Scanning tunneling microscopy and ballistic electron emission spectroscopy studies of molecular beam epitaxially grown Pt/CaF2/Si(111) structures

Abstract

The electronic and morphological properties of ultrathin Pt/CaF2/Si(111) metal insulator semiconductor structures have been characterized in situ by scanning tunneling microscopy (STM) and ballistic electron emission microscopy (BEEM). Platinum thickness from 2 Å to 20 Å grown on 5 Å epitaxial CaF2 have been characterized. The STM images of the Pt/CaF2/Si(111) structures show the atomic steps of the underlying CaF2 morphology, as well as the formation of Pt nodules. These nodules have been observed to coalesce into bigger grains when left at room temperature under ultrahigh vacuum conditions for over 24 h. For ultrathin Pt coverages (2 Å) the STM images at different tip biases reveal different topography, depending on the value of the tip bias relative to the conduction band minimum (CBM) of the CaF2 intralayer (3.3 eV). The STM images at biases well above the CBM of the CaF2 show similar features to bare CaF2/Si(111), while images at the CBM show features of the deposited Pt. BEEM spectra of the 10 Å sample show a peak at ∼4.5 eV due to the density of states of the CaF2 intralayer and an additional peak at 2 eV, which has not been observed in previous studies of metal/CaF2/Si(111) structures.