ROLE OF ATOMIC FORCE IN TUNNELING-BARRIER MEASUREMENTS

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 angstrom before the mechanical contact, and then drops to below 0.3 eV within a fraction of an angstrom. 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.