Combined voltage-dependent scanning tunneling microscopy ͑STM͒ images with atomic resolution, local scanning tunneling spectroscopy, and simulations of the potential distribution in the interface-STM tip system are used to extract the physical imaging mechanisms of GaAs p-n interfaces in STM images. It is shown that ͑i͒ the tip-induced changes of the potential near the interface result in the tunneling characteristics of the p-type (n-type͒ layer being dragged into the interfaces' depletion region at positive ͑negative͒ sample voltage. ͑ii͒ This leads to a considerable reduction of the apparent width of the image of the depletion zone in STM images. ͑iii͒ At small negative sample voltages, a pronounced depression line appears. The depression is directly correlated with the electronic interface. It arises from the interplay of competing current contributions from the valence and conduction bands. This understanding of the imaging process allows us to develop methods on how to extract accurate physical data about the properties of the electronic interfaces from scanning tunneling microscopy images.

We report on atomically resolved scanning tunneling microscopy images and tunneling spectra of ͑110͒ cleavage surfaces of semi-insulating GaAs without illumination at room temperature. With help of simple model calculations we extract the physical mechanisms involved in the tunneling processes from and into semi-insulating GaAs. Atomically resolved images can only be observed at negative voltages, while no tunneling into empty states is possible without illumination. This is explained, on the one hand, by the absence of a carrier inversion at the semiconductor surface without illumination under the nonequilibrium tunneling contact conditions. On the other hand, at negative voltages in the noncontact mode an accumulation at the surface occurs and leads to tunneling of electron from the valence band states into the empty tip states. This current is limited by the tunneling through the vacuum barrier and the scanning tunneling microscopy images are found to show the occupied dangling bond states above the arsenic atoms. In the point contact mode the current is limited by tunneling through the space charge region without and with illumination. The implications of the results for the investigation of low-conductivity materials by scanning tunneling microscopy are discussed.

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We identified p-type nanoscale dopant-induced dots that are formed by fluctuations of the dopant atom distribution in sufficiently thin GaAs p-n multilayers. Their electronic structure and the resulting potential variations were investigated by cross-sectional scanning tunneling microscopy and spectroscopy as a function of the number of dopant atoms within the dot. We find significant changes in the current-voltage characteristics of the dots compared to spatially nonconfined material, due to a reduced ability to screen the tip's electric field. This indicates a limited ability to deplete the dots of free holes arising from the presence of confining potentials surrounding the dopant-induced dots.

We identified the charge carrier compensation mechanism in Te-doped GaAs with atomically resolved scanning tunneling microscopy. Three types of defects were found: tellurium donors (TeAs), Ga vacancies (VGa), and Ga vacancy–donor complexes (VGa–TeAs). We show quantitatively that the compensation in Te-doped bulk GaAs is exclusively caused by vacancy–donor complexes in contrast to Si-doped GaAs. This is explained with the Fermi-level effect as the universal mechanism leading to Ga vacancy formation in n-doped GaAs, and a Coulomb interaction leading to the formation of the complexes. The quantification of the carrier compensation yields a −3e charge state of VGa in bulk GaAs.