Charge Transfer and Nanostructure Formation During Electroless Etching of Silicon

The Turner mechanism of porous silicon formation during stain etching was developed and accepted without surface-sensitive data and without an understanding that nanostructures are being formed. Here it is shown that an oxide intermediate does not play a role in the formation of nanocrystalline porous Si films. Furthermore, a mechanistic understanding of etching and nanostructure formation leads to the formulation of seven rules for the rational design of stain etchants. These rules are used to develop three new formulations of stain etchants containing Fe(3+), VO(2)(+), and Ce(4+), which are demonstrated to effectively produce porous silicon. These new formulations represent a significant advance in stain etching as they avoid many of the problems associated with common nitrate-/nitrite-based stain etchants including no need for "activation", short induction times, and the reproducible production of homogeneous films of unprecedented thickness.