Light emitting device with multilayer structure and method for manufacturing same

Abstract

The invention concerns a light emitting device (1), preferably µLED, comprising a semiconductor stack (2) of at least a first layer (3), a second layer (4) and a light-emitting region (5) between the first and second layers. The stack (2) has a rear surface (11a), a light emitting surface (11b) opposite the rear surface and a side surface (11c) laterally confining the semiconductor layer stack. Further the light emitting device comprises a reflector structure (14) arranged on the light emitting surface (11b), exposing a center portion (17) thereof, and comprising an inner side surface (15). The light emitting device further comprises a multilayer structure (6) comprising a plurality of sublayers (7a, 7b, 7c,...) extending parallel to the light emitting surface (11b), covering the center portion (17) and being surrounded by the reflector structure (14). A first sublayer (7a) of the multilayer structure being adjacent to the light emitting surface (11b) comprises a larger refractive index than a second sublayer (7b) of the multilayer structure being further distant from the light emitting surface, and a refractive index difference between the sublayer (7a) of the multilayer structure closest to the light emitting surface and the sublayer (e.g. 7c) of the multilayer structure furthest distant from the light emitting surface is at least 0.1.

Description

[0001] 2024PF00978

[0002] LIGHT EMITTING DEVICE WITH MULTILAYER STRUCTURE AND METHOD FOR

[0003] MANUFACTURE

[0004] The present application claims priority from German patent application DE 10 2024 135 098 . 9 filed on November 27 , 2024 , the disclosure of which is incorporated by way for reference in its entirety .

[0005] The present invention concerns a light emitting device , in particular pLED, comprising a multilayer structure in combination with a reflector structure arranged on a main light emitting surface of the light emitting device . In addition, the invention concerns a method for manuf actulring such a light emitting device .

[0006] BACKGROUND pLEDs are light emitting devices that comprise a lateral dimension in the range from a few hundred nm to about 100 pm . Such devices provide a variety of different applications , including but not limited to displays as well as AR / VR applications . pLED arrays on the other hand can for example be an array of a plurality of light emitting structures / portions each forming a pLED that comprise a lateral dimension in the range of a few pm that are arranged on a common semiconductor substrate , in particular connected by a common semiconductor layer of a semiconductor layer stack out of which they are pixelized .

[0007] In order to increase the external quantum efficiency ( EQE ) of for example pLEDs , or more general light emitting nanostructures , one can for example engineer the geometry, shape and surroundings (passivation, reflective mirror , lenses , ...) of the pLEDs in a way that the EQE in a specific solid angle is maximized .

[0008] In particular for applications like AR / VR only a ~ 15 to 18 degree escape cone is actually useful for further processing of the light which is why a high directionality of outcoupled light is a goal to be achieved . 2024PF00978

[0009] With regards to outcouplrng and collimation, known devices tend to have a separate collimator structure on top of the pLED ( e . g . a lens structure ) or outside the pLED and / or have slanted sidewalls which however only provide limited collimation . In particular, the latter approach, relies on geometrical shaping a sidewall reflector, which is essentially limited in providing outcoupling and redirection due to the fact that the quantum wells emit in a continuum of directions and over the pixel area instead of a single point . If one only relies on geometry to provide respective outcoupling and collimation, strong collimation can only happen with a very large collimator that is significantly larger in dimension than the active emission area and with a smoothly varying sidewall shape to address the angular spread of the emission ( e . g . a tall parabolic mirror collimator as can be found in car lamps ) . One can additionally, introduce an outcoupling structure / element on a main light emitting surface . However, such known approaches are limited for targeted pLED pixel sizes due to unreliability in manufacturing and alignment issues , as they can lead to large geometric deviations , which in turn can lead to some pixels having low outcoupling ef f iciency / directivity and thus low production yield .

[0010] The aim of the present application is therefore to provide a light emitting device which overcomes at least some of the aforementioned aspects , as well as to provide a method for manufacturing such a light emitting device .

[0011] SUMMARY OF THE INVENTION

[0012] This and other obj ects are addressed by the subj ect matter of the independent claims . Features and further aspects of the proposed principles are outlined in the dependent claims .

[0013] The inventors propose to provide a good outcoupling and collimation of light beyond the escape cone of the semiconductor layer stack to the final ambient material ( e . g . air ) by modifying the optical space , which is also defined by the material properties , rather than j ust the geometrical space . In most cases , by additionally tuning the material 2024PF00978 properties spatial distribution, texturing or providing a complicated geometrical shape can be avoided and uncertainties in terms of geometrical imperfection can be equalized . Instead of solely relying on a specially tuned sidewall geometry in a flat optical space ( homogeneous media ) , the idea is to incorporate a "bended" / distorted optical space in the form of a monotonously decreasing refractive index coating (whether a smooth gradient or through discrete steps over multiple layers ) of the main light emitting surface of a light emitting device paired with a reflector structure that is also arranged on the main light emitting surface and encloses the refractive index coating . Thus , instead of a smoothly varying sidewall shape or the use of different components , a "smoothly" varying material property, namely the refractive index, paired with a reflector structure is used which in turn reduces the need to have a large collimator geometry . In such a monotonously decreasing refractive index coating, purely by refraction / in-plane momentum conservation at the interface between main light emitting surface and refractive index coating, portion of the light obliquely being outcoupled from the main light emitting surface can be directed into a direction of the reflector structure of the light emitting device to be then reflected in a desired direction to in sum provide light with a high directionality outcoupled from the light emitting device .

[0014] The proposed approach thus provides an ultra high light outcoupling efficiency pLED without relying on complex lens / nanotextures but rather with an easy to fabricate reflector structure filled with a multilayer structure of layers with a monotonously decreasing refractive index . Even if there are large uncertainties at for example the sidewalls of the semiconductor layer stack due to fabrication imperfection, as long as the reflector structure filled with a multilayer structure of layers with a monotonously decreasing refractive index is well controlled, the approach promises to maintain a high outcoupling efficiency .

[0015] The core idea is thereby to exploit how light obliquely outcoupled from the main light emitting surface is refracted by and propagates as it travels into progressively lower refractive index layers of the 2024PF00978 monotonously decreasing refractive index coating to enhance the light' s interaction with a tuned reflector structure ( e . g . a simple cone / horn concentrator ) which ultimately causes a light outside a desired escape cone to be redirected to escape the light emitting device within the desired cone in a collimated manner .

[0016] Advantages that can result from such a monotonously decreasing refractive index sidewall coating can be :

[0017] - High outcoupling efficiency ( at least doubling of total light extraction efficiency as compared to state of the art ) .

[0018] - High directionality of outcoupled light . A maj ority power can be emitted within a small escape cone ( Doubling the power within the 45 -degree cone and even close to tripling the power within 18 - degree cone as compared to state of the art ) .

[0019] - High Performance robustness towards large geometrical variations within the semiconductor layer stack : e . g . Side surface of the semiconductor layer stack, or rear contact

[0020] - The decreasing refractive index coating in forms of several layers is easier to be deposited than to generate complicated 3D Lens structures for collimation and / or outcoupling efficiency .

[0021] According to a first aspect , a light emitting device , in particular pLED, comprising a semiconductor layer stack is provided . The semiconductor layer stack comprises at least a first layer of a first conductivity type , a second layer of a second conductivity type as well as an active region arranged between the first and the second layer . The active region is thereby configured to emit light of a desired wavelength, in particular first wavelength . In particular the active region of the semiconductor layer stack comprises at least one quantum well , however the active region can also comprise a multi quantum well structure comprising several quantum wells .

[0022] The semiconductor layer stack comprises a rear surface , a main light emitting surface opposite the rear surface and a side surface extending from the rear surface into the direction of the main light emitting surface . The side surface thereby in particular laterally confines a light emitting portion of the semiconductor layer stack, which in the 2024PF00978 later light emitting device is configured to emit light of a desired wavelength . The side surface extends from the rear surface through the semiconductor layer stack such that it comprises at least the second layer and the active region and in particular at least a portion of the first layer . For example , the side surface can result from a step of structuring the semiconductor layer stack thereby removing a portion of the semiconductor layer stack and remaining another portion of the semiconductor layer stack . The side surface can be a therefrom resulting side surface of the remained portion . In particular the side surface can result from an etching step for structuring the semiconductor layer stack resulting in an etched side surface extending at least through the second layer and the active region and in particular at least a portion of the first layer .

[0023] The light emitting device further comprises a reflector structure arranged on or adj acent to the main light emitting surface exposing a centre portion of the main light emitting surface and comprising an inner side surface . In addition to this a multilayer structure comprising a plurality of sublayers extending substantially parallel to the main light emitting surface covers the centre portion of the main light emitting surface being surrounded by the reflector structure . In particular the multilayer structure comprises a first sublayer that is adj acent to the main light emitting surface which comprises a larger refractive index than a second sublayer of the multilayer structure that is further distant from the main light emitting surface , and a refractive index difference between the sublayer of the multilayer structure closest to the main light emitting surface and the sublayer of the multilayer structure furthest distant from the main light emitting surface is at least 0 . 1 , at least 0 . 2 , at least 0 . 5 or at least 0 . 8 .

[0024] By means of such a reflector structure with a reflective inner side surface paired with the multilayer structure being enclosed by the reflector structure and both being arranged on the main light emitting surface of the semiconductor layer stack, light that is emitted from the semiconductor layer stack at the main light emitting surface at an angle will be deflected into the direction of the reflector structure 2024PF00978 and at the reflector structure again be reflected into a desired direction within a desired light emitting cone . Hence , an enhanced light extraction and directionality of light that is emitted from the light emitting device can be achieved without introducing a separate surface topography or optical lens on the light emitting surface from which the light is emitted .

[0025] Due to parallel momentum conservation at the interface between the main light emitting surface and the refractive index coating, light outcoupled from the main light emitting surface is refracted to more oblique angles when in coupled into the multilayer structure with a lower refractive index . Normally, this effect is shunned because it results in light being emitted from the light emitting device in a larger angular cone . However , by causing the light to propagate more obliquely, one causes more portion of the light to interact with the inner side surface of the reflector structure within a shorter depth of propagation within the lower index layers of the multilayer structure .

[0026] Naturally, total internal reflection limits the amount of power that is in coupled into the multilayer structure with a lower refractive index material . Due to the multilayer structure however comprises a refractive index gradient from a higher refractive index layer next to the main light emitting surface to a lower refractive index layer further distant from the main light emitting surface , one allows more light to be in coupled into the multilayer structure and thus more light to be redirected / ref racted to the reflector structure . If the entire multilayer structure would now however comprises of only a high refractive index material , a very tall / high multilayer structure would be needed due to a weak ref raction / redirection into the direction of the reflector structure . Therefore , the multilayer structure comprises a plurality of layers that start from a high refractive index material and progressively go down to a low refractive index material starting from the main light emitting surface , to allow more light being refracted into the direction of the reflector structure and then being redirected deep within a desired escape cone of the light emitting device in a more collimated manner . The more layers the multilayer 2024PF00978 structure has , the more portion of l ght is refracted and subsequently redirected by the combination of the multilayer structure and the reflector structure . Each sublayer of the multilayer structure can thereby in particular be responsible in sending a different angular portion of the light generated in the active region to the inner side surface of the reflector structure to be redirected in the desired direction . As a result , such a structure allows a more directional light outcoupling of a pLED with a high outcoupling efficiency as the multilayer structure together with the reflector structure redirects light outside a desired escape cone , deep within the desired escape cone in a collimated manner .

[0027] The proposed solution is in particular robust towards variation of the angle of the side surface of the semiconductor layer stack or other imperfections within the side surface of the semiconductor layer stack and a reflective coating covering the rear surface and / or the side surface of the semiconductor layer stack as the angular distribution of the light generated within the active region does not or not substantially affect the functionality of the multilayer structure together with the reflector structure and even a large angular distribution of the light generated within the active region can be redirected to result in a desired escape cone in a collimated manner .

[0028] According to some aspects , the semiconductor layer stack is of an InGaN, InGaAlP or InGaAlAs material system . For example , the semiconductor layer stack can be of an Al and / or In containing semiconductor material system. For example , the semiconductor layer stack can be of a material system comprising Indium ( In) and Aluminium (Al ) and Gallium (Ga ) and Arsenide (As ) and / or Phosphide ( P ) . The semiconductor layer stack can however also be of any other semiconductor material system.

[0029] According to some aspects , the active region comprises a multi quantum well structure . The quantum wells can thereby be substantially equal in size and / or composition, can however also vary between each other . The at least two quantum wells can for example comprise a substantially 2024PF00978 equal effective bandgap and can m particular be configured to emit light of a substantially equal wavelength.

[0030] According to some aspects, the first conductivity type is a n-type and the second conductivity type is a p-type. The first layer can thus be a n-type semiconductor layer and the second layer can be a p-type semiconductor layer.

[0031] According to some aspects, the inner side surface is tilted with regard to the main light emitting surface and in particular the inner side surface and the main light emitting surface enclose an angle different to 90°, in particular an angle between 30° and 90° excluding 90° . In particular the inner side surface and the main light emitting surface can enclose an angle of for example 60° . In particular the inner side surface can define a cone opening in a direction away from the main light emitting surface. To increase the effect of collimation by means of the reflector structure an angle of 90° between the inner side surface and the main light emitting surface is undesired. Besides a tilted inner side surface the inner side surface can also have a changing angle with regard to the main light emitting surface, either discrete or continuously changing. Hence the inner side surface can in cross section also comprise an ellipsoid shape or at least portions of an ellipsoid.

[0032] In some aspects, the sublayers of the multilayer structure are of a dielectric material. In particular the sublayers of the multilayer structure can comprise one or more materials of the following selection :

[0033] TiO2, Ta2O5, TiO2:C:H, SiNl,3:H, SiOxNy:H, PPOS, PPHC, SiO2:H, SiO2:F, PPFC, Nb2O5, ZrO2, Y2O3, A12O3, SiO2, SiN, MgF2, PET, PC, SiO2 glass, PMMA, SiO2 quartz.

[0034] The sublayers can thereby be provided using for example PECVD, PVD, or deposition of a substrate. The sublayers of the multilayer structure can however also be at least in part of a semiconductor material.

[0035] In some aspects, the sublayers of the multilayer structure are each or at least some of them of a different material with each a different 2024PF00978 refractive index, where the sublayers closer to the semiconductor layer stack comprise a larger refractive index than the layers further distant from the semiconductor layer stack . By this , a refractive index gradient results from the main light emitting surface to the outermost sublayer of the multilayer structure .

[0036] Depending on the material of the sublayers and the thicknesses of the sublayers as well as the material concentration within the sublayers , a course of the refractive index difference between the sublayer of the multilayer structure which is closest to the main light emitting surface and the sublayer of the multilayer structure which is furthest away from the main light emitting surface can either be substantially continuous , or may comprise a plurality of j umps , in particular depending on the number of sublayers plus 1 . The number of j umps can in particular depend on the interface between the semiconductor layer stack and the first sublayer, the interface between the last sublayer and a neighbouring medium and the interfaces between the other sublayers . This is however to be understood that the course of the refractive index difference between the sublayer of the multilayer structure which is closest to the main light emitting surface and the sublayer of the multilayer structure which is furthest away from the main light emitting surface does not necessarily have to decrease continuously until the sublayer of the multilayer structure which is furthest away from the main light emitting surface but due to material variations or due to thin intermediate layers for a particular different need, the course can also have deviations from a continuous decrease .

[0037] In some aspects , the multilayer structure comprises a total thickness of at most 20 times the emission wavelength of light generated in the active region, in particular first wavelength ( e . g . 12 ]im for the case of a wavelength of 600 nm) . In particular , the multilayer structure comprises a lateral thickness in a direction substantially perpendicular to the main light emitting surface of at most 20 times the first wavelength . This is because the multi-layer structure is intended to improve a small-dimensioned and tuned semiconductor layer stack in terms of coupling out and collimation of the emitted light , 2024PF00978 while at the same time avoiding enlarging the entire light-emitting device too much for this effect .

[0038] In some aspects , the multilayer structure comprises a total thickness of at least half of the emission wavelength of light generated in the active region, in particular first wavelength ( e . g . 300 nm for the case of a wavelength of 600 nm) . In particular , the multilayer structure comprises a thickness in a direction substantially perpendicular to the main light emitting surface of at least half of the first wavelength . This is because for a thinner thickness , oblique light arriving at the main light emitting surface would leave the multilayer structure either by reflection or transmission before it could reach the reflector structure as intended .

[0039] In some aspects , the sublayers of the multilayer structure each comprise a thickness of at least 20 nm, in particular between 20 nm and 1 pm, or between 50 nm and 5 ]im . In particular the minimum thickness of the sublayers of the multilayer structure is dependent on the wavelength of the light emitted from the semiconductor layer stack, for example half , a quarter , or one eighth of the first wavelength . If the sublayers would be too thin, oblique light arriving at the main light emitting surface would leave the multilayer structure either by reflection or transmission before it could reach the reflector structure as intended . In particular a sufficient thickness of each sublayer can be necessary such that light emitted from the main light emitting surface still perceive a region with an index gradient , be refracted as desired and propagate in the index gradient region sufficiently long enough to encounter the reflector structure as intended instead of being outcoupled from the light emitting device outside the desired light cone . In some aspects , the thickness of the sublayers can be substantially equal , it is however also possible that the thicknesses of the sublayers is different to each other .

[0040] According to some aspects , the side surface comprises at least a first and a second side surface portion being laterally displaced to each other, wherein the first side surface portion is directly adj acent to the rear surface . The first side surface portion thereby comprises at 2024PF00978 least the second layer and the active region whereas the second side surface portion comprises a portion of "only" the first layer . The first and the second side surface portion can in particular be connected by a substantial horizontal intermediate side surface portion . For example , the first side surface portion can result from a first etching step whereas the second side surface portion can result from a second etching step laterally displaced from the first etching step .

[0041] According to some aspects , the reflector structure comprises an electrically conductive material and is electrically contacting the first layer of the semiconductor layer stack . According to some aspects , a contact layer is arranged on the rear surface electrically contacting the second layer of the semiconductor layer stack . The first and contact layer can thereby of the same material system as the semiconductor layer stack but can also be contact layers comprising a metal and / or a transparent conductive oxide ( TCO ) such as for example indium tin oxide ( ITO ) . By means of the reflector structure and the contact layer a first and a second potential can be applied to the semiconductor layer stack to operate the light emitting device in a desired manner . For example , the reflector structure and the contact layer can be metallic and configured to besides provide electrical contracting be reflective .

[0042] According to some aspects , the light emitting device further comprises a passivation layer arranged on at least one of the following : on the side surface ; and on the rear surface , in particular exposing a contact portion of the contact layer electrically coupled to the second layer .

[0043] The passivation layer can in particular cover any side surfaces of the semiconductor layer stack . The passivation layer can in particular serve to passivate the layers / components of the light emitting device . In addition, the passivation layer can in some embodiments provide an electric isolation between the reflector structure and contact layer .

[0044] According to some aspects , the light emitting device further comprises a mirror coating arranged on at least one of the following : on the side surface ; 2024PF00978 12 on the rear surface , in particular exposing a contact portion of the contact layer electrically coupled to the second layer ; and on the passivation layer , in particular contacting a portion of the contact layer electrically coupled to the second layer .

[0045] According to some aspects , the side surface and the rear surface enclose an angle different to 90 ° . Such an angle can in particular result from a mesa etching step forming the side surface . The slanted side surface can in particular be used to form the mirror coating in such a way that the light gets collimated and redirected by the mirror coating into a desired direction . The side surface can therefore also be shaped in such a way that it follows a parabolic-like or parabolic slope .

[0046] According to some aspects , the reflector structure together with the contact layer can form a mirror , for example parabolic-like or parabolic mirror, for light generated in the semiconductor layer stack . By means of the combined mirror together with the multilayer structure , light generated within the semiconductor layer stack can be outcoupled of the light emitting device in an enhanced way, as well as a focusing of the light into a smaller solid angle is possible .

[0047] According to some aspects , the reflector structure is a metallic grid, portions of which surround the multilayer structure in the circumferential direction .

[0048] According to some aspects , the light emitting device comprises an optical element , in particular lens or plens , arranged on the multilayer structure and / or reflector structure . For example , a low index lens ( e . g . from SiO2 ) can be arranged on top of the light emitting device to further tune the directionality of the emitted light . The optical element can for example be of a dielectric material and can be formed as a lens or plens or the optical element can be in the form of a Fresnel lens or a metasurface lens .

[0049] According to a further aspect , a method for manufacturing a light emitting device is provided . The method can in particular be a method for manufacturing a light emitting device according to at least some 2024PF00978 of aforementioned aspects . Hence all aspects already described for the light emitting device can in the same way be applied to the method for manufacturing the same .

[0050] The method comprises at least the following steps :

[0051] Providing a semiconductor layer stack of at least : a first layer of a first conductivity type , a second layer of a second conductivity type , and an active region arranged between the first and the second layer being configured to emit light of a first wavelength; a rear surface ; a main light emitting surface opposite the rear surface ; and a side surface extending from the rear surface into the direction of the main light emitting surface laterally confining the semiconductor layer stack;

[0052] Providing a reflector structure on the main light emitting surface the reflector structure exposing a centre portion of the main light emitting surface and comprising an inner side surface ; and

[0053] Providing a multilayer structure comprising a plurality of sublayers extending substantially parallel to the main light emitting surface , covering the centre portion of the main light emitting surface and being surrounded by the reflector structure ; wherein a first sublayer of the multilayer structure that is adj acent to the main light emitting surface comprises a larger refractive index than a second sublayer of the multilayer structure that is further distant from the main light emitting surface ; and wherein a refractive index difference between the sublayer of the multilayer structure closest to the main light emitting surface and the sublayer of the multilayer structure furthest distant from the main light emitting surface is at least 0 . 1 , or at least 0 . 2 .

[0054] SHORT DESCRIPTION OF THE DRAWINGS

[0055] Further aspects and embodiments in accordance with the proposed principle will become apparent in relation to the various embodiments 2024PF00978 and examples described m detail m connection with the accompanying drawings in which

[0056] Figures 1A and IB each shows a cross section of a light emitting device in accordance with some aspects of the proposed principle ;

[0057] Figure 2 shows an isometric view of a light emitting device in accordance with some aspects of the proposed principle ;

[0058] Figure 3 shows a cross section of a further embodiment of light emitting devices in accordance with some aspects of the proposed principle ;

[0059] Figure 4 shows a cross section of a further embodiment of light emitting devices in accordance with some aspects of the proposed principle ; and

[0060] Figure 5 shows a cross section of a further embodiment of light emitting devices in accordance with some aspects of the proposed principle .

[0061] DETAILED DESCRIPTION

[0062] The following embodiments and examples disclose various aspects and their combinations according to the proposed principle . The embodiments and examples are not always to scale . Likewise , different elements can be displayed enlarged or reduced in size to emphasize individual aspects . It goes without saying that the individual aspects of the embodiments and examples shown in the figures can be combined with each other without further ado , without this contradicting the principle according to the invention . Some aspects show a regular structure or form . It should be noted that in practice slight differences and deviations from the ideal form may occur without , however , contradicting the inventive idea . 2024PF00978

[0063] In addition, the individual figures and aspects are not necessarily shown in the correct size , nor do the proportions between individual elements have to be essentially correct . Some aspects are highlighted by showing them enlarged . However , terms such as "above" , "over" , "below" , "under" "larger" , "smaller" and the like are correctly represented with regard to the elements in the figures . So it is possible to deduce such relations between the elements based on the figures .

[0064] Figures 1A and IB show each a cross section of an embodiment of a light emitting device 1 in accordance with some aspects of the proposed principle while Figure 2 shows a 3D drawing of the embodiment shown in Figure IB . The light emitting device comprises a semiconductor layer stack 2 with a first layer 3 of a first conductivity type , a second layer 4 of a second conductivity type as well as an active region 5 arranged between the first and the second layer 3 , 4 . The active region 5 is thereby configured to emit light of a desired wavelength .

[0065] The first conductivity type is thereby in particular an n-type and the second conductivity type is a p-type . The first layer 3 can thus be a n-type semiconductor layer and the second layer 4 can be a p-type semiconductor layer . The active region 5 and in particular the quantum well ( s ) included therein can in particular be configured to emit light of a desired wavelength when provided with a respective supply current .

[0066] The semiconductor layer stack 2 comprises a rear surface Ila , a main light emitting surface 11b opposite the rear surface Ila and a side surface 11c extending from the rear surface I la into the direction of the main light emitting surface 11b and extending through the second layer 4 , the active region 5 and a portion of the first layer 3 . The side surface 11c thereby laterally confines a light emitting portion of the semiconductor layer stack 2 , which in the light emitting device 1 is configured to emit light of the desired wavelength .

[0067] The side surface 11c and optionally the rear surface Ila of the semiconductor layer stack 2 can be overgrown with a regrowth layer to enhance the internal quantum efficiency ( IQE ) of the light emitting 2024PF00978 16 device 1 . A regrowth layer can however be understood as exemplary here and any other measures for increasing the IQE of the light emitting device 1 such as for example quantum well intermixing or the like can be applied to the semiconductor layer stack 2 .

[0068] In the embodiment shown there remains a continuous portion of the first layer 3 that defines the main light emitting surface 11b which for example may connect an array of several light emitting devices 1 or which may, as shown in figure 5 , be further pixelized to result in a single and separated light emitting device 1 .

[0069] Further a contact layer 10 is provided on the semiconductor layer stack 2 for providing an electric potential to the semiconductor layer stack and in particular to the second layer 4 . The side surface 11c is covered with a passivation layer 9 . The passivation layer 9 exposes a portion of the contact layer 10 such that at least a first potential can be applied to the semiconductor layer stack from the bottom of the light emitting device . Further a reflective embedding / mirror coating 8 is provided enclosing the underlying structure . The mirror coating 8 in the particular shown embodiment serves to reflect light generated in the active region 5 into the direction of the main light emitting surface 11b as well as can serve to provide a second electric potential to the semiconductor layer stack .

[0070] The shown rear side of the light emitting device 1 is however to be understood as exemplary and any other structure / design of the semiconductor layer stack 2 and further layers arranged on the rear and side surface I la , 11c of it is possible .

[0071] The light emitting device 1 further comprises a multilayer structure 6 as well as a reflector structure 14 laterally surrounding the multilayer structure 6 on the main light emitting surface 11b . The reflector structure 14 is arranged on the main light emitting surface 11b in form of a ring exposing a centre portion 17 of the main light emitting surface 11b directly above the active region 5 . The reflector structure 14 comprises an inner side surface 15 that faces the multilayer structure 6 and that encloses an angle of 60 ° with the main 2024PF00978 light emitting surface 11b m the embodiments shown . The angle is however to be understood as exemplary and any other angles between 45 ° and 90 ° are possible while an angle of 90 ° may however be undesired as well as a parabolic shape of the inner side surface is also possible .

[0072] The multilayer structure 6 comprises a plurality of sublayers each comprising a material with a different refractive index . In case of the embodiment of Figure 1A the multilayer structure 6 comprises two sublayers 7a, 7b while in the embodiment shown in Figure IB the multilayer structure 6 comprises three sublayers 7a , 7b , 7 c . The number of sublayers is however to be understood as exemplary and can for example as shown in Figure 3 be larger than three as well .

[0073] The multilayer structure 6 comprises a first sublayer 7a adj acent to the main light emitting surface 11b having a larger refractive index than a following second sublayer 7b , and in case of the embodiment of Figure IB the second sublayer having a larger refractive index than a following third sublayer 7 c and in Case of the embodiment of Figure 3 so on . The material of the sublayers is further chosen such that a refractive index difference between the first sublayer 7a and the outermost sublayer is at least 0 . 1 , or at least 0 . 2 . In some aspects , the refractive index difference is at least 0 . 5 or 0 . 8 . A larger number of sublayers and in particular greater refractive index difference covered by the multilayer structure thereby provides an improved redirection of light into the direction of the reflective inner side surface 15 of the reflector structure 14 and thus an improved collimation effect of the light generated in the active region 5 . The number of sublayers with each having a thickness of at least 20 nm, and in particular a thickness of for example half , a quarter, or one eighth of the effective wavelength of the light generated in the active region results in a total thickness t of the multilayer structure 6 that is in some aspects smaller than 5 pm or 20 pm. The multilayer structure 6 thereby comprises in a preferred embodiment a total thickness t of at least the effective wavelength of the light generated in the active region . 2024PF00978

[0074] The multilayer structure 6 in particular comprises a discrete number of sublayers to provide a monotonously decreasing / gradient index refractive index region covering the main light emitting surface 11b . By this , light rays emitted through the main light emitting surface 11b are deflected into the direction of the reflective inner side surface 15 of the reflector structure 14 due to refraction in the decreasing gradient index region ( indicated by means of the arrows ) .

[0075] Figure 3 shows another embodiment of a light emitting device 1 which comprises many sublayers forming a multilayer structure 6 with monotonously decreasing gradient index distribution that is arranged within the reflector structure 14 . In contrast to aforementioned embodiments the multilayer structure 6 thereby fills up the recess formed by the reflector structure 14 . Further the sublayers 7a, 7b , 7c , ... are drawn as thinner layers which shall however not be understood to show any relations between the thicknesses and the embodiments of the figures but j ust to indicate that also a greater number of sublayers can be used for the multilayer structure 6 . A larger number of sublayers and in particular greater refractive index difference covered by the multilayer structure thereby may thereby provide an improved redirection of light into the direction of the reflective inner side surface 15 of the reflector structure 14 and thus an improved collimation effect of the light generated in the active region 5 .

[0076] Figure 4 shows another embodiment of a light emitting device 1 which further comprises an additional optical element 16 , for example dielectric lens , arranged on the multilayer structure 6 and the reflector structure 14 . The optical element 16 thereby serves to further increase directionality of the light outcoupled from the light emitting device 1 .

[0077] Figure 5 shows another embodiment of a light emitting device 1 in which the semiconductor layer stack 2 is separated in such that there is no continuous residue of the first layer 3 . The side surface 11c therefore comprises a first and a second side surface portion 12a , 12b being laterally displaced to each other . The first side surface portion 12a is adj acent to the rear surface I la , comprises the second layer 4 , the 2024PF00978 19 active region 5 and a portion of the first layer 3 . The second side surface portion 12b is laterally displaced to the first side surface portion 12a and comprises a portion of the first layer 3 . The first side surface portion 12a may thereby result from a first etching step 5 for pixelating the semiconductor layer stack 2 and in particular the active region 5 while the second side surface portion 12b may result from a second etching step for separating individual semiconductor layer stack portions .

[0078] 2024PF00978 20

[0079] LIST OF REFERENCES light emitting device semiconductor layer stack first layer second layer active region multilayer structure sublayer mirror coating passivation layer contact layer rear surface main light emitting surface side surface side surface portion reflector structure inner side surface optical element centre portion thickness

Claims

2024PF00978CLAIMS1. Light emitting device (1) , in particular pLED, comprising: a semiconductor layer stack (2) of at least a first layer (3) of a first conductivity type; a second layer (4) of a second conductivity type; and an active region (5) arranged between the first and the second layer (3, 4) being configured to emit light of a first wavelength; a rear surface (Ila) ; a main light emitting surface (11b) opposite the rear surface (Ila) ; and a side surface (11c) extending from the rear surface (Ila) into the direction of the main light emitting surface (11b) laterally confining the semiconductor layer stack ( 2 ) ; a reflector structure (14) arranged on the main light emitting surface (11b) exposing a centre portion (17) of the main light emitting surface (11b) and comprising an inner side surface ( 15 ) ; and a multilayer structure (6) comprising a plurality of sublayers (7a, 7b, 7c, ...) extending substantially parallel to the main light emitting surface (11b) , covering the centre portion (17) of the main light emitting surface (11b) and being surrounded by the reflector structure (14) ; wherein a first sublayer (7a) of the multilayer structure (6) that is adjacent to the main light emitting surface (11b) comprises a larger refractive index than a second sublayer (7b) of the multilayer structure (6) that is further distant from the main light emitting surface (11b) ; and wherein a refractive index difference between the sublayer (7a) of the multilayer structure (6) closest to the main light emitting surface (11b) and the sublayer of the multilayer structure (6) furthest distant from the main light emitting surface (11b) is at least 0.1, or at least 0.2.

2. Light emitting device (1) according to claim 1,2024PF00978 wherein the inner side surface (15) and the mam light emitting surface (11b) enclose an angle different to 90°, in particular an angle between 30° and 90° excluding 90° .

3. Light emitting device (1) according to any one of the preceding claims , wherein the sublayers (7a, 7b, 7c, ...) of the multilayer structure (6) are of a dielectric material.

4. Light emitting device (1) according to any one of the preceding claims , wherein a course of the refractive index difference between the sublayer (7a) of the multilayer structure (6) closest to the main light emitting surface (11b) and the sublayer of the multilayer structure (6) furthest distant from the main light emitting surface (11b) is substantially continuous.

5. Light emitting device (1) according to any one of claims 1 to 2, wherein a course of the refractive index difference between the sublayer (7a) of the multilayer structure (6) closest to the main light emitting surface (11b) and the sublayer of the multilayer structure (6) furthest distant from the main light emitting surface (11b) comprises a plurality of jumps relating to the number of sublayers plus 1.

6. Light emitting device (1) according to any one of the preceding claims , wherein the multilayer structure (6) comprises a thickness (t) of at most 20 pm; and / or wherein the multilayer structure (6) comprises a thickness (t) of least half of the first wavelength.

7. Light emitting device (1) according to any one of the preceding claims , wherein the sublayers (7a, 7b, 7c,...) of the multilayer structure (6) each comprise a thickness of at least 1 nm.2024PF009788. Light emitting device (1) according to any one of the preceding claims , wherein the side surface (11c) comprises at least a first and a second side surface portion (12a, 12b) being laterally displaced to each other, wherein the first side surface portion (12a) is adjacent to the rear surface (Ila) , and wherein the first side surface portion (12a) comprises at least the second layer (4) and the active region (5) .

9. Light emitting device (1) according to any one of the preceding claims , wherein the reflector structure comprises an electrically conductive material electrically contacting the first layer (3) of the semiconductor layer stack (2) ; and / or wherein a contact layer (10) is arranged on the rear surface (Ila) electrically contacting the second layer (4) of the semiconductor layer stack ( 2 ) .

10. Light emitting device (1) according to any one of the preceding claims , further comprising a passivation layer (9) that is arranged on at least one of the following: on the side surface (11c) ; and on the rear surface (Ila) , in particular exposing a contact portion of the contact layer (10) .

11. Light emitting device (1) according to any one of the preceding claims , wherein the active region (5) comprises a multi quantum well structure .

12. Light emitting device (1) according to any one of the preceding claims , further comprising a mirror coating (8) that is arranged on at least one of the following: on the side surface (lie) ;2024PF00978 24 on the rear surface (Ila) , in particular exposing a contact portion of the contact layer (10) ; and on the passivation layer (9) , in particular electrically contacting the contact layer (10) .

13. Light emitting device (1) according to any one of the preceding claims , wherein the side surface (11c) and the rear surface (Ila) enclose an angle different to 90° .

14. Light emitting device (1) according to any one of the preceding claims , further comprising an optical element (16) , in particular lens, arranged on the multilayer structure (6) and / or reflector structure (14) .

15. Method for manufacturing a light emitting device (1) comprising the steps :Providing a semiconductor layer stack (2) of at least: a first layer (3) of a first conductivity type, a second layer (4) of a second conductivity type, and an active region (5) arranged between the first and the second layer (3, 4) being configured to emit light of a first wavelength; a rear surface (Ila) ; a main light emitting surface (11b) opposite the rear surface (Ila) ; and a side surface (11c) extending from the rear surface (Ila) into the direction of the main light emitting surface (11b) laterally confining the semiconductor layer stack (2) ;Providing a reflector structure (14) on the main light emitting surface (11b) the reflector structure (14) exposing a centre portion (17) of the main light emitting surface (11b) and comprising an inner side surface (15) ; andProviding a multilayer structure (6) comprising a plurality of sublayers (7a, 7b, 7c, ...) extending substantially parallel to2024PF00978 25 the main light emitting surface (11b) , covering the centre portion (17) of the main light emitting surface (11b) and being surrounded by the reflector structure (14) ; wherein a first sublayer (7a) of the multilayer structure (6) that 5 is adjacent to the main light emitting surface (11b) comprises a larger refractive index than a second sublayer (7b) of the multilayer structure (6) that is further distant from the main light emitting surface (11b) ; and wherein a refractive index difference between the sublayer of the0 multilayer structure (6) closest to the main light emitting surface(11b) and the sublayer of the multilayer structure (6) furthest distant from the main light emitting surface (11b) is at least 0.1, or at least 0.2.

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