Quantum size effects in ultrathin epitaxial Mg films on Si(111)

Highly perfect ultrathin epitaxial Mg(0001) films can be grown on Si(111)7x7 by low-temperature deposition and annealing, as demonstrated using angle-resolved photoelectron spectroscopy and low-energy electron diffraction. This is in contrast to films grown at room temperature, which present an interfacial silicide and subsequent growth of a disordered Mg metallic film. The wave-vector-dependent electronic structure of the well-ordered films is investigated in detail, as a function of overlayer thickness. The spectra exhibit a number of thickness-dependent discrete peaks in the region of the magnesium sp band for films up to 44 ML thick. These are caused by electron confinement within the Mg overlayer and are identified as quantum-well states derived from the magnesium sp band. They are interpreted in terms of the phase-accumulation model which, together with the Mg band structure, is found to account for all the main features in the spectra. An estimate of the decay length of the Mg(0001) surface state wave function is obtained from its binding energy dependence on film thickness. Sharp intensity changes at about 20 ML thickness point to a strain-driven thickness-dependent structural transition at this coverage.