Direct observation of the acoustic gaps in Ge/GexSi1-x superlattices

Raman spectra of Ge/GexSi1-x superlattices were measured, probing close to the mini-Brillouin zone edge. The data reveal peculiar features in the low- wavenumber region of the spectra. These features are observed as dips super imposed on broad bands, the latter being related to the scattering of pure longitudinal phonons, The bands are broad owing to the contribution of abso rption processes to the Raman scattering. Such processes induce strong inte nsity in a wide range around the probing wavevector, It was found that the dips can be assigned to the gaps of the dispersion relation, with the aid o f models based on the photoelastic theory for longitudinal acoustic phonons in superlattices. Copyright (C) 1999 John Whey & Sons, Ltd.