Strain reduction in supported materials; the formation of cracks and dislocations

Supported materials have been constructed by successively depositing ions onto a surface in conjunction with dynamics simulation and energy minimisation. In particular, the growth of SrO/SrO, CaO/SrO (-6.9% misfit) and BaO/SrO (+7.0% misfit) were studied as model systems to examine the influence of the misfit on the structure and stability of epitaxial thin films. At low (theoretical monolayer) coverage, the CaO formed islands on the SrO(001) surface in contrast to the strips formed by BaO. At higher coverages cracks-formed within the CaO, while dislocations appeared within the BaO thin film, in both cases to help reduce the strain within the materials. A mechanism for the formation of the dislocation is presented. For one particular CaO/SrO(001) interface, a cubic-hexagonal transformation of the CaO is observed, enabling the misfit to be perfectly accommodated.