METHOD FOR PRODUCING A GROWTH SUBSTRATE WITH DIFFERENT ELECTROCHEMICAL POROSIFICATION LEVELS WITH MESAS WITH DIFFERENT DEGREES OF PORENISATION

Description

DOMAINE TECHNIQUE

[0001] The field of the invention is that of methods for manufacturing a growth substrate comprising mesas allowing the production of a matrix of diodes adapted to emit or detect, natively, light radiation at different wavelengths. ÉTAT DE LA TECHNIQUE ANTÉRIEURE

[0002] There are methods for manufacturing a matrix of light-emitting diodes adapted to emit, natively, light radiation at different wavelengths. The matrix of diodes can then comprise diodes adapted to emit red light, other diodes green light, and finally others blue light. Such a matrix of diodes then forms a micro-screen with native RGB emission (for Red, Green, Blue, in English).

[0003] The diodes are said to be native emitting, in the sense that the active area of ​​each diode emitting at a given wavelength differs from the active areas of diodes emitting at another wavelength. In the case of diodes made from InGaN, the active areas differ from each other by the proportion of indium in the quantum wells.

[0004] Such a matrix of native emitting diodes is thus distinguished from color conversion technologies where the diodes all emit at the same wavelength, for example in blue, and are each coated with a pad comprising phosphors, for example semiconductor nanocrystals forming quantum dots, to convert at least part of the incident light into light of another wavelength.

[0005] One approach to fabricating a native emitting diode array is to use a growth substrate with mesas that have been partially rendered porous during an electrochemical porosification step. This electrochemical porosification technique is presented in the article by Griffin and Oliver entitled Porous nitride semiconductors reviewed, J. Phys. D: Appl. Phys. 53 (2020) 383002.

[0006] Document EP3840065A1 describes an example of a method for manufacturing a growth substrate and then a diode array using the electrochemical porosification technique. The method comprises producing a growth substrate (also called a pseudo-substrate) having several InGaN mesas, each formed from a doped portion of InGaN made porous during an electrochemical porosification step, and an epitaxial recovery portion of InGaN that is not intentionally doped or lightly doped so that it is not porosified (it remains intact or dense, therefore non-porous). The diodes are then produced by epitaxy from the epitaxial recovery portions.

[0007] Document DE 10 2020 128679 A1 discloses a method of manufacturing a substrate with a porous domain.

[0008] The doped portions of the mesas can have different doping levels from one mesa to another, so that the mesas have different porosification levels and therefore different relaxation rates. The diodes made from the different mesas then include quantum wells with a greater or lesser proportion of indium, thus making it possible to obtain pixels emitting at different wavelengths.

[0009] However, to obtain mesas whose InGaN-doped layers have different doping levels from one mesa to another, it is necessary to carry out, before the mesa production step, a spatially localized implantation step of dopants in a full-plate InGaN layer, which complicates the manufacturing process. EXPOSÉ DE L'INVENTION

[0010] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose a method for manufacturing a growth substrate, suitable for producing an InGaN-based diode matrix, comprising mesas having different levels of porosification, without it being necessary to carry out spatially localized implantations of dopants.

[0011] For this, the subject of the invention is a method for manufacturing a growth substrate suitable for the production by epitaxy of a matrix of diodes based on InGaN, comprising the following steps: o providing a GaN-based crystalline stack, comprising: a doped lower layer; and an upper layer made from In x2 Ga 1-x2 N with x2>0, resting on and in contact with the doped lower layer; where: the doped lower layer has a predefined proportion of indium x1≥0 such that it is non-absorbent of illuminating light radiation having a predefined central wavelength, and the upper layer has a predefined proportion of indium x2>0 such that it is absorbent of illuminating light radiation; the doped lower layer has a predefined doping level such that it is porosifiable at an electrical voltage of predefined value V ECE, and the upper layer has a predefined doping level such that it is non-porosifiable at the predefined electrical voltage V ECE;o production of mesas M (i), with i ranging from 0 to 2, by localized etching of the upper layer to form upper portions of mesas and of at least a portion of the doped lower layer to form lower portions of mesas; then o deposition of an encapsulation layer made of an electrically insulating material, covering at least one mesa denoted M (0), so that said mesa M (0) is not porosified during an electrochemical porosification of mesas denoted M (1) and M (2); then o electrochemical porosification of the lower and upper portions of at least one mesa M (2) by: photo-assisted porosification of the upper portion of the mesa M (2) by emission of illuminating light radiation; and non-photo-assisted porosification of the lower portion of the mesa M (2) by application of the electrical voltage of value V ECE;o electrochemical porosification of only one of the lower and upper portions of at least one mesa M (1) by: photo-assisted porosification of the upper portion of the mesa M (1) during that of the upper portion of the mesa M (2); or non-photo-assisted porosification of the lower portion of the mesa M (1) during that of the lower portion of the mesa M (2); o removal at least in part of the encapsulation layer, so as to free an upper surface of the mesa M (0). ;

[0012] Some preferred but non-limiting aspects of this manufacturing method are as follows.

[0013] Photo-assisted porosification and non-photo-assisted porosification can be performed in separate steps.

[0014] The method may comprise the following distinct steps: a step of photo-assisted porosification of the upper portions of the mesas M (1) and M (2), the mesas M (1) and M (2) being in contact with a liquid electrolyte; and a step of non-photo-assisted porosification of the lower portion of the mesas M (2), the mesas M (1) being covered with an encapsulation layer made of an electrically insulating material, only the mesas M (2) being in contact with a liquid electrolyte.

[0015] Alternatively, the method may comprise the following distinct steps: a step of photo-assisted porosification of the upper portion of the mesas M (2), the mesas M (1) being covered with an encapsulation layer made of an electrically insulating material, only the mesas M (2) being in contact with a liquid electrolyte; a step of non-photo-assisted porosification of the lower portion of the mesas M (1) and M (2), the mesas M (1) and M (2) being in contact with a liquid electrolyte.

[0016] Alternatively, the photo-assisted porosification and the non-photo-assisted porosification can be carried out during the same step, an opaque encapsulation layer, made of a material opaque to the illuminating light radiation, at least partially covering an upper surface of the mesas M (1) while leaving free a lateral surface of the lower portion of the mesas M (1).

[0017] The crystalline stack may comprise a continuous epitaxial recovery layer, resting on the upper layer, and made from InGaN having a predefined proportion of indium x3>0 so that it is non-absorbent of the illumination light radiation, and having a predefined doping level so that it is non-porous at the predefined electrical voltage V ECE.

[0018] The crystalline stack may not include a continuous epitaxial recovery layer, so that the upper layer has a free upper face, epitaxial recovery portions made from InGaN being made from an upper surface of the upper portions of the mesas M (1) after the porosification of the mesas M (1) and M (2).

[0019] In photo-assisted porosification, an electrical voltage can be applied between the growth substrate and a liquid electrolyte in which it is immersed, of a predefined value V PECE so that the lower portions of the mesas M (1) are not porosified.

[0020] The invention also relates to a method of manufacturing an optoelectronic device, comprising the following steps: o manufacturing the growth substrate by the method according to any one of the preceding characteristics; then o manufacturing a matrix of diodes D (i) by epitaxy from the mesas M (i) of the growth substrate, the diodes then being adapted to emit or detect light radiation at different wavelengths, the wavelength being different from one category of mesas M (i) to another.

[0021] The invention also relates to a growth substrate, suitable for the production by epitaxy of a matrix of diodes based on InGaN, comprising: o a support layer; o mesas M (i), with i ranging from 0 to 2, made from crystalline GaN, resting on the support layer, and each comprising: a doped lower portion having a proportion of indium x1≥0; and an upper portion made from In x2 Ga 1-x2 N with x2>x1 and having a doping level lower than that of the lower portion, and resting on and in contact with the lower portion; the mesas being configured so that: the lower and upper portions of the mesa M (0) are non-porous; only one of the lower and upper portions of the mesa M (1) is porous; the lower and upper portions of the mesa M (2) are porous

[0022] Each mesa M (i) may comprise an epitaxial recovery portion resting on the upper portion, made from non-porous crystalline InGaN whose lattice parameter am< cre of the relaxed material is greater than the effective lattice parameter ae< cs of the support layer: the epitaxial recovery portion of each mesa M (2) having a maximum lattice parameter ae< cre(2); the epitaxial recovery portion of each mesa M (0) having a lattice parameter ae< cre(0) less than ae< cre(2); the epitaxial recovery portion of each mesa M (1) having an intermediate lattice parameter ae< cre(1) less than ae< cre(2) and different from ae< cre(0).

[0023] The lower portions of the mesas M(i) may rest on a continuous layer made of an electrically conductive material.

[0024] The invention also relates to an optoelectronic device (1) comprising: a growth substrate according to any one of the preceding characteristics; and an array of diodes D (i) based on InGaN, epitaxially grown from the mesas of the growth substrate, the diodes being adapted to emit or detect light radiation at different wavelengths, the wavelength being different from one category of mesas M (i) to another. BRÈVE DESCRIPTION DES DESSINS

[0025] Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which: THE figures 1A à 1C are schematic and partial cross-sectional views of a growth substrate comprising mesas having different levels of porosification, according to different embodiment variants, as well as a matrix of InGaN-based diodes epitaxially grown from the growth substrate; figure 2 illustrates an example of the relationship between the doping level of a doped crystalline layer as a function of the applied electrical voltage, highlighting the domain of existence of electrochemical porosification; figures 3A à 3E illustrate different steps of a process for manufacturing a growth substrate similar to that of the fig.1A , then a diode matrix, comprising a step of photo-assisted electrochemical porosification (PECE) of the upper portions followed by a step of non-photo-assisted electrochemical porosification (ECE) of the lower portions; figures 4A à 4E illustrate different steps of a process for manufacturing a growth substrate similar to that of the fig.1B , then a diode matrix, also comprising a PECE photo-assisted electrochemical porosification step followed by an ECE non-photo-assisted electrochemical porosification step; figures 5A à 5D illustrate different steps of a process for manufacturing a growth substrate similar to that of the fig.1B , then a matrix of diodes, in which the lower portions and the upper portions are porosified concomitantly; the figures 6A à 6D illustrate different steps of a process for manufacturing a growth substrate similar to that of the fig.1C , then a matrix of diodes, in which the lower portions and the upper portions are porosified concomitantly. EXPOSÉ DÉTAILLÉ DE MODES DE RÉALISATION PARTICULIERS

[0026] In the figures and in the remainder of the description, the same references represent identical or similar elements. In addition, the different elements are not shown to scale so as to enhance the clarity of the figures. Furthermore, the different embodiments and variants are not mutually exclusive and may be combined with each other. Unless otherwise indicated, the terms "substantially", "approximately", "of the order of" mean to within 10%, and preferably to within 5%. Furthermore, the terms "between ... and ..." and equivalent mean that the limits are included, unless otherwise indicated.

[0027] The invention relates to a growth substrate and its manufacturing method. The growth substrate is suitable for the epitaxial production of an array of InGaN-based diodes, the diodes making it possible to natively emit or detect light radiation at different wavelengths.

[0028] The growth substrate comprises mesas M (i) with i ranging from 0 to 2, having different levels of porosification P (i) depending on whether the lower and upper portions of the mesas are all porous (mesas denoted M (2) ), or only one of them (mesas M (1) ), or even none (mesas M (0) ). These different levels of porosification P (i) make it possible to obtain mesas M (i) having different relaxation rates R (i) . The relaxation rates R (i) result in the fact that the epitaxial recovery portions have different values ​​of the effective lattice parameter from one category of mesa to another. Also, D(i) diodes, produced during the same epitaxy step, have different active zones in terms of the proportion of indium incorporated in the quantum wells, from one category of D(i) diodes to another (and therefore of mesas) and are therefore suitable for emitting or detecting light radiation at different wavelengths.

[0029] Note that the lower and upper portions of the mesas are in contact with each other, and are not separated from each other by an intermediate insulating layer that would allow only one or the other of said portions of the mesa to be porosified. In this configuration, the porosification of the lower and upper portions of the mesas is made possible by implementing a differentiated electrochemical porosification of the lower and upper portions of the mesas, by means of a photo-assisted porosification of the upper portion (this is then referred to as PECE porosification, for Photo ElectroChemical Etching in English) and non-photo-assisted porosification of the lower portion (we then speak of ECE porosification, for ElectroChemical Etching, in English).

[0030] For this, for each mesa, the lower portion is sufficiently doped to be porosifiable by ECE porosification for a predefined applied electrical voltage of value V ECE. On the other hand, the upper portion is unintentionally doped or insufficiently doped to be non-porosifiable during ECE porosification at the electrical voltage V ECE. Preferably, there is a difference of at least one order of magnitude, in terms of doping level, between the lower portion and the upper portion.

[0031] Furthermore, for each mesa, the upper portion has a proportion of indium x2>0 sufficient for it to absorb the illuminating light radiation of the central wavelength λ c emitted in the direction of the mesas during PECE porosification. On the other hand, the lower portion is made of a material whose proportion of indium x1≥0 is insufficient for it to absorb this illuminating light radiation during PECE porosification.

[0032] Recall that porosification by electrochemical anodization is carried out by immersing the crystalline GaN-based growth substrate in a liquid electrolyte. In ECE porosification, the growth substrate is connected to the anode of an electric generator while a counter-electrode, immersed in the electrolyte, is connected to the cathode, thus applying an electrical voltage of predefined value V ECE.

[0033] THE figures 1A à 1C are schematic and partial views, in cross-section, of an optoelectronic device 1 comprising a growth substrate 10 and a matrix of diodes D (i) based on InGaN, according to different embodiment variants. figures 1A et 1B illustrate variants where the mesas M (i) rest on the same buffer layer 12 made of a non-porosifiable material (at the porosification electrical voltage of the mesas) and electrically conductive, whereas the figure 1C illustrates a variant where such a buffer layer 12 is absent.

[0034] The optoelectronic device 1 is here a micro-screen with native RGB emission, where the pitch of the diodes D (i) can be less than or equal to approximately 10µm. The index i, ranging from 0 to 2, is relative to the categories (presented later) of the mesas M (i) and therefore of the diodes D (i).

[0035] Here and for the remainder of the description, we define a direct three-dimensional orthogonal reference frame XYZ, where the X and Y axes form a main plane of a support layer 11 and where the Z axis is oriented along the thickness of the growth substrate 10 in the direction of the diodes D (i).

[0036] The optoelectronic device 1 comprises a growth substrate 10 from which a matrix of diodes D (i) has been produced by epitaxy. The electrical polarization electrodes of the diodes are not shown here for the sake of clarity.

[0037] The growth substrate 10 comprises a support layer 11 and mesas M (i) each intended for the production by epitaxial regrowth of a diode D (i) of the matrix. A mesa (i.e. an elevation, a relief) is a part of the growth substrate 10 projecting from the support layer 11. The mesas are produced by localized etching of a crystalline stack 20 (cf. fig.3A ).

[0038] The growth substrate 10 is made from crystalline GaN, that is to say it is made of GaN and / or its compounds such as In x Ga 1-x N, Al y Ga 1-y N, or even In x Al y Ga 1-xy N. Generally speaking, by “GaN-based material”, it is meant that the material can be GaN and / or its compounds. It can thus be In x Al y Ga 1-xy N where the proportion of indium x can be zero and where the proportion of aluminum y can also be zero. And “by InGaN-based material”, it is meant that it is made of InGaN or InAlGaN, that is to say it can be made of In x Al y Ga 1-xy N where the proportion of indium x is non-zero and where the proportion of aluminum y can be zero.

[0039] The support layer 11 is made here of a non-porous material, so that it remains non-porous (dense) during the porosification of the mesas. It may be a material inert to the electrochemical porosification reaction, such as an insulating material (sapphire, etc.) or a semiconductor material (SiC, Si, etc.) that is not intentionally doped (nest) or lightly doped. It may also be a GaN-based semiconductor material whose doping level ND is lower than a minimum value ND,min (V ECE ) of the electrical voltage applied during the ECE porosification (non-photo-assisted): it may then be unintentionally doped or lightly doped. Preferably, the support layer is non-porous at the electrical voltage for porosification of the mesas. For example, the support layer may be made of sapphire, silicon, SiC, or self-supporting GaN. (freestanding in English), among others. It has a thickness for example between 200µm and 1.2mm approximately. Intermediate layers may be present between the support layer 11 and the buffer layer 12, made for example from AlN. Thus, as examples, the following stacks can be used: Si / AlN / AlGaN / GaN; sapphire / GaN nest; SiC / AlN / AlGaN etc...

[0040] A buffer layer 12 may be present, as illustrated by fig.1A et 1B . The buffer layer 12 here makes it possible to apply an electrical potential to the lower portions 13 of the mesas M (i) from a polarization electrode 3 (see for example fig.3B ). Indeed, the lower portions 13 are here distinct from each other in the XY plane (no continuity of the material of these lower portions). For this, the buffer layer 12 is made of a crystalline material based on doped GaN (here n-type) over at least part of its thickness. The buffer layer 12 is made by epitaxy from the support layer 11. It can have a thickness of between approximately 1 and 10 µm.

[0041] The buffer layer 12 may be made of a material that is not porosifiable during ECE and PECE porosification. It is then lightly doped to, on the one hand, allow the transport of charge carriers, and on the other hand, not be porosifiable during ECE porosification. More precisely, it has a doping level ND lower than a predefined minimum value ND,min (V ECE ) of the doping level ND from which, for a value V ECE of electrical voltage applied during ECE porosification, a GaN-based crystalline material is porosified. The doping level ND may be lower than 10 18< cm -3< for a value V ECE of the electrical voltage of the order of approximately 30V. In addition, in order not to be porosifiable during PECE porosification, the buffer layer 12 has an insufficient proportion of indium to absorb the illuminating light radiation.Alternatively, the buffer layer 12 may be made of a material that can be porosified during ECE polarization.

[0042] Alternatively, as illustrated by the fig.1C , the buffer layer 12 may be absent. Also, the growth substrate 10 comprises a lower continuous layer 23 from which the lower portions 13 of the mesas M (i) are formed. In other words, this lower layer 23 extends continuously in the XY plane and connects the lower portions 13 together. The polarization electrode 3 then rests in contact with the lower layer 23.

[0043] The mesas M (i) comprise two portions superimposed along the Z axis, in contact with each other, namely a lower portion 13 located on the side of the support layer 11, and an upper portion 14 which rests on and in contact with the lower portion 13. They are made of a porosifiable material in a differentiated manner, depending on whether it is a non-photo-assisted electrochemical porosification ECE or a photo-assisted electrochemical porosification PECE. An epitaxial recovery portion 15 made from InGaN rests on the upper portion 14. It can be carried out before or after the electrochemical porosification of the mesas M (i).

[0044] Each of the lower 13 and upper 14 portions is made of a GaN-based crystalline semiconductor material. The material may be chosen from InGaN, AlGaN, InAlGaN, and / or GaN. In this example, the lower 13 portions are made of GaN, and the upper 14 portions are made of InGaN. They have a thickness, for example, of between approximately 50nm and 1µm, here equal to approximately 800nm. The lower 13 and upper 14 portions of the same mesa M(i) may have the same thickness or a different thickness. The mesas M(i) may have been formed by one or more etching operations of an initial crystalline stack (see, for example, fig.3A ), for example by dry etching, so that their sides can be substantially vertical.

[0045] Let us recall here that an electrochemical porosification reaction is a selective reaction in the sense that, for the same electrical voltage E p of value V ECE , a doped crystalline semiconductor material based on GaN will be porosified if its doping level ND is greater than or equal to a predefined minimum doping level ND,min (V ECE ). Otherwise, it will not be porosified and will remain intact (dense). As such, the figure 2 illustrates an example of the domain of existence of electrochemical porosification as a function of the ND doping level (here in donors) of the GaN-based crystalline material and the applied electrical voltage E p, as described in particular in document EP3840016A1.

[0046] The minimum doping level ND,min depends on the value of the electrical voltage E p according to a decreasing function: the higher the electrical voltage E p, the lower the minimum doping level ND,min required to porosify the crystalline material. In other words, for a predefined value V ECE of the electrical voltage E p applied during the ECE porosification step, the crystalline material having a doping level ND lower than the minimum value N min (V ECE ) will not be porosified (made porous) and will therefore remain intact or dense (non-porous): this material can therefore be described as “non-porosifiable”. On the other hand, this same material, when it has a doping level ND at least equal to the minimum value ND,min (V ECE ), will be porosified (made porous) and can therefore be described as “porosifiable”. Note that the domain of existence also depends on the forbidden energy band of the material, and therefore here on the proportion of indium in the material.Indeed, the higher this proportion of indium is in the material, the lower the porosification potential. Also, it is preferable that the proportion of indium in the upper portion 14 is not much higher (e.g. of the order of 2%) than that of the lower portion 13 (e.g. of the order of 0% for GaN).

[0047] The porosification of all the lower and upper portions of the mesas M (i), or of only one of them, makes it possible to obtain a more or less significant relaxation within the mesas M (i). Thus, a porosified portion becomes deformable, and will allow a partial or total relaxation of the non-porous upper portion(s), whether it is the upper portion 14 if it is non-porous and / or the epitaxy recovery portion 15. Also, the growth substrate 10 can have the following characteristics: o in the mesas M (2) where the porosification level P (2) is maximum, the fact that all the lower 13 and upper 14 portions have become porous makes them deformable with respect to the mechanical stresses generated by the epitaxial recovery portion 15, so that the latter relaxes and presents an effective lattice parameter ae< cre(2) which can then be substantially equal to that of its bulk material am< cre: ae< cre(2) ≈ am< cre;o in the mesas M (0) where the porosification level P (0) is minimal, since none of the lower 13 and upper 14 portions have been made porous and therefore deformable, the epitaxial recovery portion 15 continues to undergo the mechanical stresses generated by the support layer 11 or by the buffer layer 12 (if applicable), and therefore has an effective lattice parameter ae < cre (0) which can be substantially equal (with a slight relaxation due to the etching of the mesa) to that of the buffer layer 12: ae < cre (0) ≈ ae < ct ;o in the mesas M (1) where the porosification level P (1) is intermediate, a porosified portion will therefore be made deformable and will allow the non-porous upper portion (the upper portion 14 and / or the epitaxial recovery portion 15) to partially relax: if the porous portion is the lower portion 13 and the upper portion 14 is non-porous and is made of InGaN, or if the porous portion is the upper portion 14, then the epitaxial recovery portion 15 has an effective lattice parameter ae< cre(1) which may be greater than ae< cre(0). We may then have a growth substrate 10 where: ae< cre(0) < ae< cre(1) < ae< cre(N). ;

[0048] The lower portions 13 of the mesas M (i) are made of a material which is porosifiable during non-photo-assisted electrochemical porosification ECE, and which is non-porosifiable during photo-assisted electrochemical porosification PECE.

[0049] Thus, to be porosifiable during ECE porosification during which an electrical voltage E p of value V ECE is applied to the growth substrate 10, the material of the lower portions 13 has a doping level ND at least equal to the minimum value VD,min (V ECE ) from which the porosification of the material takes place, for example between 3×10 18< cm -3< and 1.5×10 19< cm -3< , for example equal to approximately 6×10 18< cm -3<. The lower portions 13 are preferably doped with n-type silicon, or even germanium, but p-type doping is possible.

[0050] Furthermore, to be non-porosifiable during PECE porosification during which illuminating light radiation of wavelength λ c is emitted towards the mesas, the material has a proportion of indium x1≥0 such that the illuminating light radiation is not absorbed. The proportion of indium x1 of the lower portions 13 is such that the illuminating light radiation is not absorbed, even taking into account the Franz Keldysh effect. It is preferably lower than the proportion of indium x2 of the upper portions 14.

[0051] Furthermore, the upper portions 14 of the mesas M (i) are made of a material which is not porosifiable during non-photo-assisted electrochemical porosification ECE, but is porosifiable during photo-assisted electrochemical porosification PECE.

[0052] Thus, to be non-porosifiable during ECE porosification, the material of the upper portions 14 has a doping level ND lower than the minimum value VD,min (V ECE ) from which the porosification of the material takes place. The upper portions 14 may be unintentionally doped but are preferably lightly doped, and for example may have a doping level at most equal to 5×10 17< cm -3< . The doping may be of type n, as for the lower portions.

[0053] Furthermore, to be porosifiable during PECE porosification, the material has a sufficient proportion of indium x2>0 to absorb the illuminating light radiation at the pores. Note here that the absorption of the illuminating light radiation by the upper portions 14 may be based on the Franz-Keldysh effect, as mentioned in particular in the article by Toguchi et al. entitled Anisotropic Electrochemical Etching pf Porous Gallium Nitride by Sub-Bandgap Absorption Due to Franz-Keldysh Effect, J. Electrochem. Soc. 2019, 166, H510. Indeed, the central wavelength λ c is slightly lower than the bandgap energy of the material of the upper portions 14, but the presence of an electric field at the pores being formed allows the absorption of the illuminating light radiation. Thus, the upper portion 14 is porosified and not entirely etched.

[0054] Furthermore, each mesa M (i) of the growth substrate 10 may comprise an epitaxial recovery portion 15 which rests on the upper portion 14. It may be produced before the porosification of the mesas M (i), or after this, for example during the manufacture of the diode matrix D (i). It is made of a crystalline material based on InGaN which may be chosen from InGaN and InAlGaN, and is intended to allow the production of the diodes D (i) by epitaxial recovery. It is produced here by epitaxy from the upper portions 14. Note that, if the epitaxial recovery portion 15 is made of GaN, it may be deposited before or after the porosification of the mesas.

[0055] In the case where the epitaxial recovery portions 15 are carried out before the electrochemical porosification of the mesas M (i), it is made of a non-porosifiable material during the ECE porosification and during the PECE porosification.

[0056] Also, to be non-porosifiable during ECE porosification, the material of the epitaxial recovery portions 15 has a doping level ND lower than the minimum value VD,min (V ECE ) from which the porosification of the material takes place. Preferably, the epitaxial recovery portions 15 may be unintentionally doped (for example of the order of 10 17< cm -3< ) but preferably are weakly doped, and for example may have a doping level at most equal to 5×10 17< cm -3< . The doping may be of type n, as for the lower portions.

[0057] In addition, to be non-porosifiable during PECE porosification, the material of the epitaxial recovery portions 15 has a proportion of indium x3 such that the illumination light radiation is not absorbed. The proportion of indium x3 is lower than the proportion of indium x2 of the upper portions 14. In the case where the epitaxial recovery portions 15 are made of InGaN, they are preferably deposited after the porosification of the mesas, so as not to be constrained in terms of proportion of indium x3 with respect to PECE porosification.

[0058] Alternatively, in the case where the epitaxial recovery portions 15 are produced after the electrochemical porosification of the mesas M (i), it does not matter whether they are produced from a porosifiable material or not. Note that in the latter case, the doped portions 31 of the semiconductor junction of the diodes D (i) may correspond to the epitaxial recovery portions 15.

[0059] The epitaxial recovery portions 15 have a thickness and a non-zero proportion of indium x3 such that it contributes to obtaining the desired relaxation rates R (i) for the mesas M (1) and M (2), as explained in detail below. For example, it has a thickness of approximately 200 nm and a proportion of indium x3 at least equal to 8%. It may have a lesser thickness, for example approximately 100 nm, and a higher proportion of indium x3, for example approximately 12%.

[0060] Let us note here that the lower 13 and upper 14 portions (and the epitaxy recovery portions 15, if they are produced before the electrochemical porosification) each have a thickness less than its critical thickness at which there is a plastic relaxation of the mechanical stresses. The total thickness of the stack of these portions is also less than a predefined critical thickness. Thus, the support layer 11 generates, in the lower 13 and upper 14 portions, mechanical stresses (oriented in the XY plane) whose value is such that, before porosification, the lattice parameter at the upper face of the mesas M (i) is close to or substantially equal to the effective lattice parameter ae < cs of the support layer 11.

[0061] Since the mesas M(i) are made from the same initial crystal stack 20, each portion of a mesa M(i) is coplanar with the corresponding portions of the other mesas. Thus, the epitaxial recovery portions 15 are coplanar with each other and define upper faces that are also coplanar. The upper portions 14 are coplanar with each other. The same is true for the lower portions 13.

[0062] In addition, each portion of a mesa M (i) has a thickness identical to that of the corresponding portions of the other mesas. Thus, the epitaxial recovery portions 15 have the same thickness between them. The same is true for the lower portions 13, and for the upper portions 14.

[0063] As an illustration, in the example of the fig.1A , the growth substrate 10 comprises a buffer layer 12 made of GaN constrained by the support layer 11 made of sapphire (lattice parameter here of 3.184Â). The mesas M (i) each comprise: a lower portion 13 based on n-type doped GaN, an upper portion made of In x2 Ga 1-x2 N with x2>0. In this example, the epitaxy recovery portion 15 is made of In x3 Ga 1-x3 N with a thickness of approximately 200nm and a proportion of indium x3 equal to approximately 8%. It is carried out before the electrochemical porosification of the mesas M (i).

[0064] The mesa M (0) has a minimum relaxation rate R (0) such that the epitaxial recovery portion 15 has a lattice parameter close to or substantially equal to that (3.184Â) of the support layer 11. The mesa M (2) has a maximum relaxation rate R (2) such that the epitaxial recovery portion 15 has a lattice parameter close to or substantially equal to that of the bulk In 0.15 Ga 0.85 N (approximately 3.238Â) which would have been subsequently deposited on the mesa, due to the elastic deformation of the porous lower 13 and upper 14 portions. The mesa M (1) has an intermediate relaxation rate R (1) such that the epitaxial recovery layer 15 has a lattice parameter between 3.184Â and 3.217Å. We therefore have ae< cre(0) < ae< cre(1) < ae< cre(2) . Thus, the diode matrix can then include D(0) diodes emitting in the blue, D(1)u diodes emitting in the green, and D(N) diodes emitting in the red.

[0065] Thus, the growth substrate 10 comprises mesas of different categories M (i) having different porosification levels P (i), allowing different relaxation rates R (i). Such a growth substrate 10 then makes it possible to produce a matrix of diodes D (i) adapted to emit or receive, natively, light radiation at different wavelengths. This growth substrate 10 is obtained from an original crystalline stack 20 made of two lower 23 and upper 24 layers, in contact with each other, and not separated two by two by an insulating layer. These different porosification levels are obtained by implementing a differentiated electrochemical porosification of the lower 13 and upper 14 portions of the mesas M (i).The different categories of mesas M (i) are thus obtained, without it being necessary to carry out steps of spatially localized implantation of dopants as in the manufacturing method of the prior art mentioned previously. The absence of an insulating layer in the mesas then allows the diodes to be electrically polarized by the growth substrate 10.

[0066] THE figures 3A à 3E illustrate different steps of a method of manufacturing a growth substrate 10 similar to that of the fig.1A , then a matrix of diodes D (i) . In this example, the diodes are electroluminescent and form a micro-screen with native RGB emission. The manufacturing method implements PECE porosification of upper portions 14 of the mesas M (i) , followed by ECE porosification of lower portions 13.

[0067] In reference to the fig.3A , first of all a crystalline stack 20 is provided comprising, on a support layer 11 which extends in the main plane XY: a buffer layer 12, a doped lower continuous layer 23, an upper continuous layer 24, and here a continuous epitaxy recovery layer 25. These layers extend in the XY plane continuously over the entire extent of the support layer 11. The different layers are obtained by successive epitaxy steps from the support layer 11.

[0068] In reference to the fig.3B , here a localized etching of the crystalline stack 20 is carried out so as to form the mesas M (i). Thus, the continuous epitaxial recovery layer 25 is etched, preferably by dry etching, to form epitaxial recovery portions 15, the upper continuous layer 24 to form upper portions 14, the lower continuous layer 23 to form lower portions 13, and here open into the buffer layer 12. The lower portions 13 are therefore distinct from each other in the XY plane (no continuity of material). The etching can in particular be carried out by an ICP-RIE plasma type process in chlorine gas. The mesas M (i) are therefore separated two by two in the XY plane by a trench. An electrode 3 is then deposited on and in contact with the buffer layer 12, which makes it possible to apply an electrical potential to the mesas M (i).

[0069] Note that, as a variant (not shown), the trench can separate the mesas M (i) from one category to another. On the other hand, the mesas of the same category can be separated two by two by a shallower trench. They can thus be physically connected by an unetched part of the lower layer 23.

[0070] In reference to the fig.3C , a first step of electrochemical porosification of the mesas M (i) is carried out. This involves porosifying certain upper portions 14 in a photo-assisted manner. The lower 23 and upper 24 layers have been configured in terms of indium proportion (respectively x1 and x2) so that, during the PECE porosification where an illuminating light radiation of predefined central wavelength λ c is emitted in the direction of the mesas M (i), the upper portions 14 can be porosified, while the lower portions 13 are not porosified. In addition, in this example where the epitaxial recovery portions 15 are produced before the porosification, the material of these portions 15 has been produced in terms of indium proportion x3 so as not to be porosified during the PECE porosification.

[0071] But before that, in order to obtain mesas M (0) having a porosification level P (0), that is to say that none of the lower 13 and upper 14 portions will be porosified during the PECE and ECE porosifications, the mesas M (0) are encapsulated in an encapsulation layer 4 made of an electrically insulating material such as a photosensitive resin or a layer of a silicon oxide or nitride. Thus, the lower 13 and upper 14 portions of the mesas M (0) will not be in contact with the liquid electrolyte. The encapsulation layer 4 can be transparent to the illuminating light radiation: to the extent that the mesas M (0) will not be in contact with the liquid electrolyte, there will be no PECE porosification. On the other hand, the mesas M (1) and M (2) are not covered by this encapsulation layer 4, so as to then be in contact with the liquid electrolyte.

[0072] Then, the growth substrate is immersed in a liquid electrolyte. The polarization electrode 3 may or may not be connected to the electrical generator. It allows the photogenerated electrons to be collected. Preferably, to improve the collection of these electrons, an electrical voltage of value V PECE is applied, which is sufficiently low not to generate ECE porosification, for example 1 to 2V lower than the value V ECE. The value of the electrical voltage V PECE can be reduced by adjusting the wavelength of the incident radiation. A reference electrode may be present to precisely control the applied electrical voltage. For this, the polarization electrode 3 is connected to the anode of the electrical generator, the growth substrate 10 then forming a working electrode. A counter-electrode (here a platinum wire) is immersed in the electrolyte, and is connected to the cathode of the electrical generator.The electric generator applies an electric voltage E p of value V PECE . The liquid electrolyte can be acidic or basic, and can be oxalic acid. It can also be KOH, HF, HNO 3 , NaNO 3 , H 2 SO 4 or their mixture. Thus, a mixture of oxalic acid and NaNO 3 can also be used.

[0073] In addition, illuminating light radiation, of central wavelength λ c , is emitted in the direction of the mesas M (i). Also, the illuminating light radiation is absorbed by the upper portions 14 of the mesas M (1) and M (2) at the pores (and not massively) to obtain porosification of the portions and not total etching thereof. In this example, it can also be absorbed by the upper portion of the mesas M (0) if the encapsulation layer 4 is transparent to the wavelength λ c . Also, electron-hole pairs are photo-generated. The electrons are then collected by the polarization electrode 3, and the holes are used for the oxidation of the material of the upper portions 14 of the mesas M (1) and M (2) which are in contact with the liquid electrolyte. Also, the upper portions 14 of the mesas M (1) and M (2) are porosified in a photo-assisted manner.On the other hand, the mesas M (0) are not porosified because they are not in contact with the liquid electrolyte. Note that the lower portions 13 of the mesas M (1) and M (2) as well as the buffer layer 12 are not porosified because they are made of a material that is not porosifiable at the electrical voltage V PECE . Once the upper portions 14 of the mesas M (1) and M (2) are porosified, the growth substrate is removed from the electrolytic bath.

[0074] In reference to the fig.3D , a second step of electrochemical porosification of the mesas M (i) is carried out. This involves porosifying the lower portions 13 of the mesas M (2) in a non-photo-assisted manner, without porosifying those of the mesas M (1). The lower layer 23 has been configured in terms of doping level so that, during the ECE porosification, the lower portions 13 can be porosified if they are in contact with the liquid electrolyte.

[0075] Also, in order to preserve the mesas M (1) from further porosification, and more precisely to prevent their lower portions 13 from being porosified, the mesas M (1) are encapsulated in the encapsulation layer (the same encapsulation layer 4 or a separate encapsulation layer). Thus, the mesas M (1) will not be in contact with the liquid electrolyte during the ECE porosification. On the other hand, the mesas M (2) are not covered by this encapsulation layer 4, so as to then be in contact with the liquid electrolyte.

[0076] Then, the growth substrate 10 is again immersed in a liquid electrolyte. The polarization electrode is then connected to the electrical generator so as to apply an electrical voltage of value V ECE . This results in a porosification of the lower portions 13 of the mesas M (2), which alone are in contact with the liquid electrolyte. The lower portions 13 of the mesas M (1) and M (0) are therefore not porosified. The electrical voltage applied between the anode and the cathode can be for example between 1V and 100V. It can be applied for a period ranging from a few seconds to a few hours. A reference electrode can be used to precisely control the applied electrical voltage. The growth substrate 10 is then removed from the electrolytic bath, and the encapsulation layer 4 is removed.

[0077] A growth substrate 10 is thus obtained, comprising mesas M (i) having different porosification rates P (i), and formed from the same original crystalline stack 20. To the extent that the epitaxial recovery portions 15 are present, there is also relaxation of the mesas, so that they have different relaxation rates R (i). This relaxation of mechanical stresses differentiated from one category of mesas to another is obtained by the differentiated electrochemical porosification of the mesas M (i). These mesas M (i) all have an epitaxial recovery portion 15 in In x Ga 1-x N with the same proportion of indium x3, but with a different relaxation depending on whether the different categories of mesas M (i) are considered. They are therefore suitable for the production by epitaxy of a matrix of diodes D (i) allowing the emission or reception of light radiation at different wavelengths.

[0078] In reference to the fig.3E , the diode matrix D (i) is then manufactured by epitaxy growth from the mesas M (i) of the growth substrate 10. In this example, the diodes are light-emitting. Here, diodes D (0) are produced from the mesas M (0), diodes D (1) from the mesas M (1), and diodes D (2) from the mesas M (2).

[0079] For this, the polarization electrode 3 can be removed, then an insulating layer 6 (growth mask) can be deposited, here conformally, so as to cover the buffer layer 12 and the sides of the mesas M (i). Only the upper surfaces of the mesas M (i) are made free, so as to form a germination surface. The insulating layer 6 is made of an electrically insulating material, and can be an 80nm thick silicon oxide or nitride deposited by plasma-enhanced chemical vapor deposition (PECVD).

[0080] The diodes D (i) are produced simultaneously, by epitaxial recovery from epitaxial recovery portions 15 of the different mesas M (i). We thus form: diodes D (0), from mesas M (0), adapted to emit at a main wavelength λ 0 , for example a blue light whose wavelength λ 0 is for example between 440nm and 490nm approximately; diodes D (1), from mesas M (1), adapted to emit at a main wavelength λ 1 , for example a green light whose wavelength λ 1 is for example between 495nm and 560nm approximately; and diodes D (2), from mesas M (2), adapted to emit at a main wavelength λ 2 , for example a red light whose wavelength λ 2 is for example between 600nm and 650nm approximately.

[0081] This is possible because the epitaxial recovery portions 15 of the mesas M (0), M (1) and M (2) have a different effective lattice parameter from one category of mesas to another, due to the different relaxation rates R (0), R (1) and R (2). Also, the incorporation of indium in the diodes, and in particular in the quantum wells of the active layers 32, depends in fact on the effective lattice parameter (and therefore the relaxation rate) of the epitaxial recovery portions 15 of the different mesas. The greater the effective lattice parameter of an epitaxial recovery layer 15, the greater the main wavelength of the corresponding diode will be.

[0082] Diodes D (i) comprise a minima two doped portions 31, 33, one of n-type and the other of p-type, and an intercalary active layer 32 comprising quantum wells. The doped layers 31, 33 and the active portion 32 may be multilayers (thus the quantum wells are conventionally located between barrier sub-layers) and comprise other layers or sub-layers such as an electron blocking layer.

[0083] Furthermore, the method of manufacturing the diode matrix also includes a step of producing polarization electrodes for the diodes D (i). This step is conventional and is not described here. In the case where the mesas M (i) are electrically conductive, they can be used to polarize the diodes. Also, the polarization electrode 3 can be kept.

[0084] An optoelectronic device 1 is thus obtained, the diodes D (i) of which are adapted here to emit natively at different wavelengths, here in the three RGB colors, thanks to the growth substrate 10, the mesas M (i) of which at different levels of porosification P (i) were produced by differentiated electrochemical porosification, without having resorted to steps of spatially localized implantation of dopants, and which made it possible to produce the diodes D (i) from a single step of epitaxy recovery.

[0085] THE figures 4A à 4E illustrate different steps of a method of manufacturing a growth substrate 10 similar to that of the fig.1B , then a matrix of diodes D (i). The manufacturing method also implements a PECE porosification of upper portions 14 of the mesas M (i), followed by an ECE porosification of lower portions 13. The steps of the fig.4A, fig.4B et fig.4E are identical or similar to those of the fig.3A, fig.3B et fig.3E : they are not described again.

[0086] In reference to the fig.4C , the first step of electrochemical porosification of the mesas M (i) is carried out. This involves photo-assisted porosification of the upper portion 14 of only the mesas M (2), and not those of the mesas M (1). Also, the encapsulation layer 4 covers the mesas M (0) as well as the mesas M (1). Also, only the mesas M (2) are not covered by the encapsulation layer 4 and will be in contact with the liquid electrolyte.

[0087] Then, the growth substrate 10 is immersed in the liquid electrolyte. The polarization electrode 3 may or may not be connected to the electrical generator. The illuminating light radiation is emitted in the direction of the mesas M (i). Also, the illuminating light radiation is absorbed by the upper portion 14 of the mesas M (2). Also, the upper portion 14 of the mesas M (2) is porosified in a photo-assisted manner. On the other hand, the upper portions 14 of the mesas M (0) and M (1) are not porosified because they are not in contact with the liquid electrolyte. The buffer layer 12 is not porosified here, because it is made of a material that is not porosifiable at the electrical voltage V PECE . Once the upper portions 14 of the mesas M (2) are porosified, the growth substrate is removed from the electrolytic bath.

[0088] In reference to the fig.4D , the second step of electrochemical porosification of the mesas M (i) is carried out. This involves porosifying in a non-photo-assisted manner the lower portions 13 of the mesas M (1) and M (2), without porosifying those of the mesas M (0). For this, the part of the encapsulation layer 4 which covers the mesas M (1) is removed and the part which covers the mesas M (0) is kept.

[0089] Then, the growth substrate 10 is again immersed in a liquid electrolyte. The polarization electrode 3 is then connected to the electrical generator so as to apply an electrical voltage of value V ECE . This results in a porosification of the lower portions 13 of the mesas M (1) and M (2), which are in contact with the liquid electrolyte. The mesas M (0) are not porosified. The growth substrate is then removed from the electrolytic bath, as well as the encapsulation layer 4. This gives a growth substrate 10 similar to that of the fig.1B where the mesas M (1) comprise a porosified lower portion 13 and a non-porosified upper portion 14.

[0090] THE figures 5A à 5D illustrate different steps of a method of manufacturing a growth substrate 10 similar to that of the fig.1B , then a matrix of diodes D (i). The manufacturing method also implements a PECE porosification of upper portions 14 of the mesas and an ECE porosification of lower portions 13, these porosifications being carried out during the same step. The steps of the fig.5A, fig.5B et fig.5D are identical or similar to those of the fig.3A, fig.3B et fig.3E : they are not described again.

[0091] In reference to the fig.5C , the PECE and ECE porosifications of the mesas M (1) and M (2) are carried out during the same step, that is to say during the same immersion in the electrolytic bath. This involves porosifying in a photo-assisted manner the upper portion 14 of only the mesas M (2), and not those of the mesas M (1), and porosifying in a non-photo-assisted manner the lower portions 13 of the mesas M (1) and M (2).

[0092] To do this, the mesas M (0) are covered with an encapsulation layer 4 made of an electrically insulating material (opaque or not). In addition, the mesas M (1) are partially covered with an opaque encapsulation layer 5 made of a material opaque to the illuminating light radiation. An opaque material is a material whose transmission rate at the wavelength λ c is zero or almost zero. The opaque encapsulation layer 5 only partially covers the mesas M (1) in the sense that a part of the sides of the lower portion 13 is left free, while at least partially covering the upper surface of the mesa M (1), or even the whole surface.

[0093] Then, the growth substrate 10 is immersed in a liquid electrolyte. The polarization electrode 3 is then connected to the electrical generator so as to apply an electrical voltage of value V ECE . In addition, the illuminating light radiation is emitted in the direction of the mesas M (i). It is possible that V ECE is the same for ECE and PECE porosification, but since the proportions of indium are different, it is also possible to use a lower voltage V ECE during PECE porosification and then increase V ECE for ECE porosification.

[0094] The illumination of the mesas M (i) leads to a PECE porosification of the upper portion 14 of the mesas M (2) insofar as only this upper portion 14 is in contact with the liquid electrolyte and absorbs the illuminating light radiation. On the other hand, the upper portion 14 of the mesas M (1), although it is also in contact with the liquid electrolyte, is protected from the illuminating light radiation by the opaque encapsulation layer 5: it is substantially not (or only slightly) porosified. Finally, the upper portion 14 of the mesas M (0) is not in contact with the liquid electrolyte: it is not porosified.

[0095] Furthermore, the application of the electrical voltage E p of a value V ECE leads to a porosification of the lower portions 13 of the mesas M (1) and M (2), which alone are in contact with the liquid electrolyte. The mesas M (0) are not porosified.

[0096] The growth substrate is then removed from the electrolytic bath, as well as the encapsulation layers 4 and 5. This gives a growth substrate 10 similar to that of the fig.1B where the mesas M (1) and M (2) have been porosified by PECE and ECE porosification. We can then produce the diodes ( fig.5D ).

[0097] THE figures 6A à 6D illustrate different steps of a method of manufacturing a growth substrate 10 similar to that of the fig.1C , then a matrix of diodes D (i). The manufacturing process also implements PECE and ECE porosifications carried out during the same step, although a two-step porosification remains possible.

[0098] In reference to the fig.6A , we first provide a crystalline stack 20. This is identical to that of the fig.3A , except that it does not include the buffer layer 12, nor the continuous epitaxial recovery layer 25.

[0099] In reference to the fig.6B , the mesas M (i) are produced by localized etching of the crystal stack 20. Trenches are produced which open into the lower continuous layer 23. They could open onto the support layer 11 if it were made of an electrically conductive (non-porosifiable) material. The polarization electrode 3 would then be deposited in contact with the support layer 11. On the other hand, in this example, the support layer 11 is made of sapphire, so that the lower continuous layer 23 is not etched over its entire thickness. An unetched part connects the lower portions 13 to each other, and ensures the electrical connection to the polarization electrode 3.

[0100] In reference to the fig.6C , the PECE and ECE porosifications of the mesas M (1) and M (2) are carried out during the same step. This is identical to that of the fig.5C and is therefore not described again in detail. Note only that, during ECE polarization, the connecting part of the lower continuous layer 23 is also porosified at the same time as the lower portions 13. The part of the lower continuous layer 23 which is remote from the polarization electrode 3 may not be porosified since the series electrical resistance may be too high.

[0101] In reference to the fig.6D, the growth substrate 10 is then removed from the electrolytic bath, and the encapsulation layers 4, 5 are removed. The mesas M (i) have different levels of porosification P (i) but may not yet have the desired relaxation rates R (i) since the epitaxial recovery portions 15 have not yet been produced. After depositing a growth mask 6 (insulating layer), the epitaxial recovery portions 15 are then produced, from the upper surfaces of the upper portions 14.

[0102] A thin layer based on GaN, and preferably InGaN with an indium proportion of approximately 1%, with a thickness of between approximately 10 and 100nm, can be produced by epitaxy. This thin layer is produced on and in contact with the upper portions 14 of the mesas M (i), and makes it possible to close the pores opening at the upper face. Then, the epitaxy recovery portions 15 can be produced by epitaxy at the level of each mesa M (i). These portions 15 can have a thickness of approximately 200nm and an indium proportion of approximately 8%. They then cause the deformation of the porous portions of the mesas M (1) and M (2), which in turn allows the portions 15 to relax. The doped portions 31, 33 and the active portion 32 of the diodes are then produced. Let us note here that the doped portion 31 of the diodes D (i) can then act as an epitaxy recovery portion 15.

[0103] Particular embodiments have just been described. The invention is limited by the following claims.

Claims

1. A method for manufacturing a growth substrate (10) adapted for producing by epitaxy a matrix of diodes based on InGaN, comprising the following steps: - providing a crystalline stack (20) based on GaN, comprising: a doped lower layer (23); and an upper layer (24) made from Inx2Ga1-x2N with x2>0, resting on and in contact with the doped lower layer (23); where: - the doped lower layer (23) has a predefined proportion of indium x1≥0 such that it is non-absorbent to an illuminating light radiation having a predefined central wavelength, and the upper layer (24) has a predefined proportion of indium x2>0 such that it is absorbent of illuminating light radiation; - the doped lower layer (23) has a predefined doping level such that it is porosifiable at an electrical voltage of predefined value VECE, and the upper layer (24) has a predefined doping level such that it is non-porosifiable at the predefined electrical voltage VECE; - producing mesas M(i), with i ranging from 0 to 2, by localised etching of the upper layer (24) to form upper portions (14) of mesas and of at least a part of the lower layer (23) doped to form lower portions (13) of mesas; then - depositing an encapsulation layer (4) made of an electrically insulating material, covering at least one mesa denoted M(0), such that said mesa M(0) is not porosified during an electrochemical porosification of mesas denoted M(1) and M(2); then - electrochemically porosifying the lower (13) and upper (14) portions of at least one mesa M(2) by: photo-assisted porosification of the upper portion (14) of the mesa M(2) by emission of the illuminating light radiation; and non-photo-assisted porosification of the lower portion (13) of the mesa M(2) by application of the electrical voltage of value VECE; - electrochemically porosifying only one of the lower (13) and upper (14) portions of at least one mesa M(1) by: photo-assisted porosification of the upper portion (14) of the mesa M(1) during that of the upper portion of the mesa M(2); or non-photo-assisted porosification of the lower portion (13) of the mesa M(1) during that of the lower portion of the mesa M(2); - removing, at least in part, the encapsulation layer (4), so as to expose an upper surface of the mesa M(0).

2. The manufacturing method according to claim 1, wherein the photo-assisted porosification and the non-photo-assisted porosification are carried out in separate steps.

3. The manufacturing method according to claim 2, comprising the following distinct steps: - a step of photo-assisted porosification of the upper portions (14) of the mesas M(1) and M(2), the mesas M(1) and M(2) being in contact with a liquid electrolyte; - a step of non-photo-assisted porosification of the lower portion (13) of the mesas M(2), the mesas M(1) being covered with an encapsulation layer (4) made of an electrically insulating material, only the mesas M(2) being in contact with a liquid electrolyte.

4. The manufacturing method according to claim 2, comprising the following distinct steps: - a step of photo-assisted porosification of the upper portion (14) of the mesas M(2), the mesas M(1) being covered with an encapsulation layer (4) made of an electrically insulating material, only the mesas M(2) being in contact with a liquid electrolyte; - a step of non-photo-assisted porosification of the lower portion (13) of the mesas M(1) and M(2), the mesas M(1) and M(2) being in contact with a liquid electrolyte.

5. The manufacturing method according to claim 1, wherein the photo-assisted porosification and the non-photo-assisted porosification are carried out during the same step, an opaque encapsulation layer, made of a material opaque to the illuminating light radiation, at least partially covering an upper surface of the mesas M(1) while leaving a lateral surface of the lower portion (13) of the mesas M(1) free.

6. The manufacturing method according to any one of claims 1 to 5, wherein the crystalline stack (20) comprises a continuous epitaxial regrowth layer (25), resting on the upper layer (24), and made from InGaN having a predefined proportion of indium x3>0 such that it is non-absorbent of the illuminating light radiation, and having a predefined doping level such that it is non-porosifiable at the predefined electrical voltage VECE.

7. The manufacturing method according to any one of claims 1 to 5, wherein the crystalline stack (20) does not comprise a continuous epitaxial regrowth layer (25) so that the upper layer (24) has a free upper face, epitaxial regrowth portions (15) produced using InGaN being produced from an upper surface of the upper portions (14) of the mesas M(1) after the porosification of the mesas M(1) and M(2).

8. The manufacturing method according to any one of claims 1 to 7, wherein, during the photo-assisted porosification, an electric voltage is applied between the growth substrate (10) and a liquid electrolyte wherein it is immersed, of a predefined value VPECE so that the lower portions (13) of the mesas M(1) are not porosified.

9. A method for manufacturing an optoelectronic device (1), comprising the following steps: - manufacturing the growth substrate (10) by the method according to any one of claims 1 to 8; then - manufacturing a matrix of diodes D(i) by epitaxy from the mesas M(i) of the growth substrate (10), the diodes then being adapted to emit or detect light radiation at different wavelengths, the wavelength being different from one category of mesas M(i) to another.

10. A growth substrate (10), adapted for the production by epitaxy of a diode matrix based on InGaN, comprising: - a support layer (11); - mesas M(i), with i ranging from 0 to 2, made from crystalline GaN, resting on the support layer (11), and each comprising: a doped lower portion (13) having an indium proportion x1≥0; and an upper portion (14) made from Inx2Ga1-x2N with an indium proportion x2>x1 and having a doping level lower than that of the lower portion (13), and resting on and in contact with the lower portion (13); the mesas being configured so that: - the lower (13) and upper (14) portions of the mesa M(0) are non-porous; - only one portion among the lower (13) and upper (14) portions of the mesa M(1) is porous; - the lower (13) and upper (14) portions of the mesa M(2) are porous.

11. The growth substrate (10) according to claim 10, wherein each mesa M(i) comprises an epitaxial regrowth portion (15) resting on the upper portion (14), made from non-porous crystalline InGaN whose lattice parameter amcre of the relaxed material is greater than the effective lattice parameter aecs of the support layer (11): - the epitaxial regrowth portion (15) of each mesa M(2) having a maximum lattice parameter aecre(2); - the epitaxial regrowth portion (15) of each mesa M(0) having a lattice parameter aecre(0) less than aecre(2); - the epitaxial regrowth portion (15) of each mesa M(1) having an intermediate lattice parameter aecre(i) less than aecre(2) and different from aecre(0).

12. The growth substrate (10) according to claim 10 or 11, wherein the lower portions (13) of the mesas M(i) rest on a continuous layer (12; 23) made of an electrically conductive material.

13. An optoelectronic device (1) comprising: a growth substrate (10) according to any one of claims 10 to 12; and a matrix of diodes D(i) based on InGaN, epitaxially grown from the mesas of the growth substrate (10), the diodes being adapted to emit or detect light radiation at different wavelengths, the wavelength being different from one category of mesas M(i) to another.