BRILLOUIN SCATTERING FROM METAL SUPERLATTICES.

Abstract

Acoustic modes guided by thin-film metal superlattices have been investigated using Brillouin spectroscopy. Samples were grown on both single-crystal sapphire and fused silica substrates by alternately sputtering two different metals to yield a total thickness in the range 0.3 - 0.5 μm. Structural and chemical characterization of the polycrystalline films was performed using x-ray diffraction. Rutherford backscattering and optical interferometry. Thermally excited acoustic waves in the metal film create a surface ripple which weakly interacts with light incident from a single mode argon laser. A tandem Fabry-Perot consisting of two synchronized 3-pass cavities is used to measure the frequency shift of light which is inelastically scattered from acoustic waves. The contrast ratio of this interferometer exceeds 10¹⁰ and provides sufficient stray light rejection to detect the surface Rayleigh wave and as many as 13 higher order acoustic modes. The elastic stiffness constants of the anisotropic superlattices were estimated by fitting the measured acoustic mode velocities to a parameterized acoustic model. A comparison is made between these elastic constants and those predicted from the properties of the separate bulk constituents. The dependence of bilayer wavelength on the elastic properties of both Cu/Nb and Mo/Ta superlattices over the range of roughly 10 to 200 Å was determined. The unexpected softening of Cu/Nb superlattices within a range of bilayer wavelengths near 20 Å which was reported previously is qualitatively similar to the measurements reported here. It is shown that the elastic stiffness coefficient with the largest variation is c₄₄. The stiffness variations determined for the Mo/Ta samples are much smaller than for Cu/Nb. It is suggested that this is due to either structural differences (Cu/Nb is fcc-bcc and Mo/Ta is bcc-bcc) or the smaller interfacial lattice mismatch for Mo/Ta. Interfacial strain is found to be strongly correlated with the stiffness variations of the Mo/Ta samples. However, the underlying cause of these variations in stiffness remains anomalous. This dissertation also reports the first observations of Love waves and Stoneley waves by Brillouin scattering. The purely transverse Love waves guided by Cu/Nb films were detected by elasto-optic scattering from the evanescent acoustic strain in the sapphire substrate. The stiffness coefficient c₁₂ of the hexagonally symmetric metal film cannot be determined by the other guided acoustic waves which ripple the surface. Molybdenum in contact with fused silica is predicted to support a Stoneley wave which is guided by the interface. The lowest order Sezawa made guided by a molybdenum film was found to evolve to the Stoneley wave as the film becomes thicker. These measurements together with measurements of the surface Rayleigh wave show that the stiffness of the sputtered metal films is quite homogeneous and independent of film thickness.