Fabrication of MoS2 field-effect transistors
MoS2 and h-BN flakes were mechanically exfoliated from bulk MoS2 and h-BN crystals and transferred to a 270-nm SiO2/p++ Si substrate.
After double electron resist layers [methyl methacrylate (MMA) and poly(methyl methacrylate) (PMMA)] were spin-coated on the MoS2, the source-drain electrodes were patterned by using an electron beam lithography system. Then Ti (5 nm)/Au (45 nm) layers were deposited by using an electron-beam evaporator.
Figure 1. (A) MoS2 prepared by the mechanical exfoliation method. (B) Spin-coated electron resist double layers of MMA and PMMA, followed by hard baking (C) Electron-beam lithography for patterning four-point probe electrodes. (D) Metal electrodes deposited by an electron-beam evaporator.
Remote charge transfer doping on MoS2 FETs
The dry-transfer method was used to fabricate h-BN/MoS2 vdW heterostructures. The thin h-BN flakes were picked up by adhesive polycarbonate deposited on the dome-shaped polydimethylsiloxane stamp. Then, the h-BN flake was placed on top of the fabricated MoS2 FETs. The heterostructure was dipped into chloroform overnight to remove the remaining polymer residue.
The device was annealed at 200°C in an argon atmosphere for 2 hours to improve the interfaces.
For the preparation of the BV solutions, the BV dichloride (16.35 mg) was dissolved in deionized water (4 ml) and then toluene (4 ml) was added. Sequentially, sodium borohydride (40 mg) was added and then stirred overnight. When the color of the toluene solution was stabilized in yellow, the upper toluene layer was extracted and then diluted into 1, 2.5, and 5 mM with toluene, respectively.
20 μl of the BV solution was drop-casted and then waited for the solvent to evaporate.
Figure 2. (A) The schematic image of BV-doped h-BN/MoS2 FET with Au/Ti contacts for four-point probe measurements. (B) Optical image of h-BN/MoS2 FET before BV doping. (C) AFM image of h-BN/MoS2 FET. (D) Cs-corrected STEM image of the h-BN/MoS2 heterostructure consisting of five layers of h-BN and four layers of MoS2.
Characterization
Suitable MoS2 and h-BN flakes were located by using an optical microscope, and the thickness of the flakes were measured by an AFM system (NX 10, Park Systems). To confirm SCTD in the MoS2 devices, two-point probe electrical characterizations were performed in a temperature-variable probe station (MSTECH, M6VC) using a semiconductor parameter analyzer (Keithley 4200-SCS). Keithley 4200-SCS with a pre-amplifier was used to measure the voltage drop across the channel by applying constant dc bias for four-point probe measurements.