M-plane GaN terahertz quantum cascade laser structure design and doping effect for resonant-phonon and phonon-scattering-injection schemes.

Non-polar m-plane GaN terahertz quantum cascade laser (THz-QCL) structures have been studied. One is traditional three-well resonant-phonon (RP) design scheme. The other is two-well phonon scattering injection (PSI) design scheme. The peak gains of 41.8 and 44.2 cm-1 have been obtained at 8.2 and 7.7 THz respectively at 300 K according to the self-consistent non-equilibrium Green's function calculation. Different from the usual GaAs two-well design, the upper and lower lasing levels are both ground states in the GaN quantum wells for the PSI scheme, mitigating the severe broadening effect for the excited states in GaN. To guide the fabrication of such devices, the doping effect on the peak gain has been analyzed. The two designs have demonstrated distinct doping density dependence and it is mainly attributed to the very different doping dependent broadening behaviors. The results reveal the possibility of GaN based THz-QCL lasing at room temperature.

Optical gain reduction caused by nonrelevant subbands in narrow-period terahertz quantum cascade laser designs.

The recent designs of terahertz quantum cascade lasers usually employ the short periodic length and also the tall barriers for high-temperature operation. In this work, the effect of high-energy lying non-relevant subbands is studied based on nonequilibrium Green's function formalisms model, demonstrating those subbands are probable to play a minor role on the population inversion, but play a major role on the optical gain at high temperatures. The phenomenon can be ascribed to the appearance of leakages crossing neighboring periods via sequential resonant tunneling, and those leakages are inherently created by the specific features of the two-well configuration in this design that the phonon well should be wide enough for performing the phonon scattering to depopulate the lower-laser subband. The narrower periodic length design can strengthen this inter-period leakage. A parasitic absorption between the first high-lying nonrelevant subbands from two laser wells can closely overlap the gain shape and thus significantly reduce the peak gain.

Nonrelevant quantum levels effecting on the current in 2-well terahertz quantum cascade lasers.

Recent renewed operating temperatures in terahertz quantum cascade lasers emphasize on narrowing the periodic length in a 2-well resonant-phonon design for a clean quantum level structure, in which the depopulation energy is significantly higher than one longitudinal phonon. In this study, various depopulation energies (small and large) are engineered in a 2-well design; the effect of the high-lying nonrelevant levels on the currents are systematically studied by using the non-equilibrium Green's function method. The engineering of the depopulation energy is unable to avoid the formation of leakage channels, which are activated within at least three neighboring periods via sequential close tunneling. However, a large depopulation energy relaxes the thermal backfilling process; as a result, the net leakages at high temperatures can be significantly suppressed. In addition, pre-alignment remains a critical issue in the design when using a large depopulation energy, which requires improved engineering for the barriers to obtain better laser dynamics.

Leakages suppression by isolating the desired quantum levels for high-temperature terahertz quantum cascade lasers.

The key challenge for terahertz quantum cascade lasers (THz-QCLs) is to make it operating at room-temperature. The suppression of thermally activated leakages via high lying quantum levels is emphasized recently. In this study, we employ the advanced self-consistent method of non-equilibrium Green's function, aiming to reveal those kinds of leakages in the commonly used THz-QCL designs based on 2-, 3- and 4-quantum well. At the high temperature of 300 K, if all the confined high lying quantum levels and also the continuums are included within three neighboring periods, leakages indeed possess high fraction of the total current (21%, 30%, 50% for 2-, 3- and 4-quantum well designs, respectively). Ministep concept is introduced to weaken those leakage channels by isolating the desired levels from high lying ones, thus the leakages are well suppressed, with corresponding fractions less than 5% for all three designs.

Short-period scattering-assisted terahertz quantum cascade lasers operating at high temperatures.

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.