Kelvin Probe Force Microscopy: Recent Advances and Applications

Abstract

The Kelvin probe force microscopy technique is perhaps the most powerful tool for measuring the work function and the electric
potential distribution with nanometer resolution. The work function is one of the most important values characterizing the
property of a surface. Chemical and physical phenomena taking place at the surface are strongly affected by the work function.
Although the work function is defined as a macroscopic concept, it is necessary to consider its microscopic local variations
in understanding the behavior of semiconductor surfaces, interfaces and devices. In this chapter we describe and discuss recent
applications of Kelvin probe force microscopy in the study of semiconductors. The method is introduced in the first section,
and the second section examines the factors affecting the sensitivity and resolution of Kelvin probe force microscopy in general,
and in semiconductor measurements in particular. An efficient numerical analysis of the electrostatic interaction between
the measuring atomic force microscope tip and the semiconductor surface has allowed us to derive a point-spread function of
the measuring tip and to restore the actual surface potential from measured images in almost real time. The third section
describes the use of Kelvin probe microscopy to determine the density of surface and bulk states in inorganic and organic
semiconductors, respectively. In inorganic semiconductors the method is based on scanning a cross-sectional pn junction; as
the tip scans the junction, the position of the surface states relative to the Fermi level changes, thereby changing the surface
potential. The energy distribution is then obtained by fitting the measured surface potential. The method is applied to various
semiconductor (110) surfaces where a quantitative states distribution across most of the bandgap is obtained. In the case
of organic semiconductors the density of states in obtained by injecting charge carriers into the channel of a bottom gate
organic transistor. The measurement of the Fermi level shift together with the charge concentration allows us to derive the
density of states of the highest occupied molecular orbital band.