  We investigated electron transport through ultra small Si quantum dots. We
found that the $B$-dependence of energy levels is dominated by the Zeeman
shift, allowing us to measure the spin difference between two successive ground
states directly. In some dots the number of electrons N in the dot can be tuned
starting from zero, and the total spin of the dot can be mapped as a function
of N and B. For one of the dots we deduced that the dot becomes spontaneously
polarized at N=6 with a large spin change \Delta S=3/2, demonstrating the
essential features of spin blockade. Surprisingly, for N>20, the transitions
with \Delta S>1/2 do not lead to the suppression of the corresponding peaks at
low temperatures.
