Distance dependence and corrugation in barrier-height measurements on metal surfaces

Abstract

Scanning tunneling microscopy measurements of the derivative of the tunnel current d I/d z, performed at different distances and on differently corrugated metal surfaces, revealed that on the smooth Cu(110) surface I and d I/d z decay with distance as expected from theory, with little effect of gap width and bias voltage on the apparent barrier height derived from these measurements. On the corrugated (1×2) reconstructed Au(110) surface d I/d z measurements show anomalously low values above the troughs of the missing rows, equivalent to local barrier heights of 0.1 eV. This behavior of the d I/d z signal, which leads to an enhanced contrast in barrier-height images recorded by modulation techniques, contradicts theoretical predictions and is inconsistent also with the local barrier heights derived from constant-current measurements at different distances. We conclude that the interpretation of d I/d z measurements in terms of local barrier heights can fail on highly corrugated surfaces.