Abstract

The structural and vibrational properties of one dimensional quasiperodic Fibonacci superlattices (QPFSLs) are of great interest, since this kind of structures are an intermediate state between ordered and disordered structures.^[1] We present results obtained from measurements carried out in metallic Ta/Al QPFSLs by use of the Brillouin light scattering technique. We have used a Sanderock-type tandem six- pass Fabry-Perot interferometer, operating on a single mode of the 514.5 nm line of an Ar⁺ laser.^[2] The experimental apparatus used is sketched in Fig. 1. Measurements were performed on superiattices 1.5 µm thick, deposited by magnetron sputtering on glass substrates. The experimental points of a typical spectrum are shown in Fig. 2. Two features can be recognised in this spectrum: the pronounced peak at about 7.6 GHz, due to the surface Rayleigh wave and the dip located at about 17.5 GHz, corresponding to the frequency of longitudinal acoustic waves propagating parallel to the surface. A comparison between experimental spectra and theoretical cross section calculated according to the model of Marvin et al^[3] by taking into account only the ripple contribution, allowed us to determine the two effective elastic contats c₁₁ and c₄₄. The values obtained are 1% and 10% respectively higher than those calculated on the basis of the effective modulus model,^[4] starting from the bulk elastic contants of Ta and Al. In order to explain this, we have developed a theoretical approach which takes into account the effects of quasiperiodic modulation. This model is based on the Fouriertransform method, already used by us in previous elastic investigations of semiconductor QPFSLs.^[5] Applying this method we could explain the measured values of both c₄₄ and c₁₁, showing that a positive correction has to be made to results obtained from the effective modulus model (see Fig. 3). We stress that this modulation is associated with the large interfacial mismatch of acoustic impedance between different superlattice elements.

© 1992 IQEC