Light-emitting element comprising a standing-wave generator, and associated optoelectronic device

Abstract

The present invention relates to a light-emitting element (10) comprising: - a converting material (14) able to convert first radiation in a first spectral band into second radiation in a second spectral band, the second spectral band being distinct from the first spectral band, and - a standing-wave generator for generating a standing wave in the first spectral band, the standing-wave generator comprising a two-dimensional photonic crystal (26) configured to generate the standing wave in the first spectral band, the photonic crystal (26) being formed at least in part by light-emitting diodes (12) configured to emit in the first spectral band.

Description

[0001] Light-emitting element with standing wave generator and associated optoelectronic device

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a light emitting element and an optoelectronic device comprising such a light emitting element.

[0004] TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0005] In the field of optoelectronics, the production of very small devices is desired. This is particularly the case for pixels in color display screens.

[0006] In color displays, each pixel comprises several subpixels, each subpixel being configured to emit a specific color, so that the color emitted by the pixel can be changed by controlling which subpixel(s) is (are) activated or by changing the electrical current applied to each subpixel in order to change the relative emission intensity of each subpixel.

[0007] Semiconductor structures such as light-emitting diodes (LEDs) are commonly used for various purposes, such as lighting, due to their good potential light emission efficiency. Due to this high potential efficiency, LEDs have been suggested for the manufacture of high-efficiency display screens. LED structures generally take the form of a stack of planar semiconductor layers. Light is emitted when an electric current flows through the stack.

[0008] Thus, to reduce pixel size, it is known to grow native pixels based on LEDs made of GaN / InGaN on the same wafer. A native pixel is a pixel whose emission is natively of the desired color.

[0009] However, such pixels are not efficient because only the quantum efficiency of this type of LED for the blue color is satisfactory. In fact, the quantum efficiency for the green color is generally 30% while, for the red color, it falls to less than 5%.

[0010] Therefore, it is known to use color conversion modules that allow you to obtain other colors from a blue or UV LED. Quantum dots are common examples of converters used in these conversion modules. Quantum dots are often inserted into a matrix.

[0011] However, for pixels of the order of a few micrometers, the absorption of quantum dots is too weak to guarantee complete conversion of blue radiation to red or green radiation. This requires filtering the unconverted radiation to obtain a red or green pixel. Such filtering represents a significant loss of the radiation emitted by the LED. For example, a loss of 60% is observed for a pixel with a size of 5 pm by 5 pm.

[0012] It could be considered to compensate for this loss by increasing the number of quantum dots.

[0013] A first way to increase this number of quantum dots is to increase the concentration of quantum dots in the matrix. This proposal comes up against the fact that too high concentrations lead to losses of mechanical properties of the matrix, making it incompatible with the techniques used in pixel manufacturing.

[0014] A second way to increase the number of quantum dots is to increase the size of the matrix and in particular its thickness.

[0015] However, here again, this increase faces several problems.

[0016] The production of pixels with a very high thickness is first of all technically delicate.

[0017] A thicker matrix implies a longer optical path length of the radiation converted by the quantum dots. This increase results in greater reabsorption losses.

[0018] Another problem is related to the existence of crosstalk between two pixels. Crosstalk is usually avoided by inserting opaque walls on the edges of the color conversion module. A greater thickness of the matrix implies an increase in the height of these walls and therefore absorption losses at these walls are greater, even so great that they exceed the absorption gain linked to the increase in the number of quantum dots.

[0019] SUMMARY OF THE INVENTION

[0020] There is therefore a need for a small light emitting element capable of overcoming the aforementioned drawbacks.

[0021] For this purpose, the description describes a light-emitting element comprising a conversion material capable of converting a first radiation in a first spectral band into a second radiation in a second spectral band, the second spectral band being distinct from the first spectral band. The light-emitting element further comprises a standing wave generator in the first spectral band, the standing wave generator comprising a two-dimensional photonic crystal adapted to generate a standing wave in the first spectral band, the photonic crystal being formed at least in part by light-emitting diodes adapted to emit in the first spectral band.

[0022] Unlike the state of the art, and in particular documents FR 3 068 173 A and US 2022 / 102324 A1, the nanowires forming the electroluminescent source are part of the photonic crystal and are used to partially produce it. More precisely, the photonic crystal is here the combination of a medium and nanowires.

[0023] In addition, the photonic crystal has a different role since it serves to generate a standing wave generated efficiently thanks to the previous structural difference.

[0024] This makes it possible to produce light-emitting elements, generally pixels, of small size and with good conversion efficiency and therefore emitting a satisfactory quantity of light.

[0025] According to particular embodiments, the light-emitting element has one or more of the following characteristics, taken in isolation or in all technically possible combinations:

[0026] - the photonic crystal is formed only by light-emitting diodes and a medium surrounding the light-emitting diodes.

[0027] - light-emitting diodes have an active layer, with at least one active layer being included in the photonic crystal. This allows efficient injection of light into the photonic crystal to form the standing wave

[0028] - the photonic crystal is formed in part from each active layer of the light-emitting diodes.

[0029] - the photonic crystal includes the assembly of each light-emitting diode.

[0030] - the light-emitting diodes are in a medium, said medium being the conversion material. This makes it possible to reduce the light-emitting element.

[0031] - the conversion material is based on the photonic crystal. This facilitates the manufacture of the light-emitting element.

[0032] - the photonic crystal has multiple band gaps, the photonic crystal having a pitch and fill factor adapted to make the light-emitting diodes emit on the band edge of the first 90° band gap. This increases the probability of absorption of photons in the first spectral band by the conversion material.

[0033] - the photonic crystal has several band gaps, the photonic crystal having a pitch and a fill factor adapted to make the light-emitting diodes emit on the band edge of a band gap at 90° different from the first band gap. This makes it possible to obtain a better efficiency of the light-emitting element by an increase in the directionality of the converted radiation.

[0034] - the pitch and fill factor of the photonic crystal are also adapted to make the conversion material emit on the band edge of a 0° band gap, the band gap in which the conversion material emits being smaller than the band gap in which the light-emitting diodes emit. This allows for more precise perpendicular directional emission for the photonic crystal, resulting in better performance of the light-emitting element.

[0035] - the photonic crystal is surrounded by walls forming a cavity, at least one of the walls being made of a material chosen from the list consisting of a transparent conductive oxide such as indium tin oxide or a zinc oxide doped with gallium or aluminum, a metal such as Ag or AI, graphene and a combination of these elements. This makes it possible to obtain walls having good optical properties and therefore to improve the efficiency of the photonic crystal.

[0036] - the conversion material is a polymer matrix comprising quantum dots. This facilitates the manufacture of the light-emitting element.

[0037] - each light-emitting diode comprises an active medium made of a first material surrounded by layers made of a second material, the first material comprising InGaN, and the second material comprising GaN. This makes it possible to obtain good performance from the light-emitting element while maintaining easy manufacturing.

[0038] - the photonic crystal has a central part and a peripheral part, each part containing a set of light-emitting diodes, with only the central part of the photonic crystal being powered. This reduces energy consumption.

[0039] The description also relates to a light emitting element comprising light emitting diodes adapted to emit in a spectral band and comprising an active layer and a two-dimensional photonic crystal formed at least from a medium and active layers of the light emitting diodes, the crystal being adapted to generate a standing wave in the spectral band.

[0040] Stated differently, the description provides a light emitting element comprising a two-dimensional photonic crystal adapted to generate a standing wave, the photonic crystal comprising the emitting sources of the light emitting element.

[0041] The description also describes an optoelectronic electronic device comprising at least one light emitting element as previously described. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Characteristics and advantages of the invention will appear on reading the description which follows, given solely by way of non-limiting example, and made with reference to the appended drawings, in which:

[0043] - Figure 1 is a schematic cross-sectional representation of an example pixel,

[0044] - Figure 2 is a schematic representation of a band diagram illustrating an example of operation of the pixel of Figure 1,

[0045] - figure 3 is a schematic representation of the radiation emitted by certain elements of the pixel in the context of the operation of figure 2,

[0046] - Figure 4 is a schematic cross-sectional representation of another example pixel,

[0047] - Figure 5 is a schematic representation of a band diagram illustrating another example of operation of the pixel of Figure 1,

[0048] - Figure 6 is a schematic representation of the radiation emitted by certain elements of the pixel in the context of the operation of the figure, and

[0049] - Figure 7 is a schematic representation from above of yet another example pixel.

[0050] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] In the following, to facilitate understanding, the present invention will be explained as follows: first by explaining its general principles through a specific example and then by detailing in a second step how these same principles can be easily applied to other examples. In a third step, refinements or alternative embodiments of these examples will be described.

[0052] In addition, to simplify the description, a "definitions" section has been inserted at the end and the reader is invited to refer to it for each of the terms introduced in what follows.

[0053] Presentation of a specific example

[0054] As regards the specific example, it is proposed with reference to figure 1 to consider the case of a red sub-pixel 10, simply called red pixel 10 in the following for simplicity.

[0055] In the case of Figure 1, the red radiation of the pixel 10 is obtained by converting blue radiation from nanowires 12 using a conversion material 14. The nanowires 12 are light-emitting diodes made with a material comprising InGaN to form the active layer 16 between GaN layers 18, typically nGaN or pGaN. The nanowires 12 emit blue radiation. The emission spectrum 20 of a nanowire 12 is visible on the band diagram of Figure 2.

[0056] The nanowires 12 extend mainly in a longitudinal direction Z. The transverse directions are called respectively first transverse direction X and second transverse direction Y.

[0057] For the present example, the conversion material 14 is a matrix comprising quantum dots. Each quantum dot has the absorption spectrum 22 and the emission spectrum 24 which are illustrated in the band diagram of Figure 2.

[0058] The set of nanowires 12 is arranged in the conversion material 14 to form a photonic crystal 26 of pitch a.

[0059] This arrangement of the nanowires 12 being two-dimensional, the photonic crystal 26 is a two-dimensional photonic crystal.

[0060] This photonic crystal 26 forms a resonant cavity in a plane formed by the two transverse directions X and Y, denoted transverse plane XY in the following.

[0061] The emission curves 27 of the photonic crystal 26 corresponding to each propagation mode permitted by the photonic crystal 26 are represented schematically on the band diagram of figure 2. These emission curves 27 are curves representing the emitted wavelength as a function of the emission angle.

[0062] The propagation modes of the photonic crystal 26 are separated by band gaps. In Figure 2, two band gaps B1 and B2 are schematically represented.

[0063] The position of the two forbidden bands B1 and B2 is determined by the pitch a of the photonic crystal 26, the diameter of the nanowires 12, the indices of the materials of the nanowires 12 and of the conversion material 14 as well as the total thickness of the photonic crystal 26.

[0064] Furthermore, the photonic crystal 26 is surrounded by two walls 28 and 30, namely an upper wall 28 and a lower wall 30, the two walls 28 and 30 forming a cavity in the longitudinal direction Z.

[0065] One of the two walls 28 and 30 is a wall allowing the extraction of light while the other wall is a reflective wall.

[0066] To produce these walls 28 and 30, a stack of layers of different materials can be considered, the materials being 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 part of the nanowires 12 so as to allow electrical injection. In fact, the photonic crystal 26 here has the particularity of forming a blue standing wave generator in the transverse plane XY for wavelengths close to the 90° band gaps.

[0067] With the term “generator”, it should be understood that the photonic crystal 26 is here a primary source in the sense that the standing wave is created within the photonic crystal 26 itself. In particular, radiation sent onto such a photonic crystal does not allow the creation of a standing wave, the penetration of the radiation into the photonic crystal 26 being very low.

[0068] This peculiarity is now explained by detailing the physical phenomena involved.

[0069] In the standing wave generator, the Purcell effect causes the spontaneous emission rate of photons to increase for a material in a resonant cavity compared to a material not in a resonant cavity. This implies that the nanowires 12 emit more photons in the transverse XY plane. The Purcell effect thus results in angular and wavelength selection.

[0070] In the example described, the photonic crystal 26 operates on a specific band edge, namely the 90° band edge (corresponding to a wave vector of n / a) of the first resonance band of a nanowire 12. A band edge corresponds to a wavelength range for which the emission curve 27 of the photonic crystal flattens significantly, typically over a wavelength range of less than 25 nm, advantageously less than 10 nm and advantageously still less than 5 nm. With reference to FIG. 2, this means that the emission takes place on the first resonance band of a nanowire 12 on the part of the first resonance band in thicker line.

[0071] More precisely, the emission finally produced by the nanowire 12 in the photonic crystal 26 is limited to the portion of intersection between the emission spectrum 20 (primary emission) and the emission curve 27 of the photonic crystal 26 located below the first forbidden band B1, i.e. an angular emission according to the portion in thicker line which is mainly at 90° (corresponding to the generation of a standing wave).

[0072] This has the consequence of constraining the emission of radiation in the photonic crystal 26 only in the XY plane.

[0073] The photonic crystal 26 is thus adapted to generate a standing wave in the blue, the photonic crystal 26 being formed at least in part by nanowires 12 adapted to emit in the blue.

[0074] Obtaining such behavior of the photonic crystal 26 requires adapting its elementary mesh, its pitch and its filling factor. The elementary mesh and the pitch of the photonic crystal 26 correspond to the arrangement of the nanowires 12 while the filling factor is the ratio between the surface occupied by the nanowires 12 and the total surface of the photonic crystal 26 and therefore depends on the size of the nanowires 12 and more precisely here on their diameter.

[0075] To choose a suitable arrangement and diameter for the nanowires 12, it is possible to use the simulation technique for an easy-to-realize configuration experimentally and then use a homothety.

[0076] For example, the applicant carried out a test with a photonic crystal emitting at 0.52 pm and having a hexagonal lattice, a fill factor of 50% and extending over 1.5 pm. This led it to determine that the band edge is located at the reduced frequency of 0.43. By definition, the reduced frequency is the ratio between the pitch a and the wavelength.

[0077] This allows us to determine that for an emission at 450 nm, the appropriate step a is 193.5 nm.

[0078] Then, it is possible to carry out a numerical simulation. The applicant has thus shown that it is possible to obtain an absorption of 96% with quantum dots having an encapsulation material with an index of 1.55 while the pixel has a size of only 1.5 pm by 1.5 pm. In addition, the absorption takes place over a relatively broad band of approximately 30 nm centered around the reduced frequency of 0.43.

[0079] Thus, by reformulating what has just been indicated, by a simple choice of the arrangement of the nanowires 12 and their diameter, the blue radiation emitted by the nanowires 12 is forced to move in the transverse plane XY so that a standing wave is formed within the conversion material 14.

[0080] There is thus no blue radiation in any other direction so that the radiation is no longer Lambertian. This is clearly shown in Figure 3, which schematically represents the blue radiation emitted by the nanowires under the numerical reference 32.

[0081] As a result, the presence of the photonic crystal 26 greatly increases the length of the optical path taken by the photons emitted by the nanowires 12 in the conversion material 14 since the photons travel back and forth in the conversion material 14.

[0082] This increase in the optical path leads to a higher probability that a blue photon encounters a quantum dot, resulting in a strong increase in the absorption of blue radiation by the quantum dots. The absorption of all the light emitted by each nanowire 12 becomes almost total. In the longitudinal direction Z, the emission of red radiation is obtained, exhibiting the emission of a Lambertian as shown in dotted lines with the reference sign 36 in Figure 3.

[0083] In particular, by design, the thickness of each quantum dot is very small and therefore the path of the converted light in this conversion material 14 is very small. This significantly avoids reabsorption losses.

[0084] As a result, compared to an increase in the thickness of the conversion material, a very large increase in absorption by the quantum dots is obtained without an increase in reabsorption losses and especially without an increase in the height of the walls.

[0085] The quantum efficiency of the proposed pixel 10 is thus considerably increased.

[0086] A consequence of this good quantum efficiency is that, unlike known conversion modules, the leakage of blue photons becomes low so that the blue radiation cut-off filter has a reduced thickness, or is no longer essential if the scattering is negligible.

[0087] Also, for a suitable quantum efficiency, it can even be considered to reduce the quantity of quantum dots in the matrix. Such a reduction allows to reduce the manufacturing cost of the pixel 10 and the quantity of materials harmful to the environment.

[0088] Furthermore, the manufacture of Pixel 10 is easier than that of other known pixels, notably because fewer elements are required, such as the presence of a blue filter or high walls to avoid crosstalk.

[0089] Additionally, manufacturing can involve relatively standard techniques compatible with small 10 pixel sizes.

[0090] Extension of the principle to other examples

[0091] The principle of using a standing wave generator to improve the conversion of a pixel 10 can be applied to many other examples without changing the principle.

[0092] In particular, what has just been described remains valid for other wavelengths.

[0093] In particular, red pixel 10 may be a green pixel.

[0094] The radiation emitted by the nanowires 12 may be ultraviolet radiation.

[0095] The principle is also compatible with other conversion materials 14.

[0096] A list of possible materials can be found in the definitions section.

[0097] The materials used to form the nanowires may also be different from the GaN and InGaN pair. In particular, a semiconductor material may be considered comprising a majority of at least one element from group III and one element from group V (for example gallium nitride GaN), subsequently called a III-V compound, or comprising a majority of at least one element from group II and one element from group VI (for example zinc oxide ZnO), subsequently called an II-VI compound, or comprising a majority of at least one element from group IV.

[0098] It is also known to produce an active zone comprising confinement means, in particular a single quantum well or multiple quantum wells. A single quantum well is produced by interposing, between two layers of a first semiconductor material, for example a III-V compound, in particular GaN, respectively doped with P and N type, a layer of a second semiconductor material, for example an alloy of the III-V compound and a third element, in particular InGaN, the band gap of which is different from the first semiconductor material. A multiple quantum well structure comprises a stack of semiconductor layers forming an alternation of quantum wells and barrier layers.

[0099] Other materials can also be considered to make the upper 28 or lower 30 walls.

[0100] Thus, the upper wall 28 is made of a zinc oxide doped with gallium or aluminum (also called ZNO).

[0101] More generally, the upper wall 28 is made of transparent conductive oxide (more often referred to by the abbreviation TCO which refers to the corresponding English name of “transparent conductive oxide”).

[0102] However, other materials such as graphene can be considered.

[0103] The same materials can be used for the bottom wall 30.

[0104] Refinements or alternative embodiments

[0105] Refinements or alternative embodiments are now proposed.

[0106] With reference to Figure 4, it is possible to envisage a variant of the red pixel 10 of Figure 1 in which the conversion material 14 is positioned on the upper wall 28.

[0107] This is achieved in particular by depositing a layer over the entire upper wall. This layer is called conversion layer 15 in the following.

[0108] Alternatively, it may be envisaged to deposit quantum dots of different colors for adjacent pixels 10. Such deposition is, for example, obtained using selective lithography or etching techniques.

[0109] The medium 31 surrounding the nanowires is then, for example, SiOs. Alternatively, the medium 31 is TiOs, AI2O3 or SisN4.

[0110] More generally, the material forming the medium 31 is an oxide or a nitride transparent to the wavelengths emitted by the nanowires 12 and the quantum dots.

[0111] The operation is nevertheless different from what was described for the case of figure 1.

[0112] The absorption of the standing wave by the conversion material 14 then occurs on only one part, which is the evanescent part PE of the standing wave which excites the quantum boxes of the conversion layer 15.

[0113] This evanescent part PE exists because the standing wave has a certain extent in the Z direction.

[0114] This implies that the distance between the quantum dots and the nanowires 12 is small enough for the overlap between the PE part of the standing wave and the conversion layer 14 to be sufficiently large.

[0115] Furthermore, to advantageously promote directional emission from the quantum dots, the distance between the conversion layer 15 and the lower wall 30 is a multiple of λ / 2n with λ being the emission wavelength of a quantum dot and n being the effective index of the assembly formed by the walls 28 and 30 and the photonic crystal 26.

[0116] For example, the distance can be defined here as the distance between the center of the conversion layer 15 and the last layer of the bottom wall 30 and obtained by a simulation.

[0117] This improves the red emission from the quantum dot by increasing the quantity of photons emitted by the Purcell effect as well as the conversion efficiency while allowing more directional emission from the quantum dot.

[0118] This embodiment of a red pixel 10 is less efficient than that presented in Figure 1 but it allows the conversion layer 15 to be added apart from the generation of blue photons, which makes the process of deposition of the layer easier than incorporating a conversion matrix between the nanowires.

[0119] Furthermore, in this case, one of the last steps of the manufacturing process will be the deposition of the conversion layer 15, which implies that the quantum dots will undergo fewer technical processes that could affect their performance or reliability. The deposition of a protective layer, in particular against oxidation, can also be carried out on the conversion layer 15 in order to increase their reliability. Such a protective layer is, for example, made of SiOs, TiOs or AI2O3.

[0120] Referring to Figures 5 and 6, it is also possible to use an n-order resonance for the emission of the nanowires 12. This corresponds to using a wave vector of n*TT / a instead of n / a. In such a situation, as visible in the band diagram of Figure 5, the wavelength emitted by the quantum dots is aligned with a band edge at 0° (corresponding to the band gap denoted B2).

[0121] Such alignment makes it possible to obtain an increased internal quantum efficiency for the quantum dot (still the aforementioned Purcell effect) and to obtain a much more directional emission from each quantum dot (more centered on the Z direction).

[0122] When the emission spectrum 20 of a nanowire 12 is aligned with a 90° resonance mode (corresponding to the band gap denoted B3), the preferred emission direction of the nanowire 12 is the transverse plane XY. It can be noted that the band gap in which the conversion material 14 emits (the band B2 here) is lower than the band gap in which the nanowires 12 emit (the band B3 here).

[0123] In this case, the emission spectrum of the quantum dots being aligned with a 0° resonance mode, the preferred emission direction for the quantum dots is therefore along the Z direction. This corresponds to the presence of two standing waves, a first blue in color in the transverse XY plane and a second red along the Z axis.

[0124] The radiation pattern of a quantum dot in such use of a second-order resonance for the emission of nanowires 12 is seen in Figure 6.

[0125] Thus, advantageously, the photonic crystal 26 has a pitch a and a filling factor adapted to cause the nanofilms 12 to emit on the band edge of a forbidden band different from the first forbidden band, here the third band B3.

[0126] Such an embodiment retains the advantages of the one-pixel embodiment

[0127] 10 red according to figure 1 by adding a better directionality of the converted radiation, which contributes to further increasing the efficiency of the pixel 10 considered.

[0128] Figure 7 shows an embodiment which is compatible with all of the embodiments which have just been presented for the red pixel 10.

[0129] In this example, the photonic crystal 26 comprises a central part 42 and a peripheral part 44, each part 42 or 44 bringing together a set of nanowires 14.

[0130] The central part 42 is powered and serves to emit a standing wave as explained previously.

[0131] The peripheral part 44 surrounds the central part 42 and is not powered. The nanowires 12 of this peripheral part 44 then serve as mirrors for the standing wave.

[0132] In the peripheral part 44, if we wish to reflect several wavelengths,

[0133] 11 A variable pitch can be considered, for example an increasing pitch from the end towards the central zone. This makes it possible to do without external mirrors, which can also be used in combination if this is desirable.

[0134] A set of embodiments has therefore been presented that exploit the idea of ​​using a standing wave generator formed by a photonic crystal to better excite a conversion material. Where technically feasible, these embodiments may be combined together.

[0135] In any case, it is possible to produce small 10 pixels with good conversion efficiency and therefore emitting a satisfactory quantity of light.

[0136] This can be advantageously used for many applications.

[0137] In particular, these pixels can be used for optoelectronic devices such as a display screen, a light projector or even a pair of glasses used for virtual reality immersion.

[0138] In the case of the display screen, the optoelectronic device may be integrated into an electronic device such as a mobile phone, a tablet, or a laptop. In another embodiment, the display screen is integrated into a dedicated display device such as a television set or a desktop computer monitor.

[0139] When the screen is a polychrome screen, each pixel has several pixels of different colors. These pixels can be those just described.

[0140] However, one can also consider a coexistence of pixels according to the state of the art, particularly when efficiency is not as important (for example a screen edge).

[0141] DEFINITIONS

[0142] Blue: Blue radiation has an average wavelength between 430 nm and 470 nm.

[0143] Quantum box: A quantum box is a structure in which quantum confinement occurs in all three spatial dimensions.

[0144] An example of a quantum dot is a particle P having a maximum dimension less than or equal to the product of the electronic wavelength of the charge carriers in the conversion material times five. To give an order of magnitude, a particle P having a maximum dimension between 1 nm and 200 nm and made of a semiconductor converter material is an example of a quantum dot.

[0145] Quantum dots can be selected from group II-VI, group III-V, group IV-VI semiconductor nanocrystals or a mixture thereof.

[0146] Group II-VI semiconductor nanocrystals may include, but are not limited to: CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe.

[0147] Group III-V semiconductor nanocrystals may include, but are not limited to: GaN, GaP, GaAs, AIN, AIP, AlAs, InN, InP, InAs, InGaN, GaNP, GaNAs, GaPAs, AINP, AINAs, AlPAs and InAIPAs.

[0148] Group IV-VI semiconductor nanocrystals may include, but are not limited to: SbTe, PbSe, GaSe, PbS, PbTe, SnS, SnTe, PbSnTe. Chalcopyrite-type semiconductor nanocrystals selected from the group consisting of CulnS2, CulnSe2, CuGaS2, CuGaSe2, AglnS2, AglnSe2, AgGaS2, and AgGaSe2 could also be considered.

[0149] Another example of a quantum dot is a particle P having a core and a shell surrounding the core, the core being made of a semiconductor converter material and having a maximum dimension between 1 nm and 200 nm. The core may comprise, for example, a nanocrystal such as those described above.

[0150] The shell may be made of ZnS, CdS, ZnSe, CdSe, or any mixture thereof.

[0151] Quantum dots can also be protected against oxidation by using a metal oxide protective layer, a metal nitride protective layer, an oxynitride protective layer, or a mixture of these.

[0152] A metal oxide protective layer may be selected from, but is not limited to, the group consisting of AI2O3, SiOs, TiOs, ZrOs, B2O3, C02O3, C^Os, CuO, Pe2O3, Ga2Os, HfO2, ln2O3, MgO, Nb20s, NiO, SnO2, and Ta20s.

[0153] Metal nitrides can be for example BN, AIN, GaN, InN, ZrsN4, CuZN, etc.

[0154] An oxynitride protective layer could include, but is not limited to, SiON. The thickness of the protective layer can vary from 1 to 400 nm, preferably

[0155] I at 100 nm.

[0156] The P particles are, for example, incorporated into a photoresist. Photoresists are used in many electronic manufacturing techniques to define patterns on a semiconductor surface, in particular, since specific areas of the resist can be solidified while leaving the possibility of removing other areas, in order to define the patterns. The areas to be removed or solidified are defined by insolation using a wavelength of light to which the resist is sensitive. Such a photoresist is, in particular, used to protect the covered areas against material deposition or etching.

[0157] It should be noted that the shape of the quantum dot can vary. Examples of quantum dots of different shapes can be called nanorods, nanowires, tetrapods, nanopyramids, nanocubes, etc.

[0158] It should be noted that each particle P can comprise more than one quantum dot, for example by embedding the quantum dots in a porous silica microsphere, or by aggregating several quantum dots.

[0159] Photonic crystal: A periodic structure of dielectric, semiconductor, or metallo-dielectric materials 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 authorized and forbidden energy bands. The wavelengths that can propagate in the crystal are called modes, whose energy-wave vector representation forms bands. The absence of propagating modes of electromagnetic waves in such structures, in a range of frequencies or wavelengths, is then called a forbidden band.

[0160] Band diagram: A band diagram shows the energy of the bands expressed as a function of frequency reduced as a function of the wave vector value.

[0161] Electroluminescent Light Emitting Diode (LEP): An LED structure is a semiconductor structure comprising several semiconductor regions forming a P-N junction and configured to emit light when an electric current flows through the different semiconductor regions. A two-dimensional structure comprising an n-doped layer, a p-doped layer and at least one emitting layer is an example of an LED structure. In this case, each emitting layer is interposed, along the normal direction D, between the n-doped layer and the p-doped layer.

[0162] In one embodiment, each emitting layer has a band gap value strictly less than the band gap value of the n-doped layer and strictly less than the band gap value of the p-doped layer. For example, the n-doped layer and the p-doped layer are GaN layers, and each emitting layer is an InGaN layer.

[0163] The emitting layer is, for example, undoped. In other embodiments, the emitting layer is doped.

[0164] A quantum well is a specific example of an emitting layer having a band gap value lower than the band gap values ​​of the n- and p-doped layers.

[0165] Doping: Doping is defined as the presence of impurities in a material that provide free charge carriers. Impurities are, for example, atoms of an element that is not naturally present in the material.

[0166] When impurities increase the volume density of holes in the material, compared to undoped material, the doping is p-type. For example, a layer of gallium nitride, GaN, is p-doped by adding magnesium (Mg) atoms.

[0167] When impurities increase the volume density of free electrons in the material, compared to the undoped material, the doping is n-type. For example, a layer of gallium nitride, GaN, is n-doped by adding silicon (Si) atoms.

[0168] Conversion material: The conversion material is configured to convert the first radiation emitted by the 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.

[0169] The second radiation has a second wavelength range. The second range is distinct from the first range. In particular, the second place has a second average wavelength, the second average wavelength being different from the first average wavelength. The second average wavelength is, in particular, strictly greater than the first average wavelength.

[0170] The conversion material is, for example, a semiconductor material.

[0171] For example, the conversion material is chosen from the set consisting of: CdSe, CdTe, ZnSe, ZnTe, InP, InPZnS, Ag2S, CuInS, CuInSe, AglnS2, AglnSe2, or lnPZn x To x.ySy. However, other types of materials are conceivable. 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, including other garnets, are conceivable.

[0172] In particular, the conversion material may be an inorganic phosphorus.

[0173] Examples of inorganic phosphors are yttrium-aluminum garnet-based particles (e.g., YAG:Ce), terbium-aluminum garnet-based particles, TAG, (e.g., TAG:Ce), silicate-based particles (e.g., SrBaSiO4:Eu), sulfide-based particles (e.g., SrGa2S4:Eu, SrS:Eu, CaS:Eu, etc.), nitride-based particles (e.g., Sr2Si5N8:Eu, Ba2Si5N8:Eu, etc.), oxynitride-based particles (e.g., Ca-a-SiAION:Eu, SrSi2O2N2:Eu, etc.), fluoride-based particles (e.g., K2SiF6:Mn, Na2SiF6:Mn, etc.).

[0174] Many other conversion materials can be used, such as doped aluminates, doped nitrides, doped fluorides, doped sulfides, or doped silicates.

[0175] The conversion material is, for example, doped with rare earth elements, alkaline earth metal elements or transition metal elements. Cerium is, for example, sometimes used for doping yttrium-aluminium garnets.

[0176] The conversion material comprises, for example, a set of P particles made of the conversion material. These P particles are sometimes called "luminophores".

[0177] Semiconductor material: The expression "band gap value" should be understood as the value of the band gap between the valence band and the conduction band of the material.

[0178] The band gap value is, for example, measured in electron volts (eV). The valence band is defined as, among the energy bands that are allowed for electrons in the material, the band that has the highest energy while being completely filled at a temperature less than or equal to 20 Kelvin (K).

[0179] A first energy level is defined for each valence band. The first energy level is the highest energy level in the valence band.

[0180] The conduction band is defined as, among the energy bands that are allowed for electrons in the material, the band that has the lowest energy while not being completely filled at a temperature less than or equal to 20 K.

[0181] A second energy level is defined for each conduction band. The second energy level is the highest energy level in the conduction band.

[0182] Thus, each band gap value is measured between the first energy level and the second energy level of the material.

[0183] A semiconductor material is a material having a band gap value strictly greater than zero and less than or equal to 6.5 eV.

[0184] A direct band gap semiconductor is an example of a semiconductor material. A material is considered to have a "direct band gap" when the conduction band minimum and the valence band maximum correspond to the same charge carrier momentum value. A material is considered to have an "indirect band gap" when the conduction band minimum and the valence band maximum correspond to different charge carrier momentum values.

[0185] Each semiconductor material may be chosen, for example, from the set formed by III-V semiconductors, in particular III element nitrides, II-VI semiconductors, or IV-IV semiconductors.

[0186] III-V semiconductors include InAs, GaAs, AlAs and their alloys, InP, GaP, AIP and their alloys, and III element nitrides.

[0187] II-VI semiconductors include CdTe, HgTe, CdSe, HgSe, and their alloys.

[0188] IV-IV semiconductors include Si, Ge and their alloys.

[0189] Nanowires: A nanowire is a particular example of a three-dimensional structure. A three-dimensional structure is a structure that extends along a principal direction. The three-dimensional structure has a length measured along the principal direction. The three-dimensional structure also has a maximum lateral dimension measured along a lateral direction perpendicular to the principal direction, the lateral direction being the direction perpendicular to the principal direction along which the dimension of the structure is greatest.

[0190] The maximum lateral dimension is, for example, less than or equal to 10 micrometers (pm), and the length is greater than or equal to the maximum lateral dimension. The maximum lateral dimension is advantageously less than or equal to 2.5 pm.

[0191] The maximum lateral dimension is, in particular, greater than or equal to 10 nm.

[0192] In specific embodiments, the length is greater than or equal to twice the maximum lateral dimension, for example, it is greater than or equal to five times the maximum lateral dimension.

[0193] The principal direction is, for example, the normal direction D. In this case, the length of the three-dimensional structure is called "height" and the maximum dimension of the three-dimensional structure, in a plane perpendicular to the normal direction D, is less than or equal to 10 pm.

[0194] The maximum dimension of the three-dimensional structure, in a plane perpendicular to the normal direction D, is often called "diameter" regardless of the shape of the cross-section of the three-dimensional structure.

[0195] For example, every three-dimensional structure is a microwire. A microwire is a cylindrical three-dimensional structure.

[0196] In a specific embodiment, the microwire is a cylinder extending along the normal direction D. For example, the microwire is a cylinder with a circular base. In this case, the diameter of the base of the cylinder is less than or equal to half the length of the microwire.

[0197] A microwire with a maximum lateral dimension less than 1 pm is called a "nanowire".

[0198] Another example of a three-dimensional structure is a pyramid extending along the normal direction D from the substrate.

[0199] Another example of a three-dimensional structure is a cone extending along the normal direction D.

[0200] A truncated cone or a truncated pyramid extending along the normal direction D is yet another example of a three-dimensional structure. Standing wave: A standing wave is the phenomenon resulting from the simultaneous propagation in opposite directions of several waves of the same frequency and amplitude, in the same physical medium, which forms a figure whose certain elements are fixed in time. Instead of seeing a propagating wave, we observe a stationary vibration but of different intensity, at each observed point. The characteristic fixed points are called pressure nodes.

[0201] Pixels: Many display screens contain a set of light emitters that are used to form the image displayed on the screen. These emitters each act as a picture element or "pixel" (especially when the screen is monochrome), or a portion of such a picture element, called a "sub-pixel" (especially when the screen is a color screen, each pixel having sub-pixels of a different color whose selective lighting allows the color of the pixel to be changed). In the description, the red pixel was in this sense rather a sub-pixel.

[0202] 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 effects of quantum confinement occur when the dimension of the structure along this direction becomes comparable to or smaller than the de Broglie wavelength of the carriers, which are usually electrons and / or holes, leading to energy levels called "energy subbands".

[0203] In such a quantum well, carriers can only have discrete energy values ​​but are generally able to move within a plane perpendicular to the direction in which confinement occurs. The energy values ​​available to carriers, also called "energy levels," increase as the dimensions of the quantum well decrease along the direction in which confinement occurs.

[0204] In quantum mechanics, the "de Broglie wavelength" is the wavelength of a particle when the particle is considered a wave. The de Broglie wavelength of electrons is also called the "electronic wavelength." The de Broglie wavelength of a charge carrier depends on the material of the quantum well.

[0205] An emitting layer whose thickness is strictly less than the product of the electronic wavelength of the electrons in the semiconductor material of which the emitting layer is made and five is an example of a quantum well.

[0206] Another example of a quantum well is an emitting layer whose thickness is strictly less than the product of the De Broglie wavelength of excitons in the semiconductor material of which the emitting layer is made and five. An exciton is a quasiparticle comprising an electron and a hole.

[0207] In particular, a quantum well often has a thickness between

[0208] I nm and 50 nm.

[0209] Radiation: Radiation includes a set of electromagnetic waves.

[0210] A wavelength is defined for each electromagnetic wave.

[0211] Each set corresponds to a wavelength range or spectral band. The wavelength range is the group formed by all the wavelengths in the set of electromagnetic waves.

[0212] An average wavelength for a spectral band can be defined as the average of the ends of the spectral band.

[0213] Red: Red radiation has an average wavelength between 600 nm and 720 nm.

[0214] Ultraviolet: Ultraviolet radiation has an average wavelength between 350 nm and 430 nm.

[0215] Green: Green radiation has an average wavelength between 500 nm and 560 nm.

Claims

DEMANDS 1. Light-emitting element (10) comprising: - a conversion material (14) suitable for converting a first radiation in a first spectral band into a second radiation in a second spectral band, the second spectral band being distinct from the first spectral band, and - a standing wave generator in the first spectral band, the standing wave generator comprising a two-dimensional photonic crystal (26) adapted to generate a standing wave in the first spectral band, the photonic crystal (26) being formed at least in part by light-emitting diodes (12) adapted to emit in the first spectral band.

2. Light-emitting element according to claim 1, in which the photonic crystal (26) is formed solely by light-emitting diodes (12) and a medium (14, 31) surrounding the light-emitting diodes (12).

3. Light-emitting element according to claim 1 or 2, wherein the light-emitting diodes (12) comprise an active layer (16), at least one active layer (16) being included in the photonic crystal (26).

4. Light-emitting element according to claim 3, in which the photonic crystal (26) is formed in part from each active layer (16) of the light-emitting diodes (12).

5. Light-emitting element according to any one of claims 1 to 4, wherein the photonic crystal (26) comprises the assembly of each light-emitting diode (12).

6. Light-emitting element according to any one of claims 1 to 5, wherein the light-emitting diodes (12) are in a medium, said medium being the conversion material (14).

7. Light-emitting element according to any one of claims 1 to 5, wherein the conversion material (14) rests on the photonic crystal (26).

8. Light-emitting element according to any one of claims 1 to 7, wherein the photonic crystal (26) has several band gaps, the photonic crystal (26) having a pitch (a) and a filling factor adapted to make the light-emitting diodes (12) emit on the band edge of the first 90° band gap.

9. Light-emitting element according to any one of claims 1 to 7, wherein the photonic crystal (26) has several band gaps, the photonic crystal (26) having a pitch (a) and a filling factor adapted to make the light-emitting diodes (12) emit on the band edge of a 90° band gap different from the first band gap.

10. Light-emitting element according to claim 9, wherein the pitch (a) and the filling factor of the photonic crystal (26) are also adapted to make the conversion material emit on the band edge of a 0° band gap, the band gap in which the conversion material (14) emits being less than the band gap in which the light-emitting diodes (12) emit.

11. Light-emitting element according to any one of claims 1 to 10, wherein the photonic crystal (26) is surrounded by walls (28, 30) forming a cavity, at least one of the walls (28, 30) being made of a material chosen from the list consisting of a transparent conducting oxide such as indium tin oxide or a zinc oxide doped with gallium or aluminium, a metal such as Ag or Al, graphene and a combination of these elements.

12. Light-emitting element according to any one of claims 1 to 11, in which the conversion material (14) is a polymer matrix comprising quantum dots.

13. Light-emitting element according to any one of claims 1 to 12, in which each light-emitting diode (12) comprises an active medium (16) made of a first material surrounded by layers (18) made of a second material, the first material comprising InGaN, and the second material comprising GaN.

14. Light-emitting element according to any one of claims 1 to 13, wherein the photonic crystal (26) comprises a central part (42) and a part peripheral (44), each part (42, 44) comprising a set of light-emitting diodes (12), only the central part (42) of the photonic crystal (26) being powered.

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

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