Spectroscopic ellipsometry and band structure of Si_1-yC_y alloys grown pseudomorphically on Si (001)

We have measured the dielectric functions of three Si_1-yC_y alloys layers (y ≤ 1.4%) grown pseudomorphically on Si (001) substrates using molecular beam epitaxy at low temperatures. From the numerical derivatives of the measured spectra, we determine the critical point energies E₀′ and E₁ as a function of y (y ≤ 1.4%) using a comparison with analytical line shapes and analyze these energies in terms of the expected shifts and splittings due to negative hydrostatic pressure, shear stress, and alloying. Our data agree well with the calculated shifts for E₁, but the E₀′ energies are lower than expected. We discuss our results in comparison with recent tight-binding molecular dynamics simulations by Demkov and Sankey (Phys. Rev. B 48, 2207, 1993) predicting a total breakdown of the virtual-crystal approximation for such alloys.