Korotyeyev V. Effects of interaction of terahertz radiation with a strongly nonequilibrium electron gas in bulk and low-dimensional semiconductor structures

Thesis is devoted to theoretical studies of high-field and high-frequency electron transport in bulk samples, quantum heterostructures and diode structures based on AIIIBV semiconductor materials. In the framework of the developed theories, the effects of the high-field and magneto-transport electron kinetics, the effects of anisotropy of the high-frequency response of an electron gas induced by a strong electric and magnetic field are investigated. The considerable attention was paid to specific electron transport regimes relating to the emergence of current and plasmon instabilities which can be used for generation of THz radiation. In particular, a comprehensive analysis of the streaming effect and optical phonon transit-time resonance in compensated GaN has been carried out in the frames of Monte-Carlo simulation. It was found that streaming transport regime and associated OPTTR-phenomenon can be realized in GaN samples with moderate doping of ~1016 cm-3 and compensation degree of 90% in the range of applied electric fields of 3-10 kV/cm. It was shown that formation of the streaming transport regime can be identified as specific decreasing behavior of dependencies of the transverse-to-current diffusion coefficient. The calculations predict the effect of negative dynamic conductivity, in the frequency range of 0.5-2 THz up to temperatures of 150 K. The electrodynamic calculations of the transmission/absorption spectra reveal that single epitaxial layer of compensated GaN with thickness of 10 microns can amplify THz radiation with gain of 1-2%. The co-existence of the cyclotron and optical phonon transit-time resonances has been identified in magnetic fields of a few T applied in-parallel to electric one. It was shown that magnetic field can control polarization dependence of the amplification effects. In crossed configuration of applied electric and magnetic field, the magnetic fields of ~4 T can destroy the streaming transport regime with the effect of the collapse of dissipative current. The main magneto transport characteristics (dissipative current, Hall current, and Hall electric field) have been studied for the short and open Hall circuits. It was shown that the magneto transport measurements can provide valuable information on the main features of the electron distribution function and electron dynamics in GaN. It was established that strong dependency of the dissipative current on the parameters of the Hall circuit can be used for current modulation and current switching for high-power electronic applications. A self-consistent theory of ballistic electron transport in n+-i-n+ homo- and heterodiodes has been developed for steady-state and high-frequency regimes. The theory takes into account real injection/exclusion processes at the n+ -i interfaces, space-charge effects in the base region and diffusive electron transport in contacts. Comprehensive analysis of current-voltage characteristics and frequency dependencies of impedance was performed for InAs homodiodes and GaxIn1−xAs-GaAs-GaxIn1−x heterodiodes. It was demonstrated that widely accepted simplifications and results, such as the virtual cathode approximation, the Child law and others, cannot be applied to describe semiconductor ballistic diodes for realistic values of the applied electric biases. The negative dynamic resistance (NDR) effect is identified and studied. It was shown that the thermal spreading of injected electrons over the energy greatly affects the transit-time resonance and suppresses the NDR effect. The parameters of the diodes and working temperatures necessary to achieve the maximum NDR effect in the THz frequency range were determined. In order to increase the gain of the resonator modes, a model of a THz generator based on cascade structure of identical ballistic diodes was proposed. A theory of collective electron oscillations in two-dimensional semiconductor heterostructures subjected to a high electric field has been developed. The effect of the stationary electric field has been taken into account on both steady-state and high-frequency electron transport. The analysis has been conducted by solving Boltzmann-Vlasov equations in the collisionless approach for high-frequency electron transport. It was discovered that applied electric field induces the following effects: strong nonreciprocal behavior of both oscillation frequency and damping; interaction of plasmonic and thermal modes; instability of excitations propagating along the electron drift (effect of negative Landau damping). It was shown that the electrically induced plasmon instability provides amplification of terahertz (THz) radiation in grating-based plasmonic structures.