Optoelectronic device including nanowires with a core/shell structure

Optoelectronic device including light-emitting means in the form of nanowires (2, 3) having a core/shell-type structure and produced on a substrate (11), in which said nanowires comprise an active zone (22, 32) including at least two types of quantum wells associated with different emission wavelengths and distributed among at least two different regions (220, 221; 320, 321) of said active zone, in which the device also includes a first electrical contact zone (15) on the substrate and a second electrical contact zone (16) on the emitting means, in which said second zone is arranged so that, as the emitting means are distributed according to at least two groups, the electrical contact is achieved for each of said at least two groups at a different region of the active zone, and the electrical power supply is controlled so as to obtain the emission of a multi-wavelength light.

The invention relates to the field of optoelectronics and, in particular, light emitters based on nanowires having a core/shell-type structure.

Indeed, it is known to produce optoelectronic devices and in particular light-emitting diodes (LED) based on InGaN/GaN nanowires.

In general, nanowires can have two different structures, an axial structure or a radial structure.

In both cases, the nanowire comprises an n-doped zone, a p-doped zone and, between these two zones, an unintentionally doped active zone that comprises quantum wells.

The use of InGaN to form quantum wells is known in the prior art. By increasing the Indium concentration, it is possible to reduce the gap of this ternary material from the gap of GaN, of which the value is around 3.5 eV, to the gap of InN, of which the value is around 0.69 eV. It is thus theoretically possible to scan the entire visible spectrum.

In a nanowire with an axial structure, the active zone is oriented parallel to the nanowire growth substrate. In a nanowire with a radial structure, the active zone is epitaxially grown around the nanowire obtained from the growth substrate.

The radial structure obtained is said to be a core/shell structure. The term “nanowire” will also be used to refer to the complete structure, including the core, the axial active zone and the shell.

Nanowires with an axial structure are generally obtained by molecular beam epitaxy (MBE), whereas nanowires with a radial structure are deposited by metalorganic chemical vapor deposition (MOCVD).

Many documents describe methods enabling nanowires with a radial active zone to be obtained.

It is possible in particular to cite document U.S. Pat. No. 7,521,274, which describes a so-called pulsed growth method, or document U.S. Pat. No. 7,829,445, which describes an ammonia flow growth method.

Nanowires with a radial structure have a number of advantages over nanowires with an axial structure.

It is possible in particular to cite an increase, with respect to the same substrate surface, in the junction surface and the active zone volume, as well as a limitation of surface recombinations and therefore an increase in the electroluminescence yield.

In addition, the electrical connection of nanowires with a radial structure is generally performed according to a parallel configuration. Thus, an upper electrode enables carriers to be injected into the shell of the nanowires, while a lower electrode ensures the connection of the core of the nanowire. The difference in potential is then the same at the terminals of each nanowire.

The parallel connection diagrams are dependent on the properties of the core, the shell and the substrate used.

It is possible in particular to cite document WO 2009/087319, which describes a diagram in which the upper electrode surrounds the upper portion of the nanowires while leaving their upper face exposed. Document US 2005/0253138 describes an upper electrode that completely surrounds the upper portion of the nanowires.

In general, to ensure lighting, it is desirable to have effective systems emitting white light. However, light-emitting diodes are monochromatic.

Various solutions have therefore been proposed in the prior art in order to generate white light, based on monochromatic LEDs.

For white LEDs obtained by conventional planar technology, a first solution consists of combining a diode emitting a short wavelength λ1, coming, for example, from a blue LED, with a luminophore emitting in a long complementary wavelength λ2. The luminophore consists, for example, of YAG: Ce-type phosphor emitting primarily in the yellow range.

The color of the light perceived by the human eye (more or less pronounced white) comes from the combination of the two wavelengths λ1and λ2.

However, losses in efficacy associated with the phosphor conversion efficiency are significant and the quality of the emission is difficult to control.

For these planar LEDs, a second solution consists of using at least three LEDs emitting different visible wavelengths that combine to give a white light.

In practice, an LED emitting a blue light is associated with an LED emitting a green light and with an LED emitting a red light. These LEDs are made of different materials.

The absence of luminophore and therefore of phosphor makes it possible to overcome losses due to phosphor. However, in practice, the color rendering index and the color temperature (corresponding to the temperature of the black body emitting the same light spectrum) are dependent on the proportion of each of the components of colored light. However, this proportion is difficult to control in a device, as each of the LEDs has a different electrical behavior under electrical injection (wavelength offset, aging, etc.).

In addition, it is necessary to associate monochromatic diodes having an external quantum efficacy greater than 50% for the three colors.

However, planar technology does not make it possible to obtain this performance, in particular in the green-yellow spectral region in which the external quantum efficacy of the devices falls by about 10%.

With regard to LEDs based on nanowires, two main solutions have been proposed in the prior art for generating white light.

The first solution consists of conventionally associating phosphor with a nanowire-based structure.

It is possible, for example, to cite the document US 2005/0208302, which describes a heterogeneous structure including a nanowire and a phosphor layer over the end or over the complete surface of the nanowire.

This heterogeneous structure has the same disadvantages as a structure comprising a planar LED and a luminophore.

Another solution consists of not using phosphor and of proposing very specific structures.

Thus, document U.S. Pat. No. 7,045,375 describes a white light emitting system, in which a planar LED structure and nanowire LED structures emitting different wavelengths are associated.

This structure also has disadvantages associated with the use of different materials of which the electrical behavior may vary.

It is also possible to cite document EP 2 254 164, which describes a device in which nanowires of different heights and emitting different wavelengths are associated. It is noted that, in such a device, it is difficult to control the height of the nanowires.

Finally, in the devices described in the prior art, the nanowires used have an axial structure. There is therefore a loss in emitting surface with respect to a planar structure and therefore a loss in efficacy.

The invention is intended to overcome these disadvantages by proposing an optoelectronic device enabling the emission of a multi-wavelength light and in particular a white light with which the quality of the emission can easily be controlled.

Thus, the invention relates to an optoelectronic device including light-emitting means in the form of nanowires having a core/shell-type structure and obtained on a substrate, in which said nanowires comprise an active zone including at least two types of quantum wells associated with different emission wavelengths and distributed among at least two different regions of said active zone, in which the device also includes a first electrical contact zone on the substrate and a second electrical contact zone on the emitting means, in which said second zone is arranged so that, as the emitting means are distributed according to at least two groups, the electrical contact is achieved for each of said at least two groups at a different region of the active zone, and the electrical power supply is controlled so as to obtain the emission of a multi-wavelength light.

This optoelectronic device avoids the use of phosphor and it is obtained from the same growth substrate, with all of the nanowires being made of the same material.

This makes it possible to eliminate the problems of different aging or different behavior according to temperature, which are encountered in the known devices.

In addition, the nanowires are obtained during the same epitaxis step, which saves time when producing the device, thereby resulting in a lower cost.

Finally, the color of the light emitted by the device according to the invention is directly dependent on the number of emitting means in each of the groups present.

In an example of an embodiment of the optoelectronic device according to the invention, a first region of the active zone of the nanowires is a peripheral portion at least partially surrounding the core of the nanowires and comprising radial quantum wells and a second region of this active zone is an upper portion located on the end of the core of the nanowire, with axial quantum wells.

In another example of an embodiment of the optoelectronic device according to the invention, the active zone comprises a third region, located between the first and second regions.

Advantageously, the optoelectronic device includes a plurality of portions electrically connected in parallel and inside of which the emitting means are connected in parallel, with the electrical contact of the second electrical contact zone being produced in a region different from the active zone, for two adjacent portions.

Preferably, when the active zone of the emitting means comprises two distinct regions, the emitting means of the second group are surrounded by an electrically insulating material, with the exception of the upper face opposite the substrate, and the second electrical contact zone completely surrounds the emitting means of the second group and is provided on the periphery of the emitting means of the first group and in direct contact with same, over a height h corresponding to the first region of the active zone, so that the upper portion of these emitting means is exposed.

When the emitting means comprise an active zone with a third region, the emitting means of a third group are surrounded by an electrically insulating material over the height h, and the second electrical contact zone is provided on the periphery of the emitting means of the third group, over a height h1, greater than h, so that the upper portion of the emitting means is partially exposed.

In an alternative embodiment of the optoelectronic device according to the invention, for at least some of the emitting means, the second electrical contact zone is arranged so that the electrical contact is produced in at least two regions of the active zone.

In this alternative, the emitting means are distributed among at least two electrically independent groups, and the electrical power supply is controlled so that, for each of said at least two groups, different regions of the active zone are supplied with power.

Finally, the active zone of the nanowires is preferably made of InGaN/GaN.

The common elements in the different figures will be denoted by the same reference signs.

Throughout the description, the term “nanowire” will designate an elongate structure, i.e. a structure in which the height-to-diameter ratio (or the extension of the base width) is greater than 1.

The core10of the nanowire1consists of an actual nanowire, which has been obtained by epitaxis perpendicular to the growth substrate11.

As an example, the substrate is made of strongly n-doped silicon and the core10is made of n-doped gallium nitride.

The core10therefore has two families of distinct crystalline planes: the m-planes over the periphery100of the core and the c-plane at the upper portion101of the core, opposite the substrate.

The active zone12of the nanowire is produced around the core10. It consists of quantum wells arranged radially around the core and made of InGaN/GaN and which result from a growth in planes perpendicular to the c-plane, for example on en-planes.

It is possible to distinguish two families of quantum wells, according to the crystallographic plane on which they are epitaxially grown.

Thus, the active zone12of the nanowire1in this case comprises two regions, a first peripheral region120, in which the quantum wells are radial and a second upper region121in which the quantum wells are axial.

It has already been established that these two families of wells have emission properties (wavelength and internal quantum efficacy) that are largely dependent on the presence of an internal piezoelectric field in the quantum wells.

Reference can thus be made to the article of Lai et al., “Excitation current dependent cathodoluminescence study of InGaN/GaN quantum wells grown on m-plane and c-plane GaN substrates”, JAP 106 (2009) 113104.

In addition, the first studies of growth of quantum wells made of InGaN/GaN and having a radial structure appear to indicate that the growth mechanisms are dependent on crystallographic planes on which the wells are epitaxially grown.

Reference can be made to the article of Bergbauer W. et al, “Continuous-flux MOVPE growth of position-controlled N-face GaN nanorods and embedded InGaN quantum wells”, Nanotechnology 21 (2010) 305201.

The inventors have thus started with the hypothesis that the rate of growth and the incorporation of indium are different on a c-plane or on an m-plane. This particularity leads to quantum wells with different thicknesses and/or compositions and therefore to the emission of different wavelengths.

The inventors experimentally studied, by low-temperature cathodoluminescence, the difference in emission between axial polar quantum wells and radial non-polar quantum wells on nanowire-type LED structures obtained by MOCVD, in which said nanowires have a core/shell-type structure with an active zone formed by InGaN/GaN quantum wells.

These experiments showed that the difference in emission wavelength between the two families of quantum wells was on the order of 32 nm to 10 K.

Thus, the inventors showed that it is possible to have nanowires with a core/shell-type structure having axial quantum wells and radial quantum wells associated with emission wavelengths located in very separate spectral ranges.

In general, it is thus possible to consider the two wavelengths corresponding to the two types of quantum wells not only to be different but for there to be no significant overlap between the two spectra emitted by these two types of quantum wells. In other words, the difference between the two wavelengths is at least equal to the half-sum of the widths of the spectra at mid-height.

Finally, the nanowire comprises a shell13that is produced around the active zone12. The shell can be made of p-doped GaN.

FIG. 2diagrammatically shows an optoelectronic device according to the invention. This optoelectronic device in this case comprises two light-emitting means, of the nanowire1type shown inFIG. 1. These emitting means2and3in this case comprise a similar structure. They are distinguished only by the structure of one of their electrical contact zones.

Of course, an optoelectronic device according to the invention will, in practice, comprise more than two emitting means.

The cores20and30of the emitting means2and3are constituted by an actual nanowire that has been epitaxially grown perpendicularly to the growth substrate11, according to the insulating growth mask14.

Each emitting means2,3comprises an active zone22,32, which surrounds the core20,30, as well as a shell23,33.

Inside the active zone22,32, two regions corresponding to quantum wells associated with different emission wavelengths can be distinguished.

Thus, the peripheral region220,320comprises radial quantum wells and the upper region221,321of the active zone comprises axial quantum wells.

The axial quantum wells are characterized by a greater degree of relaxation of stresses than the radial wells. The inventors have therefore started with the hypothesis that it is theoretically possible to incorporate more indium in the axial quantum wells. This enables the axial quantum wells to generate photons of which the emission wavelength λ2is greater than the wavelength λ1of the photons generated by the radial quantum wells.

In addition, the inventors noted that the light emitted by the radial quantum wells in a core/shell-type structure is guided only slightly if at all by the wire-based geometry of the structure.

Thus, the photons of wavelength λ1leave the emitting means without reaching the axial quantum wells, which limits the photon resorption phenomena observed in conventional structures. The efficiency of the emitting means is therefore increased.

The optoelectronic device shown inFIG. 2comprises two electrical contact zones.

A first electrical contact zone15is produced on the substrate, on the side opposite the emitting means2and3.

A second electrical contact zone16is produced on the emitting means2and3.

FIG. 2shows that the emitting means3comprise, around the shell33, a sheath34made of an insulating material, except for the upper face330of the shell, which is entirely free of insulating material.

Thus, the second electrical contact zone is provided between all of the emitting means. This portion160of the second electrical contact zone is in contact with the insulating growth mask14.

In the continuity of portion160, the second electrical contact zone completely surrounds the emitting means3. The portion161of the electrical contact is therefore insulating from the peripheral portion of the means3by the insulating sheath34, but it is directly in contact with the upper face330of the shell33.

Finally, also in the extension of portion160, the second electrical contact zone16is provided on the periphery of the emitting means2and in direct contact with the shell23. However, this portion162of the second contact zone extends only over a height h of the emitting means2, so that it does not extend beyond the core20, in the axial direction of the core, or beyond the peripheral region220of the active zone22.

Owing to this particular structure of the second electrical contact zone16, when a voltage differential is applied between the first and second electrical contact zones15and16, the emitting means2will generate photons with an emission wavelength λ1, while the emitting means3will emit photons with a wavelength λ2.

Thus, the sensation of light that will be perceived by a human eye will result from the combination of wavelengths λ1and λ2.

In practice, the macroscopic spectrum of the optoelectronic device according to the invention is determined by the ratio of the number of emitting means of the type of means2over the number of emitting means of the type of means3.

Thus, by modifying this ratio, it is possible also to modify the macroscopic spectrum of the optoelectronic device.

It is therefore by choosing the structure of the second contact zone that the macroscopic spectrum of the optoelectronic device can also be adapted.

A method for obtaining the optoelectronic device shown inFIG. 2will now be described in reference toFIGS. 3ato3h.

FIG. 3ashows a first step of this method in which a growth mask is produced on a substrate.

Thus, a layer made of an electrically insulating material is deposited on substrate11, in order to prevent short-circuits between the electrical contact zone of type p, which will subsequently be formed and substrate11.

The substrate can be a strongly n-doped silicon substrate and have a thickness of 400 μm.

This layer can in particular be made of SixNyOz and, for example SiO2or SiN.

An etching mask is then applied on this insulating material layer. This etching mask can be configured by conventional nanoimprint or lithography techniques.

An etching step is then performed, which makes it possible to obtain the insulating growth mask14shown inFIG. 3a. This growth mask is characterized by zones140, without any insulating material.

FIG. 3bshows a step of localized growth of the core20,30of the nanowires that will form the emitting means2,3.

The growth is produced at zones140, according to conventional techniques, preferably non-catalyzed, such as MOCVD, MBE or HVPE (Hybrid Vapor Phase Epitaxy).

These cores can in particular be made of n-type gallium nitride.

According to a preferred means of the invention, the growth is produced by MOCVD with a low ratio V/III and under a silane flow and with nitrogen as the vector gas under high pressure (close to atmospheric pressure).

They have good electrical continuity with the silicon substrate11on which they are epitaxially grown.

In an alternative, a fine layer of Ti, Pt, W, Al, B, SiC, SiN, AlN, Th and Mg3N2 can be inserted between the substrate11and the cores20and30.

It makes it possible to protect the substrate11from any gallium attacks. It also provides a good interface for the growth of nanowires.

In this case, the layer must be fine enough for the electrical continuity between the nanowire cores and the substrate not to be modified.

FIG. 3cshows another step of the method in which the growth of the active zone and of the nanowire shell is produced.

During this step, the active zone21,31, for example consisting of a relatively thick layer of InGaN or quantum wells made of InGaN/GaN, is radially deposited.

According to the preferred growth method, the growth of quantum wells is performed by increasing the V/III ratio and by reducing the working pressure, while keeping the silane flow or not, and with nitrogen as the vector gas.

The shell23,33is then produced around the active zone. The shell is made of p-type GaN (or InGaN).

According to the preferred growth method, the p doping is performed with a high V/III ratio, under low pressure, optionally with a silane flow and with hydrogen as a vector gas.

An LED with a pn junction is thus formed.

In addition, the conditions for growth of the quantum wells of the active zones can be modified in order to change the diffusion of the species along the walls of the core and thus choose to promote the incorporation of indium in the axial or radial wells of the active zone. Indeed, the inventors have noted that the incorporation of indium could be promoted in both types of wells, after starting with the hypothesis that only the axial quantum wells were concerned.

This will make it possible to better differentiate the wavelengths λ1and λ2associated with the quantum wells.

Thus, at the end of this step, the radial quantum wells and the axial quantum wells will have a different emission wavelength.

FIG. 3dshows a step of depositing a layer17of an electrically insulating material.

This material can, for example, consist of a polymer or of SiO2.

This layer17is then etched by a conventional lithography technique in order to maintain a sheath34around the emitting means3, as shown inFIG. 3e.

FIGS. 3fand3gshow the step of formation of the electrical contact zone on the emitting means2and3.

In a first step, shown inFIG. 3f, a layer18made of a conductive material is formed over the entire surface of the growth mask14and therefore around the emitting means2and3.

The conductive material is semi-transparent so as to enable the light to be extracted from the emitting means2and3.

Thus, this layer18can, for example, be in the form of a fine metal layer (Ni/Au, Pd/Au or Pt/Au) or an ITO (Indium Tin Oxide) layer.

In a second step, the layer18is etched in the upper portion of the emitting means2. This etching can be performed during a conventional lithography step.

Thus, at the end of these two steps, the contact zone16is produced. It is a p-type electrical contact.

As explained in reference toFIG. 2, this second contact zone comprises portions160in contact with the mask14, a portion161completely surrounding the emitting means3and a portion162that extends only over the height h of the emitting means2, so that the electrical contact zone does not extend beyond the core20of the emitting means2.

Thus, the electrical contact zone16forms a so-called peripheral contact on the emitting means2, by portion162, and a so-called planar contact on the emitting means3, by portion161.

FIG. 3hshows a final step of the method in which another electrical contact zone15is produced on the substrate, on the side opposite the emitting means2and3.

This second electrical contact zone can be obtained by depositing a metal layer, for example of Ni/Au.

As indicated above, an optoelectronic device will comprise a plurality of emitting means2,3.

Two electrical connection diagram types can then be envisaged.

The different emitting means can be connected individually to one another in series.

FIG. 4shows another type of connection diagram, in which an optoelectronic device consists of nine different portions: portions40to44include only emitting means of the type of means2, i.e. nanowires for which the electrical contact zone16forms a peripheral contact and portions45to48, which include exclusively emitting means of the type of means3, i.e. nanowires comprising a planar electrical contact.

Inside each portion, the emitting means are electrically connected in parallel and the different portions are also connected to one another in parallel.

Thus, for two adjacent portions, the electrical contact of the second electrical contact zone is produced at the different regions of the active zone.

FIG. 5is an explanatory diagram of a first alternative embodiment of an optoelectronic device according to the invention.

In this alternative of the optoelectronic device shown inFIG. 2, all of the emitting means can generate photons with an emission wavelength λ1or photons with a wavelength λ2.

Thus, the optoelectronic device includes a plurality of emitting means5each including a nanowire1, as shown inFIG. 1. The nanowires are obtained by growth from the substrate11, in which a first electrical contact zone15is formed on the face of the substrate11, opposite the nanowire1.

The emitting means5are distinguished from emitting means2or3by the structure of the second electrical contact zone.

In practice, this second electrical contact zone51is made of two portions510and511.

The first portion510forms a peripheral electrical contact around the nanowires1, of the type produced for the emitting means2.

Thus, this portion510is in contact with the insulating mask14, and it is directly in contact with the peripheral portion of the shell13. In other words, portion510forms an electrical contact only over a height h of the nanowire1, so that it does not extend beyond the core10of the nanowire1.

In addition, a layer of insulating material52is present around the nanowires1, except for the upper face130of the shell13, which is entirely free of insulating material.

The second portion511covers all of the emitting means5. It is therefore insulated from the peripheral portion of the nanowire1by the insulating layer52, but it is directly in electrical contact with the upper face130of the shell. This portion511therefore produces a similar electrical contact to that present on the emitting means3.

Thus, the same group of emitting means5may generate photons with a wavelength λ1, by applying a suitable voltage differential between the electrical contact zones15and510, or photons with an emission wavelength λ2, when a suitable voltage differential is applied between the electrical contact zones15and511. The same group of emitting means5can therefore emit either only photons with an emission wavelength λ1, or only photons with an emission wavelength λ2.

An optoelectronic device according to the invention will thus be obtained by associating at least two groups of emitting means5, which two groups are electrically independent.

Such an example of an optoelectronic device according to the invention is shown inFIG. 6.

This optoelectronic device thus includes two groups60and61of emitting means5, which two groups are electrically independent. Of course, the optoelectronic device can comprise more than two groups of emitting means.

The electrical power supply of the first group60is controlled so that a voltage differential is applied between the first electrical contact zone15and the portion510of the second contact51. Thus, the emitting means5of the group60will generate photons with an emission wavelength λ1.

By contrast, the electrical power supply of the emitting means5of the second group61is controlled so that a voltage differential is applied between the first electrical contact zone and the portion511of the second electrical contact zone51. The emitting means5of the second group61will therefore emit photons with a wavelength λ2.

The macroscopic spectrum of the optoelectronic device obtained can be controlled by modulating the operating voltages applied to each of the groups60and61. Indeed, the intensity of the light emitted by each group is dependent on the current that passes through the emitting means and therefore the voltage at their terminals.

This macroscopic spectrum is also dependent on the number of emitting means present in each group.

Finally, if the number of emitting means present in each group is different, the macroscopic spectrum can also be modified by inverting the connection diagram, so that the first group60emits photons with a wavelength λ2and the second group61emits photons with a wavelength λ1.

Reference will now be made toFIG. 7, which shows a nanowire7also having a core/shell structure.

The core70of the nanowire consists of an actual nanowire, which has been obtained by epitaxis perpendicular to the growth substrate11.

In this example, the core70has three crystallographic planes: the m-planes over the periphery700of the core, the c-planes at the upper portion701of the core, opposite the substrate, as well as the r-planes, over the frusto-conical portion702of the core70, located between the periphery700and the upper portion701. This particular form of the core70can be obtained under particular growth conditions. In this regard, reference can be made to the article of Bergbauer W. et al. mentioned above.

Reference72designates the active zone of the nanowire that is produced around the core70.

This active zone comprises three families of quantum wells, according to the crystallographic plane on which they are epitaxially grown. These three families are distributed among three different regions: a peripheral region720, in which the quantum wells are radial, a second region721, in which the quantum wells are axial and a third region722, in which the quantum wells are inclined according to r-planes.

FIG. 8diagrammatically shows an optoelectronic device according to the invention that comprises three light-emitting means of the type of the nanowire7shown inFIG. 7.

These emitting means7a,7band7cdiffer from one another by the structure of their second contact zone, with the first contact zone15being produced on the face of the substrate11opposite the nanowire.

Of course, an optoelectronic device according to the invention will comprise, in practice, more than three emitting means of the type of means7a,7band7c.

Thus, each of the emitting means7ato7chas an active zone comprising three regions corresponding to quantum wells associated with different emission wavelengths. This active zone comprises a peripheral region720ato720ccapable of generating photons with an emission wavelength λ1, an upper region721ato721ccapable of generating photons with an emission wavelength λ2and an intermediate region722ato722ccapable of generating photons with an emission wavelength λ3.

The second electrical contact zone17will now be described in greater detail.

FIG. 8shows that the emitting means7bcomprise, in the peripheral portion730bof the shell73b, a sheath74bof insulating material.

In addition, the emitting means7ccomprise, around the shell73c, a sheath74cof insulating material, except for the upper face731cof the shell73c, which is entirely free of insulating material.

The second electrical contact zone17is provided on the emitting means7ato7c, and it is therefore in contact with the mask14between these means.

Thus, the second electrical contact zone completely surrounds the emitting means7c. It is therefore electrically insulated from the means7c, except for the upper face731cof the emitting means, with which it is directly in contact.

In addition, this second electrical contact zone is provided on the periphery of the peripheral portion730aof the shell73aand directly in electrical contact with same. However, the second electrical contact zone does not extend around the upper portion of the means7a, defined by the intermediate portion732aand the upper portion731aof the shell73a, or beyond the peripheral region720aof the active zone.

With regard to the emitting means7b, the second electrical contact zone extends around the insulating sheath74b. It also extends partially over the intermediate portion732bof the shell73b. In other words, the second contact zone is provided on the periphery of the emitting means7b, over a height h1, so that the upper portion of the emitting means is partially exposed. In this case, the upper portion of means7bis portion732b, which ends with the upper face731bof the shell73b.

Owing to this particular structure of the second electrical contact zone17, a voltage differential between the first and second contact zones15and17causes the generation of photons with an emission wavelength λ1by the emitting means7a, with an emission wavelength λ3by the emitting means7band with a wavelength λ2by the emitting means7c.

Thus, the sensation of light that will be perceived by a human eye will come from the combination of wavelengths λ1, λ2and λ3.

The light emitted by the device according toFIG. 8will be of higher quality than that emitted by the device according toFIG. 1, in terms of color rendering index (CRI) and correlated color temperature (CCT).

As explained in reference toFIG. 2, the macroscopic spectrum of this optoelectronic device is determined by the number of emitting means corresponding to each of types7a,7band7c.

A method for obtaining the optoelectronic device shown inFIG. 8is deduced simply from that described in reference toFIGS. 3ato3h.

Reference will now be made toFIG. 9, which shows an alternative embodiment of the optoelectronic device according toFIG. 8. This alternative is similar to that shown inFIG. 5.

Indeed,FIG. 9shows a group of emitting means that each comprise the three types of electrical contact described for means7ato7c, with regard to the second electrical contact zone81.

The first electrical contact zone15is produced on the substrate11, on the side opposite the emitting means8.

Each of the emitting means8comprises a nanowire7, as shown inFIG. 7, and which will not be described in detail again.

The second electrical contact zone81consists of three portions810,811and812.

The first portion810forms an electrical contact around the peripheral portion730of the shell73of the nanowires, as explained for the emitting means7a.

A layer of insulating material82is present on the first portion810of the second electrical contact zone, except for the intermediate portion732of the upper face731of the shell73. This layer82therefore creates an insulating sheath around the nanowires7, similar to the sheath74bdescribed for the emitting means7b.

The second portion811of the second electrical contact zone81extends over this layer82and it also extends partially over the intermediate portion732of the shell73.

In other words, the upper portion of the nanowires7, formed by the intermediate portion732and the upper face731of the shell, is partially free of any electrical contact.

Thus, this second portion811forms an electrical contact similar to that described for the emitting means7b.

Finally, a second layer83of insulating material is deposited on the second portion811. Thus, only the upper face731of the shell73of the nanowires7is free of insulating material.

The third portion812of the second electrical contact zone is deposited on the layer of insulating material83. It is therefore directly in electrical contact with the upper face731of the shell of the nanowires7.

Thus, each of the emitting means8comprises the three types of electrical contact of the second electrical contact zone that were described for means7a,7band7c.

In practice, the three emitting means8shown inFIG. 9can therefore emit photons with an emission wavelength λ1, if a voltage differential is applied between the first electrical contact zone15and the first portion810of the second electrical contact zone81. A voltage differential applied between contact zones15and811will make it possible to generate photons with an emission wavelength λ3. Finally, a voltage differential applied between the electrical contact zones15and812will make it possible to generate photons with an emission wavelength λ2.

As explained in reference toFIG. 6, an optoelectronic device according to the invention can be formed by a plurality of groups of emitting means8, in which said groups are electrically independent.

The observations made for the device shown inFIG. 6are also valid for the device shown inFIG. 9.

FIG. 10shows another alternative of the invention in which the growth of n-doped GaN nanowires is produced on an n-doped GaN substrate18.

In this case, the substrate11is not necessarily electrically conductive and can, for example, be made of sapphire.

The second electrical contact zone16, of type p, is produced as described above.

By contrast, the first electrical contact zone19, of type n, is produced on the substrate18. This requires a preliminary step of etching the growth mask14and the first electrical contact zone16.

Reference will now be made toFIG. 11, which shows an alternative of the device shown inFIG. 10, in which an electrical connection in series of the emitting means is provided.

The device includes two emitting means2and3between which a nanowire9of which the core90is optically inactive, because the current does not pass through the pn junction, is arranged.

The second electrical contact zone16forms a peripheral contact on the emitting means2, by portion162, and a planar contact on the emitting means3, by portion161.

Between these two portions161and162, the second zone16comprises a portion160in contact with the growth mask, which extends by a portion163that comes into contact with the core90of the nanowire9.

FIG. 11shows that the upper portion of the active portion92and the shell93of the nanowire9has been removed and that the insulator34is present not only around the emitting means3but also around the core90of the nanowire in this upper portion.

By contrast, the upper face900of the core90is in direct contact with the portion163of the second electrical contact zone. In this way, an electrical connection is established between the p-type contact zone162of the emitting means2and the n-type contact zone of the emitting means3. Indeed, the core90of the nanowire9is connected to the core30of the emitting means3by the substrate18. This ensures the series arrangement of the emitting means2and3.

This alternative of the device has the advantage of increasing the operating voltage and of bringing it closer to the voltage of the sector. The device is thus more efficient.

The reference signs inserted after the technical features appearing in the claims are intended solely to make it easier to understand these features and cannot limit the scope of same.