Microstructural Analysis and Modelling of Thin Porous Silicon Layers with Variable Angle Spectroscopic Ellipsometry

Abstract

AbstractThin porous silicon (PS) layers (30–1000 nm) have been produced by electrochemical anodization of highly p-doped silicon in a hydrofluoric acid electrolyte at constant current density. Variable angle spectroscopic ellipsometry was used for characterization in the spectral range 1.24–5.00 eV. Four multilayer models were developed for the PS layers. Ellipsometric spectra were fitted for three of these models by utilizing an effective medium approximation for each sublayer containing crystalline silicon and voids. A third constituent, amorphous silicon, was added in the fourth model. Excellent fits to experimental data were obtained in the entire spectral range and thickness and composition of each individual sublayer in the models were determined. The analyses revealed a compositional gradient normal to the surface. The porosity and the fraction of amorphous silicon decreased with film depth, whereas the fraction of crystalline silicon increased. The mean porosity of a PS layer was obtained as the thickness weighted average of the porosities of the sublayers. The proposed multilayer models allow a more detailed analysis of PS layers compared to single layer models.