Role of an Oxidant Mixture as Surface Modifier of Porous Silicon Microstructures Evaluated by Spectroscopic Ellipsometry

Abstract

Current research on porous silicon includes the construction of complex structures with luminescent and/or photonic properties. However, their preparation with both characteristics is still challenging. Recently, our group reported a possible method to achieve that by adding an oxidant mixture to the electrolyte used to produce porous silicon. This mixture can chemically modify their microstructure by changing the thickness and surface passivation of the pore walls. In this work, we prepared a series of samples (with and without oxidant mixture) and we evaluated the structural differences through their scanning electron micrographs and their optical properties determined by spectroscopic ellipsometry. The results showed that ellipsometry is sensitive to slight variations in the porous silicon structure, caused by changes in their preparation. The fitting process, based on models constructed from the features observed in the micrographs, allowed us to see that the mayor effect of the oxidant mixture is on samples of high porosity, where the surface oxidation strongly contributes to the skeleton thinning during the electrochemical etching. This suggests the existence of a porosity threshold for the action of the oxidant mixture. These results could have a significant impact on the design of complex porous silicon structures for different optoelectronic applications.

Thickness and tensile stress determination of black silicon layers by spectral reflectance and Raman scattering

“…Theoretical modeling can be divided into three steps: 1. dividing the measured sample into the layer system (number of layers, roughness at the interfaces, etc), 2. selecting a suitable optical function for each layer, 3. optimization of constructed model [17]. Optical parameters of pSi layers have very complex dependence on the layer microstructure and creating a suitable optical model is essential [18]. Several functions for creating a theoretical dispersion model can be used: New Amorphous Dispersion Formula (NADF, for amorphous materials exhibiting an absorption in the visible and far UV range -absorbing dielectrics, semiconductors, polymers) [19], Lorentz Dispersion Model (transparent or weakly absorbing materials -insulators, semiconductors) [20], Tauc-Lorentz dispersion formula (amorphous materials exhibiting an absorption in the visible and far UV range -absorbing dieletrics, semiconductors, polymers) [21], Briot dispersion formula (transparent materials) [22] and other.…”