Calcium fluoride as high-k dielectric for 2D electronics

Abstract

Calcium fluoride is a dielectric material with a wide bandgap ($12.1 eV) and a relatively high dielectric constant ($6.8) that forms a van der Waals interface with two-dimensional (2D) materials, meaning that it contains a very low amount of defects. Thin calcium fluoride films can be synthesized using multiple techniques that are scalable to the wafer level, including molecular beam epitaxy, atomic layer deposition, and chemical vapor deposition. However, the consolidation of calcium fluoride as dielectric for 2D electronics requires overcoming some fundamental challenges related to material quality and integration, as well as carrying out advanced characterization and computational studies to evaluate its real potential. Here, we review the status of calcium fluoride dielectric films in terms of material synthesis, fundamental electrical properties, and future applications; we also discuss the most important challenges of calcium fluoride integration in 2D materials-based, solid-state nano/micro-electronic devices, and propose several potential routes to overcome them. Our manuscript may serve as a useful guide for other scientists working on 2D electronics in general, and provides a clear pathway for calcium fluoride research in the future.

Few-Layered MnAl2S4 Dielectrics for High-Performance van der Waals Stacked Transistors

“…Among those explored candidates, the hexagonal boron nitride (h-BN) is the most widely used dielectric in building 2D FETs. , However, h-BN exhibits a relatively low dielectric constant (ε r ∼ 3.9), resulting in a weaker electrostatic control to the channel and a higher operation voltage than those of high-κ dielectrics. , Atomically thin 2D transition metal oxides (e.g., TiO 2 and others) have also been synthesized via liquid exfoliation or a direct oxidation process. − Although the obtained nanosheets exhibit large dielectric constants (ε r > 100), they usually have small sizes or lack crystallinity control. Recently, a mechanical exfoliation of 2D insulators from layered vdW crystals (e.g., α-MoO 3 and VOCl) , or a direct synthesis of large scale nonlayered high-κ 2D insulators/ionic crystals (e.g., antimony oxides and CaF 2 ) are also reported, − while a complete understanding of their properties is still at an early stage and requires more investigations.…”

Van der Waals‐Interface‐Dominated All‐2D Electronics

“…[115] As an insulator with a dielectric constant of 8.43 and a dielectric strength of 20.3 ± 0.9 MV cm −1 , CaF 2 can realize epitaxial growth of 2D channel materials on the surface and has good compatibility with 2D materials. [116] Mueller and co-workers fabricated CaF 2 with a thickness of ≈2 nm by MBE and constructed the FETs with the bottom CaF 2 gate by transferring CVD-grown MoS 2 . [115] Transmission electron microscopy (TEM) shows that CaF 2 and MoS 2 form an almost defect-free vdW interface, as shown in Figure 8b.…”

Controllable Oxidation of ZrS2 to Prepare High‐κ, Single‐Crystal m‐ZrO2 for 2D Electronics

“…The performance of our dielectric materials versus existing dielectric materials is summarized in Figure 4d, from which we can see that our m-ZrO 2 shows superior performance. [11,12,14,15,[37][38][39][40]…”