m-plane GaN split-well direct-phonon terahertz quantum cascade laser.

In this work, we theoretically investigated GaN-based terahertz quantum cascade laser (THz QCL) structure, modeled for growth along the non-polar m-plane. The design employs the split-well direct-phonon (SWDP) scheme and is analyzed using the Non-equilibrium Green's Function (NEGF) approach. The proposed design successfully addresses key limitations identified in previous studies, particularly the challenge of balancing high gain with lower current density thereby mitigating the risk of thermal damage. By introducing a thin barrier within the wider well, we achieved a substantial reduction in doping density which in turn leads to significantly lower current densities while preserving strong gain performance. As a result, the proposed design achieves optical gain comparable to state-of-the-art GaN-based THz QCLs while maintaining current densities within the same order of magnitude as mid-infrared QCLs. The device operates at a frequency of ~7.9 THz, extending beyond the typical limits of GaAs-based THz QCLs, and maintains gain above estimated waveguide losses up to room temperature. These results demonstrate that the m-plane SWDP GaN-based design offers a promising pathway toward high-frequency, high-temperature THz QCL operation and provides practical design guidelines for future experimental realization.