SEMICONDUCTIVE STRUCTURE FOR FORMING VERTICAL CAVITY LASER DIODES

Abstract

The invention relates to a semiconductor structure for forming one or more vertical cavity laser diode(s) comprising: - a stack of semiconductor layers based on III-V compounds defining an upper Bragg mirror, disposed on an active layer consisting of at least one quantum well allowing laser emission at a defined wavelength, said active layer being disposed on a lower Bragg mirror, the laser emission being intended to exit the stack through the lower mirror, - a semiconductor contact film, directly disposed under the lower Bragg mirror, the contact film having a first thickness and a first complex refractive index n1' = n1 + i.k1, with n1 being the first refractive index and k1 the first extinction coefficient for the defined wavelength; a semi-insulating gallium arsenide support substrate; and an oxide layer in contact with a front face of the support substrate and positioned between said front face and the contact film. The oxide layer has a second thickness and a second complex refractive index n2' = n2 + i.k2, with n2 the second refractive index and k2 the second extinction coefficient for the defined wavelength. In the structure, the oxide layer is in direct contact with the contact film, the first and second extinction coefficients are less than or equal to 1, and the first and second thicknesses are respectively defined by: h1 = m1*λ / (4n1) and h2 = m2*λ / (4n2), with m1 and m2 being integers. No figure.

Description

Title of the invention: SEMICONDUCTOR STRUCTURE FOR FORMING VERTICAL CAVITY LASER DIODES FIELD OF THE INVENTION

[0001] The present invention relates to the field of semiconductors and particularly optoelectronics. It relates to a semiconductor structure for forming one or more vertical cavity laser diode(s).

[0002] TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] Vertical cavity surface emitting laser diodes (VCSEL for "vertical-cavity surface-emitting laser") are experiencing increasing development in emerging consumer applications such as facial recognition in the field of mobile telephony in particular, and remote sensing (Lidar for "light detection and ranging") for automobiles.

[0004] The VCSEL 100 are produced from stacks of III-V semiconductor layers, by successive epitaxial growth ([Fig.l]). The composition, doping and thickness of each layer are finely defined to form, on the one hand, an active region 2 consisting of one or more quantum wells allowing the generation of the laser beam, and on the other hand, two Bragg mirrors 3a, 3b sandwiching the active region 2 and consisting of alternating layers with high and low refractive indices.

[0005] It is known to form the stack of layers for VCSEL 100 on a bulk substrate 1, as illustrated in [Fig.l], for example in gallium arsenide (GaAs) for laser wavelengths between 650nm and 1300nm, or in indium phosphide (InP) for laser wavelengths between 1300nm and 2000nm. Said bulk substrate 1 must have excellent quality to ensure the epitaxy seed function and guarantee the quality of the stack of layers. However, when the VCSEL(s) is(are) intended to emit from the rear face, that is to say that the laser beam will pass through the bulk substrate 1, the problem of absorption of the signal by said substrate 1 arises.For example, an n-type GaAs substrate is conventionally used as a bulk substrate 1 to build the layer stack for 100 VCSELs; such a substrate is too absorbent in the laser emission wavelength range (typically 0.5qm - 1.5qm) to ensure high performance of 100 VCSELs.

[0006] To address this problem, document WO2021 / 125005 proposes to carry out a transfer of layer(s) from a bulk substrate 1 to a support substrate whose properties are better suited to the emission constraints on the rear face of VCSELs. 100.

[0007] To carry out a layer transfer, an assembly step is required. The assembly must make it possible to preserve the quality of the transferred layer(s); it must also avoid causing disturbances in the operation of the VCSEL 100.

[0008] SUBJECT OF THE INVENTION

[0009] The present invention proposes a solution simplifying and making more reliable the manufacture of vertical cavity laser diodes (VCSEL). It relates in particular to a semiconductor structure comprising a support substrate made of semi-insulating gallium arsenide on which there is a stack of semiconductor layers based on III-V compounds, defining a lower Bragg mirror, a laser active layer and an upper Bragg mirror. A semiconductor contact film and an oxide layer, interposed between the stack of layers and the support substrate, have particular characteristics in terms of thickness and optical absorption, to give the laser diodes a high level of performance while ensuring simple and robust manufacture of the semiconductor structure.

[0010] BRIEF DESCRIPTION OF THE INVENTION

[0011] The present invention relates to a semiconductor structure for forming one or more vertical cavity laser diode(s) comprising:

[0012] - a stack of semiconductor layers based on IILV compounds defining an upper Bragg mirror, arranged on an active layer consisting of at least one quantum well allowing laser emission at a defined wavelength, said active layer being arranged on a lower Bragg mirror, the laser emission being intended to exit the stack of semiconductor layers via the lower Bragg mirror, - a semiconductor contact film, directly arranged under the lower Bragg mirror, the contact film having a first thickness and a first complex refractive index noted ni' = ni + i.ki, with ni first refractive index and ki first extinction coefficient for the defined wavelength, - a semi-insulating gallium arsenide support substrate having a front face and a rear face, - an oxide layer in contact with the front face of the support substrate, and arranged between said front face and the contact film, the oxide layer having a second thickness and a second complex refractive index noted n2' = n2 + i.k2, with n2 second refractive index and k2 second extinction coefficient for the defined wavelength.

[0013] The semiconductor structure is remarkable in that: - the oxide layer is in direct contact with the contact film, on the side opposite the lower Bragg mirror, - the first and second extinction coefficients are less than or equal to 1, and - the first and second thicknesses are respectively defined by: hi = mi*X / (4ni ) and h2 = m2*X / (4n2), with mi and m2 integers.

[0014] According to advantageous characteristics of the invention, taken alone or in any feasible combination: - the first and second extinction coefficients are less than or equal to 0.1; - the contact layer and the oxide layer have extinction coefficients over the wavelength range 0.5 pm - 2 pm, less than or equal to 1, preferably less than or equal to 0.1; - the oxide layer is formed by silicon oxide, glass or zinc oxide; - the lower Bragg mirror comprises less than seven alternations of layers of gallium arsenide and aluminum arsenide, for an X wavelength between 800 pm and 950 pm; - the semiconductor structure comprises a direct bonding interface, between the oxide layer and the contact film; - the semiconductor structure comprises a direct bonding interface, between the oxide layer and the support substrate; - the semiconductor structure includes a direct bonding interface, in the oxide layer.

[0015] The present invention also relates to a method of manufacturing a semiconductor structure such as above.

[0016] According to a first variant, the manufacturing method comprises the following steps: - the development of the stack of semiconductor layers on an initial substrate provided with a layer of van der Waals material, in the following order: the upper Bragg mirror formed on the layer of van der Waals material, the active layer formed on the upper Bragg mirror and the lower Bragg mirror formed on the active layer, - the development of the contact film on the lower Bragg mirror, thus forming a donor structure comprising the initial substrate, the layer of van der Waals material, the stack of semiconductor layers and the contact film, - the formation of the oxide layer, in whole or in part, on the contact film and / or on the support substrate, - assembly by direct bonding between the donor structure and the substrate support, along a bonding interface involving a face of the oxide layer or being included in the oxide layer, - separation at the level of the van der Waals material layer, to give rise to the semiconductor structure.

[0017] According to a second variant, the manufacturing process comprises the following steps: the development of the contact film on an initial substrate, thus forming a donor structure, the formation of the oxide layer, in whole or in part, on the contact film and / or on the support substrate, the assembly by direct bonding between the donor structure and the support substrate, along a bonding interface, involving a face of the oxide layer or being included in the oxide layer, the removal of at least a part of the initial substrate, giving rise to a free face of the contact film, the development of the stack of semiconductor layers on the free face of the contact film, in the following order: the lower Bragg mirror arranged on the contact film, the active layer arranged on the lower Bragg mirror, and the upper Bragg mirror arranged on the active layer.

[0018] Advantageously, the removal step involves separation along a buried fragile plane parallel to the bonding interface and recovery of all or part of the initial substrate. Brief description of the drawings

[0019] Other characteristics and advantages of the invention will emerge from the detailed description which follows with reference to the appended figures in which:

[0020] [Fig-1] [Fig.l] shows a semiconductor structure for the manufacture of a VCSEL, according to the state of the art;

[0021] [Fig.2a]

[0022] [Fig.2b] Figures 2a and 2b show semiconductor structures conforming to the invention;

[0023] [Fig.3a]

[0024] [Fig.3b] Figures 3a and 3b show thickness tables respectively for a contact film and for an oxide layer, in an example of a semiconductor structure according to the invention;

[0025] [Fig.4a]

[0026] [Fig.4b]

[0027] [Fig.4c]

[0028] [Fig.4d]

[0029] [Fig.4e]

[0030] [Fig.4f] Figures 4a, 4b, 4c, 4d, 4e, 4f show steps of a method of manufacturing a semiconductor structure according to a first variant embodiment of the invention;

[0031] [Fig.5a]

[0032] [Fig.5b]

[0033] [Fig.5c]

[0034] [Fig.5c']

[0035] [Fig.5d]

[0036] [Fig.5e]

[0037] [Fig.5f] Figures 5a, 5b, 5c, 5c', 5d, 5e, 5f show steps of a method of manufacturing a semiconductor structure according to a second variant embodiment of the invention;

[0038] [Fig.6] [Fig.6] shows a graph illustrating the reflectivity as a function of the number of alternating layers of gallium arsenide and aluminum arsenide, for a Bragg mirror of the state of the art and for a lower Bragg mirror of a semiconductor structure according to the invention.

[0039] Some figures are schematic representations which, for the sake of readability, are not to scale. In particular, the thicknesses of the layers along the z axis are not to scale relative to the lateral dimensions along the x and y axes.

[0040] The same references in the figures may be used for elements of the same nature. DETAILED DESCRIPTION OF THE INVENTION

[0041] The invention relates to a semiconductor structure 150, specially adapted for optoelectronic applications, for forming one or more vertical cavity laser diode(s).

[0042] The semiconductor structure 150 may be in the form of a wafer, the diameter of which is for example between 50 mm and 200 mm: in this case it is intended to accommodate a plurality of optoelectronic components (VCSEL) which may subsequently be individualized. It may alternatively be in the form of a smaller vignette, accommodating an optoelectronic component or a group of components.

[0043] As illustrated in [Fig.2a], the structure 150 comprises a stack E of semiconductor layers based on IILV compounds defining an upper Bragg mirror 30b, an active layer 20 consisting of at least one quantum well allowing laser emission at a defined wavelength X, and a Bragg mirror lower 30a. The active layer 20 has two faces, one is in contact with the upper Bragg mirror 30b, the other with the lower Bragg mirror 30a. The names "upper" and "lower" for the Bragg mirrors 30a, 30b of the stack E are used with reference to the (x,y,z) reference frame noted in [Fig.2a], the direction of the z axis indicating "the top"; along the z axis, the mirror referenced 30a is therefore at a lower level and the mirror referenced 30b is at a higher level in the structure 150. They are also used to identify the mirror through which the laser emission of wavelength X is intended to exit: this is a so-called rear-face emission, that is to say that the laser wave will exit through the lower Bragg mirror 30a, the one closest to the support substrate 10 which will be described later.

[0044] The layers 30b, 20, 30a of the stack E, and in general the layers or substrates described below, extend parallel to a main plane (x,y) and have a thickness along the z axis.

[0045] As is known per se, the active layer 20 may be formed from gallium arsenide (GaAs) and / or its ternary or quaternary compounds (such as in particular AlGaAs, InGaAs, InGaAsN, etc.); it may alternatively be formed from indium phosphide (InP) and / or its ternary or quaternary compounds (such as for example, InAsP, GaAlInP, etc.). Bragg mirrors, made up of alternating layers of low and high refractive index, are also known multilayers that can be formed from gallium arsenide (GaAs), aluminum arsenide (AlAs) and their compounds or indium phosphide (InP) and its compounds, depending on the nature of the active layer 20. Let us recall that the E stack is produced by epitaxy, which requires the use of materials whose lattice parameter and chemistry are perfectly compatible.

[0046] The thickness of the stack E can be between 1 and 15 qm.

[0047] As mentioned previously, the semiconductor structure 150 comprises a support substrate 10 made of semi-insulating gallium arsenide transparent to the wavelength of the laser, having a front face 10' and a rear face 10”. The stack of layers E is arranged on the front face 10'. The laser emission of wavelength X is intended to exit through the rear face 10” of said substrate 10. The semi-insulating gallium arsenide, which has a high optical transparency, will weakly absorb the laser wave, which is a prerequisite for efficient operation of the VCSELs. Its thickness is for example between 200 and 2000 qm.

[0048] The structure 150 also comprises a semiconductor contact film 40, directly arranged under and in contact with the lower Bragg mirror 30a of the stack E. The contact film 40 has a first thickness hi and a first complex refractive index ni', noted ni' = ni + i.ki, with ni the first refractive index and ki the first extinction coefficient, for the wavelength X.

[0049] The structure 150 finally comprises an oxide layer 50 in contact with the front face 10' of the support substrate 10 on one side, and arranged in contact with the contact film 40 on the other side. As will be described later with reference to the method for manufacturing the structure 150, the oxide layer 50 will provide a significant advantage in the assembly step because its surface preparation is easy and proven, and such a layer ensures excellent quality (low defectivity and high energy) of direct bonding (without adding adhesive material). Advantageously, the material of the oxide layer 50 is amorphous, so as to limit the stress field linked to the bonding of two materials whose crystal lattices are not aligned and / or whose lattice parameters are different, and so as to avoid the formation of nano-bubbles at the bonding interface.

[0050] The oxide layer 50 has a second thickness h2 and a second complex refractive index n2', noted n2' = n2 + i.k2, with n2 the second refractive index and k2 the second extinction coefficient, for the wavelength X.

[0051] The materials of the contact film 40 and of the oxide layer 50 are chosen so that the first ki and second k2 extinction coefficients are less than 10, less than or equal to 1, or even less than or equal to 0.1 (for the wavelength X). Advantageously, the extinction coefficients of the contact film 40 and of the oxide layer 50 are less than 10, less than or equal to 1, or even 0.1, in the wavelength range [0.5qm - 2pm]. They are chosen as close to zero as possible (at least at the wavelength X) to limit the attenuation of the light signal intended to pass through said layers 40, 50.

[0052] The contact film 40 may in particular be produced from n- or p-type GaAs in order to serve as an electrical contact layer; it then has an extinction coefficient ki equal to 0 at a wavelength X of 940 nm. In the wavelength range X=1.55 μm and on an InP-based stack, the contact film 40 may be produced from n- or p-type InP.

[0053] The oxide layer 50 can be formed by a silicon oxide (SiOx, SiO2) whose extinction coefficient k2 is equal to 1.1 103 at X=940nm. Other materials can compose the oxide layer 50, such as zinc oxide (k2 = 7.10 3 for X=0.94pm and k2 = 2.103 for X=1.55qm) or a glass.

[0054] Of course, the examples of materials given for the contact film 40 and for the oxide layer 50 are not exhaustive.

[0055] Furthermore, according to the invention, the first hi and second h2 thicknesses, respectively of the contact layer 40 and of the oxide layer 50, are defined by:

[0056] hi = mi*X / (4ni) and h2 = m2*X / (4n2),

[0057] with mi and m2 integers which may be identical or different.

[0058] These thickness rules, applied to the two layers 40,50 intercalated between the lower Bragg mirror 30a and the support substrate 10, make it possible to assimilate the contact film 40 and the oxide layer 50 to layers of the mirror 30a. This provides, on the one hand, the advantage of not disturbing the laser emission because the wave only sees, in fact, a Bragg mirror before entering and passing through the support substrate 10, and not intercalary disturbing layers. On the other hand, the number of layers necessary to produce the lower Bragg mirror 30a can be reduced since the contact film 40 and the bonding layer 50 will participate in the reflection of the laser wave.

[0059] For example, a lower Bragg mirror 30a may comprise less than seven alternations of layers (GaAs / AlAs) to achieve a reflectivity of 95%, instead of thirteen alternations ([Fig.6]). According to another example, a lower Bragg mirror 30a, formed from InP and / or its compounds, may comprise less than ten alternations of layers to achieve a reflectivity of 95% instead of more than thirty alternations.

[0060] The design rule relating the thickness to the ratio between the wavelength and the refractive index is usually applied to the multilayers forming the Bragg mirrors. On the other hand, it is never envisaged in the state of the art to apply this same rule to the underlying layers of the lower mirror. Indeed, these are usually chosen in such a way that they do not have a refractive index contrast and consequently do not contribute to the reflection of light unlike the constituent layers of the Bragg mirror.

[0061] For example, in the case of a stack E based on GaAs or its compounds (laser wavelength X=940nm), a contact film 40 in n-type GaAs and an oxide layer 50 in SiO2: - the thickness hi of the contact film 40 may be chosen from the thicknesses indicated in the table in [Fig.3a]. It is recalled that the refractive index ni is 3.5060 for X=940nm, - the thickness h2 of the oxide layer 50 can be chosen from the thicknesses indicated in the table in [Fig.3b]. It is recalled that the refractive index n2 is 1.4512 for X=940nm.

[0062] The semiconductor structure 150 according to the invention is thus particularly suitable for forming very high-quality vertical cavity laser diodes (VCSELs), both because the assembly step (required to carry out a transfer of a useful layer onto the support substrate 10) is simplified and made more reliable, and because the laser wave does not undergo any disturbances after leaving the lower Bragg mirror 30a.

[0063] The semiconductor structure 150 according to the present invention further comprises a direct bonding interface 60, extending in the main plane (x,y), said interface 60 being included in or adjacent to the oxide layer 50. The bonding interface 60 may be located between the oxide layer 50 and the contact film 40, between the oxide layer 50 and the support substrate 10, or even in the oxide layer 50.

[0064] Direct bonding means bonding that does not require an adhesive material and is based on molecular adhesion between the assembled surfaces. Several types of direct bonding exist, which differ in particular by the conditions of temperature, pressure, atmosphere or treatments prior to bringing the surfaces into contact. Examples include bonding by molecular adhesion at room temperature with or without prior plasma activation of the surfaces to be assembled, bonding by atomic diffusion ("Atomic diffusion bonding" or ADB according to English terminology), bonding with surface activation ("Surface-activated bonding" or SAB), etc.

[0065] The oxide layer 50, which is particularly favorable to bonding, can be chosen to be more or less thick according to requirements, provided that its thickness h2 satisfies the rule linking it to the ratio between the wavelength X and the refractive index n2 of said layer 50. This provides great flexibility in carrying out the assembly step, which is often easier with an oxide layer thickness greater than or equal to 150nm.

[0066] [Fig.2b] shows a semiconductor structure 150 according to the invention, which has undergone additional steps for the preparation of the optoelectronic components (VCSEL). A second contact film 41 is formed on the upper Bragg mirror 30b, an electrical contact 80,81 is formed on each of the two contact films 40,41, and a dielectric layer 90 is provided around the stack E to electrically insulate it. These steps are known and are not detailed further here.

[0067] The method for manufacturing the semiconductor structure 150 is based on state-of-the-art layer transfer technologies by bonding and thinning. Particularly suitable for the transfer of thin layers, the Smart Cut™ method can be mentioned in particular. For the transfer of thicker layers, 2DLT technology (“2D material-based Layer Transfer” in English terminology) can also be implemented.

[0068] Two variants of the manufacturing method according to the invention will now be described. First variant of the process:

[0069] A first step of the method comprises providing an initial substrate 70 suitable for producing the stack E of semiconductor layers, for example, an n-type GaAs substrate for a stack based on GaAs and / or its compounds ( [Fig.4a]). The first step also comprises providing the support substrate 10. Each of the substrates 10, 70 has a front face 10', 70' and a rear face 10”, 70”.

[0070] This first variant is based on 2DLT transfer technology, which involves a material called 2D material or van der Waals material (2D because essentially two-dimensional), interposed between the layer or stack to be transferred and the initial substrate on which said stack is formed, and at which subsequent detachment will be possible. In the present case, the van der Waals material is used to separate the stack E of semiconductor layers from the initial substrate 70.

[0071] A layer of van der Waals material 75 is formed on the initial substrate 70. A van der Waals material is defined as a material consisting of atoms strongly bonded to each other by covalent or ionic bonds only in the plane of formation of the material (here, plane (x,y)), without strong bonds perpendicular to this plane. Materials such as graphite, graphene, MoS2, WSe2, h-BN, ... are van der Waals materials or 2D materials. From a practical point of view, one or more layers of graphene (or other 2D material) can thus be used, preferably a monolayer, as detailed for example by Celesta Chang et al. in “Remote Epitaxy”, Nature Methods, June 2022, or in document WO2017 / 044577 Al. The graphene layer can be obtained, for example, by a wet transfer method of a layer obtained by CVD onto a catalytic metal substrate.It is noted that the crystal pattern of the initial substrate 70 is capable of guiding the crystal growth of the layers of the stack E through the graphene layer when it is sufficiently thin, preferably from 1 to less than 10, preferably from 1 to 3 graphene sheets.

[0072] The second step corresponds to the development of the stack E of semiconductor layers at least on the side of the front face 70' of the initial substrate 70, on the layer of van der Waals material 75, in the following order: the upper Bragg mirror 30b formed on said layer 75, the active layer 20 formed on the upper Bragg mirror 30b and the lower Bragg mirror 30a formed on the active layer 20 ([Fig.4b]). This stack is produced by successive epitaxies using the initial substrate 70 as an epitaxy seed. These steps, known from the state of the art, will not be detailed here.

[0073] The third step consists of the development of the contact film 40 on the lower Bragg mirror 30a, thus forming a donor structure 130 comprising the initial substrate 70, the layer of van der Waals material 75, the stack E of semiconductor layers and the contact film 40 ([Fig.4c]). Here again, the contact film 40 can be developed by epitaxy. It meets the conditions stated previously in terms of thickness and optical properties.

[0074] During the fourth step, the oxide layer 50 is formed, in whole or in part, on the contact film 40 and / or on the support substrate 10 ([Fig.4d]). In other words, the oxide layer 50 can be produced entirely on the side of the donor structure 130 or entirely on the side of the support substrate 10 as illustrated in [Fig.4d]; it can still be formed partly on the side of the donor structure 130 and partly on the side of the support substrate 10: after assembly (which occurs in the next step of the process), the two parts of the oxide layer 50 will be bonded along a bonding interface 60, to constitute, together, the oxide layer 50 of the semiconductor structure 150.

[0075] The oxide layer 50 may be formed by any known deposition technique, such as in particular, chemical vapor deposition (CVD) possibly with PECVD plasma (“Plasma Enhanced Chemical Vapor Deposition”), possibly with a high density plasma HDP (“High density Plasma”), physical vapor deposition (PVD), atomic deposition ALD (Atomic Layer Deposition). The deposition is typically carried out at a temperature between 100°C and 700°C. The oxide layer 50 meets the conditions stated above in terms of thickness and optical properties.

[0076] The fifth step comprises the assembly by direct bonding between the donor structure 130 and the support substrate 10, on the side of their respective front faces 10', 70', along a bonding interface 60, to form a bonded assembly 140. In the example illustrated in [Fig.4e], the direct bonding is carried out by bringing the free faces of the contact film 40 and the oxide layer 50 into intimate contact.

[0077] As mentioned previously, direct bonding by molecular adhesion at room temperature, or alternatively direct bonding under controlled atmosphere and temperature (ADB or SAB type) can be carried out. It is of course possible to provide cleaning or activation of surfaces (for example by plasma) prior to bringing the faces to be assembled into contact. These surface preparations are known and proven on an oxide layer, which is why it may be advantageous to form a portion of the oxide layer 50 on each of the surfaces to be assembled. Furthermore, the fact that the bonding interface 60 involves a face of the oxide layer 50 or is included in the oxide layer 50, said layer being able to be relatively thick (typically greater than 150 nm), ensures better management of the chemical species involved in the bonding by molecular adhesion and therefore better quality of the interface 60.

[0078] The bonded assembly 140 can advantageously undergo a heat treatment for the consolidation of the bonding interface 60, typically at a temperature between 150°C and 600°C, for a few minutes to a few hours.

[0079] A sixth step of the method corresponds to the separation of the bonded assembly 140 at the level of the layer of van der Waals material 75, to give rise to the semiconductor structure 150 according to the invention ([Fig.4f]). The separation can be caused by a mechanical stress applied in the peripheral region of the bonded assembly 140, for example at the level of the layer of van der Waals material 75, and / or by the addition of a top layer (not mentioned) presenting a constraint allowing detachment.

[0080] At the end of the sixth step, we obtain on the one hand the semiconductor structure 150, and on the other hand, the initial substrate 70 ([Fig.4f]).

[0081] Even if this first variant has been particularly described with reference to 2DLT technology, any other known transfer technique compatible with the stacking of layers to be transferred could, of course, be implemented in the method according to the invention. Second variant of the process:

[0082] The first step of the method according to this second variant comprises the provision of an initial substrate 70 suitable for the production of the stack E of semiconductor layers ([Fig.5a]). The first step also comprises the provision of the support substrate 10. Each of the substrates 10, 70 has a front face 10', 70' and a rear face 10”, 70”.

[0083] The second step corresponds to the development of the contact film 40 directly on the initial substrate 70, thus forming a donor structure 130' ([Fig.5b]). The contact film 40 is developed by epitaxy and will serve, later in the process, as a seed for the epitaxial growth of the stack E.

[0084] During a third step, the oxide layer 50 is formed, in whole or in part, on the contact film 40 and / or on the support substrate 10 ([Fig.5c]). The description given in the fourth step of the first variant of the method applies here. In the example illustrated in [Fig.5c], a part 51 of the oxide layer 50 is formed on the contact film 40 and another part 52 is formed on the support substrate 10; after assembly, these two parts 51, 52 will constitute the oxide layer 50, which must satisfy the conditions of thickness and optical properties previously stated.

[0085] The fourth step of this second variant comprises the assembly by direct bonding between the donor structure 130' and the support substrate 10, on the side of their respective front faces, along a bonding interface 60, to form a bonded assembly 140'. In the example illustrated in [Fig.5d], the direct bonding is carried out by bringing the free faces of the parts 51, 52 of the oxide layer 50 into intimate contact. The description given in the fifth step (assembly) of the first variant of the method also applies here.

[0086] A fifth step of the second variant of the method corresponds to the removal of all or part of the initial substrate 70 from the bonded assembly 140', to give rise to an intermediate structure 145' comprising the support substrate 10, the oxide layer 50, the contact film 40 and potentially a residual layer 72 of the initial substrate 70 ([Fig.5e]).

[0087] Advantageously, the removal step involves separation along a fragile plane. buried 71 parallel to the bonding interface 60 and the recovery of all or part of the initial substrate 70. For this, and as is known from the Smart Cut™ process, it is possible to provide, prior to the assembly step (fourth step), an additional step comprising the introduction of light ions into the initial substrate 70 (or alternatively into the contact film 40) so as to form a buried fragile plane 71 which delimits, with a front face of the donor structure 130', the layer which will be transferred, namely a part 51 of the bonding layer, all or part of the contact film 40, and potentially a residual layer 72 of the initial substrate 70 ([Fig.5c]').Typically, in an initial GaAs substrate, an ion implantation of helium or hydrogen or of these two ions, at a dose of 1E+16 at / cm2 to 5E+17 at / cm2 and an energy of the order of 100keV makes it possible to form the buried fragile plane 71 which will allow the transfer of 500 nm (He implantation) to 700 nm (H implantation) of layer thickness. Note that cleaning and surface preparation may be carried out beforehand and / or after implantation, so as to eliminate potential particulate, organic and / or metallic contamination.

[0088] Returning to the fifth step of the method according to the second variant, said step comprises the separation along the buried fragile plane 71, due to the presence and / or growth of cavities and microcracks in said plane. As is known per se, such a separation takes place, for example, during a heat treatment capable of causing the development of the cavities and their pressurization, and of leading to the spontaneous propagation of a fracture wave in the buried fragile plane 71. The separation heat treatment typically corresponds to an annealing at 200°C, for a duration ranging, for example, from 30 min to 2 h. Alternatively or in conjunction with the heat treatment, the separation may be caused by a mechanical stress applied to the buried fragile plane 71.

[0089] At the end of the fifth step, on the one hand, the intermediate structure 145' is obtained, and on the other hand, the remainder 73 of the initial substrate ([Fig.5e]). This step can then be completed by surface treatments (cleaning, polishing, etching) or other smoothing treatments, to improve the surface quality of the contact layer 40 or of the residual layer 72, or even to eliminate this layer 72.

[0090] Even if the fifth removal step has been particularly described with reference to the Smart Cut™ process, any other known technique for transferring monocrystalline thin layers could, of course, be implemented in the process according to the invention.

[0091] A sixth step of the second variant of the method corresponds to the production of the stack E of semiconductor layers on the intermediate structure 145'. If a residual layer 72 of the initial substrate 70 was transferred in the previous step, it can be chosen to remove it before producing the stack E. The latter is then formed in the following order: the lower Bragg mirror 30a formed on the contact film 40, the active layer 20 formed on the lower Bragg mirror 30a and the upper Bragg mirror 30b formed on the active layer 20 ([Fig.5f]). This stacking is carried out by successive epitaxies using the contact film 40 as an epitaxy seed. These steps, known from the state of the art, will not be detailed here.

[0092] At the end of this sixth step, a semiconductor structure 150 according to the present invention is obtained.

[0093] The first and second variants of the manufacturing method may comprise additional deposition and etching steps making it possible in particular to isolate from each other a plurality of laser diodes formed collectively on the same structure 150, to form a contact film 41 on the upper Bragg mirror 30b and to produce an electrical contact 80, 81 on each of the two contact films 40, 41, as illustrated in [Fig.2b],

[0094] The semiconductor structure 150 according to the invention is advantageous because it greatly facilitates the steps of preparing the surfaces before assembly and provides excellent bonding quality, due to the presence of the oxide layer 50 at the bonding interface 60. This layer 50 also eliminates the risk of dislocations forming between the crystals of the assembled materials. The design rules for the thicknesses and absorption properties, defined for the contact film 40 and the oxide layer 50, prevent these layers, interposed between the lower Bragg mirror 30a and the support substrate 10, from degrading the performance of the VCSELs, by disturbing or absorbing the outgoing laser emission, before it passes through the support substrate 10.The contact film 40 and the oxide layer 50 according to the invention also participate in the mirror function and make it possible to implement a smaller number of layer alternations for the manufacture of the lower Bragg mirror 30a, which constitutes another point of simplification of the manufacturing process.

[0095] Of course, the invention is not limited to the embodiments described and variant embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

Claims

1. Semiconductor structure (150) for forming one or more vertical cavity laser diode(s) comprising: - a stack (E) of semiconductor layers based on III-V compounds defining an upper Bragg mirror (30b), arranged on an active layer (20) consisting of at least one quantum well allowing laser emission at a defined wavelength (X), said active layer (20) being arranged on a lower Bragg mirror (30a), the laser emission being intended to exit the stack (E) of semiconductor layers via the lower Bragg mirror (30a), - a semiconductor contact film (40), directly arranged under the lower Bragg mirror (30a), the contact film (40) having a first thickness (hi) and a first complex refractive index (ni') noted ni' = ni + i.ki, with ni first refractive index and ki first extinction coefficient for the defined wavelength (X), - a support substrate (10) made of semi-insulating gallium arsenide having a front face (10') and a rear face (10”), - an oxide layer (50) in contact with the front face (10') of the support substrate (10), and arranged between said front face (10') and the contact film (40), the oxide layer (50) having a second thickness (h2) and a second complex refractive index (n2') noted n2 ' = n2 + i.k2, with n2 second refractive index and k2 second extinction coefficient for the defined wavelength (X), the semiconductor structure (150) being characterized in that: - the oxide layer (50) is in direct contact with the contact film (40), on the side opposite the lower Bragg mirror (30a), - the first (ki) and second (k2) extinction coefficients are less than or equal to 1, and - the first (hi) and second (h2) thicknesses are respectively defined by: hi = mi*X / (4ni) and h2 = m2*X / (4n2), with mi and m2 integers.

2. Semiconductor structure (150) according to the preceding claim, wherein the first (ki) and second (k2) extinction coefficients are less than or equal to 0.

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3. Semiconductor structure (150) according to one of the preceding claims, wherein the contact layer (40) and the oxide layer (50) have extinction coefficients over the length range wave 0.5 qm - 2 qm, less than or equal to 1, preferably less than or equal to 0.

1.

4. Semiconductor structure (150) according to one of the preceding claims, wherein the oxide layer (50) is formed by a silicon oxide, a glass or a zinc oxide.

5. Semiconductor structure (150) according to one of the preceding claims, in which the lower Bragg mirror (30a) comprises less than seven alternations of layers of gallium arsenide and aluminum arsenide, for a wavelength / . between 800 qm and 950 qm.

6. Semiconductor structure (150) according to one of the preceding claims, comprising a direct bonding interface (60) between the oxide layer (50) and the contact film (40).

7. Semiconductor structure (150) according to one of claims 1 to 5, comprising a direct bonding interface (60) between the oxide layer (50) and the support substrate (10).

8. Semiconductor structure (150) according to one of claims 1 to 5, comprising a direct bonding interface (60) in the oxide layer (50).

9. A method of manufacturing a semiconductor structure (150) according to one of the preceding claims, comprising the following steps: - producing the stack (E) of semiconductor layers on an initial substrate (70) provided with a layer of van der Waals material (75), in the following order: the upper Bragg mirror (30b) formed on the layer of van der Waals material (75), the active layer (20) formed on the upper Bragg mirror (30b) and the lower Bragg mirror (30a) formed on the active layer (20), - producing the contact film (40) on the lower Bragg mirror (30a), thus forming a donor structure (130) comprising the initial substrate (70), the layer of van der Waals material (75), the stack (E) of semiconductor layers and the contact film (40), - forming the layer oxide (50), in whole or in part, on the contact film (40) and / or on the support substrate (10),- direct bonding assembly between the donor structure (130) and the support substrate (10), along a bonding interface (60) involving a face of the oxide layer (50) or being included in the oxide layer (50), - separation at the level of the van der Waals material layer, (75), to give rise to the semiconductor structure (150).

10. A method of manufacturing a semiconductor structure (150) according to one of claims 1 to 8, comprising the following steps: - the development of the contact film (40) on an initial substrate (70), thus forming a donor structure (130'), - the formation of the oxide layer (50), in whole or in part, on the contact film (40) and / or on the support substrate (10), - the assembly by direct bonding between the donor structure (130') and the support substrate (10), along a bonding interface (60), involving a face of the oxide layer (50) or being included in the oxide layer (50), - the removal of at least a portion of the initial substrate (70), giving rise to a free face of the contact film (40), - the development of the stack (E) of semiconductor layers on the free face of the contact film (40), in the following order: the mirror lower Bragg (30a) arranged on the contact film (40),the active layer (20) arranged on the lower Bragg mirror (30a), and the upper Bragg mirror (30b) arranged on the active layer (20).,

11. Manufacturing method according to the preceding claim, in which the removal step involves separation along a buried fragile plane (71), parallel to the bonding interface (60), and recovery of all or part (73) of the initial substrate (70).

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