  Valley-orbit coupling is a key parameter for a silicon quantum dot in
determining its suitability for applications in quantum information processing.
In this paper we study the effect of interface steps on the magnitude and phase
of valley-orbit coupling for an electron in a silicon quantum dot. Within the
effective mass approximation, we find that the location of a step on the
interface is important in determining both the magnitude and the phase of the
valley-orbit coupling in a Si/SiGe quantum dot. Specifically, our numerical
results show that the magnitude of valley orbit coupling can be suppressed up
to 75\% by a step of one atomic monolayer, and its phase can change by almost
$\pi$. When two steps are present, the minimum value of the valley-orbit
coupling can even approach zero. We also clarify the effects of an applied
external magnetic field and the higher orbital states on the valley-orbit
coupling. Overall, our results illustrate how interface roughness affect the
valley-orbit coupling in silicon, and how spin qubits in silicon may be
affected.
