Abstract:
A method of manufacturing a solar cell by providing a first substrate; depositing on a first substrate a sequence of layers of semiconductor material forming a solar cell including at least a top subcell and a bottom subcell; mounting a surrogate substrate on top of the sequence of layers adjacent to the bottom subcell; removing the first substrate to expose the surface of the top subcell; removing the surrogate substrate; and holding the solar cell on a vacuum chuck to support it for subsequent fabrication operations, such as attaching interconnects to the solar cells to form an interconnected array.

Description:
GOVERNMENT RIGHTS STATEMENT 
     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. 
    
    
     REFERENCE TO RELATED APPLICATIONS 
     This application is related to co-pending U.S. patent application Ser. No. 12/362,213 and Ser. No. 12/362,225 filed simultaneously herewith. 
     This application is related to co-pending U.S. patent application Ser. No. 12/337,014 and Ser. No. 12/337,043 filed Dec. 17, 2008. 
     This application is related to co-pending U.S. patent application Ser. No. 12/271,127 and Ser. No. 12/271,192 filed Nov. 14, 2008. 
     This application is related to co-pending U.S. patent application Ser. No. 12/267,812 filed Nov. 10, 2008. 
     This application is related to co-pending U.S. patent application Ser. No. 12/258,190 filed Oct. 24, 2008. 
     This application is related to co-pending U.S. patent application Ser. No. 12/253,051 filed Oct. 16, 2008. 
     This application is related to co-pending U.S. patent application Ser. No. 12/190,449, filed Aug. 12, 2008. 
     This application is related to co-pending U.S. patent application Ser. No. 12/187,477, filed Aug. 7, 2008. 
     This application is related to co-pending U.S. patent application Ser. No. 12/218,558 and U.S. patent application Ser. No. 12/218,582 filed Jul. 16, 2008. 
     This application is related to co-pending U.S. patent application Ser. No. 12/123,864 filed May 20, 2008. 
     This application is related to co-pending U.S. patent application Ser. No. 12/102,550 filed Apr. 14, 2008. 
     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. 
     This application is related to co-pending U.S. patent application Ser. No. 12/023,772 filed Jan. 31, 2008. 
     This application is related to co-pending U.S. patent application Ser. No. 11/956,069 filed Dec. 13, 2007. 
     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. 
     This application is also related to co-pending U.S. patent application Ser. No. 11/836,402 filed Aug. 9, 2007. 
     This application is also related to co-pending U.S. patent application Ser. No. 11/616,596 filed Dec. 27, 2006. 
     This application is also related to co-pending U.S. patent application Ser. No. 11/614,332 filed Dec. 21, 2006. 
     This application is also related to co-pending U.S. patent application Ser. No. 11/445,793 filed Jun. 2, 2006. 
     This application is also related to co-pending U.S. patent application Ser. No. 11/500,053 filed Aug. 7, 2006. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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. 
     2. Description of the Related Art 
     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 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. 
     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, and there is increasing interest in lighter weight, “thin film” type solar cells having both high efficiency and low mass. 
     Typical III-V compound semiconductor solar cells are fabricated on a semiconductor wafer in vertical, multijunction structures. The individual solar 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. 
     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, particularly relating to the most appropriate choice of materials and fabrication steps. 
     SUMMARY OF THE INVENTION 
     Briefly, and in general terms, the present invention provides a method of forming a multijunction solar cell comprising an upper subcell, a middle subcell, and a lower subcell comprising: providing a first substrate for the epitaxial growth of semiconductor material; forming an upper first solar subcell on said first substrate having a first band gap; forming a middle second solar subcell over said first solar subcell having a second band gap smaller than said first band gap; forming a graded interlayer over said second solar cell; forming a lower third solar subcell over said graded interlayer having a fourth band gap smaller than said second band gap such that said third subcell is lattice mismatched with respect to said second subcell, and including a metal contact layer; attaching a surrogate second substrate over said third solar subcell and removing said first substrate; and etching a first trough around the periphery of said solar cell to the metal contact layer so as to form a mesa structure on said surrogate second substrate and at least one bottom contact pad on said metal layer. 
     In another aspect the present invention provides a method of manufacturing a solar cell comprising providing a first substrate; depositing on a first substrate a sequence of layers of semiconductor material forming a solar cell including at least a top subcell and a bottom subcell; mounting a surrogate substrate on top of the sequence of layers adjacent to the bottom subcell; removing the first substrate to expose the surface of the top subcell; removing the surrogate substrate; and holding the solar cell on a vacuum chuck to support it for subsequent fabrication operations. 
     Some implementations of the present invention may incorporate or implement fewer of the aspects and features noted in the foregoing summaries. 
     Additional aspects, advantages, and novel features of the present invention will become apparent to those skilled in the art from this disclosure, including the following detailed description as well as by practice of the invention. While the invention is described below with reference to preferred embodiments, it should be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional applications, modifications and embodiments in other fields, which are within the scope of the invention as disclosed and claimed herein and with respect to which the invention could be of utility. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a graph representing the band gap of certain binary materials and their lattice constants; 
         FIG. 2  is a cross-sectional view of the solar cell of the invention after the deposition of semiconductor layers on the growth substrate; 
         FIG. 3  is a cross-sectional view of the solar cell of  FIG. 2  after the next process step; 
         FIG. 4  is a cross-sectional view of the solar cell of  FIG. 3  after the next process step; 
         FIG. 5A  is a cross-sectional view of the solar cell of  FIG. 4  after the next process step in which a surrogate substrate is attached; 
         FIG. 5B  is a cross-sectional view of the solar cell of  FIG. 5A  after the next process step in which the original substrate is removed; 
         FIG. 5C  is another cross-sectional view of the solar cell of  FIG. 5B  with the surrogate substrate on the bottom of the Figure; 
         FIG. 6  is a simplified cross-sectional view of the solar cell of  FIG. 5C  after the next process step; 
         FIG. 7  is a cross-sectional view of the solar cell of  FIG. 6  after the next process step; 
         FIG. 8  is a cross-sectional view of the solar cell of  FIG. 7  after the next process step; 
         FIG. 9  is a cross-sectional view of the solar cell of  FIG. 8  after the next process step; 
         FIG. 10A  is a top plan view of a wafer in which four solar cells are fabricated; 
         FIG. 10B  is a bottom plan view of the wafer of  FIG. 10A ; 
         FIG. 10C  is a top plan view of a wafer in which two solar cells are fabricated; 
         FIG. 11  is a cross-sectional view of the solar cell of  FIG. 9  after the next process step; 
         FIG. 12A  is a cross-sectional view of the solar cell of  FIG. 11  after the next process step; 
         FIG. 12B  is a cross-sectional view of the solar cell of  FIG. 12A  after the next process step; 
         FIG. 13A  is a top plan view of the wafer of  FIG. 10A  depicting the surface view of the trench etched around the cell, after the process step depicted in  FIG. 12B ; 
         FIG. 13B  is a top plan view of the wafer of  FIG. 10C  depicting the surface view of the trench etched around the cell, after the process step depicted in  FIG. 12B ; 
         FIG. 14A  is a cross-sectional view of the solar cell of  FIG. 12B  after the next process step in a first embodiment of the present invention; 
         FIG. 14B  is a cross-sectional view of the solar cell of  FIG. 12B  after the next process step in a second embodiment of the present invention; 
         FIG. 14C  is a cross-sectional view of the solar cell of  FIG. 12B  after the next process step in a third embodiment of the present invention; 
         FIG. 14D  is a cross-sectional view of the solar cell of  FIG. 14A  after the next process step of removal of the surrogate substrate; 
         FIG. 15  is a cross-sectional view of the solar cell of  FIG. 14B  after the next process step in a third embodiment of the present invention; 
         FIG. 16  is a graph of the doping profile in the base and emitter layers of a subcell in the metamorphic solar cell according to the present invention; 
         FIG. 17  is a graph that depicts the current and voltage characteristics of an inverted metamorphic multijunction solar cell according to the present invention; 
         FIG. 18A  is a top plan view of a solar cell derived from the wafer of  FIG. 11  depicting the formation of the top contact pads and bus bar; 
         FIG. 18B  is a top plan view of a solar cell of  FIG. 18A  after etching the vias to the back metal layer; 
         FIG. 18C  is a cross sectional view of a portion of the wafer depicted in  FIG. 18B  through the B-B plane, after the process depicted in  FIG. 12A ; 
         FIG. 18D  is a cross sectional view of the solar cell of  FIG. 18B  through the B-B plane after being separated from the wafer; 
         FIG. 19A  is a top plan view of a string of three solar cells after being separated from the wafer after the cut through the channel  511  as illustrated in  FIG. 14A ; 
         FIG. 19B  is a cross sectional view of a portion of cell  1  of the string of three solar cells illustrated in  FIG. 19A  through the D-D plane of  FIG. 19A ; 
         FIG. 19C  is a cross sectional view of a portion of cells  2  and  3  of the string of three solar cells illustrated in  FIG. 19A  through the C-C plane of  FIG. 19A ; 
         FIG. 20A  is a top plan view of a string of three solar cells of  FIG. 19A  after interconnections of cell  1  and cell  2  are welded; 
         FIG. 20B  is a cross-sectional view of two of the solar cells depicted in  FIG. 20A  as seen through the E-E plane of  FIG. 20A ; 
         FIG. 21  is a diagram representing the range of band gaps of various GaInAlAs materials as a function of the relative concentration of Al, In, and Ga; 
         FIG. 22  is a graph representing the Ga mole fraction versus the Al to In mole fraction in GaInAlAs materials that is necessary to achieve a constant 1.5 eV band gap; and 
         FIG. 23  is a graph representing the mole fraction versus lattice constant in GaInAlAs materials that is necessary to achieve a constant 1.5 eV band gap. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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. 
     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 initially grown epitaxially directly on a semiconductor growth substrate, such as for example GaAs or Ge, and such subcells are consequently 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. 
     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 then attached or 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). 
     A variety of different features and aspects of inverted metamorphic multijunction solar cells are disclosed in the related applications noted above. Some or all of such features may be included in the structures and processes associated with the solar cells of the present invention. More particularly, one aspect of the present application is directed to the method of forming a bottom contact pad to the lower subcell, and another aspect is the use of a vacuum chuck to support the solar cell for certain fabrication operations. Neither, some or all of such aspects may be included in the structures and processes associated with the solar cells of the present invention. 
     It should be apparent to one skilled in the art that the inclusion of additional semiconductor layers within the cell with similar or additional functions and properties is also within the scope of the present invention. 
       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. 
     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. 
       FIG. 2  depicts the multijunction solar cell 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. Other alternative growth substrates, such as described in U.S. patent application Ser. No. 12/337,014 filed Dec. 17, 2008, may be used as well. 
     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 lattice matched to the growth substrate  101 . 
     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 band gap requirements, wherein the group III includes boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (TI). The group IV includes carbon (C), silicon (Si), germanium (Ge), and tin (Sn). The group V includes nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). 
     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. 16 . 
     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. 
     On top of the base layer  107  a back surface field (“BSF”) layer  108  preferably p+ AlGaInP is deposited and used to reduce recombination loss. 
     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, the BSF layer  108  reduces recombination loss at the backside of the solar subcell A and thereby reduces the recombination in the base. 
     On top of the BSF layer  108  is deposited a sequence of heavily doped p-type and n-type layers  109   a  and  109   b  that form a tunnel diode, i.e. an ohmic circuit element that connects subcell A to subcell B. Layer  109   a  is preferably composed of p++ AlGaAs, and layer  109   b  is preferably composed of n++ InGaP. 
     On top of the tunnel diode layers  109  a window layer  110  is deposited, preferably n+ InGaP. The advantage of utilizing InGaP as the material constituent of the window layer  110  is that it has an index of refraction that closely matches the adjacent emitter layer  111 , as more fully described in U.S. patent application Ser. No. 12/258,190, filed Oct. 24, 2008. More generally, 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. 
     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 band 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. 16 . 
     In previously disclosed implementations of an inverted metamorphic solar cell, the middle cell was a homostructure. In the present invention, similarly to the structure disclosed in U.S. patent application Ser. No. 12/023,772, the middle subcell becomes a heterostructure with an InGaP emitter and its window is converted from InAlP to InGaP. This modification eliminated the refractive index discontinuity at the window/emitter interface of the middle sub-cell. Moreover, the window layer  110  is preferably doped more than that of the emitter  111  to move the Fermi level up closer to the conduction band and therefore create band bending at the window/emitter interface which results in constraining the minority carriers to the emitter layer. 
     In one 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 emitters of middle subcell B nor the bottom subcell C will be exposed to absorbable radiation. Substantially all of the photons representing absorbable radiation will be absorbed in the bases of cells B and C, which have narrower band gaps than 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 the short circuit current J sc . 
     On top of the cell B is deposited a BSF layer  113  which performs the same function as the BSF layer  109 . The p++/n++ tunnel diode layers  114   a  and  114   b  respectively are deposited over the BSF layer  113 , similar to the layers  109   a  and  109   b , forming an ohmic circuit element to connect subcell B to subcell C. The layer  114   a  is preferably composed of p++ AlGaAs, and layer  114   b  is preferably composed of n++ InGaP. 
     A barrier layer  115 , preferably composed of n-type InGa(Al)P, is deposited over the tunnel diode  114   a / 114   b , 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 A, or in the direction of growth into the bottom subcell C, and is more particularly described in copending U.S. patent application Ser. No. 11/860,183, filed Sep. 24, 2007. 
     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 band gap of layer  116  is constant throughout its thickness, preferably approximately equal to 1.5 eV, or otherwise consistent with a value slightly greater than the band gap of the middle subcell B. One 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 or other appropriate band gap. 
     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. 
     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. 
     As an alternative to the use of non-isoelectronic surfactants 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 atom of InGaP, or the As atom in InGaAlAs, in the metamorphic buffer layer. Such Sb or Bi surfactants will not typically be incorporated into the metamorphic layer  116 . 
     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 band gap of 1.8 to 1.9 eV, then the band gap of the interlayer would remain constant at 1.9 eV. 
     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 band gap. In one 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 band gap, approximately 1.5 eV. 
     The advantage of utilizing a constant band gap 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. 
     Although one 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 second 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 band gap energy greater than that of the second solar cell. 
     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 one embodiment, barrier layer  117  is n+ type GaInP. 
     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. 
     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 other suitable materials consistent with lattice constant and band gap requirements may be used as well. The doping profile of layers  119  and  120  will be discussed in connection with  FIG. 16 . 
     A BSF layer  121 , preferably composed of InGaAlAs, is then deposited on top of the cell C, the BSF layer performing the same function as the BSF layers  108  and  113 . 
     Finally a high band gap contact layer  122 , preferably composed of InGaAlAs, is deposited on the BSF layer  121 . 
     This contact layer added to the bottom (non-illuminated) side of a lower band gap photovoltaic cell, in a single or a multijunction photovoltaic cell, can be formulated to reduce absorption of the light that passes through the cell, so that (1) an ohmic metal contact layer below (non-illuminated side) it will also act as a mirror layer, and (2) the contact layer doesn&#39;t have to be selectively etched off, to prevent absorption. 
     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. 
       FIG. 3  is a cross-sectional view of the solar cell of  FIG. 2  after the next process step in which a metal contact layer  123  is deposited over the p+ semiconductor contact layer  122 . The metal is preferably the sequence of metal layers Ti/Au/Ag/Au or Ti/Pd/Ag, although other suitable sequences and materials may be used as well. 
     Also, the metal contact scheme chosen is one that has a planar interface with the semiconductor, after heat treatment to activate the ohmic contact. This is done so that (i) a dielectric layer separating the metal from the semiconductor doesn&#39;t have to be deposited and selectively etched in the metal contact areas; and (ii) the contact layer is specularly reflective over the wavelength range of interest. 
       FIG. 4  is a cross-sectional view of the solar cell of  FIG. 3  after the next process step in which a bonding layer  124  is deposited over the metal layer  123 . In one embodiment of the present invention, the bonding layer is an adhesive, preferably Wafer Bond (manufactured by Brewer Science, Inc. of Rolla, Mo.). In other embodiments of the present invention, a solder or eutectic bonding layer  124 , such as described in U.S. patent application Ser. No. 12/271,127 filed Nov. 14, 2008, or a bonding layer  124  such as described in U.S. patent application Ser. No. 12/265,113 filed Nov. 5, 2008, may be used, where the surrogate substrate remains a permanent supporting component of the finished solar cell. 
       FIG. 5A  is a cross-sectional view of the solar cell of  FIG. 4  after the next process step in which a surrogate substrate  125 , preferably sapphire, is attached. Alternatively, the surrogate substrate may be GaAs, Ge or Si, or other suitable material. The surrogate substrate is about 40 mils in thickness, and in the case of embodiments in which the surrogate substrate is to be removed, it is perforated with holes about 1 mm in diameter, spaced 4 mm apart, to aid in subsequent removal of the adhesive and the substrate. 
       FIG. 5B  is a cross-sectional view of the solar cell of  FIG. 5A  after the next process step in which the original substrate is removed by a sequence of lapping, grinding 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. 
       FIG. 5C  is a cross-sectional view of the solar cell of  FIG. 5B  with the orientation with the surrogate substrate  125  being at the bottom of the Figure. Subsequent Figures in this application will assume such orientation. 
       FIG. 6  is a simplified cross-sectional view of the solar cell of  FIG. 5B  depicting just a few of the top layers and lower layers over the surrogate substrate  125 . 
       FIG. 7  is a cross-sectional view of the solar cell of  FIG. 6  after the next process step in which the etch stop layer  103  is removed by a HCl/H 2 O solution. 
       FIG. 8  is a cross-sectional view of the solar cell of  FIG. 7  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. 
     As more fully described in U.S. patent application Ser. No. 12/218,582 filed Jul. 16, 2008, hereby incorporated by reference, the grid lines  501  are preferably composed of the sequence of layers Pd/Ge/Ti/Pd/Au, although other suitable sequences and materials may be used as well. 
       FIG. 9  is a cross-sectional view of the solar cell of  FIG. 8  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. 
       FIG. 10A  is a top plan view of a 100 mm (or 4 inch) wafer in which four solar cells are implemented. The depiction of four cells is for illustration purposes only, and the present invention is not limited to any specific number of cells per wafer. 
     In each cell there are grid lines  501  (more particularly shown in cross-section in  FIG. 9 ), an interconnecting bus line  502 , and a contact pad  503 . The geometry and number of grid and bus lines and contact pads are illustrative, and the present invention is not limited to the illustrated embodiment. 
       FIG. 10B  is a bottom plan view of the wafer of  FIG. 10A . 
       FIG. 10C  is a top plan view of a 100 mm (or 4 inch) wafer in which two solar cells are implemented. Each solar cell has an area of 26.3 cm 2  and after fabrication will have a power/weight ratio (after separation from the growth and surrogate substrates, and including a 4 mil thick cover glass) of 945 mW/g. 
       FIG. 11  is a cross-sectional view of the solar cell of  FIG. 9  after the next process step in which an antireflective (ARC) dielectric coating layer  130  is applied over the entire surface of the “top” side of the wafer with the grid lines  501 . 
       FIG. 12A  is a cross-sectional view of the solar cell of  FIG. 11  after the next process step according to the present invention in which first and second annular channels  510  and  511 , or portion of the semiconductor structure are etched down to the metal layer  123  using phosphide and arsenide etchants. These channels, as more particularly described in U.S. patent application Ser. No. 12/190,449, filed Aug. 12, 2008, define a peripheral boundary between the cell, a surrounding mesa  516 , and a periphery mesa  517  at the edge of the wafer, and leave a mesa structure  518  which constitutes the solar cell. The cross-section depicted in  FIG. 12A  is that as seen from the A-A plane shown in  FIG. 13A   
       FIG. 12B  is a cross-sectional view of the solar cell of  FIG. 12A  after the next process step in which channel  511  is exposed to a metal etchant, layer  123  in the channel  511  is removed, and channel  511  is extended in depth approximately to the top surface of the bond layer  124 . 
       FIG. 13A  is a top plan view of the wafer of  FIG. 10A  depicting the channels  510  and  511  etched around the periphery of each cell. 
       FIG. 13B  is a top plan view of the wafer of  FIG. 10C  depicting the channels  510  and  511  etched around the periphery of each cell. 
       FIG. 14A  is a cross-sectional view of the solar cell of  FIG. 12B  after the individual solar cells (cell  1 , cell  2 , etc. shown in  FIG. 13 ) are cut or scribed from the wafer through the channel  511 , leaving a vertical edge  512  extending through the surrogate substrate  125 . In this first embodiment of the present invention, the surrogate substrate  125  forms the support for the solar cell in applications where a cover glass (such as provided in the third embodiment to be described below) is not required. In such an embodiment, electrical contact to the metal contact layer  123  may be made through the channel  510 . 
       FIG. 14B  is a cross-sectional view of the solar cell of  FIG. 12B  after the next process step in a second embodiment of the present invention in which the surrogate substrate  125  is appropriately thinned to a relatively thin layer  125   a , by grinding, lapping, or etching. In this embodiment, the thin layer  125   a  forms the support for the solar cell in applications where a cover glass, such as provided in the third embodiment to be described below, is not required. In such an embodiment, electrical contact to the metal contact layer  123  may be made through the channel  510 . 
       FIG. 14C  is a cross-sectional view of the solar cell of  FIG. 12B  after the next process step in a third embodiment of the present invention in which a cover glass  514  is secured to the top of the cell by an adhesive  513 . The cover glass  514  is typically about  4  mils thick and preferably covers the entire channel  510 , extends over a portion of the mesa  516 , but does not extend to channel  511 . Although the use of a cover glass is desirable for many environmental conditions and applications, it is not necessary for all implementations, and additional layers or structures may also be utilized for providing additional support or environmental protection to the solar cell. 
       FIG. 14D  is a cross-sectional view of the solar cell of  FIG. 14A  after the next process step in some embodiments of the present invention in which the adhesive layer  124 , the surrogate substrate  125  and the peripheral portion  517  of the wafer is entirely removed, leaving only the solar cell with the ARC layer  130  (or other layers or structures) on the top, and the metal contact layer  123  on the bottom, which forms the backside contact of the solar cell. The surrogate substrate is preferably removed by the use of a ‘Wafer Bond’ solvent. As noted above, the surrogate substrate includes perforations over its surface that allow the flow of solvent through the surrogate substrate  125  to permit its lift off. After lift off, the surrogate substrate may be reused in subsequent wafer processing operations. 
       FIG. 15  is a cross-sectional view of the solar cell of  FIG. 14C  after the next process step in some embodiments of the present invention in which the adhesive layer  124 , the surrogate substrate  125  and the peripheral portion  517  of the wafer is entirely removed, leaving only the solar cell with the cover glass  514  (or other layers or structures) on the top, and the metal contact layer  123  on the bottom, which forms the backside contact of the solar cell. The surrogate substrate is preferably removed by the use of a ‘Wafer Bond’ solvent. As noted above, the surrogate substrate includes perforations over its surface that allow the flow of solvent through the surrogate substrate  125  to permit its lift off. After lift off, the surrogate substrate may be reused in subsequent wafer processing operations. 
       FIG. 16  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. 
       FIG. 17  is a graph that depicts the current and voltage characteristics of the solar cell according to the present invention. The solar cell has an open circuit voltage (V oc ) of approximately 3.074 volts, a short circuit current of approximately 16.8 mA/cm 2 , a fill factor of approximately 85.7%, and an efficiency (at AM0) of 32.7%. 
       FIG. 18A  is a top plan view of a portion of the wafer depicted in  FIG. 10C  showing a solar cell following the process steps depicted in  FIG. 11 . As depicted in the Figure, in each solar cell there are grid lines  501  (more particularly shown in cross-section in  FIG. 9 ), an interconnecting bus line  502 , and contact pads  503 . The geometry and number of grid and bus lines and contact pads are illustrative, and the present invention is not limited to the illustrated embodiment. 
       FIG. 18B  is a top plan view of a portion of the wafer depicted in  FIG. 18A  after the processes depicted in  FIG. 12A  have been performed, depicting both the vias  519  to the back metal layer  123 , as well as the circumferential mesa channel  511 , with the metal being depicted at  519  and  511  being the back metal layer  123 . The grid lines  501 , shown in  FIG. 18A , are not depicted to simplify the drawing. 
       FIG. 18C  is a cross sectional view of a portion of the wafer depicted in  FIG. 18B  through the B-B plane, after the processes depicted in  FIG. 12A  have been performed, in particular showing one of the contact pads  519  formed at the back metal layer  123 , as well as the circumferential mesa channel  511  extending to the level of the back metal layer  123 . 
       FIG. 18D  is a cross sectional view of a solar cell of  FIG. 18B  through the B-B plane, after having been cut from the wafer through the channel  511 , leaving a vertical edge  512  extending through the surrogate substrate  125 . 
       FIG. 19A  is a top plan view of a string of three solar cells after the solar cells have been separated from the wafer (after the cut through the channel  511 ) and separated from the surrogate substrate, as illustrated in  FIG. 14D , with the three solar cells being positioned and aligned on the surface of a vacuum block to be fabricated into a finished interconnected string according to the present invention. The bottom contact pads  520   a ,  520   b ,  520   c , and  520   d  of cell  1  are depicted as being adjacent to and aligned with the top contacts  521   a ,  521   b ,  521   c , and  522   d , respectively, of cell  2 . The bottom contact pads  522   a ,  522   b ,  522   c , and  522   d  of cell  2  are depicted as being adjacent to and aligned with the top contacts  523   a ,  523   b ,  523   c , and  523   d , respectively, of cell  3 . The cross sectional view of cells  2  and  3 , through the C-C plane indicated in the Figure, is depicted in  FIG. 19C . The cross sectional view of cell  1 , through the D-D plane indicated in the Figure, is depicted in  FIG. 19B . 
       FIG. 19B  is a cross sectional view of a portion of cell  1  of the string of three solar cells illustrated in  FIG. 19A  through the D-D plane of  FIG. 19A . The bottom contact pads  520   b  and  520   c  are illustrated. 
       FIG. 19C  is a cross sectional view of a portion of cells  2  and  3  of the string of three solar cells illustrated in  FIG. 19A  through the E-E plane of  FIG. 19A  with the two cells being supported on a vacuum block  600  prior to interconnection. The vacuum block  600  includes holes  601  extending through its thickness to which a small vacuum is applied to secure the cells to the vacuum block during subsequent fabrication steps. The bottom contact pad  522   d  and the top contact pad  523   d  are illustrated. 
       FIG. 20A  is a top plan view of a string of three solar cells of  FIG. 19A  after interconnections  524   a ,  524   b ,  524   c , and  524   d  are welded between the bottom contacts  520   a ,  520   b ,  520   c , and  520   d  of cell  1  and the top contacts  521   a ,  521   b ,  521   c , and  522   d , respectively, of cell  2 . Subsequently to such welding, although not depicted in the Figure, interconnections between the bottom contacts  522   a ,  522   b ,  522   c , and  522   d  of cell  2  will be welded to the top contacts  523   a ,  523   b ,  523   c , and  523   d , respectively, of cell  3 . The cross sectional view of cells  1  and  2 , through the E-E plane indicated in the Figure, showing the interconnect  524   a , is depicted in  FIG. 20B . 
       FIG. 20B  is a cross-sectional view of two of the solar cells (cell  1  and cell  2 ) depicted in  FIG. 20A  as seen from the F-F plane indicated in  FIG. 20A , with the two cells being supported on a vacuum block  600  prior to interconnection. The vacuum block  600  includes holes  601  extending through its thickness to which a small vacuum is applied to secure the cells to the vacuum block during subsequent fabrication steps. 
       FIG. 20B  is a cross-sectional view of two of the solar cells (cell  1  and cell  2 ) depicted in  FIG. 20A  as seen from the E-E plane indicated in  FIG. 20A , with the two cells being supported on a vacuum block  600  following the welding on the interconnection  524   a  between contacts  520   a  and  521   a.    
       FIG. 21  is a diagram representing the range of band gaps of various GaInAlAs materials as a function of the relative concentration of Al, In, and Ga. This diagram illustrates how the selection of a constant band gap sequence of layers of GaInAlAs used in the metamorphic layer may be designed through the appropriate selection of the relative concentration of Al, In, and Ga to meet the different lattice constant requirements for each successive layer. Thus, whether 1.5 eV or 1.1 eV or other band gap value is the desired constant band gap, the diagram illustrates a continuous curve for each band gap, representing the incremental changes in constituent proportions as the lattice constant changes, in order for the layer to have the required band gap and lattice constant. 
       FIG. 22  is a graph that further illustrates the selection of a constant band gap sequence of layers of GaInAlAs used in the metamorphic layer by representing the Ga mole fraction versus the Al to In mole fraction in GaInAlAs materials that is necessary to achieve a constant 1.5 eV band gap. 
       FIG. 23  is a graph that further illustrates the selection of a constant band gap sequence of layers of GaInAlAs used in the metamorphic layer by representing the mole fraction versus lattice constant in GaInAlAs materials that is necessary to achieve a constant 1.5 eV band gap. 
     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. 
     Although one embodiment of the present invention utilizes a vertical stack of three subcells, the present invention can apply to stacks with fewer or greater number of subcells, i.e. two junction cells, four junction cells, five junction cells, etc. as more particularly described in U.S. patent application Ser. No. 12/267,812 filed Nov. 10, 2008. In the case of four or more junction cells, the use of more than one metamorphic grading interlayer may also be utilized, as more particularly described in U.S. patent application Ser. No. 12/271,192 filed Nov. 14, 2008. 
     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. 
     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. 
     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 or the base layer. The intrinsic layer may function 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 (“NID”) over part or all of its thickness. Some such configurations are more particularly described in copending U.S. patent application Ser. No. 12/253,051, filed Oct. 16, 2008. 
     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, AlN, GaN, InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials, and still fall within the spirit of the present invention. 
     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. 
     Thus, while the description of this invention has focused primarily on solar cells or photovoltaic devices, persons skilled in the art know that other optoelectronic devices, such as 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. 
     Without further analysis, from the foregoing others can, by applying current knowledge, readily adapt the present invention for various applications. Such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.