Ultra-low carrier concentration and surface-dominant transport in antimony-doped Bi2Se3 topological insulator nanoribbons

Abstract

A topological insulator is the state of quantum matter possessing gapless spin-locking surface states across the bulk band gap, which has created new opportunities from novel electronics to energy conversion. However, the large concentration of bulk residual carriers has been a major challenge for revealing the property of the topological surface state by electron transport measurements. Here we report the surface-state-dominant transport in antimony-doped, zinc oxide-encapsulated Bi₂ Se₃ nanoribbons with suppressed bulk electron concentration. In the nanoribbon with sub-10-nm thickness protected by a zinc oxide layer, we position the Fermi levels of the top and bottom surfaces near the Dirac point by electrostatic gating, achieving extremely low two-dimensional carrier concentration of 2×10¹¹ cm^-2 . The zinc oxide-capped, antimony-doped Bi₂ Se₃ nanostructures provide an attractive materials platform to study fundamental physics in topological insulators, as well as future applications.

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Received 8 Sep 2011 | Accepted 2 Mar 2012 | Published 27 Mar 2012 DOI: 10.1038/ncomms1771

A topological insulator is the state of quantum matter possessing gapless spin-locking surface states across the bulk band gap, which has created new opportunities from novel electronics to energy conversion. However, the large concentration of bulk residual carriers has been a major challenge for revealing the property of the topological surface state by electron transport measurements. Here we report the surface-state-dominant transport in antimony-doped, zinc oxide-encapsulated Bi2Se3 nanoribbons with suppressed bulk electron concentration. In the nanoribbon with sub-10-nm thickness protected by a zinc oxide layer, we position the Fermi levels of the top and bottom surfaces near the Dirac point by electrostatic gating, achieving extremely low two-dimensional carrier concentration of 21011 cm 2. The zinc oxide-capped,

antimony-doped Bi2Se3 nanostructures provide an attractive materials platform to study fundamental physics in topological insulators, as well as future applications.

Ultra-low carrier concentration and surface-dominant transport in antimony-doped Bi2Se3 topological insulator nanoribbons

Seung Sae Hong1, Judy J. Cha2, Desheng Kong2 & Yi Cui2,3

1 Department of Applied Physics, Stanford University, 476 Lomita Mall, McCullough 343, Stanford, California 94305, USA. 2 Department of Materials Science and Engineering, Stanford University, 476 Lomita Mall, McCullough 343, Stanford, California 94305, USA. 3 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94205, USA. Correspondence and requests for materials should be addressed to Y. C. (email: [email protected]).

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The exotic electronic properties of the surface state, due to its spin-momentum-locked Dirac cone in the electronic band structure, dene a topological insulator as a unique class of

quantum matter16. Moreover, it is predicted to oer exciting physics, such as elusive quasi-particles, spin transport and fault-tolerant quantum information processing19. Bismuth selenide (Bi2Se3) and its relative compounds are one of the most promising candidates to realize the ideal three-dimensional topological insulator due to their large bulk band gap and simple surface-band structure10. Signicant advances have been made to probe the surface state in these materials by various methods, such as angle-resolved photoemission spectroscopy1113 and scanning-tunneling microscopy1416. Transport measurements in bulk crystals have demonstrated the existence of these surface states as well1719.

Nanoscale topological insulator devices, with their large surface-to-volume ratio, oer a unique opportunity to manifest the surface eect20,21. In mesoscopic length scale, transport measurements may reect the fundamental nature of carriers, as shown in previous cases like graphene22,23. However, it is still challenging to study the topological surface states in the device level, as material imperfections in the bulk blur transport signatures of the surface state and limit further in-depth studies. The major challenge is the dominance of bulk carriers outnumbering surface-state carriers, mainly originating from intrinsic Se vacancies2427. Moreover, the material is very sensitive to environmental contaminationseveral studies observed that environmental exposure (H2O, O2) causes material degradation and additional bulk carrier generation17,28.

In this paper, we provide two eective solutions to eliminate both intrinsic and extrinsic sources of bulk carrier generation. First, we demonstrate vapour-phase antimony (Sb) doping in Bi2Se3 nanoribbons systematically suppresses the bulk conductivity. Second, we show that a zinc oxide (ZnO) capping layer on Sb-doped nanoribbons can inhibit extra carrier generation induced by extrinsic contamination. Last, in an ultrathin device with Sb doping and ZnO capping, electrostatic manipulation allows us to locate the Fermi levels of both surfaces close to the Dirac point. The eective removal of bulk carriers achieves an ideal surface-transport state that is desirable for many applications of topological insulators.

ResultsVapour-phase Sb doping and bulk carrier suppression. To suppress the bulk conductivity in topological insulator nanoribbons, we synthesize Bi2Se3 nanoribbons with Sb doping. Sb is known as an eective compensation dopant to reduce bulk electron density to below 1017 cm 3 in bulk crystals without destroying the topological surface state17,29. Bi2Se3 nanoribbons are synthesized via vapour

liquidsolid growth mechanism using gold particles as catalysts20,30, and Sb vapour is introduced by the evaporation of an Antimony selenide (Sb2Se3) powder placed at the lower temperature zone (Fig.

1a). As-grown ribbons are typically 50300 nm thick, 200 nm to several micrometres wide, and up to tens of micrometres long (Fig. 1b). Sb-doped Bi2Se3 nanoribbons are in a single crystalline rhombohedral phase (Fig. 1c), the same as undoped Bi2Se3 nanoribbons. The distribution of Sb dopants is spatially uniform in the nanoribbon, as conrmed by energy-dispersive X-ray spectroscopy (EDX; Fig. 1c,d). EDX spectra show simultaneous decreasing intensity of Bi peaks and increasing intensity of Sb peaks with higher Sb dopant concentrations (Supplementary Fig. S1), suggesting the doping mechanism is likely the substitution of the Bi atoms by the Sb atoms. Assuming of the substitutional doping, we calculated the Sb doping level, ranging from 0 to 7%, in atomic ratio.

The basic carrier types and densities of individual nanoribbons are measured by nanodevices with Hall bar geometry. Samples with dierent Sb concentrations (Table 1) all show an n-type carrier-dominant transport, as their Hall resistances are negative values. By increasing the Sb dopant concentration, the sheet resist-

a

b

Temperature gradient (C)

~400

Ar flow Au

540 ~350

Dopant (Sb2Se3)

Bi2Se3

c

d

Sb

Bi

Se Sb + Bi + Se

Figure 1 | Synthesis and material characterization of Sb-doped Bi2Se3 nanoribbons. (a) A schematic of vapourliquidsolid growth of Sb-doped

Bi2Se3 nanoribbons. By shifting the dopant source location along the temperature gradient in the tube furnace, the relative vapour pressuresof two sources and the incorporated dopant level are controlled. (b) An scanning electron microscopy image of as-grown nanoribbons. Scalebar equals 10 m. (c) A transmission electron microscopy image of a nanoribbon and its high-resolution image (inset). The rectangular box indicates the scanned area for EDX mapping. Scale bar equals 1 m (Scale bar in the inset equals 2 nm). (d) Elemental maps of Sb (red), Bi (green), Se (blue) and overlaid RGB image by scanning EDX. The overlaid map looks bluish, as the Bi and Se signals are stronger than the Sb signal. The scanning map indicates homogeneous dopant distribution of 6% (atomic ratio) Sb with 0.5% s.d.

Table 1 | Transport parameters of Bi2Se3 nanoribbon samples (not enclosed by ZnO layer) of different Sb concentrations (T=2 K, zero gate voltage).

Sample number

Sb doping (atomic %)

Sheet resistance,

Rs ()

Hall resistance,

RH (/T)

(m cm) H

(cm2 VS 1)

B1 0 43 7.4 84.5 71 0.3 1,721 B2 2 177 14.7 42.5 70 1.2 830 T1 4 422 39.9 15.7 53 2.2 948 T2 6 975 51 12.3 120 11.7 523 T3 7 1,350 63.1 9.9 83 11.2 467

Hall carrier density, nH

(1012 cm 2)

Thickness (nm)

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a b

ance increases more than an order of magnitude and the electron density is dramatically reduced, implying the signicant decrease of bulk electron contribution (Table 1). At a high Sb-doping concentration (67 atomic %), the carrier density is ~1013 cm 2. Considering the electron density of the surface states from the top and bottom surfaces near the bulk conduction band edge is ~1013 cm 2 (Supplementary Fig. S2), surface carriers are now expected to have the dominant role in transport of these Sb-doped nanoribbons. As a result, the two-dimensional (2D) carrier densities of nanoribbon devices are thickness-independent when the Sb doping is close to its maximum (67%; Supplementary Fig. S3). The surface-dominant transport is also supported by the comparable values of the averaged carrier mobility (Hall mobility) and eld-eect mobility from the surface states in low-density samples (Supplementary Table S1).

Additional electronic transport studies conrm that the bulk electron contribution is reduced signicantly by the Sb doping. In Fig. 2a, temperature-dependent resistances from low Sb concentration samples (Sb 02%) follow typical metallic behaviour. In contrast, for the samples of high Sb concentration (Sb > 4%), the resistance starts to increase and saturates at low temperature. An increase in resistance upon reducing temperature is likely due to the freeze-out of the bulk carriers. Moreover, electrostatic gating experiments in eld-eect transistor devices manifest drastic difference between low and high Sb concentration samples (Fig. 2b). Low Sb-doped samples (Sb 02%) show weak gating dependence by a bottom gate, as the ribbon thicknesses ( > 70 nm) are much larger than the depletion layer thickness (~10 nm) with a relatively high carrier concentration. However, the gating response becomes larger with the increasing Sb-doping concentration. The sample with high Sb doping (T2, T3) exhibits large increase of its resistance and signicant decrease in bulk carrier concentration.

The conductance of this ribbon decreases more than half by electrostatic manipulation via the bottom gate in spite of its large thickness (120 nm, T2), which strongly suggests the increase of the depletion layer depth (~35 nm) and the suppression of the bulk transport contribution. Its depletion-layer thickness can be estimated by two dierent ways, either by the ThomasFermi screening theory (for high carrier concentration) or solving the Poisson equation of charge-carrier density (for low carrier concentration). As the carrier density is low in most samples, we calculate the depletion layer thickness by solving band bending and do not consider the ThomasFermi screening theory. For undoped samples, the estimated depletion layer is ~10 nm, whereas for Sb-doped Bi2Se3

nanoribbons of low carrier concentration, it is ~35 nm, according to the approximated expression zd2 = 20E/e2n. In the formula,

is the dc dielectric constant of Bi2Se3 (113), E is the band-bending energy by gating, and n is the carrier density24,31. This number is based on the assumption that the carriers are distributed uniformly along the ribbon at zero gate voltage (Vg).

Magnetotransport data shows the emergence of the surface state and the suppression of bulk electrons by Sb doping. In the high-eld magnetoresistance (MR), Shubnikov-de Haas (SdH) oscillations are observed in the samples of dierent Sb concentrations (Fig. 2c). The oscillations do not depend on the Vg, which suggests that the oscillations originate from bulk electrons (Supplementary Fig. S4).

Without Sb doping, the small periodicity (BFFT = 93 T) of SdH oscillations in an inverse magnetic eld corresponds to the large cross-sectional area of the bulk Fermi surface in a highly metallic sample with excessive bulk carriers. As the Sb concentration increases, the oscillation period gets larger (BFFT = 22 T) and disappears at high doping levels, because the bulk Fermi surface eventually becomes too small to be measured in our magnetic eld range (8 T). The small cross section of the bulk Fermi surface corresponds to the decrease of the contribution of bulk electrons.

It is reasonable to ask whether the heavy Sb doping dramatically degrades bulk electron mobility, so that the oscillation disappears.

A previous study on bulk crystals showed strong bulk electron SdH oscillations observed from samples of heavy Sb doping, implying high electron mobility despite the large amount of dopants17. We also test the eect of doping on electron mobility, starting with a high Sb concentration sample (T3, without the ZnO layer initially) of low carrier concentration (1013 cm 2). Within the 8 T range, we could not see any oscillatory features comparable to the SdH oscillations. Then, we put the ZnO layer (~10 nm) to protect

1,600

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R s ()

R s ()

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570

R s () R / R (%)

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200 250

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60

80 40 20 0

Vg (V)

c d

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R/R

0.35

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1/B (T 1)

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B (T)

B cos (T)

0.5 1.0

e f

B

570

560 560

550

1.0 0.5 0

0.5 1.0

B (T)

Figure 2 | Transport in nanoribbons without ZnO layer capping. (ad) Transport measurement on nanoribbons of different doping concentration: undoped (B1, black), Sb 2% (B2, blue), Sb 4% (T1, purple), Sb 6% (T2, red), Sb 7% (T3, green). (a) The resistance prole versus temperature from Bi2Se3 nanoribbons. We note that the curves for 4 and 7% Sb doping levels are from different devices of the same growth batch. (b) Gating response of the resistance from Bi2Se3 nanoribbons (SiO2 300-nm back gate). The resistance of high Sb concentration devices (purple, red, green)

shows maximum peaks, indicating that the bottom surface switches from n-type to p-type. In general, devices of higher Sb concentrations reach the maximum peak at lower gating voltages, which suggests the Fermi level of the bottom surface to be closer to the Dirac point. (c) High-eld MR versus inverse magnetic eld (1/B). Background curve (either linear or parabolic) is subtracted from original MR curve. SdH oscillations from an undoped sample (BFFT~93 T) and 2% Sb sample (BFFT~22 T) correspond to bulk electron densities of ~61018 and 61017 cm 3, respectively. The arrow indicates 1% magnitude of total resistance. The curves are displayed with an offset for clarity. (d) MR near zero magnetic eld, showing clear feature of weak anti-localization from samples of high Sb concentration (Sb > 3%). The arrow indicates 2% magnitude of total resistance. Each curve is normalized by zero eld resistance and displayed with an offset for clarity. (e) Angle-dependent MR (~5% Sb-doped sample, nH = 21013 cm 2,d = 100 nm) with seven different eld orientations: 0 (black), 15 (red), 30 (lime), 45 (blue), 60 (orange), 75 (purple), 90 (grey). Inset: a cartoon to describe the angle denition in the experiment. (f) MRwith different eld orientations, as a function of the perpendiculareld component. All measurement are conducted at T = 2 K except temperature-dependant studies.

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the device from environmental doping during heating, and the sample was heat-treated at 100 C for 4 h in moderate vacuum (~1 Torr argon gas-lled chamber). Aer the heat treatment, carrier concentration increased a lot (2.51013 cm 2) and the SdH oscillations (BFFT = 17 T, EF = 16 meV above the conduction band edge)

emerged (Supplementary Fig. S4). A temperature of 100 C may not be regarded as a high-enough temperature to improve crystal quality, as we observed large carrier density increase from other samples, presumably by creating more Se vacancies. In this device, we also conrm that the Hall carrier density increases substantially. Therefore, we conclude that the electron mobility does not degrade by doping, and the fading of SdH oscillations is because of the elimination of bulk electrons.

Weak anti-localization (WAL), the quantum correction in the spin-orbit-coupled surface state25,32, is absent in the MR trace of the undoped sample (Fig. 2d, B1), as the large bulk electron contribution masks the surface-state transport. Samples with higher Sb concentration (Fig. 2d, T1, T2, T3), in contrast, manifest the characteristic WAL as the sharp cusp near zero magnetic eld due to the suppression of bulk conductivity. We conduct an angle-dependent study to verify the dominance of the 2D component in WAL; MR of a nanoribbon (Sb~5%) was measured at dierent eld orientations (Fig. 2e). The cusp is the sharpest with the magnetic eld perpendicular to the basal plane and becomes atter as the angle () increases. When the MR from dierent eld orientations are plotted as a function of the perpendicular eld (B cos; Fig. 2f), they overlay nicely with the 0 MR, implying the observed WAL is essentially controlled by the 2D component. Additional tting procedures are discussed in the Supplementary information (Supplementary Fig. S5, Supplementary Methods).

ZnO protective layer. So far, we have shown that Sb doping eectively reduces bulk carriers. However, nanoribbons are still exposed to extrinsic contaminations during device fabrication, which can increase the bulk carrier concentration as well17,28. Therefore, we hypothesize that the intrinsic carrier density of the Sb-doped Bi2Se3

should be much lower than that measured, and a protection layer is needed to access the intrinsic carrier density. We use ZnO for the protective layer due to the insulating properties of intrinsic ZnO, and its chemical stability against moisture and standard solvents in a wide range of temperature. Also, it can be deposited by a sputtering process without elevation of substrate temperature, avoiding additional Se vacancies generation upon heating. The sputtered ZnO layer covers the entire surface of the nanoribbons, which prevents degradation and extrinsic doping associated with the standard fabrication process. We compare carrier densities of 20 samples (10 with ZnO, 10 without ZnO capping) of dierent Sb concentrations (Supplementary Fig. S6). On average, the ZnO-capped samples exhibit lower carrier densities than uncapped devices of similar Sb concentrations. Also, ZnO capping improves both the Hall mobility and the eld-eect mobility. For samples of high Sb concentration (Sb = 67%), the average Hall mobility of uncapped devices is ~500 (cm2 V 1s 1), whereas the average Hall mobility of ZnO-protected devices is ~750 (cm2 V 1s 1).

In a bottom-gate device (Fig. 3a) fabricated on an Sb-doped, ZnO-protected nanoribbon of 200 nm in thickness, the carrier density is very low (~51012 cm 2), and the gate-dependent resistance shows that the Fermi level of the bottom surface is close to the Dirac point even without applying the Vg (Table 2). In Fig. 3b, its resistance decreases by either direction of the gating voltage, and the Hall resistance increases due to the electrostatic accumulation of charge carriers at the bottom surface (p-type carriers for negative Vg and n-type carriers for positive Vg).

The gate-dependent temperature curve study also conrms that the Fermi level of the bottom surface is close to the Dirac point. The temperature-dependent resistance shows a non-metallic behaviour

at zero Vg, with the increase of resistance by lowering the temperature, and saturating at low temperature. By introducing holes with the negative Vg (Fig. 3c), resistance starts to drop down by cooling (T < 120 K), indicating the generation of bulk carriers, whereas the general behaviour does not change by inducing more electrons (positive Vg, Fig. 3d). The asymmetric temperature-dependent transport by dierent gating reects the characteristic band structure near

the Dirac point of Bi2Se3. From the angle-resolved photoemission

a

b

Bi2Se3

SiO2 300 nm

ZnO

Rs ()

RH (/T)

(2.11013 cm2)(1.01013 cm2)(6.91012 cm2)(5.21012 cm2)

1,400

0 30 60 90 120

Si (n-type)

1,200

1,000

50 25 0 25 50

EF

EF

75

Vg (V)

c

1,400

1,200

1,000

800

1,400

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1,000

800

0 50 100 150

T (K)

200 250

Gate

Gate

d

R s ()

d

R s ()

d

0 50 100 150 T (K)

200 250

Figure 3 | Transport in a thick Sb-doped Bi2Se3 nanoribbon with ZnO layer. (a) Device (Z1) schematic (top) and optical image (bottom) of a thick sample (200 nm) of Sb 7% concentration. Scale bar equals 5 m.

(b) Gate-voltage dependence of longitudinal sheet resistance (RS)and Hall resistance (RH) measured at low eld (B < 2 T) at T = 2 K. Hall resistance increases in both direction of Vg, implying that the Fermi level of the bottom surface is near the Dirac point at zero gating voltage.

The anomalous kink in the longitudinal resistance curve is not well understood, and it is not reproducible in other samples. (c) Temperature-dependent resistance curve at different Vg (negative): 0 V (black), 10 V (blue), 20 V (teal), 40 V (green), 60 V (lime), 80 V (grey). The curve changes signicantly as more bulk holes are added, reecting the conventional metallic temperature dependence of induced carriers. A band diagram (inset) shows band bending at the bottom surface induced by gating. (d) Temperature-dependent resistance curve at different Vg

(positive): 0 V (black), + 10 V (maroon), + 20 V (red), + 40 V (pink), + 60 V (purple), + 80 V (blue). In a band diagram (inset), the Fermi level does not cross any bulk band by positive gating, explaining the qualitatively similar temperature curves over the wide range of gating voltage.

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Table 2 | Transport parameters of Bi2Se3 nanoribbon samples enclosed by ZnO layer (T=2 K, zero gate voltage)

Sample number

Sb doping (atomic %)

Sheet resistance,

Rs ()

Hall resistance,

RH (/T)

(m cm) H (cm2 V 1s 1)

Z1 7 1,350 122 5.1 200 27 903 Z2 5 4,760 280 2.8 6 2.9 470

Hall carrier density, nH

(1012 cm 2)

Thickness (nm)

spectroscopy studies11,32, the Dirac point of surface states is just above the bulk valence-band edge and far apart from the bulk conduction band edge (~0.2 eV). Within the range of Vg ( + 80 V), positive gating only accumulates more electrons from the bottom surface band of the thick nanoribbon (inset of Fig. 3d), which does not change the overall shape of the temperature curve. In contrast, bulk electronic states are easily populated from the valence band by negative gating (inset of Fig. 3c), and the ribbon is in a mixed state with the coexistence of electrons and holes (Supplementary Fig. S7). This temperature-dependent study conrms the independent tuning of the Fermi level of one surface near the Dirac point. Such a material system may allow the creation of a topological insulator junction of both types of carriers or a single surface junction to study novel proximity eects in the future8,9.

Electrostatic manipulation of the Fermi level in an ultrathin device. In addition to the manipulation of the bottom surface in the thick ribbons by substrate gating, the most attractive step is the exible manipulation of the Fermi levels of both top and bottom surfaces, to position them close to the Dirac point. Such a control by electrostatic gating requires the nanoribbons to be thinned down. We etched a thick, Sb-doped ribbon (~100 nm) by argon plasma in a sputtering machine, and in situ deposited a 15-nm thick ZnO protection layer. In Fig. 4a, the ribbon is semitransparent with a thickness of 6 nm, more than 30 times thinner than the previous device (Fig. 3). This device has a very low electron concentration of 2.81012 cm 2 at zero Vg (Fig. 4b), roughly four times lower than that (~1013 cm 2) of the maximum density for pure surface conduction in the bulk band gap (Table 2). Accordingly, the Fermi levels of both the top and bottom surfaces are completely within the bulk band gap. In other words, the Fermi level crosses only the surface Dirac cone above the Dirac point.

In this ultrathin nanoribbon with very low carrier concentration, its small thickness of 6 nm makes it possible to shi the Fermi level of the entire ribbon by electrostatic gating across the Dirac point. By sweeping Vg from positive to negative bias, its longitudinal resistance (Rxx) initially increases and reaches a peak value (~7 k) around Vg

of 50 V, and then decreases when further applying negative gate bias (black curve, Fig. 4b). The corresponding Hall slope (red curve, Fig. 4b) increases by more than ten times, and then switches the sign when Rxx reaches the peak value. These results clearly demonstrate the ambipolar eld eect and suggest that the entire sample is converted from n-type to p-type. The sample conductance depends on the gating voltage linearly, except at the plateau of minimum conductance (~3.6G0, equivalent to 2D conductivity of 0.9 G0, where

G0 is e2/h; Fig. 4c). The absence of zero conductance region during the ambipolar transition is due to the gapless surface states. The carrier density obtained by the Hall resistance, shown in Fig. 4d, also linearly depends on Vg. The sample remains purely n-type until

Vg = 50 V, switches to a mixed carrier state in the range of 65 V < Vg < 50 V, and eventually to p-type when Vg < 65 V (Fig. 4e).

Discussion

In the ultrathin device, the extremely low carrier density of nH = 21011 cm 2 observed at Vg of 51 V indicates the Fermi

a b

Bi2Se3 ZnO

8 K

2 K

(3.11011 cm2)

2 k

SiO2 300 nm

Si (n-type)

6 K

0

R xx ()

4 K

2 K

0 80 60 40 20 0

Rxy ( / T)

4 K

(1.61011 cm2)

Vg (V)

c

d 80

30

G (e2 /h)

40

60

40

11 cm2 )

20

n H(10

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Vg (V) Vg (V)

e

nH (1011 cm2)

Bottom

Top

10

1

70 65 60

1 55 50

Vg (V)

10

Figure 4 | Transport in an ultrathin Sb-doped Bi2Se3 nanoribbonwith ZnO layer. (a) Device (Z2) schematic (top) and optical image (bottom) of thin ribbon sample (6 nm) of Sb 5% concentration. Scale bar indicates 3 m. (b) Vg dependence of longitudinal resistance (Rxx) and

Hall resistance (Rxy) measured at low magnetic eld (B < 1 T), at T = 2 K. Hall resistance decreases as n-type carriers are depleted (Vg > 50 V);

increases as induced p-type carriers from the bottom surface form a mixed state with decreasing n-type carriers ( 65 V < Vg < 50 V); and decreases again as the entire sample becomes a hole conductor (Vg < 65 V).

(c) Conductance (G) versus Vg curve is linear, except near the minimum conductance (equivalent to the conductivity of 0.9 G0). (d) Electron density (nH) plot as a function of Vg. It depends on Vg linearly in the wide range of voltage ( 90 V to + 80 V). (e) Semi-log scale plot of carrier density near its charge neutrality point (Dirac point). Band diagrams of top and bottom surfaces (inset) for samples with pure p-type conduction (left), mixed conduction (middle) and pure n-type conduction (right).

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levels of both surfaces simultaneously approach the Dirac point (Supplementary Fig. S2). The removal of bulk electrons in topological insulator nanostructures achieves this stringent condition to study interesting physics near the Dirac point by transport. For example, the dimensional crossover by the top and bottom surface hybridization creates the surface band gap at the Dirac point and may possess exotic edge states13,33,34. Massive Dirac Fermions35 and a new type of Hall eect36, by incorporation of magnetic dopants, will be another important physics near the Dirac point. Our experiment opens up the possibility that such interesting physics can be studied in nanoscale device transport, providing the material tunable near the Dirac point without excessive residual carriers. We also note that some studies propose that inhomogeneous Coulomb potential near the Dirac point26,37 can contribute to the minimum carrier density, similar to the case of graphene22,23. It would be another interesting topic in topological insulator transport, and further study is expected to understand the observed minimum conductivity.

Methods

Material synthesis and device fabrication. The synthesis of Sb-doped Bi2Se3 nanoribbons was carried out in a 12-inch horizontal tube furnace with a quartz tube. Bi2Se3 source powder (99.999%) from Alfa Aesar was placed in the centreof the furnace; Sb2Se3 source powder (99.999%) was placed at an upstream lower temperature zone; and the growth substrate, silicon wafer with thermally-evaporated 10 nm Au lm, was placed at a downstream zone. High-purity argon gas was used to convey vapour from the source materials to a growth substrate at 130-s.c.c.m ow rate. During the entire growth time of 1.5 h, 1 Torr pressure and 540 C centre zone temperature were maintained. The Sb dopant concentration in nano- ribbons can be controlled by varying the Sb2Se3 source powder temperature

using the temperature gradient in the tube furnace; we placed the Sb2Se3 source powder at dierent locations of the tube furnace, resulting in variation of the

Sb2Se3 source powder temperature. The estimated temperatures of the Sb2Se3 source and the growth substrate are 450 and 350 C, respectively, for the highest

Sb concentration sample. Aer the vapourliquidsolid growth, nanoribbons were directly transferred onto a doped silicon substrate with a 300-nm silicon dioxide layer. Then, a 15-nm thick ZnO layer was deposited by sputtering to cover the nanoribbon samples. In case of the thin ribbon sample, nanoribbons were etched by argon/metal plasma and capped by the ZnO layer in situ. The chemical composition and the surface roughness did not change with the Ar plasma etching, which were checked with Auger electron spectroscopy and atomic force microscopy (AFM). The nanoribbon devices were fabricated by standard e-beam lithography, and the ZnO layer on the contact area was etched by diluted base solution (tetramethyl-ammonium hydroxide) followed by thermal evaporation of Cr/Au contact (5 nm/80 nm). The samples were stored in an N2 glovebox (H2O, O2 < 1 p.p.m.) at room temperature between processes. The device is usually measured within 1 or

2 days aer device fabrication and 2 to 3 days aer nanoribbon synthesis.

Cryogenic transport measurement. All transport measurements (except angle-dependant transport) were carried out in an Oxford 4He cryostat with superconducting magnet, using low-frequency (~2001 kHz) AC technique by digital lock-in ampliers (Stanford Research Systems SR830) with current-driven conguration. Angle-dependant transport measurements were carried out in a Quantum Design PPMS-7 instrument, Janis 9T magnet He-cryostats (base temperature 2 K, low-frequency 1 kHz). A DC sourcemeter (Keithley 2400) was used to apply back Vg with negligible leakage current ( < 1 nA). All samples were measured in either Hall-bar geometry or four-terminal conguration. The Rxx

curves were obtained by measuring the four-point resistance using the standard AC lock-in set-up while sweeping the Vg continuously using the DC sourcemeter.

Sheet resistances (Rs), conductivities and resistivities were obtained by measuring the device dimension by AFM/scanning electron microscopy. For the gate-dependent Hall measurement, we measured the Hall voltage at ve dierent elds(B = 1 T, 0.5 T, 0 T, 0.5 T, 1 T) while sweeping the gate and checked that the Hall slope is linear. The base temperature for all measurements was 2 K, except during the temperature-dependant studies.

Material characterization. For structural and elemental analysis, scanning transmission electron microscopy/transmission electron microscopy (FEI Tecnai G2 F20 X-Twin microscope, acceleration voltage 200 kV) equipped with an EDX spectrometer was used. The EDX spectra obtained from nanoribbons of dierent Sb concentrations are normalized to Se peaks (11.2 keV) for comparison. Typically, the EDX acquisition time was 300 s for all spectra for good signal-to-noise ratios (Supplementary Fig. S1).

Optical images were taken with Olympus BX51M and the scanning electron microscopy image was taken with FEI XL30 Sirion. AFM data were taken with Park Systems XE-70/XE-100.

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Acknowledgements

We thank K. Lai and J. R. Williams for the helpful discussions, and B. Weil for the help in the manuscript preparation. Y. C. acknowledges the supports from the Keck Foundation, DARPA MESO project (No. N66001-11-1-4105) and the King Abdullah University of Science and Technology (KAUST) Investigator Award (No. KUS-l1-001-12).

Author contributions

S.S.H. and Y.C. conceived the experiments. S.S.H. carried out synthesis and device fabrication, and J.J.C. carried out structural characterization. S.S.H., J.J.C. and D.K. carried out transport measurements and analysis. All authors contributed to writing of the manuscript.

Additional information

Supplementary Information accompanies this paper at http://www.nature.com/ naturecommunications

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How to cite this article: Hong, S.S. et al. Ultra-low carrier concentration and surface-dominant transport in Sb-doped Bi2Se3 topological insulator nanoribbons. Nat.

Commun. 3:757 doi: 10.1038/ncomms1771 (2012).

NATURE COMMUNICATIONS | 3:757 | DOI: 10.1038/ncomms1771 | www.nature.com/naturecommunications

2012 Macmillan Publishers Limited. All rights reserved.