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Ultrathin (Bi1−xSbx)2Se3 Field Effect Transistor with Large ON/OFF Ratio Yu-Hung Liu,† Cheong-Wei Chong,*,† Chun-Ming FanChiang,† Jung-Chun-Andrew Huang,*,†,‡,⊥ Hsieh-Cheng Han,∥ Zhongjun Li,¶ Huaili Qiu,¶ Yi-Chang Li,§ and Chuan-Pu Liu§ †
Department of Physics, ‡Advanced Optoelectronic Technology Center (AOTC), and §Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan ⊥ Taiwan Consortium of Emergent Crystalline Materials (TCECM), Ministry of Science and Technology, Taipei 10622, Taiwan ∥ Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan ¶ School of Electronic Science and Applied Physics, HeFei University of Technology, Hefei, Anhui 230009, China S Supporting Information *
ABSTRACT: Ultrathin three-dimensional topological insulator films are promising for use in field effect devices. (Bi1−xSbx)2Se3 ultrathin films were fabricated on SrTiO3 substrate, where large resistance changes of ∼25 000% could be achieved using the back gate voltage. We suggest that the large ON/OFF ratio was caused by the combined effect of Sbdoping and the reduction of film thickness down to the ultrathin regime. The crossover of different quantum transport under an electric field may form the basis for topological insulators (TI)-based spin transistors with large ON/OFF ratios in the future. KEYWORDS: (Bi1−xSbx)2Se3, ultrathin, topological insulator, field effect transistor, SrTiO3
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In doping concentration in ternary (Bi1−xInx)2Se3.9 Nevertheless, despite the importance of these systems for tunable topological physics and devices, their field-effect properties in the ultrathin regime have rarely been examined. This report describes the growth of such a system in the ultrathin limit. We recently demonstrated the strong and robust topological surface states (TSS) in (Bi1−xSbx)2Se3 ternary compound by angle-resolved photoemission spectroscopy (ARPES) and transport measurements.12 In this work, the field-effect properties of ultrathin (Bi1−xSbx)2Se3 that is grown on SrTiO3(111) using molecular beam epitaxy (MBE) are investigated. Figure 1 displays the device concept for realizing the high ON/OFF ratio on the basis of Sb-doped Bi2Se3. (Bi1−xSbx)2Se3 thin film with a thickness of less than 6 nm is fabricated. In Bi2Se3 topological insulator, the top and bottom surface states (SSs) will be hybridized and will form a surface gap when the film thickness, d, is below a critical thickness dc, where dc = 6 nm for Bi2Se3.1,2 Figure 1c shows the schematic of the band structure, where a usual Dirac cone with gapless linear dispersion will evolve into a pair of Dirac hyberbolas for the ultrathin TI with d < dc. Moreover, in such an ultrathin film, Dirac points of both the bottom and top SSs can be tuned close to the Fermi level EF under a strong electric field that is
INTRODUCTION Ultrathin films of three-dimensional (3D) topological insulators (TI) are well-known as promising for use in field-effect transistors (FET) with large ON/OFF ratios. Owing to their unique chemical potential dependent spin polarization,1,2 spincurrent transistors are expected to become a major research target in the development of nanoscaled device applications. Additionally, the crossover from 3D into 2D TIs may result in an interesting topological phase, namely, that of a quantum spin Hall insulator that supports spin-polarized 1D edge states.3,4 More interestingly, its surface band structure has been found to be tunable by applying external electric fields, leading to topological phase transitions between a trivial insulator and nontrivial topological phases.5 Several quantum transport measurements have been made, revealing, for example, crossover from weak antilocalization (WAL) to weak localization (WL)6 and Anderson localization in ultrathin 3D TI films.7 To date, most relevant studies have focused on prototype TIs of Bi2 Se3 , Sb 2Te 3 , Bi 2 Te3 , and ternary (Bi1−xSbx)2Te3.2,5−8 Other promising candidates are tunable TI systems with topological phase transitions, such as indiumdoped and antimony-doped Bi2Se3.9−11 In these systems, the large tunability of the transport properties is observed and the metal−insulator transition (MIT) is obtained by varying the doping level. Brahlek et al. demonstrated the overall transition from a nontrivial topological metal to a topological trivial band insulator via three quantum phase transitions by increasing the © XXXX American Chemical Society
Received: January 11, 2017 Accepted: March 20, 2017 Published: March 20, 2017 A
DOI: 10.1021/acsami.7b00541 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Field-effect measurement with STO(111) substrate used as back gate oxide. (b) Optical microscope image of TI film that is patterned using a photolithographic technique (scale bar: 100 μm). (c) Concept on which FET device with large ON/OFF is based: Sb-doped Bi2Se3 with d < dc is used, such that Fermi level EF can be tuned within/close to the surface gap in both bottom and top surface states (SS).
Figure 2. XRD spectrum of MBE-grown Sb-doped Bi2Se3 deposited on (a) sapphire and (d) SrTiO3(111) substrates. Right-hand figures display corresponding (b, e) RHEED patterns and (c, f) AFM images for various thicknesses, respectively.
surface gap. The obtained ON/OFF ratio is among the largest of any achieved for other systems of similar thickness. The reduction of the sheet carrier concentration by Sb doping and the ultrathin thickness are the main determinants of such a high ON/OFF ratio.
generated by the SrTiO3(111) (STO) back gate oxide. A large change in resistance is thus expected to yield a large ON/OFF ratio when the EF is close to the surface gap. Our results reveal that increasing the Sb doping level and reducing the film thickness to ∼4.2 nm causes a crossover of transport phenomena from the quantum diffusive regime to the intermediate insulating regime (kFl ∼ 1). A large ON/OFF ratio was obtained in a 5 nm FET, in which the resistance was increased by 2 orders of magnitude (∼25 000%) when the Fermi level (EF) was shifted downward by applying a negative back gate voltage, Vg. The ambipolar-like behavior and very large Rs,max with kFl ≪ 1 suggest that the EF is tuned inside the
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EXPERIMENTAL SECTION
A series of Sb-doped Bi2Se3 thin films with different thicknesses were grown on c-plane sapphire and SrTiO3(111) (STO) substrates using molecular beam epitaxy (MBE) (AdNaNo made MBE-9 system). By fixing the flux ratio of Bi:Se:Sb (1:10:0.45) (Å/min), the thicknesses of the grown films were controlled by varying the deposition duration. B
DOI: 10.1021/acsami.7b00541 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. Transport properties of BSS/sapphire at various thicknesses, measured at room temperature; (a) sheet carrier density n2D, (b) sheet resistance Rs, and (c) mobility μ as a function of film thicknesses. (d) kFl vs thickness. The thicknesses of the grown films were then determined ex situ using X-ray reflectivity (XRR). The nominal composition of the films was (Bi0.68Sb0.32)2Se3,12 and the thickness varied from ∼35 down to ∼3 nm. The film stoichiometry for the ultrathin and thicker sample is determined by transmission electron microscopy−energy-dispersive system (TEM-EDS) as shown in Figure S1. Transport properties were investigated by patterning the films into the Hall bar configuration (Figure 1b) by photolithography. To make field-effect measurements, Cr/Au films with 10/90 nm thicknesses were deposited by E-beam evaporation on the back sides of STO as gate electrodes. Longitudinal resistance (Rxx) and Hall resistance (Rxy) were measured using a closed-cycle refrigerator system.
sample (x = 0.32) exhibits the enhanced transport of topological surface states (TSS), as revealed by the high field Hall effect and weak antilocalization with a well-defined quantum diffusive regime.12 In this work, the transport properties of a film with a nominal composition of (Bi0.68Sb0.32)2Se3 was further tuned by reducing its thickness to ∼3 nm. Figure 3a plots the thickness-dependent n2D of the films that were deposited on sapphire. The n2D decreases gradually as the film thickness decreases and drops rapidly as the thickness is reduced below 6 nm. The rapid reduction of n2D is accompanied by increasing sheet resistance RS, as presented in Figure 3b, revealing that EF is within the bulk gap. The samples with a thickness (6.5 nm) of just above dc exhibit n2D ∼ 1.2 × 1013 cm−2, which is close to the conduction band edge, where n2D can be easily eliminated by the external gate voltage. Here, the dimensionless conductivity, g ≡ σ/(e2/h) = kFl with σ = 1/Rs, where kF is the Fermi wave vector and l is the mean free path, is analyzed.7,13,14 The Ioffe-Regel criterion is that, for a system to remain metallic, kFl > 1 must be satisfied. Otherwise, kFl ≪ 1 corresponds to the strong localization regime.7,13,14 Figure 3d reveals a crossover of transport phenomena from the quantum diffusive regime to the intermediate insulating regime (kFl ∼ 1−3) (shaded area) as the thickness is reduced to ∼4.2 nm. This finding clearly implies that tunable transport phenomena in different regimes can be obtained in this ternary (Bi0.68Sb0.32)2Se3 by changing the Sb doping level and film thickness, with tuning of EF as the key parameter. More importantly, crossing of the dashed line in Figure 3d may indicate the critical thickness for metal− insulator transition (when kFl < 1) that is closely related to the topological phase transition (TPT).9,10 The same is expected
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RESULTS AND DISCUSSION Figure 2a and d presents the X-ray diffraction (XRD) patterns of all of the samples that were grown on sapphire and STO(111) substrates at various thicknesses. All of the films exhibit a single-phase c-axis-oriented rhombohedral structure without any detectable second phase. Figure 2b and e shows in situ reflection high-energy electron diffraction (RHEED) patterns for some thicknesses; the streaky patterns indicate the good epitaxial quality of the films over the full range of thicknesses. Figure 2c and f displays corresponding atomic force microscopy (AFM) images. All the films are macroscopically smooth, with a root-mean-square surface roughness of approximately 0.8 nm, confirming their quality and usability in the transport measurement. (Bi1−xSbx)2Se3 (BSS) topological insulator alloys were prepared by varying the flux of Sb while fixing the Bi/Se ratio in MBE growth.12 Our previous work presented evidence of Sb substitution into the Bi atomic lattice, reducing the carrier density n2D below that of undoped Bi2Se3.12 The optimized C
DOI: 10.1021/acsami.7b00541 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a−c) Thickness-dependent transport properties of BSS/STO outgassed at 700 °C (open symbol) and 1000 °C (solid circle). All measurements were made at room temperature; (d, e) field-effect measurements for 10 nm FET at 16 K: sheet resistance Rs and carrier density n2D as a function of Vg, respectively. Open circles correspond to the sample that was outgassed at 700 °C (device A) while solid circles correspond to the sample that was outgassed at 1000 °C (device B).
Figure 5. Rs vs Vg for BSS/STO with thicknesses of (a) ∼20 nm, ∼12.3 nm, (b) and ∼5 nm; (c) first Vg sweeping curve of each device; inset plots data on logarithmic scale.
n2D, which favors field-effect measurement. Comparing the two sets of BSS/STO samples that were outgassed at various temperatures revealed no significant difference in their transport properties (with similar Hall mobilities). Nevertheless, the most remarkable difference was found in the measurements of the field-effect transistors. Figure 4d and e plots the gate voltage Vg-dependent Rs and n2D for the samples (with a thickness of 10 nm) that were outgassed at 700 and 1000 °C, denoted as devices A and B, respectively. Device A clearly exhibits a much stronger field effect, indicated by the more rapid rise (drop) of the Rs (n2D) with increasing negative gate voltage. We suggest that the better field effect performance of device A is attributable to the lower outgassing substrate temperature, which results in better dielectric properties than those of device B. Accordingly, samples that were outgassed at 700 °C were used for subsequent thickness-dependent fieldeffect measurements.
for an ultrathin BSS FET in which the transition between kFl > 1 and kFl ≪ 1 can be easily induced by varying the EF using the gate voltage. For this purpose, controlling the channel transport in the intermediate insulating regime (kFl ∼ 1−3) is of interest that may find the potential applications based on the gate-tuned quantum phase transition. BSS ultrathin films were grown on STO(111) substrates to demonstrate the field effect. Given the possibility of inducing oxygen vacancies in STO by the high-temperature outgassing step under the ultrahigh vacuum (UHV) condition (before the growth of BSS), films with various thicknesses were prepared at two outgassing temperatures. Figure 4a−c plots the transport properties of BSS/STO samples that were outgassed at 700 and 1000 °C. Generally, BSS/STO devices exhibit a similar trend as sapphire as n2D (Rs) decreases (increases) with decreasing film thickness. The films that were grown on STO exhibit a lower D
DOI: 10.1021/acsami.7b00541 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 6. kFl versus Vg for FET devices at various thicknesses. Tuning EF (using gate voltage) and BSS film thickness causes crossover of the transport phenomena from quantum diffusive to strong localization regime. The right cartoons indicate the suppression of current-induced spin polarization (arrows indicate the flow of current) when EF is tuned inside the surface gap.
Three BSS/STO devices with film thicknesses of ∼5, ∼12, and ∼20 nm were measured to elucidate the field-effect (FE) transport properties. Figure 5 plots Rs against Vg, measured at 16 K. The size of the hysteresis loop depends on the thickness of the film. Such ferroelectric-like hysteresis is commonly observed in STO substrate that is used as a back gate oxide, in which an inward surface dipole has been reported.15 The arrow that points to the right indicates the sweeping direction from −190 V to +190 V of the first sweeping trace. The field effect increased as the film thickness decreased, indicated by an increase in the dRs/dVg slope. The negative slope of dRs/dVg verified that the electrons were the majority carriers, consistent with previous reports.11,12 The FE increased gradually as the film thickness was reduced from ∼20 nm to ∼12 nm (Figure 5a). Remarkably, a large ON/OFF ratio of ∼25 000% was then observed for the 5 nm FET device as shown in Figure 5b and c, with an Rs,max of approximately 250 times larger than that at Vg = +150 V. Moreover, ambipolar-like behavior was observed with an Rs,max as large as ∼8 MΩ around −150 V, suggesting that EF is tuned within the surface gap. Both Sb-doping and reducing of film thickness could enhance the electric field effect in Bi2Se3 TI channel as reported in the literature.12,14 Shifting of EF toward the Dirac point (inside bulk band gap) depletes the transport carrier that results in the enhancement of channel resistance. However, such a large ON/OFF ratio of this ∼5 nm thick Sb-doped Bi2Se3 has never been observed in the extensively studied ternary (Bi1−xSbx)2Te3 or in pure or other doped Bi2Se3. An important question now arises: does Sb doping alter the critical thickness for the surface gap opening? Cho et al. reported a similar large FE in a 3.5 nm thick Bi2Se3 single crystal that was exfoliated on Si/SiO2 substrate.16 However, the hybridized gap in the 5QL Bi2Se3 was estimated from ∼35 meV to 41 meV.17,18 It is surprising to obtain such a large ON/OFF ratio with kFl ≪ 1 around the charge neutrality point (CNP) in our 5 nm FET device. We suggest that the anomalous FE is related to the combined effect of the finite size and antimony doping. This claim was first made by Brahlek et al.9 in the case of (Bi1−xInx)2Se3 where the penetration depth λss = ℏυF/Eg of the SS into the bulk should be considered. Here, υF is the Fermi velocity and Eg is the bulk band gap. Substitution of Bi with the lighter element In reduces the spin-orbit coupling (SOC) strength, eventually reducing Eg and increasing λss. When λss is comparable to the film thickness, coupling of the top and
bottom SS could open a surface gap. The critical thickness dc of such a system is not 6 nm, as it is for pure Bi2Se3. Very recently, Salehi et al. verified this fact by making transport measurements and by theoretical simulation.10 Sb doping in Bi2Se3, similar to that in (Bi1−xInx)2Se3, is expected to reduce the bulk band gap by weakening the SOC.19 Although no detailed study of the thickness-dependent (Bi1−xSbx)2Se3 ARPES has yet been reported, we suggest that our 5 nm FET measurements support the earlier claim owing to the large and insulating OFF states when EF is tuned in the CNP regime. Figure 6 summarizes the field-effect transport properties of this (Bi0.68Sb0.32)2Se3 by plotting kFl versus Vg for various thicknesses of BSS. The 5 nm FET exhibits the strongest FE, as the transport is tuned into the strong localization regime (kFl ≪ 1) when EF is reduced. In this regime, the transport is no longer quantum diffusive, but it is more appropriately described as nontopological normal insulating phase.10 Nevertheless, the diagram implies that by fine-tuning the film thickness and with enhanced field effect performance, the transport could be tuned from quantum diffusive regime into strong disorder regime by varying the gate voltage, as displayed in Figure 6. A gate-tunable transport scheme that is based on this ultrathin Sb-doped Bi2Se3 is proposed for future use in large-field-effect spin transistors, where crossover of the transport phenomena serves as the ingredient for the ON and OFF states of the channel spin polarization, as shown in the right cartoons of Figure 6.
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CONCLUSIONS In conclusion, an ultrathin (Bi, Sb)2Se3 topological insulator was synthesized using MBE, and its electrical transport and field-effect properties were demonstrated. A very large ON/ OFF ratio of ∼25 000% was obtained for a 5 nm BSS FET device, as a combined effect of the Sb doping and the ultrathin thickness. The evolution of the Bi2Se3 band structure as a function of Sb concentration and film thickness may require more experimental and theoretical research; this work provides a potential platform for future TI-based field effect transistor.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00541. TEM-EDS and HRTEM images for the ultrathin BSS film are shown (PDF) E
DOI: 10.1021/acsami.7b00541 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
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and Strong Surface States in Bi2‑xSbx Se3 Thin Films. Adv. Mater. 2014, 26, 7110−7115. (12) Liu, Y. H.; Chong, C. W.; Chen, W. C.; Huang, J. C. A.; Cheng, C.-M.; Tsuei, K.-D.; Li, Z.; Qiu, H.; Marchenkov, V. V. Robust Topological Insulator Surface State in MBE Grown (Bi1‑xSbx)2Se3. 2016, arXiv:1611.08395. https://arxiv.org/abs/1611.08395 (accessed Nov 25, 2016). (13) Brahlek, M.; Koirala, N.; Bansal, N.; Oh, S. Transport Properties of Topological Insulators: Band Bending, Bulk Metal-to-Insulator Transition, and Weak Anti-localization. Solid State Commun. 2015, 215−216, 54−62. (14) Liu, Y. H.; Chong, C. W.; Jheng, J. L.; Huang, S. Y.; Huang, J. C. A.; Li, Z.; Qiu, H.; Huang, S. M.; Marchenkov, V. V. Gate-Tunable Coherent Transport in Se-capped Bi2Se3 Grown on Amorphous SiO2/ Si. Appl. Phys. Lett. 2015, 107, 012106. (15) Sachs, R.; Lin, Z.; Shi, Jing Ferroelectric-like SrTiO3 Surface Dipoles Probed by Graphene. Sci. Rep. 2014, 4, 3657. (16) Cho, S.; Butch, N. P.; Paglione, J.; Fuhrer, M. S. Insulating Behavior in Ultrathin Bismuth Selenide Field Effect Transistors. Nano Lett. 2011, 11, 1925−1927. (17) Zhang, Yi; He, Ke; Chang, C. Z.; Song, C. L.; Wang, L. L.; Chen, Xi; Jia, Jin-Feng; Fang, Zhong; Dai, Xi; Shan, W. Y.; Shen, S. Q.; Niu, Qian; Qi, X. L.; Zhang, S. C.; Ma, X. C.; Xue, Q. K. Crossover of the Three-dimensional Topological Insulator Bi2Se3 to the Twodimensional limit. Nat. Phys. 2010, 6, 584−588. (18) Kim, D.; Syers, P.; Butch, N. P.; Paglione, J.; Fuhrer, M. S. Coherent Topological Transport on the Surface of Bi2Se3. Nat. Commun. 2013, 4, 2040. (19) Liu, J.; Vanderbilt, D. Topological Phase Transitions in (Bi1‑xInx)2Se3 and (Bi1‑xSbx)2Se3. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 224202.
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Cheong-Wei Chong: 0000-0002-8933-2153 Author Contributions
Y.-H. L. and C.-W. C. contributed equally. All authors contributed to the manuscript and have approved the final version. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology, R.O.C. (Grant Nos. 104-2811-M-006-029 and 104-2811-M-006-031).
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REFERENCES
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DOI: 10.1021/acsami.7b00541 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
