Patent Publication Number: US-2015059837-A1

Title: Solar cell with passivation on the contact layer

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is related to co-pending U.S. patent application Ser. No. 13/964,774 filed Aug. 12, 2013. 
     This application is related to co-pending U.S. patent application Ser. No. 13/921,756 filed Jun. 19, 2013. 
     This application is also related to co-pending U.S. patent application Ser. No. 13/768,683 filed Feb. 15, 2013. 
    
    
     GOVERNMENT RIGHTS STATEMENT 
     This invention was made with government support under Contract No. FA 9453-04-2-0041 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 disclosure relates to solar cells and the fabrication of solar cells, and more particularly the design and specification of the window layer in multijunction solar cells based on III-V semiconductor compounds. 
     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 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. 
     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. 
     The efficiency of energy conversion, which converts solar energy (or photons) to electrical energy, depends on various factors such as the design of solar cell structures, the choice of semiconductor materials, and the thickness of each cell. In short, the energy conversion efficiency for each solar cell is dependent on the optimum utilization of the available sunlight across the solar spectrum. As such, the characteristic of sunlight absorption in semiconductor material, also known as photovoltaic properties, is critical to determine the most efficient semiconductor to achieve the optimum energy conversion. 
     Typical III-V compound semiconductor solar cells are fabricated on a semiconductor wafer in vertical, multijunction structures or stacked sequence of solar subcells, each subcell formed with appropriate semiconductor layers and including a p-n photoactive junction. Each subcell is designed to convert photons over different spectral or wavelength bands to electrical current. After the sunlight impinges on the front of the solar cell, and photons pass through the subcells, the photons in a wavelength band that are not absorbed and converted to electrical energy in the region of one subcell propagate to the next subcell, where such photons are intended to be captured and converted to electrical energy, assuming the downstream subcell is designed for the photon&#39;s particular wavelength or energy band. 
     The individual solar cells or wafers are then disposed in horizontal arrays, with the individual solar cells connected together in an electrical series and/or parallel 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. 
     The energy conversion efficiency of multijunction solar cells is affected by such factors as the number of subcells, the thickness of each subcell, and the band structure, electron energy levels, conduction, and absorption of each subcell. Factors such as the short circuit current density (J sc ), the open circuit voltage (V oc ), and the fill factor are also important. 
     One of the important mechanical or structural considerations in the choice of semiconductor layers for a solar cell is the desirability of the adjacent layers of semiconductor materials in the solar cell, i.e. each layer of crystalline semiconductor material that is deposited and grown to form a solar subcell, have similar crystal lattice constants or parameters. 
     Many devices, including solar cells, are fabricated by thin epitaxial growth of III-V compound semi conductors upon a relatively thick substrate. The substrate, typically of Ge, GaAs, InP, or other bulk material, acts as a template for the formation of the deposited epitaxial layers. The atomic spacing or lattice constant in the epitaxial layers will generally conform to that of the substrate, so the choice of epitaxial materials will be limited to those having a lattice constant similar to that of the substrate material. 
     The window layer is a semiconductor layer with a thickness of between 200 and 300 Angstroms that is disposed between the surface layer (which may be the Antireflection coating layer, or the contact layer where there are grid lines over the top surface) and the emitter layer of a the top subcell, or between the tunnel diode and the emitter layer of a lower subcell. The window layer is introduced to improve subcell efficiency by preventing minority carrier recombination at the top surface of the emitter layer, thereby permitting the minority carriers present in the emitter to migrate to the pn junction of the subcell, thereby contributing to the extracted electrical current. By being identified as a distinct layer, the window layer will have a composition that differs from both the adjacent layer and the emitter layer, but will generally be lattice matched to both semiconductor layers. 
     In some embodiments, the window layer may have a higher band gap than the adjacent emitter layer, with the higher band gap tending to suppress minority-carrier injection into the window layer, and as a result tending to reduce the recombination of electron-hole pairs that would otherwise occur in the window layer, thereby decreasing the efficiency of photon conversion at that subcell, and thus the overall efficiency of the solar cell. 
     Since the window layer is directly adjacent to the emitter layer, the interface with the emitter layer is appropriately designed so as to minimize the number of minority carriers entering the window. Another characteristic is the deep energy levels in the band gap, and here again one wishes to minimize such deep energy levels which would tend to create sites that could participate in Shockley-Read-Hall (SRH) recombination of electron-hole pairs. Since crystal defects can cause these deep energy levels, the composition and morphology of the window layer should be capable of forming an interface with the emitter layer that would minimize the crystal defects at the interface. 
     However, in order to improve the efficiency of a solar cell even further, the present disclosure proposes additional design features that have heretofore not been considered. 
     The design characteristic of the window layer which has as its goal the minimization of minority-carrier recombination at the window layer/emitter layer interface is sometimes referred to as emitter “passivation”. Although “passivation” is a term in the field of semiconductor process technology that has various meanings depending on the specific materials and electrical properties and the context in which the term is used, such as the passivation approach as described in the Applicant&#39;s U.S. patent application Ser. No. 13/921,756, which is hereby incorporated by reference. In this disclosure, “passivation” will be used to have the meaning of incorporation of a passivating material onto the surface of the window layer, as described herein, unless otherwise noted. 
     SUMMARY OF THE INVENTION 
     Objects of the Invention 
     It is an object of the present invention to provide increased photoconversion efficiency in a multijunction solar cell. 
     It is another object of the present invention to provide increased fill factor in a multijunction solar cell by utilizing a passivation material on the surface of the semiconductor contact layer, adjacent to the window layer, of the top subcell. 
     It is another object of the present invention to provide improved fill factor in a multijunction solar cell by utilizing a sulfur passivation on the surface of the contact layer at the grid line/contact layer interface adjacent to a window layer of the top subcell to reduce the specific contact resistance at the metal/contact layer interface. 
     It is another object of the present disclosure to provide a method of fabricating a multijunction solar cell by application of ammonium sulphide to the top surface of the contact layer of the top subcell. 
     Some implementations of the present disclosure may incorporate or implement fewer of the aspects and features noted in the foregoing objects. 
     Features of the Invention 
     Briefly, and in general terms, the present disclosure provides a solar cell including at least one solar subcell having an emitter layer, a base layer, and a window layer adjacent to the emitter layer, and a semiconductor contact layer adjacent to the windows layer wherein the free surface of the contact layer is passivated with a passivating material. 
     In another aspect, the present disclosure provides a method of forming a solar cell including at least one solar subcell having an emitter layer, a base layer, and a window layer adjacent to the emitter layer, a contact layer adjacent to the emitter layer wherein the surface of the contact layer and the window layer are passivated with a passivating material. 
     In some embodiments, the passivating step of the contact layer or the window layer is performed by application of ammonium sulphide. 
     In some embodiments, the passivating step is performed by dipping the wafer, subsequent to the formation of the window layer, in a solution of ammonium sulphide. 
     In another aspect, the present disclosure provides a multijunction solar cell including a semiconductor contact layer; an upper first solar subcell disposed below the contact layer and being composed of a semiconductor material having a first band gap, and the first solar subcell having a base region and an emitter region; a window layer disposed directly over the emitter region of the upper first solar subcell and directly below the contact layer wherein the solar cell, including sulfur passivation on the surface of the contact layer, or the contact layer and the window layer. 
     In some embodiments, the emitter of the upper first solar subcell is composed of InGaP, and the window layer is composed of InAlP. 
     In some embodiments, the emitter of the upper first solar subcell has a thickness of 80 nm, and the window layer has a thickness of less than 220 Angstroms. 
     In some embodiments, the base of the upper first solar subcell has a thickness of less than 700 nm. 
     In some embodiments, the base of the upper first solar subcell has a thickness of 670 nm. 
     In some embodiments, the upper subcell is composed of an InGaP emitter layer and an InGaP base layer, the second subcell is composed of GaInP emitter layer and a GaAs base layer, and further comprising at least a third subcell composed of a Ge emitter layer and a Ge base layer. 
     In some embodiments, the third subcell has a band gap of 0.67 eV, the second subcell has a band gap in the range of approximately 1.35 to 1.50 eV and the upper subcell has a band gap in the range of 1.87 to 2.2 eV. 
     In another aspect, the present disclosure provides a solar cell including at least one solar subcell having an emitter layer composed of InGaP, a base layer, a window layer adjacent to the emitter layer, wherein the contact layer is composed of InGaAs and has a sulfur passivation on its surface adjacent to the layer overlying the window grid metallization. 
     In another aspect, the present disclosure provides a method of manufacturing a solar cell comprising: forming an upper first solar subcell having a first band gap under the top surface of the window layer; 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 third solar subcell adjacent to said second solar subcell and having a third band gap smaller than said second band gap; and forming a window layer over at least one of the subcells, the window layer having a sulfur passivation on the top surface thereof. 
     In another aspect, the present disclosure provides a method of manufacturing a solar cell by forming at least one solar subcell having an emitter layer, a base layer, and a window layer adjacent to the emitter layer, wherein subsequent to the window and contact layers being formed, and portions of the contact layer being removed from non-metallization regions, the wafer is dipped in a solution of ammonium sulphide. 
     In some embodiments, the emitter of the upper first solar subcell has a thickness of 80 nm. 
     In some embodiments, the base of the upper first solar subcell has a thickness of less than 400 nm. 
     In some embodiments, the base of the upper first solar subcell has a thickness of 260 nm. 
     In some embodiments, additional layer(s) may be added or deleted in the cell structure without departing from the scope of the present disclosure. 
     Some implementations of the present disclosure may incorporate or implement fewer of the aspects and features noted in the foregoing summaries. 
     Additional aspects, advantages, and novel features of the present disclosure will become apparent to those skilled in the art from this disclosure, including the following detailed description as well as by practice of the disclosure. While the disclosure is described below with reference to preferred embodiments, it should be understood that the disclosure 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 disclosure as disclosed and claimed herein and with respect to which the disclosure 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. 1A  is a perspective view of a polyhedral representation of a semiconductor lattice structure showing the crystal planes; 
         FIG. 1B  is a perspective view of the InGaAs crystal lattice showing the position of the gallium and arsenic atoms; 
         FIG. 1C  is a highly simplified depiction of the atomic structure of a layer of semiconductor material composed of In, Al and P atoms as two interpenetrating face centered cubic lattices, such structure also known as a zinc blende cubic crystal structure, such structure representing the window layer of the top subcell in the present disclosure, in which the dangling bonds of a top layer of atoms are present on the surface of the layer; 
         FIG. 1D  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 a multijunction solar cell after several stages of fabrication including the deposition of certain semiconductor layers on the growth substrate up to the grid lines, as known in the prior art; 
         FIG. 3  is a cross-sectional view of the solar cell of a multijunction solar cell after several stages of fabrication including the deposition of certain semiconductor layers on the growth substrate up to the contact layer, as known in the prior art; 
         FIG. 4A  is a highly simplified depiction of the atomic structure of the top subcell contact layer of semiconductor material composed of In, Ga and As atoms, a zinc blende cubic crystal structure, following passivation according to the present disclosure, in which the formerly dangling bonds of a top layer of atoms are bonded to sulfur atoms; 
         FIG. 4B  is a cross-sectional view of the solar cell of a multijunction solar cell of  FIG. 3  after passivation of the surface of the contact layer above the top subcell, according to the present disclosure; 
         FIG. 5  is a cross-sectional view of the solar cell of a multijunction solar cell of  FIG. 4B  after the next stage of fabrication including the deposition of a metal layer over the contact layer; 
         FIG. 6  is a cross-sectional view of the solar cell of a multijunction solar cell of  FIG. 5  after the next stage of fabrication including the lithographic processing to form grids, and the etching down of the semiconductor surface to the window layer; 
         FIG. 7  is a cross-sectional view of the solar cell of a multijunction solar cell of  FIG. 6 , in some embodiments, after passivation of the surface of the window layer above the top subcell, according to the present disclosure; and 
         FIG. 8  is a cross-sectional view of the solar cell of a multijunction solar cell of  FIG. 7  after deposition of an encapsulating layer over the top surface of the window layer and the grids, according to the present disclosure. 
     
    
    
     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. 
     A variety of different features of multijunction solar cells and inverted metamorphic multijunction solar cells are disclosed in the related applications noted above. Some, many or all of such features may be included in the structures and processes associated with the solar cells of the present disclosure. However, more particularly, the present disclosure is directed to the fabrication of a triple junction solar cell grown on a single growth substrate. More generally, however, the present disclosure may be adapted to inverted metamorphic multijunction solar cells as disclosed in the parent application and its related applications that may include three, four, five, or six subcells, with band gaps in the range of 1.8 to 2.2 eV (or higher) for the top subcell, and 1.3 to 1.8 eV, 0.9 to 1.2 eV for the middle subcells, and 0.6 to 0.8 eV, for the bottom subcell, respectively. 
     The present disclosure provides a process for the design and fabrication of a window layer in a multijunction solar cell that improves light capture in the associated subcell and thereby the overall efficiency of the solar cell. More specifically, the present disclosure intends to provide a relatively simple and reproducible technique that is suitable for use in a high volume production environment in which various semiconductor layers are deposited in an MOCVD reactor, and subsequent processing steps are defined and selected to minimize any physical damage to the quality of the deposited layers, thereby ensuring a relatively high yield of operable solar cells meeting specifications at the conclusion of the fabrication processes. 
     Prior to discussing the specific embodiments of the present disclosure, a brief discussion of some of the issues associated with the design of multijunction solar cells, and in particular inverted metamorphic solar cells, and the context of the composition or deposition of various specific layers in embodiments of the product as specified and defined by Applicant is in order. 
     There are a multitude of properties that should be considered in specifying and selecting the composition of, inter alia, a specific semiconductor layer, the back metal layer, the adhesive or bonding material, or the composition of the supporting material for mounting a solar cell thereon. For example, some of the properties that should be considered when selecting a particular layer or material are electrical properties (e.g. conductivity), optical properties (e.g., band gap, absorbance and reflectance), structural properties (e.g., thickness, strength, flexibility, Young&#39;s modulus, etc.), chemical properties (e.g., growth rates, the “sticking coefficient” or ability of one layer to adhere to another, stability of dopants and constituent materials with respect to adjacent layers and subsequent processes, etc.), thermal properties (e.g., thermal stability under temperature changes, coefficient of thermal expansion), and manufacturability (e.g., availability of materials, process complexity, process variability and tolerances, reproducibility of results over high volume, reliability and quality control issues). 
     In view of the trade-offs among these properties, it is not always evident that the selection of a material based on one of its characteristic properties is always or typically “the best” or “optimum” from a commercial standpoint or for Applicant&#39;s purposes. For example, theoretical studies may suggest the use of a quaternary material with a certain band gap for a particular subcell would be the optimum choice for that subcell layer based on fundamental semiconductor physics. As an example, the teachings of academic papers and related proposals for the design of very high efficiency (over 40%) solar cells may therefore suggest that a solar cell designer specify the use of a quaternary material (e.g., InGaAsP) for the active layer of a subcell. A few such devices may actually be fabricated by other researchers, efficiency measurements made, and the results published as an example of the ability of such researchers to advance the progress of science by increasing the demonstrated efficiency of a compound semiconductor multijunction solar cell. Although such experiments and publications are of “academic” interest, from the practical perspective of the Applicants in designing a compound semiconductor multijunction solar cell to be produced in high volume at reasonable cost and subject to manufacturing tolerances and variability inherent in the production processes, such an “optimum” design from an academic perspective is not necessarily the most desirable design in practice, and the teachings of such studies more likely than not point in the wrong direction and lead away from the proper design direction. Stated another way, such references may actually “teach away” from Applicant&#39;s research efforts and the ultimate solar cell design proposed by the Applicants. 
     In view of the foregoing, it is further evident that the identification of one particular constituent element (e.g. indium, or aluminum) in a particular subcell, or the thickness, band gap, doping, or other characteristic of the incorporation of that material in a particular subcell, is not a “result effective variable” that one skilled in the art can simply specify and incrementally adjust to a particular level and thereby increase the efficiency of a solar cell. The efficiency of a solar cell is not a simple linear algebraic equation as a function of the amount of gallium or aluminum or other element in a particular layer. The growth of each of the epitaxial layers of a solar cell in an MOCVD reactor is a non-equilibrium thermodynamic process with dynamically changing spatial and temporal boundary conditions that is not readily or predictably modeled. The formulation and solution of the relevant simultaneous partial differential equations covering such processes are not within the ambit of those of ordinary skill in the art in the field of solar cell design. 
     Even when it is known that particular variables have an impact on electrical, optical, chemical, thermal or other characteristics, the nature of the impact often cannot be predicted with much accuracy, particularly when the variables interact in complex ways, leading to unexpected results and unintended consequences. Thus, significant trial and error, which may include the fabrication and evaluative testing of many prototype devices, often over a period of time of months if not years, is required to determine whether a proposed structure with layers of particular compositions, actually will operate as intended, let alone whether it can be fabricated in a reproducible high volume manner within the manufacturing tolerances and variability inherent in the production process, and necessary for the design of a commercially viable device. 
     Furthermore, as in the case here, where multiple variables interact in unpredictable ways, the proper choice of the combination of variables can produce new and unexpected results, and constitute an “inventive step”. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
       FIG. 1A  is a perspective view of a polyhedral representation of a semiconductor lattice structure showing the crystal planes. The Miller indices are used to identify the planes, and the crystal structure is represented in the Figure by a truncated cube with the (001) plane at the top. In the case of an InGaAs compound semiconductor, which is the material of interest in the present invention, the crystal structure is known as the zinc blende structure, and is shown in  FIG. 1B , which represents a combination of two face centered cubic sublattices. The lattice constant (i.e., the distance between the arsenic atoms in the crystal) is 0.564 nm. 
       FIG. 1B  is a perspective view of the InGaAs crystal lattice showing the position of the indium, gallium and arsenic atoms, with the corresponding Miller indices identifying the lattice planes. 
       FIG. 1C  is a highly simplified depiction of the atomic structure of a layer of semiconductor material composed of In, Ga and As atoms as two interpenetrating face centered cubic lattices, such structure also known as a zinc blende cubic crystal structure, such structure representing the window layer of the top subcell in the present disclosure, in which the dangling bonds of a top layer of atoms are present on the surface of the layer. 
       FIG. 1D  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 AlGaAs 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), or other vapor deposition methods for the 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. 
     The present disclosure is directed to a growth process using a metal organic chemical vapor deposition (MOCVD) process in a standard, commercially available reactor suitable for high volume production. More particularly, the present disclosure is directed to the materials and fabrication steps that are particularly suitable for producing commercially viable multijunction solar cells or inverted metamorphic multijunction solar cells using commercially available equipment and established high-volume fabrication processes, as contrasted with merely academic expositions of laboratory or experimental results. 
     It should be noted that the layers of a certain target composition in a semiconductor structure grown in an MOCVD process are inherently physically different than the layers of an identical target composition grown by another process, e.g. Molecular Beam Epitaxy (MBE). The material quality (i.e., morphology, stoichiometry, number and location of lattice traps, impurities, and other lattice defects) of an epitaxial layer in a semiconductor structure is different depending upon the process used to grow the layer, as well as the process parameters associated with the growth. MOCVD is inherently a chemical reaction process, while MBE is a physical deposition process. The chemicals used in the MOCVD process are present in the MOCVD reactor and interact with the wafers in the reactor, and affect the composition, doping, and other physical, optical and electrical characteristics of the material. For example, the precursor gases used in an MOCVD reactor (e.g. hydrogen) are incorporated into the resulting processed wafer material, and have certain identifiable electro-optical consequences which are more advantageous in certain specific applications of the semiconductor structure, such as in photoelectric conversion in structures designed as solar cells. Such high order effects of processing technology do result in relatively minute but actually observable differences in the material quality grown or deposited according to one process technique compared to another. Thus, devices fabricated at least in part using an MOCVD reactor or using a MOCVD process have inherent different physical material characteristics, which may have an advantageous effect over the identical target material deposited using alternative processes. 
       FIG. 2  illustrates a particular example of a multijunction solar cell device  303  as known in the prior art. In the Figure, each dashed line indicates the active region junction between a base layer and emitter layer of a subcell. 
     As shown in the illustrated example of  FIG. 2 , the bottom subcell  305  includes a substrate  312  formed of p-type germanium (“Ge”) which also serves as a base layer. A contact pad  313  formed on the bottom of base layer  312  provides electrical contact to the multijunction solar cell  303 . The bottom subcell  305  further includes, for example, a highly doped n-type Ge emitter layer  314 , and an n-type indium gallium arsenide (“InGaAs”) nucleation layer  316 . The nucleation layer is deposited over the base layer  312 , and the emitter layer is formed in the substrate by diffusion of deposits into the Ge substrate, thereby forming the n-type Ge layer  314 . Heavily doped p-type aluminum gallium arsenide (“AlGaAs”) and heavily doped n-type gallium arsenide (“GaAs”) tunneling junction layers  318 ,  317  may be deposited over the nucleation layer  316  to provide a low resistance pathway between the bottom and middle subcells. 
     In the illustrated example of  FIG. 2 , the middle subcell  307  includes a highly doped p-type aluminum gallium arsenide (“AlGaAs”) back surface field (“BSF”) layer  320 , a p-type InGaAs base layer  322 , a highly doped n-type indium gallium phosphide (“InGaP2”) emitter layer  324  and a highly doped n-type indium aluminum phosphide (“AlInP2”) window layer  326 . The InGaAs base layer  322  of the middle subcell  307  can include, for example, approximately 1.5% In. Other compositions may be used as well. The base layer  322  is formed over the BSF layer  320  after the BSF layer is deposited over the tunneling junction layers  318  of the bottom subcell  304 . 
     The BSF layer  320  is provided to reduce the recombination loss in the middle subcell  307 . The BSF layer  320  drives minority carriers from a highly doped region near the back surface to minimize the effect of recombination loss. Thus, the BSF layer  320  reduces recombination loss at the backside of the solar cell and thereby reduces recombination at the base layer/BSF layer interface. The window layer  326  is deposited on the emitter layer  324  of the middle subcell B. The window layer  326  in the middle subcell B also helps reduce the recombination loss and improves passivation of the cell surface of the underlying junctions. Before depositing the layers of the top cell C, heavily doped n-type InGaP and p-type AlGaAs tunneling junction layers  327 ,  328  may be deposited over the middle subcell B. 
     In the illustrated example, the top subcell  309  includes a highly doped p-type indium gallium aluminum phosphide (“InGaAlP”) BSF layer  330 , a p-type InGaP2 base layer  332 , a highly doped n-type InGaP2 emitter layer  334  and a highly doped n-type InAlP2 window layer  336 . The base layer  332  of the top subcell  309  is deposited over the BSF layer  330  after the BSF layer  330  is formed over the tunneling junction layers  328  of the middle subcell  307 . The window layer  336  is deposited over the emitter layer  334  of the top subcell after the emitter layer  334  is formed over the base layer  332 . A cap or contact layer  338  may be deposited and patterned into separate contact regions over the window layer  336  of the top subcell  308 . The cap or contact layer  338  serves as an electrical contact from the top subcell  309  to metal grid layer  340 . The doped cap or contact layer  338  can be a semiconductor layer such as, for example, a GaAs or InGaAs layer. 
     After the cap or contact layer  338  is deposited, the grid lines  340  are formed. The grid lines  340  are deposited via evaporation and lithographically patterned and deposited over the cap or contact layer  338 . The mask is subsequently lifted off to form the finished metal grid lines  340  as depicted in the Figure, and the portion of the cap layer that has not been metallized is removed, exposing the surface  342  of the window layer  336 . In some embodiments, a trench or channel (not shown), or portion of the semiconductor structure, is also etched around each of the solar cells. These channels define a peripheral boundary between the solar cell (later to be scribed from the wafer) and the rest of the wafer, and leaves a mesa structure (or a plurality of mesas, in the case of more than one solar cell per wafer) which define and constitute the solar cells later to be scribed and diced from the wafer. 
     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  340  are preferably composed of Ti/Au/Ag/Au, although other suitable materials may be used as well. 
     Turning to the multijunction solar cell device of the present disclosure,  FIG. 3  is a cross-sectional view of the solar cell  400  of a multijunction solar cell after several stages of fabrication including the deposition of certain semiconductor layers on the growth substrate up to the contact layer  338 , similar to the structure described and depicted in  FIG. 2  as known in the prior art. 
       FIG. 4A  is a highly simplified depiction of the atomic structure of the top subcell contact layer  338  of semiconductor material composed of In, Ga and As atoms as a zinc blende cubic crystal structure, following passivation according to the present disclosure, in which the formerly dangling bonds of a top layer of atoms are bonded to sulfur atoms by the passivation process as taught by the present disclosure. 
     In some embodiments of the passivation process, the entire wafer is dipped in a solution of ammonium sulphide for a period of time at least 15 minutes. In other embodiments, the period of time may be longer depending upon the concentration of the solution. In other embodiments, the passivation of the surface may be performed by exposure to a hydrogen sulfide gas 
       FIG. 4B  is a cross-sectional view of the solar cell of a multijunction solar cell of  FIG. 3  after passivation of the surface of the contact layer  338  above the top subcell, according to the present disclosure, with the sulfur atoms being bonded and incorporated into the top surface of the contact layer  338 . The passivized surface is represented in the  FIG. 4B  by dots  343  on the top exposed surface of the contact layer  338 . 
       FIG. 5  is a cross-sectional view of the solar cell of a multijunction solar cell of  FIG. 4B  after the next stage of fabrication including the deposition of a metal layer  340  over the passivated contact layer  338 . 
       FIG. 6  is a cross-sectional view of the solar cell of a multijunction solar cell of  FIG. 5  after the next stage of fabrication including the lithographic processing to form grids  340   a  and  340   b , and subsequently using the grids as a mask to etch the semiconductor body down to the semiconductor surface  342  to the window layer  336 . 
       FIG. 7  is a cross-sectional view of the solar cell of  FIG. 6  after the next process step of surface passivation in another embodiment in which the window layer  336  is passivated, as described in U.S. patent application Ser. No. 13/954,610 and Ser. No. 13/954,630 filed Jul. 30, 2013. In this embodiment, the entire wafer is again dipped in a solution of ammonium sulphide for a period of time at least 15 minutes, thereby resulting in the passivation  344  of the surface of the window layer  336 . In other embodiments, the period of time may be longer depending upon the concentration of the solution. In other embodiments, the passivation of the surface may be performed by exposure to a hydrogen sulfide gas. The passivized surface is represented in the  FIG. 7  by dots  344  penetrating into the exposed surface of the window layer  336  and the exposed edge layers of the wafer. 
       FIG. 8  is a cross-sectional view of the solar cell of  FIG. 7  after the next process steps in which a layer  603  of silicon nitride or titanium dioxide, generally from 50 to 100 Angstroms in thickness, is deposited by plasma enhanced chemical vapor deposition. The deposition of the layer  603  should take place reasonably soon after the passivation step, e.g. after a period of time no longer than sixty minutes, to ensure the quality of the surface of the wafer. In other embodiments, the layer  603  may be deposited by other techniques known in the art, including sputtering and/or evaporation of silicon nitride or titanium dioxide. After deposition of the layer  603 , an antireflection coating layer  604  is deposited in a thickness of 800 to 1000 Angstroms over the entire top surface of the wafer. 
     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 structures or constructions differing from the types of structures or constructions described above. 
     Although described embodiments of the present disclosure utilizes a vertical stack of three subcells, various aspects and features of the present disclosure can apply to stacks with fewer or greater number of subcells, i.e. two junction cells, four junction cells, five, six, seven junction cells, etc. In the case of seven or more junction cells, the use of more than two metamorphic grading interlayer may also be utilized. 
     In addition, although the disclosed embodiments are 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 solar cell described in the present disclosure 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  309 , with p-type and n-type InGaP is one example of a homojunction subcell. 
     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, GaInP, GaInAs, GaInPAs, AlGaAs, AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb, AlGaInSb, AIN, GaN, InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials, and still fall within the spirit of the present invention. 
     While the solar cell described in the present disclosure has been illustrated and described as embodied in a conventional multijunction solar cell, it is not intended to be limited to the details shown, since it is also applicable to inverted metamorphic solar cells, and 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 the semiconductor device described in the present disclosure 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.