WO2009020875A2 - Quaternary ganasp solar cells - Google Patents

Quaternary ganasp solar cells Download PDF

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Publication number
WO2009020875A2
WO2009020875A2 PCT/US2008/071987 US2008071987W WO2009020875A2 WO 2009020875 A2 WO2009020875 A2 WO 2009020875A2 US 2008071987 W US2008071987 W US 2008071987W WO 2009020875 A2 WO2009020875 A2 WO 2009020875A2
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WIPO (PCT)
Prior art keywords
substrate
buffer layer
metamorphic buffer
forming
ganasp
Prior art date
Application number
PCT/US2008/071987
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French (fr)
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WO2009020875A3 (en
Inventor
Vladimir Odnoblyudov
Kevin A. Tetz
David Keogh
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Ybr Solar, Inc.
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Publication of WO2009020875A2 publication Critical patent/WO2009020875A2/en
Publication of WO2009020875A3 publication Critical patent/WO2009020875A3/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/184—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof the active layers comprising only AIIIBV compounds, e.g. GaAs, InP
    • H01L31/1844—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof the active layers comprising only AIIIBV compounds, e.g. GaAs, InP comprising ternary or quaternary compounds, e.g. Ga Al As, In Ga As P
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/0304—Inorganic materials including, apart from doping materials or other impurities, only AIIIBV compounds
    • H01L31/03046—Inorganic materials including, apart from doping materials or other impurities, only AIIIBV compounds including ternary or quaternary compounds, e.g. GaAlAs, InGaAs, InGaAsP
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B10/00—Integration of renewable energy sources in buildings
    • Y02B10/10—Photovoltaic [PV]
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/544—Solar cells from Group III-V materials

Definitions

  • This application relates generally to the formation of quaternary alloys over substrates. More specifically, this application relates to the formation of quaternary GaNAsP alloys such as may find utility in solar-cell applications.
  • a single band-cap material is used to capture a portion of the solar spectrum, with photons that have an energy greater than the band gap of (he material being absorbed to create an electron-bole pair that produces a dc current under the action of an electric field.
  • the conversion efficiency for a single- junction cell has a peak at the bandgap of the active region and decreases rapidly for higher energies.
  • Using a single bandgap to convert a substantial portion of the solar spectrum is therefore relatively inefficient, with a theoretical maximum efficiency of 35% but with typical efficiencies actually using this technology being on the order of 15 - 20 %.
  • a more sophisticated approach that has been explored at least theoretically is a multiple-band technique in which the number of bandgaps within a single cell is increased without the use of multiple materials.
  • Introduction of a small fraction of highly electronegative atoms into a host semiconductor material has been shown to dramatically alter the electronic band structure of the host material by splitting the conduction band into two sub bands. Because of the interaction between the two subbands, one subband is pushed to an energy higher than that of the bandgap of the host semiconductor and the other subband is pushed to a lower energy.
  • Embodiments of the invention provide material systems that use GaNAsP alloys and methods of forming such material systems
  • the material system comprises a substrate, a metamorphic buffer layer overlying the substrate, and a GaNAsP quaternary alloy overlying the metamorphic buffer layer
  • a lattice constant of the GaNAsP quaternary alloy is approximately the same as a lattice constant of the metamorphic buffer layer and substantially different from a lattice constant of the substrate
  • the substrate is GaP Suitable metamorphic buffer layers accordingly include GaAsP, InGaP, InGaAsP, InGaNP, and GaNAsP, among others
  • Each of the metamorphic buffer layer and the GaNAsP quaternary alloy may be formed using molecular-beam epitaxy or metalorganic chemical-vapor deposition When then same technique is used for the formation of both these layers, the formation of both may take place m the same processing clamber
  • the metamorphic buffer comprises a plurality of independently formed sublayers
  • Pigs IA - 1 C illustrate the electronic structure of different types of monocrystalline solar cells
  • Figs 2A and 2B compare the physical structure of layers deposited on a substrate with layers deposited over an intermediate metamorphic buffei layer
  • FIG 3 is a schematic illustration of a system that maybe used m embodiments of the invention to grow a layer over a substrate
  • Fig 4 is a flow diagram that summarizes methods of depositing a GaNAsP layer over a substrate in accordance with embodiments of the invention
  • Fig 5 compares photoluminescence spectra of a GaNAsP layer deposited directly over a GaP substiate with spectra a GaNAsP deposited over a GaP substrate with an intermediate GaAsP metamorphic buffer layer;
  • Fig 6 compares x-ray diffraction spectra of a GaAsP metamorphic buffer layer grown by molecular-beam epitaxy with spectra of a GaAsP grown by metalorganic chemical- vapor deposition
  • Figs 7A and 7B illustrate measured data and a fitted model for a GaNAsP laver grown over a GaP substrate as a function of wavelength for different incident angles
  • Fig. 8 provides a plot of an absorption coefficient as de ⁇ ved from spectroscopic elhpsometry measurements to show the existence of multiple band absorption for GaAsNP
  • Embodiments of the invention provide structures that may be used to provide efficient optical absorbing layers, particularly at wavelengths that encompass a broad range of solar illumination
  • the structure comprises a semiconductor layer formed over a substiate with an intermediate metamorphic buffer layer
  • metamorphic refers to the ability of the layer to prov ide strain relaxation between sandwiching layers that have different lattice structures
  • x andy should be selected so that there is sufficient incorporation of active nitrogen to separate the conduction band from the intermediate band This may be achieved in embodiments of the invention with x > 0 01
  • the phosphorus concentration may be selected so that it is sufficiently large to mo ⁇ e the energy levels of the electronic structure to provide good localization of conduction-band states But at the same time the phosphorus concentration may be selected to provide an indirect Xbandgap that is less than the F band gap This is achieved in specific embodiments with 0 35 ⁇ (1 -x - v) ⁇ 0 50 In particular embodiments, 0 005 ⁇ x ⁇ 0 050 and 0 3 ⁇ v ⁇ 0 7
  • Fig. 3 provides an illustration of one such structure. This structure is well suited for the use of metalorganic chemical-vapor deposition (“MOCVD”), although as explained in further detail below molecular-beam epitaxy (“MBE”) and other techniques may be used for the formation of each of the layers.
  • MOCVD metalorganic chemical-vapor deposition
  • MBE molecular-beam epitaxy
  • flows of precursors are provided to a processing chamber in which a heated substrate is disposed. The heat promotes reaction of the precursors to grow an epitaxial film of the desired composition over the substrate.
  • the basic structure of the system shown in Fig. 5 includes a processing chamber 304 within which a substrate 312 is disposed on a pedestal 308 during processing.
  • Precursors are delivered to the processing chamber 304 with a precursor delivery system 316. While the precursors may be provided as gaseous sources in some embodiments, they may alternatively be provided as liquid or solid sources. When liquid sources are used, they are vaporized by an appropriate mechanism comprised by the precursor delivery system 316, such as a bubbler.
  • the gaseous or vaporized precursors may be mixed with an inert carrier gas, as understood by those of skill in the art.
  • the drawing shows four precursors 520 that may be suitable for deposition of a GaNAsP layer: trimethylgallium (“TMG”) 320-1 , which may be suitable as a gallium precursor; dimefhylhydrazine (“DMHy”) 320-2, which may be suitable as a nitrogen precursor; arsine AsH 3 320-3, which may be suitable as an arsenic precursor; and phosphine PH 3 320-4, which may be suitable as a phosphorus precursor.
  • TMG trimethylgallium
  • DMHy dimefhylhydrazine
  • arsine AsH 3 320-3 which may be suitable as an arsenic precursor
  • phosphine PH 3 320-4 which may be suitable as a phosphorus precursor.
  • FIG. 4 A general overview of methods of forming material systems in accordance with embodiments of the invention is provided with the flow diagram of Fig. 4.
  • certain steps are illustrated specifically and are identified in one exemplary order, but this is not intended to be limiting. In other embodiments, some of the indicated steps might be omitted, some additional steps not explicitly shown maybe performed, and the order of the steps may be altered.
  • the method begins at block 404 by transferring a suitable substrate in to the processing chamber.
  • suitable substrates especially include III- V substrates like GaAs, GaP, and InP, but the invention is not limited to such substrates and elemental or II- VI substrates may be used in other embodiments.
  • the processing chamber may comprise an MOCVD or MBE chamber in certain specific embodiments, it may more generally be any chamber comprised by a system configurable for the formation of semiconductor layers over a substrate.
  • other embodiments might use plasma deposition systems.
  • the substrate may be cleaned at block 408 to prepare the substrate for deposition of overlying layers.
  • processing conditions are established within the chamber for growth of the metamorphic buffer layer.
  • processing conditions include such parameters as a temperature and pressure of the environment within the processing chamber.
  • An appropriately prepared processing chamber is thus ready to receive flow rates of precursors for formation of the metamorphic buffer layer at block 416.
  • the relative flow rates of the different precursors may be selected to achieve the desired stoichiometry of the metamorphic buffer layer, which is grown under these processing conditions at block 420.
  • GaNAsP quaternary layer may be followed by formation of the overlying GaNAsP quaternary layer by establishing processing conditions for such growth within the processing chamber at block 424.
  • flows of gallium, nitrogen, arsenic, and phosphorus precursors are provided to the processing chamber, permitting the actual growth of the quaternary semiconductor layer to occur at block 432.
  • the process is completed by terminating the precursor flows at block 436 and transferring the substrate out of the processing chamber at block 440.
  • Fig. 4 brings out a number of general aspects of certain embodiments of the invention, there are a number of variations that are possible and that remain within the intended scope of the invention.
  • formation of the metamorphic buffer layer may take place in the same or a different chamber than formation of the overlying quaternary semiconductor layer.
  • different chambers when used, they may be chambers of a similar type or may be chambers that use different semiconductor-layer fabrication processes
  • the formation of each of the layers might take place as a single substantially continuous growth process or might be performed in stages by building up successh ely deposited sublayers.
  • the metamorphic buffer layer is grown directly onto a substrate in an MOCVD reactor p ⁇ or to deposition of the GaN x As ⁇ P ⁇ x . ⁇ layer in a single growth process or in several growth processes using the same MOCVD reactor
  • the metamorphic buffer layer is grown directly onto a substrate using an MBE reactor, again prior to deposition of the GaN x AS y Fi -X y layer in a single growth process or in several growth processes using the same MBE reactor
  • the metamorphic buffer layer is grown directly onto a substrate using an MBE reactor, followed by deposition of the GaN x A s y Pi- x - y layer in a single growth process or in several growth processes with an MOCVD reactor
  • a desirable feature of the metamorphic buffer is thus that it have physical characte ⁇ stics that promote the release of strain through the formation of misfit dislocations instead of threading dislocations
  • the formation of misfit dislocations may be promoted at generally lower growth temperatures, i e. when T ⁇ 450 0 C. At temperatures higher than about 450 0 C, the foimation threading dislocations tends, to be favored.
  • low-temperature growth is somewhat easier to realize using MBE than using MOCVD because the growth of the epitaxial layer is carried out using elemental fluxes of different materials deposited onto a surface.
  • low-temperature growth is generally more difficult to realize because the growth is carried out using molecular source materials that decompose on the substrate surface in order to create reactive elements.
  • low substrate temperature thus prevents the efficient decomposition of these compound source materials on a substrate surface, making a low temperature deposition more difficult to realize.
  • the higher typical growth temperatures for compound semiconductors when using MOCVD as compared with MBE makes the use of an MOCVD reactor more appropriate for growth of the overlying quaternary semiconductor layer.
  • a typical growth temperature for GaAs using MBE is about 600 0 C while a typical growth temperature for GaAs using MOCVD is 680 0 C.
  • Higher growth temperature generally results in a better optical quality of the epitaxial layer.
  • metamorphic buffer layer There are a number of different compositions that may be used for the metamorphic buffer layer. This affects the choice of precursors and flow rates for those precursors at block 416 of the flow diagram of Fig. 4. In part, an appropriate composition for the metamorphic buffer layer depends on the particular composition of the substrate because this affects the lattice-mismatch characteristics to be accommodated. But even for a particular substrate, there are a number of different compositions that are possible for the metamorphic buffer layer.
  • Suitable metamorphic buffer layers include InGaP, InGaAsP, GaAsP, InGaNP, among others. Since GaN x As y P], x-y contains Ga, As, and P, one embodiment makes use of a GaAsP buffer layer.
  • the lattice constant is 5.5494 A.
  • the inventors have earned out a number oi experimental investigations to explore aspects of the invention dcsc ⁇ bed abo ⁇ e
  • the optical properties of GaNjAs v Pi x x layers grown ovei a GaP substrate were evaluated
  • a 0 3- ⁇ m-thick GaNo 02 AS 0 Sj 1 Po 4 layer was grown by MOCVD in one process on two different substrates (1) directly onto a GaP substrate, and (2) onto a 0 2- ⁇ m-thick GaAso 4 ⁇ Po si metamorphic buffer layer grown by MOCVD onto a GaP substrate
  • the resulting GaNo 0 2 AS 0 -, ⁇ o 4 lay er was evaluated for optical quality by conducting photolumincscence measurements In particular, a photoluminescence intensity was measured from the layer of GaNo ozAs ⁇ ssPo 4 ' n the structure without the buffer layer and m the structure with the buffer layer using a 532-nm layers Results of this investigation are shown in Fig
  • Fig 6 shows the results of (004) x-ray-diffraction measurements of the samples desc ⁇ bed above, with curve 604 showing results for the MOCVD-grown layer and curve 608 showing results for the MBE-grown layer
  • 'TWHM full width at half maximum
  • Figs 7 A and 7B also show the iesults of modeling of the GaNAsP material, with the calculated results showing good agreement with the measured data over the spectral range shown.
  • Results 820 provide data showing the absorption measured for a GaNAsP layer grown on a structure having a GaP substrate and a GaNasP buffer layer and results 824 provide data showing the absorption measured for a GaNAsP layer grown directly onto a GaP substrate. These results exhibit the characteristic absorption peaks for a multiband semiconductor material, with the direct-grown results 824 exhibiting a peak at E c ⁇ and the buffer-layer results 820 exhibiting a peak at E c2 -

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Abstract

Methods arc disclosed of fabricating a solar cell system. A solar cell is formed over a substrate. The substrate is attached to a carrier. A translucent or transparent protective cover is overlaid over the solar cell to produce the solar cell system, which is deployed onto an exterior of a building.

Description

QUATERNARY GaNAsP SOLAR CELLS
[0001] A copy of U.S. Prov. Pat. Appl. No. 60/869,984, entitled "SOLAR CELL SYSTEMS FOR USE IN BUILDINGS" ("the '984 application") is attached as an Appendix. This Appendix is incorporated herein by reference and forms part of this application.
BACKGROUND OF THE INVENTION
[0002] This application relates generally to the formation of quaternary alloys over substrates. More specifically, this application relates to the formation of quaternary GaNAsP alloys such as may find utility in solar-cell applications.
[0003] While there have long been concerns about the development of energy sources, some of these concerns have become particularly acute is the last several years. These concerns are largely twofold: there is a concern that the use of certain energy sources, particularly those that arc carbon-based, have undesirable environmental impacts. These energy sources are also largely nonrenewable, presenting concerns about the systematic depiction of them. Many alternatives have been proposed for producing energy that arc drawn from sources that have low environmental impacts and are renewable, but many of these proposals suffer from a variety of inefficiencies related to the generation techniques.
[0004] In addition, many of these proposals suffer from the fact that they require substantial modifications to existing infrastructures. While the energy generation from the techniques themselves may be attractive and generally efficient, the impact on infrastructure makes them uneconomical. In addition, there are numerous regulatory provisions that have the potential to frustrate attempts to deploy new energy-generation technologies. Navigating such a regulatory framework frequently acts to discourage large-scale implementation of many promising forms of technology.
[0005] One solution to the need for improved methods and systems of generating energy in environmentally benign ways makes use of solar systems integrated with buildings, permitting the solar cells to convert light incident onto the buildings. Such an approach is described in the '984 application, which teaches that certain solar-cell materials have particular suitability to these applications. [0006] In considering the design of photovoltaic cells such as might be used in the applications described in the '984 application, there are three predominant types of electronic structure that may be used. These are illustrated in Figs. I A - 1 C. The simplest structure, illustrated in Fig. IA, makes use of a single junction. Specifically, a single band-cap material is used to capture a portion of the solar spectrum, with photons that have an energy greater than the band gap of (he material being absorbed to create an electron-bole pair that produces a dc current under the action of an electric field. The conversion efficiency for a single- junction cell has a peak at the bandgap of the active region and decreases rapidly for higher energies. Using a single bandgap to convert a substantial portion of the solar spectrum is therefore relatively inefficient, with a theoretical maximum efficiency of 35% but with typical efficiencies actually using this technology being on the order of 15 - 20 %.
[0007] Conversion of the available solar spectrum to electrical energy may be improved by using multiple junctions. This can be accomplished by engineering multiple bandgaps into a single cell This is illustrated schematically with Fig. IB, in which individual cells with different bandgaps are grown monolithically on top of one another with the largest bandgap material located at the top of the stack. With this approach, a larger portion of the incident energy is able to be absorbed, thereby increasing the total efficiency of the cell. The most popular approach to multijunction cells currently being researched are based on lattice- matched GaInP / GaAs double-junction cells and GaInP / GaAs / Ge triple junction cells and achieve maximum efficiencies on the order of 30 - 35% in practice. The theoretical maximum efficiency for the use of two-junction cells is 50% and the theoretical maximum efficiency for the use of three-junctions cells is 56%.
[0008] A more sophisticated approach that has been explored at least theoretically is a multiple-band technique in which the number of bandgaps within a single cell is increased without the use of multiple materials. Introduction of a small fraction of highly electronegative atoms into a host semiconductor material has been shown to dramatically alter the electronic band structure of the host material by splitting the conduction band into two sub bands. Because of the interaction between the two subbands, one subband is pushed to an energy higher than that of the bandgap of the host semiconductor and the other subband is pushed to a lower energy. This results in the creation of an additional energy level in the base structure to provide for three optical transitions as shown in Fig 1C The structure is therefore functionally equivalent to a triple-junction cell The theoretical maximum efficiency using this approach is approximately 63% The inclusion of still additional bands using this technique promises e\ en higher efficiencies, with four band approaches providing a theoretical maximum efficiency of 72%.
[0009] While the multiband approach is thus promising, practical constraints have made it difficult to realize efficiencies anywhere near these theoretical maximums There is accordingly a need in the art for improved synthesis of mateπals that may be used in such applications
BRIEF SUMMARY OF THE INVENTION
[0010] Embodiments of the invention provide material systems that use GaNAsP alloys and methods of forming such material systems In specific embodiments, the material system comprises a substrate, a metamorphic buffer layer overlying the substrate, and a GaNAsP quaternary alloy overlying the metamorphic buffer layer A lattice constant of the GaNAsP quaternary alloy is approximately the same as a lattice constant of the metamorphic buffer layer and substantially different from a lattice constant of the substrate
(0011] In some embodiments the substrate is GaP Suitable metamorphic buffer layers accordingly include GaAsP, InGaP, InGaAsP, InGaNP, and GaNAsP, among others Each of the metamorphic buffer layer and the GaNAsP quaternary alloy may be formed using molecular-beam epitaxy or metalorganic chemical-vapor deposition When then same technique is used for the formation of both these layers, the formation of both may take place m the same processing clamber In some instances, the metamorphic buffer comprises a plurality of independently formed sublayers
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings wherein like reference numerals are used throughout the several drawings to refer to similar components
[0013] Pigs IA - 1 C illustrate the electronic structure of different types of monocrystalline solar cells, [0014] Figs 2A and 2B compare the physical structure of layers deposited on a substrate with layers deposited over an intermediate metamorphic buffei layer,
[0015] Fig 3 is a schematic illustration of a system that maybe used m embodiments of the invention to grow a layer over a substrate,
[0016] Fig 4 is a flow diagram that summarizes methods of depositing a GaNAsP layer over a substrate in accordance with embodiments of the invention;
|0017] Fig 5 compares photoluminescence spectra of a GaNAsP layer deposited directly over a GaP substiate with spectra a GaNAsP deposited over a GaP substrate with an intermediate GaAsP metamorphic buffer layer;
[0018] Fig 6 compares x-ray diffraction spectra of a GaAsP metamorphic buffer layer grown by molecular-beam epitaxy with spectra of a GaAsP grown by metalorganic chemical- vapor deposition,
[0019] Figs 7A and 7B illustrate measured data and a fitted model for a GaNAsP laver grown over a GaP substrate as a function of wavelength for different incident angles, and
[0020] Fig. 8 provides a plot of an absorption coefficient as deπved from spectroscopic elhpsometry measurements to show the existence of multiple band absorption for GaAsNP
DETAILED DESCRIPTION OF THE INVENTION
[0021] Embodiments of the invention provide structures that may be used to provide efficient optical absorbing layers, particularly at wavelengths that encompass a broad range of solar illumination In certain specific embodiments, the structure comprises a semiconductor layer formed over a substiate with an intermediate metamorphic buffer layer As used herein, the term "metamorphic" refers to the ability of the layer to prov ide strain relaxation between sandwiching layers that have different lattice structures
[0022] For example, consider the case where the overlying layer compπses GaNxAs1-P, x γ For such a material system to exhibit multiband properties, x andy should be selected so that there is sufficient incorporation of active nitrogen to separate the conduction band from the intermediate band This may be achieved in embodiments of the invention with x > 0 01 In addition the phosphorus concentration may be selected so that it is sufficiently large to mo\e the energy levels of the electronic structure to provide good localization of conduction-band states But at the same time the phosphorus concentration may be selected to provide an indirect Xbandgap that is less than the F band gap This is achieved in specific embodiments with 0 35 < (1 -x - v) < 0 50 In particular embodiments, 0 005 ≤x <0 050 and 0 3 < v < 0 7
[0023] These concentrations, while creating the desired multiband structure for the overlying semiconductor layer result in a relatively large mismatch between the o\ erlying layer and commercially available substrates This is generally true irrespective of the specific chemical structure of most commercially available substiates, i e is true for at least GaAs, GaP, and InP Merely by way of example consider the specific case of an o\ erlying semiconductor layer having the chemical structure GaNo oiAso ssPo 4 and a GaP substrate Corresponding free-standing lattice constants tor these materials in the zincblende crystal structure aie 5 5494 A and 5 4505 A respectively This results in a lattice mismatch of about 1 8% The relevant thickness for such a mismatch is typically on the order oi a tew hundred angstroms and a layer may be grown to a thickness of a few hundred nanometeis to a few microns to absorb a high percentage of incident photons
[0024] When grown directly onto the GaP or other substiate, such a thick layer will release the accumulated strain energy through the formation of threading and misfit dislocations This is illustrated schematically m Fig 2A where the substrate is denoted by reference number 204 and the overlying semiconductor layer is denoted by reference number 208 The dislocations 212 become centers for earner recombination and may dramatically reduce efficiency of a solar cell that uses such a structure This may be contrasted with embodiments of the invention as illustrated in Fig 2B when the substrate 220 and overlying semiconductor layer 228 are separated by a metamorphic buffer layer 224 The metamorphic buffer layer 224 may be lattice-matched to the oveilying semiconductor layer 228 While dislocations 232 are still present they occur substantially m the metamorphic buffer layer 224 allowing the strain relaxation to occur m the metamorphic buffer layer 224 The oveilying lattice-matched semiconductor 228 then has greatly reduced incidence of disclocations and may be substantially free of dislocations When used as an absorption layei for a solar-cell device, the efficiency may then be greatly enhanced [0025] There are a number of different methods that may be used to form the structures shown in Figs. 2A and 2B. And within these various methods there are a number of alternative ways of independently forming the metamorphic buffer layer and/or the overlying semiconductor layer. Furthermore, there are a variety of structures that may be used to implement such methods. Merely by way of example, Fig. 3 provides an illustration of one such structure. This structure is well suited for the use of metalorganic chemical-vapor deposition ("MOCVD"), although as explained in further detail below molecular-beam epitaxy ("MBE") and other techniques may be used for the formation of each of the layers. In MOCVD, flows of precursors are provided to a processing chamber in which a heated substrate is disposed. The heat promotes reaction of the precursors to grow an epitaxial film of the desired composition over the substrate.
[0026] The basic structure of the system shown in Fig. 5 includes a processing chamber 304 within which a substrate 312 is disposed on a pedestal 308 during processing. Precursors are delivered to the processing chamber 304 with a precursor delivery system 316. While the precursors may be provided as gaseous sources in some embodiments, they may alternatively be provided as liquid or solid sources. When liquid sources are used, they are vaporized by an appropriate mechanism comprised by the precursor delivery system 316, such as a bubbler. The gaseous or vaporized precursors may be mixed with an inert carrier gas, as understood by those of skill in the art.
[0027] The drawing shows four precursors 520 that may be suitable for deposition of a GaNAsP layer: trimethylgallium ("TMG") 320-1 , which may be suitable as a gallium precursor; dimefhylhydrazine ("DMHy") 320-2, which may be suitable as a nitrogen precursor; arsine AsH3 320-3, which may be suitable as an arsenic precursor; and phosphine PH3 320-4, which may be suitable as a phosphorus precursor. These specific precursors are identified merely by way of illustration. In other embodiments, different precursors may be used to provide sources of gallium, nitrogen, arsenic and phosphorus.
[0028] A general overview of methods of forming material systems in accordance with embodiments of the invention is provided with the flow diagram of Fig. 4. In this diagram, certain steps are illustrated specifically and are identified in one exemplary order, but this is not intended to be limiting. In other embodiments, some of the indicated steps might be omitted, some additional steps not explicitly shown maybe performed, and the order of the steps may be altered.
[0029] The method begins at block 404 by transferring a suitable substrate in to the processing chamber. Examples of suitable substrates especially include III- V substrates like GaAs, GaP, and InP, but the invention is not limited to such substrates and elemental or II- VI substrates may be used in other embodiments. In addition, while the processing chamber may comprise an MOCVD or MBE chamber in certain specific embodiments, it may more generally be any chamber comprised by a system configurable for the formation of semiconductor layers over a substrate. For example, in addition to thermal deposition systems, other embodiments might use plasma deposition systems.
[0030| The substrate may be cleaned at block 408 to prepare the substrate for deposition of overlying layers. At block 412, processing conditions are established within the chamber for growth of the metamorphic buffer layer. Such processing conditions include such parameters as a temperature and pressure of the environment within the processing chamber. An appropriately prepared processing chamber is thus ready to receive flow rates of precursors for formation of the metamorphic buffer layer at block 416. The relative flow rates of the different precursors may be selected to achieve the desired stoichiometry of the metamorphic buffer layer, which is grown under these processing conditions at block 420.
[0031] This may be followed by formation of the overlying GaNAsP quaternary layer by establishing processing conditions for such growth within the processing chamber at block 424. At block 428, flows of gallium, nitrogen, arsenic, and phosphorus precursors are provided to the processing chamber, permitting the actual growth of the quaternary semiconductor layer to occur at block 432. The process is completed by terminating the precursor flows at block 436 and transferring the substrate out of the processing chamber at block 440.
[0032] While Fig. 4 brings out a number of general aspects of certain embodiments of the invention, there are a number of variations that are possible and that remain within the intended scope of the invention. In particular, formation of the metamorphic buffer layer may take place in the same or a different chamber than formation of the overlying quaternary semiconductor layer. And when different chambers are used, they may be chambers of a similar type or may be chambers that use different semiconductor-layer fabrication processes Furthermore, the formation of each of the layers might take place as a single substantially continuous growth process or might be performed in stages by building up successh ely deposited sublayers.
[0033] For example, in one specific embodiment, the metamorphic buffer layer is grown directly onto a substrate in an MOCVD reactor pπor to deposition of the GaNxAsλPι x.} layer in a single growth process or in several growth processes using the same MOCVD reactor In another specific embodiment, the metamorphic buffer layer is grown directly onto a substrate using an MBE reactor, again prior to deposition of the GaNxASyFi-X y layer in a single growth process or in several growth processes using the same MBE reactor In a further specific embodiment, the metamorphic buffer layer is grown directly onto a substrate using an MBE reactor, followed by deposition of the GaNxA syPi-x-y layer in a single growth process or in several growth processes with an MOCVD reactor
[0034] Each of these specific examples is illustrative of the more general characteristic of the invention that foimation of the metamorphic buffer layer may sometimes be separated quite distinctly from formation of the overlying quaternary semiconductor layer. The third of these specific embodiments, in which the metamorphic buffer layer is grown using MBE while the overlying quaternary semiconductor layer is grown by MOCVD is worth some addition comment The strain relaxation in a metamorphic buffer layer occurs through the formation of threading and misfit dislocations Threading dislocations propagate through the growing layer, while misfit dislocations lay in the interface between two semiconductors Threading dislocations thus have the potential to affect device performance rather severely. Usually, misfit dislocations would have less impact on device performance because they lie outside the actual device structure and only on the interface between the substrate and the metamorphic buffer layer.
[0035] A desirable feature of the metamorphic buffer is thus that it have physical characteπstics that promote the release of strain through the formation of misfit dislocations instead of threading dislocations The formation of misfit dislocations may be promoted at generally lower growth temperatures, i e. when T < 450 0C. At temperatures higher than about 450 0C, the foimation threading dislocations tends, to be favored. [0036] Typically, low-temperature growth is somewhat easier to realize using MBE than using MOCVD because the growth of the epitaxial layer is carried out using elemental fluxes of different materials deposited onto a surface. In an MOCVD reactor, low-temperature growth is generally more difficult to realize because the growth is carried out using molecular source materials that decompose on the substrate surface in order to create reactive elements. In MOCVD processes, low substrate temperature thus prevents the efficient decomposition of these compound source materials on a substrate surface, making a low temperature deposition more difficult to realize.
[0037] At the same time, the higher typical growth temperatures for compound semiconductors when using MOCVD as compared with MBE makes the use of an MOCVD reactor more appropriate for growth of the overlying quaternary semiconductor layer. For instance, a typical growth temperature for GaAs using MBE is about 600 0C while a typical growth temperature for GaAs using MOCVD is 680 0C. Higher growth temperature generally results in a better optical quality of the epitaxial layer.
[0038] There are a number of different compositions that may be used for the metamorphic buffer layer. This affects the choice of precursors and flow rates for those precursors at block 416 of the flow diagram of Fig. 4. In part, an appropriate composition for the metamorphic buffer layer depends on the particular composition of the substrate because this affects the lattice-mismatch characteristics to be accommodated. But even for a particular substrate, there are a number of different compositions that are possible for the metamorphic buffer layer. Consider, for example, the specific case where the substrate is a GaP substrate and the overlying quaternary semiconductor layer is GaNxAsyP|-x-y, Suitable metamorphic buffer layers include InGaP, InGaAsP, GaAsP, InGaNP, among others. Since GaNxAsyP],x-y contains Ga, As, and P, one embodiment makes use of a GaAsP buffer layer. In the specific example discussed above in which the overlying quaternary semiconductor layer has a composition
Figure imgf000011_0001
0.02 and y = 0.58, the lattice constant is 5.5494 A. Substantially the same lattice constant is had by GaAs0 49P0.51, making it suitable as a metamorphic buffer layer between a GaP substrate and a GaNo o2Aso 5sPo4 overlying layer. Examples
[0039] The inventors have earned out a number oi experimental investigations to explore aspects of the invention dcscπbed abo\ e In one set of studies, the optical properties of GaNjAsvPi x x layers grown ovei a GaP substrate were evaluated A 0 3-μm-thick GaNo 02AS0 Sj1Po 4 layer was grown by MOCVD in one process on two different substrates (1) directly onto a GaP substrate, and (2) onto a 0 2-μm-thick GaAso 4ήPo si metamorphic buffer layer grown by MOCVD onto a GaP substrate The resulting GaNo 02AS0 -,^o 4 layer was evaluated for optical quality by conducting photolumincscence measurements In particular, a photoluminescence intensity was measured from the layer of GaNo ozAs^ ssPo 4 'n the structure without the buffer layer and m the structure with the buffer layer using a 532-nm layers Results of this investigation are shown in Fig 5, in which curve 504 shows the photolumincscence when the metamorphic buffer layer is present and curve 508 shows the photoluminescence when the buffer layer is absent
[0040] These results show that the photoluminescence intensity from the GaNo 02AS0 ssPo4 grown on the metamorphic buffer layer in this particular example is about 3 5 times as high as that of the GaNo 02AS0 ^Po 4 grown directly on a GaP substrate With the thicknesses of the layers and the band structure of these samples, it is believed that the higher photoluminescence intensity is due to the smallei number of dislocations in the GaNo 02As0 5sPo 4 structure when the metamorphic layer is present, but the inv ention is not limited to any particular mechanism by which increased photoluminescence intensity is achieved It is also noted from the results in Fig 5 that the wavelength of the peak photoluminescence intensity is shifted by the presence of the metamorphic buffer layer The peak wavelength seen in curve 504 when the buffer layer is present is about 10 nm shorter than that seen in curve 508 when the buffer layer is absent This is believed to be due to incomplete strain relaxation m the absence of the metamorphic buffer layer
[0041] Other experiments have been performed to compare the quality of MBE-grown and MOCVD-grown metamorphic buffer layers In one particular set of experiments, two 2 μm thick GaAsP metamorphic buffer layers were grown with concentrations of arsenic and phosphorus around 50% A first sample was grown in an MBE reactor with a substrate temperature of about 400 0C Elemental Ga and thermally cracked AsH3 and PH3 were used as precursors for MBE growth The MBE-grown sample was observed to have a mirror-like surface
[0042J \ second sample w as grown in an MOCVD reactor with a substrate temperature of about 650 0C TMGa, AsH3. and PH3 were used as precursors for MOCVD growth When growth was attempted at temperatures less than 650 0C, layers of GaAsP deposited to thicknesses greater than about 0.5 μm in an MOCVD reactor produced damaged sui faces, as evident by a clear haze over the surfaces
[0043] Fig 6 shows the results of (004) x-ray-diffraction measurements of the samples descπbed above, with curve 604 showing results for the MOCVD-grown layer and curve 608 showing results for the MBE-grown layer It is evident that the full width at half maximum ('TWHM") ot the x-iay peak for the MBE-grown spectrum 608 is substantially smaller than for the MOCVD-grown spectrum 604, with the MBE-grown FWHM being about 460 arcsec and the MOCVD-grown FWHM being about 605 arcsec. This suggests that the structural quality of the MBE-grown GaAsP metamorphic buffer layer is better than that of the MOCVD-grown GaAsP layer. It is perhaps worth noting that this comparison is provided for the same precursor sources of arsenic and phosphorus in both MBE and MOCVD layer growth It may be possible to achieve bettei structural quality with MOCVD in some embodiments by using different sources such as tertiary butyl arsine ("TBA") and tertiary butyl phosphine ("TBP") as arsenic and phosphorus sources, these sources decompose substantially better than arsme and phosphine at lower substrate temperatures, potentially permitting the MOCVD process to be conducted at such lower temperatures
[0044] Further expeπments have evaluated the optical constants of GaNAsP deposited using MOCVD hi particular, vanablc-angle spectroscopic elhpsometry was used to measure the ellipsometπc angles Δ and Ψ, the results of which are plotted m Figs 7A and 7B respectively Each measurement was made for a number of different incident angles θ[, with curves 704 and 754 showing results at 9i~ 70°, curves 708 and 758 showing results at θ[= 72°, curves 712 and 762 showing results at 0/ = 74° curves 716 and 766 showing results at θ/ =76°, curves 720 and 770 showing results at θ/ = 78°, and curves 724 and 774 showing results at θ/ = 80°. The ellipsometπc angles Δ and Ψ are related to the ratio of the amplitude reflection coefficients p and p = e tan Ϋ Figs 7 A and 7B also show the iesults of modeling of the GaNAsP material, with the calculated results showing good agreement with the measured data over the spectral range shown.
[0045] The absorption coefficient <χ which is defined by the rate at which an initial intensity IQ falls off with depth through the material z to produce an intensity I{∑) = lue'02 , is of particular interest for solar cells. This quantity was accordingly extracted from the measured data for comparison with other materials, with results provided in Fig. 8. For purposes of comparison the absorption of GaAs is shown with curve 804, the absorption of Si is shown with curve 836, and the absorption of GaP is shown with curve 812. Results 832 show data for the GaP substrate and results 828 show data for the GaAsP metamorphic buffer layer, These results were used in the layer modeling. Results 820 provide data showing the absorption measured for a GaNAsP layer grown on a structure having a GaP substrate and a GaNasP buffer layer and results 824 provide data showing the absorption measured for a GaNAsP layer grown directly onto a GaP substrate. These results exhibit the characteristic absorption peaks for a multiband semiconductor material, with the direct-grown results 824 exhibiting a peak at EcΛ and the buffer-layer results 820 exhibiting a peak at Ec2-
[0046] Thus, having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Accordingly, the above description should not be taken as limiting the scope of the invention, which is defined in the following claims.

Claims

WHAl IS CLAIMED IS-
1 A method of forming a GaNAsP quaternary alloy over a substrate, the method comprising-
forming a metamorphic buffer layer over the substrate;
disposing the substrate within a processing chamber;
flowing a gallium precursor, a nitrogen precursor, an arsenic precursor, and a phosphorus precursor into the processing chamber; and
growing the GaNAsP quaternary alloy over the metamorphic buffer layer with the gallium precursor, the nitrogen precursor, the arsenic precursor, and the phosphorus precursor
wherein a lattice constant of the GaNAsP quaternary alloy is approximately the same as a lattice constant of the metamorphic buffer layer and substantially different from a lattice constant of the substrate.
2 The method recited in claim 1 wherein the substrate is GaP.
3. The method recited in claim 2 wherein forming the metamorphic buffer layer over the substrate comprises forming a GaAsP layer over the substrate
4 The method recited in claim 2 wherein forming the metamorphic buffer layer over the substrate comprises forming an InGaP layer over the substrate.
5. The method recited in claim 2 wherein forming the metamorphic buffer layer over the substrate comprises forming an InGaAsP layer ov er the substrate.
6 The method recited in claim 2 wherein forming the metamorphic buffer layer over the substrate comprises forming an InGaNP layer over the substrate.
7. The method recited in claim 1 wherein growing the GaNAsP quaternary alloy comprises growing the GaNΛsP quaternary alloy using metalorganic chemical-\apor deposition.
26
8 The method recited in claim 7 wherein forming the metamorphic buffer layer over the substrate compπses forming the metamorphic buffer layer using molecular-beam epitaxy
9. The method recited in claim 7 wherein forming the metamorphic buffer layer over the substrate comprises forming the metamorphic buffer layer using metalorganic chemical-\apor deposition.
10. The method recited in claim 9 wherein forming the metamorphic buffer layer over the substrate is performed within the processing chamber
1 1. The method recited in claim 1 wherein growing the GaNAsP quaternary alloy comprises growing the GaNAsP quaternary alloy using molecular-beam epitaxy
12. The method recited in claim 1 1 wherein forming the metamorphic buffer layer over the substrate is performed using molecular beam epitaxy.
13. The method recited in claim 12 wherein forming the metamorphic buffer layer over the substrate is performed within the processing chamber.
14 The method recited in claim 1 wherein forming the metamorphic buffer layer over the substrate compπses independently forming a plurality of sublayers of the metamorphic buffer layer o\ er the substrate.
15. A material system comprising:
a substrate,
a metamorphic buffer layer overlying the substrate; and
a GaNAsP quaternary alloy overlying the metamorphic buffer layer,
wherein a lattice constant of the GaNAsP quaternary alloy is approximately the same as a lattice constant of the metamorphic buffer layer and substantially different from a lattice constant of the substrate.
16. The material system recited in claim 15 wherein the substrate is GaP.
27
17. The material system recited in claim 16 wherein the metamorphic buffer layer comprises an InGaP layer.
18. The material system recited in claim 16 wherein the metamorphic layer comprises an InGaP layer.
19. The material system recited in claim 16 wherein the metamorphic layer comprises an InGaAsP layer.
20. The material system recited in claim 16 wherein the metamorphic layer comprises an InGaNP layer
21 The matenal system recited in claim 16 wherein the metamorphic buffer layer comprises a plurality of independently formed sublayers
22. The material system recited in claim 15 wherein the GaNAsP quaternary alloy layer comprises a GaNAsP quaternary alloy layer grown on the metamorphic buffer layer.
23. The material system recited in claim 15 wherein the material system comprises a solar cell
24. I he method recited m claim 1 wherein forming the metamorphic buffer layer occurs at a lower temperature than forming the GaNAsP quaternary alloy layer.
25. The method recited in claim 1 wherein a solar cell device is formed.
28
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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20020195137A1 (en) * 2001-06-06 2002-12-26 King Richard Roland Lattice-matched semiconductor materials for use in electronic or optoelectronic devices
US6787385B2 (en) * 2001-05-31 2004-09-07 Midwest Research Institute Method of preparing nitrogen containing semiconductor material
US20040261837A1 (en) * 2001-12-14 2004-12-30 Friedman Daniel J Multi-junction solar cell device

Patent Citations (3)

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Publication number Priority date Publication date Assignee Title
US6787385B2 (en) * 2001-05-31 2004-09-07 Midwest Research Institute Method of preparing nitrogen containing semiconductor material
US20020195137A1 (en) * 2001-06-06 2002-12-26 King Richard Roland Lattice-matched semiconductor materials for use in electronic or optoelectronic devices
US20040261837A1 (en) * 2001-12-14 2004-12-30 Friedman Daniel J Multi-junction solar cell device

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Title
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