Quantum-size effects in ultrathin Mg films: electronic structure and collective excitations

The electronic structure of epitaxial Mg(0 0 0 1) films grown on Si(1 1 1) and Al/Si(1 1 1) is investigated in detail using angle-resolved photoelectron spectroscopy. The photoemission spectra exhibit a number of quantum-well states derived from the magnesium s–p band, caused by electron confinement within the Mg/Si overlayer for films up to more than 40 monolayers thick. This permits a parameter-free bulk band determination and the investigation of the in-plane dispersion behavior of quantum-well resonances in overlayers of different thicknesses. Strong spectral variations with photon energy are explained as effects of the interaction of the electromagnetic field with confined collective modes of the quasi-free electron gas. A non-reactive Mg/Al interface can also be prepared by low temperature deposition, permitting the creation of a double overlayer and, despite their very similar valence band structure, the observation of quantum-well states in Mg films deposited on Al layers.