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

  • 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 PtCl62– in HF matrix and multi-crystalline 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. Firstly, H2 is generated in a consecutive reaction after a monovalent hole transfer (h+) to a Si–Si bond. Secondly, H2 is produced as a result of a monovalent hole transfer to the Si–H bonds. Thirdly, H2 occurs if Si–Si back bonds of the hydrogen-terminated silicon are attacked by Cu2+ reduction resulting in the intermediate species HSiF3, which isMetal 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 PtCl62– in HF matrix and multi-crystalline 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. Firstly, H2 is generated in a consecutive reaction after a monovalent hole transfer (h+) to a Si–Si bond. Secondly, H2 is produced as a result of a monovalent hole transfer to the Si–H bonds. Thirdly, 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 SiF62–. The fourth H2-forming reaction is the reduction of 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 PtCl62– 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.show moreshow less

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Author: Stefan SchönekerlORCiD, Jörg AckerORCiDGND
URL:https://www.mdpi.com/2079-4991/11/4/982
DOI:https://doi.org/10.3390/nano11040982
ISSN:2079-4991
Title of the source (English):Nanomaterials
Document Type:Scientific journal article peer-reviewed
Language:English
Year of publication:2021
Tag:chemical analysis; dissolution; etching; hydrofluoric acid; metal deposition; molecular hydrogen; reaction kinetics; silicon
Volume/Year:11
Issue number:4
Article number:982
Faculty/Chair:Fakultät 2 Umwelt und Naturwissenschaften / FG Physikalische Chemie
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