Characterization of Thin Porous Silicon Films Formed on n + / p Silicon Junctions by Spectroscopic Ellipsometry

Abstract

Optical and structural properties of thin porous silicon (PS) layers electrochemically formed in the outermost part of 0.3 μm thin emitters of commercial silicon shallow junctions were studied by spectroscopic ellipsometry (SE). The SE data were evaluated with a multiparameter fitting procedure based on the Bruggeman effective medium approximation by using a mixture of fine‐grain polycrystalline Si and voids. The fit results show that the thicknesses of PS films grown at a constant current density of in 30% hydrofluoric acid (HF) with etching times of 1.5 and 3.5 s are 54 and 105 nm, respectively. When the formation time is less than 2.5 s, the porosity is a constant 60% throughout the entire PS layer. A porosity gradient occurs across the layer for formation times greater than 2.5 s, resulting in a layer of up to 85% porosity near the surface after 3.5 s. This increasing porosity is due to chemical dissolution by the HF electrolyte, occurring as a simultaneous reaction to the electrochemical PS formation. All the PS layers grow at a rate of , which is independent of the PS formation time and is not affected by the phosphorus doping gradient in the emitter. The calculated number of elementary charges, z, needed to dissolve one Si atom from the bulk is 3.3. Comparison of the SE fit with total reflectance measurements confirms the validity of the SE modeling. PS layer thicknesses were measured independently of the SE experiments by combining secondary ion mass spectroscopy measurements and surface profiling. © 2000 The Electrochemical Society. All rights reserved.