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

Abstract

A process for the porosification of mesas comprising the following steps: - providing a structure (100) comprising a substrate (110) covered with mesas (120) (Al,In,Ga)N / (Al,In,Ga)N, the substrate (110) comprising a support layer (114), a first undoped GaN layer (111), a second doped GaN layer (112), the mesas (120) comprising a third heavily doped (Al,In,Ga)N layer (123) 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) or a portion (123a) of the third heavily doped (Al,In,Ga)N layer (123) extending into the substrate (110), - Immerse the structure (100) and a counter electrode in an electrolytic solution, - Apply a voltage or current between the structure (100) and the counter electrode so as to porosify the third layer of heavily doped GaN (123) of the mesas (120). Figure for the abbreviation: 3A.

Description

Title of the invention: PROCESS FOR POROSIFICATION OF MESAS (Al,In,Ga)N / (Al,In,Ga)N technical field

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[0015] 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.

[0016] 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.

[0017] For example, Figures IA to IC represent different stages of such a process. More specifically, the process comprises the following stages:

[0018] - provide a stack comprising successively a support layer 10, a unintentionally doped GaN layer 11, doped or heavily doped GaN layer 12, InGaN or nested GaN layer 13 ([Fig.1A]),

[0019] - structuring mesas in this stacking, the mesas comprising 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,

[0020] - immerse the structure obtained in an electrolytic solution and apply a tension between the doped or heavily doped GaN layer 12 and a counter electrode, whereby a porosified GaN layer 12' is obtained ([Fig.lC]).

[0021] 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.

[0022] 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 becomes porous from the mesa flanks towards the center of the mesas. This layer does not porosify completely to the center of the mesas. Wafer edge / center effects are observed, for example on 2-inch wafers. This effect may be due to the degradation of the lateral conductivity of the residual doped layer at the bottom of the mesa, with polarization occurring through the periphery of the wafer, hence the wafer edge / center effect.

[0023] 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). Description of the invention

[0024] An object of the present invention is to propose a method of porosification of (Al,In,Ga)N / (Al,In,Ga)N mesas allowing not only the entire volume of the doped GaN layer of the mesas to be porosified, but also all the mesas on the wafer to be porosified in an identical (or homogeneous) manner, whether they are at the edge or in the center of the wafer, the method being advantageously usable even for large substrates (typically on substrates of at least 5 cm and preferably of at least 10 cm in diameter).

[0025] To this end, the present invention proposes a process for the porosification of (Al,In,Ga)N / (Al,In,Ga)N mesas comprising the following steps:

[0026] a) provide a structure comprising a basic substrate covered with (Al,In,Ga)N / (Al,In,Ga)N mesas,

[0027] the basic substrate comprising a support layer, a first undoped GaN layer, a second doped GaN layer,

[0028] the (Al,In,Ga)N / (Al,In,Ga)N mesas comprising a third layer of (Al,In,Ga)N heavily doped and a fourth layer of (Al,In,Ga)N undoped or weakly doped,

[0029] a portion of the second doped GaN layer extending into the mesas or a portion of the third heavily doped (Al,In,Ga)N layer extending into the basic substrate,

[0030] b) electrically connect the structure (for example at the second doped layer, the third heavily doped layer or the fourth lightly doped layer) and a counter electrode to a voltage or current generator,

[0031] c) immersing the structure and the counter electrode in an electrolytic solution,

[0032] d) apply a voltage or current between the structure and the counter electrode of in order to porosify the third layer of GaN heavily doped in the mesas.

[0033] The invention is fundamentally distinguished from the prior art by the presence of a bilayer comprising two doped GaN layers having different doping levels (the second layer doped and the third layer heavily doped). The bilayer is positioned between the first undoped GaN layer of the substrate and the fourth undoped or lightly doped (Al,In,Ga)N layer of the mesas.

[0034] The second layer has a lower doping level than the third layer and can be used to establish electrical contact. Thanks to doping-dependent porosification selectivity, the second layer is not porosified during step d) and remains intact until the end of the mesa porosification process, thus enabling the porosification of all mesas in the volume, regardless of their position in the structure. It is therefore possible to porosify the heavily doped third layer of (Al,In,Ga)N without significantly increasing the stack thickness, particularly in the case of a silicon substrate.

[0035] Advantageously, for the second layer, a sufficiently low doping level is chosen to prevent the layer from becoming porous, while still being high enough to ensure electrical conductivity. For example, the doping level of the second doped GaN layer is less than 5 x 10¹⁸ at / cm³, preferably between 5 x 10¹⁷ at / cm³ and 3 x 10¹⁸ at / cm³, and even more preferably between 5 x 10¹⁷ at / cm³ and 2 x 10¹⁸ at / cm³. Such a doping level is obtained, for example, with silicon doping. It is also possible to use germanium doping. The doping level can then be higher, for example up to 5 x 10¹⁸ at / cm³, or even up to 1.5 x 10¹⁹ at / cm³.

[0036] Advantageously, the doping of the third layer is significantly higher than that of the second layer for achieving selectivity against porosification. The greater the difference in doping between the doped second layer and the heavily doped third layer, the better the selectivity of the porosification process. The appropriate doping values ​​depend on the potential applied for the porosi- selectivity. For example, the doping level of the third layer of heavily doped GaN is between 5 x 10¹⁸ at / cm³ and 2 x 10¹⁹ at / cm³, preferably between 6 x 10¹⁸ at / cm³ and 2 x 10¹⁹ at / cm³, even more preferably between 8 x 10¹⁸ at / cm³ and 1.5 x 10¹⁹ at / cm³, and even more preferably between 8 x 10¹⁸ at / cm³ and 1 x 10¹⁹ at / cm³, for example with Si or Ge doping. It is possible to have doping levels up to 1 x 10²⁰ at / cm³ or even 1 x 10²¹ at / cm³ with Ge doping.

[0037] According to a particularly advantageous embodiment, the structure further comprises an additional layer of heavily doped GaN arranged between the first undoped GaN layer and the third doped GaN layer. This results in a trilayer comprising a heavily doped GaN layer covered by the second doped layer and the third heavily doped layer to be porosified. The trilayer is positioned between the first undoped GaN layer of the substrate and the fourth undoped or lightly doped (Al,In,Ga)N layer of the mesas.

[0038] The second doped layer provides protection for the underlying additional heavily doped layer and ensures contact. Advantageously, this layer is thin (typically between 200 nm and 1 pm, preferably between 400 and 700 nm). This layer is advantageously as thin as possible while remaining well-covering to prevent electrolyte infiltration and thus consumption of the underlying heavily doped layer during step d).

[0039] The additional heavily doped layer ensures lateral conduction of charges within the structure. For example, the doping level of the additional heavily doped GaN layer is between 5 x 10¹⁸ at / cm³ and 2 x 10¹⁹ at / cm³, preferably between 5 x 10¹⁸ at / cm³ and 1 x 5 x 10¹⁹ at / cm³, and even more preferably between 8 x 10¹⁸ at / cm³ and 1 x 10¹⁹ at / cm³. Advantageously, this layer is thick (typically between 0.5 µm and 5 µm, and preferably between 1 and 2 µm). Greater thicknesses can be achieved on sapphire. This results in a highly conductive buried layer thanks to a high doping level and a significant layer thickness. The thickness-doping ratio will be adjusted to ensure sufficient lateral conduction. In step d), conduction occurs via this additional heavily doped buried layer. Because it is highly conductive, it limits edge / center effects.

[0040] The charges pass through the second doped layer and then onto the additional highly doped layer, which then acts as a conduction highway and supplies all the mesas present on the substrate. During step d), the second doped layer protects the additional highly doped layer from porosification. Thus, each mesa is in the same electrical configuration to be porosified uniformly regardless of the size and position of the mesa on the plate (edge ​​or center).

[0041] Advantageously, the doping level of the third layer of heavily doped GaN and / or the doping level of the additional layer of heavily doped GaN is between 5.1018at / cm3 and 1.51019at / cm3.

[0042] Advantageously, the voltage applied during step e) is between 3V and 15V, preferably between 6V and 12V, even more preferably between 8V and 12V, and even more preferably between 8 and 10V.

[0043] Advantageously, the support layer is a wafer of at least 5 cm in diameter (for example 2 inches or 5.08 cm), preferably of at least 100 mm in diameter and even more preferably of at least 200 mm in diameter.

[0044] Advantageously, the support layer is made of sapphire or silicon.

[0045] Advantageously, the fourth layer of (Al,In,Ga)N, undoped or weakly doped, is a layer of GaN.

[0046] Following step d), LEDs can then be formed, for example by epitaxy, on the mesas to form pixels.

[0047] The process has many advantages:

[0048] - it is simple to implement,

[0049] - it can be applied to large wafers to manage the pro The problem of uniformity of electrochemical polarization and therefore of porosification / relaxation,

[0050] - it is compatible with the use of Si (111) substrates for growth, plus sensitive to deformation, unlike sapphire, the high doping of the buried layer allows its thickness to be reduced,

[0051] - preserving the porosity of mesa bottoms allows for better control of InGaN epitaxy, particularly in the case of selective epitaxy, the growth mask on non-porous mesa bottoms promoting species mobility towards growth sites

[0052] - it can be used for mesas of different sizes including mesas having thin thicknesses.

[0053] Furthermore, growth on such porous mesas offers the following advantages:

[0054] - the mesa structuring provides the elastic compliance effect including during growth,

[0055] - it leads to a partial or total relaxation of the stresses and reduces the polarity piezoelectric concentration by comparison to a constrained layer of the same In concentration,

[0056] - it allows a so-called "bottom up" approach for the manufacture of pLEDs and pdisplays: The growth of optical structures (N, QW, P) is achieved after mesa pixelation, regardless of pixel size, and eliminates alignment problems as with the "pick and place" process.

[0057] - there is no impact of the pixel etching process on the efficiency of the micro- LEDs, which makes it possible to create micrometric or sub-micrometric pixels.

[0058] The invention also relates to a structure obtained according to the process described above. The structure comprises a base substrate covered with porosified (Al,In,Ga)N / (Al,In,Ga)N mesas,

[0060] the basic substrate comprising a support layer, a first undoped GaN layer, a second doped GaN layer,

[0061] the (Al,In,Ga)N / (Al,In,Ga)N mesas comprising a third layer of highly doped porosified (Al,In,Ga)N and a fourth layer of undoped or lightly doped (Al,In,Ga)N,

[0062] a part of the second doped GaN layer which can also extend into the mesas.

[0063] Advantageously, the structure further comprises an additional layer of heavily doped GaN disposed between the first undoped GaN layer and the second doped GaN layer.

[0064] Advantageously, the fourth layer of (Al,In,Ga)N, undoped or weakly doped, is a layer of GaN and / or the support layer is a 2-inch slice or a slice of at least 100mm in diameter and, preferably, a slice of at least 200mm in diameter.

[0065] Other features and advantages of the invention will become apparent from the following supplementary description.

[0066] It goes without saying that this additional description is given only as an illustration of the object of the invention and should in no case be interpreted as a limitation of this object. Brief description of the drawings

[0067] 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:

[0068] [Fig.1A]

[0069] [Fig.1B]

[0070] [Fig.lC] previously described, represent schematically different stages of a porosification process according to the prior art.

[0071] [Fig.2A]

[0072] [Fig.2B]

[0073] [Fig.3A]

[0074] [Fig.3B] represent, schematically and in cross-section, different structures comprising (Al,In,Ga)N / (Al,In,Ga)N mesas according to different embodiments of the invention.

[0075] [Fig.4] is a graph representing different phenomena in intervenant (pre-porification (or "pre-breakdown"), porosification, and electropolishing) during an anodizing step, depending on the doping level of a GaN layer and the applied voltage, according to a particular embodiment of the invention; region A corresponds to the pre-porification (or 'pre-breakdown') region, region B to the porosification region, region B' to the transitional region between regions B and C, where the pores join and coalesce, and region C to the electropolishing region.

[0076] [Fig. 5] represents, schematically and in cross-section, a structure comprising mesas (Al,In,Ga)N / (Al,In,Ga)N (in particular GaN / GaN) used for comparison.

[0077] [Fig.6] is a scanning electron microscope image of a mesa (Al,In,Ga)N / (Al,In,Ga)N of the structure shown in [Fig. 5] after the porosification step, which leads to incomplete porosification,

[0078] [Fig.7A]

[0079] [Fig.7B] are dark-field optical microscope images of different (Al,In,Ga)N / (Al,In,Ga)N mesas, in top view, of the structure shown in [Fig.5] after the porosification step, respectively at the center and at an edge of the structure; 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.

[0080] [Fig.8] is a dark-field optical microscope image of mesas (Al,In,Ga)N / (Al,In,Ga)N of 3 pm width (left) and (Al,In,Ga)N / (Al,In,Ga)N mesas of 5 pm width (right), top view, after the porosification step according to different particular embodiments of the invention; the photograph is taken at the center of the structures (i.e. at the furthest point from the electrical contact which is taken at the edge of the plate, i.e. at the center where the potential loss is greatest).

[0081] [Fig.9] is a dark-field optical microscope image of mesas (Al,In,Ga)N / (Al,In,Ga)N of 3pm width (left) and (Al,In,Ga)N / (Al,In,Ga)N mesas of 5pm width (right), top view, after the porosification step according to different particular embodiments of the invention; the photograph is taken at the center of the structures.

[0082] [Fig.1OA]

[0083] [Fig.1OB] are images obtained by scanning electron microscopy of a porosified (Al,In,Ga)N / (Al,In,Ga)N mesa according to a particular embodiment of the invention.

[0084] [Fig. 11 A]

[0085] [Fig. 1 IB] are simulations representing the current densities in a structure comprising (Al,In,Ga)N / (Al,In,Ga)N mesas during a 12V porosification step, according to a particular embodiment of the invention.

[0086] The different parts represented in the figures are not necessarily shown on a uniform scale, in order to make the figures more legible.

[0087] The different possibilities (variants and embodiments) should be understood as not being mutually exclusive and can be combined with each other.

[0088] Furthermore, in the description below, orientation-dependent terms such as "above", "below", etc. of a structure apply assuming that the structure is oriented in the manner illustrated in the figures.

[0089] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0090] 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.

[0091] 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.

[0092] 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.

[0093] The process for porosifying mesas 120 of (Al,In,Ga)N / (Al,In,Ga)N comprises the following steps:

[0094] a) provide a structure 100 comprising a basic substrate 110 covered with mesas 120 (Al,In,Ga)N / (Al,In,Ga)N (figures 2A, 2B, 3A, 3B),

[0095] the basic substrate 110 comprising successively:

[0096] - a support layer 114,

[0097] - possibly a buffer layer of (Al,Ga)N, particularly in the case of a silicon support layer 114,

[0098] - a first undoped GaN layer 111,

[0099] - advantageously, an additional highly doped layer 113,

[0100] - a second layer of GaN doped 112,

[0101] the mesas 120 (Al,In,Ga)N / (Al,In,Ga)N comprising a third layer of (Al,In,Ga)N heavily doped 123 intended to be porosified and a fourth layer of (Al,In,Ga)N undoped or weakly doped 124,

[0102] a portion of the second doped GaN layer extending into the mesas or a portion of the third heavily doped (Al,In,Ga)N layer extending into the basic substrate,

[0103] the bottom of the mesas is preferably preserved from porosification during step d),

[0104] b) electrically connect structure 100 and a counter electrode to a voltage or current generator,

[0105] c) immerse the structure 100 and the counter electrode in an electrolytic solution,

[0106] d) 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.

[0107] The structure 100 provided in step a) is, for example, obtained by providing and then locally engraving a stack comprising successively:

[0108] - a support layer 114,

[0109] - possibly, a buffer layer of (Al,Ga)N, particularly in the case of a silicon support layer 114,

[0110] - a first layer of undoped gallium nitride GaN 111,

[0111] - possibly, an additional layer of heavily doped GaN 113,

[0112] - a second layer of doped GaN (GaN n) 112,

[0113] - a third layer of GaN heavily doped (GaN n+ or GaN nn) 123, and

[0114] - a fourth layer in AIN, InGaN or GaN (denoted (Al,In,Ga)N) not intended heavily doped (nest) or weakly doped 124.

[0115] Preferably, the stack consists of the layers mentioned above. In other words, it does not include any other layers.

[0116] According to an advantageous embodiment, a first part 112a of the second layer 112 forms part of the base substrate 110 and a second part 112b of the second layer 112 forms part of the mesas 120 (as shown in Figures 2A and 3A).

[0117] The mesas 120 are formed by etching the fourth layer 124, the third layer 123 and a first part 112a of the second doped layer 112 (Figures 2A and 3A). By stopping the etching in the doped layer, the entire height of the third n++ layer 123 is available for relaxation.

[0118] Each mesa 120 comprises successively from the base: the second part 112b of the doped GaN layer 112, the third heavily doped GaN layer 123 and the fourth undoped or lightly doped (Al,In,Ga)N layer 124.

[0119] The first part 112a of the second doped GaN layer 112 protects the additional layer 113 during the porosification step. Thus, the additional layer 113 is not in contact with the solution. The first part of the doped GaN layer is a common layer for all mesas.

[0120] According to another embodiment, the mesas 120 are formed by etching the fourth layer 124 and a portion of the heavily doped third layer 123 (Figures 2B and 3B). The n++ layer 123 is not completely etched, thus preserving the integrity of the n-GaN layer. This embodiment is particularly advantageous for protecting the buried n++ layer 113 ([Fig. 3B]). A first portion 123a of the layer 123 is in the base substrate 110, and a second portion 123b of the layer 123 forms part of the mesas 120.

[0121] The structuring of the stack is, for example, achieved by photolithography.

[0122] 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.

[0123] Mesas 120, also called elevations, are raised features. They are obtained, for example, by etching a continuous layer or several superimposed continuous layers, so as to leave only a certain number of "reliefs" of this layer or these layers. The etching is preferably carried out with a hard mask, for example SiO2. After etching the mesas, this hard mask is removed by a wet chemical process before porosification. It is also possible to remove this hard mask after porosification, by exposing it only in the areas used for polarization for electrochemical polarization. Advantageously, the mask is removed before the porosification step.

[0124] Preferably, the flanks of the mesas 120 are perpendicular to this stacking of layers.

[0125] The surface of the mesas can be circular, hexagonal, square or rectangular.

[0126] 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.

[0127] The thickness of the mesas corresponds to the dimension of the mesa perpendicular to the underlying stack.

[0128] The spacing between two consecutive 120 mesas ranges from 50nm to 20pm.

[0129] 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 being incorporated into the layer). Thus, after epitaxy of the complete LED structure, blue, green, and red (RGB) mesas will be obtained on the same substrate, and in a single growth step, if the difference between the relaxation levels of the mesas is sufficient.

[0130] The support layer 114 is, for example, made of sapphire or silicon.

[0131] 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.

[0132] 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.

[0133] The first layer 111 is a nested GaN layer. It is an unintentionally doped (nest) layer to avoid porosity. Unintentionally doped GaN is defined as having a concentration of less than 5e17at / cm3.

[0134] 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.

[0135] The second layer 112 is a doped GaN layer. By doped GaN, we mean a concentration greater than 5.1017 at / cm3, preferably greater than 1018 at / cm3, preferably between 1.1018 at / cm3 and 5.1018 at / cm3.

[0136] The second GaN layer 112 has, for example, a thickness ranging from 200 nm to 1 pm, preferably between 400 and 700 nm. It must be sufficiently electrically conductive to allow for contact re-establishment during the electrochemical anodizing step. The minimum thickness varies depending on the doping level. This electrically conductive layer is electrically connected to the voltage or current generator.

[0137] The third layer 123 is a heavily doped GaN layer. By heavily doped GaN, we mean a concentration greater than 5 x 10¹⁸ at / cm³, preferably greater than 8 x 10¹⁸ at / cm³, or even greater than 10¹⁹ at / cm³. It has, for example, 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.

[0138] 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 5 x 10¹⁷ at.cm³ and 2 x 10¹⁸ at.cm³. Undoped means a doping level below 5e 17at / cm3.

[0139] 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).

[0140] This layer 124 is not or only slightly affected by porification and serves as a nucleus for renewed growth. This layer 124 is continuous to ensure the quality of the repitaxed layer, for example a (In,Ga)N layer, on the structure.

[0141] The additional layer 113 has a thickness, for example, between 500 nm and 5 pm, preferably between 1 pm and 5 pm. Preferably, it has a doping concentration greater than 8.1018 at.cm³, or even greater than 10¹⁹ at.cm³, for example, 1.5 x 10¹⁹ at.cm³. The additional layer 113 of heavily doped GaN may have the same or a different doping level than the third layer of heavily doped GaN. Preferably, it has a greater thickness than the third layer to ensure good charge mobility.

[0142] The doping of the various layers mentioned above, and in particular of the second layer 112, the third layer 123 and the additional layer 113, will be chosen according to the tension applied during the porosification.

[0143] In particular, they will be chosen from the "nomogram" in [Fig. 4]. This "nomogram" allows the respective doping levels to be defined 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 must be located in region A so that the second layer 112 is not porosified during step d), and the doping level of the third layer 123 must be located in region B so that the third 123 is porosified during step d).

[0144] Subsequently, an n-type doping is described, but it could be a P-type doping

[0145] By way of illustration and not limitation, according to one embodiment, structure 100 may comprise:

[0146] - a basic substrate 110 comprising successively: a support layer 114 in sapphire or silicon, possibly a buffer layer of (Al,Ga)N, a first undoped GaN layer 111 of 4pm, a first part 112a of the second GaN layer 112 doped by 500nm (1.1018 at / cm3),

[0147] - 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 100 nm (1.1018 at / cm3), a third layer 123 of GaN heavily doped by 800 nm (1.1019 at / cm3), and a layer of (Al,In,Ga)N nest of 100nm.

[0148] According to another embodiment, structure 100 may comprise:

[0149] - a basic substrate 110 comprising successively: a support layer 114 in sapphire or silicon, possibly a buffer layer of (Al,Ga)N, a first undoped GaN layer 111 of 4pm, an additional GaN layer 113 heavily doped of 2pm (1.1019 at / cm3), a first part 112a of the second GaN layer doped 112 of 500nm (1.1018 at / cm3),

[0150] - 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 lOOnm (1.1018 at / cm3), a third layer 123 of GaN heavily doped by 800 nm (1.1019 at / cm3), and a fourth layer 124 of (Al,In,Ga)N nest of lOOnm.

[0151] In step b), the structure 100 and a counter electrode (CE) are electrically connected to a voltage or current generator. The device acts as the working electrode (WE). Hereafter, it will be referred to as a voltage generator, but it could also be a current generator that applies a current between the device and the counter electrode.

[0152] The initial contact is made on structure 100.

[0153] In particular, contact can be made on the base substrate 110. As shown in Figures 2A, 2B, 3A, and 3B, re-contact can be made on the second doped GaN layer 112 (the re-contact is represented by an arrow in Figures 2A, 2B, 3A, and 3B). Re-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 make the re-contacts.

[0154] 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, on the third layer of (Al,In,Ga)N strongly doped 123 or on the additional layer of GaN strongly doped 113. In the case of re-establishing contact on a strongly doped layer, its opening will advantageously be limited to a preserved area of ​​the electrolyte.

[0155] 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.

[0156] The counter electrode 500 is made of an electrically conductive material, such as for example a metal with a large developed surface area and inert to the chemistry of the electrolyte such as a platinum wire mesh.

[0157] In step c), the electrodes are immersed in an electrolyte, also called an electrolytic bath or electrolytic solution. The electrolyte can be acidic or basic. The electrolyte is, for example, oxalic acid. It can also be KOH, HF, HNO3, NaNO3, or H2SO4.

[0158] In step d), a voltage is applied between structure 100 and counter electrode 500. The voltage can range from 1 to 30 V, for example. Preferably, it is from 5 to 15 V, and even more preferably from 6 to 12 V, for example from 8 to 10 V. The voltage is chosen according to the doping levels of the different layers, in order to obtain the desired selectivity. It is applied, for example, for a duration ranging from a few seconds to several hours. Porification is complete when there is no longer any current at the applied potential. At that point, the entire doped structure is porosified and the electrochemical reaction stops.

[0159] The electrochemical anodizing step can be carried out under ultraviolet (UV) light.

[0160] Advantageously, the porification takes place throughout the entire volume of the third layer of heavily doped GaN 123.

[0161] 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%.

[0162] 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.

[0163] 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.

[0164] Advantageously, the process includes a subsequent step e) in which epitaxy is performed on the mesas 120, thereby obtaining an epitaxial layer that is at least partially relaxed, and preferably totally relaxed.

[0165] The percentage of relaxation corresponds to:

[0166] with a^, the lattice parameter of the starting layer on which the epitaxy is resumed (i.e., the lattice parameter of layer 124), and

[0167] ac2 the mesh parameter of the relaxed layer,

[0168] 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.

[0169] When aci=ac2 the layer is said to be constrained.

[0170] Partially relaxed means a percentage of relaxation greater than 50%.

[0171] Resumption of epitaxy can be used, for example, to form re-epitaxial LEDs.

[0172] Epitaxial regrowth is carried out on the fourth layer 124 of (Al,In,Ga)N / (Al,In,Ga)N, which is either dense or lightly doped, of the mesas 120. Since this layer is not porosified during the electrochemical anodizing step, it remains continuous and dense. Epitaxial regrowth is thus facilitated, and the epitaxially treated layer exhibits better adhesion. The formation of defects related to pore coalescence is avoided.

[0173] The epitaxial layer in this step e) is advantageously made of gallium nitride or indium gallium nitride.

[0174] Comparative examples and illustrative examples of different embodiments

[0175] Comparative example: increasing the thickness of the layer to be porosified-

[0176] In this first comparative example, the following stacking was studied ([Fig.5]):

[0177] - sapphire or silicon 114 substrate,

[0178] - 3pm thick GaN nest layer, 111

[0179] - n-GaN layer to be porosified (doping 6.1018 at / cm3) of 4pm thickness 112,

[0180] - 100 nm thick GaN nest layer 124.

[0181] Mesas of width L were formed in the stack. Each mesa comprises a portion of the n-doped GaN layer and the nest-GaN layer.

[0182] The other part of the doped GaN layer forms part of the base of the structure.

[0183] A structure with mesas 3pm wide and a structure with mesas of 5pm wide ones were manufactured.

[0184] The contacts are taken on the n-GaN layer at the base of the structure outside the mesas (the contact is represented by an arrow on [Fig.5]).

[0185] Then the anodizing step is implemented.

[0186] In this configuration, it was observed that layer 112, which serves to supply the charges, is also the site of electrochemical porosification reactions. As the process progresses, it loses its conductive properties, and the porosification of the mesas ceases. At the end of the process, the mesas were observed by scanning electron microscopy ([Fig. 6]) and by dark-field optical microscopy (Figures 7A and 7B): the mesas in the center of the structure are only very partially porosified.

[0187] In order to keep it intact, the thickness (or height) of layer 112 has been increased.

[0188] It has been observed that, in order to be able to porosify the center of nest-GaN / nGaN mesas (Doping with 6e-18 at / cm³), it is necessary to verify the relationship: L < 2 xa

[0189] . Indeed, if L > 2 xa, porosification occurs with the same kinetics on the mesa flanks and on the conductive layer. When the conductive layer becomes porosified, it can no longer supply the mesas, and porosification stops. The mesa is partially porosified: the center of the mesas is not porosified.

[0190] For L < 2 xa, the mesa is fully porosified but an edge / center effect of the The mesas are always present. Furthermore, such a configuration can lead to integration problems related to thickness. Indeed, to porosify large mesas, the contact layer must be thicker. However, for a silicon substrate, it is not possible to have a thickness of 4 µm, which leads to integration problems.

[0191] In the case of GaN / InGaN mesas, the electrolyte infiltrates through the defects ('V-pits') of the InGaN layer. Porification occurs through this layer, thus porosifying the center of the mesa regardless of the values ​​of a and L. It is also possible to have an edge / center effect of the wafer with incomplete porosification of the mesas located at the center of the wafer due to the degradation of the conductivity of the current injection layer during electrochemical polarization.

[0192] la-embodyment: stacking with a heavily doped layer-

[0193] In this first embodiment, as shown in [Fig. 2A], the structure 100 to be porosified is made from a stack comprising successively:

[0194] - a sapphire substrate 114,

[0195] - a first 111 layer of undoped GaN of 4pm,

[0196] - a second 112 layer of GaN doped at 600nm (1.1018 at / cm3),

[0197] - a third layer 123 of GaN heavily doped at 800nm ​​(1.5 x 10¹⁹ at / cm³), and

[0198] - a fourth layer 124 of (Al,In,Ga)N nest of lOOnm.

[0199] The mesas 120 are formed in the fourth layer 124 of undoped or weakly doped (Al,In,Ga)N, the third layer 123 of heavily doped GaN and part of the second layer of doped GaN 112 (over a thickness of 100nm).

[0200] A structure with 3 pm wide mesas and a structure with 5 pm wide mesas were fabricated. The porosification step is carried out by applying a voltage of 9V for 500s (non-pulsed) in a 0.3M oxalic acid solution.

[0201] All the mesas 120 of the same plate are porosified for small or medium-sized substrates. However, for very large substrates (typically greater than or equal to 200 mm), complete porosification of all the mesas of the same plate is not ensured: those at the edge are completely porosified, while those in the center, far from the contact point, are not completely porosified ([Fig.8]; on the left the 3 pm mesas and on the right the 5 pm mesas).

[0202] 2nd embodiment: stacking with two heavily doped layers arranged on either side of the doped layer-

[0203] In this other embodiment, as shown in [Fig. 3A], the structure 100 to be porosified is made from a stack comprising successively:

[0204] - a sapphire 114 support layer,

[0205] - a first 111 layer of undoped GaN of 4pm,

[0206] - an additional layer 113 of GaN heavily doped with 2pm (1.5.1019 at / cm3), said buried layer,

[0207] - a second 112 layer of GaN doped at 600nm (1.1018 at / cm3),

[0208] - a third layer 123 of GaN heavily doped at 800nm ​​(1.5.1019 at / cm3), and

[0209] - a fourth layer 124 of (Al,In,Ga)N nest of lOOnm.

[0210] The mesas 120 are formed in the fourth layer of undoped or weakly doped (Al,In,Ga)N, the third layer of heavily doped GaN and part of the second layer of doped GaN (over a thickness of lOOnm).

[0211] A structure with 3 µm wide mesas and a structure with 5 µm wide mesas were fabricated. The porosification step is carried out by applying a 9V (non-pulsed) voltage to a 0.9M oxalic acid solution for 100 to 1000 s. The process is stopped by monitoring the current drop.

[0212] With the addition of a heavily doped buried layer 113, the porosification process is rapid and all the mesas of the same plate are completely porosified not only at the edge but also in the center of the substrate ([Fig.9]; on the left the 3pm mesas and on the right the 5pm mesas).

[0213] The feet of the mesas are intact (figures 10A and 10B).

[0214] Simulation of currents during the porosification of the 2£ms-example structure-

[0215] A stack with a diameter of 50 µm was simulated. The stacks (layer thicknesses and doping) correspond to those of Example 2 above. The mesas are 3 µm wide. The simulation confirms that the current injected at the periphery passes through the buried layer, which is more heavily doped (Figures 11A and 11B). This results in a better distribution of the current within the material.

Claims

Demands

1. A process for porosifying (Al,In,Ga)N / (Al,In,Ga)N mesas comprising the following steps: a) providing a structure (100) comprising a base substrate (110) covered with (Al,In,Ga)N / (Al,In,Ga)N mesas (120), the base substrate (110) comprising a support layer (114), a first undoped GaN layer (111), a second doped GaN layer (112), and an additional heavily doped GaN layer (113) disposed between the first undoped GaN layer (111) and the second doped GaN layer (112), the (Al,In,Ga)N / (Al,In,Ga)N mesas (120) comprising a third heavily doped (Al,In,Ga)N layer (123) and a fourth layer of undoped or weakly doped (Al,In,Ga)N (124), a portion (112b) of the second layer of doped GaN (112) extending into the mesas (120) or a portion (123a) of the third layer of heavily doped (Al,In,Ga)N (123) extending into the basic substrate (110),b) electrically connect the structure (100) and a counter electrode to a voltage or current generator, c) immerse the structure (100) and the counter electrode in an electrolytic solution, d) apply a voltage or current between the structure (100) and the counter electrode so as to porosify the heavily doped third layer of (Al,In,Ga)N (123) of the mesas (120).

2. A method according to any one of claims 1, characterized in that the support layer (114) is a slice of at least 10 cm in diameter and even more preferably of at least 200 mm in diameter.

3. A method according to any one of the preceding claims, characterized in that the support layer (114) is made of sapphire or silicon.

4. A method according to any one of the preceding claims, characterized in that the fourth layer (124) of undoped or weakly doped (Al,In,Ga)N is a GaN layer.

5. A method according to any one of the preceding claims, characterized in that the doping level of the third layer of heavily doped (Al,In,Ga)N (123) and / or the doping level of the additional layer of heavily doped GaN (113) is between 5.1018 at / cm3 and 1.5.1019 at / cm3.

6. A method according to any one of the preceding claims, characterized in that the doping level of the second doped GaN layer (112) is less than 5.1018at / cm3, preferably between 5.1017 and 2.1018 at / cm3.

7. A method according to any one of claims 5 and 6, characterized in that the applied voltage is between 3V and 15V, preferably between 8V and 10V.

8. Structure (100) comprising a base substrate (110) covered with porosified (Al,In,Ga)N / (Al,In,Ga)N mesas (120), the base substrate (110) comprising a support layer (114), a first undoped GaN layer (111) and a second doped GaN layer (112), and an additional heavily doped GaN layer (113) disposed between the first undoped GaN layer (111) and the second doped GaN layer (112), the (Al,In,Ga)N / (Al,In,Ga)N mesas (120) comprising a third porosified heavily doped (Al,In,Ga)N layer (123) and a fourth undoped or lightly doped (Al,In,Ga)N layer (124), a portion (112a) of the second layer of doped GaN (112) extending into the mesas (120) or a part (123b) of the third layer of heavily doped (Al,In,Ga)N (123) extending into the basic substrate (110).

9. Structure according to claim 8, characterized in that the fourth undoped or weakly doped (Al,In,Ga)N layer (124) is a GaN layer and / or in that the support layer (114) is a slice of at least 200mm in diameter.

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