Spatially Resolved Manipulation of Single Electrons in Quantum Dots Using a Scanned Probe

The scanning metallic tip of a scanning force microscope was coupled capacitively to electrons confined in a lithographically defined gate-tunable quantum dot at a temperature of 300 mK. Single electrons were made to hop on or off the dot by moving the tip or by changing the tip bias voltage owing to the Coulomb-blockade effect. Spatial images of conductance resonances map the interaction potential between the tip and individual electronic quantum dot states. Under certain conditions this interaction is found to contain a tip-voltage induced and a tip-voltage-independent contribution.

Figure 1

(a) Topography of the structure measured at room temperature. (b) Conductance resonances measured by varying $V_{\mathrm{p}\mathrm{g}}$ at 300 mK with $20\mu \mathrm{V}$ applied between source and drain. (c) Schematic illustration of the local potential induced below the tip.

Figure 2

(a) Scanning-gate image measured with source-drain bias $V_{\mathrm{S}\mathrm{D}}=20\mu \mathrm{V}$ and $V_{\mathrm{p}\mathrm{g}}=-420\mathrm{m}\mathrm{V}$ and the tip ($V_{\mathrm{t}\mathrm{i}\mathrm{p}}=0$) scanned in feedback at a distance of a few nanometers from the surface. The oxide lines defining the structure have been drawn as white lines. Regions (I) to (IV) are described in the text. (b) Dot current obtained when scanning the tip in feedback along the horizontal dashed line in (a) for different $V_{\mathrm{p}\mathrm{g}}$. The horizontal dashed line in (b) corresponds to $V_{\mathrm{p}\mathrm{g}}$ used for the scan in (a).

Figure 3

(a) Conductance of the quantum dot vs tip bias along the horizontal dashed line in Fig. 2a at $V_{\mathrm{p}\mathrm{g}}=390\mathrm{m}\mathrm{V}$ and 200 nm tip-sample separation. (b) Scanning-gate measurements for different $V_{\mathrm{t}\mathrm{i}\mathrm{p}}$ as indicated by the dashed lines in (a) taken at a tip-sample separation of 120 nm and $V_{\mathrm{p}\mathrm{g}}=-390\mathrm{m}\mathrm{V}$. The scanned area is $6.5\times 6.5\mu {\mathrm{m}}^2$; the conductance ranges from zero (dark) to $1.1e^2/h$ (bright).

Figure 4

Coulomb-blockade diamonds as a function of tip position along the line shown in Fig. 2a and the source-drain bias voltage $V_{\mathrm{S}\mathrm{D}}$. The tip was lifted above the surface by 150 nm and the tip voltage was 0.2 V. The color scale represents the logarithm of the dot conductance.