Dielectric functions, elasto-optical effects, and critical-point parameters of biaxially stressed Si_(1-y)C_y alloys on Si (0 0 1)

摘要

The complex dielectric functions ε(hω) from 0.74 to 6.6 eV of pseudomorphically strained Si_(1-x)C_y (0 < y < 0.014) alloys grown on Si (0 0 1) were determined using spectroscopic ellipsometry. Interference effects due to surface overlayers and multiple reflections at the substrate/epilayer interface were subtracted. We also report the critical-point parameters (amplitude, energy, broadening, phase angle and dimension) of the E'_0, E_1, E_2 and E'_1 interband transitions. While the E_1 energy gap increases linearly with increasing C content, in good agreement with a continuum elasticity model (taking into account the effects of biaxial stress and alloying with C based on a linear interpolation of the Si and diamond E_1 energies), the E'_0 gap stays approximately constant and the E_2 gap shows a significant decrease. The amplitudes of all critical points decrease by about 50% and the broadenings increase by about 50-80% when adding 1.4% C. The phase angles remain approximately the same as in Si, except for E'_1. The changes in the critical-point parameters can be understood due to the lattice relaxation (four Si nearest neighbours move towards C) and the strong alloy scattering, with obvious implications for ultrafast or high-field electronic transport in such alloys. Since the Si_(1-y)C_y alloys are under a tensile biaxial stress, the measured ordinary dielectric function is also affected by piezo-optical effects, which were calculated using literature data for bulk Si.