Mechanism for Room-Temperature Single-Atom Lateral Manipulations on Semiconductors using Dynamic Force Microscopy

Vacancy-mediated lateral manipulations of intrinsic adatoms of the $\mathrm{Si}(111)\mathrm{\text{−}}(7\times 7)$ surface at room temperature are reported. The topographic signal during the manipulation combined with force spectroscopy measurements reveals that these manipulations can be ascribed to the so-called pulling mode, and that the Si adatoms were manipulated in the attractive tip-surface interaction regime at the relatively low short-range force value associated to the manipulation set point. First-principles calculations reveal that the presence of the tip induces structural relaxations that weaken the adatom surface bonds and manifests in a considerable local reduction of the natural diffusion barriers to adjacent adsorption positions. Close to the short-range forces measured in the experiments, these barriers are lowered near the limit that enables a thermally activated hopping at room temperature.

Figure 1

Topographic images, (a) to (h), from a series of vacancy-mediated manipulations of intrinsic adatoms of the $\mathrm{Si}(111)\mathrm{\text{−}}(7\times 7)$ surface—a movie is available in EPAPS 22. The vacancy pointed out by an arrow in (a) serves as a marker. First mechanical resonant frequency ($f_0$), cantilever oscillation amplitude ($A$), and cantilever spring constant ($K$) were 162295.8 Hz, 282 Å, and $28.7\mathrm{N}/\mathrm{m}$, respectively. Imaging ($\Delta f_I$) and manipulation ($\Delta f_M$) frequency shift set points were $-3.9$ and $-5.1\mathrm{Hz}$, corresponding to an attractive SR force at closest approach of $-0.15$ and $-0.5\mathrm{nN}$ [22], respectively. Image dimensions are $(8.0\times 8.0){\mathrm{nm}}^2$. In (i), a model of the atomic configuration for the highlighted unit cell in (g) is displayed.

Figure 2

(a) Ball-and-stick model of the dimer-row region of a $\mathrm{Si}(111)\mathrm{\text{−}}(7\times 7)$ faulted half-unit cell with a vacancy on a center ($\mathrm{Ce}$) adatom site. The corner ($\mathrm{Co}$) adatom sites, two equivalent $M$ sites ($M_L$ and $M_R$), and two ${\mathrm{H}}_3$ adsorption positions together with two tip locations ($P_1$ and $P_2$) are indicated. A detailed view of the $\mathrm{Co}\to \mathrm{Ce}$ manipulation path is shown in the upper part. (b) Line profiles obtained from (c), where the topography associated with the manipulation process between the images in Figs. 1d, 1e is displayed. The dotted-line rectangle corresponds to the one marked in Fig. 1d. The fast scan velocity was $8.6\mathrm{nm}/\mathrm{s}$, and the rectangle dimensions are $(4.3\times 1.5){\mathrm{nm}}^2$.

Figure 3

Calculated diffusion barriers for the adatom along the $\mathrm{Co}\to H_3$ path when the tip is located at the $P_1$ position [see Fig. 2a] at different tip-surface distances. The two insets show the tip structure (left) and the calculated SR force and related adatom vertical relaxation as a function of the tip-surface distance when the tip is over the $P_1$ position (right).