Surface atomic and electronic structure of Mn${}_5$Ge${}_3$ on Ge(111)

The atomic and electronic structure of the Mn${}_5$Ge${}_3$(001) surface grown on Ge(111) $c$($2\times 8$) has been studied in detail by angle-resolved photoelectron spectroscopy (ARPES), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy. ARPES spectra recorded from the $\overline{\Gamma }$-$\overline{K}$-$\overline{M}$ and $\overline{\Gamma }$-$\overline{M}$-$\overline{\Gamma }$ directions of the surface Brillouin zone show six surface-related features. The STM images recorded at biases higher/lower than $\pm$0.4 V always show a honeycomb pattern with two bright protrusions in each unit cell. At lower biases, a hexagonal, intermediate transition, and a honeycomb pattern are observed. These can be explained as arising from Mn and Ge atoms in the sublayer arranged in triangular structures and Mn atoms in the top layer arranged in a honeycomb structure, respectively. The photoemission and STM data from the germanide surface are discussed and compared to earlier published theoretical, photoelectron spectroscopy, and scanning tunneling microscopy studies.

Figure 1

LEED image of the Mn${}_5$Ge${}_3$ surface. The sample temperature was 100 K and the primary energy of the electrons was 46 eV. The intense $\sqrt{3}$-diffraction spots are clearly seen.

Figure 2

ARPES spectra with various emission angles recorded at 100 K along the (a) $\overline{\Gamma }$-$\overline{M}$-$\overline{\Gamma }$ and (b) $\overline{\Gamma }$-$\overline{K}$-$\overline{M}$ line of the surface Brillouin zone. The photon energy was 21.2 eV and the incidence angle for the photons was $45{}^{\circ }$. The six surface states are clearly observed and labeled as $S_1$–$S_6$. The inset shows the SBZ of the surface [same as the $\sqrt{3}\times \sqrt{3}$ surface of Ge(111)] together with the $1\times 1$ SBZ of Ge(111).

Figure 3

Band-mapping scheme showing the dispersion of the surface states along the $\overline{\Gamma }$-$\overline{M}$-$\overline{\Gamma }$ and the $\overline{\Gamma }$-$\overline{K}$-$\overline{M}$ directions of the surface Brillouin zone. Parts of the original spectra are shown in Figs. 2a and 2b. The spot size represents the intensity of the states at different emission angles. The surface states are labeled $S_1$–$S_6$. The solid line displays the upper edge of the projected bulk bands from the substrate, according to Hatta et al.[16]

Figure 4

The atomic structure of Mn${}_5$Ge${}_3$ (according to Ref. 18) (a) in top view and (b) cross section. The black rhombus and square indicate the unit cell location.

Figure 5

STM images from the Mn${}_5$Ge${}_3$ surface. (a)–(f) Filled-state images recorded at $V_S=-0.1$ to $-0.30$ V, $I=25$ pA. (g)–(l) Empty-state images from the same area as in (a)–(f),$V_S=0.1$ to 0.3 V, $I=25$ pA. The size of the images is $2.4\times 1.4$ nm. The black rhombus in all images indicates the unit cell. Circles in the figures indicate possible atom locations at the different biases.

Figure 6

Topographic STM images recorded from the Mn${}_5$Ge${}_3$ surface. (a) The honeycomb pattern with two point defects, $5.0\times 3.2$ nm, $V_S=+0.35$ V, $I=25$ pA. (b)–(d) Filled-state images recorded from one of the defects in (a), $V_S=-0.30$ to $-0.10$ V, $I=25$ pA. (e)–(g) Empty-state images recorded from the same area as in (b)–(d), $V_S=+0.30$ to $+$0.10 V, $I=25$ pA. The size of (b)–(g) is $2.2\times 1.5$ nm. The rhombus indicates the unit cell and the circle is the location of a missing atom.

Figure 7

Area-averaged STS spectra recorded from the (a) clean Ge(111) $c$(2$\times$8) surface and (b) Mn${}_5$Ge${}_3$ surface. STS spectra (numerical derivatives) were obtained within biases $\pm$2.5 V, setpoint voltage $V_S=1.5$ V, and tunneling current $I_t=50$ pA.