NANOSTRUCTURED LED ARRAY WITH COLLIMATING REFLECTORS
The present invention relates to nanostructured light emitting diodes, LEDs. The nanostructured LED device according to the invention comprises an array of a plurality of individual nanostructured LEDs. Each of the nanostructured LEDs has an active region wherein light is produced. The nanostructured device further comprise a plurality of reflectors, each associated to one individual nanostructured LED (or a group of nanostructured LEDs. The individual reflectors has a concave surface facing the active region of the respective individual nanostructured LED or active regions of group of nanostructured LEDs.
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The present invention relates to light emitting diodes, LEDs. In particular the invention relates to a nanostructured LED device comprising arrays of nanostructures.
BACKGROUNDThe today dominating type of light emitting diodes (LEDs) are built on planar technology. The PN junction is constructed as a plurality of layers on a substrate giving a device with an essentially horizontal orientation. The light-producing re-combination takes place in a subset of these layers. As the semiconductor layers have refractive indexes that are substantially higher than the refractive index of the air, a substantial portion of generated light will be reflected in the layers and not contribute to the effective luminescence of the device. In fact the layers will act as a waveguide in the horizontal plane of the LED. Measures have been suggested to mitigate the effects of the light of LED being trapped in the device and to efficiently extract the light out of the semiconductor layers. Such measures include modifying the surface in order to provide portions with varying angles to the horizontal plane. A similar approach is suggested in EP1369935, wherein nanosized particles are provided in the LED device to scatter light or alternatively absorb light and generate light of a different wavelength. In addition the planar technology imposes constrains in terms of miniaturization and choices of suitable materials, which will be further described below.
The development of nanoscale technology and in particular the ability to produce nanowires has opened up possibilities of designing structures and combining materials in ways not possible in planar technology. One basis for this development is that the 1D properties of a nanowire makes it possible to overcome the requirement of lattice matching between different materials in a device made with planar technology. It has been shown and utilized that nanowires of for example InP can be grown on InAs or Si without defects. In US 20040075464 by Samuelson et at a plurality of devices based on nanowire structures are disclosed, for example nanowire LEDs. These LEDs have an internal heterostructure giving quantum confinement effects.
US20030168964 teaches an assembly of a plurality of nanowires acting as LEDs mounted in groups between a conductive transparent substrates at the lower end of the nanowires and a transparent cover substrate at the top end, each individual nanowire having a structure of P-type, N-type and light emitting layer. The nanowires are said to be arranged to emit light through the conductive transparent substrate.
Other nanowire LED have previously been reported. Hiruma et al. fabricated vertical GaAs nanowire pn LEDs. The nanowires were embedded in an SOG and covered with an Au/Ge/Ni top contact described in “GaAs p-n junction formed in quantum crystals” by Haraguchi et al., Appl. Phys. Lett. 60 (6) 1992. These devices showed room temperature electro luminescence. GaN based nanowire LEDs have also been fabricated as described in “Core/Multishell Nanowire Heterostructure as Multicolor, High-Efficiency Light-Emitting Diodes” by Quian et al., Nanoletters.
SUMMARY OF THE INVENTIONIt has in the art been shown that nanostructures can be utilised for constructing LED devices. To fully take advantage of the possibilities offered by the nanotechnology further improvements with regards to efficiency are needed
The object of the present invention is to provide a nanostructured LED device and a method of producing such overcoming the drawbacks of the prior art devices and methods. This is achieved by the device as defined in claim 1 and the method as defined in claim 23.
The nanostructured LED device according to the invention comprises an array of a plurality of individual nanostructured LEDs. Each of the nanostructured LEDs has an active region wherein light is produced. The nanostructured device further comprise a plurality of reflectors, each associated to one individual nanostructured LED, or a group of nanostructured LEDs. The individual reflectors has a concave surface facing the active region of the respective individual nanostructured LED or active regions of group of nanostructured LEDs.
The nanostructured LED device according to the invention can be seen as comprising a LED array layer and a reflector layer. The plurality of nanostructured LEDs forms the LED array layer, with a corresponding plurality of active regions arranged in the layer The reflector layer is arranged in a plane parallel to the LED array layer and comprises a plurality of reflectors each having a concave surface facing one or a group of active regions and arranged to, direct light through the LED array. The periodicity of reflectors of the reflector layer may relate to the periodicity of the nanostructured LEDs, or their associated active regions.
In one embodiment of the invention each reflector covers the upper surface, and optionally a part of the side surface, of an elongated nanostructured LED, typically a LED form from a nanowire.
In one embodiment the nanostructured LEDs are of pyramidal shape and the reflector covers essentially all sides of the nanostructured LED except for the side facing the substrate.
The individual reflectors may be joined to form a continuous reflecting layer. In one embodiment the continuous reflecting layer covers both the upper surface of the nanostructured LEDs and a filler layer that has been provided to fill the space in between the nanostructures.
The reflectors or the continuous reflecting layer may be provided directly on the nanostructured LEDs. Alternatively a spacer material is provided there between to define the shape of the reflectors. A contact or contact layer may also be provided between the reflectors and the nanostructured LEDs. One alternative is to use the continuous reflecting layer also as a upper contact to the nanostructured LEDs.
One advantage of the present invention is that the efficiency of nanostructured LED devices can be sufficiently increased. A further advantage is that the nanostructured LED devices can be fabricated with established methods.
Yet a further advantage with the nanostructured LED according to the invention is that the fabrication can be adapted to cost efficient industrial production.
Embodiments of the invention are defined in the dependent claims. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings and claims.
Preferred embodiments of the invention will now be described with reference to the accompanying drawings, wherein:
A nanostructured light emitting diode, LED, device according to the invention comprises an upstanding nanostructured LEDs. The individual nanostructured LED are for example formed by the use of nanowires. The nanowires are either utilised as an active element in the LED or as a building block for nanostructures, wherein the nanowire makes it possible to fabricate the nanostructures with materials that otherwise would not match the materials of a substrate, for example. Suitable methods for growing nanowires on semiconductor substrates are described in US 2003010244. Methods of providing epitaxally grown nanowires with heterostructures are to be found in US 20040075464. Nanostructured LEDs may also be formed by other means, for examples as InGaN/GaN hexagonal pyramid structures on a GaN substrate as indicated in “Spatial control of InGaN luminescence by MOCVD selective epitaxy” by D. Kapolnek et al., J. of Crystal Growth 189/190 (1998) 83-86.
For the purpose of this application an upstanding nanowire should be interpreted as a nanowire protruding from the substrate at some angle, the upstanding nanowire for example grown epitaxially from the substrate. The angle with the substrate will typically be a result of the materials in the substrate and the nanowire, the surface of the substrate and growth conditions. By controlling these parameters it is possible to produce nanowires pointing in only one direction, for example vertical, or in a limited set of directions. For example nanowires and substrates of zinc-blende and diamond semiconductors composed of elements from columns III, V and IV of the periodic table, such nanowires can be grown in the [111] directions and then be grown in the normal direction to any {111} substrate surface. Other directions given as the angle between normal to the surface and the axial direction of the nanowire include 70.53° {111}, 54.73° {100}, and 35.27° and 90°, both to {110}. Thus the nanowires define one, or a limited set, of directions.
All references to upper, top, lower, downwards etc are made as considering the substrate being at the bottom and the nanowires extending upwards from the substrate. Vertical refers to a direction parallel to the longer extension of the nanowire, and horizontal to a direction parallel to the plane formed by the substrate. This nomenclature is introduced for the easy of understanding only, and should not be considered as limiting to specific assembly orientation etc.
A nanostructured LED device 101 according to the invention is schematically illustrated in
The reflector may be deposited as a highly reflective metal layer on top of the structure formed during growth and/or subsequent processing. Typical materials for the reflector include but is not limited to Ag, Al (for LEDs in green and blue color range with the wavelength λ<500 nm), same and Au for LEDs in infrared, red, orange and amber color range. Also multilayered structure comprising repeated layers of AlGaAs/GaAs or GaN/AlGaN, for example, may be used as reflectors. The deposition methods for the reflector include but are not limited to evaporation, sputtering, electrochemical or electroless plating. In order to protect the reflector from corrosion and oxidation an additional protective dielectric layer may be formed, for example from SiO2, Si3N4 or similar material. In this layer openings may be structured to provide electrical contact to the reflector.
The dimensions of an individual reflector in a nanostructured LED device according tot the invention will vary greatly depending on the implementation, not at least on the size and shape of the individual nanostructured LEDs. Typical diameters and heights range from tenth of nanometers to several micrometers in the widest parts. According to one embodiment of the invention the inner concave surface of each individual reflector 135 is defined by the contour of a at least the upper surface of respective individual nanostructured LED. A part or all of the side surfaces of the nanostructured LED may also define parts of the reflector.
The nanostructured LED device 101 may be seen as a vertically layered device with a LED array layer 180 comprising a plurality of nanostrucured LEDs 100 with a corresponding plurality of active regions 120 arranged within the LED array layer 180. In a plane parallel to the LED array layer 180 is a reflector layer 181 comprising the plurality of reflectors 135 having a concave surface facing one or a group of active regions and arranged to direct light through the LED array 180. According to one embodiment of the invention the periodicity of the individual reflectors 135 of the reflector layer 181 is related to the periodicity of the individual nanostructured LEDs. The periodicity of the reflector layer 181 may relate to the periodicity of the LED array layer 180 as a series of n or 1/n, n=1, 2, 3 . . . . As an alternative the reflector layer 181 has a random configuration or a periodicity which is not correlated to the periodicity of the LED array layer 180.
The close positioning relative the nanostructure array and the irregular shape of the reflector gives further advantages; the layer can have multiple usage. It can function as a heat conductor with a higher efficiency than in traditional planar LEDs due to the close proximity to the active area and a higher relative surface junction area of the joint semiconductor and reflector material in comparison to the area of the active region. It is also advantageous as an electrical contact to the LED array, also due to close proximity and high relative surface area of the junction. This multipurpose layer may in this way facilitate device design for LED efficiency.
Devices based on nanostructured LEDs do fundamentally differ from traditional planar LEDs as light is emitted from a number of individual light sources instead of one continuous plane. Any array design can be realized by lithographic means. The pitch and pattern of such arrays of LEDs may vary. In this invention it is advantageous to configure an array to photonic crystal character, such as triangular and hexagonal arrays where the pitch is close to ½ of the wavelength of the emitted light in order to inhibit light emission in directions close to the plane of the array. This use of a photonic crystal design in the active array is essentially different from the use of photonic crystal patterns positioned outside the active region and in the proximity of the interface where the light is aimed to be extracted from the semiconductor, as the suggested use of the photonic crystal properties is aimed to align light towards both mirror and the final light extraction interface from the semiconductor. For light ranging from UV to IR the pitch of such array can be said to roughly range within 0.1-4 μm. The specific size of the individual LEDs may often be limited by the choice of array pitch.
Nanostructured LEDs according to the invention are schematically illustrated in
A further embodiment is illustrated in
Illustrated schematically in
The thin arrows in the
In all the embodiments the contacting means are required on the volume element forming a top contact. The top contact may be positioned between the volume element and the reflector, and if so, preferably is of a transparent or semi transparent material. One alternative, illustrated in
The substrate 105 and part of the upstanding structure may be covered by a cover layer 107, for example as a thin film or as material filling the space surrounding the nanostructured LED, as illustrated in
According to the embodiments depicted in
Alternatively, as illustrated in
A further alternative is schematically illustrated in
According to one embodiment of the invention the nanowire of the nanostructured LED is used as a waveguide directing at least a portion of the light produced by the nanostructured LED in a direction given by the upstanding nanowire. The ideal waveguiding nanowire LED structure includes a high refractive index core with one or more surrounding cladding with refractive indexes less than that of the core. The structure is either circular symmetrical or close to being circular symmetrical. Light generation waveguiding in circular symmetrical structures are well know for fiber-optic applications and many parallels can be made to the area of rare-earth-doped fiber amplifiers and lasers. However, one difference is that fiber amplifier are optically pumped while the described nanowire LED structure can be seen as electrically pumped. One well know figure of merit is the so called Numerical
Aperture, NA: NA=√{square root over (n12−nn2)}, wherein n1 and n2 are the refractive indexes of the core and cladding, respectively. The NA determined the angle of light captured by the waveguide. For light generated inside the core of the waveguide the angle of capture, φ, can be determined as n1·cos(φ)=n2. The NA and angle of captured light is an important parameter in the optimization of a new LED structure.
Typical values for III-V semiconductor core material is refractive indexes in the range from 2.5 to 3.5. When combined with glass type of cladding material such as SiO2 or SiN having refractive indexes ranging from 1.4 to 2.0, the angle of capture can be as high as 65 degrees. An angle of capture of 65 degrees yield that up to 75% of the light generated can be captured and guided by the structure (both directions).
One consideration in the optimization of light extraction is to made the NA vary along the nanowire structure to optimize light extraction from the structure. In general, it is ideal to have the NA be highest when the light generation takes place furthest away from the exit location. This will maximize the light captured and guided toward the exit. In contrast, closer to the exit end of the structure, the NA can be made smaller since light generated will radiate in random directions and most of the radiate light will hit the top and side of the top part of the structure and exit. Having a lower NA in the top part of the structure also minimizes the light captures and guide back down through the structure which may not be ideal unless a reflector is inserted in the bottom of the structure. A low NA can be obtained by surrounding the III-V nanowire core with another III-V cladding of different composition with slightly less refractive index.
According to the embodiment schematically illustrated in
The waveguide 116 may be provided with one or more cladding layers. A first cladding layer 112, may be introduced to improve the surface properties of the nanowire, fore example if a GaAs nanowire is utilized it has been shown that the properties are improved by adding a cladding layer 112 of GaInP. Further cladding layers, for example an optical cladding layer 113 may be introduced specifically to improve the waveguiding properties of the waveguide 116, in manners similar to what is well established in the area of fiber optics. The optical cladding layer 113 typically has a refractive index in between the refractive index of the nanowire and the surrounding material. Alternatively the cladding layer 113 has a graded refractive index, which has been shown to improve light transmission in certain cases. If an optical cladding layer 113 is utilised the refractive index of the nanowire, nW, should define an effective refractive index for both the nanowire and the cladding layers.
The ability to grow nanowires with well defined diameters, as described in the above cited references and exemplified below, is in one embodiment of the invention utilised to optimise the waveguiding properties of the nanowire 110, or at least the waveguide 116. with regards to the wavelength of the light produced by the nanostructured LED 100. As is well known the re-combination process that is the basis for the light production of a LED, produces light in a narrow wavelength region, dependent on the material properties. In the embodiment the diameter of the nanowire 110 is chosen so as to have a favourable correspondence to the wavelength of the produced light. Preferably the dimensions of the nanowire 111 are such that an uniform optical cavity, optimised for the specific wavelength of the produced light, is provided along the nanowire. The core nanowire must be sufficiently wide to capture the light. A rule of thumb would be that diameter must be larger than λ/2nw, wherein λ is the wavelength of the produced light and nw is the refractive index of the nanowire 110.
For a nanostructured LED arranged to produce light in the visible region the diameter of the waveguiding portion of the nanowire should preferably be larger than 80 nm in order for the nanowire to be an effective waveguide. In the infra-red and near infra-red a diameter above 110 nm would be sufficient. An approximate preferred upper limit for the diameter of the nanowire is given by the growth constrains, and is in the order of 500 nm. The length of the nanowire 110 is typically and preferably in the order of 1-10 μm, providing enough volume for the active region 120, and at the same time not unnecessarily long to cause internal absorption.
According to the embodiment illustrated in
In the embodiment schematically illustrated in
In the above embodiments, for the reason of not obscuring the understanding, it has been described that light is emitted through the substrate 105. However, in a LED device comprising nanostructured LEDs, the substrate may have been removed, or provided with cut out 130 as illustrated in
The individual reflectors 135 in a nanostructured LED device 101 comprising a plurality of nanostructured LEDs 100 may conveniently be formed as a continuous reflecting layer 535 covering the plurality of nanostructured LEDs 100, as schematically illustrated in
Another realization of the current invention is depicted in
The choice of design of the nanostructured LED device for a certain application may be dependent on many parameters. In the context of light collimation and, with this also extraction and device efficiency, a pyramidal structure as descried with references to
A method of fabricating nanostructured LED is to first grow a nanowire, according to the above referred processes. Part of the nanowire is then masked and the volume element is re-grown selectively. The method is illustrated in
Considering systems where nanowire growth is locally enhanced by a substance, as VLS grown nanowires, the ability to alter between radial and axial growth by altering growth conditions enables the procedure (nanowire growth, mask formation, and subsequent selective growth) can be repeated to form nanowire/3D-sequences of higher order. For systems where nanowire growth and selective growth are not distinguished by separate growth conditions it may be better to first grow the nanowire along the length and by different selective growth steps grow different types of 3D regions or volume elements.
A method of fabricating a nanostructured LED device according to the invention comprises the basic steps of:
a)—defining growth positions on a substrate by lithography:
b)—growing nanostructured LEDs from the substrate on the defined growth positions:
c)—depositing a reflector material at least on top of the nanostructured LEDs, thereby forming individual reflectors for each nanostructured LED.
The details of the method will depend on the materials and the desired shape and functionality of the nanostructured LED device. Fabrication examples will be given below.
The method may comprise a step to be taken after the step of growing of nanostructured LEDs and prior to the step of depositing a reflector material, of forming the upper parts of the nanostructured LEDs to define the shape of the inner surface of the reflectors covering the nanostructured LEDs. Various etching or ablation methods can be utilised. Alternatively, may material be added on top of the nanostructures to define the shape for the reflectors.
Depending on the intended use of the nanostructured LED device, availability of suitable production processes, costs for materials etc, a wide range of materials can be used for the different parts of the structure. In addition the nanowire based technology allows for defect free combinations of materials that otherwise would be impossible to combine. The III-V semiconductors are of particular interest due to their properties facilitating high speed and low power electronics. Suitable materials for the substrate include, but is not limited to: Si, GaAs, GaP, GaP:Zn, GaAs, InAs, InP, GaN, Al2O3, SiC, Ge, GaSb, ZnO, InSb, SOI (silicon-on-insulator), CdS, ZnSe, CdTe. Suitable materials for the nanowire 110 and the volume element 115 include, but is not limited to: GaAs (p), InAs, Ge, ZnO, InN, GaInN, GaN AlGaInN, BN, InP, InAsP, GaInP, InGaP:Si, InGaP:Zn, GaInAs, AlInP, GaAlInP, GaAlInAsP, GaInSb, InSb, Si. Possible donor dopants for e.g. GaP are Si, Sn, Te, Se, S, etc, and acceptor dopants for the same material are Zn, Fe, Mg, Be, Cd, etc. It should be noted that the nanowire technology makes it possible to use nitrides such as GaN, InN and AlN, which gives facilitates fabrication of LEDs emitting light in wavelength regions not easily accessible by conventional technique. Other combinations of particular commercial interest include, but is not limited to GaAs, GaInP, GaAIInP, GaP systems. Typical doping levels range from 1018 to 1020. A person skilled in the art is though familiar with these and other materials and realizes that other materials and material combinations are possible.
The appropriateness of low resistivity contact materials are dependent on the material to be deposited on, but metal, metal alloys as well as non-metal compounds like Al, Al—Si, TiSi2, TiN, W, MoSi2, PtSi, CoSi2, WSi2, In, AuGa, AuSb, AuGe, PdGe, Ti/Pt/Au, Ti/Al/Ti/Au, Pd/Au, ITO (InSnO), etc. and combinations of e.g. metal and ITO can be used.
A fabrication method according to the present invention in order to fabricate a light emitting pn-diode/array with active nanowire region(s) formed of GaAs and InGaP, illustrated in
-
- 1. Defining of local catalyst/catalysts on a p+ GaP substrate 605 by lithography.
- 2. Growing GaAs nanowire 1310 from local catalyst 631. The growth parameters adjusted for catalytic wire growth.
- 3. Radial growing of thin InGaP concentric layer 612 around the nanowire (cladding layer).
- 4. Depositing of SiO2 as mask material 632,
- 5. Back etching of mask 632 to open up the upper parts of the nanowires
- 6. Selective growing of n+ InGaP volume element 615. The growth parameters adjusted to give radial growth.
- 7. Forming reflectors on the volume element by depositing reflective material at least on a portion of the volume elements 615.
The growth process can be varied in known ways to for example include heterostructures in the nanowires, provide reflective layers etc. The stem 113 utilized in some embodiment can be provided by first growing a thin nanowire (step 2), depositing a reflective layer or a selective growth mask covering the lower part, and radial growing of cladding layer or the nanowire to increase the nanowire thickness.
Further examples of realizations of the nanostructured LED utilised in the nanostructured LED device according to the present invention will be given as GaAs nanowires epitaxially grown on GaP and Si substrates. The LED functionality has been established on both kinds of substrates. The structures are evaluated in terms of temperature-dependent photoluminescence, electroluminescence, and radiation pattern.
A LED device according to the realisation comprises arrays of III-V light emitting nanowire diodes, grown and integrated on Si. Each device is built around a GaAs nanowire core, directly grown on either GaP or Si. A portion of each diode acts as the active region in these individual nanosized p-i-n light emitting structures.
The LED device 701, shown in
The fabrication process is outlined in the following. THMa metal organic sources and TMIn together with AsH3, PH3, and Si2H6 as precursor gases were used. Two growth steps were employed. First, 2 μm long GaAs/GaP nanowires were grown on p-type GaP (111)B (p=˜1018 cm−3) and Si (111) (p≈1015 cm−3) substrates by particle assisted growth using randomly deposited, 60 nm diameter nm sized Au aerosols with a particle density of 1/μm2. The nanowires were enclosed with 40 nm thick radial InGaP cladding layer, nominally lattice matched to GaAs. After this step, samples were unloaded for photoluminescence characterization or subsequent fabrication of the nano LEDs. 80 nm thick SiO2 was deposited onto the samples lined for LED fabrication. The SiO2 was back etched back to only cover the substrate surface and up to approximately 1 μm of the side wall of the nanowire. The samples were then reloaded into the MOVPE reactor and a radial Si-doped InGaP layer was selectively grown on the upper part of the GaAs/InGaP core structure. The LEDs were fully covered with 150-300 nm thick 200×200 μm2 quadratic Ni/Ce/Au contacts, each covering approximately 40000 individual nanostructured LEDs. The p-contact was fabricated on the backside of the substrate with conductive Ag paste. Other means of contacting, for example using transparent contacts are known in the art and easily adopted to the present method and device. A scanning electron microscopy (SEM) image of the structure is shown in
One important difference between the Si and the GaP device is the heterostructure sequence in the base of the nanowire, on GaP substrate being p-GaP (substrate)/i-GaP (nanowire)/i-GaAs (nanowire), while on Si substrate being p-Si (substrate)/i-GaP (nanowire)/i-GaAs (nanowire), and both hole injection conditions and internal resistance and should be expected to be appreciably different between the two structures.
LED functionality can be indicated by Photoluminescence (PL) measurements. The measurements here presented were carried out at room temperature and at a temperature of 10 K. The result is illustrate in the graphs of
To study the PL from the nanowires without influence of the substrate, the nanowires were broken off and transferred from the substrate where they were grown, and then deposited on a patterned Au surface. In this way the nanowires could also be studied individually. The PL spectra, as shown in
Both the LED on GaP and on Si demonstrated electro-luminescence (EL) when applying a forward bias, as shown in FIGs. Ta-b. The spectral peak of the light is in fair agreement with the GaAs bandgap energy.
As seen in
LED devices built on III-Nitrides, as GaN nanowires and nanostructures, are of high commercial interest due to their ability to produce light of wavelengths not accessible with other material combinations. As a further implementation example it is described to fabricate GaN nanostructures by selective area growth on GaN epitaxial films, sapphire, SiC or Si and even self-supporting GaN. On the starting substrate a layer of SiNx (30 nm in thickness) was deposited by PECVD. In a subsequent step, arrays of dot-patterned GaN openings (around 100 nm in diameter) were made by epitaxial beam lithography, EBL, and reactive ion etching, RIE. The pitch between the openings was ranged as 1.0˜3.2 μm. Then, the as-processed samples were inserted into a, horizontal MOCVD chamber to grow GaN nanowires and GaN/InGaN nanostructured LEDs. Various shapes can be formed as shown in the SEM images of
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, on the contrary, is intended to cover various modifications and equivalent arrangements within the appended claims.
Claims
1. A nanostructured LED device (101) comprising an array comprising a plurality of individual nanostructured LEDs (100), each nanostructured LED (100) of said plurality of LEDs comprising an active region (120) for light emission, and a plurality of reflectors (135), each associated to an individual nanostructured LED (100) of said plurality of LEDs or a group of nanostructured LEDs (100) of said plurality of LEDs, and each reflector (135) of said plurality of reflectors having a concave surface facing an active region of the respective individual nanostructured LED or active regions of the respective group of nanostructured LEDs.
2. The nanostructured LED device (101) according to claim 1, wherein each reflector (135) of said plurality of reflectors is positioned over a centre of the respective individual nanostructured LED (100) as seen from a top of the nanostructured LEDs.
3. The nanostructured LED device (101) according to claim 1, wherein each reflector (135) of said plurality of reflectors covers a combined upper surfaces of the respective group of nanostructured LEDs.
4. The nanostructured LED device (101) according to claim 2, wherein each reflector (135) of said plurality of reflectors covers an upper surface of the respective nanostructured LED (100).
5. The nanostructured LED device (101) according to claim 4, wherein the concave surface of each reflector (135) of said plurality of reflectors is defined by a shape of the upper surface of the respective nanostructured LED (100).
6. The nanostructured LED device (101) according to claim 5, wherein the plurality of nanostructured LEDs (100) comprise a plurality of elongated structures, each having a pointed upper part and a vertical side surface, wherein the respective reflector (135) of said plurality of reflectors covers at least the pointed upper part of the respective elongated structure.
7. The nanostructured LED device (101) according to claim 6, wherein the respective reflector (135) of said plurality of reflectors covers a portion of the vertical side surface of the respective elongated structure.
8. The nanostructured LED device (101) according to claim 5, wherein the plurality of nanostructured LEDs (100) comprise a plurality of pyramidal structures.
9. The nanostructured LED device (101) according to claim 1, wherein the reflectors (135) also are an upper contact to the respective nanostructured LED or the respective group of nanostructured LEDs.
10. The nanostructured LED device (101) according to claim 1, wherein the array forms a photonic crystal that is arranged to inhibit a wavelength of the light emitted from the active regions of the plurality of nanostructured LEDs in a direction close to a plane of the array.
11. The nanostructured LED device (101) according to claim 1, wherein the plurality of reflectors (135) are joined to form a continuous reflecting layer (535) covering the plurality of nanostructured LEDs (100).
12. The nanostructured LED device (101) according to claim 11, further comprising a fill layer (507) that covers a portion of the plurality of nanostructured LEDs, and wherein the continuous reflecting layer (535) covers at least an upper surface of the plurality of nanostructured LEDs and the fill layer in between nanostructured LEDs of said plurality of LEDs.
13. The nanostructured LED device (101) according to claim 1, wherein the plurality of nanostructured LEDs (100) comprise waveguides adapted to direct at least a portion of emitted light towards a respective reflector (135) of the plurality of reflectors.
14. The nanostructured LED device (101) according to claim 1, wherein the plurality of nanostructured LEDs forms a LED array layer, with a corresponding plurality of active regions (120) arranged in the LED array layer, and a reflector layer (181) arranged in a plane parallel to the LED array layer (180), the reflector layer comprising a plurality of reflectors (135) each having a concave surface facing one or a group of the active regions (120) and arranged to direct light through the LED array (180).
15. The nanostructured LED device (101) according to claim 14, wherein individual nanostructured LEDs in the LED array layer (180) and individual reflectors in the reflector layer (181) are periodically repeated and a periodicity of the individual reflectors (135) of the reflector layer (181) is related to a periodicity of the individual nanostructured LEDs in the LED array layer (180).
16. The nanostructured LED device (101) according to claim 14, wherein each reflector (135) of the reflector layer (181) is positioned over a centre of the respective nanostructured LED (100) of the LED array layer (180) as seen from a top of the nanostructured LEDs.
17. The nanostructured LED device (101) according to claim 15, wherein the periodicity of the reflectors (135) of the reflector layer (181) is related to the periodicity of the individual nanostructured LEDs in the LED array layer (180) as a series of n or 1/n, wherein n is an integer.
18. The nanostructured LED device (101) according to claim 14, wherein individual nanostructured LEDs in the LED array layer (180) and individual reflectors in the reflector layer (181) are periodically repeated and a periodicity of the individual reflectors (135) of the reflector layer (181) is uncorrelated to a periodicity of the individual nanostructured LEDs in the LED array layer (180).
19. The nanostructured LED device (101) according to claim 14, wherein each reflector (135) of the reflector layer (181) covers an upper surface of the respective nanostructured LED (100) of the LED array layer (180).
20. The nanostructured LED device (101) according to claim 14, wherein individual reflectors (135) of the plurality of reflectors are joined to form a continuous reflecting layer (535) covering the plurality of nanostructured LEDs (100).
21. The nanostructured LED device (101) according to claim 20, further comprising a fill layer (507) that covers a portion of the plurality of nanostructured LEDs, and wherein the continuous reflecting layer (535) covers at least an upper surface of the plurality of nanostructured LEDs and the fill layer in between nanostructured LEDs of said plurality of LEDs.
22. The nanostructured LED device (101) according to claim 14, wherein the array forms a photonic crystal in the LED array layer (180), the photonic crystal arranged to inhibit a wavelength of the light emitted from the active regions of the LED array layer (180) in a direction close to a plane of the array.
23. A method of fabricating a nanostructured LED device comprising a plurality of nanostructured LEDs, comprising the steps of:
- (a) defining growth positions on a substrate by lithography;
- (b) growing nanostructured LEDs from the substrate on the defined growth positions; and
- (c) depositing a reflector material at least on top of the grown nanostructured LEDs, thereby forming individual reflectors for each of the nanostructured LEDs.
24. The method of fabricating a nanostructured LED according to claim 23, further comprising a step, to be taken after the step of growing of nanostructured LEDs and prior to the step of depositing a reflector material, of forming upper parts of the nanostructured LEDs to define a shape of an inner surface of the reflectors covering the nanostructured LEDs.
25. The method of fabricating a nanostructured LED according to claim 24, wherein the step of forming the upper parts of the nanostructured LEDs comprises removing a material from the upper parts of the nanostructured LEDs to provide the nanostructured LEDs with a predetermined shape.
26. The method of fabricating a nanostructured LED according to claim 24, wherein the step of forming the upper parts of the nanostructured LEDs comprises adding a transparent material at least on top of the nanostructured LEDs to form a predetermined shape to define the inner surface of the reflectors.
27. The nanostructured LED device according to claim 8, wherein for an individual pyramidal structure of said plurality of pyramidal structures, an active region of a nanostructured LED is formed on the pyramidal structure, said active region comprises a p-n junction.
28. The nanostructured LED device (101) according to claim 1, wherein the plurality of nanostructured LEDs forms a LED array layer, with a corresponding plurality of active regions (120) arranged in the LED array layer, and a reflector layer (181) arranged in a plane parallel to the LED array layer (180), wherein individual reflectors of the reflector layer having a random configuration or a periodicity which is not correlated to a periodicity of individual nanostructured LEDs of the LED array layer.
Type: Application
Filed: Dec 27, 2007
Publication Date: Nov 11, 2010
Applicant:
Inventors: Lars Ivar Samuelson (Malmo), Bo Pedersen (Lund), Bjorn Jonas Ohlsson (Malmo), Yourii Martynov (Lund), Steven L. Konsek (Malmo), Peter Jesper Hanberg (Soborg)
Application Number: 12/451,911
International Classification: H01L 33/08 (20100101); H01L 33/46 (20100101);