The Role of the Molecular Hydrogen Formation in the Process of Metal-Ion Reduction on Multicrystalline Silicon in a Hydrofluoric Acid Matrix

• Main Authors: Stefan Schönekerl, Jörg Acker
• Format: Article
• Language: English
• Published: MDPI AG 2021-04-01
• Series: Nanomaterials
• Subjects: molecular hydrogen; metal deposition; silicon dissolution; hydrofluoric acid; reaction process; valence transfer
• Online Access: https://www.mdpi.com/2079-4991/11/4/982
• ISSN: 2079-4991

Metal deposition on silicon in hydrofluoric acid (HF) solutions is a well-established process for the surface patterning of silicon. The reactions behind this process, especially the formation or the absence of molecular hydrogen (H₂), are controversially discussed in the literature. In this study, several batch experiments with Ag⁺, Cu²⁺, AuCl₄^– and PtCl₆^2– in HF matrix and multicrystalline silicon were performed. The stoichiometric amounts of the metal depositions, the silicon dissolution and the molecular hydrogen formation were determined analytically. Based on these data and theoretical considerations of the valence transfer, four reasons for the formation of H₂ could be identified. First, H₂ is generated in a consecutive reaction after a monovalent hole transfer (h⁺) to a Si–Si bond. Second, H₂ is produced due to a monovalent hole transfer to the Si–H bonds. Third, H₂ occurs if Si–Si back bonds of the hydrogen-terminated silicon are attacked by Cu²⁺ reduction resulting in the intermediate species HSiF₃, which is further degraded to H₂ and SiF₆^2–. The fourth H₂-forming reaction reduces oxonium ions (H₃O⁺) on the silver/, copper/ and gold/silicon contacts via monovalent hole transfer to silicon. In the case of (cumulative) even-numbered valence transfers to silicon, no H₂ is produced. The formation of H₂ also fails to appear if the equilibrium potential of the 2H₃O⁺/H₂ half-cell does not reach the energetic level of the valence bands of the bulk or hydrogen-terminated silicon. Non-hydrogen-forming reactions in silver, copper and gold deposition always occur with at least one H₂-forming process. The PtCl₆^2– reduction to Pt proceeds exclusively via even-numbered valence transfers to silicon. This also applies to the reaction of H₃O⁺ at the platinum/silicon contact. Consequently, no H₂ is formed during platinum deposition.