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Low Ohmic contact resistance and high on/off ratio in transition metal dichalcogenides field-effect transistors via residue-free transfer

Abstract

Beyond-silicon technology demands ultrahigh performance field-effect transistors. Transition metal dichalcogenides provide an ideal material platform, but the device performances such as the contact resistance, on/off ratio and mobility are often limited by the presence of interfacial residues caused by transfer procedures. Here, we show an ideal residue-free transfer approach using polypropylene carbonate with a negligible residue coverage of ~0.08% for monolayer MoS2 at the centimetre scale. By incorporating a bismuth semimetal contact with an atomically clean monolayer MoS2 field-effect transistor on hexagonal boron nitride substrate, we obtain an ultralow Ohmic contact resistance of ~78 Ω µm, approaching the quantum limit, and a record-high on/off ratio of ~1011 at 15 K. Such an ultra-clean fabrication approach could be the ideal platform for high-performance electrical devices using large-area semiconducting transition metal dichalcogenides.

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Fig. 1: Residue-free PPC transfer versus traditional PMMA transfer.
Fig. 2: Electrical and optical effect of the residues.
Fig. 3: Ultralow contact resistance in monolayer MoS2.
Fig. 4: Ultrahigh on/off ratio in MoS2 FET.
Fig. 5: Benchmark of ultra-clean large-area monolayer MoS2 FET.

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Data availability

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

This work was supported by the Institute for Basic Science of Korea (IBS-R011-D1) and Advanced Facility Center for Quantum Technology. A.M. and C.B. acknowledge P. Ghising for the scientific discussion. We acknowledge S. G. Lee for the training on the C-AFM instrument. Theory work was supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division (M.Y.) and by the DOE, Office of Science, National Quantum Information Science Research Centers, Quantum Science Center (S.-H.K.). This research used resources of the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory, which is supported by the DOE Office of Science under contract no. DE-AC05-00OR22725, and resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the DOE Office of Science under contract no. DE-AC02-05CH11231, using National Energy Research Scientific Computing Center award BES-ERCAP0024568. K.K.K. acknowledges support from the Basic Science Research (2022R1A2C2091475) and Next-Generation Intelligence Semiconductor Program (2022M3F3A2A01072215) through the National Research Foundation of Korea, which is funded by the Ministry of Science, ICT and Future Planning.

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Contributions

C.B. and Y.H.L. supervised the project. A.M. and C.B. proposed and developed the project. A.M., C.B. and Y.H.L. designed the experiments. A.M. carried out the device fabrication and transfer of materials, and performed the all-electrical characterization, supervised by C.B.; W.C. and A.M. measured the C-AFM and AFM, respectively. All AFM data analysis was performed by W.C., A.M. and C.B.; S.P. and S.H.C. contributed to the growth of materials. Transmission electron microscopy measurements and analysis were carried out by A.M., C.B., S.H.C. and K.K.K. The DFT calculations were performed by S.-H.K. and M.Y.; A.M., C.B. and Y.H.L wrote the manuscript. A.M. and C.B. analysed all the data. All authors verified the manuscript.

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Correspondence to Chandan Biswas or Young Hee Lee.

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Nature Nanotechnology thanks Wei Chen, Jinlan Wang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Comparison between PMMA and PPC residue coverage.

a, b, AFM micrographs of the monolayer MoS2 on SiO2 substrate transferred using conventional PMMA (a) and PPC (b) methods. c, d, The contrast images of PMMA residues (black spots) extracted from the AFM micrograph taken on MoS2 (red dashed box in a) and SiO2 (green dashed box in a) regions to quantify residue coverage. PMMA residue coverage of 35.2% and 42.6% were obtained in MoS2 and SiO2 regions respectively. e, PPC residue coverage was extracted from b, and a value of 0.08% was obtained across the whole AFM image. f, AFM topography of the monolayer MoS2 on SiO2 substrate. Inset verifies the monolayer thickness (0.7 nm) from the height profile taken on the red line.

Extended Data Fig. 2 Comparative analysis of adsorption energies using various van der Waals corrections.

a, Schematic diagram illustrating the structures of PMMA and PPC trimers, along with a 7x7 MoS2. b, c, Optimized atomic structure for PMMA (b) and PPC (c) backbone and carbonyl group near the MoS2 surface with GGA+ vdW (rvv10) nonlocal correlation function calculation method. d–f, Summery of the adsorption energies obtained using different calculation methods, including LDA (d), as well as LDA, GGA+vdW (e) (opB86b, rev-vdW-DF2, rvv10) for the only carbonyl group near the MoS2 and backbone and carbonyl group near the MoS2. The adsorption energy of the PPC-carbonyl group was lower than the PMMA-carbonyl group in all calculation methods (f), suggesting efficient removal of the PPC from the MoS2 surface.

Extended Data Fig. 3 Electrical performance of different channel length devices.

a, Output characteristics (Id-Vd) of the PPC-Bi device on the h-BN substrate (h-BN/MoS2/h-BN) for different channel lengths at 300 K, Vg = 0 V. b, Id-Vd of the PPC-Bi device for different channel lengths at 15 K, Vg = 0 V. c, RT - LCH plot for different device types under different conditions at Vg = 0 V. h-BN-PPC-Bi device on the h-BN substrate (h-BN/MoS2/h-BN) shows RC of ~90 Ω-µm at 15 K and ~115 Ω-µm at 300 K. PPC-Bi device on the SiO2 substrate (SiO2/MoS2/h-BN) shows RC of ~114 Ω-µm (300 K). In contrast, the PMMA-Bi device on the SiO2 substrate (SiO2/MoS2/h-BN) shows the highest RC of ~142 Ω-µm (300 K) at Vg = 0 V condition. Inset, false color SEM image of the TLM structure with a scale bar of 1 µm.

Extended Data Fig. 4 Four-probe voltage measurement comparisons between different device types.

a, b, Schematic diagram (a) and optical micrograph (b) of the four-probe MoS2 device showing source-drain contacts (LCH ~ 7 µm) and voltage measurement probes (V1, V2). c, Four-probe resistance (R4P) – Vg plot of the PMMA-Ti and PPC-Ti devices on the SiO2 substrate (SiO2/MoS2/h-BN) at 300 K. PPC-Ti device shows significantly low resistance (4.9 × 108 Ω) compared to PMMA-Ti (8.7 × 109 Ω). d, Four-probe voltage (ΔV4P) comparison between PPC-Ti and PMMA-Ti devices at Vg = 5 V. Four-probe voltage measurement scheme was shown in the inset. e, ΔV4P comparison between different device types (PPC-Bi, PPC-Ti, PMMA-Bi, PMMA-Ti) of devices at Vg = 4 V under 300 K. Lowest R4P was observed in the PPC-Bi device of ~7.8 × 105 Ω, followed by the PMMA-Bi (~1.9 × 108 Ω), PPC-Ti (~4.9 × 108 Ω), PMMA-Ti (8.7 × 109 Ω) respectively.

Extended Data Table 1 Device performance of different contact metals for MoS2 FET

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Supplementary Information

Supplementary Figs. 1–17, Notes 1 and 2 and Table 1.

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Mondal, A., Biswas, C., Park, S. et al. Low Ohmic contact resistance and high on/off ratio in transition metal dichalcogenides field-effect transistors via residue-free transfer. Nat. Nanotechnol. 19, 34–43 (2024). https://doi.org/10.1038/s41565-023-01497-x

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