Acoustic waves in finite superlattices: Influence of buffer layers

The influence of buffer layers on acoustic waves of shear horizontal polari zation in finite superlattices is investigated here theoretically. Using a Green's-function method, we have obtained closed-form expressions of local and total densities of states as a function of the frequency omega and the wave vector k(parallel to) parallel to the layers. The expressions of the p honon trapping times are also derived and compared to the density of states . Due to the substrate/buffer layer/superlattice interaction, different kin d's of localized and resonant discrete modes are found and their properties are investigated. In particular, we show the possibility of finding well-d efined guided modes in the buffer layer or in the superlattice depending on whether the velocity of the transverse acoustic waves in the buffer is low er or higher than those in the superlattice and the substrate, respectively . These modes appear as well-defined peaks of the density of states and tra pping times. Specific applications of our analytical results are given in t his paper for a GaAs (Si) buffer layer embedded between a Si (GaAs) substra te and a finite GaAs-AlAs superlattice. [S0163-1829(99)13003-0].