Deposition of highly crystalline 2D materials

A method for providing a film of one or more monolayers of transition metal dichalcogenides on a substrate is disclosed. The method includes providing a substrate; depositing at least one monolayer of the transition metal dichalcogenides on the substrate; and selectively removing superficial islands on top of the at least one monolayer by thermal etching.

The present application claims priority from European Patent application no. 21159840.4, filed on Mar. 1, 2021, which is incorporated by reference in its entirety.

FIELD OF THE DISCLOSURE

The disclosure relates to the field of metal chalcogenide films. More specifically, it relates to a method for forming providing a film of one or more monolayers of transition metal dichalcogenides on a substrate.

BACKGROUND OF THE DISCLOSURE

Two-dimensional transition metal dichalcogenide (MX2) films attract lots of attention for next-generation electronics, optoelectronics and photonics. They could play an important role in beyond-CMoS (complementary metal-oxide-semiconductor) electronics.

The semiconducting group-VI MX2materials can possess a layered crystal structure with layer-dependent physical and electrical properties, and a chemically inert surface free of dangling bonds. As such, MX2materials can complement Silicon as channel material in nanoelectronic devices with ultra-scaled channel thickness at excellent electrostatic gate control and anticipated absence of interface defects, and decreased off-state power consumption. However, their unique properties can predominantly excel in one monolayer film and few monolayer film form, which fuels research on the large-area deposition of atomically thin MX2material.

The structural and electrical quality of two-dimensional transition metal dichalcogenide films (2D MX2films) can be highly dependent on the absence of defects and the smoothness of the film surface. The point defect, grin boundaries and surface roughness of 2D materials can make it more challenging for the integration and industrialization of 2D materials based nanoelectronics.

Towards the large-area integration of 2D materials based nanoelectronics, one of the biggest challenges can be to achieve uniform and wafer-scale single (or highly) crystalline deposition of the 2D MX2materials. The misorientation of the first monolayer crystals can result in grain boundaries, which is one kind of local defects that can degrade the advanced electronic properties of 2D materials.

There is therefore a need to achieve uniform and wafer-scale single (or highly) crystalline deposition of the 2D MX2materials.

SUMMARY OF THE DISCLOSURE

It is an object of embodiments of the present disclosure to provide a good method for providing a film of one or more monolayers of transition metal dichalcogenides on a substrate.

In embodiments of the present disclosure, this is achieved by a method in which first a substrate is provided and in which, furthermore, a sequence is provided which comprises the following steps:depositing at least one monolayer of the transition metal dichalcogenides on the substrate; andselectively removing superficial islands on top of the at least one monolayer by thermal etching.

In embodiments of the present disclosure, the sequence of depositing/selectively removing is executed at least twice. The sequence may be executed until one or more closed monolayers are obtained.

In embodiments of the present disclosure, etchants used for the etching may be selected from the group of Cl2, HCl, and CO.

In embodiments of the present disclosure, the transition metal dichalcogenides can be deposited by metal-organic chemical vapor deposition.

A method, according to embodiments of the present disclosure, may be applied on any substrate, as long as the main growth mode of the MX2layer is planar, since it is based on the presence of dangling bonds on island edges, vs. a fully saturated basal plane surface. The substrate can be, for example, a sapphire substrate.

In embodiments of the present disclosure, the transition metal dichalcogenide is MoS2. The disclosure is, however, not limited thereto. Also other transition metal dichalcogenides may be used, such as for example WS2.

In embodiments of the present disclosure, the depositing and selective removing steps are done in a same reactor. In embodiments of the present disclosure, reactivation, which may be required when moving the substrate from one reactor to another, can be avoided by growing and etching in the same reactor.

In embodiments of the present disclosure, the selective removing can be done at a temperature of at least 500° C.

In embodiments of the present disclosure, the method is suitable for selective area deposition, wherein:the provided substrate is a patterned substrate comprising a first material and a second material,the at least one monolayer of the transition metal dichalcogenides is deposited on the patterned substrate and the materials are selected such that the deposition is more inhibited on the second material than on the first material, andthe high temperature etching is such that nuclei of the transition metal dichalcogenides on the second material are removed from the second material.

In embodiments of the present disclosure, the transition metal is a metal from group VI of the periodic table of elements.

In embodiments of the present disclosure, a method is provided for manufacturing a field effect transistor. The method comprises providing a film of one or more monolayers of transition metal dichalcogenides on a substrate, using a method according to embodiments of the present disclosure.

The method, moreover, comprises forming a gate stack on top of the film of the transition metal dichalcogenides such that the film is a channel of the field effect transistor, and forming source and drain contacts at the beginning and end of the channel.

In embodiments of the present disclosure, MX2 materials replace Si as channel material. In embodiments of the present disclosure, the channel thickness can be reduced to a few or even one monolayer and that superficial islands are removed.

In embodiments of the present disclosure, the method comprises transferring the provided film from the substrate on which it is formed to a target substrate.

In summary, in embodiments of the present disclosure, by means of the (in-situ) selective etching process, an increased flexibility to tune the deposition process of epitaxial 2D MX2materials can be achieved. This enables controlling the local film thickness and thus surface smoothness and achieving uniform, wafer-scale and single (highly) crystalline 1 ML film of 2D MX2materials. The method according to embodiments of the present disclosure may, for example, be used for controlling the local film thickness variation of wafer-scale epitaxial 2D MX2materials and thus smoothening the surface. It may, for example, also be used for wafer-scale single (highly) crystalline growth of 2D MX2materials for advanced high-performance nanoelectronics and optoelectronics. Another application may be the anisotropic etching (with hard mask) techniques of 2D materials for nanoelectronics fabrication and integration.

These and other aspects of the disclosure will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE DISCLOSURE

In embodiments of the present disclosure where reference is made to a monolayer ML, the reference is made to a layer of transition metal dichalcogenides (MX2, with M a transition metal, and X a chalcogen).

In embodiments of the present disclosure where reference is made to an MX2film, the reference is made to a film on a substrate, composed of one or more monolayers of MX2.

In embodiments of the present disclosure, the thickness of a monolayer depends on the transition metal M and on the chalcogenide X. This thickness is typically around 0.6 nm.

In embodiments of the present disclosure where reference is made to the number of monolayers (NOML), the reference is made to the number of monolayers stacked on top of each other.

The total film thickness is the physical thickness of the provided MX2film. It depends on the NOML and on the transition metal M and on the chalcogenide X. For example a MX2film containing exactly 2 MLs of MX2is designated as a “2 ML film” and a MX2film of exactly 1 ML is designated as a “1 ML film or single ML film”.

Embodiments of the present disclosure relate to a method100for providing a film of one or more monolayers of transition metal dichalcogenides on a substrate. The method comprises:providing110a substrate;
the method, furthermore, comprises a sequence of:depositing120at least one monolayer of the transition metal dichalcogenides on the substrate, andselectively removing130superficial islands on top of the at least one monolayer by thermal etching.

A flow chart of an exemplary method in accordance with embodiments of the present disclosure is shown inFIG.1.

In embodiments of the present disclosure, the superficial islands are removed. Without being bound by theory, it is assumed that this is possible because of a different etch rate between the superficial islands on the monolayer, and the monolayer itself. It is assumed that the reason therefore is that the etchants first react at the edges of the MX2islands or MX2flakes due to existence of dangling bonds, rather than the planar surface which is free of dangling bonds. In embodiments of the present disclosure the etching reaction is, therefore, not conformal, but rather selective to superficial islands.

In embodiments of the present disclosure, a basal plane of the film is parallel with a surface of the wafer.

In embodiments of the present disclosure, a monolayer of transition metal dichalcogenides may be a one-molecular 2D layer of transition metal dichalcogenides (MX2, with M a transition metal, and X a chalcogen), with its basal plane parallel aligned to the wafer surface, and containing only one central layer of M-atoms, cladded above and below by a layer of X atoms.

In embodiments of the present disclosure, selective removal of the superficial islands on top of the at least one monolayer may be done by thermal etching, whereby etchants such as Cl2, HCl or CO are used.

In embodiments of the present disclosure, a setup may be used that allows the growth and etching process of 2D MX2in the same reactor. With such a setup, it is possible to etch away the undesirable superficial MX2islands in-situ and selectively after the growth, and thus smoothing the surface of MX2.

In embodiments of the present disclosure, the selective etching process can be gentle and doesn't modify the physical properties of 2D materials, which actually improves the electrical performance of MX2based transistors in terms of minimum current (i.e. the off-current, Imin) and subthreshold swing (SSmin).

In embodiments of the present disclosure, the depositing/selectively removing sequence120,130may be executed at least twice.

It is, thereby, beneficial that the number of monolayers, and the variation thereon, can be controlled by executing cycles of growth and etching. Thus, a smoothened surface can be obtained. In embodiments of the present disclosure, it is possible to selectively etch the superficial islands without damaging the underneath closed layers. When reference is made to a closed layer, reference is made to a layer without an opening in the layer. A closed layer, on the other hand, is a fully coalesced layer without any holes/openings.

By combining cycles of the selective etching process with cycles of deposition, the deposition of uniform and wafer-scale single (highly) crystalline one monolayer MX2film can be achieved. A method according to embodiments of the present disclosure can enable precise control of 2D film thickness and thus surface smoothness. Moreover, it can also provide new methods for the nanofabrication and integration process of 2D materials based nanoelectronics and optoelectronics.

The total film thickness can be controlled through tuning parameters of the growth process, such as the growth time. Moreover, a selective and gentle etching process, according to embodiments of the present disclosure, after growth, helps to etch away the superficial islands without damaging the lower-lying closed 2D layer(s).

The drawings inFIG.2schematically illustrate one cycle of growth and selective etching on wafer-scale epitaxial 2D materials. The top drawings inFIG.2show a schematic drawing of a stack of monolayers221-222,231-233, including superficial islands, on a deposition substrate210. As illustrated in the top drawings ofFIG.2, unwanted superficial islands231-233are existing on top of the lower-lying, closed 2D 1 ML film221of multi-ML film221,222after the growth, even though the deposition the deposition time is precisely controlled. Also a vertical25crystal241is grown at a discontinuity in the second monolayer222. The superficial islands are grown anisotropically on top of the lower lying layer(s) before the underlying layer(s) is closed. Therefore, in embodiments of the present disclosure, a thermal etching process is applied directly after the growth.

This etching process should be gentle and can remove the superficial islands selectively. The selectivity of the etching process can be engineered by a judicious choice of the species of the etchants, and careful tuning of process parameters such as temperature, pressure, gas flow, time, etc. In embodiments of the present disclosure, etchants used for the etching130may be, for example, selected from the group of Cl2, HCl, CO. This group is not limiting and also other etchants may be selected. In embodiments of the present disclosure, the selective removing130can be generally done at temperatures of at least 500° C. The temperatures generally range between 500° C. and 600° C. In embodiments of the present disclosure, the cycle of growth and etching can help to reduce the local NOML variation of the deposited 2D layers and thus smooth the surface. InFIG.2, the schematic drawings at the bottom show the obtained stack after selective etching.

Without being bound by theory, it is assumed that the reaction kinetics of the etchants (such as Cl2, HCl, CO, etc.) with 2D superficial islands is different from the reaction kinetics of fully closed layers of 2D materials. The etchants firstly attack the edges of 2D islands due to the existence of dangling bonds, rather than the planar surface which is free of dangling bonds.

In embodiments of the present disclosure, defects during the etching process may be avoided by the addition of H2S (or other sulphur-containing precursors) during the etching process.

In an exemplary embodiment of the present disclosure, firstly, epitaxial molybdenum disulfide (MoS2) can be deposited120on sapphire wafers210through metal-organic chemical vapor deposition (MOCVD) in an industry-compatible reactor. The sapphire wafers210can be firstly placed on a 200 mm silicon pocket wafer (structure: 100 nm ALD Al2O3/2000 nm SiO2/Si) which contains 4 pockets with size of 2-inch and then loaded to the MOCVD reactor. The MOCVD equipment for the experiment can consist of a single wafer (z 200 mm) and lamp-heated reactor with gas flow controlled by mass flow controllers, which can allow large-scale deposition of MX2on wafers with size as large as 200 mm. After loading the sapphire wafers to the reactor, the wafers can be heated to 1000° C. under high-purity N2in the reactor, then 100 standard cubic centimeter per minute (sccm) H2S (carried by 20 standard liter per minute (slm) N2) and 80 sccm Ar:Mo(CO)6gas precursor (Ar is the carrier gas for the metal precursor) can be sent to the reactor. High purity H2S, N2and Ar gas can be provided through compressed gas cylinders. The Mo(CO)6can be vaporized from the solid precursor in a metallic canister (from Air liquide) at about 26° C. under about 900 mbar. During the growth, the growth temperature can be kept at 1000° C. and the total pressure is constant at 20 Torr. For the growth of 1-2 ML (1 ML film with 2nd ML islands) and 3-5 ML (3 ML film with 4th and 5th ML islands) films of MoS2on sapphire, the growth time can be 6 min and 20 min respectively at this condition.

After the growth, the etching process can follow immediately in the same reactor. The etching process can be performed under the condition of 20 sccm Cl2and 20 slm N2at a total pressure of 20 Torr. The temperature for the etching process can be constant. Two kinds of temperature conditions may be tried, including 600° C. and 500° C. After the etching process, all the samples can be annealed in the same chamber under 100 sccm H2S (carried by 20 slm N2) at 1000° C. and at a total pressure of 90 Torr.

The selective etching of epitaxial MoS2grown on sapphire through in-situ Cl2thermal etching is demonstrated inFIG.3, parts a to i.

The graphs show:(a) thickness of 3-5 ML MoS2before etching and after different etching times at different temperatures;(b) A1gand E2gpeak frequency difference of 3-5 ML MoS2before and after etching, and as-grown 1-2 ML MoS2;(c) S/Mo ratio of 3-5 ML MoS2before etching and after different etching times at different temperatures;(d) X-ray photoelectron spectroscopy (XPS) of Mo 3d before etching and after different etching times;(e) XPS of S 2p before etching and after different etching times;(f) XPS of Cl 2p before etching and after different etching times;(g) XPS of O 1s before etching and after different etching times;(h) Raman spectroscopy of 3-5 ML MoS2before etching and after different etching times;(i) Photoluminescence (PL) spectrum of 3-5 ML MoS2before etching and after different etching times.

The temperature of etching process was constant at 600° C. forFIG.3, parts b, and d-i.

The total film thickness of the as-grown 3-5 ML MoS2film on sapphire is around 3.6 ML as calculated from Rutherford backscattering spectrometry (RBS) measurements. The Cl2etching speed of the as-grown 3.6 ML MoS2is evaluated at various etching temperatures, including 500° C. and 600° C. inFIG.3, part a. The etching speed increases with the etching temperature. Most importantly, the etching speed which is initially quite high (0.5-1 ML/minute depending on the temperature), decreases after some time of etching to ˜0.1 ML/minute (the slope inFIG.3, part a, gets less steep with the etching time), indicating etching of the closed layers is much more difficult than the superficial islands. This allows a selective etching of the superficial islands without damaging the underneath closed layer(s). The etching selection ratio can be optimized through different etchants, temperature, pressure, gas flow, time and others.

FIG.3, part b, further illustrates the selective etching through the A1gand E2gpeak frequency difference of the 3-5 ML MoS2before and after different etching time at 600° C. The A1gand E2gpeak frequency difference of MoS2decreases with the film thickness, as it can be observed for as grown 3-5 ML and 1-2 ML MoS2. Similar to that inFIG.3, part a, A1gand E2gpeak frequency difference decreases with the etching time and the decreasing slope becomes gentle with the etching time.

This Cl2thermal etching process can be gentle, which doesn't change the pristine properties of the as grown MoS2on sapphire. The measured S/Mo ratio stays around 2 for the MoS2after different etching times at both 500° C. and 600° C., as illustrated inFIG.3, part c.

Furthermore, the XPS spectra of the Mo 3d, S 2p and O 1s all stay the same before and after 1 min, 8 min and 9 min etching inFIG.3, parts e and g.

Meanwhile, the XPS spectra of Cl 2p show negligible peaks for the MoS2before and after 1 min, 8 min and 9 min etching inFIG.3, part f, indicating no Cl bonds with the MoS2after etching.

The characteristic peaks all exist in Raman and PL spectra for the MoS2after different etching time inFIG.3, parts h-i. The slightly shift of the peak position inFIG.3, part l, is due to the thickness reduction with the etching time. According to all of these physical characterizations of the epitaxial MoS2grown on sapphire before and after different etching time, the etching process doesn't change the pristine physical properties of the as-grown MoS2.

When providing a film using the exemplary method, in accordance with embodiments of the present disclosure,FIG.4illustrates that surface smoothness of the epitaxial MoS2on sapphire after Cl2etching can be achieved. This is illustrated for 3-5 ML and 1-2 ML MoS2on sapphire after Cl2etching.

FIG.4, parts a-d, show the topography obtained using AFM (atomic force microscopy) of as-grown 3-5 ML MoS2(FIG.4a) and the topography obtained after different etching times (FIG.4, parts b-d).

FIG.4, parts e-f, show the topography obtained using AFM of as-grown 1-2 ML MoS2before (FIG.4, part e) and after 0.5 min of etching (FIG.4, part f).

FIG.4, parts g-h, shows the zoom in topography maps ofFIG.4, parts e-f. The surface coverage of superficial crystals in (FIG.4, parts e, g) is around 5%, while it reduces to <1% in (FIG.4, parts f, h).

FIG.4, part i, shows the full width at half maximum (FWHM) of the E2gpeak of the as-grown 1-2 ML MoS2and the one after 0.5 min etching.

FIG.4, part j, shows the full width at half maximum (FWHM) of the A1gpeak of the as-grown 1-2 ML MoS2and the one after 0.5 min etching.

Both peaks are narrowing after etching. The temperature of etching process is constant at 600° C.

As mentioned above, the as-grown 3-5 ML MoS2comprises a closed film of 3 MLs of MoS2with 4th ML, 5th ML crystals and vertical growth on top (FIG.4, part a). The surface roughness of this as-grown 3-5 ML MoS2is around 5.76 nm. With the Cl2etching process, the vertical growth, the 5thand 4thML crystals disappear (FIG.4, parts b-d). Moreover, the surface roughness also decreases (FIG.4, parts b-d). These observations are further confirmed for the etching of as-grown 1-2 ML MoS2film (FIG.4, parts e-h). There is a clear reduction for surface coverage of superficial crystals on top of the closed monolayer MoS2before and after 0.5 min etching process (from ˜5% inFIG.4, parts e and g, to <1% inFIG.4, parts f and h. Meanwhile, the size of the superficial crystals also decreases after etching (FIG.4, parts g-h). As a result, the whole surface roughness decreases. Moreover, the E2gand A1gpeaks of the 1-2 ML MoS2become narrower after etching, indicating the sample surface after etching is more uniform.

Embodiments of the present disclosure provide a method for manufacturing a field effect transistor. The method comprises providing100a film of one or more monolayers of transition metal dichalcogenides on a substrate, using a method according to any of the previous claims. The method, moreover, comprises forming a gate stack on top of the film of the transition metal dichalcogenides such that the film is a channel of the field effect transistor, and forming source and drain contacts at the beginning and end of the channel.

FIG.8shows a schematic drawing of an exemplary FET stack obtained using a method in accordance with embodiments of the present disclosure. The stack400comprises a Si substrate410, a SiO2layer420, a first metal layer430, a high-k dielectric440, an MX2channel450, source and drain contacts460aof a second metal,460b, and metal contacts (of a third metal)470aand470bwith the source and drain contacts. This is only an exemplary FET stack, and also other FET stacks known by the person skilled in the art are possible. The channel is a MX2channel obtained using a method in accordance with embodiments of the present disclosure.

In the following paragraphs, physical properties of pristine and etched 3-5 layer MoS2based field effect transistors are evaluated.

The electrical properties of epitaxial MoS2after etching further confirm that the Cl2thermal etching process doesn't damage the closed MoS2layers under the superficial islands, and there is no doping effect induced by the Cl2etching. The typical transfer characteristic curves of pristine (reference numbers 1 and 2) and etched 3-5 layer MoS2(reference numbers 3 and 4) based field-effect transistors (FETs) inFIG.5reveal that there is no shift of the threshold voltage (Vt) for the Cl2etched MoS2based transistors at all the channel lengths (Lch), compared to that of pristine MoS2based transistors. This indicates there is no doping effect introduced by this Cl2thermal etching process. Moreover, the minimum drain current (Imin) of the pristine MoS2based transistors is larger than that of etched MoS2based transistors at all the Lch.

This is further confirmed inFIG.6, parts a-e, through comparing the electrical performance of 1080 transistors fabricated with the pristine and etched MoS2layers.FIG.6, parts a-e, shows the statistical comparison of the electrical properties of pristine and etched MoS2based transistors. From top to bottom the following graphs are shown:(a) Imax;(b) Imin;(c) SSmin;(d) Vt;(e) μFE.

The transistors are fabricated on 3-5 ML MoS2before (samples with reference numbers 1 and 2; the two left columns) and after 9 min Cl2etching (samples with reference numbers 3 and 4; the two right columns.

The maximum drain current (Imax) of the etched MoS2based transistors does not decrease inFIG.6, part a, compared to that of the pristine MoS2based transistors. This confirms that the Cl2thermal etching process is gentle, which doesn't damage the closed MoS2layers under the superficial crystals.

Similar toFIG.5,FIG.6, part b, shows that the Imin of the pristine MoS2based transistors is larger than that of the etched MoS2based transistors, especially for transistors with shorter Lch.

A similar phenomenon is also observed for minimum subthreshold swing (SSmin). As can be seen inFIG.6, part c, the SSmin of the pristine MoS2based transistors is larger than that of the etched MoS2based transistors, especially for transistors with shorter Lch.

From the graphs inFIG.6, part d, it can be seen that the threshold voltage Vtof the pristine and the etched MoS2based transistors is substantially the same, further confirming that the Cl2thermal etching doesn't introduce any doping to the MoS2layers. The calculated field effect mobility (μFE) of the pristine and etched MoS2based transistors is similar, further confirming that the Cl2thermal etching doesn't damage the MoS2layers under the superficial crystals. Thus, this in-situ Cl2thermal etching process can be gentle and can etch the superficial crystals on top of the closed 2D layers selectively.

As discussed earlier, in embodiments of the present disclosure, a uniform and wafer-scale single (or highly) crystalline epitaxial deposition of single monolayer 2D materials can be obtained. Before the closure of the 1stML of 2D materials, the coexistence of big and small 1stML crystals, and also 2ndML crystals on top of the 1stML crystals can be observed. This is illustrated inFIG.7. The left image inFIG.7shows an example of MOCVD WS2crystals grown on a SiO2/Si surface. The dashed circles outline the small crystals grown on SiO2/Si and the first ML WS2crystals. These small 1stML and 2ndML crystals can introduce more grain boundaries and local defects, which should be avoided during the deposition process. Therefore, in embodiments of the present disclosure, a process is introduced including cycles of deposition and selective etching to remove the small crystals (the superficial islands) and then regrow the layer until the 1stML closes. Through tuning the etching parameters, the etching selection ratio (SEetch) can be tuned. The etching selection ratio can be expressed as:

In this equation, [W]2ndMLrepresents the amount of material to be removed, while [W]1stMLrepresents the amount of material to be kept. When SEetchis close to zero, there are big and small MX2crystals existing simultaneously, as shown in the inset schematic310in the right graph ofFIG.7. When SEetchincreases, the small crystals are expected to be removed through etching and just the large crystals are left, as shown in the inset schematic320in the right graph ofFIG.7. In embodiments of the present disclosure, with increasing number of cycles of the deposition and selective etching process, the uniformity of the film increases. Thus, a uniform and wafer-scale single (or highly) crystalline 1 ML film can be obtained. In embodiments of the method of the present disclosure, a multi-ML film of 2D materials with a controlled NOML can be beneficially obtained.

A method according to embodiments of the present disclosure may be used for selective area deposition. In such a method, the provided110substrate can be a patterned substrate comprising a first material and a second material, and the at least one monolayer of the transition metal dichalcogenides is deposited120on the patterned substrate and the materials are selected such that the deposition is more inhibited on the second material than on the first material. Moreover, the high temperature etching130is such that nuclei of the transition metal dichalcogenides on the second material are removed from the second material.

In embodiments of the present disclosure, the transition metal dichalcogenide, the first material and the second material can be selected such that the deposition of the transition metal dichalcogenide is more inhibited on the second material than on the first material.

In an exemplary embodiment of the present disclosure, the first material of the patterned substrate can be, for example, HfO2and the second material SiO2. In the deposition step, one monolayer of MX2can be selectively deposited on HfO2as the materials are selected such that the MX2deposition is more inhibited on SiO2. However, a certain selectively loss can be expected with respect to SiO2resulting in the formation of MX2nuclei in the first monolayer also on SiO2. In embodiments of the present disclosure, these are selectively removed from the SiO2surface using thermal etch process.

In an exemplary embodiment of the present disclosure, the first material can be MoS2and the second material SiO2. The MX2material may be different from the first material. It may for example be WS2, or HfS2. The MX2material can be selectively deposited on the first material MoS2to form a heterostructure. The thermal etch process can be used to selectively etch MX2layer that nucleates on the second material SiO2to have the MX2layer only forming on the first material. As such, one can deposit heterostructure of MoS2with another MX2layer (e.g., WS2, HfS2).

In embodiments of the present disclosure, the method may comprise transferring the provided film from the substrate on which it is formed to a target substrate.

The transfer may be achieved via wet or dry transfer methods. In the wet transfer case, it may be based on water assisted delamination. A layer of PMMA may for example be firstly coated on the MX2film grown on sapphire. Then a thermal release tape (TRT) may be used for the transfer of the MX2from sapphire to other substrates. The MX2film may be delaminated from the sapphire through water intercalation in ambient. After transferring the TRT/PMMA/MX2on the target substrate (e.g. SiO2), the TRT can be released at ˜150° C. Finally, the PMMA may be removed through acetone and Isopropyl alcohol, which results in the clean MX2film on the target substrates.