Photon Factory Activity Report 2016 #34 (2017)

BL-13B/2015G697

Electronic structure of Bi(110) ultra-thin films grown on a Si(111)√3×√3-B surface
Kan Nakatsuji1,*, Tsubasa Fujiwara1, Kazuki Shishikura1, Yuri Shimokawa1, and Hiroyuki Hirayama2
1
Dept. of Materials Science and Engineering, Tokyo Institute of Technology,
J1-3, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan
2
Dept. of Physics, Tokyo Institute of Technology,
J1-3, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan
1 Introduction
Bismuth (Bi) ultra-thin film has been attracted muchinterest because of its novel spin-polarized electronicstructure originated from a large spin-orbit interaction. In
particular, bilayer Bi(111) film is expected to behave as a
two-dimensional topological insulator (2D-TI) with spinpolarized helical edge states[1]. Bilayer Bi(110) film isalso suggested to become a 2D-TI when the film has
black-phosphorus-like (BP-like) structure[2]. We havestudied the growth of Bi(110) ultra-thin films on aSi(111)√3×√3-B surface by scanning tunnelling
microscopy (STM) and found that the Bi(110) islands
stably grow up to at least 20 monolayer (ML) height on awetting layer with six rotational domains due to thecommensuration with the substrate[3]. Both the even andodd-layer-height islands from the wetting layer grow withsome preference of even-layer ones. There are two
possible internal structures of the islands; bulk-like andBP-like structures, although the structure is unclear only
from the STM observations. Recent theoretical bandcalculations for free-standing Bi(110) films below 8 ML
suggest that the bulk-like structure has a Dirac-cone-likemetallic state around the M point of the surface Brillouinzone whereas a band gap exists for the BP-likestructure[4]. In the present study, we have investigated theband structure of Bi(110) ultra-thin films below 20 ML
grown on a Si(111)√3×√3-B surface by angle-resolved
photoelectron spectroscopy (ARPES) to reveal theinternal structure of the islands.
2 Experiment
The experiments have been performed at SES200
station at BL-13B. The substrate Si(111)√3×√3-B surface
was obtained after resistive flashing and consequentannealing at 980 K and 680 K, respectively, and gradualcooling to room temperature (RT). Bi was evaporated
from a Knudsen cell. The substrate was kept at RT during
the evaporation. ARPES measurements have been
performed at RT with the incident photon energy of 51 eValong the -M line of one of the six rotational Bi(110)domains. The growth of Bi(110) islands and morphology
on the surface was confirmed by in-situ low-energy
electron diffraction and ex-situ atomic force microscopy(AFM), respectively. The total coverage of Bi andBi(110) island heights on the wetting layer wereestimated from AFM images.
3 Results and Discussion

Figure 1 shows AFM images and corresponding bandstructures of Bi(110) ultra-thin films in -M direction for
10 ML and 4.6 ML coverages, respectively. The heightsof the islands on the two samples were estimated to be
13±5 ML and 7±4 ML, respectively, from AFM
observations.

Fig. 1: AFM images and corresponding band structures ofBi(110) ultra-thin films in -M direction for 10 ML and4.6 ML coverages, respectively. The second derivativesof the photoelectron intensity maps are shown.
The thicker (10 ML) film has metallic hole-like statesaround the and M points as shown by the arrows in theband structure. The dispersion of these states are similar
to those reported on the bulk Bi(110) crystal[5] although
the Fermi wave number is smaller. These metallic states
disappear for the thinner (4.6 ML) film with less than 10ML height islands. Dirac-cone-like band characteristic forthe bulk-like structure was not observed around the M
point although small signals still exist just below theFermi energy (EF). As shown by the arrow in the figure,the uppermost band around the M point appears at about
0.3 eV from EF, which is similar to the calculated bandstructure for 8 ML height film of BP-like structure.
Considering the preferential growth of even-layer-heightislands in the STM observations, the present resultsPhoton Factory Activity Report 2016 #34 (2017)

suggest that the even-layer-height Bi(110) islands below
10 ML height have BP-like internal structure.
4 SummaryWe have investigated the electronic structure of Bi(110)
ultra-thin films grown on a Si(111)√3×√3-B surface.
Thicker films have metallic surface states similar to thosereported on the bulk Bi(110) crystal. Thinner films withless than 10 ML height islands do not have Dirac-conelike state around the M point but rather have band
structures similar to the calculated ones for BP-likestructure.
Acknowledgement
The authors acknowledge K. Mase for his helpful
supports and comments to our experiments at BL-13B.
References[1] S. Murakami, Phys. Rev. Lett. 97 236805 (2006).
[2] Y. Lu et al., Nano Lett. 15 80 (2015).
[3] I. Kokubo et al., Phys. Rev. B 91 075429 (2015).
[4] G. Bian et al., Phys. Rev. B 90 195409 (2014).
[5] Ph. Hofmann, Surf. Sci. 81 191 (2006).
* nakatsuji.k.aa@m.titech.ac.jp