Method for porosifiying a mesa for establishment of contact

Abstract

The invention relates to a method comprising the steps of: a) providing a structure that comprises a base substrate (110) covered with AlInGaN / AHnGaN mesas (120); the base substrate (110) comprising a carrier layer (114), a first undoped GaN layer (111) and a second doped GaN layer (112); the mesas (120) comprising a third heavily doped AlInGaN layer (123); b) connecting the structure and a counter electrode to a generator; c) dipping the structure and the counter electrode into an electrolytic solution; d) applying a voltage or current so as to partially porosify the third AlInGaN layer (123) of the mesas (120) to form a porosified layer (123'); at least one region (125) of the heavily doped AlInGaN layer not being porosified, the at least one unporosified region (125) forming an electrical conduction channel between the main faces of the porosified layer (1231).

Description

[0001] MESA POROSIFICATION PROCESS FACILITATING CONTACT RESUMPTION

[0002] TECHNICAL FIELD

[0003] The present invention relates to the general field of color micro-displays. The invention relates to a method for porosifying (Al, In, Ga)N / (Al,ln,Ga)N mesas. The invention also relates to a structure thus obtained comprising porosified (Al,ln,Ga)N / (Al,ln,Ga)N mesas. The invention finds applications in many industrial fields, and in particular in the field of color micro-displays based on micro-LEDs.

[0004] STATE OF THE PRIOR ART

[0005] Color microdisplays consist of pixels made up of blue, green, and red subpixels (RGB pixels). In the following description, these subpixels 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 so-called "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 not only to alignment problems, but also to the time required to perform such a technique on this scale. For displays with a large number of pixels (high definition), this "pick and place" technique is problematic in terms of time. In addition, the pixels must be picked from different wafers, which requires successive transfers. Parallel transfer techniques can also be used (mass transfer).

[0007] Another solution is to achieve color conversion with quantum dots (QDs) or nanophosphors pumped by blue pLEDs from a single wafer, either transferred or in a monolithic matrix (preferred case for micro-screens). 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 with the same family of materials and grown on the same substrate. For this, InGaN is the most promising material. This material can, in fact, theoretically cover the entire visible spectrum depending on its indium concentration. Blue micro-LEDs based on InGaN already show high luminance, much higher than their organic counterparts. To emit at wavelengths in the green, the quantum wells (QWs) of the LED must contain at least 25% indium and for emission in the red, it is necessary to have at least 35% indium. Unfortunately, the quality of the InGaN material beyond 20% In is degraded due to the low miscibility of InN in GaN, but also due to the high compressive stress inherent in the growth of the InGaN active zone 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 includes the following steps:

[0011] - provide a stack comprising a sapphire substrate covered by a layer of unintentionally doped GaN (nested GaN), a layer of Si n+ doped GaN (5el0 18 at / cm 3 ) and a layer of InGaN or GaN not intentionally doped,

[0012] - partially etch the stack to form the GaN / InGaN or GaN / GaN mesas; 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 the mesas ('ring polarization' or plate edge polarization for example)

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

[0014] The resulting porosified GaN layer can be used to grow 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 realize a native color micro-display based on all-InGaN red, green, blue (RGB) micro-LEDs with porous mesas, the method comprises, for example, the following steps:

[0017] - providing a structure comprising a substrate covered with mesas (figure 1A): the substrate comprising a support layer 14, a buffer layer 15, a non-intentionally doped GaN layer 11, a doped or heavily doped GaN layer 12, the mesas comprising a porosified GaN layer 23' and a nested GaN layer 24, the mesas being covered by a re-epitaxed LED comprising n-InGaN 30 and p-InGan 31, and more particularly at least one layer 30 of n-InGaN, an active zone with quantum wells and an upper layer 31 of p-InGaN, - forming an electrode 21 on the structure obtained: the electrode 21 comes into contact with at least a portion of the upper layer 30 of p-InGaN (if this electrode is also present on the sides of the LED, it can be isolated from the 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 (figure 1B), comprising a via (not shown), for example by metal-metal bonding;

[0018] - remove the growth substrate, then make a contact resumption, for example a cathode contact resumption 50 on the n-GaN with a transparent conductive oxide (TCO) 45 without occulting metal (figure IC); passivation layers 40 can cover the TCO 45.

[0019] However, since there is a reduction in vertical electrical conduction through the porosified mesas, it can be difficult to inject current into the n-InGaN through the porous mesas. Similarly, heat dissipation is poor. Both of these factors reduce the reliability of the resulting device.

[0020] To improve integration, it is possible to reconnect the InGaN to the cathode after transfer.

[0021] To do this, two approaches are possible.

[0022] In a first approach shown in Figures 2A to 2C, it is possible to completely remove the porous part of the mesas. After removal of the growth substrate, etching is carried out up to the n-InGaN layer 30 of the LED. It is then possible to make a metal contact 50 on the cathode. An electrically insulating layer 40 protects the layer 30 of the LED. However, this first approach has several drawbacks: after removal of the support layer 14, it is difficult to control at the scale of the substrate ('wafer') the removal of all the (Al,Ga,ln)N layers to stop in the n-InGaN layer 30 that we want 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 high thickness of the stack to be removed and the tolerance for stopping in layer 30 of n-InGaN is low due to its low thickness.In addition, the strong topography of the contact recovery is difficult to reconcile with a transparent conductive oxide electrode. Contact 50 of the metal cathode is occulting, which reduces extraction.

[0023] In a second approach shown in Figures 3A to 3C, it is possible to partially remove the porous layer 23' from the mesas and then locally etch the porous layer 23' to make contact on the non-porous InGaN layer 30 of the LED. This would improve optical extraction due to the presence of residual porous material on the surface. For this, a localized etching of the porous material must be carried out, with a stop in the thin n-InGaN layer 30. Non-uniformity problems can result, as for the first approach. The contact of the metal cathode is always occulting, which reduces extraction. In these two micro LED array manufacturing processes, a variation in the thickness (TTV: "total thickness variation") of the GaN is introduced, not only at the wafer scale but also from wafer to wafer.These thickness variations come from both variations in epitaxy thickness but especially from thinning processes or planarization processes used to remove the buffer layers of the epitaxy several microns thick, due to the non-uniformities of the processes used.

[0024] STATEMENT OF THE INVENTION

[0025] An aim of the present invention is to propose a method for manufacturing micro-LEDs which overcomes the drawbacks of the prior art, and in particular a method which makes it possible to easily make contact on the n-InGaN with the cathode while maintaining good extraction.

[0026] For this purpose, the present invention proposes a method for manufacturing and porosifying (AI,ln,Ga)N / (AI,ln,Ga)N mesas comprising the following steps: a) providing a structure comprising a base substrate covered with (AI,ln,Ga)N / (AI,ln,Ga)N mesas, the base substrate comprising a support layer, optionally a buffer layer of (AI,Ga)N, a first layer of undoped GaN, a second layer of doped GaN, the (AI,ln,Ga)N / (AI,ln,Ga)N mesas comprising a third layer of (AI,ln,Ga)N having a first main face and a second main face, the third layer of (AI,ln,Ga)N being heavily doped or the third layer of (AI,ln,Ga)N comprising a first heavily doped portion having a first electrical conductivity and a second portion formed of one or more zones, the second portion having a second electrical conductivity at least ten times lower than the first electrical conductivity,a portion of the second doped GaN layer being able to extend into the mesas, b) electrically connecting the structure and a counter electrode to a voltage or current generator, c) immersing the structure and the counter electrode in an electrolytic solution, d) applying a voltage or a current between the structure and the counter electrode so as to partially porosify the third highly doped (AI,ln,Ga)N layer of the mesas or so as to porosify the first highly doped portion of the third (AI,ln,Ga)N layer of the mesas, whereby a (AI,ln,Ga)N layer is obtained comprising a first porosified portion and a second portion formed of one or more non-porosified zones, each non-porosified zone extending from the first main face to the second main face of the partially porosified (AI,ln,Ga)N layer to form an electrical conduction channel.

[0027] The invention is fundamentally distinguished from the prior art by the presence of one or more non-porosified zones in the mesas. The non-porosified zones have a higher electrical conductivity than the porosified zones. These non-porosified zones therefore form electrical conduction channels.

[0028] Advantageously, during step a), a fourth layer of undoped or lightly doped (AI,ln,Ga)N covers the third layer of heavily doped (AI,ln,Ga)N of the (AI,ln,Ga)N / (AI,ln,Ga)N mesas or in that, after step d), a fourth layer of undoped or lightly doped (AI,ln,Ga)N is deposited on the third layer of porosified (AI,ln,Ga)N.

[0029] Advantageously, the fourth layer of undoped or lightly doped (Al,ln,Ga)N is a layer of GaN.

[0030] Advantageously, the structure further comprises an additional layer of heavily doped GaN disposed between the first layer of undoped GaN and the second layer of doped GaN.

[0031] According to a first advantageous embodiment, step d) is carried out by stopping the voltage or current before the complete porosification of the third layer of (Al,ln,Ga)N, whereby the non-porosified zone corresponds to the central part of the porosified layer of (Al,ln,Ga)N (i.e. the porosified part surrounds the porosified part). This makes it possible to preserve non-porosified 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 having porosified the core of the mesas. The conductivity of the non-porosified core remains intact and forms a privileged conduction channel. The relaxation of the edges is naturally favored by the free surfaces.

[0032] According to a second advantageous embodiment, one or more zones, having a second conductivity, are formed in the third layer of heavily doped (Al,ln,Ga)N, the second conductivity being at least ten times lower than the first conductivity, whereby during 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-dope" zones (i.e. to reduce the electrical conductivity of the zones) which will not be porosified or only slightly porosified during the anodizing process, which is very selective for doping. The reduction in conductivity makes it possible to preserve conduction channels. The zone(s) of second conductivity are not porosified or only slightly porosified during step d).

[0033] Advantageously, after step d), the method comprises a step during which a heat treatment is carried out, whereby the second electrical conductivity is increased and areas having a third electrical conductivity are obtained. This healing annealing makes it possible to at least partially recover 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.

[0034] According to another embodiment, the zone(s) of the third layer of (AI,ln,Ga)N form crowns, each crown 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 porosified and form electrical conduction channels.

[0035] The crowns extend from the first main face to the second main face of the third layer of heavily doped (Al,ln,Ga)N. This allows the creation, in the center of the crowns with degraded conductivity, of n++ channels in the mesas. The n++ GaN of the channels in the mesas thus remains intact without implantation or porosification because it is protected with the less doped, or even undoped, "shell". The shell of lower conductivity is, for example, obtained by ion implantation, in particular by He implantation. Thus, the de-doped zone is tube-shaped. This option allows the original epitaxial doping to be preserved. The electrical conductivity at the center of the crown 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 250nm. The thickness is defined by the limitations of lithography and implantation.

[0036] The third layer of (AI,ln,Ga)N from step a) can be obtained according to the following steps:

[0037] - provide a layer of heavily doped (AI,ln,Ga)N having a first electrical conductivity,

[0038] - locally reducing the electrical conductivity of the (AI,ln,Ga)N layer, to form a (AI,ln,Ga)N layer 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.

[0039] 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-dope" areas (i.e. to reduce the electrical conductivity of the areas) which will not be porosified or will be only slightly porosified during the anodizing process, which is very selective for doping. The reduction in conductivity makes it possible, in particular, to preserve conduction channels. The area(s) of second conductivity are not porosified or will be only slightly porosified during step d).

[0040] According to another advantageous embodiment variant, the third layer of (AI,ln,Ga)N of step a) is obtained according to the following steps:

[0041] - providing a layer of (AI,ln,Ga)N having a second electrical conductivity, - locally increasing the electrical conductivity of the layer of (AI,ln,Ga)N, to form a layer of (AI,ln,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.

[0042] The electrical conductivity of the (Al,ln,Ga)N layer can be increased, for example, by ion implantation of Si. A healing annealing can also be performed.

[0043] 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 mesa group having a different porosification rate or percentage of porosified surface.

[0044] The invention also relates to a method for manufacturing micro-LEDs comprising the following successive steps: i) implementing the mesa manufacturing method as defined above, ii) implementing the following steps e) to g): e) on the structure obtained in step i), performing a resumption of epitaxy to form re-epitaxed LEDs comprising layers of the 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 a so-called lower electrode after transfer), a passivation layer being able to 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 flanks of the p-doped InGaN layer, f) transferring the structure onto a substrate, for example by metal-metal bonding, g) removing the support layer,where appropriate the buffer layer in (AI,Ga)N, the first layer of undoped GaN and part or all of the second layer of doped GaN (it is possible to stop the etching in this layer so that the remaining part is part of the mesa), h) making a contact recovery on the non-porosified zone or at least on one of the non-porosified zones of the porosified (AI,ln,Ga)N layer,

[0045] Such a process is particularly advantageous because it is possible to:

[0046] - generate mesas with different relaxations to allow coepitaxy while preserving conduction;

[0047] - use ion implantation at the mesa or intra-mesa scale to vary relaxation (monolithic channel); - use ion implantation at the mesa or intra-mesa scale to vary relaxation (core-shell channel);

[0048] - choose the implanted area to have uniformity of relaxation and / or optimization for the circulation of the electrolyte;

[0049] - reduce the conductivity over the entire surface of the channel by implantation from n+4- (and possibly carry out a healing annealing);

[0050] - modulate the implantation conditions to reduce the conductivity of the canal or shell as desired.

[0051] The invention also relates to a structure comprising a base substrate covered with porosified (AI,ln,Ga)N / (AI,ln,Ga)N mesas, the base substrate comprising a support layer, optionally a (AI,Ga)N buffer layer, a first undoped GaN layer and a second doped GaN layer, the GaN / (AI,ln,Ga)N mesas comprising a third layer of partially porosified (AI,ln,Ga)N having a first main face and a second main face, and, preferably, a fourth layer of undoped or lightly doped (AI,ln,Ga)N, a portion of the second doped GaN layer extending into the mesas or a portion of the third heavily doped (AI,ln,Ga)N layer extending into the base substrate, the partially porosified (AI,ln,Ga)N layer comprising one or more non-porosified areas, each non-porosified area ranging from the first main face to the second main face of the layer of (AI,ln,Ga)N partially porosified to form an electrical conduction channel.,

[0052] Advantageously, the structure further comprises an additional layer of heavily doped GaN disposed between the first layer of undoped GaN and the second layer of doped GaN.

[0053] Such a structure has many advantages:

[0054] - better tolerance during the etching stage, and in particular for the III-N post-report etching stop,

[0055] - less topography for contact recovery for the cathode (via + metal or transparent conductive oxide (TCO)),

[0056] - a good compromise between the relaxation rate and vertical conduction.

[0057] The invention also relates to an optoelectronic device comprising successively:

[0058] - a support substrate, covered by a lower electrode, - a re-epitaxed LED comprising layers of n-InGaN layers and a layer of p-doped InGaN,

[0059] - a layer of undoped or lightly doped GaN,

[0060] - a partially porosified (AI,ln,Ga)N layer, having a first main face and a second main face, the partially porosified (AI,ln,Ga)N layer comprising one or more non-porosified zones extending from the first main face to the second main face to form an electrical conduction channel,

[0061] - a resumption of contact on the non-porosified zone or at least on one of the non-porosified zones of the partially porosified (AI,ln,Ga)N layer.

[0062] These non-porous areas limit the impact on pressure drop (higher Vf) and / or improve optical extraction by reducing or even eliminating partial occultation. In addition, the non-porous channel(s) promote heat dissipation, which increases reliability.

[0063] Thus, it is possible to eliminate the partial occultation due to metals, by making the cathode contact directly above the n-(ln,Al,Ga)N exclusively with a TCO (i.e. without occulting metal), and by adding a metallic contact recovery, for example, by a grid placed in the interpixels allowing the potential applied to the cathode to be homogenized.

[0064] In addition, the presence of porosified zones allows:

[0065] - relax the specifications for etching the thick AIGaN / GaN stack to electrically access the n-InGaN while limiting the charge loss,

[0066] - reduce the topography for the cathode contact recovery allowing the use of a TCO without occultation,

[0067] - modulate the shape and number of non-porosified injection zones to allow maximum relaxation at the center of the mesa and limit relaxation non-uniformity at the mesa scale,

[0068] - improve optical extraction (this effect is based on an optical diffusion phenomenon in the case of large pores).

[0069] Other characteristics and advantages of the invention will emerge from the additional description which follows.

[0070] It goes without saying that this additional description is given only as an illustration of the subject of the invention and should in no case be interpreted as a limitation of this subject. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] The present invention will be better understood by reading the description of exemplary embodiments given purely for informational purposes and in no way limiting, with reference to the appended drawings in which:

[0072] Figures 1A to 1C, previously described, schematically represent different steps of a method of manufacturing and transferring a micro-LED according to the prior art;

[0073] Figures 2A to 2C, previously described, represent, schematically and in section, different steps for forming a contact connection from the structure of Figure 1B according to a method of the prior art;

[0074] Figures 3A to 3C, previously described, represent, schematically and in section, different steps for forming a contact recovery from the structure of Figure 1B according to another method of the prior art;

[0075] Figures 4A to 4D schematically represent different steps of a method of manufacturing a microLED according to a particular embodiment of the invention;

[0076] Figures 5A and 5B schematically represent different steps of a method of manufacturing a mesa according to a first embodiment of the method according to the invention, the structures are represented in section along the dotted line of Figures 5C and 5D;

[0077] Figures 5C and 5D represent, in top view, the mesas of Figures 5A and 5B respectively, according to the section line shown in dotted lines in Figures 5A and 5B;

[0078] Figures 6A to 6D schematically represent different steps of a method of manufacturing a mesa according to a second embodiment of the method according to the invention, the structures are represented in section along the dotted lines of Figures 6E, 6F, 6G and 6H;

[0079] Figures 6E to 6H represent, in top view, the mesas of Figures 6A, 6B, 6C and 6D respectively, according to the section line shown in dotted lines in Figures 6A, 6B, 6C and 6D;

[0080] Figures 7A to 7D schematically represent different steps of a method of manufacturing a mesa according to a variant of the second embodiment of the method according to the invention, the structures are represented in section along the dotted lines of Figures 7E, 7F, 7G and 7H;

[0081] Figures 7E to 7H represent, in top view, the mesas of Figures 7A, 7B, 7C and 7D respectively, according to the section line shown in dotted lines in Figures 7A, 7B, 7C and 7D;

[0082] Figures 8A to 8C schematically represent different steps of a method for manufacturing a mesa according to a third embodiment of the method according to the invention, the structures are represented in section along the dotted lines of Figures 8D, 8E and 8F; Figures 8D to 8F represent, in top view, the mesas of Figures 8A, 8B and 8C respectively, along the section line shown in dotted lines in Figures 8A, 8B and 8C;

[0083] Figure 9 is a snapshot obtained using a scanning electron microscope of a mesa obtained according to the first embodiment of the invention.

[0084] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0085] The different parts represented in the figures are not necessarily on a uniform scale, to make the figures more readable.

[0086] The different possibilities (variants and embodiments) must be understood as not being mutually exclusive and can be combined with each other.

[0087] Furthermore, in the following description, terms that depend on the orientation, such as "above", "below", etc. of a structure apply with the assumption that the structure is oriented as illustrated in the figures.

[0088] Although this is in no way limiting, the invention finds particular applications in the field of color micro-screens, and more particularly for the manufacture of red green blue pixels. However, it could be used in the field of photovoltaics or even water electrolysis ("water splitting") since, on the one hand, InGaN absorbs in the entire visible spectrum and, on the other hand, its valence and conduction bands are around the stability domain of water, a thermodynamic condition necessary for the water decomposition reaction. The invention may also be of interest for the manufacture of LEDs or lasers emitting at long wavelengths.

[0089] The process is particularly interesting for manufacturing structures comprising porosified (AI,ln,Ga)N / (AI,ln,Ga)N mesas having, in particular, a pitch of less than 30 pm.

[0090] By (Al,ln,Ga)N we mean AIN, AIGaN, InGaN or GaN. Hereinafter, we refer more specifically to porous GaN, but with such a process, it is possible to have, for example, porous InGaN or AIGaN. The dense InGaN layer (in compression) or the dense AIGaN layer (in tension) will relax thanks to a porous structure regardless of its composition.

[0091] We will now describe in more detail the porosification process of (AI,ln,Ga)N / (AI,ln,Ga)N with reference to Figures 4A to 4D, 5A to 5D, 6A to 6H, 7A to 7H and 8A to 8F.

[0092] The method for porosifying mesas 120 of (AI,ln,Ga)N / (AI,ln,Ga)N comprises the following steps: a) providing a structure 100 comprising a base substrate 110 covered with mesas 120 (AI,ln,Ga)N / (AI,ln,Ga)N (Figures 5A, 6A-6B, 7A-7B, 8A-8B), the base substrate 110 successively comprising:

[0093] - a support layer 114,

[0094] - possibly a buffer layer 115 in (AI,Ga)N, in particular in the case of a support layer 114 in silicon,

[0095] - a first layer 111 of undoped GaN,

[0096] - advantageously, an additional heavily doped layer,

[0097] - a second layer 112 of doped GaN, the mesas 120 (AI,ln,Ga)N / (AI,ln,Ga)N comprising a third layer 123 of (AI,ln,Ga)N intended to be partially porosified, the third layer 123 of (AI,ln,Ga)N being heavily doped or the third layer 123 of (AI,ln,Ga)N comprising a first heavily doped portion having a first conductivity and a second portion formed of one or more zones 125, the second portion having a second conductivity at least ten times lower than the first conductivity, a portion of the second layer 112 of doped GaN being able to extend into the mesas 120, b) electrically connecting the structure 100 and a counter-electrode to a voltage or current generator, c) immersing the structure 100 and the counter-electrode in an electrolytic solution, d) applying a voltage or a current between the structure 100 and the counter-electrode so as to partially porosify the third layer 123 of (AI,ln,Ga)N heavily doped mesas 120 or so as to porosify the first heavily doped portion of the third layer 123 of (AI,ln,Ga)N of the mesas (the zone(s) 125 of the second portion not being porosified), whereby a layer of (AI,ln,Ga)N is obtained comprising a first porosified portion 123' and a second portion formed of one or more non-porosified zones 125, each non-porosified zone 125 extending from the first main face to the second main face of the partially porosified (AI,ln,Ga)N layer 123' to form an electrical conduction channel (Figures 4A, 5B, 6C, 7C, 8C).,

[0098] In step a), a fourth layer 124 of undoped or lightly doped (AI,ln,Ga)N may cover the third layer 123 of heavily doped (AI,ln,Ga)N of the mesas 120 (AI,ln,Ga)N / (AI,ln,Ga)N.

[0099] Alternatively, after step d), a fourth layer 124 of undoped or lightly doped (AI,ln,Ga)N may be deposited on the third layer 123' of porosified (AI,ln,Ga)N. After step d), the doping of the epitaxial recovery layer is not critical since the porosification has already been done. The structure 100 provided in step a) is, for example, obtained by providing and then locally etching a stack successively comprising:

[0100] - a support layer 114,

[0101] - possibly, a buffer layer 115 in (AI,Ga)N, in particular in the case of a support layer 114 in silicon,

[0102] - a first layer 111 of undoped gallium nitride GaN,

[0103] - possibly, an additional layer of heavily doped GaN (not shown in the figures),

[0104] - a second layer 112 of doped GaN (GaN n),

[0105] - a third layer 123 of heavily doped GaN (GaN n+ or GaN n++) or a third layer 123 of GaN comprising a first heavily doped part and a second less doped part 125, and

[0106] - a fourth layer 124 in AIN, InGaN or GaN (noted (AI,ln,Ga)N) not intentionally doped (nest) or lightly doped where appropriate.

[0107] Preferably, the stack consists of the layers mentioned above. In other words, it does not include any other layers.

[0108] According to an advantageous embodiment, a first part of the second layer 112 is part of the base substrate 110 and a second part of the second layer 112 is part of the mesas 120.

[0109] 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 etch in the doped layer, the entire height of the third layer 123 of heavily doped GaN is available for relaxation.

[0110] Each mesa 120 comprises successively from the base: the second part of the layer 112 of doped GaN, the third layer 123 of heavily doped GaN and the fourth layer of undoped or lightly doped (AI,ln,Ga)N 124.

[0111] The first part of the second 112 doped GaN layer protects the additional layer 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 layer common to all mesas.

[0112] The structuring of the stack is, for example, carried out by photolithography.

[0113] Thus, a structure 100 is obtained comprising a base substrate 110 surmounted by a plurality of mesas 120 made of (AI,ln,Ga)N / (AI,ln,Ga)N. The mesas 120, also called elevations, are raised elements. 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 SiCh. 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 releasing it only in the areas used for polarization for electrochemical polarization. Advantageously, the mask is removed before the porosification step.

[0114] Preferably, the sides of the mesas 120 are perpendicular to this stack of layers.

[0115] The surface of the mesas can be, for example, circular, hexagonal, square or rectangular.

[0116] The largest dimension of the surface of the mesas 120 ranges from 500nm to 500pm, preferably from 1 to 10pm and even more preferably from 3 to 5pm. For example, the largest dimension of a circular surface is the diameter.

[0117] The thickness (or depth) of the mesas corresponds to the dimension of the mesa perpendicular to the underlying stack. The depth of the mesas ranges from 0.3 to 2 pm, preferably from 0.5 to 1 pm.

[0118] The spacing between two consecutive 120 mesas ranges from 50nm to 20pm.

[0119] The 120 mesas can have identical or different dopings. The higher the doping rate, the greater the porosification will be at fixed potential. The relaxation of the fourth 124 layer of dense (Al,ln,Ga)N depends on the porosification rate of the mesas. Thus, different quantities of indium can be integrated during the re-epitaxy of InGaN on the dense 124 layer (thanks to the reduction of the "compositional pulling effect" (i.e. the pushing of the In atoms towards the surface, preventing them from being incorporated into the layer). We will thus obtain, after epitaxy of the complete LED structure, blue, green and red (RGB) mesas on the same substrate, and in a single growth step, if the difference between the relaxation levels of the mesas is sufficient.

[0120] The support layer 114 is, for example, made of sapphire or silicon.

[0121] The support layer 114 has a thickness ranging, for example, from 250 μm to 2 mm. The thickness depends on the nature of the support layer 114 and its dimensions. For example, for a 2-inch diameter sapphire support layer, the thickness may be 350 μm. For a 6-inch diameter sapphire support layer, the thickness may be 1.3 mm. For a 200 mm diameter silicon support layer, the thickness may be 1 mm.

[0122] In the case of a support layer 114 made of silicon, a buffer layer made of (AI,Ga)N is advantageously interposed between the support layer 114 and the layer 111 of GaN nest.

[0123] The first layer 111 is an undoped GaN layer. By undoped, we mean not intentionally doped (nest). It is a nest layer to avoid porosification. By unintentionally doped GaN, we mean without the intentional addition of doping species during the growth of the GaN, for example with a concentration lower than 1017 at / cm 3 .

[0124] The first layer 111 in GaN nest has, for example, a thickness ranging from 500nm to 5pm. Advantageously, its thickness is between 1 and 4pm to absorb the constraints linked to the mesh mismatch between the GaN and the substrate.

[0125] The second layer 112 is a doped GaN layer. By doped GaN we mean a concentration between 6.10 17 at / cm 3 and 5.10 18 at / cm 3 , preferably between 8.10 17 at / cm 3 and 2.10 18 at / cm 3 .

[0126] The second GaN layer 112 has a thickness ranging, for example, from 300nm to 1pm, preferably between 400 and 700nm. It must be sufficiently electrically conductive to be able to make contact on this layer during the electrochemical anodization step. The minimum thickness varies depending on the doping rate. The thickness of the layer 112 will be chosen so as to protect, if necessary, the heavily doped buried layer during anodization. This electrically conductive layer can be electrically connected to the voltage or current generator.

[0127] The third layer 123 may be a layer of heavily doped (AI,ln,Ga)N, for example GaN. By heavily doped (AI,ln,Ga)N is meant a concentration greater than 6.10 18 at / cm 3 , preferably greater than 8.10 18 at / cm 3 , or even greater than 10 19 at / cm 3. The doping level is therefore higher than that of the second layer 112. The concentration is, for example, between 6.10 18 at / cm 3 and 2.10 19 at / cm 3 , preferably between 7.10 18 at / cm 3 and 1.10 19 at / cm 3 in the case of n-doping with Si. In the case of Ge doping, for example by metal-organic vapor deposition (MOCVD), higher doping rates, typically up to 1.10 2 °at / cm 3 can be obtained. The third layer 123 has, for example, a doping ten times higher than the second layer 112. It has a thickness between 200 nm and 2 pm, preferably from 500 nm to 1 pm.

[0128] The third layer 123 may be a layer of (AI,ln,Ga)N comprising a first heavily doped part in which one or more zones 125 are arranged. The doping rate of the zone(s) 125 is at least ten times lower than the doping rate of the first heavily doped part. We will detail later the manufacture of this layer structured so as to have different doping rates.

[0129] The fourth layer 124 is an unintentionally doped or lightly doped (AI,ln,Ga)N layer. By lightly doped (AI,ln,Ga)N, we mean a doping between 2.10 17 at / cm 3 and 1.10 18 at / cm 3 . By undoped, we mean a doping rate of less than 10 17 at / cm 3 , especially for a GaN layer. For example, in the case of an InGaN layer, the doping is less than 5.10 17 at / cm 3. The porosification of a given doped layer will depend first of all on the applied potential. Moreover, if the layer to be porosified is heavily doped, lightly doped layers (typically having a doping at least a decade lower than the doping of the layer to be porosified) will not be porosified.

[0130] This can be a layer of AIN, AIGaN, InGaN or GaN. It has, for example, a thickness between 10nm and 200nm, preferably between 50 and 200nm. The doping is sufficiently low so that this layer is not porosified during step d).

[0131] This fourth layer 124 is not or only slightly impacted by porosification and serves as a seed for a resumption of growth. This fourth layer 124 is continuous to ensure the quality of the re-epitaxed layer, of a layer of (ln,Ga)N for example, on the structure.

[0132] The additional layer has a thickness of, for example, between 500nm and 5 pm, preferably between 1 pm and 3 pm. Preferably, it has a doping concentration greater than or equal to 5.10 18 at.cm 3 , preferably greater than 6.10 18 at. cm 3 , even more preferably greater than 8.10 18 at.cm 3 , or even greater than 10 19 at.cm 3 , for example 1.5.10 19 at.cm 3 . It has a doping concentration, for example, between 6.10 18 at / cm 3 and 2.10 19 at / cm 3 , preferably between 7.10 18 at / cm 3 and 1.10 19 at / cm 3. The additional heavily doped GaN layer may have the same or different doping as the third heavily doped GaN layer. The additional heavily doped GaN layer may have the same or different thickness as the third heavily doped (Al,ln,Ga)N layer 123.

[0133] The voltage applied during porosification will be chosen according to the doping of the various aforementioned layers, and in particular of the second layer 112, the third layer 123 and the additional layer, as well as the targeted doping rate.

[0134] The respective doping rates 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 layer 123 is porosified during step d).

[0135] Subsequently, n-type doping is described, but it could also be p-type doping.

[0136] By way of illustration and not limitation, according to an alternative embodiment, the structure 100 may comprise:

[0137] - a base substrate 110 successively comprising: a support layer 114 made of sapphire or silicon, possibly a buffer layer 115 made of (AI,Ga)N, a first layer 111 of undoped GaN having a thickness between 1 and 4pm, a first part of the second layer 112 of doped GaN of 500nm (1.10 18 at / cm 3 ),

[0138] - 120 GaN / (AI,ln,Ga)N mesas comprising successively: a second part of the second layer 112 of 100nm doped GaN (1.10 18 at / cm 3 ), a third layer 123 of heavily doped GaN of 800 nm (1.10 19 at / cm 3 ), and a layer of (AI,ln,Ga)N nest of 100nm. According to another embodiment variant, the structure 100 may comprise:

[0139] - a base substrate 110 comprising successively: a support layer 114 made of sapphire or silicon, possibly a buffer layer 115 made of (AI,Ga)N, a first layer 111 of undoped GaN having a thickness between 1 and 4pm, an additional layer of heavily doped GaN of 2pm (1.10 19 at / cm 3 ), a first part of the second layer 112 of 500nm doped GaN (1.10 18 at / cm 3 ),

[0140] - 120 GaN / (AI,ln,Ga)N mesas comprising successively: a second part of the second layer 112 of 100nm doped GaN (1.10 18 at / cm 3 ), a third layer 123 of heavily doped GaN of 800 nm (1.10 19 at / cm 3 ), and a fourth layer 124 of (AI,ln,Ga)N nest of 100nm.

[0141] In step b), the structure 100 and a counter electrode (CE) are electrically connected to a voltage or current generator. The device acts as a working electrode (WE). Subsequently, it will be called a voltage generator, but it could be a current generator allowing a current to be applied between the device and the counter electrode.

[0142] Contact is made on structure 100.

[0143] In particular, the contact can be made on the base substrate 110. The contact recovery can be made on the second layer 112 of doped GaN. The contact recovery 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 contact connections.

[0144] It is also possible to make contact on one of the other layers: on the fourth layer 124 of undoped or lightly doped (AI,ln,Ga)N, on the third layer 123 of heavily doped (AI,ln,Ga)N or on the additional layer of heavily doped GaN. In the case of contact resumption on a heavily doped layer, its opening will advantageously be limited to an area preserved from the electrolyte.

[0145] The contact recovery area can also be topped with a metal layer to improve contact for electrochemical polarization. This contact can be removed after porosification before epitaxy resumption.

[0146] 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 mesh.

[0147] 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, NaNCh or H2SO4. In step d), a voltage is applied between the structure 100 and the counter-electrode 500. The voltage can range from 1 to 30V for example. Preferably, it is 5 to 15V, and even more preferably from 6 to 12V, for example from 8 to 10V. The voltage is chosen according to the doping rates of the different layers, in order to obtain the desired selectivity. It is applied, for example, for a period ranging from a few seconds to several hours. Porosification is complete when there is no longer any current at imposed potential. At this point, the entire doped structure is porosified and the electrochemical reaction stops.

[0148] The electrochemical anodizing step can be carried out under ultraviolet (UV) light.

[0149] In step d), the third layer 123 of (Al,ln,Ga)N is partially porosified. Otherwise, one or more areas 125 of the heavily doped GaN layer 123 are not porosified in step d).

[0150] Each non-porous area 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 shaped like a channel or a tube, for example. Thus, it is possible to make a contact connection to the GaN layer at the level of this electrical conduction channel.

[0151] In addition, by choosing the position of the non-porous areas, it is possible to play on relaxation.

[0152] 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 (AI,ln,Ga)N layer, whereby the non-porosified zone 125 corresponds to the central part of the partially porosified (AI,ln,Ga)N layer 123'.

[0153] Porosification starts on the mesa flanks at the 123 GaN layer in contact with the electrolyte and extends towards the center of the GaN layer. As porosification progresses, the electrolyte progresses towards the core. This lateral porosification from the mesa edge is controlled by the duration of electrochemical porosification. The mesa core remains intact (i.e. it is not porosified). The mesa core forms conduction channels going from the first principal face to the second principal face of the partially porosified 123' (Al,ln,Ga)N layer. The core of the 123' layer thus created, here in n++ GaN, has intact conductivity.

[0154] This first embodiment is particularly suitable for mesas of large dimensions (for example greater than or equal to 5 pm).

[0155] According to a second advantageous embodiment, shown in FIGS. 6A to 6H, one or more zones 125 having a second electrical conductivity are formed in the layer 123 of heavily doped (Al,ln,Ga)N before the porosification step. The second conductivity is at least ten times lower than the second conductivity, whereby during step d), the zone(s) 125 of lower conductivity are not porosified.

[0156] These areas are, for example, obtained by localized ion implantation of the n++ 123 layer of the mesas in order to form non-porous pillars. The implantation leads to a degradation of the conductivity of the implanted part (this part is heavily doped before implantation and doped or even lightly doped after implantation). Preferably, the implantation makes it possible to modulate the doping by the order of a decade and in particular to reduce it by at least one decade (for example for GaN:Si, the doping rate can go from 1019 at / cm 3 at 10 18 at / cm 3 or less). During step d), this zone 125 is not or only slightly porosified because it is less conductive. The zone can be in the center of the mesa as shown in Figures 6B, 6C, 6F and 6G.

[0157] Advantageously, for this second embodiment, it is possible, after the porosification step d), to carry out a heat treatment. This is an annealing to heal the implantation defects. This annealing makes it possible to at least partially recover the conductivity of the non-porosified implanted zone and thus to form a conduction channel 125' of higher conductivity (Figures 6D and 6H).

[0158] According to an alternative embodiment, shown in Figures 7A to 7H, it is possible to implant several parts of the layer 123 of (AI,ln,Ga)N to modulate the position of the porosified zones and the non-porosified zones 125. This alternative is particularly interesting because it makes it possible to preserve the relaxation of the core and / or minimize the degradation of the re-epi seed surface.

[0159] As before, it is possible to carry out an annealing step to increase the conductivity of the pillars and have a 125' conduction zone with improved conductivity.

[0160] According to a third advantageous embodiment, shown in Figures 8A to 8F, the layer 123 of heavily doped (AI,ln,Ga)N is locally modified so as to form, in the third layer 123 of heavily doped (AI,ln,Ga)N, one or more zones 125 having a conductivity at least ten times lower than the first conductivity. These zones 125 extend from the first face to the second face of the layer 123. These zones preferably have a tube 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 layer 123 of (AI,ln,Ga)N (i.e. the inside of the tubes is heavily doped). The zones 125 form a protective barrier against anodization for the zones 127 to be protected. Thus, during step d), the core 127 of the tubes is not porosified and forms privileged conduction channels.

[0161] This third embodiment consists, for example, in carrying out a localized ion implantation of the third n++ layer 123 of the mesas, preferably in the form of crowns ('rings'), to electrically preserve n++ pillars, at the heart of the crowns, without degrading their conductivity. Thus, after step d), a non-porous implanted crown is obtained surrounding an intact non-implanted n++ core, the crown itself being in contact with the rest of the non-implanted and porosified layer. The non-porosified pillars ensure electrical conduction.

[0162] The implantation parameters are chosen so as to degrade (i.e. reduce) 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.

[0163] In Figures 8A to 8F, a single crown 125 / core 127 pattern is shown. Several patterns may advantageously be defined to form several conduction channels through the third layer 123. The positioning of the core / crown patterns will advantageously be chosen so as to obtain good relaxation.

[0164] According to the second embodiment and the third embodiment, preferably, the non-porous conduction pillars have a diameter of at least 250nm.

[0165] The channel is preferably solid (i.e. made of the same material).

[0166] The channel runs from the first principal face of the GaN layer to the second principal face of the 123 GaN layer. The channel height corresponds to the thickness of the third 123 GaN layer.

[0167] The surface of the channel can have a round, hexagonal, square, etc. shape. The largest dimension of the surface of the channel is preferably at least 0.25 μm, in particular with regard to ease of implementation (critical dimensions in photolithography).

[0168] The conduction channel(s) can be positioned in the center or at the periphery of the mesa.

[0169] 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 relaxation.

[0170] In these different embodiments, we have described that the different parts of the (AI,ln,Ga)N layer are obtained by providing a layer 123 of heavily doped (AI,ln,Ga)N having a first electrical conductivity, then locally decreasing the electrical conductivity of the (AI,ln,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.

[0171] Alternatively, it is also possible to form the different parts of the (Al,ln,Ga)N layer by providing a (Al,ln,Ga)N layer with low electrical conductivity (e.g. starting from an unintentionally doped or lightly doped layer) and then locally increasing the electrical conductivity of the (Al,ln,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 implantation of dopants (donors for n).

[0172] 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 crown 125 surrounding a conductive core 127 are dispersed.

[0173] The following table lists several data on mesas. The resistivity values ​​are taken from the literature.

[0174] From the values ​​in the table, it appears that a non-porous channel (or several non-porous channels, with an equivalent surface area of ​​lpm 2 ) can greatly reduce the vertical resistance of a highly porosified porous mesa of lOpm 2 (potential drop in the mesa reduced from 6V to 0.18V, i.e. more than a factor of 30). In this case, it is possible to keep the porous GaN in the device after transfer. The potential drop induced by the upper layer 124 of the mesa remains low compared to the potential drop of the mesa, which makes it possible to keep the mesa porous without significant degradation of the electro-optical characteristics.

[0175] The porosity rate of the porosified part of the third layer 123 of heavily doped (AI,ln,Ga)N is advantageously at least 10%. It preferably ranges from 25% to 70%, preferably from 25% to 50%, for example 45% 50%.

[0176] The largest dimension (height) of pores can vary from a few nanometers to a few micrometers. The smallest dimension (diameter) can vary from a few nanometers to a hundred nanometers, particularly from 30 to 70 nm.

[0177] The porosification obtained (porosity rate and pore size) depends on the doping of the layer and the process parameters (applied voltage, duration, nature and concentration of the electrolyte, chemical post-treatment or annealing). The variation of the porosification makes it possible to control the incorporation / segregation rate. The porosification, and in particular, the pore size, can vary subsequently, during the resumption of epitaxy depending on the applied temperature.

[0178] After step d), the method advantageously comprises the following steps: e) on the structure obtained in step i), performing a re-epitaxy to form re-epitaxed LEDs comprising layers of the layers 130 of n-InGaN and a layer 131 of p-doped InGaN (Figure 4B), then forming a contact electrode 210, a dielectric 122 being able to be deposited to protect the sides of the LED, f) transferring the structure obtained in step f) onto a substrate 200, for example by metal-metal bonding (Figure 4C), g) removing the support layer 114, where appropriate the buffer layer 115 of (Al,Ga)N, the first layer 111 of undoped GaN and all or part of the second layer 112 of doped GaN (preferably, the part which is located outside the mesas), h) performing a re-contact 150 on the non-porosified area 125 or at least on one of the non-porosified areas 125 of the layer 123' of partially porosified (AI,ln,Ga)N (figure 4D).

[0179] Advantageously, an electrically insulating layer 140 protects the porosified layer and / or prevents short circuits.

[0180] In step e), the contact electrode 210 reflector on p-InGaN is positioned on the structure. The contact electrode 210 is a so-called upper electrode before transfer and this same electrode is said to be lower after transfer.

[0181] A passivation layer 122 may 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 sides of the p-doped InGaN layer 131.

[0182] During step e), epitaxy is carried out on the mesas 120, whereby an epitaxial layer is obtained which is at least partially relaxed, and preferably completely relaxed.

[0183] For example, an all-InGaN LED structure may include:

[0184] - a 350nm n-doped InGaN layer, formed of 15 x Ino.osGao.szN / GaN (thicknesses 20nm / 1.8nm),

[0185] - multiple quantum wells (MQWs), formed of 5 x lno,4oGao,soN / Ino.osGao.ogzN (thicknesses 2, 3nm / 5, 7, 11 nm),

[0186] - a layer of Ino.osGao.szN nid (lOnm),

[0187] - an Alo.iGao.gN:Mg layer (20nm),

[0188] - a layer of Mg-doped Ino.osGao.gyN (125nm),

[0189] - a layer of p+++ doped Ino.osGao.gyN (25nm).

[0190] The percentage of relaxation corresponds to: Aa / a = (a C 2 - a c i) / a c i, with a ci the mesh parameter of the starting layer on which epitaxy is resumed (i.e. the mesh parameter of layer 124), and a C2 the lattice parameter of the relaxed layer, The layer is relaxed at 100% if ac2 corresponds to the lattice parameter of the bulk material, of the same composition as the re-epitaxed layer.

[0191] When a c i=a C 2, the layer is said to be constrained.

[0192] 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 rate of the mesas can be modulated to have different porosity rates and therefore relaxation rates depending on the mesas during the epitaxy regrowth of InGaN emitters. This makes it easier to obtain different emission colors depending on the mesas, for example, to obtain red, green, and blue emitters by growth on the same substrate. It is also possible to obtain different emission colors by changing the porosified / non-porosified surface ratio or by combining the two.

[0193] Epitaxial regrowth is preferably used to form re-epitaxial LEDs.

[0194] Epitaxial recovery is carried out on the fourth layer 124 of (AI,ln,Ga)N nest or weakly doped mesas 120. As this layer is not porosified during the electrochemical anodization step, it remains continuous and dense. Epitaxial recovery is thus facilitated and the epitaxial layer has better resistance. The creation of defects linked to the coalescence of the pores is avoided.

[0195] The epitaxial layer during this step e) is, advantageously, made of gallium nitride or indium and gallium nitride.

[0196] The In incorporation rate varies depending on the relaxation capacity (in-plane lattice parameter a). By varying the porosification rate and the porosified surface rate, the mesas can have different relaxation rates. For example, it is possible to vary the differential dedoping of GaN n++ by implantation (He).

[0197] 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 carry out successive epitaxies for each pLED.

[0198] Step f) is, for example, obtained with a metal-metal bond. It could also be a hybrid bond (metal-oxide). This step allows, in particular, the interconnection with a control circuit to dynamically modulate the emission of the microLEDs.

[0199] During step h), the contact resumption 150 can be carried out via the presence of a layer 145 of transparent conductive oxide (TCO). The TCO layer 145 is for example made of indium tin oxide (ITO). It is possible to cover the TCO layer 145 with an additional passivation layer 140 to protect it (figure 4D). e illustrative and non-limiting: A mesa comprising a layer of n++ doped GaN has been partially porosified. The porosification is stopped by stopping it before its complete completion (figure 9).

[0200] The central GaN n++ channel in the mesas exhibits intact conductivity.

Claims

CLAIMS 1. A method for porosifying (AI,ln,Ga)N / (AI,ln,Ga)N mesas comprising the following steps: a) providing a structure (100) comprising a base substrate (110) covered with (AI,ln,Ga)N / (AI,ln,Ga)N mesas (120), the base substrate (110) comprising a support layer (114), optionally a buffer layer (115) of (AI,Ga)N, a first layer (111) of undoped GaN, a second layer (112) of doped GaN, the (AI,ln,Ga)N / (AI,ln,Ga)N mesas (120) comprising a third layer (123) of (AI,ln,Ga)N having a first main face and a second main face, the third layer of (AI,ln,Ga)N (123) being heavily doped or the third layer (123) of (Al,ln,Ga)N 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 layer (112) of doped GaN being able to extend into the mesas (120), b) electrically connecting the structure (100) and a counter-electrode to a voltage or current generator, c) immersing the structure (100) and the counter-electrode in an electrolytic solution, d) applying a voltage or a current between the structure (100) and the counter-electrode so as to partially porosify the third layer (123) of heavily doped (Al,ln,Ga)N of the mesas (120) or so as to porosify the first heavily doped portion of the third layer (123) of (Al,ln,Ga)N of the mesas, whereby a layer (123') of (Al,ln,Ga)N is obtained comprising a first porosified portion and a second portion formed of one or more non-porosified zones (125), each non-porosified zone (125) extending from the first face main to the second main face of the layer (123') of (AI,ln,Ga)N partially porosified to form an electrical conduction channel., 2. Method according to claim 1, characterized in that the structure (100) further comprises an additional layer of heavily doped GaN arranged between the first layer (111) of undoped GaN and the second layer (112) of doped GaN.

3. Method according to one of claims 1 to 2, characterized in that, during step a), a fourth layer (124) of undoped or lightly doped (AI,ln,Ga)N covers the third layer (123) of heavily doped (AI,ln,Ga)N, or in that, after step d), a fourth layer (124) of undoped or lightly doped (AI,ln,Ga)N is deposited on the third layer (123') of partially porosified (AI,ln,Ga)N.

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

5. 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 (123) of (AI,ln,Ga)N, whereby the non-porosified zone (125) corresponds to the central part of the partially porosified layer (123') of (AI,ln,Ga)N.

6. Method according to any one of claims 1 to 4, characterized in that the zone(s) (125) of the third layer (123) of (AI,ln,Ga)N form crowns (125), each crown delimiting a core (127) preferably having an electrical conductivity at least ten times greater than the second electrical conductivity, whereby during step d), the cores (127) are not porosified and form electrical conduction channels.

7. Method according to any one of the preceding claims, characterized in that, after step d), the method comprises a step during which a heat treatment is carried out, whereby the second electrical conductivity is increased and zones (125') having a third electrical conductivity greater than the second electrical conductivity are obtained.

8. Method according to any one of claims 1 to 7, characterized in that the third layer (123) of (AI,ln,Ga)N of step a) is obtained according to the following steps: - provide a layer of heavily doped (AI,ln,Ga)N having a first electrical conductivity, - locally reducing the electrical conductivity of the (AI,ln,Ga)N layer, whereby a layer (123) of (AI,ln,Ga)N is formed comprising a first heavily doped portion having a first electrical conductivity and a second portion formed of one or more Tl zones (125), having a second electrical conductivity at least ten times lower than the first electrical conductivity.

9. Method according to any one of claims 1 to 7, characterized in that the third layer (123) of (AI,ln,Ga)N of step a) is obtained according to the following steps: - provide a layer of (AI,ln,Ga)N having a second electrical conductivity, - locally increasing the electrical conductivity of the (AI,ln,Ga)N layer, whereby a layer (123) of (AI,ln,Ga)N is formed 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. 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 mesa group having a different porosification rate or percentage of porosified surface.

11. A method of manufacturing micro-LEDs, the method comprising the following successive steps: i) implementing the mesa porosification method according to any one of the preceding claims, ii) implementing the following steps e) to g): e) on the structure obtained in step i), performing a resumption of epitaxy to form re-epitaxed LEDs comprising layers of the n-InGaN layers (130) and a layer (131) of p-doped InGaN, then forming a contact electrode (210), a passivation layer (122) being able to be positioned between the contact electrode (210) and the layer (131) of p-doped InGaN, the passivation layer (122) locally covering the layer (131) of p-doped InGaN, f) transferring the structure obtained in step e) onto a substrate (200), g) removing the support layer (114), where appropriate the buffer layer (115) of (AI,Ga)N, the first layer (111) of undoped GaN and the second layer (112) of doped GaN,h) making a contact resumption (150) on the non-porosified zone (125) or at least on one of the non-porosified zones (125) of the layer (123') of partially porosified (AI,ln,Ga)N., 12. Structure (100) comprising a base substrate (110) covered with porosified (AI,ln,Ga)N / (AI,ln,Ga)N mesas (120), the base substrate (110) comprising a support layer (114), optionally a buffer layer (115) of (AI,Ga)N, a first layer (111) of undoped GaN, and a second layer (112) of doped GaN, the (AI,ln,Ga)N / (AI,ln,Ga)N mesas (120) comprising a third layer (123') of partially porosified (AI,ln,Ga)N having a first main face and a second main face, and, preferably, a fourth layer (124) of undoped or lightly doped (AI,ln,Ga)N, the layer (123') of partially porosified (AI,ln,Ga)N porosified comprising one or more non-porosified zones (125, 125'), each non-porosified zone (125, 125') extending from the first main face to the second main face of the partially porosified (AI,ln,Ga)N layer (123') to form an electrical conduction channel.

13. Structure according to claim 12, characterized in that the structure (100) further comprises an additional layer of heavily doped GaN arranged between the first layer (111) of undoped GaN and the second layer (112) of doped GaN.

14. Optoelectronic device, successively comprising: - a support substrate (200), - a lower electrode (210), - a re-epitaxed LED comprising n-InGaN layers (130) and a p-doped InGaN layer (131), - a layer (124) of undoped or lightly doped (AI,ln,Ga)N, - a layer (123') of partially porosified (AI,ln,Ga)N, having a first main face and a second main face, the layer (123') of partially porosified (AI,ln,Ga)N comprising one or more non-porosified zones (125, 125') extending from the first main face to the second main face to form an electrical conduction channel, - a contact resumption (150) on the non-porosified zone (125) or at least on one of the non-porosified zones (125, 125') of the layer (123') of partially porosified (AI,ln,Ga)N.

Images