Quantum theory of transport and optical gain in quantum cascade lasers

We have developed a self-consistent non-equilibrium Green's function theory for charge transport and optical gain in quantum cascade lasers (QCL). Acoustic, polar-optical phonon scattering, impurity, Hartree electron-electron and interface roughness scattering within the self-consistent Born approximation are taken into account. The incorporation of the full momentum and energy dependence of the scattering mechanisms turns out to be vital for the QCL device characteristies. The optical gain is calculated in linear optical response, taking into account the non-equilibrium state occupations. For low-doping THz GaAs/AlGaAs QCLs, we find the carrier transport at low bias to be dominated by multi-barrier tunneling rather than by sequential tunneling. Importantly, interface roughness is shown to increase the current density drastically and up to a factor of 1.5 but suppress the gain by a factor of 10 for a realistic interface roughness. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.