Using HCl to Control Silver Dissolution in Metal-Assisted Chemical Etching of Silicon

Metal assisted chemical etching (MACE) is a viable route to cheaply and easily produce large-scale arrays of SiNW for, e.g., solar cell applications. However, control over nanostructure dimensions such as length, width, and angle is not simple, particularly with the cheaper silver-based MACE. Even an initially well-defined silver catalyst will dissolve uncontrollably under the influence of a variety of conditions acting concurrently, including H₂O₂ concentration. The authors prove here that HCl can be used, not just as an etch stop, but also as a method for controlling the nanowire etching. The authors suggest that the mechanism of this effect involves the prevention of silver catalyst dissolution, and not only the trapping of already-dissolved Ag⁺ ions via the formation of AgCl, as previously thought. The authors further highlight the importance of the silver dissolution rate as a control parameter, in contrast to an earlier focus on the initial thickness alone. This new knowledge will improve the design of etching, through the use of the independent HCl concentration parameter to moderate silver dissolution.