METHOD FOR PRODUCING A RELAXED GAN / INGAN STRUCTURE AND THE RESULTING STRUCTURE

Description

TECHNICAL FIELD

[0001] The present invention relates to the general field of color microdisplays.

[0002] The invention relates to a method for relaxing a GaN / InGaN structure.

[0003] The invention also relates to a relaxed GaN / InGaN structure.

[0004] The invention has applications in many industrial fields, and in particular in the field of micro-color displays based on micro-LEDs with a pitch of less than 10 µm. PREVIOUS STATE OF THE ART

[0005] Color microscreens include blue, green, and red pixels (RGB pixels).

[0006] Blue and green pixels can be made from nitride materials, and red pixels from phosphor 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 µm, this technique can no longer be used due to alignment problems and the time required to implement such a technique at this scale.

[0007] Another solution involves performing color conversion using quantum dots (QDs) or nanophosphors. 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 natively obtain all three RGB pixels using the same material family and 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 InGaN-based micro-LEDs already exhibit high luminance, significantly higher than 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, but also due to the high compressive stress inherent in the growth of the InGaN active region on GaN.

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

[0010] To address this problem, several solutions have been considered.

[0011] One solution involves forming nanostructures, such as nanowires or pyramids, to relax stresses at their free edges. Axial nanowires can be grown using molecular beam epitaxy (MBE). In practice, the low growth temperature used in MBE leads to low internal quantum yields (IQE). Pyramids allow for bending dislocations. In particular, full pyramids have semipolar planes that are favorable for In incorporation and for reducing the internal electric field of the active region. For truncated pyramids, the truncated faces allow quantum well growth along the c-plane, resulting in more homogeneous emission compared to emission along the semipolar planes of a full pyramid.Alternatively, growth can also occur in planar mode on planes other than the c face of the wurtzite structure, such as growth on semi-polar planes which are more favorable to the incorporation of In.

[0012] Another solution involves reducing the stresses in the active region of the LED structure by using a substrate or pseudo-substrate with a lattice parameter closer to that of the InGaN alloy of the quantum wells. Thus, even with a planar configuration, the In incorporation rate in the InGaN can be increased. It has been shown that as the substrate lattice parameter increases, the internal electric field is reduced compared to a stressed layer of the same In concentration, and the quantum well emissions are redshifted [1]. The resulting relaxed InGaN layer allows the growth of a III-N heterostructure by metal-organic vapor phase epitaxy (MOVPE). However, to date, to our knowledge, the only substrate that has enabled this demonstration is the Soitec InGaNOS pseudo-substrate obtained using the Smart Cut™ technique.

[0013] Another solution for reducing overall stress in GaN / InGaN-based LED structures is to porosify the GaN layer. In reference [2], a stack is first prepared comprising a sapphire substrate covered by an unintentionally doped GaN layer (nid GaN) and an n+ doped GaN layer. The doped GaN layer acts as the anode, and a platinum wire acts as the cathode. Electrochemical porosification is performed in a 0.2 M oxalic acid solution at 15 V for 30 min, followed by further porosification under ultraviolet radiation in a 0.06 M KOH solution at 9 V for 30 min. The porosified GaN layer thus obtained allows the growth of a multiple quantum well LED structure (MQWs for "multiple quantum wells") composed of an n+ GaN layer, five GaN / InGaN quantum wells (PQs) and a p GaN contact layer.Significant stress relaxation leads to improved electrical and optical properties, particularly with regard to photoluminescence (PL).

[0014] Reference [3] discloses methods for forming highly uniform and highly porous gallium nitride layers, with pore sizes typically less than 100 nm. Electrochemical etching of heavily doped n-type gallium nitride under low voltages in concentrated nitric acid is used to form the porous gallium nitride. The porous layers can be used in reflective structures for integrated optical devices such as VCSELs or LEDs.

[0015] Reference [4] discloses semiconductor structures, specifically LEDs, each comprising an electroluminescent layer of a group III nitride (usually quantum-well InGaN / GaN) arranged between an n-type and a p-type region, and grown on a porous region of another group III nitride (usually n-type GaN). An indium-containing group III nitride layer is disposed between the electroluminescent layer and the porous group III nitride region. Since the indium-containing group III nitride layer is deposited on the porous region, it can be at least partially relaxed; that is, the indium-containing group III nitride layer can have an in-plane lattice constant greater than that of a GaN layer conventionally deposited on a sapphire substrate.

[0016] However, the crystalline quality of the LED's GaN layer depends on the pore diameter and porosity of the porosified GaN layer, as well as the desired thickness. Therefore, it is necessary to determine the appropriate parameters each time, which complicates the industrialization of the process. DESCRIPTION OF THE INVENTION

[0017] One aim of the present invention is to propose a method which remedies the disadvantages of the prior art, and which makes it possible to obtain a GaN / InGaN structure which is at least partially or even totally relaxed, in order to manufacture, for example, red green blue pixels.

[0018] To this end, the present invention proposes a method, according to claim 1 in the appendix, for fabricating a GaN / InGaN structure that is at least partially relaxed, comprising the following steps: a) provide a device comprising: a GaN / InGaN structure including an electrically conductive layer of doped GaN locally covered by InGaN mesas, the InGaN mesas comprising a layer of doped InGaN and a layer of undoped or lightly doped InGaN, an electrically insulating layer covering the electrically conductive layer of doped GaN between the mesas, b) electrically connect the electrically conductive layer of doped GaN of the device and a counter electrode to a voltage or current generator, c) immerse the device and the counter electrode in an electrolytic solution, d) apply a voltage or current between the electrically conductive layer of doped GaN and the second electrode so as to porosify the doped InGaN layer of the mesas, e) form an InGaN layer by epitaxy on the InGaN mesas, thereby obtaining a layer d'InGaN epitaxiée relaxed.

[0019] The invention differs fundamentally from the prior art by implementing a selective porosification step of the doped InGaN layer of the mesas, via electrochemical means. The porosification takes place within the volume of the doped InGaN layer.

[0020] The undoped or lightly doped InGaN layer is electrically insulating. It is not porosified during step d). It can therefore serve as an epitaxial regrowth layer on the mesas.

[0021] It is easy to adjust the dimensions of the pores according to the doping of the mesas, the applied voltage and / or the chosen electrolyte (nature and / or concentration) to achieve the desired relaxation rate, and subsequently the desired wavelength emitted by the active zone (quantum wells), obtained by re-epitaxy.

[0022] Porification improves extraction efficiency, especially when the pore size is comparable to the target wavelength.

[0023] This step can be carried out with a low thermal budget, and in particular at ambient temperature (typically around 20-25°C), which avoids sublimating the material of the mesas.

[0024] During the resumption of epitaxy, the growth temperature used (typically from 900°C to 1000°C) allows modification of the porosified layer, notably by widening the pores of this layer, which provides an additional degree of freedom while maintaining the mesh parameter adapted to the re-epitaxial layer.

[0025] At the end of the process, we obtain a GaN / InGaN structure that is at least partially relaxed, or even totally relaxed.

[0026] Advantageously, the undoped or lightly doped InGaN layer has a thickness of 0.25 to 3nm.

[0027] Advantageously, the electrically insulating layer is made of SiO2 or SiN.

[0028] Advantageously, the indium content in InGaN mesases is greater than or equal to 8%. This ensures high-quality, In-rich re-epitaxy.

[0029] Advantageously, the mesas have a thickness of less than 100nm. Thus, the defect density in the mesas is limited despite the high In concentration.

[0030] According to a first advantageous variant, the device provided in step a) is manufactured by forming the electrically insulating layer and then the InGaN mesas.

[0031] According to another advantageous variant, the device provided in step a) is manufactured by forming the InGaN mesas and then the electrically insulating layer.

[0032] Advantageously, the electrically conductive layer of doped GaN covers a sapphire or silicon substrate.

[0033] Advantageously, the process includes a step in which organometallic doping is performed on the InGaN mesas by implantation or vapor-phase epitaxy. Implantation is selective on the InGaN mesas.

[0034] Advantageously, the doped InGaN layer of the mesas has different doping levels from one mesa to another. This results in pixels, for example three pixels, with different doping levels and therefore different relaxation percentages, and consequently different emission wavelengths. This embodiment is advantageous for creating a multispectral device, for example, simplified LEDs of different colors, or a multicolor microdisplay.

[0035] According to an advantageous embodiment, the InGaN mesas of the structure provided in step a) are obtained according to the following steps: deposit a full-plate InGaN layer, locally perform an organometallic implantation or vapor-phase epitaxial doping step in the full-plate InGaN layer so as to have doped and undoped areas, deposit an unintentionally doped or weakly doped full-plate InGaN layer on the full-plate InGaN layer, etch the full-plate InGaN layer through the undoped or weakly doped full-plate InGaN layer so as to remove the undoped areas, thereby forming locally InGaN mesas comprising a doped InGaN layer and an undoped or weakly doped InGaN layer.

[0036] Advantageously, the doped areas have different doping levels, whereby the doped InGaN layer of the mesas has different doping levels from one mesa to another.

[0037] This process offers numerous advantages: It is simple to implement, it can be used for thin mesas (typically less than 100nm), the mesa structuring provides the compliance effect, it leads to a partial or total relaxation of stresses and reduces piezoelectric polarization compared to a stressed layer of the same In concentration, it allows a so-called "bottom up" approach for the fabrication of µLEDs and µdisplays: the growth of optical structures (N, PQ, P) is carried out after pixelation in mesas, regardless of the size of the pixels, and makes it possible to overcome the alignment problems as with the "pick and place" process, there is no impact of the pixel etching process on the efficiency of the micro-LEDs, which makes it possible to produce micrometric or sub-micrometric pixels.

[0038] The invention also relates to a GaN / InGaN structure, according to claim 12 in the appendix, comprising successively: an electrically conductive layer of doped GaN, InGaN mesas comprising a porous doped InGaN layer and an undoped or lightly doped InGaN layer, preferably having a thickness ranging from 0.25nm to 3nm, an epitaxially relaxed InGaN layer.

[0039] Advantageously, the undoped or lightly doped InGaN layer has a thickness ranging from 2nm to 3nm. This layer is non-porous.

[0040] A porous layer is defined as a layer with a porosity greater than 1%. Preferably, it ranges from 5% to 70%. The upper limit value is the maximum porosity before the onset of mesas delamination.

[0041] Other features and advantages of the invention will become apparent from the supplementary description that follows.

[0042] It goes without saying that this additional description is given only as an illustration of the object of the invention and should in no way be interpreted as a limitation of this object; the object of the present invention being defined by the claims in the annex. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The present invention will be better understood upon reading the description of exemplary embodiments given by way of illustration only and in no way limiting, with reference to the accompanying drawings in which: THE Figures 1A and 1B schematically represent, in cross-section, a device comprising a GaN / InGaN structure according to various specific embodiments of the invention. figures 2A, 2B, 2C and 2D schematically represent different stages of the manufacturing process of the device comprising a GaN / InGaN structure, according to a particular embodiment of the invention. figures 3A, 3B and 3Cschematically represent different stages of the manufacturing process of the device comprising a GaN / InGaN structure, according to another particular embodiment of the invention. figure 4 schematically represents an electrochemical anodizing step of InGaN mesas, according to a particular embodiment of the invention. figure 5 is a graph representing different phenomena occurring (pre-porification (or "pre-breakdown"), porosification, and electropolishing) during an anodizing step, as a function of the doping level and the applied potential, according to a particular embodiment of the invention. Figures 6A and 6B schematically represent the resumption of epitaxial growth on InGaN mesases, according to different embodiments specific to the invention. figures 7A, 7B, 7C and 7Dare images obtained by atomic force microscopy (AFM) of a 0.8% InGaN layer before porosification, after porosification, after porosification and annealing, and finally after resumption of epitaxy, respectively.

[0044] The different parts represented in the figures are not necessarily shown on a uniform scale, in order to make the figures more legible.

[0045] The different possibilities (variants and embodiments) should be understood as not being mutually exclusive and can be combined with each other, as long as they remain covered by the scope of the present claims in the annex.

[0046] In addition, in the description below, orientation-dependent terms such as "on", "above", "below", etc. of a structure apply assuming that the structure is oriented as illustrated in the figures. DETAILED DESCRIPTION OF SPECIFIC METHODS OF IMPLEMENTATION

[0047] Although not 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 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.

[0048] The process, which allows obtaining a partially or totally relaxed GaN / InGaN structure, consists of porosifying the InGaN by electrochemical anodization according to the following steps: a) provide a device comprising: a GaN / InGaN structure including an electrically conductive layer of doped GaN 204 locally covered by InGaN mesas 100, the InGaN mesas 100 comprising a layer of doped InGaN 101 and a layer of undoped or lightly doped InGaN 102, an electrically insulating layer 300 covering the electrically conductive layer of doped GaN 204 between the mesas 100, b) electrically connect the electrically conductive layer of doped GaN 204 of the device and a counter electrode 500 to a voltage or current generator, c) immerse the device and the counter electrode 500 in an electrolytic solution, d) apply a voltage or current between the electrically conductive layer of doped GaN 204 and the second electrode 500 so as to porosify the layer of doped InGaN 101 of mesas 100, e) perform epitaxy on the InGaN 100 mesas,This results in an epitaxial InGaN layer that is at least partially relaxed, and preferably totally relaxed.

[0049] The relaxation percentage corresponds to: Δa / a ¯ = a c 2 − a c 1 / a c 1 with a c1, the mesh parameter of the starting layer, and a c2 the mesh parameter of the relaxed layer,

[0050] The layer is relaxed to 100% if ac 2 corresponds to the mesh parameter of the bulk material.

[0051] When a c1 = a c2 the layer is said to be under stress.

[0052] Partially relaxed means a percentage of relaxation greater than 50%.

[0053] The device provided in step a) comprises a GaN / InGaN structure. More specifically, the device comprises a stack 200 whose top layer is an electrically conductive layer of doped GaN 204, this layer being covered and in contact with mesas 100 of InGaN.

[0054] Advantageously, the stack of 200 layers covered by the 100 mesas of InGaN comprises: a support layer 201, or support, for example in sapphire or silicon, a first layer 202, preferably in gallium nitride (GaN) not intentionally doped (GaN nid) and / or a second layer 203, preferably in doped GaN (GaN n), a third layer corresponding to the electrically conductive layer in doped GaN 204, this layer being, preferably, heavily doped (GaN n++).

[0055] By unintentionally doped GaN, we mean a concentration less than 5.10 17< at / cm 3< .

[0056] Doped GaN means a concentration greater than 10< 18< at / cm 3< .

[0057] Highly doped GaN is defined as having a concentration greater than 10< 19< at / cm 3< .

[0058] Preferably, the 200 stack-up consists of the layers mentioned above. In other words, it does not contain any other layers.

[0059] The 201 support layer, for example, has a thickness ranging from 350 µm to 2 mm. The thickness depends on the nature of the 201 support layer and its dimensions. For example, for a 2-inch (50.8 mm) diameter sapphire support, the thickness is 350 µm, and for a 6-inch (152.4 mm) diameter sapphire support, the thickness is 1.3 mm.

[0060] The first 202 layer, for example, has a thickness ranging from 500 nm to 5 µm. This NID layer must absorb the stresses related to the lattice mismatch between the GaN and the substrate. Its thickness is generally between 1 and 4 µm.

[0061] The second layer 203, for example, has a thickness ranging from 100nm to 1µm: it is present to ensure the quality of the material of the upper layer 204.

[0062] The third 204 layer, for example, has a thickness ranging from 100nm to 500nm. It must be sufficiently conductive; the minimum thickness varies depending on the doping level.

[0063] The third electrically conductive layer 204 of the stack 200 is in direct contact with the mesas 100 to be porosified. During the electrochemical anodizing step, the electrically conductive layer 204 of the stack 200 allows for re-establishment of contact with the stack 200. This electrically conductive layer 204 is electrically connected to the voltage or current generator.

[0064] 100-mesh images, also called elevations, are raised features. They are created, for example, by etching a continuous layer or several superimposed continuous layers, leaving only a certain number of raised areas of that layer or those layers. The etching is generally plasma etching (or dry etching). These raised areas define pixels.

[0065] Preferably, the flanks of the 100 mesas are perpendicular to this stack of 200 layers.

[0066] The dimensions (width and length) of the 100 mesas range from 500nm to 500µm. By width and length, we mean the dimensions parallel to the surface of the underlying stack.

[0067] The spacing ("pitch") between two consecutive 100 mesas ranges from 50nm to 20µm.

[0068] The InGaN mesas 100 comprise a layer of doped InGaN 101 and a layer of undoped or lightly doped InGaN 102.

[0069] Preferably, the mesas are made up of these two layers 101, 102.

[0070] Doped InGaN refers to a doping concentration greater than 1018 at / cm3, or even greater than 1019 at / cm3 for a so-called heavily doped layer. The doping level will be chosen according to the porosification technique (with or without illumination). The InGaN-doped layer 101 is electrically conductive. It is porosified during step d).

[0071] By "lightly doped InGaN," we mean a doping concentration less than 5.1017 at / cm³. The doping is sufficiently low so that this layer is electrically insulating. Thus, layer 102 is not anodized, and therefore is not porosified during step d).

[0072] The doped layer 101 of the mesas 100 advantageously has a thickness ranging from 10 nm to 100 nm, preferably on the order of 30 nm. A thin layer (less than 100 nm) can contain a high In concentration while maintaining good material quality (few defects). By thickness, we mean the dimension of the mesa perpendicular to the underlying stack.

[0073] The undoped or lightly doped InGaN layer 102 has, for example, a thickness ranging from 1 nm to 5 nm, and preferably a thickness ranging from 2 to 3 nm. This layer must provide a continuous interface to ensure the resumption of coalesced epitaxy. It is advantageously chosen to be thin, for example, between 0.25 nm and 3 nm, so that it is defect-free and can relax through the subsequent porosification process of the lower layer 101 and the resumption of epitaxy on the mesia.

[0074] The electrically conductive layer 204 is covered, between the mesas 100, by an electrically insulating layer 300 so as not to be in contact with the electrolytic solution during the porosification step and thus not to be porosified.

[0075] The electrically insulating layer 300 can be made of oxide or nitride. Preferably, silicon oxide or silicon nitride is chosen. It has, for example, a thickness ranging from 5 nm to 100 nm. For a stoichiometric nitride or oxide, a thickness of 30 nm or less is chosen.

[0076] According to a first variant embodiment, represented on the figures2A to 2D Such a device can be manufactured according to the following steps: supply of a stack 200 comprising, for example, a sapphire or silicon support 201, successively covered by a first layer 202 of GaN nid and / or a second layer 203 of doped GaN, and a third electrically conductive layer 204 of strongly doped n GaN, formation of a strongly doped InGaN layer 101, then of an undoped or weakly doped InGaN layer 102, structuring, for example by photolithography, of the strongly doped InGaN layer 101 and the undoped InGaN layer 102 so as to produce mesas 100, formation of an electrically insulating layer 300, for example of SiO 2 or SiN, between the mesas 100 so as to passivate the electrically conductive layer 204.

[0077] Controlled etching of the 100 mesas allows the process to stop on or within the n++ GaN layer. By n++ GaN layer, we mean at the interface between the n++ GaN 204 layer and the doped 101 InGaN layer. In other words, the n++ GaN layer is not completely etched in order to ensure electrical conduction during electrochemical porosification.

[0078] According to another embodiment, shown on the figures 3A to 3C Such a device can be manufactured according to the following steps: supply of a stack 200 comprising, for example, a sapphire or silicon support 201, covered successively by a first layer 202 of GaN nid and / or a second layer 203 of doped GaN, and a third electrically conductive layer 204 of heavily doped n GaN, formation of an electrically insulating layer 300, for example of SiO 2 or SiN, locally covering the electrically conductive layer 204, formation of mesas 100 at the level of the areas of the electrically conductive layer 204 not covered by the electrically insulating layer 300, by forming a layer of doped InGaN 101 and then a layer of undoped or weakly doped InGaN 102.

[0079] According to this embodiment, the electrically insulating layer 300 can be obtained by depositing a continuous layer and then etching it. It is also possible to deposit the electrically insulating layer 300 through a mask. Localized growth of the mesas 100 is then carried out.

[0080] According to these embodiment variants, it is possible to carry out the doping of the doped layer 101 of the mesas 100, for example, by implantation of silicon (Si(n)) or by implantation of magnesium (Mg (p)).

[0081] The 101 layers of the 100 mesas can have identical or different doping levels. Using different doping levels allows manipulation of the porosity of the mesas depending on their initial doping level for a given applied potential. The mesas are then more or less relaxed and more or less able to incorporate indium into the layers. It is thus easy to obtain, after resuming epitaxy, pixels of different colors.

[0082] According to an advantageous embodiment, the InGaN mesas of the structure provided in step a) can be obtained by the following steps: deposit a so-called full-plate InGaN layer (i.e., a continuous layer) on the electrically conductive 204 layer of heavily n-doped GaN, locally perform an implantation step in the full-plate InGaN layer so as to have doped areas and less doped or even undoped areas; for example, for 3 levels of n-type doping, 2 lithographies and hard masks can be performed, and silicon can be implanted, advantageously, perform a so-called thermal annealing for healing, deposit an unintentionally undoped or weakly doped InGaN layer (typically with a residual between 5.1016 and 1.10 17< / cm 3< ) full plate on the full plate InGaN layer, for example by epitaxy, etch the full plate InGaN layer through the undoped or weakly doped full plate InGaN layer so as to remove the unimplanted areas, thereby forming InGaN mesas 100 comprising a doped InGaN layer 101 and an undoped or weakly doped InGaN layer 102. .

[0083] For example, a hard mask, such as a SiN or SiO₂ mask, can be used for implantation, and alignment marks (e.g., Al, Ti, TiN) can be used to match the mesas and the implanted areas. With two different masks, two areas with different implantation energies can be created, and therefore two different doping levels in addition to the initial doping of the doped wafer, for example, with MOCVD. The alignment marks allow the two masks for implantation and the mask for etching the mesas to be aligned.

[0084] The same mask set will be used for implantation and mesas.

[0085] Advantageously, for an undoped or lightly doped InGaN layer of small thickness, it is possible to implant the underlying InGaN layer through this layer.

[0086] For p-type doping, magnesium can be implanted.

[0087] Advantageously, the doped regions have different doping levels, meaning that the doped InGaN layer of the mesas also has different doping levels. The higher the doping level, the greater the porification will be at a fixed potential. The relaxation of the dense InGaN layer 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 (thanks to the reduction of the "compositional pulling effect," i.e., the pushing of In atoms towards the surface, preventing them from being incorporated into the layer). After epitaxy of the complete LED structure, blue, green, and red (RGB) mesas can be obtained on the same substrate in a single growth step, provided the difference between the relaxation levels of the mesas is sufficient.

[0088] Alternatively, in these different embodiments, implantation doping can be replaced by metal-organic vapor-phase epitaxial doping (MOCVD), using Si or Ge as the dopant, for example. It is possible, for instance, to perform three successive epitaxial steps with successive masking to obtain three different doping levels in order to form RGB mesas at the end of the process.

[0089] We will subsequently describe an n-type doping, but it could also be a p-type doping.

[0090] In step b), the device and a counter electrode (CE) 500 are electrically connected to a voltage or current generator ( figure 4 The device acts as a working electrode (WE). It will subsequently 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.

[0091] The 500 counter electrode is made of an electrically conductive material, such as a metal like platinum.

[0092] 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. For example, the electrolyte might be oxalic acid. It could also be KOH, HF, HNO3, NaNO3, or H2SO4.

[0093] It is possible to complete step c) before step b).

[0094] In step d), a voltage is applied between the device and the counter electrode 500. The voltage can range from 1 to 100 V. It is applied, for example, for a duration ranging from a few seconds to several hours. Porosification is complete when there is no longer any current at the applied potential. At this point, the entire doped structure is porosified and the electrochemical reaction stops.

[0095] The difference in doping levels allows for a high selectivity between porosification and electropolishing ( figure 5 This "abacus" allows us to define, for given conditions, the respective doping levels so that, at a given potential, there is selectivity between the heavily doped zone and the lightly doped zone.

[0096] The electrochemical anodizing step can be carried out under ultraviolet (UV) light. The process can also involve a first electrochemical anodizing step without adding ultraviolet radiation and a second electrochemical anodizing step with added ultraviolet radiation. This embodiment is particularly advantageous because, for example, initially the upper n+ doped layer can be porosified, with the porosification stopping abruptly at the GaN nid. Then, in a second step, UV light can generate charges in the GaN nid, thus allowing the porosification of undoped GaN.

[0097] Advantageously, porification takes place throughout the volume of the 101-doped InGaN layer of the 100 mesas.

[0098] At the end of the porosification step, the porosity rate of the InGaN 101 doped layer is at least 10%. It preferably ranges from 25% to 50%.

[0099] 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, particularly from 30 to 70 nm.

[0100] The resulting porosity (porosity rate and pore size) depends on the doping of the doped InGaN layer 101 of the mesas 100 and on the process parameters (applied voltage, duration, type 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.

[0101] The porosification step allows a totally or partially relaxed 101-doped InGaN layer.

[0102] Following the porosification step, we carry out a re-epitaxy on the InGaN mesas 100 (step e), for example to form re-epitaxial LEDs.

[0103] Epitaxial regrowth is performed on the undoped InGaN layer 102 of the mesas 100. Because layer 102 is not porosified during the electrochemical anodizing step, it allows for epitaxial regrowth on a continuous, non-porous, dense 2D layer. This facilitates epitaxial regrowth and results in a more stable epitaxial layer. The formation of defects due to pore coalescence is also avoided.

[0104] Various growth processes can be used to form an epitaxial layer 400.

[0105] According to a first variant embodiment, shown on the figure 6A Growth resumes laterally, and a subsequent pixelation step by etching is advantageously performed. It is also possible to passivate the edges with a thin layer (e.g., 3 nm), preferably by atomic layer deposition (ALD).

[0106] According to another embodiment, shown on the figure 6B Growth resumes vertically above the mesas. In this variant, the pixels correspond to the underlying mesas.

[0107] The epitaxial layer 400 is, in the present invention, made of indium gallium nitride.

[0108] Advantageously, a passivation layer will be deposited on the sides of the 100 mesas and re-epitaxial LEDs, for example, by atomic layer deposition (ALD). The passivation layer can be made of alumina. The passivation layer can be a few nanometers thick, for example, from 2 to 5 nm.

[0109] The re-epitaxial stacking of an all-InGaN LED can include, for example, successively from the unintentionally undoped InGaN 102 layer: an n-doped InGaN layer, preferably having the same In concentration as the InGaN layer of the substrate, an active zone with one or more InGaN / (Ga,ln)N quantum wells emitting in the red or green, a p-doped GaN or AlGaN electron barrier, a p-doped InGaN layer, preferably having an In concentration less than or equal to that of the n-InGaN layer, a p++-doped InGaN layer, preferably having the same In concentration as the p-InGaN layer.

[0110] More specifically, an all-InGaN LED structure can successively comprise: an InGaN substrate, a 350nm n-doped InGaN layer, formed from 15 x In 0.03 Ga 0.97 N / GaN (thicknesses 20nm / 1.8nm), multiple quantum wells (MQWs), formed from 5 x In 0.40 Ga 0.60 N / In 0.03 Ga 0.97 N (thicknesses 2.3nm / 5, 7, 11 nm), a nid In 0.03 Ga 0.97 N layer (10nm), an Al 0.1 Ga 0.9 N :Mg layer (20nm), a Mg-doped In 0.03 Ga 0.97 N layer (125nm), a p+++ doped In 0.03 Ga 0.97 N layer (25nm). Illustrative and non-limiting examples of one embodiment:

[0111] In this example, we use a 200 stack comprising: a sapphire support 201, a GaN layer 202 (nest), a doped GaN layer 203 (GaN n) and a heavily doped GaN layer 204 (GaN n++).

[0112] To create the InGaN mesas 100, a layer of In 0.008 Ga 0.992 N (doped n: 10 19 < at / cm 3 < ) 101 less than 100 nm thick is deposited on this stack 200. A thin layer of InGaN 102 of the same composition but undoped (2 to 3 nm) is deposited to facilitate subsequent epitaxial regrowth.

[0113] The mesas 100 are then structured using conventional photolithography techniques. The dimensions of the resulting mesas 100 range from 500 nm to 10 µm. Controlled etching of the InGaN layer allows the process to stop on or within the n++ GaN layer.

[0114] The n++ 204 GaN layer is then passivated with a 300 layer of SiO 2 or SiN for example.

[0115] The AFM image of the InGaN layer before porification is shown on the figure 7A .

[0116] The InGaN mesas are then porosified by electrochemical anodization in an electrolytic solution containing 0.1 mol / L to 0.5 mol / L of oxalic acid, for example, 0.2 mol / L. The applied voltage is 24 V. The mesas then contain an In concentration close to that of the wells and are completely or almost completely relaxed. The AFM image of the InGaN layer after porosification is shown in the image. figure 7B .

[0117] Next, annealing is carried out, for example, at 900°C ( figure 7C ).

[0118] Finally, we perform a resumption of epitaxy on the InGaN 100 mesas ( figure 7D ).

[0119] Photoluminescence (PL) characterizations were performed. Before porosification, the emission of the 0.8% InGaN sample was at 356 nm. After epitaxial resumption, the emission was at 364 nm. Samples made with 2% InGaN showed the same trend. This PL difference may indicate stress relaxation and / or the integration of more InGaN.

[0120] The object of the present invention is defined by the following claims. REFERENCES

[0121] [1] Even et al., "Enhanced In incorporation in full InGaN heterostructure grown on relaxed InGaN pseudo-substrate », Appl. Phys. Lett. 110, 262103 (2017). [2] Jang et al., "Electrical and structural properties of GaN films and GaN / InGaN light-emitting diodes grown on porous GaN templates fabricated by combined electrochemical and photoelectrochemical etching", Journal of Alloys and Compounds 589 (2014) 507-512. [3] Han et al., US 2017 / 0237234 A1, "A method for GaN vertical microcavity surface emitting laser (VCSEL)", 17 août 2017. [4] Wierer et al., US 2009 / 0140274 A1, "Ill-nitride light emitting device including porous semiconductor layer", 4 juin 2009.

Claims

1. A process comprising the following steps of: a) providing a device comprising: - a GaN / InGaN structure comprising an electrically conductive doped GaN layer (204) locally covered with InGaN mesas (100), the InGaN mesas (100) comprising a doped InGaN layer (101) and an undoped or weakly doped InGaN layer (102), - an electrically insulating layer (300) covering the electrically conductive doped GaN layer (204) between the mesas (100), b) electrically connecting the electrically conductive doped GaN layer (204) of the device and a counter-electrode (500) to a voltage or current generator, c) dipping the device and the counter-electrode (500) into an electrolyte solution, d) applying a voltage or current between the electrically conductive doped GaN layer (204) and the second electrode (500) so as to porosify the doped InGaN layer (101) of the mesas (100), e) forming an InGaN layer by epitaxy on the InGaN mesas (100), whereby a relaxed epitaxially grown InGaN layer (400) is obtained.

2. The process according to claim 1, wherein the undoped or weakly doped InGaN layer (102) has a thickness ranging from 0.25 to 3 nm.

3. The process according to one of claims 1 and 2, wherein the electrically insulating layer (300) is of SiO2 or SiN.

4. The process according to one of claims 1 to 3, wherein the device provided in step a) is manufactured by forming the electrically insulating layer (300) and then the InGaN mesas (100).

5. The process according to one of claims 1 to 3, wherein the device provided in step a) is manufactured by forming the InGaN mesas (100) and then the electrically insulating layer (300).

6. The process according to one of the preceding claims, wherein the electrically conductive doped GaN layer (204) covers a support (201) of sapphire or silicon.

7. The process according to one of the preceding claims, wherein the mesas (100) have a thickness of less than 100 nm.

8. The process according to one of claims 1 to 7, wherein the process comprises a step during which an implantation doping or metal-organic vapour phase epitaxy doping step is carried out on the InGaN mesas (100).

9. The process according to claim 8, wherein the doped InGaN layer (101) of the mesas (100) has different dopings.

10. The process according to one of claims 1 to 7, wherein the InGaN mesas (100) of the structure provided in step a) are obtained according to the following steps of: - depositing a full-plate InGaN layer, - locally performing an implantation doping or metal-organic vapour phase epitaxy doping step in the full-plate InGaN layer so as to have doped and undoped zones, - depositing an unintentionally doped or weakly doped full-plate InGaN layer onto the full-plate InGaN layer, - etching the full-plate InGaN layer through the undoped or weakly doped full-plate InGaN layer so as to remove the undoped zones, whereby InGaN mesas (100) comprising a doped InGaN layer (101) and an undoped or weakly doped InGaN layer (102) are locally formed.

11. The process according to claim 10, wherein the doped zones have different doping types.

12. A GaN / InGaN structure successively comprising: - an electrically conductive doped GaN layer (204), - InGaN mesas (100), the InGaN mesas (100) comprising a porous doped InGaN layer (101) and an undoped or weakly doped InGaN layer (102), having preferably a thickness ranging from 0.25 nm to 3 nm, - a relaxed epitaxially grown InGaN layer (400).