Electron leakage and its suppression via deep-well structures in 4.5-to 5.0-μm-emitting quantum cascade lasers

The equations for threshold-current density J(th) and external differential quantum efficiency eta(d) of quantum cascade lasers (QCLs) are modified to include electron leakage and the electron-backfilling term corrected to take into account hot electrons in the injector. We show that by introducing both deep quantum wells and tall barriers in the active regions of 4.8-mu m-emitting QCLs, and by tapering the conduction-band edge of both injector and extractor regions, one can significantly reduce electron leakage. The characteristic temperatures for J(th) and eta(d), denoted by T-0 and T-1, respectively, are found to reach values as high as 278 and 285 K over the 20 to 90 degrees C temperature range, which means that J(th) and eta(d) display approximate to 2.3 slower variation than conventional 4.5- to 5.0-mu m-emitting, high-performance QCLs over the same temperature range. A model for the thermal excitation of hot injected electrons from the upper laser level to the upper active-region energy states, wherefrom some relax to the lower active-region states and some are scattered to the upper miniband, is used to estimate the leakage current. Estimated T-0 values are in good agreement with experiment for both conventional QCLs and deep-well QCLs. The T-1 values are justified by increases in both electron leakage and waveguide loss with temperature. (C) 2010 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.3509368]