On the stability of thin epitaxial films

Abstract

The deposition of thin epitaxial layers from the vapour phase is a subject of broad and current interest. The authors have recently become aware of a particularly simple epitaxial system, (copper deposited on monocrystalline silver), for which the misfit is large, and where the two f.c.c. metals are essentially immiscible at the temperature of deposition. It is the purpose of this contribution to estimate the strain energy densities involved in the coherent deposition of copper on silver, and to evaluate the effect of coherency strains on the vapour pressure of the deposited material. The structure of copper overlays evaporated at room temperature and a pressure of about 10/sup -7/ Pa on the (111) plane of silver was investigated recently by Tyliszczak, De Crescenzi and Hitchcock. Their results indicate that layer-by-layer epitaxial growth occurs during the deposition of the first few monolayers. The copper atomic spacing is commensurate with the silver substrate. During later deposition the silver surface is covered with copper islands. At later states, the copper spacing corresponds to that of bulk copper. The lattice parameter of copper is about 0.361 nm while that of silver is 0.408 nm. Clearly, the first few layers of epitaxial copper are severelymore » strained and Hitchcock, in private communication, notes that these initial layers tend to re-evaporate readily. The authors examine a simple thermodynamic model to predict the increase in equilibrium vapour pressure arising from the epitaxial strain. The extra energy associated with this strain is calculated using linear elastic theory, with its accompanying simplifications. The calculation indicates that epitaxial growth on (100) planes would minimize this strain energy, in contrast to deposition on (111), which maximizes it.« less