EP3697940A1

EP3697940A1 - Method for synthesising two-dimensional materials

Info

Publication number: EP3697940A1
Application number: EP18786362.6A
Authority: EP
Inventors: Pierre Legagneux; Didier Pribat; Costel-Sorin Cojocaru; Bernard Servet
Original assignee: Centre National de la Recherche Scientifique CNRS; Thales SA; Ecole Polytechnique
Current assignee: Centre National de la Recherche Scientifique CNRS; Thales SA; Ecole Polytechnique
Priority date: 2017-10-20
Filing date: 2018-10-22
Publication date: 2020-08-26
Also published as: WO2019077156A1; FR3072687B1; WO2019077158A1; EP3697941A1; FR3072687A1

Abstract

The invention relates to a method of producing at least one monolayer (L) of a two-dimensional material (mat2D), said two-dimensional material comprising at least one metal element M and one chalcogen element X, and said method comprising: - a step (100) of supplying element M and a step (200) of supplying element X so as to form, in a substrate (Sub, LAu), a zone (Z[M+X]) which comprises atoms of element M and atoms of element X, and - an annealing step (300) for forming the at least one monolayer (L) of two-dimensional material (mat2D) by means of diffusing the aforementioned atoms in said substrate, wherein both supply steps (100, 200) occur by means of ion implantation.

Description

Method of synthesis of two-dimensional materials

FIELD OF THE INVENTION

The invention relates to the field of the synthesis of homogeneous layers and to large surfaces of two-dimensional materials, called 2D materials, made up of at least two different atoms, and more particularly the synthesis of transition metal dichalcogenides or DCMTs (Metal Transition). Dichalcogenides or TMDCs in English). STATE OF THE ART

Two-dimensional Mat2D materials have been developed recently. They have a planar structure and are composed of one to a few monolayers L, each monolayer comprising some atomic planes (typically 1 to 5), the number of planes being a function of the atomic structure. The chemical bonds within a monolayer are covalent.

A well-known example of this type of material are the DCMT transition metal dichalcogenides of general chemical formula MX, MX ₂ or MX ₃ with M metal and chalcogenous X such that S, Te or Se, having a 2D structure.

For example :

MX

M: Fe, Ga

X: S, Se

MX: FeSe, FeS, GaSe, GaTe

MX ₂

M: Mo, W, Zr, Hf, Pt, Nb, Ta, V, Ti, Cr

X: S, Se, Te

MX ₃

M: Ti, Zr X: S, Se, Te

For example, there are to date about 40 distinct lamellar MX ₂ compounds known that can be isolated in monolayers.

FIG. 1 illustrates an example of a monolayer L of a 2D MX _{2 material} comprising 3 atomic planes AtL, a plane of metal atoms M (clear spheres) sandwiched between two planes of atoms X (dark spheres).

FIG. 2 illustrates a stack of three monolayers L of a 2D material of DMCT type MS ₂ (S suffers). For multi-layer materials, monolayers stack up and are bonded by van der Waals forces. The electronic properties of DCMTs depend on their chemical formula and the number of layers, they can be metallic or semiconductors. It is known that the properties of semiconductor DCMTs vary according to the number of monolayers. For example the value of the band gap called GAP increases as the number of monolayers decreases. GAP is indirect for several monolayers and can become direct for a single monolayer

Two-dimensional semiconductor materials (semi2D) have the particularity of having a stable surface. These 2D materials are different from conventional solid 3D materials for which, on the surface, the atoms have unsatisfied chemical bonds, called pendent bonds. Conventionally, the properties of the surface of the 2D materials are different from the surface properties of the bulk materials. For 2D materials without crystalline defects deposited on a flat surface, all the atoms of a monolayer are connected to each other by covalent bonds, there are no bonds hanging on the surface.

The synthesis of this type of material has given rise to an abundant literature. There are currently 4 main methods of synthesis. A first method for the synthesis of DCMT type 2D material is the chalcogenation of metals (or metal oxides) in a sealed ampoule. For this purpose, it is possible to dispose in a vacuum-sealed ampoule of optionally oxidized metal layers (Mo, W, Zr, Hf, Pt, Nb, Ta, etc.) (MoO ₃ ,...) And the chalcogen (sulfur, selenium). , or tellurium). The bulb is arranged in a generally multizone oven. The chalcogen is evaporated and reacts with the optionally oxidized metal layers to form a transition metal dichalcogenide (DCMT).

A second method involves the chemical vapor deposition (CVD) of transition metal dichalcogenides (DCMTs).

The deposition of DCMT can be obtained by gas phase transport techniques, close to chemical vapor deposition (CVD). The main approaches, of which there are 3, are illustrated in Figure 3.

FIG. 3a illustrates the "chalcogenation" of a metal deposit or metal oxide: This approach, the simplest, consists in the sulfurization (or selenization or tellurization) of metal layers Prec possibly oxidized on the surface (for example Mo0 ₃ / Mo0 ₂ ). Annealing temperatures of the order of 1000 ° C. in a sulphurised atmosphere are necessary to complete the reduction of MoO ₂ (initiated at 500 ° C.) and to displace the oxygen.

Figure 3b illustrates the use of two different precursors, Pred for metal oxide (or a volatile precursor) and Prec2 for chalcogen (usually in powder form) that are vaporized and transported by a neutral gas in an area. colder of the oven where they condense in MX ₂ form. For example, for MoS ₂ , Mo0 ₃ and S are vaporized in each of the crucibles. The MoO ₃ decomposes partially during evaporation and the radicals thus formed (MoO _3-X ) then react with S on the substrate Subst to complete the decomposition and generate MoS ₂ .

FIG. 3c illustrates the use of a solid DCMT powder which is evaporated / sublimed at high temperature and is redeposited on the downstream substrate (at lower temperature, "vapor-solid growth") in the tube reaction. For example MoS ₂ (in powder form) can be directly evaporated at ~ 900 ° C under low pressure (20 Torr) and re-condensed as a film on substrates downstream of the tube in an area where the temperature is maintained at ~ 650 ° C.

For the approaches (b) and (c), the growth tends to progress laterally, because the surface of the DCMT does not comprise dangling bonds likely to fix the atoms of M or X. On the other hand, the edges of march of the clusters 2D have unsatisfied bonds, which effectively trap deposited atoms, thereby promoting 2D lateral growth.

A third method consists of CVD growth from metallo-organic precursors (MOCVD growth). "Chalcogenation" of the metal layers is carried out using a metallo-organic precursor, for example (C ₂ H ₅ ) ₂ X. X being sulfur, selenium or tellurium. Deposition can also be carried out by supplying the two metallo-organic precursors. For example, Mo (CO) ₆ for molybdenum and W (CO) ₆ for tungsten, with (C ₂ H ₅ ) ₂ X or with (CH ₃ ) ₂ X and hydrogen (X being sulfur, selenium or tellurium), as shown in Figure 4a. These elements decompose in contact with the substrate. It is also possible to use gases such as H ₂ S or H ₂ Se for the transport of chalcogen.

The result obtained is illustrated in FIG. 4b. It is a multitude of non-contiguous microcrystals on the surface of the substrate. This is because growth starts on nucleation centers and all the gaseous elements then diffuse laterally onto the surface of the substrate to form the crystals. If the growth is rapid, other centers may appear over existing crystals (vertical growth) and the final structure, locally multilayer, is inhomogeneous.

A solution for obtaining homogeneous MoS ₂ and WS ₂ layers with this method has been published in Nature's journal by Kang in 2015 (Nature, 520, 656). This publication demonstrates, as illustrated in FIG. 5, that with a growth time of t ₀ = 26 hours the structure is homogeneous, with carrier mobility in the layers of 30 cm ² V ^"1 s ^{" 1} at room temperature. The extremely slow growth avoids vertical growth on existing crystals and favors lateral growth. The different crystals coalesce and can then grow laterally until they become contiguous for a time t0 = 26 hours.

A fourth method is to use ALD ("atomic layer deposition") synthesis techniques, which conceptually lend themselves to deposition of materials of very low thickness. ALD is a deposition technique developed in the 1970s and resembles the CVD in that it also uses gaseous sources. However, the operating principle is different, and an ALD growth cycle (for example for a binary material consisting of 2 elements) consists of 4 steps, namely:

(i) injection of the precursor of the element 1,

(ii) purging the reactor with a neutral gas, in order to evacuate the excess of precursor 1 as well as the reaction products,

(iii) injection of the precursor of element 2, followed by

(iv) a second purge of the reactor.

It is therefore necessary that the precursors are adsorbed on the surface and that the adsorption is self limited to a single monolayer precursor element 1 step (i) and the element 2 step (iii). This monolayer is generally thermally decomposed (pyrolyzed) so as to deliver the element that is to be deposited. The excess precursor and the decomposition products are removed during the purge phases of the reactor. The ALD deposition of MoS ₂ can use as precursor MoCI ₅ and H ₂ S.

In general, these methods do not make it possible to obtain homogeneous layers, or else with a very long growth time. An object of the present invention is to overcome the aforementioned drawbacks by proposing a method for synthesizing 2D materials consisting of at least two different compounds, a method that is both fast and allows homogeneous layers to be obtained over large areas. DESCRIPTION OF THE INVENTION

The subject of the present invention is a process for producing at least one monolayer of a two-dimensional material, said two-dimensional material comprising at least one metal element M and a chalcogenic element X, the process comprising:

a step of supplying the element M and a step of supplying the element X so as to form, in a substrate, a zone comprising atoms of the element M and atoms of the element X,

an annealing step so as to form the at least one monolayer (L) of two-dimensional material by diffusion of said atoms in said substrate, and in which each supply step (100, 200) is carried out by ion implantation. According to one embodiment, the two-dimensional material consists of two elements, and has a chemical formula of MX or MX ₂ or MX _{3 type} with M metal and X chalcogen,

Preferably the element X is selected from sulfur, tellurium, selenium. Preferably, the annealing step is carried out at a temperature of between 600 ° and 1000 °.

According to one embodiment, the annealing step is preceded by a step of encapsulating said substrate comprising said zone.

Preferably, the total number of implanted atoms per unit area of at least one implanted element is greater than or equal to the product of the number of monolayers to be produced by an atomic density per unit area of said element implanted in a monolayer of said two-dimensional material.

According to one embodiment a temperature and an annealing time are determined so that a diffusion length of the atoms of the implanted element or elements is between 5 and 30 nm.

According to one embodiment, the element X and the element M are implanted substantially at the same depth in said substrate. According to another embodiment, the element X is implanted at a depth greater than the depth of implementation of the element M in said substrate. According to one embodiment, the method further comprises, after the annealing step, a step of selective dissolution of the upper part of the residual substrate.

According to one embodiment, the substrate is insulating and monocrystalline.

Part of the substrate not converted into a two-dimensional material after annealing is called a residual substrate. According to one embodiment, the method comprises, after the annealing step, a step of separating the at least one monolayer of two-dimensional material (mat2D) from the residual substrate and a step of recovering the at least one monolayer of two-dimensional material on a destination substrate.

According to another embodiment, the substrate consists of a metal layer deposited on a second support, the element X and the element M being implanted in said metal layer.

Part of the non-transformed metal layer in two-dimensional material after annealing is called residual substrate. According to one embodiment, the method comprises, after the annealing step, a step of separating the at least one monolayer of two-dimensional material from the residual substrate and a step of recovering the at least one monolayer of two-dimensional material on a substrate. of destination.

According to one embodiment, the separation step comprises a sub-step of detaching said residual substrate from the second support with the aid of a laser, and a selective etching step of said residual substrate.

According to another aspect the invention relates to a device comprising at least one monolayer of two-dimensional material called active layer buried in a chemically inert substrate, said material two-dimensional array comprising at least one metal element M and a chalcogenic element X.

Other features, objects and advantages of the present invention will appear on reading the detailed description which follows and with reference to the appended drawings given by way of non-limiting examples and in which:

FIG. 1, already mentioned, illustrates an example of a monolayer of a 2D material of the MX ₂ type.

FIG. 2, already mentioned, illustrates a stack of three monolayers of a 2D material of the MS ₂ type. Figure 3a, already mentioned, illustrates the "chalcogenation" of a metal deposit or metal oxide:

Figure 3b illustrates the use of two different metal precursors for the chalcogen (usually in the form of powder) which are vaporized and transported by a neutral gas in a cooler zone of the furnace where they condense in the MX ₂ form.

FIG. 3c illustrates the use of a solid DCMT powder which is evaporated / sublimed at high temperature and is redeposited on the substrate placed downstream, at a lower temperature, in the reaction tube. Figure 4a, already mentioned, illustrates a CVD growth method from metallo-organic precursors (growth MOCVD).

Figure 4b illustrates the 2D material obtained by this method, in the form of non-joined crystals. FIG. 5, already mentioned, illustrates the 2D material obtained as a function of the growth time, with a homogeneous material obtained for a growth time of t ₀ = 26 hours.

FIG. 6 illustrates the concentration of the implanted atoms as a function of the depth p, which depends on the energy of the ions E, measured in keV. Figure 7 illustrates the process according to the invention.

FIG. 7bis illustrates the surface diffusion of the metal and chalcogen atoms during the use of gaseous precursors according to the current state of the art.

Figure 7ter shows the growth of MX ₂ layers according to the invention. Fig. 7ter-a illustrates the implanted substrate and Fig. 7ter-b illustrates the 2D material layer obtained in the substrate after annealing.

FIG. 8 illustrates a first option for performing dual implantation, in which the implantation energies of elements X and M are determined so as to present their maximum concentration Rp _x and Rp _M at the same depth.

FIG. 9 shows different ways, in the case of a double implantation, to take account of the exo-diffusion of X. FIG. 10 illustrates the different steps of a first embodiment according to which the substrate according to the invention implantation is preferentially monocrystalline and insulating.

FIG. 11 illustrates an option of the process in which it is sufficient to selectively remove the upper part of the substrate by selective dissolution, to obtain a bare active layer on the lower part of the residual substrate.

FIG. 12 illustrates another option of the method according to the invention, which comprises a step of separating the two-dimensional mat2D material from the substrate, and a step of transferring mat2D onto a destination substrate.

FIG. 13 illustrates an exemplary method for producing an electronic or optoelectronic device in which the device is directly manufactured. device from the mat2D active layer. The device obtained in IV is a transistor.

Figure 14 illustrates the different steps of a second embodiment, in which the elements X and M are implanted in a substrate which is a thin layer of gold.

FIG. 15 illustrates the second embodiment of the method according to the invention comprising a step of depositing a thin layer of gold on the second support, and comprising after the annealing step, a step of separating the two-dimensional material mat2D of the residual metal layer and a step of transferring the mat2D material to a destination substrate. FIG. 16 describes an embodiment of the process described in FIG. 15 in which the separation step comprises a laser separation sub-step for separating the residual gold layer and Sub2, in order to make this residual gold layer accessible, and a selective etching step of this residual layer.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have deduced from their experiments on the synthesis of DCMT that the inhomogeneity of the layers obtained by the methods mentioned above is due, among other things, to the fact that these methods are all based on gaseous precursors which, in contact with the substrate, induce the formation of adsorbates which diffuse on the surface over "large" distances, greater than or equal to 1 micron at the beginning of growth, these adsorbates then releasing the atoms allowing the lateral growth of the crystals of the 2D material (see below figure 7a). It is noted that with this type of diffusion there are locally aggregates composed of one or more monolayers. In addition, obtaining a homogeneous layer is difficult or impossible in a reasonable time. In order to effect a homogeneous contribution of the atoms and to substantially reduce this lateral diffusion length, the 2D material synthesis method according to the invention is based on an ion implantation of at least one of the elements (or atoms) constituting the material. 2D, with an annealing of the zone comprising the different elements.

For the same temperature, the diffusion length of a previously implanted atom in a solid is much smaller (<10-30 nm) than the diffusion length of the adsorbates generated on a substrate using the growth methods of the substrate. state of the art described above (see below).

The method according to the invention is particularly applicable to the production of 2D materials comprising two types of elements, of generic formula A _x B _y : for example an element M metal and a chalcogenic element X, of formula MX, MX ₂ or MX ₃ .

Before detailing the invention, the principle of ion implantation is recalled. Ion implantation is a material engineering process. As its name suggests, it is used to implant the ions of a chemical element in a solid material (target) thereby changing some of the physicochemical properties of this solid, especially at the surface. Ion implantation is used in the manufacture of semiconductor devices, for the surface treatment of metals, as well as for research in materials science. The ions allow both to change the physico-chemical properties of the target, but also the structural properties because the crystal structure of the target can be modified, damaged or even destroyed (amorphization).

Ion implantation equipment generally consists of:

a source of ion production containing the atom to be implanted (gaseous, solid or liquid). Typically a plasma is created, and an electric field applied to the output of this source allows the extraction of ions. This ion beam then passes through a magnetic field where the ion to be implanted is selected according to its atomic mass and its charge. a particle accelerator that uses the electrostatic properties of the ion to increase its energy,

-of a room for the target. The amount of implanted atoms per unit area, called the dose, is equal to the integral over the ionic current implantation time. The dose is the amount of atoms incident on the target per unit area, and is expressed in number of atoms / cm ² .

The fluxes of atoms obtained allow to implant a small amount of ions, this is the main reason why this technique is only used in areas where the modification that is sought is weak or superficial.

Ion acceleration typically reaches energies ranging from 1 to 500 keV or more.

The introduction of dopants into a semiconductor is the most common application of ion implantation. The ions used for doping, such as boron, phosphorus or arsenic, are generally produced from a gaseous source, guaranteeing a high purity of the source. When implanted in a semiconductor, each doping atom typically creates a charge carrier thereby locally changing the conductivity of the semiconductor.

The distribution of the atoms implanted in the target material supposed to be amorphous (or slightly disoriented with respect to its directions of high symmetry in the case of a crystal) is expressed by a concentration of atoms per cm ³ C (p), p being the depth in the target material. The concentration C (p) is a function of the energy of the ions E, measured in keV, and typically takes the form of a Gaussian, illustrated in FIG. 3, of center Rp and of width at half height FWHM (full-width at half-maximum), both a function of the energy E. The higher the energy of the ions, the higher Rp is, that is to say the atoms penetrate deeper into the target material of thickness Th, and more spreading FWHM is important. The integral of C (p) corresponds to the dose of implanted atoms, expressed in number of atoms / cm ² . In general, the depth of penetration is called the average depth of implantation of the atoms. In the case of a Gaussian distribution, the penetration depth corresponds to Rp. Ion implantation equipment receives as input parameters the dose of atoms to be implanted (in number of atoms / cm ² ) and the energy ions (in keV), and then calculates the timp time required for the implantation as a function of the ionic current that can be generated. The inventors have noted that the atomic densities of the monolayers of 2D materials, depending on the size of the elementary mesh and the number of atoms of each element in these, are very well adapted to the use of ion implantation. These densities are typically a few 10 ¹⁵ atoms / cm ² per monolayer

The method according to the invention, shown schematically in FIG. 7, is a method for producing at least one monolayer L of a two-dimensional material mat2D comprising at least one element M and one element X respectively a chalcogen.

The process is particularly suitable for producing DCMT materials of chemical formula of MX or MX ₂ or MX _{3 type} with M metal and X chalcogen.

Preferably, the element X is chosen from the sulfur S, the tellurium T and the selenium Se.

The method 10 according to the invention comprises a step 100 of adding the element M and a step 200 of adding the element X so as to form, in a substrate, a zone Z [M + X] comprising atoms of the element M and the atoms of the element X. In the process according to the invention, each addition step is carried out by ion implantation.

Then annealing step 300 makes it possible to form the at least one monolayer L of two-dimensional material mat2D by diffusion of said atoms in the substrate.

Typically, the two-dimensional mat2D material produced by the process according to the invention consists of one to a few monolayers (see tens) L, and is called active layer. The mat2D active layer can be used for producing electronic or optoelectronic components or devices.

During the co-implantation of M and X elements in a chemically inert substrate. A substrate whose constituent elements do not form parasitic compounds with the elements M or X of the 2D material during annealing is called a chemically inert substrate.

For the formation of a compound of the MX, MX ₂ , MX _{3 type} , the elements X and M form in the substrate a disordered structure out of thermodynamic equilibrium after ion implantation. If a post-implantation annealing is performed at a sufficiently high temperature, the X atoms will tend to combine with the M atoms, in order to lower the total energy of the system and to reach thermodynamic equilibrium. A crystallization of the MX ₂ (or MX or MX ₃ ) crystallization compound is thus observed, which starts where there are the most atoms, ie in the region corresponding to the maximum of the implantation Gaussian. During annealing, the X atoms which are spread out in depth according to a Gaussian profile diffuse towards the peak to participate in the formation of the compound MX ₂ and make the system evolve towards a state of thermodynamic equilibrium.

The active layer thus formed, consisting of one or more monolayers of two-dimensional material according to the implanted dose, is homogeneous and crystalline.

In the case of an implantation of M and X in a metal, for example gold, the diffusion lengths of the M and X atoms are greater and these elements tend to diffuse on the surface of the gold layer.

Thus, once the zone Z [M + X] comprising X and M atoms constituted in a substrate Sub, the annealing is carried out under appropriate conditions, mainly corresponding to a suitable temperature, atmosphere and duration. Preferably, in order to obtain a better crystalline quality, the annealing is carried out at "high" temperature, typically between 600 ° C. and 1000 ° C. According to one embodiment, the annealing is carried out under vacuum or under neutral gas.

But the X element tends to exo-diffuse, that is to say to leave the substrate during annealing to the furnace atmosphere. This exo-diffusion is all the more marked as the zone Z [M + X] is closer to the surface. To limit this exo-diffusion according to one embodiment the annealing is carried out under steam pressure of the element X. According to another embodiment, the annealing is carried out by illumination of the zone Z [M + X] with a laser pulsed which allows to obtain high annealing temperatures (for better crystallization) and a very short annealing time (to limit the exo-diffusion of X atoms). It is possible to use an excimer laser (for example XeCI for a wavelength of 308 nm) or a solid state laser (for example a frequency-doubled or tripled Nd: YAG laser) with pulses lasting for a few dozen hours. ns. The fluence used is 0.1 to 1 Joule per cm ² with the use of 1 to 10 pulses. It will be noted that even if the annealing time is generally much greater (1 to qq με) than the duration of the laser pulse, the annealing times are much shorter than the annealing times in a furnace (minimum 1 minute for a furnace). fast annealing). This type of annealing can be performed under X vapor pressure.

To avoid exo-diffusion, according to another embodiment, the annealing step of the process according to the invention is preceded by a step of encapsulation of the substrate comprising the zone Z with a stable material at high temperature which is either a chemically inert dielectric such as SiO ₂ , Al ₂ O 3, HfO ₂ , Si ₃ N ₄ ..., or a thin layer of material X.

With encapsulation, the annealing step can be carried out under a neutral atmosphere.

The process according to the invention makes it possible, because of the use of ion implantation steps, to produce an active layer of homogeneous two-dimensional material over large areas. With the use of gaseous precursors according to the state of the art molecules that arrive on the surface diffuse laterally on the latter to meet a crystal germ, interact with it and help to grow it. The adsorbates typically travel several microns at the beginning of growth and the nucleation is random, which results in the creation of stacked layers, leading to inhomogeneous deposits (mixture of areas consisting of monolayers and multilayers) as shown in Figure 7bis. With these techniques, the flow of atoms / molecules impacting the substrate is homogeneous but their surface diffusion over large distances (up to a few tens of microns) combined with random nucleation, induce the growth of monolayer and multilayer zones. By way of example, FIG. 7a shows the nucleation of a second L2 monolayer of MX ₂ over the first monolayer L1, and also the presence of zones without any monolayer.

To overcome this difficulty, those skilled in the art use extremely slow growth processes (for example 26 hours for a monolayer) which make it possible to obtain homogeneous growth. Such a process is much too long to be industrialized.

This example shows a flow of atoms M and X but can be generalized. The technique of molecular beam epitaxy (MBE or MBE) generally uses a flow of M atoms and molecules X ₂ , X ₄ , ... X being a chalcogen. The formation of X ₂ is favored by cracking the molecules at high temperature (for example 900 ° C.).

CVD uses a stream of gaseous molecules that dissociate in contact with the surface, forming an intermediate compound diffusing on the surface and then releasing the M and X atoms in the crystal seed MX ₂ .

Figure 7ter shows the growth of MX ₂ layers according to the invention.

With the process according to the invention, the atoms implanted in the zone Z [M + X] are present in this zone in a very homogeneous manner over a large area. In addition they are not located on the surface but in the substrate and do not need to travel over long distances to precipitate in the form MX ₂ (or MX, or MX ₃ ) as shown in Figure 7ter-a.

The diffusion length of the atoms implanted during the annealing must be greater than or equal to the distance between the position of the implanted atom and the position of the atom in the crystallized monolayer. This distance is at most equal to the distance traveled by an implanted atom located at the end of the Gaussian to reach the place where the crystallization starts (typically the peak implantation).

A diffusion length much greater than this distance is undesirable, and therefore it is preferable to obtain a diffusion length substantially equal to this maximum path distance, which typically corresponds to a few Gaussian widths at half height FWHM. For example, 99.7% of the implanted atoms are at a distance of 2.55xFWHM. The maximum distance that must traverse the implanted atoms in order to integrate the crystalline zone MX ₂ is therefore of the order of 3xFWHM.

The use of annealing (choice of temperature and annealing time) allowing diffusion lengths of substantially 3xFWHM thus promotes the growth of monolayer crystalline zones (the number of monolayers is a function of the implanted dose) during the annealing. The result obtained after annealing is illustrated in FIG. 7ter-b for the case of a dose corresponding to an L monolayer.

Preferentially, ion implantation parameters are chosen which make it possible to obtain a FWHM value of 2-3 nm. Therefore the diffusion length necessary for the crystallization of MX ₂ layers is 5-30 nm.

The annealing parameters are chosen with respect to the element (X or M) having the greatest value of FWHM.

Thus, with a diffusion distance of the atoms implanted during the annealing, typically of the order of a few nm to 30 nm (obtained by controlling the annealing parameters such as temperature, atmosphere and duration), the atoms perform a slight displacement compared to the lengths. diffusion observed for the growth methods of those skilled in the art (up to several tens of microns). This avoids local stacks and holes in the MX ₂ layer. The use of ion implantation and suitable annealing thus favors the crystallization of homogeneous MX ₂ layers.

We have seen that the ion implantation allows to control with a great precision the dose of implanted atoms (typically 1 .5% with the rule of the 3σ). The implantation of a precise number of atoms in the substrate makes it possible to obtain in a predictive and deterministic manner after annealing one or more crystalline monolayers of mat2D in a controlled manner, because the number of atoms present in the zone Z [M + X] is substantially the number to form the corresponding number of desired monolayers.

The atomic density per unit area of the elements M and X for a 2D monolayer of material are typically of the order of a few 10 ¹⁵ atoms / cm ^2. This order of magnitude is well adapted to the doses of atoms accessible by ion implantation.

D ₀ X is the atomic density per unit area of the element X in a monolayer of mat2D material and D ₀ M the atomic density per unit area of the element M in a monolayer of mat2D material. To obtain the right number of implanted atoms, advantageously the total number of implanted atoms per unit area of DX and / or DM, corresponding to the dose as defined above, is greater than or equal to the product of the number of monolayers n to achieve by the atomic density per unit area of the implanted element in a monolayer of the two-dimensional material.

When X is implanted, then we have DX> nD ₀ X

It has been seen above that X tends to exo-diffuse. To compensate for this exo-diffusion, DX is typically implanted such that:

nD ₀ X + 10% ≤ DX <nD ₀ X + 100%

When M is implanted, we have DM> nD ₀ M and we implement DM such that:

nD ₀ M <DM <nD ₀ M + 5%

Since the M atoms do not diffuse like the X atoms, the dose of M to be implanted is close to the final number of M atoms in the mat2D material.

The atomic density per unit area D ₀ X of the element X in a monolayer of the two-dimensional material is equal to:

D ₀ X = k D ₀ M, (D ₀ M atomic density per unit area of element M in a monolayer of mat2D material)

With k stoichiometric factor equal to the number of atoms of X for an atom of M in an elemental mesh Mael ₂ D of two-dimensional material. For example, for MoS _2, the unit cell comprises D ₀ Mo = 1.16 x 10 ¹⁵ atoms / cm ² of molybdenum Mo, and therefore 2 times more, ie 2.32 x 1 0 ¹⁵ atoms / cm ² of sulfur S (k = 2 ).

For MoS ₂ : D ₀ S = 2.D ₀ Mo = 2.32 x 10 ¹⁵ atoms / cm ²

If we want to make 3 layers of MoS ₂ , we have

3DoS = 3 x 2.32 x 10 ¹⁵ = 6.96 x 10 ¹⁵ atoms / cm ² .

It is therefore necessary to implant a dose of DS sulfur greater than or equal to this value.

To implement the method according to the invention, it is first necessary to choose the implantation energy E _x and E _M as a function of the desired depth of penetration, itself a function of the targeted application. Then, in a second step, the annealing parameters are determined, mainly temperature T and time t, so that the diffusion takes place under good conditions.

Let D _{T be} the diffusion coefficient of atoms at a temperature T. The diffusion length Ld of the atoms during annealing is approximately given by the formula:

Ld = (D _T ) t ^½ with annealing time.

T and t are determined so that during annealing (see above):

Ld ~ 3xFWHM.

with FWHM being the width at half height of the concentration of the implanted element as a function of the depth.

T and t are determined so that Ld _x and Ld _M are small while respecting Ld _x > 3xFWHM _x and Ld _M ≥ 3xFWHM _M.

In addition it is necessary to limit the temperature and the annealing time so as to allow the M and X atoms to react with each other and form the desired MXn compound while avoiding exo-diffusion. There are also other options to limit the exo-diffusion (see further figure xx). Once the diffusion has been carried out and the mat2D material formed, the Sub ^{r is} the residual substrate in or on which mat2D is constituted. For certain applications according to one embodiment, the method according to the invention comprises, after the annealing step, a step of separating the at least one monolayer of mat2D two-dimensional material from the residual substrate Sub ^r and a step of transfer of mat2D on a Subd destination substrate.

When the mat2D layer is located deep in the substrate, separation is understood to mean either the separation of mat2D from the upper part of the residual substrate, or from the lower part of the residual substrate, or from the two lower and upper portions of the residual substrate, in depending on the separation method used. Examples are given below.

Preferably, the substrate in which the implantation takes place is monocrystalline. Formation of the 2D material after ion implantation of the element (s) into the substrate (which may be a thin layer) requires high temperature annealing to activate diffusion of the implanted species (or species). In order to form a uniform layer of 2D material within the substrate (of the thin layer if appropriate), well-defined diffusion coefficients (for the implanted species) are required. Gold in a thin layer or in a polyc stallin substrate, there are grain boundaries, which bypass the conventional diffusion, leading to inhomogeneous concentrations of implanted species. In other words, when a critical seed of the 2D material has been formed, the latter tends to "pump" the constituents (M and X or B and N) necessary for its growth (and this, using the diffusion the along the grain boundaries), which imbalances the concentration uniformity of the implanted species and leads to the formation of (non-contiguous) precipitates of the 2D material. It is therefore more difficult to obtain an implanted layer of uniform thickness after annealing if one is in the presence of a thin layer or a polycrystalline substrate.

According to a first option for performing double implantation, the energy E _M for implanting the element M and the energy E _x for implanting the element X are determined so as to present respectively a maximum R _pM and R _p x concentration C _M (p) and C _x (p) substantially equal to the depth p _0, as illustrated in Figure 8.

To limit the exodiffusion phenomenon of X, according to a second option, the element X is implanted at a depth greater than the depth of implementation of the element M in the Sub substrate (see FIG. 9c). Remember that the depth is a function of the implantation energy. Zone Z [M + X] here consists of atoms of X and M spatially separated before annealing.

FIG. 9 shows different ways, in the case of a double implantation, to take into account the exo-diffusion of X. The black corresponds to the element X and the dark gray to the element M. FIG. X-ray annealing, Figure 9b illustrates the X dose increase, Figure 9c illustrates the implantation at a different depth of X and M (X deeper), Figure 9d illustrates the encapsulation (before annealed) with a thin layer of X and FIG. 9e encapsulation with a "top substrate".

FIG. 10 illustrates the different steps of a first embodiment in which the Sub substrate (substrate in the sense of implantation) is insulating and preferably monocrystalline. Sub substrate is typically selected from Al ₂ 0 ₃ , MgO, .Quartz, ...

According to an option illustrated in FIG. 10, the substrate Sub is a layer epitaxied on a SubO substrate.

In general, the Sub implantation substrate is a material for which the M atoms have a low solubility and which do not form compounds with X stable at high temperature (600-1000 ° C.).

In FIG. 10a at step 100, M atoms are implanted in Sub at a mean depth p0. In Fig. 10b at step 200 X atoms are implanted in the Sub substrate at substantially the same depth p ₀ . We have seen that DX is greater than nD ₀ X, where n is the number of monolayers desired. We have DM, dose of M ions to implant, slightly greater than or equal to nD ₀ M. Indeed, the M atoms do not tend to exo-diffuse like the X atoms, the DM dose to be implanted is preferably equal to or slightly greater than nD ₀ M (see above).

The respective doses of M DM ions and X DX ions are determined so as to respect the stoichiometry of the elemental mesh of mat2D material and the desired number of layers n. The order of implantation between X and M is indifferent.

An area Z [M + X] having X atoms and implanted M atoms is created in the vicinity of the depth p ₀ in the Sub substrate.

After the annealing step 300 illustrated in FIG. 10c, an active mat2D layer is obtained.

This first embodiment has the advantage that the active layer is buried in the substrate Sub and protected from the ambient air (the layer is passivated). Moreover during the annealing, there will be less exo-diffusion of X atoms, and an encapsulation layer may become unnecessary.

According to an embodiment illustrated in FIG. 1 1 the method according to the invention further comprises after the annealing step, a step 370 of selective dissolution of the upper part of the residual substrate so as to expose the two-dimensional material mat2D. With a 2D material exposed one can easily make contacts and make devices.

According to another option illustrated in FIG. 12, the method 10 further comprises a separation step 350 of the two-dimensional mat2D material of the substrate Sub ^r (lower and upper part) and a step 360 of transfer of mat2D onto a Subd destination substrate.

According to yet another option there is no separation, and devices are directly manufactured based on this buried mat2D active layer as shown in Figure 13 for the manufacture of an electronic (opto) device (transistor, photodetector ,. ..). For example, to make a transistor, a layer of photoresist 12 is deposited, then by photolitography and ion etching ("Ion beam eching") two wells are dug in the Sub material. Then in II one fills these holes of metal and in III by a process "lift off" one keeps only the holes metallized, preferentially in contact with the layer mat2D by the wafer so as to constitute the source S and the drain D of the transistor. Then in IV the grid G is made to finalize the transistor. FIG. 14 illustrates the different steps of the method according to a second embodiment in which the elements X and M are implanted in a Sub substrate which is a thin metal layer. The metal layer is preferably gold but other metals are compatible as a substrate for the process according to the invention. For the rest, L _Au is called the metallic layer, and the examples, given with a layer of gold, are generalized to compatible metals.

An implantation of M and X elements in a gold layer is possible because gold is one of the rare metals that does not constitute chemical compounds (sulphides) with chalcogens at high temperature. Gold thus constitutes a chemically inert substrate as defined above (depending on the 2D material, other metal layers may be used).

The gold layer is itself deposited on a second support / Sub2 substrate chemically inert, insulating and preferably monocrystalline.

The term "support" is used here artificially to avoid confusion with the "substrate" in the sense of implantation, and the substrate Sub2 is a substrate in the common sense of the term.

According to the method and the deposition conditions, the L _Au layer is of polycrystalline or monocrystalline nature.

According to a preferred option, the L _Au layer is monocrystalline, and is produced by epitaxy on Sub2 (by "MBE" for Molecular Beam Epitaxy or "CVD" for Chemical Vapor Deposition) which is necessarily monocrystalline and then serves as growth germ.

The monocrystalline arrangement of the L _Au layer facilitates the organized synthesis of mat2D, i.e., monocrystalline growth (see above).

According to another option, the deposition step of the L _Au layer is carried out by an atomic layer deposition method called ALD for "Atomic Layer deposition" in English. In FIG. 14a at step 100, M atoms are implanted in the thin L _Au gold layer, at a depth pO included in the thickness of the gold layer. Typically L _Au has a thickness of between a few nm and a few tens of nanometers.

Preferably, the implantation energy of each implanted element is determined so that the location of each implanted element is effected in the deposited layer L _Au .

In FIG. 14b at step 200 X atoms are implanted in the L _Au gold layer at substantially the same depth p ₀ .

During the annealing step 300 illustrated in FIG. 14c, a layer of mat2D is preferentially formed at the interface with the air, at the surface of the gold layer. Indeed, the degree of solubility in gold of X and M is very low, the atoms are "pushed" towards the surface. The final result is then a mat2D layer on the surface of a residual metal layer L _Au ^r , itself on an insulating substrate and monocrystalline Sub2.

However, depending on the implantation conditions, the mat2D layer can also be located either inside the L _Au layer or at the interface with Sub2.

Mat2D separation of the layer L _{to the} ^R may be desired in the same manner as for the first embodiment. For some applications mat2D should be separated from the residual layer L _Au ^r .

FIG. 15 illustrates the second embodiment of the method according to the invention comprising a step 100 of deposition of a thin layer of gold on the second support Sub2, and comprising, after the annealing step, a step 310 of separation of the two-dimensional mat2D material of the residual metal layer L _Au ^r and a step 320 of transfer of mat2D material on a Subd destination substrate.

According to one embodiment, the separation is carried out by selective chemical etching of the residual material. The mat2D active layer then floats capillarity on the surface of the etching bath, and is recovered on the Subd destination substrate according to one of the methods known to those skilled in the art for the transfer of graphene, as illustrated in FIG. 12. Preferably the mat2D material forms on the surface of the gold layer or at the interface of the gold layer with Sub2.

For the case in which the mat2D layer is formed at the interface with Sub2, the layer L ^r _Au is found in the upper part and can then be etched chemically. The mat2D layer is then obtained directly (without transfer) on its substrate of insulating origin.

When, after annealing and crystallization of the mat2D layer, a portion L ^r _Au of the gold metal layer located between the mat2D layer and the Sub2 substrate remains, the separation step is not easy to achieve by etching. chemical because the point of entry of the liquid of the attack is only the slice of L ^r _Au .

To solve this problem, according to one embodiment, a separation is used as illustrated in FIG. 16, comprising a sub-step of laser separation to separate L ^r _Au and Sub2, in order to make L ^r _Au accessible, and a sub-step of selective attack of L ^r _Au (by dissolution of the metal).

In 1 6-1 is represented the initial stack consisting here of a layer of mat2D encapsulated in a layer of PMMA or other material (polymer or other) sufficiently rigid and easy to remove after transfer (this layer of PMMA will serve as intermediate support after the delamination step described later) and localized on the residual layer L ^r _Au , itself deposited on Sub2. In 16-2 is shown the sub-step of detachment using a laser beam 50 ("laser lift off" in English). For example, the rectangular beam resulting from a UV laser is transformed, with the aid of adequate optics, into a line (less than 1 mm per several cm) that is scanned on the substrate in a direction perpendicular to line.

In 1 6-3 is shown the substep of selective etching of L ^r _Au by dipping the L ^r _Au / mat2D / PMMA stack in a bath 51 selectively attacking the gold, such as Kl / I ₂ (K: Potassium and l ₂ : iodine). In 16-4 is represented the mat2D / PMMA stack once the selective attack has come to an end.

At 16-5, the mat2D / PMMA stack is transferred to the Subd destination substrate consisting of oxidized silicon (Si0 ₂ on Si) and in 1 1 -6 the PMMA layer is removed and a layer of material is obtained. h-BN on the oxidized silicon substrate.

According to another aspect, the invention relates to a typically electronic or optoelectronic device (transistor, photodetector, ...) comprising at least one monolayer L of two-dimensional material mat2D, called active layer, buried in a substrate Sub chemically inert, said two-dimensional material comprising at least one M metal element and a chalcogen X element.

An exemplary embodiment of such a transistor type device is described in FIG. 13.

The methods for producing 2D materials according to the state of the art only make it possible to produce layers on the surface (see FIG. 7bis). Only the use of ion implantation makes it possible to obtain monolayers of 2D material buried in a substrate.

Claims

1. A process for producing at least one monolayer (L) of a two-dimensional material (Mat2D), said two-dimensional material comprising at least one metal element M and a chalcogen element X, the process comprising: a supply step (100) of the element M and a step (200) of adding the element X so as to form, in a substrate (L _E D, Sub), a zone (Z [M + X]) comprising atoms of the element M and atoms of element X,

an annealing step (300) so as to form the at least one monolayer (L) of two-dimensional material (mat2D) by diffusion of said atoms into said substrate,

and wherein each delivery step (100, 200) is performed by ion implantation.

2. Method according to one of the preceding claims wherein the two-dimensional material (mat2D) has a chemical formula of type MX or MX ₂ or MX ₃ , the element X being selected from Souffre (S), Tellurium (Te), Selenium (Se).

3. Method according to one of the preceding claims wherein the annealing step is carried out at a temperature between 600 ° and 1000 °.

4. Method according to one of the preceding claims wherein the annealing step is preceded by a step of encapsulation of said substrate (Sub) comprising said zone (Z [M + X]).

5. Method according to one of the preceding claims wherein a total number (DX, DM) of implanted atoms per unit area of the at least one implanted element (X, M) is greater than or equal to the product of the number of monolayers to be made (n) by an atomic density per unit area (D ₀ X, D ₀ M) of said implanted element (X, M) in a monolayer of said two-dimensional material.

6. Method according to one of the preceding claims wherein a temperature (T) and a duration (t) of the annealing are determined so that a diffusion length of the atoms of the implanted element or elements is between 5 and 30 nm.

7. Method according to one of the preceding claims wherein the element X and the element M are implanted substantially at the same depth (pO) in said substrate (Sub).

8. Method according to one of claims 1 to 8 wherein the element X is implanted at a depth greater than the depth of implementation of the element M in said substrate (Sub).

9. Method according to one of the preceding claims further comprising after the annealing step, a step (370) of selective dissolution of the upper portion of the residual substrate.

10. Method according to one of the preceding claims wherein the substrate (Sub) is insulating and monocrystalline.

1 1. The method of claim 10 wherein a portion of the substrate not formed into a two-dimensional material after annealing is referred to as a residual substrate, and comprising, after the annealing step, a step of separating (350) the at least one monolayer of two-dimensional material (mat2D) of the residual substrate (Sub ^r ) and a step (320) of recovering said at least one monolayer of two-dimensional material (mat2D) on a destination substrate (Subd).

12. Method according to one of claims 1 to 9 wherein the substrate consists of a metal layer (L _Au ) deposited on a second support

(Sub2), the element X and the element M being implanted in said metal layer (L _Au ).

13. The method of claim 12 wherein a portion of the metal layer not converted into two-dimensional material after annealing is called residual substrate (L ^r _Au ), and comprising, after the annealing step, a step of separating (310) the at least one monolayer of two-dimensional material (mat2D) from the residual substrate (L ^r _Au ) and a step (320) recovering said at least one monolayer of two-dimensional material (mat2D) on a destination substrate (Subd).

The method of claim 13 wherein the step of separating (310) comprises a sub-step of detaching said residual substrate (L ^r _Au ) from the second support (Sub2) with a laser, and a sub-step selective etching of said residual substrate.

15. Device comprising at least one monolayer (L) of two-dimensional material (mat2D) called active layer buried in a substrate (Sub) chemically inert, said two-dimensional material comprising at least one metal element M and a chalcogen X element.