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.
The temperature dependence of threshold current density (J th ) for GaAs/Al x Ga 1Àx As terahertz quantum cascade lasers (THz-QCLs) with different Al barrier compositions is studied. We achieved a maximum operation temperature (T max ) of 143 K for a 3.7 THz QCL by employing a longitudinal-optical (LO) phonon depopulation scheme. High-Al-composition barriers are used for increasing averaged LO-phonon energy. A significant reduction in J th of approximately 30% was obtained by increasing the Al composition from 15 to 35%, when we used the same energy separation of the depopulation stats (E 21 ). T max could be increased by using high-Al-composition structures due to the reduction of thermallyactivated LO-phonon scattering. #
Terahertz quantum cascade lasers (THz QCLs) are theoretically analyzed using the non-equilibrium Green’s function method. Simulations reveal a carrier leakage channel from the upper laser level to the first high-energy state in the emitting double well of the next period. This leakage channel is due to unintentional alignment of the two states, and is distinct from the thermally activated leakage channels. By tuning the energy of this high-energy state, the leakage current is clearly suppressed. The optimized THz QCL shows significant improvement, with a peak power of 350 mW, compared with the value of 220 mW for the non-optimized structure.
In this work nonpolar (m-plane) GaN terahertz quantum cascade lasers are designed based on the non-equilibrium Green's function method. The lasing quantum-level broadening that arises from electron-longitudinal optical (LO) phonon coupling is studied by tuning the coupling strength, and the changes of optical gain due to this broadening are demonstrated in a self-consistent way. It is found this coupling process largely widens the linewidth of intersubband transition and then quenches the lasing even at a low temperature of 10 K. The electron-LO phonon coupling strength, however, can be manually engineered by tailoring the quantum well structure. As a result, the optical gain will be recovered above the cavity threshold, indicating a potential for terahertz lasing from GaN semiconductors.
Operating at high temperatures in the range of thermoelectric coolers is essential for terahertz quantum cascade lasers to real applications. The use of scattering-assisted injection scheme enables an increase in operating temperature. This concept, however, has not been implemented in a short-period structure consisting of two quantum wells. In this work, based on non-equilibrium Green’s function calculations, it emphasizes on the current leakage and parasitic absorption via high-energy states as fundamental limitations in this scheme with short-period. A new design concept employing asymmetric wells composition is proposed to suppress these limitations. A peak gain of 40 cm
−1
at 230 K is predicted in the GaAs/AlGaAs semiconductor material system with an emission frequency of 3.5 THz.
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