An EIS Investigation into the Influence of HF Concentration on Porous Silicon Formation

Electrochemical impedance spectroscopy is used to study the etching of p-type silicon in ethanolic HF over the potential region where porous silicon forms. The impedance data fits a single equivalent circuit for all HF concentrations. The appearance of a finite length Warburg element at potentials negative of the flatband potential is interpreted as evidence that a submonolayer of oxide/hydroxide islands form in the Tafel region. Although this is contrary to previous studies it is consistent with published spectroscopy data. Low values for the double layer capacitance are attributed to the hydrophobic nature of the Si-H terminated surface, whereas an inductance loop or negative differential capacitance behavior seen at low frequencies is interpreted in terms of a potential-dependent surface roughness through the formation/dissolution of a submonolayer oxide. Interpretation of the resistive components of the circuit are in line with the known mechanism of etching, that is charge transfer changing to mixed control as the potential is made more positive and dissolution becomes limited by mass transport processes. (C) 2014 The Electrochemical Society. All rights reserved.