Comparison of the quantum efficiencies of interwell and intrawell radiative transitions in quantum cascade lasers

A theoretical study has been made to compare the quantum efficiencies of interwell and intrawell radiative transitions in quantum cascade lasers based on ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{As/GaAs}$ technology. Radiative and nonradiative intersubband transition rates were calculated for a range of temperatures between 4 and 300 K at an applied electric field of 4 kV cm⁻¹ for structures designed to emit in both mid-infrared and far-infrared frequency ranges. It is found that the internal quantum efficiency of mid-infrared devices is a maximum for diagonal, or interwell, transitions. Conversely, for the far-infrared devices, the quantum efficiency is a maximum for vertical, or intrawell, transitions.