Suppression of carrier leakage in 4.8 μm - emitting quantum cascade lasers

In this work we show that by using both deep quantum wells and tall barriers in the active regions of quantum cascade (QC)-laser structures and by tapering the conduction-band edge of both injector an extractor regions one can significantly reduce the leakage of the injected carriers. Threshold-current, J(th) and differential-quantum efficiency, eta(d) characteristic temperatures, T-0 and T-1, values as high as 278 K and 285 K are obtained to 90 degrees C heatsink temperature, which means that J(th) and eta(d) vary similar to 2.5 slower over the 20-90 degrees C temperature range than in conventional QC devices. Modified equations for J(th) and eta(d) are derived. In particular, the equation for eta(d) includes, for the first time, its dependence on heatsink temperature. A model for the thermal excitation of injected carriers from the upper lasing level to upper active-region energy states from where they relax to lower active-region energy states or get scattered to the upper G miniband is employed to estimate carrier leakage. Good agreement with experiment is obtained for both conventional QC lasers and deep-well (DW)-QC lasers.