Confined LO phonons in superlattices with interfacial broadening

A 1D lattice dynamics model for superlattices (SL) is developed to investigate the consequences of interfacial broadening on the confined optical phonons. It is shown that simple results can be obtained for two cases when the alloy of the substances that construct the SL displays either explicit two-mode behaviour (for instance, AlGaAs) or an intermediate type of optical behaviour. In the latter case, the modes of two optical subbands move towards each other, and delta -like peaks of the phonon density of states broaden into the Gaussian-like curves. In contrast, for AlAs/GaAs SL, the modes of both the AlAs-like and GaAs-like subbands move downwards and broaden asymmetrically. The shape of the lines is sensitive to the composition profile.