Atomic dynamics of In nanoclusters on $\mathrm{Si}\left(100\right)$

Using scanning-tunneling microscopy and first-principles total-energy calculations, we have considered the structural properties of the so-called doped clusters formed by depositing additional $0.05$ monolayer of In onto the $4\times 3$-periodicity magic-cluster array in the $\mathrm{In}∕\mathrm{Si}\left(100\right)$ system. Low-temperature STM observations have revealed that most of the doped clusters have an asymmetric shape. According to the total-energy calculations, these clusters have plausibly ${\mathrm{Si}}_6{\mathrm{In}}_8$ composition. In such a cluster, one of the In atoms is mobile and can hop between four equivalent sites within a cluster. The hopping between sites, located in the different $2a\times 3a$ halves of the cluster, is characterized by the barrier of about $0.7\mathrm{eV},$ and this hopping becomes frozen at $55\mathrm{K}$. In contrast, the hopping between the neighboring sites within the same cluster half persists up to very low temperatures, as the barrier height here is an order of magnitude lower. Due to the above structural properties, the doped asymmetric ${\mathrm{Si}}_6{\mathrm{In}}_8$ cluster can be treated as a promising switch, logic gate, or memory cell of the atomic-scale size.

Figure 1

Room-temperature $100\times 100{\mathrm{\AA }}^2$ filled-state $(-2.0\mathrm{V})$ STM image of the $\mathrm{In}∕\mathrm{Si}\left(100\right)4\times 3$ nanocluster array with doped clusters. Scanning is from left to right with $60\mathrm{ms}$ for each line. Ordinary (nondoped) cluster is labeled 1, rigid-doped cluster is labeled 2, fuzzy-doped cluster is labeled 3, and the doped cluster undergoing conversion from fuzzy to rigid state is labeled 4. The $4\times 3$ unit cell is outlined.

Figure 2

(a) Low-temperature $\left(55\mathrm{K}\right)$ $170\times 85{\mathrm{\AA }}^2$ filled-state $(-2.0\mathrm{V})$ STM image of the $\mathrm{In}∕\mathrm{Si}\left(100\right)4\times 3$ nanocluster array with doped clusters. (b) Contour map of the image shown in (a). Most of the doped clusters are asymmetric (the shifts of their centers from the axis line are indicated by arrows), except for the one symmetric cluster in the right top corner (indicated by a circle with a cross).

Figure 3

Possible configurations occurring during hopping of the mobile In atom within a ${\mathrm{Si}}_6{\mathrm{In}}_8$ cluster, their formation energies and energetic barriers between them. (a) S1 configuration with the mobile In atom in its ground state. There are four equivalent configurations of this type (marked by squares). (b) S2 configuration with the In atom in a saddle point (marked by diamonds) while hopping to the nearest S1 site located in the same $2a\times 3a$ half of the cluster. (c) S3 configuration with the In atom in a saddle point (marked by triangles) while hopping to the other $2a\times 3a$ half of the cluster. (d) S4 configuration occurring when the mobile In atom substitutes the central In atom, pushing it to the other half of the cluster. (e) Calculated shape of the barriers for different hopping pathways: diamonds for $\mathrm{S}1\to \mathrm{S}2$, triangles for $\mathrm{S}1\to \mathrm{S}3$, and circles for $\mathrm{S}2\to \mathrm{S}4$. The inset shows the values of the barrier height $E_b$ (with respect to the S1-site level) for each pathway.

Figure 4

Tracking of the In-atom hopping between the $2a\times 3a$ halves by measuring the time dependence of the apparent tip height in the off-centered point within the ${\mathrm{Si}}_6{\mathrm{In}}_8$ cluster.