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 <i>T</i><sub>0</sub> (253 K) and the slope-efficiency characteristic temperature <i>T</i><sub>1</sub> (285 K), but, due to stronger quantum confinement, the global upper-laser-level lifetime τ<sub>4g</sub> decreases, resulting in basically the same room-temperature (RT) threshold-current density <i>J</i><sub>th</sub> 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 τ<sub>4g</sub> value while further suppressing carrier leakage. As a result, experimental RT <i>J</i><sub>th</sub> values from moderate-taper TA 4.8-μm emitting QCLs are ~14% less than for conventional QCLs and <i>T</i><sub>1</sub> 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 τ<sub>4g</sub> value, due to Stark-effect reduction and strong asymmetry. Then, the RT <i>J</i><sub>th</sub> value decreases by at least 25% compared to conventional QCLs of same geometry. In turn, single-facet, RT pulsed and continuous-wave maximum wallplug-efficiency values of 29% and 27% are projected for 4.6-4.8-μm-emitting QCLs.

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