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+D elta (1) and E-2, of relaxed Si1-xGex alloys grown epitaxially on Si(001) s ubstrates by molecular beam epitaxy have been studied as a function of Ce c omposition using their complex dielectric functions measured by spectroscop ic ellipsometry at room temperature. The interband transition edges were re solved by a line shape fitting on the numerical second derivative spectra o f the dielectric functions. The E-0('), E-1, E-1+Delta (1) and E-2(Sigma) e dges are found to shift to lower energies with increasing Ge composition wh ile 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) energ y that upward bowing exists. These behaviours of the transition edges are u nderstood by comparing the band structure of Si with that of Ge and interpr eted as due to the effect of the random potential originated by alloying di sorder.