I. V. Grekhov

Novel Closing Switches Based on Propagation of Fast Ionization Fronts in Semiconductors

New concept of triggering the fast ionization fronts in semiconductors with the field-enhanced ionization of deep electron traps is described. Closing switches designed on the base of this concept has been called the Deep Levels Dinistors, DLDs. These devises are able to form high current pulses with subnanosecond risetime and low voltage drop after switching. As an instance three generators based on DLDs are described. The possibility of picosecond switching on the base of tunnel assisted impact ionization front is discussed.

Field-enhanced ionization of deep-level centers as a triggering mechanism for superfast impact ionization fronts in Si structures

We investigate the origin of free carriers that initiate impact ionization in depleted high-voltage p-n junctions under dynamic breakdown conditions and deterministically trigger superfast ionization fronts that propagate several times faster than the saturated drift velocity. We argue that in Si structures triggering occurs due to the field-enhanced ionization of process-induced deep-level centers identified as sulfur impurities. This impurity is a double-level electron trap with low recombination activity. It is present in high-voltage Si structures due to the side effect of widely used fabrication technology. We calculate the field and temperature dependences of the ionization probability for the upper midgap level (0.28eV) and midgap level (0.54eV) in electric fields up to 5×105V∕cm as well as the occupation of these levels at different temperatures. The emission of free electrons is sufficient to trigger the ionization front from zero temperature to ∼400K, in agreement with experiments. At room tempe...

Pulse Power Generation in Nano- and Subnanosecond Range by Means of Ionizing Fronts in Semiconductors: The State of the Art and Future Prospects

Ionization front propagation in a high-voltage reversely biased silicon p-n diode is the fastest nonoptical process of electron-hole plasma generation in semiconductors. It is capable of switching a high-voltage semiconductor structure from the blocking to the conducting state within several hundreds of picoseconds. Double-donor electron traps with ionization energies of 0.28 and 0.54 eV are the main source of carriers which initiate formation and propagation of these fronts. Dinistor n+ -p-n-p+ structures with such a switching mechanism are capable of generating electric pulses with several kiloamperes of amplitude and nanosecond and even subnanosecond pulse rise time. High-power pulse generators based on these devices have been developed and found applications in many modern technologies. Our numerical simulations indicate a possibility to excite new types of impact ionization fronts which are capable of switching high-voltage devices within dozens of picoseconds.

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.

Ageing of Thin-Film Capacitor Structures Based on PZT

Thin-film Pt/PZT/Ir(Ti/SiO/Si) and Ir/PZT/PTO/Ir(Ti/SiO2/Si) capacitors, as-grown and after ageing, are investigated. A comprehensive study based on the Auger spectroscopy and measurement of electrical and ferroelectrical characteristics is performed. Ageing results in increasing of the oxygen content in PZT, broadening of interfaces owing to formation of metal oxides, diffusion of Pb and Ti towards the top electrode along the grain boundaries, and diffusion of Ti into PZT grain. These processes induce decreasing of the structure capacitance, healing of the interface traps, enhance leakage and dielectric loss.

Origin of photoresponse in heterophase ferroelectric Pt/Pb(ZrTi)O3/Ir capacitors

A steady-state short-circuit photocurrent is measured in polycrystalline Pb(ZrTi)O3(PZT) films with Pb excess. It is found that although the photocurrent is directed opposite to ferroelectric polarization, it is not the depolarization current of the ferroelectric. To explain this, the authors assume that Pt/PZT/Ir capacitor consists of PZT grains and semiconductor PbO phase segregated on PZT grain boundaries during the PZT formation. In such medium, the current across the film flows through channels formed by PbO phase, where an electric field is generated by uncompensated polarization charge of PZT grains. Using this concept, the authors described the experimental dependence of photocurrent on polarization.

Thin-film capacitor M/Pb(ZrTi)O3/M as a polarization-sensitive photocell

A steady-state short-circuit photocurrent of preliminarily polarized submicron capacitors with a polycrystalline Pb(ZrTi)O3 (PZT) film is investigated under irradiation by light with a wavelength λ > 0.4 μm. The structures with different M/PZT interfaces that differ in the leakage current by more than an order of magnitude are found to demonstrate virtually the same value of the photocurrent, which is always directed opposite to the ferroelectric polarization of the PZT film. Although the magnitude of photocurrent is determined by the degree of polarization of the film, the observed photocurrent is not a depolarization current of the ferroelectric film. Therefore, the M/PZT/M capacitor behaves like a polarization-sensitive photocell. Within the proposed theory of a heterophase medium, the dependence of the photocurrent on the magnitude of the preliminary polarization is calculated and proves to be in reasonable agreement with the experimental results.

Sub-nanosecond semiconductor opening switches based on 4H–SiC p+pon+-diodes

Abstract 4H–SiC p + n o n + - and p + p o n + -type diodes have been fabricated and evaluated as opening switch devices which are known in Si to employ the effect of fast reverse current break after switching the diodes from forward to reverse bias conditions. The recovery properties of p + p o n + -diodes were found to be drastically differ from those of p + n o n + -type ones. In p + n o n + -diodes, recombination processes predetermine soft recovery behavior in a time of approximately 16 ns. In p + p o n + -type diodes, purely drift mechanism was found to be responsible, under proper conditions, for the very fast reverse current break in a time less than 1 ns. p + p o n + -Diodes were utilized in a pulse generator which was fabricated with an inductive energy storing unit. The rise time of 400-V pulses generated, about 4 ns, was found to be equal to the quarter of the oscillation period of the LC-tank consisting of 2-μH storing inductance and 2-pF 4H–SiC diode capacitance.

A high-power semiconductor switch of high-voltage pulses with a rise time of nanosecond duration

The results of studies of new fast-acting semiconductor devices—deep-level dynistors intended for use in high-power devices of nano-and microsecond pulsed-power technology—are presented. The possibility of switching multikiloampere current pulses having a rise rate of 200 kA/μs with the use of a single device with a 12-mm-diameter structure is shown. A high-power switch based on an assembly of dynistors with an operating voltage of 12 kV connected in series is described. The switch is capable of switching current pulses with a 1200-A amplitude and a 4-ns rise time.

Auger spectroscopy and properties of Ir(Pt)/PZT(PZT/PT)/Ir nanosized thin-film structures

Studies of the elemental distribution profiles through the thickness of thin-film ferroelectric capacitor structures using Auger spectroscopy made it possible to establish the relation between the elemental and phase composition of the structures, on the one hand, and their electrophysical properties, on the other. Special features were observed in the behavior of the lead titanate underlayer both in the as-fabricated and aged structures. It is shown that the variation in the characteristics of the capacitor structures during aging is caused by diffusion of the elements at the interfaces and in the PZT film against a background of a significant increase in the oxygen concentration. As a result, interface-modifying oxide layers form and the trap density on the upper and lower interfaces decreases in all samples.