Atomic Structure of Mn Wires on Si(001) Resolved by Scanning Tunneling Microscopy

At submonolayer coverage, Mn forms atomic wires on the Si(001) surface oriented perpendicular to the underlying Si dimer rows. While many other elements form symmetric dimer wires at room temperature, we show that Mn wires have an asymmetric appearance and pin the Si dimers nearby. We find that an atomic configuration with a Mn trimer unit cell can explain these observations as due to the interplay between the Si dimer buckling phase near the wire and the orientation of the Mn trimer. We study the resulting four wire configurations in detail using high-resolution scanning tunneling microscopy (STM) imaging and compare our findings with the STM images simulated by density functional theory.

Figure 1

(a) Top and side view of the trimer wire structure (III) with an atom in the hollow site centered between two dimers of a dimer row. Mn atom positions for a dimer wire (II) and a wire with one Mn atom at the cave site (I) (we use the same terminology as in Ref. 22). (b) STM image taken at $I_t=0.15\mathrm{nA}$ and $V_s=-2.0\mathrm{V}$. The ellipse marks the accumulation of Mn wires (bright lines) at the Si step edge. The circles denote Mn clusters.

Figure 2

STM images of two wires $A$ $(a)+(b)$ and $B$ $(d)+(e)$. Tunneling parameters were $I_t=0.2\mathrm{nA}$ and $V_s=-1.7\mathrm{V}$ $(a)+(d)$, $-0.75\mathrm{V}$ $(b)+(e)$. The four wire types that occur in our experiments are shown schematically in (c).

Figure 3

(a)–(f) Drift-corrected STM images as a function of bias (a) $V_s=-1.75\mathrm{V}$, (b) $-1.00\mathrm{V}$, (c) $-0.65\mathrm{V}$, (d) $+0.65\mathrm{V}$, (e) $+1.00\mathrm{V}$, and (f) $+1.75\mathrm{V}$. The tunneling current was $I_t=74\mathrm{pA}$ for $(c)+(d)$ and 154 pA for all other images. The calculated positions of the Si dimer atoms are marked by the black overlay. (g)–(l) Simulated STM images from DFT (g) $E_B=-1.50\mathrm{eV}$, (h) $-0.75\mathrm{vV}$, (i) $-0.40\mathrm{eV}$, (j) $+0.50\mathrm{eV}$, (k) $+0.85\mathrm{eV}$, and (l) $+1.60\mathrm{eV}$. (m) Relaxed atomic positions of the $4\times 4$ supercell. Arrows to the right of panel (f) indicate the positions of the first to fourth Si dimers away from the Mn wire.

Figure 4

(a) Schematic of relaxed atom positions. The circle size indicates height of the atoms. The distortion of the Si atoms near the hollow site is exaggerated to illustrate the origin of the asymmetry. (b) and (c) Successive STM images of wire $B$ before full relaxation of the Si dimer buckling. STM parameters were $I_t=0.2\mathrm{nA}$, $V_s=-2.0\mathrm{V}$ (b) and $V_s=-1.7\mathrm{V}$ (c). The black arrows indicate the slow scan direction.