Quantum confinement effects on the electronic structure of Si(001) ultrathin films: Energy shifts of optical band edges and luminescence in crystalline and amorphous Si films

The electronic structure for Si(001) ultrathin films terminated by SiO4 species is calculated by the extended Huckel-type nonorthogonal tight-binding method and compared to experimental results for amorphous Si(a-Si)/SiO2 superlattices grown on (001)Si substrates by molecular beam epitaxy (MBE) [Lockwood et al., Phys. Rev. Lett. 76, 539 (1996)]. A remarkable coincidence in the energy shifts of the valence band maximum (VBM) and conduction band minimum (CBM) between the calculation and experiment strongly suggests that the MBE-grown a-Si well layers are almost crystalline and can have a CBM projected in the (001) growth direction and a local minimum of the conduction band as in the crystalline Si(001) films. The similarity of the thickness dependence of observed photoluminescence (PL) peak energy shifts to that of calculated energy-gap shifts indicates that the observed PL should be due to direct recombination of excitons between the VBM and the projected CBM of extended states quantized in the a-Si well layers as in the crystalline Si(001) ultrathin films. It is found that the binding energy of excitons (100 meV) related to the PL can be a combination of the increased Coulomb energy induced by a compression of wave functions for two-dimensional excitons and the one caused by a reduction in dielectric constant due to confinement in the growth direction.