Patent Application
20240410080
The present disclosure generally relates to processes for forming a two-dimensional single atomic layer transition metal dichalcogenide. The present disclosure also generally relates to a two-dimensional single atomic layer transition metal dichalcogenide formed by the process.
Two-dimensional (2D) single atomic layer (SL) transition metal dichalcogenides (TMDs) are a subject of investigation in both fundamental science and practical applications. Due to having only single atomic layer thickness, direct band structures, good carrier mobility, and high on-off ratio, SL TMDs have promising use in electronics, especially for breaking the limit on scaling-down of semiconductor transistors. For realizing industrial-level applications of SL TMDs in electronics, the growth of large-area (centimeter scale), uniform, and high-electronic quality SL TMD films is of great importance. Conventional technologies to grow high-quality semiconductor films rely on epitaxial growth. Conventional epitaxy requires a highly matched lattice structure between the substrate and grown material. The layered structure of 2D TMDs, with van der Waals (vdW) interlayer forces, enables vdW epitaxy that can break the limit of traditional epitaxy and allow for a large lattice mismatch between the substrate and 2D layers. State-of-the-art technologies have demonstrated centimeter-scale epitaxial growth of SL MoS2 film on sapphire (c-plane) substrates. However, the single grain size of the films formed by conventional technologies is small, generally less than 10 μm, because the larger the grain size the more challenging it becomes to align them in the same crystallographic orientation. In addition, the small grain size of the film formed by conventional technologies increases the chance to form defects such as grain boundary, which lowers the quality of the film.
There is a need for new and improved two-dimensional single atomic layer transition metal dichalcogenides and for new and improved processes for forming two-dimensional single atomic layer transition metal dichalcogenides.
The present disclosure generally relates to processes for forming a two-dimensional single atomic layer transition metal dichalcogenide. The present disclosure also generally relates to a two-dimensional single atomic layer transition metal dichalcogenide formed by the process. Relative to conventional technologies, aspects described herein can enable epitaxial growth of large-area, uniform, single atomic layer transition metal dichalcogenide films having larger grain sizes, less defects, and matched crystallographic orientation. In some aspects, the epitaxial growth of two-dimensional single atomic layer transition metal dichalcogenide film with larger grain sizes and less defects can be enabled by alkali metal ions serving as a surfactant.
In an aspect, a process for forming a continuous transition metal dichalcogenide film is provided. The process includes flowing a carrier gas into a processing volume of a processing chamber having a substrate positioned therein. The process further includes heating an alkali metal salt, a transition metal oxide, and a chalcogenide to form reactive species. The process further includes exposing the substrate to the reactive species to form a transition metal dichalcogenide film, wherein: the transition metal dichalcogenide film comprises crystals having an average grain size of about 50 μm to about 500 μm; and the transition metal dichalcogenide film consists of a single atomic layer of transition metal dichalcogenide.
In another aspect, a process for forming a continuous TMD film is provided. The process includes flowing a carrier gas into a processing volume of a processing chamber having a substrate positioned therein. The process further includes heating an alkali metal salt, a transition metal oxide, and a chalcogenide to form reactive species. The process further includes exposing the substrate to the reactive species to form a continuous TMD film, wherein: the continuous TMD film comprises crystals having an average grain size of about 50 μm to about 500 μm; the crystals of the continuous TMD film are aligned in the same crystallographic orientation; and the continuous TMD film consists of a single atomic layer of transition metal dichalcogenide.
In another aspect, a process for forming a continuous single atomic layer transition metal dichalcogenide film is provided. The process includes exposing a substrate positioned in a processing volume of a chamber to a vapor formed from an alkali metal salt, a transition metal oxide, and a chalcogenide to deposit a continuous single atomic layer transition metal dichalcogenide film on the substrate, the continuous single atomic layer transition metal dichalcogenide film having: a thickness of about 1.1 nm or less; a width of about 500 μm to about 15 cm; and a length of about 500 μm to about 15 cm.
In another aspect, a controller configured to perform operations of a method described herein is provided.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to aspects, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary aspects and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective aspects.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one example may be beneficially incorporated in other examples without further recitation.
The present disclosure generally relates to processes for forming a two-dimensional (2D) single atomic layer (SL) transition metal dichalcogenide (TMD). The present disclosure also generally relates to a 2D SL TMD formed by the process. The processes described herein can enable epitaxial growth of a large-single-grain size, large-area, continuous TMD film. 2D SL TMDs formed by aspects described herein can be uniform and have high electronic quality relative to those formed by conventional technologies. As a result, for example, 2D SL TMDs formed by aspects described herein can be utilized in a variety of applications, for example, electronics, optoelectronics, sensing, catalysis, energy storage, solar cells, light emitting diodes, photovoltaics, and photodetectors, among others. The 2D SL TMDs can be incorporated into devices such as transistors, for example, back-gate field effect transistors (FETs).
In some examples, the inventors show that processes described herein can enable control over the growth of the crystals and the resulting film. For example, processes of the present disclosure can enable the crystals to grow in a single crystallographic orientation. As a result, when the crystals grow together, there can be less defects such as grain boundaries in the film relative to films grown by conventional technologies. Moreover, aspects described herein can enable control over nucleation. Here, and in contrast to conventional technologies, processes of the present disclosure can enable the formation of a lower number of nucleation sites, but the crystals grown from each nucleation site are large. Such control over the growth and/or nucleation enabled by processes described herein can lead to, for example, a large-area 2D SL TMD with less defects such as grain boundaries and better matched crystallographic orientation relative to the state-of the art. Here, conventional technologies provide crystals having random crystallographic orientations, while other conventional epitaxy methods grow a large number of small-sized grains resulting in more defects (high defect density) in the film such as grain boundary. The grain boundary defect resulting from conventional technologies is a planar defect that occurs where two crystals of different orientation meet. In addition, aspects described herein can enable formation of high-electric quality films (for example, single crystal films).
The use of headings is for purposes of convenience only and does not limit the scope of the present disclosure. Embodiments described herein can be combined with other embodiments.
Aspects of the present disclosure generally relate to processes for forming 2D SL TMDs.
The apparatus 200 includes a processing chamber 207 such as a chemical vapor deposition (CVD) chamber or other suitable processing chambers. The processing chamber 207 can be a heating chamber. The processing chamber 207 can be a quartz tube. The processing chamber 207 includes an inlet 220 and an outlet 228. The processing chamber 207 is fluidly coupled to a gas tank 210 via a line 213a, junction/valve 219, and line 213b. The gas tank 210 includes a carrier gas such as hydrogen (H2), helium (He), argon (Ar), krypton (Kr), neon (Ne), xenon (Xe), nitrogen (N2), oxygen (O2), or combinations thereof, such as Ar, H2, or combinations thereof. When more than one carrier gas is utilized, more than one gas tank can be used. In some aspects, Ar can be utilized to grow TMDs where the chalcogenide includes sulfur. In at least one aspect, a mixture of carrier gases, such as Ar and H2, can be utilized to grow TMDs where the chalcogenide includes selenium.
The carrier gas flows into the processing chamber 207 through the inlet 220 via lines 213a, 213b. Although not shown, mass flow controllers can be utilized to, e.g., measure and/or control the amount of gas(es) flowing through the lines 213a, 213b and into the processing chamber 207. As an example, the carrier gas content of the gas(es) entering the processing chamber 207 can be controlled by adjusting the flow rate ratio of a first carrier gas (FRC1) and a second carrier gas (FRC2) using a mass flow controller. The flow rate ratio, FRC1/FRC2, for example, can be the flow rate of H2 to the flow rate of Ar.
The apparatus 200 further includes a first heating mechanism 211a and a second heating mechanism 211b (collectively, heating mechanisms 211) serving to heat the processing chamber 207 and components therein. The heating mechanisms 211 can include a heating wire, a heating belt, an oven, a furnace, or other suitable apparatus. The heating mechanisms 211 can be operated independently to provide different amounts of heat to different locations of the chamber, as described below. For example, the first heating mechanism 211a can be operated at a first temperature, T1, and the second heating mechanism 211b can be operated at a second temperature, T2. As shown, the first heating mechanism 211a can include a heating wire and the second heating mechanism 211b can include a heating belt.
The apparatus 200 further includes a first tray 203a and a second tray 203b (collectively, trays 203). The trays 203 can be weigh boats, crucibles, flasks, or other suitable vessels that can withstand the temperatures of the processes disclosed herein. The trays 203 can be of any suitable shape and size. The first tray 203a is utilized to hold a mixture 226 that includes a first precursor comprising a salt (for example, an alkali metal salt) and a second precursor comprising a metal oxide.
The metal oxide (MO) can be a transition metal oxide. Any suitable transition metal can be used. In some aspects, the metal of the transition metal oxide comprises a Group 6 metal, such as chromium (Cr), molybdenum (Mo), tungsten (W), or combinations thereof, such as Mo, W, or combinations thereof. In at least one aspect, the transition metal oxide comprises molybdenum dioxide (MoO2), molybdenum trioxide (MoO3), tungsten dioxide (WO2), tungsten trioxide (WO3), or combinations thereof.
The salt can include an alkali metal (Group 1 of the periodic table of elements) and a halogen (Group 17 of the periodic table of elements) such that the salt can include an alkali metal salt. Examples of alkali metal elements that can be used for the alkali metal salt can include sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), or combinations thereof, though lithium (Li), francium (Fr), or combinations thereof can optionally be used. Examples of the halogen elements that can be used for the alkali metal salt can include chlorine (Cl), bromine (Br), iodine (I), or combinations thereof, though fluorine (F), or astatine (At) can optionally be utilized. In some aspects the salt comprises an alkali metal salt made of the cation Na, K, Rb, and/or Cs, and the anion Cl, Br, and/or I. Illustrative, but non-limiting, examples of alkali metal salts can include NaCl, NaBr, NaI, KCl, KBr, KI, RbCl, RbBr, RbI, CsCl, CsBr, CsI, or combinations thereof.
The second tray 203b is utilized to hold a third precursor comprising a chalcogenide 222. The chalcogenide includes at least one chalcogen element such as sulfur (S), selenium (Se), tellurium (Te), or combinations thereof, such as S, Se, or combinations thereof. The chalcogenide 222 can be in the form of a powder such as sulfur powder or selenium powder. The second tray 203b can be upstream of the first tray 203a.
The first tray 203a is positioned between heating elements of the first heating mechanism 211a. The first heating mechanism 211a can be utilized to heat the mixture 226 of the alkali metal salt and the metal oxide disposed in the first tray 203a. During operation, the mixture 226 can be heated to a first temperature. The first temperature can be any suitable temperature, such as from about 500° C. to about 1,000° C., such as from about 600° C. to about 900° C., such as from about 650° C. to 850° C., such as from about 700° C. and 800° C. or from about 750° C. to about 800° C., though other temperatures are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range. The mixture 226 can be heated under a flow of the carrier gas.
The second tray 203b is positioned between heating elements of the second heating mechanism 211b. The second heating mechanism 211b can be utilized to heat the third precursor comprising the chalcogenide 222 disposed in the second tray 203b. During operation, the chalcogenide 222 can be heated to a second temperature. The second temperature can be any suitable temperature such as from about 100° C. to about 500° C., such as from about 150° C. to about 450° C., such as from about 200° C. to about 400° C., such as from about 250° C. to about 350° C., though other temperatures are contemplated. In at least one aspect, the second temperature can be from about 130° C. to about 250° C., such as from about 175° C. to about 225° C., such as about 200° C., or from about 250° C. to about 450° C., such as from about 350° C. to about 400° C., though other temperatures are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range. The chalcogenide 222 can be heated under a flow of the carrier gas.
The apparatus 200 further includes a controller 230 coupled to the processing chamber 207. The controller 230 can be used to regulate the heating mechanisms 211, the flow rate of carrier gas(es). The controller 230 can be configured to perform various operations of processes described herein.
The controller 230 includes a central processing unit (CPU) 232, a memory 236, and a support circuit 234 utilized to control, for example, the process sequence, regulate the gas flows from the gas tank, regulate the temperature of the heating mechanism (via, for example, a temperature sensor). The CPU 232 may be of any form of a general-purpose computer processor that may be used in an industrial setting. The software routines can be stored in the memory 236, such as random access memory, read only memory, floppy, or hard disk drive, or other form of digital storage. The support circuit 234 is conventionally coupled to the CPU 232 and may include cache, clock circuits, input/output systems, power supplies, and the like. Bi-directional communications between the controller 230 and the various components of the processing chamber 207 can be handled through numerous signal cables (not shown) referred to as signal buses.
During operation, deposition of a two-dimensional (2D) single atomic layer (SL) TMD, a substrate 201 is disposed within a processing volume 205 of the processing chamber 207. The substrate 201 is positioned above the first tray 203a and between heating elements of the first heating mechanism 211a. Examples of substrate the substrate 201 useful according to the present disclosure include, but are not limited to, substrates comprising or consisting of fluorphlogopite mica (F-mica, KMg3AlSi3O10F2, CAS No. 12003-38-2), SiO2, Si, c-sapphire, SrTiO3, hexagonal boron nitride (h-BN), highly oriented pyrolytic graphite (HOPG), or combinations thereof, such as F-mica, c-sapphire, or combinations thereof. It should be understood that while a F-mica substrate is used herein as an exemplary substrate, any suitable substrate can be used in addition to, or instead of, the same.
Returning to
The deposition/growth of operation 120 results in formation of a 2D SL TMD on the substrate 201. The 2D SL TMD can include molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2) molybdenum ditelluride (MoTe2), tungsten disulfide (WS2), tungsten diselenide (WSe2), tungsten ditelluride (WTe2), or combinations thereof.
During processes described herein, the first precursor comprising the alkali metal salt, the second precursor comprising the metal oxide, and the third precursor comprising the chalcogenide are heated such that one or more of the precursors vaporizes and forms a vapor comprising reactive species of the first, second, and/or third precursors. Reactive species include activated species, atomic species, ions, radicals, high energy molecules, or combinations thereof, among other species.
At least a portion of the reactive species meet on a surface of the substrate 201 where a chemical reaction occurs to form 2D SL TMD crystals that grow into a 2D SL TMD film. The crystals can serve as nucleation sites. Once nucleation occurs, additional atoms can incorporate into the nucleated particle. In some examples, the vapor can include an alkali metal salt, a transition metal oxide, a chalcogenide, an activated species thereof, a reactive species thereof, an ion thereof, or combinations thereof.
Although conventional technologies have demonstrated centimeter-scale epitaxial growth of SL MoS2 films on sapphire substrates, the grain size of the film is small (less than 10 μm), increasing the chance to form defects such as grain boundary leading to poorer quality films. In contrast, the crystallinity and single crystal grain sizes of TMDs grown can be controlled by aspects described herein. For example, the TMD crystal grain size can be much larger (in some examples greater than about 200 μm) than that grown by conventional technologies.
Here, the first precursor comprising the alkali metal salt can be heated to form a vapor comprising a reactive species (for example, activated species, atomic species, ions, radicals, high energy molecules, or combinations thereof) of the alkali metal salt. Such reactive species of the alkali metal salt can include an alkali metal cation, a halogen anion, or combinations thereof. In some aspects, the alkali metal salt, the alkali metal cation, halogen anion, or combinations thereof, can be deposited onto the substrate 201.
The second precursor comprising the transition metal oxide can be heated to form a vapor comprising a reactive species (for example, activated species, atomic species, ions, radicals, high energy molecules, or combinations thereof) of the transition metal oxide. The third precursor comprising the chalcogenide can be heated to form a vapor comprising a reactive species (for example, activated species, atomic species, ions, radicals, high energy molecules, or combinations thereof) of the chalcogenide. As described above, at least a portion of the reactive species in the vapor meet on the substrate surface where a chemical reaction occurs to form 2D SL TMD crystals that grow into a 2D SL TMD film.
In some embodiments, an alkali metal cation present in the vapor formed from the alkali metal salt is deposited between a surface of the substrate 201 and the 2D SL TMD. The alkali metal cation can act as a surfactant to mediate the growth of the TMD, making the TMD aligned into a 2D SL TMD.
In some aspects, TMDs comprising sulfur can be deposited/grown using a carrier gas comprising or consisting of Ar. In at least one aspect, TMDs comprising selenium can be deposited/grown using a mixture of carrier gases, such as Ar and H2.
Operation 120 can be performed under conditions effective to form the 2D SL TMD. The conditions of operation 120 can include one or more of the following operations/parameters:
(a) Heating the mixture 226 comprising the first and second precursors (the alkali metal salt and the metal oxide) at a first temperature such as from about 500° C. to about 1,000° C., such as from about 600° C. to about 900° C., such as from about 650° C. to 850° C., such as from about 700° C. and 800° C. or from about 750° C. to about 800° C., though other temperatures are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.
(b) Heating the third precursor comprising the chalcogenide 222 at a second temperature, such as from about 100° C. to about 500° C., such as from about 150° C. to about 450° C., such as from about 200° C. to about 400° C., such as from about 250° C. to about 350° C., though other temperatures are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range. In at least one aspect, and when the chalcogenide comprises sulfur, the second temperature can be from about 130° C. to about 270° C., such as from about 175° C. to about 225° C., such as about 200° C., though other temperatures are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range. In at least one aspect, and when the chalcogenide comprises selenium, the second temperature can be from about 250° C. to about 450° C., such as from about 350° C. to about 400° C., though other temperatures are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.
(c) Flowing one or more carrier gases (for example, H2, He, Ar, Kr, Ne, Xe, N2, or combinations thereof) into the processing chamber 207 before, during, and/or after heating one or more of the precursors. A flow rate of carrier gas can be from about 20 standard cubic centimeter per minute (sccm) to about 200 sccm, such as from about 50 sccm to about 150 sccm, such as from about 75 sccm to about 125 sccm, such as about 80 sccm, though other flow rates are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.
(d) When more than one carrier gas (a mixture of carrier gases) is utilized, a concentration of first carrier gas in the mixture of carrier gases can be from about 2% to about 20%, such as from about 5% to about 15%, such as from about 7% to about 10%, based on a total amount of carrier gas, though other concentrations are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range. In some examples, and when the chalcogenide 222 includes sulfur, the carrier gas comprises Ar. In some examples, and when the chalcogenide 222 includes selenium, the carrier gas comprises a mixture of Ar and H2. In some aspects, the concentration of H2 in the mixture of Ar and H2 can be from about 2% to about 20%, such as from about 5% to about 15%, such as from about 7% to about 10%, though other concentrations are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.
(e) A total growth duration of the 2D SL TMD can be any suitable period, such as from about 1 minute to about 10 minutes, such as from about 1 minute to about 5 minutes, such as from about 2 minutes to about 3 minutes, though other periods are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range. The total growth duration is the growth duration to form the 2D SL TMD film.
(f) A growth duration to form SL TMD crystals can be about 1.5 minutes or less, such as about 1.25 minutes or less, such as about 1 minute or less.
(g) A weight ratio of metal oxide to alkali metal salt can be from about 1:0.01 to about 1:0.1, such as from about 1:0.03 to about 1:0.08, such as from about 1:0.05 to about 1:0.07, though other weight ratios are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.
(h) For growth of a 1 cm×1 cm film, a total weight of the metal oxide and alkali metal salt can be from about 0.5 milligrams (mg) to about 3 mg, such as from about 1 mg to about 2 mg, such as about 1.5 mg, though other amounts are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range. In some aspects, and for growth of a 5 cm×5 cm film, a total weight of the metal oxide and alkali metal salt can be from about 2.5 mg to about 15 mg, such as from about 5 mg to about 10 mg, such as about 7.5 mg, though other amounts are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range. In at least one aspect, and for growth of a 15 cm×15 cm film, a total weight of the metal oxide and alkali metal salt can be from about 7.5 mg to about 45 mg, such as from about 15 mg to about 30 mg, such as about 7.5 mg, though other amounts are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.
(i) For growth of a 1 cm×1 cm film, and when the chalcogenide is sulfur powder, the weight of sulfur powder can be from about 100 mg to about 500 mg, such as from about 300 mg to about 450 mg, such as about 400 mg, though other amounts are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range. In some aspects, and for growth of a 5 cm×5 cm film, the amount of sulfur powder can be from about 500 mg to about 2,500 mg, such as from about 1,500 mg to about 2,250 mg, such as about 2,000 mg, though other amounts are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range. In at least one aspect, and for growth of a 15 cm×15 cm film, an amount of sulfur powder can be from about 1,500 mg to about 7,500 mg, such as from about 4,500 mg to about 6,750 mg, such as about 6,000 mg, though other amounts are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.
(j) For growth of a 1 cm×1 cm film, and when the chalcogenide is selenium powder, the weight of selenium powder can be from about 10 mg to about 200 mg, such as from about 50 mg to about 150 mg, such as about 100 mg, though other amounts are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range. In some aspects, and for growth of a 5 cm×5 cm film, the amount of selenium powder can be from about 50 mg to about 1,000 mg, such as from about 250 mg to about 750 mg, such as about 500 mg, though other amounts are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range. In at least one aspect, and for growth of a 15 cm×15 cm film, the amount of selenium powder can be from about 150 mg to about 3,000 mg, such as from about 750 mg to about 2,250 mg, such as about 1,500 mg, though other amounts are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range
Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.
In some aspects, an average grain size of a SL TMD crystal grown after about 1.5 minutes or less (such as about 1 minute or less) can be about 50 μm or more, such as about 100 μm or more, such as from about 150 μm to about 500 μm, such as from about 200 μm to about 450 μm, such as from about 250 μm to about 400 μm, such as from about 300 μm to about 350 μm, or from about 200 μm to about 250 μm, or from about 200 μm to about 225 μm, though other values are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range. Average grain size is measured by the optical microscope image. The grain size is the lateral size of each triangular crystals, for example, as shown in
If desired, The average grain size can be adjusted by the total weight of the precursors (metal oxide+alkali metal salt), weight of the chalcogens, flow rate, growth temperature, and growth duration.
The SL TMD crystals can be grown in the same crystallographic orientation by using aspects described herein. By growing the crystals such that the crystals are aligned in the same crystallographic orientation, aspects described herein can enable epitaxial growth of 2D SL TMDs. Epitaxial growth refers to the crystals of the TMD film being aligned in the same crystallographic orientation, and those crystals of the TMD film also having a certain alignment relation with the crystallographic orientation of the substrate. This further shows epitaxial growth.
When growth duration is longer than about 1 minute (for example, about 2 minutes or more), the single SL TMD crystals grow larger and merge together to form a continuous 2D SL TMD film.
While not wishing to be bound by any theory, it is believed that the alkali metal ion of the alkali metal salt can act as a surfactant, and the surfactant can mediate the growth of the TMD, making the TMD aligned into a 2D SL TMD characterized as large-single-grain size, uniform, and large-area TMD with high electronic quality (for example, an ON/OFF ratio that is greater than about 106 and a carrier mobility that is greater than about 20 cm2/Vs). Further, and while not wishing to be bound by any theory, control over the alkali metal, the substrate, the metal oxide, and/or the surface energies thereof, can enable growth of 2D SL TMD islands that then grow into the 2D SL TMD.
The 2D SL TMD film grown according to aspects described herein can have one or more of the following properties:
(a) A thickness of the 2D SL TMD film can be about 1.2 nm or less, such as about 1.1 nm or less, such as about 1 nm or less, such as about 0.9 nm or less, though other thicknesses are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range. The thickness is determined by atomic force microscopy as described in the Examples.
(b) A width (in one dimension) of the 2D SL TMD film can be about 500 μm or more, 15 centimeters (cm) or less, or combinations thereof, such as from about 500 μm to about 15 cm, such as from about 750 μm to about 12 cm, such as from about 1 cm to about 10 cm, such as from about 2 cm to about 4 cm, though other widths are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.
(c) A length (in one dimension) of the 2D SL TMD film can be about 500 μm or more, 15 centimeters (cm) or less, or combinations thereof, such as from about 500 μm to about 15 cm, such as from about 750 μm to about 12 cm, such as from about 1 cm to about 10 cm, such as from about 2 cm to about 4 cm, though other widths are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range. The width and the length of the 2D SL TMD make up an area of the 2D SL TMD.
(d) An average grain size of a SL TMD film grown after about 1.5 minutes or less (such as about 1 minute or less) can be about 50 μm or more, such as about 100 μm or more, such as from about 150 μm to about 500 μm, such as from about 200 μm to about 450 μm, such as from about 250 μm to about 400 μm, such as from about 300 μm to about 350 μm, or from about 200 μm to about 250 μm, or from about 200 μm to about 225 μm, though other values are contemplated. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.
In some aspects, the continuous TMD film comprises, consists essentially of, or consists of crystals aligned in the same crystallographic orientation. In at least one aspect, the continuous TMD film consists of crystals aligned in the same crystallographic orientation (all crystals have matched crystallographic orientation). The crystallographic orientation of the crystals in TMD film have a certain alignment relation with the crystallographic orientation of the substrate.
The inventors found that aspects of the present disclosure can enable a large average grain size of the film. Such a single crystal domain size produced by embodiments described herein is distinctive from conventional technologies and provides a variety of benefits. At the same time, the TMD film is a single atomic layer and has a large area. In contrast, and as described herein, conventional technologies have very small single grain sizes (generally less than 10 μm).
2D SL TMDs formed by aspects described herein show better uniformity and improved electronic quality relative to those formed by conventional technologies. As a result, for example, 2D SL TMDs formed by aspects described herein can be utilized in a variety of applications, for example, energy storage, electronics, optoelectronics, sensing, catalysis, solar cells, light emitting diodes, photovoltaics, and photodetectors, among others. The 2D SL TMDs can be incorporated into devices such as transistors, for example, back-gate field effect transistors (BG-FETs).
In some examples, the electrical characteristics of BG-FETs with 2D SL TMD channels formed by processes described herein are significantly improved over those TMDs made by traditional methods. In some examples, an ON/OFF current ratio of a 2D SL TMD described herein can be about 108 or more, or about 109 or more, which is a much larger ON/OFF current ratio than those TMDs formed by conventional methods. In some examples, 2D SL TMDs formed by processes described herein can have a carrier mobility that is about 20 cm2/V·s or more, representing a carrier mobility value that is much larger those TMDs formed by conventional methods. The carrier mobility value can be influenced by the channel length due to, for example, contact resistance. Such on/off current ratios and carrier mobilities for back-gate TMD FET devices demonstrate that processes described herein can form 2D SL TMDs of high electronic quality.
Unlike conventional methods, aspects of the present disclosure can enable growth of a continuous 2D SL TMD film, the continuous 2D SL TMD film comprising crystals having a large average crystal grain size (for example, from about 50 μm to about 500 μm), and the crystals of the continuous 2D SL TMD film are aligned in the same crystallographic orientation.
The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to make and use aspects of the present disclosure, and are not intended to limit the scope of aspects of the present disclosure. Efforts have been made to ensure accuracy with respect to numbers used (e.g. amounts, dimensions, etc.) but some experimental errors and deviations should be accounted for.
Optical microscope images were collected using a Renishaw in Via Raman microscope. Atomic force microscopy (AFM) images and the corresponding height profiles were collected using a Bruker Dimension Icon AFM.
Reflection high energy electron diffraction (RHEED) pattern were collecting using a home-built RHEED system attached to a molecular beam epitaxy (MBE) chamber. Photoluminescence (PL) spectra were acquired using a Renishaw in Via Raman microscope, with 1800 grooves/mm grating and a 532 nm laser, 1 mW laser power, as excitation source. Optical images were collected using a digital camera.
Growth of a continuous 2D SL TMD was performed according to the following non-limiting procedure. The 2D SL TMD was synthesized through a CVD method conducted in a tube furnace system equipped with a 1″ quartz tube.
For a typical run for the growth of epitaxial SL MoS2 crystals or continuous film, a mixture of MoO2+RbBr powder (total of about ˜1.5 mg) at a weight ratio of MoO2:RbBr of about 1:0.08 was loaded in an alumina boat (Coors high alumina combustion boat, Sigma-Aldrich), and a freshly-cleaved F-mica substrate (1×1 cm2) was placed in the same alumina boat, about 3 mm above the MoO2+RbBr mixture powder. This alumina boat is then placed at the center of the quartz tube. Sulfur powder (about 100 mg) was loaded in another alumina boat, and was placed upstream of the alumina boat with the mixture of MoO2+RbBr powder. The alumina boat holding the sulfur powder was placed about 16 inches away from the center of the quartz tube. The quartz tube was then sealed on both sides, followed by flushing with about 500 sccm Ar for about 1 hour. After flushing, the Ar flow was adjusted to about 80 sccm, and the furnace is heated to about 770° C., which is the temperature at the center of the quartz tube, and meanwhile the S powder was heated to about 200° C. with an additional heating belt. The growth time at 770° C. was about 1-2 minutes, after which the temperature was cooled down rapidly with a fan. This procedure provided the continuous 2D SL TMD MoS2 film.
The process was also repeated using various materials to provide continuous films and crystals of 2D SL MoSe2, continuous films and crystals of 2D SL WS2, and continuous films and crystals of 2D SL WSe2. Properties of the continuous films and crystals of 2D SL TMDs were determined. Characterization of the MoS2 below. The other TMDs provided similar results.
Referring back to
The left panel of
Crystal surfaces at atomic levels were investigated by reflection high energy electron diffraction (RHEED) analyses. Exemplary, but non-limiting, results are shown in
The top panel of
Optical properties of the 2D SL MoS2 film were also determined. Exemplary, but non-limiting, results are shown in
A back-gate field effect transistor (BG-FET) using a Ni/Au as source and drain electrodes were fabricated to investigate the electrical performance of SL MoS2 films. The BG-FET device was fabricated according to the following procedure. After transferring the SL MoS2 film to a 285 nm SiO2/Si (highly doped Si with an oxide layer of 285 nm) substrate, the SL MoS2 film was patterned into arrays of 10-μm-wide stripes using e-beam lithography (EBL) and reactive-ion etching (RIE, CF4/O2, 50 W RF source/OW RF bias, 20 s), followed by patterning and deposition of metal electrodes of Ni (20 nm)+Au (50 nm) using EBL, e-beam evaporation and lift-off. EBL patterning was performed by Nanobeam n64 Electron Beam Writer system, the electron beam was operated at an accelerating voltage of 80 kV.
Selected properties of the fabricated devices were measured with a Keysight B1500A parameter analyzer in a Lakeshore probe station with a vacuum level of 5×10−5 Torr at room temperature.
Exemplary, but non-limiting, data for the electrical performance of devices are shown in
In Equation (1), L/W is the ratio between channel length (Lch) and width (width is 10 μm), Vds is the source-drain voltage, Ids is the source-drain current, and Vbg is the back-gate voltage. In this model, the capacitance between channel and the back gate per unit area, Cox, is expressed as:
in which Σ0˜8.854×10−14 F/cm is the permittivity of vacuum, Σ0x˜3.9 is the relative permittivity of SiO2, and t0x=285 nm is the thickness of SiO2.
Under the conditions investigated, carrier mobility values were determined to be from about 15 cm2/V·s to about 30 cm2/V·s, with most appearing at about 20 cm2/V·s. For a short channel (for example, Lch is about 0.5 μm), it can be reasonable to have a little lower value as the mobility can be more influenced by the contact resistance. The carrier mobility values are also among the best in back-gate MoS2 FET devices on SiO2/Si reported. Overall, the electrical performance data demonstrates the high and uniform electronic quality of the epitaxial 2D SL MoS2 film made according to aspects of the present disclosure.
Aspects described herein generally relate to processes for forming a two-dimensional (2D) single atomic layer (SL) transition metal dichalcogenide (TMD) and to 2D SL TMDs formed by the process. The processes described herein can enable growth of a large-single-grain size, epitaxial-quality, large-area TMD films. Overall, processes of the present disclosure can enable control over the nucleation and/or growth of crystals and films thereof
The present disclosure provides, among others, the following aspects, each of which can be considered as optionally including any alternate aspects:
Clause A1. A process for forming a continuous transition metal dichalcogenide film, the process comprising:
Clause A2. The process of Clause A1, wherein the crystallographic orientation of the crystals of the continuous transition metal dichalcogenide film have a certain alignment relation with a crystallographic orientation of the substrate.
Clause A3. The process of Clause A1 or Clause A2, wherein:
Clause A4. The process of any one of Clauses A1-A3, wherein:
Clause A5. The process of any one of Clauses A1-A4, wherein the alkali metal salt comprises an alkali metal and a halogen, the alkali metal comprising Na, K, Rb, Cs, or combinations thereof, the halogen comprising Cl, Br, I, or combinations thereof.
Clause A6. The process of any one of Clauses A1-A5, wherein the chalcogenide comprises sulfur, selenium, or combinations thereof.
Clause A7. The process of any one of Clauses A1-A6, wherein the substrate comprises fluorphlogopite mica, SiO2, Si, c-sapphire, SrTiO3, hexagonal boron nitride (h-BN), or combinations thereof.
Clause A8. The process of any one of Clauses A1-A7, wherein the substrate comprises fluorphlogopite mica.
Clause A9. The process of any one of Clauses A1-A8, wherein the carrier gas comprises H2, Ar, He, Ne, Kr, Xe, N2, or combinations thereof.
Clause A10. The process of any one of Clauses A1-A9, wherein, when the chalcogenide comprises sulfur, the carrier gas comprises Ar.
Clause A11. The process of any one of Clauses A1-A10, wherein, when the chalcogenide comprises selenium:
Clause A12. The process of any one of Clauses A1-A11, wherein the heating the alkali metal salt, the transition metal oxide, and the chalcogenide to form the reactive species comprises:
Clause A13. The process of any one of Clauses A1-A12, wherein, when the chalcogenide comprises sulfur, the chalcogenide is heated at a temperature that is from about 130° C. to about 250° C.
Clause A14. The process of any one of Clauses A1-A13, wherein, when the chalcogenide comprises selenium, the chalcogenide is heated at a temperature that is from about 250° C. to about 450° C.
Clause B1. A process for forming a continuous single atomic layer transition metal dichalcogenide film, the process comprising:
Clause B2. The process of Clause B1, wherein the crystallographic orientation of the crystals of the continuous single atomic layer transition metal dichalcogenide film have a certain alignment relation with a crystallographic orientation of the substrate.
Clause B3. The process of Clause B1 or Clause B2, wherein:
Clause B4. The process of any one of Clauses B1-B3, wherein:
Clause B5. The process of any one of Clauses B1-B4, wherein the single atomic layer transition metal dichalcogenide film comprises MoS2, MoSe2, WS2, WSe2, or combinations thereof.
Clause B6. The process of any one of Clauses B1-B5, further comprising flowing a carrier gas comprising Ar, H2, or combinations thereof during depositing of the single atomic layer transition metal dichalcogenide film.
Clause C1. A controller configured to perform operations comprising:
Clause C2. The controller of Clause C1, wherein:
Clause C3. The process of Clause C1 or Clause C2, wherein the crystallographic orientation of the crystals of the transition metal dichalcogenide film have a certain alignment relation with the crystallographic orientation of the substrate.
Clause D1. A controller configured to perform of operations of a method described herein.
Clause E1. A continuous transition metal dichalcogenide film, comprising one or more of the following characteristics:
Clause E2. The continuous transition metal dichalcogenide film of Clause E1, wherein the continuous transition metal dichalcogenide film, comprises all the characteristics of Clause E1.
Clause E3. The continuous transition metal dichalcogenide film of Clause E1 or E2, wherein the continuous transition metal dichalcogenide film is formed according to processes described herein.
Clause E4. The continuous transition metal dichalcogenide film of any one of Clauses E1-E3, wherein the continuous transition metal dichalcogenide film is formed by the process of any one of Clauses A1-A14; the process of any one of Clauses B1-B6; the controller of any one of Clauses C1-C3; or the controller of any one of Clauses D1-D2.
Clause E5. The process of Clauses E1-E5, wherein the crystallographic orientation of the crystals of the transition metal dichalcogenide film have a certain alignment relation with the crystallographic orientation of the substrate.
As is apparent from the foregoing general description and the specific aspects, while forms of the aspects have been illustrated and described, various modifications can be made without departing from the spirit and scope of the present disclosure. Accordingly, it is not intended that the present disclosure be limited thereby. Likewise, the term “comprising” is considered synonymous with the term “including.” Likewise whenever a composition, an element or a group of elements is preceded with the transitional phrase “comprising,” it is understood that we also contemplate the same composition or group of elements with transitional phrases “consisting essentially of,” “consisting of,” “selected from the group of consisting of,” or “is” preceding the recitation of the composition, element, or elements and vice versa, e.g., the terms “comprising,” “consisting essentially of,” “consisting of” also include the product of the combinations of elements listed after the term.
For purposes of this present disclosure, and unless otherwise specified, all numerical values within the detailed description and the claims herein are modified by “about” or “approximately” the indicated value, and consider experimental error and variations that would be expected by a person having ordinary skill in the art. For the sake of brevity, only certain ranges are explicitly disclosed herein. However, ranges from any lower limit may be combined with any upper limit to recite a range not explicitly recited, as well as, ranges from any lower limit may be combined with any other lower limit to recite a range not explicitly recited, in the same way, ranges from any upper limit may be combined with any other upper limit to recite a range not explicitly recited. Additionally, within a range includes every point or individual value between its end points even though not explicitly recited. Thus, every point or individual value may serve as its own lower or upper limit combined with any other point or individual value or any other lower or upper limit, to recite a range not explicitly recited.
As used herein, the indefinite article “a” or “an” shall mean “at least one” unless specified to the contrary or the context clearly indicates otherwise. For example, aspects comprising “a precursor” include aspects comprising one, two, or more precursors, unless specified to the contrary or the context clearly indicates only one precursor is included.
While the foregoing is directed to aspects of the present disclosure, other and further aspects of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
