The temperature and energy distribution of photoexcited hot electrons

Abstract

The concept of electron temperature T _e is widely used in the description of photoexcited electrons in semiconductors. Nevertheless the photoexcited hot-electron energy distribution appears to be a Maxwell or Fermi distribution only in the case in which the electron density is very high, n ⪆ 10¹⁸ cm^-3. Usually the energy distribution may only be adequately described by temperature T _e below the threshold of the optical-phonon emission ε = ħΩ₀ (passive region ε < ħΩ₀). Above the threshold (active region ε > ħΩ₀) the distribution critically depends on the photon energy and pumping level. In the review we study how the distribution depends on the influence of the lattice and inter-electron scatterings. The electron temperature T _e is determined by the balance between the energy imparted to the electron system from the excitation source and the energy lost to the lattice owing to the phonon scattering. As a rule a minority of the electrons are in the active region rather than in the passive one but it is those electrons that are responsible for the energy losses because of the strong lattice scattering in the active region. So the energy-loss rate depends on the shape of the distribution in the active region. In the present review the question of how to calculate this energy-loss rate and to write down the correct balance equation is investigated. Experiments which allow measurement of the temperature T _e in the passive region, the shape of the distribution f(ε) in the active region, and the time evolution of these quantities are discussed.