Highly efficient ∼8 μm-emitting, step-taper active-region quantum cascade lasers

Recently, it was shown for 4.5 μm–5.0 μm-emitting, state-of-the-art quantum cascade lasers (QCLs) that the internal efficiency can be fully accounted for when considering interface-roughness (IFR)-triggered carrier leakage from both the upper-laser (ul) level and key injector states. By applying the same formalism to ∼8.0 μm-emitting QCLs of a step-taper active-region (STA) design, we find that the devices’ internal efficiency reaches a high value of ∼76%. That is partly due to a record-high injection-efficiency value (89%), as a result of strong carrier-leakage suppression, and partly due to an IFR-scattering enhanced laser-transition efficiency value of 85.2%. By comparison, when the same analysis is applied to conventional ∼8.0 μm-emitting QCLs, grown by the same crystal-growth technique: metal–organic chemical vapor deposition (MOCVD), the internal efficiency is found to be only ∼59%, typical of values extracted from experimental data of mid-infrared-emitting conventional QCLs. When further comparing the ∼8.0 μm-emitting STA QCLs with conventional QCLs, the ul-level lifetime is found to be controlled by both LO-phonon and alloy-disorder scattering, similar to what we recently found for 4.5 μm–5.0 μm-emitting QCLs. However, unlike 4.5 μm–5.0 μm-emitting QCLs, the lower-laser level lifetime is found to be controlled by both LO-phonon and IFR scattering. In addition to the high internal-efficiency value, the use of excited-state injection and a low voltage defect result in the STA QCL reaching a front-facet wall-plug efficiency value of 10.6%, a record-high, front-facet value for 8 μm–11 μm-emitting QCLs grown by MOCVD and holding potential for continuous-wave operation.