  We report Stark shift measurements for 121Sb donor electron spins in silicon
using pulsed electron spin resonance. Interdigitated metal gates on top of a
Sb-implanted 28Si epi-layer are used to apply electric fields. Two Stark
effects are resolved: a decrease of the hyperfine coupling between electron and
nuclear spins of the donor and a decrease in electron Zeeman g-factor. The
hyperfine term prevails at X-band magnetic fields of 0.35T, while the g-factor
term is expected to dominate at higher magnetic fields. A significant linear
Stark effect is also resolved presumably arising from strain.
