Effects of interface steps on the valley-orbit coupling in a Si/SiGe quantum dot

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 π. When two steps are present, the minimum value of the valley-orbit coupling can even approach zero. Our calculation can in principle be generalized to multiple steps as well, as long as the width of the regions between steps is much larger than the atomistic length scale. 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 that interface roughness can strongly affect both the magnitude and the phase of the valley-orbit coupling, which are crucial parameters for both spin and charge qubits in silicon.