Reversible phase transitions in the pseudomorphic $\sqrt{7}\times \sqrt{3}$-hex In layer on Si(111)

Using scanning tunneling microscopy, reversible phase transitions have been detected in the modulated pseudomorphic In monolayer on the Si(111) surface. It has been found that the room-temperature quasihexagonal $\sqrt{7}\times \sqrt{3}$ structure is transformed into the $\sqrt{7}\times \sqrt{7}$ structure during cooling in the temperature range from $265\text{to}225\mathrm{K}$. Further cooling results in developing long-range modulations in the In layer, including formation of the chevron-type structure with $6\sqrt{7}\times \sqrt{7}$ periodicity, the ordered arrays with regular antiphase domain boundaries with local $3\sqrt{7}\times \sqrt{7}$, and $2\sqrt{7}\times \sqrt{7}$ periodicity and the chained-ring structure with $\frac{10}{3}\sqrt{3}\times \frac{40}{3}\sqrt{3}$ periodicity, which is believed to originate from the $\frac{5}{3}\sqrt{3}\times \frac{5}{3}\sqrt{3}$ structure occurring at room temperature near surface defects and at domain boundaries of the original $\sqrt{7}\times \sqrt{3}\text{−}\mathrm{In}$ phase.

Figure 1

Structural transformation at $\mathrm{Si}\left(111\right)\sqrt{3}\times \sqrt{3}\text{−}\mathrm{In}$ surface upon RT deposition of In. (a) STM image showing the original $\sqrt{3}\times \sqrt{3}\text{−}\mathrm{In}$ surface. (b) STM image of the previous surface after deposition of additional 0.3 ML of In. The surface contains 0.63 ML of In in total and comprises a mixture of $\sqrt{3}\times \sqrt{3}$ and $2\times 2$ phases. Inset shows a $2\times 2$ phase at a greater magnification. (c) STM image of the previous surface after the deposition of additional 0.3 ML of In. The surface contains 0.93 ML of In in total and comprises a mixture of $2\times 2$ and $\sqrt{7}\times \sqrt{3}$-hex phases. Inset shows a $\sqrt{7}\times \sqrt{3}$-hex phase at a greater magnification. (d) STM image of the previous surface after the deposition of additional 0.15 ML of In. The surface contains 1.08 ML of In in total and comprises a mixture of $\sqrt{7}\times \sqrt{3}$-hex and $\sqrt{7}\times \sqrt{3}$-rec phases. Inset shows a $\sqrt{7}\times \sqrt{3}$-rec phase at a greater magnification. Scale of the images: (a) $-500\times 250{\mathrm{\AA }}^2$; (b) and (c) $-700\times 350{\mathrm{\AA }}^2$; (d) $-600\times 300{\mathrm{\AA }}^2$; insets $-70\times 50{\mathrm{\AA }}^2$.

Figure 2

$70\times 50{\mathrm{\AA }}^2$ empty-state STM images of the $\mathrm{Si}\left(111\right)\sqrt{7}\times \sqrt{3}$-hex-In surface acquired at (a) $+2.0\mathrm{V}$ and (b) $+0.5\mathrm{V}$ bias voltages. The $\sqrt{7}\times \sqrt{3}$ cell is outlined.

Figure 3

(a) $500\times 300{\mathrm{\AA }}^2$ empty-state $(+1.8\mathrm{V})$ STM image of $\mathrm{Si}\left(111\right)\sqrt{7}\times \sqrt{3}$-hex-In surface acquired at RT, which illustrates a formation of the honeycomblike structure near the surface defects and at the domain boundaries. (b) $150\times 150{\mathrm{\AA }}^2$ region at the domain boundary shown at a greater magnification. Unit cells of the $\sqrt{7}\times \sqrt{3}$ domains and that of the honeycomb structure are outlined. (c) Schematic diagram shows the honeycomblike structure to have a periodicity of $\frac{5}{3}\sqrt{3}\times \frac{5}{3}\sqrt{3}\text{−}R30°$.

Figure 4

(a) $800\times 450{\mathrm{\AA }}^2$ empty-state $(+2.3\mathrm{V})$ STM image of the surface obtained by cooling $\mathrm{Si}\left(111\right)\sqrt{7}\times \sqrt{3}$-hex surface to $225\mathrm{K}$. As a result, a honeycomblike $\sqrt{7}\times \sqrt{7}$ structure develops at the surface. In the shown region, two antiphase $\sqrt{7}\times \sqrt{7}$ domains are present, which are separated by a domain boundary seen as a linear depression. (b) $160\times 160{\mathrm{\AA }}^2$ empty-state $(+2.3\mathrm{V})$ STM image of the surface region with coexisted $\sqrt{7}\times \sqrt{3}$ and $\sqrt{7}\times \sqrt{7}$ phases. The corresponding unit cells are outlined. (c) Schematic diagram illustrating the honeycomblike structure to have a $\sqrt{7}\times \sqrt{7}\text{−}R19.1°$ lattice. (d) $300\times 300{\mathrm{\AA }}^2$ empty-state $(+2.3\mathrm{V})$ STM image showing the surface area with two $\pm 19.1°$-rotational domains of $\sqrt{7}\times \sqrt{7}$ structure. (e) FFT image of the surface shown in (d).

Figure 5

Two consequent empty-state ($+2.3\mathrm{V}$, $0.9\mathrm{nA}$) STM images of the same $680\times 560{\mathrm{\AA }}^2$ area acquired at $265\mathrm{K}$. The regions of $\sqrt{7}\times \sqrt{7}$ phase present at the first image (a) are substituted by the $\sqrt{7}\times \sqrt{3}$ structure at the second image (b).

Figure 6

(a) $900\times 450{\mathrm{\AA }}^2$ empty-state $(+2.5\mathrm{V})$ STM image acquired at $106\mathrm{K}$, showing a surface with a long-period chevronlike structure. (b) STM image of $200\times 150{\mathrm{\AA }}^2$ area with a geometric construction, illustrating the chevron structure to have a $6\sqrt{7}\times \sqrt{7}$ periodicity. (c) FFT image from the surface with a chevronlike structure.

Figure 7

(a) $900\times 600{\mathrm{\AA }}^2$ empty-state $(+1.7\mathrm{V})$ STM image acquired at $105\mathrm{K}$, showing the surface with an ordered array of the regular antiphase domain boundaries. (b) $300\times 200{\mathrm{\AA }}^2$ empty-state ($+2.7\mathrm{V}$, $0.6\mathrm{V}$) STM image acquired at $68\mathrm{K}$, illustrating that domain boundaries could change their location during image acquisition (the corresponding changeover acts are indicated by arrows).

Figure 8

(a) $900\times 450{\mathrm{\AA }}^2$ empty-state $(+2.4\mathrm{V})$ STM image acquired at $105\mathrm{K}$, showing the surface with a long-period chained-ring structure. (b) STM image of the $200\times 100{\mathrm{\AA }}^2$ area indicated with the $⟨\overline{2}11⟩$-type directions and the outlined unit cell of the chained-ring structure. The unit cell has a size of $\frac{10}{3}\sqrt{3}\times \frac{40}{3}\sqrt{3}$.