M. I. Vexler

Electrical characterization and modeling of the Au/CaF2/nSi(111) structures with high-quality tunnel-thin fluoride layer

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.

Electrical phenomena in a metal/nanooxide/p+-silicon structure during its transformation to a resonant-tunneling diode

To investigate and develop novel silicon-based electronic components, the electro-physical effects in a metal-insulator-semiconductor (MIS) structure with nanometer size parameters, gained by enhancement of the silicon doping level up to NA ∼ 1019 cm−3 and reduction of the oxide thickness down to 0.4–4.0 nm, have been studied. As a result of such changes, the MIS nanostructure satisfies necessary and sufficient conditions for the electron resonant tunneling that can be observed at relatively low (some volts) reverse biases. Thereby a MIS capacitor can be transformed into a resonant-tunneling diode with substantial extension of its properties and functions.

Static current-voltage characteristics of Au/CaF2/n-Si(111) MIS tunneling structures

Using molecular-beam epitaxy, Au/CaF2/n-Si(111) structures were fabricated that exhibit lower currents at a given fluoride film thickness (1.5–2 nm) than those of all similar structures studied. At a positive voltage at the metal, the current is in agreement with that calculated within the model with conservation of the transverse component of the wave vector during tunneling transport. Relative contributions of electron and hole components were analyzed for forward and reverse biases. The effect of the nonuniform distribution of the insulator thickness over the area on measured currents was estimated. The thin CaF2 layers that were grown are potentially applicable as barrier layers in various devices of functional electronics.

Electrical and optical characterization of Au/CaF2/p-Si(111) tunnel-injection diodes

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.

Resonant electron tunneling and related charging phenomena in metal–oxide– p + -Si nanostructures

The j–V characteristics of the Al/thermal or electrochemical SiO2(2–4 nm)/heavily doped p+-Si nanostructures operating as a resonant-tunneling diode were measured and theoretically analyzed. The characteristics have specific features in the form of current steps and peaks, which are caused by electron transport between the silicon valence band and metal through discrete levels of the quantum well formed by the p+-Si conduction band and SiO2/p+-Si interface. Resonant tunneling through the surface state levels and the appearance of a charge near this interface under certain conditions are discussed.

Features of carrier tunneling between the silicon valence band and metal in devices based on the Al/high-K oxide/SiO2/Si structure

The features of electron tunneling from or into the silicon valence band in a metal–insulator–semiconductor system with the HfO2(ZrO2)/SiO2 double-layer insulator are theoretically analyzed for different modes. It is demonstrated that the valence-band current plays a less important role in structures with HfO2(ZrO2)/SiO2 than in structures containing only silicon dioxide. In the case of a very wide-gap high-K oxide ZrO2, nonmonotonic behavior related to tunneling through the upper barrier is predicted for the valence-band–metal current component. The use of an insulator stack can offer certain advantages for some devices, including diodes, bipolar tunnel-emitter transistors, and resonant-tunneling diodes, along with the traditional use of high-K insulators in a field-effect transistor.

Determination of correlation length for thickness fluctuations in thin oxide and fluoride films

A novel technique for experimental estimation of the correlation length of insulator thickness fluctuations is proposed which is based on the statistical treatment of the results of current measurements for a random set of thin metal–insulator–semiconductor (MIS) capacitors. For testing purposes, the usual Al/SiO2/Si tunnel diodes with excessive thickness dispersion, as well as the less popular but potentially interesting Au/CaF2/Si structures, are taken. The verification is performed by a direct comparison of correlation lengths yielded by the new method with those obtained by diagnostics of the same dielectric films using atomic force microscopy.

High insulating quality CaF2 pseudomorphic films on Si(111)

Current-voltage characteristics of epitaxially grown Au∕CaF2∕Si(111) metal-insulator-semiconductor structures with thin (1.5–6nm) pseudomorphic fluoride layer have been studied. It was found that CaF2 films in these structures are of better insulating quality than those in the devices reported previously. Typical breakdown field for the fluorite layers was about 8×106V∕cm and the tunnel current did not exceed the values predicted by simulations with realistic parameters.

Quantum-well charge and voltage distribution in a metal–insulator–semiconductor structure upon resonant electron Tunneling

The prerequisites for electron storage in the quantum well of a metal–oxide–p+-Si resonant-tunneling structure and the effect of the stored charge on the voltage distribution are theoretically investigated. Systems with SiO2, HfO2, and TiO2 insulators are studied. It is demonstrated that the occurrence of a charge in the well in the case of resonant transport can be expected in structures on substrates with an acceptor concentration from (5–6) × 1018 to (2–3) × 1019 cm–3 in the range of oxide thicknesses dependent on this concentration. In particular, the oxide layer thickness in the structures with SiO2/p+-Si(1019 cm–3) should exceed ~3 nm. The electron density in the well can reach ~1012 cm–2 and higher. However, the effect of this charge on the electrostatics of the structure becomes noticeable only at relatively high voltages far above the activation of resonant transport through the first subband.

Adaptation of the model of tunneling in a metal/CaF 2 /Si(111) system for use in industrial simulators of MIS devices

An approach toward simplification of the model of the tunneling transport of electrons through a thin layer of crystalline calcium fluoride into a silicon (111) substrate with subsequent implementation in simulators of semiconductor devices is suggested. The validity of the approach is proven by comparing the results of modeling using simplified formulas with the results of precise calculations and experimental data. The approach can be applied to calculations of tunneling currents in structures with any crystalline insulators on Si (111).