Flattening the surface of caf₂si(100) structures by post-growth annealing

The surface morphologies of CaF₂films epitaxially grown on Si(100) substrates by vacuum evaporation are investigated by replica transmission electron microscopy and reflection electron diffraction analyses. It is found that (111) facets, which appear at the surface of as-grown CaF₂films, are extirpated and the surface is flattened by post-growth annealing at temperatures above 650degC. Both ex situ annealing and in situ annealing are used, and the in situ annealing is shown to be useful for flattening the surface without generating cracks. The post-growth annealing is also effective in improving the crystalline quality of the CaF₂films, as measured by ion channeling measurements. Usefulness of a two-step growth method for growing smooth CaF₂films is demonstrated. From observations of the dynamical process of the surface flattening, it is pointed out that mass transports for the flattening are dominantly progressed by surface diffusion.