Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

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


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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

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.


  1. 1.

    Guerriero, E. et al. High-gain graphene transistors with a thin AlOx top-gate oxide. Sci. Rep. 7, 2419 (2017).

    Article  Google Scholar 

  2. 2.

    Tao, L. et al. Silicene field-effect transistors operating at room temperature. Nat. Nanotechnol. 10, 227–231 (2015).

    Article  Google Scholar 

  3. 3.

    Li, L. et al. Black phosphorus field-effect transistors. Nat. Nanotechnol. 9, 372–377 (2014).

    Article  Google Scholar 

  4. 4.

    Chen, X. et al. Large-velocity saturation in thin-film black phosphorus transistors. ACS Nano 12, 5003–5010 (2018).

    Article  Google Scholar 

  5. 5.

    Kang, J., Liu, W. & Banerjee, K. High-performance MoS2 transistors with low resistance molybdenum contacts. Appl. Phys. Lett. 104, 093106 (2014).

    Article  Google Scholar 

  6. 6.

    Chuang, S. et al. MoS2 p-type transistors and diodes enabled by high work function MoOx contacts. Nano Lett. 14, 1337–1342 (2014).

    Article  Google Scholar 

  7. 7.

    Ganapathi, K., Bhattacharjee, S., Mohan, S. & Bhat, N. High-performance HfO2 back gated multilayer MoS2 transistors. IEEE Electron Dev. Lett. 37, 797–800 (2016).

    Google Scholar 

  8. 8.

    Illarionov, Y. et al. Improved hysteresis and reliability of MoS2 transistors with high-quality CVD growth and Al2O3 encapsulation. IEEE Electron Dev. Lett. 38, 1763–1766 (2017).

    Article  Google Scholar 

  9. 9.

    Smithe, K., Suryavanshi, S., Munoz-Rojo, M., Tedjarati, A. & Pop, E. Low variability in synthetic monolayer MoS2 devices. ACS Nano 11, 8456–8463 (2017).

    Article  Google Scholar 

  10. 10.

    Bolshakov, P. et al. Electrical characterization of top-gated molybdenum disulfide field-effect-transistors with high-k dielectrics. Microelectron. Eng. 178, 190–193 (2017).

    Article  Google Scholar 

  11. 11.

    Liao, W., Wei, W., Tong, Y., Chim, W. K. & Zhu, C. Electrical performance and low frequency noise in hexagonal boron nitride encapsulated MoSe2 dual-gated field effect transistors. Appl. Phys. Lett. 111, 082105 (2017).

    Article  Google Scholar 

  12. 12.

    Cho, Y. et al. Fully transparent p-MoTe2 2D transistors using ultrathin MoOx/Pt contact media for indium-tin-oxide source/drain. Adv. Funct. Mater. 28, 1801204 (2018).

    Article  Google Scholar 

  13. 13.

    Yang, L. et al. Chloride molecular doping technique on 2D materials: WS2 and MoS2. Nano Lett. 14, 6275–6280 (2014).

    Article  Google Scholar 

  14. 14.

    Liu, W. et al. Role of metal contacts in designing high-performance monolayer n-type WSe2 field effect transistors. Nano Lett. 13, 1983–1990 (2013).

    Article  Google Scholar 

  15. 15.

    Prakash, A. & Appenzeller, J. Bandgap extraction and device analysis of ionic liquid gated WSe2 Schottky barrier transistors. ACS Nano 11, 1626–1632 (2017).

    Article  Google Scholar 

  16. 16.

    Bolshakov, P. et al. Improvement in top-gate MoS2 transistor performance due to high quality backside Al2O3 layer. Appl. Phys. Lett. 111, 032110 (2017).

    Article  Google Scholar 

  17. 17.

    Wang, H. et al. Integrated circuits based on bilayer MoS2 transistors. Nano Lett. 12, 4674–4680 (2012).

    Article  Google Scholar 

  18. 18.

    Das, T. et al. Highly flexible hybrid CMOS inverter based on Si nanomembrane and molybdenum disulfide. Small 12, 5720–5727 (2016).

    Article  Google Scholar 

  19. 19.

    Wachter, S., Polyushkin, D., Bethge, O. & Mueller, T. A microprocessor based on a two-dimensional semiconductor. Nat. Commun. 8, 14948 (2017).

    Article  Google Scholar 

  20. 20.

    Xie, L. et al. Graphene-contacted ultrashort channel monolayer MoS2 transistors. Adv. Mater. 29, 1702522 (2017).

    Article  Google Scholar 

  21. 21.

    Hui, F. et al. On the use of two dimensional hexagonal boron nitride as dielectric. Microelectron. Eng. 163, 119–133 (2016).

    Article  Google Scholar 

  22. 22.

    Cassabois, G., Valvin, P. & Gil, B. Hexagonal boron nitride is an indirect bandgap semiconductor. Nat. Photon. 10, 262–266 (2016).

    Article  Google Scholar 

  23. 23.

    Geick, R., Perry, C. & Rupprecht, G. Normal modes in hexagonal boron nitride. Phys. Rev. 146, 543–547 (1966).

    Article  Google Scholar 

  24. 24.

    Hayes, W. Crystals with the Fluorite Structure (Clarendon Press, 1974).

  25. 25.

    Sugiyama, M. & Oshima, M. MBE growth of fluorides. Microelectron. J. 27, 361–382 (1996).

    Article  Google Scholar 

  26. 26.

    Illarionov, Y., Vexler, M., Fedorov, V., Suturin, S. & Sokolov, N. Electrical and optical characterization of Au/CaF2/p-Si(111) tunnel-injection diodes. J. Appl. Phys. 115, 223706 (2014).

    Article  Google Scholar 

  27. 27.

    Foster, A., Trevethan, T. & Shluger, A. Structure and diffusion of intrinsic defects, adsorbed hydrogen and water molecules at the surface of alkali-earth fluorides calculated using density functional theory. Phys. Rev. B 80, 115421 (2009).

    Article  Google Scholar 

  28. 28.

    Koma, A., Saiki, K. & Sato, Y. Heteroepitaxy of a two-dimensional material on a three-dimensional material. Appl. Surf. Sci. 41, 451–456 (1990).

    Article  Google Scholar 

  29. 29.

    Vishwanath, S. et al. Comprehensive structural and optical characterization of MBE grown MoSe2 on graphite, CaF2 and graphene. 2D Mater. 2, 024007 (2015).

    Article  Google Scholar 

  30. 30.

    Vishwanath, S. et al. MBE growth of few-layer 2H-MoTe2 on 3D substrates. J. Cryst. Growth 482, 61–69 (2018).

    Article  Google Scholar 

  31. 31.

    Banshchikov, A. et al. Epitaxial layers of nickel fluoride on Si(111): growth and stabilization of the orthorhombic phase. Phys. Solid State 57, 1647–1652 (2015).

    Article  Google Scholar 

  32. 32.

    Kaveev, A. et al. Epitaxial growth on silicon and characterization of MnF2 and ZnF2 layers with metastable orthorhombic structure. J. Appl. Phys. 98, 013519 (2005).

    Article  Google Scholar 

  33. 33.

    Ravez, J. The inorganic fluoride and oxyfluoride ferroelectrics. J. Phys. III 7, 1129–1144 (1997).

    Google Scholar 

  34. 34.

    Si, M. et al. Steep-slope hysteresis-free negative capacitance MoS2 transistors. Nat. Nanotechnol. 13, 24–28 (2018).

    Article  Google Scholar 

  35. 35.

    Watanabe, M., Funayama, T., Teraji, T. & Sakamaki, N. CaF2/CdF2 double-barrier resonant tunneling diode with high room-temperature peak-to-valley ratio. Jpn J. Appl. Phys. 39, L716 (2000).

    Article  Google Scholar 

  36. 36.

    Suturin, S. et al. Optical detection of electron transfer through interfaces in CaF2: Eu–CdF2 SLs. Appl. Surf. Sci. 162, 474–478 (2000).

    Article  Google Scholar 

  37. 37.

    Smith, T. III, Phillips, J., Augustyniak, W. & Stiles, P. Fabrication of metal–epitaxial insulator–semiconductor field-effect transistors using molecular beam epitaxy of CaF2 on Si. Appl. Phys. Lett. 45, 907–909 (1984).

    Article  Google Scholar 

  38. 38.

    Tyaginov, S. et al. Modeling of deep-submicron silicon-based MISFETs with calcium fluoride dielectric. J. Comput. Electron. 13, 733–738 (2014).

    Article  Google Scholar 

  39. 39.

    Sokolov, N., Alvarez, J. & Yakovlev, N. Fluoride layers and superlattices grown by MBE on Si(111): dynamic RHEED and Sm2+ photoluminescence studies. Appl. Surf. Sci. 60, 421–425 (1992).

    Article  Google Scholar 

  40. 40.

    Dumcenco, D. et al. Large-area epitaxial monolayer MoS2. ACS Nano 9, 4611–4620 (2015).

    Article  Google Scholar 

  41. 41.

    Gurarslan, A. et al. Surface-energy-assisted perfect transfer of centimeter-scale monolayer and few-layer MoS2 films onto arbitrary substrates. ACS Nano 8, 11522–11528 (2014).

    Article  Google Scholar 

  42. 42.

    Jiang, N. On the oxidation of CaF2 in transmission electron microscope. Micron 43, 746–754 (2012).

    Article  Google Scholar 

  43. 43.

    Knobloch, T. et al. Impact of gate dielectrics on the threshold voltage in MoS2 transistors. ECS Trans. 80, 203–217 (2017).

    Article  Google Scholar 

  44. 44.

    Knobloch, T. et al. A physical model for the hysteresis in MoS2 transistors. IEEE J. Electron Dev. Soc. 6, 972–978 (2018).

    Article  Google Scholar 

  45. 45.

    Appenzeller, J., Zhang, F., Das, S. & Knoch, J. in 2D Materials for Nanoelectronics (eds Houssa, M. et al.) 207–234 (CRC Press, 2016).

  46. 46.

    Wang, J. et al. High mobility MoS2 transistor with low Schottky barrier contact by using atomic thick h-BN as a tunneling layer. Adv. Mater. 28, 8302–8308 (2016).

    Article  Google Scholar 

  47. 47.

    Xia, P. et al. Impact and origin of interface states in MOS capacitor with monolayer MoS2 and HfO2 high-k dielectric. Sci. Rep. 7, 40669 (2017).

    Article  Google Scholar 

  48. 48.

    Li, T., Wan, B., Du, G., Zhang, B. & Zeng, Z. Electrical performance of multilayer MoS2 transistors on high-κ Al2O3 coated Si substrates. AIP Adv. 5, 057102 (2015).

    Article  Google Scholar 

  49. 49.

    Illarionov, Y. et al. The role of charge trapping in MoS2/SiO2 and MoS2/hBN field-effect transistors. 2D Mater. 3, 035004 (2016).

    Article  Google Scholar 

  50. 50.

    Wen, M., Xu, J., Liu, L., Lai, P.-T. & Tang, W.-M. Effects of annealing on electrical performance of multilayer MoS2 transistors with atomic layer deposited HfO2 gate dielectric. Appl. Phys. Express 9, 095202 (2016).

    Article  Google Scholar 

  51. 51.

    Addou, R., Colombo, L. & Wallace, R. Surface defects on natural MoS2. ACS Appl. Mater. Interfaces 7, 11921–11929 (2015).

    Article  Google Scholar 

  52. 52.

    Leong, W. et al. Tuning the threshold voltage of MoS2 field-effect transistors via surface treatment. Nanoscale 7, 10823–10831 (2015).

    Article  Google Scholar 

  53. 53.

    Yu, Z., Zhang, Y.-W. & Yakobson, B. An anomalous formation pathway for dislocation-sulfur vacancy complexes in polycrystalline monolayer MoS2. Nano Lett. 15, 6855–6861 (2015).

    Article  Google Scholar 

  54. 54.

    Di Bartolomeo, A. et al. Hysteresis in the transfer characteristics of MoS2 transistors. 2D Mater. 5, 015014 (2017).

    Article  Google Scholar 

  55. 55.

    Kaushik, N. et al. Reversible hysteresis inversion in MoS2 field effect transistors. npj 2D Mater. Appl. 1, 34 (2017).

    Article  Google Scholar 

  56. 56.

    Illarionov, Y. et al. Energetic mapping of oxide traps in MoS2 field-effect transistors. 2D Mater. 4, 025108 (2017).

    Article  Google Scholar 

  57. 57.

    Ishizaka, A. & Shiraki, Y. Low temperature surface cleaning of silicon and its application to silicon MBE. J. Electrochem. Soc. 133, 666–671 (1986).

    Article  Google Scholar 

  58. 58.

    Binder, T. et al. MINIMOS-NT User’s Guide (Institut für Mikroelektronik, 1998).

  59. 59.

    Das, S., Chen, H., Penumatcha, A. & Appenzeller, J. High performance multilayer MoS2 transistors with scandium contacts. Nano Lett. 13, 100–105 (2012).

    Article  Google Scholar 

  60. 60.

    Allain, A., Kang, J., Banerjee, K. & Kis, A. Electrical contacts to two-dimensional semiconductors. Nat. Mater. 14, 1195–1205 (2015).

    Article  Google Scholar 

Download references


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).

Author information




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.

Corresponding authors

Correspondence to Yury Yu. Illarionov or Tibor Grasser.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary sections 1–8 and Supplementary Figs. 1–14.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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).

Download citation

Further reading


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing