The doping characteristics and carrier transport in a GaAs p-n junction were visualized with a 10 nm spatial resolution, using light-modulated scanning tunneling spectroscopy. The dynamics of minority carriers under operating conditions, such as recombination, diffusion, and electric field induced drift, which had previously been analyzed on the basis of empirical electric properties, were successfully examined on the nanoscale. These results provide a solid basis for elucidating the mechanism of the carrier transport properties predicted by using the macroscopic analysis.

Probing the charge density distributions in materials at atomic scale remains an extremely demanding task, particularly in real space. However, recent advances in differential phase contrast-scanning transmission electron microscopy (DPC-STEM) bring this possibility closer by directly visualizing the atomic electric field. DPC-STEM at atomic resolutions measures how a sub-angstrom electron probe passing through a material is affected by the atomic electric field, the field between the nucleus and the surrounding electrons. Here, we perform a fully quantitative analysis which allows us to probe the charge density distributions inside atoms, including both the positive nuclear and the screening electronic charges, with subatomic resolution and in real space. By combining state-of-the-art DPC-STEM experiments with advanced electron scattering simulations we are able to map the spatial distribution of the electron cloud within individual atomic columns. This work constitutes a crucial step toward the direct atomic scale determination of the local charge redistributions and modulations taking place in materials systems.

We investigated charged defects on an n-GaAs͑110͒ surface using light-modulated scanning tunneling spectroscopy. Tunneling via a single defect-induced gap state under photoillumination was observed for the isolated atomic defects. Screened Coulomb potentials induced around a charged Ga vacancy and a step edge were visualized, for the first time, with a nanometer spatial resolution. Furthermore, the charge states of the individual defects were determined on the atomic level.

Local barrier height (LBH) of Si(001) surface was studied using light-modulated scanning tunneling spectroscopy (LM-STS), which enables the observation of the tip-sample-dependent LBH with or without photoillumination simultaneously. The bias voltage and tip-sample distance dependence of LBH were comprehensively understood by the tip-induced band bending (TIBB), which influences the scanning tunneling microscopy and spectroscopy (STM/STS) in measurement of the local electronic structures of semiconductors. A marked decrease in surface photovoltage caused by photocarrier tunneling at shorter tip-sample distance was also shown. On the basis of these results, a method to measure LBH free of TIBB is discussed.

Surface photovoltage (SPV) was visualized over the interface of a GaAs p-n junction using light modulated scanning tunneling spectroscopy. Spatially resolved SPV includes information about the built-in potential of the p-n junction as well as the photo-induced relaxation of tip-induced band bending. These two components were separately evaluated, and mapping of the built-in potential was accomplished on the nanoscale.