Session J31: Focus Session: Topological Insulators: Synthesis and Characterization - Transport

Transport in Topological Insulator Thin Films

We report on electronic transport measurements on Bi2Se3 thin-film devices and show that an ambipolar modulation can be achieved via the electric field effect by using a top-gate with a high-k dielectric insulator. By analyzing the evolution of the weak anti-localization magnetoconductance behavior with respect to gate voltage and temperature, we find that we are able to modulate the effective number of channels, demonstrating that the coherent coupling between the surface and the bulk is tunable. Moreover, we investigate the formation and behavior of tunable p-n junctions on thin-film devices with multiple local top-gates.   

Bi$_2$Se$_3$ and Bismuth: A comparison of transport in topological and trivial materials

The simple surface state structure of Bi2Se3 has made it one of the most promising materials for harnessing transport through topologically protected surface states. Identifying unambiguous signatures of transport through these states is difficult however due to residual bulk conductivity and the formation of 2D electron gases near the surface due to band bending. As in bulk bismuth these non-topological states are subject to strong-spin orbit coupling. Very thin films of bismuth are predicted to be 2D topological insulators and thus are interesting themselves for a topological to non-topological transition as a function of thickness. Analysis of signatures such as weak anti-localization and linear magnetoresistance are compared between high quality MBE grown films of these materials to determine what can and cannot be ascribed to transport through protected surface states.   

Low-temperature magnetoresistance in electrically gated Bi$_2$Se$_3$ thin films

Although transport in samples of 3D topological insulators often has a large contribution from bulk conduction, the surface transport can be studied by electrical gating of topological insulator thin films. We have measured thin films of Bi$_2$Se$_3$ grown by molecular beam epitaxy and subsequently photolithographically patterned with a high$-\kappa$ gate dielectric (HfO$_2$) using atomic layer deposition. Gate voltage-dependent Hall effect and magnetoresistance were measured over a temperature range $0.5 \rm{K} \leq T \leq 20$ K in both perpendicular and parallel magnetic fields up to 6 T. Applying a negative gate voltage forms a depletion layer at the top of the thin film and decouples the surface from bulk carriers. We use the weak antilocalization effect at low magnetic field to study the transport contribution of surface and bulk channels. We also discuss scattering mechanisms contributing to surface and bulk conduction. Supported by ONR and NSF-MRSEC.   

Thickness Independent Surface Transport of Bi$_{2}$Se$_{3}$ on Al$_{2}$O$_{3}$(0001) Substrates

The key requirement for exploiting the newly emerging three-dimensional (3D) topological insulators (TI) as a novel platform for coherent spin-polarized electronics is TI thin films with dominant surface transport properties. So far, while researchers have been able to observe the existence of surface states locally \textit{in situ}, verification over a wide thickness range outside the growth chamber has not yet been reported. Here, we report large signature of surface transport in TI Bi$_{2}$Se$_{3}$ thin films. The Bi$_{2}$Se$_{3}$ films used for this study were grown on c-axis Al$_{2}$O$_{3}$ substrates with MBE. Hall-effect measurements in the standard Van der Pauw geometry provided clear evidence of two conducting channels for 4QL-2750QL thick samples, with the transport properties for one of the channel being thickness independent and the other varying with thickness. This thickness independent carrier density of $\sim $1.5 $\times $ 10$^{13}$ cm$^{-2}$ has been observed over the entire thickness range down to 2 QL, clearly suggesting that this is due to surface states. Furthermore, another surface transport property directly related to the topological protection mechanism, the weak-antilocalization (WAL) effect, exhibited similar thickness- and bulk-independent characteristics.   

Electronic transport in MBE-grown $Bi_{2}Se_{3}$ topological insulator thin film field effect devices

Topological insulators (TI), such as $Bi_{2}Se_{3}$ and $Bi_{2}Te_{3}$, have attracted a lot of attention due to their exotic electronic properties. $Bi_{2}Se_{3}$ TI films grown by molecular beam epitaxy (MBE) are promising for studying the nature of topologically protected surface states due to their large size, high quality, the capability to tune the thickness and interface with various semiconductor substrates. In this study, thin films of $Bi_{2}Se_{3}$ have been grown on $GaAs$ (001) semi-insulating substrates in a III-V/II-VI dual chamber MBE system. To study the electronic properties, micrometer scale Hall-bar devices with high-$k$ dielectric ($Al_{2}O_{3}$) top gates have been fabricated. Systematical measurements of temperature dependent and electrical field modulated magnetro-transport are performed to exam the conduction contributed by the surface states.   

Weak anti-localization behavior in Bi2(SeTe)3 grown on GaAs (001) substrate

A series of Bi2(SeTe)3 thin films were grown by molecular beam epitaxy on GaAs(001) substrates in order to obtain ternary Te-Bi-Se-Bi-Te alloys with large bulk resistivities. X-Ray diffraction data reveals many reflections from only the {\{}003{\}}-type lattice planes, indicative of highly directed c-axis growth of these films despite the very different crystal symmetries of the film and the substrate along the film growth direction. The X-ray data reveal that a wide spectrum of mixed Bi2(SeTe)3 alloys can be obtained by this method [1]. Our studies show that the alloy films are highly uniform, and the crystallinity is comparable to that of films grown on substrates with hexagonal surface structure. In this work, we have carried out comprehensive magneto-transport measurements on a series of Bi2(SeTe)3 thin films. We find that the conductivities of the films are strongly affected by alloy composition, and that insulating samples can be obtained at Se concentrations of $\sim $30{\%}. Moreover, we observe weak anti-localization behavior in all samples, which is also composition-dependent. In order to understand diffusive transport in these topological insulator alloys, both disorder and electron-electron interaction effects are considered in our analysis. [1] X. Liu et al, J. Vac. Sci. Tech. B (submitted).   

Transport in topological insulator films

The prediction and the subsequent confirmation of topological insulators is one of the most exciting discoveries in condensed matter physics. In the transport study of topological insulator films, we demonstrate that an excellent agreement between theory and experiment is achieved when both disorder and interaction are taken into account. In addition, measurements under an in-plane magnetic field, along and perpendicular to the bias current show opposite magnetoersistance. Furthermore, Bi2Se3 topological insulator thin films contacted by superconducting (In, Al and W) electrodes show an abrupt resistance upturn when the electrodes become superconducting. Concomitant with the upturn in resistance, there is a significant weakening of the superconductivity of the electrodes.   

Magneto-transport study of the topological insulator Bi$_{2}$Te$_{3}$

Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator, but have protected surface or edge states. Many materials have been realized as topological insulators, including the HgTe/CdTe superlattice, Bi$_{1-x}$Sb$_{x}$, Bi$_{2}$Se$_{3}$, Sb$_{2}$Te$_{3}$ and Bi$_{2}$Te$_{3}$. Topological insulators are interesting not only because of their fundamental importance but also their great potential for future applications. Here, we examine the magneto-transport properties of exfoliated Bi$_{2}$Te$_{3}$ specimens prepared from Bi$_{2}$Te$_{3}$ single crystals using the scotch tape method. Indium and silver paint contacts were applied to the exfoliated specimens and magneto-transport was examined at liquid helium temperatures at moderate magnetic fields. The results of these experiments will be described here within the context of the ongoing interest in topological insulators.   

Theoretical study of carrier transport and screening in topological insulator Bi$_{2}$Se$_{3}$

This theoretical work is motivated by two recent experiments on Bi$_{2}$Se$_{3}$ examining the charge inhomogeneity [1-2] close to the topologically protected crossing point of surface bands in these bulk topological insulators. Reminiscent of graphene close to charge neutrality [3-4], the energy landscape becomes highly inhomogeneous, forming a sea of electron and hole puddles, which determine the properties at low carrier density. Here, we show that the induced carrier density fluctuations are of order 1 {\%} of the impurity density, providing a small-parameter with which we can perform a controlled perturbation theory. Analytic results are obtained for the minimum conductivity and puddle auto-correlation length. We also find that the band asymmetry between electron and holes states is a necessary ingredient to understand the aforementioned experiments. \textbf{References: } [1] H. Beidenkopf \textit{et al,} ``\textit{Spatial fluctuations of helical Dirac fermions on the surface of topological insulators},'' Nat. Phys.\textit{ online publ.}, (2011) [2] D. Kim \textit{et al.,} ``\textit{Minimum conductivity and charge inhomogeneity in Bi}$_{2}$\textit{Se}$_{3}$,'' arXiv:1105.1410. [3] S. Adam \textit{et al.}, ``\textit{A self-consistent theory for graphene transport},'' PNAS \textbf{104}, 18392 (2007). [4] S. Das Sarma \textit{et al., ``Electronic transport in 2D graphene},'' \textit{Rev. Mod. Phys.} \textbf{83}, 407 (2011).   

Minimum conductivity and charge inhomogeneity in Bi$_{2}$Se$_{3}$ in the topological regime

Using electrolytic and dielectric dual gating method, we report charge transport measurements of mechanically exfoliated Bi$_{2}$Se$_{3}$ in the topological insulator (TI) regime. We show that the surfaces of thin, low-doped Bi$_{2}$Se$_{3}$ crystals are strongly electrostatically coupled, and a gate electrode can be used to completely remove bulk charge carriers and bring both surfaces through the Dirac point nearly simultaneously with well-defined ambipolar electronic conduction of gapless surface states. In particular, we focus on linear carrier density dependent conductivity away from the Dirac point and a charge-inhomogeneous minimum conductivity region similar to that observed in graphene. An extension of the theory of charge disorder in graphene to Bi$_{2}$Se$_{3 }$explains well the mobility at high carrier density and the doping level at zero gate voltage. We show that the observed minimum conductivity is governed by induced carrier density that is self-consistently determined by the screened, charged impurity potential, as experimentally observed in recent STM study on surfaces of TIs.   

Gate-tunable electronic transport in topological insulator Bi$_{2}$Te$_{3}$ thin films synthesized by metal-organic chemical vapor deposition

Topological insulator is a new state of matter with a nominally insulating gap in the bulk and non-trivial metallic states on the surface. One of the proto-type topological insulator materials, Bi$_{2}$Te$_{3}$, can be synthesized in the form of high quality, wafer scale thin films by metal-organic chemical vapor deposition (MOCVD). Here we present an experimental study of Bi$_{2}$Te$_{3}$ thin films with thickness ranging from a few nm's to 1 $\mu $m synthesized by MOCVD on semi-insulating GaAs (001) substrates. Hall bar shaped devices using atomic layer deposition (ALD) high-k Al$_{2}$O$_{3}$ or HfO$_{2}$ as gate dielectric have been fabricated. We have measured the magneto-transport (including both R$_{xx}$, 4-terminal longitudinal resistance, and R$_{xy}$, the Hall resistance) at various temperatures and gate voltages to probe the possible transport signatures of the topological surface states. We have also studied gate-tunable weak anti-localization in R$_{xx}$(B) for ultra-thin films.   

Ambipolar field effect in the ternary topological insulator (Bi$_{x}$Sb$_{1-x})_{2}$Te$_{3}$ by composition tuning

Topological insulators exhibit a bulk energy gap and spin-polarized surface states that lead to unique electronic properties, with potential applications in spintronics and quantum information processing. However, transport measurements have typically been dominated by residual bulk charge carriers originating from crystal defects or environmental doping, and these mask the contribution of surface carriers to charge transport in these materials. Our recent work demonstrates that the ternary sesquichalcogenide (Bi$_{x}$Sb$_{1-x})_{2}$Te$_{3}$ is a tunable topological insulator system. By tuning the ratio of bismuth to antimony, we are able to reduce the bulk carrier density by over two orders of magnitude, while maintaining the topological insulator properties. As a result, we observe a clear ambipolar gating effect in (Bi$_{x}$Sb$_{1-x})_{2}$Te$_{3}$ nanoplate field-effect transistor devices, similar to that observed in graphene field-effect transistor devices. The manipulation of carrier type and density in topological insulator nanostructures demonstrated here paves the way for the implementation of topological insulators in nanoelectronics and spintronics.   

Signature of Topological Insulators in Conductance Measurements

Following the discovery of spin-polarized states at the surface of three-dimensional topological insulators (TI) like Bi$_{2}$Te$_{3}$ and Bi$_{2}$Se$_{3}$, there are intense interests in possible electrical measurements demonstrating unique signatures of these unusual states. A recent interesting proposal suggests that a signature of TI material should be a change in the conductance measured between a normal contact and a magnetic contact when the magnetization of the latter is reversed. However, the generalized Onsager relation suggests that no such change is expected in two-terminal setups and a multi-terminal set up is needed to observe the proposed effect. We present numerical results using a Non-Equilibrium Green Function (NEGF) based model capable of covering both ballistic and diffusive transport regimes seamlessly. Simple expressions based on a semi-classical picture describe some of the results quite well. Finally, we estimate the magnitude of signal expected in realistic samples that have recently been studied experimentally and have shown evidence of surface conduction.   

Coupling between 3-D topological insulator and superconductor

Topological insulators are band insulators in the bulk with gapless topologically protected surface states. Recently it has been predicted that 3-D topological insulators (TI) can host zero energy modes called Majorana fermions. Many theoretical proposals for observation of the zero energy excitations involve coupling between the surface states of TI and s-wave superconductors (SC). A prerequisite for such experiments is a highly tunable surface which is decoupled from the residual bulk carriers and well established coupling between TI and superconductor. Here we present transport measurements performed in high quality MBE grown thin films of Bi2Se3. Along with presenting evidence for significant contribution of the surface states to the electrical transport, we discuss the dependence of TI-SC coupling on temperature, gate voltage and thickness of TI films.   

Aging effect on carrier density and mobility of thin film Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ grown on Al$_{2}$O$_{3}$

Topological Insulators (TI) are materials with topologically protected metallic surface states and insulating bulk states. Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ are viable TI material as they posses bulk band gap as well as continuous surface band. However, there is bulk conduction as well in these materials due to Fermi surface lying in the bulk conduction band. Natural n-type doping due to ambient oxygen and water vapor and/or creation of Se/Te vacancies create bulk contamination, leading to change in carrier density and motility. We report transport measurement data on epitaxially grown thin films of these materials which show the dependence of charge carrier density and mobility on environmental exposure time of these samples. In addition, we will present our data on samples capped with various materials.