Strain in coherent-wave SiGe/Si superlattices

We report measurements of the Raman spectra of Si1−xGex/Si (0.4≤x≤0.6 and x=1) wavy superlattices characterized by the presence of a coherent undulated alloy-layer structure. The strain induced shifts of the alloy Si–Si, Si–Ge and Ge–Ge mode frequencies in the wavy superlattices from the corresponding unstrained bulk mode frequencies are compared with those found in Si1−xGex/Si planar superlattices. Overall, the Si–Ge and Ge–Ge mode frequencies as a function of x are found to be reduced with respect to those in the planar superlattices, indicating a reduction of average strain in the wavy superlattices compared with the built-in strain in the planar superlattices. For the Si–Si mode, the mode frequency surprisingly behaves the opposite way. We propose that a compositional fluctuation in the alloy layer of the wavy superlattices, caused by the outward diffusion of Ge into the Si layer during growth, can explain this inconsistency in the experimental results.