Structural Transitions in Thin Films
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When thin films are deposited on suitable substrate surfaces, they can exhibit crystal structures that would be metastable for the same materials in bulk form (in this article, the terms stable and metastable refer to bulk forms). The nonequilibrium crystal structure may arise when the lattice constant or symmetry of film and substrate material are different. An example of such a situation is the pseudomorphic growth. When we start depositing a thin film on a substrate, in the initial stages, the film tends to adopt the crystal structure of the substrate. When the energy gain from the lattice fit becomes smaller than the increase in the volume energy of the strained film, pseudomorphic growth breaks down and the layer usually grows in its natural crystal structure with misfit dislocations at the interface accommodating the strain. Therefore such structures which are metastable in the bulk form of a material can be grown by epitaxial growth techniques on different substrates as well as in the form of multilayers, provided the thickness of the layers is sufficiently small, typically in the nanometer or subnanometer regime. In general, metastable pseudomorphic deposits transform to their bulk-stable polymorphs upon thickening, and the transition thickness is on the order of nanometers.
With the continuing thrust toward thinner films in devices and the potential of exploiting novel properties of such dimensionally constrained systems, it is increasingly evident that such pseudomorphic phases will be continually encountered. Furthermore, because the properties of interest, electronic, magnetic, optical, and structural, are intimately linked to the crystal structure of these nanolayered films, it is important to rationalize the formation of pseudomorphic phases and, if possible, even predict their formation. Such metastable structures have been documented for many systems in the literature, including metal/metal (e.g., Refs. ) and metal/semiconductor systems (e.g., Refs. ). Experimental observations on multilayers include ones where either one (e.g., Refs. ) or even both layers can exist in metastable structures. The objective of this article is twofold: first, to briefly review the reports in the published literature of stabilization of pseudomorphic phases in nanolayered thin films, and second, to attempt rationalizing these observations based on a simple classical thermodynamic model. This model is a generic one which aids in the understanding of phase stability and structural transitions and leads to the development of a phase stability diagram for multilayered thin films.