Effects of stress on the dielectric function of strained pseudomorphic Si_(1-x)Ge_x alloys from 0 to 75% Ge grown on Si (001)

摘要

The dielectric function of bi-axially strained, intrinsic, and pseudomorphic Si_(1-x)Ge_x alloys was measured at room temperature using spectroscopic ellipsometry from 0.74 eV to 5.06 eV. Un-doped Si_(1-x)Ge_x with germanium compositions ranging from 0 to 75% was grown on Si (001) using chemical vapor deposition. High resolution x-ray diffraction was used to confirm sample composition, thickness, and strain. X-ray relaxation scans showed that all the samples were fully strained. All the alloy films in this study have low values of surface roughness, which allowed determination of the dielectric function. The presence of strain in the Si_(1-x)Ge_x alloys clearly altered the dielectric response. The bi-axial stress induced shift of the E_1 and E_1+Δ_1 critical point energies of pseudomorphic alloys can be described by the elastic response to the strain based on k*p theory [Lange et al., J. Appl. Phys. 80, 4578 (1996)]. Although the critical point energies of the alloys having higher germanium concentration showed deviations from the large shear approximation, the strain induced shift in critical point energy and the relative intensities of E_1 and E_1+Δ_1 were reasonably well described by the full elastic theory.