The Raman spectrum of nearly relaxed Ge1− ySny alloys grown on Si substrates was
investigated for Sn concentrations y< 0.2. The frequency dependence of the Ge–Ge Raman
mode was re-evaluated by carrying out careful corrections for residual strain shifts. The Ge–
Sn mode was observed for the first time in this type of samples. Its frequency appears to
decrease monotonically as a function of the Sn concentration. This is very different from the
behavior of the Si–Ge mode in Si1− yGey alloys, but a detailed analysis suggest that there is …