Applied Science and Convergence Technology 2019; 28(6): 207-212
Published online November 30, 2019
Copyright © The Korean Vacuum Society.
Jimin Chaea, Seoung-Hun Kangb, Young-Kyun Kwonb,c, and Mann-Ho Choa,*
aDepartment of Physics, Yonsei University, Seoul 03722, Republic of Korea
bKorea Institute for Advanced Study, Seoul 02455, Republic of Korea
cDepartment of Physics and Research Institute for Basic Sciences, Kyung-Hee University, Seoul 02447, Republic of Korea
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Topological insulators (TIs) have gained considerable attention owing to their topologically protected helical edge states called topological surface states. To employ TIs, it is necessary to reduce film thickness and suppress effects from the bulk carrier. When the film thickness is less than 5 quintuple layers (QLs), the top and bottom surface states overlap, thereby increasing surface bandgap. In this study, we investigate the suppression of the hybridization of surface states in a 3-QL Bi2Se3/graphene heterostructure. In the 3-QL Bi2Se3 film grown on graphene, surface states affected by strain, and band bending effects from graphene are localized to the top and bottom and possess a closed bandgap. Further, we investigated transport properties in the 3-QL Bi2Se3/graphene heterostructure and verified the independent transport channels of Bi2Se3 and graphene, and the long coherence length of 534 nm. In conclusion, the closed bandgap and long coherence length in the 3-QL Bi2Se3/graphene heterostructure implies that the proximity effect in a TI/non-TI heterostructure can be attractive for future applications, beyond the physical and topological thickness limit.
Keywords: Topological insulator, Heterostructure, Coherence length
Recently, topological insulators (TIs) have been studied intensively in condensed matter physics because of their topologically protected helical edge states, referred as topological surface states (TSSs) [1,2]. Bismuth selenide (Bi2Se3), which has a conducting, single Dirac cone of TSS at both its top and bottom surfaces and a relatively wide bulk bandgap (~0.3 eV), is an attractive candidate for future applications such as in spintronics, low-power electronics, quantum computing, and quantum mechanical platforms [3,4]. To enhance the use of TSSs, band structures of TI films with a thickness of only a few layers, wherein the bulk contribution is reduced, have been investigated theoretically and experimentally [5,6]. Because the TSSs of Bi2Se3 are distributed spatially at the top and bottom surface, and they are spread within 2.4 nm from each surface, when the film thickness is under 5 quintuple layers (QLs), the TSSs overlap each other, creating a surface bandgap . Zhang
To overcome practical obstacles such as bandgap opening and topological softening in films with only a few layers, various approaches to control the TSSs have been investigated. Among them, the application of an external electric field is an attractive method to control the TSSs. The bandgap changes when a gate bias is applied through the ferroelectric substrate . In addition, theoretical studies determined that without an additional field, the interaction at the interface (proximity effect) can modulate the spectral distribution of TSSs [9–11]. However, there are still obstacles to realize the modification of TSSs in TI films.
In this study, we investigated the suppression of the hybridization of surface states in a 3-QL Bi2Se3/graphene heterostructure. As a substrate, we used graphene, a monolayer sheet comprising a honeycomb lattice of carbon atoms, because of its low lattice match with Bi2Se3 and its proper band bending caused by the strong charge transfer at the interface [12,13]. We studied the transport properties in the 3-QL Bi2Se3/graphene heterostructure and verified the independent transport channels of Bi2Se3 and graphene
To obtain ARPES spectrum, we prepared Bi2Se3 films grown on epi-graphene that was pre-grown on a SiC substrate. We grew the Bi2Se3 film through a self-ordering process used in our previous work . Band structures of the films were obtained using synchrotron light sources in the Pohang Accelerator Laboratory (PAL) 4A1-μ ARPES beamline. We used a Scienta SES-4000 electron energy analyzer at a pressure under 5 × 10−11 Torr for detecting photoelectrons. The resolutions of energy and momentum were lower than 20 meV and 0.02 Å−1, respectively. The measurements were conducted at 50–100 K, and the size of photon beam was under 100 μm.
We performed grazing incident small angle x-ray scattering (GISAXS) with synchrotron radiation from the 3 C beamline of PAL, using a 2D detector. The incidence angle was fixed to 0.2°, and the X-ray wavelength was 1.54 Å−1. The X-ray photoelectron spectroscopy (XPS) data were obtained with high-resolution XPS (PHI 5000 Versa-probe) using a monochromatic Al Kα source (1486.6 eV). We used the
The surface band structures of Bi2Se3 films with and without monolayer graphene were calculated with the Vienna
The transport data of the Bi2Se3/graphene heterostructures were obtained using the physical property measurement system (PPMS) with a Keithley Series 2400 source meter and a nanovoltmeter unit. Devices for transport measurement were fabricated using shadow masks with a wide and long transport channel (50 × 1000 μm2) to achieve a four-point geometry. The temperature varied from 2 to 300 K and the magnetic field was increased up to 9 T.
Bi2Se3 films with thicknesses under 5 QL and their surface bandgaps have been studied with a variety of measurement tools such as ARPES, scanning tunneling microscopes, transport measurements [7,15]. In our experiment, however, the surface band structure of 3-QL Bi2Se3 on a monolayer of graphene pre-grown on a SiC substrate did not have a bandgap, as shown in Fig. 1(a). The surface band was similar to the Dirac cone of the bulk films and the bulk conduction band was not observed because of the forbidden transition by the selection rule with a photon energy of 48 eV. Although the synchrotron light source was polarized, we could not find any symmetry in the surface band. This implied that the grain size of Bi2Se3 (a few hundred nanometers to a few micrometers) was smaller than the beam size. The charge neutral point of the surface state was positioned at 0.47 eV from the Fermi level. This position implied that the charge transfer between Bi2Se3 and graphene induced strong band bending. This bandgap closing was interesting compared with other reports.
In ultrathin TI films under 5 QLs, it is well-known that the spatial distributions of the two surface states overlap with each other, and the surface bandgap opens. However, some theoretical and experimental studies have verified the spatial modulation of the TSSs. Especially, Zhang
To analyze structural effects such as strain and spin-orbit splitting (SOS) when Bi2Se3 films were grown on the graphene substrate, we employed GISAXS and XPS. We prepared two SiO2 substrates and transferred graphene sheets to one of them. In Fig. 1(b), we plot the thickness dependent GISAXS data of Bi2Se3 film on graphene and SiO2 substrates. The peak position near
To investigate the detailed changes in ultrathin films, we observed the Bi 5d spectra as a function of the thickness. As shown in Fig. 1(c), Bi 5d spectra of the 3-, 5-, and 20-QL Bi2Se3 films have clear SOS between the 5d3 and 5d5 peaks. This SOS has no exact relation with the SOC that can be explained as
We conducted an
In Fig. 2(c), the bandgap decreased abruptly but remained open, which was not consistent with our ARPES results that showed linear dispersion and no Rashba-like splitting [Fig. 1(a)]. Thus, we calculated an additional structure considering a SiC substrate. We simulated the Bi2Se3/graphene heterostructure having 2 % tensile strain and found it to have no bandgap, as shown in Fig. 2(d). There were three bands in the bandgap of Bi2Se3. Because of the strong band bending, an additional band that interacted with bulk state was formed at the middle layer . The surface state of the conduction band, which we focused on, was localized at the topmost layer more than that in the case of relaxed Bi2Se3/graphene. The clear localization of the TSS induced the suppression of band hybridization. Thus, the
We obtained temperature dependent resistance (RT) and magneto-conductance (MC) curves by using a PPMS, as shown in Fig. 3, to investigate the practical influence of bandgap closing in the 3-QL Bi2Se3/graphene heterostructure on the transport properties. In the RT curves, the resistance of 5-QL Bi2Se3/graphene was higher than that of 3-QL Bi2Se3/graphene. The difference in resistance implies that the Fermi levels of the two graphene sheets transferred onto the SiO2 substrates were different from each other. The resistances linearly decreased from 300 to 45 K for the 5-QL sample and to 26 K for the 3-QL sample; the resistances, however, exponentially increased from those temperatures to 3.5 K. In both samples, resistances below 3.5 K were saturated owing to the defect-assisted charge transports, which were related to the Se vacancy . The first region of linear decrease resulted from the thermal interference with the conducting states of both the Bi2Se3 and graphene films; the second region of exponential increase was from the two-dimensional electron–electron interaction (EEI) in both Bi2Se3 and graphene . Considering that both Bi2Se3 and graphene have a similar temperature dependency, there appeared to be no significant reason for the interaction between graphene and the Bi2Se3 film. Further, we measured the temperature dependent MCs in both heterostructures, as shown in Figs. 3(b) and 3(d). Although the two sets of data exhibited some differences, they had similar temperature dependencies. At 2 K, the MC curves had a trend containing both weak antilocalization (WAL) and weak localization (WL). Similar to how WAL is well-known to be observed in TI films because of its Berry phase, WL is well-known to be observed in the graphene film. In fact, graphene can show WAL in MC curves; however, it does this under very strict conditions, wherein the electron dephasing time is smaller than the elastic intervalley and intravalley scattering time, i.e., graphene should be almost defect-free to show WAL in the MC curves [27–29]. Our graphene was prepared by chemical vapor deposition and then transferred to the SiO2 substrate, and therefore, it was natural for our graphene to show WL in the MC curves. At 10 K, WL remained but WAL was weakened significantly. At 40 K (near the cross point between EEI and the electron–phonon region), WAL was not observed and only WL remained; at 80 K, WL also was weakened. This implies that the WAL of Bi2Se3 and WL of graphene have independent temperature dependencies, i.e., the Bi2Se3 and graphene in Bi2Se3/graphene heterostructures have independent transport channels.
To analyze WAL and WL in further detail, we fitted the two MC curves of 3-QL Bi2Se3/graphene and 5-QL Bi2Se3/graphene at 2 K, as shown in Fig. 4. We considered two independent transport channels of Bi2Se3 and graphene. For the WAL of Bi2Se3, we use the simplified Hikami–Larkin–Nagaoka equation and for the WL of graphene, we use the general equation described by McCann [30,31]. The equations are
We investigated the TSSs of Bi2Se3 and the transport properties of ultrathin Bi2Se3/graphene heterostructures through experimental and theoretical methods. We observed a closed bandgap in a 3-QL Bi2Se3 film grown on a monolayer graphene sheet, previously grown on a SiC substrate. Graphene reduced the tensile strain in the out-of-plane direction, which resulted from surface relaxation, thereby causing a bandgap reduction. From
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (Grant No. 2018R1A2A1A05023214). The authors would like to thank Byeong-Gyu Park from the Pohang Accelerator Laboratory for technical assistance with ARPES and the Korea Institute for Advanced Study for providing computing resources (KIAS Center for Advanced Computation Linux Cluster System).
Fitting values of weak antilocalization and weak localization (WL) in graphene, 3-quintuple layer (QL) Bi2Se3/graphene (BS/G), and 5-QL BS/G. We set β = 1 for the fitting of WL in bare graphene.