Atomic force microscopy study of the CaF2(111) surface: from cleavage via island to evaporation topographies

The development of (111) cleavage faces of CaF2 upon thermal treatment is studied by atomic force microscopy (AFM). Vacuum-cleaved surfaces show the atomic periodicity of the uppermost fluorine layer. Cleavage steps have heights of about 0.32 nm, corresponding to the spacing of neighboring F−–Ca2+–F− triple layers, or integer multiples thereof. The steps exhibit ‘lightning’-shaped arrangements with tips pointing in the direction of cleavage. Steps run essentially parallel to the 〈110〉, 〈211〉, and 〈321〉 directions. At about 820 K the original cleavage topography transforms into a topography with trigonal and hexagonal islands. The islands develop from the cleavage tips by a melt-off process mediated by step and/or surface diffusion. During further heat treatment at even higher temperatures the surface flattens and the formation of triangular islands with one type of 〈110〉 step is observed. The atomic configuration for this type of step is discussed. Strongly vaporized surfaces studied by an electron microscopic shadowing technique exhibit hexagonal evaporation pits. The exposure to stray electrons from the heater filament at a temperature of 820 K results in a strong surface erosion with deep triangular etch pits with edges parallel to 〈110〉. The formation of these etch pits is probably caused by a combined action of electron-stimulated desorption of fluorine, vaporization of metallic calcium clusters and a partial healing of the surface via step and/or surface diffusion.