We present a comprehensive three-dimensional analysis of Kelvin probe force microscopy of semiconductors. It is shown that high-resolution electronic defect imaging is strongly affected by free carrier electrostatic screening, and the finite size of the measuring tip. In measurements conducted under ambient conditions, defects that are not more then $2\text{nanometers}$ below the surface, and are at least $50\text{nanometers}$ apart can be imaged only if the tip-sample distance is not larger then $10\text{nanometers}$. Under ultrahigh vacuum conditions, when the tip-sample distance can be as small as $1\text{nanometer}$, it is shown that the tip-induced band bending is only around a few millivolts, and can be neglected for most practical purposes. Our model is compared to ultrahigh vacuum Kelvin probe force microscopy measurements of surface steps on GaP, and it is shown that it can be used to obtain local surface charge densities.

We present dynamic force-microscopy experiments and first-principles simulations that contribute to clarify the origin of atomic-scale contrast in Kelvin-probe force-microscopy (KPFM) images of semiconductor surfaces. By combining KPFM and bias-spectroscopy imaging with force and bias-distance spectroscopy, we show a significant drop of the local contact potential difference (LCPD) that correlates with the development of the tip-surface interatomic forces over distinct atomic positions. We suggest that variations of this drop in the LCPD over the different atomic sites are responsible for the atomic contrast in both KPFM and bias-spectroscopy imaging. Our simulations point towards a relation of this drop in the LCPD to variations of the surface local electronic structure due to a charge polarization induced by the tip-surface interatomic interaction.

Hydrostatic high-pressure studies to 17 GPa were carried out on superconducting ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ over the full range of oxygen content x. The observed bell-shaped $T_c\mathrm{}\left(P\right)\mathrm{}$ dependences are found to depend markedly on the temperature at which the pressure is changed. The time-dependent relaxation processes are studied using both $T_c$ and the electrical resistivity at 298 K as probes. The activation volume for oxygen diffusion is determined from the pressure-dependent activation energy and compared with estimates from a simple hard-sphere model.

The size reduction of modern electronic devices creates a growing demand for characterization tools to determine material properties on a nanometer scale. The Kelvin probe force microscope is designed to obtain laterally resolved images of the sample’s work function. Using a setup in ultrahigh vacuum, we were able to distinguish work function variations for differently oriented crystal facets of single grains on a semiconductor surface. For the tetragonal solar cell material ${\mathrm{CuGaSe}}_2$ the experiments demonstrate differences as low as 30 meV between (102) and (111) oriented surfaces and up to 255 meV between $\text{(1̄1̄2̄)}$ and (110) surfaces. This influences the band bending of solar cell heterostructures and consequently also the solar power conversion efficiency.

The properties and performance of polycrystalline materials depend critically on the properties of their grain boundaries. Polycrystalline photovoltaic materials – e.g. hybrid halide perovskites, copper indium gallium diselenide (CIGSe) and cadmium telluride – have already demonstrated high efficiencies and promise cost-effective electricity supply. For CIGSe-based solar cells, an efficiency above 23% has recently been achieved using an alkali-fluoride post-deposition treatment; however, its full impact and functional principle are not yet fully understood. Here, we show direct evidence for the passivation of grain boundaries in CIGSe treated with three different alkali-fluorides through a detailed study of the nanoscale optoelectronic properties. We determine a correlation of the surface potential change at grain boundaries with the open-circuit voltage, which is supported by numerical simulations. Our results suggest that heavier alkali elements might lead to better passivation by reducing the density of charged defects and increasing the formation of secondary phases at grain boundaries.