Charge Transfer and Nanostructure Formation During Electroless Etching of Silicon

Abstract

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 Fe3+, VO2 +, and Ce4+, 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.

The stoichiometry of metal assisted etching (MAE) of Si in V2O5+HF and HOOH+HF solutions

“…Understanding of the chemistry fundamental to stain etching has advanced greatly as a result of quantitative studies on charge transfer and reaction stoichiometry [39][40][41][42]. Anodic etching in the por-Si formation region occurs either by a current doubling (valence 2) or current quadrupling (valence 4) path.…”

Nanoporous silicon tubes: the role of geometry in nanostructure formation and application to light emitting diodes

“…Tube progression may stop for two possible reasons: first, the SCR at the bottom of the pillar may be influenced or screened by the bulk substrate; secondly, the transport of the species necessary for stain etching may be limited for high-aspect-ratio tubes and may deplete the solution inside. This could slow down or completely stop the advance of tube formation [66].…”

Porous Silicon Formation by Stain Etching

“…The former is used at a concentration of roughly 1 M, the latter closer to 0.1 M (Kolasinski et al 2010(Kolasinski et al , 2012Dudley and Kolasinski 2009a). Reproducible por-Si formation is possible with both oxidants when concentrated HF (aq) (49 wt% ¼ 29 mol L À1 in HF) is used as the acidic fluoride source.…”

“…Whereas ethanol addition is commonly used during anodic formation of por-Si to reduce the deleterious effects of bubbles, its use in stain etching is rather limited. The apparent reduction of bubble formation and sticking to the substrate is more closely related to a strong reduction in the etch rate, rather than a superlative surfactant action (Kolasinski et al 2010). …”

“…Through experiments on a broad range of oxidants and a better understanding of the fundamentals of the charge transfer process, Kolasinski developed a set of guidelines for the formulation of effective stain etchants (Nahidi and Kolasinski 2006; Kolasinski et al 2010Kolasinski et al , 2012Dudley and Kolasinski 2009a;Kolasinski and Gogola 2012;Kolasinski 2005Kolasinski , 2010Kolasinski , 2014). …”