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

Stefan Schönekerl,Jörg Acker

doi:10.3390/nano11040982

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.

Highlights

Silicon has a key role in various applications, for instance, in photovoltaics [1], sensors [2] or batteries [3]
(referenced to AgF) [28] and the calculation of the activity coefficients of the Ag+ ions according to Bromley’s equations [79], the initial redox strengths for the Ag+ /Ag half-cell threshold of E (Ag+ /Ag) half-cell were determined to be between 0.43 and 0.72 V vs. standard hydrogen electrode (SHE) (Figure 2a). It is assumed from the findings of our previous study [19] that one up to four holes can be transferred to the valence band of the bulk silicon during the reduction of Ag+ to
Kinetic considerations of the silver deposition in the same study led to the conclusion that valence transfer occurs via the valence band of hydrogen-terminated silicon when the redox strength of the Ag+ /Ag half-cell exceeds the amount of 0.65 V vs. SHE

Summary

Introduction

Silicon has a key role in various applications, for instance, in photovoltaics [1], sensors [2] or batteries [3]. To control the fabrication processes, thin metal films are previously deposited on the silicon surface to catalyze the consecutive etching step initiated by various oxidants (e.g., H2 O2 ) in the hydrofluoric acid (HF) matrix [7]. There are several options to create the metal coating It can be realized by atomic layer deposition [8,9], several chemical or physical deposition techniques like spin coating, sputtering or electroless metal deposition [10]. The latter method is characterized by its simplicity, rapidity, versatility and scalability [7]

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