Crystallization in Thin Amorphous Films

A general thermodynamic description of crystallization in thin amorphous films is presented. The description is based on the classical theory for heterogeneous crystal nucleation. Crystallization is assumed to start at the free surface and proceeds via nucleation and growth. The surface energy is assumed to depend on the orientation of the crystal with respect to the surface. Elastic strain energy caused by different thermal expansions of film and substrate is taken into account. It is shown that for crystals which have not yet reached the substrate minimization of both surface energy and elastic strain energy decreases the magnitude of the nucleation barrier and increases the nucleation frequency, thus leading to the preferred alignment of crystals with respect to the surface. The morphology of crystals which have already reached the substrate is approximately described by spherical discs. For this crystal geometry the influence of the surface free energy increases with decreasing film thickness while the elastic strain energy does not depend on the film thickness. This behavior results in a critical film thickness below which the surface free energy overcomes the elastic strain energy. Results from literature on crystal orientation in thin films and at the surface of thick amorphous specimens are discussed in the light of the present description.