Addressing broadening challenges in m-plane GaN two-well terahertz quantum cascade laser.

In this study, we address the challenges that result from line broadening on m-plane GaN terahertz quantum cascade lasers (THz QCLs). While past research has highlighted the difficulty of line broadening in GaN THz QCLs, our work varies from previous studies in that it questions the primary impact attributed to the strong longitudinal-optical (LO) phonon coupling. We investigate carrier transport in an m-plane GaN two-well (TW) THz QCL, using non-equilibrium Green's functions (NEGF) to quantify gain while accounting for correlation effects in level broadening. Our study reveals that LO-phonon is not the primary contributor to line broadening at relatively high doping levels in our model. Moreover, despite the observed substantial broadening, increasing the doping density by an order of magnitude over the value of GaAs-based THz QCLs leads to a substantial gain rise. These results suggest the feasibility of achieving lasing even in the presence of significant broadening mechanisms. Our findings demonstrate, for the first time, the potential of an m-plane TW GaN scheme for THz QCLs to achieve lasing up to room temperature at 7.2 THz with only 14% Al content in the barriers. Further optimizations, such as reducing leakage through increased Al content in the potential barriers or adding another barrier to the structure, could potentially lead to above room temperature performance. This work demonstrates the potential for operation with photon energies around 30 meV, which is of particular interest to the QCL community and could open avenues for GaN-based THz QCLs in diverse high-temperature applications.