  We report finite-bias characteristics of electrical transport through
phosphorus donors in silicon nanoscale transistors, in which we observe
inelastic-cotunneling current in the Coulomb blockade region. The cotunneling
current appears like a resonant-tunneling current peak emerging from the
excited state at the crossover between blockade and non-blockade regions. These
cotunneling features are unique, since the inelastic-cotunneling currents have
so far been reported either as a broader hump or as a continuous increment of
current. This finding is ascribed purely due to excitation-related inelastic
cotunneling involving the ground and excited states. Theoretical calculations
were performed for a two-level quantum dot, supporting our experimental
observation.
