Different behavior of photoluminescence anisotropy in porous silicon layers made by polarized-light-assisted electrochemical etching

Luminescent porous silicon (PSi) layers have been made on $n$-type Si wafers by anodization in a HF solution under illumination with linearly polarized light. It is observed that the photoluminescence from samples anodized at a current density of 30 $mA/cm2$ and slightly oxidized electrochemically for passivation exhibits a significant anisotropy in polarization memory (PM): the degree of PM becomes maximum when the vector electric field $(Eexc)$ of excitation laser light is parallel to that $(Eetch)$ of the light used during anodization, and minimum when $Eexc$ is perpendicular to $Eetch.$ These maxima and minima are in totally different directions from those reported previously. In addition, the anisotropy is found to be weak at low anodization current density of 2.3 $mA/cm2.$ These experimental results are discussed based on the electrochemical, rather than photochemical, thinning of Si-nanocrystal assemblies in PSi layers.