Mn${}_5$Ge${}_3$ film formation on Ge(111)$c(2\times 8)$

Thin manganese germanide films with different thicknesses on Ge(111) have been studied in detail by low-energy electron diffraction (LEED), scanning tunneling microscopy, and core-level spectroscopy (CLS). Annealing of the deposited Mn on Ge(111)$c$($2\times 8$) between 330–450${}^{\circ }$C resulted in well-ordered Mn${}_5$Ge${}_3$ surfaces as seen by intense $\sqrt{3}\times \sqrt{3}$ LEED spots. Up to a coverage of 24 monolayers (ML), island formation is favored. At a coverage of 32 ML, a well-ordered Mn${}_5$Ge${}_3$ film was found to fully cover the surface. High-resolution Ge 3$d$ CLS spectra were recorded with photon energies between 50 and 110 eV at normal and 60${}^{\circ }$ emission angles. In contrast to earlier results, three components have been used in the line-shape analysis to achieve a consistent fit over the energy and angular range. In addition, three components have been identified for the Mn $2p$ CLS spectra. The two major components fit well with a layered Mn germanide structure suggested in the literature.

Figure 1

LEED patterns from the Mn${}_5$Ge${}_3$ surfaces with different Mn coverages. (a)–(c) Annealed at 330${}^{\circ }$C and (d) at 450${}^{\circ }$C. (a) 6-ML sample showing a $1\times 1$ and weak $c$$(2\times 8)$ and $\sqrt{3}\times \sqrt{3}$ LEED diffraction spots. (b) 12-ML sample showing a $\sqrt{3}\times \sqrt{3}$ pattern. (c) 24-ML sample showing intense $\sqrt{3}\times \sqrt{3}$ pattern. (d) 32-ML sample showing intense $\sqrt{3}\times \sqrt{3}$ LEED pattern. Images (a)–(c) were recorded at 39 eV and (d) at 46 eV.

Figure 2

Topographic STM images from the Mn germanide surfaces. (a) 6-ML as-deposited sample recorded at $V_S=2.35$ V, $I=0.1$ nA, $40\times 40$ nm. (b) 6-ML sample annealed at 330${}^{\circ }$C showing large Mn${}_5$Ge${}_3$ islands, recorded at $V_S=1.6$ V, $I=10$ pA, $200\times 200$ nm. (c) 12-ML sample after annealing at $\sim$330${}^{\circ }$C, $V_S=-1.6$ V, $I=50$ pA, $300\times 300$ nm. (d) Profile along the black line in (b) showing an average island height of 2.4 nm. (e) Profile along the black line in (c) showing an average height of 3.8 nm from the large islands in (c).

Figure 3

Topographic STM images from the Mn germanide surfaces annealed at 330${}^{\circ }$C [(a), (b), and (d)] and 450${}^{\circ }$C (e). (a) A $300\times 300$ nm image from the annealed 24-ML sample, $V_S=-1.6$ V, $I=50$ pA. (b) $300\times 300$ nm image from the 32-ML sample annealed at 330${}^{\circ }$C, $V_S=-1.1$ V, $I=50$ pA. (c) Line profile along the black line in (a) showing the height difference between the substrate and Mn${}_5$Ge${}_3$. (d) A $8\times 4$ nm image from the 32-ML sample showing the honeycomb pattern, the white rhombus indicate the $\sqrt{3}\times \sqrt{3}$ unit cell, $V_S=-0.5$ V, $I=25$ pA. (e) $300\times 300$ nm image from the 32-ML sample, annealed at 450${}^{\circ }$C.

Figure 4

Calculated and measured island heights for the 6-, 12-, and 24-ML Mn, (a)–(c), respectively. Gray areas are the calculated thicknesses of Mn${}_5$Ge${}_3$ based on the structure model. The total heights are measured from STM images.

Figure 5

High-resolution CLS spectra showing the evolution of Ge $3d$ core-level spectra with Mn coverages. Recorded at normal emission with incident angle 50${}^{\circ }$ [(a)–(e)] and 45${}^{\circ }$ (f) and 90-eV photon energy. Spectra (a)–(c) recorded at RT and (d)–(f) at 100 K.

Figure 6

High-resolution CLS spectra showing the evolution of Ge $3d$ core-level spectra with photon energies. Recorded at normal emission with incident angle 50${}^{\circ }$. Spectra (a)–(d) recorded at 100 K with photon energies 50, 70, 90, and 110 eV, respectively.

Figure 7

High-resolution Ge $3d$ core-level spectra recorded at 100 K: [(a) and (c)] normal emission, and [(b) and (d)] 60${}^{\circ }$ emission. All the incident angles were 45${}^{\circ }$. The spectra were recorded with photon energies of 70 and 90 eV. The solid lines show the total contribution from the components used to fit the experimental data (circles). Underneath each spectrum, the residual lines from the fitting procedure are presented.

Figure 8

Atomic model of Mn${}_5$Ge${}_3$ exposed (0001) plane. (a) Top view with the $\sqrt{3}\times \sqrt{3}$ unit cells indicated by the black line. (b) Side view of the germanide structure in (a). Black large spheres represent Mn${}_I$ atoms in the top layer and Mn-only layers. Dark gray and light gray spheres represent Mn${}_{\mathit{II}}$ atoms in the first and deeper-lying mixed Mn/Ge layers, respectively. White spheres represent Ge atoms.

Figure 9

Mn $2p$ spectra recorded with 900-eV photon energy. (a) As-deposited 3 ML at RT. (b) 24-ML annealed sample at LN temperature. The solid lines show the total contribution from the components used to fit the experimental data (circles). Underneath the spectra, the residual line from the fitting procedure is presented.