Coexistence of Negatively and Positively Buckled Isomers on $n^{+}$-Doped $\mathrm{Si}(111)-2\times 1$

A long-standing puzzle regarding the $\mathrm{Si}(111)-2\times 1$ surface has been solved. The surface energy gap previously determined by photoemission on heavily $n$-doped crystals was not compatible with a strongly bound exciton known from other considerations to exist. New low-temperature angle-resolved photoemission and scanning tunneling microscopy data, together with theory, unambiguously reveal that isomers with opposite bucklings and different energy gaps coexist on such surfaces. The subtle energetics between the isomers, dependent on doping, leads to a reconciliation of all previous results.

Figure 1

Panels (a) and (b): EDCs near the $\overline{J}$ point of the SBZ, at (a) 100 K and (b) room temperature. When the temperature is raised, peak $A^{\prime }$ is hidden by the increased broadening, but it can be deconvolved and the result is shown below the curve. (c) A series of high angular and energy resolution EDCs (rotated by 90° counterclockwise) spanning the $\overline{\Gamma }\overline{J}$ direction of the SBZ, as a function of $k_{||}$. Spectrum (a) is the first on the right-hand side. The dispersion of the dangling-bond related surface bands is clearly seen. (d) Second derivatives of curves reported in (c), plotted on a color scale.

Figure 2

(a) and (b) STM images from cleaved Si(111), acquired at 8 K and with sample-tip voltage of $-1.0\mathrm{V}$. Image (b) shows an expanded view of the area indicated in (a). Gray scale ranges are (a) 0.22 and (b) 0.12 nm. Tunneling spectra acquired from near the N and P symbols in image (b) are shown by curves (c) and (d), respectively. Spectra acquired from neighboring surface regions are shown by (e) and (f).

Figure 3

Left panel: simulated STS of the isomers, calculated as the $GW$ density of states projected on the upper and lower chains. Right panel: surface-state $GW$ energies along $\overline{J}\overline{K}$ for the two isomers, aligned to a common vacuum level. Solid (dashed) line refers to the positive (negative) isomer.