  Demonstrating the quantum-confined Stark effect (QCSE) in silicon
nanocrystals (NCs) embedded in oxide has been rather elusive, unlike the other
materials. Here, the recent experimental data from ion-implanted Si NCs is
unambiguously explained within the context of QCSE using an atomistic
pseudopotential theory. This further reveals that the majority of the Stark
shift comes from the valence states which undergo a level crossing that leads
to a nonmonotonic radiative recombination behavior with respect to the applied
field. The polarizability of embedded Si NCs including the excitonic effects is
extracted over a diameter range of 2.5--6.5 nm, which displays a cubic scaling,
$\alpha=c D^3$, with $c=2.436\times 10^{-11}$ C/(Vm), where $D$ is the NC
diameter. Finally, based on intraband electroabsorption analysis, it is
predicted that p-doped Si NCs will show substantial voltage tunability, whereas
n-doped samples should be almost insensitive. Given the fact that bulk silicon
lacks the linear electro-optic effect as being a centrosymmetric crystal, this
may offer a viable alternative for electrical modulation using p-doped Si NCs.
