Two-dimensional material stack formation

Abstract

A method for forming a stack of two or more layers, the method comprising: a) providing: a flat surface (1), a first set of walls (3) comprising a first wall (31) and a second wall (32) in physical contact with the flat surface (1) and intersecting at a corner (312) to form an angle, and a first layer (2) formed of a two-dimensional material, the first layer (2) is in physical contact with the flat surface (1) and in physical contact with both the first wall (31) and the second wall (32) at a corner (312), where the angle is adapted to align with the crystal structure of said two-dimensional material and has a tolerance of at most 5 DEG, where the top surface (22) of the first layer (2) is exposed, where each of the first and second walls (31, 32) has a length of from 5 nm to 1000 nm, and where the first and second layers (31, 32) have a thickness of from 5 nm to 1000 nm. Wherein the height of the walls (31, 32), as measured from the exposed top surface (22) of the first layer (2) to the top (30) of the walls, is 0.6 nm to 2 nm, thereby forming a cavity delimited at least by the top surface (22) and the first set of walls (3), followed by b) forming a second layer (4, 7) in the cavity, the second layer being in physical contact with the exposed top surface (22) of the first layer (2).

Description

Technical Field

[0001] The present invention relates to the field of two-dimensional materials, such as transition metal dichalcogenide crystals, and in particular to the formation of layer stacks comprising such two-dimensional materials. Background Art

[0002] New computationally intensive applications, such as the Internet of Things (IoT) and artificial intelligence (AI), require extremely energy efficient and concomitantly improved cost-effectiveness of electronic systems. Modern computer processors are based on field effect transistors (FETs). Since the 1960s, the number of transistors in a typical microprocessor has followed an exponential growth, a trend known as Moore's Law. By shrinking transistors, more transistors are integrated into a single chip, with improved performance and reduced costs. However, as silicon transistors enter technology nodes below 10nm, channel performance decreases dramatically, limiting further device scaling.

[0003] Therefore, it is becoming increasingly important to explore alternative device geometries or new channel materials. Atomically thin two-dimensional materials, especially two-dimensional (2D) semiconductors, have attracted great interest as new channel materials that can facilitate continued transistor scaling. To benefit from continued scaling, scaled 2D transistors need to outperform silicon technology at similar device sizes.

[0004] In theory, transition metal dichalcogenides (MX2, where M and X represent transition metal and dopant, respectively) are a family of materials that could succeed in this task.

[0005] However, the deposition of 2D materials and the control of the crystalline structure, morphology, and material properties remain challenges.

[0006] Furthermore, it is expected that when the channel comprises a stack of two or more layers of a two-dimensional material, such as a transition metal dichalcogenide, the performance may be improved, particularly when the two-dimensional materials are aligned, i.e. have the same orientation. Furthermore, heterogeneous stacking comprising layers of a two-dimensional material alternating with layers of another material, such as a dielectric material, preferably having the same orientation, may improve the interface properties between the two-dimensional material layers.

[0007] Therefore, there is still a need in the art for devices and methods that solve at least some of the above problems. Summary of the invention

[0008] It is an object of the present invention to provide an excellent method for forming a two-layer or more layer stack comprising at least a first layer formed of a two-dimensional material.

[0009] Another object of the present invention is to provide a superior structure comprising a stack of two or more layers including at least a first layer formed of a two-dimensional material.

[0010] In a first aspect, the invention relates to a method for forming a stack of two or more layers, the method comprising

[0011] a) Provide:

[0012] Flat surface,

[0013] a first set of walls, the first set of walls including a first wall and a second wall, the first wall and the second wall being in physical contact with the planar surface and meeting at a corner to form an angle, and

[0014] a first layer, formed of a two-dimensional material, in physical contact with the planar surface and in physical contact with both the first wall and the second wall at the corners,

[0015] wherein the angle is adapted to align with the crystal structure of the two-dimensional material and has a tolerance of at most 5°,

[0016] wherein the top surface of the first layer is exposed,

[0017] wherein each of the walls has a length of 5 nm to 1000 nm, wherein the height of the walls as measured from the exposed top surface of the first layer to the top of the walls is 0.6 nm to 2 nm, thereby forming a cavity defined by at least the top surface and the first set of walls, and subsequently

[0018] b) forming a second layer in the cavity, the second layer in physical contact with the exposed top surface of the first layer.

[0019] In a second aspect, the present invention relates to a structure comprising:

[0020] Flat surface,

[0021] a first set of walls, the first set of walls including a first wall and a second wall, the first wall and the second wall being in physical contact with the planar surface and meeting at a corner to form an angle, and

[0022] a first layer, formed of a two-dimensional material, in physical contact with the planar surface and in physical contact with both the first wall and the second wall at the corners,

[0023] wherein the angle is adapted to align with the crystal structure of the two-dimensional material and has a tolerance of at most 5°,

[0024] wherein each of the walls has a length of 5 nm to 1000 nm, wherein the height of the walls as measured from the top surface of the first layer to the top of the walls is 0.6 nm to 2 nm, thereby forming a cavity defined by at least the top surface and the first set of walls, and

[0025] A second layer is in the cavity and in physical contact with a top surface of the first layer.

[0026] An advantage of embodiments of the present invention is that a two-dimensional material can be obtained which is contained in a stack and has a unidirectional in-plane orientation.

[0027] An advantage of embodiments of the present invention is that a precise location of a stack comprising high quality two-dimensional material crystals may be provided on a substrate, thereby enabling the fabrication of a device at that precise location.

[0028] Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate, not just where explicitly set out in a claim.

[0029] While there have been numerous improvements, variations, and developments in devices in the art, the present concept is believed to represent a substantial novel improvement involving departures from existing customary practices resulting in a more efficient, stable, and reliable device of this nature.

[0030] The above and other characteristics, features and advantages of the present invention will become apparent from the detailed description below in conjunction with the accompanying drawings, which illustrate the principles of the present invention by way of example. This description is provided for illustrative purposes only and does not limit the scope of the present invention. The reference figures cited below refer to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figures 1A to 7A and Figures 1B to 7B 3 and 4, respectively, a top view and a vertical cross-section of a structure in a subsequent step of a first example of a method according to an embodiment of the present invention.

[0032] 8A to 10A and FIG. 8B to FIG. 10B 3 are a top view and a vertical cross section, respectively, of a structure in a subsequent step of a second example of a method according to an embodiment of the present invention.

[0033] Fig.11A , Fig. 11B , Fig. 11C and Fig.11D They are respectively top views of the first layer and the first set of walls in the embodiments of the present invention.

[0034] Figures 12 to 16 is a schematic vertical cross-sectional view of a structure according to an embodiment of the present invention, in a subsequent step for forming a nanosheet field effect transistor, including a nanosheet formed by a layer stack according to an embodiment of the present invention.

[0035] The same reference numbers in different drawings identify the same or similar elements. DETAILED DESCRIPTION

[0036] The present invention will be described with reference to specific embodiments and with reference to certain drawings, but the invention is not limited thereto but only by the claims. The drawings described are only schematic and non-restrictive. In the drawings, for illustrative purposes, the size of some elements may be exaggerated and not drawn to scale. The sizes and relative sizes do not correspond to actual reductions in practicing the invention.

[0037] In addition, the terms "first", "second", "third", etc. in the specification and claims are used to distinguish similar elements and are not necessarily used to describe a sequence in time, space, order, or in any other manner. It should be understood that the terms so used are interchangeable under appropriate circumstances, and the embodiments of the invention described herein are capable of operating in other sequences than those described or illustrated herein.

[0038] Furthermore, the terms "top", "bottom", "above", "below", etc. in the specification and claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances, and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

[0039] It should be noted that the term "comprising" used in the claims should not be interpreted as being limited to the means listed thereafter; it does not exclude other elements or steps. It should therefore be interpreted as specifying the presence of the features, wholes, steps or parts described, but not excluding the presence or addition of one or more other features, wholes, steps or parts or combinations thereof. The term "comprising" covers the case where only the features described are present, as well as the case where these features and one or more other features are present. According to the present invention, the term "comprising" therefore also includes an embodiment in which no other components are present. Therefore, the scope of the expression "a device comprising means A and means B" should not be interpreted as being limited to a device consisting only of parts A and B. This means that for the purposes of the present invention, the only relevant components of the device are A and B.

[0040] References throughout this specification to "one embodiment" or "an embodiment" mean that a particular feature, structure, or characteristic associated with the description of the embodiment is included in at least one embodiment of the present invention. Thus, the phrases "in one embodiment" or "in an embodiment" appearing in various places throughout this specification do not necessarily but may refer to the same embodiment. Furthermore, it will be apparent to one of ordinary skill in the art from this disclosure that, in one or more embodiments, the particular features, structures, or characteristics may be combined in any suitable manner.

[0041] Similarly, it should be understood that in the description of exemplary embodiments of the present invention, various features of the present invention are sometimes combined together in a single embodiment, figure, or description thereof to simplify the present disclosure and aid in understanding one or more of the various inventive aspects. However, this disclosure method should not be interpreted as reflecting the following intention: the claimed invention requires more features than those expressly recited in each claim. Rather, as reflected in the appended claims, inventive aspects lie in less than all the features of a single preceding disclosed embodiment. Therefore, the claims that follow the detailed description are hereby expressly incorporated into the detailed description, with each claim itself as a separate embodiment of the present invention.

[0042] In addition, although some embodiments described herein include some features but do not include other features included in other embodiments, as will be understood by those skilled in the art, the combination of features of different embodiments is meant to be within the scope of the present invention and to form different embodiments. For example, in the appended claims, any claimed embodiment may be used in any combination.

[0043] In addition, some of the embodiments are described herein as methods or combinations of elements of methods that can be implemented by a processor of a computer system or by means of other devices that perform functions. Therefore, a processor with the necessary instructions for performing such methods or method elements forms a device for performing the methods or method elements. In addition, the elements of the device embodiments described herein are examples of devices for performing the functions performed by the elements for the purpose of implementing the present invention.

[0044] In the description provided herein, many specific details are set forth. However, it should be understood that embodiments of the present invention can be practiced without these specific details. In other cases, well-known methods, structures, and techniques are not shown in detail to avoid obscuring the understanding of this specification.

[0045] In the context of the present invention, when reference is made to the crystal structure of a material, it is meant the ordered arrangement of atoms, ions or molecules in the material. The arrangement may be independent of the type of atoms, ions or molecules: for example, both graphene and boron nitride may have a hexagonal crystal structure and may therefore have the same crystal structure, even if the atoms or ions constituting these materials are different. When referring to a specific crystal structure of a two-dimensional material, reference may be made to the corresponding two-dimensional Bravais lattice, which is grouped into five corresponding lattice systems. For example, when referring to an orthorhombic crystal structure, reference may be made to an orthorhombic lattice system including a rectangular and centered rectangular two-dimensional Bravais lattice. The angle suitable for alignment with this crystal structure is 90°. For example, when referring to a monoclinic crystal structure, reference may be made to a monoclinic lattice system including an oblique two-dimensional Bravais lattice. The angle suitable for alignment with this crystal structure is 90°. For example, when referring to a tetragonal crystal structure, reference may be made to a tetragonal lattice system including a square two-dimensional Bravais lattice. The angle suitable for alignment with this crystal structure is 90°. For example, when referring to a hexagonal crystal structure, reference is made to a hexagonal lattice system comprising a hexagonal two-dimensional Bravais lattice. An angle suitable for alignment with such a crystal structure is, for example, 60°.

[0046] As used herein, unless otherwise specified, a two-dimensional material refers to a crystalline material thin enough to exhibit properties different from its bulk material, wherein each layer is formed by a unit cell that repeats in two spatial dimensions but not in the third dimension. Sufficiently thin generally means including one to ten layers. Typically, the thickness of a two-dimensional material is at most 5nm. Two-dimensional materials are generally formed by isotopic or ectopic atomic layers arranged in layers, with a stronger connection within the layer than between material layers. Examples of two-dimensional materials are graphene, hexagonal boron nitride, phosphorene, transition metal disulfide, transition metal oxide, graphene oxide, fluorine graphene, germanene, graphane, graphene, borene, silicene, and alkanes.

[0047] In a first aspect, the invention relates to a method for forming a stack of two or more layers, the method comprising

[0048] a) Provide:

[0049] Flat surface,

[0050] a first set of walls, the first set of walls including a first wall and a second wall, the first wall and the second wall being in physical contact with the planar surface and meeting at a corner to form an angle, and

[0051] a first layer, formed of a two-dimensional material, in physical contact with the planar surface and in physical contact with both the first wall and the second wall at the corners,

[0052] wherein the angle is adapted to align with the crystal structure of the two-dimensional material and has a tolerance of at most 5°,

[0053] wherein the top surface of the first layer is exposed,

[0054] wherein each of the walls has a length of 5 nm to 1000 nm, wherein the height of the walls as measured from the exposed top surface of the first layer to the top of the walls is 0.6 nm to 2 nm, thereby forming a cavity defined by at least the top surface and the first set of walls, and subsequently

[0055] b) forming a second layer in the cavity, the second layer in physical contact with the exposed top surface of the first layer.

[0056] The corner where the first wall and the second wall meet is a corner formed by the two walls contacting each other.

[0057] In an embodiment, the radius of curvature of the corner is at most 5nm, preferably at most 1nm. The radius of curvature is the radius of the circle that best fits the surface of the corner. Ideally, the surface of the corner forms an infinitely sharp angle, i.e. the radius of curvature is 0nm. However, in practice, the surface of the corner may have a curvature. The curvature is preferably at most 5nm, and more preferably at most 1nm.

[0058] In an embodiment, at least 50%, preferably at least 60%, more preferably at least 70%, even more preferably at least 80%, yet more preferably at least 90% of the area of ​​the cross section of the first layer parallel to the flat surface intersects with a single crystal of the two-dimensional material, wherein the single crystal contacts the corner. In these embodiments, it can be ensured that the crystals in any further layer of the two-dimensional material formed on the single crystal of the two-dimensional material of the first layer are aligned. The method of the present invention is conducive to the formation of a layer stack comprising a plurality of layers of two-dimensional material, which may be alternating with layers formed of another material (e.g. an amorphous material), wherein the crystals, e.g. single crystals, of the plurality of layers of two-dimensional material have the same orientation.

[0059] In a first type of embodiment, step b comprises growing a second layer formed of a two-dimensional material in physical contact with the exposed top surface of the first layer, the second layer being in contact with the first wall and the second wall of the first set.

[0060] In a preferred embodiment, the two-dimensional material of the first layer and the second layer has the same crystal structure. In a preferred embodiment, the two-dimensional material of the second layer is the same as the two-dimensional material of the first layer. In these embodiments, the cavity is conducive to the growth of a single crystal two-dimensional material layer therein. Without being bound by theory, it is believed that the angle of the corner is suitable for aligning with the crystal structure of the two-dimensional material and the tolerance is at most 5°, which can induce the second layer of crystals to grow in the corner and / or induce seamless coalescence of crystals grown on the same wall or different walls forming the cavity. In addition, it is believed that by keeping the wall short, i.e. at most 1000nm, the atoms in contact with the surface between the walls often find themselves at a distance from the wall shorter than their diffusion length, which will be conducive to the nucleation and maturation of a few or even only one atomic nucleus. For longer walls, it is increasingly likely that the second layer of the two-dimensional material of the second layer formed between the walls is polycrystalline. In addition, it is believed that by keeping the wall low, it is possible to prevent the crystal from growing vertically, i.e. parallel to the wall and on the wall. In an embodiment, the angle suitable for aligning with the crystal structure may mean that the angle is designed to match the angle between adjacent atoms in the crystal lattice. Here, a deviation of at most 5° is allowed.

[0061] Typically, the two-dimensional material of the second layer is grown by depositing a precursor species on the top surface of the first layer, the precursor species comprising atoms that form the second layer of two-dimensional material. The precursor species is adsorbed on the top surface of the first layer and can diffuse. The two-dimensional material of the second layer can be grown by diffusing the precursor species that contact and react with each other. Alternatively, the diffuse precursor species can react in the gas phase and then be adsorbed on the top surface of the first layer and diffuse.

[0062] In embodiments, the critical dimension of the cavity is at most equal to the average diffusion length of the adsorbed precursor species in the plane of the top surface of the first layer. The critical dimension is typically the length of the longest straight line that can be contained in the cavity. For example, the critical dimension can be the distance between the distal ends of the first wall and the second wall away from the corner. In these embodiments, the growth of the two-dimensional material of the second layer can advantageously proceed from a few nuclei or even from a single nucleus. For longer critical dimensions, the probability of forming a polycrystalline two-dimensional material is increasing.

[0063] In a first type of embodiment, step b) can be performed by chemical vapor deposition, for example by metal organic chemical vapor deposition, such as metal organic chemical vapor deposition assisted by HCl, Cl2 or H2O. For example, step b can be performed by chemical vapor deposition from metal organics or metal halides (such as W(CO)6, WCl6) and sulfur precursors (such as H2S, S element). Chemical vapor deposition is advantageous because it allows the diffusion length to be adjusted in the range of 10nm to more than 1000nm across a wide range of deposition temperatures. The deposition temperature can be an important parameter because it can determine the thermal and chemical compatibility of the chemical vapor deposition with the structural component, such as the compatibility with the type and chemical composition of the flat surface and the wall.

[0064] In a first type of embodiments, step b) may be performed by atomic layer deposition. Atomic layer deposition is particularly advantageous when the length of each wall of the first set of walls is 5 nm to 50 nm.

[0065] In an embodiment, the first wall material comprises a dielectric such as Al2O3 or HfO2, a metal such as TiN or W, or a semiconductor.

[0066] In an embodiment, the chemical reactivity of one of the walls can limit the nucleation of the two-dimensional material to occur on the one of the walls. For example, different materials can be used for the first wall and the second wall, where one material forms a non-growth surface and the other material forms a growth surface. For example, the first wall can include Al2O3 and the second wall can include SiO2. As another example, the wall materials can include amorphous or crystalline wall materials: the first wall can include SiO2 and the second wall can include Si.

[0067] In an embodiment of the first type, the method may further include a step c after step b: c) depositing a second wall material on the first set of walls to form a second set of walls, so that the height from the top surface of the second layer to the top of the second set of walls is 0.6nm to 2nm. In an embodiment, step c includes selectively depositing the second wall material on the first set of walls relative to the top surface of the two-dimensional material of the second layer, thereby forming a second set of walls overlying the first set of walls and aligned with the first set of walls. In fact, the second set of walls may form a vertical extension of the first set of walls. In an embodiment, step c includes performing thermal atomic layer deposition or chemical vapor deposition. An advantage of these embodiments is that the deposition of the second wall material can be highly selective. In an embodiment, the second wall material includes a dielectric such as Al2O3 or HfO2, a metal such as TiN or W, or a semiconductor. The first set of walls and the second set of walls may be formed of the same material or different materials.

[0068] Alternatively, step c comprises forming the second set of walls by photolithography. In this case, the second set of walls may overlie the first set of walls and be aligned with the first set of walls. However, this is not required. In these embodiments, it is possible to achieve a freer adjustment of the characteristics of the second set of walls, such as: obtaining a length of the second set of walls that is different from the length of the first set of walls; obtaining an angle between the second set of walls that is different from the angle between the first set of walls; or obtaining an alignment of the second set of walls that is different from the first set of walls, so that the crystal structure of the two-dimensional material of the second layer can have a different orientation than the two-dimensional material of the layer formed on the two-dimensional material of the second layer as described below.

[0069] The top surface of the second layer may be exposed and define a further cavity together with the second set of walls. Steps corresponding to b and c may be repeated alternately to form a stack of two-dimensional material layers in contact with each other. Specifically, the method may include, after step c, alternately performing the following steps:

[0070] forming a two-dimensional material in a further cavity and in physical contact with the exposed top surface of the layer stack comprising the first layer and the second layer, and

[0071] An additional set of walls is formed overlying and aligned with the existing set of walls so that the height from the top surface of the layer stack to the top of the additional set of walls is 0.6 nm to 2 nm, thereby forming an additional cavity defined by the top surface of the layer stack and the additional set of walls.

[0072] In an embodiment, at least 50%, preferably at least 60%, more preferably at least 70%, even more preferably at least 80%, yet more preferably at least 90% of the area of ​​a cross section of the second layer parallel to the flat surface intersects with a single crystal of the two-dimensional material of the second layer, wherein said single crystal contacts a corner of the first set of walls, wherein the single crystal of the two-dimensional material of the first layer and the single crystal of the two-dimensional material of the second layer have the same orientation. These embodiments are typical embodiments, wherein at least 50% of the area of ​​a cross section of the first layer parallel to the flat surface intersects with a single crystal of the two-dimensional material of the first layer, wherein said single crystal of the first layer contacts the corner. The same orientation can come from an angle suitable for alignment with the crystal structure of the two-dimensional material of the first layer, such as with the single crystal of the first layer, and from an angle with the two-dimensional material of the second layer, such as the single crystal of the second layer, which is typically formed to also align with said corner.

[0073] In a second type of embodiment, step b comprises non-selectively depositing a second layer of material on the exposed top surface and the first set of walls of the first layer, thereby forming a second layer including a recess overlapping the first layer. In an embodiment, the second layer is a uniform layer covering both the exposed top surface and the first set of walls of the first layer. The uniformity may relate to the thickness and composition of the second layer. In an embodiment, the second layer of material may be an amorphous dielectric material, such as SiO2. In an embodiment, the non-selective deposition may include soaking, chemical vapor deposition or physical vapor deposition, preferably soaking or chemical vapor deposition. The soaking is a process in which gas phase molecules are physically adsorbed or condensed from the gas phase onto the exposed top surface to form the second layer of material. Thus, the second layer of material may partially or completely cover the underlying top surface, depending on the conditions of the soaking, such as the concentration of precursor molecules in the reactor used for soaking, static conditions, pressure in the reactor used for soaking, exposure time, and the temperature of the first layer when the soaking is performed.

[0074] In an embodiment, the method comprises, after step b, step c': c') growing a third layer formed of a two-dimensional material in the recess. Any features of the third layer of the second type of embodiment may be independently described corresponding to features of the second layer of the first type of embodiment.

[0075] In a preferred embodiment, at least 50%, preferably at least 60%, more preferably at least 70%, even more preferably at least 80%, yet more preferably at least 90% of the area of ​​the cross section of the third layer parallel to the flat surface intersects with a single crystal of the two-dimensional material of the third layer, wherein the single crystal is in contact with a corner of the recess, wherein the single crystal of the two-dimensional material of the third layer may have the same orientation as the single crystal of the two-dimensional material of the first layer. Since the third layer may be uniform over the first layer and the first set of walls, the recess typically comprises a corner having the same orientation and angle as the corner of the first set of walls. The same orientation may result from an angle of the first set of walls adapted to align with the crystal structure of the two-dimensional material of the first layer, e.g., with a single crystal of the first layer, and from an angle with the two-dimensional material of the second layer, e.g., with a single crystal of the second layer, typically formed to align with the corner of the recess.

[0076] In an embodiment, step a comprises: a1) providing a first layer formed of a two-dimensional material on a flat surface, wherein a portion of the flat surface surrounding the two-dimensional material is exposed, and a2) selectively depositing a first wall material on the exposed portion of the flat surface relative to the exposed top surface of the first layer formed of the two-dimensional material, thereby forming a first set of walls. In an embodiment, step a2 comprises performing thermal atomic layer deposition or chemical vapor deposition.

[0077] In various embodiments, step a includes: a1') obtaining a lower portion of a first set of walls formed by a first portion of a first wall material, the lower portion of the first set of walls including lower portions of a first wall and a second wall, the lower portions of the first wall and the second wall being in physical contact with a planar surface and meeting at a corner to form an angle, wherein a top surface of the planar surface is exposed, wherein a length of each of the lower portions of the first set of walls is 5 nm to 1000 nm, wherein a height of the lower portions of the first set of walls as measured from the exposed top surface of the planar surface to the top of the lower portions of the first set of walls is 0.6 nm to 2 nm, thereby forming a cavity defined at least by the top surface and the lower portions of the first set of walls. For example, step a1' may include depositing a first portion of the first wall material on the planar surface, followed by patterning the deposited first portion of the first wall material to form the lower portion of the first set of walls. In these embodiments, step a also includes: a2') forming a first layer in the cavity and in physical contact with both the planar surface and the lower portions of the first wall and the second wall at the corner. In embodiments, step a2' may be performed by chemical vapor deposition or atomic layer deposition. In these embodiments, step a may further include: a3') selectively depositing a second portion of the first wall material onto a lower portion of the first set of walls relative to the top surface of the first layer, thereby forming the first set of walls.

[0078] In an embodiment, the first set of walls comprises a third wall in physical contact with the flat surface, the third wall facing a corner, i.e. a corner of the first set of walls, wherein the shortest distance between the corner and the third wall is at most 1000 nm, wherein the length of the third wall is at least equal to the length of the longest of the first wall and the second wall. In a preferred embodiment, the first set of walls forms an enclosure.

[0079] In an embodiment, a horizontal cross-section of each of the walls parallel to the planar surface is in a straight line.

[0080] In an embodiment, the interior angle between the first wall and the second wall is 55° to 65°, and the two-dimensional material of the first layer has a hexagonal crystal structure. In these embodiments, the interior angle between the second wall and the third wall may be 55° to 65°, and the interior angle between the first wall and the third wall may be 55° to 65°.

[0081] Without being bound by theory, it is believed that since crystals with a particular symmetry, such as hexagonal symmetry, are characterized by having a particular angle, such as an angle of 60°, at the intersection of the in-plane axes of a particular crystallographic system (e.g., the three in-plane axes of hexagonal symmetry), the fact that the angle of the corner is close to the particular angle (e.g., 60°) induces crystal growth in the corner and / or induces seamless coalescence of crystals grown on the same wall or on different walls. This results in a particular alignment of the grown crystals.

[0082] In other embodiments, the internal angle between the first wall and the second wall can be 115° to 120°, and the two-dimensional material of the first layer has a hexagonal crystal structure. However, in these embodiments, the internal angle between the second wall and the third wall, and the internal angle between the first wall and the third wall will not align with the grown crystal.

[0083] The angle of the first set of walls is adapted to align with the crystal structure of the two-dimensional material of the first layer. In an embodiment, this means that the angle of the first set of walls is equal to a specific angle at the intersection of the in-plane axes of a specific crystallographic system of the two-dimensional material of the first layer. Preferably, the orientation of the two-dimensional material of the first layer is the same as if the material had originally grown from said corner. This may result in the crystal structure of the first layer having the same orientation as the crystal structure of any subsequent layer of the two-dimensional material.

[0084] In an embodiment, the two-dimensional material of the first layer may be a transition metal disulfide, a transition metal monosulfide, a transition metal halide, a transition metal carbide or a transition metal nitride.

[0085] In various embodiments, the interior angle between the first wall and the second wall is 85° to 95°, and the two-dimensional material of the first layer has an orthorhombic crystal structure or a tetragonal crystal structure. In embodiments in which the two-dimensional material has an orthorhombic crystal structure, the two-dimensional material may be a group IV transition metal monosulfide such as tin monosulfide, germanium monosulfide, or vanadium oxide V2O5 as a van der Waals oxide.

[0086] In various embodiments, the two-dimensional material of the first layer has a monoclinic crystal structure, which can be described by a lattice structure comprising a crystal angle between edges of a unit cell of the monoclinic crystal structure, wherein the internal angle between the first wall and the second wall is equal to the crystal angle and the tolerance is at most 5°. In embodiments in which the two-dimensional material has a monoclinic crystal structure, the two-dimensional material can be titanium trisulfide or tantalum trisulfide, both of which are semiconductors.

[0087] The tolerance is preferably at most 4°, more preferably at most 3°, even more preferably at most 2°, yet more preferably at most 1°, and most preferably 0°.

[0088] In an embodiment, the method comprises, after step b, a step d comprising etching the first set of walls selectively relative to the second layer. In embodiments comprising a second set of walls or any further set of walls, step d may comprise removing the second set of walls, and any further set of walls if present. After the walls are removed, the layer stack may be ready for further processing. For example, contacts may be provided in connection with each layer comprising a two-dimensional material.

[0089] In an embodiment, the method comprises, after step b, a step f of recrystallizing the second layer. Recrystallization may be applied to induce a higher level of crystallinity in the second layer. Recrystallization may result in embodiments of the second layer wherein at least 50%, preferably at least 60%, more preferably at least 70%, even more preferably at least 80%, yet more preferably at least 90% of the area of ​​a cross section of the second layer parallel to the flat surface intersects with single crystals of the two-dimensional material of the second layer, wherein the single crystals are in contact with the corners of the first set of walls, wherein the single crystals of the two-dimensional material of the first layer and the single crystals of the two-dimensional material of the second layer have the same orientation. However, the recrystallization step f is not required and these embodiments of the second layer may instead be obtained in step b of forming the second layer.

[0090] An advantage of the first and second types of embodiments of the present invention, wherein the layer stack comprises multiple layers of two-dimensional material, is that the formation of grain boundaries within each individual layer of the stack of two-dimensional material can be prevented.

[0091] An advantage of the first and second types of embodiments of the present invention is that, where the layer stack comprises multiple layers of two-dimensional materials, control over the stacking configuration (eg, AA' stacking versus AB stacking) between different layers of the stacked two-dimensional materials can be achieved.

[0092] An advantage of the first and second types of embodiments of the present invention, wherein the layer stack comprises multiple layers of two-dimensional material, is that control over the stacking angle between different layers of the stacked two-dimensional material can be achieved.

[0093] Any features of any embodiment of the first aspect may be independently and correspondingly described as being used for any embodiment of the second aspect of the present invention.

[0094] In a second aspect, the present invention relates to a structure comprising:

[0095] Flat surface,

[0096] a first set of walls, the first set of walls including a first wall and a second wall, the first wall and the second wall being in physical contact with the planar surface and meeting at a corner to form an angle, and

[0097] a first layer, formed of a two-dimensional material, in physical contact with the planar surface and in physical contact with both the first wall and the second wall at the corners,

[0098] wherein the angle is adapted to align with the crystal structure of the two-dimensional material and has a tolerance of at most 5°,

[0099] wherein each of the walls has a length of 5 nm to 1000 nm, wherein the height of the walls as measured from the top surface of the first layer to the top of the walls is 0.6 nm to 2 nm, thereby forming a cavity defined by at least the top surface and the first set of walls, and

[0100] A second layer is in the cavity and in physical contact with a top surface of the first layer.

[0101] Any features of any embodiments of the second aspect may be independently and correspondingly described as being applicable to the first aspect of the present invention.

[0102] Example 1: Forming a stack comprising multiple layers formed of two-dimensional materials

[0103] The first example is about the first type of embodiment of the present invention.

[0104] Please refer to Figure 1A and Figure 1B , which respectively show a top view and a vertical cross section of a flat surface 1 and a first layer 2 formed of a two-dimensional material in physical contact with the flat surface 1. In this example, the flat surface 1 is the top surface of a substrate, such as a semiconductor substrate. The first layer 2 formed of the two-dimensional material is parallel to the flat surface 1.

[0105] In this example, the first layer 2 has a triangular shape and includes three sides 20, each of which has the same length, which is 5nm to 1000nm. Each of the three corners 21 of the first layer 2 has an internal angle of 60° and a tolerance of at most 5°. In this example, the two-dimensional material forming the first layer 2 has a hexagonal crystal structure.

[0106] In this example, the first wall material is selectively deposited on the flat surface 1 to produce Figure 2A and Figure 2B The top view and vertical cross-section shown. The deposition is selectively performed relative to the top surface 22 of the first layer 2. Thus, the first layer 2 is used as a template for depositing the first wall material. In this way, the deposited first wall material forms a first group of three walls 3, forming a triangle. In this example, the first group of walls 3 includes a first wall 31, a second wall 32 and a third wall 33, each of which is in contact with one of the side edges 20 of the first layer 2. The height of the wall 3 as measured from the exposed top surface 22 of the first layer 2 to the top 30 of the wall 3 is 0.6nm to 2nm, thereby forming a cavity defined by at least the top surface 22 of the first layer 2 and the first group of walls 3. The first wall 31 and the second wall 32 meet at the corner 312 to form an angle suitable for alignment with the crystal structure of the two-dimensional material of the first layer 2 and a tolerance of at most 5°.

[0107] For example, the first layer 2 may have been formed in a cavity defined by a previous set of three walls forming a triangle, the previous set of three walls being located at the same position on the flat surface 1 as the current first set of walls 3, so that the crystal structure of the first layer 2 has been formed to align with the angle of the corner of the previous set of walls. As a result, the first set of walls 3 are located at the same position as the previous set of walls, and the first layer 2 is also aligned with the angle of the corner 312 of the first set of walls 3. Alternatively, the first layer 2 may be formed by patterning to arrive at a corner 312 formed at an angle suitable for aligning with the crystal structure of the two-dimensional material of the first layer 2.

[0108] Preferably, at least 50% of the area of ​​the cross section of the first layer 2 parallel to the flat surface 1 intersects with a single crystal of the two-dimensional material, wherein the single crystal is in contact with the corner 312. In this example, the first layer 2 forms a single crystal.

[0109] In this example, the first set of walls 3 forms an enclosure. The enclosure ensures that the second layer is formed on the first layer 2. If the enclosure is not formed, an attempt to deposit the second layer may result in the first layer 2 extending in a lateral direction without forming the second layer on top of the first layer 2. Enclosing the first layer 2 between the first set of walls 3 prevents the first layer 2 from extending lateral directions.

[0110] Please refer to Figure 3A and Figure 3B , and a top view and a vertical cross section shown in FIG. Subsequently, a second layer is selectively formed in the cavity. Typically, a precursor material is deposited on the exposed top surface 22 of the first layer 2, for example using atomic layer deposition or chemical vapor deposition, and the precursor material includes atoms of the two-dimensional material forming the second layer. The precursor material is selectively deposited relative to the first wall material 35 comprising the first set of walls 3. In this example, the two-dimensional material of the second layer to be formed has a hexagonal crystal structure. The precursor material is adsorbed and diffusible on the exposed top surface 22 of the first layer 2. The two-dimensional material of the second layer grows by contacting and reacting with each other by the diffused precursor materials to form a crystal nucleus 40, which can grow to form the second layer. Due to the limited size of the cavity, the diffusion length of these adsorbed precursor materials is less than the critical size of the cavity, so only a limited number of crystal nuclei 40 are formed, which are two cores 40 in this example. In addition, due to the limited size of the cavity, the crystal nucleus 40 may be located at the cavity wall 3 or in one of the corners 312. As a result, the formation of the crystal structure of the second layer may cause the crystal structure of the second layer to align with the wall 3 and the corner 312.

[0111] Please refer to Figure 4A and Figure 4B The second layer 4 is formed in the cavity in physical contact with the top surface 22 of the first layer 2. The first layer 2 and the second layer 4 are formed in parallel.

[0112] Preferably, at least 50% of the area of ​​the cross section of the second layer 4 parallel to the flat surface 1 is parallel to the top surface 22 of the first layer 2, intersecting the single crystal of the two-dimensional material of the second layer, wherein the single crystal is in contact with the corner 312. In this example, the second layer 4 forms a single crystal. This may be due to the slow growth of the second layer 4, or due to the diffusion length of the precursor species being larger than the critical size of the cavity, for example, at least 10 times larger.

[0113] Furthermore, this may be due to a recrystallization step of the second layer 4 after its formation. The recrystallization generally comprises heating the second layer 4, for example annealing.

[0114] The crystal structure of the two-dimensional material of the first layer 2 and the crystal structure of the two-dimensional material of the second layer 4 are both aligned relative to the corner 312 formed by the first wall 31 and the second wall 32. In addition, the first layer 2 and the second layer 4 have the same crystal structure, that is, a hexagonal crystal structure. For example, the first layer 2 and the second layer 4 can be formed of the same material. However, the first layer 2 and the second layer 4 can be composed of different materials having the same crystal structure. Therefore, in this example, the single crystal of the two-dimensional material of the first layer 2 and the single crystal of the two-dimensional material of the second layer 4 have the same orientation.

[0115] Please refer to Figure 5A and Figure 5B . After the second layer 4 is formed, a second wall material 55 is deposited on the first set of walls 3, i.e., on the first wall material 35 comprising the first set of walls 3. The second wall material 55 is selectively deposited relative to the top surface 42 of the two-dimensional material of the second layer 4. Thus, a second set of walls 5 is formed that overlies the first set of walls 3 and is aligned with the first set of walls 3. The height from the top surface 42 of the second layer 4 to the top 50 of the second set of walls 5 is 0.6 nm to 2 nm. The second set of walls 5 and the exposed top surface 42 of the second layer 4 define a cavity.

[0116] In fact, in this example, the second set of walls 5 forms an extension to the first set of walls 3 .

[0117] Please refer to Fig. 6A and Figure 6B . Subsequently, a third layer 6 formed of a two-dimensional material is formed on the top surface 42 of the second layer 4 and in the cavity defined by the second set of walls 5 and the exposed top surface 42 of the second layer 4. In this example, the third layer 6 has a hexagonal crystal structure and is formed of the same two-dimensional material as the first and second layers 4.

[0118] As described above, the crystal structure of the two-dimensional material of the second layer 4 is aligned relative to the corner formed by the first wall and the second wall of the first set of walls. As the second set of walls 5 covers the first set of walls 3 and is aligned with the first set of walls 3, the second set of walls 5 also forms a triangle. In this example, the length of the wall of the first set of walls 3 is the same as the length of the wall of the second set of walls 5. In addition, the corner of the second set of walls 5 has the same angle as the corner of the first set of walls 3. As a result, the crystal structure of the two-dimensional material of the third layer 6 is aligned relative to the corner 512 formed by the first wall 51 and the second wall 52 of the second set of walls 5, which overlies and is aligned with the corner formed by the first wall and the second wall of the first set of walls 3.

[0119] In this example, the single crystal of the two-dimensional material of the second layer 4 and the single crystal of the two-dimensional material of the third layer 6 have the same orientation.

[0120] By alternately repeating the above steps of selectively depositing wall material to extend the wall so that the height from the top surface of the top layer of the layer stack to the top of the wall is 0.6nm to 2nm, and then selectively depositing a two-dimensional material layer in the cavity defined by the top surface of the top layer of the layer stack and the wall, a layer stack including multiple two-dimensional material layers can be formed, and the single crystals of the layer stack have the same orientation.

[0121] Subsequently, the walls, which in this example comprise a first set of walls 3 and a second set of walls 5, may be removed selectively relative to the layer stack, which comprises a first layer 2, a second layer 4 and a third layer 6. This may be performed, for example, using an etch for selectively etching the walls. As another example, a hard mask may be applied, covering the layer stack while exposing the walls, and an anisotropic etch may be applied selectively relative to the hard mask.

[0122] In an alternative example, the walls of the second set of walls 5 overlie and are aligned with the first set of walls 3, and the length of the walls of the second set of walls 5 may be greater than the first set of walls 3. To remove the walls of the first set of walls 3 and the second set of walls 5, in the alternative example, a hard mask may be applied, covering the layer stack while exposing the walls, and then the anisotropic etch may be applied selectively to the hard mask.

[0123] Please refer to Fig. 7A and Figure 7B The result is that a layer stack comprising a first layer 2 , a second layer 4 and a third layer 6 is formed on the flat surface 1 , the single crystals of which have the same orientation.

[0124] Example 2: Forming a layer stack comprising layers formed of a two-dimensional material alternating with layers formed of another material.

[0125] The second example is about the second type of embodiment of the present invention.

[0126] Please refer to Fig. 8A and Figure 8B The top view and vertical section are shown in FIG. Figure 2A and Figure 2B As shown, in this example, starting from the first set of walls 3 and the first layer 2 on the flat surface 1, that is, in physical contact therewith, a second layer of material, such as an amorphous dielectric material, is non-selectively deposited on the exposed top surface of the first layer 2 and on the first wall material 35 including the first set of walls 3, thereby forming a second layer 7 including a recess 71 overlapping the first layer 2.

[0127] Typically, the depth of the recess 71 is 0.6 nm to 2 nm because the height of the walls 3 as measured from the top surface 22 of the first layer 2 to the top 30 of the walls 3, e.g., the top surface, is 0.6 nm to 2 nm. Due to the non-selective deposition, the width of the recess 71 is typically less than the width of the first layer 2, typically twice the thickness of the deposited second layer 7. However, for the rest, the shape of the recess 71 is typically the same as the cavity defined by at least the top surface 22 of the first layer 2 and the first set of walls 3. Thus, the corners of the recess 71 may have the same angle as the corners of the first set of walls 3 and be aligned with the corners of the first set of walls 3.

[0128] Please refer to Fig.9A and Fig. 9B . A two-dimensional material of the third layer 8 having a hexagonal crystal structure in this example is formed in the recess of the second layer 7. The crystal structure of the third layer 8 is aligned with the corner of the recess 71. Because the crystal structure of the two-dimensional material of the first layer 2 is aligned with respect to the corner formed by the first set of walls 3, the single crystals of the two-dimensional material of the first layer 2 and the single crystals of the two-dimensional material of the third layer 7 have the same orientation.

[0129] The resulting structure comprises a layer stack comprising a first layer 2 of two-dimensional material and a third layer 8 of two-dimensional material, and a portion 71 of the layer of non-selectively deposited material 7 sandwiched between the first layer 2 and the third layer 8 .

[0130] This process of non-selectively depositing a material layer and forming a two-dimensional material layer in a recess of the material layer can be repeated multiple times. Wherein, after the two-dimensional material layer is formed in the recess, a material, such as the same material as the material layer formed, can be selectively deposited on the material layer, selectively deposited relative to the exposed top surface of the two-dimensional material in the recess. This can be performed so that the height of the material selectively deposited on the material layer from the top surface of the two-dimensional material to the top surface of the material is 0.6nm to 2nm. Subsequently, another material layer can be non-selectively deposited. The selective deposition of material can ensure that the depth of the recess is 0.6nm to 2nm, which is also applicable to the subsequent formation of the material layer.

[0131] Please refer to Fig. 10A and Fig. 10B . Subsequently, a hard mask 9 may be formed overlying the layer stack comprising the two-dimensional material layer comprising at least the first layer 2 and the third layer 8, alternating with portions 72 of the non-selectively deposited material layer. Next, an anisotropic etch may be performed selectively with respect to the hard mask 9.

[0132] Although two different examples have been described according to embodiments of the present invention, which relate to first type embodiments and second type embodiments, respectively, the first type embodiments and the second type embodiments may be combined. As an example, a layer stack according to an embodiment of the present invention may include: a plurality of two-dimensional material layers formed on top of each other using a method according to an embodiment of the present invention, and on top of the two-dimensional material layer there is another material, such as a dielectric material layer, formed using a method according to an embodiment of the present invention; and another two-dimensional material layer, which may be formed on top of another material layer using a method according to an embodiment of the present invention.

[0133] Example 3: Alternative shapes formed by walls

[0134] In Examples 1 and 2, a horizontal cross section through the first set of walls has a triangular shape, that is, the first set of walls includes three walls forming a triangular enclosure. However, the present invention is not limited thereto.

[0135] refer to Fig.11A , which is a top view of a two-dimensional material having a first layer 2 of a hexagonal crystal structure on a flat surface 1. A horizontal section through the first layer 2 parallel to the flat surface 1 forms a triangle. In addition, a first wall material 35 is provided on the flat surface 1. The first wall material 35 includes a first set of walls including a first wall 341 and a second wall 342, in this example, the first wall 341 and the second wall 342 do not form an enclosure. The first layer 2 is in contact with the first wall 341 and the second wall 342 and the flat surface 1. In this example, the first wall 341 and the second wall 342 intersect at the corner 3412 to form an angle of 60°, i.e., an internal angle and a tolerance of at most 5°. However, alternatively, in particular if the crystal structure of the two-dimensional material is different from the hexagonal system, the angle may be different so as to align with the crystal structure. For example, in the case of an orthorhombic crystal structure, the angle may be 90° and the tolerance may be at most 5°.

[0136] refer to Fig. 11B , and Fig.11ASimilarly, except that the first set of walls also includes a third wall 343, and the first layer 2 is in contact with the third wall 343. The third wall 343 faces the corner 3412 where the first wall 341 and the second wall 342 meet. The shortest distance between the corner 3412 and the third wall 343 is at most 1000 nm, and the length of the third wall 343 is at least equal to the length of the longest of the first wall 341 and the second wall 342. The third wall 343 is separated from the first wall 341 and the second wall 342, so in this example, the first set of walls does not form an enclosure.

[0137] refer to Fig. 11C , which is a top view of a two-dimensional material of a first layer 23 having an orthorhombic crystal structure. A horizontal section through the first layer 23 parallel to the flat surface (i.e., in this example, completely covered and thus obscured) forms a square. In this example, the first wall material 35 includes four walls 361, 362, 363, 364, each of which contacts one side of the first layer 23. The four walls 361, 362, 363, 364 form a square, the internal angle of the corners of which is 90° and the tolerance is at most 5°, which is suitable for alignment with the crystal structure of the first layer 23 in this example, which is an orthorhombic system. For example, the first wall 361 and the second wall 362 meet at the corner to form an angle of 90° and the tolerance is at most 5°.

[0138] refer to Fig.11D , which is a top view of a two-dimensional material of a first layer 23 having an orthorhombic crystal structure. In this example, the first wall material 35 includes a first set of walls including three walls, including a first wall 361, a second wall 362, and a fourth wall 364, each of which is in contact with the side of the first layer 23. The first set of walls also includes a third wall 365 separated from the first wall 361, the second wall 362, and the fourth wall 364, so in this example, the first set of walls does not form an enclosure. The third wall 365 faces a corner 3612 between the first wall 361 and the second wall 362, and a corner 3614 between the first wall 361 and the fourth wall 364. The shortest distance between each of the corners 3612 and 3614 and the third wall 365 is at most 1000nm, and the length of the third wall 365 is at least equal to the length of the longest of the first wall 361, the second wall 362, and the fourth wall 364. The third wall 365 is separated from the first wall 361, the second wall 362 and the fourth wall 364, so in this example, the first set of walls does not form an enclosure.

[0139] Example 4: Formation of Nanosheet Field Effect Transistors

[0140] Embodiments of the present invention may be used to form nanosheets due to the excellent locality of two-dimensional material layers overlying each other. In this example, a field effect transistor is formed comprising a nanosheet formed from a layer stack according to embodiments of the present invention.

[0141] refer to Fig.12 , which is a vertical cross-section of a layer stack comprising layers formed of a dielectric sacrificial material 72 alternating with layers 2 formed of a two-dimensional material, which can be formed according to a second type of embodiment. Here, according to an embodiment of the invention, the lowest dielectric sacrificial layer 11 comprises a flat surface 110, on which the layer stack is formed using a method according to an embodiment of the invention.

[0142] refer to Fig.13 The layer formed by the dielectric sacrificial material 72 and the lowermost dielectric sacrificial layer 11 may be recessed from the side edges.

[0143] refer to Fig.14 Contacts 12 may be formed to contact opposite sides of the stacked two-dimensional material layer 2 to form a source and a drain.

[0144] refer to Fig.15 Subsequently, the dielectric sacrificial material may be removed from the layer stack by side patterning, forming a hollow portion 13 in the layer stack.

[0145] refer to Fig.16 Next, a gate oxide layer 14 may be covered on the surface of the hollow portion, covering the two-dimensional material layer and the contact portion 12, and a smaller hollow portion may be defined at the center of each hollow portion, which may be filled with a gate metal 15, thereby forming a gate stack.

[0146] It should be understood that although the preferred embodiments, specific constructions and configurations and materials of the device according to the present invention are discussed herein, various changes or modifications may be made in form and detail without departing from the scope of the present invention. Steps may be added or deleted to the methods described within the scope of the present invention.

Claims

1. A method for forming a stack of two or more layers, the method comprising: a) Provide: Flat surface (1), a first set of walls (3), the first set of walls comprising a first wall (31) and a second wall (32), the first wall (31) and the second wall (32) being in physical contact with the flat surface (1) and meeting at a corner (312) to form an angle, and a first layer (2), the first layer (2) being formed of a two-dimensional material, being in physical contact with the flat surface (1) and being in physical contact with both the first wall (31) and the second wall (32) at the corner (312), wherein the angle is adapted to align with the crystal structure of the two-dimensional material and has a tolerance of at most 5°, wherein the top surface (22) of the first layer (2) is exposed, wherein each of the first and second walls (31, 32) has a length of 5 nm to 1000 nm, wherein the height of the walls (31, 32) as measured from the exposed top surface (22) of the first layer (2) to the top (30) of the walls is 0.6 nm to 2 nm, thereby forming a cavity defined by at least the top surface (22) and the first set of walls (3), and subsequently b) forming a second layer (4, 7) in the cavity, the second layer (4, 7) being in physical contact with the exposed top surface (22) of the first layer (2).

2. The method according to claim 1, characterized in that At least 50% of the area of ​​a cross section of the first layer parallel to the flat surface (1) intersects with a single crystal of the two-dimensional material, wherein the single crystal is in contact with the corner (312).

3. The method according to claim 1 or 2, characterized in that: The second layer (4) is formed of a two-dimensional material, and wherein step b comprises growing the second layer (4) in physical contact with the exposed top surface (22) of the first layer (2), the second layer (4) being in contact with the first wall (31) and the second wall (32) of the first set of walls (3).

4. The method according to claim 3, characterized in that The method further comprises step c after step b: c) depositing a second wall material (55) on the first set of walls (3) to form a second set of walls (5) such that the height from the top surface (42) of the second layer (4) to the top (50) of the second set of walls (5) is 0.6 nm to 2 nm.

5. The method according to claim 3 or 4, characterized in that: The two-dimensional materials of the first layer (2) and the second layer (4) have the same crystal structure.

6. The method according to claim 5 based on claim 2 is characterized in that at least 50% of the area of ​​the cross section of the second layer (4) parallel to the flat surface (1) intersects with a single crystal of the two-dimensional material of the second layer (4), wherein the single crystal is in contact with a corner (312) of the first set of walls (3), in, The single crystals of the two-dimensional material of the first layer (2) and the single crystals of the two-dimensional material of the second layer (4) have the same orientation.

7. The method according to any one of claims 4 to 6, characterized in that Wherein step c comprises selectively depositing the second wall material on the first set of walls relative to the top surface of the second layer of two-dimensional material, thereby forming a second set of walls overlying the first set of walls and aligned with the first set of walls, and wherein step c preferably comprises performing thermal atomic layer deposition or chemical vapor deposition of the second wall material.

8. The method according to claim 1 or 2, characterized in that: Wherein step b comprises depositing a second layer (7) of material on the exposed top surface (22) of the first layer (2) and the first set of walls (3), thereby forming the second layer (7) comprising a recess (71) overlapping the first layer (2).

9. The method according to claim 8, further comprising step c' after step b: c′) growing a third layer (8) made of a two-dimensional material in the recess.

10. The method according to any one of the preceding claims, characterized in that The step a comprises: a1) providing the first layer (2) formed of a two-dimensional material on the flat surface (1), wherein a portion of the flat surface surrounding the two-dimensional material is exposed, and a2) depositing the first wall material (35) selectively on the exposed portion of the planar surface relative to the exposed top surface (22) of the first layer (2) formed of a two-dimensional material, thereby forming the first set of walls (3).

11. The method according to any one of the preceding claims, characterized in that The first set of walls (3) includes a third wall (33) in physical contact with the flat surface (1), the third wall (3) facing the corner (312), wherein the shortest distance between the corner (312) and the third wall (33) is at most 1000nm, and wherein the length of the third wall (33) is at least equal to the length of the longest of the first wall (31) and the second wall (32).

12. The method according to claim 11, characterized in that The first set of walls (3) forms an enclosure.

13. The method according to any one of the preceding claims, characterized in that The internal angle between the first wall (31) and the second wall (32) is 55° to 65°, and the two-dimensional material of the first layer (2) has a hexagonal crystal structure, or the internal angle between the first wall (31) and the second wall (32) is 85° to 95°, and the two-dimensional material of the first layer (2) has an orthorhombic crystal structure or a tetragonal crystal structure.

14. The method according to any one of the preceding claims based on claim 3, characterized in that The method comprises, after step b, a step f of recrystallizing the second layer (4).

15. A structure comprising: Flat surface (1), a first set of walls (3), the first set of walls comprising a first wall (31) and a second wall (32), the first wall (31) and the second wall (32) being in physical contact with the flat surface (1) and meeting at a corner (312) to form an angle, and a first layer (2), the first layer (2) being formed of a two-dimensional material, being in physical contact with the flat surface (1) and being in physical contact with both the first wall (31) and the second wall (32) at the corner (312), wherein the angle is adapted to align with the crystal structure of the two-dimensional material and has a tolerance of at most 5°, wherein the length of each of the first and second walls (31, 32) is 5 nm to 1000 nm, wherein the height of the wall (3) as measured from the exposed top surface (22) of the first layer (2) to the top (30) of the wall is 0.6 nm to 2 nm, thereby forming a cavity defined by at least the top surface (22) and the first set of walls (3), and A second layer (4, 7) is within the cavity and in physical contact with the top surface of the first layer (2).

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