Nonequilibrium optical-phonon population by sequential resonant tunneling in GaAs-AlAs superlattices

We report on the observation of nonequilibrium GaAs LO phonons generated by the relaxation of hot electrons due to sequential resonant tunneling in strongly coupled superlattices. Raman-scattering experiments were performed at 5 K with low optical excitation density. The anti-Stokes LO-phonon intensity, which gives a measure of the nonequilibrium LO-phonon population, exhibits maxima at the resonant tunneling voltages. The transport data show that the electrical heating power increases linearly with voltage. Our results suggest that hot-electron cooling via acoustical phonon emission is suppressed under sequential resonant tunneling conditions.