Effects of interface broadening on far-infrared and Raman spectra of GaAs/AlAs superlattices

A one-dimensional local-mode model is developed to describe phonons propagating normal to the layers of (GaAs${)}_{\mathit{n}1}$/(AlAs${)}_{\mathit{n}2}$ superlattices, in which the effects of broadening of the interfaces can be quantitatively described. The model is applied to the analysis of Raman scattering and far-infrared (FIR) measurements on short-period superlattices, ${\mathit{n}}_1$=${\mathit{n}}_2$=2,3,. . .,8, and is shown to give a good description of data in both the GaAs and AlAs optic-phonon regions using an interface width parameter W of 1.4 lattice units. The model also describes the intensities of Raman-scattering modes, FIR dielectric scattering strengths, and linewidths. The analysis demonstrates that the effects of interface broadening must be included in an accurate description of phonons in short-period superlattices, and gives a quantitative assessment of interface quality that is in agreement with x-ray measurements.