Observation of resonant tunneling through a compositionally graded parabolic quantum well

We report the first observation of electron resonant tunneling through parabolic quantum wells, compositionally graded by means of short-period (15Å) AlxGa1-xAs/GaAs superlattices grown by molecular beam epitaxy. In one structure, comprising a 300Å wide well compositionally graded from AlAs to GaAs, five equally spaced resonances are observed in the current voltage characteristic (I-V), in good agreement with the theory. In another structure with 432Å wide wehs graded from Al0.30Ga0.70As to GaAs, up to sixteen resonances are observed in the I-V. The first ten correspond to resonant tunneling through the quasi-bound-states of the double barrier while the other ones are ascribed to electron interference effects associated with virtual levels in the quasi-continuum energy range above the collector barrier.