Hot-electron relaxation dynamics in quantum wires

Abstract

Monte Carlo simulations of hot nonequilibrium electron relaxation in rectangular GaAs quantum wires of different cross sections are carried out. The simulations demonstrate that the initial stage of hot-electron cooling dynamics is determined by cascade emission of optical phonons and exhibits strong dependence on the excitation energy. The second (slow) relaxation stage is controlled by strongly inelastic electron interactions with acoustic phonons as well as by nonequilibrium (hot) optical phonons. The relaxation times obtained in our simulations are in good agreement with the results of recent luminescence experiments. At low electron concentrations where hot phonon effects are negligible the cascade emission of optical phonons may lead to the overcooling of the electron system to temperature below the lattice temperature. These electrons then slowly (during tens of picoseconds) relax to equilibrium due to the interaction with acoustic phonons. At certain excitation energies strong intersubband electron scattering by optical phonons leads to electron redistribution among subbands and intersubband population inversions. If the electron concentration exceeds 105 cm−1, hot phonon effects come into play. In contrast to bulk materials and quantum wells, hot phonon effects in quantum wires exhibit strong dependence on the initial broadening of the energy distribution of the electrons. The very initial electron gas relaxation stage in quantum wires is faster in the presence of hot phonons, while for t≳0.5 ps the hot phonon thermalization time defines the characteristic electron cooling time.