Atomic Dipole Moment Distribution of Si Atoms on a $\mathrm{Si}(111)\mathrm{\text{−}}(7\times 7)$ Surface Studied Using Noncontact Scanning Nonlinear Dielectric Microscopy

A local atomic electric dipole moment distribution of Si atoms on $\mathrm{Si}(111)\mathrm{\text{−}}(7\times 7)$ surface is clearly resolved by using a new technique called noncontact scanning nonlinear dielectric microscopy. The dc-bias voltage dependence of the atomic dipole moment on the $\mathrm{Si}(111)\mathrm{\text{−}}(7\times 7)$ surface is measured. At the weak applied voltage of $-0.5\mathrm{V}$, a positive dipole moment is detected on the Si adatom sites, whereas a negative dipole moment is observed at the interstitial sites of inter Si adatoms. Moreover, the quantitative dependence of the surface dipole moment as a function of the applied dc voltage is also revealed at a fixed point above the sample surface. This is the first successful demonstration of direct atomic dipole moment observation achieved in the field of capacitance measurement.

Figure 1

Bias voltage dependence of local dipole moment induced on the Si(1,1,1) surface. From left to right, the applied dc voltage was varied from $-1.0$ to 0.5 V in increments of 0.5 V. The upper images show the topography from the ${\epsilon }_{\mathrm{local}}(4)$ feedback signal in which individual Si atoms were observed. The lower images show the ${\epsilon }_{\mathrm{local}}(3)$ signal in which the red color of the ${\epsilon }_{\mathrm{local}}(3)$ signal shows that the direction of dipole moment is positive (upward), while the blue color of the ${\epsilon }_{\mathrm{local}}(3)$ signal shows that the direction of dipole moment is negative (downward).

Figure 2

(a) Topographies taken by the ${\epsilon }_{\mathrm{local}}(4)$ signal used as the feedback signal with an applied bias voltage of $-0.5\mathrm{V}$. (b) Magnified ${\epsilon }_{\mathrm{local}}(3)$ images of Si adatoms obtained simultaneously with the topography.

Figure 3

One-dimensional image of (a) topography and ${\epsilon }_{\mathrm{local}}(3)$ [both (b) $\cos \theta$ and (c) $A\cos \theta$], which were taken along the white line in Fig. 2 at a bias voltage of $V_{\mathrm{dc}}=-0.5\mathrm{V}$. The vertical axis of topography represents relative height. The arrows in Fig. 3a indicate the adatoms. The $\cos \theta$ image shows the sign of the ${\epsilon }_{\mathrm{local}}(3)$ signal to extract the polarity of dipole moment only. The average value of the ${\epsilon }_{\mathrm{local}}(3)$ signal in (c) is almost zero.

Figure 4

The dc-bias voltage dependences of the ${\epsilon }_{\mathrm{local}}(3)$ signal, ${\epsilon }_{\mathrm{local}}(4)$ signal, and tunneling current.