Light emitting device with multilayer structure and method for manufacture

Abstract

The invention concerns a light emitting device, in particular µLED, comprising 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. The semiconductor layer stack further 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 laterally confining the semiconductor layer stack. Further the light emitting device comprises a multilayer structure comprising a plurality of sublayers covering at least a portion of the side surface. A first sublayer of the multilayer structure that is adjacent to the side surface thereby comprises a larger refractive index than a second sublayer of the multilayer structure that is further distant from the side surface and a refractive index difference between the sublayer of the multilayer structure closest to the side surface and the sublayer of the multilayer structure furthest distant from the side surface is at least 0.1, or at least 0.2.

Description

[0001] LIGHT EMITTING DEVICE WITH MULTILAYER STRUCTURE AND METHOD FOR

[0002] MANUFACTURE

[0003] The present application claims priority from German patent application DE 10 2024 117 650 . 4 filed on June 21 , 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 extending along side surfaces 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 light extraction efficiency (LEE ) in a specific solid angle is maximized .

[0007] In particular for applications in AR / VR only a ~ 15 to 18 degree escape cone is actually useful for further processing of the light .

[0008] With regards to outcoupling 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 original 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 at a main light emitting surface . However, such additional structures complicate the further processing of the devices and limit the applicability of pick and place / stamping methods accessible to customers .

[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 properties spatial distribution, texturing or providing a complicated geometrical shape can be avoided . 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 sidewall coating (whether a smooth gradient or through discrete steps over multiple layers ) . Thus , instead of a smoothly varying sidewall shape , a "smoothly" varying material property is used, namely the refractive index , which in turn reduces the need to have a large collimator geometry . In such a monotonously decreasing refractive index sidewall coating , purely by refraction / in-plane momentum conservation at the interface , portion of the light obliquely coming to the sidewall can be directed into a direction of a main light emitting surface of the light emitting device .

[0013] Portion of the light that is emitted from the semiconductor material at the sidewall at an angle can therefore be deflected into the direction of a main light emitting surface of the light emitting device , in particular with a narrower angular spread and deeper within the escape cone of the main light emitting surface as it goes deeper into the monotonously decreasing refractive index sidewall coating .

[0014] Advantages that can result from such a monotonously decreasing refractive index sidewall coating can be :

[0015] - Improved light outcoupling without any outcoupling texture / structure ( e . g . lens ) at / on the light emitting surface of the light emitting device , making the light emitting device suitable for transfer stamping processes .

[0016] - Light that is outcoupled through the sidewalls of the semiconductor material of the light emitting device is automatically more collimated thus actually increasing output power in the angular range of interest .

[0017] - The decreasing refractive index sidewall coating in forms of several layers is easier to be deposited than to generate complicated 3D Lens structures for collimation and / or outcoupling efficiency .

[0018] 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 .

[0019] 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 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 .

[0020] The light emitting device further comprises a multilayer structure comprising a plurality of sublayers covering at least a portion of the side surface . The multilayer structure comprises a first sublayer that is adj acent to the side surface which comprises a larger refractive index than a second sublayer of the multilayer structure that is further distant from the side surface , and a refractive index difference between the sublayer of the multilayer structure closest to the side surface and the sublayer of the multilayer structure furthest distant from the side surface is at least 0 . 1 , at least 0 . 2 , at least 0 . 5 or at least 0 . 8 . By means of such a multilayer structure covering at least portions of the side surface of the semiconductor layer stack, light that is emitted from the semiconductor layer stack at the side surface at an angle will be deflected into the direction of the main light emitting surface and in particular with a narrower angular spread and deeper within the cone of the main light emitting surface as it goes deeper into the multilayer structure . 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 . Hence the main light emitting surface can for example be free of an optical element , in particular optical lens and can be suitable for a subsequent pick and place process .

[0021] 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 .

[0022] 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 .

[0023] 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 .

[0024] In some aspects , the multilayer structure not only covers the side surface or a portion of the side surface but also extends onto the rear surface covering a portion of the rear surface . By means of this the effect of collimation and deflection of light emitted from the semiconductor layer stack can further be improved . In addition, the multilayer structure also extending onto the rear surface can simplify the process of its manufacture and be a welcome by-product .

[0025] 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 :

[0026] Ti02 , Ta2O5 , TiO2 : C : H, SiNl , 3 : H, SiOxNy : H , PPOS , PPHC, SiO2 : H , Si02 : F, PPFC, Nb2O5 , ZrO2 , Y2O3 , A12O3 , Si02 , MgF2 , PET , PC, Si02 glass , PMMA, Si02 quartz .

[0027] The sublayers can thereby be provided using for example PECVD, PVD, or deposition of a substrate .

[0028] 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 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 side surface to the outermost sublayer of the multilayer stack .

[0029] 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 side surface and the sublayer of the multilayer structure which is furthest away from the multilayer structure 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 side surface and the sublayer of the multilayer structure which is furthest away from the multilayer structure does not necessarily have to decrease continuously until the sublayer of the multilayer structure which is furthest away from the multilayer structure 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 .

[0030] 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 pm 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 side 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 , while at the same time avoiding enlarging the entire light-emitting device too much for this effect .

[0031] In some aspects , the multilayer structure comprises a total thickness of at least 1 times the emission wavelength of light generated in the active region, in particular first wavelength ( e . g . 600 nm 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 side surface of at least 1 times the first wavelength . This is because for a thinner thickness , light arriving at the sidewalls would leave the multilayer structure either by reflection or transmission before it could reach the intended main light emitting surface .

[0032] In some aspects , the sublayers of the multilayer structure each comprise a thickness of at least 20 nm, in particular between 20 nm and 2 pm, or between 50 nm and 5 pm. 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, the light arriving at the sidewalls would leave the multilayer structure either by reflection or transmission before it could reach the intended main light emitting surface . In particular a sufficient thickness of each sublayer can be necessary such that light emitted from the side 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 intended main light emitting surface instead of being reflected back into the semiconductor layer stack at the same surface as it was emitted from. 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 .

[0033] 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 .

[0034] According to some aspects , a first contact layer is arranged on the main light emitting surface electrically contacting the first layer of the semiconductor layer stack . According to some aspects , a second contact layer is arranged on the rear surface electrically contacting the second layer of the semiconductor layer stack . The first and second 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 first and second 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 first contact layer can be of a TCO whereas the second contact layer can be metallic or vice versa .

[0035] According to some aspects , the light emitting device further comprises a dielectric layer arranged on at least one of the following : on the multilayer structure opposite the side surface ; on the multilayer structure opposite the rear surface , in particular exposing a contact portion of the second contact layer or a contact element electrically coupled to the second contact layer; and on or above the main light emitting surface .

[0036] The dielectric layer can in particular cover any side surfaces of the semiconductor layer stack not covered by the multilayer structure as well as the multilayer structure itself . The dielectric layer can in particular serve to passivate the layers / components of the light emitting device . In addition, the dielectric layer can in some embodiments provide an electric isolation between the first and second contact layer .

[0037] According to some aspects , the light emitting device further comprises a mirror coating arranged on at least one of the following : on the multilayer structure opposite the side surface ; on the multilayer structure opposite the rear surface , in particular exposing a contact portion of the second contact layer or a contact element electrically coupled to the second contact layer; and on the dielectric layer .

[0038] As a certain maximum thickness of the multilayer structure can be desired and as it can be difficult to design the multilayer stack such that its refractive index continuously decreases to a value lower than 1 ( in particular lower than 1 . 45 ) , a complete collimation and redirection of light can in addition to the monotonously decreasing index region be achieved by a sidewall reflector with an optimized slope such that the rest of the light that passes through the multilayer stack can be further collimated and redirected by the mirror coating . The multilayer stack can thus be covered with a mirror coating, in particular forming a side surface mirror and / or back side mirror , for example parabolic-like or parabolic mirror . The mirror coating can for example be a coating of a reflective material , such as for example silver, or can be a layer stack of several layers such as a DBR coating .

[0039] 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 that passes through the multilayer stack can be further 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 .

[0040] According to some aspects , the light emitting device comprises a DBR coating / angular filter on or above the main light emitting surface . The DBR coating / angular filter avoids that light emitted through the main light emitting surface under oblique angles can pass the DBR coating and is reflected back into the semiconductor layer stack . Light that is emitted through the main light emitting surface with an angle of substantially 90 ° can pass the DBR coating as it is already collimated in a desired way . The DBR coating can in particular be limited to the size of the main light emitting surface , can however also extend onto / above the multilayer stack .

[0041] According to some aspects , the light emitting device comprises a reflective contact element arranged on the multilayer structure and / or the dielectric layer electrically contacting the second contact layer . The reflective contact element together with the second 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, light generated within the semiconductor layer stack can be outcoupled of the light emitting device through the main light emitting surface in an enhanced way, as well as a focusing of the light into a smaller solid angle is possible .

[0042] According to some aspects , the reflective contact element comprises a through contact through the multilayer structure and / or the dielectric layer electrically contacting the second contact layer . The through contact can in particular be substantially centred with respect to the semiconductor layer stack or can for example due to manufacturing inaccuracies be off centred with respect to the semiconductor layer stack .

[0043] According to some aspects , the light emitting device comprises a first contact element electrically coupled to the first layer and the first contact layer respectively, wherein optionally the first contact element is of a reflective material and surrounds the semiconductor layer stack in a circumferential direction . The first contact element can for example be a metallic grid surrounding the semiconductor layer stack in the circumferential direction . Due to its conductivity, the metallic grid can together with the first contact layer and the reflective contact element and second contact layer respectively in particular be used to electrically connect the light emitting device to a current source .

[0044] 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 .

[0045] The method comprises at least the following steps : 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;

[0046] Structuring the semiconductor layer stack thereby remaining at least one first portion of the semiconductor layer stack and removing a second portion of the semiconductor layer stack adj acent to the at least one first portion, resulting in a side surface of the at least one first portion comprising the second layer and the active region; and Providing a multilayer structure comprising a plurality of sublayers covering at least a portion of the side surface ; wherein a first sublayer of the multilayer structure that is adj acent to the side surface comprises a larger refractive index than a second sublayer of the multilayer structure that is further distant from the side surface ; and wherein a refractive index difference between the sublayer of the multilayer structure closest to the side surface and the sublayer of the multilayer structure furthest distant from the side surface is at least 0 . 1 , at least 0 . 2 , at least 0 . 5 , or at least 0 . 8 .

[0047] SHORT DESCRIPTION OF THE DRAWINGS

[0048] 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

[0049] Figure 1 shows an embodiment of a light emitting device in accordance with some aspects of the proposed principle ;

[0050] Figure 2 shows a further embodiment of a light emitting device in accordance with some aspects of the proposed principle ;

[0051] Figure 3 shows an embodiment of light emitting devices in accordance with some aspects of the proposed principle on a temporary carrier substrate ;

[0052] Figures 4A to 4D show steps of a method for manufacturing another embodiment of a light emitting device in accordance with some aspects of the proposed principle ; and

[0053] Figures 5A to 51 show steps of a method for manufacturing another embodiment of a light emitting device in accordance with some aspects of the proposed principle . DETAILED DESCRIPTION

[0054] 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 .

[0055] 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 .

[0056] Figure 1 shows a first 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 .

[0057] The semiconductor layer stack 2 comprises a rear surface I la , a main light emitting surface 11b opposite the rear surface Ila and a side surface 11c extending from the rear surface Ila to the main light emitting surface 11b . 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 . The light emitting device 1 further comprises a multilayer structure 6 comprising a plurality of sublayers 7a , 7b, 7 c , 7d covering the side surface 11c . The sublayers each comprise a material with a different refractive index with a first sublayer 7a adj acent to the side surface 11c having a larger refractive index than a following second sublayer 7b, the second sublayer having a larger refractive index than a following third sublayer 7c and so on . The material of the sublayers is further chosen such a refractive index difference between the first sublayer 7a and the fourth sublayer 7d ( in this case the sublayer of the multilayer structure furthest distant from the side surface 11c ) is at least 0 . 1 , or at least 0 . 2 . in some aspects , 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 provides an improved redirection and collimation effect for a larger incoming angular cone . The number of sublayers with each having a thickness of at least 20 nm, and on 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 .

[0058] The multilayer structure 6 in particular comprises a discrete number of sublayers to provide a monotonously decreasing / gradient index refractive index region covering the side surface 11c . By this , light rays emitted through the side surface are deflected into the direction of the main light emitting surface 11b due to refraction in the decreasing gradient index region ( indicated by means of the arrows ) .

[0059] A portion of light that still passes through the multilayer structure 6 can further be redirected into the direction of the main light emitting surface 11b by an optimized slanted mirror coating 8 covering the multilayer structure 6 . In between the mirror coating 8 and the multilayer structure 6 , a dielectric layer 9 is arranged which can complement the multilayer structure 6 and also has a lower refractive index than the last sublayer 7d, or which is optimized for passivation and shaping purposes .

[0060] In the embodiment shown, not only the side surface 11c is covered by means of the multilayer structure 6 but also the rear surface Ila . This is , as during manufacturing , these layers tend to be deposited also on the rear surface Ila and can thus be regarded as a positive side product . Further in the embodiment shown, the side surface 11c is oriented in substantially vertical direction, however also different angles to adj ust to the needs of the angular emission from the semiconductor layer stack 2 are possible . A contact element 10 electrically coupled to the second layer 4 extends through the multilayer structure 6 , the dielectric layer 9 and the mirror coating 8 . Such a contacting is however to be understood as exemplary and can be varied according to contacting concepts already known . For simplicity a first contact layer on the first layer 3 is not drawn . However , any types of contacting the first layer while allowing an emission of light through the main light emitting surface 11b are thinkable .

[0061] Figure 2 shows a further embodiment of a light emitting device 1 in accordance with some aspects of the proposed principle . The light emitting device 1 further comprises a DBR coating 13 on the main light emitting surface 11b to avoid light already within the epi-ambient escape cone to be outcoupled to oblique angles outside the angular cone of interest of the application . In other words , light emitted through the main light emitting surface 11b with an angle of substantially 90 ° can pass the DBR coating 13 whereas light with angles deviating too much from a perpendicular to the main light emitting surface is reflected back into the semiconductor layer stack 2 . By this a further improved collimation of light of the light emitted from the light emitting device 1 can be achieved . In the embodiment shown, the DBR coating 13 is limited to the main light emitting surface 11b in lateral direction, however it can also be conceivable to extend the DBR coating 13 to the multilayer stack as well as onto the dielectric layer 9 and mirror coating 8 . Figure 3 shows an embodiment of light emitting devices 1 in accordance with some aspects of the proposed principle being arranged on a temporary carrier substrate 14 . In such an arrangement of the light emitting devices 1 being held by a holding structure 15 on the temporary carrier substrate 14 a simple pick and place process , for example stamping process , can be used to transfer the light emitting devices 1 to a target substrate , for example back plane for a display . The light emitting devices 1 in accordance with the proposed principle are thereby particularly suitable , as their flat main light emitting surface is maintained, and simple stamping processes can be used to connect a stamp to the main light emitting surface . This is in particular advantageous compared to light emitting devices comprising a complicated surface structure on the main light emitting surface or comprising additional lens structures / element ( s ) on the main light emitting surface 11b . Still the same optical properties can be provided by means of the light emitting devices 1 in accordance with the proposed principle in terms of collimation and directionality of emitted light without having separate optical elements arranged on the main light emitting surface 11b .

[0062] Figures 4A to 4D show steps of a method for manufacturing a further embodiment of a light emitting device 1 in accordance with some aspects of the proposed principle . In a first step , shown in Figure 4A, a semiconductor layer stack 2 with a second contact layer 20 on a rear surface Ila of the semiconductor layer stack 2 is provided . The semiconductor layer stack 2 comprises a first layer 3 of a first conductivity type , an active region 5 comprising one or more quantum wells and a second layer 4 of a second conductivity type .

[0063] 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 . The semiconductor layer stack 2 is prior to what is shown in Fig . 4A structured starting from a rear surface Ila thereby remaining a first portion of the semiconductor layer stack 2 and removing a second portion of the semiconductor layer stack 2 adj acent to the first portion . This structuring results in exposed side surface portions of the first layer 3 , the active region 5 and the second layer 4 resulting in first side surface portion 12a .

[0064] The resulting structure is then covered by means of a dielectric layer 9 and a reflective contact element 10 as well as a material forming a carrier substrate 14 so that a growth substrate , on which the semiconductor layer stack 2 has been grown, can be removed exposing a main light emitting surface 11b opposite the rear surface I la . Such a step can also be referred to as a rebonding step of the semiconductor layer stack 2 onto a carrier substrate 14 .

[0065] In a further step , as shown in Figure 4B , a second structuring using a mas k 16 is conducted into the first layer 3 thereby remaining the first portion of the semiconductor layer stack 2 and removing the second portion of the semiconductor layer stack 2 adj acent to the first portion . This structuring results in an exposed side surface portion of the semiconductor layer stack 2 resulting in a second side surface portion 12b .

[0066] Due to the rebonding step , the structuring of the semiconductor layer stack 2 comprises two steps of mesa etching from two different sides resulting in a side surface 11c comprising a first and a second side surface portion 12a, 12b laterally displaced to each other and being tilted into different directions . The first etching step is thereby provided to prestructure a core portion of a light emitting portion 6 of the semiconductor layer stack confining the active region 5 of a light emitting portion . The second etching step on the other hand is provided to pixel the semiconductor layer stack 2 e . g . spatially separate the semiconductor layer stack 2 from other portions of the semiconductor layer stack 2 . In a further step, as shown in Figure 4C, a multilayer structure 6 is grown on the resulting structure and in particular at least on the exposed side surface 11c however also covering the main light emitting surface 11b . The multilayer structure 6 comprises a plurality of sublayers 7a , 7b , 7 c, 7d with a material of each having a different refractive index with a first sublayer 7a adj acent to the side surface 11c having a larger refractive index than a following second sublayer 7b, the second sublayer having a larger refractive index than a following third sublayer 7c and so on . The material of the sublayers is further chosen such a refractive index difference between the first sublayer 7a and the fourth sublayer 7d ( in this case the sublayer of the multilayer structure furthest distant from the side surface 11c ) is at least 0 . 1 , or at least 0 . 2 . The number of sublayers with each having a thickness of at least 20 nm, and on particular a thickness of for example 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 .

[0067] In a further step , as shown in Figure 4D, the main light emitting surface 11b is freed of the multilayer structure 6 using for example a CMP or etching process . Light emitting elements 1 as shown in Fig . 4D can then be singulated, or an array of light emitting devices 1 on the carrier substrate 14 can be provided with the carrier substrate acting as a backplane .

[0068] By means of the multilayer structure 6 covering the second side surface portion 12b of the side surface 11c , light that is emitted from the semiconductor layer stack at the side surface 11c at an angle or in horizontal / lateral direction will be deflected into the direction of the main light emitting surface 11b and in particular with a narrower escape cone in vertical direction as it goes deeper into the multilayer structure 6 . 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 .

[0069] Figures 5A to 51 show steps of a method for manufacturing a further embodiment of a light emitting device 1 in accordance with some aspects of the proposed principle . In a first step , as shown in Figure 5A, a semiconductor layer stack 2 is provided on a growth substrate 17 . The semiconductor layer stack 2 comprises a first layer 3 of a first conductivity type , an active region 5 comprising one or more quantum wells and a second layer 4 of a second conductivity type . On a rear surface Ila of the semiconductor layer stack 2 portions of a contact element 10 are provided being electrically coupled to the second layer 4 each forming a contact element for a later light emitting device .

[0070] The semiconductor layer stack 2 is then, as shown in Fig . 5B , structured starting from the rear surface I la thereby remaining first portions 17a of the semiconductor layer stack 2 and removing a second portion 17b of the semiconductor layer stack 2 adj acent to the first portions 17a . This structuring results in exposed side surfaces 11c of the first layer 3 , the active region 5 and the second layer 4 .

[0071] The resulting structure is then, as shown in Fig . 5C , covered by means of a multilayer structure 6 , in particular at least on the exposed side surfaces 11c . The multilayer structure 6 comprises a plurality of sublayers 7a , 7b, 7c , 7d, 7e with a material of each having a different refractive index with a first sublayer 7a adj acent to the side surface 11c having a larger refractive index than a following second sublayer 7b, the second sublayer having a larger refractive index than a following third sublayer 7c and so on . The material of the sublayers is further chosen such a refractive index difference between the first sublayer 7a and the fifth sublayer 7e ( in this case the sublayer of the multilayer structure furthest distant from the side surface 11c ) is at least 0 . 1 , or at least 0 . 2 and in particular monotonously decreasing . The number of sublayers with each having a thickness of at least 20 nm, and on particular a thickness of for example 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 .

[0072] Excess material of the multilayer structure 6 on the contact elements 10 is then removed as shown in Fig . 5D by for example a CMP process . Then a mirror coating 8 as well as a sacrificial layer 19 is provided on the resulting structure , as shown in Fig . 5E . The sacrificial layer 19 is then structure to expose again the contact elements 10 ( Fig . 5 F) to then provide a holding structure 14 connected to the contact elements 10 as well as temporary carrier substrate 14 on the holding structure 15 ( Fig . 5G ) . The position, size and shape of the holding structure 15 shown is however only to be understood as exemplary here .

[0073] In following steps , as shown in Fig . 5H, the growth substrate 18 can then be removed ( e . g . by a laser lift-off process ) and the main light emitting surface 11b of later light emitting devices is freed using for example a CMP and / or etching process . Then the sacrificial layer 19 can be removed and singulated light emitting devices 1 remain connected to the holding structure 15 , which in a later step can then be picked from the holding structure 15 by for example means of a stamp .

[0074] LIST OF REFERENCES

[0075] 1 light emitting device

[0076] 2 semiconductor layer stack

[0077] 3 first layer

[0078] 4 second layer

[0079] 5 active region

[0080] 6 multilayer structure

[0081] 7a, 7b, 7 c, 7d sublayer

[0082] 8 mirror coating

[0083] 9 dielectric layer

[0084] 10 contact element

[0085] Ila rear surface

[0086] 11b main light emitting surface

[0087] 11c side surface

[0088] 12a , 12b side surface portion

[0089] 13 DBR coating

[0090] 14 carrier substrate

[0091] 15 holding structure

[0092] 16 mask

[0093] 17a , 17b portion

[0094] 18 growth substrate

[0095] 19 sacrificial layer

[0096] 20 second contact layer t thickness

Claims

CLAIMS1. 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) comprising a plurality of sublayers (7a, 7b, 7c, 7d) covering at least a portion of the side surface (11c) ; wherein a first sublayer (7a) of the multilayer structure (6) that is adjacent to the side surface (11c) comprises a larger refractive index than a second sublayer (7b) of the multilayer structure (6) that is further distant from the side surface (lie) ; wherein a refractive index difference between the sublayer of the multilayer structure (6) closest to the side surface (11c) and the sublayer of the multilayer structure (6) furthest distant from the side surface (11c) is at least 0.1, or at least 0.2; and wherein the multilayer structure (6) is configured to deflect at least a portion of light of the first wavelength that is emitted from the side surface (11c) into the direction of the main light emitting surface (11b) .

2. Light emitting device (1) according to claim 1, wherein the multilayer structure (6) extends onto the rear surface (Ila) covering a portion of the rear surface (Ila) .

3. Light emitting device (1) according to claim 1 or 2 ,wherein the sublayers (7a, 7b, 7c, 7d) 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 of the multilayer structure (6) closest to the side surface (11c) and the sublayer of the multilayer structure (6) furthest distant from the side surface (11c) is substantially continuous .

5. Light emitting device (1) according to any one of claims 1 to 3, wherein a course of the refractive index difference between the sublayer of the multilayer structure (6) closest to the side surface (11c) and the sublayer of the multilayer structure (6) furthest distant from the side surface (11c) 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 the first wavelength.

7. Light emitting device (1) according to any one of the preceding claims , wherein the sublayers (7a, 7b, 7c, 7d) of the multilayer structure (6) each comprise a thickness of at least 20 nm.

8. Light emitting device (1) according to any one of the preceding claims , wherein the main light emitting surface (11b) is free of an optical element, in particular optical lens.

9. Light emitting device (1) according to any one of the preceding claims ,wherein the at least one first 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) .

10. Light emitting device (1) according to any one of the preceding claims , wherein a first contact layer is arranged on the main light emitting surface (11b) electrically contacting the first layer (3) of the semiconductor layer stack (2) ; and / or wherein a second contact layer (20) is arranged on the rear surface (Ila) electrically contacting the second layer (4) of the semiconductor layer stack (2) .

11. Light emitting device (1) according to any one of the preceding claims , further comprising a dielectric layer (9) that is arranged on at least one of the following: on the multilayer structure (6) opposite the side surface (lie) ; on the multilayer structure (6) opposite the rear surface (Ila) , in particular exposing a contact portion of the second contact layer (20) or contact element (10) ; and on or above the main light emitting surface (11b) .

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

13. 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 multilayer structure (6) opposite the side surface (lie) ;on the multilayer structure (6) opposite the rear surface (Ila) , in particular exposing a contact portion of the second contact layer (20) or contact element (10) ; and on the dielectric layer.

14. 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° .

15. Light emitting device (1) according to any one of the preceding claims , further comprising a DBR coating (13) on or above the main light emitting surface (11b) .

16. 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;Structuring the semiconductor layer stack (2) thereby remaining at least one first portion (17a) of the semiconductor layer stack (2) and removing a second portion (17b) of the semiconductor layer stack (2) adjacent to the at least one first portion (17a) , resulting in a side surface (11c) of the at least one first portion (17a) comprising the second layer (4) and the active region (5) ; andProviding a multilayer structure (6) comprising a plurality of sublayers (7a, 7b, 7c, 7d) covering at least a portion of the side surface (lie) ; wherein a first sublayer (7a) of the multilayer structure (6) that is adjacent to the side surface (11c) comprises a larger refractive index than a second sublayer (7b) of the multilayer structure (6) that is further distant from the side surface (lie) ;wherein a refractive index difference between the sublayer of the multilayer structure (6) closest to the side surface (11c) and the sublayer of the multilayer structure (6) furthest distant from the side surface (11c) is at least 0.1, or at least 0.2; and wherein the multilayer structure (6) is configured to deflect at least a portion of light of the first wavelength that is emitted from the side surface (11c) into the direction of the main light emitting surface (11b) .

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