Evidence for a diamondlike electronic band structure of Si multilayers on Ag(111)

Silicon multilayers on Ag(111) have been suggested to exhibit the structure of silicene, a material that has been heralded as a novel basis for microelectronic applications. However, our angle-resolved photoemission spectra (ARPES) from silicon multilayers on Ag(111) and of the silver-induced reconstruction of Si(111) demonstrate, from the close match in the valence level band structures, that the films exhibit a $s{p}^3$ diamondlike structure. This refutes the interpretation of silicon multilayers on Ag(111) as silicene, a conclusion that is strengthened by the observation from core level photoemission that significant silver segregation occurs to the surface of these layers.

Figure 1

Angle-resolved photoemission maps for multilayer Si/Ag(111) along ${\overline{\mathrm{\Gamma }}}_{\mathrm{Ag}}-{\overline{K}}_{\mathrm{Ag}}$ measured using a photon energy of 126 eV and grown at (a) 200 °C, (b) 235 °C, and (c) 270 °C and of (d) $\left(\sqrt{3}\times \sqrt{3}\right)R{30}^{\circ }$-Ag on a Si(111) surface along the ${\overline{\mathrm{\Gamma }}}_{\mathrm{Si}}-{\overline{K}}_{\mathrm{Si}}$ symmetry direction using a photon energy of 126 eV. Momentum distribution curves (e)–(h) in extended $k_{\left|\right|}=\pm 2.5{\AA }^{-1}$ scale of (a)–(d) at 1 eV (upper panel with red line) and 2 eV (lower panel with blue line) binding energy, respectively. First derivative spectra of (a)–(d) has been shown in (i)–(l), respectively, to better resolve the weak features. The white arrow indicates the Ag $sp$ band, the yellow arrow indicates the Ag $L$-gap interface state ($I$), and the red arrows indicate different $\left(\sqrt{3}\times \sqrt{3}\right)R{30}^{\circ }$-Ag surface states.

Figure 2

Band dispersion (first derivative spectra along the energy axes to increase the contrast) of multilayer Si/Ag(111) grown at [(b), (e)] 200 °C, [(c), (f)] 235 °C, and [(d), (g)] 270 °C, around $\overline{\mathrm{\Gamma }}$ and ${\overline{\mathrm{\Gamma }}}_1$ and of (h) $\left(\sqrt{3}\times \sqrt{3}\right)R{30}^{\circ }$-Ag on a Si(111) surface around ${\overline{\mathrm{\Gamma }}}_1$ measured using a photon energy of 126 eV, along with a parabolic dispersion fitting (solid red line) of clean Ag surface state [(a)–(c)] and of $S_1$ surface state [(f), (h)]. (d) Surface Brillouin zones of (1 × 1)-Ag, (1 × 1)-Si, and $\left(\sqrt{3}\times \sqrt{3}\right)R{30}^{\circ }$-Si are shown as black, blue, and red hexagons, respectively.

Figure 3

Constant energy cuts of multilayer Si grown on Ag(111) at 235 °C and $\left(\sqrt{3}\times \sqrt{3}\right)R{30}^{\circ }$-Ag on a Si(111) surface at [(a), (b)] Fermi energy and [(c), (d)] 1.0 eV binding energy, respectively. Surface Brillouin zones of (1 × 1)-Ag and -Si are shown as blue and red hexagons, respectively.

Figure 4

(a) $\mathrm{Ag}3d_{5/2}$ core level spectra of Ag(111) covered with different amounts of Si as indicated by the colors. (b) Area under the $\mathrm{Ag}3d_{5/2}$ peak for different growth temperatures as a function of thickness. (c) Line shape analysis of the $\mathrm{Ag}3d_{5/2}$ peak, recorded for a sample grown at $235{}^{\circ }\mathrm{C}$ with a thickness of 7 monolayers. (d) Ratio of the bulk and segregated component of the $\mathrm{Ag}3d_{5/2}$ line as a function of growth temperatures.