LARGE-SCALE SYNTHESIS OF 2D SEMICONDUCTORS BY EPITAXIAL PHASE CONVERSION

There is a method for forming an oxide or chalcogenide 2D semiconductor. The method includes a step of growing on a substrate, by a deposition method, a precursor epitaxy oxide or chalcogenide film; and a step of sulfurizing the precursor epitaxy oxide or chalcogenide film, by replacing the oxygen atoms with sulfur atoms, to obtain the oxide or chalcogenide 2D semiconductor. The oxide or chalcogenide 2D semiconductor has an epitaxy structure inherent from the precursor epitaxy oxide or chalcogenide film.

BACKGROUND

Technical Field

Embodiments of the subject matter disclosed herein generally relate to fabricating large-scale oxide or chalcogenides films, and more specifically, to large-scale MoS2films that are grown through a novel epitaxial phase conversion process, involving the preparation of high-quality epitaxial MoO2films, which are then converted, by sulfurization, to epitaxial MoS2films.

Discussion of the Background

Recently, the generation of two-dimensional (2D) molybdenum disulfide (MoS2) has attracted significant attention because of its unique electrical properties that can be tuned by controlling a thickness of the film. Many reports exist on 2-dimensional (2D) MoS2for various applications such as thin film transistors, soft electronics, valley electronics, photovoltaics, photodetectors, van der Waals heterostructures, and chemical or bio-sensors.

In the meantime, a variety of preparation methods have been developed for growing MoS2flakes and films with different thicknesses in order to meet the demands of these applications. For example, MoS2flakes prepared with mechanical and chemical exfoliation methods show excellent device properties because they contain fewer defects and grain boundaries, but are limited in scalability. Chemical vapor deposition (CVD) method is a viable way to grow both flakes and large-area films, but the variation of the number of 2D layers in the large-area CVD films poses a challenge to their practical implementation. Fortunately, metalorganic CVD (MOCVD) and CVD with seed layer coating on substrates were developed to prepare uniform MoS2films on wafer scale with excellent properties. However, this wafer-scale uniformity is only feasible for preparing monolayer MoS2films.

Currently, there is no viable method for depositing high-quality few-layer MoS2films (i.e., between 5 and 10 layers) on a large scale. Such films are reported to be advantageous in some aspects, including the possibility of higher mobility (see, for example, Zheng, J. et al. High-Mobility Multilayered MoS2Flakes with Low Contact Resistance Grown by Chemical Vapor Deposition,Adv. Mater.29, 1604540 (2017)), feasibility of p-type doping through plasma (see, Nipane, A., Karmakar, D., Kaushik, N., Karande, S. & Lodha, S., Few-Layer MoS2p-Type Devices Enabled by Selective Doping Using Low Energy Phosphorus Implantation,ACS Nano10, 2128-2137 (2016)), and the ability to form Schottky diodes with a barrier height that can be adjusted by the number of MoS2layers (Kwon, J. et al., Thickness-dependent Schottky barrier height of MoS2field-effect transistors,Nanoscale9, 6151-6157 (2017)).

Thus, there is a need to develop high-quality few-layer MoS2thin films over a large area. Some efforts have already been exerted to grow large-area, few-layer MoS2films by various processes. These processes include direct pulsed laser deposition (PLD) (see, Serna, M. I. et al., Large-Area Deposition of MoS2by Pulsed Laser Deposition with In Situ Thickness Control,ACS Nano10, 6054-6061 (2016)), pulsed metalorganic CVD (PMOCVD), or two-step processes, in which sulfurization of various precursors (e.g., MoO3(see, Lin, Y. C. et al., Wafer-scale MoS2thin layers prepared by MoO3sulfurization,Nanoscale4, 6637-6641 (2012), Mo, (NH4)2MoS4, and polymer-precursor complex thin films) is carried out. However, literature reports on few-layer MoS2films made with these methods show much poorer quality compared with flakes grown by CVD or exfoliation methods.

Therefore, there is a need for a new method for growing few-layer MoS2films that are not being affected by the above discussed shortcomings.

SUMMARY

According to an embodiment, there is a method for forming an oxide or chalcogenide 2D semiconductor. The method includes a step of growing on a substrate, by a deposition method, a precursor epitaxy oxide or chalcogenide film, and a step of sulfurizing the precursor epitaxy oxide or chalcogenide film, by replacing the oxygen atoms with sulfur atoms, to obtain the oxide or chalcogenide 2D semiconductor. The oxide or chalcogenide 2D semiconductor has an epitaxy structure inherent from the precursor epitaxy oxide or chalcogenide film.

According to another embodiment, there is a MoS2electrode that includes a substrate and a single crystal MoS2film formed directly on the substrate. The single crystal MoS2film is formed by pulsed laser deposition (PLD), from a precursor single crystal MoO2film, and the precursor single crystal MoO2film is sulfurized to replace the oxygen atoms with sulfur atoms to obtain the MoS2film.

According to still another embodiment, there is a thin film transistor that includes a substrate, a single crystal MoS2film formed on the substrate, a drain electrode and a source electrode formed on the substrate and sandwiching the single crystal MoS2electrode, a dielectric layer formed over the drain electrode, the source electrode, and the single crystal MoS2electrode, and a gate electrode formed over the dielectric layer. The single crystal MoS2electrode is formed from a precursor single crystal MoO2film by pulsed laser deposition (PLD), and the precursor single crystal MoO2film is sulfurized to replace the oxygen atoms with sulfur atoms.

DETAILED DESCRIPTION

The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to a MoS22D film that may be used in a TFT transistor. However, the embodiments discussed herein are not limited to MoS2materials (as discussed later) or TFT transistors, as oxide or chalcogenides 2D films may be used for other purposes or in other electronic or photonic devices and in other devices such as a lithium-ion battery.

According to an embodiment, there is method for epitaxial two-dimensional (2D) molybdenum disulfide (MoS2) grown over a large area using a novel epitaxial phase conversion process. In this method, an epitaxial film of metallic MoO2is firstly grown by pulsed laser deposition and subsequently converted to a few-layer continuous 2D MoS2film. The term “epitaxial” is understood in the following to mean “a single crystal.” The term “film” is understood to mean a “sheet” having a certain thickness. The term “a single crystal film” is understood to mean a sheet having a certain thickness and having a structure that includes a single crystal. The term “layer” is understood to be related to the MoS2material, in the sense that a film of MoS2has a layered structure. This layered structure is defined as a plane of molybdenum atoms being sandwiched by planes of sulfide ions. These three strata form a monolayer of MoS2. Thus, a few-layer MoS2film means a sheet of a given thickness, having a structure that includes a single crystal, and the atoms forming the film are distributed in substantially parallel planes, where three planes of molybdenum atoms and sulfide ions form a layer. The same method may be applied to other oxides and chalcogenides, e.g., WO3, Ga2O3, TiO2, etc. to form sulfides, selenides, and even tellurides of the same transition metal. However, for simplicity, the following embodiments are discussed with regard to MoS2films.

In all previous methods, poor electrical switching properties were reported for thin film transistors (TFTs) using MoS2semiconductor converted from Mo-based precursors over a large area (which is called as two-step process). However, with this novel method, epitaxial MoS2films can be prepared using epitaxial MoO2precursor films, followed by sulfurization to replace oxygen with sulfur. The resulting MoS2-based TFTs prepared using this process achieve a field effect mobility as high as 10 cm2V−1s−1, which is up to six times higher than the best reported few-layer MoS2devices prepared with a two-step process. In addition, lower operation voltage (−8 to 15 V) and on current to off current ratio (Ion/Ioff) around 105are achieved. To the best of the inventor's knowledge, this process is the first MoS2growth that focuses on the quality of the precursor film MoO2(i.e., obtaining epitaxial precursors), and offers one or more of the following advantages: (1) scalable few-layer 2D film fabrication, (2) feasibility of thickness control over large area, and/or (3) possibility of epitaxial 2D MoS2growth.

This method is now discussed in more detail with regard toFIG. 1. In step100, one or more precursor single crystal (or epitaxial) MoS2films are formed. Note that by this process, the epitaxial MoO2with different thicknesses formed to be a single crystal instead of a poly-crystal as in the traditional methods. In this regard, the traditional processes use Mo-containing precursor films that are amorphous, which degrade the structure and performance of the resulting MoS2film. The precursor films in this embodiment are epitaxial (i.e., single crystal) over a large area. Further, the method used in this embodiment provides simultaneous (1) thickness control (one atomic layer at a time) and (2) growth over the large area. No other traditional process can generate MoS2films that are grain-boundary free, are grown over a large area, and show atomic thickness control (atomically flat surfaces).

In step102, the precursor epitaxial MoO2films are sulfurized through a phase conversion process. For this process, the epitaxial MoO2film(s) is placed inside a tube furnace and sulfur powder may be used as the sulfurization source. The converted MoS2film was verified to also be epitaxial.

A system for performing step100is now discussed with regard toFIG. 2. In one application, a pulse laser deposition (PLD) technique is used to form the precursor MoS2film. Although the PLD method is discussed herein, one skilled in the art would understand that other methods may be used, for example, molecular beam epitaxy (MBE) or metalorganic vapor phase epitaxy (MOCVD). As illustrated inFIG. 2, a PLD system200includes a PLD chamber202that holds a target support204. Target support204may be a plate that is attached with an axle206to a motor208. The motor208may be formed outside the PLD chamber202. Target support204holds the target material210, which in this case is MoO3. The motor208may rotate the target support204so that the target material210is rotated inside the PLD chamber202.

A pulsed laser device212, for example, a KrF2laser, may be used to generate a beam214that is directed through a port216inside the PLD chamber202. The beam214interacts with the target material210and atoms or molecules of the precursor MoO3material are ablated. The ablated atoms and/or molecules218travel to a substrate223, to form the MoO2layers220. The substrate223may be, for example, a (0001) sapphire substrate. The substrate223is attached to a holder with an axle222to a motor224. While motor224may be located outside the PLD chamber202, the substrate223and the MoO2films220of are located inside the PLD chamber and both are located above the target material210, so that the ablated atoms and/or molecules218travel vertically upwards toward the substrate223. Thus, the ablated atoms and/or molecules218deposit, layer-by-layer as the MoO2film220onto the substrate223. Substrate223may have a heater203for heating the substrate to a desired temperature (e.g., 400° C.). PLD chamber202may also have a port230through which oxygen or other gas may be inserted into the PLD chamber202. In one application, the pulsed laser energy of the laser device212is about 210 mJ, the substrate223's temperature is about 400° C., and the pressure of the oxygen gas is about 10 mtorr. It was observed that by using these parameters, it was possible to epitaxially grow the MoO2film, with not only a single crystalline structure and different thickness, but also with atomically smooth surface.

A specific implementation of step100is now discussed with regard toFIG. 3. In step300, a (001) Al2O3substrate223with, for example, an area of 10×10 mm and a thickness of 0.5 mm, was provided for the PLD system200. Before applying the PLD process to this substrate, it was cleaned sequentially with acetone, IPA, and DI water, for 5 min in each solvent, combined with sonication. Then the substrate223was attached to the corresponding axle222(seeFIG. 2).

In step302, the target material210is placed inside the PLD's PLD chamber202, on the target support204, and the vacuum inside the PLD chamber is pumped down below 10−7Torr. In this implementation, the target material210was MoO3having a purity of 99.9%. Other concentrations may be used. In step304, the laser device212was activated to achieve the deposition of the MoO2layers220on the substrate223. O2was used as the deposition atmosphere. During this step, the PLD chamber's pressure was kept at about 10 mTorr. The temperature of the substrate223during the deposition step was maintained at about 400° C. with the heater203. The KrF2laser source212, having a 248 nm wavelength, was used and set to a constant energy mode of about 210 mJ. The deposition rate of the MoO2film during step304was measured to be about 0.314 Å per shot. The substrate223was rotated during the deposition step with an angular speed between 20 and 40° per second, and more specifically, 36° per second. After deposition, in step306, the MoO2layers220were cooled down to room temperature, naturally, before being taken out of the PLD chamber202.

Next, a specific implementation of the sulfurization step102is discussed with regard toFIGS. 4 and 5. The precursor epitaxial MoO2films220were placed in step400in a cleaned quartz boat503, which was loaded into the middle zone of a three-zone tube furnace500, as shown inFIG. 5. The tube furnace500may have a body502that has an internal chamber. The internal chamber is split into three zones. The quartz boat503is placed inside the middle zone of the internal chamber.FIG. 5shows the quartz boat503holding the substrate223and the MoO2films220fromFIG. 2. Note that there is no sulfur in the substrate223or the films220when the quartz boat503is placed inside the tube furnace500. Also note that a heater504is provided around the middle zone B of the tube furnace and the middle zone B is sandwiched between the first zone A and the last zone C.

In step402, a sulfur powder530(e.g., from Fisher scientific, about 700 mg) is placed upstream of the quartz boat503, at a given distance d, in the first zone A. In one embodiment, the distance d is about 27 cm. Argon is provided in step404as the carrier gas during sulfurization. In step406, the heater504is activated for heating the middle zone B of the tube furnace500. During this high-temperature process, the sulfur powder530evaporates and its vapor is transported by the carrier gas and incorporated into the epitaxial MoO2film220. In step408, the epitaxial MoO2film220is converted to an epitaxial MoS2film, by replacing the oxygen atoms with the sulfur atoms.

Before the sulfurization process, the Ar gas was flown through the quartz tube502with 100 sccm for at least 40 min, to completely remove the oxygen from the tube. The temperature rate increase in the middle zone B of the furnace was set at 20° C./min, starting from the room temperature, up to a desired maximum value (e.g., 700, 800, or 900° C.).

After reaching to the target value, the temperature inside the middle zone B of the furnace was held for 1 hr to complete the conversion of MoO2to MoS2. Note that in one application, it is possible to convert each 0 atom to S. However, as would be understood by one skilled in the art, the conversion process does not have to convert each 0 atom. At the end of this step, the converted MoS2films520are cooled down to room temperature. The Ar flow was kept at 100 sccm throughout the process.

Some characteristics of the epitaxial growth of the MoO2 precursor films and the van der Waals MoS2converted films are now discussed. Molybdenum dioxide520(MoO2, space group: P21/c(14)) was grown on the (001) surface of a single crystal Al2O3substrate223(space group: R-3ch(167)). The MoO2film220was selected as the precursor film based on calculations showing that lattice mismatches below 2% can be achieved for the substrate and the MoO2film. The lattice parameters along the Al2O3[120] and [100] directions are 0.5720 and 0.4762 nm, respectively, and both are in the (001) Al2O3plane. In comparison, the lattice parameters along the MoO2[001] and [010] directions are 0.5628 and 0.4856 nm, respectively, both of which are in the MoO2(200) plane. Based on these values, the inventors have calculated the lattice mismatch to be −1.6% (tensile strain) for [001]MoO2/[120]Al2O3, and 2.0% (compressive strain) for [010] MoO2/[100] Al2O3. Because these strain values are negligible, it was decided to grow epitaxial (200) MoO2films on (001) Al2O3. In this respect,FIG. 6shows the θ-2θ X-ray diffraction (XRD) pattern of epitaxial MoO2films deposited on (001) Al2O3substrate using different numbers of PLD laser shots (100, 500, 1500). No diffraction peaks can be observed for films deposited with only 100 PLD shots, indicating that films deposited under these conditions are too thin. However, films deposited using 500 and 1500 shots clearly show only (200) MoO2(2θ=37.95°) and (400) (2θ=80.97°) diffraction peaks, indicating that these films are epitaxial. A phi (Φ) scan was performed to further investigate the epitaxial structure of the (200) MoO2films grown on (001) Al2O3substrate, and the results confirm the epitaxial nature of the MoO2films. The Φ-scan shows the 60° inter-spaced peaks of (011) MoO2planes, indicating their six-fold symmetry, and the 120° inter-spaced peaks of (104) Al2O3planes with three-fold symmetry. It is noted that the (011) MoO2peaks are offset by 30° relative to the (104) Al2O3peaks, confirming the epitaxial nature of the MoO2film growth.

The θ-2θ XRD scan of both precursor MoO2film (1500 PLD shots) and final MoS2film (after sulfurization) on (001) Al2O3substrate show two peaks. The peak at 16.65° can be assigned to (002) planes of 2H MoS2, and the second peak at 41.80° can be assigned to (001) planes of the Al2O3substrate. However, after sulfurization, the peak corresponding to (200) MoO2disappeared. This result indicates that the MoO2film has been completely converted to epitaxial MoS2. A Φ scan was also performed to verify the epitaxial nature of the final MoS2film. It was observed that the (107) MoS2planes exhibit a six-fold symmetry, which is offset by 30° by the (104) Al2O3peaks.

Raman spectroscopy was performed and successfully confirmed the formation of both the MoO2and MoS2structures. Actually, no MoO2Raman peaks were found in the final MoS2Raman spectra, further verifying the complete sulfurization of the precursor MoO2film. However, as previously discussed, the complete sulfurization of the precursor MoO2film is not a required condition.

In one embodiment, to achieve high-quality MoS2films, the sulfurization temperature was selected using the 100 PLD shots for the MoO2film. Specifically, three different sulfurization temperatures (700, 800, 900° C.) were evaluated during the process of forming the epitaxial MoS2film. Raman, Photoluminescence (PL) and X-ray photoelectron (XPS) spectroscopy were performed on the MoS2films to evaluate their qualities. The Raman spectra inFIG. 7show that two peak positions are always located at 384.6 and 409.6 cm−1, regardless of the sulfurization temperature, and these peaks correspond to E12gand A2gmodes of the MoS2films, respectively. However, a higher conversion temperature results in a higher intensity of both peaks.

These Raman spectra were further analyzed by Lorentz fitting. It was concluded that the full width at half maximum (FWHM) of these peaks becomes smaller as the sulfurization temperature increased, indicating that a better optical quality MoS2film is obtained at a higher temperature. In fact, when a sulfurization temperature of 900° C. was used, the FWHM of the E12gpeak was 3.74, which is close to the reported value (FWHM=3.5) for few-layer single crystalline MoS2flakes prepared by CVD. Higher PL peak intensity can also be observed in MoS2films obtained at a higher sulfurization temperature, further confirming the higher quality of MoS2at higher sulfurization temperature. XPS peaks at 232.9, 229.8 and 227, which correspond to Mo 3d3/2, Mo 3d5/2and S 2s, respectively, were observed. S 2p1/2and S 2p3/2peaks were observed at 163.8 and 162.6.

These peak positions are consistent with those reported for crystalline MoS2films. It is interesting to note that the different sulfurization temperatures that were used for obtaining the MoS2films do not cause any peak shift in the XPS spectrum, which indicates that the MoS2crystals can be obtained at all three temperatures. Further analysis was performed on XPS spectra by Lorentzian-Gaussian fitting to acquire the Mo/S ratio. It was determined that the Mo/S ratios of 1/1.88, 1/1.90 and 1/1.94 were obtained, corresponding to the sulfurization temperatures of 700, 800 and 900° C., respectively.

These results demonstrate that the most stoichiometric MoS2could be obtained using the 900° C. sulfurization process. The lowest MoS2surface roughness was also obtained at 900° C., as can be concluded from a root mean square (RMS) roughness analysis. The above analysis showed that a higher sulfurization temperature improves the 2D MoS2film's quality. However, it was also observed that there is a limit regarding the increase in temperature. By increasing the sulfurization temperature to more than 1000° C., resulted in the evaporation of the precursor MoO2film, which is undesirable. Thus, a sulfurization temperature in the range of 850 to 950° C. (referred herein to “about 900”) is believed to be preferable.

In order to investigate the sulfurization process in more detail, the surface morphology of the precursor MoO2films deposited using different PLD shots (40, 60, 80, 100, 120, 140, 160, 200, 300 shots) and the corresponding final MoS2films were studied by atomic force microscope (AFM). The typical AFM surface morphologies of MoO2films, having an RMS roughness of 0.147, 0.173 and 0.270 nm corresponding to films with 40, 100, and 300 shots, respectively, was studied. It was observed that an excellent surface smoothness of the precursor films was achieved (RMS <0.27 nm), although the RMS roughness increases slightly with the number of PLD shots.

For films having 40, 100, and 300 shots, the typical AFM surface morphologies of final MoS2films was observed. An RMS roughness of 2.554, 0.178 and 0.542 nm was observed for these MoS2films, which were converted from the MoO2films. Interestingly, the RMS roughness of the final 2D MoS2films changed significantly with the thickness (number of PLD laser shots) of MoO2films.

Precursor MoO2films deposited using 40 PLD shots resulted in isolated islands of MoS2. As the number of PLD shots of the MoO2films increased, the islands began to coalesce, but continuous MoS2films were not formed until the number of MoO2PLD shots reached 100. The RMS roughness of the 2D MoS2films decreased before the formation of the continuous films, and then increased again after the formation of continuous 2D MoS2films. Essentially, the converted MoS2films showed higher roughness than the corresponding MoO2precursor. The only exception happened when the number of MoO2laser shots was 100 (3.15 nm thick MoO2film), where the continuous MoS2film has just formed. Thicknesses of the MoS2films obtained for different MoO2precursor thicknesses (PLD shots) was studied by AFM. Before the formation of the continuous MoS2film, the thickness changed nonlinearly with the number of MoO2laser shots, but once a continuous film has formed, it began to increase linearly with the number of laser shots, as shown by curve800inFIG. 8. However, the precursor MoO2film thickness always increased linearly with the number of laser shots, as shown by curve802inFIG. 8. Note that the thickness of the 2D MoS2film, converted from the MoO2film with 100 MoO2PLD shots, is about 3 nm, which corresponds to about 4-5 layers.

The sulfurization process of MoO2precursor film was discussed above with regard toFIGS. 4 and 5. MoO2epitaxial films with 100 shots were loaded into the tube furnace500, and annealed in a mixture of Ar and S at 900° C. for 1 hr at atmospheric pressure, resulting in 2D epitaxial MoS2films520(seeFIG. 5). A low-resolution (LR) TEM image of a 3.15 nm MoO2precursor film (100 PLD shots) was obtained and the film shows an atomically flat surface, consistent with the AFM data previously discussed. A high-resolution (HR) TEM image of the same epitaxial MoO2film was generated and it was observed that the (200) plane of MoO2is parallel to the (001) surface of the Al2O3substrate. This means that excellent epitaxial growth of MoO2has been achieved, consistent with the conclusion from the XRD analysis previously discussed.

In comparison, the LR TEM image of the final MoS2film (obtained after 900° C. sulfurization) shown the thickness of the film to be 2.94 nm. The HR TEM image further shows the layer structure of the final film, with all layers perfectly parallel to the substrate surface, further confirming the van der Waals epitaxy. The Electron Energy Loss Spectroscopy (EELS) elemental mapping (S and O) of the MoO2and MoS2films were plotted together with the TEM images and by comparing the oxygen and sulfur elemental distribution, the complete sulfurization of the MoO2film was observed.

The uniformity of the epitaxial 2D MoS2films was also investigated. Optical images of the optimized MoS2film were obtained. To verify the uniformity of the MoS2films, five areas are chosen on the substrate (marked as1),2),3),4),5) inFIG. 9A) and used for the Raman and AFM line scan characterizations. The Raman spectra at the marked points are shown inFIG. 9B. It can be seen that the positions of the MoS2Raman peaks are almost the same for all five spots with fixed peak separation of around 25 cm−1. The AFM images and corresponding line profiles from these different areas further confirm the thickness uniformity of the five marked areas. The thickness of this MoS2film is around 3 nm, consistent with the TEM characterization. In order to further investigate the uniformity of the MoS2film, a randomly selected area (50×50 μm) was analyzed with the Raman mapping technique. The Raman mapping results of peak positions corresponding to E12gand A2gvibration modes and their peak difference Δω indicate that no crystal boundaries or obvious defects were detected, which is further proof of the high quality of this single crystal structure of epitaxial MoS2film. The distributions of the peak positions are very narrow: 383.8˜384.2 cm−1for E12gpeak, 408.8˜409.2 cm−1for the A2gpeak, and 24.9˜525.05 cm−1for the Δω.

To check the electrical performance of the epitaxial 2D MoS2films (which may include between 5 and 10 layers according to the method discussed above), top-gate thin film transistors1000were fabricated as illustrated inFIG. 10. Epitaxial MoS2film1002was generated on a substrate1004(e.g., sapphire) as discussed above with regard toFIGS. 4 and 5. The epitaxial MoS2film10d02was patterned using, for example, photolithography followed by a dry etching process. Au/Ti source/drain electrodes1006and1008were grown on top of the MoS2film1002by e-beam evaporation (EBE), with lift-off process. HfO2dielectric1010was grown by atomic layer deposition (ALD) (e.g., 400 cycles, 62 nm) over the epitaxial MoS2film1002and electrodes1006and1008. During the ALD process, the temperature was set as 160° C., and deionized water was used as the oxidization source with the pulse/purge time 0.015/8 s/s. Tetrakis(dimethylamido) hafnium (IV) precursor was used as Hf source, with pulse/purge time of 0.2/8 s/s. The top-gate electrode1012was grown by EBE and patterned by lift-off process. Once the device1000structure was prepared, it was annealed at 200° C. for 2 hrs in a tube furnace, for example, tube furnace500. The annealing process was protected with Ar/H2 gas at a flow rate of 40/5 sccm, with an inner pressure kept at 1 torr. Before annealing, the tube502was purged 3 times with the Ar/H2 gas. The heating rate was set as 5° C./min. The furnace was naturally cooled to room temperature after the annealing process.

The structure of the top-gated MoS2TFT1000used HfO2as dielectric, and Au/Ti as source/drain (S/D) contacts and gate (G). Linear drain current (IDS) levels (seeFIG. 11A) were observed for the gate voltage ranging from −10 to 10 V (2 V per step), indicating the Ohmic contact between the MoS2channel1002and the source/drain1006/1008, consistent with previous reports.

The capacitance per unit area is calculated to be 2.7×10-7 F/cm2, and the dielectric constant of the HfO2is determined to be 18. The field-effect mobility (μFE) is calculated to be 8.5 cm2V−1s−1. This μFE value is almost 6 times higher than the reported best few-layer MoS2device prepared from a two-step process. A detailed comparison to previous TFT devices on MoS2films obtained using the two-step process is shown in Table 1 inFIG. 12. The on-current to off-current (Ion/Ioff) ratio is determined to be 2.75×105, similar to the previous reports. The leakage current (IGS) is less than 10−11A, which is two orders of magnitude lower than the IDSused for mobility extraction, indicating that the effect of leakage current on mobility extraction is minimal. The threshold voltage (Vth) is determined to be 6.6 V, and the carrier density is calculated to be 1˜14×1012cm−2in the device operation range. Subthreshold swing (SS) is calculated (from the inverse of the maximum slope of the logarithmic transfer curve), to be 1.20 V dec−1. The interface trap density (DIT) is calculated as being 3.3×1013cm−2. The calculated DITis even higher than the carrier density, indicating that the interface between the MoS2film and the HfO2layer contains large amounts of carrier trapping centers. It is believed that if this interface can be improved, the TFT transistor could show even higher performance (such as higher μFE). The scalable increase in the IDSwith respect to the VDSindicates the modulation of the drain current is by the field-effect, instead of the contact between S/D contact and the MoS2channel.FIG. 11Bshows the distribution of μFEfrom 46 individual TFTs with different channel dimensions from the circuit shown inFIG. 10.

Table 1 inFIG. 12shows a comparison of reported large-area few-layer MoS2films and their TFT performance. It can be seen that although the methods for preparing large-area few-layer MoS2films are frequently reported in the recent two years, the device's mobility of these MoS2TFT devices are all below 2 cm2V−1s−1. In one embodiment, by using the MoS2films from the sulfurization of the epitaxial MoO2film, the TFTs show high field-effect mobility, which could even reach 10 cm2V−1s−1, smaller gate voltage operation (˜8˜15 V), which means low power consumption during the device's operation, and competitive Ion/Ioffratios.

One or more embodiments discussed above achieve high-quality, continuous, few-layer epitaxial MoS2films, which were prepared by sulfurization of ultrathin epitaxial MoO2precursor film over a large area. Compared with all previous two-step processes, which normally perform sulfurization of precursor Mo based films, this is the first time the process focuses on optimizing the precursor film's quality before sulfurization.

The sulfurization process for achieving the best quality MoS2film was optimized so that the material characterization results show that the MoS2films grow epitaxially on the (001) Al2O3substrate with excellent uniformity. Raman mapping depicted the uniformity and high optical quality with almost negligible defects and grain boundaries at the microscale. Raman and AFM measurements at several testing points further confirmed the thickness and uniformity at large area.

TFT devices fabricated using the optimized epitaxial few-layer MoS2films exhibited excellent electrical performance. Field-effect mobility values of 46 individual devices ranged from 4˜10 cm2V−1s−1, which is up to 6 times higher than the best reported MoS2FET prepared from other two-step processes. These properties are even compatible with MoS2film from a CVD process. A switching ratio of about 105was obtained, which is similar to previous reports. The used gate voltage was between −8 and 15 V, indicating that a lower power consumption is possible with these devices.

Thus, one or more of the embodiments discussed above are capable of scalable fabrication, uniformity at the wafer scale, and the possibility for layer number control.

The disclosed embodiments provide a MoS2film made of a few layers and method for making the same in which the film includes a single crystal. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.