Effects of elastic scattering on high-performance step-taper active-region quantum cascade lasers

Step-taper active-region (STA) quantum cascade lasers (QCLs) allow for both carrier-leakage suppression and ultrafast, miniband-like carrier extraction. In turn, that has led to very high internal-efficiency ni values: ~ 77 % and 80-86 % from ~ 5.0 μm- and 8-9 μm-emitting QCLs, respectively. Based on extracted parameters that characterize the interfaceroughness (IFR) scattering, a study has been performed of the effects of elastic scattering, both IFR and alloy-disorder (AD) scattering, on 5.0 μm-emitting STA-QCLs. We find that the laser-transition efficiency ntr is enhanced by ~15 % (i.e., from 83 % to ~ 95 %) due to the much stronger effect of elastic scattering on the lower-laser-level lifetime than on the effective upper-laser-level lifetime. In turn, the injection efficiency: ninj = ni /ntr , reaches ~ 81 %; that is, the highest injection-efficiency value obtained to date from QCLs. Furthermore, we find that the projected upper limit for the pulsed wall-plug efficiency can reach values as high as 44.4 % for 4.6 μm-emitting devices; thus, raising the possibility of CW operation of 4.5-5.0 μm-emitting QCLs with wallplug-efficiency values as high as 40 %.