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Received 29 Jun 2011 | Accepted 3 Nov 2011 | Published 6 Dec 2011 DOI: 10.1038/ncomms1588
Band structure engineering in (Bi1x Sb x )2Te 3
ternary topological insulators
Jinsong Zhang1,*, Cui-Zu Chang1,2,*, Zuocheng Zhang1, Jing Wen1, Xiao Feng2, Kang Li2, Minhao Liu1, Ke He2, Lili Wang2, Xi Chen1, Qi-Kun Xue1,2, Xucun Ma2 & Yayu Wang1
Topological insulators (TIs) are quantum materials with insulating bulk and topologically protected metallic surfaces with Dirac-like band structure. The most challenging problem faced by current investigations of these materials is the existence of signicant bulk conduction. Here we show how the band structure of topological insulators can be engineered by molecular beam epitaxy growth of (Bi 1x Sb x )2Te 3 ternary compounds. The topological surface states are shown to exist over the entire composition range of (Bi 1x Sb x )2Te
3, indicating the robustness of bulk Z 2 topology. Most remarkably, the band engineering leads to ideal TIs with truly insulating bulk and tunable surface states across the Dirac point that behave like one-quarter of graphene.
This work demonstrates a new route to achieving intrinsic quantum transport of the topological surface states and designing conceptually new topologically insulating devices based on well-established semiconductor technology.
1 State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University , Beijing 100084 , Peoples Republic of China.
2 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences , Beijing 100190, Peoples Republic of China.
* These authors contributed equally to this work . Correspondence and requests for materials should be addressed to K.H. (email: [email protected] ) or to Y.W. (email: [email protected] ) .
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The topological surface states of three-dimensional topological insulators (TIs) possess a single spin-polarized Dirac cone originated from strong spin orbit coupling 13. The
unique surface states are expected to host exotic topological quantum eects 46 , and nd applications in spintronics and quantum computation. Th e experimental realization of these ideas requires fabrication of versatile devices based on bulk-insulating TIs with tunable surface states. However, the currently available TI materials exemplied by Bi 2Se 3 and Bi 2Te
3 (ref. 7) always show conductive bulk states due to the defect-induced charge carriers. Tuning the band structure of the TIs to eliminate the bulk states is one of the most urgent tasks in the eld, but the problem remains unsolved despite extensive eorts involving nanostructuring 8, chemical doping 915 and electrical gating 1619.
Energy band engineering in conventional semiconductor is a powerful approach for tailoring the electronic structure of materials 20 . A notable example is the isostructural isovalent alloy of the IIIVsemiconductors Al x Ga1x As grown on GaAs by molecular beam epitaxy (MBE), in which the energy gap can be tuned continuously by the mixing ratio of AlAs and GaAs. Such energy band tuning has been essential for heterostructures, which were later used for discovery of fractional quantum Hall eect and invention of high-speed electronics.
Inspired by this idea, we conceived a new route for engineering the band structure of TIs by fabricating alloys of Bi 2Te
3 and Sb 2Te
3.
Both TIs are V VI compounds with the same crystal structure and close lattice constants 7 , making it ideal to form (Bi 1x Sb x )2Te
3 ternary compounds with arbitrary mixing ratio and negligible strain (Fig. 1a). Th e potential advantages of mixing the two TIs can be anticipated from their complementary electronic properties. Figure 1b illustrates the band structure of pure Bi 2Te
3 (refs 7, 10, 21), which reveals two major drawbacks of the surface Dirac band in Bi 2Te
3.
First, the Dirac point (DP) is buried in the bulk valence band (BVB), hence, cannot be accessed by transport experiment and, more seriously, the Fermi level ( EF ) lies in the bulk conduction band (BCB)
due to the electron-type bulk carriers induced by Te vacancies. On the other hand, the band structure of pure Sb 2Te
3 (refs 7, 21) is drastically dierent. As shown schematically in Figure 1c , here the DP lies within the bulk gap and the EF lies in the BVB due to the hole-type bulk carriers induced by Sb Te anti-site defects. Intuitively, by
mixing the two compounds one can simultaneously achieve charge compensation and tune the position of the DP, which may lead eventually to an ideal TI with exposed DP and insulating bulk.
Here we report the band structure engineering in TIs by fabricating alloys of Bi 2Te
3 andSb2Te
3 using state-of-the-art MBE. Transport and angle-resolved photoemission spectroscopy (ARPES)
measurements show that the band engineering technique allows us to achieve ideal TIs with truly insulating bulk. Th e surface states can be tuned systematically across the DP and the transport properties are consistent with that of a single spin-polarized Dirac cone.
Results
Sample structure.During theMBEgrowth of the(Bi1x Sb x )2Te
3
lms, the growth rate is calibrated by a real-time reection high-energy-electron diraction intensity oscillation measured on the (00) diraction. Supplementary Figure S1 shows a typical 1 1 reection high-energy-electron diraction pattern taken on a (Bi 1x Sb x )2Te
3
lm with ve quintuple layers (QLs) thickness. Th e sharpness of the feature provides a clear evidence for the high quality of the sample. The ve QL thickness is used for all (Bi 1x Sb x )2Te
3 lms studied in this work because in this ultrathin regime the surface states dominate charge transport, and meanwhile the lms are thick enough that the top and bottom surfaces are completely decoupled. Further discussion about the lm thickness issue can be found in the Supplementary Information.
Electronic structure. Th e electronic structures of the (Bi 1x Sb x )2Te
3
lms are measured by ARPES on a sample setup as illustrated in Supplementary Figure S2 . The ARPES band maps of eight (Bi 1x Sb x )2Te
3 lms with 0x 1 are shown in Figure 2a to h . The pure Bi 2Te
3 lm shows well-dened surface states with massless Dirac-like dispersion ( Fig. 2a ), similar to that of the cleaved Bi 2Te
3
crystal 10. With the addition of Sb, the Dirac-like topological surface states can be clearly observed in all (Bi 1x Sb x )2Te
3 lms from x=0 to 1, whereas the Dirac cone geometry changes systematically. With increasing x, the slope of the Dirac line shape becomes steeper, indicating an increase of the Dirac fermion velocity vD dened by the linear dispersion
=vD[planckover2pi]k near the DP. Meanwhile, the EF moves downwards from the BCB, indicating the reduction of the electron-type bulk carriers. Moreover, the DP moves upwards relative to the
Bi2Te3 Sb2Te3
BCB
EF
EF
DP BVB
BiSbTe M K M
K
Figure 1 | The schematic crystal and electronic structures of the (Bi1x Sb x )2Te
3 compounds. ( a) The tetradymite-type crystal structure of(Bi 1x Sb x )2Te 3 where the Bi atoms are partially substituted by Sb. ( b) The schematic electronic band structure of pure Bi 2Te 3 and ( c) pure Sb 2Te 3 based on theoretical calculations 7 and ARPES experiments 10,21.
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0.0
EF
0.1
SS
0.2
BCB
0.3
BVB
0.4
DP
0.5
0.6
Binding energy (eV)
0.2 0.1 0 0.1 0.2 0.2 0.1 0 0.1 0.2 0.2 0.1 0 0.1 0.2 0.2 0.1 0 0.1 0.2
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.2 0.1 0 0.1 0.2 0.2 0.1 0 0.1 0.2 0.2 0.1 0 0.1 0.2 0.2 0.1 0 0.1 0.2
k// (1/) k// (1/)k k// (1/) k// (1/)
Figure 2 | ARPES results on the ve QL (Bi 1x Sb x )2Te 3 lms measured along the K- [H9003]-K direction. From ( a) to ( h) the measured band structures of (Bi 1x Sb x )2Te
3 lms with x = 0, 0.25, 0.62, 0.75, 0.88, 0.94, 0.96 and 1.0, respectively. The Dirac-like topological surface states exist in all lms. The yellow dashed line indicates the position of the Fermi level ( EF). The blue and red dashed lines indicate the Dirac surface states with opposite spin polarities and they intersect at the DP.
BVB due to the increasing weight of the Sb 2Te
3 band structure.
When the Sb content is increased to x=0.88 (Fig. 2e), both the DP and EF lie within the bulk energy gap. Th e system is now an ideal TI with a truly insulating bulk and a nearly symmetric surface Dirac cone with exposed DP. Notably, when x increases from x=0.94 (Fig. 2f) to 0.96 (Fig. 2g), EF moves from above the DP to below it, indicating a crossover from electron- to hole-type Dirac fermion gas. Th e charge neutrality point (CNP) where EF meets DP can thus be identied to be located between x=0.94 and 0.96.
It is quite remarkable that the topological surface states exist in the entire composition range of (Bi 1x Sb x )2Te
3 , which implies that the nontrivial Z 2 topology of the bulk band is very robust against alloying. Th is is in contrast to the Bi 1x Sb x alloy, the rst discovered three-dimensional TI in which the topological surface states only exist within a narrow composition range near x=0.10 (refs 22, 23). Figure 3a to c summarizes the characteristics of the surface Dirac band in the (Bi 1x Sb x )2Te
3 compounds, which are extracted following the procedure presented in the Supplementary Information
and illustrated in Supplementary Figures S3 and S4 . The position of the DP rises continuously from below the top of BVB near the point at x=0 to way above that at
x=1 (Fig. 3a). Th is is accompanied by a drastic change of the relative position of EF and DP ( Fig. 3b ), which determines the type and density of Dirac fermions. Furthermore, vD increasesfrom 3.3105ms 1 at x=0to 4.1105ms 1 at
x=1 (Fig. 3c). As the three dening properties of the Dirac cone are systematically varied between that of pure Bi 2Te
3 and Sb 2Te
3,the
(Bi 1x Sb x )2Te
3 ternary compounds are eectively a series of new TIs. Th e bulk electronic structures, including the geometry of BCB and BVB as well as the energy gap between them, are also expected to change with x. Th ey are of interests in their own rights, but will not be the main focus of the current work.
Transport properties. Th e systematic Dirac band evolution also manifests itself in the transport properties. Figure 4 displays the variation of two-dimensional sheet resistance ( R [H17040] ) with temperature ( T ) for eight QL (ve) (Bi 1x Sb x )2Te
3 lms with 0 x1.
In pure Bi 2Te 3 the resistance shows metallic behaviour at high
T and becomes weakly insulating at very low T . With increasing x , the R [H17040] value keeps rising and the insulating tendency becomes stronger, reecting the depletion of electron-type bulk carriers and surface Dirac fermions. At x=0.94 when EF lies just above DP, the resistance reaches the maximum value and shows insulating behaviour over the whole T range. With further increase of Sb content from x=0.96to 1,theresistancedecreasessystematicallybecause now EF passes DP and more hole-type carriers start to populate the surface Dirac band. Th e high T metallic behaviour is recovered in pure Sb 2Te
3 when the hole-type carrier density becomes suffi ciently high.
3
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200 100 4.2
[afii9835]
EF
DP to BVB (meV)
150
100
50
DP to E F(meV)
0
100
200
300
5 m s1 )
v D(10
4.0
3.8
3.6
3.4
3.2
0
50 0.00 0.25 0.50 0.75 1.00
Sb concentration x
0.00 0.25 0.50 0.75 1.00
Sb concentration x
0.00 0.25 0.50 0.75 1.00
Sb concentration x
x obtained from the ARPES data in (Bi 1x Sb x )2Te
3. ( a) Relative position (or energy difference) between the DP and the top of BVB near the point. ( b) Relative position between the DP and the EF. (c) The Dirac fermion velocity vD ( vD ~tan) extracted from the linear dispersion near the DP. All three quantities evolve smoothly from that of pure Bi 2Te
3 (x=0) to pure Sb2Te 3 (x=1).
Figure 3 | Evolution of the surface band characteristics with
10 x = 0 x = 0.5 x = 0.75 x = 0.88 x = 0.94 x = 0.96 x = 0.98 x = 1.0
8
6
R (k)
4
2
0
0 100 200 0 100 200 0 100 200 0 100 200
0 100 200 0 100 200 0 100 200 0 100 200 300
Temperature (K)
Figure 4 | Two-dimensional sheet resistance (Rh ) versus temperature ( T) for eight ve QL (Bi 1x Sb x )2Te 3 lms. R [H17040] value keeps rising and the insulating tendency becomes stronger with increasing Sb content from x = 0 to 0.94 due to the reduction of electron-type carriers. From x=0.96 to 1 the trend is reversed, that is, R [H17040] value decreases and the insulating tendency becomes weaker with increasing Sb content due to the increasing density of hole-type carriers.
Figure 5a displays the variation of the Hall resistance ( R yx ) with magnetic eld ( H ) measured on the ve QL (Bi 1x Sb x )2Te
3 lms at
T=1.5K. For lms with x0.94, the R yx value is always negative, indicating the existence of electron-type carriers. The weak-eld slope of the Hall curves, or the Hall coeffi cient RH,increasessystematically with x in this regime. As the two-dimensional carrier density n2D can be derived from RH as n2D=1/eRH (e is the elementary charge), this trend conrms the decrease of electron-type carrier density with Sb doping. As x increases slightly from 0.94 to 0.96, the Hall curve suddenly jumps to the positive side with a very large slope, which indicates the reversal to hole-type Dirac fermions with a small carrier density. At even higher x , the slope of the positive curves decreases systematically due to the increase of hole-type carrier density.
Th e evolution of the Hall eect is totally consistent with the surface band structure revealed by ARPES in Figure 2 . To make a more quantitative comparison between the two experiments, we use the n2D derived from the Hall eect to estimate the Fermi wavevector kF of the surface Dirac band. By assuming zero bulk contribution and an isotropic circular Dirac cone structure ( Fig. 5b ), kF canbe expressed as
Dk n
F SS
2
surfaces are equivalent. Figure 5c shows that when we choose D=1, the k F values derived from the Hall eect match very well with that directly measured by ARPES. Th is remarkable agreement suggests that the transport properties of the TI surfaces are consistent with that of a single spin-polarized Dirac cone, or a quarter of graphene, as expected by theory.
Figure 5d to f summarizes the evolution of the low T transport properties with Sb content x.Th e resistance value shows a maximum at x=0.94with
R [H17040] > 10k and decreases systematically on both sides. Correspondingly, the carrier density | n2D | reaches a minimum at x=0.96with |n2D|=1.41012cm 2 and increases on both sides. Using the measured R [H17040] and | n2D |, the mobility
of the Dirac fermions can be estimated by using the Drude formula 2D=| n2D|e , where 2D=1/R [H17040] . As a function of x the mobility also peaks near the CNP and decreases rapidly on both sides.
Th e V -shaped dependence of the transport properties on the Sb content x clearly demonstrates the systematic tuning of the surface band structure across the CNP.
Discussion
Th e good agreement with ARPES suggests that the transport results are consistent with the properties of the surface Dirac fermions without bulk contribution. Moreover, the alloying allows us to approach the close vicinity of the CNP, which gives a very low | n2D| in the order of 11012cm 2.The (Bi 1x Sb x )2Te 3 compounds thus represent an
4p =| |
(1)(1)
n n
SS D
= 12 2 is the carrier density per surface if we assume that the top and bottom
Here
D is the degeneracy of the Dirac fermion and | | | |
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kF
R (k)
12
9
6
3
0 0.5 0.6 0.7Sb concentration x
Sb concentration x
6
4
2
0
0.8 0.9 1.0
10
R yx(k)
n 2D (1012 cm2)
[afii9839](cm2Vs1 )
ARPES
0.7 0.8 0.9 1.0
0.1
1 0.5 0.6 0.7Sb concentration x
0.8 0.9 1.0
2
k F(1 )
0.0
0.1
0.5 0.6Sb concentration x
Hall
4
0 0.5 0.6 0.7 0.8 0.9 1.0
0 5 10 15 [afii9839]0 H (T)
Figure 5 | The Hall effect and summary of the transport results. ( a) The eld dependence of the Hall resistance R yx for the eight (Bi 1x Sb x )2Te 3 lms measured at T = 1.5 K. From top to bottom, the curves are the Hall traces of (Bi 1x Sb x )2Te 3 lms with x = 0.96, 0.98, 1.0, 0, 0.50, 0.75, 0.88 and 0.94, respectively. The evolution of the Hall effect reveals the depletion of electron-type carriers (from x = 0 to 0.94), the reversal of carrier type (from x=0.94
to 0.96), and the increase of hole-type carrier density (from x=0.96 to 1.0). (b) Schematic sketch of an isotropic circular Dirac cone where the Fermi wavevectors kF is marked. The blue arrows indicate the helical spin texture. ( c) The kF of the Dirac cone derived from the Hall effect (black open squares)
agree well with that directly measured by ARPES (red solid circles) if we assume a single spin-polarized Dirac cone on each surface. The kF is dened to be negative for hole-type Dirac fermions. The sheet resistance R [H17040] (d), the carrier density | n2D| (e) and the mobility of the Dirac fermions ( f) measured at
T=1.5K all show V-shaped
x dependence near the CNP.
EF
EF
x = 0.94 x = 0.96 x = 0.94 x = 0.96
Figure 6 | Schematic device structures of spatially variable Dirac bands grown by CGD of (Bi 1x Sb x )2Te 3 lms. Vertical CGD TIs ( a) is an ideal system for studying the topological exciton condensation and electrical control of spin current. Horizontal CGD TIs ( b) can be used to fabricate a topological pn junction.
ideal TI system to reach the extreme quantum regime because now a strong magnetic eld can squeeze the Dirac fermions to the lowest few Landau levels. Indeed, the Hall resistance of the x=0.96 lm shown in Figure 5a is close to 7 k at 15T, which is a signicant fraction of the quantum resistance. Future transport measurements on (Bi 1x Sb x )2Te
3 lms with higher mobility to even stronger magnetic eld hold great promises for uncovering the unconventional quantum Hall eect of the topological surface states 24,25.
Th e band structure engineering oers many enticing opportunities for designing conceptually new experimental or device schemes based on the TIs. For example, we can apply the idea of compositionally graded doping (CGD) in conventional semiconductor devices 20 to
the TIs to achieve spatially variable Dirac cone structures. Figure 6a illustrates the schematic of vertical CGD TIs, in which the top and bottom surfaces have opposite types of Dirac fermions and can be used for studying the proposed topological exciton condensation 26. Th e spatial asymmetry of the surface Dirac bands can also be used to realize the electrical control of spin current by using the spin-momentum locking in the topological surfaces for spintronic applications 27. Figure 6b illustrates the schematic of horizontal CGD TIs, by which a topological pn junction between hole- and electron-type TIs can be fabricated.
Methods
MBE sample growth. Th e MBE growth of TI lms on insulating substrate has been reported before by the same group 28. The (Bi 1x Sb x )2Te 3 lms studiedhere are grown on sapphire (0001) in an ultra-high vacuum MBE-ARPES-STM combined system with a base pressure of 1 10 10Torr. Before sample growth,the sapphire substrates are rst degassed at 650 C for 90 min and then heated at 850C for 30min. High-purity Bi (99.9999%), Sb (99.9999%) and Te (99.999%) are evaporated from standard Knudsen cells. To reduce Te vacancies, the growth is kept in Te-rich condition with the substrate temperature at 180 C. Th e Bi:Sb ratio is controlled by the temperatures of the Bi and Sb Knudsen cells. The
x value in the
(Bi 1x Sb x )2Te 3 lm is determined through two independent methods, as discussed in detail in Supplementary Information .
ARPES measurements. The
in situ ARPES measurements are carried out at room temperature by using a Scienta SES2002 electron energy analyser . A Helium discharge lamp with a photon energy of h =21.218eV is used as the photon source. Th e energy resolution of the electron energy analyser is set at 15 meV. All the spectra shown in the paper are taken along the K- -K direction. To avoid sample charging during ARPES measurements due to the insulating sapphire substrate,a 300 nm-thick titanium lm is deposited at both ends of the substrate, which is connected to the sample holder. Th e sample is grounded through these contacts once a continuous lm is formed. Th e sample setup for the ARPES measurements is illustrated schematically in the Supplementary Figure S2 .
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Transport measurements. Th e transport measurements are performed ex situon the ve QL (Bi 1x Sb x )2Te 3 lms grown on sapphire (0001) substrate. To avoid possible contamination of the TI lms, a 20-nm thick amorphous Te capping layer is deposited on top of the lms before we take them out of the ultra-high vaccum growth chamber for transport measurements. Th e Hall eect and resistance are measured using standard ac lock-in method with the current owing in the lm plane and the magnetic eld applied perpendicular to the plane. The schematic device setup for the transport measurements is shown in Supplementary Figure S5 . Th e 20-nm amorphous Te capping layer causes no signicant change of the TI surface electronic structure and makes negligible contribution to the total transport signal, as shown in Supplementary Figure S6 and discussed in the Supplementary Information.
References
1. Qi, X. L. & Zhang, S. C. Th e quantum spin Hall eect and topological insulators.Phys. Today 63, 3338 (2010).
2. Moore, J. E. Th e birth of topological insulators . Nature 464, 194198 (2010).
3. Hasan, M. Z. & Kane, C. L. Topological insulators. Rev. Mod. Phys. 82, 30453067 (2010).
4. Fu, L. & Kane, C. L. Superconducting proximity eect and Majorana fermions at the surface of a topological insulator . Phys. Rev. Lett. 100, 096407 (2008).
5. Qi, X. L., Li, R., Zang, J. & Zhang, S. C. Inducing a magnetic monopole with topological surface states . Science 323, 11841187 (2009).
6. Yu, R. et al. Quantized anomalous Hall eect in magnetic topological insulators.Science 329, 6164 (2010).
7. Zhang, H. J. et al. Topological insulators in Bi 2Se 3, Bi 2Te
3 and Sb2Te 3 with a single Dirac cone on the surface . Nat. Phys. 5, 438442 (2009).
8. Peng, H. et al. Aharonov-Bohm interference in topological insulator nanoribbons.Nat. Mater. 9, 225229 (2010).
9. Hsieh, D. et al. A tunable topological insulator in the spin helical Dirac transport regime.Nature 460, 11011105 (2009).
10. Chen, Y. L. et al. Experimental realization of a three-dimensional topological insulator, Bi 2Te
3. Science 325, 178181 (2009).11. Checkelsky, J. G. et al. Quantum interference in macroscopic crystals of nonmetallic Bi 2Se 3. Phys. Rev. Lett. 103, 246601 (2009).
12. Hor, Y. S. et al. p-Type Bi 2Se 3 for topological insulator and low-temperature thermoelectric applications.Phys. Rev. B 79, 195208 (2009).
13. Analytis, J. G. et al. Two-dimensional Dirac fermions in a topological insulator: transport in the quantum limit . Nat. Phys. 6, 960964 (2010).
14. Qu, D. X., Hor, Y. S., Xiong, J., Cava, R. J. & Ong, N. P. Quantum oscillations and Hall anomaly of surface states in the topological insulator Bi 2Te 3. Science 329, 821824 (2010).
15.Taskin, A. A., Ren, Z., Sasaki, S., Segawa, K. & Ando, Y . Observation of Dirac holes and electrons in a topological insulator . Phys. Rev. Lett. 107, 016801 (2011).
16. Chen, J. et al. Gate-voltage control of chemical potential and weak antilocalization in Bi 2Se 3. Phys. Rev. Lett. 105, 176602 (2010).
17. Kong, D. S. et al. Few-Layer nanoplates of Bi 2Se 3 and Bi 2Te 3 with highly tunable chemical potential . Nano Lett. 10, 22452250 (2010).
18.Steinberg, H., Gardner, D. R., Lee, Y. S. & Jarillo-Herrero, P.Surface state transport and ambipolar electric eld eect in Bi 2Se 3 nanodevices.Nano Lett.
10, 50325036 (2010).19. Checkelsky, J. G., Hor, Y. S., Cava, R. J. & Ong, N. P.
Bulk band gap and surface state conduction observed in voltage-tuned crystals of the topological insulator Bi 2Se 3. Phys. Rev. Lett. 106, 196801 (2011).
20.Capasso, F. Band-gap engineering - from physics and materials to new semiconductor-devices.Science 235, 172176 (1987).
21. Hsieh, D. et al. Observation of time-reversal-protected single-Dirac-cone topological-insulator states in Bi 2Te 3 and Sb2Te 3. Phys. Rev. Lett. 103, 146401 (2009).
22.Fu, L. & Kane, C. L. Topological insulators with inversion symmetry.Phys. Rev. B 76, 045302 (2007).
23. Hsieh, D. et al. A topological Dirac insulator in a quantum spin Hall phase . Nature 452, 970974 (2008).
24.Qi, X. L., Hughes, T. L. & Zhang, S. C. Topological eld theory of time-reversal invariant insulators.Phys. Rev. B 78, 195424 (2008).
25. Brune, C. et al. Quantum Hall eect from the topological surface states of strained bulk HgTe.Phys. Rev. Lett. 106, 126803 (2011).
26.Seradjeh, B., Moore, J. E. & Franz, M. Exciton condensation and charge fractionalization in a topological insulator lm.Phys. Rev. Lett. 103, 066402 (2009).
27.Yazyev, O. V., Moore, J. E. & Louie, S. G. Spin polarization and transport of surface states in the topological insulators Bi 2Se 3 and Bi 2Te 3 from rst principles.Phys. Rev. Lett. 105, 266806 (2010).
28. Chang, C. Z. et al. Growth of quantum well lms of topological insulator Bi 2Se 3 on insulating substrate . SPIN 1, 2125 (2011).
Acknowledgements
We acknowledge S.C. Zhang and Y.B. Zhang for suggestions and comments. This work was supported by the National Natural Science Foundation of China, the Ministry of Science and Technology of China (grant number 2009CB929400) and the Chinese Academy of Sciences.
Author contributions
K.H., Y.W., X.C.M. and Q.K.X. designed the research. C.Z.C., J.W., X.F. and K.L. carried out the MBE growth of the samples and ARPES measurements. J.S.Z., Z.C.Z and M.H.L. carried out the transport measurements. L.L.W., X.C., and X.C.M. assisted in the experiments. K.H., Y.W. and Q.K.X. prepared the manuscript. All authors have read and approved the nal version of the manuscript.
Additional information
Supplementary Information accompanies this paper at http://www.nature.com/ naturecommunications
Competing nancial interests: Th e authors declare no competing nancial interests.
Reprints and permission information is available online at http://npg.nature.com/ reprintsandpermissions/
How to cite this article: Zhang, J. et al. Band structure engineering in (Bi 1x Sb x )2Te 3 ternary topological insulators. Nat. Commun. 2:574 doi: 10.1038/ncomms1588 (2011).
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Abstract
Topological insulators (TIs) are quantum materials with insulating bulk and topologically protected metallic surfaces with Dirac-like band structure. The most challenging problem faced by current investigations of these materials is to establish the existence of significant bulk conduction. Here we show how the band structure of topological insulators can be engineered by molecular beam epitaxy growth of (Bi1-x Sbx )2 Te3 ternary compounds. The topological surface states are shown to exist over the entire composition range of (Bi1-x Sbx )2 Te3 , indicating the robustness of bulk Z2 topology. Most remarkably, the band engineering leads to ideal TIs with truly insulating bulk and tunable surface states across the Dirac point that behaves like one-quarter of graphene. This work demonstrates a new route to achieving intrinsic quantum transport of the topological surface states and designing conceptually new topologically insulating devices based on well-established semiconductor technology.
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