Density dependence of nonresonant tunneling in asymmetric coupled quantum wells

We have measured tunneling rates in a series of asymmetric coupled quantum wells as a function of the barrier thickness and carrier density between confined levels in which optical-phonon-assisted tunneling is energetically forbidden. Our measurements have demonstrated a strong density dependence of the tunneling rates. The measured dependence is qualitatively consistent with a model of excitons diffusing to a limited number of extrinsic tunneling sites.