Patent Publication Number: US-2021193870-A1

Title: Process for manufacturing a relaxed gan/ingan structure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from French Patent Application No. 1914962 filed on Dec. 19, 2019. The content of this application is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention is concerned with the general field of colour microdisplays. 
     The invention relates to a process for relaxing a GaN/InGaN structure. 
     The invention also relates to a relaxed GaN/InGaN structure. 
     The invention finds applications in many industrial fields, especially in the field of colour micro-displays based on micro-LEDs with a pitch of less than 10 μm. 
     STATE OF PRIOR ART 
     Colour microdisplays include red, green and blue pixels (RGB pixels). 
     The blue and green pixels can be made from nitride materials and the red pixels can be made from phosphor 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 μm, this technique can no longer be used due not only to alignment problems, but also to the time required to carry out such a technique on this scale. 
     Another solution is to perform colour conversion using quantum dot (QD) or nanophosphors. However, controlling the deposition of these materials on small pixels is difficult and their flow resistance is not sufficiently robust. 
     It is therefore crucial to be able to obtain the three RGB pixels natively with the same family of materials and on the same substrate. For this, InGaN is the most promising material. This material can, indeed, theoretically cover the entire visible spectrum depending on its indium concentration. Blue InGaN-based micro-LEDs already show a high luminance, much higher than their organic counterparts. To emit at green wavelengths, the quantum wells (QWs) of the LED should contain at least 25% indium and for red emission, at least 35% indium is required. Unfortunately, the quality of the InGaN material above 20% In is degraded due to the low miscibility of InN in GaN, but also due to the strong compressive stress inherent to the growth of the InGaN active zone on GaN. 
     It is therefore essential to be able to reduce the overall stress in GaN/InGaN-based structures. 
     To address this issue, several solutions have been contemplated. 
     A first solution is to form nanostructures, such as nanowires or pyramids, to be able to relax stresses through the free edges. The growth of axial nanowires can be achieved by molecular beam epitaxy (MBE). In practice, the low growth temperature used in MBE growth leads to low Internal Quantum Efficiencies (IQE). Pyramids are used to bend dislocations. In particular, complete pyramids have semi-polar planes favourable to the In incorporation and to the reduction of the internal electric field of the active zone. For truncated pyramids, the truncated faces allow quantum well growth on the c-plane, which leads to a more homogeneous emission compared to an emission along semi-polar planes of a complete pyramid. Alternatively, growth can also take place in a planar manner on planes other than the c-plane of the wurtzite structure, such as growth on the semi-polar planes which are more favourable to In incorporation. 
     Another solution is to reduce stresses in the active zone of the LED structure by using a substrate or pseudo-substrate with a lattice parameter closer to the lattice parameter of the InGaN alloy of the quantum wells. Thus, even with a planar configuration, the In incorporation rate in InGaN can be increased. It has been shown that, as the substrate lattice parameter increases, the internal electric field is reduced as compared to a stressed layer of the same In concentration and the quantum well emissions are shifted towards the red [1]. The resulting relaxed InGaN layer allows a III-N heterostructure to be grown by metal-organic vapour phase epitaxy (MOVPE). However, to date, to our knowledge, the only substrate that enabled this demonstration is Soitec&#39;s pseudo-substrate InGaNOS obtained by the Smart Cut™ technique. Another solution to reduce overall stresses 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 with an unintentionally doped GaN layer (uid 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 carried out in an oxalic acid solution (0.2M) by applying a 15V voltage for 30 min and then under ultra-violet radiation in a KOH solution (0.06M) at 9V for 30 min. The porosified GaN layer thus obtained allows a multiple quantum well (MQWs) LED structure composed of an n+ GaN layer, five Gan/InGaN quantum wells (QWs) and a p GaN contact layer as well to be grown. Significant stress relaxation leads to better electrical and optical properties, especially with regard to photoluminescence (PL). 
     However, the crystal quality of the GaN layer of the LED depends on the pore diameter and porosity of the porosified GaN layer, as well as the desired thickness. It is therefore necessary to find suitable parameters each time, which complicates industrialisation of the process. 
     DISCLOSURE OF THE INVENTION 
     A purpose of the present invention is to provide a process which overcomes the drawbacks of prior art and makes it possible to obtain an at least partially or even totally relaxed GaN/InGaN structure, with a view to manufacturing, for example, red green blue pixels. 
     For this, the present invention provides a process for manufacturing an at least partially relaxed GaN/InGaN structure comprising the following steps of: 
     (a) providing a device comprising:
         a GaN/InGaN structure comprising an electrically conductive GaN layer locally covered with InGaN mesas, the InGaN mesas comprising a doped InGaN layer and an undoped or weakly doped InGaN layer,   an electrically insulating layer covering the electrically conductive doped GaN layer between the mesas,       

     (b) electrically connecting the electrically conductive doped GaN layer of the device and a counter-electrode to a voltage or current generator, 
     (c) dipping the device and the counter-electrode in an electrolyte solution, 
     d) applying a voltage or current between the electrically conductive doped GaN layer and the second electrode so as to porosify the doped InGaN layer of the mesas, 
     e) forming a layer of InGaN by epitaxy on the InGaN mesas, whereby a relaxed epitaxially grown InGaN layer is obtained. 
     The invention differs fundamentally from prior art in that a step of selectively porosifying the doped InGaN layer of the mesas is carried out electrochemically. Porosification takes place in the volume of the doped InGaN layer. 
     The undoped or weakly doped InGaN layer is electrically insulating. It is not porosified in step d). It can therefore be used as an epitaxial regrowth layer on mesas. 
     It is easy to adjust pore dimensions according to the mesas doping, applied voltage and/or chosen electrolyte (nature and/or concentration) to achieve the desired relaxation rate, and hence the desired wavelength emitted by the active zone (quantum wells), obtained by re-epitaxy. 
     Porosification improves extraction efficiency, in particular when the pore size is comparable with the target wavelength. 
     This step can be carried out at a low thermal expenditure, especially at room temperature (typically around 20-25° C.), which avoids sublimation of the mesa material. 
     During epitaxial regrowth, the growth temperature used (typically from 900° C. to 1000° C.) allows the porosified layer to be modified, especially by enlarging the pores of this layer, which provides an additional degree of freedom while maintaining the lattice parameter adapted to the epitaxially regrown layer. 
     At the end of the process, an at least partially relaxed or even totally relaxed GaN/InGaN structure is obtained. 
     Advantageously, the undoped or weakly doped InGaN layer has a thickness of 0.25 to 3 nm. 
     Advantageously, the electrically insulating layer is of SiO 2  or SiN. 
     Advantageously, the indium rate present in the InGaN mesas is greater than or equal to 8%. This ensures In rich, quality re-epitaxy. 
     Advantageously, the mesas have a thickness of less than 100 nm. Thus, the defect density in the mesas is limited despite the high In concentration. 
     According to a first advantageous alternative, the device provided in step a) is manufactured by forming the electrically insulating layer and then the InGaN mesas. 
     According to another advantageous alternative, the device provided in step a) is manufactured by forming the InGaN mesas and then the electrically insulating layer. 
     Advantageously, the electrically conductive doped GaN layer covers a sapphire or silicon support. 
     Advantageously, the process includes a step during which an implantation doping or metal-organic vapour phase epitaxy doping step is performed on the InGaN mesas. Implantation is selective on the InGaN mesas. 
     Advantageously, the doped InGaN layer on the mesas has different doping types from one mesa to another. This results in pixels, for example three pixels, with different doping levels and therefore different relaxation percents, hence different emission wavelengths. This alternative embodiment is advantageous for forming a multispectral device, for example LEDs of different colours in a simplified manner, or a multi-colour micro-display. 
     According to one advantageous embodiment, the InGaN mesas of the structure provided in step a) are obtained according to the following steps of:
         applying 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 are locally formed comprising a doped InGaN layer and an undoped or weakly doped InGaN layer.       

     Advantageously, the doped zones have different doping types, whereby the doped InGaN layer of the mesas has different doping types from mesa to mesa. 
     This process has many advantages:
         It is simple to implement,   it can be used for mesas with small thicknesses (typically less than 100 nm),   mesa structuring provides the compliance effect,   it leads to partial or total stress relaxation and reduces piezoelectric polarisation as compared to a stressed layer with the same In concentration,   it allows a so-called “bottom up” approach for the manufacture of μLEDs and μdisplays: the growth of optical structures (N, QW, P) is achieved after mesa pixelisation, whatever the pixel size, and enables alignment problems as for the “pick and place” process to be dispensed with,   there is no impact of the pixel etching process on the efficiency of the micro-LEDs, which makes it possible to make micrometric or sub-micrometric pixels.       

     The invention also relates to a GaN/InGaN structure successively comprising:
         an electrically conductive doped GaN layer,   InGaN mesas comprising a porous doped InGaN layer and an undoped or weakly doped InGaN layer, preferably having a thickness ranging from 0.25 nm to 3 nm,   an epitaxially grown and relaxed InGaN layer.       

     Advantageously, the undoped or weakly doped InGaN layer has a thickness ranging from 2 nm to 3 nm. This layer is non-porous. 
     By porous layer it is meant a layer with a porosity of more than 1%. Preferably it ranges from 5% to 70%. The top limit value is the maximum porosity before the start of the mesa delamination. 
     Further characteristics and advantages of the invention will appear from the following additional description. 
     Needless to say that this additional description is only given by way of illustration of the object of the invention and should in no way be construed as a limitation of that object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be better understood upon reading the description of exemplary embodiments given purely by way of indicating and in no way limiting purposes, with reference to the appended drawings in which: 
         FIGS. 1A and 1B  schematically represent, in a cross-section view, a device comprising a GaN/InGaN structure according to different particular embodiments of the invention. 
         FIGS. 2A, 2B, 2C and 2D  schematically represent different steps of the process for manufacturing the device comprising a GaN/InGaN structure, according to a particular embodiment of the invention. 
         FIGS. 3A, 3B and 3C  schematically represent different steps of the process for manufacturing the device comprising a GaN/InGaN structure, according to another particular embodiment of the invention. 
         FIG. 4  schematically represents a step of electrochemically anodising InGaN mesas in a particular embodiment of the invention. 
         FIG. 5  is a graph representing different phenomena (pre-breakdown, porosification and electropolishing) occurring during an anodisation step, as a function of the doping rate and the potential applied, according to a particular embodiment of the invention. 
         FIGS. 6A and 6B  schematically represent an epitaxial regrowth on InGaN mesas, according to different particular embodiments of the invention. 
         FIGS. 7A, 7B, 7C and 7D  are atomic force microscope (AFM) pictures of a 0.8% InGaN layer before porosification, after porosification, after porosification and annealing, and finally after epitaxial regrowth, respectively. 
     
    
    
     Different parts represented in the figures are not necessarily drawn to a uniform scale, to make the figures more legible. 
     Different possibilities (alternatives and embodiments) should be understood as being not mutually exclusive and be combinable with each other. 
     Furthermore, in the following description, terms that depend on the orientation, such as “on”, “above”, “below”, etc., of a structure apply, considering that the structure is oriented in the way illustrated in the figures. 
     DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS 
     Although this is by no means limiting, the invention finds applications particularly in the field of colour micro-displays, and more particularly in the manufacture of red green blue pixels. However, it could also be used in the field of photovoltaics or even 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 range of water stability, a thermodynamic condition necessary for the water decomposition reaction. The invention may also be of interest for the manufacture of LEDs or long wavelength emitting lasers. 
     The process, which makes it possible to obtain a partially or totally relaxed GaN/InGaN structure, consists in porosifying InGaN by electrochemical anodisation according to 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 GaN doped 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  in 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) performing epitaxy on the InGaN mesas  100 , whereby an epitaxially grown, at least partially relaxed, and preferably fully relaxed InGaN layer is obtained. 
     The relaxation percent corresponds to: 
         =( a   c2   −a   c1 ) /a   c1    
     with a c1 , the lattice parameter of the starting layer, and 
     a c2  the lattice parameter of the relaxed layer, 
     The layer is 100% relaxed if a c2  corresponds to the lattice parameter of the solid material. 
     When a c1 =a c2  the layer is said to be stressed. 
     By partially relaxed it is meant a relaxation percent of more than 50%. 
     The device provided in step a) comprises a GaN/InGaN structure. More particularly, the device comprises a stack  200 , the upper layer of which is an electrically conductive doped GaN layer  204 , this layer being covered and in contact with InGaN mesas  100 . 
     Advantageously, the layer stack  200  covered with InGaN mesas  100  comprises:
         a support layer  201 , or support, for example of sapphire or silicon,   a first layer  202 , preferably of unintentionally doped (uid GaN) gallium nitride (GaN) and/or a second layer  203 , preferably of doped GaN (n GaN),   a third layer corresponding to the electrically conductive doped GaN layer  204 , this layer preferably being highly doped (n++ GaN).       

     By unintentionally doped GaN, it is meant a concentration of less than 5 e 17 at/cm 3 . 
     By doped GaN, it is meant a concentration of more than 10 18  at/cm 3 . 
     By highly doped GaN, it is meant a concentration greater than 10 19  at/cm 3 . 
     Preferably, stack  200  consists of the aforementioned layers. In other words, it does not include any other layers. 
     The support layer  201  has, for example, a thickness ranging from 350 μm to 2 mm. The thickness depends on the nature of the support layer  201  and its dimensions. For example, for a 2-inch diameter sapphire support, the thickness is 350 μm and for a 6-inch diameter sapphire support, the thickness is 1.3 mm. 
     The first layer  202  has, for example, a thickness ranging from 500 nm to 5μm. This uid layer should absorb stresses related to the lattice mismatch between GaN and the substrate. Generally its thickness is between 1 and 4 μm. 
     The second layer  203  has, for example, a thickness ranging from 10 0nm to 1 μm: it is present to ensure quality of the material of the upper layer  204 . 
     The third layer  204  has, for example, a thickness ranging from 100 nm to 500 nm. It should be sufficiently conductive, the minimum thickness varying as a function of the doping rate. 
     The third, electrically conductive, layer  204  of stack  200  is directly in contact with the mesas  100  to be porosified. During the electrochemical anodising step, the electrically conductive layer  204  of stack  200  is used to perform recontacting on stack  200 . This electrically conductive layer  204  is electrically connected to the voltage or current generator. 
     Mesas  100 , also called elevations, are raised elements. They are obtained, for example, by etching a continuous layer or several superimposed continuous layers so that only a certain number of “reliefs” of this layer or these layers remain. Etching is generally plasma etching (or dry etching). The reliefs enable pixels to be defined. 
     Preferably, flanks of the mesas  100  are perpendicular to this layer stack  200 . 
     The dimensions (width and length) of the mesas  100  range from 500 nm to 500 μm. By width and length, it is meant dimensions parallel to the surface of the underlying stack. 
     The pitch between two consecutive mesas  100  ranges from  50  nm to 20 μm. 
     InGaN mesas  100 s consist of a doped InGaN layer  101  and an undoped or weakly doped InGaN layer  102 . 
     Preferably, the mesas comprise these two layers  101 ,  102 . 
     By doped InGaN, it is meant a doping concentration of more than 10 18  at·cm 3  , or even more than 10 19  at·cm 3  for a so-called highly doped layer. The doping will be chosen according to the porosification technique (with or without illumination). The doped InGaN layer  101  is electrically conductive. It is porosified during step d). 
     By weakly doped InGaN, it is meant a doping concentration of less than 5.10 17  at·cm 3 . The doping is low enough so that this layer is electrically insulating. Thus layer  102  is not anodised, and therefore not porosified during step d). 
     The doped layer  101  of the mesas  100  has advantageously a thickness ranging from 10 nm to 100 nm, preferably in the order of 30 nm. A layer with a small thickness (less than 100 nm) can contain a high In concentration while maintaining good material quality (few defects). By thickness, it is meant the dimension of the mesa perpendicular to the underlying stack. 
     The undoped or weakly 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 should provide a continuous interface to ensure coalesced epitaxial regrowth. It is advantageously chosen thin, for example, between 0.25 nm and 3 nm, so that it is defect-free and can be relaxed by virtue of the subsequent porosification process of the lower layer  101  and the epitaxial regrowth on the mesas. 
     The electrically conductive layer  204  is covered, between the mesas  100 , with an electrically insulating layer  300  in order not to be in contact with the electrolyte solution during the porosification step and thus not to be porosified. 
     The electrically insulating layer  300  can be of oxide or nitride. Preferably, silicon oxide or silicon nitride will be 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 will be chosen. 
     According to a first alternative embodiment, represented in  FIGS. 2A to 2D , such a device can be manufactured according to the following steps of:
         providing a stack  200  comprising, for example, a support  201  of sapphire or silicon, successively covered with a first layer  202  of uid GaN and/or a second layer  203  of doped GaN, and a third, electrically conductive, layer  204  of highly n-doped GaN,   forming a highly doped InGaN layer  101 , then an undoped or weakly doped InGaN layer  102 ,   structuring, for example by photolithography, the highly doped InGaN layer  101  and undoped InGaN layer  102  so as to make mesas  100 ,   forming 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 .       

     The controlled etching of the mesas  100  allows stopping on or in the n++ GaN layer. By “on the n++ GaN layer”, it is meant at the interface of the n++ GaN layer  204  and the doped InGaN layer  101 . In other words, the n++ GaN layer is not completely etched in such a way as to ensure electrical conduction during electrochemical porosification. 
     According to an alternative embodiment, represented in  FIGS. 3A to 3C , such a device can be manufactured according to the following steps of:
         providing a stack  200  comprising, for example, a support  201  of sapphire or silicon, successively covered with a first layer  202  of uid GaN and/or a second layer  203  of doped GaN, and a third, electrically conductive, layer  204  of highly n-doped GaN,   forming an electrically insulating layer  300 , for example of SiO 2  or SiN, locally covering the electrically conductive layer  204 ,   forming the mesas  100  in the zones of the electrically conductive layer  204  not covered with the electrically insulating layer  300 , by forming a doped InGaN layer  101  and then an undoped or weakly doped InGaN layer  102 .       

     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. 
     According to these alternative embodiments, it is possible to perform doping of the doped layer  101  of the mesas  100 , for example, by implanting silicon (Si(n)) or implanting magnesium (Mg(p)). 
     The layers  101  of the mesas  100  can have the same or different doping types. The use of different doping types makes it possible to vary the porosification rate of the mesas according to their initial doping level for a given applied potential. The mesas will be thereby more or less relaxed and more or less able to integrate indium into the layers. It is thus easy to obtain pixels of different colours after epitaxial regrowth. 
     According to an advantageous embodiment, the InGaN mesas of the structure provided in step a) can be obtained according to the following steps of:
         depositing a so-called full-plate InGaN layer (i.e. a continuous layer) on the electrically conductive highly n-doped GaN layer  204 ,   locally performing an implantation step in the full-plate InGaN layer so as to have doped zones and less doped or even undoped zones; for example, for 3 n-type doping levels, 2 lithographies and hard masks may be carried out and silicon be implanted,   advantageously, performing a so-called healing thermal annealing,   depositing an unintentionally doped or weakly doped (typically with a residual between 5.10 16  and 1.10 17 /cm 3 ) full-plate InGaN layer onto the full-plate InGaN layer, for example by epitaxy,   etching the full-plate InGaN layer through the undoped or weakly doped full-plate InGaN layer so as to remove non-implanted zones, whereby InGaN  100  mesas are formed comprising a doped InGaN layer  101  and an undoped or weakly doped InGaN layer  102 .       

     By way of illustration, it is, for example, possible to use a hard mask, such as a SiN or SiO 2  type mask, to carry out implantation and alignment marks (for example Al, Ti, TiN) to match the mesas to the implanted zones. With two different masks, two zones with different implantation energies can be made, and thus two different doping types in addition to the initial doping of the doped plate, for example by MOCVD. Alignment marks enable both masks to be aligned for implantation and the mask for mesa etching. 
     The same set of masks will be used for implantation and mesas. 
     Advantageously, for an undoped or weakly doped InGaN layer having low thickness, it is possible to perform implantation of the underlying InGaN layer through this layer. 
     For p-type doping, magnesium can be implanted. 
     Advantageously, the doped zones have different doping types, so that the doped InGaN layer of the mesas has different doping types. The higher the doping rate, the greater the porosification at a fixed potential. The relaxation of the dense InGaN layer depends on the porosification rate of the mesas. As a result, different amounts of indium may be incorporated when re-epitaxially growing InGaN on the dense layer (by virtue of reducing the “compositional pulling effect” (i.e. pushing In atoms towards the surface, preventing them from being incorporated into the layer). Thus, after epitaxy of the complete LED structure, red, green and blue (RGB) mesas will be obtained on a same substrate, as well as in a single growth step, if the deviation between the relaxation levels of the mesas is sufficient. 
     Alternatively, in these different embodiments, implantation doping can be replaced by Metal-organic Chemical Vapour Deposition (MOCVD) doping, especially with Si or Ge as dopant. It is, for example, possible to carry out three successive epitaxy steps with successive masking operations to obtain three different doping levels in order to form RGB mesas at the end of the process. 
     In what follows, an n-type doping will be described, but it could be a p-type doping. 
     In step b), the device and a counter-electrode (CE)  500  are electrically connected to a voltage or current generator ( FIG. 4 ). The device acts as a working electrode (WE). Later it will be referred to as a voltage generator, but it could be a current generator for applying a current between the device and the counter-electrode. 
     The counter-electrode  500  is of an electrically conductive material, such as for example a metal like platinum. 
     In step c), the electrodes are dipped into an electrolyte, also called an electrolyte bath or electrolyte solution. The electrolyte can be acidic or basic. The electrolyte is, for example, oxalic acid. It can also be KOH, HF, HNO 3 , NaNO 3  or H 2 SO 4 . 
     It is possible to perform step c) before step b). 
     In step d), a voltage is applied between the device and the counter-electrode  500 . The voltage can range from 1 to 100V. It is applied, for example, for a period ranging from a few seconds to several hours. Porosification is complete when there is no more current at an imposed potential. At this point, the entire doped structure is porosified and the electrochemical reaction stops. 
     The difference in the doping rate makes it possible to have a high selectivity between porosification and electropolishing ( FIG. 5 ). This “chart” makes it possible to define for given conditions, respective doping rates so that, at a given potential, there is selectivity between the highly doped zone and weakly doped zone. 
     The electrochemical anodising step can be carried out under ultraviolet (UV) light. The process can also include a first electrochemical anodisation without adding ultra-violet radiation and a second electrochemical anodisation by adding ultra-violet radiation. This embodiment is particularly interesting since, for example, firstly the upper n+ doped layer can be porosified, with the porosification stopping abruptly on the uid GaN. Then, secondly, UV light can generate charges in the uid GaN and undoped GaN can thus be porosified. 
     Advantageously, the porosification takes place in the entire volume of the doped InGaN layer  101  of the mesas  100 . 
     At the end of the porosification step, the porosity of the doped InGaN layer  101  is at least 10%. It preferably ranges from 25% to 50%. 
     The largest dimension (the height) of the pores can vary from a few nanometres to a few micrometres. The smallest dimension (the diameter) can vary from a few nanometres to a hundred nanometres, in particular from 30 to 70 nm. 
     The porosification obtained (porosity rate and pore size) depends on the doping of the doped InGaN layer  101  of the mesas  100  and the process parameters (applied voltage, period of time, nature and concentration of the electrolyte, chemical post-treatment or annealing). By varying the porosification, the rate of incorporation/segregation can be controlled. The porosification, and in particular the pore size, may vary subsequently, during epitaxial regrowth, depending on the temperature applied. 
     The porosification step provides a totally or partially relaxed doped InGaN layer  101 . 
     Following the porosification step, an epitaxial regrowth is performed on the InGaN mesas  100  (step e), for example to form epitaxially regrown LEDs. 
     Epitaxial regrowth is performed on the undoped InGaN layer  102  of the mesas  100 . As layer  102  is not porosified during the electrochemical anodisation step, it enables an epitaxial regrowth to be performed on a continuous, dense, non-porous 2D layer. The epitaxial regrowth is thus facilitated and the epitaxially grown layer has a better resistance. The creation of defects related to pore coalescence is avoided. 
     Different growth processes can be used to form an epitaxially grown layer  400 . 
     According to a first alternative embodiment, represented in  FIG. 6A , the regrowth is lateral and a subsequent pixelisation step by etching is advantageously performed. It is also possible to passivate the flanks with a layer having a small thickness (for example 3 nm), preferably by atomic layer deposition (ALD). 
     According to an alternative embodiment, represented in  FIG. 6B , the regrowth takes place vertically above the mesas. In this alternative, pixels correspond to the underlying mesas. 
     The epitaxially grown layer  400  is advantageously of gallium nitride or indium gallium nitride. 
     Advantageously, a passivation layer will be deposited onto the flanks of the mesas  100  and the epitaxially regrown LEDs, for example by atomic layer deposition (ALD). The passivation layer can be of alumina. The passivation layer can have a thickness of a few nanometres, for example 2 to 5 nm. 
     The epitaxially regrown stack of an all-InGaN LED can comprise, for example, successively from the unintentionally doped InGaN layer  102 :
         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 red or green emitting InGaN/(Ga,In)N quantum wells,   a p-doped AIGaN or GaN based 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.       

     More particularly, an all-InGaN LED structure can successively comprise:
         an InGaN substrate,   a 350 nm n-doped InGaN layer, formed by 15×In 0.03 Ga 0.97 N/GaN (thicknesses 20 nm/1.8 nm),   multiple quantum wells (MQWs), formed by 5×In 0.40 Ga 0.60 N/In 0.03 Ga 0.097 N (thicknesses 2.3 nm/5, 7, 11 nm),   a uid In 0.03 Ga 0.97 N layer (10 nm),   an Al 0.1 G 0.9 N:Mg layer (20 nm),   an Mg-doped In 0.03 Ga 0.97 N layer (125 nm),   a p++ doped In 0.03 Ga 0.97 N layer (25 nm).       

     Illustrative and Non-Limiting Examples of One Embodiment 
     In this example, a stack  200  is used, comprising: a sapphire support  201 , a (uid) GaN layer  202 , a doped GaN (n GaN) layer  203  and a highly doped GaN (n++ GaN) layer  204 . 
     To make the InGaN mesas  100 , a In 0.0008 Ga 0.992 N (n-doped: 10 19  at/cm 3 ) layer  101  less than 100 nm thick is deposited on this stack  200 . A InGaN layer  102  of the same composition, but undoped, of small thickness (from 2 to 3 nm) is deposited in order to facilitate subsequent epitaxial regrowth. 
     The mesas  100  are then structured using conventional photolithography techniques. The dimensions of the mesas  100  obtained are from 500 nm to 10 μm. The controlled etching of the InGaN layer allows stopping on or in the n++ GaN layer. 
     The n++  204  GaN layer is then passivated with a layer  300  of SiO 2  or SiN for example. 
     The AFM image of the InGaN layer before porosification is represented in  FIG. 7A . 
     The porosification is then carried out by electrochemically anodising the InGaN mesas in an electrolyte solution containing from 0.1 mol/L to 0.5 mol/L oxalic acid, for example 0.2 mol/L. The voltage applied is 24V. 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 represented in  FIG. 7B . 
     Annealing is then carried out, for example at 900° C. ( FIG. 7C ). 
     Finally, an epitaxial regrowth is performed on the InGaN mesas  100  ( FIG. 7D ). 
     Photoluminescence (PL) characterisations have been carried out. Before porosification, the emission of the 0.8% InGaN sample is at 356 nm. After the epitaxial regrowth, the emission is at 364 nm. Samples made with 2% InGaN show the same trend. This deviation in PL may be the signature of stress relaxation and/or increased In integration. 
     REFERENCES 
     [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.