Method for manufacturing a photonic device

Abstract

Method for manufacturing a photonic device This description relates to a method for manufacturing a photonic device comprising the following steps: - providing on a structure comprising a basic substrate (110) covered by mesas (120) (Al,In,Ga)N / (Al,In,Ga)N, a first mesa (120a) being entirely porosified and having flanks covered by a protective layer (140), a second mesa (120b) being non-porified and a third mesa (120c) comprising porosified flanks (121) and a non-porified central part (122), - carrying out an epitaxy of an active structure (130) comprising InGaN-based quantum wells (132) simultaneously on the first mesa (120a), the second mesa (120b) and the third mesa (120c), to form respectively a first active structure (130a) emitting at a first wavelength,a second active structure (130b) emitting at a second wavelength and a third active structure (130c) emitting at a third wavelength. Figure for the abstract: Fig. 1B,

Description

Title of the invention: Method for manufacturing a photonic device. Technical field.

[0001] This description relates generally to the general field of photonic devices, and more particularly to micro-color displays with native red, green, blue emissions as well as shortwave infrared devices.

[0002] The invention relates to such photonic devices and their manufacturing processes. Previous technique

[0003] 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.

[0004] Blue and green pixels can be made from nitride materials, and red pixels from phosphide materials. To combine these three types of pixels on the same substrate, the "pick and place" technique is generally used. However, in the case of microdisplays with pixels smaller than 10 pm, this technique can no longer be used, particularly due to alignment problems. For displays with a large number of pixels (high resolution), this "pick and place" technique is problematic in terms of time. Furthermore, the pixels must be picked from different wafers, which requires several successive transfers. Parallel transfer techniques ("mass transfer") can also be used.

[0005] Another solution involves performing color conversion using quantum dots (QDs) or nanophosphors pumped by blue pLEDs from a single wafer, either depositional 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.

[0006] It is therefore crucial to be able to obtain the three RGB pixels natively using the same family of materials grown on the same substrate. 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 emission in the red, 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 high compressive stress inherent in the growth of the active InGaN region on GaN.

[0007] It is therefore essential to be able to reduce the overall stress in GaN / InGaN based structures.

[0008] Currently, one of the most promising solutions consists of porosifying the GaN layer of mesas by electrochemical means, as described for example in the two articles by Pasayat et al. (Materials 2020, 13, 213 and Appl. Phys. Express 2021, 14, 011004).

[0009] The porous GaN layer thus obtained can be used to grow an InGaN-based nitride LED structure, thanks to the relaxation of the generated porous mesas. However, only red emission was observed.

[0010] Recently, it has been shown that relaxing the InGaN layer preceding the quantum wells is advantageous for increasing the In incorporation rate of the latter. The more relaxed the lower InGaN layer, the higher the In incorporation rate in the upper layer (EP3840065 Al and EP3840016 Al). Depending on the amount of indium incorporated into the InGaN-based quantum wells, different emitted wavelengths can be obtained.

[0011] Currently, this modification of the indium concentration is controlled during epitaxy and / or by the lattice parameter α of the substrate. To obtain different wavelengths, it is therefore necessary to perform either several successive epitaxies or to have mesas with different lattice parameters α on the same substrate.

[0012] However, to obtain the red emission, it is necessary to achieve a very large lattice parameter in the plane (typically 3.238 Å). But such a lattice parameter is difficult to achieve. Summary of the invention

[0013] There is a need to propose a method for manufacturing a photonic device that allows obtaining pixels having different wavelengths, of which at least one wavelength is preferably in the red or even in the short infrared.

[0014] This goal is achieved by a method for manufacturing a photonic device comprising the following steps: - provide a structure comprising a basic substrate covered by (Al,In,Ga)N / (Al,In,Ga)N mesas, a first mesa being entirely porosified and having flanks covered by a protective layer, a second mesa being non-porous, and a third mesa comprising porosified flanks and a non-porous central part, - to perform an epitaxy of an active structure comprising, for example, InGaN-based quantum wells, simultaneously on the first mesa, the second mesa and the third mesa, whereby the active structure on the first mesa emits at a first wavelength, the active structure on the second mesa emits at a second wavelength and the active structure on the third mesa emits at a third wavelength, the first wavelength being greater than the second and the third wavelength, the third wavelength being shorter than the second wavelength.

[0015] According to a particular embodiment, the protective layer is made of silicon nitride.

[0016] According to a particular embodiment, the pores of the first mesa and / or the third mesa have a diameter greater than 20 nm and less than 100 nm.

[0017] According to a particular embodiment, a protective layer, preferably made of silicon nitride, is disposed on the sides of the second mesa and / or on the base substrate between the mesas.

[0018] According to a particular embodiment, the structure is obtained according to the following steps: - provide a base substrate covered by (Al,In,Ga)N / (Al,In,Ga)N mesas, the mesas comprising a layer of heavily doped (Al,In,Ga)N and a layer of undoped or weakly doped (Al,In,Ga)N, - apply a first insulating layer, for example resin, to the first mesa and to the second mesa, - to partially porosify the third mesa, whereby the third mesa has porosified flanks and a non-porified central part, - remove the first insulating layer, - apply a second insulating layer, for example resin, on the second mesa and on the third mesa, - to completely porosify the first mesa, - remove the second insulating layer, - apply the protective layer to the sides of the first mesa.

[0019] According to a particular embodiment, the structure is obtained according to the following steps: - provide a basic substrate covered by a stack comprising a heavily doped (Al,In,Ga)N layer and an undoped or lightly doped (Al,In,Ga)N layer, - to apply an insulating layer, for example resin, to the stack at the level of the first mesa and the sides of the third mesa, - implant the parts of the heavily doped (Al,In,Ga)N layer not covered by the insulating layer, so as to reduce the doping of the uncovered areas, whereby the (Al,In,Ga)N layer comprises heavily doped areas and undoped or lightly doped areas, - remove the insulating layer and, preferably, perform thermal annealing, - deposit an additional insulating layer, for example of resin, openings being formed in the additional insulating layer at the first mesa, the second mesa and the third mesa, - etch the stack through the openings of the additional insulating layer, so as to form the first mesa, the second mesa and the third mesa, - optionally, remove the additional insulating layer, - porosify the heavily doped areas of the (Al,In,Ga)N layer, - deposit the protective layer on the first mesa.

[0020] This goal is also achieved by a photonic device comprising a basic substrate covered by (Al,In,Ga)N / (Al,In,Ga)N mesas, a first mesa being entirely porosified and whose sides are preferably covered by a protective layer, for example, of silicon nitride, a second mesa not being porosified and a third mesa comprising porosified sides and a non-porified central part, the first mesa, the second mesa and the third mesa being covered, respectively, by a first active structure emitting at a first wavelength, a second active structure emitting at a second wavelength and a third active structure emitting at a third wavelength, the first wavelength being greater than the second and the third wavelength, the third wavelength being less than the second wavelength.

[0021] According to a particular embodiment, the device being a micro-screen with native red, green and blue emissions.

[0022] According to a particular embodiment, the wavelengths are in the infrared.

[0023] According to a particular embodiment, the base substrate comprises a support layer, a first undoped GaN layer, optionally an additional heavily doped GaN layer, and a second doped GaN layer. Brief description of the drawings

[0024] 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:

[0025] Fig.1A and Fig.1B represent schematically different stages of a manufacturing process for a photonic device according to a particular embodiment of the invention;

[0026] Fig. 2A and Fig. 2B schematically represent different stages of a manufacturing process for a photonic device according to another particular embodiment of the invention;

[0027] [Fig.3A], [Fig.3B], [Fig.3C] and [Fig.3D] schematically represent different stages of a mesas porosification process according to another particular embodiment of the invention;

[0028] [Fig.4A], [Fig.4B], [Fig.4C], [Fig.4D], [Fig.4E] and [Fig.4F], schematically represent different stages of a mesa porosification process according to another particular embodiment of the invention;

[0029] [Fig.5] represents, schematically and in cross-section, an LED structure epitaxially mounted on a non-porous mesa, according to a particular embodiment of the invention;

[0030] [Fig.6] is a photograph obtained by scanning electron microscopy of a completely porosified mesa, obtained according to a particular embodiment of the invention;

[0031] [Fig. 7] is a scanning electron microscope image of a non-porous mesa, obtained according to a particular embodiment of the invention; and

[0032] [Fig.8] represents different photoluminescence measurements, at room temperature (typically between 20 and 25°C), on porous mesas (denoted P on [Fig.8]) and non-porous mesas (denoted NP on [Fig.8]) on which the same LED structure was epitaxially grown during the same epitaxy, according to different particular embodiments of the invention. Description of the implementation methods

[0033] 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.

[0034] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been represented and are detailed.

[0035] 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.

[0036] Unless otherwise specified, the expressions "approximately", "roughly", and "in the order of" mean within 10%, preferably within 5%.

[0037] Unless otherwise specified, between X and Y means that the terminals X and Y are included in the range.

[0038] 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.

[0039] Typically, an emission in the green is at a wavelength between 500 and 550 nm, an emission in the red is at a wavelength between 600 and 650 nm and an emission in the blue is at a wavelength between 420 and 480 nm.

[0040] The invention is particularly interesting for manufacturing micro-screens for augmented and virtual reality.

[0041] However, it could be used for the manufacture of LEDs or lasers emitting at long wavelengths. In particular, the invention finds applications for devices operating in the short-wave infrared (SWIR). Short-wave infrared is defined as wavelengths between 0.7 and 1.7 pm, and especially between 0.9 and 1.7 pm.

[0042] The invention is particularly interesting for pixels whose dimensions are less than 10 pm.

[0043] The process is particularly interesting for manufacturing structures comprising (Al,In,Ga)N / (Al,In,Ga)N mesas having, in particular, a pitch of less than 30 pm.

[0044] By (Al,In,Ga)N / (Al,In,Ga)N mesa, it is understood that the mesas comprise two (Al,In,Ga)N layers. The (Al,In,Ga)N / (Al,In,Ga)N mesas comprise a lower layer 124 of (Al,In,Ga)N and an upper layer of (Al,In,Ga)N that is undoped or lightly doped 125. The upper layer is the one on which the epitaxy is performed. It is not, or only slightly, affected by the porification step. It remains continuous and dense. It ensures the quality of the epitaxially treated layer, such as a (In,Ga)N layer.

[0045] (Al,In,Ga)N refers to AIN, AlGaN, InGaN, or GaN. Hereafter, porous GaN is specifically mentioned, but with this 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. It is also possible to replace the GaN in the mesas with a mixture of GaN and InGaN. In this case, porous GaN and porous InGaN would be produced, particularly for the mesa, which will be completely porosified.

[0046] With reference to Figures IA and IB and Figures 2A and 2B, we will now describe in more detail the manufacturing process for a photonic device. The process comprises at least the following steps: i) provide on a structure comprising a base substrate 110 covered by mesas 120 (Al,In,Ga)N / (Al,In,Ga)N, a first mesa 120a being entirely porosified and having its sides covered by a protective layer 140, a second mesa 120b not being porosified and a third mesa 120c comprising porosified sides 121 and a non-porified central part 122 ([Fig.1A], [Fig.2A]), ii) deposit by epitaxy an active structure 130, typically an LED structure, including in particular InGaN-based quantum wells 132, simultaneously on the first mesa 120a, the second mesa 120b and the third mesa 120c ([Fig.1B], [Fig.2B]).

[0047] The same active structure 130 is deposited simultaneously on the three mesas 120a, 120b, 120c.

[0048] The amount of indium incorporated on the surface of the mesas 120 varies depending not only on the relaxation level of the mesa 120 but also on the porosity of the flanks 121 of the mesas 120. Indeed, the pores have the capacity to trap indium atoms during the growth of the epitaxial layer. Controlling the exposure of the pores to the growth fluxes allows for modulation of the indium concentration on the surface of the mesas. Thus, after a single regrowth of epitaxy, three types of mesas 120a, 120b, and 120c are obtained, emitting at three different wavelengths.

[0049] By completely porosified, we mean that the lower layer 124 of the first mesa 120a is entirely porosified: its entire volume is porosified.

[0050] By non-porified, it is understood that the lower layer 124 of the second mesa 120b is not porosified.

[0051] The third mesa 120c is partially porosified. The flanks 121 of the lower layer 124 of the third mesa 120c are porosified and the core 122 of the lower layer 124 of the third mesa 120c is not porosified.

[0052] For readability, each diagram includes three mesas, one of each type. It is evident that the basic substrate 110 can be covered by three groups of mesas, each group of mesas comprising several mesas of one of the three types (i.e., a first group of completely porosified mesas 120a, a second group of non-porified mesas 120b, and a third group of partially porosified mesas 120c).

[0053] By flank, we mean the lateral part of the lower layer of the mesas 120. This part extends from the edge of the lower layer of the mesas and extends over a thickness, for example, of 50 to 200 nm towards the center of the mesa (typically for a mesa of 3 pm width). The ratio between the thickness of the flank and the width of the mesa is, for example, between 1 and 20%, preferably between 1 and 10%.

[0054] The process thus makes it possible to modulate the incorporation of indium according to the rate of porification of the flanks 121 of the mesas 120 and the accessibility of the pores on the flanks of the mesas 120.

[0055] The first mesa 120 is completely porous and its sides 121 are protected by the protective layer 140. It will give, after resumption of epitaxy of the all InGaN structure, the emission at the longer wavelength because it will allow the highest rate of relaxation of the InGaN.

[0056] The second mesa 120b is non-porous. Its sides may be protected by a protective layer or unprotected. It will give emission at an intermediate wavelength, because the structure will be less relaxed than the structure of the first mesa 120a.

[0057] The third mesa 120c has porous flanks 121 and a non-porous core 122. Its flanks 121 are not protected by a protective layer. It will give emission at the shorter wavelength because some of the incident In atoms will be captured by the apparent porous flanks 121.

[0058] Thus, it is not necessary to achieve a very large lattice parameter in the plane for red emission and it is possible to obtain the three primary colors in a single epitaxy.

[0059] Typically, a lattice parameter in the plane of 3.212Â is sufficient for the completely porous 120a mesa emitting in the red (in particular for mesas having a height of at least 800 nm and a diameter less than 3 pm) versus 3.238Â as in the prior art.

[0060] We will now describe in more detail the different elements forming the structure provided in step i) and its manufacturing process.

[0061] As shown in [Fig.5], the structure provided in step i) comprises a basic substrate 110 covered by mesas 120 (Al,In,Ga)N / (Al,In,Ga)N.

[0062] The basic substrate 110 comprises successively: - a support substrate 114, - possibly a buffer layer 115 in (Al,Ga)N, particularly in the case of a support layer 114 in silicon, - a first layer of undoped GaN 111, - possibly an additional layer of heavily doped GaN (not shown), - a second layer of doped GaN 112.

[0063] The mesas 120 (Al,In,Ga)N / (Al,In,Ga)N arranged on the base substrate 110 comprise a third layer of (Al,In,Ga)N 124 (i.e. the lower layer of the mesa) and a fourth layer of (Al,In,Ga)N undoped or weakly doped 125 (i.e. the upper layer of the mesa).

[0064] The support substrate 114 is, for example, made of sapphire or silicon.

[0065] 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.

[0066] 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.

[0067] 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 GaN, we mean a concentration of less than 117 at / cm3.

[0068] 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.

[0069] The second layer 112 is a doped GaN layer. By doped GaN, we mean a concentration, preferably greater than 1 x 10¹⁷ at / cm³ and preferably less than 5 x 10¹⁰ at / cm³ and even more preferably less than 1 x 10¹⁰ at / cm³. The presence of the electrically conductive lines allows for a GaN layer whose 173 173 17 concentration is less than 5.10 at / cm and, for example, between 1.10 at / cm and 5.10 at / cm3.

[0070] Insofar as the second doped layer 112 completely covers the additional layer, it ensures the protection of the underlying heavily doped additional layer from any porification, and also ensures contact.

[0071] This layer is advantageously as thin as possible (for example, it has a thickness between 400 and 800 nm) while remaining well covering to prevent the infiltration of the electrolyte and therefore the consumption of the additional underlying layer which is heavily doped during porosification.

[0072] The second GaN layer 112 has, for example, a thickness ranging from 200nm to Ipm, preferably between 400 and 700nm. The minimum thickness varies depending on the doping level.

[0073] The additional layer of heavily doped GaN (not shown) is arranged between the first layer 111 of undoped GaN and the second layer 112 of doped GaN. The additional heavily doped layer ensures the lateral conduction of charges in the structure.

[0074] The additional heavily doped layer is advantageously thick (typically between 0.5 µm and 5 µm and preferably between 1 and 2 µm). It has, for example, a thickness of 2 µm.

[0075] For example, the doping level of the additional layer of heavily doped GaN is between 5 x 10¹⁸ at / cm³ and 2 x 10¹⁹ at / cm³, preferably between 5 x 10¹⁸ at / cm³ and 1.5 x 10¹⁹ at / cm³, and even more preferably between 8 x 10¹⁹ at / cm³ and 1 x 10¹⁹ at / cm³. The doping level is, for example, 1 x 10¹⁹ at / cm³.

[0076] The structure thus comprises a bilayer or a trilayer comprising two or three layers based on doped GaN having different doping levels. The bilayer or trilayer is positioned between the first undoped GaN layer 111 of the substrate and the fourth undoped or lightly doped (Al,In,Ga)N layer 125 of the mesas 120. The second layer 112 has a lower doping level than the additional layer and can be used to make the electrical contact.

[0077] The third layer 124 is, at the beginning of the fabrication process of the structure provided in step i), a heavily doped GaN layer. By heavily doped GaN, we mean a concentration greater than 5 x 10¹⁸ at / cm³, preferably greater than 8 x 10¹⁸ at / cm³, or even greater than 10¹⁹ at / cm³. It has, for example, a doping level ten times higher than the second layer 112. It has a thickness of between 200 nm and 2 pm, preferably from 500 nm to 1 pm. Preferably, the doping level of the third layer 124 is 30-fold or even 100-fold higher than the doping level of the second layer 112.

[0078] As we will see later, the doping of the third layer can be reduced or even eliminated at the level of the core 122 and / or the flanks 121 of the mesa. The third layer is then completely or partially undoped or lightly doped. Lightly doped means a doping level between 1 x 10⁻¹⁷ at / cm³. Undoped means a doping level below 1 x 10¹⁷ at / cm³.

[0079] Furthermore, depending on the mesas and their manufacturing process, the third layer of (Al,In,Ga)N 124 of the structure provided in step i) can comprise: - a core 122 and highly doped porosified flanks 121 (first mesa 120a), - a core 122 and non-porified flanks 121, the core 122 and flanks being able to be highly doped or undoped or weakly doped (second mesa 120b), - a non-porified core 122 and highly doped porosified flanks 121, the core being able to be highly doped or undoped or weakly doped (third mesa).

[0080] The fourth layer 125 is an (Al,In,Ga)N layer that is either unintentionally doped or lightly doped. By lightly doped (Al,In,Ga)N, we mean a doping level between 1 "7 1 Q 1.10 at.cm and 1.10 at.cm. By undoped, we mean a doping level of less than 1.1017at / cm3.

[0081] 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).

[0082] This layer 125 is not or only slightly affected by porification and serves as a nucleation layer for resumption of growth. This layer 125 is continuous to ensure the quality of the repitaxed layer, for example a (In,Ga)N layer, on the structure.

[0083] According to another embodiment, the first part of the n-InGaN buffer layer 131 of the active structure 130 can be formed before the formation of the mesas 120. This first part can have a thickness between 50 and 200 nm. This first part is unintentionally doped (nest). According to this embodiment, the nest-GaN layer is not necessary. The second part of the n-InGaN buffer layer will be formed upon resumption of epitaxy on the mesas.

[0084] The tensions applied during the porosification will be chosen according to the doping of the different layers mentioned above, and in particular of the second layer 112 and the third layer 124.

[0085] In particular, the respective doping levels are chosen so that, at a given potential, there is selectivity between the heavily doped region and the lightly doped region. For a given potential, the doping level of the second layer 112 is chosen so that the second layer 112 is not porosified during step d), and the doping level of the third layer 124 is chosen so that the third layer 124 is porosified during step d).

[0086] The sides of the first mesa 120a are covered by a protective layer 140. This is, for example, a dielectric material. Preferably, the material is a nitride, even more preferably a silicon nitride. The thickness of the protective layer is, for example, between 20 and 100 nm, preferably between 20 and 50 nm.

[0087] Subsequently, an n-type doping is described, but it could be a p-type doping. A person skilled in the art will choose the porosification parameters according to the type of doping.

[0088] Mesas 120, also called elevations, are relief features.

[0089] Preferably, the flanks of the mesas 120 are perpendicular to this stacking of layers.

[0090] The surface of the mesas can be circular, hexagonal, square or rectangular. Preferably, it is circular. The diameter of the mesas is, for example, between 3 and 4 pm.

[0091] The height of the mesas is, for example, between 0.8 and 1.2 pm.

[0092] The thickness of the mesas corresponds to the dimension of the mesa perpendicular to the underlying stack.

[0093] The pitch (or period of the motifs) can range from a few micrometers to a few tens of micrometers. It is preferably between 50 nm and 20 pm. Even more preferably, it is between 1 and 10 pm. For example, it is 5 pm.

[0094] The structure provided in step i) can be obtained according to different embodiment variants.

[0095] According to a first embodiment, for example shown in Figures 3A to 3E, the process for porosifying mesas 120 of (Al,In,Ga)N / (Al,In,Ga)N comprises the following steps: a) provide a basic substrate 110 covered by non-porous mesas (120) (Al,In,Ga)N / (Al,In,Ga)N ([Fig.3A]), the mesas comprising a layer of doped (Al,In,Ga)N 124 covered by a layer of undoped (Al,In,Ga)N 125, b) apply a first insulating layer 201, for example resin, on the first mesa 120a and on the second mesa 120b, c) partially porosify the third mesa 120c, whereby the third mesa 120c has porosified flanks 121 and a non-porified central part 121 ([Fig.3B]), d) remove the first insulating layer 201, e) apply a second insulating layer 202, for example made of resin, on the second mesa 120b and on the third mesa 120c, f) completely porosify the first mesa 120a ([Fig.3C]), g) remove the second insulating layer 202, h) deposit the protective layer 140 on the first mesa 120a ([Fig.3D]).

[0096] This gives us the structure of [Fig. 1 A].

[0097] During step a), the mesas 120 are already formed. They are obtained, for example, by etching the fourth layer 125 and the third layer 124. The etching is carried out in such a way as to leave only a certain number of "reliefs" formed from these layers.

[0098] The etching is preferably carried out with a hard mask that exhibits favorable selectivity with the etching rate of the GaN layers (typically with an etching rate ratio > 1 / 4). The hard mask is, for example, made of SiO2. After etching the mesas, this hard mask is removed by a wet chemical process.

[0099] The etching is, for example, a chlorinated plasma etching.

[0100] 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.

[0101] In step b), a first insulating layer 201, preferably made of resin, is deposited. It is, for example, deposited in such a way as to expose the third mesa 120c. It protects the first mesa 120a and the second mesa 120b.

[0102] The resin can be replaced by any electrically insulating material, provided that it is compatible with the porosification conditions and with the level of contamination required by the epitaxy equipment.

[0103] During step c), the flanks 121 of the third mesa 120c are porosified. The duration is chosen so as to porosify only the flanks 121 of the mesa. For example, the porosification is carried out for a few seconds. This leads to the formation of large pores (typically about 100 nm long in the direction perpendicular to the c axis).

[0104] After removal of the first insulating layer 201 in step d), a second insulating layer 202 (made of resin or another insulating material) is deposited (step e)). It covers the second mesa 120b and the third mesa 120c.

[0105] During step f), the first mesa 120a is completely porosified. The duration is chosen so as to porosify the entire layer 124 of the mesa. This leads to the formation of large pores (typically about 100 nm long in the direction perpendicular to the c-axis).

[0106] During step g), the second insulating layer 202 is removed.

[0107] During step h), a protective layer 140 is deposited on the sides of the first mesa 120a and, possibly at the back of the mesa.

[0108] The order of steps g) and h) can be reversed.

[0109] The protective layer 140 is deposited on the sides of the totally porous mesa 120a, in order to maximize the incorporation of indium at the top of these mesas, via the relaxation of InGaN obtained by the porosification of the mesas.

[0110] During this step, the protective layer 140 can also be deposited on the sides of the second non-porous mesa 120b.

[0111] The protective layer 140 can be deposited on the substrate between the mesas 120. It is also possible to deposit it on the substrate at the bottom of the mesas for mesas 120a and 120b, but not for mesas 120c.

[0112] The choice of positioning of the protective layer 140 (sides and bottom of mesas) allows playing on the effect of indium capture by the pores.

[0113] With this process, the first mesa 120a comprises a highly doped porosified (Al,In,Ga)N 124 layer.

[0114] The second mesa 120b comprises a highly doped and non-porified (Al,In,Ga)N 124 layer.

[0115] The third mesa 120c is partially porosified. Part of the (Al,In,Ga)N 124 layer is heavily doped and forms the core 122, and the other part of the (Al,In,Ga)N 124 layer is heavily doped and porosified and forms the flanks 122.

[0116] According to a second embodiment, for example shown in Figures 4A to 4F, the structure is obtained according to the following steps: a') provide a basic substrate 110 covered by a stack comprising a heavily doped (Al,In,Ga)N layer 124 and an undoped (Al,In,Ga)N layer 125, b') deposit an insulating layer 211 on the undoped (Al,In,Ga)N layer 125 at the first mesa 120a and the sides 121 of the third mesa 120c ([Fig. 4A]), c') implant the areas not covered by the insulating layer 201, so as to reduce their doping ([Fig. 4B]), thereby the (Al,In,Ga)N layer 124 comprises heavily doped and undoped areas, d') perform annealing and remove the resin 201, ([Fig. 4C]), e') deposit another insulating layer 212, for example of resin, over the openings being formed in this other insulating layer 212 at the right of the first mesa 120a, the second mesa 120b and the third mesa 120c, f') etch the stack through the openings in the resin 212, so as to form the first mesa 120a, the second mesa 120b and the third mesa 120c ([Fig.4D]), g') remove the other insulating layer 212, h') porosify the heavily doped areas ([Fig.4E]), i') deposit the protective layer 140 on the first mesa 120a ([Fig.4F]).

[0117] This second variant is based on implementing an implantation to reduce or even eliminate doping in selected areas so as to obtain highly doped areas and undoped or weakly doped areas. Only the highly doped areas will be porosified.

[0118] Alternatively, an embodiment can be carried out incorporating a doping step, for example of type n, by implantation. For such an embodiment, a nest layer is implanted, for example with silicon, in the areas to be porosified. Annealing is then performed.

[0119] The number of technological steps (in particular resin deposition and removal) is reduced and the desired final structure is obtained in a single porification step.

[0120] During step b'), an insulating layer 211 is deposited. It has openings over the areas where implanting is to take place. The areas protected by the insulating layer 211 will not be implanted during step c'). This is, for example, a Si3N4 layer that may have a thickness of 20 nm. It will be removed before the epitaxial process resumes.

[0121] During step c'), an implantation is performed. The implanted areas will then be de-doped. For example, for n-doped layers, it is possible to implant helium to make these areas less doped or undoped.

[0122] During step d'), annealing is performed. Annealing helps to heal defects created during implantation. These defects could alter the porification rate during step h').

[0123] During step e'), another insulating layer 212 is deposited. Openings are formed in the resin at the first mesa 120a, the second mesa 120b and the third mesa 120c.

[0124] Then during step f'), the stack is engraved to form the mesas 120.

[0125] Step g') can be carried out after step h') or after step i').

[0126] During step h'), porosification is carried out. Thanks to the modulation of the doping by implantation, only the non-implanted parts are porosified, thus obtaining the three types of mesa 120 desired (i.e., a first mesa 120a that is completely porous, a second mesa 120b that is non-porous, and a third mesa 120c that has a non-porous center and porous flanks). The duration is chosen so as to completely porosify the first mesa 120a.

[0127] During step i'), a protective layer 140 is deposited on the sides of the first porous mesa 120a. This maximizes the incorporation of indium at the top of these mesas during the epitaxy of the active zone (LED).

[0128] A protective layer may also be deposited or not on the sides of the second non-porous mesa 120b and / or between the mesas.

[0129] This gives us the structure shown in [Fig.2A].

[0130] With this process, the (Al,In,Ga)N 124 layer of the first mesa is a porosified layer.

[0131] The (Al,In,Ga)N 124 layer of the second mesa 120b is a non-porified layer.

[0132] For the third mesa, the flanks 121 of the (Al,In,Ga)N 124 layer are porosified. The core 122 of the (Al,In,Ga)N 124 layer is not porosified.

[0133] For the different embodiments, the various porosification steps described above can be carried out according to the following substeps: - electrically connect the structure 100 and a counter electrode to a voltage or current generator, - immerse the structure 100 and the counter electrode in an electrolytic solution, - apply a voltage or current between the second layer of doped GaN 112 and the counter electrode so as to partially or totally porosify the third layer of (Al,In,Ga)N heavily doped 124 of the mesas 120.

[0134] Structure 100 and a counter electrode (CE) are electrically connected to a voltage or current generator. The device acts as a 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.

[0135] The initial contact is made on structure 100.

[0136] In particular, contact can be made on the base substrate 110. Re-establishment of contact can be made on the second doped GaN layer 112. Re-establishment of contact can be made on the bottom of the mesas, at the level of the second layer 112, which allows the etching step to also be used to establish the contacts. Alternatively, it is possible to establish the re-establishment of contact on the electrically conductive lines 200.

[0137] It is also possible to make contact on one of the other layers: on the fourth layer of (Al,In,Ga)N weakly doped 125 or on the third layer of (Al,In,Ga)N strongly doped 124. In the case of re-establishing contact on the strongly doped layer, its opening will advantageously be limited to a preserved area of ​​the electrolyte.

[0138] The contact re-establishment zone can also be covered with a metallic layer to improve contact for electrochemical polarization. This contact will be removed after porosification before epitaxial re-establishment.

[0139] 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.

[0140] The electrodes are immersed in an electrolyte, also called an electrolytic bath or electrolytic solution. The electrolyte can be acidic or basic. The electrolyte is, for example, oxalic acid. It can also be KOH, HF, HNO3, NaNO3, or H2SO4.

[0141] The voltage applied between structure 100 and the counter electrode can range from 1 to 50 V, for example. Preferably, it is from 7 to 16 V. The voltage is chosen according to the doping levels of the different layers, in order to obtain the desired selectivity. It is applied, for example, for a duration ranging from a few seconds to several hours.

[0142] Monitoring the chrono-amperometry curve during porosification shows a significant decrease in current when the layer to be porosified is completely porosified.

[0143] For example, the porosification step is carried out by applying a voltage in an oxalic acid solution. The stopping of the process is controlled by the current drop.

[0144] The electrochemical anodizing step can be carried out under ultraviolet (UV) light.

[0145] 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.

[0146] The resulting porosity (porosity rate and pore size) depends on the layer doping and the process parameters (applied voltage, duration, nature and concentration of the electrolyte, chemical post-treatment or annealing). Varying the porosity allows control of the incorporation / segregation rate. The porosity, and in particular the pore size, can vary subsequently during the resumption of epitaxy as a function of the applied temperature.

[0147] The mesas 120 of the same plate are thus porosified, whether for substrates of small or medium dimensions or for substrates of large dimensions.

[0148] The bottom of the mesas can be preserved from porosification or porosified.

[0149] In step ii), an epitaxy is performed on the mesas 120, whereby we obtain an epitaxial layer that is at least partially relaxed, and preferably totally relaxed.

[0150] The percentage of relaxation corresponds to:

[0151] Aa / a = ( a es- a ci

[0152] with a^, the lattice parameter of the starting layer on which the epitaxy is resumed (i.e., the lattice parameter of layer 125), and

[0153] ac 2 the mesh parameter of the relaxed layer,

[0154] 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.

[0155] When aci=ac2 the layer is said to be constrained.

[0156] By partially relaxed, we mean a percentage of relaxation greater than 50%.

[0157] Epitaxial resumption can be used, for example, to form re-epitaxial LEDs.

[0158] Epitaxial regrowth is performed on the fourth layer 125 of (Al,In,Ga)N / (Al,In,Ga)N nest or weakly doped mesas 120. As 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 creation of defects related to pore coalescence is avoided.

[0159] The epitaxial layer in this step iv) is advantageously made of gallium nitride or indium gallium nitride.

[0160] The same epitaxial process (i.e. the same epitaxial conditions) is applied to the 3 mesas, thus generating 3 different structures emitting at 3 different wavelengths.

[0161] The epitaxial conditions are, for example, chosen to generate an all-InGaN green LED structure on the 130b mesas. These same conditions will generate other structures on the neighboring mesas. The three mesas will not have the same final structure (different thickness, indium concentration, and emission wavelength).

[0162] This includes, in particular, an all-InGaN LED structure. The LED structure 130 comprises, for example, an n-InGaN buffer layer 131, an active region 132 of InGaN / (Ga,In)N including quantum wells (QW), a p-InGaN layer 134, and a heavily doped InGaN layer 135. The growth conditions are adapted to obtain the desired wavelength. In particular, growth conditions will be chosen to obtain green emission on the second non-porous mesa.

[0163] As shown in [Fig. 5], the LED structure of the mesa 130b comprises, for example, the following layers: - a buffer layer 131 InxGabxN doped nx between 7 and 8% (having for example a thickness of 400 nm), - an active zone 132 comprising quantum wells, the active zone comprising for example a 5 x InyGai yN / InxGai_xN multilayer with y = 25% (having for example thicknesses of 2.5nm / 6nm), - a p133 doped AlGaN layer ('electron blocking layer' (EBL)) (having for example a thickness of 20 nm), - a layer of p 134 doped InGaN (having for example a thickness of 150 nm), - a layer of p++ 135 doped InGaN (having for example a thickness of 20 nm).

[0164] The modulation of indium incorporation from one type of mesa to another is possible through the exposure or exclusion of porous zones to incident fluxes. The pores, in particular, trap some of the indium atoms.

[0165] Since, on the one hand, the percentage of porosity differs in each mesa, and, on the other hand, the accessibility of the pores differs, the amount of indium incorporated will be different. Thus, the first LED structure 130a on the first mesa 120a emits at a first wavelength, the second LED structure 130b on the second mesa 120b emits at a second wavelength, and the third LED structure 130c on the third mesa 120c emits at a third wavelength. The first wavelength is longer than the second and third wavelengths. The third wavelength is shorter than the second wavelength.

[0166] The temperatures used during epitaxy are, for example, between 700 and 850°C. Preferably, they are between 750 and 850°C for the n- InGaN and p-InGaN, between 700 and 800°C for the quantum wells of the active zone, and between 750 and 850°C for the barriers of the active zone.

[0167] By way of illustration and not limitation, the In concentration of the InGaN layer epitaxially grown on porous mesas ([Fig. 6]) and on non-porous mesas ([Fig. 7]) with visible pores in the flanks and intermesas were compared. For porous mesas, an In concentration of 3% was obtained, and for non-porous mesas, more than 6% indium. A factor of 2 was observed for the same epitaxy. This was not the case if a SiN liner protected the flanks of the porous mesas and the intermesas.

[0168] These observations were confirmed by photoluminescence measurements on both types of samples at room temperature ([Fig.8]).

[0169] 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.

[0170] 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 manufacturing a photonic device comprising the following steps: - providing a structure comprising a base substrate (110) covered by (Al,In,Ga)N / (Al,In,Ga)N mesas (120), a first mesa (120a) being fully porosified and having flanks covered by a protective layer (140), a second mesa (120b) being non-porified, and a third mesa (120c) comprising porosified flanks (121) and a non-porified central portion (122), - carrying out an epitaxy of an active structure (130) comprising, for example, InGaN-based quantum wells (132), simultaneously on the first mesa (120a), the second mesa (120b), and the third mesa (120c), thereby activating the active structure (130a) on the first mesa (120a) emits at a first wavelength,The active structure (130b) on the second mesa (120b) emits at a second wavelength and the active structure (130c) on the third mesa (120c) emits at a third wavelength, the first wavelength being greater than the second and third wavelengths, the third wavelength being less than the second wavelength.

2. Method according to claim 1, wherein the protective layer (140) is made of silicon nitride.

3. A method according to any one of claims 1 and 2, wherein the pores of the first mesa (120a) and / or the third mesa (120c) have a diameter greater than 20 nm and less than 100 nm.

4. A method according to any one of claims 1 to 3, wherein a protective layer, preferably of silicon nitride, is disposed on the sides of the second mesa (120b) and / or on the base substrate between the mesas (120).

5. A method according to any one of claims 1 to 4, wherein the structure is obtained according to the following steps: - providing a basic substrate (110) covered by mesas (120) (Al,In,Ga)N / (Al,In,Ga)N, the mesas comprising a layer of heavily doped (Al,In,Ga)N (124) and a layer of undoped or lightly doped (Al,In,Ga)N (125), - deposit a first insulating layer (201), for example of resin, on the first mesa (120a) and on the second mesa (120b), - partially porosify the third mesa (120c), whereby the third mesa (120c) has porosified sides (121) and a non-porified central part (122), - remove the first insulating layer (201), - deposit a second insulating layer (202), for example of resin, on the second mesa (120b) and on the third mesa (120c), - completely porosify the first mesa (120a), - remove the second insulating layer (202), - deposit the protective layer (140) on the sides of the first mesa (120a).

6. A method according to any one of claims 1 to 4, wherein the structure is obtained by the following steps: - providing a base substrate (110) covered by a stack comprising a heavily doped (Al,In,Ga)N layer (124) and an undoped or lightly doped (Al,In,Ga)N layer (125), - depositing an insulating layer (211), for example of resin, on the stack at the first mesa (120a) and the flanks of the third mesa (120c), - implanting the parts of the heavily doped (Al,In,Ga)N layer (124) not covered by the insulating layer (211), so as to reduce the doping of the uncovered areas, thereby the (Al,In,Ga)N layer comprises heavily doped areas and undoped or lightly doped areas, - removing the insulating layer (211) and, preferably, perform thermal annealing, - deposit an additional insulating layer (212), for example of resin,Openings being formed in the additional insulating layer (212) at the first mesa (120a), the second mesa (120b) and the third mesa (120c), - etch the stack through the openings in the additional insulating layer (212), so as to form the first mesa (120a), the second mesa (120b) and the third mesa (120c), - optionally, remove the additional insulating layer (212), - porosify the heavily doped areas of the (Al,In,Ga)N layer, - deposit the protective layer (140) on the first mesa (120a).

7. A photonic device comprising a base substrate (110) covered by mesas (120) (Al,In,Ga)N / (Al,In,Ga)N, a first mesa (120a) being entirely porosified and whose sides are preferably covered by a protective layer (140), for example, of silicon nitride, a second mesa (120b) not being porosified, and a third mesa (120c) comprising porosified sides (121) and a non-porified central portion (122), the first mesa (120a), the second mesa (120b), and the third mesa (120c) being covered, respectively, by a first active structure (130a) emitting at a first wavelength, a second active structure (130b) emitting at a second wavelength, and a third active structure (130c) emitting at a third wavelength, the first wavelength being greater than the second and third wavelengths, the third wavelength being less than the second wavelength.

8. Device according to claim 7, the device being a micro-screen with native red, green and blue emissions.

9. Device according to claim 7, wherein the wavelengths are in the infrared.

10. Device according to any one of claims 7 to 9, wherein the base substrate (110) comprises a support layer (114), a first undoped GaN layer (111), optionally an additional heavily doped GaN layer, and a second doped GaN layer (112).

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