New Interpretation Simplifies Microscopic Image Technique 

A physicist has suggested interpreting the images generated by Kelvin probe force microscopy in a new way. This special type of imaging technique was brought to the market in 1991 along with a scientific description of how to interpret the images. Christine Baumgart, a doctoral student of the nanospintronics group at Forschungszentrum Dresden-Rossendorf (FZD), has now added new features to the original description.

A Kelvin probe force microscope uses an electrically conducting tip. Therefore, it measures not only the surface topography of the sample, but also the electric force between the tip and the sample. Hence, surface phenomena like catalytic or electric activity of ion doped materials can thoroughly be investigated.

While this microscopic technique is advantageous for non-destructive investigation of electric properties, the complicated measuring procedure, which even affects the reproducibility of the scientific outcome, is considered to be its main disadvantage. Furthermore, scientists have relied on an incomplete explanation for the values they measured because it has been believed that the electric potential between the tip and the surface of the sample was measured.

Kelvin probe force microscopy has been used mainly in materials science and semiconductor physics so far. However, it is likely to become more attractive for other areas like biotechnology.

Baumgart has discovered exactly what 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. These new findings will simplify the microscopic technique itself, and will lead to unambiguous and reproducible results concerning the structure and electronic properties of samples. She recently published her insights in the journal Physical Review B.

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, 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.

In her dissertation work under the supervision of Heidemarie Schmidt, Baumgart deals with materials for future nanospintronic devices. Usually, foreign atoms are implanted into these materials. To describe doped semiconductors thoroughly, she uses various microscopic techniques like the Kelvin probe force microscope.

Baumgart explained, “I wanted to understand more precisely how this microscope works. At the Ion Beam Center of the FZD, we are able to produce especially well-defined samples. While working with such semiconducting samples, I found out what exactly the Kelvin probe force microscope measures and that the signal has not been interpreted sufficiently. The good news is that the measurement itself has always been correct.”