www.sciencemag.org/cgi/content/full/1173034/DC1

Supporting Online Material for Experimental Realization of a Three-Dimensional Topological Insulator,

Bi2Te3

Y. L. Chen, J. G. Analytis, J. H. Chu, Z. K. Liu, S.-K. Mo, X. L. Qi, H. J. Zhang, D. H. Lu , X. Dai, Z. Fang , S. C. Zhang, I. R. Fisher, Z. Hussain, Z.-X. Shen*

*To whom correspondence should be addressed. E-mail: zxshen@stanford.edu

Published 11 June 2009 on Science Express

DOI: 10.1126/science.1173034

This PDF file includes:

Materials and Methods Figs. S1 to S4 References

Supporting online materials for Experimental Realization of Three Dimensional Topological

Insulator Bi2Te3

Y. L. Chen, J. G. Analytis, J. H. Chu, Z. K. Liu, S.-K. Mo, X. L. Qi, H. J. Zhang, D.H. Lu, X. Dai, Z. Fang, S. C. Zhang, I. R. Fisher, Z. Hussain and Z.-X. Shen

This file includes:

SOM A: Materials and Methods

SOM B: Details on momentum distribution curve fitting for surface state dispersion

SOM C: Electric transport measurements

References

Figs. S1 to S4

SOM A: Materials and Methods

A1: Sample growth:

Single crystals were grown via a self flux technique. Elemental starting materials in the

molar ratio of (1-δ):δ:4 = Bi:Sn:Te were placed in alumina crucibles and sealed under vacuum in

a quartz ampoule. The mixture was heated to 700°C and held for 2 hours, then slowly cooled

down to 475°C over 90 hours, at which temperature the remaining flux was decanted using a

centrifuge. The crystals have a plate-like morphology with hexagonal edges, and grow up to

several milimeters on a side. The c-axis is perpendicular to the natural cleavage plane of the

plates.

A number of defects are common in Bi2Te3 crystals including Bi and Te vacancies,

antisite defects Bi/Te and interstitial or substitutional impurities. The defect that predominates is

very sensitive to the molar ratio of the starting materials and decanting temperature (S1). In the

present case, as-grown Bi2Te3 crystals are n-doped due to Bi vacancies and Te/Bi antisites.

Varying Sn concentration with fixed growth conditions allowed us to dope the system from n-

type to p-type conductivity. We estimate from Hall measurements shown in Fig. S4, that the holes

doped per Sn atom in the melt is approximately 1 for n-type Bi2Te3, and 0.5 for p-type Bi2Te3.

A2: Angle resolved photoemission spectroscopy (ARPES) measurement

ARPES measurements were performed at beamline 10.0.1 of the Advanced Light Source

(ALS) at Lawrence Berkeley National Laboratory and beamline 5-4 of the Stanford Synchrotron

Radiation Lightsource (SSRL). At both facilities, measurement pressure was kept <3x1011 Torr,

and data were recorded by Scienta R4000 analyzers at 15K sample temperature. The total

convolved energy and angle resolutions were 10meV/16meV and 0.2°/0.2° (i.e. <0.007(1/Å) or

<0.012(1/Å) for photoelectrons generated by 20eV and 48eV photons) at SSRL/ALS,

respectively. The ARPES measurement scheme is demonstrated in Fig. S1, with the SSRL 5-4

endstation having two sample rotational degrees of freedom (θ, φ) and the ALS 10.0.1.1

endstation having an additional azimuthal sample rotation (ω) and analyzer rotation (α). Majority

of the data were measured at ALS with a photon energy of 48eV, at which the cross-section for

both surface state and bulk bands are strong. The broad map in Fig. 1E was measured at

55eV(ALS) for larger k-space access; and the photon energy dependent measurement in Fig. 1F

was performed at SSRL with 19, 21 and 23eV photons.

The fresh surface for ARPES measurement was obtained by cleaving the sample in situ

along its natural cleavage plane (between quintuple layer sandwich structures, see Fig. 1A in

main text) (S2). Bulk and surface state bands were both seen immediately after the cleave for all

four different dopings. Absorbed residue gas molecules from the UHV environment on sample

surface gradually degraded the sample surface, introducing slight electron doping on the surface

of this material which caused rising of the Fermi energy (EF). Our observations indicated that the

degradation for the 0% and 0.27% doped samples was negligible over a 24 hr measurement

period; while the 0.67% and 0.9% doped samples showed observable surface doping within 24

hrs. Figure S2 shows an example of the aging effect on the 0.67% doped sample, where the aging

test shows that between 1hrs to 4hrs after cleaving, the EF shifted up by about 10meV; after 20

hrs, EF was raised by about 65meV and touched the bulk conduction band (BCB) minimum. This

evolution of the EF is accompanied by the corresponding evolution of the Fermi surface (FS). The

upward shift of the EF position slowed down noticeably after reaching the BCB. This behavior is

consistent with the bulk-insulating behavior of the 0.67% sample. Due to its two-dimensional

nature, the surface state has much smaller density of states compared to that of the bulk states.

Consequently, in the bulk-insulating region, EF can be effectively raised upon slight n-doping,

while after the bulk electron pocket emerges, it takes much more doping to raise EF to the same

amount due to the much larger bulk electron densities. For this reason, all the data of 0.67% and

0.9% doped samples presented in the main text of the report were acquired within 4 hrs after

sample cleave and the data of 0% and 0.27% doped samples were taken within 12 hrs after

cleaving.

We note here that the aging effect was also observed in a previous ARPES study on the

p-type sample (S3), where the author showed the surface state emerging minute after cleave, and

the up-shift of the EF was much quicker (~200meV in 1hr) than what we observed. This could be

caused by possible existence of certain residue gas molecules or active metal contamination in

their measurement environment that were more effective at doping electrons unto the sample

surface.

SOM B: Details on momentum distribution curve (MDC)

fitting for surface state dispersion

The linear dispersion of the surface state band was obtained by MDC line shape fitting

(S4) (demonstrated in Figure S3). At binding energy 0.15~0.25eV where there are only two

surface state dispersion branches without the bulk bands' interference, two Lorentzian peaks were

used to fit each MDC curve (Fig. S3A(i), b(i)), from which the fitted peak positions were

combined to obtain the band dispersions. A linear fit to the band dispersions was then used to

determine the Dirac Fermion velocity (Fig. 3A(iii),b(iii)), resulting in 2.67eV·A (4.05×105m/s)

and 2.55eV·A (3.87×105m/s) along Γ-K and Γ-M directions, respectively. The crossing point of

the two dispersion branches thus defines the Dirac point position (Eb=0.34eV).

SOM C: Electric transport measurements

The electric transport measurements were carried out on the same batches of samples as

measured by ARPES. Crystals were cut into bars with well-defined dimensions and contacts

made using 25μm gold wires and conductive silver epoxy or indium solder. For the Hall

measurements, the contact configuration is shown in the inset of Fig. S4A, though for most

samples we measured the Hall voltage in two different places to minimize contact alignment

errors. Resistivity measurements used a standard 4-point contact configuration, as shown in the

inset of Fig. S4B. From the Hall voltage we were able to extract an effective three-dimensional

carrier density ne as a function of the Sn doping δ (Fig. S4A). The carrier densities extracted for

samples above and below δ=0.67% agree well with literature results (S1,S5). Transport and

photoemission data are in broad agreement, confirming that Sn doping drives n-type Bi2Te3 into

p-type conductivity, with a greatly reduced carrier density at intermediate doping.

References:

S1 C. B. Saiterthwaite and R. W. Ure Jr., Phys. Rev., 108, 1164 (1957)

S2 R. W. G. Wyckoff, Crystal Structures Krieger, London, 1986,Vol. 2. S3 H.J. Noh, et al., Europhys. Lett. 81 57006 (2008)

S4 J. Osterwalder Surf. Rev. Lett. 4 (1997) 391

S5 M.K. Zhitinskayaa, S.A. Nemova, T.E. Svechnikovab, Mater Sci Semicond Process 6,

449 (2003)

Fig. S1: Illustration of the ARPES measurement scheme. Photons from synchrotron beamlines are

directed onto the sample surface, ejecting photoelectrons. To access band structure at different

points in k-space, the sample or analyzer is rotated so that photoelectrons from different angles

can be detected and their corresponding momentum and energy analyzed. At beamline 5-4 at

SSRL, there are two rotational degrees of freedom on the sample (θ, φ), and at beamline 10.0.1,

there are two additional rotational degrees of freedom (ω, α).

Fig. S2: Aging test of undoped Bi2Te3 sample. Panel (A~C) are data recorded at t=1hr, 4hrs, and

20hrs after sample cleave, respectively. Upper row: measured FSs (symmetrized according to

crystal structure). Bottom row: band dispersion along the Γ-K direction (marked by white dashed

lines in the corresponding FSs on the top row). The three dispersions are aligned at the Dirac

point position, with the three EF positions indicated by the solid green lines.

Fig. S3: Band dispersions extracted from MDC line shape analysis. A(i): An example of the

excellent two Lorentzian-peak fit of an MDC at Eb=0.2eV, indicated by a green dashed line in

A(ii). A(ii): Photoemission intensity plot of the band dispersion along the K-Γ-K direction. A(iii):

Both branches of dispersions extracted from MDC line shape analysis (red dots) and the linear

fits (blue line) used to determine the Dirac Fermion velocity (2.67eV·A or 4.05×105m/s) and

Dirac point energy position (0.34eV). B(i~iii): Similar results as in A(i~iii) for the dispersion

along the M-Γ-M direction.

Fig. S4: Electric transport measurements on samples of four different dopings. (A) Carrier

density determined by Hall coefficient measurement. Red symbol indicates the value for 0.67%

doped sample, for which ARPES measurements show no bulk FS pocket (see text). (B)

Resistivity measured by four-probe methods. Insets in (A), (B) show the measurement schematics.