  We investigate spin-flip processes of Si quantum dots due to spin-orbit
coupling. We utilize the spin-orbit coupling constants related to bulk and
structure inversion asymmetry obtained numerically for two dimensional
heterostructures. We find that the spin-flip rate is very sensitive to these
coupling constants. We investigate the nuclei-mediated spin-flip process and
find the level of the isotope $^{29}$Si concentration for which this mechanism
become dominant.
