Self-consistent analysis of quantum transport in a split-well resonant-phonon terahertz quantum cascade laser

The split-well resonant phonon (SWRP) THz quantum cascade laser (QCL) is a novel design scheme introduced in previous studies, demonstrating significant potential due to its reduced overlap between doped regions and active laser states. This structural advantage was expected to mitigate ionized impurity scattering (IIS) and improve overall device performance, motivating a detailed investigation of the transport mechanisms. Here, we analyze the SWRP design using nonequilibrium Green's function (NEGF) simulations. Our analysis of the SWRP-based THz QCL design reveals key mechanisms limiting its high-temperature performance and provides a pathway for significant improvement. In our study, we found that the injector level and the upper laser level (ULL) exhibit different population distributions, suggesting that injection coupling can be further enhanced to improve the temperature performance. Additionally, backfilling remains a limiting factor, which could be mitigated by increasing the energy separation between the lower laser level (LLL) and the injector level beyond 36 meV. Furthermore, our analysis highlights that interface roughness (IFR) significantly impacts optical gain and spectral broadening. We propose improving the design by reducing Al content in the barriers to reduce the interface roughness scattering, for example, by implementing mixed potential barriers, maintaining the injector at 30% aluminum while reducing other barriers to 15%. Our findings provide valuable insights into the high-temperature performance of SWRP-based THz QCLs and establish clear guidelines for further optimization, potentially pushing the design beyond the current state-of-the-art.