  Two-dimensional (2D) semiconductors have been suggested both for
ultimately-scaled field-effect transistors (FETs) and More-than-Moore
nanoelectronics. However, these targets can not be reached without accompanying
gate insulators which are scalable into the nanometer regime. Despite the
considerable progress in the search for channel materials with high mobilities
and decent bandgaps, finding high-quality insulators compatible with 2D
technologies has remained a challenge. Typically used oxides (e.g. SiO2, Al2O3
and HfO2) are amorphous when scaled, while two-dimensional hBN exhibits
excessive gate leakages. To overcome this bottleneck, we extend the natural
stacking properties of 2D heterostructures to epitaxial fluorite (CaF2), which
forms a quasi van der Waals interface with 2D semiconductors. We report
scalable single-layer MoS2 FETs with a crystalline CaF2 insulator of about 2 nm
thickness, which corresponds to an equivalent oxide thickness of less than 1
nm. While meeting the stringent requirements of low leakage currents, our
devices exhibit highly competitive performance and record-small hysteresis. As
such, our results present a breakthrough for very large scale integration
towards commercially competitive nano-electronic devices.
