Theory of superfast fronts of impact ionization in semiconductor structures

We present an analytical theory for impact ionization fronts in reversely biased p^{+}-n-n^{+} structures. The front propagates into a depleted n base with a velocity that exceeds the saturated drift velocity. The front passage generates a dense electron-hole plasma and in this way switches the structure from low to high conductivity. For a planar front we determine the concentration of the generated plasma, the maximum electric field, the front width and the voltage over the n base as functions of front velocity and doping of the n base. Theory takes into account that drift velocities and impact ionization coefficients differ between electrons and holes, and it makes quantitative predictions for any semiconductor material possible.Comment: 18 pagers, 10 figure

Dynamic avalanche breakdown of a p-n junction: deterministic triggering of a plane streamer front

We discuss the dynamic impact ionization breakdown of high voltage p-n junction which occurs when the electric field is increased above the threshold of avalanche impact ionization on a time scale smaller than the inverse thermogeneration rate. The avalanche-to-streamer transition characterized by generation of dense electron-hole plasma capable to screen the applied external electric field occurs in such regimes. We argue that the experimentally observed deterministic triggering of the plane streamer front at the electric field strength above the threshold of avalanche impact ionization but yet below the threshold of band-to-band tunneling is generally caused by field-enhanced ionization of deep-level centers. We suggest that the process-induced sulfur centers and native defects such as EL2, HB2, HB5 centers initiate the front in Si and GaAs structures, respectively. In deep-level free structures the plane streamer front is triggered by Zener band-to-band tunneling.Comment: 4 pages, 2 figure

Tunneling-assisted impact ionization fronts in semiconductors

We propose a novel type of ionization front in layered semiconductor structures. The propagation is due to the interplay of band-to-band tunneling and impact ionization. Our numerical simulations show that the front can be triggered when an extremely sharp voltage ramp ($\sim 10 {\rm kV/ns}$) is applied in reverse direction to a Si $p^+-n-n^+-$structure that is connected in series with an external load. The triggering occurs after a delay of 0.7 to 0.8 ns. The maximal electrical field at the front edge exceeds $10^6 {\rm V/cm}$. The front velocity $v_f$ is 40 times faster than the saturated drift velocity $v_s$. The front passes through the $n-$base with a thickness of $100 {\mu m}$ within approximately 30 ps, filling it with dense electron-hole plasma. This passage is accompanied by a voltage drop from 8 kV to dozens of volts. In this way a voltage pulse with a ramp up to $500 {\rm kV/ns}$ can be applied to the load. The possibility to form a kilovolt pulse with such a voltage rise rate sets new frontiers in pulse power electronics.Comment: 12 pages, 6 figure

Impact ionization fronts in Si diodes: Numerical evidence of superfast propagation due to nonlocalized preionization

We present numerical evidence of a novel propagation mode for superfast impact ionization fronts in high-voltage Si $p^+$-$n$-$n^+$ structures. In nonlinear dynamics terms, this mode corresponds to a pulled front propagating into an unstable state in the regime of nonlocalized initial conditions. Before the front starts to travel, field-ehanced emission of electrons from deep-level impurities preionizes initially depleted $n$ base creating spatially nonuniform free carriers profile. Impact ionization takes place in the whole high-field region. We find two ionizing fronts that propagate in opposite directions with velocities up to 10 times higher than the saturated drift velocity.Comment: 3 pages, 4 figure

Divergence of clinical and morphological diagnoses due to the objective complexity of differential diagnosis of adenocarcinoma

It is necessary to develop a unified approach for differential diagnosis in complex clinical cases.It is required to develop an objective method for determining the possibility of timely diagnosis in a medical organization.Продемонстрирована необходимость разработки унифицированного подхода к дифференциальной диагностике и объективному определению возможности своевременной диагностики аденокарциномы в условиях медицинской организации

Influence of film thickness on the dielectric characteristics of hafnium oxide layers

The present work focuses on the study of properties of hafnium oxide (HfO2) films, deposited by reactive magnetron sputtering of the Hf target in Ar/O2 gas mixture. The X-ray diffraction analysis of the deposited films proved the amorphous structure of the films, which was also confirmed by the Raman spectroscopy. It was determined that as the HfO2 film thickness had been decreased from 98.6 nm to 13.5 nm, film density had dropped from 9.52 to 9.26 g/cm2. At the same time, the dielectric constant dropped from 21.3 to 8.0 units, and the dielectric loss tangent rose, especially at high frequencies. At the same time, the decrease in film thickness resulted in increased leakage currents at low electric field density E, and decreased leakage currents at high E. Improvement of dielectric characteristics and increase in leakage current with film thickness growth is a consequence of film densification and the formation of a crystalline phase. Thickness dependence of the dielectric constant is associated with the violation of the ionic polarization mechanism at the film electrode interface

ИССЛЕДОВАНИЕ МОРФОЛОГИИ И СТРУКТУРЫ ТОНКИХ ПЛЕНОК 3C–SiC НА КРЕМНИИ МЕТОДАМИ ЭЛЕКТРОННОЙ МИКРОСКОПИИ И РЕНТГЕНОВСКОЙ ДИФРАКТОМЕТРИИ

Silicon carbide thin epilayers were grown on Si substrates by pulsed laser ablation of ceramic target. The influence of wafer temperature on morphological and structural properties of SiC layers was investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffractometry.Методом вакуумного лазерного испарения керамической мишени получены субмикронные эпитаксиальные пленки карбида кремния на кремнии. Исследовано влияние температуры подложки на структурные свойства и морфологию поверхности экспериментальных образцов