Vertical transport in GaAs/AlxGa1-xAs superlattices is investigated by a microwave time-of-flight technique. The velocity-field characteristics obtained from the phase data exhibit three regions: a negative differential velocity region, an oscillatory velocity region, and an increasing velocity region. The oscillations are periodic in F-1. It is shown that the F-1 oscillations are related to the resonances between the Wannier-Stark states and the continuum states above the superlattice barriers.
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