Shahin Zangooie

Vapor sensitivity of thin porous silicon layers

Spectroscopic ellipsometry has been used to study optical changes in thin porous silicon layers due to exposure to vapors from different liquids. The changes in the ellipsometric parameters Ψ and Δ were caused by changes in the total refractive index of the layers. The detection limit threshold for acetone vapors by the layers was 12 ppm. Using a 4-layer optical model, it was determined that capillary condensation was involved in the vapor sensitivity of the layers. Based on these findings, it was concluded that thin porous silicon layers can be used as an optical sensor material for gas sensing purposes.

Protein adsorption in thermally oxidized porous silicon layers

Abstract Variable angle spectroscopic ellipsometry was employed for multilayer characterization of thermally oxidized porous silicon layers and for quantitative adsorption studies of human serum albumin and fibrinogen in these layers. The oxidized samples were used either as prepared or were hydrated in a deionized water bath prior to their use. Hydration decreased the volume percentage of adsorbed albumin from 24 to 13% in the sublayer next to the ambient but resulted in an increase of the amount of adsorbed protein deeper in the film. Hydration did not, however, affect the adsorption behavior of fibrinogen which was found to adsorb only in the outermost sublayer. This phenomenon was attributed to the larger size of this protein. The amount of adsorbed fibrinogen depended on the concentration of the protein in the stock solution.

COLOR CHANGES IN THIN POROUS SILICON FILMS CAUSED BY VAPOR EXPOSURE

Thin films of porous silicon formed by electrochemically etching silicon wafers changed color when exposed to an ambient atmosphere saturated in various organic solvent vapors. The degree of the color change was related to the refractive indices of the solvents. Analysis of the data using a four‐layer optical model indicated that the film refractive index increased up to 15% when solvent molecules replaced air in the pores. Solvent condensing from the saturated atmosphere filled up to 45% of the total void volume. Thermally oxidizing the films to make them hydrophilic resulted in surfaces which changed color upon exposure to water.

Monitoring specific interaction of low molecular weight biomolecules on oxidized porous silicon using ellipsometry

Porous silicon dioxide surfaces have been used for monitoring the specific affinity binding of low molecular weight molecules to streptavidin. Streptavidin was immobilized to the porous silicon dioxide surface by spontaneous adsorption at pH 7.4. Binding of biotin and an oligopeptide synthesized by means of combinatorial chemistry were monitored with an in situ null ellipsometer. Measurements were also done with hydroxy-azobenzene-2-carboxylic acid and DL-6-8-thioctic acid amide. The performance of porous silicon dioxide as a potential surface in biosensor applications was compared with a planar silicon dioxide surface. Porous silicon dioxide showed a 10-fold amplification of the response compared to planar silicon dioxide. It was possible to monitor the binding of biotin and the oligopeptide in the concentration range 2-40 microM. A response time as low as 30 s was obtained for the oligopeptide at 40 microM.

Microstructural control of porous silicon by electrochemical etching in mixed HCl/HF solutions

Abstract The role of hydrochloric acid in the fabrication of porous silicon was investigated with atomic force microscopy (AFM) and spectroscopic ellipsometry. The study was focused on surface morphology, in terms of roughness and power spectral density (PSD), and layer microstructure and quality in terms of thickness, porosity, interface sharpness and layer homogeneity. The results show that HCl can be used as an additional component in the standard hydrofluoric acid-based etching solution in order to affect different properties of porous silicon layers such as volume porosity, thickness and pore size. In addition, HCl/HF mixtures can be used to obtain samples with sharper porous silicon/silicon interfaces and less in depth inhomogeneities as compared with porous silicon samples having similar volume porosities and thicknesses and manufactured in conventional solutions containing HF only. Whereas large variations in both thickness and volume porosity can be obtained by conventional solutions, the HCl/HF interplay yield large variations in one of the mentioned parameters only.

Electrochemical Tailoring and Optical Investigation of Advanced Refractive Index Profiles in Porous Silicon Layers

Porosity depth profiles with exponential or sinusoidal shape were fabricated electrochemically in crystalline silicon using time-variable current densities and studied employing variable angle spectroscopic ellipsometry. Since volume porosity in porous silicon depends on the current density, it was possible to electrochemically tailor porosity depth profiles, which in a first approximation resembled the time modulation of the applied current. Optical characterization of the samples were realized using multilayer optical models and the Bruggeman effective medium approximation allowing variations of the index of refraction according to the applied current density profiles. The analysis also revealed deviations from desired profiles in terms of in-depth inhomogeneities.