General Corrosion and Electrode Potential in Metal-Nanoparticle-Assisted Etching of p-Type and n-Type Silicon Using Gold and Silver
Authors: Hashiguchi Tatsuki , Azuma Kyohei , Nishinaka Rin , Matsumoto Ayumu , Yae Shinji 
Published: December 2024

Introduction
When silicon (Si) modified with noble metal catalysts is immersed in a hydrofluoric acid (HF) aqueous solution containing an oxidizing agent, such as hydrogen peroxide (H2O2), a porous layer is formed on the Si surface [1]. This phenomenon is called metal-assisted etching, and it has attracted attention as a new method for producing porous Si. The etching reaction is explained by a local cell mechanism consisting of a cathodic reaction in which the oxidizing agent is reduced on the metal catalysts and an anodic reaction in which Si is oxidized. It is known that etching behavior varies greatly depending on the type of metal catalysts and other treatment conditions, but the mechanism is not clear. We have previously reported that the entire Si surface is dissolved during metal-assisted etching, resulting in general corrosion [2]. In this study, metal-assisted etching of p-type and n-type Si was performed using gold (Au) and silver (Ag) nanoparticles, and the general corrosion depth and the potential of Si during etching were investigated.
Experimental
Single crystal p-type and n-type Si wafers (CZ, (100), 0.5~10 Ωcm) were used as the Si substrates. The pretreated Si substrate was immersed in a tetrachloroauric(III) acid aqueous solution or a silver(I) nitrate aqueous solution containing 0.15 M HF, and Au or Ag nanoparticles were deposited on Si with the metal coverage of approximately 40%. The metal-deposited Si was immersed in a 6.6 M HF aqueous solution containing 0.1 M H2O2 for 120 s in the dark. The general corrosion depth was estimated from the mass loss of the Si substrate and the depth of pores observed with scanning electron microscope (SEM) [2]. The potential of Si during etching was measured using an electrochemical cell with a platinum ring as the counter electrode and a silver/silver chloride (Ag/AgCl) electrode as the reference electrode.
Results and discussion
Figure 1 shows a schematic illustration of cross sections of Si after etching. Under all conditions, the Si surface beneath the metal particles was dissolved and vertical pores were formed. The pore depth formed by the Ag-assisted-etching was larger than that formed by the Au-assisted etching. On the other hand, the general corrosion depth in the Ag-assisted etching was smaller than that in the Au-assisted etching. In the case of Ag-assisted etching of n-Si, the general corrosion did not occur. As for the potential during etching, the potential was shifted to positive direction by the deposition of metal particles, except for the Ag-deposited n-Si. The positive shift of potential indicates that the Si substrate is anodically polarized during etching and the Si surface away from the metal catalysts can be dissolved. This corresponds to the results that the general corrosion occurred in the Au-assisted etching of p/n-Si and Ag-assisted etching of p-Si. On the other hand, the potential of n-Si was not shifted by the deposition of Ag nanoparticles, and the general corrosion did not occur in the Ag-assisted etching of n-Si. These results suggest that the potential during etching contributes to general corrosion.
[1] Z. Huang, N. Geyer, P. Werner, U. Gösele ; Adv. Mater., 23, 285 (2011).
[2] A. Matsumoto, H. Son, M. Eguchi, K. Iwamoto, Y. Shimada, K. Furukawa, S. Yae ; RSC Adv., 10, 289 (2020).

Figure 1