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Step-taper active-region (STA) quantum cascade lasers (QCLs) allow for both carrier-leakage suppression and fast, miniband-like lower-laser (ll) level depopulation. That has led to an internal-efficiency value of ~ 77 % for ~ 5 μmemitting devices, a record-high value for 4.5-6.0 μm-emitting QCLs. We have recently shown that that value can be basically accounted for when considering both LO-phonon- and interface-roughness (IFR)-triggered carrier leakage from the upper-laser (ul) level and injector states. The same carrier-leakage analysis is applied to MOCVD-grown ~ 8.0 μmemitting, STA-type QCLs, and we find that the internal efficiency reaches a high value of ~ 73.6 %, due to a record-high injection-efficiency value (86.7 %) and to an IFR-enhanced laser-transition efficiency value of ~ 85 %. In contrast, for a conventional MOCVD-grown, ~ 8.0 μm-emitting QCLs the internal efficiency is found to be only ~ 55 %, typical of values extracted from experimental data of mid-infrared-emitting conventional QCLs. The ul-level lifetime is found to be controlled by LO-phonon and alloy-disorder (AD) scattering for typical MOCVD-grown QCLs, just like for 4.5-5.0 μm-emitting QCLs. However, for typical MBE-grown QCLs the ul-level lifetime is controlled by LO-phonon, AD and IFR scattering. The ll-level lifetime is found to controlled by both LO-phonon and IFR scattering. Besides the high internal-efficiency value, the use of excited-state injection and a low voltage defect result in the STA QCL reaching a single-facet wall-plug efficiency value of 10.6 %, a record-high single-facet value for 8-11 μm-emitting QCLs grown by MOCVD and holding potential for CW operation.
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