METHOD FOR THE DEVELOPMENT AND TRANSFER OF A TWO-DIMENSIONAL MATERIAL

Abstract

The invention relates to a method for developing and transferring a two-dimensional material, comprising the following steps: growing the two-dimensional material on a surface of a growth substrate so that the two-dimensional material is bonded to the surface of the growth substrate by van der Waals forces, the surface of the growth substrate having a first contact angle with a drop of a liquid; providing a target substrate, the target substrate having a surface having a second contact angle with a drop of the liquid, the second contact angle being strictly greater than the first contact angle; joining the growth substrate and the target substrate by direct bonding between the two-dimensional material and the surface of the target substrate;rupture of the interface between the growth substrate and the two-dimensional material by propagation of an interfacial crack at the interface between the two-dimensional material and the growth substrate, the crack front being wetted by the liquid. Figure to be published with the abbreviation: Figure 1C;

Description

Title of the invention: METHOD FOR THE DEVELOPMENT AND TRANSFER OF A TWO-DIMENSIONAL MATERIAL TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to the field of two-dimensional materials. The invention relates more particularly to a method for the fabrication and transfer of a two-dimensional material. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides (MoS2, MoSe2, MoTe2, WS2, WSe2...), exist as a monolayer (atomic or molecular) or as a stack of monolayers bonded together by van der Waals forces. These materials exhibit excellent mechanical, electrical, optical, and thermal properties, making them materials of choice for numerous applications in fields as diverse as information technology, communication technology, healthcare, energy, and transportation.

[0003] 2D materials are also considered promising in the field of micro / nanoelectronics, insofar as they allow for the creation of very thin crystalline layers, typically less than a nanometer thick. The fabrication of electronic components (such as transistors or photodetectors) with nanometer dimensions on substrates of various types is thus envisioned, using the two-dimensional material as a semiconductor.

[0004] The growth of a 2D material is generally carried out at a very high temperature (800 °C - 1200 °C), which is often incompatible with the substrate onto which this two-dimensional material is to be integrated, called the target substrate or final substrate. Indeed, this target substrate may already contain components (or parts of components) that would be degraded during the synthesis of the 2D material. To overcome this problem, the growth and integration steps are separated. The 2D material is grown on a suitable growth substrate and then transferred from its growth substrate to the substrate of interest. The main difficulty lies in preserving the intrinsic properties of the 2D material after transfer.

[0005] The document [“Transfer of Large-scale two dimensional semiconductors: challenge and developments”, Watson et al., 2D materials, 8, 2021, 032001] describes several known transfer processes.

[0006] A first category of processes uses a support layer (made of polymer or metal) disposed on the 2D material. This support layer provides mechanical support of the 2D material during transfer, primarily between the moment the 2D material is detached from the growth substrate and the moment it is placed on the target substrate. This helps to reduce deformations of the 2D material during transfer.

[0007] The documents [“Surface-energy-assisted perfect transfer of centimeter-scale monolayer and few-layer MoS2 films onto arbitrary substrates”, Gurarslan et al., ACS Nano, 2014 Nov 25;8(11):11522-8] and [“Large-Area Transfer of 2D TMDCs Assisted by a Water-Soluble Layer for Potential Device Applications”, Madan Sharma et al., ACS Omega 2022, 7, 11731-11741] describe two examples of transfer processes using a polymer layer.

[0008] The documents [“Layer-engineered atomic-scale spalling of 2D van der Waals crystals”, Ji-Yun Moon et al., Matter, 5, 3935-3946, 2022] and [“Controlled crack propagation for atomic precision handling of wafer-scale two-dimensional materials”, Jaewoo Shim et al., Science 362, 665-670, 2018] describe two examples of a transfer process using a metal support layer.

[0009] Using a metal support layer rather than a polymer layer avoids leaving polymer residues on the 2D material. However, metal residues and cracks may be generated on the 2D material.

[0010] A second category of transfer processes does not use a support layer. This avoids any polymer or metal residue on the 2D material.

[0011] The document [“Centimeter-scale Green Integration of Layer-by-Layer 2D TMD vdW Heterostructures on Arbitrary Substrates by Water-Assisted Layer Transfer”, Kim et al., Scientific reports, 9, 1641, 2019] describes a process that does away with the support layer.

[0012] The 2D material placed on the growth substrate is immersed in water. The water then causes immediate delamination of the 2D material, which detaches from the growth substrate and floats to the surface of the water. The 2D material, which is then in the form of a thin film, must then be recovered and placed on the target substrate.

[0013] Allowing a film as thin as the 2D material to float and then be manipulated without mechanical support causes creases and holes in the material. These defects persist once the 2D material is bonded to the target substrate and contribute to degrading the performance of the final device. They are particularly detrimental when optical and photoluminescence applications are considered.

[0014] Furthermore, the manipulation of 2D material in water (or more generally in any type of liquid) is not suitable for the transfer of layers whose surface area is greater than a few square centimeters, nor for the cleanroom environment of the microelectronics industry. Summary of the invention

[0015] There is therefore a need to improve existing processes for transferring a 2D material onto a given target substrate.

[0016] In particular, there is a need for a process of developing and transferring a 2D material which does not generate polymer and / or metal residues, nor mechanical deformations, at the level of the 2D material.

[0017] According to the invention, this need is met by providing a process for the development and transfer of a two-dimensional material, comprising the following steps: • Growth of the two-dimensional material on a growth substrate surface such that the two-dimensional material is bound to the growth substrate surface by van der Waals forces, the growth substrate surface having a first contact angle with a droplet of liquid, • Provision of a target substrate, the target substrate having a surface having a second contact angle with a drop of the liquid, the second contact angle being strictly greater than the first contact angle; • Assembly of the growth substrate and the target substrate by direct bonding between the two-dimensional material and the surface of the target substrate; and • Rupture of the interface between the growth substrate and the two-dimensional material, by applying mechanical stress to the assembly of the growth substrate and the target substrate to generate and propagate a crack front at the interface between the growth substrate and the two-dimensional material, and by placing the assembly of the growth substrate and the target substrate in an environment such that a liquid front forms at the interface between the growth substrate and the two-dimensional material, the mechanical stress being configured so that the crack front is wetted by the liquid.

[0018] Thus, the surface of the growth substrate and the mechanical stress are configured so that the crack front is wetted by the liquid throughout the crack propagation. This allows the two-dimensional material and growth substrate to separate by adhesive rupture, since the crack is confined within the interface between the growth substrate and the two-dimensional material.

[0019] Furthermore, providing a lower wetting capacity (with respect to the liquid) of the target substrate surface than that of the growth substrate surface results in a weakened interface between the growth substrate and the two-dimensional material. Adhesive failure will occur at this interface and not at the interface between the target substrate and the two-dimensional material, which is therefore more resistant to such adhesive failure.

[0020] This allows for a relaxation of the constraints during the implementation of the request mechanical, and therefore to separate the growth substrate from the target substrate (the two-dimensional material being glued to this target substrate) easily, without requiring complex equipment, for example by performing a simple separation by wedge insertion.

[0021] The difference in contact angle, and therefore in wettability, between the surfaces of the target substrate and the growth substrate also makes it possible to obtain a process suitable for a wide variety of combinations of substrates and two-dimensional materials compared to processes based on intrinsic properties of the substrates and / or the two-dimensional material. Indeed, the wettability of substrates is not necessarily an intrinsic property of the substrate. It can be easily acquired (substrates with low wetting potential can be made highly wetting or vice versa) using, for example, physicochemical surface treatments.

[0022] Furthermore, thanks to direct bonding, the target substrate acts as a support layer for the two-dimensional material. This allows the two-dimensional material to maintain mechanical stability during transfer, while eliminating the need for the formation and removal of a support layer (because these steps are no longer required).

[0023] The absence of the support layer also avoids any residue of polymer or metal (these being linked to the support layer), nor does it cause any deterioration because no material is deposited on the 2D material.

[0024] Direct bonding, as well as the fracture step, are also compatible with the microelectronics industry and applicable to large 2D material layers. Direct bonding is indeed commonly used on a large scale and on 300 mm wafers to transfer semiconductor thin films.

[0025] In addition to the characteristics mentioned in the preceding paragraphs, the processing and transfer method according to the invention may have one or more additional characteristics from among the following, considered individually or in all technically possible combinations: • The difference between the first and second contact angles is strictly greater than 10°, preferably greater than 30°, preferably greater than 50°. • The supply of the target substrate includes a treatment of the surface of the target substrate in such a way as to increase the second contact angle. • The process includes, before the two-dimensional material growth step, a surface treatment step of the growth substrate so as to reduce the first contact angle. • The mechanical stress is configured to allow the crack front to advance at a speed less than or equal to 100 pm / s, preferably less than or equal to 10 pm / s, preferably even less than or equal to 1 pm / s. • The mechanical stress is exerted by a blade of lesser thickness or equal to 500 pm, preferably with a thickness less than or equal to 300 pm, preferably even less than or equal to 100 pm. • The blade thickness is less than or equal to 100 pm, and the blade is inserted at a first speed to initiate the crack and at a second speed to propagate the crack in the interface between the growth substrate and the two-dimensional material, the first speed being less than or equal to 1 pm / s, and the second speed being greater than the first speed and less than or equal to 100 pm / s. • During the step of breaking the interface between the growth substrate and the two-dimensional material, the assembly of the growth substrate and the target substrate is placed in the liquid. • The liquid is deionized water or an ionic solution. • During the step of breaking the interface between the growth substrate and the two-dimensional material, the assembly of the growth substrate and the target substrate is placed in a gaseous medium comprising deionized water vapor or vapor of an ionic solution. • The two-dimensional material is preferably graphene, hexagonal crystalline boron nitride (h-BN) or a transition metal dichalcogenide. • The surface of the growth substrate and the surface of the target substrate are each formed from a material chosen from silicon (Si), germanium (Ge), silicon dioxide (SiO2), silicon carbide (SiC), indium phosphide (InP), gallium arsenide (GaAs) and sapphire (Al2O3). • The surface of the growth substrate is formed of a base layer of silicon or germanium covered or not with a surface layer of a material chosen from the following materials: aluminium (Al), silicon nitride (Si3N4), copper (Cu), titanium (Ti), alumina (Al2O3), silicon dioxide (SiO2), hafnium oxide (HfO2), nickel (Ni), graphene. BRIEF DESCRIPTION OF THE FIGURES

[0026] Other features and advantages of the invention will become clear from the description given below, by way of example and not limitation, with reference to the accompanying figures, among which: • Figures IA to IC schematically represent steps in a 2D material processing and transfer method according to the invention; • Figure 2 schematically represents, on the one hand, a drop of liquid deposited on the surface of a growth substrate according to the invention and, on the other hand, a drop deposited on the surface of a substrate target conforming to the invention; • [Fig.3] represents, for different target substrates, measurements of the droplet contact angle of each surface of the target substrate as a function of a concentration of CF4 and SF6, obtained after treatment of these surfaces; • [Fig. 4] is a top-view photograph of the growth substrate obtained after a first example of the implementation of the process shown in Figures IA to IC; the untransferred two-dimensional material appears in dark grey, • [Fig.5] is a top view photograph of the growth substrate obtained from a second example of implementation of the process shown in figures IA to IC, the untransferred two-dimensional material appears in dark grey.

[0027] For clarity, identical or similar elements are identified by identical reference signs throughout the figures. DETAILED DESCRIPTION

[0028] The present invention aims to improve the processes of transferring a two-dimensional material onto a substrate of interest, also called the target substrate.

[0029] Figures IA to IC illustrate steps SI to S3 of a process for developing and transferring a two-dimensional material 10. According to this process, the two-dimensional material 10 is transferred from a growth substrate 20 to a support substrate 30, hereafter referred to as "target substrate 30".

[0030] A two-dimensional (2D) material is defined as a material composed of a single-atom or single-molecular sheet (also called a monolayer) or a stack of N identical single-atom or single-molecular sheets (N being a natural number greater than or equal to 2). Identical sheets are defined as sheets having atoms or molecules of the same type and arranged in the same way. A 2D material is said to be "monolayer" when it comprises only one sheet and "multilayer" when it comprises several sheets. Within each sheet, the atoms (or molecules) are bonded to each other by covalent bonds. The different sheets of a multilayer 2D material are bound together by van der Waals forces.

[0031] Here, the material is considered to have "2D" properties when it comprises fewer than ten single-atom or single-molecular sheets (N<10, for example N=3). Beyond that, its properties are those of a bulk material.

[0032] Step SI represented by [Fig.1A] includes the provision of the growth substrate 20, the growth of the 2D material 10 from a surface 20s of the growth substrate 20, as well as the provision of the target substrate 30.

[0033] At the atomic scale, the 2D material 10 is thus bound (or adheres) to the surface 20s of the growth substrate 20 by van der Waals forces. The interface between the 2D material and the growth substrate 20 is denoted "11" in the figures.

[0034] The 2D 10 material can be graphene, hexagonal crystal structure boron nitride (h-BN) or a transition metal dichalcogenide, such as tungsten disulfide (WS2), molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2) or tungsten diselenide (WSe2).

[0035] The growth substrate 20 is a wafer.

[0036] The growth substrate 20 has a free surface 20s (more simply denoted "surface 20s of the growth substrate" hereafter) serving as support for the growth of the 2D material 10. Preferably, the free surface 20s of the growth substrate 20 corresponds to one of its main faces.

[0037] The growth substrate 20 can be formed from a single material, as shown in [Fig. 1A]. The growth substrate 20 can thus be formed from a material selected from the following: silicon (Si), silicon dioxide (SiO2), sapphire (Al2O3), silicon carbide (SiC), germanium (Ge), gallium arsenide (GaAs), indium phosphide (InP). Alternatively, it can comprise a support layer of a first material, for example silicon, and a surface layer of a second material distinct from the first. The surface layer is disposed on the support layer 11 and forms the surface 20s of the growth substrate. The second material can be selected from the following: aluminum (Al), copper (Cu), titanium (Ti), silicon dioxide (SiO2), silicon nitride (Si3N4), alumina (Al2O3), nickel (Ni), graphene.

[0038] The diameter of the growth substrate 20 can be greater than 200 mm, for example 300 mm.

[0039] By way of example, the growth substrate 20 can be a silicon wafer of 300 mm in diameter, having a thickness generally between 300 pm and 1000 pm, preferably 775 pm and a mechanical stiffness characterized by a Young's modulus of the order of 129 GPa.

[0040] The surface 20s of the growth substrate 20 has a wettability characteristic with respect to a liquid 50 sufficient for the liquid 50 to spread over this surface 20s.

[0041] In the following description, the term "wetting" generally encompasses all phenomena (surface diffusion, mass transfer at the interface, etc.) that occur when a liquid is brought into contact with a solid surface. These phenomena depend on the combination of the nature of the liquid and the material of the solid surface.

[0042] In practice, the wettability of a solid surface is measured by the wetting angle, also called the drop contact angle, or more simply, the angle contact, what happens when a drop of liquid touches a solid surface.

[0043] This contact angle is shown in Figure 2 for the pair {surface 20s of the growth substrate 20; liquid 50], where it is noted "first contact angle #i".

[0044] As shown in Figure 2, the first contact angle is defined between the solid surface 20s and the triple line 50t (triple interface between the external gaseous medium or atmosphere ATM, the liquid 50, and the solid surface 20s) and is strictly less than 90°. Thus, the liquid 50 spreads over the surface 20s of the substrate. Note that, in the following description, the external gaseous medium ATM is considered to be air.

[0045] Preferably, the first contact angle is less than 30°. This allows for better wetting (better spreading) of the liquid 50 on the surface 20s of the growth substrate 20.

[0046] The first contact angle can also be zero. In this configuration, wetting is said to be "total," meaning that a drop of the liquid 50 spreads completely to form a film of constant thickness on the surface 20s of the growth substrate. In practice, total wetting is considered to occur when the contact angle is less than 5°.

[0047] The liquid is advantageously pure and non-volatile, with low surface tension, for example less than 80 mN / m. The liquid is preferably chosen from the following liquids: water or an ionic solution.

[0048] For example, the liquid may be deionized water. It should be noted that when the liquid is water-based, the terms "hydrophobic" and "hydrophilic" are generally used to designate, respectively, a rather unwettable surface and a rather wettable surface.

[0049] In another example, the liquid may be a solution based on potassium hydroxide (or KOH). In yet another example, the liquid may be based on sodium hydroxide (or NaOH) in solution in water.

[0050] Preferably, liquid 50 is deionized and filtered water to avoid any particulate contamination.

[0051] The growth technique used to grow the 2D material 10 on the surface 20s of the growth substrate 20 can be atomic layer deposition (ALD), vapor phase epitaxy (VPE), chemical vapor deposition (CVD) from solid or gaseous precursors, or from metal-organic precursors (MOCVD), plasma-enhanced chemical vapor deposition (PECVD), or molecular beam epitaxy (MBE). It depends on the 2D material to be grown. The technique used generates van der Waals forces between the 2D material 10 and the surface 20s of the growth substrate 20.

[0052] The target substrate 30 is also in the form of a platelet.

[0053] The diameter of the target substrate 30 can be greater than 200 mm, for example 300 mm.

[0054] The target substrate 30 may comprise a support layer 31, for example made of silicon or germanium, and a surface layer 32 disposed on the support layer 31, as represented by [Fig.1A].

[0055] The surface layer 31 is formed of a material chosen from (but not limited to) the following materials: silicon (Si), germanium (Ge), silicon dioxide (SiO2), silicon nitride (Si3N4) or sapphire (Al2O3).

[0056] The target substrate 30 may be intended for the manufacture of integrated circuits and may include electronic components or parts of electronic components, typically in the support layer 31.

[0057] Alternatively, the target substrate 30 is made of a single material, which is then chosen from the following materials: silicon (Si), germanium (Ge), silicon carbide (SiC), gallium asenide (GaAs) or sapphire (Al2O3).

[0058] The supply of the target substrate 30 can thus include a sub-step of depositing the surface layer 32 on the support layer 31.

[0059] The surface layer 32 of the target substrate 30 has a free surface forming the surface 30s of the target substrate 30.

[0060] Preferably, the 30s surface of the target substrate corresponds to one of its main faces.

[0061] The surface 30s of the target substrate 30 has a contact angle 2 between a drop of the liquid 50 and the surface 30s (hereafter referred to as the "second contact angle") strictly greater than the first contact angle (of the surface 20s of the growth substrate 20). This second contact angle 6½, as well as the gap A8 between the first and second contact angles 02, are shown in [Fig. 2].

[0062] Thus, the liquid 50 spreads and flows less well on the surface 30s of the target substrate than on that of the growth substrate 20. The second contact angle ^2 approaches, reaches or exceeds the value of 90° which corresponds to the non-wetting threshold.

[0063] Put another way, by thus providing a second contact angle ^2 greater than the first contact angle, the liquid 50 is rather wetting on the surface 20s of the growth substrate 20, while it is rather non-wetting on the surface 30s of the target substrate 30.

[0064] The greater the difference A8 between the first and second contact angles ^2, the more different the wetting regime will be between the two surfaces 20s and 30s. In this case, the greater this difference A8, the more the liquid 50 will be wetting on the surface 20s of the growth substrate and the less wetting (even non-wetting) it will be on the surface 30s. the surface 30s of the target substrate 30.

[0065] Preferably, the gap A 0 between the first and second contact angles is greater than 10°, preferably greater than 30°.

[0066] Even more advantageously, the second contact angle is more than 50° greater than the first contact angle. This makes it possible to obtain a surface 30s of the target substrate that exhibits a completely non-wetting character. In other words, this ensures that the liquid does not spread on the surface 30s of the target substrate 30.

[0067] The growth substrate 20 and / or the target substrate 30 can be surface treated to obtain the specified (desired) gap A d between the first and second contact angles.

[0068] This type of surface treatment makes it possible to modify the physicochemical wetting properties and therefore the contact angle of the substrates (specifically their surfaces). For example, a silicon or germanium substrate, naturally wettable by most liquids (it has a high-energy surface), can be made less wettable (or hydrophobic, when the liquid is water or deionized water) by these treatments. The growth substrate can also be treated to make it even more wettable: a naturally hydrophilic silicon growth substrate can, in fact, be made even more hydrophilic (or superhydrophilic).

[0069] This allows the use of growth and target substrates 20, 30 of the same nature. For example, the growth substrate 20 and the target substrate 30 can both be silicon or germanium wafers. They can even be derived from the same silicon or germanium source substrate split in two (each part providing a substrate).

[0070] This also provides the advantage of increasing the choice of possible materials for each substrate (growth, target), since their wettability (or contact angle) can be acquired a posteriori, and does not constitute an intrinsic property of the substrate(s). The process according to the invention is thus versatile, in the sense that it is able to adapt to different substrates.

[0071] The supply of the target substrate 30 can thus include a surface treatment of the target substrate intended to increase the value of the second contact angle, i.e. to reduce the wettable character of its surface 30s.

[0072] Thus, before the implementation of this surface treatment, the second contact angle 02 of the surface 30s of the target substrate 30 has an initial value, and it has a final value after the completion of this surface treatment. The final (post-treatment) value of the second contact angle 02 is greater than the initial value of the second contact angle 02 and is strictly greater than the first contact angle G.

[0073] This surface treatment (also called surface functionalization) is useful, for example, when the target substrate is made of silicon, germanium, or silicon dioxide. Indeed, as detailed previously, the target substrate 30 then has, in its natural state, a low initial value for the second contact angle, which makes it wettable with respect to most aqueous liquids.

[0074] Several processes can then be implemented. Each process is adapted for one or more material pairs of the 30s surface of the target substrate and liquid.

[0075] When the target substrate 30 is made of silicon or germanium and the liquid is water or deionized water, a first category of processes consists of coating the surface 30s of the target substrate with a hydrogenated, hydrophobic mat. This makes it possible to obtain a surface 30s of the target substrate having a second contact angle of between 70° and 80°.

[0076] According to one of the processes in this first category, the surface treatment includes a deoxidation operation consisting of removing the thin layer of oxide naturally present (in open air) on the surface of the target substrate 30 (also called native oxide), followed by a passivation operation of the surface 30s of the target substrate by hydrogen atoms.

[0077] This passivation operation of the surface 30s can be carried out by quenching the target substrate 30 in a hydrogen fluoride (HF)-based solution. The HF concentration of this solution is, by mass concentration, greater than 0.01%, preferably greater than 1%, and even more preferably greater than 48%.

[0078] Among the specified concentrations, the highest (1%, 48%) are particularly advantageous. Indeed, they have less impact on the material's roughness, which thus remains very low, on the order of 2 angstroms (the root mean square (RMS) of the asperities). This preservation of the very low surface roughness of the target substrate 30s improves the quality of the bonding performed during step S2.

[0079] Alternatively, the passivation operation can be carried out using hydrogen fluoride in vapor form.

[0080] Alternatively, the passivation operation can consist of a high-temperature vacuum treatment, above 700°C or, preferably, above 800°C. The treatment is then carried out under a reducing atmosphere containing hydrogen. These high-temperature treatments have the effect of reducing the surface roughness of the target substrate obtained after the treatment, which is favorable to good bonding quality during step S2.

[0081] According to another method belonging to the first category of treatment methods, the surface treatment comprises a silicon or germanium epitaxy operation on the surface 30s of the target substrate, so as to obtain a surface layer of silicon or germanium with a thickness greater than 10 nm and even su- greater than 50 nm. Furthermore, before the completion of this epitaxy operation, the target substrate 30 is placed under a reducing atmosphere containing hydrogen.

[0082] A second category of processes consists of coating the surface 30s of the target substrate with a fluorinated, also hydrophobic, mat. This makes it possible to obtain a surface 30s of the target substrate having a second contact angle greater than 40°. This second category of processes offers the advantage of being compatible with silicon dioxide target substrates (in addition to those made of silicon or germanium). The processes in this second category use a fluorinated plasma, for example, a nitrogen plasma comprising carbon tetrafluoride (CF4) or sulfur hexafluoride (SF6).

[0083] Figure 3 represents the contact angles measured on target silicon substrates (labeled "Si bulk" in the figure) and target silicon dioxide substrates (labeled "SiO2" in the figure) as a function of the percentage of SF6 or CF4 mixed with nitrogen.

[0084] As shown in Figure 3, when the target substrate 30 is made of SiO2, the second contact angle varies little with the concentration of SF6 or CF4, being between 45° and 50°. The second contact angle varies more widely with the concentration of SF6 or CF4 when the target substrate is silicon, but second contact angles greater than 40° can be obtained. Thus, for an SF6 concentration between 10% and 50%, the second contact angle is greater than 50°. Moreover, for a CF4 concentration greater than 70%, the second contact angle is greater than or equal to 60°.

[0085] A third category of processes consists of depositing a surface layer having high contact angles on the surface 30s of the target substrate 30. The target substrate 30 thus treated (therefore including this surface layer produced by the surface treatment) constitutes the target substrate 30 used in the next step S2.

[0086] The document “Silane Modification of Glass and Silica Surfaces to Obtain Equally Oil-Wet Surfaces in Glass-Covered Silicon Micromode Applications” by Grate et al., Water Resources Research, 2013, 49 (8), 4724, describes an example of a process belonging to this third category.

[0087] In this example, the surface treatment comprises the formation, on a target substrate 30 of silicon or silicon dioxide, of a superhydrophobic layer based on hexamethyldisilazane (or HDMS). The term “superhydrophobic” is used because of the very high contact angle, typically equal to or close to 100°, which is obtained on the surface once the treatment is complete. The document “Preparation and Characterization of Superhydrophobic Surfaces Based on Hexamethyldisilazane-Modified Nanoporous Alumina” by Tasaltin et al., Nanoscale Res Lett 2011, 6 (1), 487, describes that such an HDMS layer can also be formed on an alumina substrate.

[0088] The document “Delivering Octadecylphosphonic Acid Self-Assembled Monolayers on a Si Wafer and Other Oxide Surfaces. » de Nie et al., J. Phys. Chem. B 2006, 110 (42), 21101-21108, describes another example of a process belonging to the third category. According to this process, the surface treatment may include a self-assembled monolayer (SAM) formation operation. A self-assembled monolayer (SAM) of octadecylphosphonic acid (OPA) is applied to the surface of the target substrate using a nonpolar medium with a dielectric constant of approximately 4 (e.g., trichloroethylene). This process offers the advantage of compatibility with many oxide surfaces. It also results in a lower final residual roughness than that obtained when an HDMS layer is formed.

[0089] As mentioned previously, the growth substrate 20 can also undergo a surface treatment aimed at lowering the first contact angle. In other words, the surface 20s of the growth substrate 20 then has an initial value before the implementation of this surface treatment, and a final value after the completion of this surface treatment. The final (post-treatment) value of the first contact angle is lower than the initial value of the first contact angle and strictly lower than the second contact angle.

[0090] In step S2 of [Fig.IB], the growth substrate 20 (covered with the 2D material 10) and the target substrate 30 are assembled by direct bonding (in other words, without the addition of adhesive or metallic material) between the 2D material 10 and the surface 30s of the target substrate 30. The free surface of the 2D material 10 is thus brought into (direct) contact with the surface 30s of the target substrate 30.

[0091] S2 bonding is generally carried out at ambient temperature and pressure. However, it is possible to carry it out under vacuum and at ambient temperature. Alternatively, S2 bonding can be carried out at a temperature between 25°C and 400°C, for example, at 100°C.

[0092] The 2D material 10 is then bound by van der Waals forces to the surface 30s of the target substrate 30, which here consists of the material of the surface layer 32 (see [Fig. 1B]). The interface between the 2D material and the target substrate 30 is marked “12” in [Fig. 1B].

[0093] The bonding surfaces, namely the free surface of the 2D material 10 and the 30s surface of the target substrate 30, advantageously have a surface roughness of less than 0.5 nm and even less than 0.2 nm. This roughness value, as well as all those given subsequently, are expressed as root mean square (RMS) values. The RMS roughness (denoted Rq) is determined by statistical analysis of an atomic force microscope image, using a 1x1 pm² area as the sample.

[0094] 2D material growth techniques make it possible to obtain a roughness of surface less than 0.5 nm and even less than 0.2 nm. On the other hand, the target substrate 30 may have undergone, between the supply step SI of the target substrate 30 and the S2 step of assembly by bonding, a polishing step (for example by chemical-mechanical planarization or CMP) of its bonding surface 30s so as to obtain a surface roughness value of less than 0.5 nm and even less than 0.2 nm.

[0095] At the end of step S2, the assembly 40 represented by [Fig.1B] is obtained. As shown in [Fig.1B], this assembly 40 is a multi-material monoblock since it comprises successively, from bottom to top in the figure, the target substrate 30, the interface 12, the 2D material 10, the interface II and the growth substrate 20.

[0096] The bi-material interfaces II, 12 have a thickness (measured perpendicular to the plane of the assembly 40) less than or equal to 1 nm on average.

[0097] Finally, step S3 of [Fig.1C] (in particular the figure on the right of [Fig.1C]) consists of separating the growth substrate 20 and the target substrate 30 so that at least part of the 2D material 10 detaches from the growth substrate 20 and remains stuck to the target substrate 30.

[0098] As shown in [Fig.1C] (in particular the figure on the left), the separation of the growth substrate 20 and the target substrate 30 is accomplished by breaking the interface II between the growth substrate 20 and the 2D material 10 by propagation, in this interface II, of an interfacial crack F wetted up to its front Fa by the liquid 50.

[0099] One way to wet the crack front Fa throughout the advance of the crack F is to move the crack front Fa at a sufficiently slow speed so that the liquid 50 spreads out to this front Fa.

[0100] Keeping the crack front Fa wet with the liquid 50 ensures, in particular, that the crack F is constrained in the interface II. In other words, this allows the fracture to be an adhesive type fracture, i.e. that the fracture occurs at the interface II, the 2D material being retained on the surface 30s of the target substrate 30 and the growth substrate being detached from the assembly 40. This allows the 2D material to be transferred successfully.

[0101] Furthermore, this allows the liquid 50 (which forms a film) to be carried over the entire surface 20s of the growth substrate 20, even when the latter has a large surface area (diameter of 200 mm or even 300 mm). It should be noted that, unlike prior art solutions, the liquid 50 cannot naturally infiltrate the surface 20s of the growth substrate here, due to the rigidity of the target substrate and its surface area.

[0102] In this case, thanks to the liquid 50 which wets the crack F up to its front Fa, the growth substrate and the 2D material are interposed by this film of liquid 50 and therefore decoupled.

[0103] The wetting of the crack front Fa by the liquid 50 contributes to reducing the energy interface between the 2D material 10 and the growth substrate 20, thus facilitating the propagation of the crack F, which can then develop more easily into an interfacial crack extending over almost the entire interface II.

[0104] The S3 separation between the growth substrate 20 and the 2D material can in particular be accomplished in the manner described below.

[0105] The assembly 40 is placed in an external environment such that a liquid front 50 forms in contact with the assembly 40 (and, consequently, with the crack front Fa of the interface II).

[0106] This external environment can be a liquid medium consisting of the liquid 50 itself. In this case, the assembly is immersed in the liquid 50.

[0107] Alternatively, the external environment may be a gaseous atmosphere containing an excess of the liquid in vapor form. Thus, the liquid in vapor form condenses upon contact with the assembly 40. For example, the environment may be air with 80% or more humidity (water vapor).

[0108] In order to initiate and propagate the interfacial crack, a mechanical stress is applied to the assembly 40, from initiation sites provided by the assembly 40.

[0109] Preferably, these initiation sites are located on the outer (peripheral) surface of the 2D material 10 of the assembly 40.

[0110] In practice, a blade L, a wedge (or point), or a wire can be inserted between the growth substrate 20 and the target substrate 30, either manually or by machine. Alternatively, a force can be applied to at least one of the substrates (or wafers). Naturally, other means of mechanical constraint can be used. For example, traction can be achieved using jaws bonded to the rear faces of the substrates (growth, target) 20, 30, at the location of the first mechanical stress. It is possible to use only one jaw by clamping (pinching) the growth substrate or the target substrate to a support, for example, with a vacuum. It is possible to replace the jaw(s) with rings that utilize the chamfers of the growth and target substrates 20, 30 to apply traction in the manner described in US9583374B2.

[0111] In the following description, the constraint used to implement the first mechanical stress is considered to be a blade L such as that represented by [Fig.1C]. However, its teachings are also valid for other means of mechanical constraint, in particular the wedge and the wire.

[0112] The mechanical stress can be decomposed into two successive mechanical stresses.

[0113] Initially, a first mechanical stress produces an initial detachment of the 2D material from the growth substrate 20.

[0114] This initial mechanical stress is quite delicate because, during the insertion of the wedge or blade, the cracking speed can quickly exceed 100 pm / s. Indeed, the wedge or blade is located very close to the crack front Fa. Therefore, it is preferable, for this initial mechanical stress, to use methods that allow tensile force to be applied to one or both of the substrates (growth, target). The use of a ring in the chamfer is, for example, well-suited.

[0115] The first mechanical stress may nevertheless produce, undesirably, a crack in the interface 12 between the target substrate 30 and the 2D material 10. This possible presence of a crack in the interface 12 is not problematic, however, for the reasons given below.

[0116] Due to the difference between the first and second contact angles, the liquid 50 is wetting for the surface 20s of the growth substrate at interface II of the assembly, but is little or not wetting for the surface 30s of the target substrate at interface 12. Consequently, the liquid 50 from the external environment of the assembly 40 seeps over the edges of this crack or through some structural defects of the 2D material to return and preferentially wet interface II between the 2D material 10 and the growth substrate 20. This then allows the crack to bifurcate at the correct interface (interface II) when the opening rate falls below 100 pm / s.

[0117] On the other hand, this liquid 50 does not seep in along the side or through any defect in the 2D material 10 to reach the interface 12 between the 2D material 10 and the target substrate 30 due to the absence of wetting on the surface 30s of the target substrate 30. No process that could cause a bifurcation of the crack is therefore activated in this interface 12. As a result, this interface 12 remains intact and the 2D material remains bonded to the target substrate 30.

[0118] Thus, thanks to the wetting properties specified for the surfaces 20s, 30s of the growth and target substrates, the separation of the growth substrate and the target substrate is carried out selectively (because it concerns interface II) and simply: immersing the assembly 40 in the liquid 50 is simple to implement and the mechanical stress does not require nanometric precision equipment (on the scale of the thickness of interface II).

[0119] The thickness eL of the blade L is chosen so that the initial detachment is sufficiently slow (gentle) so that the front of this detachment advances at the wetting speed of the liquid 50 on the surface 20s of the growth substrate 20.

[0120] Preferably, a blade with a thickness of less than 500 pm, preferably less than 300 pm, and for example with a thickness of 100 pm can be used to exert the first mechanical stress.

[0121] When the blade thickness is between 500 µm and 100 µm, the speed of The propagation of the initial detachment is generally much greater than the wetting speed of the liquid 50 on the surface 20s of the growth substrate forming interface II. Detachment can then sometimes take place at interface 12 as explained previously.

[0122] To reduce the extent of this initial delamination, and thus maximize the transfer of 2D material, the thickness of the blade can advantageously be reduced to 100 pm or less. This allows control of the propagation speed of the initial delamination front Fa so that it is sufficiently low to avoid or minimize poor initial delamination. In practice, the blade L is connected to a linear drive system configured to move the blade L at a speed that is preferably less than or equal to 100 pm / s.

[0123] The insertion speed of the blade L can then be chosen to be less than or equal to 100 pm / s, preferably less than or equal to 10 pm / s, preferably even less than or equal to 1 pm / s, which corresponds, respectively, to a propagation speed of the detachment front less than or equal to 100 pm / s, preferably less than or equal to 10 pm / s, preferably even less than or equal to 1 pm / s.

[0124] Such ranges of insertion (and crack propagation) rates make it possible to increase the surface area of ​​2D material transferred, since the area corresponding to the initial delamination, where the 2D material is not transferred, is reduced. For example, this area may represent less than 10% of the surface area of ​​the 2D material.

[0125] Subsequently, mechanical stressing continues to propagate the initial detachment and thus develop it into the interfacial crack F. This second phase of mechanical stressing is denoted as the "second mechanical stressing." In other words, the second mechanical stressing "takes over" from the first mechanical stressing.

[0126] The second mechanical stress test can be identical to the first mechanical stress test in that it can be carried out with the same means of constraint (e.g., blade, wedge, wire, rings, jaws). For example, the same blade L as described above can be used and inserted with the same insertion speed as that configured for the first mechanical stress test, namely a speed less than or equal to 100 pm / s, preferably less than or equal to 10 pm / s, and even more preferably less than or equal to 1 pm / s.

[0127] Alternatively, the insertion velocity may be lower during the first mechanical stress than during the second mechanical stress. This compensates for the rapid progression rate of the initial delamination compared to that of the crack F progression. For example, the blade insertion velocity during the first mechanical stress is less than or equal to 1 pm / s, and the blade insertion velocity during the second mechanical stress is less than or equal to 100 pm / s and greater than 1 pm / s. A blade of variable thickness (thinner to initiate the crack than to propagate it) can also be considered.

[0128] This second mechanical stress is exerted on the assembly 40 until the completion of the interfacial rupture II.

[0129] The temperature of the liquid 50 can be regulated between 15 °C and 25 °C, for example to 20 °C.

[0130] Advantageously, the temperature of the liquid 50 can be lowered so as to increase in-situ the contact angle of the surface 30s of the target substrate 30. This increases the non-wetting character of this surface 30s. The interfacial rupture is then more efficient. The temperature of the liquid 50 is preferably greater than or equal to 5 °C and less than or equal to 10 °C.

[0131] Thus, thanks to the wettable character of the surface 20s of the growth substrate 20 and the non-wettable character of the surface 30s of the target substrate 30, as well as the configuration of the mechanical stress and the external environment of the assembly 40, a predominantly, typically up to 90% of the surface of the growth substrate, adhesive-type interface rupture is produced at interface II between the 2D material 10 and the growth substrate 20. This allows the 2D material 10 to be successfully transferred over a large area (typically greater than 90% of the surface of the growth substrate).

[0132] In other words, thanks to the wettable character of the surface 20s of the growth substrate 20 and the non-wettable character of the surface 30s of the target substrate 30, the configuration of the mechanical stress and the external environment of the assembly 40, an adhesive break is avoided at the interface 12 between the 2D material 10 and the target substrate 30, which would result in a lack of transfer of the 2D material.

[0133] The 2D material 10 is thus transferred from the growth substrate 20 to the target substrate 30 without the 2D material being left unsupported. Furthermore, unlike prior art transfer methods, the process avoids the use of a dedicated support layer for holding the 2D material (which must be formed and then removed). The target substrate, which is rigid (given the thickness of the wafers and their Young's modulus), replaces this support layer.

[0134] This prevents the formation of deformations such as creases or holes in the 2D material, since the 2D material is never left without sufficient mechanical support. The process also eliminates the need for the steps of forming and removing the sacrificial support layer. It does not generate any polymer or metal residue, nor does it cause any deterioration, because no material is deposited on the 2D material, unlike prior art transfer processes.

[0135] It follows that the process of Figures 1A-1C is particularly simple to implement This device is compatible with cleanroom environments in the microelectronics industry and enables the transfer of large-area 2D materials. Specifically, it can be used with wafers of 200 mm or 300 mm diameter.

[0136] The process of figures 1A-1C is applicable to both a single-layer 2D material and a multi-layer 2D material.

[0137] When the 2D material 10 is composed of a single mono-atomic or monomolecular sheet, this sheet detaches from the growth substrate 30 during the separation step.

[0138] When the 2D material 10 is composed of a stack of several (typically three) single-atom or single-molecular sheets, this stack detaches in one block from the growth substrate 30 during the separation step.

[0139] A first example of implementation of the development process will now be described.

[0140] Step IF

[0141] A stack of three MoS2 monolayers was obtained on a 200 mm diameter growth substrate, according to the method described in the document [“Development of monolayers of group (VI) transition metal dichalcogenides by surface organometallic chemistry”, S. Cadot, Materials. University of Lyon, 2016. French. NNT: 2016LYSE1075. tel-01530918]. This method includes a low-temperature ALD deposition step (approximately 100 °C) and a high-temperature (900 °C) crystallization step of the material to obtain the typical lamellar structure of 2D materials.

[0142] The growth substrate 20 is a wafer of 200 mm in diameter which comprises a support layer (or base layer) of silicon and a surface layer of SiO2, obtained for example by thermal oxidation of the silicon support layer.

[0143] According to a first embodiment variant, the surface of the SiO2 surface layer is cleaned and has a wetting contact angle of less than 10° before the growth of the 2D material.

[0144] According to a second embodiment, the surface of the surface layer is not cleaned and has (in its natural state) a wetting contact angle of between 20° and 30° before the growth of the 2D material.

[0145] The supplied target substrate 30 is entirely made of silicon.

[0146] In order to increase the droplet contact angle of the surface 30s of the target substrate 30, the target substrate is surface treated.

[0147] The surface treatment comprises silicon epitaxy on the 30s surface of the target substrate. The epitaxy is carried out using dichlorosilane (SiH2Cl2) at 950°C.

[0148] Epitaxy is described in more detail below.

[0149] The surface 30s of the target substrate 30 is first prepared according to a process called "HF last". The preparation includes a chemical cleaning based on caro acid (obtained by a mixture of phosphoric acid H2SO4 and hydrogen peroxide H2O2 at 120°C).

[0150] The cleaning is then followed by rinsing with deionized water, treatment with a mixture of ammonia, hydrogen peroxide and water in proportions of 1 / 1 / 5 at 70°C.

[0151] Rinsing is followed by deoxidation in a 0.1% mass concentration HF bath followed by rinsing with deionized water.

[0152] Each operation (cleaning, rinsing, deoxidation) lasts approximately 10 minutes.

[0153] After this cleaning and chemical preparation, the target substrate 30 is heated to 950°C under 20 mbar of hydrogen for 2 minutes. Silicon epitaxy is then performed on the prepared surface of the target substrate 30 for approximately 1 1 / 2 minutes at 950°C under 20 mbar of SiH2Cl2.

[0154] Finally, the epitaxy is completed by a smoothing anneal at 950°C under 20 mbar of hydrogen H2 for 5 min.

[0155] Images of the epitaxially coated surface of the target substrate 30 were acquired by atomic force microscopy. These images show very low surface roughness, typically less than 0.5 nm. This low roughness is perfectly compatible with direct bonding.

[0156] After epitaxy, the surface of the target substrate is passivated by hydrogen through annealing under a hydrogen-based atmosphere. This surface of the target substrate has a droplet contact angle between 70° and 90°.

[0157] Step S2

[0158] The growth substrate 20 and the target substrate 30 are then assembled by direct bonding between the 2D material 10 and the silicon of the target substrate 30, preferably at room temperature and in ambient air. The bonding is spontaneous with the propagation of a fairly rapid bond wave (approximately 20 s to travel the 200 mm diameter).

[0159] Preferably, direct bonding is carried out immediately after the growth of the 2D material to avoid any particulate contamination.

[0160] Alternatively, the growth substrate and the 2D material are stored in a clean environment free from dust and hydrocarbon contaminants. For example, they are stored in a specific container such as the one supplied by Entegris (DMS Wafer Carrier 8” black antistatic TYP A5 No. 780-550000005). The bonding remains unchanged even after a one-month waiting period.

[0161] Step S3

[0162] The separation of the growth substrate 20 and the target substrate 30 is accomplished by immersion in deionized water (EDI) regulated at 21°C.

[0163] The assembly of the growth substrate and the target substrate can remain submerged for one day before separation. It can remain submerged for a longer period, for example more than 10 days, without any degradation of the bonding being observed.

[0164] The breakdown of the interface between the growth substrate and the 2D MoS2 material (and thus the separation of the growth substrate) is initiated by introducing a blade at a speed of 1 pm / s and an acceleration of 10 pm² / s. This speed is regulated using a linear motor. The hydrophilicity of the SiO2 (on the surface 20s of the growth substrate 20) allows the advance of deionized water at the SiO2 / MoS2 interface and the separation of the substrates at this interface.

[0165] The 2D 10 material is thus transferred onto the silicon target substrate 30 because the interface between the target substrate and the hydrophobic 2D material does not undergo any interfacial fracture. As shown in [Fig. 4], all the MoS2 monolayers are transferred here, covering 90% of the surface of the target substrate (the light gray area marked T corresponds, in this photograph of the growth substrate, to the areas where the 2D 10 material has been transferred. The areas marked NT correspond, on the other hand, to the areas where the 2D 10 material appears because it has not been transferred).

[0166] A second example of implementation of the development process will now be described.

[0167] Step SI:

[0168] In this second example, the same growth substrate 20 with the same 2D MOS2 material is used as in the first implementation example. The operations for preparing the growth substrate and growing the 2D material are also identical.

[0169] The target substrate 30 supplied is another silicon wafer 200 mm in diameter.

[0170] This substrate undergoes a surface treatment that differs from that implemented in the first example.

[0171] The surface treatment here includes cleaning the target substrate 30 with caro acid (obtained by mixing phosphoric acid H2SO4 and hydrogen peroxide H2O2 at 120°C).

[0172] Next, the target substrate 30 is rinsed with deionized water.

[0173] A cleaning based on SCI composed of a mixture of water, ammonia and hydrogen peroxide in proportions of 5:1:1 at 70°C is then carried out on the rinsed target substrate.

[0174] Next, the target substrate is placed in a quench of a 10 wt% HF solution for 20s. This operation removes the native oxide and passesivates the surface with hydrogen.

[0175] It is possible to perform a rinse with deionized water for 10s without disturbing the hydrogen passivation.

[0176] Steps S2 and S3

[0177] The S2 bonding of the target substrate to the 2D material and the S3 separation of the growth substrate are identical to the first example.

[0178] As shown in [Fig. 5], the 2D material transfer is slightly less efficient, i.e., less homogeneous and over a smaller area, than in the first example (see [Fig. 4]). However, the 2D material is transferred at over 80%, which is satisfactory for many applications.

Claims

Demands

1. A method for preparing and transferring a two-dimensional material (10), comprising the following steps: - Growth (S1) of the two-dimensional material (10) on a surface (20s) of a growth substrate (20) such that the two-dimensional material (10) is bonded to the surface (20s) of the growth substrate (20) by van der Waals forces, the surface (20s) of the growth substrate having a first contact angle (θ) with a droplet of a liquid (50); - Provisioning (S1) of a target substrate (30), the target substrate (30) having a surface (30s) having a second contact angle with a droplet of the liquid (50), the second contact angle (S1) being strictly greater than the first contact angle (θ); - Bonding (S2) of the growth substrate (20) and the target substrate (30) by direct bonding between the two-dimensional material (10) and the surface of the target substrate (30s);and - Rupture (S3) of the interface (II) between the growth substrate (20) and the two-dimensional material (10), by applying a mechanical stress to the assembly (40) of the growth substrate and the target substrate to generate and propagate a crack front (Fa) at the interface (II) between the growth substrate (20) and the two-dimensional material (10), and by placing the assembly (40) of the growth substrate and the target substrate in an environment such that a liquid front (50) forms at the interface (II) between the growth substrate and the two-dimensional material, the mechanical stress being configured so that the crack front (Fa) is wetted by the liquid (50).

2. A method according to claim 1, wherein the gap (A 0) between the first and second contact angle 0 ) is strictly greater than 10°, preferably greater than 30°, preferably greater than 50°.

3. A method according to any one of claims 1 to 2, wherein the supply of the target substrate (30) comprises a surface treatment (30s) of the target substrate (30) so as to increase the second contact angle (^2)-

4. A method according to any one of claims 1 to 3, comprising, before the growth step (SI) of the two-dimensional material (10), a surface treatment step (20s) of the growth substrate (20) so as to reduce the first contact angle (#i).

5. A method according to any one of claims 1 to 4, wherein the mechanical stress is configured to allow the crack front (Fa) to advance at a rate less than or equal to 100 pm / s, preferably less than or equal to 10 pm / s, preferably even less than or equal to 1 pm / s.

6. A method according to any one of claims 1 to 5, wherein the mechanical stress is exerted by a blade (L) of thickness (eL) less than or equal to 500 pm, preferably of thickness (eL) less than or equal to 300 pm, preferably even less than or equal to 100 pm.

7. A method according to claim 6, wherein the thickness (eL) of the blade (L) is less than or equal to 100 pm, and the blade (L) is inserted at a first speed to initiate the crack (F) and at a second speed to propagate the crack (F) in the interface (II) between the growth substrate (20) and the two-dimensional material (10), the first speed being less than or equal to 1 pm / s, and the second speed being greater than the first speed and less than or equal to 100 pm / s.

8. A method according to any one of claims 1 to 7, wherein, during the step (S3) of breaking the interface (II) between the growth substrate (20) and the two-dimensional material (10), the assembly (40) of the growth substrate and the target substrate is placed in the liquid (50).

9. A method according to any one of claims 1 to 8, wherein the liquid (50) is deionized water or an ionic solution.

10. A method according to any one of claims 1 to 7, wherein, during the step (S3) of breaking the interface (II) between the growth substrate (20) and the two-dimensional material (10), the assembly (40) of the growth substrate and the target substrate is placed in a gaseous medium comprising deionized water vapor or vapor of an ionic solution.

11. A method according to any one of claims 1 to 10, wherein the two-dimensional material (10) is graphene, hexagonal crystal structure boron nitride (h-BN) or a transition metal dichalcogenide.

12. A method according to any one of claims 1 to 11, wherein the surface (20s) of the growth substrate (20) and the surface (30s) of the target substrate (30) are each formed of a material selected from silicon (Si), germanium (Ge), silicon dioxide (SiO2), silicon carbide (SiC), indium phosphide (InP), gallium arsenide (AsGa) and sapphire (Al2O3).

13. A method according to any one of claims 1 to 12, wherein the surface (20s) of the growth substrate (20) is formed of a silicon or germanium base layer covered with a surface layer of a material selected from the following materials: silicon dioxide (SiO2), silicon nitride (Si3N4), aluminium (Al), copper (Cu), titanium (Ti), alumina (Al2O3), nickel (Ni), graphene.

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