Interplay between structural changes, surface states and quantum confinement effects in semiconducting Mg₂Si and...

Ab initio techniques have been used to investigate structural changes in semiconducting Mg₂Si and Ca₂Si thin films (from 17 nm down to 0.2 nm corresponding to the 2D structure) along with band-gap variations due to quantum confinement. Cubic Mg₂Si(111) thin films being dynamically stable at thicknesses (d) larger than 0.3 nm displayed an indirect band gap, the reduction of which with increasing d could be reasonably well described by the simple effective mass approximation. Only 2D Mg₂Si has a unique structure because of the orthorhombic distortion and the direct band gap. Since the surface energy of cubic Ca₂Si(111) films was lower with respect to any surface of the orthorhombic phase, which is the ground state for the Ca₂Si bulk, the metastable in-bulk cubic phase in the form of thin films turned out to be preferable in total energy than any orthorhombic Ca₂Si thin film for d < 3 nm. Sizable structural distortion and the appearance of surface states in the gap region of Ca₂Si thin films with d < 3 nm could be the reason for an odd dependence of the band-gap variation on d.