Charge Switching of Single Atoms
Bistable charge configuration of donor systems near the GaAs(110) surfaces
[Nano Lett. 11, 3538 (2011)]
The functional element in gated semiconductor devices is a space charge layer that is located under the gate electrode. With increasing gate voltage, the microscopic process forming this space charge layer is the subsequent ionization 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 at the single donor level and visualize the charge states of individual dopants and multi donor systems. The charge configuration of isolated donors is unambiguously determined by the position of the tip and the applied gate voltage. In contrast, a two donor system with inter-donor 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.
Single Si dopants in GaAs studied by scanning tunneling microscopy and spectroscopy
[Phys. Rev. B 84, 125310 (2011)]
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.
Enhanced Donor Binding Energy Close to a Semiconductor Surface
[Phys. Rev. Lett. 102, 166101 (2009)]
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.
Controlled Charge Switching on a Single Donor with a Scanning Tunneling Microscope
[Phys. Rev. Lett. 101, 076103 (2008) ]
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 f110g 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.
Our work (Göttingen) is supported by the Deutsche Forschungsgemeinschaft (DFG) within SFB 602 (TP A7) and SPP 1285.
In collaboration with:
J. K. Garleff, A. P. Wijnheijmer, and P. M. Koenraad, Department of Semiconductor Physics, Eindhoven University of Technology, P.O. Box 513, NL-5600 MB Eindhoven, The Netherlands