Rough interfaces in THz quantum cascade lasers

The impact of interface roughness scattering on the device performance of state-of-the-art THz quantum cascade lasers is theoretically analyzed using the non-equilibrium Green´s function formalism. It is shown that for a particular direct transition QCL design style rough interfaces hardly change the electronic energy spectrum, but dramatically reduce the occupation inversion and the optical gain. A spatial separation leading to a diagonal transition laser, leads in contrast to a limited sensitivity to interface roughness. Diagonal transition QCL designs result in a better device performance and an improved reliability with respect to interface roughness induced by growth quality variations.