Scanning tunneling microscopy study of CaF₂ on Si(111): observation of metastable reconstructions

Abstract

The deposition of calcium fluoride (CaF2) on Si(111) at temperatures above 570 °C has been studied with scanning tunneling microscopy. At such temperatures, triangular calcium fluoride islands are formed both on terraces and along the phase domain boundaries of the (7 × 7) reconstruction of the Si(111) substrate. In addition to the formation of islands, we observe that CaF2 molecules react with the substrate inducing large areas of its surface to reconstruct into (√3 × √3) and c(2 × 4) phases. Upon annealing at 600 °C, the abovementioned areas of (√3 × √3) and c(2 × 4) turn into the stable (3 × 1) phase upon desorption of fluorine. Calcium fluoride islands are stable at this temperature. Depositions of calcium fluoride performed with Si substrate kept at higher temperature, namely at 680 °C, lead directly to the formation of (3 × 1) phase due to the complete desorption of fluorine, without passing through the formation of the metastable (√3 × √3) and c(2 × 4) phases. If CaF2/Si(111) is brought at even higher temperatures, Ca also starts desorbing and the (7 × 7)-Si(111) reconstruction can eventually be restored.

Calcium fluoride films with 2-10 nm thickness on Silicon-(111): growth, diagnostics, study of the through current transport

“…Thin epitaxial fluoride (specifically, calcium fluoride CaF 2 ) layers have been studied for over 30 years [1][2][3][4], although these studies were understandably less extensive than the examination of silica SiO 2 and other traditional oxide insulators [5]. The interest in calcium fluoride stems from the fact that some of its " dielectric parameters" are better than those of SiO 2 ; specifically, its permittivity is ε = 8.43 (the corresponding value for SiO 2 is 3.9), its band gap is E g = 12.1 eV (8.9 eV), and the effective electron mass in CaF 2 is m e = 1.0m 0 (0.42m 0 ) [6].…”