Since the substrate and the film are made of different materials, the ideal commensurate growth (see Fig. A) that occurs at full coincidence of lattice parameters, is unlikely. Most often, the crystal structure of the film and the substrate are different. This difference in structures is characterised by the quantitative parameter of lattice mismatch defined as a relative difference between their constants:

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Small discrepancies of lattices can be adapted due to elastic stresses, i.e. due to such deformation of the lattice that the strained lattice retains the periodicity of the substrate in the interface plane, but has a different periodicity in the perpendicular direction, while maintaining the unit cell volume. This type of growth is called pseudomorphic and is shown in Fig. b.

At large lattice mismatches the stress reaches such a value that it can be released only through epitaxial dislocations that arise at the interface, as shown in Fig. c. It is easy to prove that the distance between the dislocations is equal to