Omnipresence of weak antilocalization (WAL) in Bi2Se3 thin films: A review on its origin

nanomaterials Review Omnipresence of Weak Antilocalization (WAL) in Bi2Se3 Thin Films: A Review on Its Origin Rubén Gracia-Abad 1,2,3, Soraya Sangiao 1,2,3,* , Chiara Bigi 4, Sandeep Kumar Chaluvadi 4, Pasquale Orgiani 4and JoséMaría De Teresa 1,2,3,*   Citation: Gracia-Abad, R.; Sangiao, S.; Bigi, C.; Kumar Chaluvadi, S.; Orgiani, P.; De Teresa, J.M. Omnipresence of Weak Antilocalization (WAL) in Bi2Se3Thin Films: A Review on Its Origin. Nanomaterials 2021,11, 1077. https:// doi.org/10.3390/nano11051077 Academic Editor: Antonio Politano Received: 29 March 2021 Accepted: 19 April 2021 Published: 22 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Departamento de Física de la Materia Condensada, Universidad de Zaragoza, 50009 Zaragoza, Spain; [email protected] 2Laboratorio de Microscopías Avanzadas (LMA), Universidad de Zaragoza, 50018 Zaragoza, Spain 3Instituto de Nanociencia y Materiales de Aragón (INMA), CSIC-Universidad de Zaragoza, 50009 Zaragoza, Spain 4CNR-IOM, TASC Laboratory in Area Science Park, 34139 Triestre, Italy; [email protected] (C.B.); [email protected].it (S.K.C.); or[email protected].it (P.O.) *Correspondence: [email protected] (S.S.); [email protected] (J.M.D.T.) Abstract: Topological insulators are materials with time-reversal symmetric states of matter in which an insulating bulk is surrounded by protected Dirac-like edge or surface states. Among topological insulators, Bi2Se3 has attracted special attention due to its simple surface band structure and its relatively large band gap that should enhance the contribution of its surface to transport, which is usually masked by the appearance of defects. In order to avoid this difficulty, several features characteristic of topological insulators in the quantum regime, such as the weak-antilocalization effect, can be explored through magnetotransport experiments carried out on thin films of this material. Here, we review the existing literature on the magnetotransport properties of Bi2Se3 thin films, paying thorough attention to the weak-antilocalization effect, which is omnipresent no matter the film quality. We carefully follow the different situations found in reported experiments, from the most ideal situations, with a strong surface contribution, towards more realistic cases where the bulk contribution dominates. We have compared the transport data found in literature to shed light on the intrinsic properties of Bi2Se3 , finding a clear relationship between the mobility and the phase coherence length of the films that could trigger further experiments on transport in topological systems. Keywords: topological insulator; Berry’s phase; Bi 2 Se 3 film; weak-antilocalization; magnetotransport 1. Introduction The discovery of the quantum hall (QH) state back in 1980 opened the door to a totally new paradigm in condensed matter science [ 1 , 2 ]. Up to those days, states of matter were classified considering the symmetries they spontaneously break. For instance, crystalline solids break translational symmetry whereas ferromagnets break rotational symmetry. However, the QH state did not have any spontaneously broken symmetry, and hence, it could not be included in that classification. Instead, its novel characteristics were related to the topology of the band structure. In 2006, a new topological class known as quantum spin hall (QSH) state or topological insulators (TIs) emerged [ 3 – 5 ]. In these materials, a strong spin–orbit interaction causes band inversion, creating a non-trivial topology of the bands, which, along with the presence of time reversal symmetry (TRS), produces robust metallic states at the surfaces, whereas the bulk remains insulating. These surface states exhibit a Dirac-like dispersion relation ( Figure 1a ) with spin momentum locking, preventing the carriers from suffering backscattering against non-magnetic impurities. All these characteristics make them ideal candidates for the observation of exotic phenomena. For example, systems composed of Nanomaterials 2021,11, 1077. https://doi.org/10.3390/nano11051077 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021,11, 1077 2 of 20 a TI and a superconductor are predicted to host Majorana states, which could be implemented in the development of fault-tolerant quantum computation [ 5 ]. In addition, the spin texture of surface carriers has attracted much attention in the field of spintronics [6]. Nanomaterials 2021, 11, x FOR PEER REVIEW 2 of 20 backscattering against non-magnetic impurities. All these characteristics make them ideal candidates for the observation of exotic phenomena. For example, systems composed of a TI and a superconductor are predicted to host Majorana states, which could be implemented in the development of fault-tolerant quantum computation [5]. In addition, the spin texture of surface carriers has attracted much attention in the field of spintronics [6]. Figure 1. (a) Scheme of a Dirac cone at the surface of a Topological Insulator (TI) showing the spin-momentum locking (spin orientations are indicated by red arrows). (b) Angle-Resolved Photoemission Spectroscopy (ARPES) image of the 𝐵𝑖  𝑆𝑒  band structure showing the Dirac cone at the center of the Brillouin zone. Reprinted with permission from reference [7]. Copyright 2010 American Physical Society. TIs were first experimentally realized in two-dimensional HgTe/CdTe quantum wells [8], and then, in three dimensional systems in the alloy BiSb [9], where the topological nature was demonstrated by imaging its band structure by angle resolved photoemission spectroscopy (ARPES). Soon after that, the discovery of the topological phase in crystals of the stoichiometric materials BiSe,BiTe, and SbTe [10,11] laid the foundations for plenty of new theoretical and experimental works in this field. Many of them have focused their attention on the growth and electronic characterization of TI thin films [12–14]. However, even though the topological surface states have been demonstrated systematically through spectroscopic techniques (Figure 1b), their investigation through transport measurements has become very challenging due to the significant contribution from bulk carriers. A simple treatment of a 3D TI includes the bottom of the bulk conduction band (BCB) and the top of the bulk valence band (BVB), in parallel with the Dirac cones at each of the surfaces. In Figure 2, for the sake of simplicity, we consider the BCB and the BVB in parallel with a single Dirac cone. When the Fermi level lies within the bulk band gap, the topological regime takes place, and the transport occurs entirely through the surfaces (Figure 2b). Unfortunately, naturally formed defects combined with the degradation of the film surface caused by environmental exposure shift the chemical potential towards the BCB (Figure 2a), making the system n-type, and the significant contribution of bulk electrons masks the presence of surface states, hindering their study and manipulation [15,16]. Depletion of bulk carriers has been accomplished by using gating or doping with acceptor ions such as calcium obtaining p-type conduction (Figure 2c) [17,18], but complete elimination of the contribution of the bulk carriers is not always feasible, and is still a matter of research. In the case of narrow-gapped TIs, more difficulties come up even when the topological regime is reached, due to the thermal activation of bulk carriers when the thermal Figure 1. ( a ) Scheme of a Dirac cone at the surface of a Topological Insulator (TI) showing the spin-momentum locking (spin orientations are indicated by red arrows). ( b ) Angle-Resolved Photoemission Spectroscopy (ARPES) image of the Bi2Se3 band structure showing the Dirac cone at the center of the Brillouin zone. Reprinted with permission from Reference [ 7 ]. Copyright 2010 American Physical Society. TIs were first experimentally realized in two-dimensional HgTe/CdTe quantum wells [ 8 ], and then, in three dimensional systems in the alloy BixSb1−x [ 9 ], where the topological nature was demonstrated by imaging its band structure by angle resolved photoemission spectroscopy (ARPES). Soon after that, the discovery of the topological phase in crystals of the stoichiometric materials Bi2Se3 , Bi2Te3 , and Sb2Te3 [ 10 , 11 ] laid the foundations for plenty of new theoretical and experimental works in this field. Many of them have focused their attention on the growth and electronic characterization of TI thin films [ 12 – 14 ]. However, even though the topological surface states have been demonstrated systematically through spectroscopic techniques (Figure 1b), their investigation through transport measurements has become very challenging due to the significant contribution from bulk carriers. A simple treatment of a 3D TI includes the bottom of the bulk conduction band (BCB) and the top of the bulk valence band (BVB), in parallel with the Dirac cones at each of the surfaces. In Figure 2, for the sake of simplicity, we consider the BCB and the BVB in parallel with a single Dirac cone. When the Fermi level lies within the bulk band gap, the topological regime takes place, and the transport occurs entirely through the surfaces (Figure 2b). Unfortunately, naturally formed defects combined with the degradation of the film surface caused by environmental exposure shift the chemical potential towards the BCB ( Figure 2a ), making the system n-type, and the significant contribution of bulk electrons masks the presence of surface states, hindering their study and manipulation [ 15 , 16 ]. Depletion of bulk carriers has been accomplished by using gating or doping with acceptor ions such as calcium obtaining p-type conduction (Figure 2c) [ 17 , 18 ], but complete elimination of the contribution of the bulk carriers is not always feasible, and is still a matter of research. In the case of narrow-gapped TIs, more difficulties come up even when the topological regime is reached, due to the thermal activation of bulk carriers when the thermal energy, Nanomaterials 2021,11, 1077 3 of 20 kBT , kB being the Boltzmann’s constant and T the temperature, is significantly higher than the band-gap energy Eg(Figure 2b). Nanomaterials 2021, 11, x FOR PEER REVIEW 3 of 20 energy, kT, k being the Boltzmann’s constant and T the temperature, is significantly higher than the band-gap energy E (Figure 2b). Figure 2. Schemes showing the band structure and the different contributions to transport in TIs: (a) n-type conduction with parallel contributions of the surface and the bulk. (b) Topological regime with pure surface transport, also indicating the possibility of bulk presence due to thermal activation. (c) p-type conduction with surface and bulk contributing. Blue and yellow regions in the bands indicate electron and hole populations, respectively. Over the past decade, new groups of materials have broadened the topological classification beyond TIs. Among these groups we find topological crystalline insulators (TCI), in which surface states are protected by spatial symmetries instead of TRS [19]. An important example is the semiconductor SnTe, predicted first theoretically [20], and then experimentally confirmed [21]. In this material, the metallic surface states are topologically protected by reflection symmetry of the crystal with respect to the {110} mirror plane. The states at the (001) surface present exotic phenomena, such as a Lifshitz transition as the Fermi level passes through the Dirac point. However, the properties of the surface states are tough to observe since it is highly p-doped due to Sn vacancies. Other TCIs with similar properties but with a more accessible topological regime are the alloys PbSnTe [22], or PbSnSe [23], in which the Fermi level can be modified more easily. Another interesting group is that formed by topological Kondo insulators (TKI), in which topology and strongly correlated physics come together for the first time. In these systems, protected surface states live in the Kondo gap rather than in the Bloch gap [24]. One paradigmatic case is SmB, where, at low temperatures, the strong interaction of the localized 4f states and the dispersing 5d states of Sm, gives rise to a few meV Kondo gap with the Fermi level right in the middle. Due to this gap, SmB behaves as an insulator at low temperatures, but unlike ordinary insulators, its resistivity saturates below 4 K. This residual resistivity is attributed to the topological surface states that dominate in that regime. Surface states in SmB have shown spin polarization, demonstrating their topological nature [25]. Beyond insulators, Dirac/Weyl semimetals have attracted much attention in the last few years. They can be viewed as a 3D generalization of graphene with the conduction and valence bands in the bulk touching each other at some isolated points of momentum space called Weyl nodes [26]. These nodes come in pairs and behave as topologically protected monopoles, each of them carrying opposite monopole charge N. Apart from the so-called single-Weyl semimetals with N=±1, double-Weyl semimetals with N=±2 have also been predicted. The non-trivial characteristics of Weyl/Dirac semimetals allows for the formation of Fermi arcs at the surfaces connecting the projection Figure 2. Schemes showing the band structure and the different contributions to transport in TIs: ( a ) n-type conduction with parallel contributions of the surface and the bulk. ( b ) Topological regime with pure surface transport, also indicating the possibility of bulk presence due to thermal activation. (c) p-type conduction with surface and bulk contributing. Blue and yellow regions in the bands indicate electron and hole populations, respectively. Over the past decade, new groups of materials have broadened the topological classification beyond TIs. Among these groups we find topological crystalline insulators (TCI), in which surface states are protected by spatial symmetries instead of TRS [ 19 ]. An important example is the semiconductor SnTe , predicted first theoretically [ 20 ], and then experimentally confirmed [ 21 ]. In this material, the metallic surface states are topologically protected by reflection symmetry of the crystal with respect to the {110} mirror plane. The states at the (001) surface present exotic phenomena, such as a Lifshitz transition as the Fermi level passes through the Dirac point. However, the properties of the surface states are tough to observe since it is highly p-doped due to Sn vacancies. Other TCIs with similar properties but with a more accessible topological regime are the alloys PbxSn1−xTe [ 22 ], or PbxSn1−xSe [ 23 ], in which the Fermi level can be modified more easily. Another interesting group is that formed by topological Kondo insulators (TKI), in which topology and strongly correlated physics come together for the first time. In these systems, protected surface states live in the Kondo gap rather than in the Bloch gap [ 24 ]. One paradigmatic case is SmB6 , where, at low temperatures, the strong interaction of the localized 4 f states and the dispersing 5 d states of Sm , gives rise to a few meV Kondo gap with the Fermi level right in the middle. Due to this gap, SmB6 behaves as an insulator at low temperatures, but unlike ordinary insulators, its resistivity saturates below 4 K . This residual resistivity is attributed to the topological surface states that dominate in that regime. Surface states in SmB6 have shown spin polarization, demonstrating their topological nature [ 25 ]. Beyond insulators, Dirac/Weyl semimetals have attracted much attention in the last few years. They can be viewed as a 3D generalization of graphene with the conduction and valence bands in the bulk touching each other at some isolated points of momentum space called Weyl nodes [ 26 ]. These nodes come in pairs and behave as topologically protected monopoles, each of them carrying opposite monopole charge N . Apart from the so-called single-Weyl semimetals with N=± 1, double-Weyl semimetals with N=± 2 have also been predicted. The non-trivial characteristics of Weyl/Dirac semimetals allows for the formation of Fermi arcs at the surfaces connecting the projection of bulk Weyl nodes in Nanomaterials 2021,11, 1077 4 of 20 the surface Brillouin zone. These Fermi arcs were first mapped by ARPES in TaAs [ 27 ]. Another very promising group is that of topological superconductors (TS), formed by superconductors with a non-trivial topology of the bands. TSs present unconventional superconducting effects and are expected to host Majorana modes protected by particle–hole symmetry [ 5 ]. However, no pure TS has been discovered yet. Instead, these properties can be reached in some modified systems, such as strong-spin orbit materials like InAs in combination with a superconductor [ 28 ], or in Cu-doped TI Bi2Se3 [ 29 ]. In both cases, signs of Majorana modes have been observed as a zero-bias conductance peak in STM studies, demonstrating to be excellent platforms for this new physics. Considering all these new topological classes and their peculiar properties, it can be concluded that topology has significantly enriched the condensed matter physics field and has brought about a new era, with many technological possibilities in the coming future. Hereafter, we focus our attention on 3D TI Bi2Se3 thin films, which represent a fantastic playground in which to investigate quantum transport phenomena. This material has raised much interest due to its simple surface band structure, consisting of a single Dirac cone (Figure 1b) that allows an easier comparison between experimental and theoretical data, and to its relatively large bulk band gap of 0.3 eV that provides a larger window for the topological regime. It possesses a rhombohedral lattice structure, and it exhibits a periodic arrangement of five atomic layers known as quintuple layer (QL) (1 QL ≈1 nm) with a stacking structure Se1–Bi–Se2–Bi–Se1 (Figure 3). Three QLs form the lattice unit cell with a c-axis of 2.86 nm. Nanomaterials 2021, 11, x FOR PEER REVIEW 4 of 20 of bulk Weyl nodes in the surface Brillouin zone. These Fermi arcs were first mapped by ARPES in TaAs [27]. Another very promising group is that of topological superconductors (TS), formed by superconductors with a non-trivial topology of the bands. TSs present unconventional superconducting effects and are expected to host Majorana modes protected by particle–hole symmetry [5]. However, no pure TS has been discovered yet. Instead, these properties can be reached in some modified systems, such as strong-spin orbit materials like InAs in combination with a superconductor [28], or in Cu-doped TI BiSe [29]. In both cases, signs of Majorana modes have been observed as a zero-bias conductance peak in STM studies, demonstrating to be excellent platforms for this new physics. Considering all these new topological classes and their peculiar properties, it can be concluded that topology has significantly enriched the condensed matter physics field and has brought about a new era, with many technological possibilities in the coming future. Hereafter, we focus our attention on 3D TI BiSe thin films, which represent a fantastic playground in which to investigate quantum transport phenomena. This material has raised much interest due to its simple surface band structure, consisting of a single Dirac cone (Figure 1b) that allows an easier comparison between experimental and theoretical data, and to its relatively large bulk band gap of 0.3 eV that provides a larger window for the topological regime. It possesses a rhombohedral lattice structure, and it exhibits a periodic arrangement of five atomic layers known as quintuple layer (QL) (1 QL≈ 1 nm) with a stacking structure Se1–Bi–Se2–Bi–Se1 (Figure 3). Three QLs form the lattice unit cell with a c-axis of 2.86 nm. Figure 3. Schematic representation of the unit cell of rhombohedral Bi  Se  (blue: Bi, violet: Se1, orange: Se2) showing the Quintuple Layer (QL) arrangement and the c-axis direction. Many efforts have been devoted to the development of high-quality BiSe thin films. This is crucial for the incorporation of this material into electronic functional devices in order to carry out the aforementioned applications. Apart from this, the growth of TI thin films presents other advantages: on the one hand, the increase of surface-to-volume ratio enhances the contribution of surface carriers to the transport properties. On the other hand, unlike crystal exfoliation, the growth of films provides a high control on thickness down to few nanometers, which represents an opportunity for the observation and tuning of interesting effects. Several growth techniques such as molecular beam epitaxy (MBE) [30–32] or pulsed laser deposition (PLD) [33,34] have proven to be good tools for the study Figure 3. Schematic representation of the unit cell of rhombohedral Bi2Se3 (blue: Bi, violet: Se1, orange: Se2) showing the Quintuple Layer (QL) arrangement and the c-axis direction. Many efforts have been devoted to the development of high-quality Bi2Se3 thin films. This is crucial for the incorporation of this material into electronic functional devices in order to carry out the aforementioned applications. Apart from this, the growth of TI thin films presents other advantages: on the one hand, the increase of surface-to-volume ratio enhances the contribution of surface carriers to the transport properties. On the Nanomaterials 2021,11, 1077 5 of 20 other hand, unlike crystal exfoliation, the growth of films provides a high control on thickness down to few nanometers, which represents an opportunity for the observation and tuning of interesting effects. Several growth techniques such as molecular beam epitaxy (MBE) [ 30 – 32 ] or pulsed laser deposition (PLD) [ 33 , 34 ] have proven to be good tools for the study of TI thin films, allowing for the use of different substrates and also the growth of heterostructures. However, the number of defects in the films is still high, and the surface–carrier mobilities are still low, placing TIs far from the technological scenario. This review is organized as follows: first, an introduction to quantum transport in topological insulators is given, paying special attention to the weak-antilocalization (WAL) effect, which is frequently observed in magnetotransport measurements of Bi2Se3 thin films, as well as in other relevant materials with topological properties. Then, we will review some interesting cases reported in the literature in which the connection between the WAL effect and the role of the surface and bulk states will be evidenced. Finally, we will give some interesting insight into the intrinsic properties of Bi2Se3 by comparing the data found in the literature. 2. Weak-Antilocalization (WAL) Effect Studying surface transport in TIs is a difficult task due to the presence of bulk carriers that dominate and maintain the system out of the topological regime. The nature of surface states is different from that of bulk states due to their characteristic dispersion relation, which provide them with unique phenomena related to quantum transport and oscillations in electronic properties. These phenomena manifest themselves in transport experiments and allow us to gain insight into the study of these topological states. 2.1. Electronic Motion in the Quantum Diffusive Regime Electronic transport in materials can be classified according to comparisons among characteristic lengths of carriers. As the temperature of a system is decreased, the phase coherence length, lϕ , which defines the average distance an electron can travel until its phase is randomized, can increase and become larger than the elastic mean free path, le . In that case, in a weakly disordered system, the quantum diffusive regime, occurs, and the electrons can move coherently for a relatively long distance while bouncing off the different scattering centers. Taking this into account, the movement of an electron along a path connecting points A and B (Figure 4a), considering paths whose distance is larger than le and shorter than lϕ , can be treated quantum mechanically, and following Feynman formalism, a complex probability amplitude can be attributed to each path that the electron can follow to go from A to B: Cj=cj·eiϕj(1) where j is the label for each possible path, and ϕj is the phase an electron acquires along that path. The origin of the ϕj phase is dynamical due to the time variation, as well as geometrical, as demonstrated by Berry [ 35 ]. The probability for the electron to go from A to B is determined by the square of the total amplitude: PAB = ∑ j cj·eiϕj 2 (2) Most of time, phases associated with different paths are randomly distributed and the total probability is averaged out. However, this is different for closed paths ( A=B≡O ) (Figure 4b) forming a loop. In this situation, each possible path has a time reversal partner with the same probability amplitude, C1,2 =c1,2·eiϕ1,2 , with c1=c2 and ϕ1=ϕ2 , and we have available closed paths as long as these paths are shorter than lϕ . Then, for each loop, the contribution to the current can be expressed as: PO=|C1+C2|2=|C1|2+|C2|2+2Re(C∗ 1C2)=4|C1|2(3) Nanomaterials 2021,11, 1077 6 of 20 In the classical picture, the interference term goes away, and the total amplitude for the loop is just the sum of the individual probabilities, 2 |C1|2 . Considering each loop shorter than lϕ , and applying the same reasoning, it can be observed that quantum interferences have a strong contribution to current and are responsible for an enhancement of the probability of electron backscattering. In practice, this means that the net current is reduced, and, consequently, there is an increase of resistance compared to the classical case. This effect is called weak-localization (WL), and represents a negative correction to conductivity [36]. Nanomaterials 2021, 11, x FOR PEER REVIEW 5 of 20 of TI thin films, allowing for the use of different substrates and also the growth of heterostructures. However, the number of defects in the films is still high, and the surface–carrier mobilities are still low, placing TIs far from the technological scenario. This review is organized as follows: first, an introduction to quantum transport in topological insulators is given, paying special attention to the weak-antilocalization (WAL) effect, which is frequently observed in magnetotransport measurements of BiSe thin films, as well as in other relevant materials with topological properties. Then, we will review some interesting cases reported in the literature in which the connection between the WAL effect and the role of the surface and bulk states will be evidenced. Finally, we will give some interesting insight into the intrinsic properties of BiSe by comparing the data found in the literature. 2. Weak-Antilocalization (WAL) Effect Studying surface transport in TIs is a difficult task due to the presence of bulk carriers that dominate and maintain the system out of the topological regime. The nature of surface states is different from that of bulk states due to their characteristic dispersion relation, which provide them with unique phenomena related to quantum transport and oscillations in electronic properties. These phenomena manifest themselves in transport experiments and allow us to gain insight into the study of these topological states. 2.1. Electronic Motion in the Quantum Diffusive Regime Electronic transport in materials can be classified according to comparisons among characteristic lengths of carriers. As the temperature of a system is decreased, the phase coherence length, l, which defines the average distance an electron can travel until its phase is randomized, can increase and become larger than the elastic mean free path, l. In that case, in a weakly disordered system, the quantum diffusive regime, occurs, and the electrons can move coherently for a relatively long distance while bouncing off the different scattering centers. Taking this into account, the movement of an electron along a path connecting points A and B (Figure 4a), considering paths whose distance is larger than l and shorter than l, can be treated quantum mechanically, and following Feynman formalism, a complex probability amplitude can be attributed to each path that the electron can follow to go from A to B: C=c·e  (1) where j is the label for each possible path, and φ is the phase an electron acquires along that path. The origin of the φ phase is dynamical due to the time variation, as well as geometrical, as demonstrated by Berry [35]. The probability for the electron to go from A to B is determined by the square of the total amplitude: P=󰈏c·e  󰈏 (2) Figure 4. Sketch representing the movement of electrons through scattering centers: ( a ) two possible paths (1 and 2) for an electron going from A to B. (b) A loop formed by time reversal partners. In materials with strong spin–orbit interaction, the spin orientation is not conserved, which entails an additional phase shift in the wavefunction. In the case of closed paths (Figure 4b), this leads to time reversal partner paths that acquire opposite phases and produce destructive interference. As a result, a decrease in the probability of electron backscattering occurs, resulting in an overall decrease of electrical resistance. This is the weak-antilocalization (WAL) effect, which represents a positive correction to conductivity, as observed in thin films of materials with strong spin–orbit interaction such as Bi [ 37 ]. In the surface states of TIs, due to the momentum–spin locking, the spin of the electrons performing a closed path produces a π Berry phase in the electron wavefunction [ 38 ]. This gives rise to a purely destructive interference effect between partner paths, producing a strong WAL signature. From now on, we will be focused on this effect, whose detailed analysis provides insight into important transport parameters. Quantum corrections to resistance produced by WAL disappear in the presence of an external magnetic field that destroys TRS and introduces a phase shift between partner closed paths. This phase shift is loop dependent, since it depends on the magnetic flux piercing through the loop. As a result, the phase shift is randomly distributed among the different loops. As the magnetic field is increased, the strength of the localization effect decreases and, finally, the classical regime is recovered. Quantitatively, the WAL effect can be described by the Hikami–Larkin–Nagaoka (HLN) model [ 39 ], which gives the correction to the 2D conductance ∆Gxx in the quantum diffusive regime in the presence of a magnetic field B . In the approximation of independent transport channels, and no magnetic scattering, it is expressed as follows: ∆Gxx(B)=−αe2 2π2tsh"ln tsh 4eBl2 ϕ!−ψ 1 2+tsh 4eBl2 ϕ!# (4) where e is the electron charge, tsh is the reduced Planck’s constant, ψ is the digamma function, and α is a parameter representing the nature of the spin–orbit effect in the system, which takes the value of − 1 / 2 for each channel contributing to the WAL effect. In an ideal situation, one expects to find α=− 1 corresponding to two independent surfaces, but Nanomaterials 2021,11, 1077 7 of 20 α=− 1 / 2 is commonly found. In samples with a relevant role of the surface, it can be attributed to an indirect coupling between the top and bottom surfaces through the metallic bulk. In fact, decoupling of the surfaces has been achieved by gating and doping studies, transitioning from α=− 1 / 2 to α=− 1 [ 14 , 40 ]. This model provides a methodology for studying WAL experimentally by measuring magnetoresistance in the system and fitting the experimental data to the model, lϕ and α being the free parameters. On the one hand, the magnitude of lϕ can be strongly related to the number of defects, and can be used to probe the quality of thin films. Furthermore, lϕ decreases with increasing temperature due to the thermal activation of phonons. The temperature dependence of lϕ follows a power–law dependence lϕ∼T−p/2 , where p is positive and determined by decoherence mechanisms such as 2D (p=1) and 3D (p=2) electron–electron interaction and electron–phonon interaction (p=3) [ 41 ]. On the other hand, α provides knowledge on the channels taking part in the transport. With extra information coming from other magnetotransport measurements, α can be helpful to determine either if the surfaces are contributing to transport or even if the topological regime is taking place. A further study of the dimensionality of the transport can be carried out by studying the WAL effect in tilted magnetic fields. This can distinguish between 2D and 3D character, since 2D transport will depend only on the perpendicular component of the field [42]. In order to obtain a direct interpretation of the model HLN through the experimental magnetotransport data, a detailed simulation of Equation (4) can be carried out by varying the free parameters of the model. By considering that just an integer number of channels can contribute to the coherent transport, α can only take values that are integer multiples of 1 / 2. Among those cases, α=− 1 / 2, with a single channel, is the most frequently found in Bi2Se3 thin films. The minus sign of α appears when the spin–orbit interaction is so strong that the spin number of the carriers is not conserved anymore and, hence, a destructive interference between time reversal closed partner paths occurs (Figure 4b), giving rise to WAL and to a negative correction of the conductance as a function of the magnetic field ∆Gxx(B) . On the contrary, a positive value of α appears when the spin–orbit is weak and WL emerges, producing a positive ∆Gxx(B) . We are interested in the first case, taking place in TIs. By increasing the phase coherence length lϕ (Figure 5a) the magnitude of the WAL effect increases, which is translated to more pronounced ∆Gxx(B) . This occurs because as the value of lϕ increases, more and more paths are available to contribute coherently, and then, the weight of quantum corrections like WAL gain weight on the overall transport. For lϕ< 100 nm , ∆Gxx is just a small fraction of the spinless quantum conductance G0=e2/h = 3.35 · 10 −5S , and one can consider that the WAL is relatively weak in that system. On the other hand, going from α=− 1 / 2 to α=− 1, a new coherent channel is introduced, and, in the approximation of independent channels, this results in a doubling of the WAL correction (Figure 5b). Finally, α= 0 has been reported few times, and, in that case, the WAL effect is absent. This has been attributed either to a degradation of the topological protection in the ultrathin limit due to direct coupling between the top and bottom surface states [ 43 ], or to a motion of carriers in the strong disordered regime (kFle1, kFbeing the Fermi wave vector) [44]. 2.2. WAL in Relevant Materials The WAL effect is not an exclusive feature of TIs. It is present in materials either with strong spin-orbit interaction or with chiral carriers in the quantum diffusive regime. In the latter case, due to the presence of certain symmetries or interactions, electron states with opposite chirality are not equivalent, which imposes constraints to the type of electron scattering. In topological materials, chiral carriers appear because of the non-trivial topology of the bulk band structure. In TIs, this chirality manifests as spin-momentum locking and is protected by TRS. Nanomaterials 2021,11, 1077 8 of 20 Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of 20 the one hand, the magnitude of l can be strongly related to the number of defects, and can be used to probe the quality of thin films. Furthermore, l decreases with increasing temperature due to the thermal activation of phonons. The temperature dependence of l follows a power–law dependence l~T/, where p is positive and determined by decoherence mechanisms such as 2D (p=1) and 3D (p=2) electron–electron interaction and electron–phonon interaction (p=3) [41]. On the other hand, 𝛼 provides knowledge on the channels taking part in the transport. With extra information coming from other magnetotransport measurements, α can be helpful to determine either if the surfaces are contributing to transport or even if the topological regime is taking place. A further study of the dimensionality of the transport can be carried out by studying the WAL effect in tilted magnetic fields. This can distinguish between 2D and 3D character, since 2D transport will depend only on the perpendicular component of the field [42]. In order to obtain a direct interpretation of the model HLN through the experimental magnetotransport data, a detailed simulation of Equation (4) can be carried out by varying the free parameters of the model. By considering that just an integer number of channels can contribute to the coherent transport, α can only take values that are integer multiples of 1/2. Among those cases, α=−1/2, with a single channel, is the most frequently found in BiSe thin films. The minus sign of α appears when the spin–orbit interaction is so strong that the spin number of the carriers is not conserved anymore and, hence, a destructive interference between time reversal closed partner paths occurs (Figure 4b), giving rise to WAL and to a negative correction of the conductance as a function of the magnetic field ∆G(B). On the contrary, a positive value of α appears when the spin–orbit is weak and WL emerges, producing a positive ∆G(B). We are interested in the first case, taking place in TIs. By increasing the phase coherence length l (Figure 5a) the magnitude of the WAL effect increases, which is translated to more pronounced ∆G(B). This occurs because as the value of l increases, more and more paths are available to contribute coherently, and then, the weight of quantum corrections like WAL gain weight on the overall transport. For l<100 nm, ∆G is just a small fraction of the spinless quantum conductance G=eh ⁄=3.35·10 S, and one can consider that the WAL is relatively weak in that system. On the other hand, going from α=−1/2 to α=−1, a new coherent channel is introduced, and, in the approximation of independent channels, this results in a doubling of the WAL correction (Figure 5b). Finally, α=0 has been reported few times, and, in that case, the WAL effect is absent. This has been attributed either to a degradation of the topological protection in the ultrathin limit due to direct coupling between the top and bottom surface states [43], or to a motion of carriers in the strong disordered regime (k l≪1, k being the Fermi wave vector) [44]. (a) (b) (a) (b) Figure 5. Simulations within the HLN model of the magnetoconductance ∆Gxx(B) for different values of lϕ at fixed α : ( a ) a single coherent channel α=−1/2. (b) Two independent channels α=−1. Magnetrotransport measurements have shown corrections to magnetoresistance coming from WAL due to spin-momentum locking in TCI SnTe thin films [ 45 ]. In this material, four Dirac cones are found at each surface and then, intrasurface and intersurface scattering between the cones can appear. Determining the value of α can provide very useful information about the number of cones taking part in transport and, by changing the Fermi level, allows one to find out how these Dirac cones interact with each other. For instance, it is observed how intersurface coupling is suppressed when the Fermi level lies in the bulk band gap while intrasurface scattering is still present. On the other hand, Dirac/Weyl semimetals can also support corrections to the transport in the quantum diffusive regime. It is originated from the opposite chirality associated to each Weyl node forming the pair due to their opposite monopole charge [ 46 ]. The nature of the correction is directly connected to the Berry phase acquired by the electrons performing paths enclosing a Weyl node, and this phase is related to the monopole charge N through πN . In the case of single Weyl semimetal, N=± 1, and the Berry phase is equal to π , giving rise to a destructive interference, i.e., the WAL effect and positive magnetoresistance, whereas in the double Weyl semimetals, N=± 2 and the Berry phase equals 2 π , producing a constructive interference, i.e., the WL effect and negative magnetoresistance. On the other hand, the presence of scattering mechanisms connecting the pair of nodes contributes to localizing carriers and turning WAL into WL [ 47 ]. WAL has been confirmed experimentally in topological semimetals such as Cd3As2[48], Na3Bi [49], or TaAs [50]. These few examples reveal the narrow connection between the quantum corrections to transport in topological systems and their exotic properties, making the study of coherent transport a crucial tool for the complete understanding of these materials, as will also be shown in the particular case of Bi2Se3. 3. WAL in Bi2Se3Thin Films Bi2Se3 thin films represent an excellent platform for the observation of the WAL effect. In Bi2Se3 , the spin–orbit interaction is strong and, in the quantum diffusive regime, a positive magnetoresistance is found. In addition, the reduced dimensionality of thin films confines the motion of carriers in a plane, increasing the probability of electrons performing closed paths and then enhancing the WAL effect. The analysis of the WAL effect serves as a perfect complement for the study of electronic transport. 3.1. Growth Methods The incorporation of TI thin films in electronic devices requires high mobility of non-trivial surface carriers and low bulk carrier density. Defects in the lattice structure contribute to transport as dopants. This is the case of Bi2Se3 , in which n-type selenium vacancies populate the conduction band and make the material metallic. Furthermore, Nanomaterials 2021,11, 1077 9 of 20 even though surface states in TIs are robust, defects in the surface reduce their mobility and, hence, their contribution to transport. For all this, it is important to develop high-quality thin films to fully exploit the potential of TIs in technological applications. From this point of view, MBE has become the most powerful technique in the epitaxial growth of TIs and other 2D materials, followed by PLD. In Bi2Se3 films grown by MBE, mobilities in the range of 500 – 1000 cm2/(V·s) and sheet carrier n2D densities in the order of 10 13 cm−2 (typical densities at the topological regime are in the order of 10 12 cm−2 ) are usually reported [ 31 , 51 ]. Its low growth rate (as low as 0.2–0.3 QLs/min ) provides an accurate control on thickness, the growth of films as thin as 2 QLs being common, allowing one to explore exotic phenomena in the ultrathin regime [ 13 , 43 ]. Substrate temperature turned out to be a key parameter in the improvement of films grown by MBE. The use of a two-step growth method, where a few QLs are first deposited at lower temperatures, and then the rest of the film grown under standard conditions, has produced the best films [ 30 , 52 ]. On the other hand, even though PLD is faster than MBE, adatoms arriving onto the substrate have less time to migrate and rearrange and, hence, low defect samples are generally difficult to obtain. Typically, sheet carrier densities in the order of 10 14 cm−2 (one order of magnitude higher than in MBE-grown samples, indicating more defects) and mobilities in the range of 10100 cm2/(V·s) are measured [ 34 , 53 ]. Under certain growth conditions, films nearly as good as the ones obtained in MBE with high mobilities close to 500 cm2/(V·s) and low carrier densities have been also reported [ 54 ]. Other reported growth methods are chemical vapor deposition (CVD) and magnetron sputtering (MS). CVD has provided good thin films with mobilities around 900 cm2/(V·s) and carrier densities close to the topological regime [ 55 , 56 ], whereas MS films reported show low mobilities in the order of 10 cm2/(V·s) and sheet carrier densities in the range of 8–30·1012 cm−2[57]. Bi2Se3 TI has been grown on a wide variety of substrates, with c-plane sapphire (Al2O3) being the most common. It has the same in-plane lattice structure as Bi2Se3 , even though there is a significant lattice mismatch (∼15%) that can lead to several types of structural defects such as twin defects, antiphase domain, or mosaicity-twist. Apart from sharing the same in-plane structure, sapphire possesses many advantages that make it interesting for Bi2Se3 growth: it is low cost, it possesses surface quality, and it is chemically inert. For specific applications, other substrates can be used instead of sapphire. For example, the dielectric properties of SrTiO3(111) or Si/SiO2 make them ideal for gating purposes [ 44 , 54 ]. In Bi2Se3 films grown on SrTiO3(111) , ambipolar transport has been accomplished several times [ 12 , 32 ]. Transport studies of Bi2Se3 thin films grown on GaAs (111) [ 58 ], Si (111) [ 59 ], SiO2/graphene [ 60 ], and CdS [ 61 ] have also been reported, but most of them require surface preparation procedures such as chemical etching, ion bombardment, or temperature annealing in order to obtain atomically flat and clean substrates, which still places sapphire in a favorable position. It is important to notice that compatibility with silicon makes Bi2Se3suitable for the electronic industry. The influence of substrate choice on the transport properties of thin films has not been clearly demonstrated. In principle, the fact that interactions between QLs are governed by Van der Waals forces should relax the influence of substrate over film properties, even more as film thickness increases. However, ultrahigh mobilities up to 4000 cm2/(V·s) and up to 3500 cm2/(V·s) have been found in Bi2Se3 grown on CdS [ 62 ] and InP (111) [ 63 ], respectively. These values are several times higher than values found on standard substrates such as sapphire or SrTiO3(111) , highlighting that substrate choice might have consequences on transport properties. 3.2. Magnetotransport Properties and WAL Effect in Bi2Se3Thin Films ARPES characterization of Bi2Se3 successfully demonstrates the existence of a topological Dirac cone at the surface. Nevertheless, in transport experiments, these topological states are usually obscured by a bulk Fermi surface, which dominates in many occasions. In order to overcome this difficulty, magnetotransport measurements can provide useful information. Nanomaterials 2021,11, 1077 16 of 20 completely dominates the transport. This is reflected in the thickness dependence of transport parameters such as the mobility, µ , or the phase coherence length, lϕ , going from an independent behaviour in the first case, to a sublinear dependence, and finally to a linear dependence in the case of a very high number of defects. This is manifested clearly in the analysis of lϕ . The spin-orbit parameter α provides extra information that, together with the magnetotransport data, can help to discern the relevance of the bulk in the transport. In the literature, it is commonly observed that α=− 1 / 2, revealing a partial role of the bulk connecting the two surfaces. Even in [ 64 ], we find influence from the bulk in the value of α . This exposes the difficulties in completely avoiding the bulk states in the study of Bi2Se3 , with gating or doping being necessary to reach the topological regime and take αfrom −1/2 to −1. For a deeper analysis of the data reported in Table 1, the phase coherence length lϕ data extracted from the literature, together with that of our samples, were plotted against the mobility µ (Figure 11). Comparing them, it looks like, as µ increases, lϕ saturates, and even though there is a variability in the properties of the films, lϕ shows a behavior that can be predicted from the mobility of the carriers in the system. A theoretical study of this relation could be used as a means for a deeper understanding of transport in topological systems. Nanomaterials 2021, 11, x FOR PEER REVIEW 16 of 20 4. Remarks and Conclusions In summary, we have reviewed the study of quantum transport in BiSe thin films, paying attention to the WAL effect, omnipresent in magnetotransport measurements of these systems. We have shown how the parameters that describe this effect evolve from the most ideal case, where the number of defects is low, towards situations where the introduction of defects enhances the contribution of the bulk more and more until it completely dominates the transport. This is reflected in the thickness dependence of transport parameters such as the mobility, μ, or the phase coherence length, l, going from an independent behaviour in the first case, to a sublinear dependence, and finally to a linear dependence in the case of a very high number of defects. This is manifested clearly in the analysis of l. The spin-orbit parameter α provides extra information that, together with the magnetotransport data, can help to discern the relevance of the bulk in the transport. In the literature, it is commonly observed that α=−1/2, revealing a partial role of the bulk connecting the two surfaces. Even in [64], we find influence from the bulk in the value of α. This exposes the difficulties in completely avoiding the bulk states in the study of BiSe, with gating or doping being necessary to reach the topological regime and take α from −1/2 to − 1. For a deeper analysis of the data reported in Table 1, the phase coherence length l data extracted from the literature, together with that of our samples, were plotted against the mobility μ (Figure 11). Comparing them, it looks like, as μ increases, l saturates, and even though there is a variability in the properties of the films, l shows a behavior that can be predicted from the mobility of the carriers in the system. A theoretical study of this relation could be used as a means for a deeper understanding of transport in topological systems. Figure 11. Representation of the phase coherence length l versus mobility μ values for different films. The dashed black lines are guides for the eyes. Red empty squares correspond to data obtained in our samples. The rest have been taken from literature: Red circles, [13]; blue square, [30]; light green upwards triangle, [31]; cyan left-pointing triangle, [42]; downwards magenta triangles, [54]; right pointing orange triangle, [56]; dark green hexagon, [64]; violet pentagon, [69]; maroon star, [70]. Finally, the data presented in this work demonstrate the potential of BiSe in some of the most researched fields nowadays. The large values of the electron phase coherence length observed for BiSe, typically a few hundred nanometers, as shown in Figure 11, indicate that this material is of great interest for applications in spintronics and quantum technologies. In spintronics, a large spin-to-charge conversion has been observed in Figure 11. Representation of the phase coherence length lϕ versus mobility µ values for different films. The dashed black lines are guides for the eyes. Red empty squares correspond to data obtained in our samples. The rest have been taken from literature: Red circles, [ 13 ]; blue square, [ 30 ]; light green upwards triangle, [ 31 ]; cyan left-pointing triangle, [ 42 ]; downwards magenta triangles, [ 54 ]; right pointing orange triangle, [ 56 ]; dark green hexagon, [ 64 ]; violet pentagon, [ 69 ]; maroon star, [ 70 ]. Finally, the data presented in this work demonstrate the potential of Bi2Se3 in some of the most researched fields nowadays. The large values of the electron phase coherence length observed for Bi2Se3 , typically a few hundred nanometers, as shown in Figure 11, indicate that this material is of great interest for applications in spintronics and quantum technologies. In spintronics, a large spin-to-charge conversion has been observed in Bi2Se3 and ascribed to the presence of surface states and the inverse Rashba-Edelstein effect [ 71 ]. In addition, the existence of a large phase coherence length will allow for coherent spin and charge transport along thick Bi2Se3 spacers, paving the way for more complex spintronic devices combining Bi2Se3 and other materials [ 72 ]. Regarding quantum technologies, it has been put forward that hybrid Bi2Se3 -superconductor heterostructures can host Majorana fermions, which is of interest for quantum computing [ 73 ]. Thanks to its large phase Nanomaterials 2021,11, 1077 17 of 20 coherence length, quantum sensors based on Bi2Se3 can be built too, such as electron interferometers based on the Aharonov–Bohm effect [ 74 ] or Josephson junctions where Bi2Se3constitutes the weak link [75]. Author Contributions: S.S. and J.M.D.T. supervised the work and received the project funding. R.G.- A. performed the fabrication and electrical characterization of the electronic devices as well as the analysis of the experimental data; C.B., S.K.C., and P.O. carried out the synthesis and characterization of the thin films. R.G.-A., S.S., and J.M.D.T. discussed the results and wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: The research was funded by the Spanish Ministry of Economy and Competitiveness through the projects MAT2017-82970-C2-1-R and MAT2017-82970-C2-2-R, and the Aragon Regional Government (Construyendo Europa desde Aragón) through the project E13_20R with European Social Fund funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available from the corresponding authors upon reasonable request. Acknowledgments: R.G.-A. acknowledges the financial support coming from the Diputación General de Aragón through a doctoral fellowship. The authors acknowledge the H2020 NFFA-Europe project 654360 and the CNR-IOM TASC Laboratory. The authors acknowledge support from CSIC Research Platform PTI-001. The authors would like to acknowledge the use of Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza. Conflicts of Interest: The authors declare no conflict of interest. References 1. Von Klitzing, K. The quantized Hall effect. Rev. Mod. Phys. 1986,58, 519–531. [CrossRef] 2. Moore, J. Topological insulators: The next generation. Nat. Phys. 2009,5, 378–380. [CrossRef] 3. Kane, C.L.; Mele, E.J. Quantum Spin hall effect in graphene. Phys. Rev. Lett. 2005,95, 226801. [CrossRef] 4. Hasan, M.Z.; Kane, C.L. Colloquium: Topological insulators. Rev. Mod. Phys. 2010,82, 3045–3067. [CrossRef] 5. Fu, L.; Kane, C.L. Superconducting proximity effect and Majorana fermions at the surface of a topological insulator. Phys. Rev. Lett. 2008,100, 096407. [CrossRef] [PubMed] 6. Pesin, D.; MacDonald, A.H. Spintronics and pseudospintronics in graphene and topological insulators. Nat. Mater. 2012 ,11, 409–416. [CrossRef] [PubMed] 7. Analytis, J.G.; Chu, J.H.; Chen, Y.; Corredor, F.; McDonald, R.D.; Shen, Z.X.; Fisher, I.R. Bulk Fermi surface coexistence with Dirac surface state in Bi 2 Se 3 : A comparison of photoemission and Shubnikov-de Haas measurements. Phys. Rev. B Condens. Matter Mater. Phys. 2010,81, 205407. [CrossRef] 8. König, M.; Molenkamp, L.W.; Qi, X.; Zhang, S. Quantum Spin Hall Insulator State in HgTe Quantum Wells. Science 2007 ,766, 766–771. [CrossRef] [PubMed] 9. Hsieh, D.; Qian, D.; Wray, L.; Xia, Y.; Hor, Y.S.; Cava, R.J.; Hasan, M.Z. A topological Dirac insulator in a quantum spin Hall phase. Nature 2008,452, 970–974. [CrossRef] [PubMed] 10. Xia, Y.; Qian, D.; Hsieh, D.; Wray, L.; Pal, A.; Lin, H.; Bansil, A.; Grauer, D.; Hor, Y.S.; Cava, R.J.; et al Observation of a large-gap topological-insulator class with a single Dirac cone on the surface. Nat. Phys. 2009,5, 398–402. [CrossRef] 11. Hsieh, D.; Xia, Y.; Qian, D.; Wray, L.; Meier, F.; Dil, J.H.; Osterwalder, J.; Patthey, L.; Fedorov, A.V.; Lin, H.; et al Observation of time-reversal-protected single-dirac-cone topological-insulator states in Bi 2 Se 3 and Sb 2 Te 3 .Phys. Rev. Lett. 2009 ,103, 146401. [CrossRef] 12. Chen, J.; Qin, H.J.; Yang, F.; Liu, J.; Guan, T.; Qu, F.M.; Zhang, G.H.; Shi, J.R.; Xie, X.C.; Yang, C.L.; et al Gate-voltage control of chemical potential and weak antilocalization in Bi2Se3.Phys. Rev. Lett. 2010,105, 176602. [CrossRef] 13. Taskin, A.A.; Sasaki, S.; Segawa, K.; Ando, Y. Manifestation of Topological Protection in Transport Properties of Epitaxial Bi 2 Se 3 Thin Films. Phys. Rev. Lett. 2012,109, 066803. [CrossRef] [PubMed] 14. Steinberg, H.; Laloë, J.B.; Fatemi, V.; Moodera, J.S.; Jarillo-Herrero, P. Electrically tunable surface-to-bulk coherent coupling in topological insulator thin films. Phys. Rev. B Condens. Matter Mater. Phys. 2011,84, 233101. [CrossRef] 15. Xue, L.; Zhou, P.; Zhang, C.X.; He, C.Y.; Hao, G.L.; Sun, L.Z.; Zhong, J.X. First-principles study of native point defects in Bi 2 Se 3 . AIP Adv. 2013,3, 052105. [CrossRef] Nanomaterials 2021,11, 1077 18 of 20 16. Scanlon, D.O.; King, P.D.C.; Singh, R.P.; De La Torre, A.; Walker, S.M.K.; Balakrishnan, G.; Baumberger, F.; Catlow, C.R.A. Controlling bulk conductivity in topological insulators: Key role of anti-site defects. Adv. Mater. 2012 ,24, 2154–2158. [CrossRef] [PubMed] 17. Zhang, G.; Qin, H.; Chen, J.; He, X.; Lu, L.; Li, Y.; Wu, K. Growth of topological insulator Bi 2 Se 3 thin films on SrTiO 3 with a large tunability in chemical potential. Adv. Funct. Mater. 2011,21, 2351–2355. [CrossRef] 18. Hor, Y.S.; Richardella, A.; Roushan, P.; Xia, Y.; Checkelsky, J.G.; Yazdani, A.; Hasan, M.Z.; Ong, N.P.; Cava, R.J. P -type Bi 2 Se 3 for topological insulator and low-temperature thermoelectric applications. Phys. Rev. B Condens. Matter Mater. Phys. 2009 ,79, 2–6. [CrossRef] 19. Fu, L. Topological crystalline insulators. Phys. Rev. Lett. 2011,106, 106802. [CrossRef] [PubMed] 20. Hsieh, T.H.; Lin, H.; Liu, J.; Duan, W.; Bansil, A.; Fu, L. Topological crystalline insulators in the SnTe material class. Nat. Commun. 2012,3, 1–7. [CrossRef] 21. Tanaka, Y.; Ren, Z.; Sato, T.; Nakayama, K.; Souma, S.; Takahashi, T.; Segawa, K.; Ando, Y. Experimental realization of a topological crystalline insulator in SnTe. Nat. Phys. 2012,8, 800–803. [CrossRef] 22. Xu, S.Y.; Liu, C.; Alidoust, N.; Neupane, M.; Qian, D.; Belopolski, I.; Denlinger, J.D.; Wang, Y.J.; Lin, H.; Wray, L.A.; et al Observation of a topological crystalline insulator phase and topological phase transition in Pb 1-x Sn x Te. Nat. Commun. 2012 ,3, 1–11. [CrossRef] [PubMed] 23. Dziawa, P.; Kowalski, B.J.; Dybko, K.; Buczko, R.; Szczerbakow, A.; Szot, M.; Łusakowska, E.; Balasubramanian, T.; Wojek, B.M.; Berntsen, M.H.; et al Topological crystalline insulator states in Pb1-xSnxSe. Nat. Mater. 2012,11, 1023–1027. [CrossRef] 24. Dzero, M.; Sun, K.; Galitski, V.; Coleman, P. Topological Kondo insulators. Phys. Rev. Lett. 2010 ,104, 106408. [CrossRef] [PubMed] 25. Xu, N.; Biswas, P.K.; Dil, J.H.; Dhaka, R.S.; Landolt, G.; Muff, S.; Matt, C.E.; Shi, X.; Plumb, N.C.; Radovi´c, M.; et al Direct observation of the spin texture in SmB6as evidence of the topological Kondo insulator. Nat. Commun. 2014,5, 1–5. [CrossRef] 26. Liu, Z.K.; Zhou, B.; Zhang, Y.; Wang, Z.J.; Weng, H.M.; Prabhakaran, D.; Mo, S.K.; Shen, Z.X.; Fang, Z.; Dai, X.; et al Discovery of a three-dimensional topological dirac semimetal, Na3Bi. Science 2014,343, 864–867. [CrossRef] [PubMed] 27. Xu, S.Y.; Belopolski, I.; Alidoust, N.; Neupane, M.; Bian, G.; Zhang, C.; Sankar, R.; Chang, G.; Yuan, Z.; Lee, C.C.; et al Discovery of a Weyl fermion semimetal and topological Fermi arcs. Science 2015,349, 613–617. [CrossRef] [PubMed] 28. Das, A.; Ronen, Y.; Most, Y.; Oreg, Y.; Heiblum, M.; Shtrikman, H. Zero-bias peaks and splitting in an Al-InAs nanowire topological superconductor as a signature of Majorana fermions. Nat. Phys. 2012,8, 887–895. [CrossRef] 29. Sasaki, S.; Kriener, M.; Segawa, K.; Yada, K.; Tanaka, Y.; Sato, M.; Ando, Y.; Cu, T. Topological Superconductivity in Cu x Bi 2 Se 3 . Phys. Rev. Lett. 2011,107, 217001. [CrossRef] 30. Taskin, A.A.; Sasaki, S.; Segawa, K.; Ando, Y. Achieving surface quantum oscillations in topological insulator thin films of Bi 2 Se 3 . Adv. Mater. 2012,24, 5581–5585. [CrossRef] 31. Kim, Y.S.; Brahlek, M.; Bansal, N.; Edrey, E.; Kapilevich, G.A.; Iida, K.; Tanimura, M.; Horibe, Y.; Cheong, S.W.; Oh, S. Thicknessdependent bulk properties and weak antilocalization effect in topological insulator Bi 2 Se 3 .Phys. Rev. B Condens. Matter Mater. Phys. 2011,84, 073109. [CrossRef] 32. Chen, J.; He, X.Y.; Wu, K.H.; Ji, Z.Q.; Lu, L.; Shi, J.R.; Smet, J.H.; Li, Y.Q. Tunable surface conductivity in Bi 2 Se 3 revealed in diffusive electron transport. Phys. Rev. B Condens. Matter Mater. Phys. 2011,83, 1–5. [CrossRef] 33. Bigi, C.; Orgiani, P.; Nardi, A.; Troglia, A.; Fujii, J.; Panaccione, G.; Vobornik, I.; Rossi, G. Robustness of topological states in Bi 2 Se 3 thin film grown by Pulsed Laser Deposition on (001)-oriented SrTiO3perovskite. Appl. Surf. Sci. 2019,473, 190–193. [CrossRef] 34. Lee, Y.F.; Punugupati, S.; Wu, F.; Jin, Z.; Narayan, J.; Schwartz, J. Evidence for topological surface states in epitaxial Bi 2 Se 3 thin film grown by pulsed laser deposition through magneto-transport measurements. Curr. Opin. Solid State Mater. Sci. 2014 ,18, 279–285. [CrossRef] 35. Berry, M. Quantal phase factors accompanying adiabatic changes. Proc. R. Soc. London A. Math. Phys. Sci. 1984 ,392, 45–47. [CrossRef] 36. Bergmann, G. Weak localization in thin films: A time-of-flight experiment with conduction electrons. Phys. Rep. 1986 ,107, 1–58. [CrossRef] 37. Sangiao, S.; Marcano, N.; Fan, J.; Morellón, L.; Ibarra, M.R.; De Teresa, J.M. Quantitative analysis of the weak anti-localization effect in ultrathin Bi films. Epl 2011,95, 37002. [CrossRef] 38. Hasan, M. Berry’s phase and quantization in topological insulators. Physics 2010,3, 62. [CrossRef] 39. Hikami, S.; Larkin, A.I.; Nagaoka, Y. Spin-Orbit Interaction and Magnetoresistance in the Two Dimensional Random System. Prog. Theor. Phys. 1980,63, 707–710. [CrossRef] 40. Brahlek, M.; Koirala, N.; Salehi, M.; Bansal, N.; Oh, S. Emergence of decoupled surface transport channels in bulk insulating Bi2Se3thin films. Phys. Rev. Lett. 2014,113, 026801. [CrossRef] [PubMed] 41. Altshuler, B.L.; Aronov, A.G.; Khmelnitsky, D.E. Effects of electron-electron collisions with small energy transfers on quantum localization. J. Phys. C Solid State Phys. 1982,15, 7367–7386. [CrossRef] 42. Jing, Y.; Huang, S.; Zhang, K.; Wu, J.; Guo, Y.; Peng, H.; Liu, Z.; Xu, H.Q. Weak antilocalization and electron–electron interaction in coupled multiple-channel transport in a Bi2Se3thin film. Nanoscale 2016,18, 1879–1885. [CrossRef] [PubMed] 43. Zhang, Y.; He, K.; Chang, C.Z.; Song, C.L.; Wang, L.L.; Chen, X.; Jia, J.F.; Fang, Z.; Dai, X.; Shan, W.Y.; et al Crossover of the three-dimensional topological insulator Bi2Se3to the two-dimensional limit. Nat. Phys. 2010,6, 584–588. [CrossRef] Nanomaterials 2021,11, 1077 19 of 20 44. 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 Bi2Se3grown on amorphous SiO2/Si. Appl. Phys. Lett. 2015,107, 012106. [CrossRef] 45. Assaf, B.A.; Katmis, F.; Wei, P.; Satpati, B.; Zhang, Z.; Bennett, S.P.; Harris, V.G.; Moodera, J.S.; Heiman, D. Quantum coherent transport in SnTe topological crystalline insulator thin films. Appl. Phys. Lett. 2014,105, 102108. [CrossRef] 46. Lu, H.Z.; Shen, S.Q. Weak antilocalization and localization in disordered and interacting Weyl semimetals. Phys. Rev. B Condens. Matter Mater. Phys. 2015,92, 1–13. [CrossRef] 47. Lu, H.Z.; Shen, S.Q. Quantum transport in topological semimetals under magnetic fields. Front. Phys. 2017 ,12, 1–18. [CrossRef] 48. Zhao, B.; Cheng, P.; Pan, H.; Zhang, S.; Wang, B.; Wang, G.; Xiu, F.; Song, F. Weak antilocalization in Cd3As2thin films. Sci. Rep. 2016,6, 1–7. [CrossRef] 49. Xiong, J.; Kushwaha, S.K.; Liang, T.; Krizan, J.W.; Hirschberger, M.; Wang, W.; Cava, R.J.; Ong, N.P. Evidence for the chiral anomaly in the Dirac semimetal Na3Bi. Science 2015,350, 413–416. [CrossRef] [PubMed] 50. Huang, X.; Zhao, L.; Long, Y.; Wang, P.; Chen, D.; Yang, Z.; Liang, H.; Xue, M.; Weng, H.; Fang, Z.; et al Observation of the chiral-anomaly-induced negative magnetoresistance: In 3D Weyl semimetal TaAs. Phys. Rev. X 2015,5, 031023. [CrossRef] 51. Liu, M.; Chang, C.Z.; Zhang, Z.; Zhang, Y.; Ruan, W.; He, K.; Wang, L.L.; Chen, X.; Jia, J.F.; Zhang, S.C.; et al Electron interactiondriven insulating ground state in Bi 2 Se 3 topological insulators in the two-dimensional limit. Phys. Rev. B Condens. Matter Mater. Phys. 2011,83, 165440. [CrossRef] 52. Li, H.D.; Wang, Z.Y.; Kan, X.; Guo, X.; He, H.T.; Wang, Z.; Wang, J.N.; Wong, T.L.; Wang, N.; Xie, M.H. The van der Waals epitaxy of Bi 2 Se 3 on the vicinal Si(111) surface: An approach for preparing high-quality thin films of a topological insulator. New J. Phys. 2010,12, 103038. [CrossRef] 53. Le, P.H.; Wu, K.H.; Luo, C.W.; Leu, J. Growth and characterization of topological insulator Bi 2 Se 3 thin films on SrTiO 3 using pulsed laser deposition. Thin Solid Films 2013,534, 659–665. [CrossRef] 54. Yang, L.; Wang, Z.; Li, M.; Gao, X.P.A.; Zhang, Z. The dimensional crossover of quantum transport properties in few-layered Bi2Se3thin films. Nanoscale Adv. 2019,1, 2303–2310. [CrossRef] 55. Brom, J.E.; Ke, Y.; Du, R.; Won, D.; Weng, X.; Andre, K.; Gagnon, J.C.; Mohney, S.E.; Li, Q.; Chen, K.; et al Structural and electrical properties of epitaxial Bi 2 Se 3 thin films grown by hybrid physical-chemical vapor deposition. Appl. Phys. Lett. 2012 ,100, 162110. [CrossRef] 56. Lin, Y.C.; Chen, Y.S.; Lee, C.C.; Wu, J.K.; Lee, H.Y.; Te Liang, C.; Chang, Y.H. A study on the epitaxial Bi 2 Se 3 thin film grown by vapor phase epitaxy. AIP Adv. 2016,6, 065218. [CrossRef] 57. Wang, W.J.; Gao, K.H.; Li, Z.Q. Thickness-dependent transport channels in topological insulator Bi 2 Se 3 thin films grown by magnetron sputtering. Sci. Rep. 2016,6, 1–9. [CrossRef] 58. Richardella, A.; Zhang, D.M.; Lee, J.S.; Koser, A.; Rench, D.W.; Yeats, A.L.; Buckley, B.B.; Awschalom, D.D.; Samarth, N. Coherent heteroepitaxy of Bi2Se3on GaAs (111)B. Appl. Phys. Lett. 2010,97, 262104. [CrossRef] 59. Bansal, N.; Kim, Y.S.; Edrey, E.; Brahlek, M.; Horibe, Y.; Iida, K.; Tanimura, M.; Li, G.H.; Feng, T.; Lee, H.D.; et al Epitaxial growth of topological insulator Bi2Se3film on Si(111) with atomically sharp interface. Thin Solid Films 2011,520, 224–229. [CrossRef] 60. Kim, N.; Lee, P.; Kim, Y.; Kim, J.S.; Kim, Y.; Noh, D.Y.; Yu, S.U.; Chung, J.; Kim, K.S. Persistent topological surface state at the interface of Bi2Se3film grown on patterned graphene. ACS Nano 2014,8, 1154–1160. [CrossRef] 61. Kou, X.F.; He, L.; Xiu, F.X.; Lang, M.R.; Liao, Z.M.; Wang, Y.; Fedorov, A.V.; Yu, X.X.; Tang, J.S.; Huang, G.; et al Epitaxial growth of high mobility Bi2Se3thin films on CdS. Appl. Phys. Lett. 2011,98, 2011–2014. [CrossRef] 62. He, L.; Xiu, F.; Yu, X.; Teague, M.; Fan, Y.; Kou, X. Surface-Dominated Conduction in a 6 nm thick Bi 2 Se 3 Thin Film. Nano Lett. 2012,12, 1486–1490. [CrossRef] [PubMed] 63. Guo, X.; Xu, Z.J.; Liu, H.C.; Zhao, B.; Dai, X.Q.; He, H.T.; Wang, J.N.; Liu, H.J.; Ho, W.K.; Xie, M.H. Single domain Bi 2 Se 3 films grown on InP(111)A by molecular-beam epitaxy. Appl. Phys. Lett. 2013,102, 151604. [CrossRef] 64. Bansal, N.; Kim, Y.S.; Brahlek, M.; Edrey, E.; Oh, S. Thickness-independent transport channels in topological insulator Bi 2 Se 3 thin films. Phys. Rev. Lett. 2012,109, 116804. [CrossRef] 65. 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, 54–62. [CrossRef] 66. Orgiani, P.; Bigi, C.; Kumar Das, P.; Fujii, J.; Ciancio, R.; Gobaut, B.; Galdi, A.; Sacco, C.; Maritato, L.; Torelli, P.; et al Structural and electronic properties of Bi 2 Se 3 topological insulator thin films grown by pulsed laser deposition. Appl. Phys. Lett. 2017 ,110, 171601. [CrossRef] 67. Zhang, M.; Wei, Z.; Jin, R.; Ji, Y.; Yan, Y.; Pu, X.; Yang, X.; Zhao, Y. Electrical transport properties and morphology of topological insulator Bi 2 Se 3 thin films with different thickness prepared by magnetron sputtering. Thin Solid Films 2016 ,603, 289–293. [CrossRef] 68. Oveshnikov, L.N.; Prudkoglyad, V.A.; Nekhaeva, E.I.; Kuntsevich, A.Y.; Selivanov, Y.G.; Chizhevskii, E.G.; Aronzon, B.A. Magnetotransport in thin epitaxial Bi2Se3films. JETP Lett. 2016,104, 629–634. [CrossRef] 69. Dey, R.; Roy, A.; Pramanik, T.; Guchhait, S.; Sonde, S.; Rai, A.; Register, L.F.; Banerjee, S.K. Localization and interaction effects of epitaxial Bi2Se3bulk states in two-dimensional limit. J. Appl. Phys. 2016,120, 164301. [CrossRef] 70. You, A.; Be, M.A.Y.; In, I. Magnetotransport in Bi 2 Se 3 thin films epitaxially grown on Ge (111). AIP Adv. 2018 ,8, 115125. [CrossRef] 71. Han, W.; Otani, Y.C.; Maekawa, S. Quantum materials for spin and charge conversion. npj Quantum Mater. 2018 ,3, 1–16. [CrossRef] Nanomaterials 2021,11, 1077 20 of 20 72. Wang, H.; Kally, J.; Lee, J.S.; Liu, T.; Chang, H.; Hickey, D.R.; Mkhoyan, K.A.; Wu, M.; Richardella, A.; Samarth, N. Surface-StateDominated Spin-Charge Current Conversion in Topological-Insulator-Ferromagnetic-Insulator Heterostructures. Phys. Rev. Lett. 2016,117, 076601. [CrossRef] [PubMed] 73. Wang, M.; Liu, C.; Xu, J.; Yang, F.; Miao, L.; Yao, M.; Gao, C.L.; Shen, C.; Ma, X.; Chen, X.; et al The coexistence of superconductivity and topological order in the Bi2Se3thin films. Science 2012,336, 52–55. [CrossRef] 74. Peng, H.; Lai, K.; Kong, D.; Meister, S.; Chen, Y.; Qi, X.L.; Zhang, S.C.; Shen, Z.X.; Cui, Y. Aharonov-Bohm interference in topological insulator nanoribbons. Nat. Mater. 2010,9, 225–229. [CrossRef] [PubMed] 75. Yang, F.; Qu, F.; Shen, J.; Ding, Y.; Chen, J.; Ji, Z.; Liu, G.; Fan, J.; Yang, C.; Fu, L.; et al Proximity-effect-induced superconducting phase in the topological insulator Bi2Se3.Phys. Rev. B Condens. Matter Mater. Phys. 2012,86, 134504. [CrossRef]