Optoelectronic device and method for manufacturing same

Abstract

The invention relates to an axial 3D LED comprising, in a stack in a longitudinal direction (z): - a lower portion (21); - an active region (22) configured to emit or receive light radiation, the active region (22) bearing on the lower portion (21); - a blocking layer (31) bearing on the active region (22); - an upper portion (23) bearing on the blocking layer (31). Advantageously, the upper portion (23) is directly in contact with the flanks of the active region (22), and no blocking layer (31) is inserted at the flanks of the active region (22). The invention also relates to a method for manufacturing such an axial 3D LED.

Description

“Optoelectronic device and its manufacturing process” TECHNICAL FIELD The present invention relates to the field of optoelectronics. It finds a particularly advantageous application in the manufacture of optoelectronic devices having a three-dimensional (3D) structure, for example light-emitting diodes based on nanowires. STATE OF THE ART Nanowire-based light-emitting diodes (LEDs) can have different architectures, particularly in terms of the arrangement of the different constituent regions of the LED. An LED typically comprises carrier injection regions (electrons and holes) between which an active region is interposed. The active region is the location where radiative recombinations of electron-hole pairs occur, which allows light emission to be obtained. This active region may notably comprise quantum wells, for example based on InGaN. The LED may also include various carrier blocking layers, for example, an electron blocking layer (EBL) between the hole injection region and the active region - and conversely, a hole blocking layer (HBL) between the electron injection region and the active region. These carrier blocking layers are intended to improve the efficiency and overall performance of the LED, especially when operating the LED at high voltages, above 6 or 7 V. These different regions and layers can be arranged in a stack along a longitudinal z direction. Such an LED architecture is called axial. An axial 3D LED typically has, in a z stack, a lower part resting on a substrate, an active region resting on the lower part, and an upper part resting on the active region. The lower part is generally intended for electron injection and the upper part for hole injection. The active region typically has quantum wells extending transversely to the longitudinal z direction. An electron blocking layer may be present between the upper part and the active region. A hole blocking layer may be present between the lower part and the active region. For axial 3D LEDs based on GaN, the longitudinal z direction is typically normal to the c {0001} planes of the hexagonal crystallographic structure of GaN. Document FR3098013B1 discloses such an axial 3D LED architecture. In practice, however, it appears that this type of architecture has limited external quantum efficiency (EQE), particularly when operating the LED at medium or high voltage. Operating the LED over a wider voltage range, varying between a low voltage, for example 1V, and a high voltage, for example 5V, can be advantageous to address a contrast issue, typically depending on the external environment (day / night / sunlight). There is therefore a need to design an axial 3D LED architecture having an improved EQE at medium or high voltage or over a wider operating voltage range. The present invention aims to meet this need and / or to at least partially overcome the drawbacks mentioned above. In particular, an object of the present invention is to provide an axial 3D light-emitting diode, typically based on GaN, having an optimized EQE at medium or high operating voltage. Another object of the present invention is to provide a method for manufacturing such a light-emitting diode. Other objects, features and advantages of the present invention will become apparent from the following description and accompanying drawings. It is understood that other advantages may be incorporated. In particular, certain features and advantages of the device may apply mutatis mutandis to the method, and vice versa. SUMMARY To achieve the above-mentioned objectives, a first aspect of the invention relates to a light-emitting diode having a hexagonal crystallographic structure and comprising, stacked in a z direction normal to the c {0001} planes of the hexagonal crystallographic structure: - a first part having a first conductivity of a first type of carriers, typically of type N, and having first sides oriented according to non-polar m {10-10} planes of the hexagonal crystallographic structure, - an active region configured to emit or receive light radiation, said active region surmounting the first part, and having so-called active flanks oriented according to said non-polar m {10-10} planes, - a second part having a second conductivity of a second type of carrier, typically of type P, said second part surmounting the active region, and covering the active sides, and possibly partly the first sides, - a blocking layer of the first type of carriers intercalated between the second part and the active region. Advantageously, the second part is directly in contact with the active flanks of the active region. Thus, unlike known axial 3D LED architectures, there is no blocking layer between the second part and the sides of the active region. This improves the injection of second-type carriers from the second part to the active region. Generally, the blocking layer is useful when operating the LED at high voltage, typically above 6 or 7 V, to prevent leakage current of the first type carriers from the active region to the second part. This blocking layer, however, reduces the injection efficiency of the second type carriers. In the development of the present invention, it has been observed that the injection of carriers is preferably done via the m-planes of the active flanks, even when a blocking layer is present on the active flanks as in known axial 3D LED architectures. The m-planes, which are non-polar planes, favor the injection of carriers of the second type compared to semi-polar type planes or polar planes. The present invention makes it possible to form a direct passage for the carriers of the second type to the active region through the non-polar m-planes. The blocking layer is retained on the active region at the semi-polar and / or polar planes. Contrary to the prejudice that the absence of a blocking layer would cause leakage of carriers of the first type, it was observed during the development of the present invention that this leakage is not significant, or even non-existent, when the blocking layer is retained at the level of the semi-polar and / or polar planes only, and is suppressed on the active flanks of the active region, oriented according to the non-polar m planes. Thus, synergistically, the presence of the blocking layer on the semi-polar and / or polar planes of the active region and the absence of the blocking layer on the non-polar planes of the active region limits the leakage of carriers of the first type and improves the injection of carriers of the second type. This is particularly advantageous when operating the LED at high voltage. The LED can operate over a wider operating voltage range. A second aspect of the invention relates to a method of manufacturing a light-emitting diode according to the first aspect. This method comprises in particular the following steps: - forming the first part by axial growth, on a substrate having an orientation such that the axial growth of the first part is oriented in the direction

[0001] of the hexagonal crystallographic structure, - form the active region by axial growth, in epitaxial relation with the first part, - form the blocking layer of the first type of carriers by axial growth, in epitaxial relation with the active region, without radial growth, then - forming the second part by axial and radial growth, in epitaxial relationship with the blocking layer, so that the second part directly covers the active flanks of the active region, and possibly partly the first flanks of the first part. BRIEF DESCRIPTION OF THE FIGURES The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of embodiments thereof which are illustrated by the following accompanying drawings in which: FIGURE 1 illustrates an axial 3D LED according to a first embodiment of the present invention. FIGURE 2 illustrates an axial 3D LED according to a second embodiment of the present invention. FIGURES 3A to 3C schematically illustrate different carrier injection mechanisms, respectively through polar, semi-polar and non-polar planes of an axial 3D LED with hexagonal crystallographic structure, according to the second embodiment of the present invention. FIGURE 4A schematically illustrates a band diagram for an injection situation via polar and / or semi-polar planes of an axial 3D LED with hexagonal crystallographic structure, according to an embodiment of the present invention. FIGURE 4B schematically illustrates a band diagram for an injection situation via non-polar planes of an axial 3D LED with a hexagonal crystallographic structure, according to an embodiment of the present invention. FIGURE 5A illustrates a modeling of carrier injection through semi-polar planes of an axial 3D LED comprising a blocking layer conventionally covering all polar, semi-polar and non-polar planes, according to the prior art. FIGURE 5B illustrates a modeling of carrier injection through non-polar planes of an axial 3D LED comprising a blocking layer covering only the polar and semi-polar planes, according to an embodiment of the present invention. FIGURE 6 illustrates efficiency curves as a function of injected current density, for an LED comprising a conventional blocking layer and for an LED comprising a blocking layer according to an embodiment of the present invention. The drawings are given by way of example and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily to the scale of practical applications. In particular, the dimensions of the different parts of the 3D LED are not necessarily representative of reality. DETAILED DESCRIPTION Before beginning a detailed review of embodiments of the invention, it is recalled that the invention according to its first aspect notably comprises the following optional characteristics which can be used in combination or alternatively: According to one example, the active region comprises a first portion oriented along an axis c

[0001] of the hexagonal crystallographic structure, and a second portion oriented along an sp axis normal to semi-polar crystallographic planes of the hexagonal crystallographic structure, for example semi-polar crystallographic planes of the {10-11} or {10-12} type, said second portion having the active flanks oriented along the m planes. The parts of the diode surmounting the first portion - also called the lower portion - typically have a faceted truncated cone shape, for example a pyramid with a truncated hexagonal base. The blocking layer covers only the polar crystallographic planes of the first portion, typically the c planes of the {0001} type, and the semi-polar crystallographic planes of the second portion. The non-polar m planes of the second portion which correspond to the active flanks are not covered by the blocking layer. According to one example, the first portion is located substantially in the center and the second portion surrounds the first portion, in projection in a direction parallel to the axis c. According to one example, the diode further comprises a first quantum barrier, for example based on GaN or InGaN, intercalated between the first part and the active region, and / or a last quantum barrier, for example based on GaN or InGaN, intercalated between the active region and the second part, said first and last quantum barriers having sides oriented according to the m planes of the hexagonal crystallographic structure. The sides of the first and last quantum barriers are typically not covered by the blocking layer. The second part is typically in direct contact with the sides of the first and last quantum barriers. The blocking layer is preferably in direct contact with the last quantum barrier. In one example, the blocking layer of the first type of carriers only covers the active region at semipolar or polar crystallographic planes of the hexagonal crystallographic structure, without covering the active region at the nonpolar m {10-10} planes of the hexagonal crystallographic structure. In one example, the blocking layer is based on an intrinsic or lightly doped semiconductor material. This limits the possibility of carrier injection through the blocking layer. Carriers pass to the active region by bypassing the blocking layer. This promotes local injection of carriers through the active edges of the active region. In one example, the first and second portions are GaN-based, the active region comprises one or more InGaN-based quantum wells, and the blocking layer is AIGaN-based. In one example, the carriers of the first type are electrons and the first conductivity is N-type, the carriers of the second type are holes and the second conductivity is P-type, and the blocking layer is an electron blocking layer also referred to as EBL (acronym for "Electron Blocking Layer"). According to one example, the blocking layer has sides oriented along the m {10-10} planes. According to one example, the sides of the blocking layer, the active sides of the active region and the first sides of the first part extend substantially directly above each other. According to one possibility, the sides of the first and last quantum barriers extend substantially directly above the first sides, the active sides and the sides of the blocking layer. According to one example, the first part, the first quantum barrier if any, the active region, the last quantum barrier if any, the blocking layer, and the second part are in epitaxial relationship to each other. In one example, the axial growth formation of the blocking layer is performed by plasma-assisted molecular beam epitaxy having a nitrogen precursor flow directed along the direction

[0001] This promotes axial growth of the blocking layer. This may prevent radial growth of the blocking layer. According to one example, the blocking layer is based on AIGaN and the formation of the blocking layer is configured so that the blocking layer has an average aluminum content [Al] of less than or equal to 30% at and preferably less than or equal to 15% at. This promotes axial growth of the blocking layer. This limits or prevents, in combination with suitably chosen operating parameters, radial growth of the blocking layer. Such a blocking layer has satisfactory blocking properties. According to one example, the at least one light-emitting diode comprises a plurality of light-emitting diodes, and the formation of the first portions is such that two adjacent first portions are separated from each other by a separation distance ds of less than 180 nm, preferably less than or equal to 100 nm. The first portions of the diodes are thus distributed on the substrate with a high surface density. This promotes axial growth of the portions surmounting each first portion, in particular blocking layers. This limits or prevents radial growth of the blocking layers. In one example, the axial growth formation of the blocking layer is carried out by metal organic vapor phase epitaxy (MOVPE). In one example, the first part is supported on the substrate through a masking layer. Unless inconsistent, technical features described in detail for a given embodiment may be combined with technical features described in the context of other embodiments described by way of example and not limitation, so as to form another embodiment which is not necessarily illustrated or described. Such an embodiment is obviously not excluded from the invention. In the present invention, the method is in particular dedicated to the manufacture of light-emitting diodes (LEDs) with a 3D structure. The invention can be implemented more broadly for various optoelectronic devices with a 3D structure. The invention can therefore also be implemented in the context of laser or photovoltaic devices. Unless explicitly stated, it is specified that, in the context of the present invention, the relative arrangement of a third layer intercalated between a first layer and a second layer does not necessarily mean that the layers are in direct contact with each other, but means that the third layer is either directly in contact with the first and second layers, or separated from them by at least one other layer or at least one other element. Thus, the terms and phrases “to bear” and “to cover” or “to cover” do not necessarily mean “in contact with”. Typically, the second part bears on the active region via the electron blocking layer, which is intercalated between these two axial parts. The active region may bear on the first part via the first quantum barrier, which is intercalated between these two axial parts. The electroluminescence efficiency of diodes can be standardly evaluated by the "wall-plug efficiency" WPE (acronym for "wall-plug efficiency"). The efficiency WPE is defined as the power ratio between the optical power P op t coming out of the LED and the electrical power Peiec supplied to the LED: WPE = = EQE x ?? where ?? describes the energy of the emitted photon and EQE represents the external quantum efficiency. The external quantum efficiency can be decomposed into three components: - The injection efficiency (IE) of the carriers in the active region, - The internal quantum efficiency (IQE) which is the ratio between the number of radiative recombinations and the total number of recombinations, - The light extraction efficiency (LEE) which corresponds to the proportion of photons leaving the LED compared to the number of photons generated. The electroluminescence efficiency of diodes can also be evaluated by numerical simulations from TCAD modeling (acronym for “Technology Computer-Aided Design”). The term "3D structure" is understood in contrast to so-called planar or 2D structures, which have two dimensions in a plane much greater than the third dimension normal to the plane. Thus, the usual 3D structures targeted in the field of 3D LEDs can be in the form of a wire, a nanowire or a microwire. Such a 3D structure has an elongated shape in the longitudinal direction. The longitudinal dimension of the wire, along z in the figures, is greater, and preferably much greater, than the transverse dimensions of the wire, in the xy plane in the figures. The longitudinal dimension is for example at least five times, and preferably at least ten times, greater than the transverse dimensions. The 3D structures can also be in the form of walls. In this case, only one transverse dimension of the wall is much smaller than the other dimensions, for example at least five times, and preferably at least ten times, smaller than the other dimensions.The 3D structures of the present application preferably have substantially vertical sides or walls. The vertical walls extend. typically along m-type crystallographic planes. The 3D structures of the present application preferably have bases and vertices comprising substantially horizontal surfaces, capable of forming axial portions by axial growth. These horizontal surfaces typically extend along c-type crystallographic planes. According to the Miller-Bravais system, we will note (hkil) a plane of the hexagonal structure, {hkil} a family of planes of the hexagonal structure, [hkil] a direction or a vector of the hexagonal structure. In this patent application, the terms "light-emitting diode", "LED" or simply "diode" are used synonymously. An "LED" can also be understood as a "micro-LED". The steps of the method as claimed are understood in the broad sense and may optionally be carried out in several sub-steps. "Axial growth" means anisotropic growth occurring mainly or even only in the longitudinal direction z. “Radial growth” means isotropic growth covering in particular surfaces parallel to the longitudinal direction z. In the following, the following abbreviations relating to a material M are possibly used: aM refers to the material M in amorphous form, according to the terminology usually used in the field of microelectronics for the prefix a-, pM refers to the material M in polycrystalline form, according to the terminology usually used in the field of microelectronics for the prefix p-. Similarly, the following abbreviations relating to a material M are possibly used: Mi refers to the intrinsic or unintentionally doped material M, according to the terminology usually used in the field of microelectronics for the suffix -i. Mn refers to the material M doped N, N+ or N++, according to the terminology usually used in the field of microelectronics for the suffix -n. Mp refers to the material M doped P, P+ or P++, according to the terminology usually used in the field of microelectronics for the suffix -p. A "low-doped semiconductor material" means a semiconductor material whose doping concentration is less than 1*10E17 electrons / cm 3 . A substrate, a layer, a device, “based” on a material M, means a substrate, a layer, a device comprising this material M only or this material M and possibly other materials, for example alloying elements, impurities or doping elements. A reference frame, preferably orthonormal, comprising the x, y, z axes is shown in certain appended figures. This reference frame can be applied by extension to the other appended figures. The z axis is here parallel to the c axis, that is to say to the crystallographic direction

[0001] , In this patent application, we will preferably speak of thickness for a layer and of height for a structure or device. The thickness is taken in a direction normal to the main extension plane of the layer, and the height is taken perpendicular to the plane basal xy of the substrate. Thus, a layer typically has a thickness along z, when it extends mainly along an xy plane, and an LED has a height along z. The relative terms "on", "under", "subjacent" refer to positions taken along the z direction. A region, part or layer formed by axial growth typically has substantially the same diameter, taken in an xy plane, as the region, layer or part on which it rests. Dimensional values ​​are understood to be within manufacturing and measurement tolerances. The terms "substantially", "approximately", "of the order of" mean, when they refer to a value, "within 10%" of that value or, when they refer to an angular orientation, "within 10°" of that orientation. Thus, a direction substantially normal to a plane means a direction presenting an angle of 90±10° with respect to the plane. A first embodiment of an axial 3D LED according to the invention is illustrated in Fig. 1. The LED is here in the form of a nanowire directed along z. The LED 1a comprises successively, starting from the bottom of Fig. 1, a first part 21 based on GaN-n, a first quantum barrier 41 based on GaN-i or InGaN-i, an active region 22 based on InGaN-i, a last quantum barrier 42 based on GaN-i or InGaN-i, an electron blocking layer 31 based on AIGaN-p, and a second part 23 based on GaN-p. The different parts, layers and regions of the LED 1a typically have a hexagonal crystallographic structure. The cross-section of the nanowire, in the xy plane, can thus have a hexagonal shape. The first part 21, or lower part 21, may have a height of a few tens to a few hundreds of nanometers, for example 50 nm to 5000 nm. It may have a characteristic diameter or dimension in the xy plane of between 20 nm and 500 nm. The lower part 21 typically has a substantially horizontal upper face 211, formed by a polar plane c (0001). It typically has first flanks 210, simply called flanks 210 in the following, substantially vertical, formed by non-polar planes m {10-10}. The flanks 210 typically extend along the six sides of the hexagonal shape. The lower part 21 typically rests on a substrate, for example based on silicon, comprising a nucleation layer (not shown). The first quantum barrier 41 is only optional. It can be provided between the lower part 21 and the active region 22 to optimize the band gap engineering between the semiconductor materials of the lower part 21 and the active region 22. The first quantum barrier 41 typically has a substantially horizontal upper face, formed by a polar plane c (0001). It typically has substantially vertical sides 410, formed by non-polar planes m {10-10}. The sides 410 typically extend along the six sides of the hexagonal shape, in extension of the sides 210. The active region 22 may have substantially the same diameter or the same characteristic dimension in the xy plane as the horizontal face on which it rests. The active region 22 may have a height of a few tens of nanometers to a few hundred nanometers, for example 5 nm to 300 nm, preferably 5 nm to 100 nm. It may be based on bulk InGaN. It may alternatively comprise InGaN quantum wells. The region active 22 typically has a substantially horizontal upper face 221, formed by a polar plane c (0001). It typically has flanks 220, also called active flanks 220, substantially vertical, formed by non-polar planes m {10-10}. The active flanks 220 typically extend along the six sides of the hexagonal shape, in extension of the flanks 410. The last quantum barrier 42 is only optional. It can be provided between the active region 22 and the electron blocking layer 31 to optimize the band gap engineering between the semiconductor materials of the active region 22 and the electron blocking layer 31. The last quantum barrier 42 typically has a substantially horizontal upper face, formed by a polar plane c (0001). It typically has substantially vertical sides 420, formed by non-polar planes m {10-10}. The sides 420 typically extend along the six sides of the hexagonal shape, in extension of the sides 220. The electron blocking layer 31 may be based on an aluminum alloy, for example based on AIGaN. Its function is to prevent electron leakage from the active region 22 to the second part 23. This makes it possible to confine radiative recombinations within the active region 22. This makes it possible to increase the external quantum efficiency of the LED. The electron blocking layer 31 typically has a substantially horizontal upper face 311, formed by a polar plane c (0001). It typically has substantially vertical sides 310, formed by non-polar planes m {10-10}. The flanks 310 typically extend along the six sides of the hexagonal shape, in extension of the flanks 420. The electron blocking layer 31 may have a height of a few tens of nanometers to a few hundred nanometers, typically between 20 nm and 300 nm, for example of the order of 100 nm.The AIGaN-based electron blocking layer 31 also limits the diffusion of P-type doping into layers 42, 22 and 41. According to a preferred possibility, the flanks 310 of the electron blocking layer 31, the flanks 420 of the last quantum barrier 42, the active flanks 220, the flanks 410 of the first quantum barrier 41 and the flanks 210 of the lower part 21 are directly above each other. The second part 23, or upper part 23, may have a height of a few tens to a few hundreds of nanometers, for example 50 nm to 500 nm. The upper part 23 typically covers the upper face 311 and the flanks 310 of the electron blocking layer 31, the flanks 420 of the last quantum barrier 42, the active flanks 220, the flanks 410 of the first quantum barrier 41. It is in particular directly in contact with the active flanks 220. It may partially cover the flanks 210 of the lower part 21. The upper part 23 thus forms a cap covering the layers stacked on the lower part 21. The upper portion 23 typically comprises a central portion 23a directly above the upper face 311 of the electron blocking layer 31, and a lateral portion 23b surrounding the central portion 23a and descending on the sides 310, 420, 220, 410, 210 of the nanowire.The lateral portion 23b may have a thickness along a direction of the xy plane, for example along x, very. less than the thickness along z of the central portion 23a. The lateral portion 23b may have a thickness along x of the order of a few nanometers to a few tens of nanometers. The central portion 23a may have a thickness along z of the order of a few tens of nanometers to a few hundred nanometers. A second embodiment of an axial 3D LED according to the invention is illustrated in Fig. 2. As previously, the LED 1 b successively comprises, starting from the bottom of Fig. 2, the first part 21 based on GaN-n, the first quantum barrier 41 based on GaN-i or InGaN-i, the active region 22 based on InGaN-i, the last quantum barrier 42 based on GaN-i or InGaN-i, the electron blocking layer 31, and the second part 23 based on GaN-p. According to this second embodiment, the morphology of the different parts, layers and regions of the LED 1 b differs from that of the first embodiment. In particular, the first quantum barrier 41 is in the form of a truncated pyramid with a hexagonal base. The first quantum barrier 41 comprises a central portion having a substantially horizontal upper face, formed by a polar plane c (0001), and a lateral portion having inclined sides, formed by semi-polar planes sp, for example of type {10-11}, {11 -22} or {10-12}. The active region 22 is in the form of a layer covering the inclined sides and the upper face of the first quantum barrier 41. It comprises a central portion 22a having a substantially horizontal upper face 221, formed by a polar plane c (0001), and a lateral portion 22b having inclined faces 222, formed by semi-polar planes sp, for example of the {10-11}, {11-22} or {10-12} type. The central portion 22a is located directly above the central portion of the first quantum barrier 41. The lateral portion 22b is located directly above the lateral portion of the first quantum barrier 41. The active flanks 220 formed by non-polar planes m border the lateral portion 22b. The last quantum barrier 42 is in the form of a layer covering the inclined faces 222 and the upper face 221 of the active region 22. It comprises a central portion having a substantially horizontal upper face, formed by a polar plane c (0001), and a lateral portion having inclined faces, formed by semi-polar planes sp, for example of the {10-11}, {11-22} or {10-12} type. The central portion is located directly above the central portion 22a of the active region 22. The lateral portion is located directly above the lateral portion 22b of the active region 22. The flanks 420 formed by non-polar planes m border the lateral portion of the last quantum barrier 42. The electron blocking layer 31 is in the form of a layer covering the inclined faces and the upper face of the last quantum barrier 42. It comprises a central portion 31 a having a substantially horizontal upper face 311, formed by a polar plane c (0001), and a lateral portion 31 b having inclined faces 312, formed by semi-polar planes sp, for example of the {10-11}, {11-22} or {10-12} type. The central portion 31 a is located directly above the central portion of the last quantum barrier 42. The lateral portion 31 b is located directly above the lateral portion of the last quantum barrier 42. The flanks 310 formed by non-polar planes m border the lateral portion 31 b. As previously, the upper part 23 forms a cap covering the layers stacked on the lower part 21. The upper part 23 here comprises a central portion 23a directly above the faces 311, 312 of the electron blocking layer 31, and a lateral portion 23b surrounding the central portion 23a and descending on the sides 310, 420, 220, 410, 210 of the nanowire. It is in particular directly in contact with the active sides 220. Each of the axial portions 21, 41, 22, 42, 31 of the LED can be formed by plasma-assisted molecular beam epitaxy (MBE). Such epitaxy of III / V material requires a supply of precursor V (nitrogen precursor) and precursor III (In, Ga, Al, etc.). The nitrogen precursor is typically formed by dissociation of nitrogen N2 within the plasma. As is known, the pressure in an MBE chamber is very low, typically less than 10 -2Pa. Since the pressure is very low, the nitrogen precursor flow from such an N2 plasma source arrives at the substrate in a very directional manner, typically along the longitudinal z direction. Growth then takes place essentially axially. The axial parts formed by MBE advantageously have a high crystalline quality. The axial parts formed by MBE can also have very homogeneous doping. An ammonia-based nitrogen source (NH3) can alternatively be used for MBE. As is known, the parameters for forming the different axial parts of the LED by axial growth are adjusted according to the composition, doping and desired thickness for each of the axial parts. The top portion 23 of the LED may also be formed by plasma-assisted MBE, typically by changing the plasma density and / or polarization conditions to achieve radial growth. The radial growth forms the lateral portion 23b of the top portion 23. The radial growth formation parameters are adjusted based on the composition and desired thickness of the lateral portion 23b. Alternatively, the top portion 23 of the LED may be formed by metal-organic precursor vapor phase epitaxy (MOVPE) from an NH3 gas source. In general, a plurality of LEDs in the form of nanowires are formed on the same substrate, during the same growth cycle. According to one possibility, the distribution of nanowires on the substrate has a relatively high surface density, typically greater than or equal to 4 pm -2. The formation of nanowires densely distributed on the substrate makes it possible to promote axial growth and to limit or prevent radial growth. In particular, the lower portions 21 may be formed through a masking layer comprising openings regularly distributed in the form of a network having a pitch less than or equal to 700 nm. The formation of the lower portions 21 is preferably configured so that the tops of the lower portions are separated from each other by a separation distance ds less than 180 nm, preferably less than 150 nm, and preferably less than or equal to 100 nm. This makes it possible to limit or prevent radial growth of the axial portions 21, 41, 22, 42, 31 of the LEDs 1 a, 1 b. To favor the axial growth of the AIGaN-based electron blocking layer 31 at the expense of radial growth, it is also possible to decrease the average rate of aluminum. During the epitaxy of the electron blocking layer 31, the supply of elements III is preferably reduced. According to one possibility, the proportion of organometallic precursors in MOVPE is chosen so as to form an electron blocking layer 31 in AIGaN having an average aluminum content [Al] less than or equal to 30% at and preferably less than or equal to 15% at. According to another possibility, the atomic fluxes of the elements Al and / or Ga in MBE are chosen so as to form an electron blocking layer 31 in AIGaN having an average aluminum content [Al] less than or equal to 30% at and preferably less than or equal to 15% at. Figures 3A, 3B and 3C schematically illustrate different carrier injection mechanisms, respectively through polar, semi-polar and non-polar planes of an axial 3D LED 1 b. Figure 3A illustrates the situation where a hole populating the valence band of the second part 23 is injected into the active region 22 by crossing the electron blocking layer 31 at a polar c-plane, typically at the face 311. Figure 3B illustrates the situation where a hole populating the valence band of the second part 23 is injected into the active region 22 by crossing the electron blocking layer 31 at a semi-polar sp plane, typically at an inclined face 312. Figure 3C illustrates the situation where a hole populating the valence band of the second part 23 is injected into the active region 22 by bypassing the electron blocking layer 31, passing through the lateral portion 23b and passing at a non-polar m plane of the active region 22, typically at an active flank 220. Figure 4A schematically illustrates the band diagram of the LED in injection situations via the polar c or semipolar sp planes of the electron blocking layer 31. A hole in the valence band (VB) of the second part 23 must in this case cross a potential barrier <t>Bi to reach the active region 22. The potential barrier <t>Bi increases when the electron blocking layer 31 is not intentionally doped or lightly doped, for example when it is based on GaN-i. Figure 4B schematically illustrates the band diagram of the LED in injection situations via the non-polar m-planes of the active region 22. A hole in the valence band (VB) of the second part 23 must in this case cross a potential barrier <t>B2 to reach the active region 22. The electron blocking layer 31, which does not cover the non-polar m-planes of the active region 22, does not contribute to this potential barrier <t>B2. It appears that the potential barrier <t>B2 is significantly smaller than the potential barrier <t>Bi , typically at least two times smaller, or even five times smaller. Hole injection in injection situations via the non-polar m-planes of the active region 22 is largely favored. These injection situations are particularly advantageous when the electron blocking layer 31 is not intentionally doped or lightly doped, typically when it is based on GaN-i. Figure 5A corresponds to a modeling of carrier injection through semi-polar planes of an axial 3D LED comprising a blocking layer 31 conventionally covering all the polar, semi-polar and non-polar planes of an axial 3D LED. It appears that the carriers cross in particular the lateral portion 31b of the electron blocking layer, before reaching the edges of the central portion 22a of the active region, in which they recombine. Figure 5B corresponds to a modeling of carrier injection in an axial 3D LED 1 b comprising a blocking layer covering only the polar and semi-polar planes of the underlying LQB layer. The flanks of the active region are here directly in contact with the lateral portion 23 b. It appears that the carriers bypass the blocking layer 31 and pass through non-polar m planes of the active region, at the level of the active flanks of the active region. The carriers rise along the semi-polar planes of the lateral portion 22 b of the active region and recombine in the central portion 22 a of the active region. The carrier injection mechanisms are therefore very different for a conventional axial 3D LED structure as illustrated in Figure 5A, and for an axial 3D LED structure 1 b according to the invention. Figure 6 shows, for comparison, the WPE efficiency curve Cref as a function of the injected current density for a conventional axial 3D LED, and the WPE efficiency curve Cm as a function of the injected current density for an axial 3D LED according to the invention. The maximum efficiency is obtained for a higher current density in the case of the inventive axial 3D LED (Cm), than in the case of the conventional axial 3D LED (Cref). In particular, the following EQE values ​​are derived from the Cm and Cref curves: - EQE = 27% at 0.5 A / cm 2 for the classic axial 3D LED (Cref), - EQE = 28% at 2 A / cm 2 for the axial 3D LED according to the invention (Cm). Furthermore, the efficiency is maintained over a larger current density range in the case of the inventive axial 3D LED (Cm), compared to the case of the conventional axial 3D LED (Cref) for which the efficiency drops at high current density. In particular, the following EQE values ​​are derived from the Cm and Cref curves: - EQE = 10% at 10 A / cm 2 for the classic axial 3D LED (Cref), - EQE = 26% at 10 A / cm 2 for the axial 3D LED according to the invention (Cm), and respectively: - EQE = 2.5% at 100 A / cm 2 for the classic axial 3D LED (Cref), - EQE = 15% at 100 A / cm 2 for the axial 3D LED according to the invention (Cm). It appears that the axial 3D LED according to the invention is more efficient than the conventional axial 3D LED, in particular for high injection current densities. This is particularly advantageous for LED technologies requiring high injection current densities or operating at medium or high voltage. The invention is not limited to the embodiments previously described.< / t> < / t> < / t> < / t> < / t> < / t>

Claims

Claims 1. Light-emitting diode (1 a, 1 b) having a hexagonal crystallographic structure and comprising, stacked in a direction (z) normal to the planes c {0001} of the hexagonal crystallographic structure: - a first part (21) having a first conductivity of a first type of carriers, and having first sides (210) oriented according to non-polar m {10-10} planes of the hexagonal crystallographic structure, - an active region (22) configured to emit or receive light radiation, said active region (22) surmounting the first part (21), and having so-called active flanks (220) oriented according to said non-polar m {10-10} planes, - a second part (23, 23a, 23b) having a second conductivity of a second type of carriers, said second part (23) surmounting the active region (22), and covering the active flanks (220), - a blocking layer (31) of the first type of carriers interposed between the second part (23) and the active region (22), said light-emitting diode (1 a, 1 b) being characterized in that the second part (23, 23b) is directly in contact with the active sides (220) of the active region (22).

2. Diode (1 b) according to the preceding claim, in which the active region (22) comprises a first portion (22a) oriented along a c [0001] axis of the hexagonal crystallographic structure, and a second portion (22b) oriented along an sp axis normal to semi-polar crystallographic planes of the hexagonal crystallographic structure, for example of the {10-11} or {10-12} type, said second portion (22b) having the active flanks (220) oriented along the m planes.

3. Diode according to any one of the preceding claims further comprising a first quantum barrier (41), for example based on GaN or InGaN, intercalated between the first part (21) and the active region (22), and / or a last quantum barrier (42), for example based on GaN or InGaN, intercalated between the active region (22) and the second part (23), said first and last quantum barriers (41, 42) having sides (410, 420) oriented according to the m planes of the hexagonal crystallographic structure.

4. Diode according to the preceding claim, in which the second part (23, 23b) is directly in contact with the sides (410, 420) of the first and last quantum barriers (41, 42).

5. Diode according to any one of the preceding claims wherein the blocking layer (31) of the first type of carriers only covers the active region (22) at semi-polar or polar crystallographic planes of the hexagonal crystallographic structure, without covering the active region (22) at the non-polar m {10-10} planes of the hexagonal crystallographic structure.

6. Diode according to any one of the preceding claims in which the blocking layer (31) is based on an intrinsic or lightly doped semiconductor material.

7. Diode according to any one of the preceding claims in which the first and second parts (21, 23) are based on GaN, the active region (22) comprises one or more quantum wells based on InGaN, and the blocking layer (31) is based on AIGaN.

8. A diode according to any preceding claim wherein the carriers of the first type are electrons and the first conductivity is N-type, the carriers of the second type are holes and the second conductivity is P-type, and the blocking layer (31) is an electron blocking layer.

9. Diode according to any one of the preceding claims in which the blocking layer (31) has flanks (310) oriented according to the planes m {10-10}, and in which said flanks (310) of the blocking layer (31), the active flanks (220) of the active region (22) and the first flanks (210) of the first part (21) extend substantially in line with each other.

10. Method for producing at least one light-emitting diode (1 a, 1 b) according to any one of the preceding claims, comprising: • form the first part (21) by axial growth, on a substrate having an orientation such that the axial growth of the first part (21) is oriented along the [0001] direction of the hexagonal crystallographic structure, • form the active region (22) by axial growth, in epitaxial relation with the first part (21), • form the blocking layer (31) of the first type of carriers by axial growth, in epitaxial relation with the active region (22), without radial growth, then • form the second part (23) by axial and radial growth, in epitaxial relationship with the blocking layer (31), so that the second part (23) directly covers the active flanks (220) of the active region (22).

11. Method according to the preceding claim, in which the formation by axial growth of the blocking layer (31) is carried out by plasma-assisted molecular beam epitaxy having a flow of nitrogen precursor directed in the [0001] direction, 12. Method according to any one of claims 10 to 11, in which the blocking layer (31) is based on AIGaN and the formation of the blocking layer (31) is configured so that the blocking layer (31) has an average aluminum content [Al] less than or equal to 30% at and preferably less than or equal to 15% at.

13. Method according to any one of claims 10 to 12, wherein the at least one light-emitting diode (1 a, 1 b) comprises a plurality of light-emitting diodes, and wherein the formation of the first parts (21) is such that two adjacent first parts (21) are separated from each other by a separation distance c / s of less than 180 nm, preferably less than or equal to 100 nm.

14. Method according to the preceding claim, in which the formation by axial growth of the blocking layer (31) is carried out by organometallic precursor vapor phase epitaxy (MOVPE).

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