  The hyperfine interaction of phosphorus donors in fully strained Si thin
films grown on virtual Si$_{1-x}$Ge$_x$ substrates with $x\leq 0.3$ is
determined via electrically detected magnetic resonance. For highly strained
epilayers, hyperfine interactions as low as 0.8 mT are observed, significantly
below the limit predicted by valley repopulation. Within a Green's function
approach, density functional theory (DFT) shows that the additional reduction
is caused by the volume increase of the unit cell and a local relaxation of the
Si ligands of the P donor.
