Thickness and porosity characterization in porous silicon photonic crystals: The etch-stop effect

Abstract

One-dimensional photonic crystals and microcavities were fabricated using porous silicon via anodization. The study investigated the effect of an etch-stop layer, which is included between two adjacent porous layers to resolve the issue of HF diffusion during pore formation. Although proposed as a solution, no systematic investigation had been conducted on this parameter until now. Our study found that the etch-stop layer induces a redshift in the photonic bandgap, regardless of the electrolyte solution used. The fitting analysis of both single and multilayer structures revealed that the redshift resulted from an increase in the silicon etch rate and a decrease in porosity due to the enhancement of HF diffusion through the porous structure. Furthermore, an analysis of oxidation as a function of temperature demonstrated that the inclusion of the etch-stop layer led to the formation of porous layers with varying microstructures. This effect was particularly pronounced in devices where the first upper layer had high porosity. The thermal treatment caused a photonic bandgap inhibition, which was linked to the contraction-expansion phenomena of the low and high porosity layers, respectively. This resulted in the partial violation of Bragg's law. Overall, our study provides insights into the influence of etch-stop layers on the optical properties of porous silicon-based photonic structures.