WO2013171284A1

WO2013171284A1 - Nanostructured antireflection layer, and application of same to leds

Info

Publication number: WO2013171284A1
Application number: PCT/EP2013/060093
Authority: WO
Inventors: Haiyan Ou
Original assignee: Danmarks Tekniske Universitet
Priority date: 2012-05-15
Filing date: 2013-05-15
Publication date: 2013-11-21
Also published as: CN104487873A; CN104487873B

Abstract

An optical device having a surface in a silicon carbide or gallium nitride material is provided, the optical device having a non-periodic nano structure formed in the surface, the nano structure comprising a plurality of cone shaped structures wherein the cones are distributed non-periodically on the surface. The plurality of cone shaped structures have a random height distribution and at least a part of the cone shaped structures have a height of at least 100 nm. The nonperiodicity ensures a uniform spatial light distribution after light exits out of the chip. A method of manufacturing a non-periodic nano structured surface on an optical device is furthermore provided, the method comprising the steps of providing a silicon carbide or gallium nitride device,forming a thin film of a masking material on at least a part of the substrate, treating the thin film to form nano islands of the thin film material, etching the substrate in a mostly anisotropic etch and concurrently etching at least a part of the thin film masking material to form a non-periodical nano structure, the nano structure comprising a plurality of cone shaped surface structures. The optical device may comprise a white LED or a wavelength converter for a white light source.

Description

NANOSTRUCTURED ANTIREFLECTION LAYER, AND APPLICATION OF SAME TO LEDS

FIELD OF INVENTION The present invention relates to nano structured surfaces, especially to nano structured surfaces in silicon carbide or gallium nitride surfaces having non- periodic nano structures formed therein and to a method of fabricating such structures. The invention furthermore relates to light-emitting diodes having a non-periodic nano structure in the output surface.

BACKGROUND

Light-emitting diodes (LEDs) have attracted renewed interest in the past decades with the appearance of the world's first efficient blue-emitting GaN or InGaN light-emitting diodes which made it technical possible to provide all- solid-state white lighting for large-scale energy saving. When comparing light-emitting diodes with the traditional incandescent and fluorescent light sources, light-emitting diodes have many advantages including lower energy consumption, longer lifetime, improved robustness, smaller size, and faster switching, as well as better technical functionality for many new lighting applications. Driven by energy saving and CO2 emission reduction

requirements, high brightness light-emitting diodes are seen as

environmentally beneficial light sources and they may represent a multi- billion market.

However, there are still some challenges before the light-emitting diodes will reach the full potential. Light-emitting diodes adequate for room lighting are relatively expensive and require more precise current and heat management than compact fluorescent lamp sources of comparable output. One reason being a low extraction efficiency for semi conductor light-emitting diodes. Typically, solid-state light-emitting diodes e.g. such as GaN light-emitting diodes have been grown on sapphire substrates or silicon substrates. Lately, however, silicon carbide substrates have been preferred at least in the laboratories as silicon carbide has a better lattice match with for example the GaN and improved thermal conductivity when compared to sapphire. The substrates used are typically transparent substrates, such as sapphire or silicon carbide, allowing for transmission of the generated light.

To obtain a white light source based on the blue light-emitting diodes, wavelength converters have been employed using for example a YAG phosphor coating. The phosphors convert the blue light to yellow (down- converting) and by mixing the yellow light with blue light, light which appears white is produced. However, phosphors degrade much faster than the semiconductor light-emitting diode chips, so that there is a tendency to the white LEDs turning blue over time. Furthermore, the rare earth materials, such as Yttrium, forming part of the phosphor coating increase the price of the devices.

Recently, fluorescent silicon carbide has been found to be an interesting wavelength converter to be used in connection with the blue or near ultraviolet (UV) light-emitting semiconductor diodes as fluorescent silicon carbide has better colour rendering ability, a long life time and does not contain rare earth materials. However, the manufactured light-emitting diodes still suffer from a low extraction efficiency.

Typically, single layer quarter-wavelength thin-film antireflection coatings have been applied to enhance the lighting for a specific wavelength at very low level. Enhancing of the extraction efficiency for a broader wavelength spectrum may be achieved by applying a stack of antireflection coatings with appropriate refractive indices; this design, however, requires tightly matching of the thermal expansion coefficient. Also, periodic photonic crystals have been demonstrated as an effective way to enhance the light extraction efficiency, for example by Ou, Y et al. Optics Express, vol. 20, No. 7, 7575-7579, "Broadband and omnidirectional light harvesting enhancement of fluorescent SiC". However, these structures are made using expensive and time-consuming electron beam lithography, which brings huge extra costs and restrains the scalability.

By e.g. Song et al, in Applied physics letters 97, D931 10-1 -3, "Disordered antireflective nanostructures on GaN based light-emitting diodes using Ag nanoparticles for improved light extraction efficiency", it has been suggested to provide a disordered subwavelength structure in an ITO coating. The method includes the deposition of a silicon dioxide layer on the ITO contact layer as a durable etch mask and a buffer layer to form Ag nanoparticles. A Ag thin film layer is deposited on top of the S1O2 layer, the layer is annealed to form separated nanoparticles by self-assembled agglomeration.

Hereafter, the S1O2 is patterned using the Ag nanoscale mask whereafter, the ITO is processed in a further etching process to create subwavelength structures in the ITO.

It is a disadvantage of having the nanostructure in a coating layer in that there is always a certain loss in the transition between two materials, furthermore, the method as suggested is quite complex and requires two masking steps and two etching steps to create a subwavelength

nanostructure in the ITO layer.

It has furthermore been suggested by Dylewicz in Appl. Phys B (2012) 107: 393-399, "Nanostructured graded-index antireflection layer formation on GaN for enhancing light extraction from light-emitting diodes" to provide a random surface roughening with submicron spatial structures below 100nm. However, it is a disadvantage of the surface roughening that the

nanostructures are too small to achieve a guiding of the light, and thereby too small to efficiently increase the transmittance. SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical device having improved surface characteristics. It is a further object of the present invention to provide a light-emitting diode having a nano structure in an integrated part of the light-emitting diode.

According to the above and other objects, a device with at least one surface having a non-periodical nano structure is provided. The device may be an optical device and may have at least one surface in a material and the material may be a silicon carbide material or a gallium nitride material. The at least one surface may have the non-periodical nano structure formed in the material at the light output surface, i.e. at a refractive surface. The nano structure may comprise a plurality of cone shaped structures wherein the cones are distributed non-periodically on the surface.

The cone shaped structures, which may hereinafter be referred to as cones, may be nano-sized cones. The material may be a single crystalline material, and the non-periodical nano structures may be formed in the single crystalline material.

According to one aspect of the invention an optical device is provided, the optical device having at least one surface in a silicon carbide or gallium nitride material configured to transmit optical radiation from within the device towards the surroundings, i.e. through the output surface. The at least one surface has a subwavelength nanostructure formed in the silicon carbide or gallium nitride material. The subwavelength nanostructure comprises a plurality of cone shaped structures, wherein the plurality of cone shaped structures are distributed non-periodically over the at least one surface and may have a random height distribution. At least a part of the cone shaped structures may have a height of at least 100 nm.

According to another aspect of the present invention a silicon carbide substrate or a gallium nitride substrate configured to be used for optical devices is provided, the substrate having a first surface configured to transmit optical radiation from within the substrate towards the surroundings, a subwavelength nanostructure being provided in the first surface of the silicon carbide substrate/the gallium nitride substrate, the subwavelength

nanostructure formed in the first surface of the substrate comprises a plurality of cone shaped structures, wherein the plurality of cone shaped structures are distributed non-periodically over the at least one surface. The plurality of cone shaped structures may have a random height distribution and, at least a part of the cone shaped structures may have a height of at least 100 nm.

According to a still further aspect of the present invention, a method of manufacturing at least one subwavelength nanostructure in a surface of a silicon carbide substrate or a gallium nitride substrate is provided, to configure the silicon carbide substrate or the gallium nitride substrate for low reflectivity in a range of wavelengths, the range of wavelengths having a center wavelength, the method comprising the steps of

providing a silicon carbide or gallium nitride substrate,

providing a thin film material on at least a part of the substrate,

treating the thin film to form self-assembled nano islands of the thin film material, the nano islands being configured to mask at least a part of the substrate during at least a part of the etching,

etching the substrate at a first etch rate in a substantially anisotropic etch using the nano islands as mask, and concurrently, etching at least a part of the nano islands at a second etch rate, the second etch rate being lower than first etch rate, to thereby form a subwavelength nanostructure comprising a plurality of cone shaped surface structures, the cone shaped structures being distributed non-periodically over the surface of the substrate. The plurality of cone shaped structures may have a random height distribution and at least a part of the cone shaped structures may have a height of at least 100 nm. As a final step, the remaining thin film material may be removed.

According to a further aspect of the present invention a method of fabricating a light-emitting diode is provided, the method comprises

providing a single crystalline silicon carbide substrate, a single crystalline fluorescent silicon carbide substrate or a single crystalline gallium nitride substrate. Providing a subwavelength nanostructure according to the above described method in a first surface of the single crystalline substrate, the subwavelength nanostructure comprising a plurality of cone shaped silicon carbide/fluorescent silicon carbide/gallium nitride structures, and

on a second surface of the substrate monolithically growing a nitride based light-emitting diode comprising at least an n-doped gallium nitride layer and a p-doped gallium nitride layer. It is an advantage of distributing the plurality of cone shaped structures non- periodically on the surface in that the reflectivity of the surface is significantly reduced.

It is furthermore an advantage of the present invention that the plurality of cone shaped structures may have a random height distribution. The random height distribution ensures optimized or improved transmission and/or reflection properties for the nano structured surface over a wavelength range, such as in a wide wavelength range, for example such as for electromagnetic radiation in all or at least a part of the visible spectrum, and/or for

electromagnetic radiation ranging from infrared radiation, such as near infrared radiation, to ultraviolet radiation, such as near ultra violet radiation. It is furthermore an advantage of the present invention that the plurality of cone shaped structures may have a random height distribution, a random structure size and a random structure distance. The nonperiodicity ensures a uniform spatial light distribution after light exits out of the chip.

Typically in the prior art, nanostructures have been provided in periodic structures wherein each of the fabricated "cones" or peaks have a

substantially same height. Hereby, the reflectance and/or transmittance is improved primarily for a specific emission angle or a narrow distribution of angles.

It has furthermore been found that the reflectivity is significantly reduced also from the inner surface of the nanostructured surface of the optical device, from which direction the light experience a nanostructured surface having cone shaped indentations. Thereby, the photoluminescence of the nano structured surface is significantly increased.

In a still further aspect of the present invention, a method of increasing transmittance of at least a part of a wafer surface is provided. The method comprises manufacturing a subwavelength nanostructure in a surface of a silicon carbide or gallium nitride substrate to configure the substrate for increased transmittance in a range of wavelengths, the range of wavelengths having a center wavelength, the method comprising the steps of

providing a silicon carbide or gallium nitride wafer,

providing a thin film material on at least a part of the substrate,

treating the thin film to form self-assembled nano islands of the thin film material, the nano-islands being configured to mask at least a part of the substrate during etching,

etching the substrate at a first etch rate in a substantially anisotropic etch using the nano islands as mask, and concurrently, etching at least a part of the nano islands at a second etch rate, the second etch rate being lower than first etch rate, to thereby form a subwavelength nanostructure comprising a plurality of cone shaped surface structures, wherein the plurality of cone shaped structures are distributed non-periodically over the at least one surface. The plurality of cone shaped structures may have a random height distribution. At least a part of the cone shaped structures may have height of at least 100 nm, to thereby allow increased transmission of diffused light for a plurality of optical devices distributed on the wafer. After the nano structures are formed the thin film material may be removed. The subwavelength nanostructure may be provided in a surface of the entire wafer.

It is a significant advantage of the present invention that the method is scalable and may be used on wafer scale so that an entire wafer comprising a plurality of optical devices may be prepared in one process run. It is a further advantage of the present invention that a planar surface of the wafer may be provided for the method. Thus, a nano structure may be provided simultaneously in a plurality of optical devices having a common planar surface.

An optical device as used herein may be any device, including any surface, configured for receiving or transmitting light of any wavelength. The surface may be a refracting surface. The optical device may have at least one surface in the silicon carbide or the gallium nitride material and the at least one surface may be at least one surface of a substrate and/or at least one surface of an active element of the optical device provided in the silicon carbide or a gallium nitride material In one or more embodiments, the gallium nitride material forms part of a light- emitting diode, such as forms part of an active part of the light-emitting diode, such as for example the pn junction of the light-emitting diode. This light emitting structure may be provided on any substrate, such as sapphire, silicon carbide, silicon, etc. In another aspect of the present invention, a method of manufacturing a device having at least one nano structured surface is provided. The device may be an optical device. The method comprises providing a substrate, such as a single crystalline substrate, such as a silicon carbide substrate or a gallium nitride substrate. On at least a part of the substrate a thin film of a masking material may be formed, and the thin film may be treated to form nano islands of the thin film material. The substrate, such as the single- crystalline substrate, may be etched in a mostly anisotropic etch and at least a part of the thin film material may be etched concurrently to form a non- periodical nano structure. The nano structure may comprise a plurality of cone shaped surface structures. In a last step of the method, the thin film material may be removed, for example by etching the thin film material, for example using a wet etch.

In another aspect of the present invention, a wavelength converter is provided. The wavelength converter may have at least one surface in a single crystalline material, such as in a single crystalline silicon carbide or gallium nitride substrate. The at least one surface may have a nano structure formed in the single crystalline material and the nano structure may comprise a plurality of cone shaped structures wherein the cones are distributed non- periodically on the surface.

In one or more embodiments, the light-emitting diode structure may thus further comprise a wavelength converter. The wavelength converter may have at least one surface in silicon carbide or gallium nitride, the at least one surface having a subwavelength nanostructure formed in the silicon carbide or gallium nitride material, respectively. In a further aspect of the present invention, an optical device comprising a light-emitting diode is provided. The light-emitting diode may emit light primarily in a first wavelength range, and the optical device may further have at least one surface, such as a surface of a single crystalline material, having a nano structure as described herein. The optical device may furthermore comprise a wavelength converter as described herein.

It is an advantage of being able to provide the nanostructure directly in the single crystalline material in that the there is no difference in thermal expansion coefficients, and furthermore, that there is no difference in refractive index between the substrate material and the cone shaped nano structure, i.e. the substrate and the cone shaped nano structure has a same refractive index. Thereby, a gradient refractive index may be achieved and the light will not, or substantially not, experience any interface. For example, if the refractive index of the bottom of the nanostructures is the substrate refractive index, the effective refractive index of the nano structures will gradiently change from the substrate refractive index to the surrounding refractive index as the nanocones become narrow from bottom to top.

It is a further advantage that no materials foreign to the standard processing of the single crystalline materials, such as silicon carbide and gallium nitride, need to be introduced neither in the optical device nor in the process for manufacturing the optical device.

The plurality of cone shaped structures may have a random height distribution. Thus, the height of individual cone shaped structures forming a nano structure may vary randomly. For example, if a mean height of the plurality of cone shaped structures is 240 nm, the standard deviation of the height of the plurality of cones may be 80, if a mean height of the plurality of cone shaped structures is 500 nm, the standard deviation may be 300 nm. Thus, the standard deviation may range from 30% to 60 % of a mean height distribution.

In some embodiments, the height of the plurality of cone shaped structures may vary randomly between 100 nm and 350 nm to thereby allow

transmission of diffused light in a wavelength range, such as between 450 nm and 800 nm, such as between 390 and 700 nm.

For a plurality of cone shaped structures having a random height variation, at least a first part of the plurality of cone shaped structures may have a height in a first height interval and at least a second part of the plurality of cone shaped structures may have a height in a second height interval, different from the first height interval. The plurality of cone shaped structures may have a random distribution in among the first, second, and possibly further height intervals.

The random height distribution ensures optimized or improved transmission and/or reflection properties for the nano structured surface over a wavelength range,

The surface having a non-periodical nano structure may have a very low reflectance of light in the visible wavelength range, such as an average surface reflectance below 10 %, such as below 5 %, such as an average surface reflectance in the visible wavelength range below 2 %, such as below 1 .6 %. For some materials, due to the low reflectance, the surface may appear black.

In one or more embodiments, the material, such as the single crystalline material, may be a compound material, such as a silicon carbide material, a gallium nitride material, etc. The gallium nitride material may be any gallium nitride based material, and the gallium nitride material may comprise GaN, InGaN, etc.

The single crystalline material may have a wide bandgap and strong bonding energy.

The materials, such as a silicon carbide material or a gallium nitride material are composed of at least two compounds, and the compound material, such as the single crystalline material, may typically be characterised by a strong bond between the different compounds and the compound materials, thus, typically have a high bonding energy and typically a high chemical resistance as any chemical process will need to have an activation energy being higher than the bonding energy between the compounds. These materials therefore require either high temperature or a physical reaction to etch, and the material, such as the single crystalline material may be characterized by etching anisotropically in a reactive ion etching process, such as characterized by etching anisotropically in a reactive ion etching process using a fluoride based gas, such as SF₆. Therefore, these materials are often used in micro machining because deep structures may be etched with a minimum of undercutting.

The thin film material may be any material having the required masking capabilities, and the thin film may be made of any material comprising silver, gold, platinum, aluminum or palladium, or any combination thereof.

The thin film of masking material may have an etching rate which is much lower than the etching rate of the silicon carbide or gallium nitride material, such as an etching rate being 2, 5, or 10 times lower than the etching rate for the silicon carbide or gallium nitride material. The ratio between the first etching rate and the second etching rate may be above 1 , such as above 5, such as above 10, such as above 100.

The masking material is preferably capable of forming nano islands upon thin film treatment, thus the thin film may be treated to form nano islands by either a heating reaction, a chemical reaction, a photoreaction or any combination of these reactions to cause aggregation, nucleation or decomposition of the masking material to thereby fabricate discontinuous nano islands with half sphere-like shapes or dome shapes. The average size and density of the nano islands may be controlled by adjusting the processing parameters as well as thin film layer thickness. It is, however, an advantage of the present invention that the size of the nano islands does not need to be rigorously controlled as the nano structures, i.e. the plurality of cone shaped structures, are preferably randomly distributed over the surface, and have a random height distribution, thus the plurality of cone shaped structures do not need to be identical in neither height nor width. Thereby, any intermediate steps in the method of fabricating the nano structured surfaces, i.e. planarizing, etc. may be eliminated. In one or more embodiments, the cones may have a base width of less than 1000 nm, such as less than 800 nm, such as less than 500 nm, such as less than 400 nm, such as less than 300 nm, such as less than 200 nm, such as less than 100 nm. The base width may be between 20 nm and 1000 nm, such as between 50 and 800 nm, such as between 100 nm and 500 nm, such as between 100 nm and 300 nm.

Each of the plurality of cone shaped structures may have a height of at least 100 nm, such as at least 200 nm, such as at least 300 nm, such as at least 400 nm, such as at least 500 nm, such as at least 800 nm, such as at least 1000 nm. The cone shaped structures may have a height between 100 nm and 1000 nm, such as between 100 nm and 800 nm, such as between 100 nm and 500 nm, such as between 200 nm and 400 nm.

Any combination of cone heights and cone widths may be achievable and both the cone heights and cone widths may be tailored to achieve for example specific reflectance for a specific wavelength or a specific

wavelength range. In some embodiments, the cones may have a base width of less than 400 nm and a height of at least 400 nm. In some embodiments the cones may have a height distribution between 100 nm and 350 nm. The cones may have different base widths and different heights within the at least one surface.

The aspect ratio of the cone shaped structures may be between 2 and 15, such as between 3 and 10, between 7 and 13, etc., the aspect ratio being a height/width ratio.

The height distribution of the plurality of cone shaped structures may be selected for optimum performance in a wavelength range having a center wavelength. The height of the plurality of cone shaped structures may vary randomly between 1/3 of the center wavelength and at least half the center wavelength, such as between 100 nm and at least half the center

wavelength, such as between 100 nm and up to the center wavelength.

The plurality of cone shaped structures may be distributed non-periodically on the at least one surface. That the cone shaped structures are distributed non-periodically infers that the distribution of the cone shaped structures are not periodically and that the distance between any two cone shaped structures is not necessarily the same as the distance between any two other cone shaped structures, the non-periodical distribution may be a random, a non-periodic or a pseudo-periodic distribution. Furthermore, the individual cone shaped structures need not to be identical, the height of the cone shaped structures may vary and likewise the width of the cones may vary on a same surface, so that both the height and the width of cones may vary from 100 nm to 1000 nm, such as from 100 nm to 800 nm, such as from 100 nm to 500 nm. Thus, the size distribution for the cones may extend over more than 1000 nm and a mean value may be given with regards to the cone width and height with a possible height and/or width variation of 900 nm, such as of 500 nm, such as of 300 nm, etc.

Typically, the height is measured from a selected base plane, such a base plane comprising a lowest etch point, such as a base plane proving a base plane for a plurality of cone shaped structures. Typically, also the width of a cone shaped structure is measured along a selected base plane.

The cones may be distributed with an average of 1 .0E8 to 2.0E1 1 cones pr cm² or with an average of up to 1 .0E12.

The density of the self-assembled nano islands may be between 1 and 2000 nanoparticles/ μηη2, such as between 100 and 200 nanoparticles/μηη². The nano island particle area coverage may be between 20 % and 40 %, such as between 25 % and 35 %. Typically, the density of the plurality of cone shaped nano structures may correspond to the density of the self-assembled nano islands, and thus the density of the plurality of cone shaped structures may be between 1 and 2000 cone shaped structures/ μηη2, such as from 100 and 200 cone shaped structures/μηη².

In one or more embodiments, the self-assembled nano islands may have an average particle size of 10 nm to 380 nm and/or an average interval between the self-assembled nano islands may be between 10 nm and 380 nm. In one or more embodiments of the present invention, the single crystalline material is silicon carbide or gallium nitride and the nano structure comprising a plurality of cone shaped structures may also be silicon carbide or gallium nitride, so that the cone shaped structures are fabricated in silicon carbide or gallium nitride, respectively. The silicon carbide may be used as a substrate forming the basis for the light-emitting diode. However, silicon carbide, as well as silicon and sapphire, have very high refractive indices. Thus, much light will remain trapped inside the diode, i.e. is reflected back into the material at the material/air surface interface reducing the light extraction efficiency for the light-emitting diodes. The same applies mutatis mutandis for GaN.

Silicon carbide is normally a transparent material, and is as such used as material for optical devices allowing for transmission of the light. Silicon carbide having a periodic nano-structure manufactured in the surface layer is also a transparent material, however, providing a non-periodic nano structure in a silicon carbide substrate renders the silicon carbide to appear black. The surface having a non-periodical nano structure has a very low reflectance of light in the visible wavelength range and therefore the transmittance will be increased. Typically, the surface reflection of silicon carbide is about 20% for light in the visible wavelength range, however, by applying a nano structured surface on the silicon carbide, the reflectance of the surface material may be reduced by about a factor 15 (from 20.5% to 1 .62%) and the extraction efficiency may be increased by as much as 70%, such as by 60 % depending on the cone distribution, the cone width and the cone height.

In one or more embodiments of the present invention the single crystalline material is used in the manufacturing of light-emitting diodes. The optical device may comprise a light-emitting diode structure provided at least partly in a gallium nitride material, and the optical device may have at least one gallium nitride surface, the light-emitting diode structure being configured to emit light through the at least one gallium nitride surface, wherein the subwavelength nanostructure is provided in the gallium nitride surface.

In a further aspect of the present invention, a light-emitting diode for emission of white light is provided. The light-emitting diode comprises a substrate having a nano structure on a first side, and a light-emitting diode structure provided on a second side of the substrate. The light-emitting diode structure is configured for emission of light through the substrate and the nano structure formed in the substrate may comprise a plurality of cone shaped structures wherein the cones are distributed non-periodically on the surface. The cone shaped structures may be formed in the substrate, i.e. in the substrate material. The height of the cone shaped structures may vary randomly.

In a still further aspect of the present invention, a method of fabricating a light-emitting diode is provided, the method comprises providing a substrate, providing a non-periodic nano structure on a first side of the substrate, and providing a light-emitting diode on a second side of the substrate, the light- emitting diode being configured to emit light through the substrate.

The substrate may be a high crystalline quality material in for example silicon carbide, or gallium nitride or any other high crystalline quality material being transparent for the white light. The high crystalline quality material may be a single crystalline material. Preferably, the high crystalline quality material has a low dislocation density and/or a high purity.

In one or more embodiments, the light-emitting diode structure is fabricated primarily in a silicon carbide material, a gallium nitride material or any combination thereof. The light-emitting diode structure may comprise fluorescent silicon carbide and the fluorescent silicon carbide may comprise a first layer of fluorescent silicon carbide being n-type doped, for example by doping with nitrogen and boron and a second layer of fluorescent silicon carbide being p-type doped for example by doping with nitrogen and aluminium. The thickness of the layers may be selected so that the respective layers are sufficiently thick so that the volume is sufficient to produce a strong light emission. The p-type doped layer, such as the nitrogen and aluminium doped layers, yields a broad donor-to-acceptor band luminescence, which together with the broad donor-to-acceptor band luminescence in the n-type doped layer, such as in the nitrogen and boron doped layer, typically provide light having

wavelengths in the visible region with a broad full width half maximum curve. Especially, the nitrogen and boron doped silicon carbide layer may emit warm white light with a peak wavelength of around 600 nm. The spectrum from nitrogen and aluminium doped silicon carbide may exhibit blue-green emission. By combining these two fluorescent layers and the two broad wavelength light outputs, a pure white light is obtained covering the at least a large part of the visible spectrum.

The light-emitting structure may further comprise a light-emitting diode configured to excite the substrate, such as the fluorescent silicon carbide, and the light-emitting diode may for example be a nitride based near ultraviolet stack or any other light-emitting diode capable of exiting the substrate or the fluorescent silicon carbide.

The nitride based near ultra violet stack may be grown on the fluorescent silicon carbide and may for example be monolithically grown. It is an advantage of providing a silicon carbide substrate, a monolithically grown fluorescent silicon carbide, and a monolithically grown light-emitting diode on top of the fluorescent silicon carbide, in that the entire optically device may be fabricated in one process without bonding, etc, of different substrates. Thus, the fabrication process may be automated so that the time and the costs of the fabrication process may be significantly reduced.

It is a further advantage of using a single crystalline material as a substrate for growing light-emitting diodes that the light-emitting diode may be grown directly on the single crystalline material. For example using a single crystalline silicon carbide substrate provides a better lattice match with for example the GaN and improved thermal conductivity when compared to sapphire, so does a single crystalline gallium nitride material.

In a further aspect of the invention, a method of fabricating a light-emitting diode is provided, the method comprises providing a high crystalline quality silicon carbide or gallium nitride substrate, growing. On a first side of the silicon carbide substrate or gallium nitride substrate, a silicon carbide layer may be grown. The method may further comprise monolithically growing an n-type doped layer and a p-type doped layer of silicon carbide, such as a nitrogen and boron doped layer of fluorescent silicon carbide, and a nitrogen and aluminium doped layer fluorescent silicon carbide. On top of the fluorescent silicon carbide, a nitride based near ultra violet light-emitting diode comprising at least an n-doped GaN layer and a p-doped GaN layer may be monolithically grown. A contact area may be provided to the n-doped GaN layer and the p-doped GaN layer, respectively. On a second side of the silicon carbide layer a non-periodic nano structure may be provided. The nano structure may comprise a plurality of cone shaped silicon carbide structures wherein the silicon carbide cones are distributed non-periodically on the surface. In one or more embodiments, the nano structures may comprise a plurality of pyramid shaped structures, or the nano structures may comprise a plurality of pyramid and/or cone shaped structures.

The surroundings may encompass any material surrounding the optical device and/or the nano structured surface, such as air, such as ambient air, such as a protected environment, such as a liquid, such as water, etc. The light may thus be emitted from within a device towards the surroundings, such as through an output surface. The output surface may be a

nanostructured silicon carbide or gallium nitride surface. The person skilled in the art will understand that the process of monolithically growing a first doped layer and a second doped layer, such as a first doped layer comprising a nitrogen and boron doped layer of fluorescent silicon carbide, and a second doped layer comprising a nitrogen and aluminium doped layer of fluorescent silicon carbide, respectively, may be any known process. Furthermore, also the process of monolithically growing on top of the fluorescent silicon carbide a nitride based near ultra violet light-emitting diode comprising at least an n-doped GaN layer and a p-doped GaN layer may be performed using any conventionally known or state of the art process. Providing a contact area to the n-doped GaN layer and the p-doped GaN layer may be performed by deposition of a selected contact material or by providing a contact material by any other known process.

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout. Like elements will, thus, not be described in detail with respect to the description of each figure. BRIEF DESCRIPTION OF THE DRAWING

Fig. 1 a shows schematic illustrations of a non-periodic antireflective sub- wavelength structures fabrication process, Fig. 1 c shows a SEM picture of the metal nano islands formed, Fig. 1 b, 1 d and 1 e show SEM pictures of the nano structures formed, and Fig. 1f shows schematically the nano structures as seen in the SEM picture in Fig. 1 d, Figs. 2a and b shows the water droplet contact angle on a bare silicon carbide substrate,

Figs. 3a and b shows the water droplet contact angle on a silicon carbide substrate having non-periodic nanostructure,

Fig. 4 shows the surface reflectance of the bare silicon carbide and a silicon carbide substrate having a non-periodic nanostructure,

Fig. 5 shows the photo luminescence improvement for bare silicon carbide and a silicon carbide substrate having a non-periodic nanostructure,

Fig. 6 illustrates schematically the light emission from a bare silicon structure versus and a silicon carbide substrate having a non-periodic nanostructure, Fig. 7 shows the angle-resolved emission intensities for a bare silicon carbide and a silicon carbide substrate having a non-periodic nanostructure,

Fig. 8 shows the luminescence enhancement of the silicon carbide substrate having a non-periodic nano structure at different emission angles, Fig. 9 shows schematically a light-emitting diode on a silicon carbide substrate,

Fig. 10 shows a GaN light-emitting diode on a silicon carbide substrate,

Fig. 1 1 shows schematically a process for fabricating a non-periodic nanostructure,

Fig. 12 shows SEM pictures of samples with Au thickness of 5 nm and 7 nm, respectively,

Fig. 13 shows the relation between Au thickness and particle diameter, NP density and area coverage, respectively, Fig. 14 shows SEM pictures of the antireflective structures formed with Au film thickness of 3nm, 5 nm and 7 nm, respectively,

Fig. 15 shows measured transmittance and reflectance as a function of wavelength, and calculated absorbance as a function of wavelength.

Fig. 16 shows the measured average reflectance and transmittance and calculated average absorbance as a function of Au thickness.

DETAILED DESCRIPTION OF THE DRAWING

In the present invention an optical device is fabricated, the device has a substrate 2 with a surface 3. The substrate is a silicon carbide substrate or a gallium nitride substrate. In Fig. 1 a, the process of fabricating a nanostructure comprising a plurality of cone shaped structures on the surface wherein the cones are distributed non-periodically on the surface is briefly illustrated. Firstly, a thin film layer of a metal, typically Au, is deposited on the single crystalline substrate 2 and the thin film is treated to form nano islands by either a heating reaction, a chemical reaction, a photoreaction or any combination of these reactions to cause aggregation, nucleation or decomposition of the masking material to thereby fabricate discontinuous nano-islands 4 with half sphere-like shape or dome like shapes. (Note, however, that periodic structures are schematically drawn in steps (ii) and (iii) to simplify the illustrations). The average size and density of the nano islands may be controlled by adjusting the processing parameters as well thin film layer thickness. In step (iii) reactive-ion etching (RIE) is applied with SF₆ and O2 gases mixture, and the non-periodic cone-shaped nano structures are formed on the substrate using the thin film nano-islands as a mask layer. The residual thin film is removed to get an optical device 1 having a substrate with non-periodical cone shaped nano structures. In the present description of the drawing, the material in which the non- periodic nano structure is formed is a single crystalline material, however, it is envisaged that for applications in which the single crystalline properties are not exploited, a non single crystalline material, such as a polycrystalline substrate or an amorphous substrate may be used.

In a specific example, non-periodic cone-shaped anti reflective nano structure are formed on a N-B doped fluorescent 6H-S1C by using self- assembled etch mask. An exemplary sample using a silicon carbide substrate, and the intermediate thin film nano-islands have been characterized by scanning electron microscope, SEM.

In Figs. 1 b and 1 c, the nano-islands 4 are seen in the SEM picture. Different processing parameters have been used, and it is seen that the density, size and distribution of the nano islands 4 differs between Fig. 1 b and Fig. 1 c. The nano islands 4 are seen as bright spots against the dark substrate 2.

Figs. 1 d and 1 e, show a same sample from different angles. In Fig. 1 d, the non-periodic nano structures 5 are shown from the side, and in Fig. 1 e, the non-periodic nano structures 5 are shown from an elevated angle. It is seen that the nano-structures are cone shaped; the top of the cone may be slightly rounded. It is further seen that the height and width of the structures differ so that not two cones may be identical. Fig. 1f shows schematically the non- periodic nano structures as seen in Fig. 1d with cones 5 on substrate 2.

The non-periodic or pseudo-periodic nano structure has a mean pitch of approximately 1 15 to 230 nm, that is the mean distance between successive cone shaped nano structures, and the structure height varies from 400 to 850 nm.

It is envisaged that although silicon carbide and gallium nitride are used to describe the effects and devices herein, also other materials may be used preferably being a single crystalline material having a high chemical resistance, such as for example Sapphire.

The nano structured surface has been characterized, and Figs. 2a and 2b show the water contact angle measurements realized by using a drop shape analyzer (Kruss DSA 100S). The bare substrate 2 with a droplet 6 is shown schematically in Fig. 2a, and as DSA picture in Fig. 2b. In this case the substrate is fluorescent silicon carbide, and it is seen to be hydrophilic with a contact angle of 49°. After providing the fluorescent silicon carbide substrate 2 with the non-periodic nano structure 7, it is seen schematically in Fig. 3a, and in the form of a DSA picture in Fig. 3b, that the surface turns

hydrophobic with a contact angle of 98°. The nano structure 7 is not visible in the DSA picture. It is an advantage of being able to provide a hydrophobic surface especially for LED applications used at low temperature and/or in a humid environment.

The antireflection properties of the nano structure surface is shown in Fig. 4, wherein a bare silicon carbide surface is compared with the nano structured silicon carbide surface. The antireflection properties may depend on structure height and typically at least 100 nm high structures are required to achieve fairly good antireflection performance and in the present case the average height of the non-periodic nano-structures is controlled to be larger than 400 nm. This may be obtained when using a reactive ion etch, RIE, for etching the nano structures in the silicon carbide or gallium nitride substrate. For silicon carbide, the RIE conditions may for example be: process pressure of 30 mT, RF power of 100 W, gases flow rates of 24 seem SF6 and 6 seem O2, and process time of 15 minutes.

The surface reflectance obtained is illustrated in Fig. 4, where the reflection has been measured by using a calibrated goniometer system (Instrument Systems GON360) at near-normal incidence of 6° over a wavelength range of 390-785 nm which covers the entire visible spectral range (typically from 390 to 750 nm). The reflectance spectra are shown in Fig. 4, the bare silicon carbide substrate, i.e. the bare SiC, having a reflectance curve 8 showing a reflectance of about 20 % and the substrate surface having a non-periodic nano structure, i.e. the ARS SiC, has a reflectance curve 9 showing a reflectance of between 0.1 to a few percent reflection. It may be seen that the average surface reflectance may be significantly suppressed from 20.5 % to 1 .62 % by a factor of 1 1 .6 after introducing the non-periodic nano structure. It is seen that the reflectance at the luminescence peak (576 nm) is lower than 2 % and the minimum value of 0.05 % is obtained at 405 nm. Although the reflectance starts to increase at longer wavelengths, the value through the whole measured spectral range is below 4 %. It may be seen that the fluorescent silicon carbide surface turns from shiny light green color (transparent) to dark greenblack color (black, transparent) after introducing the non-periodic nano structure on the surface.

Especially, for the use of the non-periodic nano structured surfaces in the light-emitting diode industry, the photoluminescence, PL, is an important factor. Fig. 5 shows the angle-resolved photoluminescence (PL)

measurement which has been performed by using the same goniometer system as above, and a 377 nm laser beam from a diode laser has been used as the excitation source. The sample was optically excited from its back side and the emission angle-resolved photoluminescence was measured from 0 (normal to the sample front surface) to 90° in a step of 10°. The photoluminescence spectra of the bare and ARS SiC measured at 0° are shown in Fig. 5. Broad DAP band luminescence with a peak wavelength of 576 nm and a full width at half maximum (FWHM) of around 1 10 nm are observed for both samples, curve 10 and 1 1 , respectively, and it is seen that the non-periodic nano structured silicon carbide has a luminescence enhancement of 55 % at the emission angle of 0° which at least indicates that a higher light extraction efficiency may be obtained. Although the light transmission are not simply governed by the Snell's law in the nano-scale structures, a simple schematic illustration in Fig. 6

demonstrates a general idea of how the non-periodic nano structure improve the light extraction efficiency. For a substrate 2 without a non-periodic nano structure coating, light with an incident angle larger than the critical angle will not escape the substrate, however, when the substrate 2 has a non-periodic nano structure 7, emitted light with an emission angle larger than the critical angle could escape the substrate 2 through the non-periodic nano structure, which leads to an enhanced light extraction efficiency. It is an advantage of providing a light-emitting diode on top of a silicon carbide substrate in that the GaN light-emitting diode may be grown monolithically on the silicon carbide substrate, using standard processes. It is a further advantage that the SiC surface is well adapted for harsh

environments. Emitting the generated light through the non-periodic nano structured surface of the silicon carbide substrate substantially increases the extraction efficiency for the diode.

In Fig. 7, the angle resolved emission intensities, or the spatial emission patterns, for the bare substrate and the non-periodic nano structured surface are shown. In Fig. 8, the luminescence enhancement of a fluorescent silicon carbide substrate is shown, i.e. the enhancement provided by the fluorescent silicon carbide substrate having a non-periodic nano structure when compared with the bare substrate, at different emission angles. It is seen that the enhancement increased from 55 % at 0° to 186 % at 90°, and the overall luminescence enhancement in the whole range is 66.3 %.

In Fig. 9, a light-emitting diode on a silicon carbide substrate 24 is shown. The silicon carbide substrate is of a high crystalline quality, and may be a single crystalline silicon carbide substrate 24. On a first surface 26 of the silicon carbide substrate 24, a non-periodic nano structure 25 is provided in the silicon carbide substrate 24. On the other or second side of the silicon carbide substrate 24, a buffer layer 23 is provided. An n-doped gallium nitride layer 22 is provided next to the buffer layer 23. Multiple quantum wells are provided in a stack 21 , and adjacent the quantum wells, a p-doped gallium nitride layer 20 is provided, thus the multiple quantum wells are provided between the p-doped GaN layer 20 and the n-doped GaN layer 22. The light is emitted from the light-emitting diode through the first surface 26 and is indicated by arrows 27.

It is envisaged that a mirror may be provided on the bottom surface to allow for reflection of radiation through the GaN layer 20. Thus, a nanostructure may be provided in the GaN layer 20, in alternative to, or in addition to the nanostructure in the SiC layer 24.

In Fig. 10, another light-emitting diode structure is shown. The silicon carbide substrate 31 is a fluorescent silicon carbide structure doped with boron and nitride. On a first surface 30 of the substrate 31 is a non-periodic nano structure 32 provided. On the other side of the substrate 31 , a thin AIN buffer layer 34 is provided on top of which an n-doped GaN layer 35 is grown. Multiple quantum wells 36 of alternating layers of GaN and GalnN (not shown) are provided on the layer 35, and another buffer layer of AIGaN 37 may be grown before a p-doped GaN layer 38 is provided. The stack comprising the layers 34 through 38 are called a Nitride based near ultra violet stack, a NUV stack, and contacts 39, 40 may be provided to the n- doped GaN layer 35 and the p-doped GaN layer 38, respectively. The contacts may for example be gold contacts.

It is an advantage of providing a GaN light-emitting diode on top of a silicon carbide substrate in that the GaN light-emitting diode may be grown monolithically on the silicon carbide substrate, using standard processes. Emitting the generated light through the non-periodic nano structured surface of the silicon carbide substrate substantially increases the extraction efficiency for the diode.

It should be mentioned that also a refracting GaN surface may be provided with a nanostructure as described above.

In Fig. 1 1 , a process or a method for fabricating the non-periodic

nanostructure is provided. The substrate 42 is fabricated of a single crystalline material such as silicon carbide or gallium nitride, but it may also be a microcrystalline material, having a high chemical resistance. A thin film, such as a metal thin film, such as a Au thin film. 41 is provided on top of the substrate 42, e.g. by e-beam evaporation, step (a). The thin film 41 may be between 1 and 50 nm, such as between 3 nm and 20 nm, such as between 5 nm and 10 nm, such as between 5 nm and 10 nm, such as 7 nm thick. The thin film may be an Au film and in step (b), the thin film is treated so as to form self-assembled nano islands 43 on the surface of the substrate 42. In the present example, the thin film is treated with rapid thermal processing at 350°C for 5 minutes in an N₂ ambient. Hereby, the thin film layer turns into discontinuous self-assembled nano islands with half sphere like or dome like shapes. The size and shape of the nano islands may be controlled by adjusting the annealing conditions as well as the layer thickness of the thin film 41 . In steps (c), (d) and (e) reactive-ion etching (RIE) 44 is applied with SF₆ and O2 gases in a mixture of 4:1 . It is seen that the RIE etches trenches 47 in the silicon carbide substrate 42, and furthermore that while the nano islands 43 are used as a mask, the nano islands 43 are being gradually etched and over etching of at least some of the nano islands may occur so that at least some of the nano islands are etched away during the process. As the silicon carbide substrate is chemically resistant to the SF₆ and O2 gases, substantially no undercutting of the thin film nano islands 43 occurs and the etching is thus anisotropic. The total etching time may wary depending on the thickness of the thin film 41 , the predetermined height of cone structures to be reached etc, and may be between 5 and 20 minutes, such as 15 minutes. After the etching non-periodic cone-shaped nano structures on the fluorescent SiC surface are formed. In step (f), the residual nano islands, such as the residual Au islands, are removed by using an iodine based solution of KI:I₂:H₂O-100 g:25 g:500 ml. Hereafter, the substrate 42 has a surface 48 having a nano structure 46 formed in the single crystalline material. It is seen that the nanostructure comprises a plurality of cone shaped structures 49 wherein the cones are distributed non-periodically on the surface. Thus, the nano islands are configured to mask the silicon carbide substrate during at least a part of the etching. It is seen that the silicon carbide substrate is etched at a first etch rate in a substantially anisotropic etch using the nano islands as mask, and concurrently, at least a part of the nano islands are etched at a second etch rate, the second etch rate being lower than first etch rate.

A thin film of Au has been deposited on silicon carbide wafers, the thickness of the film ranging from 3 to 21 nm (see table 1 ). The silicon carbide wafers are double-side polished 6H-SiC samples and the thin film has been deposited by using e-beam deposition (Alcatel) with a deposition rate of 1 A s. The samples were treated using thermally annealing to form self- assembled nano islands of the thin film material.

A first annealing process included thermally annealing the samples for 3 minutes at 650 degree Celsius, and for samples with a Au thin film thickness from 3 to 1 1 nm, this annealing step was sufficient to form self-assembled Au nano-islands. A second annealing process included thermally annealing the samples for 33 minutes at 650 degree Celsius, was needed to form Au nano- islands on samples with a Au thin film thickness from 13 to 21 .

The self-assembled Au nano-islands have been observed by SEM and related calculations of particle density, particle area coverage, mean effective diameter and spread in diameter have been performed for the samples with different Au thin film thickness, see table 1 .

Table 1

Nano islands comprising Au nanoparticles (i.e. Au particles size range is between 1 nm and 100 nm) were formed when the Au thin film thickness was below 13 nm and nano islands comprising Au nanoclusters (i.e. Au cluster size range is between sub-nanometer and 10 nm) were formed when the Au thickness was above 13 nm. Fig. 12 shows SEM pictures of samples with a Au thin film thickness of 3 nm, 5 nm and 7 nm, respectively. It is seen that by increasing the Au thin film thickness, the nano structure particle density decreases from approximately 1900 to approximately 90 particles pr. μιτι². Fig. 13a shows the relation between particle diameter and Au thin film thickness. According to the figure, it is seen that when increasing the Au thickness the particle diameter increases almost proportionally. For a Au thickness of between 3 nm and 7 nm, the particle diameter is varying between 20 and 50 μιτι². Fig 13b shows the relation between particle density and Au thickness. According to the figure, it is seen that when increasing the Au thickness the particle density decreases abruptly when the Au thickness is changed from 3 nm to 5 nm, and when the Au thickness increases above 5 nm the particle density decreases almost proportionally with the Au thickness. For thin film thickness between 3 and 7nm, it is seen that the particle density is varying between 90 and 2000 particles pr. μηη². Fig 13c shows the relation between area coverage and Au thickness. Within the area of interest, i.e. thin film thickness between 3 and 7 nm, the particle area coverage is varying between 30 % and 40 %,

In table 2(a) samples are etched by RIE (reactive ion etch) with different etching time according to the estimated mask thickness. Samples are afterwards cleaned by Iodine solutions to remove the residual Au, i.e. the remaining thin film, or the remaining nano islands. The table includes calculations of particle area coverage, mean effective diameter, estimated mask thickness and RIE etching time according to different Au film thickness. The estimated mask thickness is calculated by dividing Au thickness x 100 % with particle area coverage. Table 2(b) includes measured average cone height for samples with different Au thickness. When the Au thickness is between 3 nm and 7 nm the measured average cone height is varying between 83 and 315 nm, i.e. the height distribution of the plurality of cone shaped structures varies from 83 nm to 315 nm.

Estimated mask thickness = Au thickness x 100% / particle area coverage

Table 2

Fig. 14 shows SEM pictures of the antireflective structures formed with Au film thickness of 3 nm, 5 nm and 7 nm, respectively. For Au thin film thickness of 3 nm , 5 nm and 7 nm, it is seen in Fig. 14, , that the cone shaped structures are distributed non-periodically and that the height of the cones are varying, and according to table 2(b), the cone height is varying between 83 and 315 nm when the Au thickness is between 3 nm and 7 nm.

Fig. 15(a) and (b), shows measured surface diffuse reflectance and transmittance, respectively. The reflectance and transmittance were measured by using a 6-inch integrating sphere (OL 700-71 from Gooch&Housego) together with a Xenon lamp.The absorbance as a function of wavelength has been calculated by;

Absorbance(A) = 1 - Transmittance(A) - Reflectance(A). Figure 15(c) shows the calculated absorbance as a function of wavelength.

Fig. 16 shows an average measured reflectance and transmittance as a function of Au thickness, and an average calculated absorbance as a function of Au thickness, the measurements being averaged over the wavelength range in question, i.e. from 370 nm to 770 nm.

Claims

1 . An optical device having at least one surface in a silicon carbide or galliunn nitride material configured to transmit optical radiation from within the device towards the surroundings, the at least one surface having a subwavelength nanostructure formed in the silicon carbide or gallium nitride material, the subwavelength nanostructure comprising a plurality of cone shaped structures, wherein the plurality of cone shaped structures have a random height distribution and are distributed non-periodically over the at least one surface, at least a part of the cone shaped structures having a height of at least 100 nm.

2. An optical device according to claim 1 , wherein the at least one surface in the silicon carbide or the gallium nitride material is at least one surface of a substrate and/or at least one surface of an active element of the optical device provided in the silicon carbide or a gallium nitride material.

3. An optical device according to claim 1 or 2, wherein the optical device comprises a light-emitting diode structure provided at least partly in a gallium nitride material, the optical device having at least one gallium nitride surface, the light-emitting diode structure being configured to emit light through the at least one gallium nitride surface, wherein the subwavelength nanostructure is provided in the gallium nitride surface.

4. An optical device according to claim 1 , wherein the optical device comprises a silicon carbide substrate or a gallium nitride substrate and wherein the subwavelength nanostructure is provided in a first surface of the substrate, and a light-emitting diode structure is provided on a second surface (opposite the first surface) of the substrate, the light-emitting diode structure being configured to generate light in a wavelength range having a center wavelength and the light-emitting diode structure being configured for emission of light through the nanostructured first surface of the substrate.

5. An optical device according to any of claims 1 -4, wherein the height of the plurality of cone shaped structures varies randomly between 100 nm and 350 nm to thereby allow transmission of diffused light in a wavelength range, such as between 450 nm and 800 nm, such as between 390 and 700 nm.

6. An optical device according to any of the previous claims, wherein the material is a single crystalline material.

7. An optical device according to any of the previous claims, wherein each of the plurality of cone shaped structures has a base width of less than 400 nm.

8. An optical device according to any of the previous claims, wherein the light-emitting diode structure further comprises a wavelength converter, the wavelength converter having at least one surface in silicon carbide or gallium nitride, the at least one surface having a subwavelength nanostructure formed in the silicon carbide or gallium nitride material, respectively.

9. An optical device according to claim 8, wherein the wavelength converter comprises a fluorescent silicon carbide layer.

10. An optical device according to claims 8 - 9, wherein the light-emitting diode structure further comprises a nitride based near ultra violet (NUV) stack configured to excite the fluorescent silicon carbide layer.

1 1 . An optical device according to any of claims 8 - 10, wherein the optical device comprises a single crystalline silicon carbide substrate and wherein the subwavelength nanostructure is provided in a first surface of the single crystalline silicon carbide substrate, and wherein the fluorescent silicon carbide is provided on a second surface of the single crystalline silicon carbide substrate,

12. An optical device according to claim 10 or 1 1 , wherein the nitride based NUV (near ultra violet) stack is subsequently monolithically grown on the fluorescent silicon carbide.

13. An optical device according to any of the previous claims, wherein the aspect ratio of the cone shaped structures is between 3 and 15.

14. An optical device according to any of the previous claims, wherein the height distribution of the plurality of cone shaped structures is selected for optimum performance in a wavelength range having a center wavelength, and wherein the height of the plurality of cone shaped structures varies randomly between 1 /3 of the center wavelength and at least half the center wavelength.

15. A silicon carbide substrate or a gallium nitride substrate configured to be used for optical devices, the substrate having a first surface configured to transmit optical radiation from within the substrate towards the surroundings, a subwavelength nanostructure being provided in the first surface of the silicon carbide substrate and/or gallium nitride substrate, the subwavelength nanostructure formed in the first surface of the substrate comprises a plurality of cone shaped structures, wherein the plurality of cone shaped structures have a random height distribution and are distributed non-periodically over the at least one surface, at least a part of the cone shaped structures having a height of at least 100 nm.

16. A method of manufacturing at least one subwavelength nanostructure in a surface of a silicon carbide substrate or a gallium nitride substrate to configure the silicon carbide or gallium nitride substrate for low reflectivity in a range of wavelengths, the range of wavelengths having a center wavelength, the method comprising the steps of

providing a silicon carbide or gallium nitride substrate,

providing a thin film material on at least a part of the substrate,

etching the substrate at a first etch rate in a substantially anisotropic etch using the nano islands as mask, and concurrently, etching at least a part of the nano islands at a second etch rate, the second etch rate being lower than first etch rate,

to thereby form a subwavelength nanostructure comprising a plurality of cone shaped surface structures, the cone shaped structures being distributed non- periodically over the surface of the substrate, wherein the plurality of cone shaped structures have a random height distribution and, at least a part of the cone shaped structures having a height of at least 100 nm, and removing the thin film material.

17. A method according to claim 16, wherein the ratio between the first etching rate and the second etching rate above 1 .

18. A method according to any of claims 16-17, wherein the density of the self-assembled nano islands is between 1 and 2000 nanoparticles/ μηη2, such as from 100 and 200 nanoparticles/μηη², and/or wherein the particle area coverage is between 20% and 35 %.

19. A method according to any of claims 16-17, wherein the silicon carbide substrate is a single crystalline silicon carbide substrate.

20. A method according to any of claims 16-19, wherein the step of treating the thin film to form self-assembled nano islands comprises using a heating reaction, a chemical reaction, a photoreaction or any combination of these reactions to cause aggregation, nucleation or decomposition of the masking material.

21 . A method according to any of claims 16-20, wherein the thin film material is a material comprising silver, gold, platinum, aluminum or palladium.

22. A method according to any of claims 16-21 , wherein the self-assembled nano islands have an average particle size of 10 nm to 380 nm and/or wherein an average interval between the self-assembled nano islands is between 10 nm and 380 nm.

23. A method of fabricating a light-emitting diode, the method comprises providing a single crystalline silicon carbide substrate, a single crystalline fluorescent silicon carbide substrate or a single crystalline gallium nitride substrate,

providing a subwavelength nanostructure according to any of claims 16-22 in a first surface of the single crystalline substrate, the subwavelength nanostructure comprising a plurality of cone shaped silicon

carbide/fluorescent silicon carbide/gallium nitride structures, and

on a second surface of the substrate monolithically growing a nitride based light-emitting diode comprising at least an n-doped gallium nitride layer and a p-doped gallium nitride layer.

24. A method according to claim 23, the method comprises

providing a single crystalline silicon carbide substrate,

monolithically growing, on a second surface of the substrate, a fluorescent silicon carbide layer, the fluorescent silicon carbide layer comprising:

a fluorescent silicon carbide layer co-doped with nitrogen and boron, and a fluorescent silicon carbide layer co-doped with nitrogen and aluminium, on top of the fluorescent silicon carbide layers monolithically growing a nitride based near ultra violet light-emitting diode comprising at least an n-doped gallium nitride layer and a p-doped gallium nitride layer, and

providing a non-periodic subwavelength nanostructure according to any of claims 16-22, in a first surface of the single crystalline silicon carbide substrate, or in the p-doped gallium nitride layer, wherein the plurality of cone shaped structures have a random height distribution and are distributed non- periodically over the at least one surface.

25. A method of increasing transmittance of at least a part of a wafer surface, the method comprising manufacturing a subwavelength nanostructure in a surface of a silicon carbide or gallium nitride substrate to configure the substrate for increased transmittance in a range of wavelengths, the range of wavelengths having a center wavelength, the method comprising the steps of providing a silicon carbide or gallium nitride wafer,

providing a thin film material on at least a part of the substrate,

to thereby form a subwavelength nanostructure comprising a plurality of cone shaped surface structures, wherein the plurality of cone shaped structures have a random height distribution and are distributed non-periodically over the at least one surface, at least a part of the cone shaped structures having a height of at least 100 nm, to thereby allow increased transmission of diffused light for a plurality of optical devices distributed on the wafer, and removing the thin film material.

26. A method according to claim 25, wherein a subwavelength nanostructure is provided in a surface of the entire wafer.