Self-consistent gain calculations and carrier transport analysis for split-well direct-phonon terahertz quantum cascade lasers

A split-well direct phonon THz quantum cascade laser scheme supporting a clean three-level system was analyzed using nonequilibrium Green's functions. In this system, most of the electrons occupy the lower three levels, and thermally activated leakage channels are suppressed almost entirely. The simulations showed that the gain values were deeply affected by the changes in the devices' interface roughness and impurity scattering strengths. We concluded that in addition to the low oscillator strength and relatively low doping level, these are the mechanisms responsible for the temperature performance limitations of this scheme.