Kelvin probe force microscopy: Recent advances and applications

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.