for photovoltaic cells. 
         [0144]    Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.