Folded acoustic and confined optical phonons in a (Si15Ge4)(50) atomic-layer superlattice

A Raman-scattering intensity analysis of folded acoustic modes in a high-qu ality (Si15Ge4)(50) superlattice has allowed a detailed;comparison with var ious theoretical models. The modulus of the Ge/Si photoelastic coefficient ratio is found to be similar to 1 at 457.9 nm, but a satisfactory simultane ous fit to all the intensity information could not be obtained with any of the models. The role of an interface translational displacement contributio n to the scattering cross section is shown to be significant. Odd and, for the first time, even index confined optic modes are observed for the Si lay ers in the polarized Raman spectrum. Lower-frequency quasiconfined optic mo des exhibit dispersion that is consistent with previous model calculations. [S0163-1829(99)05203-0].