EFFECT OF FREE-CARRIER SCREENING ON THE ULTRAFAST RELAXATION KINETICS IN HOT POLAR SEMICONDUCTORS

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Ultrafast kinetics of evolution of optical phonons in a photoinjected highly excited plasma in semiconductors

We consider the ultrafast kinetics of evolution of optical phonons in a photoinjected highly excited plasma in semiconductors. The state of the nonequilibrium ('hot') phonon system is described in terms of the concept of a nonequilibrium temperature, referred to as quasitemperature, per mode, which can be experimentally characterized and measured. The phonon emission time shows that optical phonons are preferentially produced, well in excess of equilibrium, in a reduced off-center region of the Brillouin zone. The phonons in this region are responsible for the phenomenon referred to as 'hot-phonon temperature overshoot.' Most of the phonons, namely, those outside such a region, are only weakly to moderately excited, and mutual thermalization of the nonequilibrium carriers and optical phonons follows, typically, in the tenfold picosecond scale. All these results are influenced by the experimental conditions, which we discuss on the basis of calculations specialized for GaAs. Comparison with experimental data is presented.5416113111131

NONLINEAR TRANSPORT IN PHOTOEXCITED PLASMA IN SEMICONDUCTORS - NONOHMIC MOBILITY AND A GENERALIZED EINSTEIN RELATION

Resorting to a theory of responses to thermal and mechanical perturbations, based on statistical irreversible thermodynamics for systems arbitrarily away from equilibrium, we obtain the diffusion and mobility coefficients in a highly photoexcited plasma in semiconductors in the presence of an electric field. They are dependent on the evolution of the nonequilibrium thermodynamic state of the system. From these transport coefficients we derived a generalized Einstein relation for ultrafast transient regimes and for non-Ohmic conditions. In all cases this generalized Einstein law acquires values that are field dependent and larger than those in its original form only valid in steady-state conditions and the limit of weak fields. Numerical results appropriate for the case of a GaAs sample are presented.5219139361394

DIFFUSION OF PHOTOINJECTED CARRIERS IN NONEQUILIBRIUM POLAR SEMICONDUCTORS

Ambipolar diffusion of photoinjected carriers in nonequilibrium polar semiconductor plays, in conjunction with other effects, an important role in the relaxation processes in such hot carrier systems, providing a mechanism for a slowing down of the relaxation processes. Diffusion processes are governed by an ambipolar diffusion coefficient. In these far-from-equilibrium semiconductors such transport coefficient depends on the nonequilibrium macroscopic state of the sample, as it evolves in time. Resorting to an informational statistical thermodynamics, we derive an expression for this ambipolar diffusion coefficient accounting for its dependence on the time evolution of the dissipative processes that develop in the medium. The theoretical results are compared with experimental data obtaining a good agreement.87429930

Relaxation dynamics of hot carriers and phonons in semiconductors: Influence of the excitation conditions

The so-called hot-phonon effect that accompanies the rapid relaxation processes in the photoinjected plasma in semiconductors is analyzed, resorting to a nonlinear quantum kinetic theory based on a nonequilibrium ensemble formalism. We concentrate the study on the process of generation and decay of the nonequilibrium longitudinal optical phonon population per mode. Particular attention is paid to the question of the influence of the conditions of excitation imposed on the system, which determine the occurrence of different regimes of relaxation by means of carrier-phonon interactions. Comparison of the relaxation dynamics in the case of several semiconductors with different polar strengths is done. (C) 2001 American Institute of Physics.9083973397