Patent Application
20240274747
The present invention relates to the field of optoelectronics. It finds a particularly advantageous application in the manufacture of optoelectronic devices having a three-dimensional structure, for example light-emitting diodes based on nanowires.
Light-emitting diodes (LEDs) based on nanowires may have different architectures, in particular with regards to the arrangement of the different constituent regions of the LED.
A LED typically comprises carrier injection regions (electrons and holes) between which an active region is interposed. The active region is the place where radiative recombinations of electron-hole pairs occur, which allow obtaining a light emission. In particular, this active region may comprise quantum wells, for example based on InGaN.
The LED may also comprise different carrier blocking layers, for example an electron blocking layer at the hole injection region—and vice versa, intended to improve the overall efficiency and performances of the LED.
These different regions and layers may be arranged in a stack according to a longitudinal direction z. Such an LED architecture is called axial. An axial 3D LED typically has, stacked according to z, a lower portion bearing on a substrate, an active region bearing on the lower portion, and an upper portion bearing on the active region. In general, the lower portion is intended for the injection of electrons and the upper portion to hole injection. The active region typically has quantum wells extending transversely to the longitudinal direction z. An electron blocking layer may be present between the upper portion and the active region. A hole blocking layer may be present between the lower portion and the active region.
Such an axial LED may typically be made by molecular beam epitaxy MBE (acronym for Molecular Beam Epitaxy). In the case of a GaN-based LED, the molecular flow of the nitrogen precursor is primarily oriented according to the longitudinal direction z, as illustrated in the document “Galopin et al., Nanotechnology 22, 245606 (2011)”. Indeed, for low pressures such as those implemented in MBE, the molecular flows are essentially ballistic. Hence, the orientation of the flows according to z allows promoting the formation of the LED according to the axial architecture.
Alternatively, the different regions and layers of the LED may be arranged radially around the longitudinal direction z. Such a LED architecture is called radial or core-shell. A radial 3D LED typically has an elongate inner portion (the core) according to z and bearing on a substrate, an active region surrounding the inner portion, and an outer portion (the shell) surrounding the active region. In general, the inner portion is intended for the injection of electrons and the outer portion for the injection of holes. The active region typically has quantum wells extending parallel to the longitudinal direction z. An electron blocking layer may be present between the outer portion and the active region. A hole blocking layer may be present between the inner portion and the active region.
Such a radial LED may also be made by MBE, by modifying the main orientation of the molecular flow, as illustrated in the document “Galopin et al., Nanotechnology 22, 245606 (2011)”. Alternatively, a radial LED may be formed by Metal Organic Vapour Phase Epitaxy MOVPE (acronym for Metal Organic Vapour Phase Epitaxy) using gaseous precursors at higher pressures.
Regardless of the targeted LED architecture, parasitic growth might occur—for example the formation of a partial shell when making an axial LED, as illustrated in the document “Galopin et al., Nanotechnology 22, 245606 (2011)”. Carrier leaks might then occur at the LED. This deteriorates the performances of the LED.
The present invention aims to at least partially overcome the above-mentioned drawbacks.
In particular, an object of the present invention is to provide a method for manufacturing a light-emitting diode having an optimised architecture. Another object of the present invention is to provide such a light-emitting diode, in particular limiting carrier leaks.
The other objects, features and advantages of the present invention will appear upon examining the following description and the appended drawings. It is understood that other advantages can be incorporated therein. In particular, some features and some advantages of the method may be applied mutatis mutandis to the device, and vice versa.
To achieve the aforementioned objectives, a first aspect of the invention relates to a method for manufacturing a GaN-based light-emitting diode having a three-dimensional (3D) structure, the method comprising successive formations by axial growth of so-called axial portions.
The axial portions comprise at least, stacked according to a longitudinal direction z:
Preferably, the bases and the tops each extend transversely to the longitudinal direction z. The axial portions respectively have walls parallel to the longitudinal direction z.
Advantageously, the method further comprises at least one formation by radial growth of at least one so-called radial portion, said at least one radial portion comprising:
Advantageously, the at least one formation by radial growth is interposed between two successive formations by axial growth.
Thus, the method provides for interposing at least one radial growth from the axial growths. This allows deliberately forming a radial portion—typically a shell—surrounding an entire axial portion. Advantageously, this shell is a carrier blocking layer, typically an electron blocking layer or a hole blocking layer. This allows limiting and possibly suppressing carrier leaks in the 3D LED.
In contrast with the principle of an architecture that is either only axial or only radial advocated by the prior art, and which in practice turns out to be a poorly controlled mixed architecture, the method according to the invention intentionally introduces axial growth steps alternating with or interspersed by at least one radial growth step. This allows controlling the formation of at least one radial portion, which may be in the form of an integral shell, in contrast with the partial shells obtained unintentionally according to the prior art.
In general, the formations by axial growth require specific techniques distinct from the techniques required for the formations by radial growth. According to a technical prejudice, it is difficult and even impossible to implement these two types of formations by axial growth and by radial growth in the same method. The invention overcomes this prejudice in order to provide a manufacturing method allowing optimising the architecture of a light-emitting diode. The method according to the invention allows considering various morphologies and architectures of 3D LEDs.
A second aspect of the invention relates to a GaN-based light-emitting diode having a 3D three-dimensional structure and comprising so-called axial portions, said axial portions comprising at least, stacked according to a longitudinal direction z:
Preferably, the bases and the tops each extend transversely to the longitudinal direction z. The axial portions respectively have walls parallel to the longitudinal direction z.
Advantageously, the light-emitting diode further comprises at least one so-called radial portion comprising a carrier blocking layer extending in contact with at least one of the base and the top of the active region, and completely covering walls of at least one axial portion.
Thus, the light-emitting diode according to the invention has a mixed axial and radial architecture. In particular, the portions and regions transporting and using the carriers are formed axially, and the portions and regions passivating or blocking the carriers are formed radially. This allows optimising the operation of the 3D LED. Thus, the axial portions may be viewed as active portions, and the radial portions may be viewed as passive portions. Thus, the active portions benefit from excellent crystalline quality related to axial growth. The internal quantum efficiency is improved. Thus, the passive portion benefit from an excellent radial coverage related to radial growth. Carrier leaks are considerably limited and possibly suppressed. The overall efficiency of the 3D LED is improved.
Advantageously, such a 3D LED may be obtained by the method according to the first aspect of the invention.
The aims, objects, as well as the features and advantages of the invention will appear better from the detailed description of embodiments of the latter which are illustrated by the following appended drawings wherein:
The drawings are provided by way of example and are not intended to limit the scope of the invention. They constitute diagrammatic views intended to ease the understanding of the invention and are not necessarily to the scale of practical applications. In particular, the dimensions of the different portions of the 3D LED do not necessarily represent reality.
Before starting a detailed review of embodiments of the invention, it is recalled that the invention according to its first aspect comprises in particular the optional features hereinafter which can be used in combination or alternatively:
According to one example, the at least one radial portion comprises a first radial portion comprising an electron blocking layer, and wherein the respective formations of the axial portions and of the at least one radial portion follow the following sequence of steps:
According to one example, the at least one radial portion further comprises a second radial portion comprising a hole-blocking layer and wherein the sequence of steps further comprises forming the hole-blocking layer by radial growth, after formation of the lower portion and before formation of the active region, so that said hole blocking layer extends in contact with the base of the active region, and completely covers the walls of the lower portion.
According to one example, the method further comprises, after formation of the upper portion by axial growth, passivating the walls of said upper portion.
According to one example, each formation by axial growth comprises a plasma-assisted molecular beam epitaxy having a flow of nitrogen precursor directed according to a first direction forming an angle α1 with the longitudinal direction z, such that 0°<α1<30°.
According to one example, the formations by axial growth are implemented in a first chamber and the at least one formation by radial growth is implemented in a second chamber.
According to one example, the formations by axial growth and by radial growth are implemented successively in the same chamber.
According to one example, the at least one formation by radial growth comprises vapour phase epitaxy with organometallic precursors.
According to one example, the at least one formation by radial growth is followed by purging the chamber before implementation of the next formation by axial growth.
According to one example, the axial portions of the 3D LED are formed from a nitrogen plasma source.
According to one example, the radial portions of the 3D LED are formed from a nitrogen gas source.
According to one example, the 3D LED is partly formed from a nitrogen plasma source, and partly formed from a nitrogen gas source.
According to one example, the at least one formation by radial growth comprises plasma-assisted molecular beam epitaxy having a flow of nitrogen precursor directed according a second direction forming an angle α2 with the longitudinal direction z, such that α2>30°.
According to one example, the axial portions and the at least one radial portion are formed alternately. If the number of formations by axial growth is greater than the number of formations by radial growth, this alternation is not necessarily strict and two formations by axial growth could immediately follow one another.
The invention, according to the second aspect thereof, comprises in particular the optional features hereinafter which may be used in combination or alternatively:
According to one example, the at least one radial portion comprises a first radial portion comprising an electron blocking layer extending over the walls and the top of the active region, and/or a second radial portion comprising a hole blocking layer extending over the walls and the top of the lower portion.
According to one example, the lower portion bears on the substrate throughout a masking layer, and wherein the at least one radial portion bears on said masking layer.
According to one example, the walls of the upper portion are covered by a passivation layer. Unless incompatibility, technical features described in detail for a given embodiment may be combined with the technical features described in the context of other embodiments described as a non-limiting example, so as to form another embodiment which is not necessarily illustrated or described. Of course, such an embodiment is not excluded from the invention.
In the present invention, the method is dedicated in particular to the manufacture of light-emitting diodes (LEDs) with a 3D structure.
The invention may be implemented more broadly for different optoelectronic devices with a 3D structure.
The invention may therefore also be implemented in the context of laser or photovoltaic devices. Unless stated explicitly, it is specified that, in the context of the present invention, the relative arrangement of a third layer interposed between a first layer and a second layer does not necessarily mean that the layers are directly in contact with each other, but means that the third layer is either directly in contact with the first and second layers, or separated from these by at least one other layer or at least one other element.
Thus, the terms and expressions “Bear” and “cover” do not necessarily mean “in contact with”. Typically, the upper portion bears on the active region via the electron blocking layer, which is interposed between these two axial portions. The active region may bear on the lower portion via the hole blocking layer, which is interposed between these two axial portions.
The steps of the method as claimed should be understood in a broad sense and may possibly by carried out into several sub-steps.
The term “3D structure” should be understood in contrast with so-called planar or 2D structures, which have two dimensions in a plane larger than the third dimension normal to the plane. Thus, the usual 3D structures targeted in the field of 3D LEDs may be in the form of wire, nanowire or microwire. Such a 3D structure has an elongated shape according to the longitudinal direction. The longitudinal dimension of the wire, according to z in the figures, is larger, and preferably much larger, than the transverse dimensions of the wire, in the plane xy in the figures. For example, the longitudinal dimension is at least five times, and preferably at least ten times, larger than the transverse dimensions. The 3D structures may also be in the form of walls. In this case, only one transverse dimension of the wall is quite smaller than the other dimensions, for example at least five times, and preferably at least ten times, as small as the other dimensions. Preferably, the 3D structures of the present application have substantially vertical walls, capable of forming radial portions by radial growth. The vertical walls typically extend according to m-type crystallographic planes. Preferably, the 3D structures of the present application have substantially horizontal bases and tops, suitable for the formation of axial portions by axial growth. Horizontal bases and tops typically extend according to c-type crystallographic planes.
An “axial” architecture or growth is commonly understood by a person skilled in the art as a stack of layers formed according to z in the figures. During an axial growth, the successive layers grow over the upper surface of the preceding layer, so that the different layers are substantially planar and delimited by the upper surface of the layer on which they rest. For example, for an axial architecture with a cylindrical geometry, the different layers form cylinder portions stacked on top of one another, according to z in the figures.
Conversely, a “radial” architecture or growth is commonly understood by a person skilled in the art as a series of layers “stacked” radially according to a plurality of directions of the plane xy, and forming a so-called “core-shell” geometry. In this architecture, the different layers wrap the core concentrically, like the rings of a tree. For example, for a radial architecture with a cylindrical geometry, the different layers form tubes directed according to z in the figures, with different radii and nested within each other. The “tubes” may be closed on one side without this being considered as an axial structure.
The “axial” and “radial” architectures are structurally perfectly distinct from each other for a person skilled in the art. They typically require very different growth conditions. Hence, a person skilled in the art would distinctly and deliberately select either one of these structures, without assimilating the characteristics of one with those of the other.
In the present patent application, the terms “light-emitting diode”, “LED” or simply “diode” are used as synonyms. A “LED” may also be understood as a “micro-LED”.
By “axial growth”, it should be understood an essentially anisotropic growth occurring according to the longitudinal direction z.
By “radial growth”, it should be understood an essentially isotropic growth covering in particular the surfaces parallel to the longitudinal direction z.
Hereinafter, the following abbreviations relating to a material M are optionally used:
Similarly, the following abbreviations relating to a material M are possibly used:
By a substrate, a layer, a device, “based” on a material M, it should be understood a substrate, a layer, a device comprising this material M alone or this material M and possibly other materials, for example, alloy elements, impurities or doping elements.
A reference frame, preferably orthonormal, comprising the axes x, y, z is represented in some appended figures. This reference frame can be applied by extension to the other appended figures.
In the present patent application, we will preferably talk about thickness for a layer and height for a structure or a device. The thickness is considered according to a direction normal to the main plane of extension of the layer, and the height is considered perpendicularly to the base plane xy of the substrate. Thus, a layer typically has a thickness according to z, when it extends mainly along a plane xy, and a LED has a height according to z. The relative terms “over”, “under”, “underlying” refer to positions considered according to the direction z.
The dimensional values should be understood within manufacturing and measurement tolerances.
The terms “substantially”, “about”, “in the range of” mean, when they relate to a value, “within 10%” of this value or, when they relate to an angular orientation, “within 10°” of this orientation. Thus, a direction substantially normal to a plane means a direction having an angle of 90±10° with respect to the plane.
A first embodiment of the method according to the invention is illustrated in
In practice, it is not possible to form radial portions by simply changing the conditions of growth by MBE of the axial portions. Henceforth, the radial portions of the LED are formed by changing the nitrogen precursor source. According to this embodiment, the nitrogen precursor is typically supplied in the form of an NH3 gas. The NH3 gas pressure is several orders of magnitude higher than the pressure implemented in plasma-assisted MBE. The dissociation of the nitrogen precursor takes place at the substrate, in contact with the surfaces to be covered, in a manner similar to the principle of chemical vapour deposition. This allows depositing a conformal layer of material over a 3D structure. Thus, it is possible to form radial portions covering the substantially vertical walls of the axial portions. Thus, the hole blocking layer 32, the electron blocking layer 31 and/or the passivation layer 33 are advantageously formed radially by metal-organic precursor vapour deposition (MOCVD).
A principle of the method according to the invention is to alternate the formations of the axial 2 and radial 3 portions of the diode 1. This is achieved in this first embodiment by using different nitrogen precursor sources. The axial portions are formed by MBE from an N2 plasma source. The radial portions are formed by MOCVD from an NH3 gas source. This allows combining the relative advantages of these two techniques for the manufacture of a LED with an optimised mixed architecture.
In known manner, the parameters of formation by axial growth of the different axial portions of the LED are adjusted according to the composition, the doping and the thickness desired for each of the axial portions.
Similarly, the parameters of formation by radial growth of the different radial portions of the LED are adjusted according to the composition and the thickness desired for each of the radial portions.
According to one possibility, the axial formations by MBE and the radial formations by MOCVD are done in the same chamber of a deposition frame. This allows avoiding transporting the different portions of the LED between different rooms during manufacturing.
According to one possibility, the axial formations by MBE are done in a first chamber and the radial formations by MOCVD are done in a second chamber of the same deposition frame or of two different deposition frames. This allows avoiding purging the chamber. This reduces the duration of the process.
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Preferably, the top 231 of the upper portion 23 is cleared (
Advantageously, this first embodiment of the method allows forming an LED 1 comprising alternating axial 2, 21, 22, 23 and radial 3, 31, 32, 33 portions.
A second embodiment of the method may be considered.
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According to one possibility, the axial formations by MBE and the radial formations by MBE are carried out in the same chamber of a deposition frame comprising two nozzles of the same nitrogen precursor oriented differently, or one single nozzle that can be oriented during the manufacturing process according to at least two different angles. This allows avoiding transporting the different portions of the LED between different rooms during manufacturing.
According to one possibility, the axial formations by MBE are carried out in a first chamber and the radial formations by MBE are carried out in a second chamber of the same deposition frame or of two different deposition frames. This avoids the need for an orientable nozzle system or multiple nozzles in the same chamber. This improves the robustness of the method. This allows forming different portions of different LEDs in parallel.
This second embodiment of the method according to the invention is illustrated in
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Advantageously, this second embodiment of the method allows forming a LED 1 comprising alternating axial portions 2, 21, 22, 23 and radial portions 3, 31, 32, 33. This second embodiment also allows forming 3D LEDs according to different networks of varied steps. This second embodiment of the method has a reduced cost and duration compared to alternative structure passivation techniques.
The invention also relates to a LED as described and illustrated throughout the above-described method steps.
The invention is not limited to the previously-described embodiments.
According to one example, several angles larger than 30°, for example α2, α3, α4 may be implemented in the formation of the different radial portions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2106198 | Jun 2021 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/065627 | 6/9/2022 | WO |
