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High barrier AlxGa1–xAs terahertz quantum cascade structures with AlAs compositions up to 30% are studied. The influence of thermally excited leakage scattering to higher bound energy states and decoherence effects are investigated using a density matrix method where the pure dephasing time is self-consistently solved. The lattice temperature dependence of the light output in diagonal optical transition structures is calculated and shown to be consistent with experiment. Scattering from the upper lasing state to higher bound energy states is found to have minimal effects, and rather the decoherence from the calculated reduction of the pure dephasing time due to the impurity interaction is primarily responsible for the temperature dependence of a recently reported structure. This shows that the effects from an increased impurity interaction due to thinner barriers and different well dimensions can dominate over that from the increased interface roughness interaction due to higher potentials.

Terahertz quantum cascade structures using double longitudinal-optical phonon intrawell scattering for depopulation are theoretically studied. A density matrix Monte Carlo method is used to calculate the temperature dependent optical power, in double phonon structures with diagonal optical transitions. It is shown that using depopulation transitions greater than the resonant longitudinal-optical phonon energy ΔE > ℏωLO reduces the phonon absorption thermal backscatter, allowing for higher operating temperatures, with prospects for 300 K room temperature and beyond. Furthermore, results indicate that the temperature limit may also be improved in single phonon structures, by similarly increasing the depopulation transition.

The nonequilibrium absorption of longitudinal-optical phonons by hot electrons are studied in terahertz quantum cascade structures. We present a method for including electron leakage to the continuum that takes into account the mobility of the electrons. This is incorporated into a density matrix Monte Carlo method that includes the optical field within the resonant cavity. The effects of electron leakage to the continuum as a function of lattice temperature are discussed. Results are compared with experiment and found to be consistent. It is shown that using only confined wave functions and thereby neglecting the leakage via tunneling is inadequate for describing the electron transport.