Characterization of Bi2Te3 and Bi2Se3 topological insulators grown by MBE on (001)
GaAs substrates
Xinyu Liu, David J. Smith, Helin Cao, Yong P. Chen, Jin Fan, Yong-Hang Zhang, Richard E. Pimpinella,
Malgorzata Dobrowolska, and Jacek K. Furdyna
Citation: Journal of Vacuum Science & Technology B 30, 02B103 (2012); doi: 10.1116/1.3668082
View online: http://dx.doi.org/10.1116/1.3668082
View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/30/2?ver=pdfcov
Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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Characterization of Bi2Te3 and Bi2Se3 topological insulators grown by MBE
on (001) GaAs substrates
Xinyu Liua)
Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556

David J. Smith
Department of Physics, Arizona State University, Tempe, Arizona 85287

Helin Cao and Yong P. Chen
Department of Physics, Purdue University, West Lafayette, Indiana 47907

Jin Fan
Department of Physics, Arizona State University, Tempe, Arizona 85287 and Center for Photonics Innovation,
Arizona State University, Tempe, Arizona 85287

Yong-Hang Zhang
Center for Photonics Innovation, Arizona State University, Tempe, Arizona 85287 and School of Electrical,
Computer and Energy Engineering, Arizona State University, Tempe, Arizona 85287

Richard E. Pimpinella, Malgorzata Dobrowolska, and Jacek K. Furdyna
Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556

(Received 16 September 2011; accepted 20 November 2011; published 9 December 2011)
Films of pseudohexagonal Bi2Te3, Bi2Se3 and their alloys were successfully grown by molecular
beam epitaxy on GaAs (001) substrates. The growth mechanism and structural properties of
these films were investigated by reflection high-energy electron diffraction, atomic force microscopy, x-ray diffraction (XRD), high-resolution transmission electron microscopy, and Raman
spectroscopy and mapping. The results indicate that the epitaxial films are highly uniform and are
C 2012 American Vacuum Society. [DOI: 10.1116/1.3668082]
of high crystalline quality. V

I. INTRODUCTION
The recent discovery of quantum spin Hall effect (QSHE)
in two-dimensional (2D) HgTe quantum wells1 has stimulated
an intensive search for three-dimensional (3D) topological
insulators (TI), a new state of matter with topologically nontrivial band structures originating from strong spin-orbit coupling (SOC).2,3 Angle-resolved photoelectron spectroscopy
(ARPES) measurements have confirmed the 3D TI behavior
in a number of materials—Bi1-xSbx,4 Bi2Se3,5 Bi2Te3,6 and
Sb2Te3,7 —all of which show an insulating energy gap in the
bulk and gapless surface state(s) with Diraclike linear band
dispersion. Theoretical models predict that these TI surface
states are “topologically protected” and are characterized by
extremely high mobilities and spin-locked transport,3 thus
opening up interesting opportunities for applications in
spintronics.8
In order to study fundamental TI properties, high quality
TI films need to be interfaced with superconductors, ferromagnets or other materials. For this reason, molecular beam
epitaxy (MBE) is especially attractive because of its capability
for growing multilayer heterostructures under highly controlled conditions, so that defect formation is minimized during growth. Most efforts to fabricate TI films by MBE have so
far been carried out using substrates with a hexagonal or
three-fold symmetric surface structure, such as Si (111),9,10
sapphire11 or SrTiO3 (111) (Ref. 12) substrates, with some
a)

Author to whom correspondence should be addressed; electronic mail:
xliu2@nd.edu

02B103-1

J. Vac. Sci. Technol. B 30(2), Mar/Apr 2012

limited work done on GaAs (111) substrates.13 Because the
representative spintronic materials, such as GaMnAs, are usually grown on GaAs (001) substrates,14 and Fe films of very
high crystalline perfection can also be grown on GaAs (001)
and (110) surfaces,15 in this work we have extended MBE
growth of Bi2Te3, Bi2Se3 and their alloys to deposition on the
symmetrically-mismatched GaAs (001) substrates. Such novel
growth mode may enable one to combine almost any pair of
layered materials together; thus allowing us to produce a variety of new high quality semiconductor heterostructures. Our
work reveals unique layer-by-layer growth of these materials
in a pseudohexagonal layered structure—a crystalline structure that involves sequences of five atomic layers [quintuple
layers (QLs), e.g., Te(1)-Bi-Te(2)-Bi-Te(1) or Se(1)-Bi-Se(2)Bi-Se(1)], with each atomic Te/Se or Bi layer within the QL
forming a 2D hexagonal lattice perpendicular to the c-axis.
Our observations suggest a powerful new possibility for incorporating the highly attractive properties of TI materials with
traditional electronic materials that are more compatible with
the cubic structure, to construct novel multifunctional device
configurations.
II. FABRICATION AND EXPERIMENTAL DETAILS
TI films, including Bi2Te3, Bi2Se3 and their ternary alloys,
were grown using a dual-chamber Riber 32 solid-source MBE
system. The Bi, Te2 and Se2 fluxes were generated by standard effusion cells installed in the II-VI MBE chamber. The
structure and thickness of the films were monitored in situ by
reflection high-energy electron diffraction (RHEED). The

1071-1023/2012/30(2)/02B103/4/$30.00

C 2012 American Vacuum Society
V

02B103-1

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02B103-2 Liu et al.: Characterization of Bi2Te3 and Bi2Se3 topological insulators grown by MBE

02B103-2

FIG. 1. (Color online) RHEED intensity of the specular point vs growth time
under different Bi cell temperatures: (a) Bi2Te3 and (b) Bi2Se3. Inset:
RHEED patterns observed for ½1
10 direction of the GaAs (001) substrate
during MBE growth of: (a) Bi2Te3 and (b) Bi2Se3.

growth sequence was as follows. First, an epi-ready GaAs
(001) substrate was heated to 600  C for deoxidation in the
III-V MBE chamber. This was followed by deposition of a
100 nm GaAs buffer layer. This modified substrate was then
transferred to the II-VI MBE chamber through an ultra-high
vacuum connection. The growth of the TI film is initiated by
deposition of a series of monolayers of Te-Bi-Te-Bi-Te or SeBi-Se-Bi-Se—a quintuple layer (QL)—in atomic layer epitaxy (ALE) fashion at room temperature. The substrate was
gradually heated to 300  C, and a streaky RHEED unreconstructed pattern then appeared (see insets of Fig. 1). The MBE
growth of Bi2Te3, Bi2Se3, or their alloys was then performed
under Te2 or Se2 rich conditions with Tsubstrate ¼ 300  C. The
RHEED patterns shown in insets of Fig. 1 were maintained
throughout the entire growth process. It is important to note
that the RHEED pattern showed recurrences six times during
each rotation of the substrate, which confirms the c-axis
growth of the TI films, with the a-axis lying along either the
[110] or the ½1
10 direction of the GaAs (001) substrate.
At the beginning of growth, RHEED oscillations of the
specular spot were observed, with each oscillation corresponding to the growth of one QL. Figure 1 shows RHEED
oscillations observed with different temperatures of the Bi
cell. As the Bi temperature was increased, the period of the
oscillations decreased, indicating that the growth rate was
directly controlled by the Bi flux, and that the growth of the
TI films proceeded in a QL-by-QL mode. The TI samples
grown in this manner were then characterized ex situ by
atomic force microscopy (AFM), high resolution x-ray diffraction (XRD), Raman spectroscopy and mapping, and transmission electron microscopy (TEM).
III. RESULTS AND DISCUSSION

FIG. 2. (Color online) AFM images of Bi2Te3 and Bi2Se3 samples grown
with the Te2/Bi BEP ratio of ten and Se2/Bi BEP ratio of 20, respectively.
(a) 210-nm-thick Bi2Te3; (b) 215-nm-thick Bi2Se3; (c) 15-nm-thick Bi2Te3;
(d) 15-nm-thick Bi2Se3.

Figure 2 shows AFM images of Bi2Te3 and Bi2Se3 films
deposited at a growth rate of 2 nm/min, and at the Te2/Bi
beam equivalent pressure (BEP) ratio of ten and Se2/Bi ratio
of 20, respectively. The thicknesses of the Bi2Te3 and
Bi2Se3 layers shown in Figs. 2(a) and 2(b) are 210 and 215

nm, respectively. The thicknesses of the films shown in Figs.
2(c) and 2(d) are 15 nm. The images show many hills of triangular shape aligned along specific orientations. Our results
agree with earlier reports on Bi2Te3,16 and Bi2Se3 films,12

J. Vac. Sci. Technol. B, Vol. 30, No. 2, Mar/Apr 2012

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02B103-3 Liu et al.: Characterization of Bi2Te3 and Bi2Se3 topological insulators grown by MBE

02B103-3

FIG. 3. X-ray diffraction data obtained for a 220-nm-thick Bi2(TeSe)3 film
grown on a GaAs (001) substrate. The (003) family of reflections are labeled, together with (002) and (004) reflections from the GaAs (001) substrate. Inset: QL thicknesses dQL calculated from XRD data for a series of
Bi2(TeSe)3 films plotted as a function of Te2/(Te2 þ Se2) BEP ratio. The
curve is a guide for the eye.

suggesting that the growth of TI films takes place by a stepflow growth mode, with strongly anisotropic Bi adatom diffusion. In addition, as shown in Fig. 2(d), for a 15 nm thick
Bi2Se3 layer, many small triangular terraces are clearly
observed, indicating islandlike growth for very thin films,13
and suggesting that the mobility of Bi adatoms is much
slower on the Bi2Se3 surface than on Bi2Te3 due to different
chemical bond strengths of Bi-Te and Bi-Se. However, as
growth proceeds, the surface morphology of Bi2Se3 eventually becomes similar to Bi2Te3. It is already known that the
surface morphology of TI films is dramatically affected by
the group-VI/Bi BEP ratio and the growth rate.16 In fact, in
the case of Bi2Se3, as we decreased the Bi flux, the surface
of thin Bi2Se3 layers became much smoother and Bi2Te3like.
The high crystalline quality of the TI films was confirmed
by high resolution XRD measurements on a series of
Bi2(TeSe)3 alloy films grown on GaAs (100) substrates with
various Te2/(Te2 þ Se2) BEP ratios. The ternary films were
grown in a Te-rich regime by varying Se2 flux, with a constant
of Te2/Bi BEP of around ten. Representative XRD spectra
taken on a 220-nm-thick Bi2(TeSe)3 alloy film shown in
Fig. 3 reveals many reflections from only {003}-type lattice
planes, which is indicative of highly directed c-axis growth of
the TI films.17 X-ray rocking curves yielded a full-width-athalf maximum of 0.2  0.5 . The QL thicknesses dQL were
calculated from the XRD data. As shown in the inset of
Fig. 3, the film composition of Bi2(TeSe)3 based on dQL does
not linearly depends on the Te2/(Te2 þ Se2) BEP ratio, which
suggests that Bi favors bonding with Se rather than with Te.
This result agrees with our AFM measurements.
Raman spectroscopy and mapping of the TI films was also
performed using a 532 nm laser for excitation (at power
0.8 mW). The results show two characteristic peaks for
Bi2Te3 [at 102 cm1 (E2g) and 134 cm1 (A21g)], and three
peaks for Bi2Se3 [at 71 cm1 (A11g), 131 cm1 (E2g) and
174 cm1 (A21g)].17 These peaks are consistent with the lattice vibration modes reported earlier for corresponding bulk

FIG. 4. (Color online) Representative Raman maps (the position differences
of the Raman peak E2g) measured within a scan area of 15 lm  15 lm for
(a) 136 nm thick Bi2Te3 and (b) 150 nm thick Bi2Se3. The unit for the scale
bars is cm1.

materials.18 In Fig. 4, representative Raman maps (showing
position differences of the Raman peak E2g) are plotted for
Bi2Te3 [Fig. 4(a)] and Bi2Se3 [Fig. 4(b)]. These Raman maps
show that the position differences of the Raman peaks are less
than 1 cm1 within a scan area of 15 lm  15 lm, indicating a high uniformity of the films.
The microstructure of the films was determined using
cross-section transmission electron microscopy (XTEM).
Samples were prepared for TEM examination using standard
mechanical polishing and argon-ion-milling, with the sample
held at liquid-nitrogen temperature during the latter process
in order to avoid unintentional ion-milling artifacts. In
Fig. 5, XTEM images of Bi2Te3 and Bi2Se3 layers grown on
GaAs (100) buffers show the lattice structure of both the TI

JVST B - Microelectronics and Nanometer Structures

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02B103-4 Liu et al.: Characterization of Bi2Te3 and Bi2Se3 topological insulators grown by MBE

02B103-4

symmetry of the GaAs (001) surface, we have successfully
grown high quality epitaxial films of Bi2Te3, Bi2Se3 and
their alloys on GaAs (001) substrates. The films are highly
uniform and the crystallinity is comparable to that of films
grown on substrates with hexagonal surface structure. We
observed a step flow mode of growth, with strongly anisotropic Bi adatom diffusion, the same as reported previously
for TI films. Future studies of TI films grown on GaAs (001)
substrates should contribute towards a better knowledge of
the MBE growth of TI layered structures; at the same
time opening up an opportunity for future spin-based devices
that combine topological insulators with ferromagnetic
semiconductors.

ACKNOWLEDGMENTS

FIG. 5. High-resolution transmission electron microscopy images showing
cross sections of topological insulator (a) Bi2Te3 and (b) Bi2Se3 grown by
MBE on a GaAs (001) substrate. The distances between QLs (1 nm) are
shown as “I.”

films and the GaAs substrate at their interfaces. Clean interfaces without any amorphous phases are observed, as
reported for films grown on GaAs (111) substrates.13 The
highly parallel QLs—Te(Se)-Bi-Te(Se)-Bi-Te(Se)—are visible in both Bi2Te3 and Bi2Se3 films, marked by the symbol
“I” in the figure. Figure 5 suggests that the highly parallel
QLs in Bi2Se3 film extend over a significantly longer range
than those in Bi2Te3 films, indicating a particularly strong internal self-correction process in Bi2Se3 films that is occurring as the growth proceeds.17 In addition, despite the
symmetry mismatch between the hexagonal lattices of the TI
films and the four-fold cubic symmetry of the GaAs (001)
surface, the TEM images show that the TI films are highly
uniform, and that their crystallinity is comparable to that of
films grown on substrates with hexagonal surface structure.
Earlier studies of MBE growth of Bi2Te3 on cubic Si (001)
substrates16 appeared to suggest that a hexagonal structure of
the substrate surface was essential for epitaxial growth of
Bi2Te3 film to succeed. In contrast, our work shows that high
quality Bi2Te3, Bi2Se3 and their alloys can form on GaAs
(001) substrates with well-defined crystal orientations. This
result suggests that the problems encountered in the MBE
growth of Bi2Te3 films on Si (001) substrates could be caused
by the reactivity of Te with Si,19 rather than being a result of
mismatched symmetries at the substrate-TI interface. Our discovery shows that self-correction process during growth of
these layered honeycomb materials may play an important
role in overcoming differences between crystal arrangements
at interfaces during epitaxy.
IV. SUMMARY
In summary, even though there is a mismatch between the
hexagonal lattices of Bi2Te3 and Bi2Se3 TI films and the cubic

This work was supported by NSF Grant No. DMR10-05851
(ND); NSF Grant No. ECCS10-02114 and an AFOSR Grant
No. FA9550-10-1-0129 (ASU); and DARPA MESO program
(Purdue). The authors acknowledge use of facilities in the John
M. Cowley Center for High Resolution Electron Microscopy at
Arizona State University.
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