Porous silicon films and multilayers were studied using Brillouin spectroscopy. Acoustic wave velocities and elastic constants were determined for films with porosity ∼0.60 formed from p–, p+ and n+ crystalline silicon. The velocities and elastic constants depend on the pore/crystallite geometry and morphology. Porous silicon multilayers were fabricated from (100) p+ crystalline silicon. For a multilayer with 250 nm layer thickness and layer porosity sequence of 0.45/0.65/0.45/0.65/0.45, the Rayleigh surface phonon velocity was found to be 2850 m/s. The origin(s) of other peaks in the multilayer spectra are unknown but it is unlikely that they are surface modes because their shift(s) are independent of incident angle. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ordered carbon nanotube arrays fabricated on alumina templates were studied using inelastic laser light scattering spectroscopy. Multiple peaks were observed in spectra from an array where the nanotubes extend ∼150nm beyond the template surface. These protruding segments are modeled as hollow cylinders clamped at one end. Good agreement is obtained between the predicted vibrational frequencies and the experimental peak frequency shifts. The peaks are thus attributed to the transverse, longitudinal, and torsional vibrations of the protruding nanotube segments. This assignment yields values of 26 and 20GPa for the Young’s and shear moduli of the nanotubes, respectively.

Porous silicon superlattices with constituent layer porosities of ∼30% and ∼60% and modulation wavelengths of 17 nm, 20 nm and 34 nm were fabricated by electrochemical etching in HF‐based electrolytes. The elastic constants of the superlattices were estimated using Brillouin light scattering spectroscopy and compared to those calculated from an effective elastic constants model (M. Grimsditch and F. Nizzoli, Phys. Rev. B 33, 5891 (1986) [1]). The fair agreement between the experimental and calculated constants suggests that the model holds promise for predicting the elastic properties of these and possibly other porous multilayered systems. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Brillouin spectra have been recorded for a series of supported films of p-type porous 6H-SiC with a branched morphology and porosities in the range from 30% to 58%. Complex spectra comprising up to 7 identifiable components were observed in some cases. An effective medium model is being developed as an aid in interpreting the spectra, and preliminary results are presented.

An overview of the technique of Brillouin light scattering spectroscopy and its application to the study of hypersound in micro- and mesoporous materials systems will be presented. Particular emphasis will be placed on results obtained from light scattering experiments on porous silicon-based structures. For porous silicon films, it was found that the acoustic phonon velocities and elastic properties depend strongly on the film porosity and morphology. Brillouin studies of porous silicon superlattices with periodicity on the order of the hypersound wavelength reveal that these structures behave as hypersonic phononic crystals, while those with smaller modulation wavelengths act as effective elastic media. New results on localized acoustic modes in porous silicon multilayers will also be discussed. Collectively, these studies provide a detailed picture of hypersound propagation in porous silicon systems and demonstrate the utility of Brillouin spectroscopy for probing acoustic phonon behavior in this challenging class of materials. [This work was supported by the Canada Foundation for Innovation, Memorial University of Newfoundland, and the Natural Sciences and Engineering Research Council of Canada.]