Epitaxial growth mechanisms and structure of CaF2/Si(111).

Abstract

The early stages of interface formation between ${\mathrm{CaF}}_{2}$ and Si(111) have been studied, in situ, by a combination of reflection high-energy electron diffraction, x-ray diffraction, and core-level photoemission. The results are combined with ex situ transmission-electron-microscopy measurements to show that the initial growth mode changes from Volmer-Weber to Stranski-Krastanow, depending on the substrate temperature. The crossover is correlated with a submonolayer transition from the Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) to a (3\ifmmode\times\else\texttimes\fi{}1) reconstruction. This is accompanied by fluorine dissociation at the interface. Both initial growth modes can lead to a uniform ${\mathrm{CaF}}_{2}$ epilayer and subsequent growth on this surface is layer by layer. Using x-ray crystal truncation-rod analysis, we have examined the ${\mathrm{CaF}}_{2}$/Si(111) surface and interface structure. For films grown at temperatures above the (7\ifmmode\times\else\texttimes\fi{}7)\ensuremath{\rightarrow}(3\ifmmode\times\else\texttimes\fi{}1) transition, the Ca atom in the CaF layer at the interface is located in a single ${\mathit{T}}_{4}$ bonding site. Finally, we have observed a structural transition at the interface from the as-grown structure to a (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} reconstruction, which appears to be incommensurate. The dynamics of this transition and the possible mechanisms will be discussed.