OPTOELECTRONIC DEVICE AND METHOD FOR PRODUCING THEREOF

Abstract

An optoelectronic device includes at least one optoelectronic light source having an active region for generating light and a light emitting surface for emitting the generated light, an electrically conductive layer extending between the light emitting surface and the active region, and a photonic crystal structure. The photonic crystal structure is arranged in the electrically conductive layer. The at least one optoelectronic light source is a plurality of optoelectronic light sources. The optoelectronic light sources of the plurality of optoelectronic light sources are arranged in an array-like structure. Each of the optoelectronic light sources has an active region which is separate from the active regions of the other optoelectronic light sources. The electrically conductive structure extends over all optoelectronic light sources of the plurality of optoelectronic light sources.

Claims

1. An optoelectronic device comprising: at least one optoelectronic light source having an active region for generating light and a light emitting surface for emitting the generated light, an electrically conductive layer extending between the light emitting surface and the active region, and a photonic crystal structure arranged in the electrically conductive layer, wherein the at least one optoelectronic light source is a plurality of optoelectronic light sources, the optoelectronic light sources of the plurality of optoelectronic light sources are arranged in an array-like structure, and each of the optoelectronic light sources has an active region which is separate from the active regions of the other optoelectronic light sources, and wherein the electrically conductive structure extends over all optoelectronic light sources of the plurality of optoelectronic light sources.

2. The optoelectronic device in accordance with claim 1, wherein the photonic crystal structure is a 2-dimensional photonic crystal structure.

3. The optoelectronic device in accordance with claim 1, wherein the photonic crystal structure provides a photonic bandgap or a partial photonic bandgap at least for electromagnetic radiation of a given wavelength or in a given wavelength regime that propagates in a lateral direction, the lateral direction being parallel to the light emitting surface.

4-5. (canceled)

6. The optoelectronic device in accordance with claim 1, wherein the photonic crystal structure comprises a plurality of holes formed in the electrically conductive layer, the holes forming a lattice, such as a hexagonal lattice.

7. The optoelectronic device in accordance with claim 6, wherein the holes are filled with a dielectric material having an index of refraction that differs from the index of refraction of the electrically conductive layer (25).

8. The optoelectronic device in accordance with claim 6, wherein each hole comprises a center axis, the center axes of the plurality of holes are arranged in parallel to each other and/or the center axes of the plurality of holes are arranged orthogonal to the light emitting surface.

9. The optoelectronic device in accordance with claim 6, wherein each hole has a circular cross section.

10. The optoelectronic device in accordance with claim 1, wherein the at least one optoelectronic light source is a μLED, with a size of 1 μm to 10 μm.

11. The optoelectronic device in accordance with claim 1, wherein the electrically conductive layer has a height of 140 nm to 350 nm.

12. The optoelectronic device in accordance with claim 1, wherein the electrically conductive layer is made of Indium tin oxide (ITO).

13. The optoelectronic device in accordance with claim 1, wherein the photonic crystal structure is formed by a hexagonal pattern of holes in the electrically conductive layer and has a thickness of 300 nm, a hole radius of 178.2 nm, and a pitch of 540 nm.

14. The optoelectronic device in accordance with claim 1, wherein the photonic crystal structure has a thickness, and a pitch and radius of holes, such that trapped modes and/or guided modes couple to an extraction cone.

15. A method for producing an optoelectronic device, the method comprising: providing at least one optoelectronic light source having an active region for generating light, providing an electrically conductive layer above the active region, and generating a photonic crystal structure in the electrically conductive layer, wherein the at least one optoelectronic light source is a plurality of optoelectronic light sources, and wherein the electrically conductive layer is provided on the optoelectronic light sources of the plurality of optoelectronic light sources such as to extend over all the optoelectronic light sources.

16. The method in accordance with claim 15, wherein the method further comprises: arranging the optoelectronic light sources of the plurality of optoelectronic light sources in an array-like structure, each of the optoelectronic light sources having an active region which is separate from the active regions of the other optoelectronic light sources.

17. The method in accordance with claim 16, wherein the optoelectronic light sources are individual μLEDs arranged in the array-like structure.

18. (canceled)

19. The method in accordance with claim 15, wherein the optoelectronic light sources are individual μLEDs arranged in an array-like structure.

20. The optoelectronic device in accordance with claim 1, wherein the at least one optoelectronic light source is a μLED, with a size of 1 μm to 5 μm.

21. The optoelectronic device in accordance with claim 1, wherein the at least one optoelectronic light source is a μLED, with a size of 1 μm to 2 μm.

22. The optoelectronic device in accordance with claim 1, wherein the electrically conductive layer has a height of 150 nm to 300 nm.

Description

[0038] Preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings in which:

[0039] FIG. 1 shows schematically a cross-sectional view of an exemplary embodiment of an optoelectronic device in accordance with the present invention.

[0040] FIG. 2 shows schematically an electrically conductive layer with a photonic crystal structure arranged in the device of FIG. 1.

[0041] FIG. 3 shows schematically the radiant power per solid angle as a function of the direction of emission of the device of FIG. 1 and of a corresponding device, but which has an unstructured electrically conductive layer.

[0042] FIG. 4 shows a diagram that illustrates the electrical resistance of a conductive layer with holes as a function of the ratio between hole radius and pitch.

[0043] FIG. 5a shows a photonic bandstructure diagram.

[0044] FIG. 5b shows another photonic bandstructure diagram.

[0045] FIG. 1 shows schematically a cross-sectional view of an exemplary embodiment of an optoelectronic device in accordance with the present invention. The view was obtained from a numerical simulation. Outer frame 11 limits the calculation area and serves to define boundary conditions for the numerical simulation. The frame 11 is not a component of the physical optoelectronic device.

[0046] The optoelectronic device shown in FIG. 1 is preferably a PLED and comprises an optoelectronic light source 13 having an active region 15 for generating light. The active region 15 comprises a quantum well emission region in which light in a defined wavelength region is generated.

[0047] Emission of the active region 15 can have a directivity that corresponds to dipole emission. The layer structure that forms the active region 15 is embedded in a funnel-like mesa boundary 19, which has a metal and passivation layer 21 on its inner surface. The metal and passivation layer 21 provides some reflectivity that directs the light upwards in the z-direction.

[0048] An optional reflecting element (not shown), for example a layered structure that forms a Bragg reflector, can be arranged below the active region 15 to reflect light upwards in the z-direction.

[0049] One or more epitaxial layers 23 are arranged on top of the active region 15. An electrically conductive layer 25 is arranged on top of the epitaxial layers 25. The epitaxial layers 23 and the electrically conductive layer 25 are transparent or at least partly transparent for the light emitted in the active region 15. Furthermore, a photonic crystal structure 27 is arranged in the electrically conductive layer 25.

[0050] Above the photonic crystal structure 27, one or more additional layers can be arranged, and the top surface of these layers can form the light emitting surface. Alternatively, the top side of the electrically conductive layer 25 can serve as light emitting surface 29 through which the generated light can leave the device. On top of the light emitting surface 29 can be air 31.

[0051] The electrically conductive layer 25 serves as a current spreading layer to provide an electric current to an n-contact of the active region 15. Furthermore, a p-contact of the active region 15 is provided with electricity via a bottom metallic layer 33.

[0052] By introducing the photonic crystal structure 27 into the electrically conductive layer 25 the light extraction and the directionality of the extracted light is enhanced while the capability of providing electricity to the active region 15 of the optoelectronic light source 13 is maintained. This can for example be achieved by designing the photonic crystal structure 27 such that trapped modes and/or guided modes of the structure 27 are coupled to an extraction cone of the emitted light. Furthermore, as will be explained further below, the height of the electrically conductive layer 25 can be increased due to the photonic crystal structure 27, resulting in further improvement of the electric conductivity of the layer 25.

[0053] The optoelectronic light source 13 can be arranged in an array-like structure of a plurality of optoelectronic light sources 13. The photonic crystal structure 27 helps to avoid or reduce crosstalk between the optoelectronic light sources 13 of the array of light sources. Crosstalk mainly occurs when light travels along the electrically conductive layer 25 from one light source 13 to a neighboring light source 13. There, the light may disturb the light generating process in the active zone 15 of the neighboring light source 13.

[0054] As shown in FIG. 2, the photonic crystal structure 27 in the electrically conductive layer 25 is a 2-dimensional hexagonal photonic crystal structure. The structure has a periodic or quasi-periodic variation of the index of refraction along the x-axis and along the y-axis. The x-axis and the y-axis are perpendicular to each other and they lie in a plane that is parallel to the light emitting surface 29. In other words, the x-axis and the y-axis are orthogonal to the z-axis, which is orthogonal to the light emitting surface 29. The z-axis corresponds to the main direction of the light that is emitted by the optoelectronic device.

[0055] The photonic crystal structure 27 comprises a plurality of holes 35 arranged in a hexagonal lattice structure. The holes 35 may have been etched into the electrically conductive layer 25, in particular such that the dimensioning of the lattice causes the creation of a bandgap (not shown) that covers the wavelengths of the light emitted by the optoelectronic light source 13. The holes 35 can be filled with a dielectric material having an index of refraction that differs from the index of refraction of the electrically conductive layer 25. Alternatively, the holes 35 can contain air.

[0056] As the periodicity of the structure extends in the xy-plane, the bandgap is in particular relevant for the propagation of light in the conductive layer 25 along a lateral direction, i.e. a propagation direction in the xy-plane. The bandgap can be a partial bandgap as engineering a complete bandgap for the emission wavelength might not be feasible due to restriction in dimension and materials. The bandgap can prevent light from traveling in the lateral direction within the conductive layer 25 if the wavelength of the light is in the bandgap. Thereby, crosstalk between neighboring light sources of an array of light sources 13 that share the electrically conductive layer 25 can be prevented or reduced.

[0057] FIG. 3 shows radiant power per solid angle as a function of the direction of emission. The direction of the emission is given by an angle which is measured with respect to a normal axis (see the z-axis in FIG. 1) that is perpendicular to the light emitting surface. Specifically, FIG. 3 shows a first curve 37 that relates to the radiant power per solid angle as a function of the direction of emission for a device with a photonic crystal structure 27 as shown in FIG. 1. Furthermore, FIG. 3 shows a second curve 39 of a corresponding device, but which does not have the photonic crystal structure 27 in its electrically conductive layer 25. Both profiles are shown for the farfield.

[0058] As shown in FIG. 3, the first curve 37 is more confined than the second curve 39. The photonic crystal structure 27 therefore helps with coupling guided modes in the photonic crystal structure 27 to an extraction cone as represented by first curve 37 resulting in enhanced light extraction and in an improved directionality.

[0059] FIG. 4 shows a diagram that illustrates the electrical resistance of a conductive layer 25 with a thickness of 300 nm and with holes 35 (see FIG. 2) forming a photonic crystal structure as a function of the ratio between hole radius and pitch. As an example, a 150 nm thick electrically conductive layer, which does not have any holes has a resistance of about 155 Ohm (see the horizontal line in FIG. 4). The electrically conductive layer 25 with holes in a hexagonal structure has double the thickness. Yet at a radius/pitch lower than 0.33 it shows a lower resistance. The resistance of the conductive layer 25 with holes can further drop if the thickness of the layer is increased. Thus, a thicker conductive layer 25 with holes 35 in addition to the presence of a photonic bandgap which prevents any crosstalk can have a lower resistance than a thinner unstructured layer, as the thicker structured layer 25 can consist of more conductive material.

[0060] In a preferred embodiment using ITO as material for the electrically conductive layer 25, the following dimensions apply: a=520 nm, r/a=0.30, d=150 mm, Po=0.27, Pc=0.01. In a further preferred embodiment, the following dimensions apply: [0061] a=540 nm, r/a=0.33, d=300 nm, Po=0.30, Pc=0.01.

[0062] “a” refers to the pitch, “r” refers to the radius of the holes, “d” refers to the thickness of the layer 25 measured along the z-axis. Po is the cumulative light output of a light source that is emitted out-of-plane divided by the emitted power from the source in the active zone. Pc is the cumulative light output of a light source that is transferred to the adjacent cells divided by the emitted power.

[0063] The pitch “a” is the center-to-center distance between two neighboring holes in a hexagonal lattice.

[0064] With the given dimensions, a tolerance of ±10 nm in the radius does not influence the out-couple power as well as the crosstalk via the electrically conductive layer 25. This makes the photonic structure resilient to fabrication tolerances.

[0065] The thickness of the electrically conductive layer 25 is of importance in optoelectronic light sources that are μLEDs. On one hand making it less than 200 nm can be challenging and will reduce the conductivity of such a layer. On the other hand, a thick unstructured conductive layer 25 will result in crosstalk. Patterning the conductive layer 25 with a photonic crystal structure that has a photonic bandgap at the emission line of the light sources can result in total suppression of crosstalk and at the same time can allow electric current to flow.

[0066] From the viewpoint of optimizing the photonic bandgap the thicker the conductive layer 25 the better it would be. This is because the 2D photonic structure will give a broad frequency bandgap when the thickness is equal or less than the lattice constant. In an example, an optimal lattice constant for light extraction is calculated at 540 nm so the closer to this thickness the broader the bandgap.

[0067] A PLED as optoelectronic light source 13 can have a size in the range between 10 μm and 1 μm, preferably between 5 μm and 1 μm and further preferably between 2 μm and 1 μm. The size is measured along the x-axis or along the y-axis. The height is measured along the z-axis.

[0068] FIG. 5a shows a photonic bandstructure diagram for a conductive layer 25 with a photonic crystal structure realized by a triangular lattice of holes 35 with a pitch of 540 nm and a hole radius of 178.2 nm. The layer has a thickness of 150 nm. A bandstructure diagram corresponds to a dispersion diagram showing frequency over wavenumber for a photonic structure, and the bandstructure diagram depicts which modes are allowed to propagate and which modes are prohibited. The center of the radiation cone coincides with the Gamma “Γ”-point, the high symmetry point in the Brillouin zone where kx=ky=0. At this point radiation modes radiate out-of-plane in the z-direction.

[0069] FIG. 5b shows a photonic bandstructure diagram for a conductive layer 25 similar to the one of FIG. 5a, but the conductive layer 25 related to the bandstructure diagram of FIG. 5b has a thickness of 300 nm. A high concentration of modes at the Gamma point around the emission frequency means higher out-of-plane emission or higher light extraction efficiency. Thus, a high concentration of optical modes at the high symmetry Gamma “Γ”-point and around the emission frequency of the light source 13 results in enhanced light extraction and of a coupling of the optical mode to the light extraction cone.

[0070] The conductive layer related to FIG. 5b is twice as thick as the conductive layer related to FIG. 5a, but it gives due to the bandstructure around 36% more output power emission compared to FIG. 5a. In addition to creating a bandgap for crosstalk suppression, a photonic crystal structure can couple guided modes and trapped modes to an extraction cone of the emitted light. Modes in the extraction cone are present near the r-point.

[0071] In view of the above, some advantages of a photonic crystal structured conductive layer 25 on microLEDs are: [0072] 1. Light extraction efficiency (LEE) enhancement up to 50% [0073] 2. Suppressing crosstalk [0074] 3. Enhanced current spreading [0075] 4. Directionality [0076] 5. Increasing pixel density—due to suppression of crosstalk [0077] 6. Helping with the geometry—μLEDs are very sensitive to resonances and a LEE layer would allow for more relaxed fabrication tolerances and different geometries.

LIST OF REFERENCE SIGNS

[0078] 11 outer frame [0079] 13 optoelectronic light source [0080] 15 active region, active zone [0081] 19 mesa boundary [0082] 21 metal and passivation layer [0083] 23 epitaxial layers [0084] 25 electrically conductive layer [0085] 27 photonic crystal structure [0086] 29 light emitting surface [0087] 31 air [0088] 33 metallic layer [0089] 35 holes [0090] 37 first curve [0091] 39 second curve [0092] 41 photonic bands