Metal-assisted chemical etching of silicon is an electroless method that can produce porous silicon by immersing metal-modified silicon in a hydrofluoric acid solution without electrical bias. We have been studying the metal-assisted hydrofluoric acid etching of silicon using dissolved oxygen as an oxidizing agent. Three major factors control the etching reaction and the porous silicon structure: photoillumination during etching, oxidizing agents, and metal particles. In this study, the influence of noble metal particles, silver, gold, platinum, and rhodium, on this etching is investigated under dark conditions: the absence of photogenerated charges in the silicon. The silicon dissolution is localized under the particles, and nanopores are formed whose diameters resemble the size of the metal nanoparticles. The etching rate of the silicon and the catalytic activity of the metals for the cathodic reduction of oxygen in the hydrofluoric acid solution increase in the order of silver, gold, platinum, and rhodium.

Metal-assisted chemical etching of silicon is an electroless method that can produce porous silicon by immersing metal-modified silicon in a HF solution without electrical bias. We have been studying this etching using dissolved oxygen as an oxidizing agent and recently reported that the catalytic activity of noble metal particles including silver, gold, platinum, and rhodium, influences the cathodic reduction of oxygen and controls the etching rate. In this paper, we investigate the influence of the HF concentration on the etching rate and the pore morphology. In the case of high HF concentration, the etching rate is independent of the HF concentration and depends on the oxygen concentration and the catalytic activity of the metal particles. In the low HF concentration case, the etching rate depends on the HF concentration and is independent of the oxygen concentration and the catalytic activity. The pore morphology is also changed by the HF concentration.

We deposit fine metal particles on silicon (Si) by a displacement reaction, which is the immersion of Si wafers into a metal-salt solution containing hydrofluoric acid, that consists of a local cathodic reduction of metal ions and a local anodic dissolution of Si. In this study, the displacement deposition of silver (Ag) nanoparticles on the Si(111) surface with an atomic step-terrace structure is investigated by atomic force microscopy. Ag particles are uniformly deposited on the Si surface without influence of the step-terrace structure. The particle density of the deposited Ag decreases and then increases with immersion times between 1 and 15 s. The step-terrace structure disappears and nanoholes are formed by an immersion time of 15 s. We propose a model of Ag particle density and Si surface changes with time.

Metal-assisted HF etching of Si has attracted considerable attention as a new electroless method that can produce porous Si by immersing metal-modified Si in a HF solution without bias. Such etching generally uses not only metal-modified Si but also an oxidizing agent. Palladium exhibits high activity in assisted etching under dissolved-oxygen-free and dark conditions. In this study, we investigate the Pd assisted HF etching of n-Si by electrochemical measurements. The potential of Pd metal on Si is more negative than the potential of hydrogen evolution at open circuit conditions. Anodic current generation of Pd-modified Si electrodes at positive bias and the localization of etching under Pd films at low thickness indicate that Pd catalyzes the anodic dissolution of Si and the cathodic hydrogen evolution.