Applied Surface Science 398 (2017) 125–129

Contents lists available at ScienceDirect

Applied Surface Sciencejournal homepage: www.elsevier.com/locate/apsusc

Growth and atomic structure of tellurium thin ﬁlms grown on Bi2 Te3Yuma Okuyama a , Yuya Sugiyama a , Shin-ichiro Ideta b , Kiyohisa Tanaka b ,Toru Hirahara a,∗ab

Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, JapanUVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japana r t i c l e

i n f oArticle history:
Received 16 October 2016
Received in revised form
21 November 2016
Accepted 24 November 2016
Available online 29 November 2016
Keywords:
Tellurium
Thin ﬁlm
Epitaxial growth
Straina b s t r a c t
We have grown tellurium (Te) thin ﬁlms on Bi2 Te3 and investigated the atomic structure. From lowenergy electron diffraction (LEED) measurements, we found that the Te ﬁlms are [101̄0]-oriented with six
domains. A detailed analysis of the reﬂection high-energy electron diffraction (RHEED) pattern revealed
that the ﬁlms are strained with the in-plane lattice constant compressed by ∼1.5% compared to the bulk
value due to the epitaxy between Te and Bi2 Te3 . These ﬁlms will be interesting systems to investigate
the predicted topological phases that occur in strained Te.
© 2016 Elsevier B.V. All rights reserved.

1. Introduction
Chirality is a geometric property of materials and is mostly
discussed in chemistry or biology [1]. A chiral molecule/ion is nonsuperposable on its mirror image (enantiomers) and individual
enantiomers are often called as either “right-” or “left-handed”.
Some solid crystals also have chirality such as elements tellurium(Te) and selenium (Se). As shown in Fig. 1(a), they are trigonal structures that can be regarded as a hexagonal array of helical chains atambient pressure. They lack inversion symmetry and possess chirality depending on the rotation direction of the screw axis. Namely,
right- and left-handed chirality correspond to the structures shownin Fig. 1(b) and (c). Due to this peculiar crystal structure, Te shows
intriguing properties such as the circular photon drag effect [2] orthe current-induced spin polarization [3,4].
It is also well known that the physical properties of Te and Se canbe tuned by controlling the lattice constant, which can be achieved
by applying pressure [5,6]. They will eventually undergo a structural transition to the monoclinic or the triclinic phases at very
high pressure in the order of ∼10 GPa. The electronic structure canalso be changed from a semiconductor (the band gap is 0.33 eV
and 2.0 eV for Te and Se, respectively) to a metal by applying pressure. Recently, based on ab initio calculations, it was predicted that

∗ Corresponding author.
E-mail address: hirahara@phys.titech.ac.jp (T. Hirahara).
http://dx.doi.org/10.1016/j.apsusc.2016.11.196
0169-4332/© 2016 Elsevier B.V. All rights reserved.

Te undergoes a trivial insulator to a strong topological insulator(metal) transition under shear (hydrostatic or uniaxial) strain in
the trigonal phase [7]. Furthermore, it was predicted that the lackof inversion symmetry and spin–orbit interaction leads to the existence of Dirac points or Weyl nodes in the band structure of Te
or Se [8]. Such states possess an unconventional spin texture andfurthermore, if one can ﬁne-tune the lattice constant by carefully
controlling the applied pressure, Te and Se are expected to show
the Weyl semimetal phase [9].
An alternative way to change the lattice constant of a material is
to grow thin ﬁlms and utilize the epitaxy with the substrate. Suchmethod has been applied to Bi ﬁlms grown on Bi2 Te3 and showeda change in their topological properties [10]. Thin ﬁlm growth of
Te has actually been performed from the monolayer regime [11]to relatively thick ﬁlms [12,13]. The main purpose to grow thickTe ﬁlms was to use them as capping layers to protect the topological surface states in air. Thus, it did not matter whether thegrown Te was amorphous or granular, but it turned out that thegrown Te ﬁlms were crystalline. Tellurium ﬁlms grown on Bi2 Te3showed clear low-energy electron diffraction (LEED) and reﬂection
high-energy electron diffraction (RHEED) patterns [12], and X-ray
diffraction measurements showed that Te ﬁlms grown on Cr-doped(Bi,Sb)2 Te3 (CBST) are oriented in the [101̄0] direction [13].
In the present study, we performed a detailed investigation of
the atomic structure of Te ﬁlms grown on Bi2 Te3 to see if these ﬁlms
are strained or not. To the best of our knowledge, such experimental
effort has not been performed previously. From RHEED and LEED126

Y. Okuyama et al. / Applied Surface Science 398 (2017) 125–129

Fig. 1. (a) Schematic drawing of the chiral crystal structure of Te and Se. Right- and left-handed chiral structures are shown in (b) and (c), respectively.

measurements, we found that the grown Te ﬁlms are six-domain(101̄0) ﬁlms and the lattice constant is compressed by ∼1.5% compared to the bulk values due to the epitaxial growth and the slightly
different lattice constants between Te and Bi2 Te3 . Thus, they should
be an interesting system to study the possible realization of topological phases in Te.
2. Experimental
The experiments were conducted in ultrahigh vacuum chambers and RHEED was used to monitor the sample growth.
Molecular beam epitaxy was employed as the deposition method.
The fabrication of the substrate Bi2 Te3 was the same asthat reported previously. First, a clean Si(111)-7 × 7 surface(3 mm × 15 mm × 0.5 mm) was prepared on an n-type substrate (Pdoped, 1–10  cm at room temperature) by a cycle of resistive heat
treatments. Then Bi was deposited on the 7 × 7 surface at 200 ◦ C ina Te-rich condition. This results in a quintuple-layer-by-quintuplelayer [(QL), 1 QL = 10.2 Å] Bi2 Te3 (111) epitaxial ﬁlm formation
with bulk-like crystal structure [10,14,15]. The thickness of the
Bi2 Te3 ﬁlms used in the present study was ∼15 QL. Tellurium wasdeposited on Bi2 Te3 at room temperature. The deposition rate of Te
was calibrated by the time it took to cover the Si(111)-7 × 7 surface
and was determined as 0.2 ML/min (1 ML = 3.8 × 1014 atoms/cm2 ,the atomic density of the Te(101̄0) surface). Judging from theatomic structure of bulk Te shown in Fig. 1, we believe that Te grows
in a trilayer-by-trilayer mode for the [101̄0] orientation and one trilayer (3 ML) corresponds to a thickness of ∼3.9 Å. LEED observations
were performed after the sample fabrication at room temperature.
3. Results and discussion
Figure 2(a) shows the RHEED pattern of a 15 QL thick
Bi2 Te3 (111) ﬁlm grown on Si(111). The incident energy of the electron beam was 15 keV. The 1 × 1 periodicity can be observed clearly
showing both the zeroth and ﬁrst Laue zones. Note that due to thelimited domain size of the ﬁlms, the spots are elongated in thevertical direction and look more like streaks. This kind of streakfeature is quite often found in RHEED patterns of thin ﬁlms, in contrast to the sharp RHEED spots of semiconductor clean surfaces
such as Si(111)7 × 7. Fig. 2(b), (c), and (d) show the RHEED patterns after 12 ML (16 Å) (b), 36 ML (47 Å) (c), and 60 ML (78 Å)(d) of Te deposition on the Bi2 Te3 substrate at room temperature,
respectively. One can see that the streaks of the ﬁrst Laue zoneof Bi2 Te3 have disappeared completely and more streaks can befound around the zeroth Laue zone. There are no signiﬁcant differences between Fig. 2(b)–(d) although slight changes in the spot
intensity can be found. So we think that the structure of the Te ﬁlm
does not change for different ﬁlm thickness. In fact, the RHEED patterns for the ﬁlm thinner than 12 ML showed a pattern that can beinterpreted as an overlap of the streaks of Bi2 Te3 and Te. Thus wesuspect that the Te ﬁlm has the same structure already from theinitial stage of ﬁlm growth. This is in sharp contrast to other casessuch as Bi ﬁlms grown on Si(111)7 × 7, where an allotropic transformation has been conﬁrmed [17]. The overall features shown in
Fig. 2(b)–(d) are similar to that reported in Ref. [12].
Since it seemed difﬁcult to determine the surface periodicity
from the complicated RHEED patterns shown in Fig. 2(b)–(d), we
performed LEED observation of the grown Te ﬁlms. Fig. 3(a) shows
the LEED pattern of the 60 ML Te ﬁlm grown on Bi2 Te3 . The incident
energy was 21.5 eV. Although the background level is quite high,one can notice spot structures which are similar to that reported inRef. [12]. In Ref. [12], no detailed structure determination based on
LEED was performed and the complicated pattern was just interpreted as a result of the multidomain growth mode. Fig. 3(b) shows
the spot positions of the LEED pattern in Fig. 3(a) for clarity. We ﬁrst
tried to assign a reciprocal lattice assuming only one domain, but
it was not successful. We thus assumed a six-domain structure of arectangular lattice. The different colors and symbols in Fig. 3(b) correspond to the different domains and the reciprocal unit cell for oneof the domains is also indicated in Fig. 3(b). As shown in Fig. 3(c),one can explain almost all the observed spots assuming this structure (the open circles are the observed ones and the ﬁlled circles
are the ones expected from the structure assumed). However, wefound that the positions for the spots located at the periphery of the
LEED screen showed a large deviation from their expected position.
We think this is due to the ﬂat MCP screen used in the LEED observation which can have a large inﬂuence on the spots far from thecenter, while those near the center are not much affected. Therefore, we believe that our structural analysis is correct and Te likely
grows in a rectangular lattice. Roughly judging from the ratio of thelong and short sides of the rectangle in Fig. 3, the ﬁlm orientationis [101̄0] (or any other equivalent direction).
In order to conﬁrm our anticipation, we have analyzed the
RHEED patterns of the Te ﬁlms. We have found that when we
assume that the electron beam is incident from the direction shownin Fig. 3(b), all the streaks in the RHEED pattern can be explained
nicely as shown in Fig. 4(a). (As mentioned before, note that theY. Okuyama et al. / Applied Surface Science 398 (2017) 125–129

127

Fig. 2. RHEED patterns of the substrate Bi2 Te3 (a), 16 Å thick Te ﬁlm (b), 47 Å thick Te ﬁlm (c), and 78 Å thick Te ﬁlm (d), grown on Bi2 Te3 , respectively.

Fig. 3. (a) LEED pattern of the 78 Å thick Te ﬁlm grown on Bi2 Te3 . (b) The reciprocal lattice derived from the LEED pattern in (a). The electron incident direction of the RHEED
patterns is also indicated. Different colors represent different domains and the rectangular reciprocal unit cell for one of the domains is explicitly shown. A and B correspondto the spots in the RHEED pattern of Fig. 4(b). The angles between different domains are also shown. (c) Same as (a) but with the reciprocal lattice overlapped for clarity. (Forinterpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of the article.)vertical position is not so important due to the elongation of the
spots.) A and B in Figs. 3(b) and 4(b) correspond to each other.
Since we know the electron incident direction of the RHEED patterns with respect to the Bi2 Te3 (111) surface, we can determine
the in-plane stacking direction between the multidomain Te(101̄0)
and Bi2 Te3 (111). Furthermore, by comparing the streak spacing
between the Bi2 Te3 substrate and the grown Te ﬁlms, the lattice
constant of Te can be determined. Fig. 4(b) shows the line proﬁles of the RHEED streaks derived from the patterns in Fig. 2(a)and (d). For a quantitative analysis, ﬁrst, a peak ﬁt was performed
to deduce the length of the arrow shown in Fig. 4(b) for Bi2 Te3(topmost arrow). This corresponds to 4/aBi2 Te3 . Then a peak ﬁt

for the line proﬁle of Te line 2 was performed and the arrowlength was determined, which corresponds to (4/a) cos23◦ (a isthe short side of the rectangular unit cell). By taking the ratio of
the length of the two arrows and using the lattice constant of
Bi2 Te3 (aBi2 Te3 = 4.38 Å), one obtains a = 4.39 ± 0.025 Å. Similarly, apeak ﬁt for the line proﬁle of Te line 1 was performed and the lengthof the top arrow was deduced, which corresponds to (4/c) cos7◦(c is the long side of the rectangular unit cell). This results in
c = 5.85 ± 0.052 Å. The length of the remaining arrows in Fig. 4(b)was determined and we cross checked if the above analyses werereasonable using reciprocal spots from all the domains. As a result,
we determined the in-plane lattice constants of the Te(101̄0) as

128

Y. Okuyama et al. / Applied Surface Science 398 (2017) 125–129

Fig. 4. (a) RHEED pattern of the 78 Å thick Te ﬁlm grown on Bi2 Te3 with the reciprocal lattice overlapped. Different colors represent different domains. A and B correspondto the reciprocal lattice shown in Fig. 3(b). (b) The RHEED spot intensity proﬁle for the lines shown in Fig. 2. The distance of the indicated arrows was used in the quantitative
analysis to deduce the real-space lattice constants (see the main text for details). (For interpretation of the references to color in this ﬁgure legend, the reader is referred to
the web version of the article.)

Fig. 5. (a) The real space lattice of the grown Te(101̄0) ﬁlms together with that of substrate Bi2 Te3 . The lattice constants are shown explicitly. (b) The magic matchingcondition between Te and Bi2 Te3 lattices (5aBiTe ∼3

a2 + c 2 ).

a = 4.39 ± 0.025 Åand c = 5.85 ± 0.052 Å, respectively. Compared tothe bulk values a = 4.46 Å and c = 5.92 Å, they are compressed by
1.6% and 1.2%.
Figure 5(a) summarizes the structure in real space we havedetermined from LEED and RHEED concerning the Te(101̄0) ﬁlm
grown on Bi2 Te3 (111). A six-domain rectangular Te lattice grows
on the single-domain hexagonal Bi2 Te3 lattice. One can notice thatthe nearest-neighbor position is quite similar between the two
with only ∼±7◦ rotation. Compared to the Te(101̄0) ﬁlm grown on
CBST, the Te domains of our ﬁlms are rotated by 30◦ [13]. Since
the lattice constant of CBST is aCBST = 4.28 Å, which is only 0.1 Å
(2.3%) different with Bi2 Te3 , it is quite interesting that such a small
can induce a large difference in the in-plane orientation. In Ref.
[13], it was shown that there is a lattice matching condition of√
3aCBST ∼ a2 + c 2 between CBST and Te(101̄0). In the present caseof Te/Bi2 Te3 , we found a matching condition of 5aBi2 Te3 ∼3 a2 + c 2
as shown in Fig. 5(b). Such commensurate relation√ is √similar tothat reported for Bi(110) ﬁlm growth on Si(111) 3 × 3-B [16].
It would be interesting to tune the lattice constant of the substrateprecisely and see how the Te ﬁlm orientation, as well as the lattice
constants, changes according to it.
Another interesting point that should be mentioned is thatthe lattice constant of the present Te ﬁlms is different from thebulk value even for a relatively large ﬁlm thickness (60 ML = 78 Å)and compressed by ∼1.5%. Intuitively, the lattice constant of athin ﬁlm should relax to its bulk value even when the ﬁrst several layers grow epitaxially with the substrate. However, there aresome cases where the matching condition makes the ﬁlm strainedeven for relatively thick ﬁlms such as Bi ﬁlms grown on Si(111)7 × 7 (the experimentally determined in-plane lattice constant was
4.48 Å, which was 1.3% smaller than the bulk value and this wasinterpreted as due to the “magic mismatch” between Bi and the7 × 7 surface) [17]. We believe that the matching condition of

5aBi2 Te3 ∼3 a2 + c 2 is also quite dominant in determining the lattice constant in the present case. It has been shown that when ahydrostatic pressure is applied to Te, a decreases while c increases
slightly [5]. Therefore we can also say that our ﬁlms are strainedin a unique manner that is not possible by just applying pressure,again suggesting the strong role of the substrate. Thus theoretical
calculations for Te slabs strained in this intriguing way will be helpful in identifying the realization of topological phases in these ﬁlms
as well as experimental veriﬁcation of the band structure. Furthermore, these ﬁlms should possess chirality as shown in Fig. 1 andexperimentally determining such property will be an importanttask as well as ﬁnding a way to grow the desired chirality.
4. Conclusions
In conclusion, we have fabricated Te thin ﬁlms on Bi2 Te3 .
Through LEED and RHEED observations, we have determined thatthey are six-domain [101̄0]-oriented ﬁlms with the lattice constantcontracted by ∼1.5% comparedto the bulk value due to the epitax

ial relation of 5aBi2 Te3 ∼3 a2 + c 2 . These ﬁlms should be intriguingin terms of verifying the predicted topological phases in strained
Te. It would also be interesting to identify and control the chiralityof the ﬁlms and investigate how they affect the physical properties.
Acknowledgements
We acknowledge discussions with Motoaki Hirayama andShuichi Murakami. This work has been supported by Grants-In-Aid
from Japan Society for the Promotion of Science (Nos. 15H05453

Y. Okuyama et al. / Applied Surface Science 398 (2017) 125–129

and 16K13683) and the Toray Science Foundation. The LEED experiments were performed under the UVSOR Proposal Nos. 27-533,
28-526, and S-15-MS-0034.
References[1] J. Smith, Organic Chemistry, 4th ed., McGraw-Hill Education, 2013.
[2] V.A. Shalygin, M.D. Moldavskaya, S.N. Danilov, I.I. Farbshtein, L.E. Golub, Phys.
Rev. B 93 (2016) 045207.
[3] L.E. Vorob’ev, E.L. Ivchnko, G.E. Pikus, I.I. Farbshtein, V.A. Shalygin, A.V.
Shturbin, JETP Lett. 29 (1979) 441.
[4] V.A. Shalygin, A.N. Sofronov, L.E. Vorob’ev, I.I. Farbshtein, Phys. Solid State 54(2012) 2362.
[5] R. Keller, W.B. Holzapfel, H. Schulz, Phys. Rev. B 16 (1977) 4404.
[6] C. Hejny, M.I. McMahon, Phys. Rev. B 70 (2004) 184109.
[7] L.A. Agapito, N. Kioussis, W.A. Goddard, N.P. Ong, Phys. Rev. Lett. 110 (2013)
176401.
[8] M. Hirayama, R. Okugawa, S. Ishibashi, S. Murakami, T. Miyake, JPS Conf. Proc.
5 (2015) 011024.

129[9] M. Hirayama, R. Okugawa, S. Ishibashi, S. Murakami, T. Miyake, Phys. Rev.
Lett. 114 (2015) 206401.
[10] T. Hirahara, N. Fukui, T. Shirasawa, M. Yamada, M. Aitani, H. Miyazaki, M.
Matsunami, S. Kimura, T. Takahashi, S. Hasegawa, K. Kobayashi, Phys. Rev.
Lett. 109 (2012) 227401.
[11] J. Ren, L. Fu, G. Bian, J. Su, H. Zhang, S. Velury, R. Yukawa, L. Zhang, T. Wang, G.
Zha, R. Guo, T. Miller, M.Z. Hasan, T.-C. Chiang, ACS Appl. Mater. Interfaces 8(2016) 726.
[12] K. Hoefer, C. Becker, S. Wirth, L.H. Tjengb, AIP Adv. 5 (2015) 097139.
[13] J. Park, Y.-A. Soh, G. Aeppli, X. Feng, Y. Ou, K. He, Q.-K. Xue, Sci. Rep. 5 (2015)
11595.
[14] G. Wang, X.-G. Zhu, Y.-Y. Sun, Y.-Y. Li, T. Zhang, J. Wen, X. Chen, K. He, L.-L.
Wang, X.-C. Ma, J.-F. Jia, S.B. Zhang, Q.-K. Xue, Adv. Mater. 23 (2011) 2929.
[15] T. Shirasawa, J. Tsunoda, T. Hirahara, T. Takahashi, Phys. Rev. B 87 (2013)075449.
[16] I. Kokubo, Y. Yoshiike, K. Nakatsuji, H. Hirayama, Phys. Rev. B 91 (2015)
075429.
[17] T. Nagao, J. Sadowski, M. Saito, S. Yaginuma, Y. Fujikawa, T. Kogure, T. Ohno, Y.
Hasegawa, S. Hasegawa, T. Sakurai, Phys. Rev. Lett. 93 (2004) 105501.