WO2011126528A1 - Substrats virtuels pour croissance épitaxiale et leurs procédés de fabrication - Google Patents

Substrats virtuels pour croissance épitaxiale et leurs procédés de fabrication Download PDF

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Publication number
WO2011126528A1
WO2011126528A1 PCT/US2010/061666 US2010061666W WO2011126528A1 WO 2011126528 A1 WO2011126528 A1 WO 2011126528A1 US 2010061666 W US2010061666 W US 2010061666W WO 2011126528 A1 WO2011126528 A1 WO 2011126528A1
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single crystalline
crystalline layer
substrate
film
coherently
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PCT/US2010/061666
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English (en)
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Harry A. Atwater
Marina S. Leite
Emily C. Warmann
Dennis Callahan
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California Institute Of Technology
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/20—Deposition of semiconductor materials on a substrate, e.g. epitaxial growth solid phase epitaxy
    • H01L21/2003—Deposition of semiconductor materials on a substrate, e.g. epitaxial growth solid phase epitaxy characterised by the substrate
    • H01L21/2007—Bonding of semiconductor wafers to insulating substrates or to semiconducting substrates using an intermediate insulating layer
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/10—Inorganic compounds or compositions
    • C30B29/40—AIIIBV compounds wherein A is B, Al, Ga, In or Tl and B is N, P, As, Sb or Bi
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/10—Inorganic compounds or compositions
    • C30B29/46—Sulfur-, selenium- or tellurium-containing compounds
    • C30B29/48—AIIBVI compounds wherein A is Zn, Cd or Hg, and B is S, Se or Te
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B33/00—After-treatment of single crystals or homogeneous polycrystalline material with defined structure
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02367—Substrates
    • H01L21/0237—Materials
    • H01L21/02387—Group 13/15 materials
    • H01L21/02392—Phosphides
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/02521—Materials
    • H01L21/02538—Group 13/15 materials
    • H01L21/02546—Arsenides

Definitions

  • This application is directed to virtual substrates for epitaxial growth and methods of making the virtual substrates.
  • semiconductor epitaxial alloys are widely used in solid state applications. For example, defect-free III-V compound semiconductor alloys are responsible for the widespread application of a vast array of optoelectronic devices, including semiconductor lasers, light-emitting diodes, and solar cells.
  • One of the key technological requirements for achieving high quality optoelectronic materials and devices is the epitaxial crystal growth.
  • the versatility of semiconductor epitaxial growth is restricted by the limited number of single crystal substrates available in bulk form. Indeed, the limited number of single crystal substrates available in bulk form is currently an obstacle to the development of high quality semiconductor heterostructures as the variety of compositions and lattice parameters is restricted to those available as single crystal substrates in bulk form.
  • the deposited material assumes the substrate crystal structure and lattice parameter, and the grown film is free of strain-relieving dislocations.
  • the lattice parameter of the deposited film in the parallel direction (a // ) is equal to that of the substrate (a su b), and the lattice parameter in the perpendicular direction (a ⁇ ) is free to expand or contract due to the existence of a traction-free top surface and the Poisson effect, which preserves the material unit cell volume ViVlG. 1 A).
  • i m is larger that that of the substrate (a su b), it is compressed with respect to the substrate, and therefore the strain value is less than zero (i.e., ⁇ // ⁇ 0).
  • Any material that is lattice-mismatched to the substrate will be strained, causing an increase in the elastic energy of the system. If the elastic energy exceeds the energy associated with the introduction of defects such as dislocations, these defects can be introduced into the film to minimize the overall energy. However, the defects compromise the quality of the crystal, and consequently the device's performance.
  • these structures can enable control of silicon band offsets
  • these templates consist of a combination of strained layers in a strain-balanced laminate, and none of the layers relax to their native lattice parameters nor is the laminate a single crystal substrate or a template for epitaxial growth.
  • a virtual substrate for use as a template for epitaxial growth of optoelectronic devices and the like includes a handle support and a substantially strain-relieved single crystalline layer on the handle support.
  • the single crystalline layer may include any suitable material, for example, the material may be selected from III-V semiconductor materials, II-VI semiconductor materials, IV- IV semiconductor materials, and ceramic semiconductor materials.
  • the single crystalline layer includes a III-V semiconductor material, e.g., an alloy of In, Ga and As.
  • a method of forming a virtual substrate includes growing a coherently-strained single crystalline layer on an initial growth substrate, removing the initial growth substrate to form a substantially strain-relieved single crystalline layer, and applying the substantially strain-relieved single crystalline layer to a handle support.
  • the method may further include applying a transfer support to the coherently- strained single crystalline layer prior to removing the initial growth substrate, and the transfer support may include any suitable material, such as, for example, waxes, greases, polymers, adhesives and adhesive tapes.
  • the transfer support is removed after applying the
  • the substantially strain-relieved single crystalline layer may be applied to the handle support by applying water to the substantially strain-relieved single crystalline layer and/or the handle support, and allowing a van der Waals bond to form between the substantially strain-relieved single crystalline layer and the handle support.
  • the substantially strain-relieved single crystalline layer and/or the handle support may also be heated during formation of the van der Waals bond to convert the van der Waals bond to a covalent bond.
  • the method includes applying a sacrificial layer on the initial growth substrate and growing the coherently- strained single crystalline layer on the sacrificial layer.
  • the initial growth substrate is removed from the coherently-strained single crystalline layer by removing (e.g. by selective etching) the sacrificial layer.
  • This method may also include applying a transfer support to the coherently- strained single crystalline layer prior to removing the sacrificial layer, and then removing the transfer support after applying the substantially strain-relieved single crystalline layer to the handle support.
  • the method includes implanting H + or He + ions in the initial growth substrate after growing the coherently- strained film.
  • the initial growth substrate is removed from the coherently-strained single crystalline layer by heating the initial growth substrate and coherently-strained single crystalline layer after implanting the ions.
  • the heat triggers exfoliation of the initial growth substrate, which results in removal of all but a thin layer of the initial growth substrate from the coherently- strained single crystalline layer.
  • the thin layer of the initial growth substrate may then be removed from the coherently-strained single crystalline layer by, e.g., selective etching.
  • This method may also include applying a transfer support to the coherently-strained single crystalline layer prior to heating the initial growth substrate and coherently-strained single crystalline layer, and then removing the transfer support after applying the substantially strain-relieved single crystalline layer to the handle support.
  • FIG. 1A is a schematic illustrating the in-plane and out-of-plane strain in an In x Gai -x As film grown on an InP substrate and the relaxation of the strain in the In x Gai -x As film when transferred to a Si0 2 /Si handle support according to an embodiment of the present invention.
  • FIG. IB is a photograph of a 50 mm diameter Ino .43 Gao.57As virtual substrate template according to an embodiment of the present invention.
  • FIG. ID is an electron diffraction pattern of the region shown in FIG. 1C.
  • FIG. IE is an atomic resolution plane view transmission electron microscopy image of the region shown in FIG. 1C.
  • FIG. 2 is a schematic illustrating a process by which a strain-relieved single crystalline layer is applied to a handle support according to an embodiment of the present invention.
  • FIG. 2A depicts X-ray diffraction ⁇ -2 ⁇ (0 0 4) scans of 40nm thick In x Gai -x As films (having varying In contents) on InP substrates (black line) and after bonding to Si0 2 /Si handle wafers (blue line).
  • FIG. 2B depicts reciprocal space maps for the In x Gai -x As films of FIG. 2A before substrate removal. The color scale refers to the diffracted intensity.
  • FIG. 2C depicts reciprocal space maps for the In x Gai -x As films of FIG. 2A after the In x Gai_ x As films are transferred to a Si0 2 /Si handle support.
  • the color scale refers to the diffracted intensity.
  • FIG. 3 is a graph of the material unit cell reconstructions showing the in-plane (a / /) and out-of-plane (aj_) lattice parameters for strained In x Gai -x As:InP films and elastically- relaxed virtual substrates as a function of In content inferred from the X-ray diffraction measurements of FIG. 2.
  • FIG. 4A is a graph of the room temperature photoluminescence (PL)
  • the inset depicts a scheme of the light- and heavy-hole valence band splitting caused by tensile strain.
  • FIG. 4B is a zoom-in of the measurements in FIG. 4A with fits (solid black, orange and green lines) for the low energy portion of the spectrum (temperature independent) used to estimate E g for each strain configuration (values are shown on Table in inset).
  • FIG. 5 is an illustration of a method of fabricating a virtual substrate according to an embodiment of the present invention.
  • FIG. 6A is an illustration of a method of fabricating a virtual substrate according to another embodiment of the present invention.
  • FIG. 6B is an illustration of a method of fabricating a virtual substrate according to another embodiment of the present invention.
  • FIG. 7 A is a photograph of an Apiezon wax/In x Gai -x As/InP system according to an embodiment of the present invention.
  • FIG. 7B is a photograph of the In x Gai -x As film of FIG. 7A after substrate removal according to an embodiment of the present invention.
  • FIG. 7C is a photograph of a 50mm diameter In x Gai -x As film after transfer of the film to a Si0 2 /Si handle support to fabricate a virtual substrate according to an embodiment of the present invention.
  • FIG. 7D is a photograph of a PDMS wax/In 0 . 4 5Gao.5 5 As/InP system according to another embodiment of the present invention.
  • FIG. 8 is a scheme of X-ray diffraction geometry for reciprocal space maps measurements used to calculate the material unit cell before and after strain relaxation.
  • FIG. 9 A depicts: 1) X-ray diffraction ⁇ -2 ⁇ (0 0 4) measurement of an
  • FIG. 9B is a reciprocal space map on (2 2 4) reflection showing a coherently- strained In 0 4 3Gao.5 7 As layer under Bragg diffraction conditions, and an InP substrate peak as a reference.
  • FIG. 9C is a reciprocal space map on (2 2 4) reflection of the elastically-relaxed Ino.4 3 Gao.5 7 As film of FIG. 9B obtained after film transfer, where the small InP peak refers to a substrate residue, used as a reference.
  • FIG. 10A is a graph comparing the X-ray diffraction data of a coherently- strained Ino .55 Gao.45As film grown on an InP initial growth substrate to elastically-relaxed
  • Ino.s5Gao.45As films transferred to a handle support using Apiezon® W, W40 and W100 transfer support materials.
  • FIG. 10B is a graph comparing the X-ray diffraction data of a coherently-strained Ino.43Gao. 57 As film grown on an InP initial growth substrate to elastically-relaxed
  • a virtual substrate includes a handle support and a single crystalline layer.
  • the single crystalline layer is substantially free of strain, and the substrate can be used as a template for epitaxial growth.
  • inventive substrates may be tenned "virtual substrates," and as used herein, the term “virtual substrate” refers to a substrate including a single crystalline layer that is substantially free of strain and has its natural lattice parameter.
  • single crystalline layer refers to a single layer of crystalline material having a lattice parameter distinct from the wafer substrate (also termed “handle support,” “handling support,” or “handle substrate,” all of which terms are used interchangeably herein).
  • single crystalline layer refers to a single layer of a single material, although the single material may be a compound alloy or other compound system of elements.
  • the term “substantially” is used to convey the inherent degree of potential error or deviation in the calculations and measurements used to arrive at the reported values for the identified parameters. For example, the term “substantially free of strain” indicates that the material has no strain, or if there is some strain, it is negligible. Similarly, the term “single crystalline layer” refers to a single layer of a single material, although the single material may be a compound alloy or other compound system of elements.
  • the term “substantially” is used to convey the inherent degree of potential error or deviation in the calculations and measurements used to arrive at the reported values for the identified parameters. For example, the term “substantially free of strain” indicates that the material has no strain, or if there is some strain, it is negligible. Similarly, the term
  • the handle support can be any material suitable for mechanically supporting the single crystalline layer.
  • the handle support should be compatible with the device and/or reactor used for the epitaxial growth process.
  • the handle support is compatible for use in chemical vapor deposition apparatuses and/or in molecular beam and/or vapor phase epitaxial equipment, the handle support may also be selected based on cost considerations.
  • a nonlimiting example of a suitable handle support is a Si0 2 /Si wafer (i.e., a Si wafer coated with a layer of Si0 2 ).
  • Si0 2 /Si wafers are also relatively inexpensive.
  • handle support is dependent on the growth reactor used to effect subsequent epitaxial growth of the single crystalline layer of the virtual substrate.
  • the various handle supports also termed “handling supports” or “handle substrates" for different growth reactors are well known in the relevant field, and the selection of an appropriate such handle support is within the knowledge and skill of one of ordinary skill in this field.
  • a crystalline wafer such as GaAs, Si, etc, has been used for the epitaxial growth, which is one of the main limitations of epitaxial growth.
  • any support that is inert upon in-situ heating used during cleaning procedures and that can fit the sample holder in a reactor can be used as the handle substrate.
  • the handle substrate should be flat to prevent virtual substrate buckling.
  • the handle material may be selected based on the coefficient of thermal expansion, cost, and optical or electronic properties.
  • the inventive virtual substrates can be used to integrate different material systems on the same wafer with high quality.
  • a handle substrate would be selected based on its suitability for this application.
  • the handle substrate should be substantially planar, compatible with the growth/process temperatures, a material that won't contaminate the growth or other processes, and have a thermal expansion coefficient that is reasonably close to the single crystalline layer.
  • suitable handle substrates include sapphire, quartz, GaP or Ge.
  • the material for the single crystalline layer is not particularly limited, and can be selected based on the desired properties of the layer. Specifically, the material of the single crystalline layer may be selected based on the properties specifications required by the intended application of the layer. This ability is a unique and highly desirable aspect of the virtual substrates of the present invention. In particular, the ability to individually select, or tune, the lattice parameters, band gaps or band offsets of the single crystalline layer used for later epitaxial growth enables the production of high quality optoelectronic devices with optical characteristics not available by using conventional substrates as a starting point for the epitaxial growth. Although the techniques of the present invention are applicable across the semiconductor material spectrum, in some embodiments, the single crystalline layer may include a III-V, II-VI or IV- IV compound semiconductor material or ceramic.
  • the single crystalline layer is formed by first fabricating a coherently-strained semiconductor layer by known methods (e.g., by epitaxial growth on a bulk crystal substrate) and transferring the layer from the growth substrate to a handle support, thereby relieving the strain on the layer.
  • the material of the single crystalline layer is limited only to those materials that can be grown as a coherently-strained film. Such materials are well known in the semiconductor field, and would be understood by those of ordinary skill in the art. As noted above, some such materials include III-V, II-VI or IV- IV compound semiconductor materials and ceramics, but the present invention is not limited to these materials, and any material capable of being grown in a strained state can be used to form the single crystalline layer. Indeed, growth of coherently-strained semiconductor films has been widely demonstrated with a wide range of materials, such that the grown coherently-strained film can be achieved with a wide range of materials, as would be understood by those of ordinary skill in the art.
  • the single crystalline layer is substantially free of strain and becomes a substantially strain-relieve single crystalline layer.
  • substantially strain-relieved single crystalline layer refers to a single crystalline layer that is grown under biaxial strain to form an initial coherently-strained layer, and then relieved of the strain by transferring the initial grown layer to a separate handle support.
  • the single crystalline layer is substantially relaxed such that the material of the layer assumes its bulk properties, as predicted by Vegard's law. Vegard's law predicts the lattice parameter of a compound semiconductor material based on the constituent elements of the semiconductor alloy.
  • Vegard's law establishes a linear relationship between the lattice parameter of the alloy and the concentrations of the constituent elements.
  • the lattice parameter (a) of an alloy of InGaAs follows the formula:
  • the single crystalline layer includes a compound semiconductor material that is substantially free of strain, and has a lattice parameter that substantially matches the one predicted by Vegard's law.
  • the thickness of the single crystalline layer is not particularly limited. However, the layer should be thin enough to resist cracking or deformation. Indeed, the more highly strained the layer is, the thinner the layer must be to avoid the formation of dislocations or defects. Also, the tendency of the layer to form cracks will increase with increasing thickness. Consequently, the thinnest possible layer is the most desirable, and also reduces costs.
  • the layer has a thickness up to about 3 microns.
  • the layer may have thickness ranging from about 30nm to about 1 micron.
  • the single crystalline layer has a thickness ranging from about 40nm to about 80, and in some embodiments, the thickness is about 40nm.
  • the single crystalline layer may have thicknesses of 30nm or less, handling such ultrathin films without damaging them can be challenging.
  • the single crystalline layer may also have thicknesses up to about 3 microns, fabricating an original coherently-strained film substantially free of dislocations at that thickness can be challenging. This fabrication depends on the kinetic growth conditions, which can be achieved by tuning the deposition rate and the temperature during the epitaxial growth, and strongly depends on the components of the alloy forming the film.
  • the virtual substrates of the present invention can be scaled up to wafer-size, as shown in FIG. I B. Indeed, the virtual substrates can be made in any desired size or shape.
  • the size of the virtual substrate is, of course, dictated by the size of the initial growth substrate, and consequently the size of the grown film.
  • the virtual substrates can have single crystalline layers with diameters (or dimensions) up to about 50 mm or about 2 inches. In other exemplary embodiments, the virtual substrates can have single crystalline layers with diameters or dimensions measuring from about 1 cm to about 50mm. In some embodiments, for example, the virtual substrates can have single crystalline layers with a diameter or dimension of about 2mm.
  • a method of fabricating a virtual substrate for epitaxial growth enables the production of large areas (e.g., 20 mm in diameter or greater, such as 50 mm) of substantially dislocation-free, fully or substantially relaxed single crystalline compound semiconductor (e.g., In x Gai_ x As) layers with lattice parameters equal to the bulk value predicted by Vegard's law (e.g., the bulk In x Gai_ x As value).
  • a method of making a virtual substrate includes growing a coherently-strained film on an initial growth substrate (shown in box 1 ).
  • the film is distorted to match the parallel lattice parameter of the initial growth substrate, yielding a strain value for the grown film that is not equal to zero (also shown in box 1).
  • the initial growth substrate is then removed to relax the grown film (shown in boxes 2-4), and then the relaxed film is applied on a separate handle support (shown in boxes 5-9).
  • FIG. 1A shows that the strain value of the grown film while it remains on the initial growth substrate is not equal to zero, and shows that upon removal from the initial growth substrate and placement on a handle support, the grown film relaxes to a state in which the strain is zero (or about zero).
  • the biaxial in-plane distortion affects the energy band gap of the alloy, as observed in optical measurements, such as those shown in FIGs. 4A and 4B.
  • a layer of the desired material is grown on an initial growth substrate by any suitable method, as would be known to those of skill in the art.
  • the film may be heteroepitaxially grown using chemical vapor deposition, molecular beam epitaxy, etc. Any desired material may be grown in this manner, and the layer grown on the initial growth substrate serves as the single crystalline layer in the resultant virtual substrate. Accordingly, the materials of the coherently-strained film are the same as the materials of the single crystalline layer, discussed above. Additionally, as the coherently-strained film is subsequently removed from the initial growth substrate and applied to a separate handle support, the thickness of the coherently-strained film is substantially the same as that of the single crystalline layer.
  • the film grown on the initial growth substrate takes on the parallel lattice parameter (a // ) of the initial growth substrate.
  • the initial growth substrate has a lattice parameter different from the one predicted by Vegard's law (the predicted bulk crystal lattice parameter) for the grown film.
  • the difference in the actual lattice parameter of the film and the predicted bulk crystal lattice parameter results in biaxial strain in the grown film. This strain occurs due to the compensation of deformations in the in-plane lattice parameter (a // ) by distortion of the out-of-plane parameter (cij) to conserve the unit cell volume. This phenomenon is illustrated in FIG.
  • the out-of-plane lattice parameter (a j) compensates for the deformation of the in-plane lattice parameter (a // ), thereby conserving the unit cell volume of the grown In x Gai -x As film, but resulting in biaxial strain (i.e, ⁇ //, ⁇ 0). Accordingly, growth of the film on the initial growth substrate results in a coherently-strained film on the initial growth substrate.
  • the materials of the coherently- strained film are the same as the materials of the single crystalline layer.
  • growth of coherently- strained semiconductor films has been widely demonstrated with a wide range of materials, such that the grown coherently-strained film can be achieved with a wide range of materials, as would be understood by those of ordinary skill in the art.
  • a few, nonlimiting examples of these include Si x Gej -x layers grown on bulk Si, In x Gai -x P and In x Ga ]-x As alloys on GaAs substrates, In x Ali -x As alloys in other fields, etc.
  • the initial growth substrate is then removed from the film to relieve the strain on the film.
  • the strained film relaxes to its bulk value as predicted by Vegard's law, and can then be transferred to a handle support to create the virtual substrate.
  • the initial growth substrate may be removed by any suitable method, for example by etching with any suitable material, e.g., room temperature, concentrated HC1.
  • any suitable material e.g., room temperature, concentrated HC1.
  • the selection of the initial growth substrate depends on the desired material of the grown film, and it is within the skill of those of ordinary skill in the art to select an appropriate initial growth substrate for the desired grown film.
  • etch selectivity which allows selective removal of the initial growth substrate without affecting the grown film
  • selective etching is a commonly used tool in semiconductor manufacture.
  • a transfer support may be first applied to the surface of the film before removing the initial growth substrate.
  • the transfer support can be any material sufficient to provide support for the free film and enable handling of the thin film, and sufficiently flexible to allow the film to relax from the strained state to the relaxed state, in which the lattice parameters of the film relax to the bulk values predicted by Vegard's law.
  • Suitable materials for the transfer support include flexible polymers and like substances, which allow enough movement to allow relaxation of the free film, but enough support to allow mechanical handling.
  • suitable materials for the transfer support include waxes and greases, such as Apiezon® W, Apiezon® W40 (thin wax or heavy grease) and Apiezon® W100 (somewhat thicker grease than Apiezon® W40) (available from M&I Materials, Ltd.) and paraffin, as well as flexible polymers, such as polydimethylsiloxane (PDMS) and polyimide, adhesives (such as CrystalbondTM 509 (C509), a hard adhesive available from SPI Supplies) and adhesive tapes (such as Kapton® (available from E.I. du Pont de Nemours and Company) and vinyl tapes, and the like).
  • PDMS polydimethylsiloxane
  • adhesives such as CrystalbondTM 509 (C509), a hard adhesive available from SPI Supplies
  • adhesive tapes such as Kapton® (available from E.I. du Pont de Nemours and Company) and vinyl tapes, and the like).
  • a combination of transfer support materials may also be used, e.g., a combination of one or more of the Apiezon® materials (i.e., Apiezon® W, W40 and/or W100) and PDMS.
  • a mechanical support e.g., glass or the like
  • the transfer support is placed on the grown film, and the mechanical support is placed on the transfer support.
  • the transfer support enables relaxation of the grown film, and the mechanical support enables easy handling of the free film.
  • FIG. 7B shows an In x Gai -x As film after removal of the InP initial growth substrate and including a glass mechanical support.
  • the method used to apply the transfer support to the grown film may vary as necessary depending on the material used for the transfer support, and any suitable method may be used. In some embodiments, however, the transfer support material is dissolved in a solvent prior to application to the grown film, then the solution is drop cast onto the film and the solvent is allowed to evaporate away, leaving a relatively thin layer of the transfer support. Any suitable solvent may used that is capable of dissolving the transfer support material.
  • a suitable solvent includes trichloroethylene (TCE).
  • TCE trichloroethylene
  • the transfer support may be applied by melting it directly onto the grown film.
  • the material may be desirable to melt the material directly onto the grown film, while in other embodiments, for example those using an Apiezon wax/grease, it may be desirable to dissolve the material in a solvent and apply the solution to the grown film.
  • an adhesive tape when using an adhesive tape, however, the material can be simply applied to the grown film without dissolution in a solvent or melting.
  • the transfer support material may be cured after application.
  • the curing may be conducted at any suitable temperature and for any suitable amount of time to effect the cure.
  • the curing temperature and timing will depend on the selected transfer support material and its thermal properties. In some embodiments, for example, those structures including an Apiezon wax/grease as the transfer support material, the curing is performed at a temperature of about 100°C for about 90 minutes.
  • FIGs. 10A and 10B demonstrate that a wide variety of transfer support materials and methods of application for the transfer support material can be used to successfully relax the coherently-strained film upon removal of the initial growth substrate.
  • FIG. 1 OA depicts X-ray diffraction data showing the successful transfer of 40nm thick
  • FIG. 10A also depicts X-ray data for a coherently-strained Ino. 55 Gao. 45 As film on an InP substrate (labeled Ino .55 Gao. 45 As as grown).
  • FIG. 1 OA each of the Ino.5 5 Gao. 45 As films were successfully transferred from the InP substrate to a handle support (i.e., each of the films relaxed upon transfer). Accordingly, FIG. 10A shows that a variety of different transfer support materials can be used to successfully transfer (and relax) the grown films from the initial growth substrate to the handle support.
  • FIG. 10B depicts X-ray diffraction data showing the successful transfer of Ino .43 Gao.57As films (grown on InP initial growth substrates) using Apiezon® W and C509 applied to the grown film using different techniques.
  • FIG. 10B shows the successful transfer of a film using Apiezon® W that was applied by first dissolving it in TCE and then applying it to the film and allowing the TCE to evaporate.
  • FIG. 1 OB also shows the successful transfer of films using Apiezon® W and C509 that were applied by melting directly onto the grown film.
  • FIG. 10B depicts X-ray data for a coherently-strained In 0 43 Gao.57As film on an InP substrate (labeled Ino.
  • FIG. 10B shows that different methods of applying the transfer support materials can be used to successfully transfer (and relax) the grown films from the initial growth substrate to the handle support.
  • FIG. 10B appears to show a very slightly shifted peak for the film using the C509 transfer support, the date presented in the graph for that film does show that the film is substantially strain-relieved. Accordingly, while the results may appear slightly superior in the films using the Apiezon wax, the results also establish that the C509 transfer support also works.
  • FIGs. 10A and 10B show the versatility of the transfer technique with at least Ino. 43 Gao.5 7 As and Ino.55Gao.45As films.
  • the mechanical difficulty of this process is governed by the fragility and brittle nature of semiconductor films, and In x Gai_ x As films, such as those used in these examples are not especially or unusually strong or resistant to cracking compared to other semiconductor materials, as would be recognized by those of ordinary skill in the art.
  • the inventive method is applicable across the spectrum of semiconductor materials, and is not limited to the specific examples listed, as would be understood and expected by those of ordinary skill in the art.
  • the transfer support material may be applied on the grown film in a single layer, or in multiple layers.
  • the transfer support material may be a wax/grease, adhesive or polymer that is applied in multiple layers to a desired thickness.
  • the thickness of the applied transfer support is also not particularly limited, but is generally used in an amount and thickness sufficient to provide mechanical support for the handling of the film and to allow for relaxation of the film.
  • the transfer support has a thickness ranging from about 0.5mm to about 6mm. In particular, in some embodiments in which the transfer support material is melted on the film, the thickness of the transfer support may range from about 1mm to about 5mm.
  • the thickness of the transfer support may range from about 0.5mm to about 3mm. Additionally, in some embodiments in which an Apiezon material is used, the thickness of the transfer support may be from about 1 to about 3mm, and the transfer support may have a dome shape, as shown in FIG. 7A. In other embodiments, for example embodiments using PDMS, the transfer support may have a thickness from about 1mm to about 6mm, and the transfer support may be uniform in thickness along the wafer, which is due to the curing process. See FIG. 7D. In some exemplary embodiments, the transfer support has a thickness of about 2mm.
  • the relaxed film/transfer support structure may take a curved configuration, as shown in FIG. 2.
  • the curved configuration occurs because the grown film is very thin, but aids in the application of the film to the handle support.
  • a thin coating of deionized water is applied to the film (e.g., by rinsing the film in the water) and then the coated film is applied to the handle support.
  • the water can be applied to the handle support, or to both the film and the handle support.
  • van der Waals bonding occurs at room temperature between the film and the handle support (as shown in FIG. 2), and after a suitable period of time to allow bonding, the film is sufficiently bonded to the handle support.
  • the van der Waals bonding is allowed to occur for any amount of time suitable to complete the bonding procedure, for example, the bonding can be allowed to occur for a period of time of about 5 hours or longer, for example from about 5 hours to about 30 hours. In some exemplary embodiments, the bonding is allowed to occur for about 24 hours, but there is no particular upper limit to the amount of time for the bonding. In particular, no damage would be caused to the resulting structure from allowing the bond to cure for extended periods of time, and allowing the bond to form for a matter of days would also be suitable.
  • the curved configuration of the film/transfer support structure aids the bonding procedure.
  • the curved section of the film initially contacts the support and as the bonding continues, capillary forces pull the film toward the support from the curved region radially outward, as depicted in FIG. 2. This allows the film to bond to the handle support by gradually increasing the surface area of the contact, thereby substantially preventing the creation of trapped bubbles.
  • the transfer support is removed from the structure to yield a virtual substrate including a handle support and a substantially strain-relieved single crystalline layer, as shown in FIG. 7C. Removal of the transfer support can be accomplished using any suitable technique. In some
  • the transfer support is removed by chemical etching using a suitable solvent.
  • a suitable solvent e.g., a wax (e.g., Apiezon W, W40 or W100)
  • the etching solvent may be trichloroethylene (TCE) or the like.
  • TCE trichloroethylene
  • the transfer support material may be removed by simply peeling off the PDMS layer.
  • the solvent may be acetone or the like.
  • the solvent may be hexane or other hydrocarbon solvent or like chemical.
  • the solvent will be specific to the adhesive of the tape, but in some examples may be HC1, acetone, limonene or other suitable chemical.
  • the removal may include plasma ashing or other high temperature process to degrade the polymer. After removal of the transfer support, the resulting film/handle support structure can be used as a template for further epitaxial growth.
  • a sacrificial layer is grown on the initial growth substrate prior to growing the coherently-strained film which is then grown on the sacrificial layer.
  • the transfer support is applied on the film, and then the sacrificial layer is removed by selective chemical etching.
  • This method may be particularly desirable when using a initial growth substrate that is expensive because this method, termed "epitaxial liftoff," enables reuse of the initial growth substrate.
  • the selection of the sacrificial layer depends on the initial growth substrate and the coherently-strained film to be grown.
  • the sacrificial layer can be any layer that can be grown lattice-matched to the initial growth substrate, or any layer that is strained without causing dislocations in the crystal layer that will ultimately form the virtual substrate.
  • the sacrificial layer can be any material that can be selectively etched, without damaging either the single crystalline layer or the initial growth substrate.
  • the Clawson article discussed above and incorporated herein by reference, discusses several wet chemical etching solutions that can be used for III-V semiconductor alloys.
  • an AlAs sacrificial layer may be used in a structure using a GaAs substrate and a In x Gai -x P film to be grown on the substrate.
  • This method may also be used to make Si/Ge virtual substrates, for example by using a Ge rich SiGe alloy as the sacrificial layer, which can be selectively etched by hydrogen peroxide (H 2 0 2 ).
  • a Si0 2 sacrificial layer can be selectively etched by 10% HF from a SIM OX substrate.
  • a coherently-strained epitaxial thin layer (a "template layer") is grown on the initial growth substrate, as shown at 1 in FIG. 6B.
  • Thin film growth may be accomplished by any epitaxial technique, such as metal organic chemical vapor deposition or molecular beam epitaxy.
  • H + or He + ions are implanted in the substrate for the eventual exfoliation of the template layer and a small amount of the substrate material, as shown at 2 in FIG. 6B.
  • the implantation energy determines the substrate film thickness that will be exfoliated.
  • the transfer support material e.g.
  • a polymer is applied (e.g., by spin-coating) on the template layer's surface at a thickness of about 600 to about 700 nm, as shown at 3 in FIG. 6B.
  • curing parameters will depend on the materials used. However, in some embodiments, curing may be performed at about 300°C for about 30 minutes to allow layer exfoliation and reuse of the original initial growth substrate.
  • a mechanical support, such as glass or another polymer is placed against the transfer support material, and the system is heated to cure the transfer support material and trigger layer exfoliation, a result of ion implantation. The result is a manageable ensemble formed by the handle + transfer support material + template layer + thin film of exfoliated initial growth substrate material.
  • the initial growth substrate can be reused, decreasing considerably the cost of the virtual substrate.
  • the gray region is the initial growth substrate
  • the orange region is the coherently-strained layer (below critical thickness)
  • the green region is the transfer support material (e.g., polyimide)
  • the purple region is the mechanical handle (e.g., glass)
  • the blue region is the handle support (i.e., inexpensive substrate that is highly vacuum compatible).
  • the initial growth substrate film may be removed using a selective chemical etch to expose the template layer and elastically relax its strain.
  • the template layer may then be bonded to an inexpensive handle support suitable for growth reactor processes, such as silicon.
  • the transfer support material e.g., polymeric membrane
  • This semiconductor layer can be used for the defect-free growth of additional epitaxial layers with this distinct lattice parameter.
  • the mechanical difficulty of the inventive methods is governed by the fragility and brittle nature of semiconductor films, and In x Ga 1-x As films, such as those used in these examples are not especially or unusually strong or resistant to cracking compared to other semiconductor materials, as would be recognized by those of ordinary skill in the art.
  • the inventive methods are applicable across the spectrum of semiconductor materials, and are not limited to the specific examples listed, as would be understood and expected by those of ordinary skill in the art.
  • the in-plane strain (s // ) between the films and the InP ranged from + 0.3% to - 0.7%, representing both tensile and compressive, coherently-strained films.
  • the 0.58 In content film had a strain value of -0.33%
  • the 0.48 In content film had a strain value of +0.35%
  • the 0.45 In content film had a strain value of +0.56%>
  • the 0.13 In content film had a strain value of +0.70%.
  • the In x Gai -x As films were coated by a thermoplastic wax support to relax and handle the film. Once the film was coated with the wax, the InP was selectively etched, allowing for film relaxation.
  • the In x Gai_ x As films are compressively strained with respect to the wax, and are thus more stable against crack formation, as shown in FIG. IB.
  • the relaxed films were then bonded to a handle support, e.g. Si0 2 /Si, as shown in FIG. 1 A. Upon removal of the wax, the templates were ready for epitaxial growth.
  • the epitaxial growth was accomplished by metalorganic chemical vapor deposition at 550°C and a 3 A/min growth rate, as shown in FIG. 5, step A. These conditions correspond to a kinetically controlled epitaxial growth regime, resulting in dislocation-free films.
  • the strained In x Gai_ x As films were initially coated with Apiezon W wax, which is dissolved in trichloroethylene (TCE) and applied in solution.
  • step B Many layers of the wax were applied to the sample surface to create, in aggregate, a 2 mm thick wax layer, as shown in FIG. 5, step B.
  • the wax was then cured at 100°C for 90 min, and a glass slide was used to support the wax for handling purposes.
  • the InP substrate was selectively etched at 5 ⁇ / ⁇ using room temperature concentrated HC1, as shown in FIG. 5, step C. Strain relaxation occurred when the substrate was completely removed, and the In x Gai_ x As crystals assumed their bulk value, as predicted by Vegard's law.
  • the thin film was rinsed in DI H 2 0 and immediately placed into contact with the transfer support, e.g. Si0 2 /Si, as shown in FIG. 5, step D.
  • Van der Waals bonding was allowed to take place between the In x Gai -x As film and the handle support for about 24 hours, after which time, the two materials were bonded. Because the wax has a thermal expansion coefficient (6.20x 10 "4 °C " ') two orders of magnitude larger than the semiconductor film (5.66x10 "6 °C " '), it is under tension with respect to the film - ideal for layer relaxation independent of the original film strain configuration. The wax was then removed by trichloroethylene (TCE), as shown in FIG. 5, step E. After an appropriate cleaning procedure, the elastically-relaxed single crystal film was ready for epitaxial growth, as shown in FIG. 5, step F.
  • TCE trichloroethylene
  • the virtual substrate fabrication procedure can be scaled up to entire wafers, as illustrated in FIG. I B (depicting an entire wafer).
  • the plan view transmission electron microscopy image in FIG. 1 C shows a free-standing piece of the relaxed In x Gai -x As film free of cracks or dislocations. The absence of dislocations indicates that the relaxed film preserves the high quality of the original material.
  • the well defined spots of the electron diffraction pattern in FIG. ID confirm the good crystalline quality of the relaxed film, i.e., that the elastically-relaxed film is a defect-free single crystal.
  • FIG. 1 E displays an atomic resolution transmission electron microscopy image of a representative region of the virtual substrate; no defects were observed.
  • the distance between the (001) projected atomic columns points out that the lattice parameter is equal to the bulk value, corresponding to relaxed Ino.45Gao.57As.
  • the overall crystalline quality of the film is preserved by the inventive methods to achieve elastically-relaxed single crystalline layers.
  • FIG. 2A shows (004) ⁇ -2 ⁇ scans for all samples before and after film transfer to a Si0 2 /Si handle support.
  • the distance between the InP (appearing as a sharp peak) and the 40 nm thick film (appearing as a broad peak) confirms that the In x Gai -x As is originally coherently-strained with respect to the substrate.
  • After transfer a significant peak shift is observed, resulting from strain relief. The observed peak shift for all samples is due to material full relaxation.
  • FIG. 2B shows reciprocal space maps for the coherently-strained In x Gai_ x As on InP, taken near the asymmetric (224) reflection.
  • the 40 nm thick In x Gai -x As:InP film is coherently-strained in all cases.
  • the intense red peak in FIG. 2B corresponds to the substrate and appears in the same position for all samples.
  • the strained layers appear shifted in both ⁇ and 2 ⁇ directions.
  • the position of the film peak is used to infer the original in-plane strain values, as well as strained a / / and a ⁇ for all samples.
  • FIG. 3 is a graphical representation of the inferred strain values.
  • the strained and relaxed lattice parameters were quantified.
  • the virtual substrate lattice parameter and film relaxation can be calculated from the measurements in FIG. 2C.
  • strained an was equal to a / foundedp, as a result of coherent epitaxial growth.
  • the material unit cell also deforms in the out-of-plane direction, as shown in the graph of FIG. 3. After strain relief, the unit cells of the fabricated virtual substrates were reconstructed.
  • the relaxed lattice parameters (a // , a ) and the films natural lattice parameter (predicted by Vegard's law), as illustrated in FIG. 3.
  • the material's natural lattice parameter (o ? w ) is given by the average of the lattice parameters of the elements constituting the alloy, compensated by the molar fraction of each species.
  • FIG. 8 shows the reciprocal space geometry for glancing incidence scans, performed to reconstruct the virtual substrate unit cell.
  • A3 ⁇ 4 and k s are the entrance and exit wave vectors
  • q is the difference between t and k s
  • k / / and k ⁇ are the parallel and perpendicular components of q
  • 26 is the diffracting angle
  • ⁇ is the angle between the incident X-ray beam and the Bragg planes.
  • a (224) map is enough to reconstruct the crystal unit cell.
  • the in-plane and out-of-plane components of the wave vector q can be calculated using Equations 1 and 2:
  • Equation 3 0 ub hu is the substrate Bragg angle, ⁇ is the angular separation between the substrate and the film at the (224) reflection ( ⁇ 0 for compressive films and > 0 for tensile films), ⁇ is the angle between the (224) and (004) planes, AOO MI is the separation in ⁇ between substrate and film peaks for a ⁇ h k ) Bragg reflection (equal to zero for films with no tilt).
  • Both In x Gai_ x As in-plane and out-of-plane lattice spacings (a // and a J) can be calculated for a certain (h k ⁇ ) Bragg condition using Equation 3 :
  • FIG. 4A shows the room temperature, steady-state photoluminescence observed from coherent In x Gai_ x As films grown on InP and the corresponding relaxed In x Gai_ x As films after transfer to Si0 2 /Si.
  • the readily observable photoluminescence signal of the 40 nm In x Gai_ x As layers at room temperature confirmed the high quality of the films.
  • Lattice- matched mo.5 3 Gao. 47 As films on InP showed nearly identical band edge photoluminescence intensities after transfer to Si0 2 /Si, demonstrating that the optoelectronic properties of the film are preserved throughout the layer transfer process.
  • Ino.53Gao.47As film is lattice-matched to InP, no peak shift is observed.
  • FIG. 4B presents the fitting results from the low-energy portion of the PL signal using Equation 5:
  • Equation 5 A is a proportionality constant, E g is the band gap, and a is a tail fit parameter.
  • E g is the band gap
  • a is a tail fit parameter.
  • Both coherently- grown and transferred Ino.53Gao.47As films show values of E g and comparable to the expected bulk parameters.
  • PL spectra of strained In x Gai_ x As films collected at room temperature show the effects of film relaxation upon layer transfer to Si0 2 /Si substrates.
  • both the tensile and compressed In x Gai -x As virtual substrates were fabricated and measured by X-ray diffraction. This technique allows measurement of the in-plane and out-of-plane strains and reconstruction of the material unit cell.
  • FIGs. 9A, 9B and 9C show these measurements for an Ino.
  • Virtual substrate in-plane and out-of-plane lattice parameters (a. / / and a ⁇ , respectively) can be calculated by measuring reciprocal space maps at an asymmetric reflection, such as (2 2 4).
  • FIGs. 9B and 9C show the film peak shift in both ⁇ and 2 ⁇ directions, indicating again that the film relaxed.
  • Full relaxation was also achieved for In x Gai -x As films with compressive and different tensile strain values and their unit cells are in agreement with Vegard's Law prediction, as shown in FIG. 3 (depicting that different In x Gai -x As alloy compositions (and strain values) completely relax, assuming the lattice parameter predicted by Vegard's Law).
  • the search for a dislocation-free crystalline template has been a "holy grail" objective in the field of materials science for more than 20 years because success in this endeavor enables the synthesis of a very large variety of strain-free crystals at lattice parameters other than those achievable via bulk crystal growth.
  • the inventive virtual substrates are applicable across a wide spectrum of semiconductor materials, including III-V, II-VI and IV-IV semiconductors and ceramics.
  • the creation of a template that can be used for epitaxial growth has the potential to enable new crystalline material growth that can be used as a building block for innovative optoelectronic designs for semiconductor
  • heterostructures that require tunable lattice parameters and band structures, such as light- emitting diodes, photodetectors and semiconductor lasers with frequency operation that are currently difficult to fabricate, thin film solar cells that absorb photons with specific energies, flexible electronics and woodpile-structure three-dimensional photonic crystals.
  • inventive virtual substrates and methods have promising commercial utility for III-V multi-junction solar cells and other applications.
  • the virtual substrates allow selection of a lattice constant that supports defect-free growth of

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Abstract

Le substrat virtuel ci-décrit comprend un support de traitement et une couche monocristalline relaxée sur ledit support de traitement. Un procédé de fabrication du substrat virtuel selon l'invention comprend la croissance d'une couche monocristalline contrainte de manière cohérente sur un substrat de croissance initial, le retrait du substrat de croissance initial pour relaxer la contrainte exercée sur la couche monocristalline, et l'application de la couche monocristalline relaxée sur un support de traitement.
PCT/US2010/061666 2010-04-08 2010-12-21 Substrats virtuels pour croissance épitaxiale et leurs procédés de fabrication WO2011126528A1 (fr)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20050130424A1 (en) * 2002-07-16 2005-06-16 International Business Machines Corporation Use of hydrogen implantation to improve material properties of silicon-germanium-on-insulator material made by thermal diffusion
US6921914B2 (en) * 2000-08-16 2005-07-26 Massachusetts Institute Of Technology Process for producing semiconductor article using graded epitaxial growth
US20100025728A1 (en) * 2008-05-15 2010-02-04 Bruce Faure Relaxation and transfer of strained layers

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6921914B2 (en) * 2000-08-16 2005-07-26 Massachusetts Institute Of Technology Process for producing semiconductor article using graded epitaxial growth
US20050130424A1 (en) * 2002-07-16 2005-06-16 International Business Machines Corporation Use of hydrogen implantation to improve material properties of silicon-germanium-on-insulator material made by thermal diffusion
US20100025728A1 (en) * 2008-05-15 2010-02-04 Bruce Faure Relaxation and transfer of strained layers

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