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Graphene is a promising material for applications as a channel in graphene field-effect 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 SiO2 and Al2O3, however, typically suffer from oxide dangling bonds at the interface, high surface roughness and numerous border oxide traps. In order to address these challenges, here we use 2 nm thick epitaxial CaF2 as a gate insulator in GFETs. By analyzing device-to-device variability for about 200 devices fabricated in two batches, we find that tens of them show similar gate transfer characteristics. Our statistical analysis of the hysteresis up to 175oC has revealed that while an ambient-sensitive counterclockwise hysteresis can be present in some devices, the dominant mechanism is thermally activated charge trapping by border defects in CaF2 which results in the conventional clockwise hysteresis. We demonstrate that both the hysteresis and bias-temperature instabilities in our GFETs with CaF2 are comparable to similar devices with SiO2 and Al2O3. In particular, we achieve a small hysteresis below 0.01 V for equivalent oxide thickness (EOT) of about 1 nm at the electric fields up to 15 MV cm−1 and sweep times in the kilosecond range. Thus, our results demonstrate that crystalline CaF2 is a promising insulator for highly-stable GFETs. [ABSTRACT FROM AUTHOR]

The effect of enhancement of the electron concentration in the near-surface quantum well owing to a resonant tunneling between the semiconductor and the metal is analyzed theoretically. The range of parameters (acceptor concentration and insulator thickness) wherein such an electron-supplying mechanism becomes plausible is defined. Generally, the supply effect can be expected for a doping between (5-6)×1018 and (2-3)×1019 cm-3, e.g., for the SiO2/p+-Si(1019 cm-3) structures with the oxide thickness exceeding ~3 nm. The electron density provided due to a resonant tunneling can reach rather high values (~1012 cm-2 and more). Consequently, the quasi-Fermi level in the well is substantially shifted approaching that of the semiconductor bulk. However, a deformation of the band diagram due to electron charge in the quantum well in the regimes close to activation of a resonant transport is small. [ABSTRACT FROM AUTHOR]

Metal/CaF2/p-Si(111) capacitors with the improved-quality several-nanometer-thick epitaxial fluorite films are examined, aiming at solidifying a candidacy of this material for barrier layers in silicon devices. Structural and transport properties of a thin crystalline dielectric are characterized by different experimental techniques. The measured current-voltage characteristics accompanied with simulation results demonstrate that the elastic tunneling electron injection takes place in the considered structures. The same result follows from the behavior of hot-electron-injection-related electroluminescence within the selected spectral intervals. The result is important considering a perspective of using the epitaxial fluorides as barrier layers in resonant tunneling diodes. [ABSTRACT FROM AUTHOR]

Voltage distribution in a spherically symmetrical metal-insulator-semiconductor structure with a tunnel-thin insulating layer has been analyzed using the proposed analytic model and compared with that in a regular planar structure. In many electrostatic respects, the spherical system was shown to behave like a planar one with higher than specified semiconductor doping level. Special attention is paid to the reverse bias polarity. With a decrease of the insulator-semiconductor interface radius, for a fixed terminal bias, the insulator voltage becomes higher and the band bending smaller. The geometry effects are revealed for the radii below 100-200 nm, i.e., by far not in the extreme quantum limit. The elaborated model is useful for treatment of non-planar metal-insulator-semiconductor structures in the cases when the consideration of an exact topology is problematic or too speculative. [ABSTRACT FROM AUTHOR]

Au/CaF2/nSi(111) 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 mF∼1.2m0 for carriers in the forbidden band of CaF2 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 me=1.0m0, 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. [ABSTRACT FROM AUTHOR]

We present both theoretical and experimental results of our investigation into hot-electron luminescence in p-Si tunnel metal-oxide-semiconductor structures. The luminescence spectra of such structures contrast considerably with those of other silicon-based devices due to the fundamentally different method of charge injection, and we discuss several of their interesting features. The potential for direct modulation of low-intensity, low-efficiency infrared emission between 1.24 and 1.6 μm is also demonstrated. Additionally, we investigate the role of self-heating, the influence of crystal axis orientation, and the possibility of anisotropy, and find the effect of each to be relatively small. [ABSTRACT FROM AUTHOR]

Bidirectional electric stresses are performed in metal—oxide—silicon capacitors with ultrathin (2.5 nm) oxides and with highly doped substrates. The observed effects are compared to those occurring under the conventional unidirectional constant-current overload. The effects observed after stress of ultrathin oxides bring into play defects of comparable nature to those noted for thick oxides, namely, neutral traps. Furthermore, soft breakdown events are scrutinized based on defect creation throughout the oxide and modulation of charge state trapping at the interfaces. [ABSTRACT FROM AUTHOR]

The effect of increasing the tunnel current in a metal-calcium fluoride-silicon structure with addition of a silicon dioxide layer between fluoride and metal (which seems paradoxical at first glance) has been considered. This effect of nonmonotonic change in the tunnel conductivity with an increase in the insulator thickness may occur at a relatively high bias at the structure and is related to the tunnel-barrier deformation, at which electrons are tunneling through its part formed by the oxide. At low biases, the occurrence/ thickening of an additional layer leads to a natural decrease in the current. Similar behavior is possible in principle for some other combinations of materials. [ABSTRACT FROM AUTHOR]