Effects of stress on the dielectric function of strained pseudomorphic Si1−xGex alloys from 0 to 75% Ge grown on Si (001)

Abstract

The dielectric function of bi-axially strained, intrinsic, and pseudomorphic Si1−xGex alloys was measured at room temperature using spectroscopic ellipsometry from 0.74 eV to 5.06 eV. Un-doped Si1−xGex 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 Si1−xGex alloys clearly altered the dielectric response. The bi-axial stress induced shift of the E1 and El + Δ1 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 E1 and El + Δ1 were reasonably well described by the full elastic theory.