Light-emitting element equipped with a standing-wave generator and related optoelectronic devices

Abstract

The present invention relates to a light-emitting element (10), - a conversion material (14) suitable for converting a first emission in a first spectral band into a second emission in a second spectral band, the second spectral band being distinct from the first spectral band, - a standing wave generator in a first spectral band, comprising a two-dimensional photonic crystal (26) suitable for generating a standing wave in the first spectral band, the photonic crystal (26) being at least partially formed by a light-emitting diode (12) suitable for emitting in the first spectral band, and relates to a light-emitting element (10) comprising the same.

Description

Technical Field

[0001] The present invention relates to a light-emitting element and an optoelectronic device including such a light-emitting element.

Background Art

[0002] In the field of optoelectronics, it is desired to fabricate very small devices. This is particularly the case for pixels in color display devices.

[0003] In a color display device, each pixel includes a plurality of sub-pixels, and each sub-pixel is configured to emit a specific color. Thereby, the color emitted from the pixel can be changed by controlling which sub-pixels are activated or by changing the current applied to each sub-pixel to change the relative emission intensity of each sub-pixel.

[0004] Semiconductor structures such as light-emitting diodes (LEDs) are generally used for various purposes such as lighting due to their potentially good light-emitting efficiency. LEDs have been proposed for the manufacture of high-efficiency display devices due to their potentially high efficiency. An LED structure usually takes the form of a stack of flat semiconductor layers. When current flows through the stack, light is emitted.

[0005] Regarding this, a method of growing native pixels containing LEDs made of GaN / InGaN on the same wafer to reduce the size of pixels is known. A native pixel is a pixel whose emission is inherently of a desired color.

[0006] However, such pixels are not efficient because only the blue quantum efficiency of such types of LEDs is satisfactory. In fact, the green quantum efficiency is generally 30%, while for red, the quantum efficiency drops below 5%.

[0007] Therefore, methods of using a color conversion module to obtain other colors from blue LEDs or UV LEDs are known. Quantum dots are a common example of a converter used in the conversion module. Quantum dots are often inserted into a matrix.

[0008] However, in pixels on the order of several micrometers, the absorption of quantum dots is too low to guarantee complete conversion from blue emission to red or green emission. As a result, unconverted emission must be filtered to obtain red or green pixels. What such filtering means is that the emission released from the LED is greatly lost. For example, in pixels with a size of 5μm×5μm, a 60% loss is observed.

[0009] It is possible to attempt to compensate for such losses by increasing the number of quantum dots.

[0010] The first method of increasing the number of quantum dots is to increase the concentration of quantum dots in the matrix. Such a proposal runs into the fact that too high a concentration causes loss of the mechanical properties of the matrix, making the matrix incompatible with the techniques used in the fabrication of the pixels.

[0011] The second method of increasing the number of quantum dots is to increase the size of the matrix, particularly its thickness.

[0012] However, in this case as well, such an increase runs into multiple problems.

[0013] First, fabricating pixels with a very large thickness is technically difficult.

[0014] The fact that the matrix becomes thicker means that the optical path length of the emission converted by the quantum dots becomes longer. As a result of such an increase, the reabsorption loss becomes larger.

[0015] Another problem is related to the existence of crosstalk between two pixels. Crosstalk is usually prevented by inserting an opaque wall at the edge of the color conversion module. As the thickness of the matrix increases, the height of the wall increases, and thus the absorption loss in the enlarged wall increases. Furthermore, it becomes too large and the absorption gain associated with the increase in the number of quantum dots is exceeded by the loss.

Prior Art Documents

Patent Documents

[0016]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0017] Therefore, there is a need for a small-sized light-emitting element to overcome the above-mentioned drawbacks.

Means for Solving the Problems

[0018] For this purpose, in this specification, a light-emitting element is described, which includes a conversion material suitable for converting the first radiation in the first spectral band into the second radiation in the second spectral band, and the second spectral band is separate from the first spectral band. The light-emitting element further includes a standing wave generator in the first spectral band, which includes a two-dimensional photonic crystal suitable for generating a standing wave in the first spectral band, and the photonic crystal is at least partially formed by a light-emitting diode suitable for emitting in the first spectral band.

[0019] In contrast to the prior art, in particular document FR3068173A and US patent application No. 2022 / 102324 (A1), the nanowires forming the light-emitting source are part of a photonic crystal and are used to fabricate part of the photonic crystal. More precisely, in this case, the photonic crystal is a combination of a medium and the nanowires.

[0020] In addition, this photonic crystal has a different role because it serves to generate a standing wave that is efficiently generated due to the differences in the previous structure.

[0021] In this way, it is possible to fabricate a light-emitting element, generally a pixel, that is small in size, has good conversion efficiency, and thus emits a satisfactory amount of light.

[0022] According to other specific embodiments, the light-emitting element has one or more of the following features, which are selected individually or according to all technically possible combinations. - The photonic crystal is formed only by the light-emitting diode and the medium surrounding the light-emitting diode. - The light-emitting diode includes an active layer, and at least one active layer is included in the photonic crystal. This makes it possible to efficiently inject light into the photonic crystal to form a standing wave. - The photonic crystal is partially formed from each active layer of the light-emitting diode. - The photonic crystal includes all of each light-emitting diode. - The light-emitting diode is in a medium, and the medium is a conversion material. The above content is used to miniaturize the light-emitting element. - The conversion material is based on a photonic crystal. The above content facilitates the fabrication of the light-emitting element. - The photonic crystal has a plurality of band gaps, and the photonic crystal has a pitch and a filling factor suitable for causing the light-emitting diode to emit at 90° at a band edge of the first band gap. According to the foregoing, the probability of absorption of photons within the first spectral band by the conversion material is increased. - The photonic crystal has several band gaps, and the photonic crystal has a pitch and a filling factor suitable for causing the light-emitting diode to emit at 90° at a band edge of a band gap different from the first band gap. In this way, better efficiency of the light-emitting element can be obtained by enhancing the directivity of the converted radiation. - The pitch and the filling factor of the photonic crystal are also suitable for causing the conversion material to emit at a band edge of the 0° band gap, and the band gap emitted by the conversion material is smaller than the band gap emitted by the light-emitting diode. According to the foregoing, it becomes possible to achieve more accurate vertical emission of the photonic crystal, and as a result, to bring about better performance of the light-emitting element. - The photonic crystal is surrounded by a wall forming a cavity, and at least one of the walls is made of a material selected from the list consisting of transparent conductive oxides such as indium tin oxide or zinc oxide doped with gallium or aluminum, metals such as Ag or Al, graphene, and combinations of the foregoing elements. In this way, it is possible to obtain a wall having good optical properties, and thus to improve the efficiency of the photonic crystal. - The conversion material is a polymer matrix including quantum dots. According to the foregoing, the fabrication of the light-emitting element is facilitated. - Each light-emitting diode includes an active medium made of a first material surrounded by a layer made of a second material, the first material includes InGaN, and the second material includes GaN. In this way, it is possible to obtain good performance of the light-emitting element while maintaining easy fabrication. - The photonic crystal includes a central portion and a peripheral portion, each portion being a collection of a set of light-emitting diodes, and energy is supplied only to the central portion of the photonic crystal. In this way, it is possible to reduce energy consumption.

[0023] This specification also relates to a light-emitting device including a light-emitting diode adapted to emit within a certain spectral band and having an active layer, and a two-dimensional photonic crystal formed from at least a medium and the active layer of the light-emitting diode, the two-dimensional photonic crystal being adapted to generate a standing wave within that spectral band.

[0024] Stated explicitly in another form, this specification proposes a light-emitting device including a two-dimensional photonic crystal adapted to generate a standing wave, the photonic crystal including a light-emitting source of the light-emitting device.

[0025] This specification also describes an optoelectronic device including at least one of the above-described light-emitting devices.

[0026] The features and advantages of the present invention are given by way of example only and are not limited thereto. They will become apparent upon reading the following description and referring to the accompanying drawings.

Brief Description of the Drawings

[0027]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0028] Hereinafter, for ease of understanding, the present invention will be described as follows. First, its general principle will be explained through specific examples, and then second, details on how the same principle can be easily applied to other examples will be given. Third, improved forms or alternative embodiments of those examples will be described.

[0029] Furthermore, for simplicity of explanation, a "Definition" section is inserted at the end, and the reader is referred to the "Definition" section for each of the terms introduced hereinafter.

[0030] Presentation of Specific Examples Regarding the specific example, with reference to FIG. 1, it is proposed to consider the case of the red sub-pixel 10, which will be simply referred to as the red pixel 10 hereinafter (for simplicity).

[0031] In the case of FIG. 1, the red radiation of the pixel 10 is obtained by converting the blue radiation from the nanowire 12 using the conversion material 14.

[0032] The nanowire 12 is typically a light-emitting diode made of a material containing InGaN for forming the active layer 16 between layers 18 of GaN, such as nGaN or pGaN. The nanowire 12 emits blue radiation. The emission spectrum 20 from the nanowire 12 can be seen in the band diagram shown in FIG. 2.

[0033] The nanowire 12 mainly extends along the longitudinal direction Z. The transverse directions are respectively referred to as the first transverse direction X and the second transverse direction Y.

[0034] In this example, the conversion material 14 is a matrix including quantum dots. Each quantum dot has an absorption spectrum 22 and an emission spectrum 24 shown in the band diagram of FIG. 2.

[0035] The set of nanowires 12 is arranged in the conversion material 14 so as to form a photonic crystal 26 with a pitch a.

[0036] Since such an arrangement of the nanowires 12 is two-dimensional, the photonic crystal 26 is a two-dimensional photonic crystal.

[0037] The photonic crystal 26 forms a resonant cavity in a plane formed by two transverse directions X and Y, hereinafter denoted as the transverse plane XY.

[0038] The emission curve 27 of the photonic crystal 26 corresponding to each propagation mode enabled by the photonic crystal 26 is schematically shown on the band diagram of FIG. 2. The emission curve 27 is a curve representing the wavelength emitted as a function of the emission angle.

[0039] The propagation modes of the photonic crystal 26 are separated by band gaps. In FIG. 2, two band gaps B1 and B2 are schematically shown.

[0040] The positions of the two band gaps B1 and B2 are determined by the pitch a of the photonic crystal 26, the diameter of the nanowires 12, the refractive index of the material of the nanowires 12 and the refractive index of the conversion material 14, and the total thickness of the photonic crystal 26.

[0041] Furthermore, the photonic crystal 26 is surrounded by two walls 28 and 30, i.e., an upper wall 28 and a lower wall 30, and the two walls 28 and 30 form a cavity along the longitudinal direction Z.

[0042] One of the two walls 28 and 30 is a wall that enables light extraction, while the other wall is a reflective wall.

[0043] To fabricate the walls 28 and 30, it is possible to contemplate a stack of layers of different materials that are metals and / or dielectrics. Advantageously, a conductive material such as indium tin oxide (also called ITO), a metal such as Ag or Al, is in direct contact with the upper and lower parts of the nanowire 12 so as to enable electrical injection.

[0044] In fact, the photonic crystal 26 has, in this specification, the particularity of forming a blue standing wave generator in the transverse XY plane for wavelengths close to the bandgap at 90°.

[0045] Regarding the term "generator", it should be understood that the photonic crystal 26 is, in this specification, a primary source in the sense that a standing wave is created within the photonic crystal 26 itself. More specifically, since the entry of radiation into the photonic crystal 26 is very small, the radiation sent onto such a photonic crystal does not enable the creation of a standing wave.

[0046] Such particularities will be explained in detail below by discussing the physical phenomena involved.

[0047] In a standing wave generator, due to the Purcell effect, the rate of spontaneous photon emission of the material within the resonant cavity increases compared to the material outside the resonant cavity. What the foregoing means is that the nanowire 12 emits more photons in the transverse plane XY. Thereby, the selection of the angle and wavelength is brought about as a result of the Purcell effect.

[0048] In the example described here, the photonic crystal 26 functions at a specific band edge, that is, at the band edge at 90° (corresponding to the wave vector π / a) of the first resonance band of the nanowire 12. The band edge corresponds to a wavelength range over which the emission curve 27 of the photonic crystal flattens significantly, typically within a range of less than 25 nm, preferably less than 10 nm, and more preferably still less than 5 nm. Referring to FIG. 2, what the foregoing content means is that emission occurs on the portion of the first resonance band of the nanowire 12 indicated by the thicker line of the first resonance band.

[0049] More precisely, the emission ultimately generated within the photonic crystal 26 by the nanowire 12 is restricted to the intersection of the emission spectrum 20 (primary emission), which is located below the first band gap B1, and the emission curve 27 of the photonic crystal 26, that is, mainly to the angular emission at 90° (corresponding to the generation of standing waves) by the thicker line portion.

[0050] The result thereof is that it restricts the emission of radiation within the photonic crystal 26 only to the XY plane.

[0051] The photonic crystal 26 is thereby suitable for generating standing waves in the blue [range], and the photonic crystal 26 is at least partially formed by the nanowire 12 suitable for emitting in the blue [range].

[0052] To obtain such behavior of the photonic crystal 26, it is a prerequisite to adapt the elementary unit cell, pitch, and filling rate of the photonic crystal.

[0053] The elementary lattice and pitch of the photonic crystal 26 correspond to the arrangement of the nanowires 12, while the filling factor is the ratio of the surface area occupied by the nanowires 12 to the total surface area of the photonic crystal 26, and thus is determined according to the size of the nanowires 12, or more precisely, in this specification, according to the diameter of the nanowires.

[0054] To select an appropriate arrangement and diameter of the nanowires 12, simulation techniques can be used to obtain a configuration that is easy to implement experimentally, and then homothety is used.

[0055] For example, the applicant conducted tests using a photonic crystal that emits at 0.52 μm, has a hexagonal lattice, a 50% filling factor, and extends over 1.5 μm. Based on the previous content, the applicant has determined that the band edge is located at a reduced frequency of 0.43. By definition, the reduced frequency is the ratio of the pitch a to the wavelength.

[0056] In this way, for emission at 450 nm, it is possible to determine that the appropriate pitch a is 193.5 nm.

[0057] Then, numerical simulations can be performed. Thus, the applicant has shown that although the pixels have a size of only 1.5 μm × 1.5 μm, 96% absorption can be obtained with quantum dots having an encapsulating material with a refractive index of 1.55. In addition, the absorption occurs over a relatively wide band of approximately 30 nm centered at a reduced frequency of 0.43.

[0058] Thereby, by reconstructing what has just been shown by a simple selection of the arrangement of the nanowires 12 and the diameter of the nanowires 12, the blue emission emitted from the nanowires 12 is forced to move within the transverse plane XY, thereby generating a standing wave in the conversion material 14.

[0059] Thus, there is no blue emission in another direction, whereby the emission is no longer Lambertian. The foregoing is clearly shown in FIG. 3, which schematically represents the blue emission emitted from the nanowire under reference numeral 32.

[0060] Thereby, due to the presence of the photonic crystal 26, the length of the optical path through which photons emitted from the nanowire 12 into the conversion material 14 travel increases considerably because the photons travel to and fro in the conversion material 14.

[0061] Such an increase in the optical path enhances the probability of blue photons encountering the quantum dots, resulting in a considerable increase in the absorption of blue emission by the quantum dots. The absorption of the entire light emitted from each nanowire 12 becomes almost complete.

[0062] Inside the dotted line in FIG. 3, as indicated by reference numeral 36, red emission exhibiting Lambertian emission is obtained along the longitudinal direction Z.

[0063] More specifically, by design, the thickness of each quantum dot is very small, and thus the path of the converted light in the conversion material 14 is very small. According to the foregoing, reabsorption loss is greatly prevented.

[0064] Thereby, compared with an increase in the thickness of the conversion material, no increase in reabsorption loss occurs, especially no increase in the height of the wall, resulting in a very large increase in absorption by the quantum dots.

[0065] Thereby, the quantum efficiency of the pixel 10 proposed herein increases considerably.

[0066] Such a result of good quantum efficiency means that, unlike known conversion modules, the leakage of blue photons is reduced, whereby the thickness of the blue emission cut-off filter is reduced or the cut-off filter is no longer essential when the diffusion can be ignored.

[0067] Also, in the case of appropriate quantum efficiency, it is even possible to attempt to reduce the amount of quantum dots in the matrix. Such a reduction will result in a reduction in the manufacturing cost of pixel 10 and the amount of environmentally harmful materials.

[0068] Furthermore, the manufacture of pixel 10 is easier than that of other known pixels, especially insofar as fewer elements are required, such as the presence of, for example, a blue filter or the presence of high walls to prevent crosstalk.

[0069] In addition, the manufacturing may involve relatively standard techniques compatible with the small-sized pixel 10.

[0070] Expansion of the present principle to other examples The present principle of using a standing wave generator to improve the conversion of pixel 10 can have versions for many other examples without changing the principle.

[0071] In particular, what has been described just now remains valid for other wavelengths.

[0072] In particular, the red pixel 10 can be a green pixel.

[0073] The radiation emitted from the nanowire 12 can be ultraviolet radiation.

[0074] The present principle is also compatible with other conversion materials 14.

[0075] The list of possible materials can be found in the section on definitions.

[0076] The materials used for the formation of the nanowire can also be different from the pair of GaN and InGaN.

[0077] In particular, it is possible to contemplate semiconductor materials that mainly contain at least one element from Group III and one element from Group V (hereinafter referred to as III-V compounds, such as gallium nitride GaN), or semiconductor materials that mainly contain at least one element from Group II and one element from Group VI (hereinafter referred to as II-VI compounds, such as zinc oxide ZnO), or semiconductor materials that mainly contain at least one element from Group IV.

[0078] Methods for creating an active zone with means for confinement, in particular for creating a single quantum well or multiple quantum wells, are also known. A single quantum well is created by inserting a layer of a second semiconductor material, for example an alloy of a III-V compound and a third element, in particular InGaN, the bandgap of which is different from that of the first semiconductor material, between two layers of a first semiconductor material, for example a III-V compound, in particular GaN, each doped p-type or n-type. A multiple quantum well structure comprises a stack of semiconductor layers forming an alternating arrangement of quantum wells and barrier layers.

[0079] Other materials for creating the upper wall 28 or the lower wall 30 can also be contemplated.

[0080] Thereby, the upper wall 28 is made of zinc oxide doped with gallium or aluminum (also called ZNO).

[0081] More generally, the upper wall 28 is made of a transparent conductive oxide (often abbreviated as TCO).

[0082] However, other materials such as graphene can be considered.

[0083] The same material can also be used for the bottom wall 30.

[0084] Improved forms or alternative embodiments Next, improved forms or alternative embodiments are proposed.

[0085] Referring to FIG. 4, it is possible to contemplate a deformed form of the red pixel 10 shown in FIG. 1, where the conversion material 14 is positioned on the upper wall 28.

[0086] The foregoing is achieved, in particular, by depositing a layer over the entire upper wall. Hereinafter, the layer will be referred to as the conversion layer 15.

[0087] In a deformed form, it is possible to contemplate depositing quantum dots of different colors for adjacent pixels 10. Such deposits can be obtained, for example, using selective lithography techniques or selective etching techniques.

[0088] In that case, the medium 31 surrounding the nanowire is, for example, SiO2.

[0089] In a deformed form, the medium 31 is TiO2, Al2O3, or Si3N4.

[0090] More generally, the material forming the medium 31 is an oxide or nitride that transmits the wavelengths emitted from the nanowire 12 and the quantum dots.

[0091] Nevertheless, it has a different function from that described for the case of FIG. 1.

[0092] In that case, the absorption of the standing wave by the conversion material 14 occurs on only one part, which is the evanescent part PE of the standing wave, and this evanescent part PE excites the quantum dots of the conversion layer 15.

[0093] The evanescent part PE exists because the standing wave has a certain extent along the direction Z.

[0094] What the foregoing means is that the distance between the quantum dots and the nanowire 12 is small enough that the overlap between the PE part of the standing wave and the conversion layer 14 becomes large enough.

[0095] Furthermore, in order to advantageously improve the directional emission of the quantum dots, the distance between the conversion layer 15 and the lower wall 30 is a multiple of λ / 2n, where λ is the emission wavelength of the quantum dots and n is the effective refractive index of the set formed by the walls 28 and 30 and the photonic crystal 26.

[0096] For example, this distance can be defined herein as the distance between the center of the conversion layer 15 and the final layer of the bottom wall 30 and can be obtained by simulation.

[0097] According to the foregoing, the red emission by the quantum dots is improved by increasing the number of photons emitted by the Purcell effect, enhancing the conversion efficiency, and enabling more directional emission of the quantum dots.

[0098] Such an embodiment of the red pixel 10 is not as efficient as the embodiment shown in FIG. 1, but it allows the conversion layer 15 to be added separately from the generation of blue photons, thereby making the process of depositing this layer easier than incorporating a conversion matrix between the nanowires.

[0099] Furthermore, in such a case, one of the final steps of the fabrication process is the deposition of the conversion layer 15, which means that the technical processes that the quantum dots undergo and that can affect the performance or reliability of the quantum dots are fewer. Further, in order to enhance the reliability of the conversion layer 15, a protective layer deposition, particularly against oxidation, can be carried out on the conversion layer 15. Such a protective layer is made of, for example, SiO2, TiO2, or Al2O3.

[0100] Referring to FIGS. 5 and 6, it is also possible to use resonance of order n for the emission of the nanowires 12. The foregoing corresponds to using the wave vector n*π / a instead of the wave vector π / a.

[0101] In such a situation, as can be seen in the band diagram shown in FIG. 5, the wavelength emitted from the quantum dot is position-aligned with the band edge at 0° (corresponding to the band gap denoted as B2).

[0102] Using such position alignment, it is possible to obtain an increase in the internal quantum efficiency of the quantum dot (again, the aforementioned parcel effect), and also to obtain a much more directional emission (concentrated by the direction Z) of each quantum dot.

[0103] When the emission spectrum 20 of the nanowire 12 is position-aligned with the 90° resonance mode (corresponding to the band gap denoted as B3), the preferred emission direction of the nanowire 12 is the transverse plane XY. It should be noted that the band gap (band B2 in this specification) emitted by the conversion material 14 is narrower than the band gap (band B3 in this specification) emitted by the nanowire 12.

[0104] In the current case, since the emission spectrum of the quantum dot is position-aligned with the 0° resonance mode, the preferred emission direction of the quantum dot is thus along the direction Z. The previous content corresponds to the existence of two standing waves, a first blue one in the transverse plane XY and a second red one along the axis Z.

[0105] The radiation pattern of the quantum dot when the secondary resonance is used in this way for the emission of the nanowire 12 can be seen in FIG. 6.

[0106] Therefore, advantageously, the photonic crystal 26 has a pitch a and a filling rate suitable for causing emission at the band edge of a band gap different from the first band gap, here the third band B3, in the nanowire 12.

[0107] Such an embodiment retains the advantages of the embodiment of the red pixel 10 according to FIG. 1 by adding better directivity of the converted radiation, which contributes to a further increase in the efficiency of the pixel 10 under consideration.

[0108] FIG. 7 shows an embodiment that is compatible with all the embodiments presented just now for the red pixel 10.

[0109] In the above example, the photonic crystal 26 includes a central portion 42 and a peripheral portion 44, and each portion 42 or 44 is a collection of a set of nanowires 14.

[0110] The central portion 42 is powered, and as described above, the central portion 42 functions to emit a standing wave.

[0111] The peripheral portion 44 surrounds the central portion 42 and the peripheral portion 44 is not powered. In that case, the nanowires 12 of the peripheral portion 44 function as a mirror for the standing wave.

[0112] In the peripheral portion 44, when it is desired to reflect a plurality of wavelengths, a variable pitch, for example, a pitch that increases from the edge towards the central zone, can be considered.

[0113] According to the foregoing, it becomes possible to dispense with an external mirror that can also be used in combination if desired for combination.

[0114] As described above, a set of embodiments has been presented that utilize the idea of using a standing wave generator formed by a photonic crystal in order to better excite the conversion material. If technically possible, such embodiments can be combined together.

[0115] In any case, it is possible to fabricate a small pixel 10 that has good conversion efficiency and thus emits a satisfactory amount of light.

[0116] The foregoing can advantageously be used in many application fields.

[0117] More specifically, this pixel can be used in optoelectronic devices such as display screens, light projectors, or even glasses used for virtual reality immersion.

[0118] In the case of a display screen, the optoelectronic device can be incorporated into an electronic device such as a mobile phone, a tablet device, or a laptop device. In another embodiment, the display screen is incorporated into a dedicated display device such as a television receiver or a desktop computer monitor.

[0119] When the screen is a full-color screen, each pixel includes a plurality of pixels of different colors. Such a pixel can be the pixel described just now.

[0120] However, when efficiency is not particularly important (for example, at the edge of the screen), it is also possible to contemplate the coexistence of pixels according to the prior art.

[0121] Definition Blue: Blue emission has an average wavelength included between 430 nm and 470 nm.

[0122] Quantum dot: A quantum dot is a structure in which quantum confinement occurs in all three spatial dimensions.

[0123] An example of a quantum dot is a particle P having a maximum dimension less than or equal to five times the electron wavelength of the charge carriers in the conversion material.

[0124] By way of example of the size range, a particle P having a maximum dimension included between 1 nm and 200 nm and made of a semiconductor conversion material is an example of a quantum dot.

[0125] Quantum dots can be selected from II-VI semiconductor nanocrystals, III-V semiconductor nanocrystals, IV-VI semiconductor nanocrystals, or mixtures thereof.

[0126] II-VI semiconductor nanocrystals can include, but are not limited to, CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe.

[0127] III-V semiconductor nanocrystals can include, but are not limited to, GaN, GaP, GaAs, AlN, AlP, AlAs, InN, InP, InAs, InGaN, GaNP, GaNAs, GaPAs, AlNP, AlNAs, AlPAs, and InAlPAs.

[0128] IV-VI semiconductor nanocrystals can include, but are not limited to, SbTe, PbSe, GaSe, PbS, PbTe, SnS, SnTe, PbSnTe. Chalcopyrite semiconductor nanocrystals selected from the group consisting of CuInS2, CuInSe2, CuGaS2, CuGaSe2, AgInS2, AgInSe2, AgGaS2, and AgGaSe2 can also be considered.

[0129] Another example of a quantum dot is particle P, which has a core and a shell surrounding the core, where the core is made of a semiconductor conversion material and has a maximum dimension included between 1 nm and 200 nm.

[0130] The core can comprise nanocrystals such as the nanocrystals described above.

[0131] The shell can consist of ZnS, CDS, ZnSe, CdSe, or any mixture thereof.

[0132] Quantum dots can also be protected from oxidation by using a protective layer of metal oxide, a protective layer of metal nitride, a protective layer of oxynitride, or a mixture thereof.

[0133] The protective layer of metal oxide can be selected from the group consisting of, but not limited to, Al2O3, SiO2, TiO2, ZrO2, B2O3, Co2O3, Cr2O3, CuO, Fe2O3, Ga2O3, HfO2, ln2O3, MgO, Nb2O5, NiO, SnO2, and Ta2O5.

[0134] The metal nitride can be, for example, BN, AlN, GaN, lnN, Zr3N4, CuZN, etc.

[0135] The oxynitride protective layer can include, but not limited to, SiON.

[0136] The thickness of the protective layer ranges from 1 to 400 nm, preferably from 1 to 100 nm.

[0137] The particle P is incorporated into, for example, a photosensitive resin. The photosensitive resin is used to define a pattern on the semiconductor surface in many electronics manufacturing techniques. More specifically, to define a pattern, a specific zone of the resin can be solidified while leaving the possibility of removing other zones. This is because the zones to be removed or solidified are defined by exposure using the wavelength of light to which the resin is sensitive. Such a photosensitive resin is more specifically used to protect the coated zones from material deposition or etching.

[0138] It should be noted that the shape of the quantum dots can be various. Examples of quantum dots with different shapes may be called nanorods, nanowires, tetrapods, nanopyramids, nanocubes, etc.

[0139] It should be noted that each particle P can be provided with more than one quantum dot, for example, by integrating quantum dots into porous silica microspheres or by aggregating a plurality of quantum dots.

[0140] Photonic crystal: A periodic structure of a dielectric material, semiconductor material, or metal-dielectric material that modifies the propagation of electromagnetic waves in the same way that a periodic potential in a semiconductor crystal affects the movement of electrons by creating allowed and forbidden energy bands. The wavelengths that can propagate within the crystal are called modes, and the representation of these modes as energy wave number vectors forms bands. In such a structure, a bandgap is said to exist when there are no propagation modes for electromagnetic waves within a certain range of frequencies or wavelengths.

[0141] Band diagram: A band diagram shows the energy of the bands expressed as a function of the reduced frequency as a function of the value of the wave number vector.

[0142] Light-emitting diode (LED): An LED structure is a semiconductor structure that includes a plurality of semiconductor regions forming a P-N junction and is configured to emit light when current flows through different semiconductor zones.

[0143] A two-dimensional structure including an n-type doped layer, a p-type doped layer, and at least one emissive layer is an example of an LED structure. In such a case, each emissive layer is inserted between the n-type doped layer and the p-type doped layer along the normal direction D.

[0144] In one embodiment, each emissive layer has a bandgap value that is strictly smaller than the bandgap value of the n-type doped layer and strictly smaller than the bandgap value of the p-type doped layer. For example, the n-type doped layer and the p-type doped layer are GaN layers, and each emissive layer is an InGaN layer.

[0145] The emissive layer is, for example, undoped. In other embodiments, the emissive layer is doped.

[0146] A quantum well is a specific example of an emissive layer having a bandgap value smaller than the bandgap values of the n-type doped layer and the p-type doped layer.

[0147] Doping: Doping is defined as the presence of impurities in a material that result in free charge carriers. The impurities are, for example, atoms of elements that are not originally present in the material.

[0148] When the impurities increase the density of holes in the material compared to the undoped material, the doping is p-type. For example, a layer of gallium nitride GaN is p-type doped by adding magnesium atoms (Mg).

[0149] When the impurities increase the volume density of free electrons in the material compared to the undoped material, the doping is n-type, and for example, a layer of gallium nitride GaN is n-doped by adding silicon (Si) atoms.

[0150] Conversion material: A conversion material is configured to convert a first radiation emitted from a light emitter into a second radiation. In other words, the conversion material is configured to be excited by the first radiation and to emit the second radiation in response thereto.

[0151] The second radiation has a second range of wavelengths. The second range is distinct from the first range. More specifically, the second position has a second average wavelength, and the second average wavelength is different from the first average wavelength. The second average wavelength is, in particular, strictly greater than the first average wavelength.

[0152] The conversion material is, for example, a semiconductor material.

[0153] For example, the conversion material is selected from the group consisting of CdSe, CdTe, ZnSe, ZnTe, InP, InPZnS, Ag2S, CuInS, CuInSe, AgInS2, AgInSe2, or even InPZn x Se x-y S y selected from the group consisting of. However, other types of materials are also conceivable.

[0154] In other embodiments, the conversion material is a non-semiconductor material such as an inorganic garnet. For example, the conversion material is a doped yttrium aluminum garnet. However, other types of non-semiconductor conversion materials, particularly other garnets, are also conceivable.

[0155] More specifically, the conversion material can be an inorganic phosphor.

[0156] Examples of inorganic phosphors are yttrium aluminum garnet particles (e.g., YAG:Ce), terbium aluminum garnet particles TAG (e.g., TAG:Ce), silicate particles (e.g., SrBaSiO4:Eu), sulfide particles (e.g., SrGa2S4:Eu, SrS:Eu, CaS:Eu, etc.), nitride particles (e.g., Sr2Si5N8:Eu, Ba2Si5N8:Eu, etc.), oxynitride particles (e.g., Ca-α-SiAlON:Eu, SrSi2O2N2:Eu, etc.), and fluoride particles (e.g., K2SiF6:Mn, Na2SiF6:Mn, etc.).

[0157] Many other conversion materials can be used, such as doped aluminates, doped nitrides, doped fluorides, doped sulfides, or doped silicates.

[0158] The conversion material is doped, for example, with a rare earth element, an alkaline earth element, or a transition metal element. Cerium, for example, may be used to dope yttrium aluminum garnet.

[0159] The conversion material comprises, for example, a set of particles P made of the conversion material. Those particles P may be called "lumophores".

[0160] Semiconductor material: The term "bandgap value" should be understood as the bandgap value between the valence band and the conduction band of the material.

[0161] The bandgap value is measured, for example, in units of electron volts (eV).

[0162] Of the allowed energy bands of electrons in a material, the valence band is defined as the band having the highest energy when it is completely filled at a temperature of 20 kelvin (K) or less.

[0163] For all valence bands, a first energy level is defined. The first energy level is the highest energy level in the valence band.

[0164] Of the allowed energy bands of electrons in a material, the conduction band is defined as the band having the lowest energy when it is not completely filled at a temperature of 20 K or less.

[0165] For all conduction bands, a second energy level is defined. The second energy level is the highest energy level in the conduction band.

[0166] Therefore, all band gap values are measured between the first energy level and the second energy level of the material.

[0167] A semiconductor material is a material having a band gap value greater than exactly zero and less than or equal to 6.5 eV.

[0168] A direct band gap semiconductor is an example of a semiconductor material. A material is considered to have a "direct band gap" when the minimum value of the conduction band and the maximum value of the valence band correspond to the same value of the momentum of the charge carriers. A material is considered to have an "indirect band gap" when the minimum value of the conduction band and the maximum value of the valence band correspond to different values of the momentum of the charge carriers.

[0169] Each semiconductor material can be selected from the group consisting of, for example, III-V semiconductors, particularly nitrides of group III elements, II-VI semiconductors, or even IV-IV semiconductors.

[0170] III-V semiconductors include, in particular, InAs, GaAs, AlAs, and their alloys, InP, GaP, AlP, and their alloys, and nitrides of group III elements.

[0171] II-VI semiconductors include CdTe, HgTe, CdSe, HgSe, and their alloys.

[0172] IV-IV semiconductors include, in particular, Si, Ge, and their alloys.

[0173] Nanowire: A nanowire is a specific example of a three-dimensional structure.

[0174] A three-dimensional structure is a structure that extends along a main direction. The three-dimensional structure has a length measured along the main direction. The three-dimensional structure also has a maximum lateral dimension measured along a lateral direction perpendicular to the main direction, and the lateral direction is the direction perpendicular to the main direction along which the dimensions of the structure are largest.

[0175] The maximum lateral dimension is, for example, 10 micrometers (μm) or less, and the length is greater than or equal to the maximum lateral dimension. The maximum lateral dimension is preferably 2.5 μm or less.

[0176] The maximum lateral dimension is in particular 10 nm or more.

[0177] In certain embodiments, the length is at least twice the maximum lateral dimension, for example, at least five times the maximum lateral dimension.

[0178] The main direction is, for example, the vertical direction D. In such a case, the length of the three-dimensional structure is called the "height", and the maximum dimension of the three-dimensional structure in a plane perpendicular to the vertical direction D is 10 μm or less.

[0179] The maximum dimension of the three-dimensional structure in a plane perpendicular to the vertical direction D is often called the "diameter" regardless of the cross-sectional shape of the three-dimensional structure.

[0180] All three-dimensional structures are, for example, micro-wires. The micro-wires are three-dimensional cylindrical structures.

[0181] In a particular embodiment, the micro-wire is a cylinder extending along the vertical direction D. The micro-wire is, for example, a cylinder having a circular bottom surface. In such a case, the diameter of the bottom surface of the cylinder is less than or equal to half of the length of the micro-wire.

[0182] A micro-wire with a maximum lateral dimension of less than 1 μm is called a "nanowire".

[0183] A pyramid extending along the vertical direction D from the substrate is another example of a three-dimensional structure.

[0184] A cone extending along the vertical direction D is another example of a three-dimensional structure.

[0185] A frustum of a cone or a frustum of a pyramid extending along the vertical direction D is yet another example of a three-dimensional structure.

[0186] Stationary wave: A stationary wave is a phenomenon that occurs as a result of multiple waves of the same frequency and the same amplitude propagating simultaneously in opposite directions in the same physical medium, thereby forming a pattern in which the components of the wave are fixed in time. In the pattern, instead of seeing the propagating waves, stationary vibrations of different intensities are observed at each observation point. These characteristic fixed points are called pressure nodes.

[0187] Pixels: Many display screens have a set of light emitters used to form the images displayed on the screen. Such light emitters each serve the role of an image element, or "pixel" which is derived from the English "Picture Element" (especially when the screen is monochromatic), or serve the role of a part of such an image element called a "subpixel" (especially when the screen is a color screen and each pixel contains subpixels of different colors and the color of the pixel can be changed by selectively illuminating the subpixels). In this specification, the red pixel was rather a subpixel in the above sense.

[0188] Quantum well: A quantum well is a structure in which quantum confinement occurs in one direction for at least one type of charge carrier. The quantum confinement effect occurs when the size of the structure along such a direction becomes comparable to or smaller than the de Broglie wavelength of the carriers, which are usually electrons and / or holes, thereby giving rise to energy levels called "energy subbands".

[0189] In such a quantum well, the carriers can only have discrete energy values, but generally tend to move within a plane perpendicular to the direction in which the confinement occurs. When the dimension of the quantum well decreases along the direction in which the confinement occurs, the energy values available to the carriers, also known as "energy levels", increase.

[0190] In quantum mechanics, the "de Broglie wavelength" is the wavelength of a particle when the particle is regarded as a wave. The de Broglie wavelength of an electron is also called the "electron wavelength". The de Broglie wavelength of a charge carrier is determined according to the material in which the quantum well is made.

[0191] A light-emitting layer having a thickness strictly smaller than the product of the electron wavelength of the electrons in the semiconductor material forming the light-emitting layer and 5 is an example of a quantum well.

[0192] Another example of a quantum well is a light-emitting layer having a thickness that is strictly less than the product of the de Broglie wavelength of the excitons in the semiconductor forming the light-emitting layer and 5. An exciton is a quasiparticle comprising an electron and a hole.

[0193] In particular, quantum wells often have a thickness included between 1 nm and 50 nm.

[0194] Emission: All emissions include a set of electromagnetic waves.

[0195] For all electromagnetic waves, the wavelength is defined.

[0196] All sets correspond to a wavelength range or spectral band. The wavelength range is a group consisting of the set of wavelengths of the set of electromagnetic waves.

[0197] The average wavelength of a spectral band can be defined as the average of both ends of the spectral band.

[0198] Red: Red emission has an average wavelength included between 600 nm and 720 nm.

[0199] Ultraviolet: Ultraviolet emission has an average wavelength included between 350 nm and 430 nm.

[0200] Green: Green emission has an average wavelength included between 500 nm and 560 nm.

Explanation of Signs

[0201] 10 Red subpixel, red pixel 12 Nanowire 14 Conversion material 15 Conversion layer 16 Active layer 18 Layer made of GaN 20 Emission spectrum 22 Absorption spectrum 24 Emission spectrum 26 Photonic crystal 27 Emission curve 28 Upper wall 30 Lower wall, bottom wall 31 Medium 42 Central part 44 Peripheral part a Pitch B1 First band gap B2 Band gap, band B3 Third band

Claims

**Claim 1** A conversion material (14) suitable for converting a first emission in a first spectral band into a second emission in a second spectral band, the second spectral band being separate from the first spectral band, the conversion material (14); A standing wave generator in the first spectral band, comprising a two-dimensional photonic crystal (26) suitable for generating a standing wave in the first spectral band, the photonic crystal (26) being at least partially formed by a light-emitting diode (12) suitable for emitting in the first spectral band, the standing wave generator; A light-emitting element (10) including the above. **Claim 2** The light-emitting element according to claim 1, wherein the photonic crystal (26) is formed only by a light-emitting diode (12) and a medium (14, 31) surrounding the light-emitting diode (12). **Claim 3** The light-emitting element according to claim 1 or 2, wherein the light-emitting diode (12) includes an active layer (16), and at least one active layer (16) is included in the photonic crystal (26). **Claim 4** The light-emitting element according to claim 3, wherein the photonic crystal (26) is partially formed from each active layer (16) of the light-emitting diode (12). **Claim 5** The light-emitting element according to any one of claims 1 to 4, wherein the photonic crystal (26) includes all of the light-emitting diodes (12). **Claim 6** The light-emitting element according to any one of claims 1 to 5, wherein the light-emitting diode (12) is in a medium, and the medium is the conversion material (14). **Claim 7** The light-emitting element according to any one of claims 1 to 5, wherein the conversion material (14) is on the photonic crystal (26). **Claim 8** The light-emitting element according to any one of claims 1 to 7, wherein the photonic crystal (26) has a plurality of band gaps, and the photonic crystal (26) has a pitch (a) and a filling ratio suitable for causing the light-emitting diode (12) to emit at a 90° angle at a band edge of a first band gap. **Claim 9** The photonic crystal (26) has a plurality of band gaps, and the photonic crystal (26) has a pitch (a) and a filling ratio suitable for causing emission at 90° at a band edge of a band gap different from the first band gap in the light-emitting diode (12). The light-emitting element according to any one of claims 1 to 7.

10. The pitch (a) and the filling ratio of the photonic crystal (26) are also suitable for causing emission at the band edge of the 0° band gap in the conversion material, and the band gap emitted by the conversion material (14) is smaller than the band gap emitted by the light-emitting diode (12). The light-emitting element according to claim 9.

11. The photonic crystal (26) is surrounded by walls (28, 30) that form cavities, and at least one of the walls (28, 30) is made of a material selected from the list consisting of transparent conductive oxides such as indium tin oxide or zinc oxide doped with gallium or aluminum, metals such as Ag or Al, graphene, and combinations of the elements. The light-emitting element according to any one of claims 1 to 10.

12. The conversion material (14) is a polymer matrix comprising quantum dots. The light-emitting element according to any one of claims 1 to 11.

13. Each light-emitting diode (12) includes an active medium (16) made of a first material surrounded by a layer (18) made of a second material, the first material includes InGaN, and the second material includes GaN. The light-emitting element according to any one of claims 1 to 12.

14. The photonic crystal (26) has a central portion (42) and a peripheral portion (44), each portion (42, 44) is a collection of a plurality of light-emitting diodes (12), and energy is supplied only to the central portion (42) of the photonic crystal (26). The light-emitting element according to any one of claims 1 to 13.

15. An optoelectronic device comprising at least one light-emitting element (10) according to any one of claims 1 to 14.

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