Modeling with graded interfaces: Tool for understanding and designing record-high power and efficiency mid-infrared quantum cascade lasers

By employing a graded-interfaces model based on a generalized formalism for interface-roughness (IFR) scattering that was modified for mid-infrared emitting quantum cascade lasers (QCLs), we have accurately reproduced the electro-optical characteristics of published record-performance 4.9 mu m- and 8.3 mu m-emitting QCLs. The IFR-scattering parameters at various interfaces were obtained from measured values and trends found via atom-probe tomography analysis of one of our 4.6 mu m-emitting QCL structures with variable barrier heights. Those values and trends, when used for designing a graded-interface, 4.6 mu m-emitting QCL, led to experimental device characteristics in very good agreement with calculated ones. We find that the published record-high performance values are mainly due to both injection from a prior-stage low-energy (active-region) state directly into the upper-laser (ul) level, thus at low field-strength values, as well as to strong photon-induced carrier transport. However, the normalized leakage-current density J(leak)/J is found to be quite high: 26-28 % and 23.3 %, respectively, mainly because of IFR-triggered shunt-type leakage through high-energy active-region states, in the presence of high average electron temperatures in the ul laser level and an energy state adjacent to it: 1060 K and 466 K for 4.9 mu m- and 8.3 mu m-emitting QCLs, respectively. Then, modeling with graded interfaces becomes a tool for designing devices of performances superior to the best reported to date, thus closing in on fundamental limits. The model is employed to design a graded-interface 8.1 mu m-emitting QCL with suppressed carrier leakage via conduction-band engineering, which reaches a maximum front-facet wall-plug efficiency value of 22.2 %, significantly higher than the current record (17 %); thus, a value close to the fundamental front-facet, upper limit (i.e., 25 %) for similar to 8 mu m-emitting QCLs.