  The valley splitting in Si two-dimensional electron systems is studied using
Si/SiGe single quantum wells (QWs) with different well widths. The energy gaps
for 4 and 5.3 nm QWs, obtained from the temperature dependence of the
longitudinal resistivity at the Landau level filling factor $\nu=1$, are much
larger than those for 10 and 20 nm QWs. This is consistent with the well-width
dependence of the bare valley splitting estimated from the comparison with the
Zeeman splitting in the Shubnikov-de Haas oscillations.
