G. Socino

Brillouin scattering determination of the whole set of elastic constants of a single transparent film of hexagonal symmetry

In this work it is shown that the Brillouin light scattering technique can be successfully applied for determining the five effective elastic constants of a single transparent film of hexagonal symmetry, in the micron range of thicknesses. Measurements have been performed on a polycrystalline ZnO film, about 1.3 mu m thick, supported by a Si substrate. A major result of this work is that the elastic constant c66 is selectively determined for the first time from detection of the shear horizontal mode travelling parallel to the film surface. Similarly, a selective determination of c11 is attained from observation of the longitudinal mode guided by the film. The three remaining elastic constants, namely c13, c33, and c44, can be then obtained from detection of the Rayleigh surface mode and of the longitudinal bulk wave propagating at different angles from the surface normal.

Brillouin scattering by surface acoustic modes for elastic characterization of ZnO films

Brillouin scattering from surface phonons was used for determining the dispersion curves of guided acoustic modes propagating along piezoelectric ZnO films. Measurements were performed on films of different thicknesses in the range between 20 and 320 nm, deposited by RF magnetron sputtering on Si and SiO/sub 2/ substrates. Brillouin spectra from Rayleigh acoustic modes are taken in the backscattering geometry at different incidence angles between 30 degrees and 70 degrees . The experimental data for the ZnO/Si films fit the expected theoretical dispersion curves fairly well for film thicknesses greater than 150 nm, while they appreciably depart from the same curves for smaller thicknesses. This behavior is interpreted in terms of a reduction of the effective elastic constants of the film in a layer near the interface, due to the lattice misfit between the film and the substrate. This effect was not observed in the case of ZnO films deposited on fused quartz substrates.<<ETX>>

Comparative study of the elastic properties of polycrystalline aluminum nitride films on silicon by Brillouin light scattering

Abstract Brillouin light scattering (BLS) has been used to investigate the elastic properties of polycrystalline AIN films, about 1 μ m thick, grown by DC-reactive magnetron sputtering on Si 3 N 4 coated (100)-Si substrates. Taking advantage from the detection of a number of different acoustic modes, a complete elastic characterization of the films has been achieved. The elastic constants c 11 and c 66 have been selectively determined from detection of the longitudinal and of the shear horizontal bulk modes, respectively, travelling parallel to the film surface. The three remaining elastic constants, namely c 44 , c 33 and c 13 , have been obtained from detection of the Rayleigh surface wave and of the longitudinal bulk wave propagating at different angles from the surface normal. The values of the elastic constants of these sputtered AIN films depend on the deposition conditions and on the microstructural properties of the films, especially oxygen contamination and quality of texture. In the case of the films with the best degree of texture and the lowest oxygen content, the values of the elastic constants are rather close to those previously determined in epitaxial A1N films grown at high temperature by MOCVD. This demonstrates that sputter deposition at relatively low temperature can be used to grow high quality A1N films on Si and is of great importance in view of the integration of these films in the technology of IC semiconductors.

Incidence angle and polarization dependence of light diffracted by acoustic surface waves

A theoretical analysis of the diffracting effects produced by ASW on light beams contained on the sagittal plane of the wave is presented. The separate contributions have been explicitly studied, produced by periodical changes of the refractive index and by corrugation of the surface in cases of x‐ and y ‐cut crystalline quartz and y ‐cut LiNbO3 for both states of polarization of the incident light. In all cases analyzed, measurements of light‐deflection efficiency have been performed as a function of the angle of incidence in light‐transmission experiments.

The dynamics of poly(n-butyl acrylate) above the glass transition

Measurements of the dielectric constant of poly(n-butyl acrylate) have been taken in the frequency range 100 Hz-3 GHz, for temperatures between 254 and 359 K. Analysis of dielectric data and their comparison with results recently obtained on the same system by Brillouin light scattering have evidenced a two-step relaxation process. Dielectric spectra account for the behaviour of the slower process, i.e. the structural relaxation, while Brillouin spectra are particularly sensitive to the faster process, i.e. the secondary relaxation. The temperature behaviour of these processes is not in accordance with the predictions of the mode-coupling theory, while it can be suitably interpreted by a model that relates the high-frequency dependence of the dielectric loss curve to the local chain dynamics and the low-frequency dependence to the intermolecular correlation.

Interface effects and elastic constants of Ag/Ni superlattices studied by Brillouin scattering

The elastic properties of sputter‐deposited Ag/Ni superlattices have been studied by means of the Brillouin light scattering technique. Measurements have been taken on both free‐standing and glass‐supported superlattices, the former deposited at 100 K, the latter at 300 K. The two effective elastic constants c11 and c44 have been determined on specimens 5 μm thick for different superlattice periods; c44 decreases by almost 40% as the bilayer periodicity decreases from 19 to 1.3 nm, while c11 shows no appreciable variation. Measurements performed on supported specimens, 0.5 μm thick, enabled us to get information also on the constant c33, which exhibits a monotonic increase with period. The elastic anomalies found can be related to the appreciable lattice distortion observed by x‐ray diffraction experiments. Both Brillouin scattering and x‐ray analysis have shown that the interface quality improves appreciably when the temperature of deposition is lowered from 300 to 100 K.

Propagating volume and localized spin wave modes on a lattice of circular magnetic antidots

The spectrum of spin wave excitations on a nanometric two-dimensional periodical array of circular holes in a magnetic film was measured using the Brillouin light scattering technique. Two modes with positive group velocity in the frequency range between 4 and 7GHz were observed. Our calculations show that these correspond to the two lowest modes propagating along the edges of an effective stripe waveguide, perpendicular to the applied field, whose width is equal to the interhole distance. Moreover, a number of higher-frequency modes has been measured and identified as volume excitations of the same effective stripe.

Acoustic Lamb wave‐electric field nonlinear interaction in YZ LiNbO3 plates

The nonlinear electroacoustic effect has been investigated for acoustic Lamb wave propagation in a YZ LiNbO3 plate under a uniform bias electric field. A theoretical model is briefly outlined, which shows how the effect can be interpreted in terms of second and third order material constants. The strength of the nonlinear interaction has been experimentally evaluated through measurements of the change in phase velocity of Lamb modes produced by the bias field. The experiments were performed in the frequency range 8–33 MHz on most of the Lamb modes excited in a 1.25‐mm‐thick plate by an interdigital transducer with a periodicity λ=0.396 mm. The results evidence a strong dependence of the nonlinear effect on the order and symmetry of the analyzed modes.

The elastic constants of sputtered aluminum nitride films

Brillouin light scattering has been used for determining the five independent elastic constants of polycrystalline AlN films grown by DC magnetron sputtering on Si/sub 3/N/sub 4/ coated (100)-Si substrates. The elastic constants c/sub 66/ and c/sub 11/ have been selectively determined from detection of the shear horizontal mode and of the longitudinal mode travelling parallel to the film surface, respectively. The three remaining elastic constants, namely c/sub 13/, c/sub 33/ and c/sub 44/, have been obtained from detection of the Rayleigh surface mode and of the longitudinal bulk wave propagating at different angles from the surface normal. The values of the elastic constants of these sputtered AlN films exhibit an appreciable dependence on the microstructural properties of the films, which are strongly affected by the deposition conditions. However, they are rather close to those previously determined in epitaxial AlN films grown at high temperature by MOCVD.

Brillouin scattering by pseudosurface acoustic modes on (11(mean)1) GaAs

The authors have studied the propagation of pseudosurface acoustic modes on the (11(mean)1) plane of GaAs. From a calculation of the surface density of phonon states, two resonances are found in the continuum of modes, corresponding to the pseudosurface acoustic modes, called PSM and HFPSM. Experimental evidence for these modes is given by Brillouin spectra taken for different directions on the (11(mean)1) plane of a GaAs crystal. They found a good agreement between the experimental spectra and the theoretical cross sections, calculated taking into account both the ripple and the elasto-optic contributions to the scattering process.