Theoretical Concepts of Photoexcited Hot Carriers

Abstract

Most of the extensive literature on hot carriers refers to carrier heating by an electric field. This lecture concerns a totally different mechanism for producing a non-equilibrium carrier distribution. Carriers are photoexcited into a band at a steady rate with a mean energy in excess of the thermal energy. If the recombination lifetime T_r is sufficiently short in relation to the thermalization time τ_th the carriers spend insufficient time in the band to completely thermalize via the electron-phonon interaction prior to their recombination and a hot steady-state carrier distribution is obtained. The thermalization time is not a very well defined quantity and like the recombination time is dependent on carrier energy, but broadly speaking we can use these quantities to classify the types of heating that we can expect to find. If τ_r << τ_th the distribution is wholly recombination-controlled with effectively no thermalization. If τ_r >> τ_th the majority of carriers are thermalized and the distribution function only differs from equilibrium by having a high-energy tail containing a small minority of unthermalized carriers. The tailed-thermal distribution approaches the equilibrium limit for large τ_r by the height of the tail decreasing to zero.