  Shifts from the expected nuclear magnetic resonance frequencies of antimony
and bismuth donors in silicon of greater than a megahertz are observed in
electrically detected magnetic resonance spectra. Defects created by ion
implantation of the donors are discussed as the source of effective electric
field gradients generating these shifts via quadrupole interaction with the
nuclear spins. The experimental results are modeled quantitatively by molecular
orbital theory for a coupled pair consisting of a donor and a spin-dependent
recombination readout center.
