Metal-Assisted Etching of Silicon: Activity of Metal Catalysts and Control of Porous Structure

Metal-assisted etching of silicon has attracted considerable attention as an electroless method that can produce porous silicon and silicon nanowires by simply immersing metal-modified silicon in a hydrofluoric acid solution without electrical bias (1). Such etching generally uses not only metal-modified silicon wafer but also an oxidizing agent. The common oxidizing agent is hydrogen peroxide. We have been using dissolved oxygen in hydrofluoric acid solution as the oxidizing agent, which promotes much slower etching than that using hydrogen peroxide (2, 3). The metal-assisted etching proceeds by a local galvanic cell mechanism consisting of local anodic dissolution of silicon and local cathodic reduction of the oxidizing agent on metal. We have been studying the metal-assisted etching of silicon to produce porous structures for antireflection of solar cells for photovoltaics and photoelectrochemical solar hydrogen production (2-4), and for autocatalytic electroless formation of metal-nanorods and adhesive metal films on Si surfaces (5). In this paper, we present various porous structures produced by the metal-assisted etching using several kinds of noble metals (6-8), and discuss control factors and formation mechanisms of the structures.

Noble metal nanoparticles were deposited on crystalline silicon wafers by electroless displacement deposition immersing the wafers into a metal salt solution containing hydrofluoric acid (9). The size and particle density of deposited metal particles were controlled by deposition conditions such as the concentration of metal salts and deposition time. The metal-particle-deposited silicon wafers were immersed in a hydrofluoric acid solution including oxygen or hydrogen peroxide as an oxidizing agent.

Figure 1 shows porous structures having variety of pore-size between nanometers and sub-millimeters. Photoillumination during etching generates electron-hole pairs in silicon, and thus enhances and de-localizes the etching reaction (Fig. 1c). Metals on silicon catalyze the local cathodic reaction thus their kind, size, and particle density change the etching rate. Palladium and ruthenium have high catalytic activity for the etching of silicon (6, 8). Under suitable conditions, the anodic dissolution of silicon is localized under the metals (Fig. 1b and d). Oxidizing agents promote the local cathodic reaction thus their kind and concentration change the etching rate and nanopore shape. Electrolessly deposited silver nanoparticles and hydrogen peroxide produce a uniform thin porous layer, which works as an optical antireflection film for solar cells, consisting of straight nanopores having metal nanoparticle on each bottom (4).

ACKNOWLEDGEMENTS

The present work was partly supported by JSPS KAKENHI (26289276).

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