Abstract
Two-dimensional (2D) semiconductors are currently considered a very promising alternative to Si for channel applications in next-generation field-effect transistors of sub-5 nm designs. However, their huge potential cannot be fully exploited owing to a lack of competitive insulators which are required to effectively separate the channel from the gate, while being scalable down to few nanometers thicknesses. Recently we have made an attempt at addressing this issue by using crystalline CaF2 insulators and demonstrated competitive MoS2 devices with the insulator thickness of only about 2 nm. Here we report a detailed study of the performance, reliability and thermal stability of these devices. We demonstrate that, in contrast to SiO2 and other amorphous oxides, CaF2 has a very low density of insulator defects which are responsible for the hysteresis and long-term drifts of the transistor characteristics. At the same time, CaF2 exhibits smaller leakage currents and higher electric stability compared to hBN. By comparing our MoS2 transistors with CaF2 fabricated using MoS2 films of different quality, we show that the major limitations on the performance and reliability of these devices come from the bare channel rather than from the superior CaF2 insulator. Finally, we perform the first study of degradation mechanisms only observed for tunnel-thin gate insulators in 2D devices. While these degradation mechanisms are similar for hBN and CaF2, they are less pronounced in CaF2. Our results therefore present a solution to a very important roadblock on the way towards fully scalable 2D nanoelectronics with competitive performance and reliability.