Effects of doping on terahertz quantum-cascade lasers

We present the influence of doping on terahertz quantum-cascade lasers. The active region is based on the longitudinal-optical phonon depopulation of the lower laser state. A linear dependency between the threshold current density and the doping is observed. We contribute that to the fact that quantum-cascade lasers require a sub-band alignment prior to lasing. Non-linear effects like electron-electron scattering or changes in the upper state life time play only a minor role. Concerning the maximum lasing temperature no optimum doping concentration could be found.