Method for selectively growing van der Waals heterostructures on graphene substrates by chemical vapor deposition using electron beam irradiation and heterostructures produced by this method

Abstract

The invention relates to a method for the manufacture of van der Waals heterostructures based on transition metal dichalcogenides selectively grown by chemical vapor deposition on a graphene substrate, characterized in that the surface of the graphene substrate is irradiated with an electron beam and the growth of the transition metal dichalcogenides in the areas irradiated with the electron beam is controlled by exposing the irradiated substrate to the atmosphere.The invention also relates to heterostructures manufactured by the described method, characterized in that a two-dimensional structure in the form of a monolayer is grown on the graphene substrate.

Description

[Technical field]

[0001] The object of the present invention is a method for growing van der Waals heterostructures based on transition metal sulfides on graphene using electron beam irradiation while maintaining selectivity control of the growth area, and the heterostructures produced by this method.

[0002] Van der Waals heterostructures are stacks of layered materials, i.e., ionic and covalent bonds exist only in the plane of the atomic layers. The van der Waals heterostructures are characterized in that the individual layers of the stack forming the heterostructure are interconnected by weak atomic interactions (van der Waals interactions). The van der Waals heterostructures often take the form of two-dimensional (2D) material stacks, i.e., stacks of materials with a thickness of a single atomic or molecular layer.

[0003] An example of a two-dimensional material is graphene, a single layer made up of a hexagonal network of carbon atoms. Graphene is characterized by a unit cell made up of two atoms, each with four valence electrons. One of these valence electrons lies in an atomic orbital perpendicular to the graphene plane, allowing weak van der Waals bonds to form, so that many layers of graphene can be stacked together to form the bulk material graphite. If another two-dimensional material is placed on top of graphene in place of the carbon atom layer, these bonds form van der Waals heterostructures.

[0004] Another class of 2D materials are the transition metal dichalcogenides (TMDs), which consist of atoms from the D block of the periodic table bonded to chalcogens (elements in group 16 of the periodic table) and have the formula MX2, where M is a transition metal (Mo, W, Pt, etc.) and X is a chalcogen (S, Se, Te, etc.). The most frequently studied monolayers of transition metal dichalcogenides are molybdenum disulfide (MoS2) and tungsten disulfide (WS2).

[0005] All the above mentioned materials (graphene, MoS2, WS2) in the form of single atomic layers have very interesting properties when it comes to their use in nanoelectronics. Graphene is a semimetal that conducts electricity perfectly, which makes it an excellent material for manufacturing electrodes in electronic devices. MoS2 and WS2 are often of the 2H polytype, and these materials are semiconductors with a direct band gap (in the case of single layers). Direct band gap semiconductors are ideal for use in devices such as transistors, because in the case of these two materials, the energy of the band gap corresponds to energies in the range of visible light, allowing the generation of electric carriers under illumination (applications such as photodetection).

[0006] Combining two 2D materials (e.g. graphene and MoS2) to form van der Waals heterostructures enables many important applications in modern electronics. Using these materials in heterostructures, it is possible to obtain photosensitive field effect transistors with advantageously designed electrical contacts (graphene-MoS2 contacts). Such devices can also be used as photodetectors and optical storage, storing information about the illumination of the sample for longer periods of time (from seconds to months). However, the fabrication of van der Waals heterostructures and the production of devices based on these materials remain challenges.

[0007] Heterostructures are often produced in two main ways. One of them is mechanical exfoliation (delamination), which involves the separation of weakly bonded layers from a bulk crystal and the placement of these layers in a suitable way on a selected substrate. This method is inefficient and practically impossible to adapt to industrial scale, due to the small size, the randomness of the exfoliation process and the difficulty of precisely placing the layers on one another on the substrate. The second way to realize heterostructures is layer-by-layer growth by chemical vapor deposition (CVD). In this method, precursors (substances involved in the chemical reaction that leads to the formation of the desired chemical compounds) are supplied in gaseous form to the substrate at a suitable high temperature (up to 400 degrees Celsius) and under a suitable pressure (up to 1000 hPa), which then react with each other or undergo pyrolysis, and the transformed compounds are deposited on the loaded substrate in the form of the desired material, most often in the form of thin-layer crystals. Growth by CVD allows a very large coverage (0-100%) of the selected substrate with layers of two-dimensional materials, if optimal process parameters are in place.

[0008] Although CVD is a scalable and reproducible method, the process itself is highly dependent on the selection and preparation of the substrate. Moreover, the method does not allow for selective growth of selected materials in specific regions of the substrate. The CVD method requires additional steps, such as plasma etching, to confine the area of ​​the material on the substrate to only the designed channels of the device, which requires the use of special equipment and increases the manufacturing costs. Therefore, the development of modern electronics could be greatly benefited by the CVD method that provides optimized, preferably selective, growth of van der Waals heterostructures.

[0009] Patent specification CN111146079A and the publication "General synthesis of two-dimensional van der Waals heterostructure arrays" (Li et al., Nature 579, 368-374 (2020)) present the growth of vertical van der Waals heterostructures (i.e., individual layers stacked on top of each other) made of materials with metallic and semiconducting properties. In this method, a laser is irradiated on the allocated areas of the semiconductor layer (MoS2, WS2, etc.) that etches defects. On such defective substrates, the CVD process is carried out to obtain vertical TMD / TMD heterostructures such as VSe2 / WSe2 and CoTe2 / WSe2. This method requires the production of multilayer substrate materials and cannot be used for single-layer materials.

[0010] Patent specification US10465276B2 presents the growth process of two-dimensional materials and their vertical heterostructures (TMD / graphene), where selectivity is reached by using lithographic resist to mask the appropriate areas of the sample. In this process, it is possible to mask several times to obtain the pn junction. This is a resist-based process, which is spread over the substrate surface and then removed, which may leave behind chemical contamination.

[0011] Patent specification US10872973B2 presents a process for growing antimonene, an allotrope of antimony atoms, on a pre-etched substrate with a monolayer of a two-dimensional material, such as WS2 or MoS2, to form vertical van der Waals heterostructures. This growth selectivity is based on the energetically favorable nucleation of antimonene on the two-dimensional material, but not on the surrounding substrate, such as sapphire or silicon. However, this method is only adapted to the formation of antimonene layers, which must be further adapted to the assigned properties, such as thinning by plasma etching to obtain the semiconducting properties of the material.

[0012] The patent specification US2017025505A1 and the publication "Patterned arrays of lateral heterojunction within monolayer two-dimensional semiconductor conductors" (Mahjouri-Samani et al., Nature Coommunications 6, 7749 (2015)) present a process for selectively growing lateral heterostructures (i.e., those in contact with the edge of a layer) by sulfurizing the areas that are not lithographically masked. Herein, a process is presented in which, in a lithographic process, a monolayer of a starting material (e.g., MoSe2) is prepared by masking the assigned areas, which are then sulfurized to form a sulfur-based material (MoS2), resulting in a lateral heterostructure.

[0013] Patent specification WO2018005838A1 and the publication "Large-scale chemical assembly of atomically thin trasistors and circuits" (Zhao et al., Nature Nanotechnology 11, 954-959 (2016)) present a method for growing lateral heterostructures based on graphene. In this method, graphene is plasma etched and then the etched area is subjected to a nucleation process of a MoS2 layer. On the sample produced in this way, a MoS2 monolayer is grown and a graphene-MoS2 heterojunction is formed. The method uses lithography to produce the lateral structures, and the plasma etching contaminates the surface. The method itself shows process residues as nucleation sites for the next material layer.

[0014] Patent specification CN113327840A and the publication "Grain-Boundary Engineering of Monolayer MoS2 for Energy-Efficient Lateral Synaptic Devices" (Wang et al., Advanced Materials 33(32), 219243(2021)) disclose a suitable transition metal oxide (MoO x , WO x presents a method for the growth of lateral heterostructures by oxidizing a substrate layer (e.g. MoS2, WS2) with laser radiation to produce lateral heterostructures. This oxide is then reduced with another chalcogen, e.g. selenium. In this way, heterostructures such as MoS2-MoSe2 or WS2-WSe2 can be realized on the substrate produced by the laser radiation. The limitations of this method are due to the oxidation process. Since the method is based on the oxidation of the base layer, the possible heterostructure configurations are limited to the same transition metal.

[0015] The paper "Endoepitaxial growth of monolayer mosaic heterostructures" (Zhang et al., Nature Nanotechnology 17, 493-499 (2022)) presents selective growth of lateral heterostructures by laser and thermal etching. In this method, a laser is used to introduce point defects into a continuous TMD monolayer, which are then expanded by thermal etching. Other TMDs are grown on such defective material to obtain lateral heterostructures.

[0016] The selectivity in most of the above mentioned methods is achieved by lithographic methods, which can lead to contamination or doping of the underlying layer, which makes it necessary to cover said layer with a polymeric material. In the case of laser methods, the layer is most often removed or significantly deteriorated, which is undesirable if the physical and electrical properties of the underlying layer need to be maintained. The presented methods are very diverse, but none of them have a selective method using an electron beam to control the selective growth area. The present invention relates to a process for selectively growing van der Waals heterostructures of transition metal dichalcogenides on graphene by chemical vapor deposition. The selectivity of this process is ensured by irradiating the sample with a very powerful electron beam, but without using resists or other chemical reagents that are commonly used in lithographic processes. It was found that the irradiation of a very powerful electron beam on a graphene sample synthesized by chemical vapor deposition on an assigned substrate or on a graphene sample transferred to another substrate causes structural defects in the irradiated parts, but does not lead to a complete degradation of the layer. It should be noted that the substrate irradiated in this way is indistinguishable from non-irradiated areas when viewed with optical microscopy, which is important for optoelectronic applications: irradiated and non-irradiated areas can only be distinguished using a scanning electron microscope in in-lens imaging mode to distinguish between areas with different work functions, or by Raman spectroscopy, which can measure the intensity of the D peak, which is an indicator of graphene defects.

[0017] The sample thus prepared is used to produce, by chemical vapor deposition, a second monolayer which forms a heterostructure with the graphene substrate. Depending on how the sample is stored after the electron beam irradiation, positive growth (a monolayer of transition metal dichalcogenide grows only at the site of the electron beam irradiation) or negative growth (no synthesized crystals of transition metal dichalcogenide are observed only at the site of the electron beam irradiation, these crystals grow around the site of the electron beam irradiation) can be obtained. This method and the heterostructure produced by this method are being supported by the Graphene Flagship Project of the Horizon 2020 Programme of the European Commission (Project No. Graphene Flagship COre3N).o It was developed as a result of the joint research project (881603).

[0018] The present invention relates to a method for producing van der Waals heterostructures based on transition metal dichalcogenides selectively grown by chemical vapor deposition on a graphene substrate, by irradiating the surface of the graphene substrate with an electron beam and exposing the irradiated substrate to the atmosphere, thereby controlling the growth of the transition metal dichalcogenide in the regions irradiated with the electron beam.

[0019] Preferably, the minimum dose of the electron beam is 1,000 μC / cm 2 It is.

[0020] Preferably, the maximum dose of the electron beam is 1,000,000 μC / cm 2 The higher the dose, the greater the difference in synthesis of the transition metal dichalcogenides between the irradiated and non-irradiated areas.

[0021] Preferably, for normal growth, the exposure time of the irradiated substrate to air is at least 24 hours at a temperature of 20 degrees Celsius to obtain growth of transition metal dichalcogenides in the electron beam irradiated areas.

[0022] Preferably, for normal growth, the exposure time of the irradiated substrate to air is at least 6 hours and the irradiated graphene is simultaneously placed on a heating plate at a temperature of at least 70 degrees Celsius.

[0023] Preferably, in the case of negative growth, the exposure time of the irradiated substrate to air is at most 20 minutes, so that the electron beam does not cause the growth of the transition metal dichalcogenide in the irradiated areas. The contact of the sample with air should be limited to the moment when the sample is transferred from the electron lithography chamber to a furnace to carry out the CVD growth process.

[0024] The invention also relates to heterostructures produced by the method described, characterized in that they are synthesized in the form of monolayers of two-dimensional structures on a graphene substrate.

[0025] Preferably, the two-dimensional structure comprises a transition metal dichalcogenide.

[0026] Preferably, the transition metal dichalcogenide is a compound of the MX2 type, where M is a transition metal which is molybdenum, tungsten, platinum or vanadium, and X is a chalcogen which is sulfur, selenium or tellurium.

[0027] Preferably, the graphene substrate layer is slightly degraded after the irradiation step, as measured by Raman spectroscopy using a green laser with a wavelength of 532 nm, defined as the graphene D peak amplitude not exceeding the graphene 2D peak amplitude.

[0028] The selective growth of two-dimensional transition metal dichalcogenides on graphene substrates is characterized by the high precision of the desired shape of the target electronic device (irradiation is performed with an accuracy of 100 nm) and the ability to produce precisely defined shapes (0.2x0.2 μm 2 Very small heterostructures with dimensions of less than 100 mm (or less) as well as large area structures (limited only by the size of the electron lithography chamber, typically tens of cm 2 ) can also be manufactured. [Brief description of the drawings]

[0029] The invention in embodiments is illustrated in the drawings and is as follows: [Figure 1] FIG. 1 illustrates the growth of negative heterostructures, positive heterostructures, and heterostructures with random distribution of transition metal dichalcogenide (TMD) crystallites. [Diagram 2] Figure 2 shows graphene irradiated with an electron beam: a) optical microscope image, b) scanning electron microscope image with in-lens contrast, c) spatial mapping of graphene defect peak intensity (D) by Raman spectroscopy (the brighter the area, the more defects in the graphene), and d) defect levels in graphene irradiated with the maximum dose of an electron beam (dashed line) compared to non-irradiated graphene (continuous line). [Diagram 3] Figure 3 shows a) optical micrographs showing the growth of the negative heterostructure, b) photoluminescence spectra from irradiated areas of the negative heterostructure, and c) photoluminescence spectra from non-irradiated areas of the negative heterostructure. [Figure 4] Figure 4 shows a) an optical micrograph showing the growth of a positive heterostructure, b) a photoluminescence spectrum from an irradiated area of ​​the positive heterostructure, and c) a photoluminescence spectrum from a non-irradiated area of ​​the positive heterostructure. [Diagram 5] Figure 5 shows a) an optical micrograph showing the lack of growth selectivity when not irradiated with an electron beam, and b) a photoluminescence spectrum of a transition metal dichalcogenide crystal in the area not irradiated with an electron beam. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The method according to the invention is further illustrated in the following examples, the results of which identify the efficiency of the selective growth method and are shown in detail in the figures.

[0031] Example 1: Demonstrating the growth of negative heterostructures A sample of single-layer graphene, previously synthesized on a sapphire substrate by CVD, is placed in an electron lithography chamber. The sample does not require any special preparation before the irradiation step. In the electron lithography, the area of ​​the substrate (graphene) to be irradiated with an electron beam is specified. The beam parameters are 100,000 μC / cm 2 The exposure step is set to 0.2 μm, which is the distance between successive exposure points of the electron beam in the x-axis and y-axis of the substrate plane. The exposure is performed with an aperture of 120 μm. The exposure dose is 100,000 μC / cm 2 This results in a clean area on the sample that is free of impurities due to excessive particle deposition on defects.

[0032] After the irradiation of the graphene, the sample is removed from the chamber and placed without undue delay in the process chamber of the furnace, where the step of synthesizing the transition metal dichalcogenide on top is carried out. The residence time of the sample in air between the removal of the sample from the lithography chamber and the placement in the process chamber of the furnace is a total of 10 minutes. In the case of negative growth, the sample should not be exposed to air for more than 20 minutes after the irradiation. The contact of the sample with air should be limited to the moment of transferring the sample from the electron lithography chamber to the furnace to carry out the CVD growth process.

[0033] The growth process of transition metal dichalcogenide monolayers (such as MoS2 or WS2) is carried out in a furnace. The growth process is carried out in an argon atmosphere at a pressure of 20 mbar, using sublimated sulfur, tungsten oxide, and sodium chloride as precursors, at 900 degrees Celsius. As an effect of the CVD process, the WS2 crystals cover the entire substrate, except for the areas determined by the electron beam irradiation. After removing the sample from the furnace, the process is completed and the van der Waals heterostructure is produced. The bottom layer is graphene, and the WS2 layer is synthesized on it in the places where the electron beam was not irradiated. The places where the electron beam was irradiated are left clean, covered only with a graphene monolayer.

[0034] Example 2: Demonstrating the growth of positive heterostructures A sample of single-layer graphene previously synthesized on a sapphire substrate by the CVD method is placed in an electron lithography chamber. The sample does not require any special preparation before irradiation with electron beam lithography. In the electron lithography, the area of ​​the substrate (graphene) to be irradiated with an electron beam is specified. The beam parameters are 100,000 μC / cm 2 The exposure step is set to 0.2 μm, which is the distance between successive exposure points of the electron beam in the x-axis and y-axis of the substrate plane. The exposure is performed with an aperture of 120 μm. The exposure dose is 100,000 μC / cm 2This allows for a clean area on the sample to be obtained without adding impurities due to excessive particle deposition on the defects.

[0035] After graphene irradiation, the sample is removed from the chamber and left on a heating table at an atmospheric temperature of 70 degrees Celsius for 65 hours. The sample is then loaded into the process chamber of the furnace and the growth process of transition metal dichalcogenide monolayers (such as MoS2 and WS2) is carried out. The growth process is carried out at 900 degrees Celsius in an argon atmosphere at a pressure of 20 mbar using sublimated sulfur, tungsten oxide and sodium chloride as precursors, and the amount of precursors is less than that in Example 1. As an effect of the CVD process, the WS2 crystals cover the area determined by the irradiation of the electron beam with a much larger extent (more than 30%) and larger size (more than 3 μm) than the non-irradiated areas (5% and 0.5 μm, respectively). After removing the sample from the furnace, the van der Waals heterostructure is created where the electron beam irradiated the sample surface, with the WS2 layer synthesized on top of the graphene underlayer.

[0036] Example 3: Demonstrating the growth of heterostructures without controlling the growth selectivity of transition metal dichalcogenides The sample of monolayer graphene previously synthesized on a sapphire substrate using the CVD method is placed in the process chamber of the furnace, and a process of synthesizing a transition metal dichalcogenide thereon is carried out. The growth process of the transition metal dichalcogenide monolayer (such as MoS2 or WS2) is carried out in the furnace. The growth process is carried out at 900 degrees Celsius in an argon atmosphere at a pressure of 20 mbar using sublimated sulfur, tungsten oxide, and sodium chloride as precursors. As a result, dichalcogenide crystals cover random areas of the substrate. After removing the sample from the furnace, the process is completed and the van der Waals heterostructure is produced, with the bottom layer being graphene and WS2 synthesized on top of it in random areas of graphene. In this process, the substrate is not irradiated with an electron beam, so the growth area of ​​the transition metal dichalcogenide cannot be controlled and grows in random places.

Claims

1. A method for the fabrication of van der Waals heterostructures based on transition metal dichalcogenides selectively grown by chemical vapor deposition on a graphene substrate, comprising the steps of: irradiating a surface of the graphene substrate with an electron beam and exposing the irradiated substrate to air to control the growth of a transition metal dichalcogenide in the area irradiated with the electron beam.

2. 2. The method of claim 1, wherein the minimum dose of the electron beam is 1,000 μC / cm ２ The method according to claim 1,

3. 3. The method according to claim 1, wherein the maximum dose of the electron beam is 100,000 μC / cm ２ The method according to claim 1,

4. 4. The method according to claim 1, characterized in that in the case of positive growth, the exposure time of the irradiated substrate to air is at least 24 hours at a temperature of 20 degrees Celsius in order to obtain a growth of transition metal dichalcogenide in the area irradiated with the electron beam.

5. 4. The method according to any one of claims 1 to 3, characterized in that in case of positive growth, the exposure time of the irradiated substrate to air is at least 6 hours and the irradiated graphene is simultaneously placed on a heating plate at a temperature of at least 70 degrees Celsius.

6. 6. The method according to claim 1, characterized in that in the case of negative growth, the exposure time of the irradiated substrate to air is at most 20 minutes, so that no growth of transition metal dichalcogenides is obtained in the areas irradiated with the electron beam.

7. A heterostructure produced by the method of claims 1 to 6, characterized in that a two-dimensional structure in the form of a monolayer is synthesized on the graphene substrate.

8. 8. The heterostructure of claim 7, wherein said two-dimensional structure comprises a transition metal dichalcogenide.

9. 9. The heterostructure of claim 8, wherein the transition metal dichalcogenide is MX ２ Compounds of the type, wherein M is a transition metal which is molybdenum, tungsten, platinum or vanadium, and X is a chalcogen which is sulfur, selenium or tellurium, heterostructures.

10. 10. The heterostructure according to any one of claims 7 to 9, characterized in that the graphene substrate layer is slightly degraded after the irradiation step, as measured by Raman spectroscopy with a green laser of wavelength 532 nm, defined as the graphene D peak amplitude not exceeding the graphene 2D peak amplitude.

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