Structural transitions of theCaF2/Si(111) interface

We have used x-ray reflectivity and transmission electron microscopy to study the ${\mathrm{CaF}}_2$/Si(111) interface. The results are consistent with a reconstructed two-layer CaF interface which can be irreversibly transformed to a different structure simply by increasing the thickness of the ${\mathrm{CaF}}_2$ overlayer. We are able to reconcile previous measurements of the interface structure and gain insight into the rich variety of phenomena that may be observed at heteroepitaxial interfaces.