Patent Publication Number: US-8969712-B2

Title: Four junction inverted metamorphic multijunction solar cell with a single metamorphic layer

Description:
REFERENCE TO RELATED APPLICATIONS 
     This is a divisional of patent application Ser. No. 12/401,157, filed Mar. 10, 2009, pending, which is incorporated herein by reference in its entirety. 
     This application is related to co-pending U.S. patent application Ser. No. 12/389,053 filed Feb. 19, 2009. 
     This application is related to co-pending U.S. patent application Ser. No. 12/367,991 filed Feb. 9, 2009. 
     This application is related to co-pending U.S. patent application Ser. No. 12/362,201, Ser. No. 12/362,213, and Ser. No. 12/362,225 filed Jan. 29, 2009. 
     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. 12/267,812 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. 12/218,582 filed Jul. 18, 2008. 
     This application is related to co-pending U.S. patent application Ser. No. 12/218,558 filed Jul. 17, 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. 8, 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. 
    
    
     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. 
    
    
     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 multifunction 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. 
     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 relating to the appropriate choice of materials and fabrication steps. 
     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 
     Briefly, and in general terms, the present invention provides a multijunction solar cell including an upper first solar subcell having a first band gap; a second solar subcell adjacent to said first solar subcell and having a second band gap smaller than said first band gap; a graded interlayer adjacent to said second solar subcell, said graded interlayer having a third band gap greater than said second band gap; a third solar subcell adjacent to said graded interlayer, said third subcell 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 a lower fourth solar subcell adjacent to said third solar subcell, said lower fourth subcell having a fifth band gap smaller than said fourth band gap, wherein said lower fourth subcell is lattice matched with respect to said third subcell. 
     In another aspect the present invention provides a method of manufacturing a solar cell by providing a first substrate; forming an upper first solar subcell having a first band gap on said first substrate; forming a second solar subcell adjacent to said first solar subcell and having a second band gap smaller than said first band gap; forming a graded interlayer adjacent to said second solar subcell, said graded interlayer having a third band gap greater than said second band gap; forming a third solar subcell adjacent to said graded interlayer, said third subcell 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; forming a lower fourth solar subcell adjacent to said third solar subcell, said lower fourth subcell having a fifth band gap smaller than said fourth band gap, wherein said lower fourth subcell is lattice matched with respect to said third subcell; mounting a surrogate substrate on top of said lower fourth solar subcell; and removing the first substrate. 
    
    
     
       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 bandgap of certain binary materials and their lattice constants; 
         FIG. 2  is a cross-sectional view of the solar cell of the present invention after an initial stage of fabrication including the deposition of certain semiconductor layers on the growth substrate; 
         FIG. 3  is a cross-sectional view of the solar cell of  FIG. 2  after the next sequence of process steps; 
         FIG. 4  is a cross-sectional view of the solar cell of  FIG. 3  after the next sequence of process steps; 
         FIG. 5  is a cross-sectional view of the solar cell of  FIG. 4  after the next sequence of process steps; 
         FIG. 6  is a cross-sectional view of the solar cell of  FIG. 5  after the next process step; 
         FIG. 7  is a cross-sectional view of the solar cell of  FIG. 6  after the next process step in which a surrogate substrate is attached; 
         FIG. 8A  is a cross-sectional view of the solar cell of  FIG. 7  after the next process step in which the original substrate is unloved; 
         FIG. 8B  is another cross-sectional view of the solar cell of  FIG. 8A  with the surrogate substrate on the bottom of the Figure; 
         FIG. 9  is a simplified cross-sectional view of the solar cell of  FIG. 8B  after the next process step; 
         FIG. 10  is a cross-sectional view of the solar cell of  FIG. 9  after the next process step; 
         FIG. 11  is a cross-sectional view of the solar cell of  FIG. 10  after the next process step; 
         FIG. 12  is a cross-sectional view of the solar cell of  FIG. 11  after the next process step; 
         FIG. 13A  is a top plan view of a wafer in which the solar cells are fabricated; 
         FIG. 13B  is a bottom plan view of a wafer in which the solar cells are fabricated; 
         FIG. 14  is a cross-sectional view of the solar cell of  FIG. 12  after the next process step; 
         FIG. 15  is a cross-sectional view of the solar cell of  FIG. 14  after the next process step; 
         FIG. 16  is a cross-sectional view of the solar cell of  FIG. 15  after the next process step 
         FIG. 17  is a top plan view of the wafer of  FIG. 16  depicting the surface view of the trench etched around the cell; 
         FIG. 18A  is a cross-sectional view of the solar cell of  FIG. 16  after the next process step in a first embodiment of the present invention; 
         FIG. 18B  is a cross-sectional view of the solar cell of  FIG. 16  after the next process step in a second embodiment of the present invention; 
         FIG. 19  is a cross-sectional view of the solar cell of  FIG. 18  after the next process step; 
         FIG. 20  is a graph of the doping profile in a base and emitter layers of a subcell in the metamorphic solar cell according to the present invention; 
         FIG. 21  is a graph that depicts the current and voltage characteristics of an inverted metamorphic multijunction solar cell according to the present invention; 
         FIG. 22  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. 23  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; 
         FIG. 24  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 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. 
     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 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. However, more particularly, the present invention is directed to the fabrication of a four junction inverted metamorphic solar cell using two different metamorphic layers, all grown on a single growth substrate. In the present invention, the resulting construction includes four subcells, with band gaps in the range of 1.8 to 2.1 eV, 1.3 to 1.5 eV, 0.9 to 1.1 eV, and 0.6 to 0.8 eV respectively. 
       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. 
     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 . 
     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). 
     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. 20 . 
     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, a BSF layer  18  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 forms 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. The window layer  110  used in the subcell B also 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 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. 20 . 
     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 subcell, as more fully described in U.S. patent application Ser. No. 12/258,190, filed Oct. 24, 2008. Moreover, the window layer  110  is preferably is doped three times 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 the preferred embodiment of the present invention, the middle subcell emitter has a band gap equal to the top subcell emitter, and the third 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 third 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 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 . 
     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(ADP, 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 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. 
     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 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 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 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 InGaAIAs, 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 bandgap 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 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 band gap, approximately 1.5 eV. 
     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. 
     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 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 bandgap 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 InGnAlAs metamorphic layer  116 . The second bather layer  117  will typically have a different composition than that of bather layer  115 , and performs essentially the same function of preventing threading dislocations from propagating. In the preferred embodiment, bather layer  117  is n+ type GaInP. 
     A window layer  118  preferably composed of n+ type GaInP is then deposited over the bather 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 another suitable materials consistent with lattice constant and bandgap requirements may be used as well. For example, these layers may be composed of n+ type GaInAsP and p-type GaInAsP, respectively. The doping profile of layers  119  and  120  will be discussed in connection with  FIG. 20 . 
     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 . 
     The p++/n++ tunnel diode layers  122   a  and  122   b  respectively are deposited over the BSF layer  121 , similar to the layers  114   a  and  114   b , forming an ohmic circuit element to connect subcell C to subcell D. The layer  122   a  is preferably composed of p++ InGaAlAs, and layer  122   b  is preferably composed of n++ InGaAlAs. 
       FIG. 3  depicts a cross-sectional view of the solar cell of  FIG. 2  after the next sequence of process steps. A barrier layer  123 , preferably composed of n-type GaInP, is deposited over the tunnel diode  122   a / 122   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 top and middle subcells A, B and C, or in the direction of growth into the subcell D, 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)  124  is deposited over the barrier layer  123  using a surfactant. Layer  124  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 C to subcell D while minimizing threading dislocations from occurring. The band gap of layer  124  is constant throughout its thickness, preferably approximately equal to 1.1 eV, or otherwise consistent with a value slightly greater than the band gap of the middle subcell C. 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 1.1 eV or other appropriate band gap. 
     In the surfactant assisted growth of the metamorphic layer  124 , a suitable chemical element is introduced into the reactor during the growth of layer  124  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  124 , 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. 
     A window layer  125  preferably composed of n+ type InGaAlAs is then deposited over layer  124  (or over a second barrier layer, if there is one, disposed over layer  124 ). This window layer operates to reduce the recombination loss in the fourth subcell “D”. 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. 
       FIG. 4  depicts a cross-sectional view of the solar cell of  FIG. 3  after the next sequence of process steps. On top of the window layer  125 , the layers of cell D are deposited: the n+ emitter layer  126 , and the p-type base layer  127 . 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 material consistent with lattice constant and bandgap requirements may be used as well. The doping profile of layers  126  and  127  will be discussed in connection with  FIG. 20 . 
     Turning next to  FIG. 5 , a BSF layer  128 , preferably composed of p+ type InGaAlAs, is then deposited on top of the cell D, the BSF layer performing the same function as the BSF layers  108 ,  113  and  121 . 
     Finally a high band gap contact layer  129 , preferably composed of p++ type InGaAlAs, is deposited on the BSF layer  128 . 
     The composition of this contact layer  129  located at the bottom (non-illuminated) side of the lowest band gap photovoltaic cell (i.e., subcell “D” in the depicted embodiment) in a multijunction photovoltaic cell, can be formulated to reduce absorption of the light that passes through the cell, so that (i) the backside ohmic metal contact layer below it (on the non-illuminated side) will also act as a mirror layer, and (ii) 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. 6  is a cross-sectional view of the solar cell of  FIG. 5  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. 
     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 (1) 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 (2) the contact layer is specularly reflective over the wavelength range of interest. 
       FIG. 7  is a cross-sectional view of the solar cell of  FIG. 3  after the next process step in which an adhesive layer  131  is deposited over the metal layer  130 . The adhesive is preferably Wafer Bond (manufactured by Brewer Science, Inc. of Rolla, Mo.). 
     In the next process step, a surrogate substrate  132 , preferably sapphire, is attached. Alternative, the surrogate substrate may be GaAs, Ge or Si, or other suitable material. The surrogate substrate is 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 and the substrate. As an alternative to using an adhesive layer  131 , a suitable substrate (e.g., GaAs) may be eutectically or permanently bonded to the metal layer  130 . 
       FIG. 8A  is a cross-sectional view of the solar cell of  FIG. 7  after the next process step in which the original substrate is removed by a sequence of lapping and/or etching steps in which the substrate  101 , and the buffer layer  103  are removed. The choice of a particular etchant is growth substrate dependent. 
       FIG. 8B  is a cross-sectional view of the solar cell of  FIG. 8A  with the orientation with the surrogate substrate  132  being at the bottom of the Figure. Subsequent Figures in this application will assume such orientation. 
       FIG. 9  is a simplified cross-sectional view of the solar cell of  FIG. 8B  depicting just a few of the top layers and lower layers over the surrogate substrate  132 . 
       FIG. 10  is a cross-sectional view of the solar cell of  FIG. 9  after the next process step in which the etch stop layer  103  is removed by a HCl/H 2 O solution. 
       FIG. 11  is a cross-sectional view of the solar cell of  FIG. 10  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. 18, 2008, hereby incorporated by reference, the grid lines  501  are preferably composed of Pd/Ge/Ti/Pd/Au, although other suitable materials may be used as well. 
       FIG. 12  is a cross-sectional view of the solar cell of  FIG. 11  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. 13A  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. 
     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 the contact pad are illustrative and the present invention is not limited to the illustrated embodiment. 
       FIG. 13B  is a bottom plan view of the wafer with four solar cells shown in  FIG. 13A . 
       FIG. 14  is a cross-sectional view of the solar cell of  FIG. 12  after the next process step in which an antirefiective (ARC) dielectric coating layer  130  is applied over the entire surface of the “bottom” side of the wafer with the grid lines  501 . 
       FIG. 15  is a cross-sectional view of the solar cell of  FIG. 14  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  130  using phosphide and arsenide etchants. These channels define a peripheral boundary between the cell and the rest of the wafer, and leave a mesa structure which constitutes the solar cell. The cross-section depicted in  FIG. 15  is that as seen from the A-A plane shown in  FIG. 17 . In a preferred embodiment, channel  510  is substantially wider than that of channel  511 . 
       FIG. 16  is a cross-sectional view of the solar cell of  FIG. 15  after the next process step in which channel  511  is exposed to a metal etchant, and that portion of the metal layer  130  located at the bottom of the channel  511  is removed. The depth of the channel  511  is thereby extended to approximately to the top surface of the adhesive layer  131 . 
       FIG. 17  is a top plan view of the wafer of  FIG. 16  depicting the channels  510  and  511  etched around the periphery of each cell. 
       FIG. 18A  is a cross-sectional view of the solar cell of  FIG. 16  after the next process step in a first embodiment of the present invention in which the surrogate substrate  132  is appropriately thinned to a relatively thin layer  132   a , by grinding, lapping, or etching. In this embodiment, the thin layer  132   a  forms the support for the solar cell in applications where a cover glass, such as provided in the second embodiment to be described below, is not required. In such an embodiment, electrical contact to the metal contact layer  130  may be made through the channel  510  or by other via structures. 
       FIG. 18B  is a cross-sectional view of the solar cell of  FIG. 16  after the next process step in a second 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  preferably covers the entire channel  510 , but does not extend to the periphery of the cell near the channel  511 . Although the use of a cover glass is the preferred embodiment, 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. 19  is a cross-sectional view of the solar cell of  FIG. 18B  after the next process step of the present invention in which the adhesive layer  131 , the surrogate substrate  132  and the peripheral portion  512  of the wafer is entirely removed, breaking off in the region of the channel  510 , leaving only the solar cell with the cover glass  514  (or other layers or structures) on the top, and the metal contact layer  130  on the bottom, which forms the backside contact of the solar cell. The surrogate substrate is preferably removed by the use of the etchant EKC  922 . As noted above, the surrogate substrate includes perforations over its surface that allow the flow of etchant through the surrogate substrate  132  to permit its lift off. The surrogate substrate may be reused in subsequent wafer processing operations. 
       FIG. 20  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. 21  is a graph that depicts the current and voltage characteristics of one of the test solar cells fabricated according to the present invention. In this test cell, the lower fourth subcell had a band gap in the range of approximately 0.6 to 0.8 eV, the third subcell had a band gap in the range of approximately 0.9 to 1.1 eV, the second subcell had a band gap in the range of approximately 1.35 to 1.45 eV and the upper subcell had a band gap in the range of 1.8 to 2.1 eV. The solar cell was measured to have an open circuit voltage (V oc ) of approximately 3.265 volts, a short circuit current of approximately 16.26 mA/cm 2 , a fill factor of approximately 82%, and an efficiency of 32.2%. 
       FIG. 22  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. 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 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. 24  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 the preferred embodiment of the present invention utilizes a vertical stack of four subcells, the present invention can apply to stacks with fewer or greater number of subcells, i.e. two junction cells, three junction cells, five junction cells, etc. In the case of four or more junction cells, the use of more than one metamorphic grading interlayer may also be utilized. 
     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. 
     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, GaTnP, 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. 
     While the invention has been illustrated and described as embodied in a 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, 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.