  Double quantum dots are convenient solid-state platforms to encode quantum
information. Two-electron spin states can be conveniently detected and
manipulated using strong quantum selection rules based on the Pauli exclusion
principle, leading to the well-know Pauli spin blockade of electron transport
for triplet states. Coherent spin states would be optimally preserved in an
environment free of nuclear spins, which is achievable in silicon by isotopic
purification. Here we report on a deliberately engineered, gate-defined silicon
metal-oxide-semiconductor double quantum dot system. The electron occupancy of
each dot and the inter-dot tunnel coupling are independently tunable by
electrostatic gates. At weak inter-dot coupling we clearly observe Pauli spin
blockade and measure a large intra-dot singlet-triplet splitting $>$ 1 meV. The
leakage current in spin blockade has a peculiar magnetic field dependence,
unrelated to electron-nuclear effects and consistent with the effect of
spin-flip cotunneling processes. The results obtained here provide excellent
prospects for realizing singlet-triplet qubits in silicon.
