Controlled Charge Switching on a Single Donor with a Scanning Tunneling Microscope

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.

Figure 1

(a) Shows a constant current topography image at 2 V and 100 pA. Two donors are surrounded by a disk of enhanced topographic height. The atomic corrugation is not disturbed by the disks. In (b) cross sections through another donor at different voltages are shown. At the edge of the disk a jump in the topographic height is seen, indicated by the arrows.

Figure 2

Schematic representation of the ionization mechanism. When the tip is laterally far away from the donor (a), the bands on top of the donors are flat (b) and the donor will be neutral. As the tip approaches the donor with a positive sample bias (c), the bands are lifted due to the TIBB (d). When the donor level aligns with the conduction band in the bulk, the electron escapes.

Figure 3

(a) Topography image of a donor at a voltage of 2.5 V and current 500 pA. The dashed line indicates the position of the spectrum section in (d). (b),(c) Spatially resolved $dI/dV$ maps at different voltages. Higher differential conductivity is seen as a ring around the donor center, the ring diameter increase with voltage. (d) Shows a $dI/dV$ section along the dashed line in (a). The hyperbola corresponds to the ring in (b) and (c). A second measurement, acquired with a different tip, is shown in (e), where we averaged in angular direction. Contour lines of constant TIBB are added in both images. The details of the fitting are given in the text. (f) The measured position of the ring (dots) as a function of current setpoint and calculated contour lines of the TIBB.

Figure 4

A spectrum taken directly above the donor (dotted line) and on the free surface (solid line) are shown in (a). At voltages lower than 1.3 V the curves overlap. The curves overlap for higher voltages as well, by shifting the spectrum above the donor by a certain voltage (dashed line). The situation is schematically shown in the upper inset. The voltage shift directly gives the Coulomb potential originating from the ionized donor. We measured the voltage shift for different lateral distances to the donor center. The results are the dots in (b). The data are fitted with the Coulomb potential directly at the surface; see solid line in the lower inset. The three curves correspond to different donor depths, given in the text.