Process for producing a continuous nitride layer

Abstract

Title: Method for producing a continuous nitride layer The invention relates to a method for obtaining a layer made at least in part of a nitride (N), comprising firstly providing a stack comprising at least one set of pads (1000A1-1000B4) extending from a substrate (100). Each pad comprises at least one creep section (220A1-220A5) and a crystalline section (300A1, 300A5) surmounting the creep section (200A1-200A5). A crystallite (510A1-510A5) is then grown by epitaxy on at least some of said pads until the crystallites coalesce so as to form a layer (550A) of nitride.The pads of the assembly are distributed on the substrate so that the relative arrangement of the pads of the assembly is such that during the epitaxy of the crystallites, the progressive coalescence of the crystallites always takes place between, on the one hand, a crystallite or a plurality of coalesced crystallites and, on the other hand, an isolated crystallite. Figure for abstract: Fig. 3H.

Description

Title of the invention: Method for producing a continuous nitride layer Technical field

[0001] The present invention relates to the production of continuous layers made of a nitride (N) preferably obtained from at least one of gallium (Ga), indium (In) and aluminum (Al). The invention finds application, for example, in the field of optoelectronic devices comprising a plurality of micrometric-sized light-emitting diodes (LEDs), generally called micro-LEDs. In this field, a particularly advantageous use of the invention relates to the production of small-sized screens generally called micro-screens or micro-displays. STATE OF THE ART

[0002] For many microelectronic or optoelectronic applications, it is desired to produce nitride layers from at least one of gallium, indium and aluminum. Specific applications concern, for example, the production of micro-LEDs.

[0003] A nitride layer is conventionally obtained by epitaxial growth from a crystalline layer covering a plate.

[0004] A major challenge is to minimize the density of defects in the nitride layer obtained by epitaxy. Indeed, the performance of microelectronic or optoelectronic devices made from these nitride layers is very sensitive to the density of structural defects such as dislocations.

[0005] These dislocations originate from the difference in lattice parameter between the epitaxial layer and the substrate, as well as the coalescence of the small-sized grains which are formed at the beginning of growth; these grains are slightly disoriented with respect to each other and they join together, forming structural defects at the coalescence joint, including dislocations, which can then cross the entire epitaxial structure.

[0006] The most direct way to solve these problems is to use substrates of the same nature as the layers that are to be epitaxially grown (homo-substrates). However, these substrates are not commercially available or are still only of small dimensions, and are very expensive, which does not allow substrates of sufficient dimensions to be cut from them for the envisaged industrial applications.

[0007] The solutions envisaged to date for industrial applications are therefore mainly based on the use of hetero-substrates in combination with methods known as “lateral regrowth” or ELOG, an acronym for “lateral epitaxial” overgrowth”. This method, based on the use of a mask to block dislocations, makes it possible to reduce their density. However, these dislocations are distributed non-uniformly, which can pose a problem during device manufacturing.

[0008] Another solution consists of regrowing the material by epitaxy on pre-existing pads of this material: this is the so-called pendeo-epitaxy process which makes it possible to avoid regrowth on the mask. On the other hand, conventional pendeo-epitaxy solutions do not make it possible to eliminate, or even significantly reduce, the appearance of defects generated by the coalescence of adjacent seeds.

[0009] Patent application WO2019122461 describes a solution illustrated in Figures 1A to 1D: a. A first step is to provide a stack comprising a finishing layer 200, a crystalline layer 300 and a nitride layer 500 or seed layer 500, typically a gallium nitride (GaN) layer. A buffer layer 400 may also be present between the finishing layer and the nitride layer 500. This step is illustrated in [Fig.lA]. b. A second step consists of forming pads 1000A1-1000A4, 1000B1-1000B4 by etching the crystalline layer 300, possibly the buffer layer 400, and at least a portion of the finishing layer 200 ( [Fig.lB]). Each pad thus comprises a finishing section 220A1-220A4, 220B1-220B4 originating from the finishing layer 200 and a crystalline section 300A1-300A4, 300B1-300B4 originating from the crystalline layer 300. c. Epitaxially grow crystallites 510A1-510A4, 510B1-510B4 on pads 1000A1-1000A4, 1000B1-1000B4 until they coalesce (Figures 1C and 1D). The layer thus formed can continue its growth by thickening.

[0010] During their epitaxial growth, the crystallites formed at the top of the pads coalesce to form a continuous layer intended to form an optoelectronic or electronic device. During epitaxy, the portion of the pad that is formed by the fining section reaches (or exceeds) its glass transition temperature or a temperature very close to the latter. Under the force of a mechanical stress, this portion of pad can thus deform ([Fig.lE]). Thus, when two crystallites come into contact and coalesce, the mechanical stresses generated by this contact are transferred to the pads and therefore to the fining sections supporting them. The latter deform, thereby absorbing part of the mechanical stresses.This mobility of the plots due to the presence of the fining sections makes it possible in particular to accommodate relative disorientations of the crystallites between them, whether in the plane in which the substrate mainly extends ("twist") or out of plane ("tilt").

[0011] However, it appears that this method does not make it possible to obtain satisfactory defect densities. As illustrated in FIGS. 2A and 2B, continuous layers obtained by such a method have in places very high orientation angles of the material at the base of the nitride layer, reflecting the existence of structural defects which are detrimental to the manufacture of devices from these continuous layers.

[0012] There is therefore a need to limit or even eliminate the disadvantages of known solutions. The present invention aims in particular to propose a solution to meet this need.

[0013] An objective of the present invention is to propose a solution for obtaining a nitride (N) layer obtained from at least one of gallium (Ga), indium (In) and aluminum (Al), having a significantly reduced defect density. SUMMARY

[0014] To achieve this objective, according to one embodiment, a method is provided for obtaining a layer made at least in part of a nitride (N) preferably obtained from at least one of gallium (Ga), indium (In) and aluminum (Al), comprising the following steps: a. providing a stack comprising at least one set of pads extending from a substrate, each pad comprising at least: i. a first section, called the finishing section, formed in an amorphous material having a glass transition temperature Glass transition? ü. a second, crystalline section, called the crystalline section, surmounting the finishing section, b. growing a crystallite by epitaxy on at least some of said pads and continuing the epitaxial growth of the crystallites until coalescence of the crystallites carried by the pads of the set of pads, so as to form a nitride layer,

[0015] The method is characterized in that the pads of said set of pads are distributed on the substrate so that the relative arrangement of the pads of the set of pads is such that during the epitaxy of the crystallites, the progressive coalescence of the crystallites always takes place between on the one hand a set, called a coalesced set, comprising one crystallite or a plurality of coalesced crystallites and on the other hand at least one crystallite, called an isolated crystallite, which has not already coalesced with other crystallites.

[0016] Thus, at a coalescence point, the progressive coalescence of the crystallites always takes place between, on the one hand, a crystallite or a plurality of coalesced crystallites and, on the other hand, a single crystallite, which can also be described as an isolated crystallite since it has not previously coalesced with one or more other crystallites. The section of fining underlying said isolated crystallite has all the mobility necessary necessary to compensate for any misalignments and disorientations between the isolated crystallite and the crystallite or plurality of coalesced crystallites to which it coalesces. This prevents the formation of structural defects within the crystallites and therefore the continuous layer during successive coalescences, which ultimately results in a continuous layer of excellent quality.

[0017] Electronic and optoelectronic devices, for example LEDs or vertical devices such as vertical transistors or lateral components such as HEMT transistors, having very good performances can thus be manufactured from these layers having low densities of structural defects. BRIEF DESCRIPTION OF THE FIGURES

[0018] The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of an embodiment thereof which is illustrated by the following accompanying drawings in which:

[0019] [Fig. 1A] Figures 1A to 1E illustrate steps of a method of forming a continuous layer according to the prior art.

[0020] [Fig.lB]

[0021] [Fig.lC]

[0022] [Fig.lD]

[0023] [Fig.lE]

[0024] [Fig.2A] [Fig.2A] is a scanning electron microscope (SEM) photograph illustrating continuous layers obtained by the process illustrated in Figures 1A to 1E.

[0025] [Fig.2B] [Fig.2B] is an image obtained by dark field X-ray microscopy showing the variations in orientation of the crystallites obtained by the process illustrated in Figures 1A to 1E.

[0026] [Fig.3A] Figures 3A to 3H illustrate steps of a method according to a non-limiting example of the method according to the present invention. [Fig.3A] illustrates an example of a stack from which an example of the method according to the invention can be implemented.

[0027] [Fig.3B] [Fig.3B] illustrates the stack of [Fig.3A] on which a seed layer is formed.

[0028] [Fig.3C] [Fig.3C] illustrates the result of a step consisting of forming a set of pads from the stack of [Fig.3A] or that of [Fig.3B].

[0029] [Fig.3D] [Fig.3D] illustrates a phase of epitaxial growth of crystallites on the top of the pads, this growth phase not being completed.

[0030] [Fig.3E] Figures 3E to 3H illustrate the successive coalescences of the crystallites to form a continuous layer.

[0031] [Fig.3F]

[0032] [Fig.3G]

[0033] [Fig.3H]

[0034] [Fig.4A] Figures 4A to 4F illustrate examples of pad arrangements allowing the fabrication of continuous layers having a low defect density. Figures 4A and 4B illustrate an example in which the pads are arranged in a logarithmic spiral.

[0035] [Fig.4B]

[0036] [Fig.4C] [Fig.4C] illustrates an example in which some of the pads are arranged along several parallel alignment axes.

[0037] [Fig.4D] [Fig.4D] illustrates an example in which the pads are arranged along an alignment axis.

[0038] [Fig.4E] Figures 4E and 4F illustrate an example in which the pads are arranged along five concurrent alignment axes.

[0039] [Fig.4F]

[0040] [Fig.5A] Figures 5A and 5B represent the coalescence of two continuous layers themselves previously formed by the coalescence of crystallites.

[0041] [Fig.5B]

[0042] The drawings are given as examples and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily on the scale of practical applications. In particular the relative thicknesses of the different layers, sections, crystallites and continuous layers are not representative of reality. DETAILED DESCRIPTION

[0043] Before beginning a detailed review of embodiments of the invention, optional features which may possibly be used in combination or alternatively are set out below:

[0044] According to an advantageous embodiment, the pads carrying the crystallites forming the nitride layer are substantially arranged, in projection in a horizontal plane in which the upper face of the substrate mainly extends, according to a logarithmic spiral.

[0045] According to one embodiment, the pads carrying the crystallites forming the continuous nitride layer are, in projection in a horizontal plane in which the upper face of the substrate mainly extends, substantially aligned along at least one so-called alignment axis.

[0046] According to one embodiment, the pads arranged on the same alignment axis are spaced apart by an increasing distance along the alignment axis in a direction given from a given point, called the initial point.

[0047] According to one example, the pads carrying the crystallites forming the continuous layer are separated by a distance varying logarithmically from near to near, said distance being measured along their alignment axis.

[0048] According to one example, the pads carrying the crystallites forming the continuous nitride layer are substantially aligned, in projection in a horizontal plane in which the upper face of the substrate mainly extends, along concurrent axes, the concurrent axes verifying between them a rotational symmetry around a central point in projection in the horizontal plane.

[0049] According to one example, the plots are aligned along five concurrent axes.

[0050] According to one example, the epitaxial growth is carried out at a temperature Tepitaxy, such that Tepitaxy > ki x with kj > 0.8.

[0051] According to one example, ki > 1, and preferably ki > 1.5.

[0052] Advantageously, the set of pads comprises at least 3 pads.

[0053] According to one embodiment, the stack comprises a plurality of sets of pads on the substrate, a continuous layer of nitride being formed from each set of pads, the epitaxial growth of the crystallites being interrupted before the crystallites belonging to two distinct sets of pads coalesce, so that the continuous layers formed from each set of pads are distant from each other.

[0054] In this embodiment in which several sets make it possible to form several continuous and mutually disjointed layers on the same plate, these different continuous layers are typically designated "vignettes". Each of these vignettes can then be involved in the production of an electronic or optoelectronic arrangement, such as an LED or micro-LED or a vertical device such as a vertical transistor. The characteristics described below with reference to a set of pads, its crystallites and the continuous layer formed from this set of pads apply mutatis mutandis to the other sets of pads, the crystallites they carry and the continuous layers formed by coalescence of these crystallites.

[0055] According to one example, each pad has an upper face and in which the epitaxial growth of the crystallites is carried out at least in part and preferably only from said upper face.

[0056] According to one example, the method further comprises a step of forming in the continuous layer a device, the device being taken from an LED and a transistor, for example a vertical transistor, for example a HEMT type transistor.

[0057] According to one embodiment, the pads of said set of pads are distributed on the substrate so that the relative arrangement of the pads of the set of pads is such that during the epitaxy of crystallites, at least at certain times the progressive coalescence of crystallites takes place between on the one hand said coalesced set and on the other hand several isolated crystallites.

[0058] Alternatively, the pads of said set of pads are distributed on the substrate so that the relative arrangement of the pads of the set of pads is such that during the epitaxy of the crystallites, the progressive coalescence of the crystallites always takes place between, on the one hand, said coalesced set and, on the other hand, a single isolated crystallite.

[0059] According to one example, each pad has a section whose maximum dimension dpiot is between 10 and 500 nm (109 meters), the maximum dimension dpiot being measured in a plane parallel to a horizontal plane in which the upper face of the substrate mainly extends, preferably 20 nm < dpiot < 200 nm and preferably 50 nm < dpiot <100 nm.

[0060] According to a preferred example, the continuous layer has a section whose maximum dimension dlayer is between 0.5 and 10 qm (106 meters), the maximum dimension dlayer being measured in a plane parallel to a horizontal plane in which the upper face of the substrate mainly extends, preferably 0.8 qm < dlayer < 3 qm and preferably 1 qm < dlayer < 2 qm.

[0061] Preferably, the finishing layer is made of a material taken from: a. a silicon oxide SixOy, x and y being integers, and preferably the finishing layer is SiO2, b. a glass, c. a borosilicate glass, d. borophosphosilicate glass (BPSG).

[0062] Advantageously, Tepitaxy < k2 x Tfusionmin, Tfusionmin being the lowest melting temperature among the melting temperatures of the sections forming the pads, with k2 < 0.9 and preferably k2 < 0.8.

[0063] According to an advantageous embodiment, the pads comprise at least one buffer layer overlying the crystalline layer, and made of a material different from that of the continuous nitride layer, and preferably the continuous nitride layer is made of gallium nitride (GaN) and the buffer layer is made of aluminum nitride (AIN). This makes it possible to avoid the phenomena of alteration of the GaN layer by reaction with the silicon layer, phenomena usually referred to as "melt-back etching").

[0064] According to one example, the pads comprise, before the step of epitaxial growth of the continuous nitride layers, at least one seed layer, overlying said buffer layer and made of gallium nitride (GaN).

[0065] According to an advantageous example, providing said stack comprises providing a substrate developed of the silicon on insulator (SOI) type comprising a base substrate successively surmounted by an oxide layer forming said finishing layer and a semi-conductor layer forming said crystalline layer.

[0066] According to an advantageous example, the finishing section has a height e22o such that c22o > 0.1xdpiot, dpiot being the diameter of the pad or more generally the edge-to-edge distance of the pad taken, at the level of the finishing section and in a direction parallel to a horizontal plane in which mainly an upper face of the substrate extends, preferably e220 > lxdpiot.

[0067] According to one example, the step of forming the pads comprises etching the crystalline layer and etching only a portion of the finishing layer so as to retain a portion of the finishing layer between the pads.

[0068] In the remainder of the description, the terms crystals and crystallites will be considered equivalent.

[0069] It is specified that in the context of the present invention, the terms “on”, “overcomes”, “covers” or “underlying” or their equivalents do not mean “in contact with”. Thus, for example, “the deposition of a first layer on a second layer” does not necessarily mean that the two layers are in direct contact with each other, but it does mean that the first layer at least partially covers the second layer by being either directly in contact with it or by being separated from it by at least one other layer or at least one other element, including air. Similarly, “a pad overcoming a first layer” does not mean that the pad is necessarily in contact with this first layer, but means that the pad is either in contact with this first layer or in contact with one or more layers arranged between the first layer and the pad.

[0070] The stages of formation of the different layers and regions are understood in the broad sense: they can be carried out in several sub-stages which are not necessarily strictly successive.

[0071] In the following description, the thickness or height is taken along a direction perpendicular to the main faces of the different layers. In the figures, the thickness or height is taken along the vertical or along the z axis of the orthogonal reference frame illustrated in figures 1A to 1E, 3A to 3H and 5A and 5B.

[0072] Similarly, when it is indicated that an element is located to the right of another element, this means that these two elements are both located on the same line perpendicular to the main plane of the substrate, or on the same line oriented vertically (z axis) in the figures.

[0073] A substrate, a layer, a device, “based” on a material M, is understood to mean a substrate, a layer, a device comprising this material M only or this material M and possibly other materials, for example alloying elements, impurities or doping elements.

[0074] The terms “substantially”, “approximately”, “of the order of” mean “to within 10%” or, when it comes to an angular orientation, “to within 10°”. Thus, a direction substantially normal to a plane means a direction having an angle of 90+10° relative to the plane.

[0075] The term “micro-LED” designates an LED of which at least one dimension taken in a plane parallel to the main plane in which the substrate supporting the micro-LED extends (i.e., the XY plane of the orthogonal reference frame referenced in the figures) is micrometric, i.e. strictly less than 1 mm (103 meters) and preferably less than or equal to 100 pm (1 pm = 106 meters). In the context of the invention, the micro-LEDs have, in projection in a main extension plane parallel to the main faces of the micro-LEDs, i.e., parallel to an upper face of the substrate, maximum dimensions of micrometric dimension in the plane. Preferably, these maximum dimensions are less than a few hundred micrometers. Preferably, these maximum dimensions are less than 100 pm.

[0076] In the present invention, the term "vertical devices" means devices in which the charge carriers move in the thickness of the layer, i.e. in a direction perpendicular to the planes in which the main faces of this layer extend.

[0077] In the present invention, the term “HEMT” (High Electron Mobility Transistor) type transistors refers to field effect transistors with high electron mobility, sometimes also referred to as heterostructure field effect transistors. Such a transistor includes the superposition of two semiconductor layers having different band gaps which form a quantum well at their interface. Electrons are confined in this quantum well to form a two-dimensional gas of electrons. For reasons of high voltage and temperature resistance, the materials of these transistors are chosen so as to have a wide energy band gap.

[0078] An example of a method for forming a continuous layer according to the invention will now be described with reference to FIGS. 3A to 3H.

[0079] It will be noted that, according to one example, the method described below makes it possible to form a single continuous layer on a substrate 100 or on a plate. According to another example, the method described below makes it possible to form a plurality of continuous layers on the same substrate 100 or on the same plate. In the latter case, each layer is separate from the others and can be referred to as a vignette. Each vignette can then be used to produce a device such as an LED or a micro-LED or any other vertical nitride-based device such as a HEMT type transistor.

[0080] As illustrated in [Fig.3A], a stack is provided comprising at least one substrate 100, successively surmounted by a finishing layer 200 and a crystalline layer 300. Thus the finishing layer 200 is arranged between the substrate 100 and the crystalline layer 300.

[0081] According to an exemplary embodiment, the substrate 100 is based on silicon, amorphous or crystalline. It ensures the mechanical strength of the stack.

[0082] The crystalline layer 300 has a lower face 302 facing the finishing layer 200 and an upper face 301 whose function is to serve as a base layer for epitaxially growing the nitride of the continuous layer 550A that is ultimately desired to be obtained. For example, the layer that is ultimately desired to be obtained is a layer of gallium nitride GaN. According to an exemplary embodiment, the crystalline layer 300 is based on monocrystalline silicon. Alternatively, the crystalline layer 300 may be based on SiC or Al203.

[0083] Preferably, the fining layer 200 is made of a viscous material. The fining layer 200 has a glass transition temperature. It therefore has a glass transition and exhibits the behavior of glass transition materials. Like all materials having a glass transition temperature, the fining layer 200, under the effect of a rise in temperature, deforms without breaking and without returning to its initial position after a drop in temperature. On the contrary, the crystalline layer 300 does not naturally have a glass transition. The crystalline layer 300 deforms, then dislocates and can break. Consequently, the fining layer 200 and the crystalline layer 300 are different. The fining layer 200 is not crystalline.

[0084] The finishing layer 200 is made of an amorphous material such as an oxide, preferably a silicon oxide SixOy, such as SiO2. The role of this layer will be explained in the remainder of the description.

[0085] Advantageously but not limitingly, this stack comprising the substrate 100, the finishing layer 200 and the crystalline layer 300 constitutes a substrate of the semiconductor on insulator type, preferably silicon on insulator (SOI). In this case, the finishing layer 200 is formed by the buried oxide layer (BOX) of the SOI substrate.

[0086] According to an advantageous embodiment illustrated in [Fig.3A], a buffer layer 400 is deposited by epitaxy on the upper face 301 of the crystalline layer 300. When the continuous layer 550A that is ultimately desired to be obtained is formed of GaN and the crystalline layer 300 is a silicon-based layer, this buffer layer 400 is typically made of aluminum nitride (AIN). This makes it possible to avoid the phenomenon known as “Melt-back etching”, generated by the very high reactivity between silicon and gallium at the usual epitaxy temperatures (1000 / 1100°C) and which leads to very significant degradation of the continuous layer 550A of GaN.

[0087] Typically, the thickness of the AIN layer is between 10 and 200 nanometers (109 meters).

[0088] As illustrated in [Fig.3B], it is also possible to deposit by epitaxy, on the upper face 401 of the buffer layer 400, a seed layer 500. This seed layer 500 has the function of facilitating the resumption of growth of the crystallites 510A1-510A5 during the following steps. In this case, it is from an upper face of the seed layer 500 that at least part of the epitaxial growth of the crystallites 510A1-510A5 occurs, the crystallites being illustrated in [Fig.3D]. This seed layer 500 is preferably made of the same material as that of the continuous layer 550A that it is desired to obtain in the end. Typically, when the material of the continuous layer 550A is gallium nitride GaN, the seed layer 500 is also made of GaN. This seed layer 500 typically has a thickness of between 50 and 200 nanometers.

[0089] For the sake of brevity and clarity, only five pads 1000A1-1000A5 are shown in the figures to support a continuous layer 550A. Naturally, a continuous layer 550A can be formed on a greater number of pads. As will be described later, the number of pads as well as their period will be adapted according to the desired size for the micro-LED.

[0090] It will be noted that the buffer layer 400 and the priming layer 500 are only optional. Thus, according to embodiments not illustrated in FIGS. 3A to 3H, only the buffer layer 400 or only the priming layer 500, or even neither of these two layers 400 and 500, may be provided.

[0091] As illustrated in [Fig.3C], pads 1000A1-1000A5 are then formed from the stack. These pads are obtained by etching the stack into the finishing layer 200, at least part of the etching extending within the finishing layer 200.

[0092] To form the pads by etching, numerous etching techniques known to those skilled in the art may be used. In particular, conventional lithography techniques may be used, such as photolithography techniques comprising the formation of a mask, for example in resin, then the transfer of the mask patterns into the stack. Electron beam lithography (e-beam) techniques or nanometric printing techniques may also be used.

[0093] These pads 1000A1-1000A5 are of small dimensions and can be described as nano-pads. Typically, the maximum dimension of the section of the pads, taken in a plane parallel to the horizontal plane XY of the orthogonal reference XYZ or to the plane of the upper face 101 of the substrate 100, and referenced dpiot in [Fig.3C], is between a few tens and a few hundreds of nanometers. More precisely, dpiot is between 10 and 500 nanometers and preferably between 20 and 150 nm and preferably between 50 and 100 nm for example of the order of 50 nm or 100 nm. If the pads are of circular section, this maximum dimension dpiot corresponds to the diameter of the pads. If the pads are of hexagonal section, this maximum dimension d piot corresponds to the diagonal or the diameter of the circle passing through the angles of the hexagon. If these pads are of rectangular or square section this maximum dimension dpiot corresponds to the largest diagonal. It is understood that the section of the pads may have a shape other than circular, hexagonal, rectangular or square.

[0094] In the context of the present invention, the pads 1000A1-1000A5 are distributed in a particular way in projection in the XY plane: their arrangement is such that, as will be illustrated further on, the coalescences occurring successively during the implementation of the method involve on the one hand a single crystallite 510A1 (during the first coalescence which will be described later with reference to [Fig. 3E]) or a plurality of already coalesced crystallites 510A1-510A4 (during the following coalescences, FIGS. 3F to 3H), and on the other hand at least one isolated crystallite 510A2-510A5. In other words, the arrangement of the pads 1000A1-1000A5 makes it possible to avoid any coalescence between, on the one hand, a first plurality of coalesced crystallites and, on the other hand, a second plurality of coalesced crystallites. To do this, the pads 1000A1-1000A5 do not all have the same distance from their first neighbors.The arrangement of the 1000A1-1000A5 pads must in particular take into account the increase in the projection dimensions in the XY plane of the 510A1-510A5 crystallites as they grow by epitaxy. Examples of arrangement of the 1000A1-1000A5 pads will be given later with reference to Figures 4A to 4F.

[0095] As illustrated in [Fig.3C], the pads are etched through the entire seed layer 500, the entire buffer layer 400 (when the latter are present) and the entire crystalline layer 300. Preferably, only a portion 220 of the finishing layer 200 is etched. This embodiment has the advantage of preventing the nitride of the continuous layer 550A from growing on the finishing sections 220 during epitaxy. This selectivity of the epitaxy is encountered in particular when the continuous layer 550A of nitride that is grown by epitaxy is GaN and the finishing sections are SiO2. On the contrary, if, with these same materials, the finishing layer 200 is etched over its entire thickness, then, during epitaxy, the nitride of the continuous layer 550A develops from the upper face 101 of the substrate 100, usually formed of silicon. This situation is obviously not desirable.

[0096] Furthermore, it has been observed that retaining an unetched portion 210 of the creep layer 200 facilitates creep of section 220, in particular when the crystallites are twist-disoriented, i.e. in the main extension planes of the continuous layer 550A that it is desired to obtain. These main extension planes of the continuous layer 550A are parallel to the horizontal plane XY of the XYZ reference frame.

[0097] Preferably, the etched thickness e22o, and therefore forming the height of the creep section 220, is equal to half the thickness of the creep layer 200. This makes it possible to have very good reorientation of the crystallites during the formation of grain boundaries.

[0098] [Fig.3D] illustrates the formation of crystallites 510A1-510A5 by epitaxial growth from the seed layer 500 (or from the upper face 301 of the crystalline layer 300 when the layers 400 and 500 are absent).

[0099] As illustrated in this [Fig.3D], the pads 1000A1-1000A5 each support a crystallite 510A1-510A5 carried by a stack of sections 400A1-400A5, 300A1-300A5, 220A1-220A5. The sections extend along the main direction of extension of the pad, i.e. vertically (Z) in Figures 3A to 3H.

[0100] The sections form circular cylinders if the cross-section of the pads is mainly circular. If the cross-section of the pads 1000A1-1000A5 is polygonal, for example hexagonal, the sections then form cylinders of hexagonal cross-section. Preferably, the sections are solid. The cross-section of the pads is taken parallel to the horizontal plane XY, i.e. parallel to the planes in which the creep layer 220 and the crystalline layer 300 mainly extend.

[0101] Whatever the embodiment chosen, that is to say with or without seed layer 500 and with or without buffer layer 400, the epitaxial growth of the crystallites 510A1-510A5 is carried out at least in part or only from the upper face 1001A1-1001A5 of the pad 1000A1-1000A5. Thus, this upper face 1001A1-1001A5 is formed either by the crystalline section 300A1-300A5, or by the section 500A1-500A5 formed by the seed layer 500, or by the section 400A1-400A5 formed by the buffer layer 400. This makes it possible in particular to quickly obtain crystallites 510A1-510A5 of significant thickness.

[0102] It will be noted that the upper faces 401, 501 of the buffer layer 400 and of the priming layer 500, that is to say the faces facing the continuous layer 550A that it is desired to grow, have polarities of the Gallium (Ga) type, and not nitrogen (N), which considerably facilitates the obtaining of a continuous layer 550A of high-quality epitaxial nitride.

[0103] The growth of the 510A1-510A5 crystallites continues and extends laterally, in particular along planes parallel to the xy plane. The 510A1-510A5 crystallites grow until those carried by the two pads closest to each other the other, referenced 1000A1 and 1000A2 in Figures 3C to 3H and separated by a distance Pb coalesces ([Fig.3E]). This coalescence takes place without dislocations or with few dislocations within the crystallites. It allows the formation of a continuous layer 500A from crystallites 510A1 and 510A2. This continuous layer 500A constitutes a coalesced assembly.

[0104] The growth of the crystallites involved in this first coalescence 510A1, 510A2 as well as the other crystallites 510A3-510A5 continues until the continuous layer 500A formed by the crystallites 510A1 and 510A2 coalesces with the crystallite 510A3 carried by the nearest pad 1000A3. The crystallite 510A3 is thus added to the coalesced whole. It should be noted that, due to the distribution of the pads 1000A1-1000A5 on the substrate, from the beginning of its growth ([Fig.3D]) and until this step of the process ([Fig.3F]), the crystallite 510A3 being coalesced with the continuous layer 550A has never coalesced with another crystallite. Its 1000A3 pad therefore has all the mobility necessary to accommodate alignment defects in the XY plane or out of plane with the continuous layer 550A formed by the first two coalesced crystallites 510A1, 510A2. In [Fig.3F], a deformation of the 220A3 fining section according to its “tilt” angle is illustrated.A deformation of this 220A3 fining section according to the “twist” angle can occur alternately or simultaneously. The properties of its 220A3 fining section make it possible to significantly reduce defects at the interface between the continuous layer 550A and the 510A3 crystallite. Explanations on the interest of this precaution are given further on.

[0105] The continuous layer 500A is then formed by the crystallites 510A1, 510A2 and 510A3.

[0106] The growth of the continuous layer 500A continues, as does that of the not yet coalesced crystallites 510A4, 510A5. A new coalescence occurs when the continuous layer 500A comes into contact with the crystallite 510A4 ([Fig.3G]). The fining section 220A4 of the latter deforms so that the pad 1000A4 in its entirety deforms and thus accommodates any misalignment and any disorientation of the continuous layer 500A with the crystallite 510A4. The crystallites 510A4 and 510A5 are thus added to the coalesced whole.

[0107] Finally, [Fig.3H] illustrates a coalescence allowing the 510A5 crystallite to be integrated into the continuous layer 500A (i.e. into the coalesced assembly).

[0108] As is clear from the figures, the continuous layer 550A extends at the end of the process between several pads 1000A1-1000A5 and forms a continuous layer.

[0109] It is understood that Figures 3A to 3H illustrate the example of a continuous layer 500A formed by adding a single crystallite 510 at each coalescence step, but that it is perfectly conceivable that several crystallites 510A1-510A5 coalesce simultaneously with the continuous layer 500A, provided that these are not themselves themselves already coalesced between themselves or with one or more other crystallites. Such an example will be described further with reference to [Fig.4F].

[0110] Figures 3A to 3H are not intended to provide a visual representation close to reality concerning the deformation of the sections of the fining. These figures are intended to provide a schematic representation to easily understand the principle of coalescence of the crystallites without dislocation at the coalescence joints. In particular, the deformations of the fining sections are not shown in all the planes for the sake of conciseness. Furthermore, the deformations of the pads have been exaggerated for explanatory purposes. The upper surface of the continuous layer obtained at the end of the process has a much less rough appearance than that shown, or even smooth.

[0111] The growth of the 510A1-510A5 crystallites does not extend downwards. Furthermore, this growth is selective in that it does not take place on the finishing layer 200, typically made of an oxide. In this sense, the growth of the 510A1-510A5 crystallites takes place according to the principle of pendeo-epitaxy.

[0112] It will be noted that it is particularly advantageous to etch the pads 1000A1-1000A5 after epitaxial formation of the buffer layer 400 and the seed layer 500 (when these layers are present). Indeed, if one of these layers 400, 500 were deposited after etching, it would form at least partly between the pads 1000A1-1000A5 on the upper face of the finishing layer 200. In the case where the epitaxially grown nitride is GaN, and the finishing layer 200 is SiO2, then, at the epitaxial deposition temperature, the epitaxial growth of the continuous nitride layer 550A would not be carried out selectively but would on the contrary also take place between the pads 1000A1-1000A5, which is naturally not desirable.

[0113] Particularly advantageously, the epitaxy temperature at which the epitaxy is carried out is higher than or of the order of the temperature of glass transition of the 200 fining layer. Thus, during epitaxy, the 220A1-220A5 fining sections are brought to a temperature which allows them to deform.

[0114] Consequently, if the crystallites 510A11-510A12 carried by two adjacent pads 1000A1-1000A2 are disoriented relative to each other, during the coalescence of these two crystallites, the joint formed at their interface, usually called grain boundary or coalescence joint, will form without dislocation to compensate for these disorientations. The deformation of the fining sections 220 thus makes it possible to compensate for these disorientations and to obtain a continuous layer 550A without or with very few dislocations at the coalescence joints.

[0115] We denote by dlayer the maximum dimension of a continuous layer measured parallel to the xy plane. Thus, dlayer corresponds to the maximum dimension of a projection of the continuous layer in a plane parallel to the xy plane. Preferably 0.8 pm < dlayer < 3 pm and preferably 1 pm < dlayer < 2 pm. dlayer depends on the speed and duration of epitaxial growth as well as the number and size of the pads.

[0116] The distances (for example referenced Pb P2, P3, P4... in the figures) separating two pads whose overlying crystallites are successively coalesced to the continuous layer are typically greater than 300 nm, and for example between 300 nm and 10 pm, for example between 300 nm and 5 pm. These distances can in particular be chosen according to the size of the component that one wishes to manufacture from the continuous layer formed at the end of the process. The increase in the distance between pads as one considers pads of increasingly higher index is typically done more quickly for a small component than for a large component. Indeed, for a large component, one could then end up with pads that are very far apart. However, it can be complex to achieve the coalescence of crystallites belonging to very distant plots, particularly because at an advanced stage of the process, the continuous layer is quite thick.It should be noted, however, that a significant thickness of the continuous layer may be required for the manufacture of certain devices, for example for power electronics. It is therefore not excluded to achieve coalescence between crystallites and the continuous layer when the thickness of nitride is significant (possibly up to several tens of micrometers).

[0117] The process for producing the continuous layer 550A can be stopped at the end of [Fig.3H]. Alternatively, this process can be continued to form a micro-LED from the continuous layer 550A.

[0118] According to a non-limiting embodiment not illustrated, quantum wells are produced within the continuous layer 550A. This embodiment advantageously makes it possible to directly produce a micro-LED of a size corresponding to the initial size of the continuous layer. To produce quantum wells within the continuous layer 550A, those skilled in the art will be able to implement the solutions known from the state of the art. Thus, once the crystallites 510A1-510A5 have coalesced, the same growth conditions are adopted for the wells as during conventional two-dimensional growth.

[0119] The smallest possible dimension for micro-LEDs depends on the ultimate resolution of the chosen structuring methods: for example, for arrays of pads produced by nanoimprinting, pad sizes of 50 nm and inter-pad distances of 100 to 150 nm are achieved. This means that continuous layer dimensions of 1 to 2 pm are obtained. This is therefore in the order of the pixel sizes sought for high-resolution p-displays.

[0120] Explanation of the principle of coalescence without dislocation or with few dislocations (fs) within continuous layers

[0121] As indicated above with reference to Figures 3E to 3H, the formation of the continuous layer 500A involves the successive coalescences of isolated crystallites 510A1-510A5.

[0122] As illustrated in Figures 5A and 5B, the coalescence of sets of already coalesced crystallites induces defects at the coalescence boundaries.

[0123] Figures 5A and 5B illustrate the example of two continuous layers 500A', 500B' each having been obtained by coalescence of two crystallites (510A1' and 510A2' on the one hand, 510B1' and 510B2' on the other hand), each supported by a pad (1000A1' and 1000A2' on the one hand, 1000B1' and 1000B2' on the other hand). The crystal planes of these two layers have an angular orientation defect identified by the angle [3.

[0124] It might be expected that, as in the case of the coalescence of two isolated crystallites, this orientation defect would be compensated for by the properties of the fining sections of each of the plots. It has been noted, however, that compensation did not take place, or at least took place in insufficient proportions to limit the coalescence defects satisfactorily.

[0125] This is explained by the following elements. During a first coalescence of a crystallite with a neighboring crystallite, the underlying fining section is deformed according to the relative misorientations of these two first crystallites. After this first coalescence, the fining section is therefore mechanically constrained in a direction dependent on these misorientations.

[0126] If a new coalescence occurs, we then distinguish two cases: a. The coalescence of the continuous layer occurs with an isolated crystallite (a crystallite that has not yet coalesced with one or more other crystallites is called an isolated or single crystallite): the fining section underlying this isolated crystallite has great mechanical freedom and can alone compensate for the misorientations between the crystal lattices of the continuous layer and the isolated crystallite. It is understood that deformation of the fining sections underlying the continuous layer is also possible, in a much smaller proportion than the deformation of the fining section underlying the isolated crystallite. b. The coalescence of the continuous layer occurs with another continuous layer, also obtained by the coalescence of crystallites carried by several pads (case illustrated in [Fig.5B]): the different sections of fining are then too constrained due to their mechanical dependence on the continuous layer and the other sections of fining. Their presence does not allow the misalignments between the two continuous layers to be compensated for and these coalesce, causing numerous defects within the crystallites, particularly at the level of the coalescence joint. It is understood that the underlying sections of fining to the two continuous layers can deform, but to an extent too small to compensate on their own for the misorientations of the continuous layers.

[0127] In other words, the fining sections of the pads 1000A1', 1000A2', 1000B1', 1000B2' supporting the crystallites involved 510A1', 510A2', 510B1', 510B2' in a coalescence between already coalesced crystallites have a much lower mobility than when each of them supports an isolated crystallite. It is this difference in mobility which explains why the principle of coalescence of raw crystallites by pendeo-epitaxy does not apply satisfactorily to a network of pads with any relative arrangements.

[0128] Figures 5A and 5B illustrate the fact that an out-of-plane misalignment of the continuous layers 500A', 500B' is not compensated by the presence of the finishing sections, but it is understood that the same observation can be made for an orientation defect in the xy plane.

[0129] As explained above, it has been noted that if an isolated crystallite is involved in the coalescence, its fining section alone makes it possible to compensate for its misalignments with the continuous layer. Thus, the invention proposes to arrange the pads on the surface of the substrate in such a way that only this situation is encountered during the progressive formation of the continuous layer, and that the coalescence of two continuous layers each resulting from several crystallites is avoided. Thus, according to the invention, at a coalescence point, the progressive coalescence of the crystallites 510A1-510A5 always takes place between, on the one hand, a single crystallite 510A1 or a plurality of coalesced crystallites 510A1-510A4 and, on the other hand, an isolated crystallite 510A2-510A5.This avoids the formation of structural defects within the crystallites and therefore within the continuous layer during successive coalescences, which ultimately makes it possible to obtain a continuous layer of excellent quality.

[0130] As previously expressed, this does not prevent several coalescences from occurring simultaneously at the level of the set of plots. In this case, a plurality of crystallites that have already coalesced beforehand can coalesce simultaneously with a plurality of crystallites isolated from each other, i.e. not having coalesced beforehand with any other crystallite.

[0131] It is understood that two crystallites forming part of the continuous layer formed by successive coalescences are considered to be coalesced with each other, whether or not they were in direct contact with each other during the coalescence of one of them with the continuous layer. Generally, any crystallite of the continuous layer is considered to be coalesced with all the other crystallites forming the continuous layer, and this from its integration by coalescence with the continuous layer. The continuous layer in fact forms a coalesced whole within which the limits between crystallites cannot be truly defined, in particular as the epitaxy progresses. Thus, the coalescence progressive coalescence of crystallites which is controlled during the process according to the invention is understood as the succession of additions of one or more crystallites to the continuous layer. In particular, it is perfectly conceivable that during the process according to the invention, two crystallites forming part of the continuous layer come into contact and coalesce again with each other, this time directly. These coalescences can be called internal coalescences. Internal coalescences are not taken into account for the progressive coalescence of crystallites, since during internal coalescence, no crystallites are added to the continuous layer. As will be discussed further below, precautions in terms of the arrangement of the pads can be taken in order to optimize these internal coalescences.

[0132] It should be noted that a variation in growth rate may occur during the process due to inevitable variations in the process (for example due to the injection of precursors, variation in temperature or pressure, etc.). Thus, advantageously, it is provided that the variation in the distance between pads from a center of symmetry of the coalescence makes it possible to compensate for this variation in coalescence rate for the same set of pads. In this way, even under the effect of a variation in the coalescence rate, the principle of formation of the continuous layer by successive coalescences of isolated crystallites is verified.

[0133] Examples of arrangement of the pads allowing coalescence without dislocation or with few dislocations (fs) within the continuous layers

[0134] Different examples of arrangement of the pads on the substrate 100 making it possible to obtain a continuous layer having a low defect density will now be described with reference to FIGS. 4A to 4F.

[0135] It is understood that the arrangement of the pads is understood as their distribution in space in projection in the xy plane, and in particular in projection on the upper face 101 of the substrate 100.

[0136] The distance between two neighboring plots is measured from one edge of one of these two plots to one edge of the other of these two plots, in projection in the xy plane, as illustrated in [Fig.3C].

[0137] A first example of arrangement of the pads 1000A1-1000A5 on the substrate 100 is illustrated in FIGS. 4A and 4B. In this example, the pads 1000A1-1000A5 are arranged according to a logarithmic spiral. Such a spiral is a curve whose polar equation is of the form r=abü, with a and b positive real numbers and r the radius measured between the center of the spiral and the point of the curve forming an angle 0 with an arbitrary origin axis of the horizontal plane XY. Advantageously, therefore, the pads 1000A1-1000A5 are arranged on a theoretical curve of this form. The distribution of the pads 1000A1-1000A5 on this theoretical curve is notably chosen according to the level of control of the processes involved, in particular of the process epitaxy, and their variability. The better the control, the more the spacing between two consecutive plots on the curve can increase slightly as the curve is traversed from the origin.

[0138] [Fig.4A] illustrates a theoretical arrangement according to such a model, while [Fig.4B] is a top view SEM photograph of pads arranged according to this embodiment.

[0139] This embodiment is particularly advantageous because it allows a high density of pads on the surface of the substrate, which results in rapid coalescence of the crystallites. Indeed, for identical epitaxial growth parameters, the greater the density of pads, the higher the coalescence speed. The density of pads on the surface of the substrate is defined in projection in the horizontal plane XY as the ratio between the surface occupied by the pads on the upper face 101 of the substrate 100 and the surface of this upper face 101 of the substrate 100.

[0140] According to another embodiment illustrated in [Fig.4D], the pads are aligned along a single axis, called the alignment axis. An alignment direction of this alignment axis is defined as well as an initial point positioned on this same axis. A first pad 1000A1 is located on this alignment axis, for example on the initial point. The other pads are also located on this alignment axis, and the distance separating two consecutive pads 1000A1-1000A6 increases as the alignment axis is traveled in the alignment direction. Noting Pb P2, P3, P4 and P5 the distances separating respectively the first 1000Al and the second 1000A2 plots, the second 1000A2 and the third 1000A3 plots, the third 1000A3 and the fourth 1000A4 plots, the fourth 1000A4 and the fifth 1000A5 plots and finally the fifth 1000A5 and the sixth 1000A6 plots, the order of the plots being given according to the direction of alignment, we thus have: Pi <P2<P3<P4<P5.By noting i the index of the plot on the alignment axis, with i ranging from 1 to N, N being the number of plots arranged along the alignment axis, we preferably have, for i>2, Pi>l,l*Pi b Advantageously, the distance between plots evolves, on the alignment axis and according to the alignment direction, according to a geometric sequence: for i>l, Pi=aqi, a and q being positive real numbers, with preferably 2 < q and / or q > 1,1. .

[0141] This embodiment can be described as an “in-line” arrangement.

[0142] According to another embodiment illustrated in [Fig.4C], some of the pads are arranged along alignment axes as defined previously with reference to [Fig.4D]. These alignment axes are parallel to each other. In order to allow good initiation of coalescence at the start of the process, it is also provided that other pads are not necessarily aligned on one of the alignment axes.

[0143] The pads may for example be arranged along axes parallel to each other and perpendicular to the previously mentioned alignment axes. These axes are designated secondary alignment axes. The pads arranged on the same axis secondary alignment pins are preferably arranged at a constant pitch. Furthermore, by considering all the secondary alignment axes in the direction parallel to all the alignment axes and in the same alignment direction as these axes, the pitch between pads aligned on the same secondary alignment axis increases. In this example, care is taken to ensure that the distance separating two successive alignment axes (which can also be seen as the pitch between the pads on the secondary alignment axes) is always greater than the pitch between pads aligned along the alignment axes. This example is illustrated in [Fig.4C].

[0144] According to an embodiment illustrated in Figures 4E and 4F, the pads are arranged along a plurality of concurrent alignment axes, for example five as shown. A central point can be defined, at which a pad may or may not be located, corresponding to the point of intersection of these alignment axes. The pads are then distributed on branches each starting from the central point.

[0145] Advantageously, and as is highlighted in [Fig.4F], the pads are distributed on concentric circles.

[0146] On each of the branches, the pads can be arranged as described for a single alignment axis, the initial point of the alignment axis as defined in the previous embodiment then being the point of intersection of the alignment axes defined here.

[0147] This embodiment can be described as a “star” arrangement.

[0148] [Fig.4F] highlights the order of coalescence of the crystallites in an example of this embodiment. It is a top view of a set of pads at different stages of progress in the epitaxial growth of the crystallites. The pads are represented by the black dots. The different concentric discs around the same pad correspond to different stages of progress in the growth of the crystallite supported by said pad. More precisely, a disc corresponds to a stage of progress in the growth at which a coalescence (or several simultaneous coalescences) occurs (occur). This figure takes into account internal coalescences as defined above.

[0149] In this example, a first pad 1000A1 is located at the central point of the set of pads and five second pads 1000A2 are located on a first circle having as its center the central point of the set of pads. A first growth phase takes place and ends with the coalescence of the first pad 1000A1 with the five second pads 1000A2 (these five coalescences taking place substantially simultaneously). This coalescence makes it possible to form a continuous layer.

[0150] It should be noted that during the first growth phase, the distance between the second pads 1000A2 and the third pads 1000A3 (and, more generally, any other higher pad) is large enough for their crystallites to remain pectives do not coalesce at this stage of the process (i.e., P2>Pi). This allows the crystallites supported by the second 1000A2 pads to be isolated crystallites during their coalescence with the first 1000A1 pad.

[0151] During a second growth phase, all of the crystallites continue to grow, until the continuous layer coalesces with the crystallites carried by third pads 1000A3 (here five in number). It should be noted here again that during the second growth phase, the distance between the third pads 1000A3 and the fourth pads 1000A4 (and, more generally, any other higher pad) is large enough so that their respective crystallites do not coalesce at this stage of the process (i.e., P3>P2). This allows the crystallites supported by the third pads 1000A3 to be isolated crystallites during their coalescence with the continuous layer.

[0152] The same observations can then be made for the coalescence of the third plots 1000A3 on the one hand and of the fourth plots 1000A4 on the other hand, respectively during a third and a fourth growth phase.

[0153] It is understood that the different phases of epitaxial growth typically take place continuously and that they are represented and described here separately for explanatory purposes only. In particular, the concentric circles are only schematic representations of the progress of the growth at different times of the process but have no physical reality at the end of the process.

[0154] The various examples of arrangement of the pads 1000A1-1000A5 on the substrate 100 described above have made it possible to obtain continuous layers of very good quality, greatly reducing the density of defects compared to layers obtained without precautions on the arrangement of the pads. In particular, the rotational symmetry allowed by the distribution of the pads around the first pad 1000A1 makes it possible to limit or even eliminate the potential defects generated by internal coalescences in the continuous layer. It also makes it easier to manufacture the pads.

[0155] It is understood, however, that any arrangement of the pads allowing the formation of a continuous layer solely by adding isolated crystallites to a cluster of crystallites (or possibly, at the start of the process, to another isolated crystallite) makes it possible to obtain equally satisfactory results. It is, for example, possible to remove the central pad.

[0156] The non-limiting example described above refers to the use of the nitride layer to form an LED or a micro-LED. As previously indicated, this example is not limiting. The method described can also be used to produce other nitride-based devices, for example vertical devices such as HEMT transistors.

[0157] The invention is not limited to the embodiments previously described and extends to all embodiments covered by the invention.

Claims

Claims

1. A method of obtaining a layer made at least in part of a nitride (N) preferably obtained from at least one of gallium (Ga), indium (In) and aluminum (Al), the method comprising the following steps: • provide a stack comprising at least one set of pads (1000A1-1000B4) extending from a substrate (100), each pad (1000A1-1000B4) comprising at least: i. a first section, called the finishing section (220A1-220A5), formed in an amorphous material having a glass transition temperature ii. a second, crystalline section, called the crystalline section (300A1-300A5), surmounting the finishing section (200A1-200A5), • growing by epitaxy a crystallite (510A1-510A5) on at least some of said pads (1000A1-1000A5) and continuing the epitaxial growth of the crystallites (510A1-510A5) until coalescence of the crystallites (510A1-510A5) carried by the pads (1000A1-1000A5) of the set of pads, so as to form a continuous layer (550A) of nitride, the method being characterized in that the pads (1000A1-1000A5) of said set of pads are distributed on the substrate (100) so that the relative arrangement of the pads of the set of pads is such that during the epitaxy of the crystallites (510A1-510A5), the progressive coalescence of the crystallites (510A1-510A5) always takes place between on the one hand a set, called a coalesced set, comprising a crystallite (510A1) or a plurality of crystallites (510A1-510A4) coalesced and on the other hand at least one crystallite, called an isolated crystallite (510A2-510A5), which has not already coalesced with other crystallites.

2. Method according to the preceding claim, in which the pads (1000A1-1000A5) carrying the crystallites forming the continuous layer (550A) of nitride are substantially arranged, in projection in a horizontal plane (XY) in which the upper face (101) of the substrate (100) mainly extends, according to a logarithmic spiral.

3. Method according to claim 1, in which the pads (1000A1-1000A5) carrying the crystallites forming the continuous layer (550A) of nitride are, in projection in a horizontal plane (XY) in which the upper face (101) of the substrate (100) mainly extends, substantially aligned along at least one so-called alignment axis.

4. Method according to the preceding claim, in which the pads (1000A1-1000A5) arranged on the same alignment axis are spaced apart by an increasing distance while traveling along the alignment axis in a given direction from a given point, called the initial point.

5. Method according to any one of the two preceding claims, in which the pads (1000A1-1000A5) carrying the crystallites forming the continuous layer (550A) are separated by a distance varying logarithmically from near to near, said distance being measured along their alignment axis.

6. Method according to claim 1, in which the pads (1000A1-1000A5) carrying the crystallites forming the continuous layer (550A) of nitride are substantially aligned, in projection in a horizontal plane (XY) in which the upper face (101) of the substrate (100) mainly extends, along concurrent axes, the concurrent axes verifying between them a rotational symmetry around a central point in projection in the horizontal plane (XY).

7. Method according to the preceding claim, in which the pads (1000A1-1000A5) are aligned along five concurrent axes.

8. A method according to any one of the preceding claims wherein the epitaxial growth is carried out at a temperature Tepitaxy, such that Tepitaxy > ki x TtransiPonvitreuse, with k| > 0.

8.

9. Method according to the preceding claim, in which ki > 1, and preferably ki > 1.

5.

10. A method according to any preceding claim wherein the set of pads comprises at least 3 pads.

11. A method according to any preceding claim wherein the stack comprises a plurality of sets of pads on the substrate, a continuous layer of nitride being formed from each set of pads, the epitaxial growth of the crystallites being interrupted before the crystallites belonging to two distinct sets of pads coalesce, so that the continuous layers formed from each set of pads are spaced apart from each other.

12. Method according to any one of the preceding claims, in which each pad (1000A1-1000A5) has an upper face (1001A1) and in which the epitaxial growth of the crystallites (510A1-510A5) is carried out at least in part and preferably only from said upper face (1001A1).

13. A method according to any preceding claim further comprising a step of forming in the continuous layer (550A) a device, the device being taken from an LED and a transistor, for example a vertical transistor, for example a HEMT type transistor.

14. Method according to any one of the preceding claims in which the pads (1000A1-1000A5) of said set of pads are distributed on the substrate (100) so that the relative arrangement of the pads of the set of pads is such that during the epitaxy of the crystallites (510A1-510A5), at least at certain times the progressive coalescence of the crystallites (510A1-510A5) takes place between on the one hand said coalesced set and on the other hand several isolated crystallites (510A2-510A5).

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