  With any roughness at the interface of an indirect-bandgap semiconducting
dot, the phase of the valley-orbit coupling can take on a random value. This
random value, in double quantum dots, causes a large change in the exchange
splitting. We demonstrate a simple analytical method to calculate the phase,
and thus the exchange splitting and singlet-triplet qubit frequency, for an
arbitrary interface. We then show that, with lateral control of the position of
a quantum dot using a gate voltage, the valley-orbit phase can be controlled
over a wide range, so that variations in the exchange splitting can be
controlled for individual devices. Finally, we suggest experiments to measure
the valley phase and the concomitant gate voltage control.
