  We calculate the diffusion thermopower of the degenerate two-dimensional hole
gas in a $p-$type Si/Si$_{1-x}$Ge$_x$ lattice mismatched heterostructure at low
temperatures and zero magnetic field. The effects of possible scatterings, e.g.
remote impurity, alloy disorder, interface roughness, deformation potential,
and random piezoelectric on the hole mobility and the diffusion thermopower are
examined. Calculated results are well fitted to the experimental data recently
reported. In addition, we predict a possibility for the diffusion thermopower
to change its sign as the SiGe layer thickness changes, the effect has not been
discussed yet.
