  We examine energy spectra of Si quantum dots embedded into Si_{0.75}Ge_{0.25}
buffers using atomistic numerical calculations for dimensions relevant to qubit
implementations. The valley degeneracy of the lowest orbital state is lifted
and valley splitting fluctuates with monolayer frequency as a function of the
dot thickness. For dot thicknesses <6 nm valley splitting is found to be >150
ueV. Using the unique advantage of atomistic calculations we analyze the effect
of buffer disorder on valley splitting. Disorder in the buffer leads to the
suppression of valley splitting by a factor of 2.5, the splitting fluctuates
with ~20 ueV for different disorder realizations. Through these simulations we
can guide future experiments into regions of low device-to-device fluctuations.
