ELECTRONIC-PROPERTIES OF NANOMETER-SIZE METAL-SEMICONDUCTOR POINT CONTACTS STUDIED BY STM

Using the scanning tunneling microscope (STM), we made and studied in UHV metal-semiconductor point contacts with contact areas in a nanomet er range. Current-voltage (I-V) curves measured as a function of the t ip-sample distance show that the conductance increases exponentially a nd is saturated at contact. The value of the saturated conductance str ongly depends on the structural and electronic properties of the semic onductor surfaces. Thus, the conductance at contact taken on the Si(11 1)7 X 7 surface is highly reduced by oxygen adsorption on the surface, but does not depend on bulk properties, such as dopant concentration or the type of dopant of the semiconductor samples. We conclude that s urface conductivity via surface states is a significant component of t his saturated conductance. As a result, the conductance at contact dep ends on morphology of semiconductor surface surrounding the contact ar ea. The saturated conductance measured on the top of Si islands grown on the 7 X 7 reconstructed surface is lower than that measured on 7 X 7 terraces, suggesting the importance of electron scattering at steps.