Silicon shows unique electrochemical properties in solutions containing hydrofluoric acid.[1±4] Cylindrical holes electrochemically formed in silicon in these solutions have recently attracted much attention due to their possible applications in photonic crystals, [5] micromachining, [6] etc. These nanoholes are generated in the <100> direction at those positions where small pits were preformed by photolithography. [7,8] Here we report that, without using an electrochemical method, cylindrical nanoholes can be generated by immersing silicon wafers loaded with Ag nanoparticles in a solution containing hydrofluoric acid and hydrogen peroxide. We have been studying the chemical etching of silicon for making an antireflective surface structure (textured structure) in order to increase the efficiency of multi-crystalline Si solar cells. [9] In the course of this study, we found that the use of Ag nanoparticles as a catalyst is especially effective for making the textured structure. We also found that the catalysis by Ag nanoparticles results in the formation of cylindrical nanoholes in single-crystalline Si wafers when the treatment is continued for a longer time. During the process, the Ag particles gradually sink into the silicon, forming nanoholes as deep as 500 lm and with diameters of about 50 nm. Figure 1 shows a scanning electron microscope (SEM) image of Ag particles deposited on the surface of a p-type Si(100) wafer by electroless plating (see Experimental).[10]The sizes of the particles are mostly between 30 nm and 100 nm. There was no significant difference between the p-type and n-type wafers in densities and size distributions of the deposited Ag particles. In addition, the (100) and (111) orientations did not affect the Ag deposition. Other metal particles were also deposited on Si wafers from solutions containing metal ions, such as Pt, Pd, and Cu.After the deposition of these metal particles, the Si wafers were etched in a mixed solution of 10 % HF and 30 % H 2 O 2 (10:1 v/v) for 30 min in the dark at room temperature. The etched surface of the Si wafers was covered with a microporous Si layer. The microporous Si layer became as thick as 3 lm when Pt particles were used as the catalyst, and showed orange photoluminescence under UV irradiation at 265 nm. Since Pt has a strong catalytic activity in the reduction of hydrogen peroxide, [11] it is assumed that valence-band electrons in Si are drawn through the Si/metal interface as hydrogen peroxide is reduced. As a result, positively charged holes are generated in the Si, leading to the formation of a microporous

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