Elastic properties of porous silicon superlattices

Polomska, Anna Maria (2010) Elastic properties of porous silicon superlattices. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The elastic properties of porous silicon single layers and superlattices were determined by means of Brillouin light scattering. The quality of the Brillouin spectra dependent on the porosity of the porous layer and significant improvement was observed with the increase of porosity. The morphology and thicknesses of the porous silicon films were studied using scanning electron microscope. The porosity of both single and multilayered films was calculated using gravimetric method. -- The elastic constants of p-Si superlattices composed of layers of various porosity were compared to values of effective elastic constants obtained from the model proposed by Grimsditch and Nizzoli [M. Grimsditch and F. Nizzoli, Phys. Rev. B, 33, 8, 5891, 1986] which was reported to be applicable for other types of semiconducting superlattices. As the model requires the elastic constants of the constituent layers of the superlattice as input, the set of elastic constants was determined for each single porous silicon layer assuming cubic symmetry. -- The bulk phonon velocities and respective elastic constants of single layers and superlattices decreased with increasing average porosity of the film. The effective elastic constants of the superlattices were calculated directly from the spectra collected at smallest incident angle as well as obtained through the fitting of data with expressions for angular dependence of the velocities of the bulk phonons. The fitting was done with and without constraints on the values of elastic constants (c₁₁ > c₃₃). The longitudinal and transverse elastic constants of the superlattice (c₃₃ and c₄₄) showed excellent agreement with the model for all the approaches, while the values of c₁₁ and c₁₃ agreed only when the constraints were imposed. -- Based on the results only partial agreement with a model may be concluded. An excellent agreement for two elastic constants for all superlattices may either mean the other two cannot be determined due to uncertainties involved in the experiments or that the model needs corrections in order to work for superlattices created by electrochemical etching of a crystalline parent wafer.

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