  We theoretically model the spin-orbit interaction in silicon quantum dot
devices, relevant for quantum computation and spintronics. Our model is based
on a modified effective mass approach with spin-valley boundary conditions,
derived from the interface symmetry under presence of perpendicular to the
interface electric field. The g-factor renormalization in the two lowest valley
states is explained by the interface-induced spin-orbit 2D (3D) interaction,
favoring intervalley spin-flip tunneling over intravalley processes. We show
that the quantum dot level structure makes only negligible higher order effects
to the g-factor. We calculate the g-factor as a function of the magnetic field
direction, which is sensitive to the interface symmetry. We identify spin-qubit
dephasing sweet spots at certain directions of the magnetic field, where the
g-factor renormalization is zeroed: these include perpendicular to the
interface magnetic field, and also in-plain directions, the latter being
defined by the interface-induced spin-orbit constants. The g-factor dependence
on electric field opens the possibility for fast all-electric manipulation of
an encoded, few electron spin-qubit, without the need of a nanomagnet or a
nuclear spin-background. Our approach of an almost fully analytic theory allows
for a deeper physical understanding of the importance of spin-orbit coupling to
silicon spin qubits.
