Evolution of optical constants and electronic structure of disordered Si1-xGex alloys

The energy shifts of optical interband transition edges, E-0('), E-1, E-1+Delta (1) and E-2, of relaxed Si1-xGex alloys grown epitaxially on Si(001) substrates by molecular beam epitaxy have been studied as a function of Ce composition using their complex dielectric functions measured by spectroscopic ellipsometry at room temperature. The interband transition edges were resolved by a line shape fitting on the numerical second derivative spectra of the dielectric functions. The E-0('), E-1, E-1+Delta (1) and E-2(Sigma) edges are found to shift to lower energies with increasing Ge composition while the E-2(X) edge shifts to higher energies. Also it is found for E-1 and E-1+Delta (1) energies that downward bowing exists and for Delta (1) energy that upward bowing exists. These behaviours of the transition edges are understood by comparing the band structure of Si with that of Ge and interpreted as due to the effect of the random potential originated by alloying disorder.