Thickness-dependent strain relaxation in Si0.8Ge0.2/Si(001) epitaxial films studied by spectroscopic ellipsometry

Variation of dielectric response function of Si0.8Ge0.2/Si(0 0 1) epitaxial films for varying layer thickness was detected by spectroscopic ellipsometry measurements. As the epilayer thickness increases, interband-transition structures near 3.3 eV by E-0', E-1 and E-1 + Delta(1) edges and near 4.2 eV by E-2 edges of Si0.8Ge0.2 are found to shift gradually to lower energies indicating gradual relaxation of lattice-mismatch-induced strain in the epilayer. Result of a line-shape analysis on the dielectric functions of the alloys indicates that the rates of the strain-induced energy shifts of the E-1 and E-1 + Delta(1) edges are larger than those of the E-0' and E-2 and also the energy splitting between the E-1 and E-1 + Delta(1) increases as the strain in the film increases. The strain dependence of the E-1 and E-1 + Delta(1) energies of the alloys agrees fairly with the result of a theoretical estimation based on the deformation potential theory. (C) 1998 Elsevier Science Ltd. All rights reserved.