Atomic force microscopy is well-known even in the public as a versatile tool for the production of images on the nanoscale level. Kelvin probe force microscopy is a special type of this imaging technique named after Lord Kelvin. When brought to the market in 1991, a scientific description of how to interprete the images was delivered. To this, physicist Christine Baumgart, a doctoral student of the nanospintronics group at the research center Forschungszentrum Dresden-Rossendorf (FZD), has now added new features.
Christine Baumgart now discovered what exactly is measured by Kelvin probe force microscopy. It is the electric potential which is needed to move electrons or holes from the inside to the surface of a semiconductor. Her new findings will simplify the microscopic technique itself, and will lead to unambiguous and reproducible results concerning the structure and electronic properties of samples. Also, Kelvin probe force microscopy, which has been used mainly in materials science and semiconductor physics so far, is likely to become more attractive for other areas like biotechnology.
But how exactly does a Kelvin probe force microscope work? The tip is deflected by the electrostatic force between cantilever and sample when moved over the sample. By applying bias to the sample, electrons and holes are moved to the surface of the semiconductor and the electrostatic force decreases. The cantilever moves back to its original position and the applied bias is stored as the signal measured. To be more precise, there is a quantitative relation between the measured Kelvin bias and the difference between the calculated Fermi energy and respective semiconductor band edge independent of the work function of the probing microscope tip. Thus, Christine Baumgart’s novel explanation of how the Kelvin probe force microscope works elucidates why the signal depends on the bias necessary for injecting majority charge carriers towards the interface between insulator and semiconductor.
Baumgart C., Helm M., Schmidt H., (2009): Quantitativ dopant profiling in semiconducters: A Kelvin probe force microscopy model. DOI: 10.1103/PhysRevB.80.085305.