Light-emitting diodes are semiconductor devices comprising a semiconductor layer stack including a sequence of a first semiconductor layer of a first conductivity type , e . g . p-type , and a second semiconductor layer of a second conductivity type , e . g . n-type . When a voltage is applied to the semiconductor layer stack, photons are emitted due to the recombination of electrons and holes . Generally, such a LED implements a Lambertian emitter, which emits electromagnetic radiation via a main surface of the semiconductor layer stack at an intensity, which varies in dependence from an emission angle .

Generally, a great surface area is desired in order to achieve a large electrical contact area . On the other side , for many applications , a spot-like light source is desired .

Therefore , concepts are being developed, by which the trade-off between optical and electrical performance may be overcome .

It is an obj ect of the present invention to provide an improved light emitting diode .

According to embodiments , the above obj ect is achieved by the claimed matter according to the independent claims . Further developments are defined in the dependent claims .

A light-emitting diode comprises a semiconductor layer stack comprising a first semiconductor layer of a first conductivity type , a second semiconductor layer of a second conductivity type , and an active zone arranged between the first semiconductor layer and the second semiconductor layer . The first semiconductor layer, the active zone and the second semiconductor layer are stacked along a vertical direction so that the active zone is adj acent to a first main surface of the second semiconductor layer . The light-emitting diode further comprises an ordered photonic structure arranged in a second main surface of the second semiconductor layer at a horizontally central position of the second semiconductor layer . The ordered photonic structure is absent from edge portions of the second semiconductor layer .

For example , a thickness of the semiconductor layer stack is selected so that a discrete amount of waveguide modes is generated in the semiconductor layer stack . According to embodiments , the thickness may be selected so that less than 20 waveguide modes are generated in the semiconductor layer stack .

By way of example , the thickness of the semiconductor layer stack may be less than 10 *Xeff , wherein Xeff denotes an effective wavelength of electromagnetic radiation emitted by the active zone .

According to embodiments , the active zone may comprise GaN . In this case , a thickness of the semiconductor layer stack may be less than 2 pm .

The light-emitting diode may further comprise a first mirror arranged at a first side face of the semiconductor layer stack, and a second mirror arranged at a second side face of the semiconductor layer stack .

For example , at least one of the first and second mirrors may be implemented as a distributed Bragg reflector .

According to embodiments , a horizontal dimension of the ordered photonic structure may be less than 2 /3 of the horizontal dimension of the semiconductor layer stack .

The ordered photonic structure may comprise holes formed in the second semiconductor layer . A depth of the holes may decrease from a central portion towards an edge portion of the semiconductor layer stack . According to embodiments , a lighting device comprises the lightemitting diode as described above . The lighting device may be selected from an automotive lighting device , a proj ector , or a display .

The accompanying drawings are included to provide a further understanding of embodiments of the invention and are incorporated in and constitute a part of this specification . The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles . Other embodiments of the invention and many of the intended advantages will be readily appreciated, as they become better understood by reference to the following detailed description . The elements of the drawings are not necessarily to scale relative to each other . Like reference numbers designate corresponding similar parts .

Fig . 1 shows a schematic cross-sectional view of a light-emitting diode according to embodiments .

Fig . 2 is a cross-sectional view of a light-emitting diode according to further embodiments .

Fig . 3 is a schematic view of a lighting device according to embodiments .

In the following detailed description reference is made to the accompanying drawings , which form a part hereof and in which are illustrated by way of illustration specific embodiments in which the invention may be practiced . In this regard, directional terminology such as "top" , "bottom" , "front" , "back" , "over" , "on" , "above" , "leading" , "trailing" etc . is used with reference to the orientation of the Figures being described . Since components of embodiments of the invention can be positioned in a number of different orientations , the directional terminology is used for purposes of illustration and is in no way limiting . It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope defined by the claims . The description of the embodiments is not limiting . In particular, elements of the embodiments described hereinafter may be combined with elements of different embodiments .

The terms "wafer" or "semiconductor substrate" used in the following description may include any semiconductor-based structure that has a semiconductor surface . Wafer and structure are to be understood to include doped and undoped semiconductors , epitaxial semiconductor layers , e . g . supported by a base semiconductor foundation, and other semiconductor structures . For example , a layer of a first semiconductor material may be grown on a growth substrate of e . g . a second semiconductor material . According to further embodiments , the growth substrate may be an insulating substrate such as a sapphire substrate . Depending on the purpose of use , the semiconductor may be based on a direct or an indirect semiconductor material . Examples of semiconductor materials particularly suitable for generation of electromagnetic radiation comprise nitride-compound semiconductors , by which e . g . ultraviolet or blue light or longer wavelength light may be generated, such as GaN, InGaN, AIN, AlGaN, AlGalnN, phosphide- compound semiconductors , by which e . g . green or longer wavelength light may be generated such as GaAsP, AlGalnP , GaP , AlGaP, as well as further semiconductor materials including AlGaAs , SiC, ZnSe , GaAs , ZnO , Ga2O3, diamond, hexagonal BN und combinations of these materials . Further examples of semiconductor materials may as well be silicon, silicon-germanium and germanium . The stoichiometric ratio of the compound semiconductor materials may vary . In the context of the present specification, the term "semiconductor" further encompasses organic semiconductor materials .

The term "vertical" as used in this specification intends to describe an orientation which is arranged perpendicular to the first surface of a substrate or semiconductor body .

The terms "lateral" and "horizontal" as used in this specification intends to describe an orientation parallel to a first surface of a substrate or semiconductor body . This can be for instance the surface of a wafer or a die .

As employed in this specification, the terms "coupled" and/or "electrically coupled" are not meant to mean that the elements must be directly coupled together - intervening elements may be provided between the "coupled" or "electrically coupled" elements . The term "electrically connected" may describe a low-ohmic electric connection between the elements electrically connected together .

The term "electrically connected" further comprises tunneling contacts between connected elements .

According to further embodiments and where appropriate , the term "electrically connected" may mean that the respective elements are "directly connected" or are "directly and permanently connected" .

The term "electrically connected" may describe a permanent low- resistive connection between electrically connected elements , for example a direct contact between the concerned elements or a low- resistive connection via a metal and/or heavily doped semiconductor material . The term "electrically coupled" may include that one or more intervening element ( s ) adapted for signal and/or power transmission may be connected between the electrically coupled elements , for example , elements that are controllable to temporarily provide a low- resistive connection in a first state and a high-resistive electric decoupling in a second state . An ohmic contact may be a non-rectifying electrical j unction .

Fig . 1 shows a schematic cross-sectional view of a light-emitting diode , LED 10 according to embodiments . The light-emitting diode 10 comprises a semiconductor layer stack 100 . The semiconductor layer stack 100 comprises a first semiconductor layer 110 of a first conductivity type , e . g . p-type , a second semiconductor layer 120 of a second conductivity type , e . g . n-type , and an active zone 115 arranged between the first semiconductor layer 110 and the second semiconductor layer 120 . The first semiconductor layer 110 , the active zone 115 and the second semiconductor layer 120 are stacked along a vertical direction, e.g. the z-direction. The active zone 115 is adjacent to a first main surface 121 of the second semiconductor layer 120.

The active zone 115 may, for example, comprise a pn junction, a double heterostructure, a single quantum well (SQW) structure or a multi quantum well (MQW) structure for generating radiation. In this context, the term "quantum well structure" has no meaning with regard to the dimensionality of the quantization. Thus, it includes, among other things, quantum wells, quantum wires and quantum dots, as well as any combination of these layers .

The LED 10 further comprises an ordered photonic structure 130, which is arranged in a second main surface 122 of the second semiconductor layer 120. The ordered photonic structure 130 is arranged at a horizontally central position of the second semiconductor layer 120. Further, the ordered photonic structure 130 is absent from edge portions 107 of the second semiconductor layer 120.

In the context of the present disclosure, the term "ordered photonic structure" means a structure the structural elements of which are arranged at predetermined locations. The arrangement pattern of the structural elements is subject to a specific order. The functionality of the ordered photonic structure results from the arrangement of the structural elements. The structural elements are, for example, arranged such that diffraction effects occur. The structural elements may be implemented in the form of a nanotexture. The structural elements may be in a submicron size range. For example, a dimension of the structural elements, e.g. a horizontal dimension may be smaller than 10 pm. The structural elements may be arranged periodically, for example, so that a photonic crystal is realized. According to further embodiments, the structural elements may be arranged such that they represent deterministic aperiodic structures, for example bird spirals. According to further embodiments, the structural elements may be arranged such that they realize a quasi-periodic crystal, for example an Archimedean lattice . For example , the LED 10 may comprise a conductive carrier 140 . A rear mirror stack 137 may be formed between the conductive carrier 140 and the semiconductor layer stack 100 . For example , the rear mirror stack 137 may comprise an alternating sequence of layers having comparably high and low refractive indices . For example , the layers may be dielectric layers . The alternating sequence of layers may form a DBR ("distributed Bragg reflector" ) mirror . Accordingly, the rear mirror stack 137 may be insulating and may have a high reflectivity .

A transparent conductive oxide layer 135 may be arranged over the rear mirror stack 137 . For example , the transparent conductive oxide may comprise ITO ( indium tin oxide ) or any other suitable transparent conductive material .

As is illustrated in Fig . 1 , first contact elements 112 may be arranged to electrically connect the transparent conductive oxide layer 135 to the conductive carrier 140 . As is indicated by broken lines , second contact elements 113 may be arranged to electrically connect the second semiconductor layer 120 with a second terminal . A first terminal may be electrically connected to the conductive carrier 140 . According to further implementations , the first contact elements 112 may be electrically connected to the first terminal by a different wiring structure .

The semiconductor layer stack 100 may be arranged over the transparent conductive oxide layer 135 . The transparent conductive oxide layer 135 serves as a current spreading layer .

As is illustrated in Fig . 1 , the ordered photonic structure 130 is arranged in a central portion of the LED . For example , a horizontal extension of the ordered photonic structure 130 depends on the number of waveguide modes , which may be present in the semiconductor layer stack 100 . This will be explained herein below in more detail . For example , the term "central portion 105" refers to less than 2 /3 of the surface area of the LED . Further, the term "central portion 105" comprises a center position of the semiconductor layer stack 100 . As is further shown in Fig . 1 , due to the presence of the ordered photonic structure 130 , electromagnetic radiation is emitted from the central portion 105 of the LED . In more detail , the electromagnetic radiation 15 is emitted via the ordered photonic structure 130 . Accordingly, the LED 10 implements a point-like light source . Due to the presence of the ordered photonic structure 130 , the emitted radiation 15 may be guided to a lens 133 so that the central ray 16 is emitted in a vertical direction . Further , edge rays 17 may be emitted towards the lens so that they are deflected by the lens 133 in a vertical direction . As a consequence , highly collimated radiation 15 may be emitted by the LED 10 .

Due to the presence of the ordered photonic structure 130 , a small light escape area from the LED 10 may be realized . Further, the active zone 115 and the conductive carrier 140 have a larger horizontal extension than the size of the light escape area . Hence , a high wallplug efficiency, i . e . a low forward voltage and a high IQE ( internal quantum efficiency) may be achieved by the light-emitting diode . Further , a high external package and optical system efficiency may be achieved . In more detail , due to the small light escape area , light losses in external optical systems and packages may be reduced . Accordingly, the light-emitting diode 10 described hereinabove resolves the trade-off between a large surface which enables a high IQE and low forward voltage and a larger light escape area and thus larger external systems .

Fig . 1 further shows waveguide modes 125 that may be present in the semiconductor layer stack 100 . For example , the thickness of the semiconductor layer stack may made very small . Differently stated, the semiconductor layer stack is made wave-optically thin so that only a discrete amount of waveguide modes may be present in the semiconductor layer stack . In particular, due to the small thickness of the semiconductor layer stack 100 , the emission of the active zone is forced into low order waveguide modes . For example , the number of waveguide modes that may be present in the semiconductor layer stack 100 may be made very small . For example , a thickness of the semiconductor layer stack may be less than 10 times the effective wavelength emitted by the LED . The term "effective wavelength" refers to the wavelength in the material of the semiconductor layer stack 100 .

The semiconductor layer stack 100 is also referred to as an "epitaxial layer stack" meaning that the semiconductor layers of the semiconductor layer stack 100 may have been grown by epitaxial methods . For example , a number of waveguide modes that may be present in the semiconductor layer stack may be less than 20 or less than 15 . For example , when the LED is a blue emitting LED, a thickness of the semiconductor layer stack 100 may be less than 2 pm .

Due to the wave-optically thin semiconductor layer stack 100 , less waveguide modes may be present in the semiconductor layer stack 100 . In this case outcoupling by the ordered photonic structure 130 may be more effective . Further, less losses due to other structures outside the semiconductor layer stack may be suffered . Moreover , longer propagation lengths in the semiconductor layer stack 100 may be achieved which increases the probability of light reaching the central outcoupling region . Further, more efficient coupling of the quantum well emission to the intended low order waveguide modes may be accomplished . Moreover , emission outside the intended light escape area may be reduced .

Due to the wave-optically thin semiconductor layer stack 100 , the ordered photonic structure 130 only needs to mainly outcouple a limited amount of waveguide modes thus resulting in less design requirements . Accordingly, the ordered photonic structure 130 may be engineered to perform very efficiently .

Due to the presence of the ordered photonic structure 130 , emission may be effected in the central portion 105 of the LED . Further, due to the use of periodic/quasi-periodic structure , a more directional outcoupled light or beaming functionality may be implemented . As a result , the system efficiency may be improved . Further , surface areas of the semiconductor layer stack without outcoupling textures may be more reflective . Hence , due to the presence of the ordered photonic structure , losses of light due to back ref lection/scattering from the package/external system to the LED chip may be reduced .

The ordered photonic structure 130 may comprise a plurality of holes 131 that are formed in the second main surface 122 of the second semiconductor layer 120 . The second main surface 122 may implement a horizontal surface . For example , the holes 131 may be columnar or conical or may have a shape of a pyramid . A horizontal cross-section, of the holes may be arbitrary . For example , the horizontal crosssection may comprise circular shapes , elliptical shapes and others . For example , the ordered photonic structure may be formed at a period p . The period p may be determined as p = 2n/K _p wherein K _p denotes a momentum of a waveguide mode to be outcoupled .

As is for example shown in Fig . 1 , the depth of the single holes 131 may increase from a region facing the edge portion 107 to the central portion 105 . Due to this specific shape , a gradual amplitude of the ordered photonic structure or varying corrugation depth may be achieved . In particular, the increasing depth leads to the effect that the waveguide modes are not reflected at the edge of the ordered photonic structure 130 due to perceiving a sudden large change of the refractive index . Due to this specific implementation, the waveguide modes are transmitted towards the central portion and are outcoupled . The single holes 131 are formed in the second main surface 122 of the second semiconductor layer . The second main surface 122 may be a horizontal surface . Accordingly, the single holes 131 may extend to different vertical positions of the light-emitting diode 10 . As is shown, the emission area may be small , thus resulting in a dot-like emission source . At the same time , a horizontal extension of the LED may be made large so as to increase the electrical performance of the device . For example , the single holes 131 may extend to a depth of less than half the thickness of the second semiconductor layer 120 . The specific shape of the ordered photonic structure may be selected depending on the number of waveguide modes that are present in the semiconductor layer stack 100 . By increasing the refractive index contrast or the depth of the single holes 131 , the characteristics may be set . For example , the ordered photonic structure 130 needs to have a minimum area so as to show a "crystal-like" behavior . For example , the central ray 16 may refer to a reflected low order mode , whereas the edge rays 17 refer to higher order modes .

The LED 10 shown in Fig . 1 further comprises a first mirror 127 and a second mirror 128 which are arranged at side faces 108 of the semiconductor layer stack 100 . Due to the presence of the first mirror 127 and the second mirror 128 , reflection in this region may be improved . Further, the generated electromagnetic radiation may be redirected back to the central portion 105 to be outcoupled . Further , a concentration of light within the semiconductor layer stack 100 is achieved . Accordingly, the light may be effectively out-coupled via the ordered photonic structure .

Fig . 2 shows a further example of an LED 10 according to embodiments . Components of the LED shown in Fig . 2 are identical to those illustrated in Fig . 1 . Differing from embodiments that are illustrated in Fig . 1 , the first and the second mirror 127 , 128 are implemented as Bragg reflectors . Thereby, the sidewall reflectance may be improved .

Fig . 3 shows a lighting device 20 according to embodiments . The lighting device 20 comprises the above-described light-emitting diode 10 . For example , the lighting device 20 may comprise an array of the above-described light-emitting diodes 10 . The lighting device 20 may be selected from an automotive lighting device , a proj ector, or a display . The light-emitting diode ( s ) 10 may be implemented as high power LEDs or MicroLEDs .

While embodiments of the invention have been described above , it is obvious that further embodiments may be implemented . For example , further embodiments may comprise any subcombination of features recited in the claims or any subcombination of elements described in the examples given above . Accordingly, this spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein .

LIST OF REFERENCES light-emitting diode emitted radiation central ray edge ray lighting device semiconductor layer stack central portion edge portion side face first semiconductor layer first contact element second contact element active zone second semiconductor layer first main surface of the second semiconductor layer second main surface of the second semiconductor layer waveguide mode first mirror second mirror ordered photonic structure hole lens transparent conductive oxide layer rear mirror stack conductive carrier