Thin Porous Silicon Films Displaying a Near-Surface Dip in Porosity

Studies of the depth profile of Porous Silicon (PS) structures etched under a wide span of different formation parameters have been performed using Spectroscopic Ellipsometry (SE) and Transmission Electron Microscopy (TEM). Incorporation of a sharp dip in the porosity profiles close to the sample surface was required for several types of PS films in order to successfully fit ellipsometric data to a Bruggeman Effective Medium Model (BEMA). TEM studies show a corresponding lower porosity region near the sample surface. Investigations have been performed in order to understand the origin of this dip and to explore the parameter space where it is detected. The detected low porosity region is discussed in the context of current knowledge on pore nucleation and growth mechanisms of PS.