Using time-resolved transconductance spectroscopy, we study the tunneling
dynamics between a two-dimensional electron gas (2DEG) and self-assembled
quantum dots (QDs), embedded in a field-effect transistor structure. We find
that the tunneling of electrons from the 2DEG into the QDs is governed by a
different time constant than the reverse process, i.e., tunneling from the QDs
to the 2DEG. This asymmetry is a clear signature of Coulomb interaction and
makes it possible to determine the degeneracy of the quantum dot orbitals even
when the individual states cannot be resolved energetically because of
inhomogeneous broadening. Our experimental data can be qualitatively explained
within a master-equation approach
