  In this work, we study hole transport in a planar silicon
metal-oxide-semiconductor based double quantum dot. We demonstrate Pauli spin
blockade in the few hole regime and map the spin relaxation induced leakage
current as a function of inter-dot level spacing and magnetic field. With
varied inter-dot tunnel coupling we can identify different dominant spin
relaxation mechanisms. Applying a strong out-of-plane magnetic field causes an
avoided singlet-triplet level crossing, from which the heavy hole g-factor
$\sim$ 0.93, and the strength of spin-orbit interaction $\sim$ 110 $\mu$eV, can
be obtained. The demonstrated strong spin-orbit interaction of heavy hole
promises fast local spin manipulation using only electrical fields, which is of
great interest for quantum information processing.
