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Published March 15, 1989 | public
Journal Article Open

Band bending and the apparent barrier height in scanning tunneling microscopy


We consider the influence of tip-induced band bending on the apparent barrier height deduced from scanning tunneling microscopy (STM) experiments at unpinned semiconductor surfaces. Any voltage applied to a probe tip appears partly in the vacuum gap as an electric field at the semiconductor surface and partly in the semiconductor interior as band bending. The fraction appearing in each region is a function of gap spacing so that modulation of the tip-sample separation inevitably modulates the induced surface potential in the semiconductor. At finite temperature, the height and shape of this barrier determine the probability that an electron will reach the semiconductor surface where it can subsequently tunnel through the vacuum gap. Since the surface potential decreases with increasing tip-sample separation, STM measurements of the tunneling barrier at unpinned semiconductor surfaces will yield unusually low values. Detailed numerical calculations of the effect for passivated n-type Si(111) show it to be of observable magnitude. This mechanism may be distinguished from other recently proposed barrier-lowering mechanisms in that it is doping dependent, potentially long range, and possesses a unique voltage signature.

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©1989 The American Physical Society Received 21 November 1988 The authors are grateful to W.J. Kaiser for stimulating their interest in the Schottky barrier problem, as well as for many fruitful discussions. One of us (M.W.) also wishes to acknowledge the helpful comments of J. Stroscio. This work was supported by the Office of Naval Research under Contract No. N00014-86-K-0214, and by the Shell Company Foundation.


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