The charge state of individually addressable impurities in semiconductor material was manipulated with a scanning tunneling microscope. The manipulation was fully controlled by the position of the tip and the voltage applied between tip and sample. The experiments were performed at low temperature on the (110) surface of silicon doped GaAs. Silicon donors up to 1 nm below the surface can be reversibly switched between their neutral and ionized state by the local potential induced by the tip. By using ultrasharp tips, the switching process occurs close enough to the impurity to be observed as a sharp circular feature surrounding the donor. By utilizing the controlled manipulation, we were able to map the Coulomb potential of a single donor at the semiconductor-vacuum interface.

We measured the ionization threshold voltage of individual impurities close to a semiconductor-vacuum interface, where we use the STM tip to ionize individual donors. We observe a reversed order of ionization with depth below the surface, which proves that the binding energy is enhanced towards the surface. This is in contrast to the predicted reduction for a Coulombic impurity in the effective mass approach. We can estimate the binding energy from the ionization threshold and show experimentally that in the case of silicon doped gallium arsenide the binding energy gradually increases over the last 1.2 nm below the (110) surface.

Scanning tunneling spectroscopy was performed at low temperature on buried manganese ͑Mn͒ acceptors below the ͑110͒ surface of gallium arsenide. The main Mn-induced features consisted of a number of dI / dV peaks in the band gap of the host material. The peaks in the band gap are followed by negative differential conductivity, which can be understood in terms of an energy-filter mechanism. The spectroscopic features detected on the Mn atoms clearly depend on the depth of the addressed acceptor below the surface. Combining the depth dependence of the positions of the Mn-induced peaks and using the energy-filter model to explain the negative resistance qualitatively proves that the binding energy of the hole bound to the Mn atom increases for Mn acceptors closer to the surface.

The electronic structure of the Si(111)-2 ϫ 1 surface has been studied with scanning tunneling spectroscopy (STS). A large experimental local density of states data set of LDOS͑x , y , E͒ with subatomic resolution has been compared with ab initio calculated LDOS distributions. The influence of the tunneling tip DOS has been eliminated by repeated measurements with different tips. The experimentally determined shape of the LDOS͑x , y , E͒ agrees very well with the calculated results based on the -bonded-chain model for both the surface valence and the conduction band. The good agreement with ab initio calculations of the electronic structure of the Si(111)-2 ϫ 1 surface shows that STS provides reliable information of the sample LDOS even with subatomic resolution.

No abstract

No abstract

In gated semiconductor devices, the space charge layer that is located under the gate electrode acts as the functional element. With increasing gate voltage, the microscopic process forming this space charge layer involves the subsequent ionization or electron capture of individual dopants within the semiconductor. In this Letter, a scanning tunneling microscope tip is used as a movable gate above the (110) surface of n-doped GaAs. We study the build-up process of the space charge region considering donors and visualize the charge states of individual and multi donor systems. The charge configuration of single donors is determined by the position of the tip and the applied gate voltage. In contrast, a two donor system with interdonor distances smaller than 10 nm shows a more complex behavior. The electrostatic interaction between the donors in combination with the modification of their electronic properties close to the surface results in ionization gaps and bistable charge switching behavior.

We present a comprehensive scanning tunneling microscopy and spectroscopy study of individual Si dopants in GaAs. We explain all the spectroscopic peaks and their voltage dependence in the band gap and in the conduction band. We observe both the filled and empty donor state. Donors close to the surface, which have an enhanced binding energy, show a second ionization ring, corresponding to the negatively charged donor D − . The observation of all predicted features at the expected spectral position and with the expected voltage-distance dependence confirms their correct identification and the semiquantitative analyses of their energetic positions.