No 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.

Thin (4–20 nm) yttrium iron garnet (Y3Fe5O12, YIG) layers have been grown on gadolinium gallium garnet (Gd3Ga5O12, GGG) 111-oriented substrates by laser molecular beam epitaxy in 700–1000 °C growth temperature range. The layers were found to have atomically flat step-and-terrace surface morphology with step height of 1.8 Å characteristic for YIG(111) surface. As the growth temperature is increased from 700 to 1000 °C the terraces become wider and the growth gradually changes from layer by layer to step-flow regime. Crystal structure studied by electron and X-ray diffraction showed that YIG lattice is co-oriented and laterally pseudomorphic to GGG with small rhombohedral distortion present perpendicular to the surface. Measurements of magnetic moment, magneto-optical polar and longitudinal Kerr effect (MOKE), and X-ray magnetic circular dichroism (XMCD) were used for study of magnetization reversal for different orientations of magnetic field. These methods and ferromagnetic resonance studies have shown that in zero magnetic field magnetization lies in the film plane due to both shape and induced anisotropies. Vectorial MOKE studies have revealed the presence of an in-plane easy magnetization axis. In-plane magnetization reversal was shown to occur through combination of reversible rotation and abrupt irreversible magnetization jump, the latter caused by domain wall nucleation and propagation. The field at which the flip takes place depends on the angle between the applied magnetic field and the easy magnetization axis and can be described by the modified Stoner–Wohlfarth model taking into account magnetic field dependence of the domain wall energy. Magnetization curves of individual tetrahedral and octahedral magnetic Fe3+ sublattices were studied by XMCD.

Graphene is a promising material for applications as a channel in graphene fieldeffect transistors (GFETs) which may be used as a building block for optoelectronics, high-frequency devices and sensors. However, these devices require gate insulators which ideally should form atomically flat interfaces with graphene and at the same time contain small densities of traps to maintain high device stability. Previously used amorphous oxides, such as SiO 2 and Al 2 O 3 , however, typically suffer from oxide

Au/ CaF 2 / nSi͑111͒ structures with 4-5 monolayers of epitaxial fluoride are fabricated and electrically tested. The leakage current in these structures was substantially smaller than in similar samples reported previously. Simulations adopting a Franz-type dispersion relation with Franz mass of m F ϳ 1.2m 0 for carriers in the forbidden band of CaF 2 reproduced the measured current-voltage curves quite satisfactorily. Roughly, these curves could also be reproduced using the parabolic dispersion law with the electron mass of m e = 1.0m 0 , which is a material constant rather than a fitting parameter. Experimental facts and their comparison to modeling results allow qualification of the crystalline quality of fabricated structures as sufficient for device applications.