Tuning the Dirac Point Position in ${\mathrm{Bi}}_2{\mathrm{Se}}_3(0001)$ via Surface Carbon Doping

Angular resolved photoemission spectroscopy in combination with ab initio calculations show that trace amounts of carbon doping of the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ surface allows the controlled shift of the Dirac point within the bulk band gap. In contrast to expectation, no Rashba-split two-dimensional electron gas states appear. This unique electronic modification is related to surface structural modification characterized by an expansion of the top Se-Bi spacing of $\approx 11\%$ as evidenced by surface x-ray diffraction. Our results provide new ways to tune the surface band structure of topological insulators.

Figure 1

Experimental (symbols) and calculated (lines) structure factor amplitudes $|F(hk\ell )|$ along four CTRs, $10\ell$, $01\ell$, $20\ell$, and $11\ell$, for the clean ${\mathrm{Bi}}_2{\mathrm{Se}}_3(0001)$, as measured at the ESRF (a) and in the LAB (b), as well as those for carbon doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3(0001)$ as acquired in the LAB (c). Curves are shifted for clarity.

Figure 2

Model of the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ near-surface structure. Red (small) and gray (large) spheres represent Se and Bi atoms, respectively. Interlayer spacings relative to the bulk are labeled on the right for the different data sets. Carbon (small black sphere) is placed in the most favorable position according to the ab initio calculations.

Figure 3

(a) Differential AES spectrum showing the Bi-NOO (96 and 101 eV) and the Bi-NNO transitions (249 and 268 eV) of the doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ sample. (b) Zoom into the 250 to 270 eV regime: (I) reference reproduced from Ref. [22], (II) undoped sample, and (III) carbon-doped sample.

Figure 4

Momentum-resolved photoemission measurement: band dispersion of undoped (a) and carbon-doped (b) ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ along the $\overline{\mathrm{K}}-\overline{\mathrm{\Gamma }}$ and $\overline{\mathrm{\Gamma }}-\overline{\mathrm{M}}$ direction. Dotted straight lines serve as a guide to the eye to the band dispersion in the vicinity of the DP. Binding energies are given relative to the Fermi level ($E_F$). Horizontal lines indicate the energy of the DP and the $k_{\parallel }$ contour for the undoped (c) and doped (d) samples. (e) Intensity profiles along the dashed line in (c) and (d).

Figure 5

Band structure of ${\mathrm{Bi}}_2{\mathrm{Se}}_3(0001)$ calculated for several expansions, $\mathrm{\Delta }{d}_{12}/d_{12}$. (a): Structure from SXRD, $\mathrm{\Delta }{d}_{12}/d_{12}=3\%$, (b): 10%, (c): 20%. Red and blue lines indicate spin up and spin down states, respectively. The shaded area corresponds to the bulk states.