MESA POROSIFICATION PROCESS FACILITATING RE-ESTABLISHMENT OF CONTACT

Abstract

A method comprising the following steps: a) providing a structure (100) comprising a base substrate (110) covered with (Al,In,Ga)N / (Al,In,Ga)N mesas (120), the base substrate (110) comprising a support layer (114), a first undoped GaN layer (111), a second doped GaN layer (112), the mesas (120) comprising a third heavily doped (Al,In,Ga)N layer (123), b) connecting the structure (100) and a counter electrode to a generator, c) immersing the structure (100) and the counter electrode in a solution, d) applying a voltage or current so as to partially porosify the third GaN layer (123') of the mesas (120), at least one region of the heavily doped GaN layer not being porosified, each non-porified region forming an electrical conduction channel between the main faces of the third porosified GaN layer (123'). Figure for the abstract: 5B.

Description

Title of the invention: MESA POROSIFICATION METHOD FACILITATING CONTACT RE-ESTABLISHMENT technical field

[0001] The present invention relates to the general field of color micro-displays.

[0002] The invention relates to a method for porosifying (Al,In, Ga)N / (Al,In,Ga)N mesas.

[0003] The invention also relates to a structure thus obtained comprising porosified (Al,In,Ga)N / (Al,In,Ga)N mesas.

[0004] The invention has applications in numerous industrial fields, particularly in the field of micro-color displays based on micro-LEDs. PRIOR TECHNOLOGY

[0005] Color microdisplays comprise pixels made up of blue, green, and red sub-pixels (RGB pixels). In the following description, these sub-pixels will be referred to simply as pixels for the sake of brevity.

[0006] Blue and green pixels can be made from nitride materials, and red pixels from phosphide materials. To combine these three types of pixels on the same substrate, the "pick and place" technique is generally used. However, in the case of microdisplays with pixels smaller than 10 pm, this technique can no longer be used due to alignment problems, as well as the time required to implement such a technique at this scale. For displays with a large number of pixels (high resolution), this "pick and place" technique is problematic in terms of time. Furthermore, the pixels must be picked from different wafers, which necessitates successive transfers. Parallel transfer techniques ("mass transfer") can also be used.

[0007] Another solution involves performing color conversion using quantum dots (QDs) or nanophosphors pumped by blue pLEDs from a single wafer, either depositionalized or in a monolithic matrix (the preferred case for microdisplays). However, controlling the deposition of these materials on small pixels is difficult, and their resistance to flux is not sufficiently robust.

[0008] It is therefore crucial to be able to obtain the three RGB pixels natively using the same family of materials grown on the same substrate. For this purpose, InGaN is the most promising material. This material can, in fact, theoretically cover the entire visible spectrum depending on its indium concentration. Blue InGaN-based micro-LEDs already exhibit high luminance, significantly exceeding that of their organic counterparts. To emit at wavelengths in the green range, the LED's quantum wells (PQs) must contain at least 25% indium, and for red emission, at least 35% indium is required. Unfortunately, the quality of the InGaN material beyond 20% In is degraded due to the low miscibility of InN in GaN, as well as the high compressive stress inherent in the growth of the InGaN active region on GaN.

[0009] It is therefore essential to be able to reduce the overall stress in GaN / InGaN based structures.

[0010] Currently, one of the most promising solutions is to porosify the GaN layer, as described, for example, in the two articles by Pasayat et al. (Materials 2020, 13, 213; Appl. Phys. Lett. 116 111101 (2020)). The process described in these articles comprises the following steps:

[0011] - provide a stack comprising a sapphire substrate covered by a layer of unintentionally doped GaN (GaN nid), a layer of Si n+ doped GaN (5eL018 at / cm3) and a layer of InGaN or unintentionally doped GaN,

[0012] - partially etch the stack to form the GaN / InGaN or GaN / mesas GaN; the thickness of the doped layer is only partially etched so that the residual doped layer at the bottom of the mesa allows the polarization of the doped layer of all mesas ('ring polarization' or plate edge polarization for example)

[0013] - carry out an electrochemical porosification step in an acid solution oxalic (0.3M), the doped GaN layer acting as the anode and a platinum wire acting as the cathode.

[0014] The porous layer of GaN thus obtained can allow the growth of an InGaN-based nitride LED structure of better crystalline quality, thanks to the relaxation of the generated porous mesas.

[0015] Other articles cite the use of porous GaN for optoelectronic devices, for example the article by Zhang et al. ('A resonant cavity blue-violet light-emitting diode with conductive nanoporous distributed Bragg reflector', Phys. Status Solidi A 214, 1600866 (2017)) and the article by Zhou et al. ('Thermal transport of nanoporous gallium nitride for photonic applications', Journal of Applied Physics 125, 155106 (2019)).

[0016] To produce a native color micro-display based on all InGaN red, green, blue (RGB) micro-LEDs with porous mesas, the process includes, for example, the following steps:

[0017] - provide a structure comprising a substrate covered with mesas ([Fig.1A]):

[0018] the substrate comprising a support layer 14, a buffer layer 15, an unintentionally doped GaN layer 11, a doped or heavily doped GaN layer 12,

[0019] the mesas comprising a porosified GaN layer 23' and a nest GaN layer 24

[0020] the mesas being covered by a re-epitaxial LED 30 comprising n-InGaN 30 and p-InGaN 31, and more particularly at least one n-InGaN 30 layer, an active region with quantum wells and a top layer of p-InGaN 31,

[0021] - to form an electrode 21 on the structure obtained: the electrode 21 comes into contact with the- minus a portion of the upper p-InGaN layer 30 (if this electrode is also present on the sides of the LED, it can be isolated from electrode 21 by a dielectric layer 22 which can cover the sides of layers 30 and 31 and / or part of layer 31, for example); then transfer the assembly onto a final substrate 20 ([Fig. 1B]), having a via (not shown), for example by metal-to-metal bonding,

[0022] - remove the growth substrate, then re-establish contact, for example a cathode contact 50 on the n-GaN with a transparent conductive oxide (TCO) 45 without occulting metal ([Fig.lC]); passivation layers 40 can cover the TCO 45.

[0023] However, since there is a reduction in vertical electrical conduction through the porous mesas, it can be difficult to inject current into the n-InGaN through the porous mesas. Similarly, heat dissipation is poor. These two factors reduce the reliability of the resulting device.

[0024] In order to improve integration, it is possible to re-establish contact of ITnGaN on the cathode after reporting.

[0025] For this, two approaches are possible.

[0026] In a first approach ([Fig. 2A] to 2C), it is possible to completely remove the porous part of the mesas. After removal of the growth substrate, an etch is made down to the n-InGaN of the LED 30. It is then possible to re-establish contact 50 with the metal on the cathode. An electrically insulating layer 40 protects the LED 30. However, this first approach has several drawbacks: after removal of the support 14, it is difficult to control, at the substrate ('wafer') scale, the removal of all the (Al,Ga,In)N layers to stop at the n-InGaN layer 30 that one wants to contact, without reaching the active layer of the LED. Indeed, the epitaxy and especially the layer removal process(es) used generate a non-uniformity linked to the large thickness of the stack to be removed and the tolerance to stop in the n-InGaN layer 30 is low due to its small thickness.Furthermore, the pronounced topography of the contact re-establishment is hardly compatible with a conductive transparent oxide electrode. The contact 50 of the metallic cathode is occulting, which reduces extraction.

[0027] In a second approach shown in Figures 3A to 3C, it is possible to partially remove the porous portion 23' of the mesas and then locally etch the porous layer 23' to make contact with the non-porous InGaN layer of the LED 30. This would improve optical extraction thanks to the presence of residual pores on the surface. To achieve this, localized etching of the pores must be performed, stopping within the thin n-InGaN layer. As with the first approach, non-uniformity problems may arise. The contact with the metal cathode remains occulting, which reduces extraction.

[0028] In these two micro LED matrix fabrication processes, a variation in the thickness (TTV: "total thickness variation") of the GaN is introduced, not only at the wafer level but also from wafer to wafer. These thickness variations arise both from variations in the thickness of the epitaxy and, more importantly, from the thinning or planarization processes used to remove the epitaxial buffer layers, which are several microns thick, due to the non-uniformities of the processes used. Description of the invention

[0029] An object of the present invention is to propose a method for manufacturing micro-LEDs which remedies the drawbacks of the prior art, and in particular a method which makes it easy to make contact on the n-InGaN with the cathode while maintaining good extraction.

[0030] To this end, the present invention proposes a process for manufacturing and porosifying (Al,In,Ga)N / (Al,In,Ga)N mesas comprising the following steps:

[0031] a) provide a structure comprising a basic substrate covered with (Al,In,Ga)N / (Al,In,Ga)N mesas,

[0032] the basic substrate comprising a support layer, optionally a buffer layer of (Al,Ga)N, a first undoped GaN layer, a second doped GaN layer,

[0033] (Al,In,Ga)N / (Al,In,Ga)N mesas comprising a third layer of (Al,In,Ga)N having a first principal face and a second principal face, the third layer of (Al,In,Ga)N being heavily doped or the third layer of (Al,In,Ga)N comprising a first heavily doped part having a first electrical conductivity and a second part formed of one or more zones, the second part having a second electrical conductivity at least ten times lower than the first electrical conductivity,

[0034] a portion of the second doped GaN layer that can extend into the mesas,

[0035] b) electrically connect the structure and a counter electrode to a voltage or current generator,

[0036] c) immersing the structure and the counter electrode in an electrolytic solution,

[0037] d) apply a voltage or current between the structure and the counter electrode of in order to partially porosify the third layer of (Al,In,Ga)N heavily doped mesas or in order to porosify the first heavily doped part of the third layer of (Al,In,Ga)N mesas, thereby obtaining a layer of (Al,In,Ga)N comprising a first porosified part and a second part formed of one or more non-porified zones, each non-porified zone extending from the first main face to the second main face of the partially porosified (Al,In,Ga)N layer to form an electrical conduction channel.

[0038] The invention is fundamentally distinguished from the prior art by the presence of one or more non-porous zones in the mesas. The non-porous zones have a higher electrical conductivity than the porous zones. These non-porous zones therefore form electrical conduction channels.

[0039] Advantageously, in step a), a fourth layer of undoped or weakly doped (Al,In,Ga)N covers the third layer of heavily doped (Al,In,Ga)N of the (Al,In,Ga)N / (Al,In,Ga)N mesas or in that, after step d), a fourth layer of undoped or weakly doped (Al,In,Ga)N is deposited on the porosified third layer of (Al,In,Ga)N.

[0040] Advantageously, the fourth layer of (Al,In,Ga)N, undoped or weakly doped, is a layer of GaN.

[0041] Advantageously, the structure further comprises an additional layer of heavily doped GaN disposed between the first undoped GaN layer and the second doped GaN layer.

[0042] According to a first advantageous embodiment, step d) is carried out by stopping the voltage or current before the complete porosification of the third (Al,In,Ga)N layer, whereby the unporified zone corresponds to the central part of the porosified (Al,In,Ga)N layer (i.e., the porosified part surrounds the porosified part). This makes it possible to preserve unporified zones in the core of the mesas by interrupting the electrochemical porosification before completion. Thus, by choosing the electrochemical porosification conditions, the anodization is partial: it stops before porosifying the core of the mesas. The conductivity of the unporified core remains intact and forms a preferential conduction channel. The relaxation of the edges is naturally favored by the free surfaces.

[0043] According to a second advantageous embodiment, one or more zones, having a second conductivity, are formed in the third layer of heavily doped (Al,In,Ga)N, the second conductivity being at least ten times lower than the first conductivity, whereby, in step d), the zone(s) are not porosified and form electrical conduction channels. The second conductivity is obtained, for example, by locally degrading the conductivity of the mesas by ion implantation. Ion implantation makes it possible to "de-dop" zones (i.e., of decrease the electrical conductivity of the areas that will not be or only slightly porosified during the anodizing process, which is highly selective for doping. The decrease in conductivity helps to preserve conduction channels. The area(s) of secondary conductivity are not or only slightly porosified during step d).

[0044] Advantageously, after step d), the process includes a step in which a heat treatment is carried out, thereby increasing the second electrical conductivity and obtaining areas with a third electrical conductivity. This curing annealing allows at least partial recovery of the conductivity of the implanted area. The third electrical conductivity is greater than the second electrical conductivity. It is less than or equal to the first electrical conductivity.

[0045] According to another embodiment, the zone or zones of the third layer of (Al,In,Ga)N form rings, each ring delimiting a core preferably having an electrical conductivity at least ten times greater than the second electrical conductivity, whereby during step d), the cores are not porous and form electrical conduction channels.

[0046] The rings extend from the first principal face to the second principal face of the third layer of heavily doped (Al,In,Ga)N. This allows the creation of n++ channels in the mesas at the center of the rings, which have degraded conductivity. The n++ GaN in the mesas channels thus remains intact without implantation or porification because it is protected by the less doped, or even undoped, "shell." The shell with lower conductivity is obtained, for example, by ion implantation, particularly by He implantation. Thus, the de-doped region is tube-shaped. This option allows the original epitaxial doping to be preserved. The electrical conductivity at the center of the ring is, for example, identical to the first electrical conductivity. The thickness of the shell (i.e., the thickness of the "walls" of the tube) is preferably at least 250 nm. The thickness is defined by the limitations of lithography and implantation.

[0047] The third (Al,In,Ga)N layer of step a) can be obtained according to the following steps:

[0048] - provide a heavily doped (Al,In,Ga)N layer having a first electrical conductivity,

[0049] - locally decrease the electrical conductivity of the (Al,In,Ga)N layer, to form a layer of (Al,In,Ga)N comprising a first heavily doped part having a first electrical conductivity and a second part formed of one or more zones, having a second electrical conductivity at least ten times lower than the first electrical conductivity.

[0050] The second conductivity is obtained, for example, by locally degrading the Conductivity of mesas by ion implantation. Ion implantation allows for the "de-doping" of areas (i.e., the reduction of the electrical conductivity of the areas) that will not be, or only slightly, porosified during the anodizing process, which is highly selective for doping. The decrease in conductivity allows, in particular, the preservation of conduction channels. The area(s) of secondary conductivity are not, or only slightly, porosified during step d).

[0051] According to another advantageous embodiment, the third (Al,In,Ga)N layer of step a) is obtained according to the following steps:

[0052] - provide a layer of (Al,In,Ga)N having a second electrical conductivity,

[0053] - locally increase the electrical conductivity of the (Al,In,Ga)N layer, for form a layer of (Al,In,Ga)N comprising a first heavily doped part having a first electrical conductivity and a second part formed of one or more zones, having a second electrical conductivity at least ten times lower than the first electrical conductivity.

[0054] The electrical conductivity of the (Al,In,Ga)N layer can be increased, for example, by ion implantation of Si. A healing annealing can also be carried out.

[0055] Advantageously, the first stack comprises three groups of mesas, each group of mesas being intended to form a red, green or blue micro-LED, each group of mesas having a different porification rate or percentage of porosified surface.

[0056] The invention also relates to a method for manufacturing micro-LEDs comprising the following successive steps:

[0057] i) implementation of the mesas manufacturing process as defined above,

[0058] ii) implementation of the following steps e) to g):

[0059] e) on the structure obtained in step i), perform a resumption of epitaxy to form re-epitaxial LEDs comprising layers of n-InGaN layers and a p-doped InGaN layer, then forming a contact electrode (the contact electrode being a so-called upper electrode before transfer and this same electrode being said to be lower after transfer), a passivation layer that can be positioned between the contact electrode (anode) and the p-doped InGaN layer, the passivation layer locally covering the p-doped InGaN layer, for example at the edges of the p-doped InGaN layer,

[0060] f) transfer the structure onto a substrate, for example by metal-to-metal bonding,

[0061] g) remove the support layer, where applicable the (Al,Ga)N buffer layer, the first undoped GaN layer and part or all of the second doped GaN layer (it is possible to stop the etching in this layer so that the remaining part is part of the mesa),

[0062] h) to re-establish contact on the non-porous zone or at least on one of the non-porous zones of the porous (Al,In,Ga)N layer

[0063] Such a process is particularly advantageous because it is possible to:

[0064] - generate mesas with different relaxations to allow coepitaxy in preserving conduction

[0065] - use ion implantation at the mesa or intra-mesa scale to vary the re laxation (monolithic canal)

[0066] - use ion implantation at the mesa or intra-mesa scale to vary the re laxation (heart-shell canal)

[0067] - choose the implanted area to have uniformity of relaxation and / or optimization setting for electrolyte circulation

[0068] - decrease the conductivity over the entire surface of the channel by implantation from n++ (and possibly perform a healing anneal)

[0069] - modulate the implantation conditions to decrease the conductivity of the canal or shell.

[0070] The invention also relates to a structure comprising a basic substrate covered with porosified (Al,In,Ga)N / (Al,In,Ga)N mesas,

[0071] the basic substrate comprising a support layer, optionally a buffer layer of (Al,Ga)N, a first undoped GaN layer and a second doped GaN layer,

[0072] GaN / (Al,In,Ga)N mesas comprising a third partially porosified (Al,In,Ga)N layer having a first principal face and a second principal face, and, preferably, a fourth undoped or weakly doped (Al,In,Ga)N layer,

[0073] a portion of the second doped GaN layer extending into the mesas or a portion of the third heavily doped (Al,In,Ga)N layer extending into the basic substrate,

[0074] the partially porosified (Al,In,Ga)N layer comprising one or more non-porified zones, each non-porified zone extending from the first main face to the second main face of the partially porosified (Al,In,Ga)N layer to form an electrical conduction channel.

[0075] Advantageously, the structure further comprises an additional layer of heavily doped GaN disposed between the first undoped GaN layer and the second doped GaN layer.

[0076] Such a structure offers many advantages:

[0077] - improved tolerance during the etching step, and in particular for stopping engraving IIL post report

[0078] - a lesser topography for re-establishing contact for the cathode (via + metal or transparent conductive oxide (TCO)),

[0079] - a good compromise between the relaxation rate and vertical conduction.

[0080] The invention also relates to an optoelectronic device comprising successively:

[0081] - a support substrate, covered by a lower electrode,

[0082] - a re-epitaxial LED comprising layers of n-InGaN layers and a layer of p-doped InGaN,

[0083] - an undoped or weakly doped GaN layer,

[0084] - a partially porosified (Al,In,Ga)N layer, having a first face main face and a second main face,

[0085] the partially porosified (Al,In,Ga)N layer comprising one or more non-porified zones extending from the first main face to the second main face to form an electrical conduction channel,

[0086] - a re-establishment of contact on the non-porous zone or at least on one of the non-porous zones porosified layer of (Al,In,Ga)N partially porosified.

[0087] These non-porous zones limit the impact on pressure drop (higher Vf) and / or improve optical extraction by reducing or even eliminating partial occlusion. Furthermore, the non-porous channel(s) promote heat dissipation, thereby increasing reliability.

[0088] Thus, it is possible to eliminate the partial occultation due to metals, by taking the cathode contact directly above the n-(In,Al,Ga)N exclusively with a TCO (i.e. without occulting metal), and by adding a metallic contact resumption, for example, by a grid placed in the interpixels allowing to homogenize the potential applied on the cathode.

[0089] Furthermore, the presence of porous zones allows for:

[0090] - to release the specifications for etching the thick AlGaN / GaN stack for to access the n-InGaN electrically while limiting the charge loss,

[0091] - Reducing the topography for cathode contact resumption allowing the use of a TCO without obscuration,

[0092] - modulate the shape and number of non-porous injection zones to allow maximum relaxation at the center of the mesa and limiting the non-uniformity of relaxation across the mesa scale,

[0093] - improve optical extraction (this effect is based on a diffusion phenomenon optical (in the case of large pores).

[0094] Other features and advantages of the invention will become apparent from the following supplementary description.

[0095] It goes without saying that this additional description is given only as an illustration of the object of the invention and should in no way be interpreted as a limitation of this object. Brief description of the drawings

[0096] The present invention will be better understood upon reading the description of exemplary embodiments given by way of illustration only and in no way limiting, with reference to the accompanying drawings in which:

[0097] The [Fig. 1 A]

[0098] The [Fig.1B]

[0099] The [Fig.lC], previously described, schematically represent different stages of a process for manufacturing and transferring a micro-LED according to the prior art.

[0100] The [Fig.2A]

[0101] The [Fig.2B]

[0102] The [Fig.2C], previously described, represent schematically and in cross-section, different steps to form a re-contact from the structure of the [Fig.1B] according to a prior art process.

[0103] The [Fig.3A]

[0104] The [Fig.3B]

[0105] Figure [Fig. 3C], described above, schematically and in cross-section, represents different stages for forming a contact re-establishment from the structure of Figure [Fig. 1B] according to another prior art process,

[0106] The [Fig.4A]

[0107] The [Fig.4B]

[0108] The [Fig.4C]

[0109] Figure 4D schematically represents different stages of a process of manufacturing a microLED according to a particular embodiment of the invention.

[0110] The [Fig.5A]

[0111] Fig. 5B schematically represents different stages of a manufacturing process for a mesa according to a first embodiment of the process according to the invention; the structures are shown in cross-section along the dotted line in Figures 5C and 5D.

[0112] The [Fig.5C]

[0113] Figure [5D] shows, in top view, the mesas of Figures 5A and 5B respectively, along the cutting line shown in dotted lines on Figures 5A and 5B,

[0114] The [Fig.6A]

[0115] The [Fig.6B]

[0116] The [Fig.6C]

[0117] Figure 6D schematically represents different stages of a process of manufacturing a mesa according to a second embodiment of the process according to the invention, the structures are represented in section along the dotted lines of figures 6E, 6F, 6G and 6H.

[0118] The [Fig.6E]

[0119] The [Fig.6F]

[0120] The [Fig.6G]

[0121] Figure [6H] shows, in top view, the mesas of Figures 6A, 6B, 6C and 6D respectively, along the cutting line shown in dotted lines in figures 6A, 6B, 6C and 6D.

[0122] The [Fig.7A]

[0123] The [Fig.7B]

[0124] The [Fig.7C]

[0125] Figure 7D schematically represents different stages of a process of manufacturing a mesa according to a variant of the second embodiment of the process according to the invention, the structures are represented in section along the dotted lines of figures 7E, 7F, 7G and 7H.

[0126] The [Fig.7E]

[0127] The [Fig.7F]

[0128] The [Fig.7G]

[0129] Figure [7H] shows, in top view, the mesas of figures 7A, 7B, 7C and 7D respectively, along the cutting line shown in dotted lines in figures 7A, 7B, 7C and 7D,

[0130] The [Fig.8A]

[0131] The [Fig.8B]

[0132] Figure 8C schematically represents different stages of a manufacturing process for a mesa according to a third embodiment of the process according to the invention; the structures are shown in cross-section along the dotted lines of Figures 8D, 8E and 8F.

[0133] The [Fig.8D]

[0134] The [Fig.8E]

[0135] The [Fig.8F], represent in top view, the mesas of figures 8A, 8B and 8C respectively, along the cutting line shown in dotted lines on figures 8A, 8B and 8C.

[0136] The [Fig.9] is a scanning electron microscope image of a mesa obtained according to the first embodiment of the invention.

[0137] The different parts represented in the figures are not necessarily shown on a uniform scale, in order to make the figures more legible.

[0138] The different possibilities (variants and embodiments) must be understood as not being mutually exclusive and can be combined with each other.

[0139] Furthermore, in the description below, terms which depend on the orientation, such as "above", "below", etc. of a structure apply assuming that the structure is oriented in the manner illustrated in the figures.

[0140] DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0141] Although this is by no means limiting, the invention finds particular applications in the field of color microdisplays, and more specifically for the fabrication of red, green, and blue pixels. However, it could also be used in the field of photovoltaics or water electrolysis ("water splitting") since, on the one hand, InGaN absorbs across the entire visible spectrum and, on the other hand, its valence and conduction bands are around the stability range of water, a thermodynamic condition necessary for the water decomposition reaction. The invention may also be of interest for the fabrication of LEDs or lasers emitting at long wavelengths.

[0142] The process is particularly interesting for manufacturing structures comprising porosified (Al,In,Ga)N / (Al,In,Ga)N mesas having, in particular, a pitch of less than 30 pm.

[0143] By (Al,In,Ga)N, we mean AIN, AlGaN, InGaN, or GaN. Hereafter, we refer more specifically to porous GaN, but with such a process, it is possible to obtain, for example, porous InGaN or AlGaN. The dense InGaN layer (under compression) or the dense AlGaN layer (under tension) will relax thanks to a porous structure, regardless of its composition.

[0144] We will now describe in more detail the porosification process of (Al,In,Ga)N / (Al,In,Ga)N with reference to figures 4A to 4D, 5A to 5D, 6A to 6H, 7A to 7H and 8A to 8F.

[0145] The process for porosifying mesas 120 of (Al,In,Ga)N / (Al,In,Ga)N comprises the following steps:

[0146] a) provide a structure 100 comprising a basic substrate 110 covered with mesas 120 (Al,In,Ga)N / (Al,In,Ga)N (figures 5A, 6A-6B, 7A-7B, 8A-8B),

[0147] the basic substrate 110 comprising successively:

[0148] - a support layer 114,

[0149] - possibly a buffer layer 115 in (Al,Ga)N, particularly in the case of a silicon support layer 114,

[0150] - a first undoped GaN layer 111,

[0151] - advantageously, an additional heavily doped layer,

[0152] - a second layer of GaN doped 112,

[0153] the 120 (Al,In,Ga)N / (Al,In,Ga)N mesas comprising a third layer of (Al,In,Ga)N intended to be partially porosified, the third layer of (Al,In,Ga)N 123 being heavily doped or the third layer of (Al,In,Ga)N 123 comprising a first heavily doped part having a first conductivity and a second part formed of one or more zones 125, the second part having a second conductivity at least ten times lower than the first conductivity,

[0154] a part of the second doped GaN layer 112 being able to extend into the mesas 120,

[0155] b) electrically connect structure 100 and a counter electrode to a voltage or current generator,

[0156] c) immersing the structure 100 and the counter electrode in an electrolytic solution,

[0157] d) applying a voltage or current between the structure 100 and the counter electrode so as to partially porosify the third heavily doped (Al,In,Ga)N layer 123 of the mesas 120 or so as to porosify the first heavily doped part of the third (Al,In,Ga)N layer 123 of the mesas (the zone(s) 125 of the second part not being porosified), thereby obtaining an (Al,In,Ga)N layer comprising a first porosified part 123' and a second part formed of one or more non-porified zones 125, each non-porified zone 125 extending from the first main face to the second main face of the partially porosified (Al,In,Ga)N layer 123' to form an electrical conduction channel (Figures 4A, 5B, 6C, 7C, 8C).

[0158] During step a), a fourth layer of undoped or weakly doped (Al,In,Ga)N 124 can cover the third layer of heavily doped (Al,In,Ga)N 123 of the mesas 120 (Al,In,Ga)N / (Al,In,Ga)N.

[0159] Alternatively, after step d), a fourth undoped or weakly doped (Al,In,Ga)N layer 124 can be deposited on the third porosified (Al,In,Ga)N layer 123'. After step d), doping the epitaxial recovery layer is not critical since the porosification has already been done.

[0160] The structure 100 provided in step a) is, for example, obtained by providing and then locally engraving a stack comprising successively:

[0161] - a support layer 114,

[0162] - possibly, a buffer layer 115 in (Al,Ga)N, particularly in the case of a silicon support layer 114,

[0163] - a first layer of undoped gallium nitride GaN 111,

[0164] - possibly, an additional layer of heavily doped GaN (not shown (on the figures),

[0165] - a second layer of doped GaN (GaN n) 112,

[0166] - a third layer of heavily doped GaN (GaN n+ or GaN n++) 123 or a third layer of GaN comprising a first heavily doped part and a second less doped part 125, and

[0167] - a fourth layer in AIN, InGaN or GaN (noted (Al,In,Ga)N) non-intention neatly doped (nest) or weakly doped 124 as appropriate.

[0168] Preferably, the stack consists of the layers mentioned above. In other words, it does not include any other layers.

[0169] According to an advantageous embodiment, a first part of the second layer 112 forms part of the base substrate 110 and a second part of the second layer 112 forms part of the mesas 120.

[0170] The mesas 120 are formed by etching a portion of the fourth layer 124, the third layer 123, and a first portion of the second doped layer 112 (Figures 4A, 5A, 6A, and 7A). By stopping the etching in the doped layer, the entire height of the third n++ layer 123 is available for relaxation.

[0171] Each mesa 120 comprises successively from the base: the second part of the doped GaN layer 112, the third GaN layer 123 and the fourth (Al,In,Ga)N layer undoped or weakly doped 124.

[0172] The first part of the second doped GaN layer 112 protects the additional layer 113 during the porosification step. Thus, the additional layer is not in contact with the solution. The first part of the doped GaN layer is a common layer for all mesas.

[0173] The structuring of the stack is, for example, achieved by photolithography.

[0174] Thus, we obtain a structure 100 comprising a basic substrate 110 surmounted by a plurality of mesas 120 in (Al,In,Ga)N / (Al,In,Ga)N.

[0175] Mesas 120, also called elevations, are raised features. They are obtained, for example, by etching a continuous layer or several superimposed continuous layers, so as to leave only a certain number of "reliefs" of this layer or these layers. The etching is preferably carried out with a hard mask, for example SiO2. After etching the mesas, this hard mask is removed by a wet chemical process before porosification. It is also possible to remove this hard mask after porosification, by exposing it only in the areas used for polarization for electrochemical polarization. Advantageously, the mask is removed before the porosification step.

[0176] Preferably, the flanks of the mesas 120 are perpendicular to this stack of layers.

[0177] The surface of the mesas can be, for example, circular, hexagonal, square or rectangular.

[0178] The largest dimension of the surface of the 120 mesas ranges from 500 nm to 500 pm, preferably from 1 to 1 pm and even more preferably from 3 to 5 pm. For example, the largest dimension of a circular surface is the diameter.

[0179] The thickness (or depth) of the mesas corresponds to the dimension of the perpendicular mesa dicular to the underlying stacking. The depth of the mesas ranges from 0.3 to 2pm, preferably from 0.5 to 1pm.

[0180] The spacing between two consecutive 120 mesas ranges from 50nm to 20pm.

[0181] The mesas 120 can have identical or different doping levels. The higher the doping level, the greater the porification will be at a fixed potential. The relaxation of the fourth layer 124 of dense (Al,In,Ga)N depends on the porification level of the mesas. Thus, different amounts of indium can be incorporated during the re-epitaxy of InGaN on the dense layer 124 (thanks to the reduction of the "compositional pulling effect" (i.e., the pushing of In atoms towards the surface, preventing them from incorporating into the layer)). After epitaxy of the complete LED structure, blue, green, and red (RGB) mesas will be obtained on the same substrate in a single growth step, provided the difference between the relaxation levels of the mesas is sufficient.

[0182] The support layer 114 is, for example, made of sapphire or silicon.

[0183] The support layer 114 has, for example, a thickness ranging from 250pm to 2mm. The thickness depends on the type and dimensions of the support layer 114. For example, a 2-inch diameter sapphire support layer might be 350 µm thick. A 6-inch diameter sapphire support layer might be 1.3 mm thick. A 200 mm diameter silicon support layer might be 1 mm thick.

[0184] In the case of a silicon support layer 114, a buffer layer ('buffer') of (Al,Ga)N is advantageously interposed between the support layer 114 and the GaN nest layer 111.

[0185] The first layer 111 is an undoped GaN layer. Undoped means unintentionally doped (nest). It is a nest layer to prevent porosity. Unintentionally doped GaN means, without the deliberate addition of a doping species during GaN growth, a concentration below 117 at / cm³.

[0186] The first layer 111 in GaN nid has, for example, a thickness ranging from 500nm to 5pm. Advantageously, its thickness is between 1 and 4pm to absorb the stresses related to the lattice mismatch between the GaN and the substrate.

[0187] The second layer 112 is a doped GaN layer. By doped GaN, we mean a concentration between 6.1017 at / cm3 and 5.1018 at / cm3, preferably between 8.1017 at / cm3 and 2.1018 at / cm3.

[0188] The second GaN layer 112 has, for example, a thickness ranging from 300 nm to 1 pm, preferably between 400 and 700 nm. It must be sufficiently electrically conductive to allow for contact re-establishment on this layer during the electrochemical anodizing step. The minimum thickness varies depending on the doping level. The thickness of layer 112 will be chosen so as to protect the If necessary, the buried layer is heavily doped during anodizing. This electrically conductive layer can be electrically connected to the voltage or current generator.

[0189] The third layer 123 can be a heavily doped GaN layer. By heavily doped GaN, we mean a concentration greater than 6 x 10¹⁸ at / cm³, preferably greater than 8 x 10¹⁸ at / cm³, or even greater than 10¹⁹ at / cm³. The concentration is, for example, between 6 x 10¹⁸ at / cm³ and 2 x 10¹⁹ at / cm³, preferably between 7 x 10¹⁸ at / cm³ and 1 x 10¹⁹ at / cm³ in the case of n-type doping with Si. In the case of Ge doping, for example by metal-organic vapor deposition (MOCVD), higher doping levels, typically up to 1 x 10²⁰ at / cm³, can be achieved. The third layer 123 has, for example, a doping ten times higher than the second layer 112. It has a thickness between 200nm and 2pm, preferably from 500nm to Ipm.

[0190] The third layer can be a GaN 123 layer comprising a first heavily doped portion in which one or more 125 regions are arranged. The doping level of the 125 region(s) is at least ten times lower than the doping level of the first heavily doped portion. We will subsequently detail the fabrication of this layer structured to achieve different doping levels.

[0191] The fourth layer 124 is an unintentionally doped or lightly doped (Al,In,Ga)N layer. Lightly doped (Al,In,Ga)N means a doping level between 2 × 10¹⁷ at / cm³ and 1 × 10¹⁸ at / cm³. Undoped means a doping level below 1 × 10¹⁷ at / cm³, particularly for a GaN layer. For example, in the case of an InGaN layer, the doping level is less than 5 × 10¹⁷ at / cm³. The porosification of a given doped layer will depend primarily on the applied potential. Furthermore, if the layer to be porosified is heavily doped, the lightly doped layers (typically having a doping level at least a decade lower than that of the layer to be porosified) will not be porosified.

[0192] This can be a layer of AIN, AlGaN, InGaN or GaN. For example, it has a thickness between 100 nm and 200 nm, preferably between 50 and 200 nm. The doping is sufficiently low so that this layer is not porosified during step d).

[0193] This layer 124 is not or only slightly impacted by porification and serves as a seed for regrowth. This layer 124 is continuous to ensure the quality of the re-epitaxial layer, for example a (In,Ga)N layer, on the structure.

[0194] The additional layer has a thickness of, for example, between 500 nm and 5 pm, preferably between 1 pm and 3 pm. Preferably, it has a doping concentration greater than or equal to 5 x 10¹⁸ at.cm³, preferably greater than 6 x 10¹⁸ at.cm³, even more preferably greater than 8 x 10¹⁸ at.cm³, or even greater than 10¹⁹ at.cm³, per For example, 1.5 x 10¹⁹ at / cm³. It has a doping concentration, for example, between 6 x 10¹⁸ at / cm³ and 2 x 10¹⁹ at / cm³, preferably between 7 x 10¹⁸ at / cm³ and 1 x 10¹⁹ at / cm³. The additional heavily doped GaN layer may have the same or a different doping level than the third heavily doped GaN layer. The additional heavily doped GaN layer may have the same or a different thickness than the third heavily doped GaN layer.

[0195] The tension applied during porosification will be chosen according to the doping of the different layers mentioned above, and in particular of the second layer 112, the third layer 123 and the additional layer, as well as the target doping rate.

[0196] The respective doping levels are chosen so that at a given potential, there is selectivity between the heavily doped zone and the lightly doped zone, i.e. so that the second layer 112 is not porosified during step d) and so that the third 123 is porosified during step d).

[0197] Subsequently, an n-type doping is described, but it could be a p-type doping.

[0198] By way of illustration and not limitation, according to one embodiment, structure 100 may comprise:

[0199] - a basic substrate 110 comprising successively: a support layer 114 in sapphire or silicon, possibly a buffer layer of (Al,Ga)N, a first undoped GaN layer 111 with a thickness between 1 and 4pm, a first part of the second GaN layer 112 doped with 500nm (1.1018 at / cm3),

[0200] - 120 GaN / (Al,In,Ga)N mesas comprising successively: a second part of the second layer 112 of GaN doped by lOOnm (1.1018 at / cm3), a third layer 123 of GaN heavily doped by 800 nm (1.1019 at / cm3), and a nest layer (Al,In,Ga)N of lOOnm.

[0201] According to another embodiment, structure 100 may comprise:

[0202] - a basic substrate 110 comprising successively: a support layer 114 in sapphire or silicon, possibly a buffer layer of (Al,Ga)N, a first undoped GaN layer 111 with a thickness between 1 and 4pm, an additional heavily doped GaN layer of 2pm (1.1019 at / cm3), a first part of the second doped GaN layer of 500nm (1.1018 at / cm3),

[0203] - 120 GaN / (Al,In,Ga)N mesas comprising successively: a second part of the second layer 112 of GaN doped by lOOnm (1.1018 at / cm3), a third layer 123 of GaN heavily doped by 800 nm (1.1019 at / cm3), and a fourth layer 124 of (Al,In,Ga)N nest of lOOnm.

[0204] In step b), the structure 100 and a counter electrode (CE) are electrically connected to a voltage or current generator. The device acts as the working electrode (WE). Hereafter, it will be referred to as the voltage generator, but it could This involves a current generator that allows a current to be applied between the device and the counter electrode.

[0205] The initial contact is made on structure 100.

[0206] In particular, the contact can be made on the base substrate 110. The re-contact can be made on the second layer of doped GaN 112. The re-contact can be made on the bottom of the mesas, at the level of the second layer 112, which makes it possible to use the etching step to also make the contacts.

[0207] Contact can also be made on one of the other layers: on the fourth undoped or lightly doped (Al,In,Ga)N layer 124, on the third heavily doped (Al,In,Ga)N layer 123, or on the additional heavily doped GaN layer. In the case of re-establishing contact on a heavily doped layer, its opening will advantageously be limited to a region protected from the electrolyte.

[0208] The contact re-establishment zone can also be covered with a metallic layer to improve contact for electrochemical polarization. This contact can be removed after porosification before epitaxial re-establishment.

[0209] The counter electrode 500 is made of an electrically conductive material, such as for example a metal with a large developed surface area and inert to the chemistry of the electrolyte such as a platinum wire mesh.

[0210] In step c), the electrodes are immersed in an electrolyte, also called an electrolytic bath or electrolytic solution. The electrolyte can be acidic or basic. The electrolyte is, for example, oxalic acid. It can also be KOH, HF, HNO3, NaNO3, or H2SO4.

[0211] In step d), a voltage is applied between structure 100 and counter electrode 500. The voltage can range from 1 to 30 V, for example. Preferably, it is from 5 to 15 V, and even more preferably from 6 to 12 V, for example from 8 to 10 V. The voltage is chosen according to the doping levels of the different layers, in order to obtain the desired selectivity. It is applied, for example, for a duration ranging from a few seconds to several hours. Porification is complete when there is no longer any current at the applied potential. At that point, the entire doped structure is porosified and the electrochemical reaction stops.

[0212] The electrochemical anodizing step can be carried out under ultraviolet (UV) light.

[0213] During step d), the third layer of (Al,In,Ga)N 123 is partially porosified. Alternatively, one or more zones 125 of the heavily doped GaN layer are not porosified during step d).

[0214] Each non-porified zone extends from the first main face to the second main face to form an electrical conduction channel through the GaN layer. The electrical conduction channel can be in the form of a channel or a tube, for example. This allows for re-establishment of contact with the GaN layer at the level of this electrical conduction channel.

[0215] Furthermore, by choosing the position of the non-porous areas, it is possible to play on relaxation.

[0216] According to a first advantageous embodiment, shown in Figures 5A to 5C, step d) is an incomplete porosification step: step d) is carried out by stopping the voltage or current before the complete porosification of the (Al,In,Ga)N layer, whereby the non-porified zone 125 corresponds to the central part of the partially porosified (Al,In,Ga)N layer 123'.

[0217] Porosification begins on the mesa flanks at the level of the GaN 123 layer in contact with the electrolyte and extends towards the center of the GaN layer. As porosification progresses, the electrolyte moves towards the core. This lateral porosification from the mesa edge is controlled by the duration of the electrochemical porosification. The mesa core remains intact (i.e., it is not porosified). The mesa core forms conduction channels extending from the first principal face to the second principal face of the partially porosified (Al,In,Ga)N 123 layer. The core of the n++ GaN layer thus created exhibits intact conductivity.

[0218] This first embodiment is particularly suitable for mesas of large dimensions (for example greater than or equal to 5pm).

[0219] According to a second advantageous embodiment, shown in Figures 6A to 6H, one or more zones 125 having a second electrical conductivity are formed in the heavily doped (Al,In,Ga)N layer 123 before the porosification step. The second conductivity is at least ten times lower than the second conductivity, whereby in step d), the zone(s) 125 with the lowest conductivity are not porosified.

[0220] These zones are, for example, obtained by localized ion implantation of the n++ 123 layer of the mesas to form non-porous pillars. Implantation leads to a degradation of the conductivity of the implanted portion (this portion is heavily doped before implantation and doped or even lightly doped after implantation). Preferably, implantation allows the doping to be modulated on the order of a decade and, in particular, to be reduced by at least a decade (for example, for GaN:Si, the doping level can decrease from 1E19 at / cm3 to 1E18 at / cm3 or less). During step d), this zone 125 is not or only slightly porous because it is less conductive. The zone can be located at the center of the mesa as shown in Figures 6B, 6C, 6F, and 6G.

[0221] Advantageously, for this second embodiment, it is possible, after step d) of porosification, to carry out a heat treatment. This consists of annealing of healing of implantation defects. This annealing allows at least partial recovery of the conductivity of the non-porified implanted area and thus to form a 125' conduction channel of higher conductivity (figures 6D and 6H).

[0222] According to an alternative embodiment, shown in Figures 7A to 7H, it is possible to implant several parts of the (Al,In,Ga)N layer 123 to modulate the position of the porosified and non-porified areas 125. This alternative is particularly interesting because it allows to preserve the relaxation of the core and / or minimize the degradation of the re-epi germ surface.

[0223] As before, it is possible to carry out an annealing step to increase the conductivity of the pillars and have a conduction zone with improved conductivity 125'.

[0224] According to a third advantageous embodiment, shown in Figures 8A to 8F, the heavily doped (Al,In,Ga)N layer 123 is locally modified so as to form, in the third heavily doped (Al,In,Ga)N layer 123, one or more regions 125 having a conductivity at least ten times lower than the first conductivity. These regions 125 extend from the first face to the second face of the layer 123. These regions preferably have a tube-like shape and protect a core 127 located inside the tube (this is the central part of the tube). The central part of the tube has, for example, the same conductivity as the first part of the (Al,In,Ga)N layer 123 (i.e., the inside of the tubes is heavily doped). The 125 zones form a protective barrier against anodizing for the 127 zones to be protected. Thus, during step d), the core 127 of the tubes is not porosified and forms preferential conduction channels.

[0225] This third embodiment consists, for example, of localized ion implantation of the n++ 123 layer of the mesas, preferably in the form of rings, to electrically preserve the n++ pillars at the core of the rings without degrading their conductivity. Thus, after step d), a non-porous implanted ring is obtained surrounding an intact, unimplanted n++ core, the ring itself being in contact with the remaining unimplanted and porosified layer. The non-porified pillars ensure electrical conduction.

[0226] The implantation parameters are chosen so as to degrade (i.e. decrease) the conductivity in the implanted areas by at least a factor of 10. A lower value does not prevent good conductivity through the channels that remain intact.

[0227] In Figures 8A to 8F, a single crown 125 / core 127 motif is shown. Several motifs may advantageously be defined to form several conduction channels through layer 123. The positioning of the core / crown motifs will advantageously be chosen so as to obtain good relaxation.

[0228] According to the second embodiment and the third embodiment, of Preferably, non-porous conductive pillars have a diameter of at least 250nm.

[0229] The channel is preferably solid (i.e. made of the same material).

[0230] The channel goes from the first main face of the GaN layer to the second face main layer of GaN 123. The height of the channel corresponds to the thickness of the GaN 123 layer.

[0231] The channel surface can have a round, hexagonal, square, etc. shape. The largest dimension of the channel surface is preferably at least 0.25 pm, particularly with regard to implementation facilities (critical dimensions in photolithography).

[0232] The conduction channel(s) may be positioned in the center or at the periphery of the mesa.

[0233] The surface area of ​​the non-porous channel or the total surface area of ​​the non-porous channels as well as the position of the non-porous channel(s) impact the relaxation.

[0234] In these different embodiments, we have described that the different parts of the (Al,In,Ga)N layer are obtained by providing a heavily doped (Al,In,Ga)N layer 123 having a first electrical conductivity, then locally decreasing the electrical conductivity of the (Al,In,Ga)N layer, to form a layer comprising a first heavily doped part and a second part formed of one or more zones 125 of lower conductivity.

[0235] Alternatively, it is also possible to form the different parts of the (Al,In,Ga)N layer by providing an (Al,In,Ga)N layer having low electrical conductivity (for example, starting from an unintentionally doped or lightly doped layer) and then locally increasing the electrical conductivity of the (Al,In,Ga)N layer by at least a factor of 10, to form a heavily doped part. The increase in conductivity is, for example, achieved by implanting dopants (n-donors).

[0236] For each of the variants, it is possible to obtain a first heavily doped part in which one or more less conductive pads 125 are dispersed or in which one or more structures comprising a less conductive ring 125 surrounding a conductive core 127 are dispersed.

[0237] The following table lists several data on the mesas. The resistivity values ​​are taken from the literature. Non-porous channel 125: 1E19 at / cm3 Non-porous mesa: 1E19 at / cm3 Fully porous mesa Top layer n-GaN 124 mesa Top layer GaN 124 nest mesa Ipm2 0.lpm2 lOpm2 Poro 10% Poro 70% 1E18 cm 3 1E17 cm 3 1 1 1 1 1 0.1 0.1 1 0.1 10 10 10 10 10 3.0E-03 3.0E-0 3 3.0E-03 5.0E-02 l.0E+0 0 l.6E-02 l.3E-01 3.0E+01 3.0E+0 2 3.0E+00 5.0E+01 l.0E+0 3 l.6E+00 l.3E+01 100 100 100 100 100 100 100 6 6 6 6 6 6 6 6.0E-06 6.0E-0 6 6.0E-06 6.0E-06 6.0E-06 6.0E-06 6.0E-06 0.18 1.8 0.018 0.3 6 0.0096 0.078

[0238] According to the values ​​in the table, it appears that a non-porous channel (or several non-porous channels, with an equivalent surface area of ​​Ipm²) can significantly reduce the vertical resistance of a highly porous 10pm² porous mesa (potential drop in the mesa reduced from 6V to 0.18V, i.e., by more than a factor of 30). In this case, it is possible to retain the porous GaN in the device after transfer. The potential drop induced by the upper layer 124 of the mesa remains small compared to the mesa potential drop, which allows the porous mesa to be retained without significant degradation of the electro-optical characteristics.

[0239] The porosity of the porosified portion of the third layer of highly doped (Al,In,Ga)N 123 is advantageously at least 10%. It preferably ranges from 25% to 70%, preferably from 25% to 50%, for example 45% to 50%.

[0240] The largest dimension (the height) of the pores can vary from a few nanometers to a few micrometers. The smallest dimension (the diameter) can vary from a few nanometers to a hundred nanometers, in particular from 30 to 70 nm.

[0241] The resulting porosity (porosity rate and pore size) depends on the layer doping and the process parameters (applied voltage, duration, nature and concentration of the electrolyte, chemical post-treatment or annealing). Varying the porosity allows control of the incorporation / segregation rate. The porosity, and in particular the pore size, may vary subsequently during the resumption of epitaxy as a function of the applied temperature.

[0242] After step d), the process advantageously comprises the following steps:

[0243] e) on the structure obtained in step i), perform a second epitaxial reaction to form re-epitaxial LEDs comprising layers of n-InGaN layers 130 and a p-doped InGaN layer 131 ([Fig.4B]), then forming a contact electrode 210, a dielectric 122 being able to be deposited to protect the sides of the LED,

[0244] f) transferring the structure obtained in step f) onto a substrate 200, for example by metal-to-metal bonding ([Fig.4C]),

[0245] g) remove the support layer 114, where applicable the (Al,Ga)N buffer layer 115, the first undoped GaN layer 111 and all or part of the second doped GaN layer 112 (preferably the part which is located outside the mesas),

[0246] h) make a re-contact 150 on the non-porified zone 125 or at least on one of the non-porified zones 125 of the partially porosified (Al,In,Ga)N layer 123' ([Fig.4D]).

[0247] Advantageously, an electrically insulating layer 140 protects the porosified layer and / or prevents short circuits.

[0248] In step e), the contact electrode 210 reflector on p-InGaN is positioned on the structure. The contact electrode 210 is referred to as the upper electrode before transfer and this same electrode is referred to as the lower electrode after transfer.

[0249] A passivation layer 122 can be positioned between the contact electrode 210 (anode) and the p-doped InGaN layer 131. The passivation layer 122 locally covers the p-doped InGaN layer 131, in particular on the inclined flanks of the p-doped InGaN layer 131.

[0250] During step e), an epitaxy is performed on the mesas 120, thereby obtaining an epitaxial layer that is at least partially relaxed, and preferably totally relaxed.

[0251] For example, an all-InGaN LED structure may include:

[0252] - a 350nm n-doped InGaN layer, formed of 15 x Ino.o3Gao.97N / GaN (thicknesses 20nm / l,8nm)

[0253] - multiple quantum wells (MQWs), formed of 5 x Ino.4oGao.6oN / In0.03Ga0.097N (thicknesses 2.3nm / 5, 7, 11 nm),

[0254] - a layer of Ino.o3Gao.97N nest (lOnm),

[0255] - an Alo.1Gao.9N :Mg (20nm) layer,

[0256] - a layer of Ino.o3Gao.97N doped with Mg (125nm),

[0257] - a layer of Ino.o3Gao.97N doped p+++ (25nm).

[0258] The percentage of relaxation corresponds to:

[0259] with a^, the lattice parameter of the starting layer on which the epitaxy is resumed (i.e., the lattice parameter of layer 124), and

[0260] ac2 the mesh parameter of the relaxed layer,

[0261] The layer is 100% relaxed if ac2 corresponds to the lattice parameter of the bulk material, of the same composition as the re-epitaxial layer.

[0262] When aci=ac2 the layer is said to be constrained.

[0263] Partially relaxed means, for example, a relaxation percentage greater than 50%. The relaxation percentage will depend on the final mesa (e.g., blue, green, or red mesa). The doping level of the mesas can be modulated to obtain different porosity levels, and therefore different relaxation levels, for each mesa during the resumption of InGaN emitter epitaxy. This facilitates obtaining different emission colors for each mesa, for example, to obtain red, green, and blue emitters by growing them on the same substrate. It is also possible to obtain different emission colors by changing the ratio of porosified to non-porous surface area or by combining the two.

[0264] Epitaxial resumption is preferably used to form re-epitaxial LEDs.

[0265] Epitaxial regrowth is carried out on the fourth layer 124 of (Al,In,Ga)N, which is either dense or lightly doped, of the mesas 120. Since this layer is not porosified during the electrochemical anodizing step, it remains continuous and dense. Epitaxial regrowth is thus facilitated, and the epitaxially treated layer exhibits better adhesion. The formation of defects related to pore coalescence is avoided.

[0266] The epitaxial layer in this step e) is advantageously made of gallium nitride or indium gallium nitride.

[0267] The In incorporation rate varies according to the relaxation capacity (lattice parameter a in the plane). By manipulating the porification rate and the porified surface area, the mesas can have different relaxation rates. For example, it is possible to manipulate the differential n++ GaN dedoping by implantation (He).

[0268] The process is thus simpler to implement because a single epitaxy is sufficient to form the red, green, and blue pLEDs. There is no need to perform successive epitaxies for each pLED.

[0269] Step f) is, for example, achieved with metal-to-metal bonding. It could also be a hybrid (metal-oxide) bond. This step allows, in particular, interconnection with a control circuit to dynamically modulate the emission of the microLEDs.

[0270] During step h), the re-establishment of contact 150 can be achieved via the presence of a transparent conductive oxide (TCO) layer 145. The TCO layer 145 is, for example, indium tin oxide (ITO). It is possible to cover the TCO layer 145 with an additional passivation layer 140 for protection ([Fig. 4D]). Illustrative and non-limiting example:

[0271] A mesa comprising an n++ doped GaN layer has been partially porosified. Porosification is stopped by halting it before its complete completion ([Fig.9]).

[0272] The central GaN n++ channel in the mesas exhibits intact conductivity.

Claims

Demands

1. A process for the porosification of (Al,In,Ga)N / (Al,In,Ga)N mesas comprising the following steps: a) provide a structure (100) comprising a base substrate (110) covered with mesas (120) (Al,In,Ga)N / (Al,In,Ga)N, the base substrate (110) comprising a support layer (114), optionally a buffer layer of (Al,Ga)N (115), a first undoped GaN layer (111), a second doped GaN layer (112), the mesas (120) (Al,In,Ga)N / (Al,In,Ga)N comprising a third layer of (Al,In,Ga)N (123) having a first principal face and a second principal face, the third layer of (Al,In,Ga)N (123) comprising a first heavily doped portion having a first electrical conductivity and a second portion formed of one or more zones (125), the second portion having a second electrical conductivity at least ten times lower than the first electrical conductivity, a portion of the second a layer of doped GaN 112 that can extend into the mesas 120, b) electrically connect the structure (100) and a counter electrode to a voltage or current generator,c) immerse the structure (100) and the counter electrode in an electrolytic solution, d) apply a voltage or current between the structure (100) and the counter electrode so as to partially porosify the third heavily doped (Al,In,Ga)N layer (123) of the mesas (120) or so as to porosify the first heavily doped part of the third (Al,In,Ga)N layer (123) of the mesas, thereby obtaining an (Al,In,Ga)N layer comprising a first porosified part (123') and a second part formed of one or more non-porified zones (125), each non-porified zone (125) extending from the first main face to the second main face of the partially porosified (Al,In,Ga)N layer (123') to form an electrical conduction channel.

2. Method according to claim 1, characterized in that the structure further comprises an additional layer of heavily doped GaN disposed between the first undoped GaN layer (111) and the second doped GaN layer (112).

3. A method according to any one of claims 1 to 2, characterized in that, in step a), a fourth layer of undoped or weakly doped (Al,In,Ga)N (124) covers the third layer of heavily doped (Al,In,Ga)N (123) or in that, after step d), a fourth layer of undoped or weakly doped (Al,In,Ga)N (124) is deposited on the partially porosified third layer of (Al,In,Ga)N (123').

4. Method according to claim 3, characterized in that the fourth layer (124) of undoped or weakly doped (Al,In,Ga)N is a layer of GaN.

5. A method according to any one of claims 1 to 4, characterized in that step d) is carried out by stopping the voltage or current before the complete porosification of the third layer of (Al,In,Ga)N (123), whereby the non-porified zone (125) corresponds to the central part of the partially porosified (Al,In,Ga)N layer (123').

6. A method according to any one of claims 1 to 4, characterized in that the zone(s) (125) of the third layer of (Al,In,Ga)N (123) form rings (125), each ring delimiting a core (127) preferably having an electrical conductivity at least ten times greater than the second electrical conductivity, wherein in step d), the cores (127) are not porosified and form electrical conduction channels.

7. A method according to any one of the preceding claims, characterized in that, after step d), the method comprises a step in which a heat treatment is carried out, thereby increasing the second electrical conductivity and obtaining zones (125') having a third electrical conductivity greater than the second electrical conductivity.

8. A method according to any one of claims 1 to 7, characterized in that the third (Al,In,Ga)N layer (123) of step a) is obtained by the following steps: - providing a heavily doped (Al,In,Ga)N layer having a first electrical conductivity, - locally decreasing the electrical conductivity of the (Al,In,Ga)N layer, thereby forming an (Al,In,Ga)N layer (123) comprising a first heavily doped portion having a first electrical conductivity and a second portion formed of one or more zones (125), having a second electrical conductivity. less than ten times lower than the first electrical conductivity.

9. A method according to any one of claims 1 to 7, characterized in that the third layer of (Al,In,Ga)N (123) of step a) is obtained according to the following steps: - providing a layer of (Al,In,Ga)N having a second electrical conductivity, - locally increasing the electrical conductivity of the layer of (Al,In,Ga)N, thereby forming a layer of (Al,In,Ga)N (123) comprising a first heavily doped part having a first electrical conductivity and a second part formed of one or more zones (125), having a second electrical conductivity at least ten times lower than the first electrical conductivity.

10. A method according to any one of the preceding claims, characterized in that three groups of mesas are formed, each group of mesas being intended to form a red, green or blue micro-LED, each group of mesas having a different porification rate or percentage of porosified surface.

11. A method for manufacturing micro-LEDs, the method comprising the following successive steps: i) carrying out the mesa porosification process according to any one of the preceding claims, ii) carrying out the following steps e) to g): e) on the structure obtained in step i), perform a re-epitaxial process to form re-epitaxial LEDs comprising layers of n-InGaN layers (130) and a p-doped InGaN layer (131), then form a contact electrode (210), a passivation layer (122) that can be positioned between the contact electrode (210) and the p-doped InGaN layer (131), the passivation layer (122) locally covering the p-doped InGaN layer (131), f) transfer the structure obtained in step e) onto a substrate (200), g) remove the support layer (114), where applicable the (Al,Ga)N buffer layer (115), the first undoped GaN layer (111) and the second doped GaN layer (112),h) to re-establish contact (150) on the non-porous zone (125) or at least on one of the non-porous zones (125) of the partially porosified (Al,In,Ga)N layer (123').

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