The Role of the Molecular Hydrogen Formation in the Process of Metal-Ion Reduction on Multicrystalline 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 (H), are controversially discussed in the literature. In this study, several batch experiments with Ag, Cu, AuCl and PtCl 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 H could be identified. First, H is generated in a consecutive reaction after a monovalent hole transfer (h) to a Si-Si bond. Second, H is produced due to a monovalent hole transfer to the Si-H bonds. Third, H occurs if Si-Si back bonds of the hydrogen-terminated silicon are attacked by Cu reduction resulting in the intermediate species HSiF, which is further degraded to H and SiF. The fourth H-forming reaction reduces oxonium ions (HO) 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 H is produced. The formation of H also fails to appear if the equilibrium potential of the 2HO/H 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 H-forming process. The PtCl reduction to Pt proceeds exclusively via even-numbered valence transfers to silicon. This also applies to the reaction of HO at the platinum/silicon contact. Consequently, no H is formed during platinum deposition.