High Φ values in scanning tunneling microscopy: Field emission and tunnel regimes

Abstract

Local barrier height (Φ) images on a Ni (110) sample in ultrahigh vacuum (UHV) conditions show the coexistence of small regions (50 Å) with low and high Φ values. Thus, we measured I–s and s–V plots at a fixed point and several tip–sample voltages in order to understand the behavior of the barrier height with different gap distances. A good fitting of these plots with theoretical expressions is obtained for high Φ values, and we evaluate the contribution of the image potential to the tunnel barrier. Anomalously low Φ values in the tunnel regime correspond to quite high values in field emission. The data are consistent with the suggestion that apparently low barrier heights in tunneling are due to forces between tip and sample, resulting in changes of the real gap distance.