Acoustic phonon propagation in superlattices: Group velocities and angular dependence of stop bands

Abstract

The field of group velocities in systems of elastically isotropic layers is calculated from dispersion surfaces. Numerical results are presented for the a-Si:H/a-siN x:H superlattice investigated in Ref. 1. The magnitude of the group velocity is used to classify modes. Furthermore we obtained, for different types of vibrations, the dependence of the frequency intervals corresponding to nonpropagating modes (stop bands) on the direction of incidence. It is shown that the stop band widths can be adjusted by introducing different outoffs of the group velocity. Finally, the dependence of the direction of propagation of frequency is derived for different angles of incidence.