  We analyze the effects of valley interference on the quantum control and
manipulation of an electron bound to a donor close to a Si/SiO2 interface as a
function of the valley-orbit coupling at the interface. We find that, for
finite valley-orbit coupling, the tunneling times involved in shuttling the
electron between the donor and the interface oscillate with the interface/donor
distance in much the same way as the exchange coupling oscillates with the
interdonor distance. These oscillations disappear when the ground state at the
interface is degenerate (corresponding to zero valley-orbit coupling).
