Energy And Momentum Relaxation Of Hot Electrons By Acoustic Phonon Emission

Abstract

Abstract At low electron temperatures the dominant process of energy relaxation by hot electrons in semiconductors is by acoustic phonon emission. The rate of energy loss due to electromagnetic (far infra-red) radiation is absolutely tiny by comparison. At higher electron temperatures optic phonon emission takes over. The carrier temperature at which the changeover from acoustic to optic phonon emission takes place depends on the optic phonon energy. In gallium arsenide the optic phonon energy is about 36 meV and the changeover is at between 30 and 50 K (Hawker et al. 1992). In gallium nitride, which has a much larger optic phonon energy( 100 meV), the changeover is expected to occur at a higher temperature. At room temperature optic phonon emission is dominant. This is due to an exponential dependence of the optic phonon emission rate on carrier temperature, while the energy relaxation rate through acoustic phonon emission saturates because emission of large wave vector (high-energy) acoustic phonons is forbidden by momentum conservation considerations. This effect is particularly important in low-dimensional electron systems, in which the reduction of phase space for electron scattering can significantly suppress the energy relaxation by acoustic phonon emission.