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Research Paper

Applied Science and Convergence Technology 2021; 30(5): 156-158

Published online September 30, 2021

https://doi.org/10.5757/ASCT.2021.30.5.156

Copyright © The Korean Vacuum Society.

Comparison of Ionic Liquid and Ion-Gel Top-Gate MoS2 Field-Effect Transistors

Guen Hyung Oh and TaeWan Kim*

Department of Electrical Engineering and Smart Grid Research Center, Jeonbuk National University, Jeonju 54896, Republic of Korea

Correspondence to:E-mail: twkim@jbnu.ac.kr

Received: August 9, 2021; Accepted: August 24, 2021

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Polymer electrolytes and ionic liquids (ILs) have attracted significant interest in applications as gate dielectrics. In this study, we fabricated top-gated molybdenum disulfide (MoS2) thin-film transistors using IL and ion-gel (IG) gate dielectrics. Room-temperature Raman spectra measurements indicated a dominant peak spectral emission at 358 cm−1 (E12g) and 406.44 cm−1 (A1g) associated with bilayer MoS2 films. The fabricated thin-film field-effect transistors (FET) with IG gate dielectric exhibited band transport with a highest mobility of 0.5 cm2/V⋅s, and a poor ION/IOFF ratio of ~10. By contrast, the FET with IL gate dielectric exhibited a 3400 % improvement in terms of the mobility (17.9 cm2/V⋅s), and a 1000 % improvement of the ION/IOFF ratio (~100).

Keywords: MoS2, Top-gate, Field-effect transistor, Ionic liquid, Ion-gel

The field effect in top-gate field-effect transistor (FET) configurations comprised of ion-gels (IGs) and ionic liquids (ILs) as gate dielectrics have attracted considerable attention, owing to their flexibility, printability, and high ionic conductivity. Moreover, these devices have the potential to be implemented as two-dimensional materials. IL-gated two-dimensional (2D) molybdenum disulphide (MoS2) FETs have been fabricated previously [13]. Despite the excellent performance of IL-gated FETs, ILs have significant limitations for practical device applications, owing to the volatility. However, IGs can be suitably implemented in applications, as the gel state is significantly less volatile compared to ILs. IG is advantageous in that it has the flexibility of a polymer dielectric and the high capacitance of an ionic liquid. In this study, we report the growth of bilayer MoS2 thin films on silicon dioxide/silicon (SiO2/Si) substrates using metal–organic vapor phase epitaxy (MOVPE). Furthermore, to compare the IG and IL dielectrics, we fabricated top-gate MoS2 FETs with IG and IL as gate dielectrics by forming metal electrodes through an electron beam evaporator using a shadow mask on the obtained MoS2 thin film.

MoS2 thin films with thickness of a few atomic layers were grown on a SiO2/p-Si substrate via MOVPE, with a chamber pressure of 10 Torr and susceptor temperature of 400 °C. Hydrogen sulphide and molybdenum hexacarbonyl were used as sources of S and Mo, respectively. The Raman spectra of the bilayer MoS2 films were recorded using a dispersive Raman microscope (Thermo Scientific). Excitation was induced using a 532-nm laser diode. Moreover, X-ray photoemission spectroscopy (XPS, SES-100, VG-Scienta) was used to record the Mo and S binding energies in the bilayer MoS2 films.

The ILs considered in this study were 1-Ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide (EMIM-TFSI), purchased from Sigma Aldrich. Poly (styrene-block-methyl methacrylate-block-styrene)(PS-PMMA-PS) triblock copolymer was purchased from Polymer Source, Inc. The IG solution was fabricated by mixing the copolymer PS-PMMA-PS with the IL EMIM-TFSI in dichloromethane as a solvent. Then, the mixtures were baked at 70 °C in a vacuum chamber at a pressure of 10−6 Torr for 24 h, in order to evaporate the molecular moisture of IL and IG solvent. Then, the IL was immediately transferred for storage in a glove box sustained under a controlled argon atmosphere. All experiments in this study were conducted under dry atmospheres of either argon (glove box), oxygen, or vacuum.

Source and drain electrodes were deposited on the multilayer MoS2 film using an electron beam evaporator. Further, the 5-nm Ti and 50-nm Au films were deposited separately using a Kapton tape and copper line mask to fabricate a channel of area of 50 × 2,000 µm2. The weight ratio of the polymer, IL, and solvent was sustained at 0.7:9.3:20. These solutions were drop-cast onto a SiO2/Si substrate with the MoS2 film and electrodes. A top-gate electrode comprised of a gold/tungsten tip directly in contact with the IL and IG was used to measure the electrical transport properties. Electrical measurements of the device were performed using an in-house four-probe station with a commercial parameter analyzer (2636 B, Keithley Inc.) at 300 K.

Figure 1 shows the Raman spectrum of the MoS2 thin films deposited using MOVPE, which confirms the number of atomic layers. In particular, the peak positions of the observed 358 cm−1 (E2g1), 406.44 cm−1 (A1g), and the Raman peak difference (Δ ∼ 21.21cm−1), establish our films as bilayer MoS2 thin films, when compared with previously reported results [13]. The results of the XPS measurement confirmed the existence of S and Mo. The 162.05 and 160.85 eV peaks observed in the S2p-scan results in Fig. 2(a), are associated with S2p1/2 and S2p3/2, respectively, and are distinctive properties of MoS2. The peak observed at 223.5 eV is generated by S2s. As shown in Fig. 2(b), the Mo3d5/2 and Mo3d3/2 peaks appear at 228 and 231.15 eV, respectively, which is in good agreement with previously reported results on MoS2 [35].

Figure 1. Raman spectra results of the MoS2 bilayer film.

Figure 2. XPS results of (a) S2p-scan and (b) Mo3d-scan obtained for a MoS2 bilayer sample.

We fabricated IL and IG-gated FET devices to study the electrical characteristics of the bilayer MoS2 thin film, as shown in the schematic diagram in Fig. 3(a). The transfer properties of a top-gate MoS2 FET with IL and IG are plotted in Figs. 3(c) and 3(d), respectively. Here, the electrical transports of the MoS2 thin films exhibit the characteristics of n-type metal-oxide semiconductor-FETs operating at relatively low gate voltages (~1 V). However, in the case of MoS2 thin films, the device on-off current ratio, ION/IOFF, is very low (~2.4). Furthermore, we observed a large hysteresis effect for both FETs with IL and IG. This hysteresis in the counter-clockwise direction can be attributed to the slow movement of ions in the ionic liquid [68]. We calculated the mobility of our MoS2 device with IL and IG dielectrics as 17.9 and 0.5 cm2/V⋅s, respectively. In these calculations, we considered the slope of the transfer properties for the gate voltage in the range of 1 – 2 V using the conventional equation for a linear area

Figure 3. (a) Schematic image of a top-gate FET with MoS2. (b) I ds-V ds character-istics of the MoS2 FET. Electrical transport characteristics of top-gate MoS2 FET designed using (c) IL and (d) IG dielectrics at room temperature (300 K).

1D=μWVDCiL(VGVth)

where µ is the field-effect mobility; ID is the drain current; W and L are the channel width and length, respectively; Ci is the specific capacitance of the dielectric; which was 4.24 and 1.66 µF/cm2 for the IL and IG, respectively; VD is the drain voltage; VG and Vth are the gate and threshold voltages, respectively. As shown in Fig. 3(b), poor mobility arises from the high contact resistance. Notably, the mobility of the MoS2 FET with IL dielectric is significantly improved compared to that of the IG dielectric. This behavior can be attributed to the decrease in the capacitance of the IG dielectric with increasing polymer content [9].

In conclusion, we fabricated atomic-layer MoS2-based top-gate FET devices using IL and IG dielectrics. Raman spectroscopy and XPS measurements confirmed the atomic bilayer MoS2 structure. The MoS2 FET with IL indicated the best performance in terms of mobility and

ION/IOFF ratio between the IL and IG dielectrics, which stems from the low capacitance of the IG dielectric compared to that of the IL. The MoS2-based FET with IL contact exhibits a mobility of 17.9 cm2/V⋅s and an ION/IOFF ratio of 102. Our results indicate that the selection of dielectrics can dramatically affect the electrical transport characteristics of 2D MoS2-based top-gate FET devices. Thus, our results can act as benchmarks for the development of high-performance 2D-MoS2-based flexible and printable electronic devices.

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