Role of atomic force in tunneling-barrier measurements

Abstract

Experimental measurements of the apparent barrier height as a function of tip-sample separation using a scanning tunneling microscope (with clean W tips and clean Si surfaces in ultra high-vacuum) show that the barrier height starts at 3.5 eV at large separations, increases to 4.8 eV at about 1.5 Å before the mechanical contact, and then drops to below 0.3 eV within a fraction of an ångström. At the distances encountered in scanning tunneling microscopy, forces between sample and tip can be significant. Using a simple model of this system including tip-sample forces leads to a calculated apparent barrier height which quantitatively reproduces the observed variation in apparent barrier height over the entire range of tip-sample separations.