Termination-dependent surface properties in the giant-Rashba semiconductors $\text{BiTe}X$ ($X=\text{Cl}$, Br, I)

The noncentrosymmetric semiconductors $\mathrm{BiTe}X(X=\text{Cl},\text{Br},\text{I})$ show large Rashba-type spin-orbit splittings in their electronic structure making them candidate materials for spin-based electronics. However, BiTeI(0001) single-crystal surfaces usually consist of stacking-fault-induced domains of Te and I terminations implying a spatially inhomogeneous electronic structure. Here we combine scanning tunneling microscopy, photoelectron spectroscopy (ARPES, XPS), and density functional theory calculations to systematically investigate the structural and electronic properties of $\mathrm{BiTe}X\left(0001\right)$ surfaces. For $X=\text{Cl}$, Br we observe macroscopic single-terminated surfaces. We discuss chemical characteristics among the three materials in terms of bonding character, surface electronic structure, and surface morphology.

Figure 1

Crystal structure and room-temperature STM measurements for $\mathrm{BiTe}X$. (a) Bulk unit cells of BiTeBr/I and BiTeCl and the resulting surface terminations after cleaving. The inset sketches the situation for an ideal crystal mounted between the two sample holders (black) after the cleave. STM measurements $(500\mathrm{nm}\times 500\text{nm})$ of (b) BiTeI, (c) Te-terminated and (d) Br-terminated BiTeBr, as well as (e) Te-terminated and (f) Cl-terminated BiTeCl. The gap voltage is varied from $-0.05$ V to $-1$ V and the tunneling current was 0.1 nA in (b) and 0.2 nA in (c)–(f). The outermost parts of the images are $dI/dV$ maps of the areas between the lines.

Figure 2

X-ray photoemission data for BiTeBr in (a) and BiTeCl in (b), (c). Characteristic intensity differences in the Te and Br/Cl core level signals are observed for the different surface terminations, reflecting the changed atomic stacking orders and the finite probing depth of the experiment. Furthermore, band bending gives rise to small energy shifts between the spectra for Te-terminated and Br/Cl-terminated surfaces.

Figure 3

Angle-resolved photoemission data for BiTeBr in (a) and (b), and for BiTeCl in (c) and (d) $(h\nu =21.2\text{eV})$. The contrast between $E_{\mathrm{F}}$ and 0.5 eV (indicated by horizontal lines) has been increased in (a) and (c) for better visibility of the Rashba-split band near $E_{\mathrm{F}}$. The red dotted lines serve as guides to the eye.

Figure 4

Effect of Cs adsorption on BiTeCl(0001). Panels (a) and (d) show core level spectra measured before and after Cs deposition on a Te- and a Cl-terminated surface, respectively. Corresponding valence band spectra taken at the $\overline{\mathrm{\Gamma }}$ point are shown in (b) and (e) $(h\nu =21.2\text{eV})$. STM images and $dI/dV$ maps acquired after deposition of Cs on Te- and Cl-terminated BiTeCl are given in (c) and (f).

Figure 5

LT-STM measurements; all scans are performed at $T=5\text{K},V=1$ V, and $I=10$ pA. (a) The Te termination of BiTeBr shows the lowest defect density of the $\mathrm{BiTe}X$ family. Panel (b) is the zoom-in of (a) at the green square. We can find mainly one type of surface defect and three different types of third-layer defects with an additional variation, but no second-layer defects could be found. Panel (c) shows the Te termination of BiTeCl. The defect density is the highest of the $\mathrm{BiTe}X$ compounds. Panel (d) shows the zoom-in of (c). One can find at least two different types of defects; others might be covered. (e) Side view of a hard ball sketch of BiTeBr-Te. Second (2nd) and third (3rd) layer defects and their effect on nearest-neighbor atoms are indicated schematically. (f) Top view of a hard ball sketch of BiTeBr-Te. Second-layer defects would result in three neighboring Te atoms with different contrast while third-layer defects mainly affect three next-nearest-neighbor Te atoms, as can be seen in defect C in panel (b).