Multi-colour electroluminescent display device and method for manufacturing such a device

A device including first and second light-emitting cells respectively emitting in first and second wavelength ranges, wherein:

This application is a national stage filing under 35 U.S.C. § 371 of International Patent Application Serial No. PCT/FR2019/052907, filed Dec. 3, 2019, which claims priority to French patent application FR18/72366, filed Dec. 5, 2018. The contents of these applications are incorporated herein by reference in their entireties.

TECHNICAL BACKGROUND

The present disclosure generally concerns light-emitting devices based on semiconductor materials and methods of manufacturing the same. It more particularly aims at the forming of a multi-color light-emitting display device, that is, comprising a plurality of pixels, among which pixels of different types are respectively capable of emitting light in different wavelength ranges.

PRIOR ART

A light-emitting display device conventionally comprises a plurality of pixels, each pixel being individually controllable to convert an electric signal into a light radiation. More particularly, each pixel comprises a light-emitting cell comprising a stack of a first doped semiconductor layer of a first conductivity type, of an active layer, and of a second doped semiconductor layer of the second conductivity type. In operation, an electric current is applied between the first and second semiconductor layers of the cell. Under the effect of this current, the active layer emits a light ray in a wavelength range which essentially depends on its composition. Each pixel may further comprise a control circuit, for example comprising one or a plurality of transistors, enabling to control the intensity of the current applied to the light-emitting cell of the pixel, and accordingly the intensity of the light radiation emitted by the pixel.

To enable to display multi-color images, the display device should comprise a plurality of pixels of different types capable of respectively emitting in different wavelength ranges.

To form a multi-color display device, a possibility is to transfer onto a same substrate pixels formed separately based on different semiconductor materials. The bonding and the alignment of the pixels on the transfer substrate may however be difficult to achieve. In particular, such a technique is not adapted to the forming of display devices having a small pitch between pixels, for example, smaller than 10 μm.

Another possibility is to form a display device where all the light-emitting cells of the pixels all emit in the same wavelength range, the light-emitting cells of certain pixels being coated with a color conversion layer, for example, a layer incorporating quantum dots or nanophosphors, capable of converting the light radiation emitted by the light-emitting cell into a light radiation in another wavelength range. The local deposition of color conversion layers may however be difficult to perform. Further, the lifetime of certain materials is relatively limited.

Another possibility is to successively form the light-emitting cells of the pixels of different types during distinct local epitaxy sequences, by modifying for each sequence the composition of the deposited layers to obtain pixels with a direct emission in different wavelength ranges. A disadvantage of this method however is its high cost, due to the implementation of a plurality of successive epitaxy sequences to successively form the light-emitting cells of the different types of pixels.

SUMMARY

There is a need for a multi-color light-emitting display device and for a method of manufacturing such a device, overcoming all or part of the disadvantages of known solutions.

Thus, an embodiment provides a light-emitting device comprising first and second light-emitting cells capable of respectively emitting in first and second wavelength ranges, wherein:each light-emitting cell comprises a stack of a first layer of a first semiconductor material and of a second layer of a second semiconductor material having a mesh parameter different from that of the first material;in the first cell, the first layer is in contact with the second layer across substantially the entire surface of the cell; andin the second cell, a mask provided with a plurality of through nano-openings regularly distributed across the entire surface of the cell forms an interface between the first layer and the second layer, the second layer comprising a plurality of nanopillars of the second material arranged in the nano-openings of the mask, in contact, by a first surface, with the first layer, and a coalesced layer of the second material in contact with a second surface of the nanopillars opposite to the first layer, extending across substantially the entire surface of the cell on the side of the mask opposite to the first layer.

According to an embodiment, in each cell, the first layer is doped with a first conductivity type, the second layer is topped with an active layer, and the active layer is topped with a doped semiconductor layer of the second conductivity type.

According to an embodiment, the first layer is made of gallium nitride and the second layer is made of indium gallium nitride.

According to an embodiment, the device comprises a third light-emitting cell identical to the first cell, wherein the third cell is topped with a wavelength conversion element, and the first and second cells are not topped with wavelength conversion elements.

According to an embodiment, the device further comprises a third light-emitting cell adapted to emitting in a third wavelength range, wherein:the third light-emitting cell comprises a stack of a first layer of the first semiconductor material and of a second layer of the second semiconductor material;in the third cell, a mask provided with a plurality of through nano-openings regularly distributed across the entire surface of the cell forms an interface between the first layer and the second layer;the thickness of the mask is larger in the third cell than in the second cell; andthe thickness of the second layer is greater than the thickness of the mask in the second cell and smaller than the thickness of the mask in the third cell.

According to an embodiment, each cell further comprises a third layer of a third semiconductor material topping the second layer, wherein:in each of the first and second cells, the third layer is in contact with the second layer across substantially the entire surface of the cell; andin the third cell, the third layer comprises a plurality of nanopillars arranged in the nano-openings of the mask, in contact, by a first surface, with the second layer, and a coalesced layer of the material of the third layer in contact with a second surface of the nanopillars opposite to the second layer, extending across substantially the entire surface of the cell on the side of the mask opposite to the first layer.

According to an embodiment, the third material has a mesh parameter difference with the second material, the mesh parameter difference between the second material and the third material being greater than the mesh parameter difference between the first material and the third material.

According to an embodiment, the first, second, and third layers are respectively made of gallium nitride, of aluminum gallium nitride, and of indium gallium nitride.

According to an embodiment, the first, second, and third layers are respectively made of gallium nitride, of indium gallium nitride, and of indium gallium nitride, the compositions of the second and third indium gallium nitride layers being different.

According to an embodiment, the device further comprises a third light-emitting cell adapted to emitting in a third wavelength range, wherein:the third light-emitting cell comprises a stack of a first layer of the first semiconductor material and of a second layer of the second semiconductor material;in the third cell, a mask provided with a plurality of through nano-openings regularly distributed across the entire surface of the cell forms an interface between the first layer and the second layer;the dimensions of the nano-openings of the mask are greater in the third cell than in the first cell.

Another embodiment provides a method of manufacturing a device such as defined hereabove, wherein the second layers of the different cells are simultaneously formed during a same epitaxy step.

According to an embodiment, the active layers of the different cells are formed simultaneously during a same epitaxy step.

DESCRIPTION OF THE EMBODIMENTS

For the sake of clarity, only the steps and elements that are useful for an understanding of the embodiments described herein have been illustrated and described in detail. In particular, the forming of the light-emitting cells of the pixels of a multi-color light-emitting display device is more particularly considered. The forming of the pixel control circuits and of possible structures of insulation between the light-emitting cells of the pixels and/or of connection of the light-emitting cells to the pixel control circuits has not been detailed, the described embodiments being compatible with the usual forming of these elements.

FIGS.1to6illustrate successive steps of an example of a method of manufacturing a multi-color light-emitting display device according to a first embodiment.

The forming of a display device comprising three types of different pixels, capable of respectively emitting in three different wavelength ranges, is considered herein as an example. More particularly, it is desired in the present example to form a display device comprising one or a plurality of pixels of a first type, called blue pixels, capable of mainly emitting blue light, for example, in a wavelength range from 400 to 490 nm, one or a plurality of pixels of a second type, called green pixels, capable of mainly emitting green light, for example, in a wavelength range from 490 to 570 nm, and one or a plurality of pixels of a third type, called red pixels, capable of mainly emitting red light, for example, in a wavelength range from 570 to 710 nm. InFIGS.1to6, a single blue pixel B, a single green pixel G, and a single red pixel R are shown, it being understood that, in practice, the display device may comprise a plurality of pixels of each type, the pixels of a same type being identical or similar to within manufacturing dispersions. As an example, the light-emitting cells of pixels R, G, and B have, in top view, the same general shape, for example, a square or rectangular shape, and the same lateral dimensions, for example, in the range from 1 to 20 μm, and preferably from 5 to 10 μm. The described embodiments are however not limited to this specific case.

In this example, in each pixel, the active layer of the light-emitting cell of the pixel comprises confinement means corresponding to multiple quantum wells. More particularly, the active layer comprises an alternation of semiconductor layers of a first material and of semiconductor layers of a second material, each layer of the first material being sandwiched between two layers of the second material, the first material having a bandgap narrower than that of the second material.

As an example, each of the first and second materials mainly comprises a III-V compound comprising at least a first group-III element, a second group-V element, and, possibly, a third element, for example, a group-III element other than the first element.

Examples of group-III elements comprise gallium (Ga), indium (In), or aluminum (Al). Examples of group-V elements comprise nitrogen, phosphorus, or arsenic. Examples of binary and ternary III-V compounds are GaN, AlN, InN, InGaN, AlGaN, or AlInGaN. Generally, the elements in the III-V compound may be combined with different molar fractions.

In a preferred embodiment, the first material is InGaN and the second material is GaN or InGaN having an indium concentration lower than that of the first material.

According to an aspect of an embodiment, it is provided to simultaneously form the light-emitting cells of the three types of pixels, during a same epitaxy step sequence.

FIG.1comprises a top view (A) and a cross-section view (B) along plane B-B of view (A) of an initial structure comprising a growth substrate101, for example, made of sapphire (Al2O3), of silicon carbide (SiC), of silicon (Si), of gallium nitride (GaN), or of aluminum nitride (AlN), a stack103of one or a plurality of buffer layers on top of and in contact with the upper surface of growth substrate101, and a crystalline N-type doped gallium nitride (GaN) layer105on top of and in contact with the upper surface of stack103. As an example, the stack of buffer layers103is formed by epitaxy on the upper surface of substrate101, and layer105is formed by epitaxy on the upper surface of stack103. In this example, stack103extends continuously and has a substantially uniform thickness over the entire surface of substrate101, and layer105extends continuously and has a substantially uniform thickness over the entire surface of stack103. The thickness of stack103is for example in the range from 0.5 to 10 μm. The thickness of layer105is for example in the range from 0.5 to 10 μm.

FIG.1more particularly illustrates a step of forming of a mask107on the upper surface of layer103. The mask is for example made of a dielectric material, for example, of silicon nitride or of silicon oxide. In this example, mask107is located opposite the green pixels G of the device only, that is, it does not cover the light-emitting cells of the red pixels R or the light-emitting cells of the blue pixels B of the device. Opposite each green pixel G, mask107comprises a plurality of through nano-openings109, identical or similar, for example, of cylindrical shape, regularly distributed across the entire surface of the light-emitting cell of the pixel. Nan-openings here means openings having lateral dimensions (width or diameter) smaller than one micrometer. Through here means that openings109vertically thoroughly cross mask107. As an example, each of openings109has a lateral dimension (width or diameter) in the range from 10 to 100 nm, preferably from 20 to 50 nm. The repetition pitch of openings109(distance from center to center between two neighboring openings in top view) is for example smaller than 200 nm, preferably smaller than 100 nm, for example, in the range from 20 to 80 nm. As an example, openings109each have a lateral dimension in the order of 25 nm, and the repetition pitch of the openings is in the order of 40 nm. The thickness of mask107is for example in the range from 10 to 100 nm, preferably from 10 to 30 nm, for example in the order of 20 nm. In the show example, mask107has, in top view, a generally square shape and has a square symmetry. The described embodiments are not limited to this specific example. As a variant, mask107may have, in top view, a generally hexagonal shape and have a hexagonal symmetry.

In a preferred embodiment, to form openings109of mask107, a film (not shown) of controlled porosity based on block copolymers is deposited on the upper surface of a continuous layer of the masking material, after which mask107is etched opposite the pores of the copolymer film to form openings109. The copolymer film is thus used as a mask for the etching of openings109. Once the etching of openings109has been performed, the copolymer film may be removed. An example of implementation of such a method is for example described in the article entitled “Density Multiplication and Improved Lithography by Directed Block Copolymer Assembly” of Ricardo Ruiz et al (Science 15 Aug. 2008: Vol. 321, Issue 5891, pp. 936-939). An advantage of such a method is that it enables to rapidly form a large number of openings109of small dimensions spaced apart in mask107.

As a variant, the openings109of mask107are formed by electron beam lithography.

As an example, the forming of mask107may comprise a step of deposition of a masking layer extending continuously over the entire surface of the device, followed by a step of forming, in the masking material, of openings109regularly distributed across the entire surface of the device, followed by a step of local removal of the mask to only keep the mask opposite the green pixels G of the device.

FIG.2is a cross-section view in the same plane as view (B) ofFIG.1, illustrating a step of deposition, by epitaxy, of a layer111of an N-type doped semiconductor material on the upper surface of the structure obtained at the end of the steps ofFIG.1. During this step, the epitaxial growth is performed from the unmasked portions of the upper surface of layer105. Thus, in red and blue pixels R and B, the epitaxial growth is performed uniformly across the entire surface of the light-emitting cells of the pixels, and, in green pixels G, the epitaxial growth is only performed from the portions of the upper surface of layer105located at the bottom of the openings109of mask107. The epitaxy is performed simultaneously in all the pixels of the device. The epitaxy is for example performed by MOCVD (“MetalOrganic Chemical Vapor Deposition”) or by MBE (“Molecular Beam Epitaxy”).

Layer111is deposited across a thickness greater than the thickness of mask107, so that, opposite each green pixel G, layer111comprises, in its upper portion, a coalesced layer113continuously extending over the entire surface of the light-emitting cell of the pixel. As an example, the thickness of layer111at the end of the deposition step ofFIG.2is in the range from 20 to 100 nm. At the end of the deposition step ofFIG.2, in each red R or blue B pixel of the device, layer111is a substantially planar layer continuously extending across a substantially uniform thickness over the entire surface of the light-emitting cell of the pixel and, in each green pixel G of the device, layer111comprises:a lower portion formed of nanopillars115of the semiconductor material, filling openings109of mask107, each nanopillar115being in contact, by its lower surface, with layer105; andan upper portion formed by the coalesced layer113of the semiconductor material, layer113being substantially planar and continuously extending across a substantially uniform thickness over the entire surface of the light-emitting cell of the pixel, and being in contact, by its lower surface, with the upper surface of nanopillars115and with the upper surface of mask107.

The semiconductor material of layer111has a mesh parameter different from that of layer105. In this example, layer105is made of gallium nitride (GaN) and layer111is made of indium gallium nitride (InGanN). Due to the mesh parameter difference between the materials of layers105and111, and due to the relatively small thickness of layer111with respect to layer105, layer111is, on its upper surface side, relatively strongly strained opposite the red R and blue B pixels of the device. In the green pixels G of the device, due to the presence of nanostructures on the lower surface side of layer111, at the interface with layer105, layer111is however relatively lightly strained or even relaxed on its upper surface side.

FIG.3is a cross-section view in the same plane as view (B) ofFIG.2, illustrating a step of deposition, by epitaxy, of an active layer117in the red, green, and blue, R, G, and B pixels of the device. During this step, the epitaxial growth is performed from the upper surface of layer111. The epitaxy is performed simultaneously in all the device pixels. The epitaxy is for example performed by MOCVD. During this step, an alternation of semiconductor layers of a first material, for example, InGaN, and of semiconductor layers of a second material having a wider bandgap than that of the first material, for example, GaN or InGaN having an indium concentration lower than that of the first material, is deposited, to form a stack of quantum wells, for example, a stack of from 3 to 10 quantum wells. As an example, the total thickness of active layer117is in the range from 40 to 150 nm. In each pixel of the device, active layer117is a substantially planar layer continuously extending across a substantially uniform thickness over the entire surface of the light-emitting cell of the pixel. The thickness of active layer117is for example substantially the same in all the pixels of the device. However, due to the strain difference in the upper portion of layer111between red and blue pixels R and B on the one hand, and green pixels G on the other hand, the indium concentration in each InGaN layer of active layer117is not the same in pixels R and B as in pixels G. More particularly, a lower indium concentration is obtained in pixels R and B than in pixels G. This concentration difference originates from the fact that, during a single epitaxy step, indium incorporates more easily in a relatively lightly-strained or even relaxed layer than in a relatively strongly strained layer. This enables to obtain different emission wavelength ranges in the light-emitting cells of pixels R and B on the one hand, and in the light-emitting cells of pixels G on the other hand. More particularly, in this example, the growth conditions of active layer117are selected to obtain an emission mainly in blue in the light-emitting cells of pixels R and B, and an emission mainly in green in the light-emitting cells of pixels G.

FIG.3further illustrates an optional step of deposition, in each pixel, of a layer119forming an electron barrier on top of and in contact with the upper surface of active layer117. Layer119is for example a P-type doped aluminum gallium nitride layer (AlGaN). Layer119may be simultaneously deposited in all the pixels of the device. In each pixel of the device, layer119is a substantially planar layer continuously extending across a substantially uniform thickness over the entire surface of the light-emitting cell of the pixel. The thickness of layer119is for example substantially the same in all the device pixels, for example, in the range from 10 to 50 nm, for example in the order of 20 nm.

FIG.3further illustrates a step of deposition of a layer121of a P-type doped semiconductor material on top of and in contact with the upper surface of barrier layer119. Layer121is for example a P-type doped gallium nitride layer (GaN). Layer121may be deposited simultaneously in all the device pixels. In each pixel of the device, layer121is a substantially planar layer continuously extending across a substantially uniform thickness over the entire surface of the light-emitting cell of the pixel. The thickness of layer121is for example substantially the same in all the pixels of the device, for example, in the range from 100 to 300 nm.

FIG.3further illustrates a step of deposition, in each pixel, of a conductive layer123on top of and in contact with the upper surface of P-type layer121. Layer123may be simultaneously deposited in all the pixels of the device. In each pixel of the device, layer123is for example a substantially planar layer continuously extending across a substantially uniform thickness over the entire surface of the light-emitting cell of the pixel. The thickness of layer123is for example substantially the same in all the pixels of the device. In this example, layer123is a metal layer continuously extending over substantially the entire upper surface of the device obtained at the end of the previous steps.

FIG.4illustrates a step of transfer of the structure150obtained at the end of the steps ofFIGS.1to3onto an integrated circuit200comprising, for each pixel of the device, an elementary control circuit, for example, based on MOS transistors, enabling to individually control the current flowing through the light-emitting cell of the pixel, and accordingly the light intensity emitted by the cell. It should be noted that inFIG.4, structure150is flipped with respect to the orientation ofFIG.3.

Integrated circuit200is formed inside and on top of a semiconductor substrate201, for example, made of silicon, for example in CMOS technology. In this example, integrated circuit200comprises a stack203of insulating and conductive interconnection layers coating the upper surface of substrate201. Interconnection stack203particularly comprises, for each pixel, a metal connection pad205flush with the upper surface of stack203, connected to the elementary pixel control circuit. In this example, integrated circuit200further comprises a metal layer207extending continuously over substantially the entire upper surface of interconnection stack203.

During the transfer step ofFIG.4, the lower surface (according to the orientation ofFIG.4) of the metal layer123of structure150is placed into contact with the upper surface of the metal layer207of circuit200. Structure150and circuit200are for example fastened to each other by direct bonding or molecular bonding of metal layer123to metal layer207.

FIG.5illustrates a step subsequent to the transfer of structure150onto control circuit200, during which growth substrate101and the stack of buffer layers103of structure150are removed to free the access to the upper surface (that is, the surface opposite to control circuit200) of layer105.

FIG.5further illustrates a step, subsequent to the removal of substrate101and of stack103, of forming of trenches220insulating from one another the light-emitting cells of the different pixels of the device. In this example, trenches220extend vertically from the upper surface of layer105to the upper surface of interconnection stack203. Trenches220may be totally or partially filled with an electrically-insulating material, for example, silicon oxide.

FIG.6illustrates a subsequent step of deposition of a conductive layer223onto the upper surface of the structure obtained at the end of the steps ofFIGS.1to5. In this example, layer223is a transparent conductive layer, for example, made of indium tin oxide (ITO), extending continuously over substantially the entire surface of the device. Layer223is in particular in contact, in each pixel, with the upper surface of the N-type semiconductor layer105of the light-emitting cell of the pixel. In this example, layer223enables to take a collective contact on the layers105of all the light-emitting cells of the device, an individual contact being taken on the P-type semiconductor layer121of each cell via pads205and metal layers207and123.

FIG.6further illustrates a step of local deposition or transfer, opposite each red pixel R of the device, onto the upper surface of transparent conductive layer223, of a color conversion element225adapted to converting the light radiation emitted by the light-emitting cell of pixel R into a light radiation in another wavelength range. In this example, element225is adapted to converting the blue light emitted by the light-emitting cell of pixel R into red light. Element225is for example formed of a layer incorporating quantum dots or nanophosphors.

An advantage of the method described in relation withFIGS.1to6is that it enables to simultaneously form, during a same sequence of epitaxy steps, two types of light-emitting cells adapted to respectively emitting in two different wavebands. This particularly enables to decrease the number of color conversion elements of different types to be provided to form a multi-color display device. In particular, in the above-described example, a single type of color conversion element is sufficient to form a three-color display device, which decreases the complexity of the device.

FIGS.7to11are cross-section views illustrating successive steps of an example of a method of manufacturing a multi-color light-emitting display device according to a second embodiment.

The forming of a display device comprising three types of different pixels, capable of respectively emitting in three different wavelength ranges, is considered herein as in the example ofFIGS.1to6. More particularly, it is desired in the example ofFIGS.1to6to form a display device comprising one or a plurality of blue pixels B, adapted to mainly emitting blue light, one or a plurality of green pixels G, adapted to mainly emitting green light, and one or a plurality of red pixels R, adapted to mainly emitting red light.

FIG.7shows an initial structure identical or similar to the structure ofFIG.1, comprising a growth substrate101, a stack103of one or a plurality of buffer layers on top of and in contact with the upper surface of growth substrate101, and an N-type doped gallium nitride (GaN) layer105on top of and in contact with the upper surface of stack103.

FIG.7more particularly illustrates a step of forming of a mask307on the upper surface of layer103. Mask307is for example made of a dielectric material, for example, of silicon nitride or of silicon oxide. Mask307is for example identical or similar to the mask107ofFIG.1, but for the fact that, in the example ofFIG.7, mask307extends on all the pixels R, G, and B of the device, and not only opposite green pixels G. Mask307comprises a plurality of through nano-openings309, for example, identical or similar to the openings109of the mask107ofFIG.1, regularly distributed across the entire surface of the device.

FIG.8illustrates a step of local thinning of mask307opposite the blue pixels B of the device. As an example, the thinning of mask307is performed by local vertical anisotropic etching. At the end of this step, the thickness of mask307opposite the blue pixels B is for example smaller by at least 10 nm than the thickness of mask307opposite pixels R and G.

FIG.9illustrates a step of local removal307opposite the green pixels G of the device to keep the mask only opposite the red and blue pixels R and B of the device.

FIG.10illustrates a step of deposition by epitaxy of a layer311of an N-type doped semiconductor material onto the upper surface of the structure obtained at the end of the steps ofFIGS.7to9. During this step, the epitaxial growth is performed from the unmasked portions of the upper surface of layer105. Thus, in green pixels G, the epitaxial growth is performed uniformly over the entire surface of the light-emitting cells of the pixels and, in the red and blue pixels R and B, the epitaxial growth is only performed from the portions of the upper surface of layer105located at the bottom of the openings309of mask307. The epitaxy is performed simultaneously in all the pixels of the device, during a single epitaxy step. The epitaxy is for example performed by MOCVD.

Layer311is deposited across a thickness greater than the thickness of mask307opposite the blue pixels B of the device, and smaller than the thickness of mask307opposite the red pixels R. Thus, opposite each blue pixel B, layer311comprises, in its upper portion, a coalesced layer313extending continuously over the entire surface of the light-emitting cell of the pixel and, in its lower portion, a plurality of nanopillars315arranged in the openings309of mask307and coupling coalesced layer313to the upper surface of layer105. In each green pixel G of the device, layer311is a substantially planar layer continuously extending across a substantially uniform thickness over the entire surface of the light-emitting cell of the pixel. In each red pixel R of the device, layer311is a discontinuous layer formed of nanopillars317arranged in the openings309of mask307, extending from the upper surface of layer105and only partially filling openings309.

The semiconductor material of layer311has a mesh parameter different from that of layer105. In this example, layer105is made of gallium nitride (GaN) and layer311is made of aluminum gallium nitride (AlGanN). Due to the mesh parameter difference between the materials of layers105and311, and due to the relatively small thickness of layer311with respect to layer105, layer311is, on its upper surface side, relatively strongly strained opposite the green pixels G of the device. In the red and blue pixels R and B of the device, due to the presence of nanostructures on the lower surface side of layer311, layer311is however relatively lightly strained or even relaxed on its upper surface side.

FIG.11illustrates a step of deposition by epitaxy of a layer319of an N-type doped semiconductor material onto the upper surface of the structure obtained at the end of the steps ofFIGS.7to10. During this step, the epitaxial growth is performed from the upper surface of layer311. Thus, in the green and blue pixels G and B, the epitaxial growth is performed uniformly over the entire surface of the light-emitting cells of the pixels and, in the red pixels R, the epitaxial growth is only performed from the upper surface of nanopillars317located in the openings309of mask307. The epitaxy is performed simultaneously in all the pixels of the device. The epitaxy is for example performed by MOCVD.

Layer319is deposited across a thickness greater than the height of the portions of the cavities309of mask307which are not filled with nanopillars317in the red pixels R of the device. Thus, opposite each red pixel R, layer319comprises, in its upper portion, a coalesced layer321extending continuously over the entire surface of the light-emitting cell of the pixel and, in its lower portion, a plurality of nanopillars323arranged in the openings309of mask307and coupling coalesced layer321to the upper surface of the nanopillars317of layer311. In each green or blue pixel G or B of the device, layer319is a substantially planar layer continuously extending across a substantially uniform thickness over the entire surface of the light-emitting cell of the pixel.

The semiconductor material of layer319has a mesh parameter different from that of layer311and, preferably, different from that of layer105. In this example, the mesh parameter difference between the material of layer319and the material of layer311is greater than between the material of layer319and the material of layer105. In this example, layer319is made of gallium-indium nitride (InGaN).

In the blue pixels B of the device, layer311is lightly strained or even relaxed on its upper surface side. Due to the relatively high mesh parameter difference between the material of layer311and the material of layer319, layer319is highly strained on its upper surface side in the blue pixels B of the device.

In the green pixels G of the device, since layer311is already strained on the mesh of layer105, and since the mesh parameter difference between the material of layer319and the material of layer105is smaller than the mesh parameter difference between the material of layer319and the material of layer311, layer319has, on its upper surface side, a lighter strain than in the blue pixels B of the device.

In the red pixels R of the device, due to the presence of nanostructures on the lower surface side of layer319, at the interface with layer311, layer319is however relatively lightly strained or even relaxed on its upper surface side.

The next steps of the display device manufacturing method (not detailed herein) are for example similar to what has been previously described in relation withFIGS.3to6. Due to the strain differences of the upper portion of layer319between the red, green, and blue pixels R, G, and B, the indium concentration in each InGaN layer of the active layer is not the same in the R, G, and B pixels. More particularly, a lighter indium concentration is obtained in blue pixels B than in green pixels G, and in green pixels G than in red pixels R. This enables to obtain different wavelength ranges in the light-emitting cells of the different types of pixels. More particularly, in this example, the growth conditions of the active layer are selected to obtain an emission mainly in green in the light-emitting cells of pixels G, an emission mainly in blue in the light-emitting cells of pixels B, and an emission mainly in red in the light-emitting cells of pixels R.

Thus, an advantage of the manufacturing method described in relation withFIGS.7to11is that it enables to simultaneously form, during a same sequence of epitaxy steps, three types of light-emitting cells adapted to respectively emitting in three different wavebands. This particularly enables to totally do away with color conversion elements in the case of a three-color display device.

FIG.12is a cross-section view illustrating steps of an example of a method of manufacturing a multi-color light-emitting display device according to a third embodiment.

The embodiment ofFIG.12differs from the embodiment ofFIGS.7to11in that, in the embodiment ofFIG.12, Mask307is thinned opposite the green pixels G of the device, and it totally removed opposite the blue pixels B of the device.

In the embodiment ofFIG.12, layers311and319are made of N-type doped semiconductor materials having different compositions, having different mesh parameters. More particularly, the material of layer319has a mesh parameter greater than that of the material of layer311, which is itself greater than that of layer105. Layer311is deposited across a thickness greater than the thickness of mask307opposite green pixels G, and smaller than the thickness of mask307opposite red pixels R. Layer319is deposited across a thickness greater than the height of the portions of the cavities309of mask307which are not filled with nanopillars317in the red pixels R of the device.

In this example, layer105is made of gallium nitride (GaN) and layers311and319are made of indium gallium nitride (InGaN). Due to the mesh parameter difference between the materials of layers105and311, and due to the relatively low thickness of layer311with respect to layer105, layer311is, on its upper surface side, relatively strongly strained opposite the blue B pixels of the device. In the green and red pixels G and R of the device, due to the presence of nanopillars on the lower surface side of layer311, layer311is however relatively lightly strained or even relaxed on its upper surface side.

In the blue pixels B of the device, due to the relatively high strain of layer311on the mesh of layer105, layer319is also relatively strongly strained on its upper surface side.

In the green pixels G of the device, due to the mesh parameter difference between the material of layer311and the material of layer319, layer319has, on its upper surface side, a relatively high strain, smaller however than the strain of layer319in the blue pixels B of the device.

In the red pixels R of the device, due to the presence of nanostructures on the lower surface side of layer319, at the interface with layer311, layer319is relatively lightly strained (that is, less strained than in green pixels G) or even relaxed on its upper surface side.

The next steps of the display device manufacturing method (not detailed herein) are for example similar to what has been previously described. Similarly to what has been described in the example ofFIGS.7to11, due to the strain differences in the upper portion of layer319between the red, green, and blue pixels R, G, and B, the indium concentration in each InGaN layer of the active layer is not the same in the R, G, and B pixels. More particularly, a lower indium concentration is obtained in blue pixels B than in green pixels G, and in green pixels G than in red pixels R. This enables to obtain different emission wavelength ranges in the light-emitting cells of the different types of pixels. More particularly, in this example, the growth conditions of the active layer are selected to obtain an emission mainly in green in the light-emitting cells of pixels G, an emission mainly in blue in the light-emitting cells of pixels B, and an emission mainly in red in the light-emitting cells of pixels R.

FIG.13is a cross-section view illustrating steps of an example of a method of manufacturing a multi-color light-emitting device according to a fourth embodiment.

The embodiment ofFIG.13differs from the previous embodiments in that, in the embodiment ofFIG.13, the masks107or307of the previous examples are replaced with a mask407. Mask407comprises, opposite the red pixels R of the device, openings identical or similar to the openings109of the mask107ofFIG.1or to the openings309of the mask307ofFIG.7. Mask407further comprises, opposite the green pixels G of the device, similar openings having greater dimensions, for example, a greater width, and optionally, a greater pitch than in red pixels R. In this example, the thickness of mask407is substantially the same opposite the red pixels R and opposite the green pixels G of the device. Mask407is further totally removed opposite the blue pixels B of the device.

In the embodiment ofFIG.13, after the forming of mask407, a layer411of an N-type doped semiconductor material is deposited on the upper surface of the structure. During this step, the epitaxial growth is performed from the unmasked portions of the upper surface of layer105. As in the previous embodiments, the epitaxy is performed simultaneously in all the pixels of the device. Layer411is deposited across a thickness greater than the thickness of mask407so that, opposite each red and green pixel R and G of the device, layer411comprises, in its upper portion, a coalesced layer313extending continuously over the entire surface of the light-emitting cell of the pixel.

The semiconductor material of layer411has a mesh parameter different from that of layer105. In this example, layer105is made of gallium nitride (GaN) and layer411is made of indium gallium nitride (InGanN).

In the blue pixels B of the device, due to the mesh parameter difference between the materials of layers105and411, layer411is relatively highly strained on its upper surface side.

In the red pixels R of the device, due to the presence of nanostructures on the lower surface side of layer411, at the interface with layer105, layer411is however relatively lightly strained or even relaxed on its upper surface side.

In the green pixels G of the device, due to the larger dimensions of the nanostructures on the lower surface side of layer411, the strain of layer411is greater than the strain of layer411opposite red pixels R, but smaller than the strain of layer411opposite blue pixels B.

As in the examples ofFIGS.7to11and12, this enables to obtain an indium concentration lower in the active layer of the blue pixels B than in the active layer of the green pixels G, and lower in the active layer of the green pixels G than in the active layer of the red pixels R, and thus different emission wavelengths in the light-emitting cells of the different types of pixels. More particularly, in this example, the growth conditions of the active layer are selected to obtain an emission mainly in green in the light-emitting cells of pixels G, an emission mainly in blue in the light-emitting cells of pixels B, and an emission mainly in red in the light-emitting cells of pixels R.

Various embodiments and variants have been described. It will be understood by those skilled in the art that certain features of these various embodiments and variants may be combined, and other variants will occur to those skilled in the art. In particular, the described embodiments are not limited to the above-described embodiment of the insulating structures of the elementary light-emitting cells of the pixels, and of the connections between elementary light-emitting cells and a circuit for controlling the device.

Further, the described embodiments are not limited to the examples of materials and of dimensions indicated in the present disclosure.

Further, although preferred embodiments enabling to simultaneously form three types of pixels adapted to respectively emitting in blue, in green, and in red have been described, the described embodiments are not limited to this specific case. In particular, by varying the compositions of the semiconductor materials and/or the dimensions and/or the spacing of the openings of masks107,307, and/or407, light-emitting cells emitting in other wavebands than those mentioned hereabove may be formed.

Further, to limit the strain and the risk of bowing of substrate101during the implementation of the different steps of deposition of the above-described method, trenches (not shown) delimiting the different pixels of the device may be etched in semiconductor layer105, for example across the entire height of layer105, before the forming of mask107,307, or407.

It should further be noted that all the conductivity types of the above-described semiconductor layers may be inverted.