Reprinted from

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Fermi-Level Tuning of Topological Insulator Thin Films
Masaki Aitani, Yusuke Sakamoto, Toru Hirahara, Manabu Yamada, Hidetoshi Miyazaki,
Masaharu Matsunami, Shin-ichi Kimura, and Shuji Hasegawa

Jpn. J. Appl. Phys. 52 (2013) 110112# 2013 The Japan Society of Applied Physics

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Japanese Journal of Applied Physics 52 (2013) 110112

Nano Electronics and Devices

http://dx.doi.org/10.7567/JJAP.52.110112

Fermi-Level Tuning of Topological Insulator Thin Films
Masaki Aitani1 , Yusuke Sakamoto1 , Toru Hirahara1 , Manabu Yamada1 , Hidetoshi Miyazaki2 y,
Masaharu Matsunami2 , Shin-ichi Kimura2 z, and Shuji Hasegawa1
1

Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, JapanUVSOR Facility, Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan
E-mail: hirahara@surface.phys.s.u-tokyo.ac.jp
2

Received April 24, 2013; revised June 15, 2013; accepted July 17, 2013; published online October 16, 2013
Topological insulators are insulating materials but have metallic edge states with peculiar properties. They are considered to be promising for thedevelopment of future low energy consumption nano-electronic devices. However, there is a major problem: Naturally grown materials are nottruly insulating owing to defects in their crystal structure. In the present study, we have examined the electronic structure and transport propertiesof topological insulator ultrathin Bi2 Te3 films by angle-resolved photoemission spectroscopy and in situ transport measurements. To realize a truly
bulk insulating film, we tried to tune the Fermi-level position using two methods. The first of these, i.e., changing the Si substrate temperatureduring film growth (350–450 K) to reduce the defects in the grown films, had some effect in reducing the bulk residual carriers, but we could not
fabricate a film that showed only the surface states crossing the Fermi level. The second method we employed was to incorporate Pb atoms
during film growth since Pb has one less electron than Bi. When the films were grown at around 350 K, we observed a systematic shift in the Fermi
level and obtained a bulk insulating film, although it was not possible to move the Dirac point just at the Fermi level. The change in the measured
film conductivity was consistent with the shift in the Fermi level and suggested the detection of the surface-state conductivity. For films grown at ahigher substrate temperature (450 K), the Fermi level could be tuned only slightly and a bulk n-type film was obtained. Pb incorporation changes
the shape of the Dirac cone, suggesting the formation of a stoichiometric ternary alloy of Bi, Pb, and Te, which is another topological insulator.
# 2013 The Japan Society of Applied Physics

1. Introduction

Topological insulators have attracted much attention from awide variety of researchers in condensed matter physics aswell as device physics for spintronics. They are mathematically characterized by the so-called Z2 topologicalnumber.1) While the bulk is insulating, there is a metallicedge or surface state that is topologically protected andhence robust against weak perturbation or disorder. Thesesurface states also have a spin-helical structure; i.e., the spin
is locked with the momentum. Angle-resolved photoemission spectroscopy (ARPES) measurements have directly
shown the presence of such surface states, and their spin
structure has been resolved for a variety of materials.2–5)
Owing to the spin-helical nature of their surface states,
topological insulators can exhibit spin polarization by applying an electric ﬁeld even in the absence of a magnetic ﬁeld.6)
When doped with ferromagnets, a quantum anomalous Halleﬀect will arise that is also controllable by the electric ﬁeld.7)
Moreover, it has been predicted that when topologicalinsulators are placed into contact with superconductors,
Majorana fermions will emerge.8) Because of their nonAbelian nature, these Majorana fermions are said to be applicable in topological quantum computation.9) Also, since
typical examples such as Bi2 Te3 and Bi2 Se3 are known to be
thermoelectric materials, topological insulators are expected
to be utilized in high-performance thermoelectric devices.10)
All these unique properties make topological insulators promising candidates for fabricating high-speed, low-energyconsumption, next-generation, nano-scale electronic devices.
One critical issue in the study of topological insulators is
the realization of a true bulk insulator. In principle, when asample is cooled to low temperatures, the insulating bulkyPresent address: Department of Environmental and Materials Engineering,
Nagoya Institute of Technology, Nagoya 466-8555, Japan.
zPresent address: Graduate School of Frontier Biosciences, and Departmentof Physics, Osaka University, Suita, Osaka 565-0871, Japan.

carriers will freeze out, and thus only the metallic surface
states should contribute to the physical properties. But it
has been known that some unwanted doping occurs, and inreality, it is diﬃcult to make the bulk ‘‘insulating’’. ARPES
studies have shown that naturally grown stoichiometricsamples are actually n-type semiconductor owing toinevitable defects or substitutions in the lattice.2,11) Many
attempts have been made to perform the opposite dopingwith intentional substitution of atoms, and these havebeen partly successful.12) Recently, two new materials,
BiSbTeSe13) or Sn-doped Bi2 Te2 Se14) were shown to be bulk
insulating crystals.
Another interesting question is how the surface states will
be aﬀected in ambient conditions. In ARPES, the experiments are all performed under ultrahigh vacuum conditions
where the crystals are cleaved in situ prior to the measurement. In contrast, transport measurements are performed
ex situ in air. Whether the same surface states are present inboth environments is an open question. Some reports havefocused on the oxidation of these materials.15) There are
actually some transport measurements that show that thesurface states degrade due to oxidation.16) Others claim thatkeeping the topological insulator crystals in air will just
induce carrier doping.13) In this respect, it is desirable toperform in situ transport measurements17,18) that would befree of the complicating eﬀect of oxidation.
In the present work, we have attempted to tune the Fermilevel (EF ) position of a Bi2 Te3 ﬁlm grown on Si(111). The
advantage of using an ultrathin ﬁlm is to enhance the surface/volume ratio by decreasing the sample thickness and making
the surface-state transport dominant. However, if the ﬁlm is
too thin, there will be no topological surface states.19) Therefore, we have fabricated three quintuple layer (1 QL =
 ﬁlms and measured the band dispersion as well as
10.2 A)
performed in situ transport measurements. For the doping, wehave employed two methods. The ﬁrst of these is to change
the substrate temperature during ﬁlm growth and control thedefect density. ARPES measurements showed that Fermi-

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Jpn. J. Appl. Phys. 52 (2013) 110112

Fig. 1. RHEED pattern of a 3 QL Bi2 Te3 ﬁlm. Downward-pointing, white
triangles indicate the 1  1 spots, while upward-pointing, black triangles
indicate the Kikuchi lines.

level tuning was possible to some extent, but the doping wascomplicated. In situ transport measurements did not show
systematic change with the band dispersion. As a second,
alternative method, we have incorporated Pb into the Bi2 Te3ﬁlm. We found that for rather low substrate temperaturesduring ﬁlm growth (350 K), it was possible to realize a bulk
insulator when 10% Pb was alloyed with Bi; ARPES showedthat only the surface states crossed the Fermi level. In
contrast, it was not possible to realize a bulk insulating state
when the substrate temperature was high (450 K). In bothcases, we found a change in the shape of the Dirac cone thatsuggested the formation of another stoichiometric alloy of Bi,
Pb, and Te, which are also known as topological insulators.
Furthermore, the measured ﬁlm conductivity was in accordance with the change in the band dispersion. Thus, theseﬁlms may enable the fabrication of devices utilizing the
intriguing properties of topological surface states.
2. Experimental Methods

The ultrathin Bi2 Te3 ﬁlms were fabricated in situ by amethod similar to that in Ref. 20. First, a clean Si(111)7  7 surface was prepared on an n-type substrate (P-doped,
1–10 cm at room temperature) by a cycle of resistive heat
treatments. Then Bi was deposited on the 7  7 structure at
350–450 K under Te-rich conditions. This procedure isreported to result in a smooth epitaxial ﬁlm, as can be clearlyseen in the RHEED pattern in Fig. 1, indicating the 1  1
periodicity of the Bi2 Te3 (111) surface as well as the Kikuchilines. Scanning tunneling microscopy measurements havealso conﬁrmed the formation of high-quality ﬁlms with an
atomically ﬂat surface.21) It is also known that the minimum
ﬁlm thickness that can be achieved by this method is a
 and the ﬁlms grow in aquintuple layer (1 QL = 10.2 A),
QL-by-QL fashion as revealed by the RHEED intensity
oscillation.22) For carrier doping, we have alloyed Pb with Bi
and co-deposited them with ﬁxed ﬂux ratios under Te-richconditions. This co-deposition also resulted in a smoothepitaxial ﬁlm with no noticeable change in lattice constant as
far as revealed by the RHEED pattern.
In situ conductivity measurements were performed withmonolithic micro four-point probes (MFPP) in a custommade UHV chamber17) at room temperature without applying a magnetic ﬁeld. The conductivity of the underlying
substrate Si(111)-7  7 is much smaller than the measured

values and can be neglected.23) Furthermore, it is likely thatthe 7  7 structure is destroyed by the ﬁlm formation. The
vertical error bars in the conductivity measurements represent the data scattering caused by measuring diﬀerent positions of the sample surface. Since the probe spacing (20 m)
is much smaller than the sample size (typically 2  10 mm2 ),such scattering reﬂects the inhomogeneity of the ultrathin
ﬁlms (nonuniform distribution of defects and steps, etc.).
The scattering is not so large as can be seen in Figs. 3, 5, and9, meaning that the ﬁlms are rather homogeneous, which has
also been conﬁrmed by measuring the core-level photoemission spectra of Bi, Te, and Pb at various parts of the sample.
The only exception is at the sample edge near the clamps
where the underlying substrate is contaminated.
ARPES experiments were performed at BL-5U of
UVSOR-II using an electron analyzer of MBS-Toyama
A-1. The energy and angular resolutions were 20 meV and
0.2 , respectively. The photon energy used was h ¼ 21 eV
and the measurements were done at 10 K. We emphasizethat both the conductivity and ARPES measurements wereconducted in situ under UHV conditions, ensuring that thesame surface states were present in both measurements.
3. Results and Discussion

Experimentally, it is known that Bi2 Te3 ﬁlms can exhibit
Dirac-like surface-state band dispersion from 2 QL. For the
1 QL ﬁlm, the top and bottom surface Dirac cones interact,which results in a massive Dirac cone (parabolic band
dispersion).20) Theoretically, it has been pointed out thatthree or more QL ﬁlms have topologically nontrivial surface
states.24) Therefore, in the following, we will discuss theband dispersion and ﬁlm conductivity of a 3 QL Bi2 Te3 ﬁlm.
3.1 Substrate temperature dependenceFirst, we show how the band dispersion is aﬀected bychanging the substrate temperature during the growth of the
Bi2 Te3 ﬁlms. Figure 2 shows the Fermi surface (top row) andthe band dispersion (bottom row) of the 3 QL Bi2 Te3 ﬁlm
when the substrate temperature (TSi ) during the growth is 350(a), 421 (b), 431 (c), and 449 K (d). The outer hexagons andinner circles in the Fermi surface of Fig. 2(a) indicate theDirac-cone surface state and bulk conduction band, respectively. For the other data, the distinction between the bulk andsurface is rather unclear, but by analyzing the photoemission
spectra in detail, we can outline the individual components
and their probable schematic shape is shown in Figs. 2(b)–2(d). This indicates that all of the ﬁlms are electron-doped(n-type), which means the bottom of the bulk conduction
band is below EF . The red line shows the position of the
Dirac point (DP). It can be seen that the DP shifts towards EF
as TSi is increased. This is basically consistent with the resultsof a previous study showing the Fermi-level tuning of a
60 QL Bi2 Te3 ﬁlm.25) However, although they succeeded inchanging the bulk carrier from electrons (n-type) to holes (ptype), Fig. 2(d) shows that the ﬁlms remain n-type throughout the temperature range we have studied; the intensity at
the center of the Fermi surface in Figs. 2(b)–2(d) means thatthe bulk conduction band remains below EF .26) Furthermore,
we note that the energy positions of the surface and the bulk
states shifted diﬀerently: While the surface states shifted byabout 0.2 eV towards the Fermi level from (a) to (d), the bulk

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K(a)350K(b)421K(c)431K

B

B

B(d)449K

0.1
0.0

_
K

k (Å_ -1)_

Jpn. J. Appl. Phys. 52 (2013) 110112

S

-0.1

B
S

S

S

Binding energy (eV)-0.1_ 0.0
_ _0.1
M
M
EF

0.2

-0.1

0.0 0.1

-0.1

-0.1

0.0 0.1

0.0 0.1-1k (Å )

B

S

0.4-0.2 0.0 0.2
_ _ _
K
K

-0.2 0.0 0.2
_ _ _
M
M

-0.2 0.0 0.2
_ _ _
M
M

-0.2 0.0 0.2
_ _ _
M
M

k (Å-1)
 point (bottom row) of the 3 QL Bi2 Te3 ﬁlm when the substrate
Fig. 2. (Color online) Fermi surface (top row) and the band dispersion around the temperature (TSi ) during growth was 350 (a), 421 (b), 431 (c), and 449 K (d). The red line indicates the position of the Dirac point. B and S denote bulk andsurface states, respectively, and the hexagons and circles indicate the shapes of the Fermi surface.

Bulk carrier denstiy (10 19 cm-3)(a)4Surface carrier density
Bulk carrier density

1.5

3

1.0

2

0.5

1

0.0

0

340

360

380

400

420

440

460

Surface state carrier density (1013 cm-2)

states appear to have shifted by less than 0.1 eV. The originof this behavior is not clear, but this shows that the slightchange in substrate temperature as well as ﬁlm thickness canhave a diﬀerent inﬂuence on the surface and bulk band
structure.
Figure 3(a) shows the change in the bulk and surface-state
carrier density as a function of the substrate temperature
derived from the area of the Fermi surface in Fig. 2.
Figure 3(b) compares the change in DP energy position (redﬁlled squares) with measured ﬁlm two-dimensional (2D)
conductivity (blue open diamonds). As discussed above,
the DP shifts systematically towards EF for higher TSi . On
the other hand, the 2D conductivity is 65 S for the ﬁlm
grown at 350 K, and increases to 250 S for ﬁlms with
TSi ¼ 405{430 K, then returns to 80 S for the ﬁlm with
TSi ¼ 450 K. Thus, we conclude that there is no correspondence between the ﬁlm conductivity and the Dirac point
energy. This is not surprising since we showed that theenergy shifts of the surface and bulk seem to be diﬀerent.
Furthermore, since conductivity is given by  ¼ en, where
n is carrier density and  is mobility, conductivity is not
only determined by n (the Fermi-level tuning). Since
mobility is inﬂuenced by defect density, we need to know
how the defect distribution varies with TSi . Obtaining thisinformation requires a detailed study using diﬀerent experimental techniques.
In summary, we attempted to fabricate a truly bulk
insulating 3 QL Bi2 Te3 ﬁlm by tuning the Fermi-level
position through changing TSi . The approach was, to someextent, successful; however, the bulk and surface states
were aﬀected diﬀerently. Furthermore, there was no clear
relationship between the ﬁlm conductivity and the EF
position. This suggests that changing TSi is not enough tomake a bulk insulating ﬁlm.

(b)Fig. 3. (Color online) (a) Change of the bulk and surface-state carrier
density for the 3 QL Bi2 Te3 ﬁlms formed at diﬀerent substrate temperaturesfrom the Fermi surface of Fig. 2. These values represent the values for zerokelvin. (b) Comparison of the change in the Dirac point position (fromFig. 2) with the measured ﬁlm 2D conductivity.

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K(a)0.0%(b)1.3%(c)3.2%(d)8.7%

B

B

B

B

0.0

S

-0.1

(f)22%-0.1_ 0.0
_ _0.1
M
M
EF

-0.1

0.0 0.1-0.1

0.0 0.1

0.0 0.1-0.1

0.0 0.1-0.1

0.0 0.1k (Å-1)
B

B

B

B

S

S

S

S
0.2

-0.1

S

S

S

S

S

Binding energy (eV)(e)14%

0.1_
K

k (Å_-1) _Jpn. J. Appl. Phys. 52 (2013) 110112

S

S

0.4
-0.2 0.0 0.2
_ _ _
K
K

-0.2

0.0 0.2-0.2

0.0 0.2-0.2

0.0 0.2-0.2

0.0 0.2-0.2

0.0 0.2k (Å-1)

Fig. 4. (Color online) Fermi surface (top row) and band dispersion (bottom row) of the 3 QL Bi2 Te3 ﬁlm grown at TSi ¼ 350 K with a Pb concentration of
0 (a), 1.3 (b), 3.2 (c), 8.7 (d), 14 (e), and 22% (f ). The red line indicates the position of the Dirac point. B and S represent the bulk and surface states,
respectively.

Bulk carrier denstiy (10 19 cm-3)(a)

An alternative way to tune the Fermi level of Bi2 Te3 ﬁlms
would be to replace Bi with Pb. Since Pb has one lesselectron than Bi, this would act as hole doping and decrease
the bulk electron density. First, we show the results of the Pb
doping eﬀect when the Si temperature during growth is
350 K. Figure 4 shows the Fermi surface (top row) and theband dispersion (bottom row) of the 3 QL Bi2 Te3 ﬁlm when
TSi ¼ 350 K with a Pb concentration of 0 (a), 1.3 (b), 3.2 (c),8.7 (d), 14 (e), and 22% (f ). For the undoped and 1.3–8.7%
doped ﬁlms [(a)–(d)], there are two (bulk and surface) Fermi
surfaces. Their size shrinks gradually, and at 14% doping,
the bulk Fermi surface seems to disappear; we can only
recognize a single Fermi ring of the Dirac cone in (e). The
features are more or less the same for the 22%-doped ﬁlm in(f ), though the Dirac-cone Fermi ring has shrunk. The DP
position (red line) shifts systematically towards the Fermi
level, but seems to saturate around 0.12 eV below EF [(f )].
From the area of the Fermi surface, it is possible to estimate
the carrier density, which is shown in Fig. 5(a). In contrast to
the discussion in the previous section where the changes
in the energy positions of the bulk and surface states
were diﬀerent, they seem to shift together in the present
case. This can also be gleaned from the conductivity
measurements. Figure 5(b) compares the measured ﬁlm
conductivity (blue open diamonds) to the shift in the
Dirac point (red ﬁlled squares). The conductivity gradually
decreases as Pb is doped from its initial value of 65 S
and saturates at around 20 S for doping levels higher than
10%. We can say that there is a systematic relationshipbetween the DP energy and the conductivity, unlike inthe results shown in the previous section. Thus, the change
in carrier concentration presumably corresponds to thechange in conductivity. Since only the Dirac-cone surface
state crosses the Fermi level with a Pb density of more than
10%, the saturated value of 20 S likely corresponds tothe surface state contribution, but further study is neededto draw a deﬁnite conclusion. As the measurements were

1.6

Surface carrier density
Bulk carrier density

3

1.2
1.0
2

0.8
0.6

1

0.4
0.2
0.0

0

(b)

1.4

0

5

10

15

20

Surface state carrier density (1013 cm-2)

3.2 Pb doping dependence: low TSi

Fig. 5. (Color online) (a) Change in the bulk and surface-state carrier
density for 3 QL Bi2 Te3 ﬁlms doped with diﬀerent Pb concentrations fromthe Fermi surface of Fig. 4. These values represent the values at zero kelvin.
(b) Comparison of the change in the Dirac point position (from Fig. 4) withthe measured ﬁlm 2D conductivity.

performed at room temperature, there should still be 1017
cm3 bulk carriers present, considering the relatively small
activation gap (0.15 eV for undoped Bi2 Te3 ) for the bulk
carriers. Since information concerning the mobility cannot

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Jpn. J. Appl. Phys. 52 (2013) 110112

00Spot intensity (arb. units)

x = 10.0 %x=0%0high Tsi
low Tsi

0
1
2

0
1

3

2

3

0

x = 16.7 %x = 13.2 %

3

1

2

x = 6.8 %
3

1

2

x=0%

Time (sec)Fig. 6. (Color online) Specular RHEED spot intensity oscillation during
ﬁlm growth for varying Pb-doping concentration, x. Low TSi corresponds to350 K and high TSi corresponds to 450 K.

be determined from the present measurement, we cannot
distinguish the bulk and surface contributions. If we assume
that only the surface state contributes to the measured
conductivity, the mobility at room temperature can be
estimated to be 22 cm2 V1 s1 .
We have found that Pb does not actually donate one hole
per atom. The ratio (induced hole carriers)/(incorporated
Pb atoms) changes from 0.2 for the 1.3%-doped ﬁlm to 0.05
for the 22%-doped ﬁlm. One report shows that part of thePb atoms in Bi2 Te3 are electrically inactive, forming aseven-layer lamellae of Te–Bi–Te–Pb–Te–Bi–Te (septuple
layer, SL).27) Furthermore, it has recently been reported
in Refs. 28–31 that Pb, Bi, and Te can be used to fabricate
other topological insulator materials such as PbBi2 Te4 ,PbBi4 Te7 , and PbBi6 Te10 . These materials are formed byrepeating not only QLs, but a combination of QLs and SLs.
Thus, it can be concluded that Pb-alloying does not only
aﬀect the carrier density of Bi2 Te3 , but may also be changing the crystal structure itself. Figure 6 shows the RHEED
oscillation during ﬁlm growth. After the completion of a
wetting layer (0), QL-by-QL growth occurs for the undopedﬁlm. We note that the interval between the ﬁrst peak (1)and the second peak (2) decreases with increasing Pb
concentration. For the 16.7%-doped ﬁlm, the ratio of
the time it takes for the ﬁrst, second, and third peakK(b)1.3%

3.3 Pb doping dependence: high TSi

Let us discuss the eﬀect of Pb doping when the ﬁlms were
fabricated at a high substrate temperature (TSi ¼ 449 K).
Figure 8 shows the evolution of the Fermi surface (top row)and the band dispersion (bottom row) for 3 QL Bi2 Te3 ﬁlms
with a Pb doping concentration of 0 (a), 2.5 (b), 4.4 (c), 10.0(d), 15.2 (e), and 20.3% (f ) (only the band dispersion). The
red line indicates the position of the DP. We can see that thesize of the Fermi surface changes only slightly. Since theintensity at the center of the Fermi surface remains, thebulk conduction band crosses the Fermi level for all Pb
concentrations, although the distinction between the surface
and bulk states is extremely diﬃcult to see. Thus, it is notpossible to estimate the respective carrier densities. The
DP seems to shift slightly towards EF for the 2.5 (b) and
4.4% (c) doped samples, but moves away from EF to higher
binding energy for the 10.0–20.3% samples in (d)–(f ).
Furthermore, the shape of the Dirac cone appears to change
as Pb is incorporated. The velocity (slope) becomes smaller
as Pb is increased. This suggests the formation of another
topological insulator material made up of Pb, Bi, and Te.
To investigate this eﬀect, we have examined the RHEED
intensity oscillation. The results are shown in Fig. 6. Since(c)3.2%(d)8.7%(e)14%(f)22%

0.1
0.0

_
K

k (Å_-1)_(a)0.0%structureðpeak1{peak0 : peak2{peak1 : peak3{peak2Þ 
7 : 3 : 7. This suggests the formation of a SL–QL–SL
structure, and hence, a PbBi4 Te7 ﬁlm. This can also beseen from the change in the shape of the Dirac cone.
Figure 7 shows the constant energy contour (CEC) mappingat the DP. While the CEC for the pristine ﬁlm has a rather
large and fuzzy circular feature around the  point owing tothe overlap with the bulk valence band, this circle shrinks as
the Pb concentration increases. Once the bulk conduction
band exceeds EF , the CEC at the DP becomes a ‘‘real point’’[Figs. 7(e) and 7(f )], implying that the overlap between
the surface states and bulk valence band has been eliminatedand the Dirac cone has become isolated. While a precise
description of the structure would require further investigation, it is clear that Pb-doped Bi2 Te3 ﬁlms have an ideal,well-isolated Dirac cone in contrast to the undoped Bi2 Te3shown in Fig. 2.
In summary, we found that it is possible to fabricate atruly bulk insulating Bi2 Te3 ﬁlm by 10% Pb incorporation
during ﬁlm growth at TSi ¼ 350 K. The Dirac cone becomeswell isolated from the bulk states and is more ideal than thatof the pristine Bi2 Te3 . The measured ﬁlm conductivity is
consistent with the change in Fermi level revealed by
ARPES.

-0.1-0.1 0.0 0.1
_ _ _
M
M

-0.1

0.0 0.1-0.1

0.0 0.1-0.1

0.0 0.1-0.1

0.0 0.1-0.1

0.0 0.1k (Å-1)

Fig. 7. Constant energy contour mapping at the Dirac point energy for 3 QL Bi2 Te3 ﬁlms with varying Pb doping concentration.

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K(b)2.5%(c)4.4%(d)10.0%(e)15.2%-0.1

0.0 0.1-0.1

0.0 0.1-0.2

0.0 0.2-0.2

0.0 0.2(f)20.3%

0.1
0.0

_
K

k (Å_-1)_(a)0.0%Binding energy (eV)-0.1
-0.1 0.0 0.1
_ _ _
M
M

-0.1

-0.2 0.0 0.2
_ _ _
M
M

-0.2

0.0 0.1-0.1

0.0 0.1k (Å-1)

EF

0.2

0.4
0.0 0.2-0.2

0.0 0.2-0.2

0.0 0.2k (Å-1)

Fig. 8. (Color online) Fermi surface (top row) and band dispersion (bottom row) for 3 QL Bi2 Te3 ﬁlms with a Pb doping concentration of 0 (a), 2.5 (b),4.4 (c), 10.0 (d), 15.2 (e), and 20.3% (f ) (only the band dispersion). The red line indicates the position of the Dirac point. The distinction between the bulk
and surface states is not clear for these ﬁlms, but the dashed lines near EF in (a) and (f ) illustrate the dispersion of the Dirac cone.

To summarize, we have found that it is not possible tofabricate a truly bulk insulating Bi2 Te3 ﬁlm by Pb incorporation during ﬁlm growth at TSi ¼ 450 K, although there issome shift. It is likely that a diﬀerent structure composed ofPb, Bi, and Te is formed, judging by the change in the shapeof the Dirac cone.
4. SummaryFig. 9. (Color online) Variation of the Dirac point position (from Fig. 8)
with the measured ﬁlm 2D conductivity.

the growth mode at high substrate temperature is of a stepﬂow type,25) we could not observe any oscillatory behavior,and it was diﬃcult to determine which alloy was formed.
The question of how the diﬀerence in TSi inﬂuences the
Fermi-level tuning of Bi2 Te3 ﬁlms as well as the actual
structure of the Pb-doped ﬁlms remain unclear. Further
measurements are needed to determine the real atomic
structure of the ﬁlms shown in Figs. 8(d)–8(f ).
Figure 9 compares the measured ﬁlm conductivity (blueopen diamonds) and the shift in the DP (red ﬁlled squares).
It can be seen that the two follow the same trend: A slight
drop is observed at the low-concentration Pb doping, thenboth start to increase with further Pb incorporation. Thissuggests that the conductivity is basically determined by the
carrier density, which was also discussed in the previous
section. However, since these ﬁlms are not bulk insulatingﬁlms, the absolute values of the conductivity shown in Fig. 9
are higher than 20 S (Fig. 5), which is the value measured
for the ﬁlm that the bulk band does not cross EF [Figs. 4(e)and 4(f )].

In conclusion, we tried to tune the Fermi level of a 3 QL
Bi2 Te3 ﬁlm to realize a bulk insulating material using one oftwo methods. The ﬁrst method, which involved changing the
Si substrate temperature during ﬁlm deposition (350–449 K),
had some eﬀect in reducing the bulk residual carriers. However, we could not fabricate a ﬁlm in which only the surfacestate bands crossed the Fermi level. The second method was
to incorporate Pb during ﬁlm growth by co-deposition of Bi,
Pb, and Te with controlled ratios of deposition rates. Whenthe ﬁlms were grown at a relatively low substrate temperature, we observed a systematic shift in the Fermi leveland obtained a bulk insulating ﬁlm. The measured ﬁlm conductivity was consistent with the change in DP position.
These ﬁlms can hopefully be utilized in electronic device
applications. For the ﬁlms grown at a high substrate temperature, the Fermi level could be changed slightly, but
only a bulk n-type ﬁlm was obtained. Pb incorporation
changed the shape of the Dirac cone, which likely suggests
the formation of a stoichiometric ternary alloy of Bi, Pb,
and Te.
Acknowledgments

This work was supported by Grants-in-Aid from theJapan Society for the Promotion of Science (Nos. 22656011
and 23686007), the JGC-S Scholarship Foundation,
the Kao Foundation for Arts and Sciences, and the Support
Center for Advanced Telecommunications Technology
Research. The ARPES experiments were performed
under the UVSOR Proposal Nos. 22-521, 23-515, and 24521.

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Jpn. J. Appl. Phys. 52 (2013) 110112
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Masaki Aitani received his master’s degree inphysics from University of Tokyo in 2013. He iscurrently working at Fujitsu Limited.

Manabu Yamada received his master’s degree inphysics from the University of Tokyo in 2012. He iscurrently working at Toshiba Corporation.

Yusuke Sakamoto received his master’s degree inphysics from University of Tokyo in 2011. He iscurrently working at Mitsubishi Electric Corporation.

Hidetoshi Miyazaki received his Ph. D. degree inEngineering from Nagoya University in 2009. From2009 to 2011, he worked at the UVSOR Facility,
Institute for Molecular Science, as a Postdoctral
Fellow. Since 2011, he has been working at theCenter for Fostering Young and Innovative Researchers, Nagoya Institute of Technology, as an
Assistant Professor.

Toru Hirahara obtained his Ph. D. degree inscience from the department of physics, Universityof Tokyo in 2008. Since 2007, he is an assistantprofessor at the department of physics, University ofTokyo. His major interest is experimental surface
physics, especially exploring novel spin–orbit
coupling eﬀects at nonmagnetic surfaces.

Masaharu Matsunami received Ph. D. degree inPhysics from Kobe University in 2004. From 2004
to 2007, he worked at RIKEN SPring-8 Center as aSpecial Postdoctoral Researcher. From 2007 to
2010, he worked at the Institute for Solid State
Physics (ISSP), the University of Tokyo, as a
Postdoctoral Fellow of the Japan Society for the
Promotion of Science (JSPS). Since 2010, he has
worked at UVSOR Facility in the Institute for
Molecular Science, as an Assistant Professor.

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SELECTED TOPICS IN APPLIED PHYSICS

Jpn. J. Appl. Phys. 52 (2013) 110112
Shin-ichi Kimura was born in Fukushima Prefecture, Japan in 1966. He obtained his B. Sc.
(1988), M. Sc. (1990), and D. Sc. (1991) degrees
from Tohoku University. He was a postdoctoral
fellow at JSPS (1991–1993), a research associate at
Kobe University (1993) and at Institute for Molecular Science (1993–1998), and an associate
professor at Kobe University (1998–2002) and at
Institute for Molecular Science (2002–2013). Since
2013, he has been a professor at Osaka University.
He has worked on studies of electronic structure of correlated materials by
an infrared/terahertz spectroscopy and an angle-resolved photoemission
spectroscopy and synchrotron radiation instrumentations. His research isalso now focused on the investigation of electronic structure of solids andsoft-matters at multi-extreme conditions.

110112-8

Shuji Hasegawa obtained the master’s degree inscience from the department of physics, Universityof Tokyo in 1985. He has worked in the Advanced
Research Laboratory, Hitachi, Ltd. from 1985 to
1990, and since 1990 he has been an assistant/associate/full professor at the department of physics, University of Tokyo. His major interest isexperimental surface physics, including surface
transport, microscopy, spectroscopy, and diﬀraction.

# 2013 The Japan Society of Applied Physics