Modeling with graded interfaces: Tool for understanding and designing record‐high power and efficiency mid‐infrared quantum cascade lasers
Suraj Suri, B. Knipfer, Thomas Grange, Huilong Gao, Jeremy Kirch, L. J. Mawst, Robert A. Marsland, D. Botez
Abstract
Abstract 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 µm‐ and 8.3 µm‐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 μm‐emitting QCL structures with variable barrier heights. Those values and trends, when used for designing a graded‐interface, 4.6 μm‐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 µm‐ and 8.3 µm‐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 µm‐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 ∼8 µm‐emitting QCLs.