Developing antiphase boundaries in one-monolayer $\mathrm{Tl}∕\mathrm{Ge}\left(100\right)$ system by re-bonding of underlying Ge dimers

Using scanning tunneling microscopy, it has been found that a one-monolayer $\mathrm{Tl}∕\mathrm{Ge}\left(100\right)2\times 1$ surface demonstrates a fascinating possibility to form antiphase boundaries (APB’s) by re-bonding underlying Ge dimers. This is in contrast to adsorbate-free Si(100) and Ge(100) surfaces and one-monolayer $\mathrm{Tl}∕\mathrm{Si}\left(100\right)$ system, where APB formation does not occur. Energetics of APB formation in various systems, including adsorbate-free Si(100) and Ge(100) surfaces and $\mathrm{Tl}∕\mathrm{Si}\left(100\right)$ and $\mathrm{Tl}∕\mathrm{Ge}\left(100\right)$ systems, have been evaluated using first-principles total-energy calculations, which reveal that $\mathrm{Tl}∕\mathrm{Ge}\left(100\right)$ is the only system where APB formation is the energetically favorable process.

Figure 1

(a) High-resolution $(23\times 23{\mathrm{\AA }}^2)$ filled-state STM image of the one-monolayer $\mathrm{Tl}∕\mathrm{Ge}\left(100\right)2\times 1$ surface. (b) Double-layer structural model of the surface. Tl atoms are shown by large gray circles, and Si atoms by small white circles.

Figure 2

(a) $165\times 130{\mathrm{\AA }}^2$ filled-state STM image of the $\mathrm{Tl}∕\mathrm{Ge}\left(100\right)2\times 1$ surface area containing two antiphase boundaries (indicated APB). (b) Experimental and (c) simulated high-resolution STM images of the APB region. (d) Optimized structural model of the APB region. Tl atoms are shown by large gray circles, and Si atoms by small white circles.

Figure 3

[(a) and (b)] Two successive filled-state STM images of the same $115\times 115{\mathrm{\AA }}^2$ region of the $\mathrm{Tl}∕\mathrm{Ge}\left(100\right)2\times 1$ surface with APB (indicated by arrows). One can see that APB has moved by about $20\mathrm{\AA }$. (c) Magnified STM image of the area around APB, illustrating continuous Ge-dimer re-bonding occurring during STM imaging.

Figure 4

Surface configurations with APB’s for which formation energies have been calculated (see Table ).

Figure 5

$125\times 125{\mathrm{\AA }}^2$ STM image of the $c(2\times 8)$ reconstruction formed in $\mathrm{Tl}∕\mathrm{Ge}\left(100\right)$ system at Tl coverage beyond 1 ML. This reconstruction shows up much like an ordered APB array.