  We observe conductivity oscillations with aperiodic spacing to only one side
of the tunneling current in a dual-gated graphene field effect transistor with
an n-p-n type potential barrier. The spacing and width of these oscillatoins
were found to be inconsistent with pure Farbry-Perot-type interferences, but
are in quantitative agreement with theoretical predictions that attribute them
to resonant tunneling through quasi-bound impurity states. This observation may
be understood as another signature of Klein tunneling in graphene
heterojunctions and is of importance for future development and modeling of
graphene based nanoelectronic devices.
