Laser-Assisted Formation of Porous Si in Diverse Fluoride Solutions: Reaction Kinetics and Mechanistic Implications

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.