Multidimensional Conduction-Band Engineering for Maximizing the Continuous-Wave (CW) Wallplug Efficiencies of Mid-Infrared Quantum Cascade Lasers

By tailoring the active-region quantum wells and barriers of 4.5–5.0-μm-emitting quantum cascade lasers (QCLs), the device performances dramatically improve. Deep-well QCLs significantly suppress carrier leakage, as evidenced by high values for the threshold-current characteristic temperature $T_{0}$ (253 K) and the slope-efficiency characteristic temperature $T_{1}$ (285 K), but, due to stronger quantum confinement, the global upper-laser-level lifetime ${tau}_{rm 4g}$ decreases, resulting in basically the same room-temperature (RT) threshold-current density $J_{rm th}$ as conventional QCLs. Tapered active-region (TA) QCLs, devices for which the active-region barrier heights increase in energy from the injection to the exit barriers, lead to recovery of the ${tau}_{rm 4g}$ value while further suppressing carrier leakage. As a result, experimental RT $J_{rm th}$ values from moderate-taper TA 4.8-μm emitting QCLs are ∼14% less than for conventional QCLs and $T_{1}$ reaches values as high as 797 K. A step-taper TA (STA) QCL design provides both complete carrier-leakage suppression and an increase in the ${tau}_{rm 4g}$ value, due to Stark-effect reduction and strong asymmetry. Then, the RT $J_{rm th}$ value decreases by at least 25% compared to conventional QCLs of same geometry. In turn, single-facet, R- pulsed and continuous-wave maximum wallplug-efficiency values of 29% and 27% are projected for 4.6–4.8-μm-emitting QCLs.