Multijunction solar cells on bulk GeSi substrate

Abstract

A solar cell comprising a bulk germanium silicon growth substrate; a diffused photoactive junction in the germanium silicon substrate; and a sequence of subcells grown over the substrate, with the first grown subcell either being lattice matched or lattice mis-matched to the growth substrate.

Claims

1. A four-junction space-qualified solar cell assembly designed for operation at AMO and at a 1 MeV electron equivalent fluence of at least 110.sup.14 e/cm.sup.2, the solar cell comprising subcells, wherein a combination of compositions and band gaps of the subcells is designed to maximize efficiency of the solar cell at a predetermined time, after initial deployment, when the solar cell is deployed in space at AMO and at an operational temperature in the range of 40 to 70 degrees Centigrade, the predetermined time being at least five years and referred to as the end-of-life (EOL), the solar cell comprising: an upper first solar subcell composed of indium gallium aluminum phosphide and having a first band gap in the range of 2.0 to 2.2 eV; a second solar subcell adjacent to said upper first solar subcell and including an emitter layer composed of indium gallium phosphide or aluminum indium gallium arsenide, and a base layer composed of aluminum indium gallium arsenide and having a second band gap in the range of approximately 1.55 to 1.8 eV and being lattice matched with the upper first solar subcell, wherein the emitter and base layers of the second solar subcell form a photoelectric junction; a third solar subcell adjacent to said second solar subcell and composed of indium gallium arsenide and having a third band gap less than that of the second solar subcell and being lattice matched with the second solar subcell; and a fourth solar subcell adjacent to said third solar subcell and composed of germanium silicon and has an indirect band gap in the range of 0.7 to 1.1 eV, or 0.85 to 1.05 eV; wherein each of the upper first solar subcell, the second solar subcell and the third solar subcell is lattice-mismatched to the fourth solar subcell, and wherein a numerical sum of the band gaps of the four solar subcells, divided by four, is equal to 1.35 eV.

2. The four junction solar cell assembly as defined in claim 1, wherein the upper first solar subcell has a band gap of less than 2.15, the second solar subcell has a band gap of less than 1.73 eV; and the third solar subcell has a band gap in the range of 1.15 to 1.2 eV.

3. The four junction solar cell assembly as defined in claim 1, the upper first solar subcell has a band gap of 2.05 eV.

4. The four junction solar cell assembly as defined in claim 1, wherein the band gap of the third solar subcell is less than 1.41 eV, and greater than that of the fourth solar subcell.

5. The four junction solar cell assembly as defined in claim 1, further comprising: a distributed Bragg reflector (DBR) layer adjacent to and disposed between the third and the fourth solar subcells and arranged so that light can enter and pass through the third solar subcell and at least a portion of which can be reflected back into the third solar subcell by the DBR layer, and is composed of a plurality of alternating sublayers of lattice matched materials with discontinuities in their respective indices of refraction; and wherein the difference in refractive indices between alternating sublayers is maximized in order to minimize the number of periods required to achieve a given reflectivity, and the thickness and refractive index of each period determines the stop band and its limiting wavelength.

6. The four junction solar cell assembly as defined in claim 5, wherein the DBR layer includes a first DBR layer composed of a plurality of p type ln.sub.zAl.sub.xGa.sub.1-x-zAs sublayers, and a second DBR layer disposed over and adjacent to the first DBR layer and composed of a plurality of p type ln.sub.wAl.sub.yGa.sub.1-y-wAs sublayers, where 0<w<1, 0<x<1, 0<y<1, 0<z<1 and y is greater than x, thereby increasing the reflection bandwidth of the DBR layer.

7. The four junction solar cell assembly as defined in claim 1, wherein the fourth solar subcell is lattice mismatched with respect to the third solar subcell, and has a band gap between 0.83 and 0.88 eV as measured at 300 degrees Kelvin, corresponding to a percentage of Si in the GeSi substrate ranging between 13.0 and 15.0 percent by mole fraction.

8. The four junction solar cell assembly as defined in claim 1, wherein the top subcell is composed of a base layer of (ln.sub.xGa.sub.1-x).sub.1-yAl.sub.yP where x is 0.505, and y is 0.142, corresponding to a band gap of2.10 eV, and an emitter layer of (ln.sub.xGa.sub.1-x).sub.1-yAl.sub.yP where x is 0.505, and y is 0.107, corresponding to a band gap of 2.05 eV.

9. The four junction solar cell assembly as defined in claim 1, further comprising a tunnel diode disposed over the fourth solar subcell, and intermediate layer disposed between the third solar subcell and the tunnel diode wherein the intermediate layer is compositionally graded to lattice match the third solar subcell on one side and the tunnel diode on the other side and is 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 than or equal to that of the third solar subcell and different than that of the tunnel diode, and having a band gap energy greater than that of the fourth solar subcell.

10. The four junction solar cell assembly as defined in claim 1, further comprising an intermediate layer disposed between the third solar subcell and the fourth solar subcell wherein the intermediate layer is compositionally step-graded with between one and four steps to lattice match the fourth solar subcell on one side and composed of ln.sub.xGa.sub.1-xAs or (ln.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs with 0<x<1, 0<y<1, and x and y selected such that the band gap is in the range of 1.15 to 1.41 eV throughout its thickness.

11. The four junction solar cell assembly as defined in claim 10, wherein the intermediate layer has a graded band gap in the range of 1.15 to 1.41 eV, or 1.2 to 1.35 eV, or 1.25 to 1.30 eV.

12. The four junction solar cell assembly as defined in claim 1, wherein either (i) the emitter layer; or (ii) the base layer and emitter layer, of the upper first subcell have different lattice constants from the lattice constant of the second subcell.

13. The four junction solar cell assembly as defined in claim 1, further comprising: a distributed Bragg reflector (DBR) layer adjacent to and beneath the third solar subcell and arranged so that light can enter and pass through the third solar subcell and at least a portion of which can be reflected back into the third solar subcell by the DBR layer, wherein the distributed Bragg reflector layer is composed of a plurality of alternating layers of lattice matched materials with discontinuities in their respective indices of refraction, wherein the difference in refractive indices between alternating layers is maximized in order to minimize the number of periods required to achieve a given reflectivity, and the thickness and refractive index of each period determines the stop band and its limiting wavelength, and wherein the DBR layer includes a first DBR layer composed of a plurality of p type ln.sub.zAl.sub.xGa.sub.1-x-zAs layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality ofp type ln.sub.wAl.sub.yGa.sub.1-y-wAs layers, where O<w<1, 0<x<1, 0<y<1, 0<z<1 and y is greater than x; and an intermediate layer disposed between the DBR layer and the fourth solar subcell, wherein the intermediate layer is compositionally step-graded to lattice match the DBR layer on one side and the fourth solar subcell on the other side, and is composed of any of the As, P, N, Sb based III-V compound semiconductors subject to the constraints ofhaving the in-plane lattice parameter greater than or equal to that of the DBR layer and less than or equal to that of the lower fourth solar subcell, and having a band gap energy greater than that of the fourth solar subcell.

14. The four junction solar cell assembly as defined in claim 1, wherein each subcell includes an emitter region and a base region, and one or more of the subcells have a base region having a gradation in doping that increases exponentially from 110.sup.15 atoms per cubic centimeter adjacent the p-n junction to 410.sup.18 atoms per cubic centimeter adjacent to the adjoining layer at the rear of the base, and an emitter region having a gradation in doping that decreases from approximately 510.sup.18 atoms per cubic centimeter in the region immediately adjacent the adjoining layer to 510.sup.17 atoms per cubic centimeter in the region adjacent to the p-n junction.

15. The four junction solar cell assembly as defined in claim 9, wherein at least one of the upper sublayers of the graded interlayer has a larger lattice constant than the adjacent layers to the upper sublayer disposed above the grading interlayer.

16. The four junction solar cell assembly as defined in claim 1, wherein the difference in lattice constant between the adjacent third and fourth subcells is in the range of 0.1 to 0.2 Angstroms.

17. The four junction solar cell assembly as defined in claim 1, further comprising a inactive majority carrier layer (window, BSF, tunnel diode) disposed over the second or third solar subcell and having a lattice constant that is greater than that of the third and fourth solar subcells so that the tunnel diode layers are strained in tension.

18. The four junction solar cell assembly as defined in claim 1, further comprising a first threading dislocation inhibition layer having a thickness in the range of 0.10 to 1.0 microns and disposed over said second solar subcell.

19. The four junction solar cell assembly as defined in claim 18, further comprising a second threading dislocation inhibition layer having a thickness in the range of 0.10 to 1.0 micron and composed of InGa(Al)P, the second threading dislocation inhibition layer being disposed over and directly adjacent to said grading interlayer for reducing the propagation of threading dislocations, said second threading dislocation inhibition layer having a composition different from a composition of the first threading dislocation inhibition layer.

20. The four junction solar cell assembly as defined in claim 1 including an intermediate layer disposed between the third solar subcell and the fourth solar subcell and providing a gradual transition in lattice constant in semiconductor structure from the third solar subcell to the fourth solar subcell, wherein the intermediate layer has a band gap that is constant throughout its thickness.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIG. 1 is a graph representing the band gap of GeSi and their lattice constants;

(3) FIG. 2A is a cross-sectional view of a bulk GeSi substrate;

(4) FIG. 2B is a cross-sectional view of an embodiment of a GeSi solar cell after several stages of fabrication including the deposition of certain semiconductor layers on the growth substrate of FIG. 2A, according to the present disclosure;

(5) FIG. 2C is a cross-sectional view of an embodiment of a GeSi solar cell of FIG. 2B after the diffusion of dopant elements into the growth substrate, according to the present disclosure; and

(6) FIG. 3 is a highly simplified cross-sectional view of a portion of an embodiment of a multijunction solar cell grown on a GeSi substrate according to the present disclosure.

GLOSSARY OF TERMS

(7) III-V compound semiconductor refers to a compound semiconductor formed using at least one elements from group III of the periodic table and at least one element from group V of the periodic table. III-V compound semiconductors include binary, tertiary and quaternary compounds. Group III includes boron (B), aluminum (Al), gallium (Ga), indium (In) and thallium (T). Group V includes nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb) and bismuth (Bi).

(8) Band gap refers to an energy difference (e.g., in electron volts (eV)) separating the top of the valence band and the bottom of the conduction band of a semiconductor material. More particularly, the expression band gap of a solar subcell, which internally has layers of different band gaps shall be defined to mean the band gap of the layer of the solar subcell in which the majority of the charge carriers are generated (such sublayer typically being the p-type base semiconductor layer of the base/emitter photovoltaic junction of such subcell). In the event such layer in turn has sublayers with different band gaps (such as the case of a base layer having a graded composition and more particularly a graded band gap), the sublayer of that solar subcell with the lowest band gap shall be taken as defining the band gap of such a subcell. Apart from a solar subcell, and more generally in the case of a specifically designated semiconductor region (such as a metamorphic layer), in which that semiconductor region has sublayers or subregions with different band gaps (such as the case of a semiconductor region having a graded composition and more particularly a graded band gap), the sublayer or subregion of that semiconductor region with the lowest band gap shall be taken as defining the band gap of that semiconductor region.

(9) Beginning of Life (BOL) refers to the time at which a photovoltaic power system is initially deployed in operation.

(10) Bottom subcell refers to the subcell in a multijunction solar cell which is furthest from the primary light source for the solar cell.

(11) Compound semiconductor refers to a semiconductor formed using two or more chemical elements.

(12) Current density refers to the short circuit current density Jsc through a solar subcell through a given planar area, or volume, of semiconductor material constituting the solar subcell.

(13) Deposited, with respect to a layer of semiconductor material, refers to a layer of material which is epitaxially grown over another semiconductor layer.

(14) Dopant refers to a trace impurity element that is contained within a semiconductor material to affect the electrical or optical characteristics of that material. As used in the context of the present disclosure, typical dopant levels in semiconductor materials are in the 1016 to 1019 atoms per cubic centimeter range. The standard notation or nomenclature, when a particular identified dopant is proscribed, is to use, for example, the expression GaAs:Se or GaAs:C for selenium or carbon doped gallium arsenide respectively. Whenever a ternary or quaternary compound semiconductor is expressed as AlGaAs or GaInAsP, it is understood that all three or four of the constituent elements are much higher in mole concentration, say on the 1% level or above, which is in the 1021 atoms/cm-3 or larger range. Such constituent elements are not considered dopants by those skilled in the art since the atoms of the constituent element form part of the crystal structure of the compound semiconductor. In addition, a further distinction is that a dopant has a different valence number than the constituent component elements. In a commonly implemented III-V compound semiconductor such as AlGaInAs, none of the individual elements Al, Ga, In, or As are considered to be dopants since they have the same valence as the component atoms that make up the crystal lattice.

(15) End of Life (EOL) refers to a predetermined time or times after the Beginning of Life, during which the photovoltaic power system has been deployed and has been operational. The EOL time or times may, for example, be specified by the customer as part of the required technical performance specifications of the photovoltaic power system to allow the solar cell designer to define the solar cell subcells and sublayer compositions of the solar cell to meet the technical performance requirement at the specified time or times, in addition to other design objectives. The terminology EOL is not meant to suggest that the photovoltaic power system is not operational or does not produce power after the EOL time.

(16) Graded interlayer (or grading interlayer)see metamorphic layer.

(17) Inverted metamorphic multijunction solar cell or IMM solar cell refers to a solar cell in which the subcells are deposited or grown on a substrate in a reverse sequence such that the higher band gap subcells, which would normally be the top subcells facing the solar radiation in the final deployment configuration, are deposited or grown on a growth substrate prior to depositing or growing the lower band gap subcells.

(18) Layer refers to a relatively planar sheet or thickness of semiconductor or other material. The layer may be deposited or grown, e.g., by epitaxial or other techniques.

(19) Lattice mismatched refers to two adjacently disposed materials or layers (with thicknesses of greater than 100 nm) having inplane lattice constants of the materials in their fully relaxed state differing from one another by less than 0.02% in lattice constant. (Applicant expressly adopts this definition for the purpose of this disclosure, and notes that this definition is considerably more stringent than that proposed, for example, in U.S. Pat. No. 8,962,993, which suggests less than 0.6% lattice constant difference).

(20) Metamorphic layer or graded interlayer refers to a layer that achieves a gradual transition in lattice constant generally throughout its thickness in a semiconductor structure.

(21) Middle subcell refers to a subcell in a multijunction solar cell which is neither a Top Subcell (as defined herein) nor a Bottom Subcell (as defined herein).

(22) Short circuit current (I.sub.sc) refers to the amount of electrical current through a solar cell or solar subcell when the voltage across the solar cell is zero volts, as represented and measured, for example, in units of milliamps.

(23) Short circuit current densitysee current density.

(24) Solar cell refers to an electronic device operable to convert the energy of light directly into electricity by the photovoltaic effect.

(25) Solar cell assembly refers to two or more solar cell subassemblies interconnected electrically with one another.

(26) Solar cell subassembly refers to a stacked sequence of layers including one or more solar subcells.

(27) Solar subcell refers to a stacked sequence of layers including a p-n photoactive junction composed of semiconductor materials. A solar subcell is designed to convert photons over different spectral or wavelength bands to electrical current.

(28) Space qualified refers to an electronic component (e.g., as used in this disclosure, to a solar cell) provides satisfactory operation under the high temperature and thermal cycling test protocols that establish typical qualification requirements for use by customers who utilize such components in the outer space environment. The exemplary conditions for such qualifications include (i) vacuum bake-out testing that includes exposure to a temperature of +100 C. to +135 C. (e.g., about +100 C., +110 C., +120 C., +125 C., +135 C.) for 2 hours to 24 hours, 48 hours, 72 hours, or 96 hours; and (ii) TVAC and/or APTC test that includes cycling between temperature extremes of 180 C. (e.g., about 180 C., 175 C., 170 C., 165 C., 150 C., 140 C., 128 C., 110 C., 100 C., 75 C., or 70 C.) to +145 C. (e.g., about +70 C., +80 C., +90 C., +100 C., +110 C., +120 C., +130 C., +135 C., or +145 C.) for 600 to 32,000 cycles (e.g., about 600, 700, 1500, 2000, 4000, 5000, 7500, 22000, 25000, or 32000 cycles), and in some space missions up to +180 C. See, for example, Fatemi et al., Qualification and Production of Emcore ZTJ Solar Panels for Space Missions, Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th (DOI: 10.1109/PVSC 2013 6745052).

(29) Substantially current matched refers to the short circuit current through adjacent solar subcells being substantially identical (i.e. within plus or minus 1%).

(30) Top subcell or upper subcell refers to the subcell in a multijunction solar cell which is closest to the primary light source for the solar cell.

(31) Upright multijunction solar cell refers to a solar cell in which the subcells are deposited or grown on a substrate in a sequence such that the lower band gap subcells are deposited or grown on a growth substrate prior to depositing or growing the higher band gap subcells.

(32) ZTJ refers to the product designation of a commercially available SolAero Technologies Corp. triple junction solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(33) 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.

(34) A variety of different features of multijunction solar cells (as well as 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 upright solar cells of the present disclosure. However, more particularly, some embodiments of the present disclosure are directed to the fabrication of a multijunction solar cell grown on a germanium silicon growth substrate.

(35) The novel proposal for the use of germanium silicon instead of germanium as the growth substrate and bottom solar subcell is the cornerstone of the present disclosure. Following on Applicant's earlier advances as represented by U.S. Pat. No. 7,339,109 and subsequent proposals and developments, the formation of a photoelectric junction in such a germanium silicon substrate is an improvement that extends the spectral band that can be captured by the bottom subcell in a multijunction solar cell to be an indirect band gap in the range of 0.7 to 1.1 eV. In addition to the specification of a germanium silicon bottom subcell, the present disclosure also provides for an embodiment of a multijunction solar cell in which the two lower subcells (e.g., the third and fourth subcells in a four junction solar cell) are lattice mismatched. More specifically, in some embodiments, the present disclosure relates to four junction solar cells with direct band gaps in the range of 2.0 to 2.15 eV (or higher) for the top subcell, and (i) 1.65 to 1.8 eV, and (ii) 1.41 eV or less, for the middle subcells, and (iii) 0.7 to 1.1 eV indirect bandgaps for the bottom subcell, respectively.

(36) The present disclosure, similar to the related applications of Applicant, provides an unconventional four junction design (with three grown lattice matched subcells, which are lattice mismatched to the GeSi substrate) that leads to a surprising significant performance improvement over that of traditional three junction solar cell on Ge despite the substantial current mismatch present between the top three junctions and the bottom Ge junction. This performance gain is especially realized at high temperature and after high exposure to space radiation by the proposal of incorporating high band gap semiconductors that are inherently more resistant to radiation and temperature, thus specifically addressing the problem of ensuring continues adequate efficiency and power output at the end-of-life.

(37) In some embodiments, the fourth subcell is germanium silicon, while in other embodiments the fourth subcell is InGaAs, GaAsSb, InAsP, InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN. InGaSbBiN or other III-V or II-VI compound semiconductor material.

(38) The indirect band gap of germanium at room temperature is about 0.66 eV, while the direct band gap of germanium at room temperature is 0.8 eV. Those skilled in the art will normally refer to the band gap of germanium as 0.67 eV, since it is lower than the direct band gap value of 0.8 eV.

(39) In the present disclosure, the indirect band gap of the germanium silicon growth substrate would broadly be in the range of 0.7 to 1.1 eV, or for certain applications in the range of 0.85 to 1.05 eV.

(40) More specifically, the present disclosure intends to provide a relatively simple and reproducible technique for upright processing of metamorphic multijunction solar cells, that is suitable for use in a high volume production environment in which various semiconductor layers are grown on a growth substrate in an MOCVD reactor, and subsequent processing steps are defined and selected to minimize the physical damage to the quality of the deposited layers, thereby simplifying wafer handling and ensuring a relatively high yield of operable solar cells meeting specifications at the conclusion of the fabrication processes.

(41) As suggested above, incremental improvements in the design of multijunction solar cells are made in view of a variety of new space missions and application requirements. Moreover, although such improvements may be relatively minute quantitative modifications in the composition, lattice constant, or band gap of certain subcells or adjoining layers, as we noted above, such minute parametric changes can provide substantial improvements in efficiency that specifically address the problems that have been identified associated with the existing current commercial multijunction solar cells, and provide a solution that represents an inventive step in the design process.

(42) 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 in 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.

(43) 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'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).

(44) 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'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's research efforts and the ultimate solar cell design proposed by the Applicants.

(45) 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 a 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.

(46) 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.

(47) Furthermore, as in the case here, where multiple design variables interact in unpredictable ways, the proper choice of the combination of variables can produce new and unexpected results, and constitute an inventive step.

(48) 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 deposition method, such as Molecular Beam Epitaxy (MBE), 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, and are within the scope of the present disclosure.

(49) The present disclosure is in one embodiment 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. Other embodiments may use other growth technique, such as MBE. More particularly, regardless of the growth technique, 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.

(50) Some comments about MOCVD processes used in one embodiment are in order here.

(51) 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.

(52) FIG. 1 is a graph representing the band gap of GeSi and their lattice constants in the range of interest of the present disclosure, i.e, a Ge concentration by mole fraction of over 85%, and in some embodiments in the range of 85% to 87%.

(53) FIG. 2A is a cross-sectional view of a GeSi growth substrate 100 which as bulk semiconductor material may be between 50 and 600 microns in thickness, or in some embodiments between 100 and 200 microns.

(54) FIG. 2B depicts the epitaxial growth of a nucleation layer 102 and a buffer layer 103 on top of the GeSi growth substrate 100. The buffer layer 103 may be composed of GaAs.

(55) FIG. 2C depicts the result of diffusion of As and P into the growth substrate 100, and the formation of a photovoltaic junction depicted by the dashed line in the interior of the substrate 100. As a result of the diffusion, the upper portion 101 of the substrate 100 is converted into an n+ type semiconductor which forms the emitter of the solar subcell formed in the growth substrate 100, which in the embodiments discussed below will be the bottom subcell D of a multijunction solar cell.

(56) Turning to a multijunction solar cell device of the present disclosure, FIG. 3 is a cross-sectional view of an embodiment of a four junction solar cell 450 after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer 322 according to the present disclosure.

(57) As shown in the illustrated example of FIG. 4, the bottom or fourth subcell D includes a growth substrate 300 formed of p-type germanium silicon (GeSi) which also serves as a base layer. A back metal contact pad 350 formed on the bottom of base layer 300 provides electrical contact to the multijunction solar cell 200. The bottom subcell D, further includes, for example, an n+ type nucleation and buffer layer 302. The nucleation layer is deposited over the growth substrate, and the emitter layer is formed in the substrate by diffusion of dopants into the GeSi substrate, thereby forming the n+ type GeSi layer 301. Heavily doped p-type aluminum indium gallium arsenide (AlGaAs) and heavily doped n-type gallium arsenide (GaAs) tunneling junction layers 304, 303 may be deposited over the buffer layer to provide a low resistance pathway between the bottom and middle subcells.

(58) In the embodiment depicted, an intermediate graded interlayer 506, comprising in one embodiment step-graded sublayers 505a through 505zz, is disposed over the tunnel diode layer 303/304. In particular, the graded interlayer provides a transition in the in-plane lattice constant from the lattice constant of the substrate subcell D to the larger lattice constant of the middle and upper subcells C, B and A.

(59) The graph on the left side of FIG. 3 depicts the in-plane lattice constant being incrementally monotonically increased from sublayer 505a through sublayer 505zz, such sublayers being fully relaxed.

(60) At least a first alpha or threading dislocation inhibition layer 504, preferably composed of p-type InGaP, is deposited over the tunnel diode 303/304, to a thickness of from 0.10 to about 1.0 micron. Such an alpha layer is intended to prevent threading dislocations from propagating, either opposite to the direction of growth into the bottom subcell D, or in the direction of growth into the subcell C, and is more particularly described in U.S. Patent Application Pub. No. 2009/0078309 A1 (Cornfeld et al.). More generally, the alpha layer has a different composition than the adjacent layers above and below it.

(61) The metamorphic layer (or graded interlayer) 506 is deposited over the alpha layer 504 using a surfactant. Layer 505 is preferably a compositionally step-graded series of p-type InGaAs or InGaAlAs layers, preferably with monotonically changing lattice constant, so as to achieve a gradual transition in lattice constant in the semiconductor structure from subcell D to subcell C while minimizing threading dislocations from occurring. In one embodiment, the band gap of layer 506 is constant throughout its thickness, preferably approximately equal to 1.22 to 1.34 eV, or otherwise consistent with a value slightly greater than the band gap of the middle subcell C. In another embodiment, the band gap of the sublayers of layer 506 vary in the range of 1.22 to 1.34 eV, with the first layer having a relatively high band gap, and subsequent layers incrementally lower band gaps. One embodiment of the graded interlayer may also be expressed as being composed of InxGa1-xAs, with 0<x<1, 0<y<1, and x and y selected such that the band gap of the interlayer remains constant at approximately 1.22 to 1.34 eV or other appropriate band gap.

(62) In one embodiment, aluminum is added to one sublayer to make one particular sublayer harder than another, thereby forcing dislocations in the softer material.

(63) In the surfactant assisted growth of the metamorphic layer 506, a suitable chemical element is introduced into the reactor during the growth of layer 506 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 506, 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.

(64) 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.

(65) 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 InGaAlAs, in the metamorphic buffer layer. Such Sb or Bi surfactants will not typically be incorporated into the metamorphic layer 506.

(66) In one embodiment of the present disclosure, the layer 506 is composed of a plurality of layers of InGaAs, with monotonically changing lattice constant, each layer having a band gap in the range of 1.22 to 1.34 eV. In some embodiments, the band gap is constant in the range of 1.27 to 1.31 eV through the thickness of layer 505. In some embodiments, the constant band gap is in the range of 1.28 to 1.29 eV.

(67) The advantage of utilizing a constant bandgap material such as InGaAs is that arsenide-based semiconductor material is much easier to process in standard commercial MOCVD reactors.

(68) Although the described embodiment of the present disclosure utilizes a plurality of layers of InGaAs for the metamorphic layer 506 for reasons of manufacturability and radiation transparency, other embodiments of the present disclosure may utilize different material systems to achieve a change in lattice constant from subcell C to subcell D. Other embodiments of the present disclosure 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 less than or equal to that of the third solar subcell C and greater than or equal to that of the fourth solar subcell D. In some embodiments, the layer 505 has a band gap energy greater than that of the third solar subcell C, and in other embodiments has a band gap energy level less than that of the third solar subcell C.

(69) In some embodiments, a second alpha or threading dislocation inhibition layer 507, preferably composed of p type GaInP, is deposited over metamorphic buffer layer 506, to a thickness of from 0.10 to about 1.0 micron. Such an alpha layer is intended to prevent threading dislocations from propagating, either opposite to the direction of growth into the fourth subcell D, or in the direction of growth into the third subcell C, and is more particularly described in U.S. patent application Pub. No. 2009/0078309 A1 (Cornfeld et al.).

(70) In the specific embodiment depicted in FIG. 3, the top or uppermost sublayer 505zz of the graded interlayer 506 is strained or only partially relaxed, and has a lattice constant which is greater than that of the layer above it, i.e., the alpha layer 507 (should there be a second alpha layer) or the BSF layer 306. In short, in this embodiment, there is an overshoot of the last one sublayer 505zz of the grading sublayers, as depicted on the left hand side of FIG. 4B, which shows the step-grading of the lattice constant becoming larger from layer 505a to 505zz, and then decreasing back to the lattice constant of the upper layers 507 through 322.

(71) In the illustrated example of FIG. 3, the third subcell C includes a highly doped p-type aluminum indium gallium arsenide (AlInGaAs) back surface field (BSF) layer 306, a p-type InGaAs base layer 307, a highly doped n-type indium gallium arsenide (InGaAs) emitter layer 308 and a highly doped n-type indium aluminum phosphide (AlInP.sub.2) or indium gallium phosphide (GaInP) window layer 309. The InGaAs base layer 307 of the subcell C can include, for example, approximately 1.5% In. Other compositions may be used as well. The base layer 307 is formed over the BSF layer 306 after the BSF layer is deposited over the DBR layers 305.

(72) The window layer 309 is deposited on the emitter layer 308 of the subcell C. The window layer 309 in the subcell C also helps reduce the recombination loss and improves passivation of the cell surface of the underlying junctions. Before depositing the layers of the subcell B, heavily doped n-type InGaP and p-type AlGaAs (or other suitable compositions) tunneling junction layers 310, 311 may be deposited over the subcell C.

(73) The second subcell B includes a highly doped p-type aluminum indium gallium arsenide (AlInGaAs) back surface field (BSF) layer 312, a p-type AlInGaAs base layer 313, a highly doped n-type indium gallium phosphide (InGaP.sub.2) or AlInGaAs layer 314 and a highly doped n-type indium gallium aluminum phosphide (AlGaAlP) window layer 315. The InGaP emitter layer 314 of the subcell B can include, for example, approximately 50% In. Other compositions may be used as well.

(74) Before depositing the layers of the top or upper first cell A, heavily doped n-type InGaP and p-type AlGaAs tunneling junction layers 316, 317 may be deposited over the subcell B.

(75) In the illustrated example, the top subcell A includes a highly doped p-type indium aluminum phosphide (InAlP.sub.2) BSF layer 318, a p-type InGaAlP base layer 319, a highly doped n-type InGaAlP emitter layer 320 and a highly doped n-type InAlP.sub.2 window layer 321. The base layer 319 of the top subcell A is deposited over the BSF layer 318 after the BSF layer 318 is formed.

(76) After the cap or contact layer 322 is deposited, the grid lines are formed via evaporation and lithographically patterned and deposited over the cap or contact layer 322.

(77) 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.

(78) Although described embodiments of the present disclosure utilizes a vertical stack of four 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, three junction cells, five, six, seven junction cells, etc.

(79) 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.

(80) 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 C, with p-type and n-type InGaAs is one example of a homojunction subcell.

(81) 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.

(82) The composition of the window or BSF layers may utilize other semiconductor compounds, subject to lattice constant and band gap requirements, and may include AlInP, AlAs, AlP, AlGaInP, AlGaAsP, AlGaInAs, AlGaInPAs, GaInP, GaInAs, GaInPAs, AlGaAs, AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb, AlGaInSb, AlN, GaN, InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials, and still fall within the spirit of the present invention.

(83) While the solar cell described in the present disclosure has been illustrated and described as embodied in a conventional upright 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.

(84) 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.

(85) 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.