Growth of thallium overlayers on a Si(100) surface

Formation of the thallium (Tl) overlayers on the $\mathrm{Si}\left(100\right)2\times 1$ surface has been studied using scanning tunneling microscopy (STM) and first-principles total-energy calculations. It has been found that adsorption of Tl atom involves a charge transfer leading to the development of a static dipole which is responsible for the field-assisted migration of the Tl adsorbate on the surface. When the STM tip bias voltage is positive, Tl atoms are repelled out from the region underneath the tip apex. In the case of the negative bias voltage, Tl is accumulated underneath the tip. Four principal ordered $\mathrm{Tl}∕\mathrm{Si}\left(100\right)$ reconstructions have been found, namely, three $2\times 2\text{−}\mathrm{Tl}$ reconstructions with 0.25, 0.50, and 0.75 ML of Tl and $2\times 1\text{−}\mathrm{Tl}$ reconstruction with 1.0 ML of Tl. The possible atomic arrangement of these reconstructions has been evaluated using first-principles total-energy calculations.

Figure 1

Schematic diagram summarizing the results of the LEED and STM observations: observed structures as a function of the STM bias polarity and Tl dose. Not that for the Tl dose beyond 0.5 ML a more expanded scale is used.

Figure 2

(a) Empty-state ($V_t=-2.2\mathrm{V}$, $I=0.4\mathrm{nA}$) and (b) filled-state ($V_t=+1.7\mathrm{V}$, $I=0.4\mathrm{nA}$) STM images of the same region $(190\times 120{\mathrm{\AA }}^2)$ of the Si(100) surface after deposition of 0.1 ML of Tl.

Figure 3

Determination of the location of the low-coverage $(\sim 0.1\mathrm{ML})$ Tl-associated empty-state protrusions with respect to the $\mathrm{Si}\left(100\right)2\times 1$ surface. In the empty states ($V_t=-1.9\mathrm{V}$, $I=0.4\mathrm{nA}$), one sees the Tl overlayer; in the filled states ($V_t=+1.9\mathrm{V}$, $I=0.4\mathrm{nA}$), one probes the Si(100) substrate. (a) The Tl protrusions lie in troughs between the Si-dimer rows. (b) The line drawn through the midpoint between the two neighboring Tl rows in the $3\times 2$ structure crosses the location of the missing-Si dimer defect. Scale of the images is $150\times 110{\mathrm{\AA }}^2$. The lines drawn in (a) and (b) are reproduced in schematic diagram in (c), which shows that Tl protrusions occupy valley-bridge sites.

Figure 4

Empty-state ($V_t=-1.9\mathrm{V}$, $I=0.4\mathrm{nA}$) STM image $(100\times 50{\mathrm{\AA }}^2)$ illustrating the hopping of the low-coverage $(\sim 0.1\mathrm{ML})$ Tl features in the course of STM imaging.

Figure 5

Successive empty-state ($V_t=-2.3\mathrm{V}$, $I=0.14\mathrm{nA}$) STM imaging at $200\mathrm{K}$ of the same region $(950\times 800{\mathrm{\AA }}^2)$ results in gradual accumulation of Tl in it. Image in (b) was acquired $17\min$ after image in (a). Deposited amount of Tl is about 0.15 ML.

Figure 6

When STM imaging $(V_t=\pm 1.9\mathrm{V},I=0.13\mathrm{nA})$ is conducted at a reduced temperature of $215\mathrm{K}$, the removal (appearance) of Tl features occurs at a “finite” time of several seconds after switching the bias polarity. The scale of the image is $170\times 140{\mathrm{\AA }}^2$. Deposited amount of Tl is about 0.15 ML.

Figure 7

Schematic diagram illustrating the behavior of the polarized adsorbate in the nonuniform field generated in the tip-sample gap.

Figure 8

STM observation of the $\beta \text{−}2\times 2\text{−}\mathrm{Tl}$ reconstruction with $\sim 0.5\mathrm{ML}$ of Tl. (a) Dual-polarity $(V_t=\pm 0.8\mathrm{V},I_t=0.1\mathrm{nA})$ STM image. (b) Determination of location of the empty-state $\beta \text{−}2\times 2\text{−}\mathrm{Tl}$ protrusions with respect to the $\mathrm{Si}\left(100\right)2\times 1$ substrate. Under relatively “tough” tunneling conditions $(+1.5\mathrm{V},3.0\mathrm{nA})$, the Tl overlayer is removed and the $\mathrm{Si}\left(100\right)2\times 1$ substrate becomes bare (upper half of the image). The scale of the images is $150\times 100{\mathrm{\AA }}^2$. The lines drawn in (b) are reproduced in the schematic diagram in (c) indicating location of the empty-state $\beta \text{−}2\times 2\text{−}\mathrm{Tl}$ protrusions. (d) Schematic diagram summarizing the results of the evaluation for the $\beta \text{−}2\times 2\text{−}\mathrm{Tl}$ reconstruction: empty-state protrusions (gray ovals) occupy the pedestal sites, filled-state protrusions (black circles) are in the valley-bridge sites.

Figure 9

STM observation of the $\mathrm{Si}\left(100\right)\gamma \text{−}2\times 2\text{−}\mathrm{Tl}$ structure at $\sim 0.75\mathrm{ML}$ Tl coverage. (a) Dual-polarity $(V_t=\pm 0.4\mathrm{V},I_t=0.3\mathrm{nA})$ STM image of the $\gamma \text{−}2\times 2\text{−}\mathrm{Tl}$ (upper half: filled states, lower half: empty states). The scale of the image is $135\times 85{\mathrm{\AA }}^2$. (b) Bias-dependent STM appearance of the $\gamma \text{−}2\times 2\text{−}\mathrm{Tl}$ structure in empty states (upper panel: $V_t=-0.8\mathrm{V}$, $I_t=0.3\mathrm{nA}$, lower panel: $V_t=-1.5\mathrm{V}$, $I_t=0.3\mathrm{nA}$). The scale of the image is $135\times 85{\mathrm{\AA }}^2$. Changing of the bias voltage results in shifting protrusions along the rows. The lines are drawn to guide the eye.

Figure 10

(a) $135\times 95{\mathrm{\AA }}^2$ filled-state STM image showing the field-induced removal of the Tl layer from the $\gamma \text{−}2\times 2\text{−}\mathrm{Tl}$ structure with $\sim 0.75\mathrm{ML}$ of Tl (upper half: $\gamma \text{−}2\times 2\text{−}\mathrm{Tl}$ at $V_t=+1.5\mathrm{V}$, $I_t=0.8\mathrm{nA}$, lower half: $2\times 1\text{−}\mathrm{Si}$ at $V_t=+1.5\mathrm{V}$, $I_t=50\mathrm{nA}$). Locations of the Tl-associated protrusion and Si-dimers are indicated by open circle and ovals, respectively. (b) Schematic diagram illustrating location of the $\gamma \text{−}2\times 2\text{−}\mathrm{Tl}$ STM protrusions as determined from (a) and Fig. 9: the filled-state protrusions occupy the pedestal sites, the empty-state protrusions occupy the pedestal sites at absolute biases below $\sim 1\mathrm{V}$ and the valley-bridge sites at higher voltages.

Figure 11

Empty-state $(V_t=-0.4\mathrm{V})$ STM observation of the Si(100) surface after deposition of $\sim 1\mathrm{ML}$ of Tl. At relatively high tunneling current $(I_t=1.1\mathrm{nA})$ the surface displays the $2\times 1\text{−}\mathrm{Tl}$ structure (upper half), while at low current $(I_t=0.33\mathrm{nA})$ it displays the $\gamma \text{−}2\times 2\text{−}\mathrm{Tl}$ structure (lower half). The scale of the image is $135\times 85{\mathrm{\AA }}^2$.

Figure 12

(a) Dual-polarity $(V_t=\pm 0.13\mathrm{V},I_t=1.0\mathrm{nA})$ STM image of the $2\times 1\text{−}\mathrm{Tl}$ structure prepared by deposition of 2 ML of Tl onto Si(100) surface. The scale of the image is $120\times 75{\mathrm{\AA }}^2$. (b) Bias-dependent STM appearance of the $2\times 1\text{−}\mathrm{Tl}$ structure in empty states. The blurred rows seen at absolute biases above $\sim 0.7\mathrm{V}$ are shifted by a distance of $a=3.84\mathrm{\AA }$ with respect to the rows of the oval-shaped protrusions seen at lower biases. The scale of the image is $150\times 160{\mathrm{\AA }}^2$.

Figure 13

(a) $75\times 60{\mathrm{\AA }}^2$ filled-state STM image showing the field-induced removal of the Tl layer from the $2\times 1\text{−}\mathrm{Tl}$ structure with $\sim 1\mathrm{ML}$ of Tl (lower half: $2\times 1\text{−}\mathrm{Tl}$ at $V_t=+0.3\mathrm{V}$, $I_t=1.0\mathrm{nA}$, upper half: $2\times 1\text{−}\mathrm{Si}$ at $V_t=+1.5\mathrm{V}$, $I_t=50\mathrm{nA}$). (b) Schematic diagram illustrating the determined location of the $2\times 1\text{−}\mathrm{Tl}$ STM protrusions: the filled-state protrusions occupy the pedestal sites, the empty-state protrusions occupy the pedestal sites at absolute biases below $\sim 0.7\mathrm{V}$ and show up as blurred rows along the trenches between Si-dimer rows at greater voltages.