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 nanometer range. Current-voltage (I-V) curves measured as a function of the tip-sample distance show that the conductance increases exponentially and is saturated at contact. The value of the saturated conductance strongly depends on the structural and electronic properties of the semiconductor surfaces. Thus, the conductance at contact taken on the Si(111)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 surface conductivity via surface states is a significant component of this saturated conductance. As a result, the conductance at contact depends on morphology of semiconductor surface surrounding the contact area. 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.