Elastic and structural properties of supported porous silicon layers

Andrews, Gordon Todd (1999) Elastic and structural properties of supported porous silicon layers. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Brillouin spectroscopy has been used to investigate the elastic properties of (111)-oriented π - Si samples formed from p⁻ and p⁺ type c - Si substrates. In the frequency regime studied, Brillouin spectra of low porosity (28%-40%) p⁺ samples consist of a single set of inelastic peaks due to the surface acoustic wave. These surface waves were found to have velocities which are significantly lower than the corresponding c - Si velocity, and, for samples of a given set, decreased with increasing porosity, ξ. Removal of the thin native oxide film present on low porosity p⁺ type samples by an HF dip, results in a 3%-6% decrease in surface acoustic wave velocity compared to pre-dip values. Adsorbed water on the porous silicon surface appears to have no measurable effect on the surface acoustic wave velocity. -- Complete sets of elastic constants for several low porosity layers formed from p⁺ type substrates were determined from the directional dependence of the surface acoustic wave velocity in the (111) plane. The elastic constants Cij, were found to be much smaller than those for c - Si and, for a given set, decreased with increasing porosity. Empirical fitting of expressions of the form -- [special characters omitted] -- to the experimental data leads to the following relations for the porosity dependence of the elastic constants: -- C11π-Si = 168.5(1-ξ)3.00 -- [special characters omitted] -- The exponents m₁₁, m₁₂ and m₁₄ differ from those of porous samples formed from p⁻ type substrates. This is attributed to microstructural differences between porous layers formed from p⁻ and p⁺ type substrates. In addition, Young's modulus values were calculated from the elastic constants and compared with those determined in other experiments. In contrast to other studies, elastic anisotropy is taken into account. -- A limited number of Brillouin spectra was also obtained from intermediate porosity (50% and 60%) samples fabricated from p⁻ type substrates. These spectra exhibited multiple broad Brillouin peaks at relatively low frequency shifts (< 15 GHz) and are qualitatively similar to those collected by Beghi et al. from samples with similar porosity formed from p⁻ type substrates. -- The structural and light-emitting properties of π - Si prepared from p⁻ and p⁺ type (111)-oriented c - Si substrates have been studied using Raman scattering. A detailed analysis of the Raman lineshapes was performed using a phonon confinement model with realistic longitudinal and transverse optic phonon dispersion curves. This model basically explains the reduced Raman shifts and asymmetric broadening of the Raman peaks in the porous silicon samples of the present work. Characteristic nanocrystallite sizes and shapes were determined for samples with porosities in the range 35% to 80%. The highly porous samples consist of fine Si spheres, while those of lower porosity are primarily wire-like. The photoluminescence spectra are less size-sensitive than the Raman spectra and no clear correlation between the Raman scattering structural information and the photoluminescence spectra has been observed.

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