Control of the surface morphology of solid electrolyte films during field-driven growth in a reactive plasma

Abstract

Experiments on the generation of non-equilibrium surface patterns on ion-conducting thin films (AgCl, silver chloride) during field-driven growth in chlorine rf plasmas are reported. The growth experiments are performed in a vacuum flow reactor in the temperature range between 320 K and 450 K, at chlorine gas pressures in the order of 100 Pa, and by applying current densities in the order of 1 mA cm⁻². The surface morphology of product films is documented by scanning electron microscopy and analysed in terms of typical surface patterns. A large variety of different surface morphologies (often finger-like or dendritic) is prepared with a high lateral uniformity and good reproducibility. A theoretical stability criterion is derived on the basis of a linear stability analysis. The model-type experiments confirm this formal stability criterion: (a) The growth of the product surface is morphologically stable, if the plasma exhibits a higher conductivity than the growing solid. (b) The typical wavelength of the surface patterns that are formed during growth is proportional to the reciprocal root of the electric current. (c) The periodicity of the surface patterns decreases when the temperature is increased, and the growth rate of a surface instability grows with increasing temperature. All theoretical predictions are confirmed by corresponding experimental results.