Direct measurement of forces during scanning tunneling microscopy imaging of silicon pn junctions

Abstract

We investigated the forces acting between tip and surface during scanning tunneling microscopy (STM) imaging of a silicon pn junction. Using a conductive and stiff atomic force microscopy (AFM) cantilever, the current between the tip and sample, and the normal force (or lever bending) were measured independently. This method allows us to use either AFM or STM, depending on the feedback signal. By comparing topographic images of the pn junction acquired in contact AFM mode with the STM images, large variations of STM topography and normal force across the junction could be observed. We find that at reverse bias the tip presses against the surface to draw the set-point current, while it is in noncontact tunneling regime at the forward bias. The current measured as a function of tip-sample distance shows a strong dependence on polarity of the bias in the p,n, and inverted regions, consistent with the force measurements during constant current STM mode.