P18S

23AG036

BL-7

Attempt to Control the Anisotropy of Topological Surface
StatesR. Yamamotoa, K. Sumidab, K. Miyamotob, and T. Okudab.
aGraduate School of Advanced Science and Engineering Hiroshima University, 1-3-1 Kagamiyama
Higashi-Hiroshima 739-8526, JapanbHiroshima Synchrotron Radiation Center (HSRC), Hiroshima University 2-313 Kagamiyama,Higashi-Hiroshima 739-0046, Japan

Keywords: Topological Surface States, Silicon Vicinal Surface, Anisotropy of Band Structure, Bi2Te3 film

Topological insulators have spin-polarized conduction states on their surfaces (= topological surfacestates), where the spin direction of electrons is locked by their momentum resulting the helical spin-texture.
The unique helical spin-texture is considered to prohibit complete backscattering by non-magnetic impurities.
Thus, the surface of topological insulators is expected to be promising platform for spintronics devices.
However, other backscattering passes except for the complete backscattering are not prohibited [Fig.1(a)].
One solution to suppress the backscattering is forming an anisotropic Fermi surface, ideally a onedimensional Fermi surface[Figs. 1(b) and 1(c)].
In previous research, it has been reported that Ag films on Si(111)-(4×1)-In, which has an array of In
chains, exhibit a quasi-one-dimensional band structure [1]. This result is attributed to the confinement ofelectrons in the quasi-one-dimensional Ag films caused by the step and terrace structure, and a similar effectcan be expected even on vicinal surface. The vicinal silicon surface has atomically regular step arrays andelectron motion in the film grown on the surface might be restricted only one direction parallel to the step.
From this analogy, we considered that it might be possible to obtain the anisotropic spin-polarized band
structure by growing Bi2Te3 films on silicon vicinal surface. However, there is no report of the fabricationof Bi2Te3 film on silicon vicinal surface so far. Therefore, the purposes of this research are to fabricate Bi2Te3
film on silicon vicinal surface, measure the band structure, and evaluate the band anisotropy.
In our study, we grew Bi2Te3 ultrathin films on Si(111) and Si(557) by molecular beam epitaxy, whichwere used as flat and vicinal surface substrate, respectively. Si(557) is a surface tilted by 9.5° from Si(111),
and its terraces have the same plane as Si(111). We checked the quality of surface structure by low energy
electron diffraction (LEED) and auger electron spectroscopy (AES). Figure 2 shows LEED patterns and thespectra of AES of Bi2Te3 film on each substrate. As in Fig. 2, we can see clear spots in LEED and peak ofBi and Te in AES indicating that we could succeed to grow Bi2Te3 film on vicinal silicon surface.
To observe the band structure of the fabricated films, angle-resolved photoemission spectroscopy(ARPES) measurements were done at BL-7 in HiSOR. Figure 3(a) shows the wide energy range ARPES
images on Si(111) (top) and Si(557) (bottom). Clear bulk bands are observed on Si(111). On Si(557) substratefaint but similar bands that are shifted by 9.5° from the results of Si(111) are observed. Figure 3(b) shows
the magnified band structure near the Fermi energy of each sample. A clear V-shaped topological surface
state is visualized on Si(111). The observed feature is very similar to the previously reported band structureof Bi2Te3/Si(111) films with one quintuple layer (QL) thickness [2]. Thus, the thickness of our film isestimated to be ~1 QL. On Si(557) substrate, similar V-shaped surface state crossing the Fermi energy canbe seen, but it was hard to estimate the Fermi momenta (𝑘 ) of surface bands due to the weak intensity.
Although we attempted to evaluate the anisotropy of the surface bands by Fermi surface in Fig. 3(c),unfortunately, it was also difficult.
In conclusion, we succeeded to grow Bi2Te3 film on silicon vicinal surface and to measure the bandstructure. However it was difficult to evaluate the band isotropy because of the poor intensity of topologicalsurface states on Bi2Te3/Si(557). Further experiment with the better sample quality is expected in the future.

FIGURE 1. Schematic illustrations of isotropic (a), anisotropic (b), and ideally anisotropic (one-dimensional) Fermi
surfaces (c) of topological insulator.

FIGURE 2. (a) AES spectrum of Bi2Te3/Si(111) (b)LEED pattern of Bi2Te3/Si(111) taken at 55 eV. (c) AES spectrum ofBi2Te3/Si(557) (d) LEED pattern of Bi2Te3/Si(557) taken at 39 eV.

FIGURE 3. (a) Wide range ARPES images of Bi2Te3/Si(111) and Bi2Te3/Si(557) acquired at hν = 35 and 70 eV. (b)Magnified ARPES images around the Fermi energy of (a). (c) Fermi surface of Bi2Te3/Si(111) and Bi2Te3/Si(557)

REFERENCES
1. N.Nagamura et al., Phys. Rev. Lett. 96, 256801 (2006).
2. S. Hatta et al., Sci. Rep. 11, 5742 (2021).