Resonance effects in the carrier-tunneling dynamics in asymmetric coupled quantum wells

We present a detailed experimental study of the tunneling times, after picosecond excitation, of electrons and holes in a large set of unbiased GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As asymmetric double-quantum-well structures, pointing out the role played by resonances in the tunneling process. In particular, fast hole tunneling is found when the quantum wells show band alignment both at ${\mathit{k}}_{\mathrm{?}}$=0 and at ${\mathit{k}}_{\mathrm{?}}$≠0. Referring to electrons, a resonance is found in the tunneling time, which corresponds to phonon-assisted processes. Experimental data at high excitation intensity are also reported, suggesting band-filling effects in the wider well.