Brillouin light scattering from porous silicon films and multilayers

Abstract

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)

Porosity dependence of the acoustic longitudinal velocity in heavily doped p++ porous silicon layers

“…It is notable that as we have shown, in agreement with the work of Andrews [3], that for samples of identical porosity different longitudinal acoustic velocities will be found dependent on the doping of the silicon wafer in which the porous layer was etched. The velocity depends on a currently unquantified parameter of the PSi layer.…”

“…The velocity of sound in porous material depends on its microstructure [2,3] which for PSi strongly depends on the level of doping of the original silicon wafer. Higher doping levels are known to give a more ordered nanocrystal and pore morphology [3][4][5]. The majority of existing data on elastic properties is tabulated in the 1997 review of the properties of porous silicon [6].…”

Off-axis phonon and photon propagation in porous silicon superlattices studied by Brillouin spectroscopy and optical reflectance

“…Theoretical simulations of phonon and photon dispersion in multilayered porous silicon have been done to study propagation at an angle to the modulation axis; 11 these structures exhibited phononic and photonic band gaps at oblique angles. While oblique incidence 180 -backscattering geometry Brillouin light scattering experiments have been conducted on porous silicon superlattices, [12][13][14] no phononic crystal effects were reported. In particular, experiments were carried out to investigate the applicability of an effective elastic medium model to p-Si structures with modulation lengths much smaller than the probed phonon wavelength, 12,13 while other work has focused on surface-propagating modes with only a qualitative discussion of the observed bulk modes.…”

Elastic constants of porous silicon superlattices

“…Discrete layer formation was, however, observed in SEM images of multilayers with 100 nm constituent layer thickness. Moreover, the process used to fabricate the SLs was similar to that used in our previous work to make π-Si multilayers with layer thicknesses of ~0.5 μm [5] in which clear evidence of discrete layer formation was observed in SEM micrographs of cleaved cross-sections. As a further check that SLs were formed as prescribed, a second Λ = 17 nm SL and a second Λ = 20 nm SL were synthesized using conditions identical to those described above and subjected to a 3% KOH etch.…”

“…1 Introduction The elastic properties of supported porous silicon (π-Si) films have been studied in some detail over the past few years [2][3][4][5]. In contrast, the elastic properties of π-Si superlattices (SLs) have received little attention.…”