Subpicosecond luminescence study of tunneling and relaxation in coupled quantum wells

We study, by time-resolved luminescence with subpicosecond resolution, the tunneling of electrons out of a narrow well (NW) coupled to a wider well (WW) by a thin barrier. The relative energy positions of the NW ground state and of the WW first excited level are changed by adjusting the WW width. Near resonance, and for narrow enough barriers, the transfer time of the electrons to the WW ground state is close to 2 ps and is analogous to LO-phonon-assisted intersubband relaxation. When the barrier thickness increases above 40 Å, this transfer time increases exponentially with the barrier thickness. The barrier thickness above which the variation becomes exponential strongly depends on the energy mismatch of the levels in the isolated wells and is indicative of interface roughness. The dependence of the decay times on barrier thickness for purposely misaligned samples is always exponential.