Ambipolar Surface Conduction in Ternary Topological Insulator Bi_2(Te_{1-x}Se_x)_3 Nanoribbons
ZhenHua Wang, Richard L. J. Qiu, Chee Huei Lee, ZhiDong Zhang, Xuan P. A. Gao
AAmbipolar Surface Conduction in Ternary Topological Insulator Bi (Te x Se x ) Nanoribbons
ZhenHua Wang , †, ‡ Richard L. J. Qiu , ‡ Chee Huei Lee , ‡ ZhiDong Zhang † and Xuan P. A. Gao *, ‡ † Shenyang National Laboratory for Materials Science, Institute of Metal Research, and International Centre for Materials Physics, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China ‡ Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106, United States * To whom correspondence should be addressed. Email: [email protected]
ABSTRACT
We report the composition and gate voltage induced tuning of transport properties in chemically synthesized Bi (Te x Se x ) nanoribbons. It is found that increasing Se concentration effectively suppresses the bulk carrier transport and induces semiconducting behavior in the temperature dependent resistance of Bi (Te x Se x ) nanoribbons when x is greater than ~10%. In Bi (Te x Se x ) nanoribbons with x ~20%, gate voltage enables ambipolar modulation of resistance (or conductance) in samples with thickness around or larger than 100nm, indicating significantly enhanced contribution in transport from the gapless surface states. KEYWORDS: Topological insulator, ambipolar conduction, nanoribbon, bismuth selenide, bismuth telluride hree-dimensional (3D) topological insulators (TIs) with both metallic surface states and insulating bulk states have attracted enormous research attention. The metallic surface states in 3D TIs consist of a single Dirac cone at the Г point which gives rise to interesting surface transport phenomena. Binary topological insulators Bi Te , Bi Se and Sb Te have been confirmed to have robust surface states by surface sensitive probes such as angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS). The charge carriers originating from surface states also have been identified by magneto-transport experiments.
In addition, electrical gating was used as a means to manipulate the gapless surface state conduction in 3D TIs in Bi Te and Bi Se . However, the topological surface contribution in transport is hindered by residual bulk carriers from environmental doping or crystal defects.
Therefore, to exploit the surface transport properties of topological insulators, it is crucial to achieve a bulk-insulating state in a topological insulator material. Recently, the ternary and quaternary topological insulators have been synthesized to maximize the surface properties. The Shubnikov-de Haas (SdH) oscillations coming from the surface states were observed in bulk Bi x Sb x Se , Cd-doped Bi Se and Bi Te Se crystals and nanopletes. However, no SdH oscillation was observed in Bi x Ca x Se due to the disorder from the low level substitution of Ca for Bi . Bi Te Se presented a high resistivity and a variable-range hopping behavior, which is crucial to achieve the surface transport properties. Cd-doped Bi Se showed a transition from n-type to p-type conversion upon increasing Se vacancies by post annealing. The solid solution Bi x Sb x Te y Se y showed a series of “intrinsic” compositions and presented a maximally bulk-insulating behavior, which can achieve a surface-dominated transport. The SdH oscillation of both Dirac holes and electrons was observed in he compound Bi Sb Te Se . Besides studies on bulk materials, topological insulator nanoribbons and nanoplates are expected to have significantly enhanced surface conduction because of their large surface-to-volume ratios. (Bi x Sb x ) Te nanoplates with thickness less than 10nm have been shown to exhibit ambipolar field effect. Alternatively, the Fermi level of Bi Te nanoplates could be tuned by Na doping and enhanced surface states conduction was observed, in which the ambipolar surface transport could be achieved by applying a back-gate voltage. Bi (Se x Te x ) is a topological insulator for all atomic ratio x , similar to (Bi x Sb x ) Te . In Bi (Se x Te x ) nanoribbons and nanoplates with high carrier density, two-dimensional (2D) weak anti-localization was clearly observed. Similar 2D weak anti-localization effect was also observed in Bi Te Se nanoplates. However, ambipolar field effect has never been reported in the ternary Bi (Se x Te x ) TI materials. Here, we report the synthesis and transport studies of Bi (Te x Se x ) nanoribbons. For nanoribbons with thickness ~100-300nm, we show that increasing the Se concentration, x , gradually tunes the material from metallic to semiconducting. Moreover, ambipolar field effect from surface state conduction was achieved by applying a back gate voltage for semiconducting compound with x ~20%. The thickness of nanoribbons is also found to be important for observing enhanced ambipolar field effect. Results and Discussion
Vapor-liquid-solid (VLS) process was used to synthesize Bi (Te x Se x ) nanoribbons in a vapor transport setup (see Methods section). Figure 1A shows scanning electron microscope (SEM) images of the as synthesized ribbons, the length of the ribbons ranges from several m to tens of m and the width ranges from a few hundred nm to a few m. The wide ribbons display 120 o facet angles reflecting the crystal structure. The concentrations of Bi, Te and Se in Bi (Te x Se x ) nanoribbons were estimated by energy-dispersive X-ray spectroscopy (EDS) on individual anoribbons. An example of EDS data corresponding to Bi (Te x Se x ) nanoribbon with x=20±5% is shown in Figure 1B. To investigate the transport properties and explore the topological surface conduction in Bi (Te x Se x ) nanoribbons, back-gated field effect transistor (FET) devices were fabricated on doped Si substrates with 300nm thick SiO on surface and measured in a cryostat (see Methods Section). Temperature dependent resistance measurements of Bi (Te x Se x ) nanoribbons with different x show that substituting Te with Se can indeed suppress the residual bulk carriers and tune the Fermi level into bulk bandgap, in a similar way to the studies in bulk crystals. In Figure 2, we compare R vs . T for a series of Bi (Te x Se x ) nanoribbons with different x . To compare the temperature dependence directly, the resistances of different samples are normalized by the values at 300K. A clear trend can be seen in Figure 2 that as more Se is incorporated into the Bi Te , the material turns from metallic into semiconducting at x > ~10%. Atomic force microscope (AFM) measurements revealed that the thickness of device R ( T ) shows an insulating behavior, but it saturates below ~75 K due to the metallic surface transport as well as the bulk impurity-band transport. The nonmetallic R vs . T observed in these nanoribbons is consistent with those reported in the bulk crystals of Bi Sb x Te Se y , and the similar nonmetallic properties were also observed in Na-doped Bi Te nanoplates and Bi (Se x Te x ) nanoribbons/plates. Figure 1. Morphological and composition data of Bi (Te x Se x ) nanoribbons. (A) SEM images of Bi (Te x Se x ) nanoribbons with length from several m to tens of m and width from several hundred nm to several m. (B) EDX data of a Bi (Te x Se x ) nanoribbon with x ≈ A μ m μ m μ m B x =20 x =20 x =20 Te nanoribbondevice x =8 Bi (Te x Se x ) nanoribbon R / R ( T = K ) T (K) Figure 2. Composition tuned temperature dependent transport in Bi (Te x Se x ) nanoribbons. The dependence of resistance with temperature showing the transport property changes from metallic into semiconducting as the concentration of Se increases in Bi (Te x Se x ) nanoribbons. The resistances at 300K are 77 , 970 , 4440 , 6120 , 49.2k , and 4.3k for device
16, 17, 27
For Bi (Te x Se x ) nanoribbons or Bi Te Se nanoplates, either very high electron density was observed or only the n-type conduction was achieved in the gating experiment, meaning the Fermi level was still above the Dirac point. In our experiments, we found that with the bulk carrier conduction suppressed by the incorporation of Se, nanoribbons of ternary Bi (Te x Se x ) could start to exhibit ambipolar field effect in samples with thickness ~100nm or even larger. Figure 3A-B shows the zero-field resistance as a function of the back gate voltage ( V g ) at various temperatures for device x is ~20% and the sample thickness is ~105nm. It is interesting to note that the gate odulated resistance showed distinct behaviors at different temperature. At 300 K, the resistance increased with decreasing the gate voltage, exhibiting a clear n-type feature. When the temperature was decreased to 200 K, the device kept weak n-type characteristic with high gate voltage, however the resistance started to decrease when the applied gate voltage was below about +20 V. This behavior indicates a transition from n-type to p-type conduction. When the temperature dropped further down to 2 K, an ambipolar characteristic became more evident in the R ( V g ) curve and the turning point moved to ~-20V, as shown in Figure 3B. This behavior in the temperature effect on the ambipolar field effect can be understood by considering the relative contributions from surface and bulk states at different T : since the semiconducting R ( T ) of Bi (Te .8 Se .2 ) samples in Figure 2 suggests that the Fermi level resides in the bulk band gap in these samples, there are exponentially more thermally excited electrons in the bulk at high temperatures which may smear out the ambipolar field effect from the gapless surface states. Further studies on the gate tuned Hall resistivity at different temperatures would be required to quantitatively distinguish the surface carrier density and mobility from the bulk states by a multi-band transport analysis, as previously done on bulk or exfoliated flakes of topological insulator materials.
17, 21, 22, 32
Unfortunately, making Hall measurement has been difficult for our nanowires/nanoribbons. Such measurement and analysis will be a subject of future study. In addition to ambipolar field effect, we observed that 2D weak anti-localization effect also evolves with the gate voltage. Figure 3c shows the conductivity xx in units of e /h versus perpendicular magnetic field ( B ) at 2 K at various V g . The sharply peaked behavior near B =0 indicates weak antilocalization effect. We fit the conductivity data to 2D weak antilocalization model Δσ xx = α e /(2 π ħ )[ln( B / B ) − (1/2 + ( B / B ))], where B = ħ /(4e L φ ) is a characteristic field efined by the dephasing length L , is the digamma function and is a constant.
17, 18, 33
Correlating with the crossover from n- to p-type conduction at V g ~-20V, we observe that increases as V g becomes smaller than -20V. Previous measurements on molecular beam epitaxy grown Bi Se thin films with thickness of 20nm or thinner showed that increased from ~0.5 or 0.7 toward 1, concomitant with the ambipolar field effect in the gate tuned conductance.
18, 33
It is known that both conventional 2D films with strong spin-orbit coupling and a single layer of TI surface can contribute weak antilocalization correction with =1/2. So this gate tuned change of from 0.5 to 1 was attributed to the gate induced separation of bulk states from surface states when one of the TI surfaces was gated to the Dirac point, leaving the bulk states and the top TI surface each contributing 0.5 to the total .
18, 33
Although we have obtained a similar increasing trend of at V g <-20V accompanying the ambipolar conduction, the magnitude of enhancement in is smaller, and itself (~0.3-0.34) is smaller than 0.5. This discrepancy in the quantitative magnitude of may lie in the fact that for our sample, the conductivity xx is only 3-4×e /h. Strictly speaking, the weak antilocalization fitting only works in the diffusive metallic transport regime where xx >>e /h. For xx comparable to quantum conductance e /h, it is reasonable to expect that the standard weak antilocalization model does not give a quantitatively accurate fitting result although it captures the qualitative physics.
100 -80 -60 -40 -20 0 20 40 606.06.36.66.97.27.5 -100 -80 -60 -40 -20 0 20 40 603.803.853.903.954.704.754.804.85 R ( k s qu a r e ) Vg (V) B Bi (Te Se ) nanoribbon, device Temperature 200K 300K R ( k / s qu a r e ) Vg (V) A -1.0 -0.5 0.0 0.5 1.03.103.203.303.403.503.60 -60 -30 0 30 600.280.300.320.340.36 Vg 60V 40V 20V 0V -20V -40V -60V xx ( e / h ) Magnetic Field (Tesla)
T=2K C Vg (V) D igure 3. Gate voltage tuned ambipolar surface conduction and weak anti-localization in Bi (Te Se ) nanoribbon. (A, B) The dependence of resistance per square vs . gate voltage at different temperatures for a 105nm thick Bi (Te Se ) nanoribbon (device T <200K. (C) Magneto-conductivity of the device at different gate voltage showing weak antilocalization effect. The symbols are data and solid lines are fits. (C) The fitting parameter in the weak antilocalization fit vs . gate voltage. It is remarkable that clear ambipolar field effect is observed in nanoribbons with thickness ~100nm, attesting to the effectiveness of including Se to compensate the defect states in Bi Te to achieve an insulating bulk behavior. Previously, gate voltage induced ambipolar conduction was observed in Bi Se flakes or films with thickness less than 50nm. In (Bi x Sb x ) Te nanoplates, the ambipolar field effect was very weak once the plate thickness increased above 9nm. Indeed, comparing our Bi (Se x Te x ) nanoribbons with the same x but different thickness we found thicker samples exhibited weaker ambipolar field effect, due to the more dominant bulk contribution to the overall conductance. For 105nm thick device R on V g shows a weaker tunability, as shown in Figure 4. Moreover, the ambipolar surface conduction effect disappeared at 20 and 10 K for device -120 -100 -80 -60 -40 -20 0 20 40 60 device device R ( k ) Vg (V)
Figure 4. Ambipolar conduction in Bi (Te x Se x ) nanoribbons with large thickness. Gate voltage tuned ambipolar conduction at various temperatures in Bi (Te x Se x ) nanoribbons with x ~0.2 and the thickness about 205 nm (device CONCLUSION
In summary, we realized efficient suppression of bulk carrier transport in Bi (Se x Te x ) nanoribbons/nanowires with increasing Se concentration x . Semiconducting temperature dependent resistivity was observed in Bi (Se x Te x ) nanoribbons/nanowires with thickness of 100-300nm at x ~20%, indicating that the Fermi level was tuned into bulk band gap. Moreover, as result of reduced bulk conduction, ambipolar field effect from the gapless surface states was observed in Bi (Se .8 Te .2 ) nanoribbons/nanowires with thickness more than 100nm. This study illustrates the usefulness of composition tuned ternary topological insulator nanomaterials in transport studies of topological insulators. METHODS
Vapor-liquid-solid (VLS) growth method was used to synthesize ternary Bi (Te x Se x ) nanoribbons, using 20 nm gold nanoparticles (Ted Pella, Inc) as metal catalyst on Si substrates. Pure Bi Te and Bi Se powders (99.999%, Alfa Aesar) were used as precursor. Bi Te was placed in the center of a one inch diameter quartz tube inside a furnace (Lindberg/Blue M) and Bi Se was placed at different sites far from the center at low temperature area, the atomic ratio of Te/Se in these nanoribbons was controlled by changing the distance of Bi Te and Bi Se precursors. The growth conditions for Bi (Te x Se x ) nanoribbons were similar to the conditions for Bi Se nanoribbons. The concentrations of Bi, Te and Se in Bi (Te x Se x ) nanoribbons were estimated by energy-dispersive X-ray spectroscopy (EDS) equipped in HITACHI S4500 SEM. Back-gated Bi (Te x Se x ) nanoribbon field effect transistor (FET) devices were fabricated by standard photolithography patterning and liftoff process, with doped Si wafers as substrates. The Si substrates had 300nm thick SiO on surface as gate dielectric. The transport properties of Bi (Te x Se x ) nanoribbon devices were measured in a Quantum Design PPMS (physical property measurement system) using low frequency lock-in technique. The resistance R was measured either in a four-probe or two-probe configuration. We found that the contact resistance was small, and four-probe measurement gave result similar to that of a simple two-probe configuration. cknowledgments Z.H.W. acknowledges China Scholarship Council for a scholarship supporting her visit to CWRU. X. P. A. G acknowledges the NSF CAREER Award program (grant number DMR-1151534) for financial support of research at CWRU.
Z.D.Z acknowledges the National Natural Science Foundation of China with Grant No. 50831006.
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50K 20K 10K 2K R ( k s qu a r e ) Vg (V) Bi (Te Se ) nanoribbon Bi (Te x Se x ) nanoribbon R / R ( T = K ) T (K) increasingincreasing