Vertical transport in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As 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 ${\mathit{F}}^{\mathrm{-}1}$. It is shown that the ${\mathit{F}}^{\mathrm{-}1}$ oscillations are related to the resonances between the Wannier-Stark states and the continuum states above the superlattice barriers.

• Received 10 November 1994

DOI:https://doi.org/10.1103/PhysRevB.51.17614