Romanets P. Non equilibrium processes in semiconductor structures and graphene, resulting from photoexcitation or carrier heating electric field

The thesis deals with the nonlinear kinetic processes, accompanied by a modification of the distribution of carriers (carrier heating) in an intrinsic graphene. Specifically investigated the interaction of charge carriers in graphene with photonic thermostat, which has a temperature T2 different from the phonon thermostat temperature T1 , i.e. temperature of graphene structure; the distribution of nonequilibrium electron-hole pairs was obtained for the cases when the interparticle scattering is unessential and when the Coulomb scattering dominates. For the first case, the distribution function is determined by the interplay of intraband relaxation of energy due to acoustic phonons and interband radiative transitions caused by the thermal radiation. For the alter case, the quasiequilibrium distribution with effective temperature and non-equilibrium concentration, determined through balance equations, is realized. Due to the effect of thermal radiation with temperature concentration and conductivity of carriers in graphene is modified essentially. It is demonstrated that at T2-T1>65K, the negative interband absorption caused by the inversion of carriers distribution, may occur. Also, the nonlinear current-voltage characteristics of intrinsic graphene is analyzed. We describe the transient process of carriers heating in graphene by a strong dc electric field, when it is abruptly turned on. At room temperature, a nearly-linear current-voltage characteristic and a slowly-varied concentration take place for fields up to 12 kV/cm. Since a predominant recombination of high-energy carriers due to optical phonon emission at low temperatures, a depletion of concentration takes place below 250 K. For lower temperatures the current tends to be saturated and a negative differential conductivity appears below 175 K in the region of fields 10 V/cm. In addition, we study transient kinetic processes in intrinsic graphene arising as the result of photoexcitation of carriers by ultrashort optical pulse. The transient evolution of carriers in an intrinsic graphene under ultrafast excitation, which is caused by the collisionless interband transitions, is investigated. The energy relaxation due to the quasielastic acoustic phonon scattering and the interband generation-recombination transitions due to thermal radiation are analyzed. Moreover, the Rabi oscillations and optical nutation of response are considered in graphene. The photoexcited distribution is calculated versus time and energy taking into account the effects of energy relaxation and dephasing. Spectral and temporal dependencies of the response on a probe radiation (transmission and reflection coefficients) are considered for different pumping intensities and the Rabi oscillations versus time and intensity are analyzed. The optical nutations are predicted to be easily observable through the corresponding modulation of the transmission and reflection coefficients for the pulse durations < 80 fs and the pulse energies per area corresponding to W>200 nJ/cm2. The transient phenomena related to the magnetotransport in classical semiconductor structures A3B5 are also considered. The transient magnetooptical response of electrons with partly inverted initial distribution produced by an ultrashort optical pulse near the optical phonon energy is studied theoretically. Transient cyclotron absorption and Faraday rotation of polarization plane are considered for bulk semiconductors (GaAs,InAs, and InSb) as well as for a GaAs-based quantum well. Transient negative absorption in the cyclotron resonance conditions and peculiarities of Faraday rotation of the polarization plane associated with partial inversion of the initial distribution are considered. The possibility of transient enhancement of the probe field under cyclotron resonance conditions is indicated. The nonlinear transient magnetoresponse of two-dimensional (2D) electrons is studied theoretically. Transient conductivity is calculated under exact cyclotron resonance condition. The negative cyclotron absorption caused by the transient absolute negative conductivity effect is predicted for a certain range of parameters.