Carbon dependence of the dielectric response function in epitaxial SiGeC layers grown on Si

The influence of carbon on optical transitions in SiGeC layers grown pseudo morphically on Si(100) substrate by rapid thermal chemical vapor deposition was investigated. We used spectroscopic ellipsometry to extract the pseudo dielectric function for two sets of layers, with Ge-content of 8 and 18% an d with C-content between 0 and 1.8%. From the numerical derivatives of the measured dielectric functions, we determined the critical point energies E- 1, E-1 + Delta(1), and E-2 as a function of the C-content. A weak linear de pendence on the C-content was observed for all transitions. The experimenta l results could not be described either by considering solely the alloying effect, nor by linear addition of the strain-induced contributions. For a c omplete description of the observed energy shifts, a detailed band structur e calculations and further experimental data are necessary. (C) 2000 Elsevi er Science S.A. All rights reserved.