Web Release Date: February 10,
Laser-Assisted Formation of Porous Si in Diverse Fluoride Solutions: Reaction Kinetics and
Mechanistic Implications
School of Chemistry, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom
Received: September 11, 2000
In Final Form: December 12, 2000
Abstract:
The formation rate of porous silicon by photoelectrochemical etching of n-type silicon is measured in situ by
a novel technique. The reflection of the laser beam used to drive the reaction contains circular patterns, the
outer radius of which bears a linear relationship to the depth of etching. The use of the radius to measure etch
rate and the absolute depth of etching is described and justified. The rates are analyzed in terms of the calculated
activities of the different species in solution. A rate equation, which is first order in HF and 
is deduced
and rate coefficients calculated. It is concluded that the etching mechanism includes two parallel
rate-determining steps involving HF and 
and that the 
channel is roughly 20 times faster than the
HF channel. In light of our data, previously proposed mechanisms of Si dissolution in fluoride solutions
should be reevaluated.
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