EPITAXIAL-GROWTH MECHANISMS AND STRUCTURE OF CAF2 SI(111)/

The early stages of interface formation between CaF2 and Si(111) have been studied, in situ, by a combination of reflection high-energy elec tron diffraction, x-ray diffraction, and core-level photoemission. The results are combined with ex situ transmission-electron-microscopy me asurements to show that the initial growth mode changes from Volmer-We ber to Stranski-Krastanow, depending on the substrate temperature. The crossover is correlated with a submonolayer transition from the Si(11 1)-(7X7) to a (3X1) reconstruction. This is accompanied by fluorine di ssociation at the interface. Both initial growth modes can lead to a u niform CaF2 epilayer and subsequent growth on this surface is layer by layer. Using x-ray crystal truncation-rod analysis, we have examined the CaF2/Si(111) surface and interface structure. For films grown at t emperatures above the (7X7-->(3X1) transition, the Ca atom in the CaF layer at the interface is located in a single T-4 bonding site. Finall y, we have observed a structural transition at the interface from the as-grown structure to a (root 3X root 3)R 30 degrees reconstruction, w hich appears to be incommensurate. The dynamics of this transition and the possible mechanisms will be discussed.