Abstract
In silicon field-effect transistors (FETs), degenerate doping of the channel beneath the source and drain regions is used to create high-performance n- and p-type devices by reducing the contact resistance. Two-dimensional semiconductors have, in contrast, relied on metal-work-function engineering. This approach has led to the development of effective n-type 2D FETs due to the Fermi-level pinning occurring near the conduction band, but it is challenging with p-type FETs. Here we show that the degenerate p-type doping of molybdenum diselenide and tungsten diselenide—achieved through substitutional doping with vanadium, niobium and tantalum—can reduce the contact resistance to as low as 95 Ω µm in multilayers. This, though, comes at the cost of poor electrostatic control, and we find that the doping effectiveness—and its impact on electrostatic control—is reduced in thinner layers due to strong quantum confinement effects. We, therefore, develop a high-performance p-type 2D molybdenum diselenide FET using a layer-by-layer thinning method to create a device with thin layers at the channel and thick doped layers at the contact regions.
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Data availability
The data that support the findings of this study are available from the corresponding authors upon reasonable request.
Code availability
The codes used for plotting the data are available from the corresponding authors upon reasonable request.
Change history
14 November 2024
A Correction to this paper has been published: https://doi.org/10.1038/s41928-024-01309-7
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Acknowledgements
We thank L. J. Liermann for conducting the ICP-AES analysis, J. M. Anderson for conducting the SEM-EDS analysis on the doped crystals and Y. Zheng for assisting in the oxygen plasma treatment of the fabricated devices. Our sincere gratitude also goes to the Penn State Nanofabrication staff and the cleanroom facility at the Materials Research Institute (MRI), Penn State, where all the device fabrication was carried out. The work was supported by the National Science Foundation (NSF) through a CAREER Award under grant no. ECCS-2042154. Z.S. was supported by ERC-CZ programme (project LL2101) from Ministry of Education Youth and Sports (MEYS) and by the project Advanced Functional Nanorobots (reg. no. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the ERDF). A.P. was supported by a NASA Space Technology Graduate Research Opportunity grant (no. 80NSSC23K1197). K.J.S. was supported by the Johannes Amos Comenius Programme, European Structural and Investment Funds, project CHEMFELLS VI (no. CZ.02.01.01/00/22_010/0008122).
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S.D. conceived the idea and designed the experiments. M.D., D. Sen., N.U.S., H.R., P.V., M.U.K.S. and S.S.R. fabricated and measured the devices. Y.S., Z.Z., Y.H. and Y.Y. obtained and analysed the high-resolution cross-sectional TEM data. D.E.S performed the c-axis high-resolution TEM on the doped MoSe2 crystals. A.S., Z.Y. and M.T. grew the Nb-doped MoSe2. K.J.S., K.M. and Z.S. grew the Nb-, V- and Ta-doped MoSe2 and WSe2 crystals. A.P. helped with the AFM measurements. S.G. and D. Somvanshi. performed the DFT simulations. All authors contributed to the preparation of the paper.
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Extended data
Extended Data Fig. 1 Dopant concentration in V-, Nb-, and Ta-doped MoSe2 and WSe2 crystals.
Bar plots showing the dopant concentrations for V, Nb, and Ta, determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES) for a) MoSe2 and b) WSe2. Dopant concentrations for V, Nb, and Ta, extracted using scanning electron microscopy-electron dispersive X-ray spectroscopy (SEM-EDS) for c) MoSe2 and d) WSe2. Bulk carrier concentrations (NB) at room temperature, obtained from Hall measurements for e) MoSe2 and f) WSe2 doped with V, Nb, and Ta.
Extended Data Fig. 2 Electrical characterization of pristine MoSe2 and WSe2 FETs.
Transfer characteristics for a) two thick (~4–6 monolayers) and b) two thin (~1–3 monolayers) MoSe2 FETs. Transfer characteristics for c) two thick (~4–6 monolayers) and d) two thin (~1–3 monolayers) WSe2 FETs at \({V}_{{DS}}\) = 1 V.
Extended Data Fig. 3 Substitutionally doped p-type WSe2 FETs.
Transfer characteristics, that is, source-to-drain current (\({I}_{{DS}}\)) as a function of the back-gate voltage (\({V}_{{BG}}\)) for a constant source-to-drain bias of \({V}_{{DS}}\) = 1 V for a) V-, b) Nb-, and c) Ta-doped WSe2 FETs with thick (~4–6 monolayers) channels. These FETs show high on-state current (ION) (except for the case of V-doped WSe2) and poor electrostatic gate control (low ION/IOFF) confirming degenerate p-type doping. In contrast, transfer characteristics for d) V-, e) Nb-, and f) Ta-doped WSe2 FETs with thin (~1–3 monolayers) channels retained high ION/IOFF more than 105. However, ION values were significantly lower. Corresponding scatter plots with ION measured at VBG = −15 V and ION/IOFF as the two axes for g) V-, h) Nb-, and i) Ta-doped FETs with thick and thin WSe2 channels. All FETs have a channel length (\({L}_{{CH}}\)) of 500 nm.
Extended Data Fig. 4 Band alignment of pristine and doped1L, 2L and 8L-MoSe2.
In this observation, it is evident that the introduction of V, Nb, and Ta doping results in a reduction of the band gap, and there is also a notable shift of the Fermi level below the valence band.
Extended Data Fig. 5 Field effect mobility (μFE) and 4-point probe mobility (μ4PP) for MoSe2 and WSe2 FETs.
Hole field-effect mobility (\({\mu }_{{FE}}\)), extracted from peak transconductance for a) thick channel and b) thin channel FETs based on V-, Nb-, and Ta-doped MoSe2 and for c) thick channel and d) thin channel FETs based on V-, Nb-, and Ta-doped WSe2. Bar plots showing the 4-point probe mobility (\({\mu }_{4{PP}}\)) extracted from Hall measurement for holes for e) MoSe2 and f) WSe2 specimens with dopants V, Nb, and Ta.
Extended Data Fig. 6 Layer by layer thinning of multilayer MoSe2 flake.
a) Optical images of MoSe2 flakes taken before and after successive treatment with mild oxygen plasma followed by DI water rinse to remove the top-layer of MoOx. The layer-by-layer thinning is evident from the contrast of the flakes that transitions from a deep brown hue to a lighter shade of brown. b) AFM images of the evolution of layer-by-layer thinning of MoSe2 flakes subjected to O2 plasma. Each plasma step is followed by a DI water dip before subsequent AFM measurements. The height profile of the flake along line 1 is also depicted and reveals a reduction of about 0.8 nm after each plasma step suggesting the removal of a single layer of MoSe2 flake.
Extended Data Fig. 7 As-fabricated and post-processed Nb-doped MoSe2 FETs.
Transfer characteristics of a) as-fabricated and b) post-processed Nb-doped MoSe2 FETs with \({L}_{{CH}}\) = 50 nm, 100 nm, 150 nm, 200 nm, and 500 nm. Three successive rounds of oxygen plasma treatment, followed by immersion in DI water, were carried out to reduce the thickness of the channel between the contacts while keeping the channel thicker underneath the contacts. c) Corresponding total resistance \({R}_{T}\) (normalized by width) measured at \({V}_{{BG}}\) = −15 V versus \({L}_{{CH}}\) for as-fabricated and post-processed Nb-doped MoSe2 FETs at \({V}_{{DS}}\) = 1 V.
Extended Data Fig. 8 Resistor network for carrier transport in our proposed 2D FET.
(a) A resistor network with essential resistance components associated with carrier transport in our proposed 2D FET structure. Schematic and corresponding band-diagrams for MoSe2 based 2D FETs with thick channels underneath the contacts and (b) multilayer, (c) bilayer and (d) monolayer channels between the contacts. The bandgap mismatch between thick and thin MoSe2 layers give rise to an additional energy barrier at the interface between the thinner channel and thicker contact regions. This energy barrier remains relatively small for channels over 2 layers, as the MoSe2 bandgap stays nearly constant. However, the barrier height can increase markedly when the channel becomes monolayer, due to the transition from indirect to direct bandgap between bi-layer and monolayer. This alteration can lead to a considerable increase in RC. (e) Transfer characteristics of a multilayer Nb-doped MoSe2 FET after each etch cycle, and (f) corresponding bar plot of the \({R}_{C}\) values obtained from TLM measurements.
Extended Data Fig. 9 Extraction of Schottky barrier (ϕSB-P) heights for doped MoSe2 and WSe2 FETs and temperature dependence of output characteristics for Nb-doped MoSe2 DGFETs.
\({\phi }_{{SB}-P}\) values were obtained for pristine, V-, Nb-, and Ta-doped a) MoSe2 and b) WSe2 FETs. c-d) Schematic of band diagrams for intrinsic and heavily p-type doped semiconductors in contact with a metal. e) Band diagram portraying the broadening of the Fermi-Dirac tail with increasing temperature, providing additional holes for tunnelling through the narrower region of the Schottky barrier. The enhanced tunnelling of holes results in a Ohmic like characteristics at higher temperatures. Output characteristics measured at a constant VBG of −8 V as the temperature is increased from 75 K to 300 K. The output characteristics transitioned from mostly Schottky type to nearly Ohmic-like.
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Das, M., Sen, D., Sakib, N.U. et al. High-performance p-type field-effect transistors using substitutional doping and thickness control of two-dimensional materials. Nat Electron 8, 24–35 (2025). https://doi.org/10.1038/s41928-024-01265-2
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DOI: https://doi.org/10.1038/s41928-024-01265-2
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