Patent Application
20240213398
This application claims the priority of French Patent Application No. FR2214167, filed Dec. 21, 2023, which is incorporated herein by reference in its entirety.
The field of the invention is that of methods for manufacturing a growth substrate including mesas, and for producing a matrix of diodes adapted to emit or detect, natively, light radiation at various wavelengths.
Methods exist for manufacturing a matrix of light-emitting diodes adapted to emit, natively, light radiation at various wavelengths. The matrix of diodes can then include diodes adapted to emit a red light, other diodes a green light, and also others a blue light. Such a matrix of diodes then forms a microscreen with native RGB (standing for Red, Green, Blue) emission.
The diodes are said to have native emission, in that the active zone of each diode emitting at a given wavelength differs from the active zones of the diodes emitting at another wavelength. In the case of diodes produced based on InGaN, the active zones differ from one another through the proportion of indium in the quantum wells.
Such a matrix of native-emission diodes is thus distinguished from colour-conversion technologies where the diodes all emit at the same wavelength, for example in the blue, and are each covered with a pad including luminophores, for example semiconductor nanocrystals forming quantum boxes, to at least partly convert the incident light into a light of another wavelength.
To manufacture a matrix of native-emission diodes, one approach consists in using a growth substrate having mesas having been partly made porous during an electrochemical porosification step. This electrochemical porosification technique is presented in particular in the article by Griffin and Oliver entitled Porous nitride semiconductors reviewed, J. Phys. D: Appl. Phys. 53 (2020) 383002.
The document EP3840065A1 describes an example of a method for manufacturing a growth substrate and then a matrix of diodes using the electrochemical porosification technique. The method includes producing a growth substrate (also called pseudo-substrate) having a plurality of mesas made from InGaN, each formed by a doped portion of InGaN made porous during an electrochemical porosification step, and an InGaN epitaxial regrowth portion not intentionally doped or weakly doped so that it is not porosified (it remains integral or dense, and therefore non-porous). The diodes are next produced by epitaxy from epitaxial regrowth portions.
The doped portions of the mesas can have different doping levels from one mesa to another, so that the mesas have different porosification levels and therefore different degrees of relaxation. The diodes produced from the various mesas then include quantum wells having a greater or lesser proportion of indium, thus making it possible to obtain emissive pixels at various wavelengths.
However, to obtain mesas the doped InGaN layers of which have various doping levels from one mesa to another, it is necessary to carry out, before the step of producing the mesas, a step of spatially localised implantation of dopants in a full layer of InGaN, which makes the manufacturing method more complex.
The objective of the invention is to at least partly remedy the drawbacks of the prior art, and more particularly to propose a method for manufacturing a growth substrate, adapted to produce a matrix of diodes based on InGaN, and including mesas of different deformabilities, without it being necessary to perform spatially localised implantations of dopants. These mesas of various deformabilities thus make it possible to natively produce diodes based on InGaN that can each emit or detect light radiation at various wavelengths, for example in the three RGB colours.
For this purpose, the object of the invention is a method for manufacturing a growth substrate adapted to produce by epitaxy a matrix of diodes based on InGaN, including the following steps of:
Some preferred yet non-limiting aspects of this manufacturing method are as follows.
The elimination step can be performed by photoelectrochemically etching the separation intermediate portion of at least the mesas M3, the mesas M1 being covered by an encapsulation layer.
The elimination step can be performed by dry etching the upper portion of at least the mesas M3, with etch stop on the separation intermediate portion, the mesas M1 being covered by an etching mask.
After the step of producing the mesas, the separation intermediate portion can be, in each mesa, on and in contact with the lower portion, the method then including a step of producing epitaxial regrowth portions produced based on InGaN, carried out after the electrochemical porosification step, resting on the upper portion in the mesas M1, and resting on the lower portion in the mesas M3.
The method may include, after the elimination and porosification steps and before the step of producing the epitaxial regrowth portion, a step of producing a sealing portion deposited at least on and in contact with the lower portion of the mesas M3 then porosified.
During the step of producing the crystalline stack, an epitaxial regrowth layer may be located on and in contact with the lower layer, so that, after the elimination and porosification steps, the mesas M3 have an upper face formed by an epitaxial regrowth portion from the epitaxial regrowth layer.
The invention also relates to a method for manufacturing a matrix of diodes of categories D1, D2, D3 from a growth substrate, including the following steps of: manufacturing the growth substrate by the method according to any one of the preceding features; then depositing a growth mask, leaving free an upper surface of the mesas M1, M2, M3; then producing the matrix of diodes D1, D2, D3, respectively from the mesas M1, M2, M3.
The invention also relates to a growth substrate adapted to produce by epitaxy a matrix of diodes based on InGaN, including:
Each mesa M1, M2, M3 may include a non-porous epitaxial regrowth portion produced based on doped InGaN, resting, in the mesas M1, on the upper portion, and, in the mesas M3, on the lower portion.
Each mesa M1, M2, M3 may include a non-porous sealing portion produced based on GaN, located, in the mesas M1, between and in contact with the upper portion and with the epitaxial regrowth portion, and in the mesas M3, between and in contact with the lower portion and with the epitaxial regrowth portion.
Each mesa M1, M2, M3 may include an epitaxial regrowth intermediate portion produced based on InGaN and located, in the mesas M1, between the lower portion and the separation intermediate portion, in the mesas M3, on the lower portion.
The epitaxial regrowth intermediate portion of each mesa M1, M2, M3 may be non-porous.
The epitaxial regrowth intermediate portion may be porous in the mesas M1, and non-porous in the mesas M2 and M3.
Other aspects, aims, advantages and features of the invention will appear better upon reading the following detailed description of preferred embodiments thereof, given as a non-limiting example, and made with reference to the appended drawings, wherein:
In the figures and in the following the description, the same references represent identical or similar elements. In addition, the various elements are not shown to scale in such a way as to promote clarity of the figures. Moreover, the different embodiments and variants are not mutually exclusive and could be combined together. Unless indicated otherwise, the terms “substantially”, “approximately”, “in the order of” mean within a 10% margin, and preferably within a 5% margin. Moreover, the terms “between . . . and . . . ” and equivalents mean that the bounds are included, unless stated otherwise.
The invention relates to a growth substrate and the method for manufacturing same. The growth substrate is adapted to produce by epitaxy a matrix of diodes based on InGaN, the diodes making it possible to emit or to detect, natively, light radiation at various wavelengths, for example of RGB (red, green, blue) type.
The growth substrate includes mesas of at least three different categories, noted M1, M2 and M3, having various deformabilities d1, d2, d3, depending on whether or not the mesas include an upper portion based on AlGaN, and depending on whether they include a lower portion based on GaN and/or a porosified or non-porosified epitaxial regrowth portion based on InGaN. Generally, deformability is the ability of the mesa to deform during the production of the diode.
The various deformabilities d1, d2, d3 of the mesas result in the fact that the mesas M1, M2, M3 then have various values of the effective mesh parameter, from one category of mesa to another, at their upper face, during the growth of the diodes. The effective mesh parameter is the mesh parameter, defined in a main XY plane (orthogonal to the growth axis of the layers), of the layer or layer portion considered. In addition, the diodes of categories D1, D2 and D3, produced during the same epitaxy step from the growth substrate, will have different active zones in terms of proportion of indium incorporated into the quantum wells, and therefore will be adapted to emit or detect light radiation at various wavelengths.
The manufacturing method proposes to firstly produce mesas M1, M2, M3 based on GaN, each including a lower portion based on GaN, a so-called separation intermediate portion based on InGaN, and an upper portion based on AlGaN. Then, the upper portion based on AlGaN of at least the mesas M3, and possibly also of the mesas M2, are eliminated by etching, whereas they are kept in the mesas M1. This elimination step may be, in a first embodiment, carried out by photoelectrochemically etching the separation intermediate portion based on InGaN, or, in a second embodiment, carried out by dry etching the upper portions based on AlGaN with etch stop on the separation intermediate portions based on InGaN. The method also envisages porosifying by electrochemical porosification the lower portions of only the mesas M1 and M3, whereas they are not porosified in the mesas M2.
It should be stated that the photoelectrochemical etching is carried out by immersing the growth substrate in a liquid electrolyte, and by illuminating the mesas with an excitation light beam capable of being absorbed only by the separation intermediate portions. The material of these intermediate portions is then oxidised and dissolved by the liquid electrolyte. A low electrical voltage VPECE can be applied to collect the photogenerated electrons. Moreover, it should also be stated that the electrochemical porosification (porosification by electrochemical anodisation) is carried out also by immersing the growth substrate in a liquid electrolyte. An electrical voltage of a higher value VECE is applied, causing the porosification of these desired layer portions. Within the scope of the invention, the electrochemical porosification is non-photo-assisted.
It should be stated here that an electrochemical porosification reaction is a selective reaction in that, for the same electrical voltage Ep of value VECE, a crystalline semiconductor material based on GaN will be porosified if its doping level ND is higher than or equal to a predefined minimum doping level ND,min(VECE). Otherwise, it will not be porosified and will remain integral (dense). In this regard, the document EP3840016A1 illustrates an example of the domain of existence of electrochemical porosification as a function of the doping level ND (here in donors) of the crystalline material based on GaN and of the electrical voltage VECE applied.
Thus, a growth substrate is obtained, where:
An orthogonal three-dimensional direct reference frame XYZ is defined here and in the following description, where the X and Y axes form a main plane of the support substrate 2 and where the Z axis is oriented across the thickness of the growth substrate 20 in the direction of the mesas.
With reference to
The support layer 2 is here produced from a non-porosifiable material, so that it remains non-porous (dense) during the subsequent porosification of the mesas M1 and M3. It may be a material inert to the electrochemical porosification reaction, such as an insulating material (sapphire, etc.) or a semiconductor material (SiC, Si, etc.) not intentionally doped (nid) or weakly doped. It may also be a semiconductor material based on GaN not intentionally doped or weakly doped so as not to be porosifiable at the porosification voltage VECE. By way of example, the support layer can be produced from freestanding sapphire, silicon, SiC or GaN, among others. It has a thickness for example between approximately 200 μm and approximately 1.2 mm. Intermediate layers can be present between the support substrate 2 and the conductive buffer layer 11, produced for example based on AlN. Thus, by way of examples, the following stacks can be used: Si/AlN/AlGaN/GaN; sapphire/nid GaN; SiC/AlN/AlGaN etc., the support substrate 2 may be absent if the conductive buffer layer 11 has a thickness that is sufficient to ensure the mechanical strength of the growth substrate 20.
The crystalline stack 10 includes a conductive buffer layer 11, whereon the mesas M1, M2, M3 rest and that will make it possible to apply an electrical potential to the lower portions 22 of the mesas from a biasing electrode 3. For this purpose, the conductive buffer layer 11 is produced from a crystalline material based on doped GaN (here n-type) over at least one part of its thickness, with for example a doping level of 1018 cm−3. The conductive buffer layer 11 is produced by epitaxy from the support substrate 2. It can have a thickness of between approximately 1 and 10 μm. With this doping level, the conductive buffer layer 11 is not porosified during the subsequent electronic porosification step (with the electrical voltage in the order of approximately 15 V), and makes the circulation of the charge carriers possible.
The lower layer 12 rests on the conductive buffer layer 11 and is in electrical contact with it (and here in physical contact). It is intended to form the lower portions 22 of the mesas, of which only the lower portions 22 of the mesas M1 and M3 will be porosified. It is produced based on GaN, and can, in general, be produced in InxGa1-xN with a proportion of indium x, positive or zero, lower than that of the separation intermediate layer 13 so as not to absorb the excitation light beam during the subsequent photoelectrochemical etching step. It is sufficiently doped in order that the lower portions 22 of the mesas M1 and M3 are porosified during the subsequent electrochemical porosification step. In this example, it is produced from n-type doped GaN with a doping level of approximately 6×1018 cm−3. Its thickness is in the order of several hundreds of nanometres to make a good relaxation of the mechanical stresses of the mesas M1 and M3 possible, for example at least equal to 500 nm, and here is equal to approximately 1000 nm.
The separation intermediate layer 13 rests on the lower layer 12, and here is in contact with it. It is intended to be etched, during the subsequent photoelectrochemical etching step, to ensure the removal of the upper portions 24 of only the mesas M2 and M3 (and not of the mesas M1). Therefore, it is a sacrificial layer. It is produced based on InxGa1-xN, with a non-zero proportion of indium x greater than those of the lower 12 and upper layers 14, for example between 10 and 15%, to be the only one to absorb the excitation light beam during the subsequent photoelectrochemical etching step. In other words, its band gap energy is lower than that of the lower 12 and upper layers 14. In this example, it is produced from InGaN not intentionally doped, with a proportion of indium x of approximately 15%. Its thickness is in the order of a few nanometres, for example of approximately 3 nm. It can thus form a single quantum well or a plurality of quantum wells. It should be noted that InGaN porosifies at an electrical voltage VECE lower than that of GaN, with equivalent doping level. The proportion of indium x and the doping level are selected here in order that the separation intermediate portion 23 is not porosified during the porosification step of
The upper layer 14 rests on the separation intermediate layer 13, and here is in contact with it. It is intended to form the upper portions 24 of the mesas, of which that of the mesas M3 will be eliminated (and here also that of the mesas M2) during the subsequent photoelectrochemical etching step, and that of the mesas M1 will be kept. It is produced based on AlyGa1-yN, with a non-zero proportion of aluminium y, greater than those of the lower 12 and intermediate layers 13. The doping level is selected in order that the upper portion 24 of the mesas M1 is not porosified during the subsequent electrochemical porosification step. Therefore, the material may be not intentionally doped or weakly doped. In this example, it is produced from AlGaN not intentionally doped with a proportion of aluminium y of approximately 10%. Its thickness is in the order of around one hundred nanometres, for example of approximately 100 nm. It should be noted that the upper layer 14 can be covered with a thin protective layer (not shown), for example made from GaN or from InGaN of a thickness in the order of 1 to 3 nm, to avoid the oxidation of AlGaN.
Here, it should be noted that the lower 12, intermediate 13 and upper layers 14 each have a thickness lower than their critical thickness at which there is a plastic relaxation of the mechanical stresses. The total thickness of the crystalline stack 10 is also lower than a predefined critical thickness. Thus, the conductive buffer layer 11 generates, in the lower 12, intermediate 13 and upper layers 14, mechanical stresses (oriented in the XY plane) the value of which is such that, before porosification, the effective mesh parameter is close or substantially equal to the effective mesh parameter of the conductive buffer layer 11, here substantially equal to that of the relaxed GaN (or slightly in compression if it is produced from sapphire).
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Next, a biasing electrode 3 is deposited on and in contact with the conductive buffer layer 11, which makes it possible to apply an electrical potential to the mesas.
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For this purpose, firstly the encapsulation layer 4 is deposited on the mesas M1, in such a way as to completely cover them. The mesas M2 and M3 are not covered by this layer 4. This may be a photosensitive resin, or even a conformal layer of an oxide. The intermediate portion 23 of the mesas M1 will therefore not be in contact with the liquid electrolyte.
Next, the etching of the intermediate portions 23 of the mesas M2 and M3 is carried out. The growth substrate 20 is immersed in a liquid electrolyte making it possible to dissolve the oxidised InGaN formed by the absorption of the excitation light beam. The liquid electrolyte can be acidic or basic, and can be oxalic acid. It can also be KOH, HF, HNO3, NaNO3, H2SO4 or a mixture thereof. It is thus possible also to use a mixture of oxalic acid and NaNO3. The growth substrate 20 is also subjected to an excitation light beam the spectrum of which makes it possible to only excite the intermediate portion 23. It may be a white light lamp associated with a filter, an LED, or even a laser source. Thus, a laser at 405 nm can be used to only excite the intermediate portions 23 made from InGaN with 15% of indium. Moreover, an electrical voltage VPECE can be applied by an electrical generator between the biasing electrode and a counter-electrode inserted in the electrolyte (for example, a platinum grid or wire). It can also be for example at 2 V. This electrical voltage VPECE makes it possible to improve the collection of photogenerated electrons.
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Next, the growth substrate 20 is once again immersed in a liquid electrolyte. The biasing electrode 3 is then connected to the electrical generator in such a way as to apply an electrical voltage of value VECE, for example here equal to 15 V. This results in a porosification of the lower portions 22 of the mesas M1 and M3, which alone are in contact with the liquid electrolyte. The lower portion 22 of the mesa M2 therefore is not porosified. The electrical voltage can be applied for a period ranging from a few seconds to a few hours. A reference electrode can be used to precisely control the electrical voltage applied. During this step, there is no emission of the excitation light beam in the direction of the mesas. Next, the growth substrate 20 is removed from the electrolytic bath, and the encapsulation layer 5 is removed.
Thus, a growth substrate 20 is obtained, of which the mesas, which have various deformabilities, have been obtained from the same initial crystalline stack 10. It is therefore adapted to produce by epitaxy a matrix of diodes making it possible to emit or to receive light radiation at various wavelengths natively, in particular after having produced epitaxial regrowth portions 27 based on InGaN.
Thus, the mesas M2 have a minimal deformability d2, due to the fact that it is not porosified. In this example, it no longer includes the upper portion 24 based on AlGaN. Alternatively, as indicated above, it may have kept it, in which case the upper portion 24 would not be porosified (like the lower portion 22). In any case, the mesas M2 have an effective mesh parameter that is substantially equal to that of the conductive buffer layer 11, or even is substantially equal to the mesh parameter of the relaxed material of the layer 11, due to the low relaxation of the mesas during their production (
The mesas M1 have a deformability d1 greater than d2 due to the porosification of the lower portion 22. In addition, the porosified lower portion 22 makes it possible for the mesas M1 to relax under the effect of the mechanical compressive stresses generated by the non-porosified upper portion 24 based on AlGaN, so that the effective mesh parameter is then lower than that of the mesas M2. However, after this relaxation, the portion 24 will limit the deformation of the mesa M1 during the production of the diodes, and therefore limit the incorporation of indium. The mesas M1 will make it possible to produce diodes D1 emitting for example in blue.
Finally, the mesas M3 have a deformability d3 greater than d2 and different from d1, due to the porosification of the lower portion 22 and to the absence of the upper portion 24 based on AlGaN. The mesas M3 may have, during the production of the epitaxial regrowth portion 27 based on InGaN, an effective mesh parameter greater than that of the mesas M2 due to the mechanical tensile stresses generated by the portion 27. The mesas M3 will make it possible to produce diodes D3 emitting for example in red.
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Next, epitaxial regrowth portions 27 are produced on the mesas M1, M2 and M3 and in particular on that of the mesas M3. These portions 27 can have a thickness of approximately 200 nm and can be produced from InGaN with a proportion of indium in the order of approximately 8%. The epitaxial regrowth portion 27 then causes the deformation of the porosified lower portion 22 of the mesas M3 (tensile stresses), which in return make it possible for the epitaxial regrowth portions 27 to relax. In addition, the mesh parameter at the upper face of the mesas M3 is different from that of the mesas M1.
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Thus, the manufacturing method makes it possible to natively produce diodes D1, D2, D3 that will emit at different wavelengths, here in the three RGB colours. This is possible due to the fact that the mesas M1, M2, M3 have different deformabilities d1, d2, d3. In addition, incorporating indium into the diodes, and in particular into the first n-doped portions then into the quantum wells of the active zones, depends indeed on the deformability of the mesas and of the effective mesh parameter. The higher the effective mesh parameter of an epitaxial regrowth portion (after producing the diodes), the longer the principal wavelength of the corresponding diode. The method for manufacturing the growth substrate 20 thus brings into play a photoelectrochemical etching step, and at least one electrochemical porosification step. Therefore, there is no need to carry out, as in the prior art mentioned above, the steps of localised ion implantation of dopants to obtain the various types of mesas.
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Next, a second dry etching step, for example by RIE etching, is carried out with a second etching agent. An etching mask 7 (hard mask) is deposited to cover the mesas M1 and leave free the upper face of at least the mesas M3, and here also that of the mesas M2. It may also cover the free face of the conductive buffer layer 11. Here, the second etching agent is selected to be able to locally etch the upper portions 24 based on AlGaN of the mesas M2 and M3, and to stop on the intermediate portions 23. Next, the hard mask 7 is removed. After this step, the upper face of the mesas M1 is formed by the non-porosified upper portion 24 based on AlGaN, and those of the mesas M2 and M3 are formed by the intermediate portion 23 still present (which can be etched on a part of its thickness, depending on the operating conditions of the dry etching).
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Particular embodiments have just been described. Various variants and modifications will appear to the person skilled in the art.
Thus, as described above, the upper portion 24 of the mesas M2 can be kept, as for the mesas M1. Moreover, the non-photo-assisted electrochemical porosification steps can be performed before the step of eliminating the upper portion 24 of at least the mesa M3. Finally, the epitaxial regrowth portions 25, 27 can be produced before or during the step of producing the diodes. When they are produced before that of the diodes, they may be porosified, in which case they have a low thickness, or they may not be porosified, in which case they may have a greater thickness.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2214167 | Dec 2022 | FR | national |
