Strong photoluminescence anisotropy in porous silicon layers prepared by polarized-light assisted anodization

Linearly-polarized infrared (1.06 μm) laser light with intensities ranging from 5.3 to 97 mW/cm2 has been used to obtain anisotropically luminescent porous silicon (PSi) layers by photoanodic etching in a hydrofluoric acid solution. Remarkably large photoluminescence (PL) anisotropy has been observed in samples prepared with the highest illumination intensity. These samples show very low degrees of linear polarization when the PL excitation light is polarized parallel to the polarization direction of the etching light. When the excitation light is polarized perpendicular to that, we obtain usual degrees of linear polarization of several percent. This result indicates that anisotropic Si nanostructures in PSi layers can be made isotropic with high orientation selectivity by the polarized-light assisted technique. A simple two-dimensional model is presented to explain the observed prominent anisotropy.