Fabrication and electrical properties of MoS2 nanodisc-based back-gated field effect transistors
© Gu et al.; licensee Springer. 2014
Received: 28 January 2014
Accepted: 19 February 2014
Published: 28 February 2014
Two-dimensional (2D) molybdenum disulfide (MoS2) is an attractive alternative semiconductor material for next-generation low-power nanoelectronic applications, due to its special structure and large bandgap. Here, we report the fabrication of large-area MoS2 nanodiscs and their incorporation into back-gated field effect transistors (FETs) whose electrical properties we characterize. The MoS2 nanodiscs, fabricated via chemical vapor deposition (CVD), are homogeneous and continuous, and their thickness of around 5 nm is equal to a few layers of MoS2. In addition, we find that the MoS2 nanodisc-based back-gated field effect transistors with nickel electrodes achieve very high performance. The transistors exhibit an on/off current ratio of up to 1.9 × 105, and a maximum transconductance of up to 27 μS (5.4 μS/μm). Moreover, their mobility is as high as 368 cm2/Vs. Furthermore, the transistors have good output characteristics and can be easily modulated by the back gate. The electrical properties of the MoS2 nanodisc transistors are better than or comparable to those values extracted from single and multilayer MoS2 FETs.
The structure of molybdenum disulfide (MoS2), a layered transition metal dichalcogenide (TMD), comprises S-Mo-S in a hexagonal close-packed arrangement. Covalent bonds exist between the atoms in each layer, while the layers interact via weak van der Waals forces. Similar to extracting graphene from graphite , bulk MoS2 is easily split into single-layer (SL) or few-layer (FL) MoS2 sheets. Compared with graphene, single and multilayer MoS2 have a larger bandgap [2–6]. The presence of a large bandgap makes MoS2 more attractive than gapless graphene for logic circuits and amplifier devices. Single and multilayer MoS2 field effect transistors (FETs) have been prepared with on/off current ratio exceeding 108 at room temperature, effective mobility as high as 700 cm2/Vs and steep subthreshold swing (74 mV/decade) [7–13]. MoS2 also shows great promise for optoelectronics [14, 15] and energy harvesting [16, 17] and other nanoelectronic applications.
MoS2 sheets are most commonly fabricated by micromechanical exfoliation (Scotch-tape peeling) [18, 19]. Lithium-based intercalation [20, 21], liquid-phase exfoliation , and other methods [23–25] have also been used to synthesize single-layer and few-layer MoS2. However, the yield and reproducibility of micromechanical exfoliation are poor, and the complexity of the other methods presents disadvantages to their use. Chemical vapor deposition (CVD) is a simple and scalable method for the synthesis of transition metal dichalcogenide thin films having large area. Liu et al. and Zhan et al. have successfully synthesized large-area MoS2 films via CVD [26, 27].
Much research has been done on single and multilayer MoS2 FETs where the MoS2 layer is fabricated by micromechanical exfoliation then transferred to Si substrates. However, few studies have addressed the electrical properties of back-gated MoS2 field effect transistors with Ni as contact electrodes. This study is the first to report back-gated FETs based on MoS2 nanodiscs synthesized directly using CVD. The MoS2 nanodiscs fabricated via CVD are large and uniform. We herein report upon their surface morphologies, structures, carrier concentration, and mobility, as well as the output characteristics and transfer characteristics of FETs based on these obtained MoS2 nanodiscs, with Ni as contact electrodes.
Figure 1b is a schematic of a MoS2 back-gated FET. The source and drain electrodes were formed by lithographic patterning, and Ni electrodes were sputtered onto them using magnetron sputtering technology. The MoS2 nanodiscs serve as the channel, whose length and width are 1.5 and 5 μm, respectively. The back gate of the FET was completed by sputtering a 50-nm-thick Ni layer on the back of the Si substrate.
The surface morphology and crystalline structure of the MoS2 discs were analyzed by atomic force microscopy (AFM) and X-ray diffraction (XRD), respectively. The electrical properties of the samples were measured using a Hall Effect Measurement System (HMS-3000, Ecopia, Anyang, South Korea) at room temperature. The electrical properties of the MoS2 nanodisc-based FETs, configured as shown in Figure 1b, were measured using a Keithley 4200 semiconductor characterization system (Cleveland, OH, USA).
Results and discussion
The surface current-voltage (I-V) properties, surface carrier concentration and mobility of the obtained MoS2 nanodiscs are very sensitive to the quality of the film. Figure 3b shows the surface I-V properties of the MoS2 nanodisc film. The inset shows the layout of the four measurement points on the MoS2 nanodisc film. The I-V curves measured between any two points show a perfect linear dependence, which indicates that the deposited MoS2 nanodiscs have good conductivity. The measured carrier concentration of the MoS2 discs is about 3.412 × 106 cm−2, and their electron mobility is as high as 6.42 × 102 cm2/Vs. This mobility value is higher than previously reported values (2 to 3 × 102 cm2/Vs) for single and multilayer MoS2[19, 28]. This significant increase of room-temperature mobility value in our MoS2 may result from the MoS2 nanodisc structure. The mobility of SL MoS2 is generally smaller than bulk MoS2 because of the larger phonon scattering . However, FL MoS2 exhibits fewer dangling bonds and defect states than does SL MoS2, significantly decreasing the phonon scattering. The lattice scattering in the two-dimensional (2D) nanodiscs should be even lower, due to their surface roughness and boundaries. The above findings clearly demonstrate that the MoS2 nanodiscs fabricated via CVD have uniform morphologies, structures, and electrical properties.
Figure 4b displays the output characteristics (drain current IDS versus drain voltage VDS) of back-gated MoS2 transistors at room temperature for VGS = 0, 5, 10, 15, and 20 V. For small VGS, the current IDS shows an exponential dependence on VDS at low VDS values, which results from the presence of a sizable Schottky barrier at the Ni-MoS2 interface . Then, for larger values of VGS, the relation between IDS and VDS becomes linear as VDS increases, which is consistent with the previously reported findings . The barrier height at larger VGS is lower that has been previously demonstrated in greater detail [12, 30, 31]. Thus, the channel can give rise to thermally assisted tunneling, which is responsible for the linear relationship between IDS and VDS. Finally, when VDS increases above a certain value, the current IDS becomes saturated, achieving the output properties of a traditional FET.
Using CVD, we have fabricated uniform MoS2 nanodiscs, organized into thin films with large area and having good electrical properties. The nanodiscs were incorporated into high-performance back-gated field effect transistors with Ni as contact electrodes. The transistors have good output characteristics and exhibit typical n-type behavior, with a maximum transconductance of approximately 27 μS (5.4 μS/μm), an on/off current ratio of up to 1.9 × 105 and a mobility as high as 368 cm2/Vs, comparable to that of FETs based on single and multilayer MoS2. These promising values along with the very good electrical characteristics, MoS2 transistors will be the attractive candidates for future low-power applications.
WG is a graduate student major in fabrication of new semiconductor nanometer materials. JS is a lecturer and PhD-degree holder specializing in semiconductor devices. XM is a professor and PhD-degree holder specializing in semiconductor materials and devices, especially expert in nanoscaled optical-electronic materials and optoelectronic devices.
This work was supported in part by the National Natural Science Foundation of China (no. 60976071) and the Innovation Program for Postgraduate of Suzhou University of Science and Technology (No. SKCX13S_053).
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