The apparent barrier height in the scanning tunneling microscope, defined in terms of the rate of change (at constant bias) of the logarithm of current with tip-sample separation, is calculated as a function of separation. It is found to be substantially lower than the sample work function in the range of tip-sample separations commonly used experimentally, and becomes very small in the region just before contact between tip and sample.

• Received 25 January 1988

DOI:https://doi.org/10.1103/PhysRevB.37.10395