Nanogroove Formation By Ag-Assisted Chemical Etching to Separate Neighboring Terraces on Vicinal Si(111)
Authors: Arima Kenta , Masumoto Seiya , Ma Zhida , Kawai Kentaro , Yamamura Kazuya 
Published: December 2020

We aim to form nanogrooves to separate neighboring terraces on a Si surface for the fabrication of functional devices such as nanogap electrodes. For this purpose, we propose a self-assembled process composed of multiple wet-chemical treatments of a vicinal Si(111) surface. In this scheme, we used p-type Si(111) wafers with resistivity 10–20 Ω･cm. The miscut angle was 0.2°. After both metallic and organic contaminants were removed from the Si wafer by wet cleaning, it was dipped into a dilute HF solution to passivate the surface with H atoms. The sample was then immersed for 25 minutes in water with an ultralow level of dissolved oxygen, which is referred to hereafter as LOW. LOW was obtained by adding the deoxygenation agent ammonium sulfite monohydrate to deionized water [1] in which the O2 concentration was less than 3 ppb. This treatment with LOW led to anisotropic etching of Si(111) to form atomically flat terraces together with atomic steps. Each terrace width was approximately 90 nm. Next, the flattened Si sample was immersed for 2–5 seconds in LOW containing Ag+ ions at a concentration of 5 ppm. Atomic force microscopy (AFM) observations revealed that Ag atoms were selectively adsorbed at the edges of atomic steps on Si(111). In other words, Ag nanowires were formed along the step edges. Nanowire width and height were approximately 10 and 3 nm, respectively. Based on previous reports [2,3], it is speculated that the immersion of a flattened Si(111) surface into LOW caused the decoration of the step edge with hydroxyl (OH ) species. The oxygen atom in OH strongly interacted with Ag+ ions, promoting the transfer of electrons from the Si substrate to the ions. As a result, reduced Ag atoms were adsorbed at the step edges, and a Si dioxide (SiO2) film was formed locally underneath the Ag nanowires. Finally, the Si sample with Ag nanowires was immersed into a mixture of HF, H2O2, and deionized water for a few minutes. The HF and H2O2 concentrations were 4.6 and 0.005 M, respectively. AFM images indicated that adsorbed Ag atoms at step edges were replaced by perforated grooves. Groove width and depth were about 10 and 2 nm, respectively. The Ag nanowires are interpreted as acting as a catalyst to enhance chemical etching of the Si surface underneath, thus forming the nanogrooves. It is supposed that the Ag atoms dissolved gradually during the immersion into the solution containing H2O2. Finally, a likely explanation for formation of a “perforated” groove, rather than a continuous one, is that Ag atoms did not align to form a continuous wire during the deposition process; this finding indicates that the deposition process needs improvement. Nevertheless, these results present a possibility in which nanogrooves can be formed along a step edge on a Si(111) surface by catalyst-assisted chemical etching using a template of Ag nanowires. Because this process enables the separation of neighboring Si terraces, it is expected to be of use for future Si-based nanodevices.
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