Hot-optical-phonon effects on electron relaxation in an AlGaAs/GaAs quantum cascade laser structure

The influence of hot-phonon effects on coupled electron-phonon system relaxation dynamics in an AlGaAs/GaAs quantum cascade laser structure at 10 K has been investigated by the ensemble Monte Carlo technique. The GaAs quantum well laser system considered herein supports lasing between two electron subbands separated by 295 meV. After injection into the upper energy level, electrons transit to the lower subband by means of light emission or phonon-assisted scattering processes. Optical-phonon emission dominates among the latter radiationless electron relaxation channels making the carrier lifetime in the upper subband very short. Therefore, large threshold injection currents are required to create the electron population inversion necessary for lasing; this is one of the most significant shortcomings of quantum cascade lasers. The possibility of increasing the effective lifetime of carriers in the upper laser subband as a result of their return there from the lower subband by means of induced hot-optical-phonon reabsorption was proposed in the literature. However, our simulation results demonstrate that under realistic conditions the role of hot phonons is the opposite: substantial electron heating in the subbands and significant induced optical-phonon emission lead to a reduction in the electron population inversion causing an additional increase in the threshold currents.