CaF2-Si(111) as a model ionic-covalent system: Transition from chemisorption to epitaxy

The demands of chemisorption and epitaxy are quite different for electronically dissimilar systems. The transition between these two regimes in ${\mathrm{CaF}}_2$-Si(111) is studied with transmission-electron microscopy and photoemission. Changes in the electronic structure of the evolving growth surface are expressed in the composite growth mode, a Stranski-Krastanow pathway to layer-by-layer growth, which begins with ${\mathrm{CaF}}_2$ coherent island formation on a Si-CaF layer. After this transition, layer-by-layer ${\mathrm{CaF}}_2$ homoepitaxy is possible even at room temperature, and the critical thickness can be extended.