Porosification process of (Al,In, Ga)N / (Al,In,Ga)N

Abstract

Process for porosifying (Al,In,Ga)N / (Al,In,Ga)N mesas. This description relates to a process for porosifying a structure (100) comprising a base substrate (110) covered with (Al,In,Ga)N / (Al,In,Ga)N mesas (120), comprising a porosification step in which the (Al,In,Ga)N / (Al,In,Ga)N mesas are electrochemically porosified. During the porosification step, the structure (100) further comprises, between the mesas (120) or between groups of mesas (120), electrically conductive lines (200) covered by an electrically insulating element (300). Figure for the abstract: Fig. 3A

Description

Title of the invention: Process for the porosification of (AlJn,Ga)N / (AlJn,Ga)N mesas technical field

[0001] This description relates generally to the general field of color microdisplays, and in particular to the field of color microdisplays based on micro-LEDs.

[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. Previous technique

[0004] 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.

[0005] 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.

[0006] 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.

[0007] 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.

[0008] It is therefore essential to be able to reduce the overall stress in GaN / InGaN based structures.

[0009] 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:

[0010] - provide a stack comprising a sapphire substrate covered by a layer of unintentionally doped GaN (nid GaN), a layer of Si n+ doped GaN (5eL018 at / cm3) and a layer of InGaN or unintentionally doped GaN,

[0011] - 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)

[0012] - 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.

[0013] The porous layer of GaN thus obtained can allow the growth of a better crystalline quality InGaN-based nitride LED structure, thanks to the relaxation of the generated porous mesas.

[0014] However, the quality of the LED depends not only on the pore diameter and the porosity of the porosified GaN layer. It is therefore necessary to be able to porosify all the mesas of the same plate homogeneously.

[0015] It is also possible to have a common layer for all the mesas in these stacks. The porosification step is then carried out by polarizing this common layer with an anodic potential.

[0016] For example, Figures IA to IC represent different stages of such a process. More specifically, the process comprises the following stages: - provide a stack comprising successively a support layer 10, an unintentionally doped GaN layer 11, a doped or heavily doped GaN layer 12, an InGaN or nested GaN layer 13 ([Fig.1A]), - to structure mesas in this stacking, the mesas including the InGaN layer or of GaN nid 13 and part of the doped or heavily doped GaN layer 12 ( [Fig.1B]), the other part of the doped GaN layer playing the role of a common mesas layer, - immerse the structure obtained in an electrolytic solution and apply a voltage between the doped or heavily doped GaN layer 12 and a counter electrode, thereby obtaining a porosified GaN layer 12' ([Fig. IC]).

[0017] It is known that the InGaN nid layers have intrinsic defects ('V-pits'), which facilitates the porosification of the mesas because the electrolytic solution can infiltrate from this upper layer 13 of the mesas to the doped or heavily doped layer 12 through these defects at the same time as through the flanks of the doped or heavily doped layer of the mesas 12.

[0018] However, the GaN nid layers do not exhibit such defects: they are dense layers. Thus, in the GaN / GaN configuration, the doped or heavily doped layer of the mesas 12 porosifies from the mesa flanks towards the center of the mesas. This layer does not porosify completely to the center of the mesas. Edge / center effects of the wafer are observed, for example on 2-inch wafers. Figures 2A and 2B show the mesas at the center of such a structure, after porosification, in cross-section and top view, respectively. The mesas are only very partially porosified.

[0019] This effect may be due to the degradation of the lateral conductivity of the residual doped layer at the bottom of the mesa, the polarization taking place through the periphery of the plate ('wafer') hence the edge / center effect of the plate.

[0020] There is therefore a real need for a process to fully porosify the mesas and to limit the edge-center effects on the plates, especially on 2-inch (5.08cm) plates and even more so on larger plates (200mm and 300mm). Summary of the invention

[0021] There is a need to propose a process for porosifying (Al,In,Ga)N / (Al,In,Ga)N mesas, allowing for the identical (or homogeneous) porosification of all mesas on the wafer, whether they are at the edge or in the center of the wafer, the process being advantageously usable even for large substrates (typically on substrates of at least 5 cm and preferably of at least 10 cm in diameter). This goal is achieved by a porosification process of a structure comprising a basic substrate covered with (Al,In,Ga)N / (Al,In,Ga)N mesas, The process includes a porosification step during which the (Al,In,Ga)N / (Al,In,Ga)N mesas are electrochemically porosified, during the porosification stage, electrically conductive lines covered by an electrically insulating element are arranged on the base substrate between mesas or between groups of mesas.

[0022] According to an advantageous embodiment, the basic substrate comprises a support layer, a first undoped GaN layer, a second doped GaN layer.

[0023] Preferably, the (Al,In,Ga)N / (Al,In,Ga)N mesas comprise a third layer of heavily doped (Al,In,Ga)N and a fourth layer of undoped or lightly doped (Al,In,Ga)N. A portion of the second doped GaN layer extends into the mesas. During the porosification step, the electrically conductive lines are in direct contact with the second doped GaN layer.

[0024] The third layer of highly doped (Al,In,Ga)N of the mesas is porosified during the porosification step. The third layer of highly doped (Al,In,Ga)N of the mesas may be partially or totally porosified during the porosification step.

[0025] According to an advantageous embodiment, the electrically conductive lines are made of a metal, preferably titanium, or a conductive polymer.

[0026] According to another advantageous variant, the electrically conductive lines are made of heavily doped GaN.

[0027] According to this other advantageous variant, the process may include a step in which the electrically conductive lines are formed by epitaxy.

[0028] Advantageously, the electrically conductive lines form a grid.

[0029] Advantageously, the electrically conductive lines locally cover the base substrate and the space between the electrically conductive lines and the mesas is between 100 nm and 1 pm, preferably between 200 nm and 500 nm.

[0030] One embodiment provides a structure comprising a base substrate covered with porosified (Al,In,Ga)N / (Al,In,Ga)N mesas. The structure further comprises electrically conductive lines, covered by an electrically insulating element, arranged on the base substrate between the mesas or between groups of mesas.

[0031] Advantageously, the basic substrate comprises a support layer, a first undoped GaN layer and a second doped GaN layer.

[0032] Preferably, the (Al,In,Ga)N / (Al,In,Ga)N mesas comprise a third, heavily doped, porosified GaN layer and a fourth, undoped or lightly doped (Al,In,Ga)N layer. A portion of the second doped GaN layer extends into the mesas.

[0033] The third layer of (Al,In,Ga)N heavily doped from the mesas can be porosified in part or totally.

[0034] According to one embodiment, the electrically conductive lines are made of heavily doped GaN, metal or conductive polymer. Brief description of the drawings

[0035] These features and advantages, as well as others, will be described in detail in the following description of particular embodiments, given by way of non-limiting example, in relation to the accompanying figures, among which:

[0036] Figures IA, IB and IC, previously described, schematically represent different stages of a porosification process according to the prior art;

[0037] The [Fig.2A], previously described, is a scanning electron microscope image of a (Al,In,Ga)N / (Al,In,Ga)N mesa, according to the prior art, after the porosification step: the porosification is incomplete,

[0038] Fig. 2B, previously described, is a dark-field optical microscope image of different (Al,In,Ga)N / (Al,In,Ga)N mesas, in top view, according to the prior art, after the porosification step, at the center of the plate; the part on the left of the figures corresponds to (Al,In,Ga)N / (Al,In,Ga)N mesas of 3 pm width and the part on the right of the figures corresponds to (Al,In,Ga)N / (Al,In,Ga)N mesas of 5 pm width,

[0039] Figures 3A, 3B, 3C schematically represent different stages of a mesas porosification process according to a particular embodiment of the invention;

[0040] Figure 4 schematically and in cross-section represents a structure comprising porosified (Al,In,Ga)N / (Al,In,Ga)N mesas and electrically conductive lines, according to an embodiment of the invention,

[0041] Figures 5A, 5B, 5C, 5D schematically represent different stages of a process for manufacturing a metal grid according to a particular embodiment of the invention;

[0042] Figures 6A, 6B, 6C, 6D schematically represent different stages of a process for manufacturing a metal grid according to another particular embodiment of the invention;

[0043] Figures 7A, 7B, 7C schematically represent different stages of a process for coating a metal grid with an electrically insulating element according to a particular embodiment of the invention; and

[0044] Figures 8A, 8B, 8C, 8D and 8E schematically represent different steps of a manufacturing process for a heavily doped GaN grid, covered by an electrically insulating element, according to a particular embodiment of the invention. Description of the implementation methods

[0045] The same elements have been designated by the same reference numerals in the different figures. In particular, the structural and / or functional elements common to the Different embodiments may have the same references and may have identical structural, dimensional and material properties.

[0046] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been represented and are detailed.

[0047] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "superior", "inferior", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures or to a ... in a normal position of use.

[0048] Unless otherwise specified, the expressions "approximately", "roughly", and "in the order of" mean within 10%, preferably within 5%.

[0049] 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.

[0050] 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.

[0051] By (Al,In,Ga)N, we mean AIN, AlGaN, InGaN, or GaN. Hereafter, we will 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.

[0052] With reference to Figures 3A, 3B and 3C, we will now describe in more detail the process of porosification of mesas 120 of (Al,In,Ga)N / (Al,In,Ga)N, which comprises the following steps:

[0053] i) provide a structure 100 comprising a basic substrate 110 covered with mesas 120 (Al,In,Ga)N / (Al,In,Ga)N, electrically conductive lines 200 covered by an electrically insulating element 300 being arranged on the basic substrate 110 between groups of mesas or between the mesas 120 ([Fig.3A]),

[0054] ii) porosify the mesas 120 ([Fig.3B]),

[0055] iii) optionally remove the electrically insulating element 300 or remove the lines electrically conductive 200 and the insulating element 300 ([Fig.3C]).

[0056] The use of electrically conductive lines 300 positioned between the mesas 120 makes it possible to obtain a structure in which each mesa 120 has an equivalent electrical configuration (the potential loss between the mesas is sufficiently low). The polarization of the mesas 120 is homogeneous, leading to good control of the porosity of the mesas on each motif. The process makes it possible not only to porosify all the mesas 120 on the plate identically (or homogeneously), whether they are at the edge or in the center of the plate, but also to porosify the entire volume of the doped layer of mesas 120. It is also possible to control the porosity of the mesas 120 in order to leave, for each mesa 120, a central unporified zone forming an electrical conduction channel within the mesas.

[0057] There are no edge-center effects for porification. The process is perfectly suited to patterns with dimensions greater than 5 pm and / or for large substrates (typically on substrates of at least 5 cm and preferably of at least 10 cm in diameter).

[0058] Furthermore, the upper layer of the substrate 110 in contact with the electrically conductive lines 200 can have less doping since electrical conduction is carried out by means of the electrically conductive lines 200. Thus, not only is the layer less exposed to undesired porification but also the layer is less stressed.

[0059] With such a process, the resulting structure exhibits good crystalline quality. There is no risk of porification of the underlying layers due to mesa toe defects. Re-epitaxial layers can be easily deposited on the structure without risk of stress leading to crack formation. The total re-epitaxial thickness can be high.

[0060] These lines are covered with an electrically insulating element that completely covers them and prevents them from coming into contact with the electrolyte during the porosification step. Thus, the lines are not damaged during this step; there is no loss of conductivity.

[0061] We will now describe in more detail the different stages of the process.

[0062] As shown in [Fig.3A] and [Fig.4], the basic substrate 110 of the structure 100 provided in step i) comprises successively: - a support layer 114, - possibly a buffer layer of (Al,Ga)N, particularly in the case of a silicon support layer 114, - a first layer of undoped GaN 111, - a second layer of GaN doped 112.

[0063] The 120 (Al,In,Ga)N / (Al,In,Ga)N mesas comprise a third layer of (Al,In,Ga)N heavily doped 123 and a fourth layer of (Al,In,Ga)N undoped or lightly doped 124.

[0064] At the end of step ii) of porosification, the third layer of (Al,In,Ga)N 123' of the mesas is porosified at least in part (i.e. in part or totally).

[0065] A portion 112b of the second layer of doped GaN 112 extends into the mesas 120. The other portion 112a of the second layer of doped GaN 112 forms the upper part of the substrate of the base 110. This portion is in direct contact with the electrically conductive lines 200. In other words, there is no element between the portion 112a of the second layer of doped GaN 112 and the electrically conductive lines 200. The portion 112a of the second layer of doped GaN 112 forms the bottom of the mesas.

[0066] The bottom of the mesas is preferably preserved from porification during step ii).

[0067] The structure 100 provided in step i) is, for example, obtained by providing then locally engraving a stack comprising successively: - a support layer 114, - possibly, a buffer layer of (Al,Ga)N, particularly in the case of a silicon support layer 114, - a first layer of undoped gallium nitride (GaN) 111, - a second layer of doped GaN (GaN n) 112, - a third, heavily doped GaN layer (GaN n+ or GaN nn) 123, and - a fourth layer in AIN, InGaN or GaN (noted (Al,In,Ga)N) unintentionally doped (nest) or weakly doped 124.

[0068] Preferably, the stack consists of the layers mentioned above. In other words, it does not include any other layers.

[0069] The support layer 114 is, for example, made of sapphire or silicon.

[0070] 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.

[0071] By "from X to Y", it is meant that the value is between X and Y, inclusive.

[0072] 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.

[0073] The first layer 111 is a GaN nest layer. It is an unintentionally doped (nest) layer so as not to be porosified. By GaN unintentionally Doped, we mean a concentration less than 17at / cm3.

[0074] 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.

[0075] The second layer 112 is a doped GaN layer. By doped GaN, we mean a concentration, preferably greater than 1 x 10¹⁷ at / cm³ and preferably less than 5 x 10¹⁸ at / cm³ and even more preferably less than 1 x 10¹⁸ at / cm³. The presence of electrically conductive lines makes it possible to have a GaN layer with a concentration less than 5 x 10¹⁷ at / cm³ and, for example, between 1 x 10¹⁷ at / cm³ and 5 x 10¹⁷ at / cm³.

[0076] The second GaN layer 112 has, for example, a thickness ranging from 200nm to 1 pm, preferably between 400 and 700nm. The minimum thickness varies depending on the doping level.

[0077] The third layer 123 is a heavily doped GaN layer. Heavily doped GaN is defined as having a concentration greater than 5 x 10¹⁸ at / cm³, preferably greater than 8 x 10¹⁸ at / cm³, or even greater than 10¹⁹ at / cm³. For example, it has a doping level 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. Preferably, the doping level of the third layer 123 is 30-fold or even 100-fold higher than that of the second layer 112.

[0078] The fourth layer 124 is an (Al,In,Ga)N layer that is either unintentionally doped or lightly doped. Lightly doped (Al,In,Ga)N means a doping level between 1 x 10¹⁷ at / cm³ and 5 x 10¹⁷ at / cm³. Undoped means a doping level below 1 x 10¹⁷ at / cm³.

[0079] 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 electrically insulating. It is not porosified in step d).

[0080] This layer 124 is not or only slightly affected by porification and serves as a nucleation layer for resumption of growth. This layer 124 is continuous to ensure the quality of the repitaxed layer, for example a (In,Ga)N layer, on the structure.

[0081] The doping of the various layers mentioned above, and in particular of the second layer 112 and the third layer 123, will be chosen according to the tension applied during the porosification.

[0082] In particular, the respective doping levels are chosen so that, at a given potential, there is selectivity between the heavily doped region and the lightly doped region. For a given potential, the doping level of the second layer 112 is chosen so as to that the second layer 112 is not porosified during step d) and the doping rate of the third layer 123 is chosen so that the third 123 is porosified during step d).

[0083] Subsequently, an n-type doping is described, but it could be a p-type doping.

[0084] The stacking described above is structured to form the mesas 120, for example, by photolithography.

[0085] 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 that exhibits favorable selectivity with the etching rate of the GaN layers (typically with an etching rate ratio > 1 / 4). The hard mask is, for example, made of 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 in electrochemical polarization. Advantageously, the mask is removed before the porosification step.

[0086] Preferably, the flanks of the mesas 120 are perpendicular to this stacking of layers.

[0087] The surface of the mesas can be circular, hexagonal, square or rectangular.

[0088] The largest dimension of the surface of the 120 mesas ranges from 500nm to 500pm. By For example, the largest dimension of a circular surface is the diameter.

[0089] The thickness of the mesas corresponds to the dimension of the mesa perpendicular to the underlying stack.

[0090] The spacing between two consecutive mesas 120 can range from a few micrometers to a few tens of micrometers. The spacing preferably ranges from 50 nm to 20 pm. For example, it is 5 pm. Even more preferably, it is between 1 and 2 pm.

[0091] 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 thus be obtained on the same substrate, and in a single growth step, if the difference between the relaxation levels Mesas are sufficient.

[0092] The mesas 120 are formed by etching the stack, and more particularly, by etching the fourth layer 124, the third layer 123 and a first part 112a of the second doped layer 112. By stopping the etching in the doped layer, the entire height of the third layer n++ 123 is available for relaxation.

[0093] 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.

[0094] Each mesa 120 comprises successively from the base: the second part 112b of the doped GaN layer 112, the third layer of heavily doped GaN 123 and the fourth layer of undoped or weakly doped (Al,In,Ga)N 124.

[0095] By way of illustration and not limitation, structure 100 may include: - a basic substrate 110 comprising successively: a support layer 114 in sapphire or silicon, possibly a buffer layer in (Al,Ga)N, a first layer 111 of GaN undoped by 4pm, a first part 112a of the second layer 112 of GaN doped by 500nm (1.1018 at / cm3), - 120 (Al,In,Ga)N / (Al,In,Ga)N mesas comprising successively: a second part 112b of the second layer 112 of GaN doped by 100nm (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 100nm.

[0096] The structure 100 further comprises electrically conductive lines 200 positioned between the mesas 120 on the base substrate 110. During porosification, the charges pass through the electrically conductive lines 200 and are injected near each mesa 120. The distance between the electrically conductive lines 200 and the mesas 120 will be chosen to be sufficiently small to avoid porosification of the second doped layer 112. It can be adjusted to control the desired degree of porification of the mesas.

[0097] By electrically conductive, we mean a conductivity that can allow, for example, a current density of the order of 8mA / cm2.

[0098] The electrically conductive lines 200 follow the bottom of the mesas 120. The electrically conductive lines 200 can be straight lines (especially in the case of square mesas) or broken lines (for example in the case of hexagonal mesas).

[0099] The electrically conductive lines 200 advantageously form a grid, i.e., the lines intersect. Each mesa 120 is advantageously positioned next to one or more electrically conductive lines 200. By next to, it is meant that at least one intermesa space adjacent to a mesa 120 is covered by an electrically conductive line 200. Preferably, at least two spaces, and even more preferably at least four (for example, six) intermesa spaces. adjacent to a mesa are covered by lines.

[0100] It is also possible to position the electrically conductive lines 200 around groups formed of several mesas. It is, for example, possible to form groups of 4 or more mesas.

[0101] The electrically conductive lines 200 may locally or totally cover the bottom of the mesas. They are not part of the mesas 120 nor of the basic substrate 110.

[0102] Electrically conductive lines 200 have a width, for example, between 750 nm and 2 pm.

[0103] The width of the electrically conductive lines 200 is chosen according to the space between the mesas. For example, for an inter-mesa space ('trench') of 1.5 pm, lines 1 pm wide may be chosen.

[0104] The thickness of electrically conductive lines 200 is, for example, between 100 and 200 nm.

[0105] The dimensions and thickness will be chosen according to the material and the desired conductivity.

[0106] The electrically conductive lines 200 locally cover the basic substrate 110 and the space between the electrically conductive lines 200 and the mesas 120 is between 100 nm and 1 pm, preferably between 200 nm and 500 nm.

[0107] According to a first advantageous embodiment, the electrically conductive lines 200 are made of metal, for example selected from titanium and aluminum. Preferably, the electrically conductive lines 200 are made of titanium.

[0108] According to a second embodiment, the electrically conductive lines 200 are made of heavily doped (Al,In,Ga)N, preferably heavily doped GaN. Heavily doped means a concentration greater than 5 x 10¹⁸ at / cm³.

[0109] According to a third embodiment, the electrically conductive lines 200 are made of an electrically conductive polymer.

[0110] The electrically conductive lines 200 are covered by an electrically insulating element 300. The electrically insulating element 300 is chemically inert in the solution used for porosification. The electrically insulating element 300 completely covers the electrically conductive lines 200 so that they are not in contact with the electrolyte during the porosification step. The electrically insulating element 300 is not porosified. The electrically insulating element 300 is preferably a dielectric material. For example, the electrically insulating element 300 is made of silicon dioxide (SiO2) or silicon nitride (Si3N4). It can also be made of a polymer, for example, a photosensitive polymer.

[0111] Depending on the nature of the electrically conductive lines 200, several manufacturing steps can be implemented.

[0112] By way of illustration, two alternative embodiments for forming electrically conductive metal lines 200 will be described.

[0113] Figures 5A to 5D represent a first embodiment. The first embodiment comprises the following steps: - provide structure 100 ([Fig.5A]), - form a full plate metal layer 202 (i.e., the metal layer 202 covers the mesas 120 and the space between the mesas 120), then deposit a resin 201 between the mesas 120, the resin layer 201 having the shape of the lines that we wish to form ([Fig. 5B]), - etch the metal layer 202 not covered by the resin layer 201 ([Fig.5C] ), - remove the resin 201 (“stripping”), thereby obtaining electrically conductive lines 200 of metal located between the mesas 120 ([Fig.5D]).

[0114] Resin 201 defines the grid pattern. It can be formed by photolithography or by screen printing.

[0115] Figures 6A to 6D represent a second embodiment. The second embodiment comprises the following steps: - provide structure 100 ([Fig.6A]), - to form by lithography a layer of resin 203 on the mesas 120 and locally between the mesas 120, so as to have an area between the mesas not covered by the resin 203, the uncovered area having a form of lines, in particular lines that intersect, preferably a grid shape ([Fig.6B]), - deposit a metallic layer 200 on the area not covered by the resin 203, the metallic layer 200 thus having the form of lines, preferably a grid shape ([Fig.6C]), - remove the resin 203 (“lift off metal by stripping”), thereby obtaining electrically conductive lines 200 of metal located between the mesas 120 ( [Fig.6D]).

[0116] The metallic layer can be deposited full plate.

[0117] According to another variant not shown, the electrically conductive lines 200 are made of conductive polymer. The conductive polymer can be deposited by screen printing, lithography (exposure-development), or by any other means enabling the formation of lines with this material.

[0118] After porosification, the electrically conductive lines 200 made of conductive polymer can be removed by wet etching and / or by dry etching.

[0119] Regardless of the embodiment implemented, an electrically insulating element 300 is then deposited on the electrically conductive lines 200 in order to protect them during the anodizing step.

[0120] To achieve this, as shown in Figures 7A to 7C, the following steps can be carried out: - deposit a full layer of electrically insulating material 301 ([Fig.7A]), - locally deposit a resin 203 opposite the electrically conductive lines 200 ([Fig.7B]), - etch the electrically insulating layer 301 not covered by the resin 203, then remove the resin 203 ([Fig.7C]).

[0121] To fabricate electrically conductive lines 200 of doped GaN (typically with a doping level greater than 1 x 10¹⁸ at / cm³), a selective area growth (SAG) can be formed, for example. The growth area is delimited by a SAG layer made of a material unfavorable to GaN growth. The SAG layer covers the mesas 120 and locally covers the space between the mesas. The SAG layer has an opening in the form of lines, preferably in the form of a grid, delimiting an uncovered area on which the doped GaN grid is to be formed. The SAG layer has a shape complementary to the grid.

[0122] By unfavorable to the growth of GaN, it is understood that the GaN cannot grow on this layer or that the growth kinetics of GaN on this material is very slow compared to the growth kinetics of GaN on the material accessible through the openings of the SAG layer (i.e. the material of the intermesa space, i.e. the material forming the second layer 112: the doped GaN).

[0123] More specifically, to form electrically conductive lines 200 in doped GaN, it is possible to carry out the following steps (figures 8A to 8D): - deposit a full plate layer 315 of a material unfavorable to GaN growth, for example a nitride or an oxide, in particular a silicon nitride and / or a silicon oxide ([Fig.8A]), - locally deposit a resin 316 by lithography, so as to leave accessible a part of the layer 315 in material unfavorable to the growth of GaN between the mesas ([Fig.8B]), - then engrave said material 315, and remove the resin 316, thereby obtaining a zone of selective growth in the form of lines between the mesas ([Fig.8C]), - perform an epitaxy of n+ doped GaN to form electrically conductive lines 200 in n+ doped GaN between the mesas ([Fig.8D]).

[0124] Layer 315 is then removed.

[0125] As before, an electrically insulating element 300 is then deposited on the electrically conductive lines 200 to protect them during the anodizing step ([Fig. 8E]). For example, the steps described above, shown in particular in Figures 7A to 7C, can be carried out.

[0126] In particular, it may involve the following steps: - deposit a full layer of electrically insulating material (for example, a dielectric material), - deposit resin locally opposite the electrically conductive lines, - etch the electrically insulating layer not covered by the resin, - remove the resin.

[0127] In step ii), the structure is porosified. The porosification can be carried out according to the following substeps: - electrically connect structure 100 and a counter electrode to a voltage or current generator, - Immerse structure 100 and the counter electrode in an electrolytic solution, - apply a voltage or current between the second layer of doped GaN 112 and the counter electrode so as to porosify the third layer of heavily doped (Al,In,Ga)N 123 of the mesas 120.

[0128] Structure 100 and a counter electrode (CE) are electrically connected to a voltage or current generator. The device acts as a working electrode (WE). Hereafter, it will be referred to as a voltage generator, but it could also be a current generator that applies a current between the device and the counter electrode.

[0129] The initial contact is made on structure 100.

[0130] In particular, the contact can be made on the base substrate 110. The re-establishment of contact can be made on the second doped GaN layer 112. The re-establishment of contact can be made on the bottom of the mesas, at the level of the second layer 112, which allows the etching step to also be used to establish the contacts. Alternatively, it is possible to establish the re-establishment of contact on the electrically conductive lines 200.

[0131] It is also possible to make contact on one of the other layers: on the fourth layer of (Al,In,Ga)N weakly doped 124 or on the third layer of (Al,In,Ga)N strongly doped 123. In the case of re-establishing contact on the strongly doped layer, its opening will advantageously be limited to a preserved area of ​​the electrolyte.

[0132] 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.

[0133] The counter electrode 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.

[0134] The electrodes are immersed in an electrolyte, also called an electrolytic bath or Electrolyte 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.

[0135] The voltage applied between structure 100 and the counter electrode 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 a given potential. At that point, the entire doped structure is porosified and the electrochemical reaction stops.

[0136] For example, the porosification step is carried out by applying a 9V (non-pulsed) voltage to an oxalic acid solution. The process is stopped by monitoring the current drop.

[0137] The electrochemical anodizing step can be carried out under ultraviolet light (UV).

[0138] Advantageously, the porification takes place throughout the entire volume of the third layer of heavily doped GaN 123.

[0139] According to another advantageous embodiment, only a portion of the volume of the third layer of heavily doped GaN 123 is porosified. It is possible to stop the porosification step before the third layer of heavily doped GaN 123 is completely porosified. For example, it is possible to leave a conducting channel in the center of the mesas in this way.

[0140] At the end of the porosification step, the porosity of the third layer of heavily doped GaN 123 is advantageously at least 10%. It preferably ranges from 25% to 70%, preferably from 25% to 50%, for example 45% to 50%.

[0141] 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.

[0142] 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, can vary subsequently during the resumption of epitaxy as a function of the applied temperature.

[0143] The mesas 120 of the same plate are thus porosified, whether for substrates of small or medium dimensions or for substrates of large dimensions.

[0144] The feet of the mesas are intact.

[0145] The structure obtained is shown in [Fig.3B] and in [Fig.4].

[0146] The process may include a step iii) in which the electrically conductive lines and the electrically insulating element are removed ( [Fig.3C]).

[0147] Advantageously, electrically conductive lines are removed when they are made of metal or conductive polymer. This prevents any contamination during a subsequent epitaxial retreatment.

[0148] According to an advantageous embodiment, the lines are made of doped GaN and are retained. The insulating element can be retained or removed. The presence of electrically conductive lines is particularly advantageous in the case of a large-scale structure. They can advantageously play a role in the final device, for example, in the operation of a large display. Indeed, this prevents potential loss between the edge and the center and provides a homogeneous contact layer over the entire surface of the structure.

[0149] Advantageously, the process includes a subsequent step iv) in which epitaxy is performed on the mesas 120, thereby obtaining an epitaxial layer that is at least partially relaxed, and preferably totally relaxed.

[0150] The percentage of relaxation corresponds to:

[0151] — Sa - -s

[0152] with a^, the lattice parameter of the starting layer on which the epitaxy is resumed (i.e., the lattice parameter of layer 124), and

[0153] ac2 the mesh parameter of the relaxed layer,

[0154] 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.

[0155] When aci=ac2 the layer is said to be constrained.

[0156] By partially relaxed, we mean a percentage of relaxation greater than 50%.

[0157] Epitaxial reactivation can be used, for example, to form re-epitaxial LEDs.

[0158] Epitaxial reactivation is performed on the fourth layer 124 of (Al,In,Ga)N / (Al,In,Ga)N nest or weakly doped mesas 120. Because this layer is not porosified during the electrochemical anodizing step, it remains continuous and dense. Epitaxial regrowth is thus facilitated, and the epitaxial layer exhibits better adhesion. The creation of defects related to pore coalescence is avoided.

[0159] The epitaxial layer in this step iv) is advantageously made of gallium nitride or indium gallium nitride.

[0160] Various embodiments and variants have been described. The person skilled in the art will understand that certain features of these various modes of embodiment and variants could be combined, and other variants will appear to the person of the trade.

[0161] Finally, the practical implementation of the embodiments and variants described is within the reach of a person skilled in the art, based on the functional indications given above.

Claims

Claims

1. A method of porosifying a structure (100) comprising a base substrate (110) covered with mesas (120) (Al,In,Ga)N / (Al,In,Ga)N), comprising a porosification step during which the mesas (Al,In,Ga)N / (Al,In,Ga)N are porosified electrochemically, characterized in that, during the porosification step, electrically conductive lines (200) covered by an electrically insulating element (300) are arranged on the base substrate (110) between the mesas (120) or between groups of mesas (120).

2. Method according to claim 1, characterized in that the base substrate (110) comprises a support layer (114), 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 heavily doped (Al,In,Ga)N layer (123) and a fourth undoped or lightly doped (Al,In,Ga)N layer (124), a portion (112b) of the second doped GaN layer (112) extending into the mesas (120), during the porosification step, the electrically conductive lines (200) being in direct contact with the second doped GaN layer (112) the third heavily doped (Al,In,Ga)N layer (123) being porosified during the porosification step.

3. Method according to one of claims 1 and 2, characterized in that the electrically conductive lines (200) are made of a metal, preferably titanium, or a conductive polymer.

4. Method according to one of claims 1 and 2, characterized in that the electrically conductive lines (200) are made of heavily doped GaN.

5. Method according to the preceding claim, characterized in that the method comprises a step during which the electrically conductive lines (200) are formed by epitaxy.

6. Method according to any one of the preceding claims, characterized in that the electrically conductive lines (200) form a grid.

7. Method according to any one of the preceding claims, characterized in that the electrically conductive lines (200) locally cover the base substrate (110) and in that the space between the electrically conductive lines (200) and the mesas (120) is between 100 nm and 1 pm, preferably between 200 nm and 500 nm.

8. Structure (100) comprising a base substrate (110) covered with porosified (Al,In,Ga)N / (Al,In,Ga)N mesas (120), the structure (100) further comprising electrically conductive lines (200), covered by an electrically insulating element (300), arranged on the base substrate (110) between the mesas (120) or between groups of mesas (120).

9. Structure (100) according to claim 8, characterized in that the base substrate (110) comprises a support layer (114), a first undoped GaN layer (111) and a second doped GaN layer (112), the mesas (120) (Al,In,Ga)N / (Al,In,Ga)N comprising a third porosified heavily doped GaN layer (123) and a fourth undoped or lightly doped (Al,In,Ga)N layer (124), a portion (112a) of the second doped GaN layer (112) extending into the mesas (120).

10. Structure according to one of claims 8 and 9, characterized in that the electrically conductive lines (200) are made of heavily doped GaN, metal or conductive polymer.

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