Three-dimensional structure of (110) porous silicon with in-plane optical birefringence

Abstract

Electrochemical etching of a (110) oriented Si wafer results in a porous silicon (PSi) layer which exhibits a strong in-plane optical birefringence. We study the refractive index ellipsoid of (110) PSi by angle-resolved optical transmittance measurements and reveal that it is a biaxial crystal. The angle-resolved transmission electron microscope observations demonstrate that pores grow along the directions in between the <100> crystal axes and the etching current flow and these directions depend on the etching current density. The etching current density dependence of the pore direction indicates that the shape of the index ellipsoid can be controlled by the etching condition.