Scanning tunnelling microscopy studies of heteroepitaxy: CaF₂ on Si(111)

Abstract

We apply scanning tunnelling microscopy and tunnelling spectroscopy to study the initial stages of CaF2 epitaxy on Si(111)‐(7×7) for a range of deposition temperatures. We find that deposition at surface temperatures <700°C leads to CaF2 clustering. At temperatures between 750 and 800°C two new structures, a 2×3 and a (√3 X √3***)R30°, are observed. Tunnelling spectra and topographs obtained at different energies allow us to measure interface band gaps, and determine the valence state of Ca bonded directly to silicon and the valence state of Ca in the second layer. More importantly, we demonstrate that STM can be used to image thick insulating layers of CaF2 by tunnelling into the conduction band of the films. One of our goals in this study has been to test the ability of STM to image insulating multilayers. It is generally believed that STM cannot be applied to insulators. We find, however, that despite the large band gap of ∼12 eV, the CaF2 conduction band is accessible, and that insulating films can be imaged by STM. 1988 Blackwell Science Ltd