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

The deposition of calcium fluoride (CaF2) on Si(111) at temperatures above 570 °C has been studied with scanning tunnelling microscopy (STM). At such temperatures, triangular calcium fluoride islands are formed both on terraces and along the phase domain boundaries of the (7x7) reconstruction of the Si(111) substrate. In addition to the formation of islands, we observe that CaF2 molecules react with the substrate inducing larges areas of its surface to reconstruct into (√3x√3) and c(2x4) phases. Upon annealing at 600 °C, the abovementioned areas of (√3x√3) and c(2x4) turn into the stables (3x1) phase due to desorption of fluorine. Calcium fluoride islands are stable at this temperature. Depositions of calcium fluoride performed with Si substrate kept at higher temperatures, namely at 680 °C, lead directly to the formation of the (3x1) phase, without passing though the formation of the metastable (√3x√3) and c(2x4) phases. If CaF2/Si(111) is brought at even higher temperatures, Ca also starts desorbing and the (7x7)-Si(111) reconstruction can eventually be restored.