PROBING THE CAF2 DENSITY-OF-STATES AT AU/CAF2/N-SI(111) INTERFACES WITH PHOTOELECTRON-SPECTROSCOPY AND BALLISTIC-ELECTRON-EMISSION MICROSCOPY

The electronic properties, chemistry, and spatial structure of Au/CaF2/n-Si(111) metal-insulator-semiconductor (MIS) structures, with epitaxially grown CaF2 layers of a few monolayers (ML) thickness, have been studied by photoelectron spectroscopy, scanning-tunneling microscopy, and ballistic-electron emission microscopy. CaF2 films on Si are characterized by flat surfaces with defect lines about 500 angstrom apart, and band bending in Si reduces gradually with increasing CaF2 layer thickness. Au grows on top of the CaF2 layer in the form of hexagonal terraces. A Si segregation to the surface, as observed in case of the bare Au/Si interface, is strongly reduced by the CaF2 intralayer. Ballistic-electron emission microscopy shows a strong influence of the CaF2 density of states for electron transport through the intralayer. For a 4 ML thick CaF2 intralayer, the position of the CaF2 conduction-band minimum is derived from the onset of the collector current at 3.3 V The valence-band offset at the CaF2/Si interface is derived from the valance-band edge observed in photoelectron spectroscopy.