  We present an experimental and theoretical study of the conduction states of
crystalline Si films confined within amorphous SiO_2 barriers, using the Si-2p
core-level excitations. The spectral peaks near the conduction band minimum are
compared with the bulk silicon spectrum. In the Si quantum wells, it is found
that the conduction band minimum and the low-lying peaks undergo a blue shift
while all higher peaks remain unshifted. The experimental results are supported
by calculations using recent first-principles structural models for Si/SiO_2
superlattices. The experimental results suggest that all conduction states up
to a given conduction band offset become confined and blue-shifted while those
at higher energies are not confined and undergo no shift. These results provide
unambiguous evidence that the visible-light emitting properties of Si/SiO_2
structures depend strongly on electron confinement effects.
