Crystalline Calcium Fluoride: A Record-Thin Insulator for Nanoscale 2D Electronics

Two-dimensional (2D) electronics can enable FETs down to a few nanometers. However, these devices require scalable insulators which should form high-quality interfaces with 2D channels and maintain low gate leakage currents for sub-1nm equivalent oxide thickness (EOT). Previously used amorphous oxides result in poor interfaces with 2D materials, while hBN has mediocre dielectric properties ( ε < 5, E g = 6eV) [1] . As a promising alternative, we suggest the use of the crystalline ionic insulator CaF 2 ( ε = 8.43, E g = 12.1eV) which forms van der Waals interfaces with 2D semiconductors [2] . At the moment, CaF 2 can be grown by molecular-beam epitaxy (MBE) down to a few nanometers thickness [3] and appears promising for chemical vapour deposition (CVD) [4] and atomic-layer deposition (ALD) [5] . Here we discuss our recent progress [3] , [6] , [7] on ultra-thin CaF 2 which presents a universal platform for 2D devices. In particular, we demonstrate nanoscale MoS 2 FETs with L =50-60nm and a record-thin ~ 2nm CaF 2 insulator (EOT~ 0.9nm) which exhibits near-ideal subthreshold swing (SS).