STM investigation of the charge transport mechanisms to nanoscale metallic islands on a semiconductor substrate

Scanning tunneling spectroscopy (STS) was performed at room temperature on disconnected two-dimensional nanoscale metallic islands made of erbium disilicide (ErSi2) grown on Si(111). Our aim was to investigate how the charges injected by the STM tip were transferred from the metallic island to the semiconductor substrate, and how the charge transport mechanisms were reflected in the STS data. For that purpose, the relative importance of the two possible conduction paths, parallel or perpendicular to the surface, was modified by oxygen adsorption. The spectra of oxygen-dosed samples were found to change with setpoint current on p-type substrates, whereas no shift was observed for clean samples. Since oxygen exposure only affects the Si surface, this effect was interpreted as a change in the easy conduction path between the islands and the bulk substrate from parallel to perpendicular to the surface (i.e., through the metal-semiconductor interface). We propose and detail a method based on the analysis of STS spectra to determine the characteristics of the nanoscale metal-semiconductor contact. The results are in agreement with previous point contact experiments, which validates our analysis.