Light emitting device with multilayer structure and method for manufacture

Abstract

The invention concerns a light emitting device (1), in particular microLED, 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 being configured to emit light. The semiconductor layer stack further comprises a rear surface (11a), a main light emitting surface (11b) opposite the rear surface (11a) and a side surface (11c) extending from the rear surface into the direction of the main light emitting surface laterally confining the semiconductor layer stack (2). Further the light emitting device comprises a multilayer structure (6) covering at least a centre portion (17) of the main light emitting surface (11b), the multilayer stack (6) comprising a core layer (7a) and at least one cladding layer (7b, 7c) laterally enclosing the core layer (7a). The core layer (7a) comprises a larger refractive index than the at least one first cladding layer (7b, 7c) and a refractive index difference between the core layer (7a) and a cladding layer (7b, 7c) of the multilayer structure (6) furthest distant from the core layer (7a) is at least 0.1, or at least 0.2.

Description

[0001] 2024PF00996

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

[0003] The present application claims priority from German patent application DE 10 2024 135 093 . 8 filed on November 27 , 2024 , the disclosure of which is incorporated by way for reference in its entirety .

[0004] The present invention concerns a light emitting device , in particular pLED, comprising a multilayer 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 .

[0005] BACKGROUND pLEDs are light emitting devices that comprise a lateral dimension in the range from a few pm to about 40 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 .

[0006] In order to increase the external quantum efficiency ( EQE ) of for example pLEDs , or more general light emitting nanostructures , one can 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 .

[0007] 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 . 2024PF00996

[0008] 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 .

[0009] 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 .

[0010] SUMMARY OF THE INVENTION

[0011] 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 .

[0012] 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 2024PF00996 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 ) arranged on the main light emitting surface of a light emitting device . The coating is thereby arranged and designed to comprise a core layer arranged on the main light emitting surface laterally enclosed by at least one cladding layer wherein the monotonously decreasing refractive index is provided from the center axis of the core layer in the lateral direction, so in particular in a direction substantially parallel to the main light emitting surface . Thus , instead of a smoothly varying sidewall shape or the use of different components , a "smoothly" varying material property, namely the refractive index , 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 / ref racted in a desired direction to in sum provide light with a high directionality outcoupled from the light emitting device .

[0013] 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 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 multilayer structure of layers with a monotonously decreasing refractive index is well controlled, the approach promises to maintain a high outcoupling efficiency .

[0014] The core idea is thereby to exploit how light obliquely outcoupled from the main light emitting surface is refracted by and propagates as it 2024PF00996 travels into progressively lower refractive index layers of the monotonously decreasing refractive index coating to enhance the light outside a desired escape cone to be ref racted / redirected to escape the light emitting device within the desired cone in a collimated manner . Advantages that can result from such a monotonously decreasing refractive index sidewall coating can be :

[0015] - High outcoupling efficiency .

[0016] - High directionality of outcoupled light . A maj ority power can be emitted within a small escape cone .

[0017] - High Performance robustness towards large geometrical variations within the semiconductor layer stack : e . g . Side surface of the semiconductor layer stack, or rear contact

[0018] - 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 .

[0019] 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 .

[0020] 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 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 2024PF00996 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 .

[0021] The light emitting device further comprises a multilayer structure covering at least a centre portion of the main light emitting surface . The multilayer stack comprises a core layer and at least one cladding layer laterally enclosing the core layer , wherein the core layer can in particular be in the form of a cylinder or cone , in particular frustum of a cone , and wherein the at least one cladding layer can form a shell layer , in particular hollow cylinder or cone , around the core layer which envelops the core layer . The core layer can thereby in particular be arranged on the main light emitting surface such that its top surface opposite the floor is arranged on the on the main light emitting surface wherein the at least one cladding layer can be arranged on the main light emitting surface such that its top surface opposite the floor is arranged on the on the main light emitting surface adj oining the shell surface of the core layer .

[0022] The core layer comprises a larger refractive index than the at least one first cladding layer and a refractive index difference between the core layer and a cladding layer of the multilayer structure furthest distant from the core layer is at least 0 . 1 , at least 0 . 2 , at least 0 . 5 or at least 0 . 8 .

[0023] By means of such a multilayer structure 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 first be ref racted / def lected into the direction of the at least one cladding layer and then ref racted / ref lected into the direction of the height of the at least one cladding layer such that more light generated within the active 2024PF00996 region is emitted from the light emitting device 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 .

[0024] Due to parallel momentum conservation at the interface between the main light emitting surface and the core layer as well as due to parallel momentum conservation at the interface between the core layer and the at least one cladding layer , light outcoupled from the main light emitting surface is refracted to firstly more oblique angles when in coupled into the core layer with a lower refractive index and then refracted to steeper angles when in coupled into the at least one cladding layer with an even lower refractive index .

[0025] 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 core layer next to the main light emitting surface to at least one lower refractive index cladding layer further distant from the core layer and thus 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 at least one cladding layer . If the entire multilayer structure would now however comprise of only a high refractive index material , more light would be outcoupled from the light emitting device than without the multilayer structure but the light outcoupled from the multilayer structure would be within an even larger light emitting cone as without the multilayer structure . Therefore , the multilayer structure comprises at least one cladding layer laterally eclosing a core layer, with the core layer starting from a high refractive index material and progressively go down to a low refractive index material starting from the center axis of the core layer in the lateral direction, so in particular in a direction substantially parallel to the main light emitting surface . This allows in first place more light being in coupled into the core layer and being refracted into the direction of the at least one cladding layer 2024PF00996 and then being ref racted / redirected deep within a desired escape cone of the light emitting device in a more collimated manner by means of the at least one cladding layer . The more cladding layers the multilayer structure has , the more portion of light is refracted into a desired escape cone of the light emitting device in a more collimated manner . Each cladding layer of the multilayer structure can thereby in particular be responsible in sending a different angular portion of the light generated in the active region and being in coupled into the core layer 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 redirects light outside a desired escape cone , deep within the desired escape cone in a collimated manner .

[0026] 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 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 .

[0027] 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.

[0028] According to some aspects , the light emitting device further comprises a reflector structure arranged on the main light emitting surface exposing the centre portion of the main light emitting surface and 2024PF00996 comprising an inner side surface which is tilted compared to the mam light emitting surface . The reflector structure laterally surrounds the multilayer structure .

[0029] According to some aspects , the cladding layer of the multilayer structure furthest distant from the core layer adj oins the inner side surface .

[0030] By means of such a reflector structure with a tilted inner side surface paired with the multilayer structure being surrounded / 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 and that is not already deflected into the desired light cone by the at least one cladding layer will be deflected into the direction of the reflector structure and at the reflector structure be reflected into the desired direction within the desired light emitting cone . Hence , an enhanced light extraction and directionality of light that is emitted from the light emitting device can be achieved with further providing a reflector structure laterally enclosing the multilayer structure and in particular the at least one cladding layer .

[0031] 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 contacting be reflective . However , the reflector structure can also be electrically isolated from the semiconductor layer stack / does not serve as an electrical contact besides being reflective . 2024PF00996

[0032] According to some aspects , the reflector structure is a metallic grid, portions of which surround the multilayer structure in the circumferential direction .

[0033] According to some aspects , the inner side surface and the main light emitting surface enclose an angle different to 90 ° , in particular an angle between 45 ° and 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 .

[0034] According to some aspects , a shell surface of the core layer and the main light emitting surface enclose an angle different to 90 ° , in particular an angle between 45 ° and 135 ° excluding 90 ° . In particular the shell surface and the main light emitting surface can enclose an angle of for example 60 ° or 110 ° . In particular the shell surface can define a cone opening in a direction away from the main light emitting surface , however the shell surface can also define a cone that is opening in the direction of the main light emitting surface . In particular the at least one cladding layer and in particular a shell surface of the at least one cladding layer can enclose a substantial equal angle with the main light emitting surface as the shell surface of the core layer . This is as the at least one cladding layer may enclose the core layer in the lateral direction with a substantially equal thickness such that the shell surfaces of the core layer and the at least one cladding layer eclose a substantial equal angle with the main light emitting surface . To increase the effect of collimation by means of the multilayer structure an angle of 90 ° between the shell surfaces and the main light emitting surface may however be undesired .

[0035] According to some aspects , a shell surface of the core layer and / or a shell surface of the cladding layer ( s ) is axisymmetric with regard to a centre axis of the light emitting device that is perpendicular to the main light emitting surface . In particular the shell surface ( s ) 2024PF00996 can define a cone opening m a direction away from the mam light emitting surface, or define a cone that is opening in the direction of the main light emitting surface, wherein the cone is axisymmetric with regard to a centre axis of the light emitting device that is perpendicular to the main light emitting surface. By this a light output out of the light emitting device around the centre axis within for example a 15-18 degree cone can be achieved.

[0036] According to some aspects, the core layer can be an oblique cylinder or an oblique cone with regard to a centre axis of the light emitting device that is perpendicular to the main light emitting surface. The cladding layer (s) can then be defined by an oblique hollow cylinder or an oblique hollow cone. By this a light output out of the light emitting device around an axis tilted with regard to the main light emitting surface within for example a 15-18 degree cone can be achieved. The light can thus be concentrated around a desired polar angle and azimuth different to the centre axis .

[0037] 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 equal effective bandgap and can in particular be configured to emit light of a substantially equal wavelength.

[0038] 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.

[0039] In some aspects, the core layer and the at least one cladding layer are of a dielectric material. In particular the core layer and the at least one cladding layer of the multilayer structure can comprise one or more materials of the following selection:

[0040] Ti02, Ta2O5, TiO2:C:H, SiNl,3:H, SiOxNy:H, PPOS, PPHC, SiO2:H, SiO2:F, PPFC, Nb2O5, ZrO2, Y2O3, A12O3, Si02, SiN, MgF2, PET, PC, Si02 glass, PMMA, Si02 quartz. 2024PF00996

[0041] The core layer and the at least one cladd ng layer can thereby be provided using for example PECVD, PVD, or deposition of a substrate . The core layer and the at least one cladding layer or at least part of them can however also be od a semiconductor material .

[0042] In some aspects , the lateral dimensions of a proj ection of the multilayer structure are larger as the lateral dimensions of a proj ection of the active region when viewed in a direction perpendicular to the main light emitting surface . In particular when viewed in a direction perpendicular to the main light emitting surface a proj ection of the active region is completely covered by a proj ection of the multilayer structure . The multilayer structure may in particular cover a centre portion of the main light emitting surface that is larger than a proj ection of the active region when viewed in a direction perpendicular to the main light emitting surface . Thus , light being generated within the active region and outcoupled from the main light emitting surface is mostly or even completely in coupled into the multilayer structure to be redirected / ref racted into a desired direction .

[0043] In some aspects , the lateral dimensions of a proj ection of the core layer are at least of the size as the lateral dimensions of a proj ection of the active region when viewed in a direction perpendicular to the main light emitting surface . In particular when viewed in a direction perpendicular to the main light emitting surface a proj ection of the at least one cladding layer may lie outside a proj ection of the active region . The core layer may in particular cover a portion of the main light emitting surface and in particular a central portion of the centre portion of the main light emitting surface that is larger than a proj ection of the active region when viewed in a direction perpendicular to the main light emitting surface . Thus , light being generated within the active region and outcoupled from the main light emitting surface into the multilayer structure is mostly or even completely in coupled into the core layer first before being redirected / ref racted to the at least one cladding layer . 2024PF00996

[0044] In some aspects , the core layer and the at least one cladd ng layer of the multilayer structure are each or at least some of them of a different material with each a different refractive index , where the core layer comprises a larger refractive index than the cladding layer ( s ) further distant from the core layer . By this , a refractive index gradient results from the main light emitting surface to core layer in a vertical direction and from the core layer to the outermost cladding layer of the multilayer structure in a lateral direction .

[0045] Depending on the material of the core layer and the at least one cladding layer and the thicknesses of the core layer and the at least one cladding layer as well as the material concentration within the core layer and the at least one cladding layer , a course of the refractive index difference between the core layer and the cladding layer of the multilayer structure which is furthest away from the core layer can either be substantially continuous , or may comprise a plurality of j umps , in particular depending on the number of cladding layers plus 1 . The number of j umps can in particular depend on the interface between the semiconductor layer stack and the core layer , the interface between the core layer and a cladding layer enclosing the core layer , the interface between the outermost cladding layer and a neighbouring medium ( for example air, or a reflector structure ) and the interfaces between possible further other cladding layers . This is however to be understood that the course of the refractive index difference between the core layer and the cladding layer of the multilayer structure which is furthest away from the core layer does not necessarily have to decrease continuously until the cladding layer of the multilayer structure which is furthest away from the core layer 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 .

[0046] In some aspects , a proj ection of the multilayer structure comprises when viewed in a direction perpendicular to the main light emitting surface a diameter of at most 20 times the lateral dimensions of a proj ection of the active region when viewed in a direction perpendicular to the main light emitting surface . In particular , the 2024PF00996 multilayer structure comprises lateral dimensions m a direction substantially parallel to the main light emitting surface of at most 20 times the lateral dimensions of a proj ection of the active region when viewed in a direction perpendicular to the main light emitting surface . This is because the multilayer structure is intended to improve a small-dimensioned and tuned semiconductor layer stack in terms of coupling out and collimation of the emitted light , while at the same time avoiding enlarging the entire light emitting device too much for this effect .

[0047] In some aspects , the cladding layers of the multilayer stack comprise a 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 cladding layers of the multilayer stack comprise a thickness in a direction substantially parallel 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 cladding layers enclosing the core layer would leave the multilayer structure either by reflection or transmission before it could be refracted as intended .

[0048] In some aspects , the at least one cladding layer of the multilayer structure comprises a thickness of at least 1 nm, in particular between 20 nm and 1 pm, or between 50 nm and 5 pm . In particular the minimum thickness of the at least one cladding layer 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 at least one cladding layer would be too thin, oblique light arriving at the cladding layers enclosing the core layer would leave the multilayer structure either by reflection or transmission before it could be refracted as intended . In particular a sufficient thickness of each cladding layer can be necessary such that light emitted from the core layer still perceives a region with an index gradient , be refracted as desired and propagate in the index gradient region sufficiently long enough for being outcoupled from the light emitting device within the desired light cone . In some aspects , the thickness of the cladding layers can be substantially equal , it is 2024PF00996 however also possible that the thicknesses of the cladding layers is different to each other .

[0049] In some aspects , the light emitting device further comprises a further multilayer structure arranged on the side surface , the further multilayer structure comprises at least two sublayers laterally surrounding the semiconductor layer stack . The sublayer adj acent to the side surface comprises a larger refractive index than the active region and a refractive index difference between the sublayer adj acent to the side surface and the sublayer furthest distant from the side surface is at least 0 . 1 , or at least 0 . 2 . In particular the further multilayer structure can be designed in the same way as the cladding layer ( s ) enclosing the core layer according to some of aforementioned aspects .

[0050] 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 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 .

[0051] 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 .

[0052] 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 . 2024PF00996

[0053] 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 ; 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 .

[0054] 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 .

[0055] 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 .

[0056] 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 , metasurface lens , micro lens , a grating structure , a DBR filter , or another multilayer structure as already described . For example , the optical element can be formed of the material of the core layer . 2024PF00996 16

[0057] 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 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 .

[0058] The method comprises at least the following steps :

[0059] 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;

[0060] Providing a multilayer structure covering at least a centre portion of the main light emitting surface , the multilayer stack comprising a core layer and at least one cladding layer laterally enclosing the core layer ; wherein the core layer comprises a larger refractive index than the at least one first cladding layer; and wherein a refractive index difference between the core layer and a cladding layer of the multilayer structure furthest distant from the core layer is at least 0 . 1 , or at least 0 . 2 .

[0061] SHORT DESCRIPTION OF THE DRAWINGS

[0062] Further aspects and embodiments in accordance with the proposed principle will become apparent in relation to the various embodiments and examples described in detail in connection with the accompanying drawings in which 2024PF00996

[0063] Figure 1 shows a cross section of a light emitting device in accordance with some aspects of the proposed principle ;

[0064] Figure 2A and 2B show each a cross section of a further embodiment of light emitting devices in accordance with some aspects of the proposed principle ;

[0065] Figure 3 shows a cross section of a further embodiment of light emitting devices in accordance with some aspects of the proposed principle ;

[0066] Figure 4 shows a cross section of a further embodiment of light emitting devices in accordance with some aspects of the proposed principle ; and

[0067] Figure 5 shows a cross section of a further embodiment of light emitting devices in accordance with some aspects of the proposed principle .

[0068] DETAILED DESCRIPTION

[0069] 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 .

[0070] 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" , 2024PF00996

[0071] "below" , "under" "larger" , "smaller" and the I ke 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 .

[0072] Figure 1 shows a cross section of an embodiment of a light emitting device 1 in accordance with some aspects of the proposed principle . 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 . The first layer 3 comprises in the embodiment shown a remaining continuous portion that defines the main light emitting surface 11b which for example may connect an array of several light emitting devices 1 or which may be further pixelized to result in a single and separated light emitting device 1 .

[0073] 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 .

[0074] 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 .

[0075] The side surface 11c and optionally the rear surface Ila of the semiconductor layer stack 2 can for example be overgrown with a regrowth layer to enhance the internal quantum efficiency ( IQE ) of the light 2024PF00996 19 emitting device 1 . A regrowth layer is however only to 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 .

[0076] 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 further covered with a passivation layer 9 . The passivation layer 9 exposes a portion of the contact layer 10 such that a first potential can be applied to it to drive the light emitting device . A reflective embedding / mirror coating 8 encloses 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 and a second electric potential can be applied to the semiconductor layer stack via the mirror coating 8 .

[0077] 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 .

[0078] 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 . 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 light emitting surface 11b in the embodiments shown . The angle is however to be understood as exemplary and any other angles between 45 ° and 135 ° are possible while an angle of 90 ° may however be undesired .

[0079] The multilayer structure 6 comprises a core layer 7a as well as at least one cladding layer 7b, 7 c laterally enclosing the core layer 7a each comprising a material with a different refractive index . In case 2024PF00996 20 of the embodiment of Figure 1 the multilayer structure 6 comprises a core layer 7a and two cladding layers 7a , 7b . The number of cladding layers is however to be understood as exemplary and can be larger than two as well .

[0080] The multilayer structure 6 comprises a core layer 7a covering a central portion of the centre portion 17 of the main light emitting surface 11b having a larger refractive index than a first cladding layer 7b laterally enclosing the core layer 7a, and the second cladding layer 7b having a larger refractive index than a third cladding layer 7 c laterally enclosing the first cladding layer 7b . The material of the core layer and the cladding layers is chosen such that a refractive index difference between the core layer 7a and the outermost cladding layer 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 cladding layers and in particular greater refractive index difference covered by the multilayer structure thereby provides an improved redirection and refraction of light and thus an improved collimation effect of the light generated in the active region 5 . Light that is not already redirected and refracted by the cladding layers such that it is already within a desired light cone when being outcoupled from the multilayer structure is by the cladding layers ref racted / def lected into the direction of the reflective inner side surface 15 of the reflector structure 14 and there reflected into a desired direction . The reflector structure 14 is thereby however to be understood as optional and a desired effect can also already be provided with only the multilayer structure 6 . The number of cladding layers with each having a thickness of at least 1 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 diameter D of the multilayer structure 6 that is in some aspects smaller than 0 , 5 pm, 1 pm, 5 pm or 20 pm.

[0081] The multilayer structure 6 in particular comprises a discrete number of cladding layers in combination with the core layer to provide a monotonously decreasing refractive index region . By this , light rays emitted through the main light emitting surface 11b into the core layer 2024PF00996 21 are deflected into a direction substantially perpendicular to the mam light emitting surface due to refraction in the decreasing gradient index region ( indicated by means of the arrows ) .

[0082] Figure 2A shows another embodiment of an array of light emitting devices 1 ( indicated by the wave lines on the right and left ) which are connected by a continuous first layer as well as reflective embedding 8 . The rear side of the light emitting devices 1 is thereby shown in a simplified way to illustrate that it is not limited to the design shown in Figure 1 but can be modified in a desired manner .

[0083] Figure 2B 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 . The optical element 16 can for example be formed of the material of the core layer 7a and be one piece with the core layer 7a .

[0084] Figure 3 shows another embodiment of a light emitting device 1 which further comprises a further multilayer structure 13 . The further multilayer structure 13 comprises a plurality of sublayers 12a, 12b , 12 c laterally surrounding the semiconductor layer stack 2 . In the embodiment shown only three sublayers are depicted which is however to be understood as exemplary and the number can also be larger . The sublayer 12a adj acent to the side surface 11c comprises a larger refractive index than the semiconductor layer stack 2 and a refractive index difference between the sublayer 12a adj acent to the side surface 11c and the sublayer 12 c furthest distant from the side surface 11c is at least 0 . 1 , or at least 0 . 2 . The further multilayer structure can be designed in the same way as the cladding layer ( s ) enclosing the core layer according to some of aforementioned aspects . Further in the embodiment shown, the reflector structure is dispensed with to indicate that the reflector structure is optional . In addition the thickness of the remaining continuous portion of the semiconductor layer stack 2 forming the main light emitting surface 11b is to be understood as 2024PF00996 exemplary as it is shown and can also be thicker as shown m the figures .

[0085] Figure 4 shows another embodiment of a light emitting device 1 in which the multilayer structure 6 is enclosed by the reflector structure 14 , however a gap is formed between the multilayer structure 6 and the inner side surface 15 of the reflector structure 14 . The inner side surface 15 that faces the multilayer structure 6 encloses an angle of 60 ° with the main light emitting surface 11b in the embodiments shown . The core layer 7a as well as the cladding layers 7b , 7c however comprise a shell surface that enclose an angle of 60 ° in the reverse direction with the main light emitting surface 11b in the embodiments shown . Hence the shell surfaces are tilted into another direction than the inner side surface 15 . In particular the inner side surface 15 defines a cone opening into a direction facing away from the main light emitting surface 11b, whereas the shell surfaces of the core layer 7a and the cladding layers 7b, 7c define a cone opening into the direction of the main light emitting surface 11b . In particular the cones are each axisymmetric with regard to a centre axis of the light emitting device that is perpendicular to the main light emitting surface 11b . By such a combination it is possible to maximize the light output out of the light emitting device around the center axis within for example a 15 - 18 degree cone .

[0086] Figure 5 shows another embodiment of a light emitting device 1 in which compared to aforementioned multilayer structure 6 the shell surfaces of the core layer 7a and the cladding layers 7b, 7 c define an oblique cone / oblique hollow cones opening into the direction of the main light emitting surface 11b , wherein the oblique cones are not axisymmetric with regard to the centre axis of the light emitting device that is perpendicular to the main light emitting surface 11b but to an axis tilted with regard to the main light emitting surface 11b . By such a design it is possible to maximize the light output out of the light emitting device around this tilted axis within for example a 15 -18 degree cone . 2024PF00996

[0087] LIST OF REFERENCES

[0088] 1 light emitting device

[0089] 2 semiconductor layer stack

[0090] 3 first layer

[0091] 4 second layer

[0092] 5 active region

[0093] 6 multilayer structure

[0094] 7a , 7b , 7c sublayer

[0095] 8 mirror coating

[0096] 9 passivation layer

[0097] 10 contact layer

[0098] Ila rear surface

[0099] 11b main light emitting surface

[0100] 11c side surface

[0101] 12a, 12b , 12 c sublayer

[0102] 13 further multilayer structure

[0103] 14 reflector structure

[0104] 15 inner side surface

[0105] 16 optical element

[0106] 17 centre portion

Claims

2024PF00996 24CLAIMS1. 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 ) ; and a multilayer structure (6) covering at least a centre portion (17) of the main light emitting surface (11b) , the multilayer stack (6) comprising a core layer (7a) and at least one cladding layer (7b, 7c) laterally enclosing the core layer (7a) ; wherein the core layer (7a) comprises a larger refractive index than the at least one first cladding layer (7b, 7c) ; and wherein a refractive index difference between the core layer (7a) and a cladding layer (7b, 7c) of the multilayer structure (6) furthest distant from the core layer (7a) is at least 0.1, or at least 0.2.

2. Light emitting device (1) according to claim 1, further comprising a reflector structure (14) arranged on the main light emitting surface (11b) exposing the centre portion (17) of the main light emitting surface (11b) and comprising an inner side surface (15) which is tilted compared to the main light emitting surface (11b) , wherein the reflector structure (14) laterally surrounds the multilayer structure (6) .

3. Light emitting device (1) according to claim 2,2024PF00996 wherein the cladding layer (7b, 7c) of the multilayer structure (6) furthest distant from the core layer (7a) adjoins the inner side surface (15) .

4. Light emitting device (1) according to claim 2 or 3, wherein the reflector structure (14) comprises an electrically conductive material electrically contacting the first layer (3) ; and / or wherein a contact layer (10) is arranged on the rear surface (Ila) electrically contacting the second layer (4) .

5. Light emitting device (1) according to any one of claims 2 to 4, wherein the inner side surface (15) and the main light emitting surface (11b) enclose an angle different to 90°, in particular an angle between 45° and 90° .

6. Light emitting device (1) according to any one of the preceding claims , wherein the core layer (7a) and the at least one cladding layer (7b, 7c) are of a dielectric material.

7. Light emitting device (1) according to any one of the preceding claims , wherein a course of the refractive index difference between the core layer (7a) and the cladding layer (7b, 7c) of the multilayer structure (6) furthest distant from the core layer (7a) is substantially continuous .

8. Light emitting device (1) according to any one of claims 1 to 6, wherein a course of the refractive index difference between the core layer (7a) and the cladding layer (7b, 7c) of the multilayer structure (6) furthest distant from the core layer (7a) comprises a plurality of jumps relating to the number of cladding layers (7b, 7c) .

9. Light emitting device (1) according to any one of the preceding claims ,2024PF00996 26 wherein when viewed m a direction perpendicular to the main light emitting surface (11b) the lateral dimensions of a projection of the core layer (7a) are at least of the size as the lateral dimensions of a projection of the active region (5) .

10. Light emitting device (1) according to any one of the preceding claims , wherein when viewed in a direction perpendicular to the main light emitting surface (11b) the lateral dimensions of a projection of the multilayer stack (6) are larger as the lateral dimensions of a projection of the active region (5) .

11. Light emitting device (1) according to any one of the preceding claims , wherein when viewed in a direction perpendicular to the main light emitting surface (11b) a projection of the multilayer structure (6) comprises a diameter (D) of at most 20 times the lateral dimensions of a projection of the active region (5) .

12. Light emitting device (1) according to any one of the preceding claims , wherein the at least one cladding layer (7b, 7c) comprises a thickness of at least 1 nm.

13. Light emitting device (1) according to any one of the preceding claims , further comprising a further multilayer structure (13) arranged on the side surface (11c) the further multilayer structure comprising at least two sublayers (12a, 12b) laterally surrounding the semiconductor layer stack (2) , wherein a refractive index difference between the sublayer (12a) adjacent to the side surface (11c) and the sublayer furthest distant from the side surface (11c) is at least 0.1, or at least 0.2.

14. Light emitting device (1) according to any one of the preceding claims ,2024PF00996 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) .

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

16. 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) ; 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) .

17. 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° .

18. 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) .

19. 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, and2024PF00996 28 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) ;5 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 stack0 (2) ;Providing a multilayer structure (6) covering at least a centre portion (17) of the main light emitting surface (11b) , the multilayer stack (6) comprising a core layer (7a) and at least one cladding layer (7b, 7c) laterally enclosing the core layer (7a) ;5 wherein the core layer (7a) comprises a larger refractive index than the at least one first cladding layer (7b, 7c) ; and wherein a refractive index difference between the core layer (7a) and a cladding layer (7b, 7c) of the multilayer structure (6) furthest distant from the core layer (7a) is at least 0.1, or at0 least 0.2.

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