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Physica E

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journal homepage: www.elsevier.com/locate/physe

Robust topological surface state against direct surface contamination

Z.K. Liu a,b, Y.L. Chen a,b,c,n, J.G. Analytis a,b, S.K. Mo c, D.H. Lu a, R.G. Moore a, I.R. Fisher a,b,Z. Hussain c, Z.X. Shen a,b

a Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USAb Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, CA 94305, USAc Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

a r t i c l e i n f o

Article history:

Received 20 September 2011

Accepted 24 October 2011

77/$ - see front matter Published by Elsevier

016/j.physe.2011.10.023

esponding author at: Geballe Laboratory for Adv

cs and Applied Physics, Stanford University, Sta

650 521 4420.

ail address: chenyl@stanford.edu (Y.L. Chen).

e cite this article as: Z.K. Liu, et al.,

a b s t r a c t

Angle resolved photoemission spectroscopy (ARPES) study were performed on the (111) surface of

topological insulators Bi2Te3, Bi2Se3 and semimetals Bi and Sb. In all four materials, we observed clear

surface states centered at the G point of the surface Brillouin zone. We further studied the evolution of

these surface states under controlled surface contamination of CO and O2 gas molecules, and found that

the surface states in Bi2Te3 and Bi2Se3 were both robust against the direct influence of these gas

molecules, while the surface states of Bi and Sb could be easily destroyed under the same condition. Our

finding provides direct experimental evidence confirming the unusual robustness of the topological

surface state of topological insulators with the presence of the time reversal symmetry.

Published by Elsevier B.V.

1. Introduction

Topological insulators represent a new state of quantum matterwith a bulk gap and an odd number of relativistic Dirac fermions onthe surface. With many novel physical properties and applicationpotentials [1–5], topological insulators have grown as one of the mostintensively studied fields in condensed matter physics. In particular,the intriguing topological surface state with unique helical spinstructure [6–9] is at the focus of current researches. Arising fromthe non-trivial topology of the bulk band structure and protected bythe bulk energy gap, this unusual surface state is immune to surfaceimpurities as long as the disorder potential does not violate the timereversal symmetry [10–13], contrast to the ordinary surface states oftopologically trivial materials, which are very sensitive to the surfacedisorder and chemical environments [14].

In this work, we use ARPES to directly study the surface stateof Bi2Te3, Bi2Se3, Sb and Bi, and investigate their evolution underdirect surface influence—CO and O2 surface doping. As CO is apolarized molecule and O2 is a strong oxidizing agent, they areeffective in modifying the surface chemical environments andintroducing disorders. Given that there is no magnetic impuritiesinvolved, the topological surface states of Bi2Te3 and Bi2Se3 areexpected to be robust against these surface contaminations,contrast to those of Bi and Sb [15–17].

B.V.

anced Materials, Departments

nford, CA 94305, USA.

Physica E (2011), doi:10.10

2. Experiment details

High quality single crystals Bi2Te3 and Bi2Se3 were grown byslowly cooling a ternary melt. The mixture of Bismuth andTellurium/Selenium were sealed in the evacuated quartzampoules, with excess amount of Tellurium/Selenium to reducethe anion vacancy [18,19]. The mixture was raised to 750 1C andcooled slowly to 550 1C, then annealed for extended period. Thecrystals were cleaved perpendicular to the c-axis. High quality Biand Sb single crystals were purchased from MaTeck GmbH. Thedoping gases used in this study (O2 and CO) are of ultra-highpurity (grade 4.4) from Airgas Inc.

The ARPES measurements were carried out at Beamline 10.0.1of Advanced Light Source in Lawrence Berkeley National Lab andBeamline 5–4 in Stanford Synchrotron Radiation Lab. At bothfacilities, measurement pressure was kept o3�10�11 Torr,except during the gas doping process. ARPES data were recordedby Scienta R4000 analyzers at 15 K sample temperature; and thetotal convolved energy and angle resolutions were 10 meV/16 meV and 0.21/0.21 (i.e. o0.007(1/A) or o0.012(1/A) forphotoelectrons generated by 20 eV and 48 eV photons) at SSRL/ALS, respectively.

3. Results and discussion

Intrinsic electronic structures of the four materials studied inthis work (Fig. 1) were measured on freshly cleaved samplesurfaces. For all materials, sharp surface state band (SSB) can beclearly seen around the G point in both the Fermi surface maps

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Fig. 1. ARPES spectra of the electronic structure of various samples. (a) Fermi surface map and (b) Intensity plot of high symmetry G–K cut of Bi2Se3. (c) Fermi surface map

and (d) Intensity plot of high symmetry G–K cut of Bi2Te3. (e) Fermi surface map and (f) Intensity plot of high symmetry G–M cut of Sb. (g) Fermi surface map and

(h) Intensity plot of high symmetry G–M cut of Bi. Different types of bands (Bulk Conduction Band, Bulk Valence Band and Surface State Band) are labeled.

Z.K. Liu et al. / Physica E ] (]]]]) ]]]–]]]2

(Fig. 1(a), (c), (e), (g)) and the band dispersion plots (Fig. 1(b), (d),(f), (h)). Their bulk bands, both bulk conduction band (BCB) andbulk valence band (BVB), are characterized by the broaderdispersions due to their 3D nature (thus their spectra arebroadened due to the kz dispersions) [6,20]. It’s also obvious fromthe measurements that the SSB of topological insulators Bi2Se3

and Bi2Te3 comprises of a single Dirac cone (Fig. 1(b), (d)) whilethe SSB of Sb and Bi has the regular parabolic dispersion withrashba splitting for different electron spin polarizations (Fig. 1(f),(h)). The existence of clear SSBs in freshly cleaved surfaces of allfour materials provides a great opportunity for the comparison oftheir evolution under direct surface influence, such as surfacecontamination from gas molecules.

As the contaminants on the surface not only modify thesurface chemical environment (thus may affect the surface state),but also scatter the photoelectrons (from both the surface and thebulk states [21]); we can use the relative evolution of ARPESsignal of the SSB and BCB/BVB to differentiate these two effects: Ifthe surface states are intrinsically altered, the ARPES spectra fromthem should deteriorate faster than those from the bulk states;while if the evolution of the ARPES spectra of the surface states isdominated by the trivial scattering process, it should be similar tothe evolution of the bulk states’ spectra.

In Fig. 2, we show clearly different evolutions of the surfaceelectronic structures of Sb and Bi2Se3 under controlled CO surfacecontamination for comparison. In both cases, the freshly cleavedsample surfaces were contaminated in situ by high purity polarmolecule CO (gas pressure �10�9 Torr) introduced into the UHVchamber (Fig. 2(b)). ARPES measurements along high symmetrydirections that show both the surface and bulk band dispersionsare used for the comparison. As demonstrated in Fig. 2(a), for bothsamples, the trivial scatting effect of the adsorbed CO molecules

Please cite this article as: Z.K. Liu, et al., Physica E (2011), doi:10.10

can be seen from the broadened line-shape of both the surfaceand bulk dispersions upon increasing CO dosage.

The difference between the two samples, however, lies in thedistinct behavior of the relative evolution between the surfacestate and the bulk ones. For Sb, when the CO dosage goes beyond2L (Langmuirs, marked by the red arrow on the dosage axis), theSSB completely disappears while the BVB clearly persists—thisdifference rules out the possibility that the disappearance of SSBis simply due to the scattering of photoelectrons by adsorbed COmolecules (as the photoelectrons from BVB would experience thesame scattering as photoelectrons from SSB). In contrast to thecase of Sb, the SSB of Bi2Se3 persists at much higher COcontamination level (�7 L CO dosage), even when both the SSBand BCB/BVB dispersions are significantly broadened (and theintensity are weakened) due to severe impurity scattering fromthe large amount of CO molecules adsorbed on the surface. Thisdifference between the SSB spectra evolutions in Bi2Se3 and Sbclearly indicates the robustness of the SSB in Bi2Se3.

One may raise a question that the different behaviors betweenthe Sb and Bi2Se3 SSBs under CO contamination may result fromthe different CO adsorption coefficients on their surfaces—theactual CO molecules deposited on the Sb surface may be largerthan that on the Bi2Se3. This possibility can be easily ruled out byinvestigating the impurity photoemission spectra. In Fig. 2(c), onesees that the spectra peaks from CO around 7 and 10 eV bindingenergy grow faster in the Bi2Se3 case, indicating that the adsorbedCO molecules on the Bi2Se3 surface are actually much more thanthose on the Sb surface at the same exposing dosage.

With similar experimental setup and conditions, we show inFig. 3 the different evolutions of the SSB of Bi and Bi2Te3 undercontrolled O2 surface contamination. Despite the different origi-nal dispersions and types of gas contaminant used, the band

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Fig. 2. ARPES data of electronic structure evolution of Bi2Se3 and Sb under CO surface contamination. (a) Intensity plot of high symmetry G–M cut of Sb (upper panels) and

G–K cut of Bi2Se3 (lower panels). CO dosage in units of Langmuir is labeled on the axis of each plot. (b) Experimental setup for controlled CO surface contamination.

(c) Integrated Energy Distribution Curves (EDCs) of features along the G–M direction of Sb (upper panel) and G–K direction of Bi2Se3 (lower panel) with binding energy less

than 12 eV at the corresponding CO doping levels. Peaks at 7 and 10 eV binding energy are CO impurity bands. Red arrows in (a) and (c) mark the dosage when the SSB in

Sb disappears.

Fig. 3. ARPES data of electronic structure evolution of Bi2Te3 and Bi under O2 surface contamination. (a) Intensity plot of high symmetry G–M cut of Bi (upper panels) and

G–K cut of Bi2Te3 (lower panels). O2 dosage in units of Langmuir is labeled on the axis of each plot. (b) Experimental setup for controlled O2 surface contamination.

(c) Integrated EDCs of features along the G–M direction of Bi (upper panel) and G–K direction of Bi2Te3 (lower panel) with binding energy less than 12 eV at the

corresponding O2 doping levels. Peaks at 4 eV binding energy are O2 impurity band. Red arrows in (a) and (c) mark the dosage when SSB in Bi disappears.

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structure of Bi evolves similarly as that of Sb in Fig. 2: thedispersion from both SSB and BVB broadens as the dosageincreases and the SSB vanishes above a �4.5 L O2 dosage whilethe BVB persists. The contrasting evolution of the SSB in Bi2Te3

that persists to the much higher dosage (�24 L, and it vanishestogether with the BVB due to the scattering effect from surface O2

molecules) again illustrates the robustness of the SSB in Bi2Te3.Our observation of the unusual robustness of the surface state in

topological insulators Bi2Se3 and Bi2Te3, while surprising, is consis-tent with the general theory of topological insulators—as long as thebulk electronic structure of the topological insulator is not altered,the topological surface state has to exist on the interface between atopological insulator and an ordinary insulator or vacuum [10–13].

Practically, the robustness of the surface state enables the tuningof the surface carrier density with ease (e.g. the tuning of the surfacecarrier density by NO [7] and O2 [15] molecules), which makes therealization of topological insulator applications possible.

4. Summary

By directly monitoring the evolution of the surface state ARPESspectra of topological insulators Bi2Te3, Bi2Se3 and semimetals Bi

Please cite this article as: Z.K. Liu, et al., Physica E (2011), doi:10.10

and Sb, we confirmed the robustness of the topological surfacestate of Bi2Te3 and Bi2Se3 against direct surface contamination withthe presence of the time reversal symmetry, contrast to thevulnerable surface state of Sb and Bi, which can easily be destroyedby the contaminant. This unique robustness of the topologicalsurface state further provides the opportunity for the realization oftopological insulator applications by enabling the tuning of thesurface carrier density with direct surface modification.

Acknowledgments

This work was supported by DOE–BES, DMS & E at SLAC(DE–AC02–76SF00515) and ALS (DE–AC02–05CH1 1231).

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