A field effect transistor (FET) device of a MoS2/graphene vertical heterostructure material combined with the high carrier mobility of graphene material with a permanent band gap of monolayer MoS2 material, can realize the application of digital circuit. In this paper, the high-quality MoS2/graphene vertical heterostructure material can be synthesized by chemical vapor deposition (CVD) and wet transfer methods. The electron transfer, photoluminescence (PL) spectrum and carrier transport of MoS2/graphene heterostructure material can be characterized by microscope, Raman spectrometer, SEM, AFM and XPS, which can be used to judge the quality of the material. Besides, monolayer MoS2 and MoS2/graphene FETs can be prepared by lithography and electron beam evaporation, and the electrical performance is also tested and analyzed. The results show that the Schottky barrier height can be adjusted by the gate voltage and doped graphene. The photoexcited electron-hole pairs of monolayer MoS2 material can be separated by MoS2/graphene heterojunction when the gate voltage is zero, which would greatly decrease the PL characteristics. Compared to the MoS2/metal FET, the on-state current, switching ratio ,and drain current of MoS2/graphene FET are increased by an order of magnitude, the mobility and transconductance can also be increased, which can be explained by the smaller Schottky barrier height and the energy band rearrangement. Meanwhile, drain current exceeds 4×10−6A, which can also confirm the enhanced electronic characteristics of MoS2/graphene FET. Therefore, MoS2/graphene vertical heterostructure material can be applied to the ultra-high electronic device fields.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The size of silicon-based devices continues to decrease according to the Moore’s Law, which is close to the physical limit. At present, many scientists are committed to the future electronic/optoelectronic devices development of two-dimensional (2D) materials. Due to the unique physical and structural characteristics, graphene and other 2D materials can provide many new research directions in nanoscience and technology field, for example, MoS2/graphene heterostructure material, is widely used in the photo-catalyst, microwave absorption, diode and supercapacitor fields [1–4]. As we all know, h-BN material can be used as the insulating substrate and gate dielectric layer, graphene material is used as the interconnect, and MoS2 or other 2D semiconductor can serve as the semiconductor channel materials [5,6]. Monolayer 2D material is the thinnest material with electronic properties, which represents the scaling ultimate limit of vertical direction. Meanwhile, it can also effectively decrease the power consumption of device due to the short channel effect. Graphene is the single-atom-thick 2D sp2 hybrid carbon flake material, which play the vital role in new nano-scale electronics . The customization and design of electrical properties are difficult for graphene material, which is not conducive to the preparation of new electronic products with extremely small dimensions. Without sacrificing the inherent electron and hole mobility, graphene material needs to rely on the advanced methods to open the significant transmission band gap . The transition metal dichalcogenides (TMDs) materials have the unique structure, electronic properties, suitable band gap and mobility, so it can help the realization of small size and unique performance devices . As the most concerned TMDs materials, MoS2 material has the interesting semiconductor properties and reasonable mobility, and the band gap depends on the layer number . MoS2 material is not conducive to the application of high-performance digital circuits, which can be explained by the heavier electronic mass and lower carrier mobility. Due to the specific energy band arrangement, structural features and electronic properties, a variety of 2D layered semiconductor can be flexibly combined to form various van der Waals (vdWs) heterostructure materials [11,12]. Meanwhile, CVD-growth of the 2D ternary alloyed TMDC-based heterostructure materials is also extensively studied [13,14]. These heterostructure materials have the clear atomic-level interfaces and tunable band structures, which can open the broad opportunities of the new electronic and optical properties. Compared with the conventional semiconductor heterostructure, the vdWs heterostructures have no the limitation of atomic lattice matching due to the lack of dangling bonds, so there exists the high-quality interface . Therefore, these ultra-thin vdWs heterostructures with different energy band structures can provide the new inspiration for the electronic and optoelectronic fields, such as field effect transistor , photodetector [17,18], logic circuit, and flexible electronic device .
The development of new electronic devices is driving by the combination of graphene and other 2D materials, the preparation method of vdWs heterostructure materials mainly includes the mechanically peeled and secondary stacked sheets methods . The preparation processes of secondary stacked sheets are relatively complicated, and the success rate is lower, which is not conducive to large-scale preparation. Recent research indicates that vdWs heterostructure materials can also be directly prepared by chemical vapor deposition (CVD) method [21,22]. Compared to the mechanical peeled method, CVD preparation method have the obvious advantages, such as easy control of size and clean interface, which is expected to realize the large-scale preparation of vdWs heterostructure materials . In addition, due to the existence of Fermi level pinning, Schottky barrier would be formed when metals are in contact with MoS2 materials, which would lead to the large contact resistance. For the larger contact resistance, reason is that Fermi level is pinned to MoS2 material near the edge of conduction band. The improvement methods of MoS2 contact mainly include (1) contact metal with lower work function, (2) gas doping and (3) molecular or solid doping on MoS2 film . MoS2/graphene heterostructure material can effectively combine high electron mobility of graphene material and band gap structure of MoS2 material, which can provide the new research perspective about contact improvement.
In this paper, MoS2 FET with graphene/metal contact transistor can be constructed, and the electrical performance was also tested. The main research contents of this paper are as follows: First, the large-scale high-quality MoS2/graphene vdWs vertical heterostructure material can be synthesized by CVD and wet transfer methods, graphene material is used between metal contact and n-type monolayer MoS2 material, which can enhance the electronic coupling and promote the electron injection. Next, the morphology, charge transfer, Raman and photoluminescence (PL) spectroscopies of MoS2/graphene heterostructure material were characterized by optical microscope and Raman spectrometer. Due to the interaction between electrons, the photogenerated electron-hole pairs can be effectively separated, and there exist the significant charge transfer. MoS2/graphene heterostructure material has the atomically flat interface, and layers can be connected by the vdWs force. Subsequently, MoS2/graphene FET was prepared, and the electron transport characteristics of device were also studied. Compared to MoS2/metal FET, MoS2/graphene FET has the relatively excellent electron transport performance, and graphene/metal heterogeneous contact can decrease the contact resistance and barrier height, which can provide the guidance for the preparation of high-performance integrated optoelectronics.
2. Results and discussion
2.1 Test and characterization of Raman spectroscopy
To further explore the crystal quality and spectral properties of MoS2/graphene heterostructure material, Raman and PL spectrum of MoS2/graphene heterostructure material were tested by 532 nm laser under the different position and laser power, which can confirm the existence of graphene and MoS2 materials. Due to vdWs force, Raman spectroscopy can also indicate the changes of charge doping, strain or electronic band structure.
The layer number of MoS2 material can be determined by the contrast and Raman characteristic spectrum. As shown in Fig. 1(a), E12g and A1g Raman characteristic peaks of MoS2 material are located at 383.2 cm-1 and 402.1 cm-1, respectively. The Raman characteristic peak difference is 18.9 cm-1, which indicates the existence of monolayer MoS2 material. Figure 1(b) shows the photoluminescence (PL) spectrum of MoS2 material under the different point, and the strong background comes from PL spectrum of MoS2 material. There are two strongest characteristic peaks, and the peak positions are located at 631.3 nm and 668.7 nm, respectively. According to the conversion relationship between wavelength and photon energy, photon energy of two characteristic peaks are 1.969 eV and 1.859 eV, respectively. The peak position of 1.859 eV (A) remains unchanged while peak position of 1.969 eV (B) fluctuates within a certain range. This is because MoS2 material has the different strains, defect contents as well as crystallinity at the different positions, which would affect the peak position of B exciton. Figure 1(c) shows the power Raman spectroscopy of MoS2 material, Raman spectrum intensity increases with laser power increases. The peak spacing can reflect the layer number of films, and the Raman characteristic peak position of monolayer MoS2 material were approximately at 384.7 cm-1 and 403.8 cm-1, which correspond to the in-plane vibration E12g and out-of-plane vibration A1g characteristic peak modes, respectively. The power PL spectrum of MoS2/graphene heterostructure material is shown in Fig. 1(d). The characteristic peaks of 673.4 nm (1.846 eV) and 633.6 nm (1.962 eV) can approximately match the optical transitions of direct gap between A and B excitons. The peak position of red line emission peak shifts significantly, which can be explained by the recombination of spatially separated carriers and strain during thermal growth. Besides, there was the larger peak separation phenomenon of MoS2/graphene heterostructure stacking region, and PL quenching represents energy transfer of charged carriers, which is induced by the interaction between graphene and MoS2 material . In the Raman spectrum of graphene material, the intensity ratio (2D/G) from 2D band to G band is as high as 5.3, and graphene is monolayer material, as shown in Fig. 1(e). As an indicator of graphene defects, the intensity ratio (D/G) of D band to G band is also considered. The D band of 1337 cm-1 is easily affected by point defects of graphene impurities and interaction of dangling bonds . The D characteristic peak of graphene material indicates the presence of defects, edge states or vacancies. In the Raman spectrum of graphene material at 8 points, the intensity of D band is relatively small, and the quality of graphene material is good, which can be used to fabricate transistor and circuits of MoS2/Graphene heterostructure material. In Fig. 1(f), the intensity of D peak was relatively low, and the peak position of 2D peak can confirm the existence of monolayer graphene material. The ratio of D/G characteristic peak increases slightly with the increases of laser power, and both 2D (2678.2 cm-1) and G peak (1593.4 cm-1) shifted to higher wavenumbers. Both charge doping and strain would shift the 2D and G peak positions of graphene material, and the peak shift degree of each effect is different. MoS2 material has the effect on Raman signal intensity of graphene material to a certain extent, which can decrease the signal-to-noise ratio of graphene spectrum.
In Fig. 1 g, E12g and A1g Raman characteristic peak positions of MoS2 material decrease with the increase of laser power, and the peak positions of strongest PL spectrum also decrease. The reason is that MoS2 material on SiO2/Si substrate has the n-type doping. Figure 1 h shows the power-dependent Raman peaks variation of graphene material, peak positions of D and 2D peaks decrease when the laser power increases, and the peak position of G peak increases, which can be explained by the charge doping and strain.
2.2 SEM and EDX element distribution images of MoS2/graphene heterostructure
The particle size, dispersibility and morphology of nanomaterial can be distinguished and obtained by SEM. The layer number of materials can be roughly reflected by the color and surface wrinkles of SEM images. The morphology of MoS2, graphene and MoS2/graphene heterostructure materials on SiO2/Si substrate can be clearly seen by SEM, as shown in Fig. 2. At the growth boundary of MoS2 material, graphene material can cross grain boundary to form the MoS2/graphene continuous film.
The Energy-dispersive X-ray spectroscopy (EDX) data of material can be obtained and analyzed by the characteristic X-ray wavelength and elements intensity, which can determine the elements of sample . Figure 3 is the EDX element distribution image of MoS2/graphene heterostructure material, the molybdenum, sulfur and carbon elements are uniformly distributed on the surface of SiO2/Si substrate, which also illustrates the preparation of MoS2/graphene heterostructure material.
2.3 XPS and AFM of MoS2/graphene heterostructure material
The surface composition and chemical state of sample can be determined by XPS spectrum, which can be obtained by the monochromatic Al Ka radiation with a 400μm aperture. The chemical states and composition of bottom MoS2 and top graphene materials are deeply analyzed and detected by XPS, and XPS energy spectrum of MoS2/graphene heterostructure material is shown in Fig. 4(a). Figure 4(b) and 4(c) respectively show the XPS spectrum of Mo 3d and S 2p, Mo 3d3/2 and Mo 3d5/2 are respectively located at 233.2 eV and 229.9 eV, and 163.9 eV and 162.7 eV correspond to S 2P1/2 and S 2P3/2, which can determine elemental composition of MoS2 material [28,29]. The atomic ratio of S to Mo is approximately 1.93, which can determine the expected charge state of S 2− and Mo 4+ of MoS2 material. In Fig. 4(d), binding energies of C1s core energy level spectral region are respectively at 284.6 eV, 285.2 eV and 287.1 eV, which can be attributed to sp2 C-C bond, sp3 C-C bond and C-O bond, respectively . The carbon atoms maintain the honeycomb structure, which cannot be chemically bonded to Mo and S atoms.
The morphology and thickness of MoS2, graphene and MoS2/graphene heterostructure materials can also be tested and confirmed by AFM, which can measure the height gradient difference between material and underlying substrate. Figure 5(a), 5(b) and 5(c) show the AFM images of MoS2, graphene and MoS2/graphene heterostructure materials, respectively. Graphene material can fill any shape patterns of MoS2 material, which can form the MoS2/graphene vertical heterostructure material. MoS2/graphene heterostructure material show the relatively uniform color, which can establish the relationship between optical contrast and thickness. It is impossible to form the atomically sharp interface due to the lattice mismatch. The surface of MoS2/graphene heterostructure is flat, and it has the continuous, large and uniform film. The layers number of MoS2, graphene and MoS2/graphene heterostructure materials can be determined by AFM image. In Fig. 5(d) and 5(e), the thickness of MoS2 and graphene materials are 0.83 nm and 0.35 nm, respectively. Meanwhile, the layer number of MoS2/graphene heterostructure material is double layer, the test result is show in Fig. 5(f).
3. FET device of MoS2/graphene heterostructure material
3.1 Schematic diagram of MoS2/graphene FET
As the important core device, field effect transistors (FETs) are used in the modern integrated circuits and microelectronics field. Figures 6(a) and 6(b) show the schematic diagram and optical microscope image of MoS2/graphene FET, respectively. Our research mainly includes the following aspects: First, MoS2/graphene heterostructure material can be grown by CVD growth and wet transfer methods. Next, MoS2/graphene FET can be prepared by the photolithography, electron beam evaporation and acetone stripping processes, Ti/Au metal layers were used as the source and drain electrodes, and silicon substrate was used as back gate electrode. Finally, the electronic transmission performance of device was tested at room temperature and atmospheric pressure.
3.2 Electrical performance of MoS2/Graphene FET
Figures 7(a) and 7(b) show the transfer characteristics of MoS2/graphene and MoS2 FETs under the linear and logarithmic conditions, respectively. It can be found by observing the transfer characteristic curve that source-drain current (Ids) increases monotonously when gate voltage (Vg) increases from -40 V to 40 V. The transfer characteristic curve initially shows the second-order correlation at lower gate bias voltage, and the increase speed of Ids becomes slower when Vg is greater than 10 V. The on-state currents of MoS2/graphene and MoS2 FET devices are 6.2×10−7A and 3.7×10−8A, respectively. Meanwhile, the switching ratio of MoS2/graphene and MoS2 FET devices can respectively reach 105 and 104. Compared to MoS2 FET, on-state current and switching ratio of MoS2/graphene FET device are both improved by an order of magnitude.
In formula (1), gm, L and W symbols respectively represent the transconductance, channel length and channel width, Cox is the gate dielectric capacitance, and VDS is the drain-to-source bias voltage. Mobility of device is proportional to transconductance when other conditions are the same parameters, so the mobility and transconductance of MoS2/graphene FET are higher than that of MoS2 FET. In summary, MoS2/graphene FET has the higher on-state current, switching ratio, transconductance and mobility, which means that it has the better gate electric field control effect. The charge injection of MoS2/graphene heterostructure material can be promoted by improving bending stability and decreasing the charge capture and release process. Meanwhile, the phenomenon is also mainly due to the decrease of interface Coulomb scattering and contact barrier.
The output characteristic IDS-VDS curves of MoS2/graphene and MoS2 FETs under the different gate control voltage are shown in Fig. 7(c) and 7(d), respectively. The maximum drain current of MoS2/graphene and MoS2 FETs are 4.3×10−6A and 2.4×10−7A when Vg is 10 V, and the drain current is increased by an order of magnitude. The drain current decreases when Vg decreases from 10 V to -10 V, and Ids-Vds curve shows the inflection point near Vds=0 V, so MoS2/graphene heterostructure and MoS2 materials have the slight Schottky barrier in contact with Ti/Au metals. Graphene is p-type semiconductor, and MoS2 material is n-type semiconductor material. The band arrangement at interface and electrostatic band bending would promote the electrons of electron-rich MoS2 material transfer to p-type graphene material When MoS2 and graphene materials are stacked on each other, which is attributed to the high electrical conductivity of graphene material and the synergistic effect of MoS2 and graphene layers. The information storage capacity of device can be judged from the transmission characteristic curve. Figures 7(e) and 7(f) show bidirectional transfer characteristic curves of MoS2/graphene and MoS2 FETs at two scanning directions, respectively. The voltage offset of MoS2/graphene FET is greater than that of MoS2 FET, and voltage offset represents the hysteresis and response of device, which is related to the charging and discharging effect of gate voltage.
In our research, MoS2 FET with graphene/metal contact transistor was constructed. First, MoS2/graphene vdWs vertical heterostructure material can be prepared by CVD and wet transfer methods. Then, the charge transfer, Raman and PL spectrum characteristics of MoS2/graphene heterostructure material can be tested and characterized by microscope, Raman spectrometer, SEM, XPS and AFM. The large electrons from MoS2 material can be transferred to graphene material, which is due to the difference of work function. The charge transfer also results in the formation of Schottky barrier at interface, which can increase the interlayer resistance. The built-in potential of Schottky barrier can split the light-excited electron-hole pairs of monolayer MoS2 material, which would significantly decrease the PL phenomenon. Afterwards, MoS2/graphene FET can be fabricated, and the electron transmission performance of MoS2/graphene and MoS2 devices were also be tested and analyzed. Compared to MoS2 FET device, the on-state current, switching ratio and maximum drain current of MoS2/graphene FET were all increased by an order of magnitude, and the mobility and gate electric field control can be also effectively improved. Finally, the interlayer impedance of MoS2/graphene FET can also be better controlled by gate and drain voltages. Our research demonstrates the new type of 2D heterostructure and device architecture, which would greatly promote the electrons transmission, and expand the research scope of optoelectronic devices based on van der Waals forces.
China Scholarship Council (202006960029); Fundamental Research Funds for the Central Universities (XJS211109); National Natural Science Foundation of China (61904136, U1866212).
Tao Han: conceptualization, writing—original draft preparation; Hongxia Liu: Writing—Reviewing and Editing, Funding acquisition; Shulong Wang: methodology; Shupeng Chen: validation; Kun Yang: writing—original draft preparation. This work is also supported by The Laboratory Open Fund of Beijing Smart-chip Microelectronics Technology Co., Ltd.
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
See Supplement 1 for supporting content.
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