Dielectric Properties of Ultrathin CaF₂ Ionic Crystals

Abstract

Mechanically exfoliated 2D hexagonal boron nitride (h‐BN) is currently the preferred dielectric material to interact with graphene and 2D transition metal dichalcogenides in nanoelectronic devices, as they form a clean van der Waals interface. However, h‐BN has a low dielectric constant (≈3.9), which in ultrascaled devices results in high leakage current and premature dielectric breakdown. Furthermore, the synthesis of h‐BN using scalable methods, such as chemical vapor deposition, requires very high temperatures (>900 °C) , and the resulting h‐BN stacks contain abundant few‐atoms‐wide amorphous regions that decrease its homogeneity and dielectric strength. Here it is shown that ultrathin calcium fluoride (CaF2) ionic crystals could be an excellent solution to mitigate these problems. By applying >3000 ramped voltage stresses and several current maps at different locations of the samples via conductive atomic force microscopy, it is statistically demonstrated that ultrathin CaF2 shows much better dielectric performance (i.e., homogeneity, leakage current, and dielectric strength) than SiO2, TiO2, and h‐BN. The main reason behind this behavior is that the cubic crystalline structure of CaF2 is continuous and free of defects over large regions, which prevents the formation of electrically weak spots.

Variability and Yield in h‐BN‐Based Memristive Circuits: The Role of Each Type of Defect

“…To find out which types of defects are responsible for the RS in the Au/h-BN/Au memristors, we scan the surface of the h-BN films (on their Cu growth substrate) with the tip of a conductive AFM (i.e., CAFM) by applying a low voltage of 0.2 V. The simultaneously collected topographic and current maps (Figure 4a,b) indicate that the leakage current flows preferentially across three sites: i) h-BN grown on Cu GB, i.e., junction between Cu grains; ii) GBs within the polycrystalline h-BN film (i.e., junction between h-BN grains); and iii) random locations within the h-BN grains. The higher currents registered with the CAFM at those locations are related to defective atomic bonding [26] (i.e., missing atoms, pentagonal and heptagonal bonding, interlayer bonding, impurities, strained bonding, twin boundaries [27,28] ). By collecting small-size (1 µm × 1 µm) CAFM current maps inside the h-BN grains we can determine that the density of conductive spots distortions is ≈250 spots µm −2 , although interdefect distances down to 2 nm have been observed in TEM images.…”

Calcium fluoride as high-k dielectric for 2D electronics

“…The method used to clean the surface of the Si (111) substrate prior to the CaF 2 growth consisted of: (i) wet chemical processes to eliminate contaminants on the Si substrate using deionized water, methyl alcohol, trichlorethylene, nitric acid, hydrogen peroxide, ammonium hydroxide and hydrogen fluoride; (ii) thin oxide film formation in hydrogen chloride: hydrogen peroxide: de-ionized water (3:1:1) at 90 C for 10 min on the Si surface to avoid contamination; (iii) thermal desorption of a thin oxide film under ultra-high vacuum (UHV) by annealing the sample for 2 min at 1200 C. This method had been used previously 33 to clean the surface of Si (100) and Si (111) substrates prior to MBE synthesis of Si, and it leads to a low density of dislocations below 1000/cm 2 in the substrates and high quality epitaxial Si. The ultra-thin CaF 2 films synthesized by MBE on such clean surfaces 34 exhibited an excellent single-crystalline structure when analyzed via cross-sectional TEM [see Fig. smooth transitions from one pattern to another were always observed, indicating the absence of defect clusters or grain boundaries.…”

A First-Principles Study on the Electronic, Thermodynamic and Dielectric Properties of Monolayer Ca(OH)2 and Mg(OH)2

“…In this second work, a channel with HfNCl as a gate dielectric was predicted to be the best combination for a p -MOS transistor. Furthermore, there have also been experimental efforts attempting to resolve the issue, as recently reported in Nature Electronics regarding CaF and Bi SeO [ 39 , 40 , 41 ].…”