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

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 wave-length 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.