WO2024042221A1 - Nanostructure/microstructure device - Google Patents
Nanostructure/microstructure device Download PDFInfo
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- WO2024042221A1 WO2024042221A1 PCT/EP2023/073388 EP2023073388W WO2024042221A1 WO 2024042221 A1 WO2024042221 A1 WO 2024042221A1 EP 2023073388 W EP2023073388 W EP 2023073388W WO 2024042221 A1 WO2024042221 A1 WO 2024042221A1
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- layer
- microstructures
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- cores
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Classifications
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- H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L33/005—Processes
- H01L33/0062—Processes for devices with an active region comprising only III-V compounds
- H01L33/0066—Processes for devices with an active region comprising only III-V compounds with a substrate not being a III-V compound
- H01L33/007—Processes for devices with an active region comprising only III-V compounds with a substrate not being a III-V compound comprising nitride compounds
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- H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
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- H01L21/02636—Selective deposition, e.g. simultaneous growth of mono- and non-monocrystalline semiconductor materials
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- H—ELECTRICITY
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- H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L33/02—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
- H01L33/08—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a plurality of light emitting regions, e.g. laterally discontinuous light emitting layer or photoluminescent region integrated within the semiconductor body
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- H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L33/02—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
- H01L33/20—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a particular shape, e.g. curved or truncated substrate
- H01L33/24—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a particular shape, e.g. curved or truncated substrate of the light emitting region, e.g. non-planar junction
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- H—ELECTRICITY
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- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L33/02—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
- H01L33/26—Materials of the light emitting region
- H01L33/30—Materials of the light emitting region containing only elements of Group III and Group V of the Periodic Table
- H01L33/32—Materials of the light emitting region containing only elements of Group III and Group V of the Periodic Table containing nitrogen
Definitions
- compositions comprising nanostructures or microstructures grown on a substrate.
- the nanostructures or microstructures comprise at least one lll-V layer positioned on top of the nanostructure cores or microstructure cores, and at least part of said at least one lll-V layer is semipolar.
- the compositions of matter that are formed can be used in an electronic device such as an LED, solar cell, transistor, laser or photodetector.
- Nanowires which are also referred to as nanowhiskers, nanorods, nanopillars, nanocolumns, etc. by some authors, have found important applications in a variety of electrical devices such as sensors, solar cells, transistors and LED's.
- the nanowires in such devices can act as light emitters (e.g. in the case of LEDs), or as light absorbers (e.g. in the case of photodetectors).
- UV LED devices in particular have found promising applications in antibacterial and antiviral applications.
- the present inventors have surprisingly found that by positioning semipolar layers on top of a nanostructure/microstructure core, beneficial light extraction or absorption is obtained.
- composition of matter comprising: a substrate; a plurality of nanostructures or microstructures grown from said substrate, wherein said nanostructures or microstructures comprise: nanostructure cores or microstructure cores respectively, comprising at least one semiconducting group lll-V compound, at least one lll-V layer positioned on top of said nanostructure cores or microstructure cores, wherein at least part of said at least one lll-V layer is semipolar; and wherein said nanostructures or microstructures comprise a p-n or p-i-n junction.
- composition of matter comprising a substrate; a plurality of nanostructures or microstructures grown from said substrate, wherein said nanostructures or microstructures comprise: nanostructure cores or microstructure cores respectively, comprising at least one semiconducting group lll-V compound, at least one lll-V layer positioned on top of said nanostructure cores or microstructure cores, wherein at least part of said at least one lll-V layer is a pyramidal layer; and wherein said nanostructures or microstructures comprise a p-n or p-i-n junction.
- composition of matter comprising: a substrate; a non-planar coalesced structure on said substrate, e.g. formed from a plurality of coalesced nanostructures or microstructures, said non-planar coalesced structure and/or nanostructures or microstructures comprising at least one semiconducting group lll-V compound; wherein said non-planar coalesced structure comprises at least one lll-V layer, typically positioned on top of coalesced nanostructure cores or microstructure cores, wherein at least part of said at least one lll-V layer is semipolar; and preferably wherein said at least one lll-V layer extends continuously over the plurality of coalesced nanostructure cores or microstructure cores.
- a device such as an electronic or opto-electronic device, comprising a composition of matter as defined herein, e.g. wherein said device is a solar cell, photodetector, transistor, laser or LED, preferably an LED, more preferably a UV LED, more preferably a UV-C LED.
- a process for preparing a composition of matter as defined herein comprising: (I) growing a plurality of nanostructure cores or microstructures cores from a substrate, e.g. via MOCVD, said nanostructure cores or microstructures cores comprising at least one semiconducting group 11 l-V compound;
- a method of emitting light from a device (or absorbing light by a device) as defined herein comprising emitting (or absorbing) transverse magnetic (TM) polarized light via the side and/or bottom of the device, preferably via: a) emission (or absorption) of TM polarized light substantially parallel to the II l-V layer and in a direction substantially towards the substrate-side of the device (or from the substrate side of the device for a light absorbing device) and/or b) emission (or absorption) of TM polarized light substantially parallel to the II l-V and in (or from) a direction substantially opposite to the substrate-side of the device, wherein said light reflects off a reflective layer positioned on top of said nanostructures or microstructures
- group 11 l-V pyramidal semipolar layers for increasing the transverse magnetic (TM) light emission from LEDs comprising nanostructures and/or microstructures on a substrate.
- a group lll-V compound semiconductor is meant one comprising at least one element from group III and at least one element from group V. There may be more than one element present from each group, e.g. InGaAs, AIGaN (i.e. a ternary compound), AllnGaN (i.e. a quaternary compound) and so on, as well as dopants of elements from other groups.
- the term lll-V semiconducting nanostructure or microstructure is meant nanostructure or microstructure made of semiconducting materials from group lll-V elements.
- Nanostructure may herein mean a nanowire (also termed a nanorod, nanopillar, nanocolumn or nanowhisker), a nanopyramid, or a nanoribbon
- microstructure may herein mean a microwire (also termed a microrod, micropillar, microcolumn or microwhisker), a micropyramid, or a microribbon.
- Nanowire is used herein to describe a solid, wire-like structure of nanometer dimensions.
- Nanowires preferably have an even diameter throughout the majority of the nanowire, e.g. at least 75% of its length.
- Nanowires may have tapered end structures.
- the nanowires can be said to be in essentially in onedimensional form with nanometer dimensions in their width or diameter and their length typically in the range of 100 nm to a few (e.g. 5) pm.
- the nanowires are at least 1 micrometer in length.
- at least 90% of the nanowires grown on a substrate will be at least 1 micrometer in length.
- substantially all the nanowires will be at least 1 micrometer in length.
- the nanowire diameter/width is not greater than 1000 nm.
- the nanowire diameter/width is between 10 and 1000 nm, e.g. 50 and 500 nm.
- the nanostructures or microstructures grown have the same dimensions, e.g. to within 10% of each other.
- at least 90% (preferably substantially all) of the nanostructures or microstructures on a substrate will preferably be of the same diameter/width and/or the same length (i.e. to within 10% of the diameter/length of each other).
- the skilled person is looking for homogeneity and nanostructures or microstructures that are substantially the same in terms of dimensions.
- the length of the nanostructures or microstructures is often controlled by the length of time for which the growing process runs. A longer process typically leads to a (much) longer nanostructure or microstructure. Ideally, the diameter at the base of the nanowire and at the top of the nanowire should remain about the same (e.g. within 20% of each other).
- microwire is used herein to describe a solid, wire-like structure of micrometer dimensions. The same considerations as for nanowires apply here, the difference lying in the dimensions.
- microwire is intended to cover the use of microrods, micropillars, microcolumns or micro whiskers some of which may have tapered end structures.
- the microwires can be said to be in essentially in onedimensional form with micrometer dimensions in their width or diameter, and their length typically in the range of a 1-10,000 pm, i.e. at least 1 pm.
- the microwire diameter/width is typically greater than 1 pm. Ideally the microwire diameter/width is between 1 pm and 500 pm.
- the diameter/width at the base of the microwire and at the top of the microwire should remain about the same (e.g. within 20% of each other).
- nanopyramid refers to a solid pyramidal type structure.
- pyramidal is used herein to define a structure with a base whose sides taper to a single point generally above the centre of the base. It will be appreciated that the single vertex point may appear chamfered, e.g. such that the pyramid has a flat top.
- the chamfered portion is equivalent to less than 50%, e.g. less than 40%, e.g. less than 30%, e.g. less than 20%, e.g. less than 10%, e.g. less than 5% of the total length of the nanopyramid edge.
- the nanopyramids may have multiple faces, such as 3 to 8 faces, or 4 to 7 faces.
- the base of the nanopyramids might be a triangle, square, pentagonal, hexagonal, heptagonal, octagonal and so on.
- the pyramid is formed as the faces taper from the base to a central point (forming therefore triangular faces).
- the triangular faces are normally terminated with ⁇ 1- 101 ⁇ or ⁇ 1-102 ⁇ planes.
- the triangular side surfaces with ⁇ 1-101 ⁇ facets could either converge to a single point at the tip or could form a new facets ( ⁇ 1-102 ⁇ planes) before converging at the tip.
- These 4-digit indices are typically most appropriate for hexagonal crystals.
- the nanopyramids are truncated with its top terminated with ⁇ 0001 ⁇ planes.
- the base itself may comprise a portion of even cross-section before tapering to form a pyramidal structure begins.
- the thickness of the base may therefore be up to 500 nm, e.g. up to 200 nm, such as 50
- the base of the nanopyramids can be between 50 and 500 nm in diameter across its widest point. In another embodiment, the base of the nanopyramids can be 200 nm to one micrometer in diameter across its widest point.
- the height of the nanopyramids may be 200 nm to a few (e.g. 5) micrometers, such as 400 nm to 1 micrometer in height.
- micropyramid refers to a solid pyramidal type structure of micrometer dimensions.
- the above discussion on the structure of nanopyramids apply also here for micropyramids, the only difference being the dimensions.
- the thickness of the base may be at least 1 pm, and up to 500 pm, e.g. up to 200 pm, such as 50 pm.
- the base of the micropyramids can be between 50 and 500 pm in diameter/width across its widest point.
- the base of the micropyramids can be 200 pm to one micrometer in diameter/width across its widest point.
- the height of the micropyramids may be at least 1 pm, e.g. 1-10,000 pm, such as 1 pm to 500 pm in length.
- nanoribbon refers to a nanostructure in the form of a ribbon where the lateral dimensions are considerably different, e.g. in a first lateral dimension the width of the ribbon is between 10 nm -1000 nm and in a second lateral dimension the width of the nanoribbon is more than 100 times greater, e.g. a few hundred times greater than in the first dimension.
- microribbon refers to a microstructure in the form of a ribbon where the lateral dimensions are considerably different, e.g. in a first lateral dimension the width of the ribbon is between 1pm - 1000 pm and in a second lateral dimension the width of the microribbon is more than a 100 times greater, e.g. a few hundred times greater than in the first dimension.
- the substrate comprises a plurality of nanostructures or microstructures. This may be called an array of nanostructures or microstructures.
- the plurality of nanostructures or microstructures is a plurality of nanowires or nanopyramids, preferably a plurality of nanowires.
- epitaxy comes from the Greek roots epi, meaning “above”, and taxis, meaning “in ordered manner”.
- the atomic arrangement of the nanostructure or microstructure is typically based on the crystallographic structure of the substrate.
- Epitaxial growth means herein the growth on the substrate of a nanostructure or microstructures that mimics the orientation of the substrate.
- bottom refers to the substrate side of the nanostructures or microstructures
- top refers to the side of the nanostructures or microstructures that is opposite to the substrate.
- the nanostructure or microstructures typically grow initially from the substrate and hence it is preferred that the substrate is a crystalline substrate.
- the substrate is typically semiconducting.
- the substrate may have a crystal orientation of [111], [110], [0001] or [100] perpendicular to the surface. [0001] is preferred.
- the substrate is a [0001] lll-N substrate.
- the substrate may comprise silicon, sapphire, SiC, Ga2Oa, graphene or group lll-V substrates, preferably group lll-V substrates.
- group lll-V substrates preferably group lll-V substrates.
- V substrates and thus the definition of lll-V materials for the nanostructures/microstructures also apply for lll-V substrates.
- Ill-N substrates are particularly preferred.
- the substrates may be formed from at least one lll-V compound.
- Group III options are B, Al, Ga, In, and Tl.
- Preferred options here are Ga, Al, B and In, preferably Ga, Al, and In.
- Group V options are N, P, As, Sb. N is preferred.
- V compounds may be binary, ternary, quaternary, quintinary etc.
- AIN is a typical binary compound for the substrate.
- Ternary compounds may be of formula XYZ wherein X is a group III element, Y is a group III different from X, and Z is a group V element.
- the X to Y molar ratio in XYZ is preferably 0.01-0.99, e.g. 0.1 to 0.9, i.e. the formula is preferably X x Yi. x Z where subscript x is 0.01-0.99, e.g. 0.1 to 0.9.
- Quaternary systems might also be used and may e.g. be represented by the formula A x Bi- x C y Di- y where A and B are group III elements and C and D are group V elements or A x B y Ci- x.y D where A, B and C are group III elements and D is a group V element.
- subscripts x and y are typically 0.01-0.99, e.g. 0.1 to 0.9. Other options will be clear to the skilled person.
- the substrate preferably is or comprises a Ill-N compound.
- the substrate preferably is or comprises AIGaN (e.g. n-AIGaN or p-AIGaN) or AIN.
- the substrate in the present invention is typically doped, i.e. n-doped or p- doped, typically n-doped.
- the doped substrate typically acts as an injector of current.
- the doped substrate can be an injector of electrons (if n-doped) or injector of holes (if p-doped).
- Doping of the substrate typically involves the introduction of impurity ions.
- the doping level can be controlled from ⁇ 10 15 /cm 3 to 10 22 /cm 3 , preferably 10 16 /cm 3 to 10 21 /cm 3 , preferably 10 17 /cm 3 to 10 2 °/cm 3 , preferably 10 18 /cm 3 to 10 19 /cm 3 , (these numbers refer to the number of doping/impurity ions per cm 3 ).
- the substrate may be highly doped, e.g. has a doping level of at least 10 15 /cm 3 , preferably of at least 10 16 /cm 3 , preferably of at least 10 17 /cm 3 , preferably of at least 10 18 /cm 3 .
- the substrate is preferably n-type doped.
- Suitable acceptors for the substrate can be Be, Mg and Zn, when the substrate is p-type doped.
- the substrate may therefore be doped with at least one of Be, Mg and Zn.
- suitable donors can be Te, Sn, Si, Ge and C.
- the substrate may therefore be doped with at least one of Te, Sn, Si, Ge and C. Dopants can be introduced during the growth process or by ion implantation after formation of the substrate.
- the doped substrate facilitates the electrical functionality of a device, i.e., transport of charge carriers and injection/extraction at the functional components, e.g., metal contacts or active region.
- doped substrates have a low sheet resistance as well as low contact resistance which enables efficient devices, in particular in terms of low voltages, low heat generation, and high wall-plug efficiency.
- layers with different properties can be stacked to improve different aspects, e.g., two layers with different doping levels optimized to improve sheet resistance and contact resistance, respectively. In such a way key properties can be decoupled to independently improve multiple aspects of efficiency.
- thermal conductivity will be affected by doping. So a doped substrate will most likely improve heat conductivity and extraction.
- a high degree of doping can be beneficial as this can improve device efficiency. It can also be beneficial for device efficiency to have similar doping levels for the substrate and the nano-/microstructures.
- the substrate may therefore be provided with an electrical contact (typically metallic), preferably a contact on the top side of the substrate (i.e. same side as the nanostructures/microstructures). A portion of the top surface of the substrate can be etched away to connect an electrical contact.
- the electrical contact enables vertical hole or electron injection.
- the substrate may be a multi-layer substrate comprising stacked layers of different materials.
- the substrate may comprise, for example, two or more layers selected from silicon, sapphire, SiC, Ga2Os, graphene or group lll-V layers.
- the substrate may comprise two or more layers of a lll-N material, e.g. AIGaN (p-type or n-type) on AIN.
- the substrate typically comprises at least a lll-N layer, preferably as the top layer.
- the compositions/device of the invention may comprise a planar group lll-V layer (e.g.
- top layer in contact with the top layer of the substrate, on the opposite side of the substrate to the nanostructures or microstructures, preferably wherein said planar group lll-V layer is AIN.
- Such an additional layer may be beneficial as it can provide strain moderation and ensure compositional uniformity of the layer on top of it (e.g. AIGaN).
- Top when referring to the substrate herein refers to the layer of the substrate in contact with the nanostructure or microstructure cores.
- Additional layers e.g. sapphire, may be present in the substrate (e.g. under the other layers), and may be desirable to act as a support and add rigidity to the structure.
- a sacrificial layer may also be present (see Figure 10).
- the substrate may comprise a sacrificial layer of GaN or highly doped n-AIGaN, for example.
- the whole top heterostructure can be lifted off using chemical, electrochemical or laser lift off techniques.
- the advantage of this set up is that the substrate becomes thinner, and thus light extraction/absorption efficiency can be improved, and/or more flexible devices can be obtained.
- the substrate is transparent, e.g. to any light emitted by or detected by the nanostructures or microstructures (e.g. UV light or UVC light).
- any light emitted by or detected by the nanostructures or microstructures e.g. UV light or UVC light.
- the substrate typically has a thickness of 0.1-2000 pm, e.g. 0.1-1000 pm, e.g. 0.1-100 pm, e.g. 0.5-50 pm, e.g. 1-10 pm.
- nanostructures/microstructures can refer to the nanostructure/microstructure cores, or the cores in combination with additional layers positioned thereon.
- the core is herein defined as the (innermost) part of the nanostructure/microstructure which grows first on the substrate, and/or which is in physical contact with the substrate.
- the term ‘horizontal’ refers herein to the plane of the substrate, and the term ‘vertical’ typically herein refers to the growth direction of the nano/microstructures, or an axis along the longest dimensions thereof (i.e. perpendicular to substrate plane)
- nanostructures/microstructures In order to prepare nanostructures/microstructures of commercial importance, it is preferred that these grow epitaxially on the substrate. It is also ideal if growth occurs perpendicular to the substrate and ideally therefore e.g. in the [111 ]-direction for cubic crystal structure or e.g. in the [0001]-direction for hexagonal crystal structure. Ill-N materials, which are preferred, are typically hexagonal and therefore the [0001] direction is preferred.
- the wording ‘grown from said substrate’ means that the nanostructures/microstructures are located on the substrate, and preferably form an epitaxial relationship with the substrate (i.e. top layer of the substrate if the substrate is a multi-layer substrate).
- the nanostructures/microstructures are typically grown from the substrate through the holes of the mask layer. This means that the nano-/microstructures are located over the holes, and there is part of the nanostructure/microstructure that extends (downwards) in the hole to the substrate, and preferably the nanostructure/microstructure forms an epitaxial relationship with the substrate. Alternatively put, the nanostructure/microstructure are present in the holes of the mask layer. Alternatively put, nanostructures/microstructures are nucleated from the holes in the mask layer, or extend therefrom.
- the nanostructures or microstructures typically extend over the surface of the mask layer, if present.
- the nanostructures or microstructures can grow out of, or nucleate in, the hole of the mask, and then grow/extend sideways (i.e. laterally/radially) to cover at least a portion of the top surface of the mask layer (i.e. they ‘mushroom’ out of the holes). They typically grow or extend laterally/radially (i.e. in the same plane as the substrate/mask), such that only the inner portion of the nanostructure/microstructure core is formed over the hole in the mask layer mask.
- the nanostructure cores or microstructure cores have pyramidal tips.
- the overall nanostructures or microstructures have pyramidal tips.
- the tips are hexagonal pyramidal. Growth of nanostructure or microstructure can be controlled through the MOCVD growth conditions. Pyramids are encouraged, for example if high group V flux is employed.
- nanostructures or microstructures can accommodate much more lattice mismatch than thin films for example.
- the nanostructures or microstructures have typically a hexagonal cross sectional shape.
- the nanowire may have a cross sectional diameter/width of 25 nm to several micrometers (i.e. its thickness). As noted above, the diameter is ideally constant throughout the majority of the nanowire. Nanostructure/microstructure diameter/width can be controlled by the manipulation of the growth parameters such as the substrate temperature and/or the ratio of the atoms used to make the nanowire as described further below.
- the length and diameter of the nanostructures or microstructures can be affected by the temperature at which they are formed. Higher temperatures encourage high aspect ratios (i.e. longer and/or thinner nanowires/microwires).
- the skilled person is able to manipulate the growing process to design nanostructures or microstructures of desired dimensions.
- the nanostructures or microstructures of the invention comprise at least one lll-V compound, preferably lll-N.
- the nanostructures or microstructures comprise an Al-containing lll-V compound, e.g. AIGaN, AllnGaN, or AIN.
- the nanostructure/microstructure cores comprise (or are made of) an Al-containing lll-V compound, e.g. AIGaN, AllnGaN, or AIN, preferably AIGaN.
- Al-containing lll-V compounds are most suitable for UV applications.
- Group III options are B, Al, Ga, In, and Tl. Preferred options here are Ga, Al, B and In, preferably Ga, Al and In. Group V options are N, P, As, Sb. N is preferred.
- the compounds may be binary, ternary, quaternary, quinary etc.
- the above and below applies to both the nanostructure or microstructure cores but also to any additional lll-V layers on top of the cores.
- Suitable binary compounds for nanostructure or microstructure manufacture include AlAs, GaSb, GaP, GaN, AIN, GaAs, InP, InN, InSb, InAs. Compounds based on Al, Ga and In in combination with N are one preferred option.
- the use of GaN, AIN is highly preferred for binary compounds. It is most preferred if the nanostructures or microstructures comprise or consist of Ga, Al, In and N (along with any doping atoms as discussed below) (NB: any electrical contacts are not typically considered part of the nano- /microstructures).
- Ternary compounds may be of formula XYZ wherein X is a group III element, Y is a group III different from X, and Z is a group V element.
- the X to Y molar ratio in XYZ is preferably 0.01 to 0.99, e.g. 0.1 to 0.9, i.e. the formula is preferably X x Yi. x Z where subscript x is 0.01 to 0.99, e.g. 0.1 to 0.9.
- AIGaN, InGaN, InAIN, InGaAs, or AIGaAs are preferred ternary compounds.
- AIGaN, InAIN, and InGaN are most preferred, especially AIGaN.
- the nanostructures or microstructures do not comprise InGaN, since InGaN is not suitable for UV/LIVC applications.
- Quaternary systems might also be used and may e.g. be represented by the formula A x Bi- x C y Di- y where A and B are group III elements and C and D are group V elements or A x B y Ci- x.y D where A, B and C are group III elements and D is a group V element.
- subscripts x and y are typically 0.01 to 0.99, e.g. 0.1 to 0.9.
- AIGalnN is particularly preferred compound.
- AllnGaBN is an example of a suitable quinary compound.
- ternary/quaternary materials for at least one lll-V layer, the core, and/or the substrate is preferred.
- AIGaN, AI(ln)GaN are most preferred for the cores and layers. It is preferred if the nanostructure cores or microstructure cores comprise, or consist of, AIGaN, preferably n-AIGaN or p-AIGaN, preferably n- AIGaN.
- AI(ln)GaN both n-type and p-type is also highly suitable.
- the Al content in the nano-/microstructures is of at least 60% (by atom ratio of the group-ill component, usually given by AI/(AI+Ga)), since this increases transparency to UVC light.
- the use of Al is advantageous as high Al content leads to high band gaps, enabling IIV-C LED emission from the active layer(s) of nano-/microstructures and/or avoiding absorption of the emitted light in the at least one lll-V layers.
- the use therefore of AIN or AIGaN in nano- /microstructures is preferred.
- the preferred Al content may apply to the nanostructure cores as well as any lll-V layers.
- compositions of the present invention comprise at least one lll-V layer positioned on top of the nanostructure cores or microstructure cores, wherein at least part of said at least one lll-V layer is semipolar.
- the at least one lll-V layer is semipolar. In other words, preferably, the entirety of the lll-V layers are semipolar or pyramidal, i.e. each lll-V layer is completely semipolar/pyramidal.
- the composition or device comprises a top GaN (e.g. p-GaN) layer, which absorbs UV light
- having semipolar group lll-V layers ensures that UV absorption is minimized, since there are no wrap-around shell layers, for example, which would absorb UV light and reduce the efficiency of the device.
- a core-shell structure which may have semipolar as well as vertical parts, undesirably high levels of absorption can occur. This is especially the case in devices where light is emitted/absorbed via the bottom of the device. Minimising absorption by a UV-opaque top layer is important.
- a polar structure can be seen as a structure without space inversion symmetry, and of the 31 crystal classes 21 of them have this property. All polar crystal structures, such as Wurtzite, have a specific polar axis, along where the space inversion symmetry is broken. Semipolar planes are planes that have a normal vector at an angle which is non-0°, non-90° and non-180° with respect to the polar axis of the crystal structure. Semipolar planes/layers are therefore planes/layers which are not perpendicular to, nor parallel to, the polar axis of the crystal structure.
- the at least one lll-V layer is therefore typically formed of a compound with a semipolar crystal structure, and the lll-V layer is neither perpendicular nor parallel to the polar axis of said polar crystal structure.
- the at least one lll-V layer is therefore typically formed of a compound with a semipolar crystal structure, with the lll-V layer having an angle of 5-85° relative to the polar axis of said polar crystal structure.
- the lll-V layer is typically neither perpendicular nor parallel to the growth axis of the nano/microstructure.
- the lll-V layer is typically neither perpendicular nor parallel to the substrate plane.
- the at least one lll-V layer has an orientation which is not perpendicular to, nor parallel to, a low index (e.g. [001], [010], [100]) axis of that substrate.
- the core is grown/positioned on the patterned substrate.
- the core’s upper surface is typically composed of six equivalent surface parts arranged in a pyramidical shape merging at a common tip.
- each of the six equivalent surface parts forms a semipolar plane, characterized by a specific angle and an equivalent set of Miller’s indices, e.g., 62° with respect to the horizontal plane and ⁇ 10-11 ⁇ .
- the angle and indices of the semipolar planes are reproduced.
- multiple semipolar interfaces can be created.
- such semipolar interfaces provide special physical properties in terms of charge carrier distribution and light-matter-interaction.
- highly TM-polarized light can result from the semipolar interfaces.
- polar surfaces induce an effect called the quantum confined stark effect, which is an undesired effect in LEDs.
- the semipolar plane will, due to the reduced polarity, also have a reduced quantum confined stark effect.
- the at least one 11 l-V layer is typically a pyramidal layer, preferably a hexagonal pyramidal layer (since lll-N nanostructure or microstructure cores are preferred and these typically have hexagonal symmetry).
- the at least one 11 l-V layer comprises tilted sidewalls extending from a pyramid peak, wherein the sidewalls are semipolar.
- the tips of the semipolar (i.e. pyramidal) layers can be seen as the meeting point of 6 semipolar planes (for hexagonal symmetry).
- the at least one 11 l-V layer typically has six equivalent planes/surface parts arranged in a pyramidical shape merging at a common tip. Each surface part preferably is a ⁇ 10- 11 ⁇ facet, but a range of similar planes can be considered herein. Generally speaking, therefore, the at least one 11 l-V layer typically has ⁇ 10-11 ⁇ facets, but higher index facets are also possible.
- the internal pyramidal angle of said at least one 11 l-V layer is in the range 5-175°, preferably 40-70°, preferably 45-65°, preferably 50-60°, e.g. 56°.
- the ⁇ 10-11 ⁇ plane family typically shares an angle of approximately 62° with the horizontal plane, in other words approximately 28° to the vertical (e.g. growth axis of the nano-/microstructures).
- the sidewalls of the lll-V layer therefore typically have an angle of 5-85° with the horizontal (i.e. substrate) plane, preferably 45-75°, preferably 50-70°, preferably 55-65° with the horizontal plane.
- the sidewalls of the lll-V layer therefore typically have an angle of 5-85° with the vertical axis (e.g. polar axis or growth axis), preferably 15-45°, e.g. 20-40° or 25-35° with the vertical axis.
- the exact value will depend on the c/a ratio of the crystal structure (and hence the strain and the Al content, if present).
- the nanostructure or microstructure cores There is preferably a plurality of group lll-V layers on top of the nanostructure or microstructure cores. If there is more than one lll-V layer on top of the cores, then the layers are preferably stacked vertically, i.e. on top of each other along the vertical axis (i.e. growth axis or longest dimension of the nanostructures or microstructures). The stacking could be considered to be vertical stacking, or pyramidical stacking, therefore. These terms are considered to be equivalent.
- the nanostructures or microstructures are preferably axially heterostructured. In a particular embodiment, the nano-/microstructures are not radially heterostructured.
- compositions of the invention typically comprise at least one optional intrinsic lll-V semiconducting layer, and at least one n-type doped lll-V layer if the nanostructure or microstructure cores are p-type doped, or at least one p-type doped lll-V layer if the nanostructure or microstructure cores are n-type doped.
- the nanostructures or microstructures of the invention can contain a p-n, n- p, n-i-p, or p-i-n junction, e.g. to enable their use in LEDs.
- the nanostructure or microstructure cores typically have the same doping type as the substrate (e.g. if the substrate is n-type doped, then the nanostructure or microstructure core will be n-type, and vice versa).
- the junction can be in the form of additional layers on the cores.
- Nanostructures or microstructures of the invention are therefore optionally provided with an undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region.
- the intrinsic region may consist of a single layer of material or a heterostructure consisting of multiple quantum wells and barriers.
- the intrinsic layer is a multiple quantum well.
- the intrinsic layer is positioned directly on the core.
- the nanostructures or microstructures are doped.
- the core is doped as well as at least one additional layer on top of the core.
- Doping typically involves the introduction of impurity ions into the nanowire, e.g. during epitaxial growth.
- the doping level can be controlled from ⁇ 10 15 /cm 3 to 10 22 /cm 3 .
- the same doping characteristics as discussed above for the substrates apply here also.
- the doping level can be controlled from ⁇ 10 15 /cm 3 to 10 22 /cm 3 , preferably 10 16 /cm 3 to 10 21 /cm 3 , preferably 10 17 /cm 3 to 10 2 °/cm 3 , preferably 10 18 /cm 3 to 10 19 /cm 3 .
- the n(p)-type semiconductors have a larger electron (hole) concentration than hole (electron) concentration by doping an intrinsic semiconductor with donor (acceptor) impurities.
- Suitable donor (acceptors) for lll-V compounds can be Te, Sn, Si, Ge and C (Be, Mg and Zn).
- Si can be amphoteric, either donor or acceptor depending on the site where Si goes to, depending on the orientation of the growing surface and the growth conditions. Dopants can be introduced during the growth process or by ion implantation of the nanowires or nanopyramids after their formation.
- the nanostructures/microstructures may comprise additional n, i-n, p-, i-p, n-i-p or p-i-n layers, preferably additional i-p or i-n layers, e.g. positioned on top of the cores.
- the nanostructures/microstructures may comprise an i-layer directly positioned on the core and one or more (e.g. 2-4) p-layers on top of the i- layer (if the core is n-doped) or one or more (e.g. 2-4) n-layers on top of the i-layer if the core is p-doped.
- the nanostructures/microstructures may additionally comprise p-AIGaN, i-AIGaN, and/or n-AIGaN layers for example.
- AI(ln)GaN is also suitable.
- the ‘(In)’ indicates that small amounts of In may optionally be present. Such nomenclature is well known in the art.
- the nanostructures or microstructures comprise or consist of an AIGaN (e.g. n-AIGaN) core, and additionally have at least n-AIGaN, i-AIGaN, and/or p-AIGaN layers thereon, and optionally a GaN (e.g. p-GaN) layer.
- the composition may comprise at least one i-AIGaN layer, at least one p-AIGaN layer (preferably two p-AIGaN layers, optionally differentiated by different doping, different Al percentage, and/or different thickness), and optionally at least one p-GaN layer, preferably in that order.
- AIGa(ln)BN systems are also suitable herein (i.e. AIGa(ln)BN core and one or more AIGa(ln)BN layers).
- Group III options for lll-V layer(s) are B, Al, Ga, In, and Tl. Preferred options here are Ga, Al, B and In, preferably Ga, Al and In. Group V options are N, P, As, Sb. N is preferred. 11 I-N layers are particularly preferred.
- the 11 l-V layer(s) may be a ternary compounds of formula XYZ wherein X is a group III element, Y is a group III different from X, and Z is a group V element.
- the X to Y molar ratio in XYZ is preferably 0.01-0.99, e.g. 0.1 to 0.9, i.e. the formula is preferably X x Yi-xZ where subscript x is 0.01-0.99, e.g. 0.1 to 0.9.
- Quaternary systems might also be used and may e.g. be represented by the formula A x Bi-xC y Di-y where A and B are group III elements and C and D are group V elements or A x B y Ci-x-yD where A, B and C are group III elements and D is a group V element.
- subscripts x and y are typically 0.01-0.99, e.g. 0.1 to 0.9. Other options will be clear to the skilled man.
- a tunnel junction layer may be present, for example.
- a tunnel junction can be composed of a layer stack with multiple components.
- the tunnel junction is formed by two highly doped layers made of lll-N material, one with p-doping and one with n-doping.
- doping levels are in the range of at least 10 19 /cm 3 , e.g. 10 19 /cm 3 - 10 21 /cm 3 , e.g., 10 2 °/cm 3 - 10 21 /cm 3 .
- the tunnel junction can contain an undoped layer made of lll-N material, positioned in between two doped layers.
- the entire tunnel junction layer stack can have dimensions (i.e. thickness) of less than 50nm, e.g. in the range 20-50nm. It is preferred if the active part of the tunnel junction is very thin, e.g. less than 10 nm in thickness to facilitate efficient tunnelling.
- the functionality as ‘active part’ is determined by the combination of doping levels, material molar ratios, and thicknesses of all the layers involved in the tunnel junction layer stack.
- a tunnel junction layer may be different from other layers as a result of a different doping level and/or Al mole ratio.
- an n-type doped region and a p-type doped region are separated by an intrinsic region which acts as a multiple quantum well, and said p-type doped region comprises an electron blocking layer.
- the at least one 11 l-V layer is transparent to the light being emitted by (or absorbed by) the device.
- Suitable structures include the following.
- the nanostructures or microstructures may comprise or consist of, from bottom to top (i.e. starting from the core in contact with the substrate upwards) the following: an n-AIGaN core, an i-AIGaN layer, a p-AIGaN layer (which optionally acts as an electron blocking layer), a different p-AIGaN layer, a p-GaN layer; an n-AIGaN core, an i-AIGaN layer, a p-AIGaN layer (which optionally acts as an electron blocking layer), a different p-AIGaN layer; or an n-AIGaN core, an i-AIGaN layer, a p-AIGaN layer (which optionally acts as an electron blocking layer), a different p-AIGaN layer, an undoped (In, Al, Ga)N interlayer acting as a tunnel junction, an n-AIGaN layer;
- the nano-/microstructures typically have an Al-containing layer.
- the increasing ionization energy of Mg acceptors with increasing Al content in AIGaN alloys makes it difficult to obtain higher hole concentration in AIGaN alloys with higher Al content.
- To obtain higher hole injection efficiency (especially in the cladding/barrier layers consisting of high Al content), the inventors have devised a number of strategies which can be used individually or together.
- SPSL short period superlattice
- electrical conductivity i.e. maximise hole concentration
- a superlattice structure is grown consisting of alternating layers with different Al content instead of a homogeneous AIGaN layer with higher Al composition.
- the low ionization energy of acceptors in layers with lower Al composition leads to improved hole injection efficiency without compromising on the barrier height in the cladding layer. This effect is additionally enhanced by the polarization fields at the interfaces.
- the SPSL can be followed with a highly p-doped GaN:Mg layer for better hole injection.
- the inventors propose to introduce a p-type doped ALGa?. xN/AlyGav.yN short period superlattice (i.e. alternating thin layers of ALGav-xN and AlyGav.yN) into (or onto) the nano-/microstructure structure, where the Al mole fraction x is less than y, instead of a p-type doped Al z Ga7. z N alloy where x ⁇ z ⁇ y. It is appreciated that x could be as low as 0 (i.e. GaN) and y could be as high as 1 (i.e. AIN).
- the superlattice period should preferably be 5 nm or less, such as 2 nm, in which case the superlattice will act as a single Al z Ga7.
- z N alloy (with z being a layer thickness weighted average of x and y) but with a higher electrical conductivity than that of the Al z Ga7.
- z N alloy due to the higher p-type doping efficiency for the lower Al content AlxGa ⁇ -xN layers.
- the p-type dopant is an alkali earth metal such as Mg or Be.
- a nanostructure can be designed containing a gradient of Al content (mole fraction) in the growth direction of the AIGaN within the nano-/microstructures.
- the layers are graded either from GaN to AIN or AIN to GaN.
- the graded region from GaN to AIN and AIN to GaN may lead to n-type and p-type conduction, respectively. This can happen due to the presence of dipoles with different magnitude compared to its neighbouring dipoles.
- the GaN to AIN and AIN to GaN graded regions can be additionally doped with n-type dopant and p-type dopant respectively.
- p-type doping is used in AIGaN nanowires using Be as a dopant.
- one option would be to start with a GaN nano-/microstructure cores and increase Al and decrease Ga content gradually to form AIN, perhaps over a growth thickness of 100 nm.
- This graded region could act as a p- or n-type region, depending on the crystal plane, polarity and whether the Al content is decreasing or increasing in the graded region, respectively.
- the opposite process is effected to produce GaN once more to create an n- or p-type region (opposite to that previously prepared).
- These graded regions could be additionally doped with n-type dopants such as Si and p-type dopants such as Mg or Be to obtain n- or p-type regions with high charge carrier density, respectively.
- the crystal planes and polarity is governed by the type of nano-/microstructure as is known in the art.
- the nano-/microstructures of the invention comprise Al, Ga and N atoms wherein during the growth of the nano- /microstructures the concentration of Al is varied to create an Al concentration gradient within the nanowires or nanopyramids.
- a tunnel junction is a layer stack comprising a barrier, such as a thin layer, between two electrically conducting materials.
- the barrier functions as an ohmic electrical contact in the middle of a semiconductor device.
- a thin electron blocking layer is inserted immediately after the active region, which is followed by a p-type doped AIGaN cladding layer with Al content higher than the Al content used in the active layers.
- the p-type doped cladding layer is followed by a highly p-type doped cladding layer and a very thin tunnel junction barrier layer followed by an n-type doped AIGaN layer.
- the tunnel junction layer stack is chosen such that the electrons tunnel from the valence band in p-AIGaN to the conduction band in the n-AIGaN, creating holes that are injected into the p-AIGaN layer.
- the lll-V layer(s) may be 500nm in thickness or less, e.g. 200 nm in thickness or less, e.g. 100 nm in thickness or less, e.g. 20 nm in thickness or less, e.g. 10 nm or less in thickness.
- the lll-V layer(s) may be 0.5 nm in thickness or more, e.g. 1 nm in thickness or more, e.g. 2 nm in thickness or more.
- Suitable thickness ranges for the lll-V layer include 0.5-500 nm or 1-200 nm.
- Particularly preferred thicknesses for the lll-V layer include 200nm, 80nm, 50nm, 20nm, 10nm, 5nm, 2nm.
- the lll-V layers may be 100 pm in thickness or less, e.g. 20 pm in thickness or less, e.g. 10 pm or less in thickness. Suitable thickness ranges include 0.5-100 pm, e.g. 1-20 pm.
- edges of the at least one lll-V layer(s) preferably form a vertical side wall with the nanostructure/microstructure core. If there are multiple group lll-V layers on the cores, preferably the edges of all of these form a vertical side wall with the nanostructure/microstructure core. Alternatively put, the edges of the at least one group lll-V layers are vertically aligned with each other, and preferably also with the sides of the cores. This has benefits in terms of light emission/absorption, and/or in terms of light guiding. If one of the lll-V layers absorbs light at a wavelength of interest, then such a construction has benefits since absorption is minimized, especially compared to core-shell structures.
- the composition comprises a top GaN (e.g. p-GaN) layer, which absorbs UV light
- a top GaN e.g. p-GaN
- having the edges of the at least one group lll-V layers (including the GaN layer) vertically aligned with each other ensures that UV absorption is minimized, since there are no wrap-around shell layers, for example, which would absorb UV light and reduce the efficiency of the device.
- a core-shell structure which may have semipolar as well as vertical parts, undesirably high levels of absorption can occur.
- the cores have pyramidal tips, and any additional p/i/n layers mirror the topography (i.e. shape) of the underlying cores.
- the additional layers can either be limited to the width of the nanostructures/microstructures, or they may be in the form of a layer continuously covering at least a portion of the plurality of nanostructures/microstructures.
- at least one of the lll-V layers continuously covers at least a portion of the nanostructures/microstructures (i.e. of the cores, with potential intermediate layers between the core and the top continuous layer).
- the layer that continuously covers at least a portion of the nanostructures/microstructures may be continuous in its upper region, but may have voids in its lower region.
- the top layer e.g. the top AIGaN or GaN layer
- the doped top layer e.g. n- layer
- the doped top layer can act as a transparent current spreader with an acceptable sheet resistance without the continuous layer being too thick.
- Layers beneath the top layer e.g. the i-layer and below
- an electrical contact layer i.e. metallic contact layer
- an electrical contact layer may continuously extend over the nanostructures or microstructures.
- such a top layer will typically be ridged/corrugated, i.e. comprising a plurality of protrusions, wherein the protrusions (e.g. pyramidal protrusions) are positioned on top of the nanostructure/microstructure tips
- nanostructures or microstructures may be that as numerous nanostructures or microstructures grow from the substrate that the nanostructures or microstructures coalesce at a certain distance from the substrate, or directly on top of the mask layer. It can be beneficial to form large area structures through coalescence of positioned nanostructures or microstructures, or through coalescence of one of the top layers (e.g. p-AIGaN, p- GaN etc.). The nanostructures or microstructures may be coalesced, therefore. The coalescence of nanowires may appear almost film like (e.g. like a corrugated film if the nanostructures or microstructures have pyramidal tips).
- Coalescence refers to the side-on joining of two or more nanostructures during growth. This results in a 2D or 3D structure.
- the nanostructures or microstructures must preferably have their crystal lattices in the same orientation, such that the formation of gaps and dislocations can largely be eliminated, i.e. the coalescing nanostructures or microstructures or lll-V layers thereof must preferably have nearly identical epitaxial relationship with respect to the substrate.
- the nano-/microstructures may be coalesced with amorphous, polycrystalline or a different crystalline material in between them.
- the region between the nanostructures and microstructures may be amorphous lll-V material or consist of multiple lll-V grains that are not epitaxially related to the nano- /microstructures.
- the regions between the nanostructures may be lll-V material (e.g. the same as used for nanostructure or microstructure growth), air, vacuum, or a dielectric material.
- Any lll-V material between the nanostructures/microstructures may be amorphous, polycrystalline or crystalline. The coalescence may occur via the cores, the at least one lll-V layer, or both.
- a completely coalesced structure i.e. a composition of matter comprising: a substrate; a non-planar (e.g. corrugated or ridged) coalesced structure on said substrate, e.g.
- non-planar coalesced structure formed from a plurality of coalesced nanostructures or microstructures, said non-planar coalesced structure and/or nanostructures or microstructures comprising at least one semiconducting group lll-V compound; wherein said non-planar coalesced structure comprises at least one lll-V layer, typically positioned on top of coalesced nanostructure cores or microstructure cores, wherein at least part of said at least one lll-V layer is semipolar; and preferably wherein said at least one lll-V layer extends continuously over the plurality of coalesced nanostructure cores or microstructure cores.
- the coalesced structure typically comprises a plurality of protrusions, wherein the protrusions (e.g. pyramidal protrusions) are tips of the nanostructures/microstructures.
- the non-planar/corrugated structure is beneficial as it can enable good light extraction for transverse-magnetic (TM) polarisation (see Figure 4).
- manufacturers of nanostructure devices etch corrugated/ridged designs to improve light extraction.
- pyramidal tipped nano- /microstructures are used, the corrugation is obtained without having to perform any subsequent etching step.
- pyramidal tipped nano-/microstructure shape benefits in terms of ease of manufacture and light extraction (or absorption) are obtained, therefore.
- any discussion of nano- /microstructures or lll-V layers for separate nano-/microstructures also applied to structures in which the nano-/microstructures or additional layers are coalesced.
- Any of the definitions for compositions comprising nano-/microstructures are applicable here, where technically viable.
- the materials for the nano-/microstructures are applicable to the corrugated films.
- the corrugated films may comprise the same layers that the nano-/microstructures comprise.
- the top layer (e.g. the uppermost 11 l-V layer) can act as a top-emitting transparent electrode.
- transparent is hereby meant transparent to the light emitted by the nano-/microstructures, e.g. in the case of a UV-C LED, then the layer is transparent at least to IIV-C light.
- photodetectors by transparent is meant transparent to any incoming light. E.g. if the composition/device is a solar cell, then transparent means transparent at least to solar light.
- compositions or devices of the invention may comprise on the substrate a mask layer through which the nanostructures or microstructures are grown.
- Such a mask layer serves several purposes. It enables the positioning (i.e. SAG) of the nano-/microstructures.
- the holes in the mask also allow the nanostructure or microstructure material to be epitaxial with the substrate.
- the mask layer may be a dielectric material, such as silicon dioxide or silicon nitride, or graphene. In a particular embodiment, the mask is not graphitic/graphene.
- the mask may also be multi-layer, i.e. have two or more different materials stacked on top of each other, e.g. graphene and SiC>2.
- graphene refers to a planar sheet of sp 2 -bonded carbon atoms that are densely packed in a honeycomb (hexagonal) crystal lattice.
- graphene also means structures having a small number of graphene sheets.
- the interplanar spacing in graphene is 0.335 nm.
- the graphene layer is well known for its superior optical, electrical, thermal and mechanical properties. Graphene is very thin but very strong, light, flexible, and impermeable.
- the mask layer in general is 200 nm or less, e.g. 100 nm in thickness or less, e.g. 50 nm or less.
- the mask layer may be 1 nm in thickness or more, e.g. 5 nm in thickness or more, e.g. 10 nm in thickness or more.
- Preferred thickness ranges for the mask layer include 1-200 nm, preferably 5-100 nm, preferably 10-50 nm.
- Some materials such as silicon oxide, are relatively transparent to UVC and so a wide range of thicknesses can be used (for UVC applications).
- Other materials such as silicon nitride have a lower band gap, and are hence less transparent. In these instances, the mask thickness should preferably be kept low.
- the mask layer in general is 200 pm or less, e.g. 100 pm in thickness or less, e.g. 50 pm or less.
- the mask layer may be 1 pm in thickness or more, e.g. 5 pm in thickness or more, e.g. 10 pm in thickness or more.
- Preferred thickness ranges for the mask layer include 1-200 pm, preferably 5-100 pm, preferably 10-50 pm.
- the area of the mask layer in general is not limited. This might be as much as 0.5 mm 2 or more, e.g. up to 5 mm 2 or more such as up to 10 cm 2 .
- the wafers i.e. substrate + mask
- the wafers typically have a diameter or maximum width of 1-20 inches, e.g. 2 inch, or 4 inch, or 6 inch, or 8 inch, or 12 inch.
- the area of the mask layer is thus only limited by practicalities.
- the diameter of the holes is preferably up to 500 nm, such as up to 100 nm, ideally up to 20 to 200 nm.
- the diameter of the holes is preferably up to 500 pm, such as up to 100 pm, ideally 20 to 200 pm.
- the diameter of the hole sets a maximum diameter/width for the size of the nanostructures/microstructures during initial growth.
- nanostructure/microstructure diameter larger than the hole size can be achieved by changing the growth parameters or by adopting a core-shell nanostructure or microstructure geometry.
- the nanostructures or microstructures extend laterally (i.e. radially) over any mask outside of the holes.
- the number of holes is a function of the area of the mask layer and desired nanostructure or microstructure density.
- the shape of the holes is not limited. Whilst these may be circular, holes may also be in other shapes, such as triangular, rectangles, oval etc.
- the mask layer may be in electrical contact with the base of the nanostructures or microstructures, which is useful in the case of the mask layer acting as an electrode. As the nanostructures or microstructures begin growing within a hole, this tends to ensure that the initial growth of the nanostructures or microstructures is epitaxial and substantially perpendicular to the substrate. This is a further preferred feature of the invention.
- One nanostructure or microstructure preferably grows per hole.
- the mask layer may be doped to improve its electrical conductivity. This may be beneficial if the mask layer acts as an electrode (e.g. as a gate for a vertical transistor).
- the nano-/microstructures extend laterally over the mask outside of the holes, there is good electrical contact between the nano-/microstructures and the substrate below.
- the nano-/microstructures typically nucleate in the holes of the patterned mask on the substrate, and then grow laterally, i.e. radially, over the top surface of the mask layer (i.e. they ‘mushroom’ out of the holes).
- the mask layer is not an electrode, or does not act as an electrode. In a particular embodiment, there is no electrical contact on the mask layer. For certain applications, e.g. transistors, it may be beneficial for the mask layer to act as an electrode. In a particular embodiment, therefore, the mask layer acts (or is) an electrode.
- the mask layer is typically positioned directly on the substrate.
- the composition does not comprise a holed mask layer on top of the substrate. Nucleation sites can be provided by etching holes in the substrate, for example.
- Semiconductor nanostructures or microstructures have wide ranging utility. They are semiconductors so can be expected to offer applications in any field where semiconductor technology is useful. They are primarily of use in integrated nanoelectronics and nano-optoelectronic applications.
- An ideal device for their deployment might be a solar cell, transistor, laser LED or photodetector.
- the semiconductor nanostructures or microstructures have utility in LEDs, in particular UV LEDs and especially UV-A, UV-B, or UV-C LEDs, more preferably UV-C LEDs.
- the invention therefore provides a device, such as an electronic or opto-electronic device, comprising a composition as defined herein, e.g. a solar cell, photodetector, transistor, laser, or LED, preferably an LED, more preferably a UV LED, more preferably a UV-C LED, comprising a composition as defined herein.
- UV (preferably UVC) devices are typically preferred, e.g. UV LED, UV photodetector, UV laser.
- the composition of matter is an electronic or optoelectronic device, preferably a solar cell, transistor, laser, photodetector or LED, preferably a LED, preferably a UV LED, preferably a UVC LED.
- the present devices (whether LED or other) emit or absorb light in the UV region, preferably UV-C region.
- the present devices emit or absorb light with a wavelength of 280nm or less, e.g. in the range 200-280 nm.
- each individual nanostructure or microstructure can be seen as an individual LED nanostructure or microstructure (or individual photodetector/solar cell etc.).
- the nanostructures or microstructures comprise a light generating (or light absorbing) region.
- the top of the nanostructures or microstructures preferably comprise a top contact.
- a conventional top contact is positioned on the top layer of the nanostructures or microstructures, e.g. on a top AIGaN or GaN layer (e.g. p-AIGaN or p-GaN) (this top layer may extend continuously over the underlying layers and nanostructure/microstructure cores, as previously discussed).
- This top contact may have a finger design in order to reduce the amount of the contact that blocks light from exiting or entering the device (e.g.
- the area covered by the contact is typically 50% or less, preferably 20% or less of the top surface area of the nanostructures or microstructures), or may be in the form of a continuous layer covering a plurality (e.g. at least 50%, at least 90% or substantially all) of nanostructures or microstructures.
- a conventional top contact metal layer stack can be used.
- An electrode may alternatively or additionally be positioned on the bottom region of the nano- /microstructures (e.g. bottom 20% of the length of the structures).
- the contacts described herein are typically metallic. They are chosen to have an ohmic behaviour with the top layer.
- the contacts/contact pads can be electrically connected to the appropriate power supply lead of the device package.
- the compositions/devices of the invention may further comprise electrical contacts, preferably wherein one electrical contact is a layer positioned on top of the nanostructures or microstructures, and/or preferably wherein one electrical contact is positioned on said substrate.
- the composition/device is configured or arranged to emit/absorb light in said direction.
- light may be emitted from (or absorbed by) said device omnidirectionally (i.e. in all directions, i.e. through the top of the device, through the substrate and through the side).
- the light may also be emitted (or absorbed) in a direction substantially opposite to a light reflective layer positioned on top of the nanostructures or microstructures.
- the light may therefore be emitted (or absorbed) through the substrate.
- the light may also be emitted sideways from the nanostructures or microstructures (or absorbed sideways into the nanostructures or microstructures). Any combination of the above may be contemplated and will depend on the choice of substrate and top layers in particular.
- the device would not typically be a flip chip device, or is not in a flip chip configuration.
- the nanostructures/microstructures directing all light back towards (and through) the substrate.
- the nanostructures/microstructures do not comprise a reflective layer continuously covering the top of the nanostructures/microstructures.
- GaN is often used as a lll-V layer, but it has limited transparency. If none of the at least one lll-V layers are GaN, then omnidirectional light emission (or absorption for photodetectors) is typically preferred. In other words, for omnidirectional light emission/detection, GaN as a lll-V layer material is typically avoided.
- Figure 4 shows a TM mode simulation result of a composition according to the present invention.
- the TM light is emitted substantially parallel to the intrinsic (emitting) layer, or parallel to the sidewalls of the at least one lll-V layer.
- the particular geometry of the nano-/microstructures with the semipolar (and/or pyramidal) layers provides efficient light emission from the device in a number of directions.
- the at least one lll-V layer may be configured to emit (or absorb) TM polarized light via the side and/or bottom of the device, preferably via: a) emission (or absorption) of TM polarized light substantially parallel to the lll-V layer and in a direction substantially towards the substrate-side of the device (or from the substrate side of the device for a light absorbing device); and/or b) emission (or absorption) of TM polarized light substantially parallel to the lll-V layer and in (or from) a direction substantially opposite to the substrate-side of the device, wherein said light reflects off a reflective layer positioned on top of said nanostructures or microstructures.
- Substantially parallel typically means herein that at least 50%, at least 75% at least 90% or at least 99% of the light is emitted (or absorbed) within ⁇ 10° of a plane of the lll-V layer.
- TM light emitted from (or absorbed by) the device can be significantly stronger than the TE light.
- the advantage of having increased TM light emission is that one can get more total light out of the device if the light generated in the active area has a higher degree of TM-polarization. This is a typical effect when increasing the Al- content of an active Al x Gai- x N layer to reduce the wavelength of the emitted (or absorbed) light.
- Another advantage with TM over TE polarized light is that there exists a so-called Brewster angle for TM polarized light where there is no reflection, hence potentially increasing the light output.
- the light emitted from (or absorbed by) the device comprises at least 50% TM light, e.g. at least 75% TM light, e.g. at least 90% TM light, e.g. at least 95% TM light, e.g. at least 99% TM light.
- the upper limit could be 100% or 99% TM light for example.
- a particular advantage of the present compositions is the ability to emit (or absorb) more TM light than what is possible for a thin film.
- the present disclosure also provides method of emitting light from a device as defined herein (or absorbing light by a device as defined herein), said method comprising emitting (or absorbing) transverse magnetic (TM) polarized light via the side and/or bottom of the device, preferably via: a) emission (or absorption) of TM polarized light parallel to the lll-V layer and in a direction substantially towards the substrate-side of the device; and/or b) emission (or absorption) of TM polarized light parallel to the lll-V and in a direction substantially opposite to the substrate-side of the device, wherein said light reflects off a reflective layer positioned on top of said nanostructures or microstructures
- the present disclosure also provides the use of group lll-V semipolar layers for increasing the transverse magnetic (TM) light emission from LEDs comprising nanostructures and/or microstructures on a substrate.
- TM transverse magnetic
- the doped substrate can act as an active injector of current.
- the conductive pathway is typically vertical (up or down), i.e. in the same axial direction as the nano-/microstructures. If the substrate is p-doped, the conductive pathway of the holes (which is the same as the direction of the current) is from bottom to top. For n-doped substrate the conductive pathway of the electrons is from bottom to top (NB: the direction of the current is defined as opposite to the direction of the electrons).
- the positioned nanostructures or microstructures grow initially, or nucleate, from the substrate. If a mask layer is used, that means that holes need to be patterned through the mask layer. Making of these holes is a well-known process and can be carried out using e-beam lithography and etching or any other known techniques.
- the hole patterns in a mask can be easily fabricated using conventional lithography techniques such as photo/e-beam lithography, nanoimprinting, and so on. Focussed ion beam technology may also be used in order to create a regular array of nucleation sites on the substrate surface for the nanostructures or microstructures growth.
- the holes created in any mask layer can be arranged in any pattern which is desired.
- the positioning of the nanostructures or microstructures can be controlled by growing them from holes, defects or hillocks in the substrate. These can be produced by selective etching away of the substrate (e.g. using reactive ion etching or chemical wet etching). Direct hole patterning can be performed on any substrate (e.g. directly on AIGaN or AIN substrate).
- MBE Molecular beam epitaxy
- Metalorganic vapour phase epitaxy also called as metalorganic chemical vapour deposition (MOCVD) is an alternative method to MBE for forming depositions on crystalline substrates.
- MOVPE/MOCVD Metalorganic vapour phase epitaxy
- the deposition material is supplied in the form of metalorganic precursors, which on reaching the high temperature substrate decomposes leaving atoms on the substrate surface.
- this method requires a carrier gas (typically H2 and/or N2) to transport deposition materials (atoms/molecules) across the substrate surface. These atoms reacting with other atoms form an epitaxial layer on the substrate surface. Choosing the deposition parameters carefully results in the formation of a nanowire.
- the nanostructures or microstructures of the invention may be grown by selective area growth (SAG) method.
- SAG selective area growth
- the substrate temperature can be set to a temperature suitable for the growth of the nanostructures or microstructures in question.
- the growth temperature may be in the range 300 to 1000 °C.
- the temperature employed is, however, specific to the nature of the material in the nanowire.
- a preferred temperature is 700 to 950 °C, e.g. 800 to 900 °C, such as 810 °C.
- AIGaN the range is slightly higher, for example 800 to 980 °C, such as 830 to 950 °C, e.g. 850 °C.
- the nanostructure or microstructures are formed by MOCVD.
- the MOCVD is typically used for growing the structure cores, as well as any additional lll-V layers on top of the cores.
- the nanostructures or microstructures can comprise different group lll-V semiconductors within the nanostructure or microstructure, e.g. starting with an AIGaN core followed by an AIGaN layer component and so on.
- Nanowire MBE growth can be initiated by opening the shutter of the Ga effusion cell, the nitrogen plasma cell, and the dopant cell simultaneously initiating the growth of doped GaN nanostructures or microstructures, hereby called as stem.
- the length of the GaN stem can be kept between 10 nm to several 100s of nanometers. Subsequently, one could increase the substrate temperature if needed, and open the Al shutter to initiate the growth of AIGaN nanostructures or microstructures.
- n- and p- doped nanostructures or microstructures can be obtained by opening the shutter of the n-dopant cell and p-dopant cell, respectively, during the growth of the nanostructures or microstructures.
- Si dopant cell for n-doping of nanostructures or microstructures and Mg dopant cell for p-doping of nanostructures or microstructures.
- the temperature of the effusion cells can be used to control growth rate.
- Convenient growth rates, as measured during conventional planar (layer by layer) growth, are 0.05 to 2 pm per hour, e.g. 0.1 pm per hour.
- the ratio of Al/Ga can be varied by changing the temperature of the effusion cells.
- the pressure of the molecular beams can also be adjusted depending on the nature of the nanostructures or microstructures being grown. Suitable levels for beam equivalent pressures are between 1 x 10' 7 and 1 x 10' 4 Torr.
- the beam flux ratio between reactants can be varied, the preferred flux ratio being dependent on other growth parameters and on the nature of the nano-/microstructures being grown.
- reactants e.g. group III atoms and group V molecules
- the preferred flux ratio being dependent on other growth parameters and on the nature of the nano-/microstructures being grown.
- nitrides which is preferred, nanostructures or microstructures are always grown under nitrogen rich conditions.
- a multistep, such as two step, growth procedure is employed, e.g. to separately optimize the nucleation and growth.
- nanostructures or microstructures can be grown at a much faster growth rate.
- This method allows the growth of axial heterostructured nanostructures or microstructures using techniques such as pulsed growth technique or continuous growth mode with modified growth parameters for e.g., lower V/lll molar ratio and higher substrate temperature.
- the reactor must be evacuated after placing the sample, and is purged with N2 to remove oxygen and water in the reactor. This is to avoid any damage to any substrate or mask layer, if present, at the growth temperatures, and to avoid unwanted reactions of oxygen and water with the precursors.
- the total pressure is set to be between 20 and 400 Torr.
- the substrate After purging the reactor with N2, the substrate is thermally cleaned under H2 atmosphere at a substrate temperature of about 1200 °C.
- a substrate conditioning step can be performed to make the substrate surface more favourable for the subsequent material growth (e.g. AIGaN).
- the conditioning step one group of precursors is switched on while the other group of precursors remains switched off so that no material growth starts.
- Mentioned groups of precursors are either group-ill metalorganics (e.g. TMAI, TMGa, TMIn) or group-V hydrides (e.g. NH3).
- group-V hydrides are switched on in the conditioning step, more preferably NH3.
- the substrate temperature can then be set to a temperature suitable for the growth of the nanostructures or microstructures in question.
- the growth temperature may be in the range 700 to 1250°C.
- the temperature employed is, however, specific to the nature of the material in the nanostructures or microstructures.
- a preferred temperature is 800 to 1150°C, e.g. 900 to 1100 °C, such as 1100°C or 1000°C.
- the range is slightly higher, for example 900 to 1250°C, such as 1050 to 1250°C, e.g. 1250°C or 1150°C.
- the metal organic precursors for the nano-/microstructures growth can be either trimethylgallium (TMGa), or triethylgallium (TEGa) for Ga, trimethylalumnium (TMAI) or triethylalumnium (TEAI) for Al, and trimethylindium (TMIn) or triethylindium (TEIn) for In.
- the precursors for dopants can be SiF for silicon and bis(cyclopentadienyl)magnesium (Cp2Mg) or bis(methylcyclopentadienyl)magnesium ((MeCp)2Mg) for Mg.
- the flow rate of TMGa, TMAI and TMIn can be maintained between 5 and 2000 seem.
- the NH3 flow rate can be varied between 10 and 20000 seem.
- Growth rate is a result of different reactor parameters, mainly precursor flows and temperature.
- the growth rate can be intentionally adjusted and used as a tool to tune not only the nanostructures or microstructures dimensions, but also shape, defects, or electrical properties. Growth rates can range from 0.05 pm/h to 5 pm/h, typically 100 nm/h for thinner layers and 2 pm/h for thicker layers.
- the pressure in the MOCVD reactor is given by the carrier gas and can be individually adjusted for each growth step.
- the pressure will influence the gas flow pattern and can be used to achieve good uniformity of nanostructures or microstructures over a large substrate area of one or multiple substrates.
- Reactor pressures can range from 20 Torr to 400 Torr, e.g., 100 Torr.
- the V/l 11 ratio is given by the partial pressure ratio of group-V over group-ill precursors and determined by the precursor flows.
- the V/l 11 ratio can influence the growth rate, material molar ratio, morphology, or electrical properties.
- the skilled worker utilizes the V/lll ratio to tune the growth process to the desired outcome.
- the V/lll ratio can range from 5 to 10000, preferably from 50 to 2000, e.g. 100 or 1000.
- vapour-solid growth may enable nanostructures or microstructures growth.
- simple application of the reactants, e.g. In and N, to the substrate without any catalyst can result in the formation of nanostructures or microstructures.
- This forms a further aspect of the invention which therefore provides the direct growth of a semiconductor nanostructure or microstructure formed from the elements described above on a substrate.
- the term direct implies therefore the absence of a film of catalyst to enable growth.
- the nanostructures or microstructures of the invention may also be grown in the presence of a catalyst.
- a catalyst can be introduced into holes (of the mask or of the substrate) to provide nucleating sites for nanostructure or microstructure growth.
- the catalyst can be one of the elements making up the nanostructure or microstructure so-called self-catalysed, or different from any of the elements making up the nanowire (e.g. Au or Ag).
- Nanowire growth can be initiated by opening the shutter of the group III effusion cell and the counter ion effusion cell, simultaneously once a catalyst film has been deposited and melted.
- the temperature of the effusion cells can be used to control growth rate.
- Convenient growth rates, as measured during conventional planar (layer by layer) growth, are 0.05 to 2 pm per hour, e.g. 0.1 pm per hour.
- the pressure of the molecular beams can also be adjusted depending on the nature of the nanostructures or microstructures being grown. Suitable levels for beam equivalent pressures are between 1 x 10' 7 and 1 x 10' 5 Torr.
- the beam flux ratio between reactants can be varied, the preferred flux ratio being dependent on other growth parameters and on the nature of the nano-/microstructures being grown. It is known that the beam flux ratio between reactants can affect crystal structure of the nanostructures or microstructures.
- Nanostructure or microstructure diameter can in some cases be varied by changing the growth parameters, e.g. the flux ratio of the 11 l-V precursors.
- the skilled worker is therefore able to manipulate the nanostructures or microstructures in a number of ways.
- the size of the holes can be controlled to ensure that only one nanostructure or microstructure can grow in each hole (i.e. holes in a mask or holes in a substrate). It is therefore preferred if only one nanostructure or microstructure grows per hole in the mask.
- the holes can be made of a size where the droplet of any catalyst that forms within the hole is sufficiently large to allow nanostructure or microstructure growth. In this way, a regular array of nanostructures or microstructures can be grown.
- Figure 1 shows a first composition according to the invention.
- Figure 2 shows a plurality of nano-/microstructures as shown in Figure 1 , in which the nano-/microstructures are joined together by regions of amorphous/polycrystalline II l-V material.
- Figure 3 shows emission of TM light from the intrinsic layer of the composition of Figure 2 (in the case of a light emitter)
- Figure 4 is a simulation of TM emitted light from the composition of Figure 2.
- Figure 5 is a design variation of Figure 1 , in which there is no p-GaN layer on the p- side.
- the p-contact (or reflector) is on the top p-AIGaN layer.
- the absence of p- GaN increases emission of light via the top of the device (in a top-emitting LED for example), or via the sides/bottom for bottom or omnidirectional light extraction.
- the absence of p-GaN increases absorption of light via these pathways.
- Figure 6 shows a design variation of Figure 1, in which there is a tunnel junction comprising an (un)doped (In, Al, Ga)N interlayer.
- the tunnel junction is beneficial for electrical and optical properties. It enables efficient current injection by highly conductive n-AIGaN as well as reduced absorption by avoiding p-GaN.
- Figure 7 shows a design variation of Figure 1, in which the substrate comprises a top graphene layer.
- the high electrical conductivity of graphene can enable parallel current spreading in addition to the n-AIGaN.
- the high thermal conductivity of graphene can enable better heat dissipation, thereby increasing the internal quantum efficiency and the total wall-plug efficiency (WPE).
- Figure 8 shows a design variation of Figure 1, in which the mask layer is a multilayer mask comprising SiC>2 and a graphene layer.
- Figure 9 shows a design variation of Figure 2, in which patterns of holes are created on the substrate directly, which are used for the selective growth of the nano-/microstructures. This can increase light extraction efficiency by removing reflections or interference at the mask layer. The refractive index remains high.
- Figure 10 shows a design variation of Figure 2, in which a sacrificial layer is positioned within the substrate. The whole top heterostructure can be lifted off from the sapphire using chemical, electrochemical or laser lift off techniques. The sacrificial layer could be GaN or highly doped n-AIGaN, for example. An advantage is that the remaining structure with the thinner substrate may have higher light extraction efficiency, and the thin structures may be suitable for flexible devices.
- the sacrificial layer is typically selectively destroyed/removed, leaving the top and bottom parts separated.
- the sacrificial layer could also be positioned between AIN and n-AIGaN, leaving the bottom (AIN+sapphire) and the top (nAIGaN+nanowires) separated.
- Figure 11a-c shows SEM images at various stages prior to nanostructure growth.
- Figure 12a shows a TEM cross-section image of the full nanostructure with SiO2 mask on a n-AIGaN/AIN/sapphire substrate.
- Figure 12b shows the electroluminescence spectra of the nanostructure LEDs at different current levels.
- Figure 13 shows emission spectra (Fig. 13a) and LI characteristics (Fig. 13b) of semipolar versus polar UV LEDs fabricated on different substrates. This comparison illustrates the advantages of semipolar UVC LEDs.
- the 2D-simulations were done in a wave optics approach in the COMSOL Multiphysics software suite.
- the optical material properties (refractive index and extinction coefficient) were assigned to the different areas as described in the text.
- the model was excited by a point source, emitting TM-polarized light in one of the nanowires.
- Figure 4 shows the absolute value of the Pointing’s vector in dB, which is equal to the intensity, for one specific position of the point source. To get the full intensity output, one would need to integrate/sum this contribution for point sources over all positions in the active region.
- 2 nd order scattering conditions were used to reduce the amount of light reflected back into the model.
- Fig. 12a shows a cross-section TEM (transmission electron microscopy) image of an epitaxial nanowire structure which enables a functional UVC LED with electro-optical performance data shown in Fig. 12b.
- the substrate consists of a 1-pm-thick n-type doped AIGaN layer grown on AIN on 2- inch sapphire wafer.
- the substrate surface was patterned using a 35-nm-thin SiO2 mask with hexagonal hole pattern, fabricated by oxide coating and subsequent resist deposition, lithographic exposure, and etching followed by standard cleaning procedures.
- the AIGaN-based LED heterostructure was grown by MOCVD under nitrogen-rich conditions.
- the n-AIGaN buffer pyramid was nucleated at a temperature of > 1000 degree Celsius and extended until full development of the semipolar [10-11] pyramid facets near coalescence.
- Manifold functional AIGaN layers were grown subsequentially with specified composition and doping. These include Si-doped layers in the n-side and Mg-doped layers in the p-side, as well as quantum well layers with low Al-content (-50%), n-AIGaN layers with intermediate Al-content (-60%), and electron blocking layers with high Al-content (-80%).
- the last epitaxial layer in the p-side is a Mg-doped GaN layer. Preliminary emission spectra and intensity data were collected in a quick test electroluminescence setup with non-optimized metal contacts as shown in Fig. 12b.
- Figure 13 shows a comparison of emission spectra (Fig. 13a) and LI (light intensity vs. current) characteristics (Fig. 13b) from UV LEDs fabricated with semipolar versus polar structure.
- the identical layer stack was grown on two kinds of substrate: On patterned n-AIGaN as semipolar NW and on unpatterned n- AIGaN as polar heterostructure film.
- the electro-optical characteristics of both LEDs were determined in a quick test electroluminescence setup. Both integrated intensity and spectral intensity are superior for the semipolar LED in this comparison.
- the emission wavelength of the polar LED is red-shifted towards 300 nm, whereas the semipolar UVC LED shows the desired emission near 260 nm.
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Abstract
A composition of matter comprising a substrate; a plurality of nanostructures or microstructures grown from said substrate, wherein said nanostructures or microstructures comprise: nanostructure cores or microstructure cores respectively, comprising at least one semiconducting group lll-V compound, at least one lll-V layer positioned on top of said nanostructure cores or microstructure cores, wherein at least part of said at least one lll-V layer is semipolar; and wherein said nanostructures or microstructures comprise a p-n or p-i-n junction.
Description
Nanostructure/Microstructure device
This invention concerns compositions comprising nanostructures or microstructures grown on a substrate. The nanostructures or microstructures comprise at least one lll-V layer positioned on top of the nanostructure cores or microstructure cores, and at least part of said at least one lll-V layer is semipolar. The compositions of matter that are formed can be used in an electronic device such as an LED, solar cell, transistor, laser or photodetector.
Background
Over recent years, the interest in semiconductor nanowires has intensified as nanotechnology becomes an important engineering discipline. Nanowires, which are also referred to as nanowhiskers, nanorods, nanopillars, nanocolumns, etc. by some authors, have found important applications in a variety of electrical devices such as sensors, solar cells, transistors and LED's.
The nanowires in such devices can act as light emitters (e.g. in the case of LEDs), or as light absorbers (e.g. in the case of photodetectors). UV LED devices in particular have found promising applications in antibacterial and antiviral applications.
There is a constant drive to improve light extraction for light emitters and to increase light absorption for light absorbers.
The present inventors have surprisingly found that by positioning semipolar layers on top of a nanostructure/microstructure core, beneficial light extraction or absorption is obtained.
Summary of invention
Thus viewed from one aspect there is provided a composition of matter comprising: a substrate; a plurality of nanostructures or microstructures grown from said substrate, wherein said nanostructures or microstructures comprise: nanostructure cores or microstructure cores respectively, comprising at least one semiconducting group lll-V compound,
at least one lll-V layer positioned on top of said nanostructure cores or microstructure cores, wherein at least part of said at least one lll-V layer is semipolar; and wherein said nanostructures or microstructures comprise a p-n or p-i-n junction.
Viewed from a further aspect, there is provided a composition of matter comprising a substrate; a plurality of nanostructures or microstructures grown from said substrate, wherein said nanostructures or microstructures comprise: nanostructure cores or microstructure cores respectively, comprising at least one semiconducting group lll-V compound, at least one lll-V layer positioned on top of said nanostructure cores or microstructure cores, wherein at least part of said at least one lll-V layer is a pyramidal layer; and wherein said nanostructures or microstructures comprise a p-n or p-i-n junction.
Viewed from another aspect, there is provided a composition of matter comprising: a substrate; a non-planar coalesced structure on said substrate, e.g. formed from a plurality of coalesced nanostructures or microstructures, said non-planar coalesced structure and/or nanostructures or microstructures comprising at least one semiconducting group lll-V compound; wherein said non-planar coalesced structure comprises at least one lll-V layer, typically positioned on top of coalesced nanostructure cores or microstructure cores, wherein at least part of said at least one lll-V layer is semipolar; and preferably wherein said at least one lll-V layer extends continuously over the plurality of coalesced nanostructure cores or microstructure cores.
Viewed from another aspect, there is provided a device, such as an electronic or opto-electronic device, comprising a composition of matter as defined herein, e.g. wherein said device is a solar cell, photodetector, transistor, laser or LED, preferably an LED, more preferably a UV LED, more preferably a UV-C LED.
Viewed from another aspect, there is provided a process for preparing a composition of matter as defined herein, comprising:
(I) growing a plurality of nanostructure cores or microstructures cores from a substrate, e.g. via MOCVD, said nanostructure cores or microstructures cores comprising at least one semiconducting group 11 l-V compound;
(II) growing said least one 11 l-V layer on top of said nanostructure cores or microstructures cores.
Viewed from another aspect, there is provided a process for preparing a composition of matter as defined herein, comprising:
(I) growing a plurality of nanostructure cores or microstructures cores from a substrate, e.g. via MOCVD, said nanostructure cores or microstructures cores comprising at least one semiconducting group 11 l-V compound;
(II) growing said least one 11 l-V player on top of said nanostructure cores or microstructures cores; to the point that the nanostructure cores or microstructures cores are coalesced, and/or to the point that the at least one 11 l-V layer extends continuously over the plurality of nanostructure cores or microstructure cores
Viewed from another aspect, there is provided a method of emitting light from a device (or absorbing light by a device) as defined herein, said method comprising emitting (or absorbing) transverse magnetic (TM) polarized light via the side and/or bottom of the device, preferably via: a) emission (or absorption) of TM polarized light substantially parallel to the II l-V layer and in a direction substantially towards the substrate-side of the device (or from the substrate side of the device for a light absorbing device) and/or b) emission (or absorption) of TM polarized light substantially parallel to the II l-V and in (or from) a direction substantially opposite to the substrate-side of the device, wherein said light reflects off a reflective layer positioned on top of said nanostructures or microstructures
Viewed from another aspect, there is provided a use of group 11 l-V pyramidal semipolar layers for increasing the transverse magnetic (TM) light emission from LEDs comprising nanostructures and/or microstructures on a substrate.
The features of the aspects and/or embodiments indicated herein are usable individually and in combination in all aspects and embodiments of the invention where technically viable, unless otherwise indicated.
Definitions
By a group lll-V compound semiconductor is meant one comprising at least one element from group III and at least one element from group V. There may be more than one element present from each group, e.g. InGaAs, AIGaN (i.e. a ternary compound), AllnGaN (i.e. a quaternary compound) and so on, as well as dopants of elements from other groups. The term lll-V semiconducting nanostructure or microstructure is meant nanostructure or microstructure made of semiconducting materials from group lll-V elements.
‘Nanostructure’ may herein mean a nanowire (also termed a nanorod, nanopillar, nanocolumn or nanowhisker), a nanopyramid, or a nanoribbon, and microstructure may herein mean a microwire (also termed a microrod, micropillar, microcolumn or microwhisker), a micropyramid, or a microribbon.
The term nanowire is used herein to describe a solid, wire-like structure of nanometer dimensions. Nanowires preferably have an even diameter throughout the majority of the nanowire, e.g. at least 75% of its length. Nanowires may have tapered end structures. The nanowires can be said to be in essentially in onedimensional form with nanometer dimensions in their width or diameter and their length typically in the range of 100 nm to a few (e.g. 5) pm. Preferably the nanowires are at least 1 micrometer in length. Where a plurality of nanowires are grown, it is preferred if at least 90%, preferably all, meet these dimension requirements. Ideally, at least 90% of the nanowires grown on a substrate will be at least 1 micrometer in length. Preferably substantially all the nanowires will be at least 1 micrometer in length. Ideally the nanowire diameter/width is not greater than 1000 nm. Ideally the nanowire diameter/width is between 10 and 1000 nm, e.g. 50 and 500 nm.
Moreover, it will be preferred if the nanostructures or microstructures grown have the same dimensions, e.g. to within 10% of each other. Thus, at least 90% (preferably substantially all) of the nanostructures or microstructures on a substrate will preferably be of the same diameter/width and/or the same length (i.e. to within 10% of the diameter/length of each other). Essentially, therefore the skilled person is looking for homogeneity and nanostructures or microstructures that are substantially the same in terms of dimensions.
The length of the nanostructures or microstructures is often controlled by the length of time for which the growing process runs. A longer process typically leads to a (much) longer nanostructure or microstructure.
Ideally, the diameter at the base of the nanowire and at the top of the nanowire should remain about the same (e.g. within 20% of each other).
The term microwire is used herein to describe a solid, wire-like structure of micrometer dimensions. The same considerations as for nanowires apply here, the difference lying in the dimensions. The term microwire is intended to cover the use of microrods, micropillars, microcolumns or micro whiskers some of which may have tapered end structures. The microwires can be said to be in essentially in onedimensional form with micrometer dimensions in their width or diameter, and their length typically in the range of a 1-10,000 pm, i.e. at least 1 pm. The microwire diameter/width is typically greater than 1 pm. Ideally the microwire diameter/width is between 1 pm and 500 pm.
Ideally, the diameter/width at the base of the microwire and at the top of the microwire should remain about the same (e.g. within 20% of each other).
The term nanopyramid refers to a solid pyramidal type structure. The term pyramidal is used herein to define a structure with a base whose sides taper to a single point generally above the centre of the base. It will be appreciated that the single vertex point may appear chamfered, e.g. such that the pyramid has a flat top. Typically, the chamfered portion is equivalent to less than 50%, e.g. less than 40%, e.g. less than 30%, e.g. less than 20%, e.g. less than 10%, e.g. less than 5% of the total length of the nanopyramid edge. The nanopyramids may have multiple faces, such as 3 to 8 faces, or 4 to 7 faces. Thus, the base of the nanopyramids might be a triangle, square, pentagonal, hexagonal, heptagonal, octagonal and so on. The pyramid is formed as the faces taper from the base to a central point (forming therefore triangular faces). The triangular faces are normally terminated with {1- 101} or {1-102} planes. The triangular side surfaces with {1-101} facets could either converge to a single point at the tip or could form a new facets ({1-102} planes) before converging at the tip. These 4-digit indices are typically most appropriate for hexagonal crystals. In some cases, the nanopyramids are truncated with its top terminated with {0001} planes. The base itself may comprise a portion of even cross-section before tapering to form a pyramidal structure begins. The thickness of the base may therefore be up to 500 nm, e.g. up to 200 nm, such as 50 nm.
The base of the nanopyramids can be between 50 and 500 nm in diameter across its widest point. In another embodiment, the base of the nanopyramids can be 200 nm to one micrometer in diameter across its widest point. The height of the
nanopyramids may be 200 nm to a few (e.g. 5) micrometers, such as 400 nm to 1 micrometer in height.
The term micropyramid refers to a solid pyramidal type structure of micrometer dimensions. The above discussion on the structure of nanopyramids apply also here for micropyramids, the only difference being the dimensions. The thickness of the base may be at least 1 pm, and up to 500 pm, e.g. up to 200 pm, such as 50 pm.
The base of the micropyramids can be between 50 and 500 pm in diameter/width across its widest point. The base of the micropyramids can be 200 pm to one micrometer in diameter/width across its widest point. The height of the micropyramids may be at least 1 pm, e.g. 1-10,000 pm, such as 1 pm to 500 pm in length.
The term nanoribbon refers to a nanostructure in the form of a ribbon where the lateral dimensions are considerably different, e.g. in a first lateral dimension the width of the ribbon is between 10 nm -1000 nm and in a second lateral dimension the width of the nanoribbon is more than 100 times greater, e.g. a few hundred times greater than in the first dimension. The term microribbon refers to a microstructure in the form of a ribbon where the lateral dimensions are considerably different, e.g. in a first lateral dimension the width of the ribbon is between 1pm - 1000 pm and in a second lateral dimension the width of the microribbon is more than a 100 times greater, e.g. a few hundred times greater than in the first dimension.
It will be appreciated that the substrate comprises a plurality of nanostructures or microstructures. This may be called an array of nanostructures or microstructures. Preferably, the plurality of nanostructures or microstructures is a plurality of nanowires or nanopyramids, preferably a plurality of nanowires.
The term epitaxy comes from the Greek roots epi, meaning "above", and taxis, meaning "in ordered manner". The atomic arrangement of the nanostructure or microstructure is typically based on the crystallographic structure of the substrate. Epitaxial growth means herein the growth on the substrate of a nanostructure or microstructures that mimics the orientation of the substrate.
The term carried ‘directly’ implies that the layers in question are adjacent.
Unless otherwise stated, the term ‘bottom’ refers to the substrate side of the nanostructures or microstructures, and the term ‘top’ refers to the side of the nanostructures or microstructures that is opposite to the substrate.
Substrate
The nanostructure or microstructures typically grow initially from the substrate and hence it is preferred that the substrate is a crystalline substrate. The substrate is typically semiconducting.
The substrate may have a crystal orientation of [111], [110], [0001] or [100] perpendicular to the surface. [0001] is preferred. Preferably, the substrate is a [0001] lll-N substrate.
The substrate may comprise silicon, sapphire, SiC, Ga2Oa, graphene or group lll-V substrates, preferably group lll-V substrates. The lll-V materials described for the nanostructures/microstructures below are also suitable for the III-
V substrates, and thus the definition of lll-V materials for the nanostructures/microstructures also apply for lll-V substrates. Ill-N substrates are particularly preferred.
The substrates may be formed from at least one lll-V compound. Group III options are B, Al, Ga, In, and Tl. Preferred options here are Ga, Al, B and In, preferably Ga, Al, and In. Group V options are N, P, As, Sb. N is preferred. The III-
V compounds may be binary, ternary, quaternary, quintinary etc.
AIN is a typical binary compound for the substrate.
Ternary compounds may be of formula XYZ wherein X is a group III element, Y is a group III different from X, and Z is a group V element. The X to Y molar ratio in XYZ is preferably 0.01-0.99, e.g. 0.1 to 0.9, i.e. the formula is preferably XxYi.xZ where subscript x is 0.01-0.99, e.g. 0.1 to 0.9.
Quaternary systems might also be used and may e.g. be represented by the formula AxBi-xCyDi-y where A and B are group III elements and C and D are group V elements or AxByCi-x.yD where A, B and C are group III elements and D is a group V element. Again subscripts x and y are typically 0.01-0.99, e.g. 0.1 to 0.9. Other options will be clear to the skilled person.
The substrate preferably is or comprises a Ill-N compound. The substrate preferably is or comprises AIGaN (e.g. n-AIGaN or p-AIGaN) or AIN.
The substrate in the present invention is typically doped, i.e. n-doped or p- doped, typically n-doped. The doped substrate typically acts as an injector of current. The doped substrate can be an injector of electrons (if n-doped) or injector of holes (if p-doped).
Doping of the substrate typically involves the introduction of impurity ions. The doping level can be controlled from ~ 1015/cm3to 1022/cm3, preferably 1016/cm3 to 1021/cm3, preferably 1017/cm3to 102°/cm3, preferably 1018/cm3to 1019/cm3, (these numbers refer to the number of doping/impurity ions per cm3). The substrate may be highly doped, e.g. has a doping level of at least 1015/cm3, preferably of at least 1016/cm3, preferably of at least 1017/cm3, preferably of at least 1018/cm3. For applications such as UVC LED, the substrate is preferably n-type doped.
Suitable acceptors for the substrate can be Be, Mg and Zn, when the substrate is p-type doped. The substrate may therefore be doped with at least one of Be, Mg and Zn. For n-type substrates, suitable donors can be Te, Sn, Si, Ge and C. The substrate may therefore be doped with at least one of Te, Sn, Si, Ge and C. Dopants can be introduced during the growth process or by ion implantation after formation of the substrate.
The advantages of having a doped substrate are the following.
The doped substrate facilitates the electrical functionality of a device, i.e., transport of charge carriers and injection/extraction at the functional components, e.g., metal contacts or active region.
Preferably, doped substrates have a low sheet resistance as well as low contact resistance which enables efficient devices, in particular in terms of low voltages, low heat generation, and high wall-plug efficiency.
In case of a multi-layer doped substrate, layers with different properties can be stacked to improve different aspects, e.g., two layers with different doping levels optimized to improve sheet resistance and contact resistance, respectively. In such a way key properties can be decoupled to independently improve multiple aspects of efficiency.
In addition, thermal conductivity will be affected by doping. So a doped substrate will most likely improve heat conductivity and extraction.
A high degree of doping can be beneficial as this can improve device efficiency. It can also be beneficial for device efficiency to have similar doping levels for the substrate and the nano-/microstructures.
It can be easier to make electrical contact on a lll-N substrate, for example, than on graphene or graphene-like materials. One can simply etch away some of the substrate and attach a metal as an electrical contact, or connect the metal to a surface of the substrate. The substrate may therefore be provided with an electrical contact (typically metallic), preferably a contact on the top side of the substrate (i.e.
same side as the nanostructures/microstructures). A portion of the top surface of the substrate can be etched away to connect an electrical contact. The electrical contact enables vertical hole or electron injection.
The substrate may be a multi-layer substrate comprising stacked layers of different materials. The substrate may comprise, for example, two or more layers selected from silicon, sapphire, SiC, Ga2Os, graphene or group lll-V layers. The substrate may comprise two or more layers of a lll-N material, e.g. AIGaN (p-type or n-type) on AIN. The substrate typically comprises at least a lll-N layer, preferably as the top layer. The compositions/device of the invention may comprise a planar group lll-V layer (e.g. lll-N) in contact with the top layer of the substrate, on the opposite side of the substrate to the nanostructures or microstructures, preferably wherein said planar group lll-V layer is AIN. Such an additional layer may be beneficial as it can provide strain moderation and ensure compositional uniformity of the layer on top of it (e.g. AIGaN). ‘Top’ layer when referring to the substrate herein refers to the layer of the substrate in contact with the nanostructure or microstructure cores.
Additional layers, e.g. sapphire, may be present in the substrate (e.g. under the other layers), and may be desirable to act as a support and add rigidity to the structure.
A sacrificial layer may also be present (see Figure 10). The substrate may comprise a sacrificial layer of GaN or highly doped n-AIGaN, for example. In such a scenario, the whole top heterostructure can be lifted off using chemical, electrochemical or laser lift off techniques. The advantage of this set up is that the substrate becomes thinner, and thus light extraction/absorption efficiency can be improved, and/or more flexible devices can be obtained.
Preferably, the substrate is transparent, e.g. to any light emitted by or detected by the nanostructures or microstructures (e.g. UV light or UVC light).
The substrate typically has a thickness of 0.1-2000 pm, e.g. 0.1-1000 pm, e.g. 0.1-100 pm, e.g. 0.5-50 pm, e.g. 1-10 pm.
Nanostructures and microstructures
Any discussion of nanostructures herein equally applies to microstructures, where technically viable, and vice-versa. Discussion of nanostructures/microstructures can refer to the nanostructure/microstructure cores,
or the cores in combination with additional layers positioned thereon. The core is herein defined as the (innermost) part of the nanostructure/microstructure which grows first on the substrate, and/or which is in physical contact with the substrate. The term ‘horizontal’ refers herein to the plane of the substrate, and the term ‘vertical’ typically herein refers to the growth direction of the nano/microstructures, or an axis along the longest dimensions thereof (i.e. perpendicular to substrate plane)
In order to prepare nanostructures/microstructures of commercial importance, it is preferred that these grow epitaxially on the substrate. It is also ideal if growth occurs perpendicular to the substrate and ideally therefore e.g. in the [111 ]-direction for cubic crystal structure or e.g. in the [0001]-direction for hexagonal crystal structure. Ill-N materials, which are preferred, are typically hexagonal and therefore the [0001] direction is preferred. The wording ‘grown from said substrate’ means that the nanostructures/microstructures are located on the substrate, and preferably form an epitaxial relationship with the substrate (i.e. top layer of the substrate if the substrate is a multi-layer substrate).
If a mask layer is present, then the nanostructures/microstructures are typically grown from the substrate through the holes of the mask layer. This means that the nano-/microstructures are located over the holes, and there is part of the nanostructure/microstructure that extends (downwards) in the hole to the substrate, and preferably the nanostructure/microstructure forms an epitaxial relationship with the substrate. Alternatively put, the nanostructure/microstructure are present in the holes of the mask layer. Alternatively put, nanostructures/microstructures are nucleated from the holes in the mask layer, or extend therefrom.
The nanostructures or microstructures typically extend over the surface of the mask layer, if present. In other words, the nanostructures or microstructures can grow out of, or nucleate in, the hole of the mask, and then grow/extend sideways (i.e. laterally/radially) to cover at least a portion of the top surface of the mask layer (i.e. they ‘mushroom’ out of the holes). They typically grow or extend laterally/radially (i.e. in the same plane as the substrate/mask), such that only the inner portion of the nanostructure/microstructure core is formed over the hole in the mask layer mask.
Preferably, the nanostructure cores or microstructure cores have pyramidal tips. Typically, therefore, the overall nanostructures or microstructures have pyramidal tips. Preferably, the tips are hexagonal pyramidal. Growth of
nanostructure or microstructure can be controlled through the MOCVD growth conditions. Pyramids are encouraged, for example if high group V flux is employed.
Whilst it is ideal that there is no lattice mismatch between a growing nanostructure or microstructure and the substrate layer, nanostructures or microstructures can accommodate much more lattice mismatch than thin films for example.
The nanostructures or microstructures have typically a hexagonal cross sectional shape. The nanowire may have a cross sectional diameter/width of 25 nm to several micrometers (i.e. its thickness). As noted above, the diameter is ideally constant throughout the majority of the nanowire. Nanostructure/microstructure diameter/width can be controlled by the manipulation of the growth parameters such as the substrate temperature and/or the ratio of the atoms used to make the nanowire as described further below.
Moreover, the length and diameter of the nanostructures or microstructures can be affected by the temperature at which they are formed. Higher temperatures encourage high aspect ratios (i.e. longer and/or thinner nanowires/microwires). The skilled person is able to manipulate the growing process to design nanostructures or microstructures of desired dimensions.
The nanostructures or microstructures of the invention comprise at least one lll-V compound, preferably lll-N. Preferably, the nanostructures or microstructures comprise an Al-containing lll-V compound, e.g. AIGaN, AllnGaN, or AIN. Preferably the nanostructure/microstructure cores comprise (or are made of) an Al-containing lll-V compound, e.g. AIGaN, AllnGaN, or AIN, preferably AIGaN. Al-containing lll-V compounds are most suitable for UV applications.
Group III options are B, Al, Ga, In, and Tl. Preferred options here are Ga, Al, B and In, preferably Ga, Al and In. Group V options are N, P, As, Sb. N is preferred.
It is of course possible to use more than one element from group III and/or more than one element from group V. The compounds may be binary, ternary, quaternary, quinary etc. The above and below applies to both the nanostructure or microstructure cores but also to any additional lll-V layers on top of the cores.
Suitable binary compounds for nanostructure or microstructure manufacture include AlAs, GaSb, GaP, GaN, AIN, GaAs, InP, InN, InSb, InAs. Compounds based on Al, Ga and In in combination with N are one preferred option. The use of GaN, AIN is highly preferred for binary compounds.
It is most preferred if the nanostructures or microstructures comprise or consist of Ga, Al, In and N (along with any doping atoms as discussed below) (NB: any electrical contacts are not typically considered part of the nano- /microstructures).
Ternary compounds may be of formula XYZ wherein X is a group III element, Y is a group III different from X, and Z is a group V element. The X to Y molar ratio in XYZ is preferably 0.01 to 0.99, e.g. 0.1 to 0.9, i.e. the formula is preferably XxYi.xZ where subscript x is 0.01 to 0.99, e.g. 0.1 to 0.9. AIGaN, InGaN, InAIN, InGaAs, or AIGaAs are preferred ternary compounds. AIGaN, InAIN, and InGaN are most preferred, especially AIGaN. Preferably, for UV applications in particular, the nanostructures or microstructures do not comprise InGaN, since InGaN is not suitable for UV/LIVC applications.
Quaternary systems might also be used and may e.g. be represented by the formula AxBi-xCyDi-y where A and B are group III elements and C and D are group V elements or AxByCi-x.yD where A, B and C are group III elements and D is a group V element. Again subscripts x and y are typically 0.01 to 0.99, e.g. 0.1 to 0.9. Other options will be clear to the skilled man. AIGalnN is particularly preferred compound.
AllnGaBN is an example of a suitable quinary compound.
The use of ternary/quaternary materials for at least one lll-V layer, the core, and/or the substrate is preferred. AIGaN, AI(ln)GaN are most preferred for the cores and layers. It is preferred if the nanostructure cores or microstructure cores comprise, or consist of, AIGaN, preferably n-AIGaN or p-AIGaN, preferably n- AIGaN. AI(ln)GaN (both n-type and p-type) is also highly suitable.
In general, it is preferred if the Al content in the nano-/microstructures is of at least 60% (by atom ratio of the group-ill component, usually given by AI/(AI+Ga)), since this increases transparency to UVC light. The use of Al is advantageous as high Al content leads to high band gaps, enabling IIV-C LED emission from the active layer(s) of nano-/microstructures and/or avoiding absorption of the emitted light in the at least one lll-V layers. The use therefore of AIN or AIGaN in nano- /microstructures is preferred. The preferred Al content may apply to the nanostructure cores as well as any lll-V layers. lll-V layer(s) and doping
The compositions of the present invention comprise at least one lll-V layer positioned on top of the nanostructure cores or microstructure cores, wherein at least part of said at least one lll-V layer is semipolar. The at least one lll-V layer is also typically semiconducting. Generally speaking, planes that are not vertical (=non-polar) or horizontal (=polar) are considered semipolar. Semipolar herein means having semipolar crystallinity. Preferably the at least one lll-V layer is semipolar. In other words, preferably, the entirety of the lll-V layers are semipolar or pyramidal, i.e. each lll-V layer is completely semipolar/pyramidal. Compared to core-shell structures which might have layers which have vertical as well as semipolar parts, having completely semipolar/pyramidal layers is beneficial since undesired absorption of light by one or more of the lll-V layers is minimised. For example, if the composition or device comprises a top GaN (e.g. p-GaN) layer, which absorbs UV light, then having semipolar group lll-V layers (including the GaN layer) ensures that UV absorption is minimized, since there are no wrap-around shell layers, for example, which would absorb UV light and reduce the efficiency of the device. In a core-shell structure, which may have semipolar as well as vertical parts, undesirably high levels of absorption can occur. This is especially the case in devices where light is emitted/absorbed via the bottom of the device. Minimising absorption by a UV-opaque top layer is important.
A polar structure can be seen as a structure without space inversion symmetry, and of the 31 crystal classes 21 of them have this property. All polar crystal structures, such as Wurtzite, have a specific polar axis, along where the space inversion symmetry is broken. Semipolar planes are planes that have a normal vector at an angle which is non-0°, non-90° and non-180° with respect to the polar axis of the crystal structure. Semipolar planes/layers are therefore planes/layers which are not perpendicular to, nor parallel to, the polar axis of the crystal structure. The at least one lll-V layer is therefore typically formed of a compound with a semipolar crystal structure, and the lll-V layer is neither perpendicular nor parallel to the polar axis of said polar crystal structure. The at least one lll-V layer is therefore typically formed of a compound with a semipolar crystal structure, with the lll-V layer having an angle of 5-85° relative to the polar axis of said polar crystal structure. The lll-V layer is typically neither perpendicular nor parallel to the growth axis of the nano/microstructure. The lll-V layer is typically neither perpendicular nor parallel to the substrate plane. In the case of substrates without a horizontal c-plane surface or polar axis the at least one lll-V layer has an
orientation which is not perpendicular to, nor parallel to, a low index (e.g. [001], [010], [100]) axis of that substrate.
Growth of the nanostructures or microstructures is typically performed step by step. In one of the first steps the core is grown/positioned on the patterned substrate. The core’s upper surface is typically composed of six equivalent surface parts arranged in a pyramidical shape merging at a common tip. Considering the polarity properties of the lll-N wurtzite crystal structure, for example, each of the six equivalent surface parts forms a semipolar plane, characterized by a specific angle and an equivalent set of Miller’s indices, e.g., 62° with respect to the horizontal plane and {10-11}.
By stacking additional 11 l-V (preferably lll-N) layers on top, the angle and indices of the semipolar planes are reproduced. In this way, multiple semipolar interfaces can be created. In contrast to other types of interfaces (e.g., polar or nonpolar) such semipolar interfaces provide special physical properties in terms of charge carrier distribution and light-matter-interaction. In case of UV LEDs, highly TM-polarized light can result from the semipolar interfaces. In addition, polar surfaces induce an effect called the quantum confined stark effect, which is an undesired effect in LEDs. The semipolar plane will, due to the reduced polarity, also have a reduced quantum confined stark effect. These physical properties can be exploited to improve radiative recombination efficiency and light extraction efficiency in LEDs in particular.
The at least one 11 l-V layer is typically a pyramidal layer, preferably a hexagonal pyramidal layer (since lll-N nanostructure or microstructure cores are preferred and these typically have hexagonal symmetry).
Alternatively put, the at least one 11 l-V layer comprises tilted sidewalls extending from a pyramid peak, wherein the sidewalls are semipolar.
The tips of the semipolar (i.e. pyramidal) layers can be seen as the meeting point of 6 semipolar planes (for hexagonal symmetry). As mentioned above, the at least one 11 l-V layer typically has six equivalent planes/surface parts arranged in a pyramidical shape merging at a common tip. Each surface part preferably is a {10- 11} facet, but a range of similar planes can be considered herein. Generally speaking, therefore, the at least one 11 l-V layer typically has {10-11} facets, but higher index facets are also possible.
Typically, the internal pyramidal angle of said at least one 11 l-V layer is in the range 5-175°, preferably 40-70°, preferably 45-65°, preferably 50-60°, e.g. 56°. The
{10-11} plane family typically shares an angle of approximately 62° with the horizontal plane, in other words approximately 28° to the vertical (e.g. growth axis of the nano-/microstructures). The sidewalls of the lll-V layer therefore typically have an angle of 5-85° with the horizontal (i.e. substrate) plane, preferably 45-75°, preferably 50-70°, preferably 55-65° with the horizontal plane. The sidewalls of the lll-V layer therefore typically have an angle of 5-85° with the vertical axis (e.g. polar axis or growth axis), preferably 15-45°, e.g. 20-40° or 25-35° with the vertical axis. The exact value will depend on the c/a ratio of the crystal structure (and hence the strain and the Al content, if present).
There is preferably a plurality of group lll-V layers on top of the nanostructure or microstructure cores. If there is more than one lll-V layer on top of the cores, then the layers are preferably stacked vertically, i.e. on top of each other along the vertical axis (i.e. growth axis or longest dimension of the nanostructures or microstructures). The stacking could be considered to be vertical stacking, or pyramidical stacking, therefore. These terms are considered to be equivalent. The nanostructures or microstructures are preferably axially heterostructured. In a particular embodiment, the nano-/microstructures are not radially heterostructured.
The compositions of the invention typically comprise at least one optional intrinsic lll-V semiconducting layer, and at least one n-type doped lll-V layer if the nanostructure or microstructure cores are p-type doped, or at least one p-type doped lll-V layer if the nanostructure or microstructure cores are n-type doped.
The nanostructures or microstructures of the invention can contain a p-n, n- p, n-i-p, or p-i-n junction, e.g. to enable their use in LEDs. The nanostructure or microstructure cores typically have the same doping type as the substrate (e.g. if the substrate is n-type doped, then the nanostructure or microstructure core will be n-type, and vice versa). The junction can be in the form of additional layers on the cores. Nanostructures or microstructures of the invention are therefore optionally provided with an undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The intrinsic region may consist of a single layer of material or a heterostructure consisting of multiple quantum wells and barriers. Typically, the intrinsic layer is a multiple quantum well. Typically, the intrinsic layer is positioned directly on the core.
It is therefore preferred if the nanostructures or microstructures are doped. Preferably, the core is doped as well as at least one additional layer on top of the core. Doping typically involves the introduction of impurity ions into the nanowire,
e.g. during epitaxial growth. The doping level can be controlled from ~ 1015/cm3to 1022/cm3. The same doping characteristics as discussed above for the substrates apply here also. The doping level can be controlled from ~ 1015/cm3to 1022/cm3, preferably 1016/cm3to 1021/cm3, preferably 1017/cm3to 102°/cm3, preferably 1018/cm3 to 1019/cm3.
The n(p)-type semiconductors have a larger electron (hole) concentration than hole (electron) concentration by doping an intrinsic semiconductor with donor (acceptor) impurities. Suitable donor (acceptors) for lll-V compounds can be Te, Sn, Si, Ge and C (Be, Mg and Zn). Si can be amphoteric, either donor or acceptor depending on the site where Si goes to, depending on the orientation of the growing surface and the growth conditions. Dopants can be introduced during the growth process or by ion implantation of the nanowires or nanopyramids after their formation.
The nanostructures/microstructures may comprise additional n, i-n, p-, i-p, n-i-p or p-i-n layers, preferably additional i-p or i-n layers, e.g. positioned on top of the cores. Preferably the nanostructures/microstructures may comprise an i-layer directly positioned on the core and one or more (e.g. 2-4) p-layers on top of the i- layer (if the core is n-doped) or one or more (e.g. 2-4) n-layers on top of the i-layer if the core is p-doped. The nanostructures/microstructures may additionally comprise p-AIGaN, i-AIGaN, and/or n-AIGaN layers for example. AI(ln)GaN is also suitable. The ‘(In)’ indicates that small amounts of In may optionally be present. Such nomenclature is well known in the art. In a particular embodiment, therefore, the nanostructures or microstructures comprise or consist of an AIGaN (e.g. n-AIGaN) core, and additionally have at least n-AIGaN, i-AIGaN, and/or p-AIGaN layers thereon, and optionally a GaN (e.g. p-GaN) layer. The composition may comprise at least one i-AIGaN layer, at least one p-AIGaN layer (preferably two p-AIGaN layers, optionally differentiated by different doping, different Al percentage, and/or different thickness), and optionally at least one p-GaN layer, preferably in that order. AIGa(ln)BN systems are also suitable herein (i.e. AIGa(ln)BN core and one or more AIGa(ln)BN layers).
Generally speaking, the above-described binary, ternary, quaternary or quinary compounds are suitable for the lll-V layers and also apply here.
Group III options for lll-V layer(s) are B, Al, Ga, In, and Tl. Preferred options here are Ga, Al, B and In, preferably Ga, Al and In. Group V options are N, P, As, Sb. N is preferred. 11 I-N layers are particularly preferred.
The 11 l-V layer(s) may be a ternary compounds of formula XYZ wherein X is a group III element, Y is a group III different from X, and Z is a group V element. The X to Y molar ratio in XYZ is preferably 0.01-0.99, e.g. 0.1 to 0.9, i.e. the formula is preferably XxYi-xZ where subscript x is 0.01-0.99, e.g. 0.1 to 0.9.
Quaternary systems might also be used and may e.g. be represented by the formula AxBi-xCyDi-y where A and B are group III elements and C and D are group V elements or AxByCi-x-yD where A, B and C are group III elements and D is a group V element. Again subscripts x and y are typically 0.01-0.99, e.g. 0.1 to 0.9. Other options will be clear to the skilled man.
Additional layers may be present. A tunnel junction layer may be present, for example. A tunnel junction can be composed of a layer stack with multiple components. Typically, the tunnel junction is formed by two highly doped layers made of lll-N material, one with p-doping and one with n-doping. Preferably, doping levels are in the range of at least 1019/cm3, e.g. 1019/cm3 - 1021/cm3, e.g., 102°/cm3 - 1021/cm3. In addition, the tunnel junction can contain an undoped layer made of lll-N material, positioned in between two doped layers. Also, a grading in the material molar ratio can be implemented. Typically, the entire tunnel junction layer stack can have dimensions (i.e. thickness) of less than 50nm, e.g. in the range 20-50nm. It is preferred if the active part of the tunnel junction is very thin, e.g. less than 10 nm in thickness to facilitate efficient tunnelling. The functionality as ‘active part’ is determined by the combination of doping levels, material molar ratios, and thicknesses of all the layers involved in the tunnel junction layer stack.
A tunnel junction layer may be different from other layers as a result of a different doping level and/or Al mole ratio.
Typically, an n-type doped region and a p-type doped region are separated by an intrinsic region which acts as a multiple quantum well, and said p-type doped region comprises an electron blocking layer.
It is preferred if the at least one 11 l-V layer is transparent to the light being emitted by (or absorbed by) the device.
Suitable structures include the following. The nanostructures or microstructures may comprise or consist of, from bottom to top (i.e. starting from the core in contact with the substrate upwards) the following: an n-AIGaN core, an i-AIGaN layer, a p-AIGaN layer (which optionally acts as an electron blocking layer), a different p-AIGaN layer, a p-GaN layer;
an n-AIGaN core, an i-AIGaN layer, a p-AIGaN layer (which optionally acts as an electron blocking layer), a different p-AIGaN layer; or an n-AIGaN core, an i-AIGaN layer, a p-AIGaN layer (which optionally acts as an electron blocking layer), a different p-AIGaN layer, an undoped (In, Al, Ga)N interlayer acting as a tunnel junction, an n-AIGaN layer;
The nano-/microstructures typically have an Al-containing layer. The increasing ionization energy of Mg acceptors with increasing Al content in AIGaN alloys makes it difficult to obtain higher hole concentration in AIGaN alloys with higher Al content. To obtain higher hole injection efficiency (especially in the cladding/barrier layers consisting of high Al content), the inventors have devised a number of strategies which can be used individually or together.
Using a short period superlattice (SPSL) can maximise electrical conductivity (i.e. maximise hole concentration) in AIGaN alloys with higher average Al content. In this method, a superlattice structure is grown consisting of alternating layers with different Al content instead of a homogeneous AIGaN layer with higher Al composition. For example, the cladding layer with 35% Al content could be replaced with a 1.8 to 2.0 nm thick SPSL consisting of, for example, alternating AlxGai-xN:Mg I AlyGai.yN:Mg with x=0.30/y=0.40. The low ionization energy of acceptors in layers with lower Al composition leads to improved hole injection efficiency without compromising on the barrier height in the cladding layer. This effect is additionally enhanced by the polarization fields at the interfaces. The SPSL can be followed with a highly p-doped GaN:Mg layer for better hole injection.
More generally, the inventors propose to introduce a p-type doped ALGa?. xN/AlyGav.yN short period superlattice (i.e. alternating thin layers of ALGav-xN and AlyGav.yN) into (or onto) the nano-/microstructure structure, where the Al mole fraction x is less than y, instead of a p-type doped AlzGa7.zN alloy where x < z < y. It is appreciated that x could be as low as 0 (i.e. GaN) and y could be as high as 1 (i.e. AIN). The superlattice period should preferably be 5 nm or less, such as 2 nm, in which case the superlattice will act as a single AlzGa7.zN alloy (with z being a layer thickness weighted average of x and y) but with a higher electrical conductivity than that of the AlzGa7.zN alloy, due to the higher p-type doping efficiency for the lower Al content AlxGa^-xN layers.
In the nano-/microstructures comprising a p-type doped superlattice, it is preferred if the p-type dopant is an alkali earth metal such as Mg or Be.
Instead of a superlattice containing thin AIGaN layers with low or no Al content, a nanostructure can be designed containing a gradient of Al content (mole fraction) in the growth direction of the AIGaN within the nano-/microstructures. Thus, as the nano-/microstructures grow, the Al content is reduced/increased and then increased/reduced again to create an Al content gradient within the nanowires or nanopyramids.
This may be called polarization doping. In one embodiment, the layers are graded either from GaN to AIN or AIN to GaN. The graded region from GaN to AIN and AIN to GaN may lead to n-type and p-type conduction, respectively. This can happen due to the presence of dipoles with different magnitude compared to its neighbouring dipoles. The GaN to AIN and AIN to GaN graded regions can be additionally doped with n-type dopant and p-type dopant respectively.
In a preferred embodiment, p-type doping is used in AIGaN nanowires using Be as a dopant.
Thus, one option would be to start with a GaN nano-/microstructure cores and increase Al and decrease Ga content gradually to form AIN, perhaps over a growth thickness of 100 nm. This graded region could act as a p- or n-type region, depending on the crystal plane, polarity and whether the Al content is decreasing or increasing in the graded region, respectively. Then the opposite process is effected to produce GaN once more to create an n- or p-type region (opposite to that previously prepared). These graded regions could be additionally doped with n-type dopants such as Si and p-type dopants such as Mg or Be to obtain n- or p-type regions with high charge carrier density, respectively. The crystal planes and polarity is governed by the type of nano-/microstructure as is known in the art.
Viewed from another aspect therefore, the nano-/microstructures of the invention comprise Al, Ga and N atoms wherein during the growth of the nano- /microstructures the concentration of Al is varied to create an Al concentration gradient within the nanowires or nanopyramids.
In a third embodiment, the problem of doping in an Al containing nano- /microstructure is addressed using a tunnel junction. A tunnel junction is a layer stack comprising a barrier, such as a thin layer, between two electrically conducting materials. In the context of the present invention, the barrier functions as an ohmic electrical contact in the middle of a semiconductor device.
In one method, a thin electron blocking layer is inserted immediately after the active region, which is followed by a p-type doped AIGaN cladding layer with Al
content higher than the Al content used in the active layers. The p-type doped cladding layer is followed by a highly p-type doped cladding layer and a very thin tunnel junction barrier layer followed by an n-type doped AIGaN layer. The tunnel junction layer stack is chosen such that the electrons tunnel from the valence band in p-AIGaN to the conduction band in the n-AIGaN, creating holes that are injected into the p-AIGaN layer.
For nanostructures, the lll-V layer(s) may be 500nm in thickness or less, e.g. 200 nm in thickness or less, e.g. 100 nm in thickness or less, e.g. 20 nm in thickness or less, e.g. 10 nm or less in thickness. The lll-V layer(s) may be 0.5 nm in thickness or more, e.g. 1 nm in thickness or more, e.g. 2 nm in thickness or more. Suitable thickness ranges for the lll-V layer include 0.5-500 nm or 1-200 nm. Particularly preferred thicknesses for the lll-V layer include 200nm, 80nm, 50nm, 20nm, 10nm, 5nm, 2nm.
For microstructures, the lll-V layers may be 100 pm in thickness or less, e.g. 20 pm in thickness or less, e.g. 10 pm or less in thickness. Suitable thickness ranges include 0.5-100 pm, e.g. 1-20 pm.
Structure
The edges of the at least one lll-V layer(s) preferably form a vertical side wall with the nanostructure/microstructure core. If there are multiple group lll-V layers on the cores, preferably the edges of all of these form a vertical side wall with the nanostructure/microstructure core. Alternatively put, the edges of the at least one group lll-V layers are vertically aligned with each other, and preferably also with the sides of the cores. This has benefits in terms of light emission/absorption, and/or in terms of light guiding. If one of the lll-V layers absorbs light at a wavelength of interest, then such a construction has benefits since absorption is minimized, especially compared to core-shell structures. For example, if the composition comprises a top GaN (e.g. p-GaN) layer, which absorbs UV light, then having the edges of the at least one group lll-V layers (including the GaN layer) vertically aligned with each other ensures that UV absorption is minimized, since there are no wrap-around shell layers, for example, which would absorb UV light and reduce the efficiency of the device. In a core-shell structure, which may have semipolar as well as vertical parts, undesirably high levels of absorption can occur.
As mentioned above, it is particularly preferred if the cores have pyramidal tips, and any additional p/i/n layers mirror the topography (i.e. shape) of the underlying cores. The additional layers (which can be considered to form part of the nanostructures/microstructures), can either be limited to the width of the nanostructures/microstructures, or they may be in the form of a layer continuously covering at least a portion of the plurality of nanostructures/microstructures. In a particular embodiment, at least one of the lll-V layers continuously covers at least a portion of the nanostructures/microstructures (i.e. of the cores, with potential intermediate layers between the core and the top continuous layer). The layer that continuously covers at least a portion of the nanostructures/microstructures may be continuous in its upper region, but may have voids in its lower region. The top layer (e.g. the top AIGaN or GaN layer) may be continuous, e.g. it may cover at least 50% of the nanostructures or microstructures, e.g. at least 75%, at least 90%, or at least 99% of the nanowires. This is beneficial since the doped top layer (e.g. n- layer) can act as a transparent current spreader with an acceptable sheet resistance without the continuous layer being too thick. Layers beneath the top layer (e.g. the i-layer and below) may be continuous or they may be non-continuous (i.e. limited to the width of the nanostructures/microstructures).
Alternatively or additionally, an electrical contact layer (i.e. metallic contact layer) may continuously extend over the nanostructures or microstructures.
In either case, such a top layer will typically be ridged/corrugated, i.e. comprising a plurality of protrusions, wherein the protrusions (e.g. pyramidal protrusions) are positioned on top of the nanostructure/microstructure tips
It may be that as numerous nanostructures or microstructures grow from the substrate that the nanostructures or microstructures coalesce at a certain distance from the substrate, or directly on top of the mask layer. It can be beneficial to form large area structures through coalescence of positioned nanostructures or microstructures, or through coalescence of one of the top layers (e.g. p-AIGaN, p- GaN etc.). The nanostructures or microstructures may be coalesced, therefore. The coalescence of nanowires may appear almost film like (e.g. like a corrugated film if the nanostructures or microstructures have pyramidal tips).
Coalescence refers to the side-on joining of two or more nanostructures during growth. This results in a 2D or 3D structure. For coalescence, the nanostructures or microstructures must preferably have their crystal lattices in the same orientation, such that the formation of gaps and dislocations can largely be
eliminated, i.e. the coalescing nanostructures or microstructures or lll-V layers thereof must preferably have nearly identical epitaxial relationship with respect to the substrate. However, the nano-/microstructures may be coalesced with amorphous, polycrystalline or a different crystalline material in between them. The region between the nanostructures and microstructures may be amorphous lll-V material or consist of multiple lll-V grains that are not epitaxially related to the nano- /microstructures. The regions between the nanostructures may be lll-V material (e.g. the same as used for nanostructure or microstructure growth), air, vacuum, or a dielectric material. There may be a grain boundary between the structures. Any lll-V material between the nanostructures/microstructures may be amorphous, polycrystalline or crystalline. The coalescence may occur via the cores, the at least one lll-V layer, or both.
Alternatively, there is also provided a completely coalesced structure, i.e. a composition of matter comprising: a substrate; a non-planar (e.g. corrugated or ridged) coalesced structure on said substrate, e.g. formed from a plurality of coalesced nanostructures or microstructures, said non-planar coalesced structure and/or nanostructures or microstructures comprising at least one semiconducting group lll-V compound; wherein said non-planar coalesced structure comprises at least one lll-V layer, typically positioned on top of coalesced nanostructure cores or microstructure cores, wherein at least part of said at least one lll-V layer is semipolar; and preferably wherein said at least one lll-V layer extends continuously over the plurality of coalesced nanostructure cores or microstructure cores.
The coalesced structure typically comprises a plurality of protrusions, wherein the protrusions (e.g. pyramidal protrusions) are tips of the nanostructures/microstructures. The non-planar/corrugated structure is beneficial as it can enable good light extraction for transverse-magnetic (TM) polarisation (see Figure 4). Often, manufacturers of nanostructure devices etch corrugated/ridged designs to improve light extraction. In the present case, if pyramidal tipped nano- /microstructures are used, the corrugation is obtained without having to perform any subsequent etching step. By using pyramidal tipped nano-/microstructure shape, benefits in terms of ease of manufacture and light extraction (or absorption) are obtained, therefore. It will be appreciated that any discussion of nano- /microstructures or lll-V layers for separate nano-/microstructures also applied to
structures in which the nano-/microstructures or additional layers are coalesced. Any of the definitions for compositions comprising nano-/microstructures (e.g. in relation to the nature of the materials) are applicable here, where technically viable. The materials for the nano-/microstructures are applicable to the corrugated films. The corrugated films may comprise the same layers that the nano-/microstructures comprise.
The top layer (e.g. the uppermost 11 l-V layer) can act as a top-emitting transparent electrode. As discussed above, by transparent is hereby meant transparent to the light emitted by the nano-/microstructures, e.g. in the case of a UV-C LED, then the layer is transparent at least to IIV-C light. In the case of photodetectors, by transparent is meant transparent to any incoming light. E.g. if the composition/device is a solar cell, then transparent means transparent at least to solar light.
Mask layer
The compositions or devices of the invention may comprise on the substrate a mask layer through which the nanostructures or microstructures are grown.
Such a mask layer serves several purposes. It enables the positioning (i.e. SAG) of the nano-/microstructures. The holes in the mask also allow the nanostructure or microstructure material to be epitaxial with the substrate.
The mask layer may be a dielectric material, such as silicon dioxide or silicon nitride, or graphene. In a particular embodiment, the mask is not graphitic/graphene.
The mask may also be multi-layer, i.e. have two or more different materials stacked on top of each other, e.g. graphene and SiC>2.
As used herein, the term graphene refers to a planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb (hexagonal) crystal lattice. The term ‘graphene’ also means structures having a small number of graphene sheets. The interplanar spacing in graphene is 0.335 nm. The graphene layer is well known for its superior optical, electrical, thermal and mechanical properties. Graphene is very thin but very strong, light, flexible, and impermeable.
In terms of thicknesses, for nanostructures it is preferred if the mask layer in general is 200 nm or less, e.g. 100 nm in thickness or less, e.g. 50 nm or less. The mask layer may be 1 nm in thickness or more, e.g. 5 nm in thickness or more, e.g.
10 nm in thickness or more. Preferred thickness ranges for the mask layer include 1-200 nm, preferably 5-100 nm, preferably 10-50 nm.
Some materials, such as silicon oxide, are relatively transparent to UVC and so a wide range of thicknesses can be used (for UVC applications). Other materials such as silicon nitride have a lower band gap, and are hence less transparent. In these instances, the mask thickness should preferably be kept low.
For microstructures, it is preferred if the mask layer in general is 200 pm or less, e.g. 100 pm in thickness or less, e.g. 50 pm or less. The mask layer may be 1 pm in thickness or more, e.g. 5 pm in thickness or more, e.g. 10 pm in thickness or more. Preferred thickness ranges for the mask layer include 1-200 pm, preferably 5-100 pm, preferably 10-50 pm.
For nanostructures, the area of the mask layer in general is not limited. This might be as much as 0.5 mm2 or more, e.g. up to 5 mm2 or more such as up to 10 cm2. The wafers (i.e. substrate + mask) typically have a diameter or maximum width of 1-20 inches, e.g. 2 inch, or 4 inch, or 6 inch, or 8 inch, or 12 inch. The area of the mask layer is thus only limited by practicalities.
For nanostructures, the diameter of the holes is preferably up to 500 nm, such as up to 100 nm, ideally up to 20 to 200 nm. For microstructures, the diameter of the holes is preferably up to 500 pm, such as up to 100 pm, ideally 20 to 200 pm. The diameter of the hole sets a maximum diameter/width for the size of the nanostructures/microstructures during initial growth. However, nanostructure/microstructure diameter larger than the hole size can be achieved by changing the growth parameters or by adopting a core-shell nanostructure or microstructure geometry. Typically, the nanostructures or microstructures extend laterally (i.e. radially) over any mask outside of the holes.
The number of holes is a function of the area of the mask layer and desired nanostructure or microstructure density.
The shape of the holes is not limited. Whilst these may be circular, holes may also be in other shapes, such as triangular, rectangles, oval etc. The mask layer may be in electrical contact with the base of the nanostructures or microstructures, which is useful in the case of the mask layer acting as an electrode. As the nanostructures or microstructures begin growing within a hole, this tends to ensure that the initial growth of the nanostructures or microstructures is epitaxial and substantially perpendicular to the substrate. This is
a further preferred feature of the invention. One nanostructure or microstructure preferably grows per hole.
The mask layer may be doped to improve its electrical conductivity. This may be beneficial if the mask layer acts as an electrode (e.g. as a gate for a vertical transistor).
Where the nano-/microstructures extend laterally over the mask outside of the holes, there is good electrical contact between the nano-/microstructures and the substrate below. As discussed above, the nano-/microstructures typically nucleate in the holes of the patterned mask on the substrate, and then grow laterally, i.e. radially, over the top surface of the mask layer (i.e. they ‘mushroom’ out of the holes).
It is preferred herein if the mask layer is not an electrode, or does not act as an electrode. In a particular embodiment, there is no electrical contact on the mask layer. For certain applications, e.g. transistors, it may be beneficial for the mask layer to act as an electrode. In a particular embodiment, therefore, the mask layer acts (or is) an electrode.
The mask layer is typically positioned directly on the substrate.
In a further embodiment, the composition does not comprise a holed mask layer on top of the substrate. Nucleation sites can be provided by etching holes in the substrate, for example.
Devices/Applications
Semiconductor nanostructures or microstructures have wide ranging utility. They are semiconductors so can be expected to offer applications in any field where semiconductor technology is useful. They are primarily of use in integrated nanoelectronics and nano-optoelectronic applications.
An ideal device for their deployment might be a solar cell, transistor, laser LED or photodetector.
The semiconductor nanostructures or microstructures have utility in LEDs, in particular UV LEDs and especially UV-A, UV-B, or UV-C LEDs, more preferably UV-C LEDs. The invention therefore provides a device, such as an electronic or opto-electronic device, comprising a composition as defined herein, e.g. a solar cell, photodetector, transistor, laser, or LED, preferably an LED, more preferably a UV LED, more preferably a UV-C LED, comprising a composition as defined herein. UV
(preferably UVC) devices are typically preferred, e.g. UV LED, UV photodetector, UV laser. Alternatively viewed, the composition of matter is an electronic or optoelectronic device, preferably a solar cell, transistor, laser, photodetector or LED, preferably a LED, preferably a UV LED, preferably a UVC LED. Preferably, the present devices (whether LED or other) emit or absorb light in the UV region, preferably UV-C region. Preferably, the present devices emit or absorb light with a wavelength of 280nm or less, e.g. in the range 200-280 nm.
In the present disclosure, any discussion about emission of light, in the context of light emitters such as UV LEDs, also applies to the absorption of light in the case of light absorbers. When discussing emission of light in particular direction, for example, the same applies to absorption of light from a particular direction. In the compositions/devices of the invention, each individual nanostructure or microstructure can be seen as an individual LED nanostructure or microstructure (or individual photodetector/solar cell etc.). The nanostructures or microstructures comprise a light generating (or light absorbing) region.
It will be appreciated that devices of the invention are provided with electrodes to enable charge to be passed into the device. In order to create an optoelectronic device, the top of the nanostructures or microstructures preferably comprise a top contact. In one embodiment, a conventional top contact is positioned on the top layer of the nanostructures or microstructures, e.g. on a top AIGaN or GaN layer (e.g. p-AIGaN or p-GaN) (this top layer may extend continuously over the underlying layers and nanostructure/microstructure cores, as previously discussed). This top contact may have a finger design in order to reduce the amount of the contact that blocks light from exiting or entering the device (e.g. the area covered by the contact is typically 50% or less, preferably 20% or less of the top surface area of the nanostructures or microstructures), or may be in the form of a continuous layer covering a plurality (e.g. at least 50%, at least 90% or substantially all) of nanostructures or microstructures. In one embodiment, a conventional top contact metal layer stack can be used. An electrode may alternatively or additionally be positioned on the bottom region of the nano- /microstructures (e.g. bottom 20% of the length of the structures). The contacts described herein are typically metallic. They are chosen to have an ohmic behaviour with the top layer. The contacts/contact pads can be electrically connected to the appropriate power supply lead of the device package.
Generally speaking, the compositions/devices of the invention may further comprise electrical contacts, preferably wherein one electrical contact is a layer positioned on top of the nanostructures or microstructures, and/or preferably wherein one electrical contact is positioned on said substrate.
In the present disclosure, where a particular light direction is referred to ‘in use’, it may equally be said that the composition/device is configured or arranged to emit/absorb light in said direction. In use, light may be emitted from (or absorbed by) said device omnidirectionally (i.e. in all directions, i.e. through the top of the device, through the substrate and through the side). The light may also be emitted (or absorbed) in a direction substantially opposite to a light reflective layer positioned on top of the nanostructures or microstructures. The light may therefore be emitted (or absorbed) through the substrate. The light may also be emitted sideways from the nanostructures or microstructures (or absorbed sideways into the nanostructures or microstructures). Any combination of the above may be contemplated and will depend on the choice of substrate and top layers in particular.
If light is emitted (or absorbed in the case of photodetectors) omnidirectionally, which includes via the top of the device (i.e. in a direction substantially opposite to the substrate), the device would not typically be a flip chip device, or is not in a flip chip configuration. Preferably, in this case there is no reflective layer at the top of the nanostructures/microstructures directing all light back towards (and through) the substrate. In a particular embodiment, therefore, the nanostructures/microstructures do not comprise a reflective layer continuously covering the top of the nanostructures/microstructures.
GaN is often used as a lll-V layer, but it has limited transparency. If none of the at least one lll-V layers are GaN, then omnidirectional light emission (or absorption for photodetectors) is typically preferred. In other words, for omnidirectional light emission/detection, GaN as a lll-V layer material is typically avoided.
Figure 4 shows a TM mode simulation result of a composition according to the present invention. The TM light is emitted substantially parallel to the intrinsic (emitting) layer, or parallel to the sidewalls of the at least one lll-V layer. The particular geometry of the nano-/microstructures with the semipolar (and/or pyramidal) layers provides efficient light emission from the device in a number of directions.
The at least one lll-V layer (preferably an intrinsic layer such as i-AIGaN) may be configured to emit (or absorb) TM polarized light via the side and/or bottom of the device, preferably via: a) emission (or absorption) of TM polarized light substantially parallel to the lll-V layer and in a direction substantially towards the substrate-side of the device (or from the substrate side of the device for a light absorbing device); and/or b) emission (or absorption) of TM polarized light substantially parallel to the lll-V layer and in (or from) a direction substantially opposite to the substrate-side of the device, wherein said light reflects off a reflective layer positioned on top of said nanostructures or microstructures.
Substantially parallel typically means herein that at least 50%, at least 75% at least 90% or at least 99% of the light is emitted (or absorbed) within ±10° of a plane of the lll-V layer.
In practice, a mix of TE and TM polarization light is produced. However, the TM light emitted from (or absorbed by) the device can be significantly stronger than the TE light. The advantage of having increased TM light emission is that one can get more total light out of the device if the light generated in the active area has a higher degree of TM-polarization. This is a typical effect when increasing the Al- content of an active AlxGai-xN layer to reduce the wavelength of the emitted (or absorbed) light. Another advantage with TM over TE polarized light is that there exists a so-called Brewster angle for TM polarized light where there is no reflection, hence potentially increasing the light output. Preferably, the light emitted from (or absorbed by) the device comprises at least 50% TM light, e.g. at least 75% TM light, e.g. at least 90% TM light, e.g. at least 95% TM light, e.g. at least 99% TM light. The upper limit could be 100% or 99% TM light for example. A particular advantage of the present compositions is the ability to emit (or absorb) more TM light than what is possible for a thin film.
The present disclosure also provides method of emitting light from a device as defined herein (or absorbing light by a device as defined herein), said method comprising emitting (or absorbing) transverse magnetic (TM) polarized light via the side and/or bottom of the device, preferably via: a) emission (or absorption) of TM polarized light parallel to the lll-V layer and in a direction substantially towards the substrate-side of the device; and/or
b) emission (or absorption) of TM polarized light parallel to the lll-V and in a direction substantially opposite to the substrate-side of the device, wherein said light reflects off a reflective layer positioned on top of said nanostructures or microstructures
The present disclosure also provides the use of group lll-V semipolar layers for increasing the transverse magnetic (TM) light emission from LEDs comprising nanostructures and/or microstructures on a substrate.
As mentioned above, the doped substrate can act as an active injector of current. The conductive pathway is typically vertical (up or down), i.e. in the same axial direction as the nano-/microstructures. If the substrate is p-doped, the conductive pathway of the holes (which is the same as the direction of the current) is from bottom to top. For n-doped substrate the conductive pathway of the electrons is from bottom to top (NB: the direction of the current is defined as opposite to the direction of the electrons).
Patterning
The positioned nanostructures or microstructures grow initially, or nucleate, from the substrate. If a mask layer is used, that means that holes need to be patterned through the mask layer. Making of these holes is a well-known process and can be carried out using e-beam lithography and etching or any other known techniques. The hole patterns in a mask can be easily fabricated using conventional lithography techniques such as photo/e-beam lithography, nanoimprinting, and so on. Focussed ion beam technology may also be used in order to create a regular array of nucleation sites on the substrate surface for the nanostructures or microstructures growth. The holes created in any mask layer can be arranged in any pattern which is desired.
Alternatively, the positioning of the nanostructures or microstructures can be controlled by growing them from holes, defects or hillocks in the substrate. These can be produced by selective etching away of the substrate (e.g. using reactive ion etching or chemical wet etching). Direct hole patterning can be performed on any substrate (e.g. directly on AIGaN or AIN substrate).
Selective area growth of nanostructures or microstructures
Molecular beam epitaxy (MBE) is a method of forming depositions on crystalline substrates. The MBE process is performed by heating a crystalline substrate in a vacuum so as to energize the substrate's lattice structure. Then, an atomic or molecular mass beam(s) is directed onto the substrate's surface. The term element used above is intended to cover application of atoms, molecules or ions of that element. When the directed atoms or molecules arrive at the substrate's surface, the directed atoms or molecules encounter the substrate's energized lattice structure or a catalyst droplet as described in detail below. Over time, the oncoming atoms form a nanowire.
Metalorganic vapour phase epitaxy (MOVPE) also called as metalorganic chemical vapour deposition (MOCVD) is an alternative method to MBE for forming depositions on crystalline substrates. In case of MOVPE/MOCVD, the deposition material is supplied in the form of metalorganic precursors, which on reaching the high temperature substrate decomposes leaving atoms on the substrate surface. In addition, this method requires a carrier gas (typically H2 and/or N2) to transport deposition materials (atoms/molecules) across the substrate surface. These atoms reacting with other atoms form an epitaxial layer on the substrate surface. Choosing the deposition parameters carefully results in the formation of a nanowire.
The nanostructures or microstructures of the invention may be grown by selective area growth (SAG) method. Inside the growth chamber in case of MBE or inside the reactor in case of MOVPE/MOCVD, the substrate temperature can be set to a temperature suitable for the growth of the nanostructures or microstructures in question. In case of MBE, the growth temperature may be in the range 300 to 1000 °C. The temperature employed is, however, specific to the nature of the material in the nanowire. For GaN, a preferred temperature is 700 to 950 °C, e.g. 800 to 900 °C, such as 810 °C. For AIGaN the range is slightly higher, for example 800 to 980 °C, such as 830 to 950 °C, e.g. 850 °C.
Preferably, the nanostructure or microstructures are formed by MOCVD. The MOCVD is typically used for growing the structure cores, as well as any additional lll-V layers on top of the cores.
As previously discussed, it will be appreciated therefore that the nanostructures or microstructures can comprise different group lll-V semiconductors within the nanostructure or microstructure, e.g. starting with an AIGaN core followed by an AIGaN layer component and so on.
Nanowire MBE growth can be initiated by opening the shutter of the Ga effusion cell, the nitrogen plasma cell, and the dopant cell simultaneously initiating the growth of doped GaN nanostructures or microstructures, hereby called as stem. The length of the GaN stem can be kept between 10 nm to several 100s of nanometers. Subsequently, one could increase the substrate temperature if needed, and open the Al shutter to initiate the growth of AIGaN nanostructures or microstructures. One could initiate the growth of AIGaN nanostructures or microstructures on the substrate without the growth of GaN stem, but a GaN stem is preferred, n- and p- doped nanostructures or microstructures can be obtained by opening the shutter of the n-dopant cell and p-dopant cell, respectively, during the growth of the nanostructures or microstructures. For ex: Si dopant cell for n-doping of nanostructures or microstructures, and Mg dopant cell for p-doping of nanostructures or microstructures.
The temperature of the effusion cells can be used to control growth rate. Convenient growth rates, as measured during conventional planar (layer by layer) growth, are 0.05 to 2 pm per hour, e.g. 0.1 pm per hour. The ratio of Al/Ga can be varied by changing the temperature of the effusion cells.
The pressure of the molecular beams can also be adjusted depending on the nature of the nanostructures or microstructures being grown. Suitable levels for beam equivalent pressures are between 1 x 10'7 and 1 x 10'4 Torr.
The beam flux ratio between reactants (e.g. group III atoms and group V molecules) can be varied, the preferred flux ratio being dependent on other growth parameters and on the nature of the nano-/microstructures being grown. In the case of nitrides, which is preferred, nanostructures or microstructures are always grown under nitrogen rich conditions.
It is an embodiment of the invention, a multistep, such as two step, growth procedure is employed, e.g. to separately optimize the nucleation and growth.
In case of MOVPE, a significant benefit is that the nanostructures or microstructures can be grown at a much faster growth rate. This method allows the growth of axial heterostructured nanostructures or microstructures using techniques such as pulsed growth technique or continuous growth mode with modified growth parameters for e.g., lower V/lll molar ratio and higher substrate temperature.
In more detail, the reactor must be evacuated after placing the sample, and is purged with N2 to remove oxygen and water in the reactor. This is to avoid any damage to any substrate or mask layer, if present, at the growth temperatures, and
to avoid unwanted reactions of oxygen and water with the precursors. The total pressure is set to be between 20 and 400 Torr.
After purging the reactor with N2, the substrate is thermally cleaned under H2 atmosphere at a substrate temperature of about 1200 °C. After thermal cleaning in H2 a substrate conditioning step can be performed to make the substrate surface more favourable for the subsequent material growth (e.g. AIGaN). In the conditioning step one group of precursors is switched on while the other group of precursors remains switched off so that no material growth starts. Mentioned groups of precursors are either group-ill metalorganics (e.g. TMAI, TMGa, TMIn) or group-V hydrides (e.g. NH3). Preferably, group-V hydrides are switched on in the conditioning step, more preferably NH3.
The substrate temperature can then be set to a temperature suitable for the growth of the nanostructures or microstructures in question. The growth temperature may be in the range 700 to 1250°C. The temperature employed is, however, specific to the nature of the material in the nanostructures or microstructures. For GaN, a preferred temperature is 800 to 1150°C, e.g. 900 to 1100 °C, such as 1100°C or 1000°C. For AIGaN the range is slightly higher, for example 900 to 1250°C, such as 1050 to 1250°C, e.g. 1250°C or 1150°C.
The metal organic precursors for the nano-/microstructures growth can be either trimethylgallium (TMGa), or triethylgallium (TEGa) for Ga, trimethylalumnium (TMAI) or triethylalumnium (TEAI) for Al, and trimethylindium (TMIn) or triethylindium (TEIn) for In. The precursors for dopants can be SiF for silicon and bis(cyclopentadienyl)magnesium (Cp2Mg) or bis(methylcyclopentadienyl)magnesium ((MeCp)2Mg) for Mg. The flow rate of TMGa, TMAI and TMIn can be maintained between 5 and 2000 seem. The NH3 flow rate can be varied between 10 and 20000 seem.
Growth rate is a result of different reactor parameters, mainly precursor flows and temperature. The growth rate can be intentionally adjusted and used as a tool to tune not only the nanostructures or microstructures dimensions, but also shape, defects, or electrical properties. Growth rates can range from 0.05 pm/h to 5 pm/h, typically 100 nm/h for thinner layers and 2 pm/h for thicker layers.
The pressure in the MOCVD reactor is given by the carrier gas and can be individually adjusted for each growth step. The pressure will influence the gas flow pattern and can be used to achieve good uniformity of nanostructures or
microstructures over a large substrate area of one or multiple substrates. Reactor pressures can range from 20 Torr to 400 Torr, e.g., 100 Torr.
The V/l 11 ratio is given by the partial pressure ratio of group-V over group-ill precursors and determined by the precursor flows. The V/l 11 ratio can influence the growth rate, material molar ratio, morphology, or electrical properties. The skilled worker utilizes the V/lll ratio to tune the growth process to the desired outcome. In MOCVD, the V/lll ratio can range from 5 to 10000, preferably from 50 to 2000, e.g. 100 or 1000.
In particular, the simple use of vapour-solid growth may enable nanostructures or microstructures growth. Thus, in the context of MBE, simple application of the reactants, e.g. In and N, to the substrate without any catalyst can result in the formation of nanostructures or microstructures. This forms a further aspect of the invention which therefore provides the direct growth of a semiconductor nanostructure or microstructure formed from the elements described above on a substrate. The term direct implies therefore the absence of a film of catalyst to enable growth.
The nanostructures or microstructures of the invention may also be grown in the presence of a catalyst. A catalyst can be introduced into holes (of the mask or of the substrate) to provide nucleating sites for nanostructure or microstructure growth. The catalyst can be one of the elements making up the nanostructure or microstructure so-called self-catalysed, or different from any of the elements making up the nanowire (e.g. Au or Ag).
The use of catalyst assisted-growth is well-established in the field. Suitable methods are described in W02021/009325 for example.
Nanowire growth can be initiated by opening the shutter of the group III effusion cell and the counter ion effusion cell, simultaneously once a catalyst film has been deposited and melted.
The temperature of the effusion cells can be used to control growth rate. Convenient growth rates, as measured during conventional planar (layer by layer) growth, are 0.05 to 2 pm per hour, e.g. 0.1 pm per hour.
The pressure of the molecular beams can also be adjusted depending on the nature of the nanostructures or microstructures being grown. Suitable levels for beam equivalent pressures are between 1 x 10'7 and 1 x 10'5 Torr.
The beam flux ratio between reactants (e.g. group III atoms and group V molecules) can be varied, the preferred flux ratio being dependent on other growth
parameters and on the nature of the nano-/microstructures being grown. It is known that the beam flux ratio between reactants can affect crystal structure of the nanostructures or microstructures.
Nanostructure or microstructure diameter can in some cases be varied by changing the growth parameters, e.g. the flux ratio of the 11 l-V precursors. The skilled worker is therefore able to manipulate the nanostructures or microstructures in a number of ways.
Moreover, the size of the holes can be controlled to ensure that only one nanostructure or microstructure can grow in each hole (i.e. holes in a mask or holes in a substrate). It is therefore preferred if only one nanostructure or microstructure grows per hole in the mask. Finally, the holes can be made of a size where the droplet of any catalyst that forms within the hole is sufficiently large to allow nanostructure or microstructure growth. In this way, a regular array of nanostructures or microstructures can be grown.
Brief description of Figures
Figure 1 shows a first composition according to the invention.
Figure 2 shows a plurality of nano-/microstructures as shown in Figure 1 , in which the nano-/microstructures are joined together by regions of amorphous/polycrystalline II l-V material.
Figure 3 shows emission of TM light from the intrinsic layer of the composition of Figure 2 (in the case of a light emitter)
Figure 4 is a simulation of TM emitted light from the composition of Figure 2.
Figure 5 is a design variation of Figure 1 , in which there is no p-GaN layer on the p- side. The p-contact (or reflector) is on the top p-AIGaN layer. The absence of p- GaN increases emission of light via the top of the device (in a top-emitting LED for example), or via the sides/bottom for bottom or omnidirectional light extraction. For photodetectors, the absence of p-GaN increases absorption of light via these pathways.
Figure 6 shows a design variation of Figure 1, in which there is a tunnel junction comprising an (un)doped (In, Al, Ga)N interlayer. The tunnel junction is beneficial for electrical and optical properties. It enables efficient current injection by highly conductive n-AIGaN as well as reduced absorption by avoiding p-GaN.
Figure 7 shows a design variation of Figure 1, in which the substrate comprises a top graphene layer. The high electrical conductivity of graphene can enable parallel current spreading in addition to the n-AIGaN. The high thermal conductivity of graphene can enable better heat dissipation, thereby increasing the internal quantum efficiency and the total wall-plug efficiency (WPE).
Figure 8 shows a design variation of Figure 1, in which the mask layer is a multilayer mask comprising SiC>2 and a graphene layer.
Figure 9 shows a design variation of Figure 2, in which patterns of holes are created on the substrate directly, which are used for the selective growth of the nano-/microstructures. This can increase light extraction efficiency by removing reflections or interference at the mask layer. The refractive index remains high. Figure 10 shows a design variation of Figure 2, in which a sacrificial layer is positioned within the substrate. The whole top heterostructure can be lifted off from the sapphire using chemical, electrochemical or laser lift off techniques. The sacrificial layer could be GaN or highly doped n-AIGaN, for example. An advantage is that the remaining structure with the thinner substrate may have higher light extraction efficiency, and the thin structures may be suitable for flexible devices. During lift off, the sacrificial layer is typically selectively destroyed/removed, leaving the top and bottom parts separated. The sacrificial layer could also be positioned between AIN and n-AIGaN, leaving the bottom (AIN+sapphire) and the top (nAIGaN+nanowires) separated.
Figure 11a-c shows SEM images at various stages prior to nanostructure growth. Figure 12a shows a TEM cross-section image of the full nanostructure with SiO2 mask on a n-AIGaN/AIN/sapphire substrate. Figure 12b shows the electroluminescence spectra of the nanostructure LEDs at different current levels. Figure 13 shows emission spectra (Fig. 13a) and LI characteristics (Fig. 13b) of semipolar versus polar UV LEDs fabricated on different substrates. This comparison illustrates the advantages of semipolar UVC LEDs.
Examples
The 2D-simulations were done in a wave optics approach in the COMSOL Multiphysics software suite. The optical material properties (refractive index and extinction coefficient) were assigned to the different areas as described in the text.
The model was excited by a point source, emitting TM-polarized light in one of the nanowires. Figure 4 shows the absolute value of the Pointing’s vector in dB, which is equal to the intensity, for one specific position of the point source. To get the full intensity output, one would need to integrate/sum this contribution for point sources over all positions in the active region. At the boundaries of the model 2nd order scattering conditions were used to reduce the amount of light reflected back into the model.
Fig. 12a shows a cross-section TEM (transmission electron microscopy) image of an epitaxial nanowire structure which enables a functional UVC LED with electro-optical performance data shown in Fig. 12b. In this embodiment, the substrate consists of a 1-pm-thick n-type doped AIGaN layer grown on AIN on 2- inch sapphire wafer. The substrate surface was patterned using a 35-nm-thin SiO2 mask with hexagonal hole pattern, fabricated by oxide coating and subsequent resist deposition, lithographic exposure, and etching followed by standard cleaning procedures. The AIGaN-based LED heterostructure was grown by MOCVD under nitrogen-rich conditions. The n-AIGaN buffer pyramid was nucleated at a temperature of > 1000 degree Celsius and extended until full development of the semipolar [10-11] pyramid facets near coalescence. Manifold functional AIGaN layers were grown subsequentially with specified composition and doping. These include Si-doped layers in the n-side and Mg-doped layers in the p-side, as well as quantum well layers with low Al-content (-50%), n-AIGaN layers with intermediate Al-content (-60%), and electron blocking layers with high Al-content (-80%). The last epitaxial layer in the p-side is a Mg-doped GaN layer. Preliminary emission spectra and intensity data were collected in a quick test electroluminescence setup with non-optimized metal contacts as shown in Fig. 12b.
Figure 13 shows a comparison of emission spectra (Fig. 13a) and LI (light intensity vs. current) characteristics (Fig. 13b) from UV LEDs fabricated with semipolar versus polar structure. To illustrate the advantages of the semipolar design over conventional polar designs, the identical layer stack was grown on two kinds of substrate: On patterned n-AIGaN as semipolar NW and on unpatterned n- AIGaN as polar heterostructure film. The electro-optical characteristics of both LEDs were determined in a quick test electroluminescence setup. Both integrated intensity and spectral intensity are superior for the semipolar LED in this comparison. In addition, the emission wavelength of the polar LED is red-shifted
towards 300 nm, whereas the semipolar UVC LED shows the desired emission near 260 nm.
Claims
1. A composition of matter comprising a substrate; a plurality of nanostructures or microstructures grown from said substrate, wherein said nanostructures or microstructures comprise: nanostructure cores or microstructure cores respectively, comprising at least one semiconducting group lll-V compound, at least one lll-V layer positioned on top of said nanostructure cores or microstructure cores, wherein at least part of said at least one lll-V layer is semipolar; and wherein said nanostructures or microstructures comprise a p-n or p-i-n junction.
2. A composition of matter comprising a substrate; a plurality of nanostructures or microstructures grown from said substrate, wherein said nanostructures or microstructures comprise: nanostructure cores or microstructure cores respectively, comprising at least one semiconducting group lll-V compound, at least one lll-V layer positioned on top of said nanostructure cores or microstructure cores, wherein at least part of said at least one lll-V layer is a pyramidal layer; and wherein said nanostructures or microstructures comprise a p-n or p-i-n junction.
3. The composition of matter as claimed in any preceding claim, wherein said nanostructures are nanowires or nanopyramids, and wherein said microstructures are microwires or micropyramids, preferably wherein said plurality of nanostructures or microstructures is a plurality of nanowires or nanopyramids, and wherein said nanostructure cores or microstructure cores are nanowire cores or nanopyramid cores.
4. The composition of matter as claimed in any preceding claim, wherein said substrate is a doped substrate, preferably an n-doped substrate.
5. The composition of matter as claimed in any preceding claim, wherein said substrate comprises a semiconducting group lll-V compound, e.g. AIGaN (preferably n-AIGaN) and/or AIN.
6. The composition of matter as claimed in any preceding claim, wherein said at least one lll-V layer has {10-11} crystallinity.
7. The composition of matter as claimed in any preceding claim, wherein the lll-V layer comprises sidewalls extending from a pyramid peak, and wherein said sidewalls are semipolar.
8. The composition of matter as claimed in any preceding claim, wherein the internal pyramidal angle of said at least one lll-V layer is in the range 5-175°, preferably 40-70°, preferably 45-65°, preferably 50-60°, e.g. 56°.
9. The composition of matter as claimed in any preceding claim, further comprising a mask layer on top of said substrate; and wherein the nanostructures or microstructures are grown through a plurality of holes in the mask layer.
10. The composition of matter as claimed in any preceding claim, wherein the nanostructure cores or microstructure cores comprise, or consist of, AIGaN, preferably n-AIGaN or p-AIGaN, preferably n-AIGaN.
11. The composition of matter as claimed in any preceding claim, wherein the at least one lll-V layer comprises, or consists of, AIGaN, preferably i-AIGaN and n- AIGaN layers or i-AIGaN and p-AIGaN layers.
12. The composition of matter as claimed in any preceding claim, comprising at least one optional intrinsic lll-V semiconducting layer, and at least one n-type doped lll-V layer if the nanostructure or microstructure cores are p-type doped, or at least one p-type doped lll-V layer if the nanostructure or microstructure cores are n-type doped.
13. The composition of matter as claimed in any preceding claim, wherein the edges of the at least one lll-V layer(s) form a vertical side wall with the nanostructure/microstructure core.
14. A composition as claimed in any preceding claim in which said nanostructures or microstructures are grown epitaxially from the substrate, preferably through the holes in a mask.
15. The composition of matter as claimed in any preceding claim, comprising a layer (e.g. a lll-V layer and/or a top electrode layer) continuously covering at least a portion of the plurality of nanostructures or microstructures, e.g. at least 50%, at least 75%, at least 90%, or at least 99% of the nanostructures or microstructures.
16. The composition of matter as claimed in any preceding claim, wherein said composition of matter further comprises a planar group lll-V layer in contact with the substrate, on the opposite side of the substrate to the nanostructures or microstructures, preferably wherein said planar group lll-V layer is AIN.
17. A composition of matter comprising: a substrate; a non-planar coalesced structure on said substrate, e.g. formed from a plurality of coalesced nanostructures or microstructures, said non-planar coalesced structure and/or nanostructures or microstructures comprising at least one semiconducting group lll-V compound; wherein said non-planar coalesced structure comprises at least one lll-V layer, typically positioned on top of coalesced nanostructure cores or microstructure cores, wherein at least part of said at least one lll-V layer is semipolar; and preferably wherein said at least one lll-V layer extends continuously over the plurality of coalesced nanostructure cores or microstructure cores.
18. A device, such as an electronic or opto-electronic device, comprising a composition of matter as claimed in any of claims 1-17, e.g. wherein said device is a solar cell, photodetector, transistor, laser or LED, preferably an LED, more preferably a UV LED, more preferably a UV-C LED.
19. A device as claimed in claim 18, wherein in use light is emitted from (or absorbed by) said device in at least one of the following directions: omnidirectionally (i.e. in all directions), in a direction substantially opposite to a light reflective layer positioned on top of the nanostructures or microstructures, through the substrate; and/or sideways from the nanostructures or microstructures (or sideways into/by the nanostructures or microstructures).
20. A device as claimed in claim 18 or 19, wherein in use TM polarized light/photons are emitted (or absorbed) substantially parallel to the at least one lll-V layer, e.g. parallel to sidewalls of the at least one pyramidal lll-V layer
21. A device as claimed in any of claims 18-20, wherein the at least one lll-V layer (preferably an intrinsic layer such as i-AIGaN), is configured to emit (or absorb) TM polarized light via the side and/or bottom of the device, preferably via: a) emission (or absorption) of TM polarized light substantially parallel to the lll-V layer and in a direction substantially towards the substrate-side of the device (or from the substrate side of the device for a light absorbing device); and/or b) emission (or absorption) of TM polarized light substantially parallel to the lll-V and in (or from) a direction substantially opposite to the substrate-side of the device, wherein said light reflects off a reflective layer positioned on top of said nanostructures or microstructures.
22. A process for preparing a composition of matter as claimed in any of claims 1-17, comprising:
(I) growing a plurality of nanostructure cores or microstructures cores from a substrate, e.g. via MOCVD, said nanostructure cores or microstructures cores comprising at least one semiconducting group lll-V compound;
(II) growing said least one lll-V layer on top of said nanostructure cores or microstructures cores.
23. A process for preparing a composition of matter as claimed in claim 17 comprising:
(I) growing a plurality of nanostructure cores or microstructures cores from a substrate, e.g. via MOCVD, said nanostructure cores or microstructures cores comprising at least one semiconducting group lll-V compound;
(II) growing said least one lll-V player on top of said nanostructure cores or microstructures cores; to the point that the nanostructure cores or microstructures cores are coalesced, and/or to the point that the at least one lll-V layer extends continuously over the plurality of nanostructure cores or microstructure cores
24. A method of emitting light from a device (or absorbing light by a device) as claimed in any of claims 18-21 , said method comprising emitting (or absorbing) transverse magnetic (TM) polarized light via the side and/or bottom of the device, preferably via: a) emission (or absorption) of TM polarized light substantially parallel to the lll-V layer and in a direction substantially towards the substrate-side of the device (or from the substrate side of the device for a light absorbing device) and/or b) emission (or absorption) of TM polarized light substantially parallel to the lll-V and in (or from) a direction substantially opposite to the substrate-side of the device, wherein said light reflects off a reflective layer positioned on top of said nanostructures or microstructures
25. The use of group lll-V pyramidal semipolar layers for increasing the transverse magnetic (TM) light emission from LEDs comprising nanostructures and/or microstructures on a substrate.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GBGB2212395.4A GB202212395D0 (en) | 2022-08-25 | 2022-08-25 | Nanostructure/Microstructure device |
| GB2212395.4 | 2022-08-25 |
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| Publication Number | Publication Date |
|---|---|
| WO2024042221A1 true WO2024042221A1 (en) | 2024-02-29 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| PCT/EP2023/073388 WO2024042221A1 (en) | 2022-08-25 | 2023-08-25 | Nanostructure/microstructure device |
Country Status (3)
| Country | Link |
|---|---|
| GB (1) | GB202212395D0 (en) |
| TW (1) | TW202425077A (en) |
| WO (1) | WO2024042221A1 (en) |
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|---|---|---|---|---|
| WO2016099491A1 (en) * | 2014-12-17 | 2016-06-23 | Intel Corporation | Integrated circuit die having reduced defect group iii-nitride structures and methods associated therewith |
| WO2017192667A1 (en) * | 2016-05-04 | 2017-11-09 | Schneider Jr Richard P | Monolithic multicolor direct view display containing different color leds and method of making thereof |
| EP3413412A2 (en) * | 2017-05-19 | 2018-12-12 | Palo Alto Research Center, Incorporated | Electron beam pumped non-c-plane uv emitters |
| WO2021009325A1 (en) | 2019-07-16 | 2021-01-21 | Crayonano As | Nanowire device |
| US20210119420A1 (en) * | 2019-10-15 | 2021-04-22 | The Regents Of The University Of Michigan | Nanocrystal surface-emitting lasers |
-
2022
- 2022-08-25 GB GBGB2212395.4A patent/GB202212395D0/en not_active Ceased
-
2023
- 2023-08-25 TW TW112132094A patent/TW202425077A/en unknown
- 2023-08-25 WO PCT/EP2023/073388 patent/WO2024042221A1/en unknown
Patent Citations (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2016099491A1 (en) * | 2014-12-17 | 2016-06-23 | Intel Corporation | Integrated circuit die having reduced defect group iii-nitride structures and methods associated therewith |
| WO2017192667A1 (en) * | 2016-05-04 | 2017-11-09 | Schneider Jr Richard P | Monolithic multicolor direct view display containing different color leds and method of making thereof |
| EP3413412A2 (en) * | 2017-05-19 | 2018-12-12 | Palo Alto Research Center, Incorporated | Electron beam pumped non-c-plane uv emitters |
| WO2021009325A1 (en) | 2019-07-16 | 2021-01-21 | Crayonano As | Nanowire device |
| US20210119420A1 (en) * | 2019-10-15 | 2021-04-22 | The Regents Of The University Of Michigan | Nanocrystal surface-emitting lasers |
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| TW202425077A (en) | 2024-06-16 |
| GB202212395D0 (en) | 2022-10-12 |
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