The relative importance of electron–electron and electron–phonon scattering in terahertz quantum cascade lasers

Abstract

This theoretical work explores the carrier dynamical issues that must be confronted if the recent successes of the mid-infrared quantum cascade lasers [1] are to be translated into longer wavelength devices. Whilst the small subband separations lead to a partial suppression of longitudinal optic (LO) phonon emission, which is the dominant non-radiative loss mechanism in quantum cascade lasers, it also encourages intersubband electron–electron scattering, the rate of which can be comparable with LO phonon scattering. A triple quantum well active layer design is advanced which exhibits population inversion and operates at 7.7 THz (39 μm), it is shown that intersubband electron–electron scattering can dominate over phonon scattering for depopulating the lower laser level. The role of intrasubband electron–electron scattering on the non-equilibrium carrier distributions and its potential influence on the phonon scattering rate is also discussed. It is argued that only a full Monte Carlo simulation will describe completely the carrier dynamics of quantum cascade lasers.