The Kinetics and Stoichiometry of Metal Cation Reduction on Multi-Crystalline Silicon in a Dilute Hydrofluoric Acid Matrix

Abstract

In this study, the process of metal cation reduction on multi-crystalline silicon in a dilute hydrofluoric acid (HF) matrix is described using Ag(I), Cu(II), Au(III) and Pt(IV). The experimental basis utilized batch tests with various solutions of different metal cation and HF concentrations and multi-crystalline silicon wafers. The metal deposition kinetics and the stoichiometry of metal deposition and silicon dissolution were calculated by means of consecutive sampling and analysis of the solutions. Several reaction mechanisms and reaction steps of the process were discussed by overlaying the results with theoretical considerations. It was deduced that the metal deposition was fastest if the holes formed during metal ion reduction could be transferred to the valence bands of the bulk and surface silicon with hydrogen termination. By contrast, the kinetics were lowest when the redox levels of the metal ion/metal half-cells were weak and the equilibrium potential of the H3O+/H2 half-cells was high. Further minima were identified at the thresholds where H3O+ reduction was inhibited, the valence transfer via valence band mechanism was limited by a Schottky barrier and the dissolution of oxidized silicon was restricted by the activity of the HF species F−, HF2− and H2F3−. The findings of the stoichiometric conditions provided further indications of the involvement of H3O+ and H2O as oxidizing agents in addition to metal ions, and the hydrogen of the surface silicon termination as a reducing agent in addition to the silicon. The H3O+ reduction is the predominant process in dilute metal ion solutions unless it is disabled due to the metal-dependent equilibrium potential of the H3O+/H2 half-cell and the energetic level of the valence bands of the silicon. As silicon is not oxidized up to the oxidation state +IV by the reduction of the metal ions and H3O+, water is suspected of acting as a secondary oxidant. The stoichiometric ratios increased up to a maximum with higher molalities of the metal ions, in the manner of a sigmoidal function. If, owing to the redox level of the metal half-cells and the energetic level of the valence band at the metal–silicon contact, the surface silicon can be oxidized, the hydrogen of the termination is the further reducing agent.

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

“…The cathodic reaction in our previous study [ 26 ] was interpreted as the reduction of the oxonium ion (H 3 O + ) or the ion pair H 3 O + F − to molecular hydrogen at the metal/silicon contact. According to this model, the process occurs via the intermediate formation of hydrogen radicals (∙H), hydroxyl radicals (∙OH), water (H 2 O) and fluoride (F − ).…”

“…The basic requirement for this process is the metal deposition on the silicon after the initial metal-ion reduction, i.e., forming a metal/silicon contact ( Figure 1 a,b) [ 26 ]. The Fermi energy equilibration ( E F ) at the metal/silicon contact occurs close to the energetic level of the standard hydrogen half-cell [ 13 , 27 ] at E (2H 3 O + /H 2 ) = 0 V vs. the standard hydrogen electrode (SHE) [ 28 ].…”

“…The Fermi energy equilibration ( E F ) at the metal/silicon contact occurs close to the energetic level of the standard hydrogen half-cell [ 13 , 27 ] at E (2H 3 O + /H 2 ) = 0 V vs. the standard hydrogen electrode (SHE) [ 28 ]. Furthermore, the valence ( E V ) and the conduction band ( E C ) bending of the bulk silicon [ 19 , 24 , 29 , 30 , 31 , 32 ] and that of the hydrogen-terminated silicon results in the order of the difference of the work functions of the respective metal ( Φ Me ) and that of the hydrogen-terminated silicon ( Φ Si-Hx ) ( Figure 1 b) [ 26 ]. Depending on the current density J of the metal ions on the metal/silicon surface, either an underpotential or an overvoltage of the hydrogen electrode is established on the metal surface [ 33 ].…”

“…It was found in our previous study [ 26 ] that stoichiometric ratios of metal deposition on silicon change gradually as a function of the initial metal ion molality. The ratios increased with the increasing metal ion concentration from just above 0:1 mol:mol on metal-dependent maxima, indicating a rising proportion of silicon oxidation up to the oxidation state of +IV by metal-ion reduction.…”

“…Furthermore, it was concluded that the kinetics of metal deposition became maximal when the redox half-cell of the metal ions (Me z+ /Me) was sufficiently strong to reach both the valence’s energetic level band of the bulk silicon and that of the hydrogen-terminated silicon. The metal ion concentration thresholds, matching the energetic level of the hydrogen-terminated silicon, depending on the amount of band bending at the metal/silicon contact [ 26 ].…”