Band engineering in an epitaxial two-dimensional honeycomb ${\mathrm{Si}}_{6-x}{\mathrm{Ge}}_x$ alloy

In this Letter, we demonstrate that it is possible to form a two-dimensional (2D) silicenelike ${\mathrm{Si}}_5\mathrm{Ge}$ compound by replacing the Si atoms occupying on-top sites in the planarlike structure of epitaxial silicene on ${\mathrm{ZrB}}_2$(0001) by deposited Ge atoms. For coverages below 1/6 monolayer, the Ge deposition gives rise to a ${\mathrm{Si}}_{6-x}{\mathrm{Ge}}_x$ alloy (with $x$ between 0 and 1) in which the on-top sites are randomly occupied by Si or Ge atoms. The progressive increase of the valence band maximum with $x$ observed experimentally originates from a selective charge transfer from Ge atoms to Si atoms. These achievements provide evidence for the possibility of engineering the band structure in 2D SiGe alloys in a way that is similar for their bulk counterpart.

Figure 1

Deposition of Ge on silicene on ${\mathrm{ZrB}}_2$(0001). (a) and (b) Top and side views of the epitaxial planarlike structure of silicene on ${\mathrm{ZrB}}_2$(0001) as determined by DFT calculations. Si, Zr, and B atoms are, respectively, dark-blue, red, and purple colored. The green rhombus indicates the ($\sqrt{3}\times \sqrt{3}$)-reconstructed unit cell. (c) STM image (30 $\mathrm{nm}\times$ 30 nm, $V=1.0$ V, $I=100$ pA) of epitaxial silicene after deposition of 0.057 ML Ge. The STM image (33 $\mathrm{nm}\times 23$ nm) in the inset shows the silicene-Ge alloy (top left) and Si bilayer islands side-by-side. (d) Photoelectron spectrum recorded with $h\nu =80$ eV in the Ge $3d$ and Zr $4p$ regions after deposition of 0.090 ML Ge. The experimental spectrum is indicated by the black line. Yellow- and purple-filled curves are the contribution of Ge $3d$ and Zr $4p$ core levels determined by fitting and the green-filled curve is their sum. The full widths at half maximum are 270 and 290 meV, respectively, for the Ge $3d_{3/2}$ and Ge $3d_{5/2}$ peaks. (e) and (f) Top and side views of the structure of epitaxial ${\mathrm{Si}}_5\mathrm{Ge}$ on ${\mathrm{ZrB}}_2$(0001) as determined by DFT calculations. Si, Zr, and B atoms are colored in the same way as in (a) and Ge atoms are light-blue colored.

Figure 2

${\mathrm{Si}}_{6-x}{\mathrm{Ge}}_x$ alloy. (a)–(d) STM images ($V=1.0$ V, $I=100$ pA) and profiles along the red lines after deposition of (a) 0.030 ML Si and (b)–(d) 0.057, 0.113, and 0.167 ML Ge. Their common color-coded $z$ scale bar is shown. (e) Spectra recorded in the Ge $3d$ and Zr $4p$ core-level regions recorded for different Ge coverages with a photon energy of 80 eV. (f) Integrated intensity of the photoelectron spectra and percentage of protruding Ge atoms as functions of the Ge coverage.

Figure 3

Valence band of ${\mathrm{Si}}_{6-x}{\mathrm{Ge}}_x$ alloy. The inset of (a) shows the region around the $K$ point of the Brillouin zone of unreconstructed silicene in which the spectra were recorded. Brillouin zones of ($\sqrt{3}\times \sqrt{3}$)-reconstructed and unreconstructed silicene are respectively indicated by black and red lines. (a)–(e) ARPES spectra recorded with a photon energy of $h\nu =45$ eV on ${\mathrm{Si}}_{6-x}{\mathrm{Ge}}_x$ alloys for $x=0$, 0.25, 0.5, 0.75, and 1. The horizontal lines indicate $E_{VBM}$. The inset of (e) shows $\mathrm{\Delta }{E}_{VBM}=E_{VBM}-[E_{VBM}^{\text{silicene}}x+E_{VBM}^{{\text{Si}}_5\text{Ge}}(1-x)]$ as a function of $x$. Its fitting with $bx(1-x)$ and $b=-125$ meV is indicated by a dashed red line.

Figure 4

Calculated band structures of silicene and ${\mathrm{Si}}_5\mathrm{Ge}$. (a) and (b) Band structures of silicene and ${\mathrm{Si}}_5\mathrm{Ge}$ in the region indicated in the inset of Fig. 3. The spectral weight of the combined contribution of Si and Ge atoms is indicated by the size of the circles. (c) Lattice parameter dependence of the energy per atom of freestanding planarlike structures of silicene (red) and ${\mathrm{Si}}_5\mathrm{Ge}$ (blue). (d) Schematics of the planarlike structure on Zr-terminated ${\mathrm{ZrB}}_2$(0001). Zr, on-top, bridge, and hollow atoms are respectively black, red, green, and blue colored. (e). Schematics of the $k$ space. Black and red lines indicate the ($\sqrt{3}\times \sqrt{3}$) and ($1\times 1$) Brillouin zones of silicene. (f) and (g) Calculated band structures for silicene and ${\mathrm{Si}}_5\mathrm{Ge}$ along the path indicated in (e). The contribution of the on-top, bridge, and hollow site atoms are respectively red, green, and blue colored in agreement with (d). The size of the circles represents the spectral weight of the combined contribution of Si and Ge atoms.