Electrochemical and Chemical Behavior of Porous Silicon Layers: The Role of the Material Wettability and its High Specific Surface Area

Abstract

Anodic oxidation in hydrochloric (HC1) acid and chemical dissolution in hydrofluoric (HF) acid solutions of porous silicon (PS) layers obtained from lightly doped p-type substrate have been investigated. It is shown that the wettability of PS, which is a highly hydrophobic and organophilic material, strongly affects its behavior. When the solutions do not penetrate the pores, a very weak electroluminescence is obtained during the anodic oxidation process and furthermore the chemical etch rate in HF is strongly reduced. This is attributed to the fact that only the top surface of the layers is involved. However when the solutions infiltrate the pores, both the EL intensity and chemical etch rate are drastically increased since the whole volume of PS layers is involved. The etch rate of PS in HF solution is quantitatively compared to that of bulk Si. The high etch rate of PS is attributed to the high surface area contained in the material. The specific surface area of PS as a function of the porosity is deduced from the chemical dissolution data. It is also shown that the etch rate of PS increases with decreasing HF concentration. A model is tentatively suggested for this surprising result.