Coverage dependence and surface atomic structure of $\text{Mn}/\text{Si}\left(111\right)\text{−}\sqrt{3}\times \sqrt{3}$ studied by scanning tunneling microscopy and spectroscopy

Thin manganese silicide films of different thicknesses on Si(111) have been studied in detail by low-energy electron diffraction (LEED), scanning tunneling microscopy, and scanning tunneling spectroscopy (STM/STS). Up to a Mn coverage of 3–4 monolayers (ML), island formation is favored. For higher Mn coverages up to 12 ML uniform film growth is found. The silicide film morphology at low coverages supports a layered Mn-Si film structure. The silicide surfaces displayed a $\sqrt{3}\times \sqrt{3}$ LEED pattern. STM images recorded from the $\sqrt{3}\times \sqrt{3}$ surfaces mostly show a hexagonal pattern but a honeycomb pattern has also been observed. A surface atomic structure based on chained Mn triangles is proposed. Our STM results are in good agreement with a recent theoretical model. The high-quality STS spectra recorded from the different surfaces show a clear metallic character at 1.5 ML and higher coverages. The filled-state features in the STS spectra at surfaces with 3–4 ML Mn coverages are similar to earlier published angle-resolved photoelectron spectroscopy data.

Figure 1

LEED patterns of Mn/Si(111) surfaces as a function of annealing temperature and Mn coverages; (a) 1.5 ML annealed at $300°\text{C}$ showing a $1\times 1$ pattern; (b) 1.5 ML annealed at $400°\text{C}$ for 5 min showing a weak $\sqrt{3}\times \sqrt{3}$ pattern; and (c) 4 ML annealed at $400°\text{C}$ for 5 min showing a strong $\sqrt{3}\times \sqrt{3}$ pattern. All images were recorded at 46 eV.

Figure 2

(Color online) Topographic STM images from the 1.5 ML sample. (a) and (b) show large and small scale images from the as-deposited sample, $100\times 100{\text{nm}}^2$ and $20\times 20{\text{nm}}^2$, respectively. Both images were recorded at $V_S=1.6\text{V}$ and $I=0.2\text{nA}$. (c) Image after anneal at $\sim 300°\text{C}$, $V_S=-1.6\text{V}$, $I=1\text{nA}$, and $100\times 100{\text{nm}}^2$. (d) Closeup image of one of the islands seen in (c), $V_S=-1.5\text{V}$, $I=0.2\text{nA}$, and $25\times 25{\text{nm}}^2$. (e) A $100\times 100{\text{nm}}^2$ image showing MnSi islands from the 1.5 ML sample annealed at $400°\text{C}$. (f) An $8\times 8{\text{nm}}^2$ image showing the $2\times$ regions from the areas between the islands in (e). [(e) and (f)] $V_S=1.5\text{V}$ and $I=0.1\text{nA}$.

Figure 3

(Color online) Topographic STM images from the Mn/Si(111) surfaces. (a) A $200\times 200{\text{nm}}^2$ image from the annealed 3 ML sample, $V_S=1.6\text{V}$ and $I=0.5\text{nA}$. (b) $8\times 4{\text{nm}}^2$ image from the $\sqrt{3}\times \sqrt{3}$ surface in (a), $V_S=-1.4\text{V}$ and $I=0.05\text{nA}$. (c) Line profile along the black line in (a) showing the height difference between the layers. (d) A $200\times 200{\text{nm}}^2$ image from the 4 ML sample, $V_S=-0.9\text{V}$ and $I=0.2\text{nA}$. (e) A $14\times 7{\text{nm}}^2$ image from the surface in (d), $V_S=0.9\text{V}$ and $I=0.3\text{nA}$. (f) Line profile along the black line in (e) showing the island height.

Figure 4

(Color online) STM images from the $\text{Mn}/\text{Si}\left(111\right)\text{−}\sqrt{3}\times \sqrt{3}$ surface with 3 ML coverage. (a) Empty-state image recorded at $V_S=1.6\text{V}$, $I=1.5\text{nA}$, and $9\times 6{\text{nm}}^2$. (b) Filled-state image from the same area as in (a), $V_S=-1.6\text{V}$, $I=1.5\text{nA}$, and $9\times 6{\text{nm}}^2$. (c) Empty-state image recorded from the same area and at the same conditions as in (a), $9\times 7{\text{nm}}^2$. (d) Top view of the IET atomic structure of the $\sqrt{3}\times \sqrt{3}$ surface, $1.8\times 1.2{\text{nm}}^2$. White and black circles correspond to Mn and Si atoms, respectively.

Figure 5

(Color online) Top view of the atomic-structure model of the MnSi film with B20 structure according to Ref. 4. The $\sqrt{3}\times \sqrt{3}$ unit cell is indicated by the dashed line. Small light gray spheres (connected with black lines) represent Si atoms in the top-layer and large dark gray spheres (connected with blue lines) represent Mn atoms in the first Mn layer. The hexagonal bonding pattern displays the Si substrate lattice.

Figure 6

Area averages STS spectra recorded from the (a) clean $\text{Si}\left(111\right)\text{−}7\times 7$ surface, (b) the 1.5 ML as-deposited film, (c) the 1.5 ML film annealed at $300°\text{C}$, (d) the 1.5 ML film annealed at $400°\text{C}$, (e) 3ML film annealed at $400°\text{C}$, and (f) the 4 ML film annealed at $400°\text{C}$.