Optical properties of Ge and Si nanosheets - confinement and symmetry effects

Comparison between silicon and germanium quantum sheets has been made regarding their electronic and optical properties. Ab initio calculations have been applied for this purpose by mean of the linearized augmented plane wave method. Quantum confinement is found to shift the band gap of both Si and Ge bulk to the blue, however this upshift depends strongly on the surface orientation. Moreover also the nature of the band gap is related to the surface symmetry: for (100)-oriented films the band gap becomes direct for both semiconductors, whereas the (110)- and (111)-oriented films show a different behavior. In the first case the band gap is direct for Si and indirect for Ge, in the second it is direct for Ge and indirect for Si. Concerning the optical properties, since in the Si films the folded bulk band energies are found to retain their original indirect character, all the Si films have an intense absorption peak only at high energies corresponding to the blueshifted direct band gap of Si bulk. In Ge films the conduction band minimum retains a strong F component. Therefore, dielectric function calculations clearly show that Ge films have a strong optical absorption in the visual energy region. Analysis of the squared optical matrix elements is also presented and the data are compared to the results for GaAs. (C) 2003 Elsevier Science B.V. All rights reserved