We describe photoluminescence and admittance spectroscopy of p-type Si/Si0.25Ge0.25/Si quantum well structures with the SiGe quantum well surrounded by undoped Si spacer layers of various thickness. Holes confined in the SiGe quantum well create a local electric field, which induces potential barriers for holes in the surrounding Si : and a potential well for electrons in the vicinity oi:he Sice quantum-well region. Decreasing the thickness of one of the Si spacers from 30 nm to 5 nm increases the local electric field and shifts the SiGe-related near-band-edge photoluminescence spectrum to higher photon energies. This can be explained by a reduced exciton binding energy due to exciton polarization. The polarization is caused by the increasingly asymmetrical potential well for electrons and holes for the thinner Si spacer layers. In addition, admittance spectroscopy was carried out in order to measure the potential barriers for the confined holes for various thicknesses of the Si spacer layers. For thicker Si spacer layers, the results are in agreement with the photoluminescence data. For thinner Si spacer layers, thermally activated tunnelling of holes via the potential barrier was observed, Our interpretations are supported by theoretical calculations.