Composition and strain in thin Si1−xGex virtual substrates measured by micro-Raman spectroscopy and x-ray diffraction

Abstract

Micro-Raman spectroscopy was employed for the determination of the germanium content, x and strain, ε, in ultrathin SiGe virtual substrates grown directly on Si by molecular beam epitaxy. The growth of highly relaxed SiGe layers was achieved by the introduction of point defects at a very low temperature during the initial stage of growth. SiGe virtual substrates with thicknesses in the range 40–200 nm with a high Ge content (up to 50%) and degree of relaxation, r, in the range 20%–100% were investigated using micro-Raman spectroscopy and x-ray diffraction (XRD) techniques. The Ge content, x, and strain, ε, were estimated from equations describing Si–Si, Si–Ge, and Ge–Ge Raman vibrational modes, modified in this study for application to thin SiGe layers. The alteration of the experimentally derived equations from previous studies was performed using independent data for x and r obtained from XRD reciprocal space maps. A number of samples consisting of a strained-silicon (s-Si) layer deposited on a SiGe virtual substrate were also analyzed. The stress value for the s-Si varied from 0.54 to 2.75 GPa, depending on the Ge-content in the virtual substrates. These results are in good agreement with theoretically predicted values.