Intervalley scattering in GaAs-AlAs quantum cascade lasers

We have investigated the importance of intervalley (Gamma-X) electron transfer between Gamma-point quantum well states and X-point barrier states in GaAs-based quantum cascade lasers with indirect band gap AlAs barriers. A series of samples has been studied in which the energy separation between the coupled injector/upper laser levels and the lowest confined X state in the injection barrier is varied. We demonstrate that for lasing to occur, electron injection into the upper laser level must proceed via Gamma states confined below the lowest X state in the injection barrier. The limit this places on the minimum operating wavelength (lambdaapproximate to8 mum) for the present laser design is overcome by utilizing a double injection barrier to achieve lasing at lambda=7.2 mum. (C) 2002 American Institute of Physics.