Applied Science and Convergence Technology 2021; 30(5): 156-158
Published online September 30, 2021
Copyright © The Korean Vacuum Society.
Department of Electrical Engineering and Smart Grid Research Center, Jeonbuk National University, Jeonju 54896, Republic of Korea
Correspondence to:E-mail: email@example.com
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 [1–3]. 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 (
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,
where µ is the field-effect mobility;
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
The authors declare no conflicts of interest.