In-plane dispersion of the quantum-well states (QWS’s) associated with the electron confinement in metastable epitaxial Ag films grown on the $\mathrm{Si}\mathrm{}\left(111\right)7\times 7$ and $\mathrm{Si}\mathrm{}\left(001\right)2\times 1$ surfaces is investigated by angle-resolved photoemission using synchrotron radiation. In contrast to the free-electron-like behavior expected, these QWS’s show intriguing dispersions such as (i) a significant enhancement of the in-plane effective mass with decreasing binding energy and (ii) a splitting of a QWS into two electronic states with different dispersions at off-normal emission. Such unexpected electronic properties of a QWS are obviously related to the substrate band structure. Further the QWS splitting is explained by the energy-dependent phase shift of the film-substrate interface occurring at the substrate band edge.

• Received 27 July 2001

DOI:https://doi.org/10.1103/PhysRevB.65.085327