Effects of stress on the dielectric function of strained pseudomorphic Si _1-xGe _x alloys from 0 to 75 Ge grown on Si (001)

Abstract

The dielectric function of bi-axially strained, intrinsic, and pseudomorphic Si 1-xGe x alloys was measured at room temperature using spectroscopic ellipsometry from 0.74 eV to 5.06 eV. Un-doped Si 1-xGe 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-xGe x alloys clearly altered the dielectric response. The bi-axial stress induced shift of the E 1 and E l Δ 1 critical point energies of pseudomorphic alloys can be described by the elastic response to the strain based on kp theory [Lange, 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 l Δ 1 were reasonably well described by the full elastic theory. © 2012 American Institute of Physics.