Mesas porosification process

Abstract

Process for porosifying mesas. This description relates to a process for porosifying a structure (100) comprising a base substrate (110) covered with mesas (120), the mesas (120) being (Al,In,Ga)N or InP mesas or (Al,In,Ga)N / (Al,In,Ga)N or InP / InP mesas, the mesas (120) being porosified electrochemically according to the following step cycle: i) applying a first potential for a first time, ii) applying a second potential for a second time, thereby obtaining porosified mesas (120) comprising, from the lateral faces of the mesas (120) towards the center of the mesas (120) or vice versa, a first part (123a) having a first degree of porification and a second part (123b), the second part (123b) having a second degree of porification. of porification greater than the first porification rate or the second part (123b) being hollowed out. Figure for the abstract: Fig. 4

Description

Title of the invention: Mesas porosification process technical field

[0001] This description relates generally to the general field of micro-color displays based on micro-LEDs.

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

[0003] The invention also relates to a structure thus obtained comprising porosified mesas. Previous technique

[0004] Color microdisplays comprise pixels made up of blue, green, and red sub-pixels (RGB pixels). In the following description, these sub-pixels will be referred to simply as pixels for the sake of brevity.

[0005] Blue and green pixels can be made from nitride materials, and red pixels from phosphide materials. To combine these three types of pixels on the same substrate, the "pick and place" technique is generally used. However, in the case of microdisplays with pixels smaller than 10 pm and / or displays with a large number of pixels (high resolution), this technique can no longer be used due to alignment problems and / or the time required. Furthermore, the pixels must be picked from different wafers, which necessitates successive transfers. Parallel transfer techniques ("mass transfer") can also be used.

[0006] Another solution involves performing color conversion using quantum dots (QDs) or nanophosphors pumped by blue pLEDs from a single wafer, either depositionalized or in a monolithic matrix (the preferred case for microdisplays). However, controlling the deposition of these materials on small pixels is difficult, and their resistance to flux is not sufficiently robust.

[0007] One solution is to natively form the three RGB pixels using the same family of materials, so that they can be grown on the same substrate. InGaN is the most promising material for this purpose. This material can, in fact, theoretically cover the entire visible spectrum depending on its indium concentration. Blue InGaN-based micro-LEDs already exhibit high luminance, significantly higher than their organic counterparts. To emit at wavelengths in the green range, the LED's quantum wells (PQs) must contain at least 25% indium, and for red emission, at least 35% indium is required. Unfortunately, the quality of InGaN material beyond 20% In is degraded due to the low miscibility of InN in GaN, but also due to the strong compressive stress inherent in the growth of the active InGaN zone on GaN.

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

[0009] Currently, one of the most promising solutions consists of electrochemically porosifying the GaN layer of the mesas by applying a potential for a specific duration. The resulting porosified GaN layer can be used to grow an InGaN-based nitride LED structure with improved crystalline quality, thanks to the relaxation of the generated porous mesas.

[0010] However, it has been observed that the reactivation of epitaxial processes by metal-organic vapor deposition (MOCVD) from precursors on porous InGaN / GaN mesas appears to be disrupted by the presence of porosity on the mesa flanks. A liner layer must therefore be deposited on the flanks to neutralize the porosity and prevent epitaxial growth on the flanks. However, when the pixel pitch decreases, or when the intermesa distance decreases, the deposition of the liner layer becomes more challenging. Summary of the invention

[0011] There is a need for a manufacturing process for a device comprising porous mesas, with re-epitaxy being able to be easily performed on the mesas obtained.

[0012] This goal is achieved by a porosification process of a structure comprising a basic substrate covered with mesas, the mesas being (Al,In,Ga)N or InP mesas or (Al,In,Ga)N / (Al,In,Ga)N or InP / InP mesas, the mesas being porosified electrochemically according to the following step cycle: i) apply a first potential for a first duration, ii) apply a second potential for a second duration, whereby we obtain porosified mesas comprising, from the lateral faces of the mesas towards the center of the mesas or vice versa, a first part having a first rate of porification and a second part, the second part having a second rate of porification greater than the first rate of porification or the second part being hollowed out.

[0013] According to a particular embodiment, the first potential is lower than the second potential.

[0014] According to a particular embodiment, the first potential is between 3 V and 12 V and / or the second potential is between 5 and 20 V.

[0015] According to a particular embodiment, step i) or the cycle of steps i) and ii) is repeated at least once so as to form an alternation of first parts and second parts.

[0016] This goal is also achieved by a structure comprising a basic substrate covered with porosified mesas, the porosified mesas being (Al,In,Ga)N or InP mesas or (Al,In,Ga)N / (Al,In,Ga)N or InP / InP mesas, the mesas comprising, from the lateral faces of the mesas towards the center of the mesas or vice versa, a first part having a first degree of porification and a second part, the second part having a second degree of porification greater than the first degree of porification or the second part being hollowed out.

[0017] According to a particular embodiment, the first part corresponds to the flank of the mesas and the second part corresponds to the core of the mesas.

[0018] According to a particular embodiment, the second part corresponds to the flank of the mesas and the first part corresponds to the core of the mesas.

[0019] According to a particular embodiment, the mesas comprise, from the lateral faces of the mesas towards the center of the mesas or vice versa, an alternation of first parts and second parts.

[0020] According to a particular embodiment, the basic substrate comprises a support layer, a first undoped GaN layer, a second doped GaN layer, a part of the second doped GaN layer extending into the mesas, the basic substrate being able to further comprise one or more additional conductive layers, preferably in heavily doped GaN, arranged between the first undoped GaN layer and the second doped GaN layer.

[0021] According to a particular embodiment, the first porification rate is less than 10% and / or the second porification rate is greater than or equal to 40%, preferably between 40 and 70%. Brief description of the drawings

[0022] These features and advantages, as well as others, will be described in detail in the following description of particular embodiments, given by way of non-limiting example, in relation to the accompanying figures, among which:

[0023] [Fig.1] represents, schematically and in cross-section, a structure comprising a mesa (Al,In,Ga)N / (Al,In,Ga)N or InP / InP, before porosification, according to a particular embodiment of the invention;

[0024] [Fig.2] schematically represents, in cross-section, a structure comprising a mesa (Al,In,Ga)N or InP, before porosification, according to another particular embodiment of the invention;

[0025] Fig. 3, Fig. 4, Fig. 5, Fig. 6 and Fig. 7 represent, schematically and in cross-section, different structures comprising (Al,In,Ga)N or InP mesas or (Al,In,Ga)N / (Al,In,Ga)N or InP / InP mesas, after porosification, according to different particular embodiments of the invention;

[0026] [Fig.8] is a graph representing different phenomena occurring (pre-porification (or “pre-breakdown”), porosification and electropolishing) during an anodizing step, depending on the doping rate and the applied potential, according to a particular embodiment of the invention;

[0027] [Fig.9A] and [Fig.9B] are images obtained by scanning electron microscopy of a porosified (Al,In,Ga)N / (Al,In,Ga)N mesa, respectively, in side view and in section, according to another particular embodiment of the invention;

[0028] [Fig. 10] is a scan electron microscope image of a porosified (Al,In,Ga)N mesa, seen in section, according to another particular embodiment of the invention;

[0029] [Fig. 11] and [Fig. 12] are images obtained by scanning electron microscopy of different porosified (Al,In,Ga)N / (Al,In,Ga)N mesas, in cross-sectional view, according to different particular embodiments of the invention. Description of the implementation methods

[0030] The same elements have been designated by the same reference numerals in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same reference numerals and may have identical structural, dimensional and material properties.

[0031] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been represented and are detailed.

[0032] Unless otherwise specified, when referring to two elements connected together, this means directly connected without intermediate elements other than conductors, and when referring to two elements connected (in English "coupled") together, this means that these two elements can be connected or linked through one or more other elements.

[0033] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "superior", "inferior", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures.

[0034] Unless otherwise specified, the expressions "approximately", "roughly", and "on the order of" mean to within 10% or 10°, preferably to within 5% or 5°.

[0035] By included between X and Y, it is meant that the bounds X and Y are included.

[0036] The porosity rate of a material is the ratio between the volume of the pores (volume of voids) and the total volume of the material.

[0037] Although this is not limiting in any way, the invention finds particular applications in the field of color micro-screens, and more particularly for the manufacture of red green blue pixels.

[0038] However, it could be used in the field of photovoltaics or even 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. Furthermore, it has a large specific surface area, which is particularly advantageous.

[0039] The invention may also be of interest for the manufacture of LEDs or lasers emitting at long wavelengths.

[0040] The porosification process which will be described in more detail later can be implemented on structures 100 having mesas 120 (Al,In,Ga)N / (Al,In,Ga)N ([Fig.1]) or mesas 120 (Al,In,Ga)N ([Fig.2]).

[0041] 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.

[0042] By (Al,In,Ga)N / (Al,In,Ga)N mesa, it is understood that the mesas comprise a highly doped (Al,In,Ga)N layer 123 to be porosified, covered by an undoped or lightly doped (Al,In,Ga)N layer 124 ([Fig. 1]). According to this configuration, the resumption of epitaxy is carried out on the undoped or lightly doped layer 124. Similarly, by InP / InP mesa, it is understood that the mesas comprise a highly doped InP layer 123 to be porosified, covered by an undoped or lightly doped InP layer 124.

[0043] By (Al,In,Ga)N mesa, it is understood that the mesas comprise a layer of (Al,In,Ga)N heavily doped 123 to be porosified. The layer of (Al,In,Ga)N heavily doped 123 to be porosified is not covered by a layer of (Al,In,Ga)N undoped or lightly doped. According to this configuration, the resumption of epitaxy is carried out directly on the porosified layer of (Al,In,Ga)N heavily doped 123. Similarly, by InP mesa, it is understood that the mesas comprise a layer of InP heavily doped 123 to be porosified. The layer of InP heavily doped 123 to be porosified is not covered by a layer of InP undoped or lightly doped.

[0044] Subsequently, the process and structure will be described more particularly for (Al,In,Ga)N / (Al,In,Ga)N mesas or (Al,In,Ga)N mesas, but the invention can be carried out on InP / InP mesas or InP mesas.

[0045] The porosification process comprises the following steps: a) provide a structure 100 comprising a basic substrate 110 covered with mesas 120, the mesas 120 being (Al,In,Ga)N / (Al,In,Ga)N mesas ([Fig.1]) or (Al,In,Ga)N mesas ([Fig.2]), b) electrically connect structure 100 and a counter electrode to a voltage or current generator, c) immerse structure 100 and the counter electrode in an electrolytic solution, d) porosify the mesas 120 electrochemically by carrying out one or more times the cycle formed by the following steps i) and ii): i) apply a first potential for a first duration between structure 100 and the counter electrode, ii) apply a second potential for a second duration between structure 100 and the counter electrode.

[0046] Implementing a multi-stage porosification process with different potentials allows for the production of structured mesas exhibiting several differently porosified parts in the xOy plane, i.e., parallel to the stacking formed by the base substrate and the mesa (Figures 2 to 7). For each mesa, the differently porosified parts proceed from the lateral faces of the mesa 120 towards its center.

[0047] Different doping levels in the mesa are not required to modulate porosity. Porosity is modulated according to the different applied potentials. Indeed, at a constant doping level, different porosity levels, pore sizes, and densities are obtained depending on the applied potential (see the "nomogram" in [Fig. 8]).

[0048] At low potential, the porosity rate and pore size are small. This is a nucleation regime ('pre-breakdown'), leading to the creation of channels. The channels are created from: - the sides of mesas 120, when the mesas include an undoped upper layer 124, or - the sides and upper part of the mesas 120 (i.e. at the level of the sufficiently doped parts in contact with the electrolytic solution) when the mesas 120 are not covered by the undoped upper layer.

[0049] At higher potential, the porosification regime is reached: porosification spreads in the most conductive zones (in other words, in the parts of the mesa that have not yet been involved in the electrochemical, charge-consuming reactions).

[0050] Several porous structures can be obtained.

[0051] According to an alternative embodiment, as shown for example in [Fig. 3] or [Fig. 4], the mesa 120 has a first porous portion 123a, having a first degree of porosity, and a second porous portion 123b, having a second degree of porosity, the first degree of porosity being lower than the second degree of porosity. Preferably, the flanks of the mesa 120 correspond to the first portion 123a. In other words, the core 123b of the mesa is more porous than the flanks 123a of the mesa 120. This allows for less porosity on the surface of the flanks while maintaining high central porosity.The lower porosity on the flanks limits the roughness of their surface and will therefore have less impact on the resumption of epitaxy, thus facilitating the implementation of all the necessary technological steps (a smaller volume of chemical solution that penetrates the porous structure, less release, better integrity of the structure which is more adapted to receive a conforming deposit, etc).

[0052] Such structures are particularly advantageous because they allow: - modulation of the porosity which plays a strong role on the incorporation of In during the resumption of InGaN epitaxial growth, - preservation of the compliance effect provided by the high porosity at the center of the mesa and allow an incorporation of In adapted to the emission of quantum wells in InGaN in the red (typically greater than 620 nm).

[0053] In the case where the mesas 120 are (Al,In,Ga)N mesas (i.e., in the case where the doped layer 123 is in contact with the electrolyte), the upper part 123c of the doped layer 123 is also porosified along an axis perpendicular to the mesa / substrate stacking ([Fig. 4]). The degree of porosification of the upper part 123c of the mesa 120 corresponds to the degree of porosification of the flanks of the mesas 120.

[0054] One advantage of this structure is that the initial epitaxy is simpler since it is mono-material (in this case the GaN or InGaN uid layer is no longer necessary).

[0055] According to another embodiment, for example shown in figures 6 and 7, the mesa 120 has a first porosified part 123a and a second hollowed (i.e. devoid of material) part 123b.

[0056] The hollowed-out part 123b can correspond to the central part of the mesa, the porosified part 123a then forming the sides of the mesa. The mesa has a 'drum' shape ([Fig.6]).

[0057] Alternatively, as shown in [Fig. 7], the hollowed-out portion 123b may correspond to the initial location of the flanks of the mesa 120 and the first porosified portion 123a to the core of the mesa 120. For example, to obtain such a structure, the first potential is located in the electropolishing zone to etch the flanks and reduce the width of the mesa structure. The second applied potential is lower than the first potential to porosify the core of the mesas.

[0058] According to another embodiment, as, for example, shown in [Fig. 5], it is possible to form a structure having mesas 120 exhibiting alternating first parts 123a and second parts 123b when the cycle of steps i) and ii) is repeated several times. The second parts 123b can be highly porous or devoid of material. Such a structure 100 is of interest for microdisplays ('p-displays') because the different porosities induce varying relaxation rates of the upper layer. Such a structure 100 can also be of interest for other photonic applications.

[0059] We will now describe in more detail the different stages of the process.

[0060] The structure 100 provided in step a) comprises a base substrate 110 covered with mesas 120.

[0061] The basic substrate 110 comprises successively (Figures 1 and 2): - a support layer 114, - possibly a buffer layer of (Al,Ga)N (not shown), particularly in the case of a silicon support layer 114, - a first layer of undoped GaN 111, - possibly, an additional layer of heavily doped GaN 113, - a second layer of doped GaN 112, a first part 112a of the layer of doped GaN 112 extending into the mesas 120 and a second part 112b of the second layer of doped GaN forming part of the substrate 110.

[0062] The second layer of doped GaN 112 may be unintentionally doped if the structure includes the additional layer of heavily doped GaN 113.

[0063] The first part 112b of the doped GaN layer is a layer common to all mesas.

[0064] Each mesa 120 comprises successively from the base: the second part 112a of the doped GaN layer 112, the third layer of heavily doped GaN 123 and, where applicable, the fourth layer of undoped or weakly doped (Al,In,Ga)N 124.

[0065] The structure 100 provided in step a) is, for example, obtained by providing and then locally engraving a stack comprising successively: - a support layer 114 (also called substrate), - possibly, a buffer layer of (Al,Ga)N, particularly in the case of a silicon or SiC support layer 114, - a first layer of undoped gallium nitride (GaN) 111, - possibly, an additional layer of heavily doped GaN 113, - a second layer of GaN 112 doped (GaN n) or unintentionally doped (if the structure includes the additional layer of heavily doped GaN 113), - a third layer of heavily doped GaN (GaN n+ or GaN nn) 123, and - where appropriate, a fourth layer of AIN, InGaN or GaN (noted (Al,In,Ga)N) unintentionally doped (nested) or weakly doped 124.

[0066] Preferably, the stack consists of the layers mentioned above. In other words, it does not include any other layers.

[0067] The structuring of the stack is, for example, achieved by photolithography.

[0068] 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.

[0069] 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, made of SiO2. After etching the mesas, this hard mask is removed by a wet chemical process before porosification. It is also possible to remove this hard mask after porosification, by exposing it only in the areas used for electrochemical polarization. Advantageously, the mask is removed before the porosification step.

[0070] Preferably, the lateral faces and flanks (lateral parts) of the mesas 120 are perpendicular to this stacking of layers.

[0071] The surface of the mesas can be circular, hexagonal, square or rectangular.

[0072] 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.

[0073] The thickness of the mesas corresponds to the dimension of the mesa perpendicular to the underlying stack.

[0074] The 120 mesas can have a pitch of less than 30 pm. The spacing between two consecutive 120 mesas ranges from 50 nm to 20 pm.

[0075] 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). After epitaxy of the complete LED structure, blue, green, and red (RGB) mesas will thus be obtained on the same substrate, and in a single growth step, if the difference between the relaxation levels of the mesas is sufficient.

[0076] The support layer 114 is, for example, made of sapphire, SiC or silicon. It could also be made of GaN ('GaN free standing').

[0077] The support layer 114 has, for example, a thickness ranging from 250 µm to 2 mm. The thickness depends on the nature of the support layer 114 and its dimensions. For example, for a sapphire support layer 2 inches in diameter, the thickness can be 350 µm. For a sapphire support layer 6 inches in diameter, the thickness can be 1.3 mm. For a silicon support layer 200 mm in diameter, the thickness can be 1 mm.

[0078] 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.

[0079] The first layer 111 is a nested GaN layer. It is an unintentionally doped (nest) layer so as not to be porous. By unintentionally doped (In)GaN, we mean a concentration of less than 5e17at / cm3 for InGaN and 5e17at / cm3 for GaN.

[0080] 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.

[0081] For InP, the undoped or weakly doped InP layer 124 (n-) has, for example, a doping level below 117 at.cm 3 and the heavily doped InP layer 123 (n+) has, for example, a doping level above 5e 18 at.cm 3.

[0082] According to a particularly advantageous embodiment, the structure 100 comprises the additional heavily doped GaN layer 113 (shown only in [Fig. 2] but which may be present in all structures) arranged between the first undoped GaN layer 111 and the second doped GaN layer 112. This results in a three-layer structure comprising a heavily doped GaN layer covered by the second doped layer 112 and the third heavily doped GaN layer 123 to be porosified. The second doped layer 112 provides protection for the underlying additional heavily doped layer 113 and ensures contact during porosification. Thus, the additional layer 113 is not in contact with the solution.

[0083] The additional heavily doped layer 113 ensures the lateral conduction of charges in the structure. For example, the doping level of the additional heavily doped GaN layer is between 5 × 10⁻¹⁸ at / cm² and 2 × 10⁻¹⁹ at / cm², preferably between 5 × 10¹⁸ at / cm³ and 1 × 5 × 10¹⁹ at / cm³, and even more preferably between 8 × 10¹⁸ at / cm³ and 1 × 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 obtained on sapphire. A highly conductive buried layer is thus obtained thanks to with a high doping level and a significant layer thickness. The thickness-doping ratio will be adjusted to ensure sufficient lateral conduction. During step d), conduction occurs via this additional, heavily doped, buried layer. Because it is highly conductive, it minimizes edge / center effects.

[0084] The charges pass through the second doped layer 112 and then onto the additional, highly doped layer 113, which then acts as a conduction highway and supplies all the mesas present on the substrate. During step d), the second doped layer 112 protects the additional, highly doped layer 113 from porosification. Thus, each mesa 120 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).

[0085] The second layer 112 can be a doped or lightly doped GaN layer, depending on the architecture of the structure. Doped GaN is defined as a concentration greater than 1.10 at / cm³, preferably greater than 5.10 at / cm³, and preferably between 5.10 at / cm³ and 2.10 at / cm³. As previously stated, in the case of a trilayer (i.e., if layer 113 is present), layer 112 can be doped GaN or Nid-GaN.

[0086] The second GaN layer 112 has, for example, a thickness ranging from 200 nm to Ipm, 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.

[0087] 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 10¹⁰ at / cm³. It has, for example, a doping level ten times higher than the second layer 112. It has a thickness, for example, of between 200 nm and 2 pm. Preferably from 500 nm to 1 pm.

[0088] 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 10⁻¹⁰ at¹ cm³ and 1 x 10⁻¹⁰ at¹ cm³. Undoped means a doping level below 5 x 10⁻¹⁷ at / cm³ or even below 1 x 10⁻¹⁷ at / cm³.

[0089] 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).

[0090] 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.

[0091] The doping of the different layers mentioned above will be chosen according to the tension applied during the porosification.

[0092] In particular, they will be chosen from a "nomogram" such as that in [Fig. 8]. This "nomogram" makes it possible to define the respective doping levels so that, at a given potential, there is selectivity between the heavily doped area and the lightly doped area. For a given potential, the doping level of the second layer 112 must be in the 'pre-breakdown' region so that the second layer 112 is not porosified during step d). The doping level of the third layer 123 must be in the 'porification' region so that the third 123 is porosified or in the 'electropolishing' region so that the third layer 123 is etched.

[0093] Subsequently, an n-type doping is described, but it could be a p-type doping. The electrochemical conditions (for example the potential) will be chosen for such a doping.

[0094] By way of illustration and not limitation, according to one embodiment, the structure 100 to be porosified may comprise: - a basic substrate 110 comprising successively: a support layer 114 in sapphire or silicon, possibly a buffer layer in (Al,Ga)N, a first layer 111 of GaN undoped by 4pm, a first part 112a of the second layer 112 of GaN doped by 500nm (1.1018 at / cm3), - 120 (Al,In,Ga)N / (Al,In,Ga)N mesas comprising successively: a second part 112b of the second layer 112 of GaN doped by 100nm (1.1018 at / cm3), a third layer 123 of GaN heavily doped by 800 nm (1.1019 at / cm3), and, where applicable, a nest layer (Al,In,Ga)N of 100nm.

[0095] An additional layer 113 of GaN heavily doped with 2pm (1.1019 at / cm3) can be positioned between the first layer 111 of undoped GaN and the first part 112a of the second layer 112 of doped GaN.

[0096] 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.

[0097] The initial contact is made on structure 100.

[0098] In particular, contact can be made on the base substrate 110, especially on the second doped GaN layer 112. Preferably, the contact contact can be made on the bottom of the mesas 120, at the level of the second part 112b of the second layer 112, which allows the etching step to be used to also make the contacts.

[0099] 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 and before epitaxial re-establishment.

[0100] The counter electrode is made of an electrically conductive material, such as for example a metal with a large developed surface area and inert to the chemistry of the electrolyte such as a platinum wire mesh.

[0101] 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, KOH, HF, HNO3, NaNO3, or H2SO4. It can also be a mixture of these, for example, a mixture of oxalic acid and NaNO3 to promote kinetics.

[0102] During step d), the mesas are porosified.

[0103] The first potential El is different from the second potential E2. The first potential El is, preferably, lower than the second potential E2.

[0104] The potential modulation during anodizing allows, initially, for very slight porosification of the mesa flanks and subsequently for more pronounced porosification of the mesa center, even etching it. Thus, the first low-potential step (E1) creates channels. These channels then conduct the electrolyte towards the mesa center, which, under the effect of a second potential (E2 with E2>E1), becomes more extensively porous than the flanks, or is even electropolished. This results in mesas with a highly porous or material-free central portion and relatively low-porosity flanks.

[0105] During the iteration of steps i) and ii), it would also be possible to use different potentials and / or durations than those used during the first cycle of steps i) and ii). Preferably, the same potentials are used.

[0106] Depending on the width of the mesas, it is possible to adapt the number of iterations, the potentials, and the application times of the potentials. It is possible to modulate the number of first parts 123a (low porosity) and second parts 123b (highly porous or hollowed out) as well as their widths.

[0107] The applied potentials can be between 1 and 50V. Preferably, they are between 3 and 20V.

[0108] For example, the first potential is between 3 V and 12 V and / or the second potential is between 5 and 20 V.

[0109] On sapphire, the first potential is preferably between 3.5 and 4.5 V and / or the second potential is between 7 and 10 V.

[0110] On silicon, the first potential is preferably between 7 and 10 V and / or the second potential is preferably between 14 and 17 V.

[0111] The potential is chosen according to the doping levels of the different layers, in order to to obtain the desired selectivity. It is applied, for example, for a duration ranging from a few seconds to several hours.

[0112] The porosification is complete when there is no longer any current at the imposed potential. At that moment, the entire doped structure is porosified and the electrochemical reaction stops.

[0113] It is also possible to achieve incomplete porosification and retain a non-porous and unetched core. The non-porous core is surrounded, on both sides, successively by the second part forming a highly porous intermediate zone and then by the first part forming slightly porous flanks.

[0114] The electrochemical anodizing step can be activated by irradiation at the wavelength corresponding to the band gap of the material (for example, UVA for GaN, UVB and UVC for AlGAN depending on the Al content).

[0115] Advantageously, at the end of the porosification step the entire volume of the third layer of heavily doped GaN 123 is porosified.

[0116] The first part 123a is slightly porous. The first degree of porosity is preferably less than 10%. It is, for example, between 2 and 10%.

[0117] The second part 123b is hollowed out or highly porous. The second degree of porification is preferably at least 40%. It is, for example, between 40% and 70%.

[0118] 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.

[0119] The resulting porosity (porosity rate, also called porosity rate, and pore size) depends on the layer doping and the process parameters (applied voltages, durations, 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 and morphology, can vary subsequently during the resumption of epitaxy as a function of the applied temperature.

[0120] The process may also include a step in which the highly porous part 123b is etched by a solution, in particular an alkaline solution, preferably KOH or tetramethylammonium hydroxide (TMAH).

[0121] 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.

[0122] The percentage of relaxation corresponds to: Aa / a = (ac2-ac J / a^

[0123] with a^, the lattice parameter of the starting layer on which the epitaxy is resumed (i.e., the lattice parameter of layer 124), and

[0124] ac 2 the mesh parameter of the relaxed layer,

[0125] 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.

[0126] When aci=ac2 the layer is said to be constrained.

[0127] Partially relaxed means a percentage of relaxation greater than 50%.

[0128] Resumption of epitaxy can be used, for example, to form re-epitaxial LEDs.

[0129] Epitaxial regrowth can be carried out on the fourth layer 124 nest or weakly The mesas are doped at 120. Because this layer is not porosified during the electrochemical anodizing step, it remains continuous and dense. Epitaxial regrowth is thus facilitated, and the epitaxial layer exhibits better adhesion. The formation of defects related to pore coalescence is avoided.

[0130] Epitaxial regrowth can also be performed on the upper part 123c of the porosified doped layer 123 of the mesas. As this layer is weakly porosified, epitaxial regrowth can be easily achieved.

[0131] The epitaxial layer in this step e) is advantageously made of gallium nitride or indium gallium nitride.

[0132] Illustrative and non-limiting examples of different embodiments

[0133] Several structures have been created.

[0134] 1st structure

[0135] The first structure to be porosified is a structure such as that shown in [Fig.1]. It has (Al,In,Ga)N / (Al,In,Ga)N mesas, and more particularly InGaN / GaN or GaN / GaN mesas.

[0136] The first low-potential step (E1 = 4.5 V for 300 s) allows the creation of channels at the mesa flanks, which become less conductive. These channels conduct the electrolyte towards the center of the mesa when the second potential is applied. Under the effect of the second potential E2, which is stronger than the first potential (E2 = 11 V for 250 s), the center of the mesa becomes more porous than the flanks. The porosification reaction takes place at the interface between the electrolyte and the highly conductive or highly doped region, i.e., in the central region of the mesa.

[0137] The potential modulation during anodizing allows the sides of the mesa to be porosified very slightly and then the center of the mesa to be porosified more (figures 9A and 9B).

[0138] 2nd structure

[0139] The second structure to be porosified corresponds to a structure such as that shown in [Fig. 2]. The structure comprises (Al,In,Ga)N mesas, and more particularly Si-doped GaN mesas. In this structure, there is no nest layer covering the mesas.

[0140] As in the case of the first structure, two different potentials are applied. The first potential, E1 = 3V, is applied for 130 s and the second potential, E2, is applied for 100 s. The application of two different potentials allows the formation of a very low-porosity layer on the upper part and on the sides of the mesas. The pore size is very small (typically less than 100 µm) and compatible with defect-free GaN epitaxial resumption (since lateral GaN epitaxy is strong). The multi-potential process here makes it possible, starting from an n-doped GaN mesa, to obtain a mesa with a "core / shell" structure having a very porous core and a very low-porosity shell ([Fig. 10]).

[0141] The thickness of the low-porosity envelope depends on the application time of the first potential El < E2. In the case of structure No. 2, 130 s at El produces an envelope of 200 to 250 nm. This time can be reduced, and it is possible to produce thinner envelopes on the order of 50 to 100 nm.

[0142] This process allows for vertical and homogeneous porosification over the entire mesa except for the low-porosity envelope. The core exhibits a porosity of at least 40% with a dendritic morphology.

[0143] Such vertical relaxation is favorable to relaxation. It can improve the relaxation of the upper epitaxial layers, whether it is in InGaN for red emission or in AlGaN for UV applications.

[0144] 3rd structure

[0145] The third structure to be porosified corresponds to a structure such as that shown in [Fig. 1]. The structure has (Al,In,Ga)N / (Al,In,Ga)N mesas, and more particularly GaN / GaN mesas.

[0146] Two potentials are applied to the structure. The potential E2 is higher than the potential EL. The potential E2 is located in the electropolishing zone of the material. Thus, the center of the mesa is not porosified but etched. An additional chemical etching step, in particular an alkaline chemical etch (for example, TMAH or KOH), can be used to etch any remaining material at the center of the mesa, thus forming a cavity.

[0147] An InGaN or GaN membrane (but it could also be an AlGaN membrane) of thickness varying between 50nm and 200nm is obtained suspended above a cavity with slightly porous GaN walls of dimension varying between Ipm and a few pm, 3pm for example ([Fig.1 1]).

[0148] Stress relaxation on this GaN or InGaN layer is maximal since the layer on which epitaxial regrowth takes place is decoupled from its growth substrate, which imposed its lattice parameter. This new type of structure (a kind of drum) is particularly interesting for photonic devices but also for MEMS-type devices.

[0149] 4th structure

[0150] The fourth structure to be porosified corresponds to a structure such as that shown in [Fig. 1]. The structure has (Al,In,Ga)N / (Al,In,Ga)N mesas, and more particularly GaN / GaN mesas.

[0151] In this example, the first potential El is applied for 200s and the second potential E2 is applied for 120s. The cycle of applying the potentials El and E2 is repeated once. The resulting structure is shown in [Fig. 12].

[0152] Depending on the width of the mesa, it is possible to adapt the number of iterations and the application time of the potentials to modulate the number of alternations of low-porosity and high-porosity zones as well as their width.

[0153] Various embodiments and variations have been described. A person skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will become apparent to a person skilled in the art.

[0154] Finally, the practical implementation of the embodiments and variants described is within the reach of a person skilled in the art, based on the functional indications given above.

Claims

Demands

1. A method for porosifying a structure (100) comprising a base substrate (110) covered with mesas (120), the mesas (120) being (Al,In,Ga)N or InP mesas or (Al,In,Ga)N / (Al,In,Ga)N or InP / InP mesas, the mesas (120) being electrochemically porosified according to the following step cycle: i) applying a first potential for a first time, ii) applying a second potential for a second time, thereby obtaining porosified mesas (120) comprising, from the lateral faces of the mesas (120) towards the center of the mesas (120) or vice versa, a first portion (123a) having a first degree of porification and a second portion (123b), the second portion (123b) having a second degree of porification greater than the first degree of porification or the second part (123b) being hollowed out.

2. The method according to claim 1, wherein the first potential is less than the second potential.

3. A method according to any one of claims 1 and 2, wherein the first potential is between 3 V and 12 V and / or wherein the second potential is between 5 and 20 V.

4. A method according to any one of the preceding claims, wherein step i) or the cycle of steps i) and ii) is repeated at least once so as to form an alternation of first parts (123a) and second parts (123b).

5. Structure (100) comprising a basic substrate (110) covered with porosified mesas (120), the porosified mesas (120) being (Al,In,Ga)N or InP mesas or (Al,In,Ga)N / (Al,In,Ga)N or InP / InP mesas, the mesas (120) comprising, from the lateral faces of the mesas (120) towards the center of the mesas (120) or vice versa, a first part (123a) having a first degree of porification and a second part (123b), the second part (123b) having a second degree of porification greater than the first degree of porification or the second part (123b) being hollowed out.

6. Structure according to claim 5, wherein the first part (123a) corresponds to the flank of the mesas (120) and the second part (123b) corresponds to the core of the mesas (120).

7. Structure according to claim 5, wherein the second part (123b) corresponds to the flank of the mesas and the first part (123a) corresponds to the core of the mesas.

8. Structure according to claim 5, wherein the mesas (120) comprise, from the lateral faces of the mesas (120) towards the center of the mesas (120) or vice versa, an alternation of first parts (123a) and second parts (123b).

9. Structure (100) according to any one of claims 5 to 8, wherein the base substrate (110) comprises a support layer (114), a first undoped GaN layer (111), a second doped GaN layer (112), a portion (112a) of the second doped GaN layer (112) extending into the mesas (120), the base substrate (110) being able to further comprise one or more additional conducting layers, preferably of heavily doped GaN, disposed between the first undoped GaN layer (111) and the second doped GaN layer (112).

10. Structure according to any one of claims 5 to 9, wherein the first porification rate is less than 10% and / or wherein the second porification rate is greater than or equal to 40%, preferably between 40 and 70%.

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