Symmetrically strained Si/Ge superlattices on Si substrates

Symmetrically strained Si/Ge superlattices with an overall thickness of 0.2 m, well above the critical thickness (10 nm) of unsymmetrically strained superlattices of the same composition, are studied. Strain adjustment is obtained by growing thin homogeneous Si0.4Ge0.6 buffer layers (20 nm) on Si substrates. The period lengths vary in the range 0.7-2.8 nm. Raman scattering experiments confirm quantitatively the strain distribution and the superlattice periodicity. Observed zone-folded acoustic-phonon energies agree well with theoretically expected dispersion relations.