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Ultrathin calcium fluoride insulators for two-dimensional field-effect transistors

Abstract

Two-dimensional semiconductors could be used to fabricate ultimately scaled field-effect transistors and more-than-Moore nanoelectronic devices. However, these targets cannot be reached without appropriate gate insulators that are scalable to the nanometre range. Typically used oxides such as SiO2, Al2O3 and HfO2 are, however, amorphous when scaled, and 2D hexagonal boron nitride exhibits excessive gate leakage currents. Here, we show that epitaxial calcium fluoride (CaF2), which can form a quasi van der Waals interface with 2D semiconductors, can serve as an ultrathin gate insulator for 2D devices. We fabricate scalable bilayer MoS2 field-effect transistors with a crystalline CaF2 insulator of ~2 nm thickness, which corresponds to an equivalent oxide thickness of less than 1 nm. Our devices exhibit low leakage currents and competitive device performance characteristics, including subthreshold swings down to 90 mV dec−1, on/off current ratios up to 107 and a small hysteresis.

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Fig. 1: Bilayer MoS2 FETs with 2-nm-thick CaF2 insulators.
Fig. 2: Device-to-device variability.
Fig. 3: Best transistor performance achieved for our CaF2/MoS2 FETs.
Fig. 4: Hysteresis in our CaF2/MoS2 FETs.

Data availability

The data that support the graphs within this Article and further details of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

The authors acknowledge financial support through the Austrian Science Fund FWF grant no. I2606-N30. T.M., D.K.P. and S.W. acknowledge financial support by the Austrian Science Fund FWF (START Y 539-N16) and the European Union (grant agreement no. 785219 Graphene Flagship). This work was partly supported by the Russian Foundation for Basic Research (grant no. 18-57-80006 BRICS_t). We also gratefully acknowledge useful discussions with M. Jech and technical assistance from B. Stampfer. M.P. acknowledges financial support from the doctoral college programme TU-D funded by TU Wien. Y.Y.I. is a member of the Mediterranean Institute of Fundamental Physics (MIFP).

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Y.Y.I. introduced the idea of MoS2 FETs with an ultrathin CaF2 insulator, performed their characterization and prepared the manuscript. A.G.B. performed MBE growth of CaF2 and provided the substrates. D.K.P. and S.W. fabricated MoS2 FETs. T.K. performed TCAD simulations. M.T. contributed to preparation of figures. L.M. and M.P. performed SHG and Raman measurements, respectively. M.S.-P. and A.S.-T. performed TEM measurements and sample preparation, respectively. M.I.V. performed quantitative analysis of gate leakage currents using tunnel models. M.W. programmed electrical measurements. N.S.S., T.M. and T.G. supervised this work. All authors regularly discussed the results and commented on the manuscript.

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Correspondence to Yury Yu. Illarionov or Tibor Grasser.

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Illarionov, Y.Y., Banshchikov, A.G., Polyushkin, D.K. et al. Ultrathin calcium fluoride insulators for two-dimensional field-effect transistors. Nat Electron 2, 230–235 (2019). https://doi.org/10.1038/s41928-019-0256-8

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