Initial surface topography changes during divalent dissolution of silicon electrodes

The changes of the surface topography of float zone FZ n Si 111 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 0.2M NH4F electrolyte at pH 4 , of the potential and of the charge passed on the topography is investigated. The dissolution charges Qdiss ranged from 0.28 mCcm 2 to 10.6 mCcm 2 corresponding to 0.5 bilayers BL and 21 BL, respectively. The root mean square roughness Rq changes from Rq 0.2 nm for the H terminated surface to 2.9 nm for a charge passed of 10.6 mC cm2 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 Qdiss , these islands are beginning to show corrosion. An exponential relation between Rq and Qdiss is found by evaluation of the three dimensional roughness. The slope, i.e. the increase of ln Rq with Qdiss 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