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

Abstract

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 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 SiF62–. 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 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.

Metal Assisted Catalytic Etching (MACE) for Nanofabrication of Semiconductor Powders

“…As explained above, the kinetics of Cu deposition are slow on ideal Si surfaces because the standard reduction potential of Cu is less positive than the Si VBM. It has been confirmed experimentally [4] that Cu deposition occurs initially on defect sites in contrast to Ag, Au and Pt deposition which occurs on both ideal and defect sites. Preferential deposition at step defects will lead to step-flow etching if the Cu assemblies are unable to expand onto the terraces before being dissolved.…”

“…In contrast, the metals Hg, Ag, Au, Pd, Pt, Rh, Ir and Tl that have E° > 0.6 V have facile kinetics for metal deposition onto Si particles. Substantial differences in the kinetics of Cu deposition as compared to Ag, Au and Pt have been verified experimentally by Schönekerl and Acker [4].…”

“…It has long been known that metals can spontaneously deposit on semiconductors out of solutions containing dissolved ions [1][2][3]. The kinetics of this process are complex, only partially understood, and depend on the concentration of the metal ion as well as the available counter reaction [4,5]. Thermodynamically, at standard concentration with evolution of H2 as a counter reaction, any metal with a positive reduction potential, e.g., W, Re, Bi, Cu, Po, Ru, Hg, Ag, Au, Pd, Pt, Rh, Ir and Tl, could deposit spontaneously on a semiconductor.…”

Metal-Assisted Catalytic Etching (MACE) for Nanofabrication of Semiconductor Powders

“…In contrast, the metals Hg, Ag, Au, Pd, Pt, Rh, Ir, and Tl that have E ° > 0.6 V have facile kinetics for metal deposition onto Si particles. Substantial differences in the kinetics of Cu deposition as compared with Ag, Au, and Pt have been verified experimentally by Schönekerl and Acker [ 4 ].…”

“…It has long been known that metals can spontaneously deposit on semiconductors out of solutions containing dissolved ions [ 1 , 2 , 3 ]. The kinetics of this process are complex, only partially understood, and depend on the concentration of the metal ion as well as the available counter reaction [ 4 , 5 ]. Thermodynamically, at a standard concentration with the evolution of H 2 as a counter reaction, any metal with a positive reduction potential, for example, W, Re, Bi, Cu, Po, Ru, Hg, Ag, Au, Pd, Pt, Rh, Ir, and Tl, can deposit spontaneously on a semiconductor.…”