Prediction of two-dimensional topological insulator by forming a surface alloy on Au/Si(111) substrate

Two-dimensional (2D) topological insulators (TIs), which can be integrated into the modern silicon industry, are highly desirable for spintronics applications. Here, using first-principles electronic structure calculations, we show that the Au/Si(111)-$\sqrt{3}$ substrate can provide a platform for hosting 2D TIs obtained through the formation of surface alloys with a honeycomb pattern of adsorbed atoms. We systematically examined elements from groups III to VI of the periodic table at 2/3 monolayer coverage on Au/Si(111)-$\sqrt{3}$, and found that In, Tl, Ge, and Sn adsorbates result in topologically nontrivial phases with band gaps varying from 0 to 50 meV. Our scanning tunneling microscopy and low-energy electron diffraction experiments confirm the presence of the honeycomb pattern when Bi atoms are deposited on Au/Si(111)-$\sqrt{3}$, in accord with our theoretical predictions. Our findings pave the way for using surface alloys as a potential route for obtaining viable 2D TI platforms.

Figure 1

Side and top views of two structural models used for metal adsorbates on an Au/Si(111)-$\sqrt{3}$ substrate: (a) planar and (b) buckled honeycomb. The orange colored atoms are placed slightly higher than the purple colored atoms. The red dashed line marks the unit cell of the $\sqrt{3}$ structure.

Figure 2

Representative band structures for adsorbates from groups III-VI on an Au/Si(111)-$\sqrt{3}$ substrate, assuming the planar honeycomb structure of Fig. 1: (a) Tl (group III), (b) Sn (group IV), (c) Bi (group V), and (d) Te (group VI). Red and blue circles identify metal-derived states with opposite spins. A 2D Brillouin zone of Au/Si(111)-$\sqrt{3}$ with specific symmetry points labeled is shown in (d).

Figure 3

(a) The structural model and (b) the band structure of a nanoribbon of 2/3 ML Bi on Au/Si(111)-$\sqrt{3}$ with a zigzag edge. The contribution from the edge on the right (left) hand side is marked with red crosses (blue circles); symbol sizes are proportional to the contribution from the Bi atoms on the edges. The yellow region denotes the projected bulk bands.

Figure 4

Honeycomb pattern formed by depositing Bi at 2/3 ML on Au/Si(111)-$\sqrt{3}$. (a) LEED pattern taken at 48 eV with $1\times 1$ and $\sqrt{3}\times \sqrt{3}$ lattice spots marked by red and green arrows, respectively. (b) $40\times 20{\mathrm{nm}}^2$ STM image of 0.66 ML Bi on Au/Si(111)-$\sqrt{3}$ followed by annealing at $\sim 280$ ${}^{\circ }\mathrm{C}$. (c) Atom-resolved STM image of the area marked by the red square in (b). The image was acquired with a sample bias of 1.5 V at 0.1 nA. The honeycomb pattern and the $\sqrt{3}\times \sqrt{3}$ lattice are shown.