Initial surface topography changes during divalent dissolution of silicon electrodes

The changes of the surface topography of float zone (FZ) n-Si(1 1 1) upon conditioning of the electrodes at potentials slightly anodic of the rest potential are monitored with atomic force microscopy (AFM) in the contact mode. The influence of the composition of the used 0.1 and 0.2 M NH₄F electrolyte at pH 4, of the potential and of the charge passed on the topography is investigated. The dissolution charges Q_diss ranged from 0.28 to 10.6 mC cm⁻² corresponding to ∼0.5 and ∼21 bilayers (BL), respectively. The root mean square roughness R_q changes from R_q=0.2 nm for the H-terminated surface to 2.9 nm for a charge passed of 10.6 mC cm⁻² at an electrode potential of 0.1 V positive of the rest potential. The evaluation of height, deflection and line scan AFM data shows pitting to originate at edges of oriented steps which separate atomically smooth terraces. Upon increased dissolution charge, island-type smooth and rather circular features form. Only for the highest Q_diss, these islands are beginning to show corrosion. An exponential relation between R_q and Q_diss is found by evaluation of the three-dimensional roughness. The slope, i.e. the increase of ln R_q with Q_diss depends on the composition of the electrolyte and is higher for the 0.1 M NH₄F solution. From these data, a branching of the dissolution reaction between charge going into terrace removal or pit formation is obtained. Synchrotron radiation photoelectron spectroscopy (SRPES) is used to identify chemical products of the dissolution process.Comparison of data obtained at 0.15 V anodic of the electrode rest potential with an elaborate model of Gerischer and coworkers (which, however, only describes terrace dissolution) yields partial agreement with the predictions.