Interplay between structural changes, surface states and quantum confinement effects in semiconducting MgSi and CaSi thin films.

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