In-plane refractive-index anisotropy in porous silicon layers induced by polarized illumination during electrochemical etching

Abstract

Porous silicon (PSi) layers have been anodically etched under polarized illumination, and the degree of linear polarization of their photoluminescence (PL) was measured. The etching conditions were chosen such that the resulting PSi layers were thin enough for interference fringes to appear in their PL spectra. Experimental results show a sinusoidal variation in the degree of linear polarization as a function of the emission photon energy. The amplitude and phase of the sinusoidal variation depend largely on the polarization direction of the excitation light. These observations give strong evidence that there is a significant in-plane anisotropy of the refractive indices for these PSi samples. Values ranging from 0.1% to 0.67% have been obtained for the magnitude of the birefringence. The maximum refractive index is obtained when the excitation-light polarization direction is perpendicular to that of the polarized illumination used during etching. This can be explained by assuming that the polarized photoelectrochemical etching causes the Si structures to be thinner in the polarization direction.