Roles of an ultrathin Si insertion layer in AlAs/Si/GaAs system are studied by using in situ x-ray photoemission spectroscopy. It is found that a main role of the Si layer is not a control of a valence band offset as was proposed by Sorba et al, but an introduction of band bending in an overgrown layer. The proposed band-bending model explains all the experimental observations in the x-ray photoemission spectra; an Al 2p line broadening and the dependence of a peak energy shift on the crystallographic orientation and an overgrown-layer thickness.