Carrier transport in GaN terahertz (THz) quantum cascade laser (QCL) structures is theoretically investigated using a non-equilibrium Green's function method. Although scattering due to polar optical phonons in GaN is greatly enhanced with respect to GaAs/AlGaAs THz QCLs, the phonon-induced broadening of the laser levels is found to remain much smaller than other sources of broadening arising from impurity and electron-electron scattering. The gain is calculated self-consistently accounting for the correlation effects in level broadening. Three-well based design with resonant-phonon scheme shows a peak gain of 88/cm at 10 K, and 34/cm at 280 K, which remains above the calculated loss of a double metal waveguide. The results suggest that lasing at 6.6 THz, which is beyond the traditional GaAs THz QCLs, is possible up to 280 K.

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Only the intrasubband contribution of the gain broadening as defined in Ref. 39 is discussed here, which adds to the intersubband contribution (i.e., lifetime-limited part).

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