An Interface-Free-Energy Approach to Semiconductor Electrode Chemistry: Examples of Si and Ge

At a solid electrode, the solid/liquid interface free energy affects the wetting behavior of the liquid on the solid surface, This property may be used as a tool for monitoring changes in the electrode surface chemistry. Here such investigations have been carried out using contact angle measurements. The variations of the advancing and receding contact angles during a potential scan have been measured by a gravimetric method for Si and Ge electrodes, whose surfaces can be turned into a reduced or an oxidized state. For p-Si in dilute fluoride electrolyte, the change from the reduced state to the oxidized state (electropolishing regime) is associated with a large decrease of the contact angles. For Ge in H₂SO₄ or HClO₄ electrolyte, the change of the contact angles between the reduced state and the oxidized state is much smaller than for silicon, and is dependent upon crystal orientation, the (111) orientation actually giving an increase of the contact angles when the surface becomes oxidized. This strikingly different behavior for Si and Ge can be related to differences in the chemistry of these two electrodes in the oxidized state, namely the presence of a hydrated oxide on Si, as opposed to a hydroxyl monolayer on Ge. The present results demonstrate that gravimetric measurements of the contact angles can be turned into a convenient and sensitive technique for monitoring changes in interface chemistry.