Silicon electrochemistry related to the formation of porous silicon

The authors have examined in detail the electrochemistry of both n- and p-type single-crystal (100) silicon in the porous-silicon-formation regime, using a rotating Si disk apparatus with an Ag/AgCl reference electrode. For p-type Si with doping levels between 10¹⁵ and 10¹⁹/cm³, the current-potential (I-E) characteristics are independent of doping level for all electrolyte compositions examined. This independence precludes the use of p-type layers for self-stopping porous Si-based SOI wafer synthesis. At low (<10 wt.%) HF electrolyte concentrations, I-E curves exhibit peaks rather than the plateau expected for a rotating-disk voltammogram (RDV). These are attributed to a chemical passivation/depassivation mechanism involving a competition between SiO₂ formation and dissolution by F-ions. For n-type Si with doping levels of 10¹⁵ to 10¹⁹/cm³, I-E characteristics depend on the dopant concentration as well as the electrolyte composition. The strong dopant dependence is well known and is the basis for controlled potential anodization of n^-/n⁺/n ^- structures for SOI wafer synthesis. It is shown that in these layered structures, however, stray dendrite-like pores can form in the n^- overlayer as well as in the n^-substrate. For buried layer porous silicon formation in n-Si, proper selection of both the cell potential and electrolyte composition are necessary to reduce stray pore formation