S. E. Tyaginov

Transient-current measurement of the trap charge density at interfaces of a thin-film metal∕ferroelectric∕metal structure

A method providing estimation of the trap density at metal∕ferroelectric interfaces of a depleted ferroelectric film located between back-to-back Schottky barriers has been developed. The method is based on the recharge of interface traps induced by external bias pulse applied to the metal∕ferroelectric∕metal structure. It is shown that the transient current under bias pulse can be controlled by the trap recharge on the reverse-biased interface. Using the method, the trap charge density on interfaces of the Pt∕PZT∕Ir(Ti∕SiO2∕Si) capacitor was found from transient-current measurements to be two orders of magnitude less than the remnant polarization of PbZrxTi1−xO3 film.

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.

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.

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.

Application of an MOS tunnel transistor for measurements of the tunneling parameters and of the parameters of electron energy relaxation in silicon.

Abstract Measurements of some barrier parameters – thickness, hole effective mass – in the Al/SiO2/Si system, are carried out on the Metal-Oxide-Semiconductor (MOS) tunnel emitter transistors. For the high-doping case, the effect of resonant carrier transport via discrete levels in a quantum well of the depletion layer is considered and used for thickness estimation. Degradation of an MOS emitter is paid attention to. Hot electron injection related phenomena in silicon, namely electron–hole pair generation and photon emission, are quantitatively studied.

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.

A MOS tunnel emitter transistor as a tool for determining the effective hole mass in a thin film of SiO2

The effective mass of holes in a tunnel-thin (2–3 nm) SiO2 layer was experimentally determined: mh = (0.32–0.33)m0. The use of this value enables an adequate simulation of a direct-tunneling hole current in MOS devices. The mathematical processing of characteristics of a MOS tunnel emitter transistor has been applied for the first time in the effective mass determination; this method makes it possible to obtain the precise value of the effective thickness of the oxide, because the electron effective mass for SiO2 is well known from literature. The calculations were performed under the assumption that the probability of tunneling across the barrier depends only on the Ez component of the particle energy, related to the motion in the direction of tunneling.

Current-voltage characteristics of Al/SiO2/p-Si MOS tunnel diodes with a spatially nonuniform oxide thickness

The effect of nonuniform distribution of the insulator thickness on the behavior of Al/SiO2/p-Si MOS tunnel structures with a (1–4)-nm-thick insulator is studied. The character and magnitude of the effect depend on the applied bias. In any conditions, the nonuniformity of the SiO2 thickness enhances the total through currents as compared to those flowing across a uniform oxide layer of the same nominal thickness. Further, the potential of the inversion layer changes in the inversion mode. The calculations performed take into account the tunnel transport between the Si conduction band and the metal, that between the Si valence band and the metal (including in the inversion mode, the resonant transport, which is less clearly pronounced because of the thickness nonuniformity), and the band-to-band tunneling in the semiconductor.

Influence of insulator thickness nonuniformity on the switching of the Al/SiO2/n-Si tunnel MOS structure at reverse bias

Current-voltage characteristics of the reversely biased Al/SiO2/n-Si MOS structure are calculated taking into account the nonuniformity of oxide thickness distribution over an area at a nominal thickness of 1–3 nm. It is known that the characteristics are S-shaped in a certain range of average SiO2 thickness, which suggests that a device is bistable. Holding and threshold voltage shifts, caused by statistical thickness variations, were predicted. In response to electrical stress, the root-mean-square deviation of the SiO2 thickness increases, which results in a shift of the threshold voltage to higher values. The calculations are complemented by experimental data.

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).