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Tuning of carrier type, enhancement of Linear magnetoresistance and inducing ferromagnetism at room temperature with Cu doping in Bi2Te3 Topological Insulators
Accepted Manuscript Title: Tuning of carrier type, enhancement of Linear magnetoresistance and inducing ferromagnetism at room temperature with Cu doping in Bi2 Te3 Topological Insulators Authors: Abhishek Singh, Rahul Singh, T. Patel, G.S. Okram, A. Lakhani, V. Ganeshan, A.K. Ghosh, S.N. Jha, Swapnil Patil, Sandip Chatterjee PII: DOI: Reference:
S0025-5408(17)32792-7 https://doi.org/10.1016/j.materresbull.2017.09.060 MRB 9601
To appear in:
MRB
Received date: Revised date: Accepted date:
18-7-2017 21-9-2017 27-9-2017
Please cite this article as: Abhishek Singh, Rahul Singh, T.Patel, G.S.Okram, A.Lakhani, V.Ganeshan, A.K.Ghosh, S.N.Jha, Swapnil Patil, Sandip Chatterjee, Tuning of carrier type, enhancement of Linear magnetoresistance and inducing ferromagnetism at room temperature with Cu doping in Bi2Te3 Topological Insulators, Materials Research Bulletin https://doi.org/10.1016/j.materresbull.2017.09.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Tuning
of
carrier
type,
enhancement
of
Linear
magnetoresistance and inducing ferromagnetism at room temperature with Cu doping in Bi2Te3 Topological Insulators Abhishek Singh1, Rahul Singh1, T. Patel2, G. S. Okram2, A. Lakhani2, V. Ganeshan2, A. K. Ghosh3, S. N. Jha4, Swapnil Patil1, Sandip Chatterjee1,* 1
Department of Physics, Indian Institute of Technology (Banaras Hindu University), Varanasi-
221005, India 2
UGC-DAE Consortium for Scientific Research, Indore, Madhya Pradesh 452001, India
3
Department of Physics, Banaras Hindu University, Varanasi-221005, India
4
Raja-Ramanna center for advanced technology, Indore, Madhya Pradesh 452001, India
*corresponding author e-mail id: [email protected]
Graphical abstract
(a)
xx(m ohm-cm)
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Bi2Te3 Bi2Cu0.15Te2.85
2
1
0
0
100
200
T(K)
0
300
Bi2Te3
(b)
150
Bi2Cu0.15Te2.85
-30
60
25
2
6
10 2 5 0 0
-150
30
-4
4
-4
-120
2
Bi2Cu0.15Te2.85
15
Power Factor(10 W/K m)
8
Bi2Te3 20
100
200
Seebeck Cofficient(V/K)
90
-90 PF(10 W/K m)
Seebeck Cofficient(V/K)
120
-60
0
300
T(K)
0
100
200
300
T(K)
Clear observation of charge carrier tuning and large thermoelectric power factor
1
Highlights
With Cu doping in Bi2Te3, resistivity increases as Fermi level is shifted into valence band with extra scattering centers.
Cu doping tunes the carrier from n to p type, which is attributed to the presence of TeBi and BiTe antisites.
The observed Linear Magneto resistance is believed to be of quantum origin and associated with gapless linear energy spectrum of surface Dirac Fermions along with disorderness in the sample.
Cu doping induces room temperature ferromagnetism.
Abstract Structural, resistivity, thermoelectric power, magneto-transport and magnetic properties of Cu doped Bi2Te3 topological insulators have been investigated. The tuning of charge carriers from n to p type by Cu doping at Te sites of Bi2Te3 is observed both from Hall effect and thermoelectric power measurements. Carrier mobility decreases with the doping of Cu which provides evidence of the movement of Fermi level from bulk conduction band to the bulk valence band. Thermoelectric power also increases with doping of Cu. Moreover, linear magnetoresistance (LMR) is observed at high magnetic field in pure Bi2Te3 which is associated to the gapless topological surface states protected by time reversal symmetry (TRS). Furthermore, doping of non-magnetic Cu induces ferromagnetism at room temperature and also enhances the magnetoresistance.
2
Introduction: Topological insulators (TIs) are a new class of materials, which are insulating in bulk but conducting at the surface. This is due to the gapless edge or spin resolved surface states (SS), which are topologically protected by time reversal symmetry. The spins are locked in the perpendicular direction of momentum due to the strong spin-orbit interaction. As a matter of fact, electrical conduction is robust against backscattering at the edge states or on the surfaces in TIs. These special helical spin properties of electrons make TIs interesting. Since the locking of spin and orbital states is protected by time reversal symmetry [1], the delocalized surface states are unaffected from nonmagnetic dopants and defects. The possibility of Majorana Fermions [2], Topological superconductivity [3,4], novel magnetoelecric quantum states [5], the absence of backscattering from nonmagnetic impurities [6], exciton condensation [7], magnetic monopole [8], and anomalous quantum Hall effect [9,10] types of exotic properties in TIs are very promising in the application of spintronic devices and quantum computing. Topological surface states in Bi2Te3 and Bi2Se3 with only one massless Dirac cone on each surface were studied using Angle-resolved photoemission spectroscopy (ARPES) [11,12]. Quantum magneto-transport phenomenon such as weak antilocalization [13-15], Aharonov-Bohm oscillations [16], and quantum conductance fluctuations [17], are associated with surface states. The time reversal symmetry protection of the Dirac point can be lifted by magnetic dopants, resulting in a band gap due to the separation in the upper and lower branches of the Dirac cone [18]. Moreover, materials with large magnetoresistance (MR) are of great interest from the application point of view as well as for fundamental research. According to Abrikosov's theory [19], a quantum linear magneto resistance, is expected in the materials having zero band gap with linear energy spectrum whereas Parish and Littlewood (PL) have proposed a classical model for
3
the explanation of MR in the inhomogeneous system having strong disorder such as nonmagnetic graphene, Ag2±δSe and Ag2±δTe, InSb system [20-22] which was based on two-dimensional random register network. Materials with linear MR might be used in magneto electronic applications. Recently, a giant and linear MR in TIs have been reported [23-30]. Zhang et al. showed that on ultra thin film of Bi2Se3 a surface state gap opened at the Dirac point because of tunneling between the top and bottom surface states [31]. Interestingly, while most of the reports in Bi2Te3 samples were on doping on Bi site [32-36], doping on Te site has not yet been reported to the best of our knowledge, which however might be equally interesting to study. Here we report a study of 5% Cu doping on Te site in Bi2Te3. It is observed from Hall resistivity and thermoelectric data that type of carrier changes from n type to p type with Cu doping. Furthermore, beside the interesting features in TIs, breaking of time reversal symmetry on the surface may open a new way for applications. Therefore, magnetically doped TIs have recently been attracted great interest for their promising applications in the emerging field of spintronics and many other novel phenomena discussed above. Jo et al. [37] reported study on Fe doped Bi2Te3 in paramagnetic state, Kulbachinskii [38] have observed that Bi2Te3 becomes ferromagnetic by Fe doping with c axis as the easy magnetization axis and Curie temperature (TC) below 12K. Watson et al.[32] found that Mn doped Bi2Te3 are soft ferromagnets with small coercive field and TC ≈ 9.4K whereas Hor et al. [33] demonstrated that very weak ferromagnetic exchange energy may responsible for the small coercivity and weak ferromagnetism in Mn doped Bi2Te3 which showed second order ferromagnetic transition with a TC of approximately 12K, but ferromagnetic ordering at room temperature is required for the spin based applications. In this present work, we investigate the structural, magneto-transport and magnetic properties of Cu doped Bi2Te3 which shows tuning of carrier type from n to p type, high carrier concentration
4
and mobility, large magnetoresistance and room temperature ferromagnetism which are important for quantum computing, spintronic devices, multifunctional electromagnetic application and disc reading heads.
Experiment: Single crystals of Bi2CuxTe3-x(x=0, 0.15) were grown by modified Bridgman method. High purity powder of Bi (99.999%), Te (99.999%) and Cu (99.999%) were mixed uniformly in their stoichiometric ratios. The mixture was sealed in quartz tube after evacuating down to ~10 -6 torr. The crystal growth involves cooling from 9500C to 5500C in 24 hours and annealing at this temperature for 72 hours. Thus obtained silver colored single crystals were then cooled down to room temperature slowly. For phase identification X-ray powder diffraction (XRD) measurements were carried out with Cu Kα radiation using a Rigaku (MiniFlex II DEXTOP) powder diffractometer (the cleaved surface is shiny and ~5 mm in size). Hall effect and magnetotransport measurements were performed on single crystals by standard four probe technique. These measurement were carried out as a function of magnetic field and temperature by using Quantum Design Physical Property Measurement system (PPMS). Thermopower was measured using a standard home-made set up. Sample was sandwiched between two cylindrical oxygenfree highly conducting copper blocks in vacuum. The voltage difference between the cold and the hot ends was measured at each chosen temperature (details are given in Ref. 39). Magnetic properties were measured using the Quantum Design Magnetic Property Measurement system (MPMS). To study the chemical states, X-ray photoelectron spectroscopy (XPS) investigation were carried out using AlKα radiation, the samples were sputtered with 1.5KeV argon ions.
5
Results & Discussion: The XRD pattern of Bi2CuxTe3-x (x=0, 0.15) samples are shown in Fig. 1. X-ray diffracted 5 beam directions showed only the (00L) with space group 𝐷3𝑑 (R-3m). All the peaks of the as
obtained XRD patterns are indexed using the standard JCPDS file (JCPDS#15-0863). Moreover, the peak positions are consistent with those of already reported data [37]. All peaks positions of pure and Cu doped Bi2Te3 well matched to the standard Bragg’s positions of Rhombohedral structure of Bi2Te3. It is obvious from the XRD patterns that the Cu doping does not lead to the presence of any extra peaks or absence of any peaks of the rhombohedral structure of pure Bi2Te3 indicating the structure of the Cu doped Bi2Te3 retains the rhombohedral crystal structure 5 belonging to the space group 𝐷3𝑑 (R-3m). Laue pattern of Bi2Cu0.15Te2.85 sample is shown in
inset of Fig. 1. The Full Width Half Maxima (FWHM) of the (003) peak is 0.1150 and 0.1250 whereas for (006) peak it is 0.1310 and 0.1370 for pure and Cu doped samples respectively, which are very small indicating that as prepared samples have excellent single crystallinity in long range. Around 2θ=540 and 640, a small splitting due to the CuKβ radiation is observed. An additional peak around 2θ=27.60 and 400 for both the samples is observed which might be due to the slight misalignment or slight tilt angle of the crystallinity of as cleaved single crystals. A small shift of ~0.150±0.020 in every peak position towards lower angle in 2θ value is seen in the doped sample. As a matter of fact, according to Bragg’s law, interplaner spacing (d) increases with Cu doping. The obtained lattice parameters for pure Bi2Te3 sample is a=b=4.55Å and c=30.52 Å and the cell volume V is 547.72 Å3 whereas the lattice parameters for Cu doped sample is a=b=4.46Å and c=30.61 Å and the obtained V is 528.27 Å3, which are consistent with the reported values [31]. Shift in Bragg’s angle and the change in the lattice parameters also support the doping of Cu at Te site. The values of a, b parameters and the cell volume of doped
6
sample are lower than those values for undoped sample which is attributed to the smaller ionic radius of Cu than that of Te, The slightly elongated c parameter in doped sample do not however compensate the decreased a and b parameters in maintaining same lattice volume, leading to smaller cell volume. The valence band spectra was collected for Bi2Te3 and Bi2Cu0.15Te2.85 for a range of photon energies from 16 eV to 27 eV in the angle integrated mode of photoelectron detection (AIPES). Such a range of photon energy was chosen for these compounds based on previous experiments [40]. Based on the inelastic mean free path (IMFP) of the photoelectrons [41] it is concluded that the surface probing depth for h=16 eV becomes almost twice as large as that for h=27 eV. Hence we demonstrate in fig.2 the evolution of the electronic structure across bulk and surface for both the compounds, by comparing the density of states (DOS) obtained at the two extreme photon energies. The DOS was obtained by the well known procedures of division of the valence band spectrum by the Fermi distribution function and the symmetrization procedure [42] (shown in the inset of fig.2(a) and fig. 2(b)). Both the procedures agree with each other in that and both show the surface is more metallic than the bulk which is expected from a topological insulator. This holds true for both the compounds studied. Thus the photoemission results allow us to characterize the surface metallicity of the topological insulator surface. Therefore, the photoemission results together with bulk measurements indicate a reliable topological insulating behavior of Bi2Te3 and Bi2Cu0.15Te2.85 single crystals. In order to see the electrical transport behavior of the materials, temperature variation of resistivity under zero magnetic field for pure and Cu doped Bi2Te3 samples were carried out [Fig. 3 (a)]. It is observed that the resistivity increases with temperature for both pure and doped Bi2Te3 samples indicating typical metallic behavior. Figure 3(b) shows the variation of Seebeck
7
coefficient with temperature for both the Bi2Te3 and Bi2Cu0.15Te2.85 samples in the temperature range of 10K-300K. Seebeck coefficient for pure Bi2Te3 sample shows negative slope whereas Bi2Cu0.15Te2.85 sample shows positive slope in the whole range of temperature indicating a change-over in carrier type from n to p with Cu doping in Bi2Te3. Moreover, the maximum absolute value of Seebeck coefficient in case of Bi2Te3 is 142.8µv/K whereas in doped sample it is around 159.8µV/K. The values are consistent with those reported by Cao et.al. [43]. Recently Seebeck coefficient of Bi2Te3/Cu composite has been reported. It is observed as the Cu content increases the value of the Seebeck coefficient increases but it remains negative [44]. In the present investigation doping of Cu is not only tuning the carrier type but also increasing absolute value of Seebeck coefficient. It has been reported [37] that if bulk single crystal Bi2Te3 are prepared using stoichiometric melting, Tellurium rich composition generates n type charge carriers because of TeBi antisite defects i.e. some of the Te atoms occupy the Bi vacancies. As a consequence, n type charge carriers are generated such that Fermi level touches the bulk conduction band. In the same way, when an excess Bi is incorporated in Te sites, then Bi Te antisite defects become dominant and Bi atoms occupy the Te sites. Bi rich composition thus becomes p type and Fermi level touches the valence band. It is clear that the Cu doped Bi2Cu0.15Te
2.85
is Te deficient. Furthermore, the valence states of Bi and Te is +3 and -2
respectively. Moreover, we have shown below from the XPS measurement that Cu is in Cu0 state. Therefore, Cu0 substitutes the Te which is in -2 states leading to hole doping and hence the Cu doped sample becomes p type. Also, Fermi level moves from the conduction band to bulk valence band. This gives rise to the suppression in carrier mobility and increment in electrical resistivity.
8
To determine the efficiency of thermoelectric power we have calculated the power factor [shown in the inset of Fig. 3(b)] of doped and undoped samples using the formula Power factor (PF) =S2/ρ where ρ is the electrical resistivity and S is the Seebeck coefficient. It has already been shown that with increase of the temperature electrical resistivity increases in both pure and doped samples but the increase in Seebeck coefficient (S) is much larger than the increase of electrical resistivity which gives rise to the increase of power factor with increasing temperature. In order to determine the carrier concentration (n or p) and mobility (μ) of the pure and Cu doped Bi2Te3, we have also carried out the Hall measurement. The carrier concentration has been calculated using the slope of the Hall resistivity. Using the data of carrier concentration and electrical resistivity we determined carrier mobility of the samples. Figure 4(a) shows the variation of Hall resistivity as a function of applied magnetic fields at temperatures 200K and 300K for pure Bi2Te3. Slope of the curve is negative which shows that carriers in pure Bi2Te3 are n type for the entire range of temperature of measurement which is also consistent with the negative Seebeck coefficient of the sample. Calculated carrier concentration (n) of the pure Bi2Te3 is shown in the inset 1 of Fig. 4(a) which comes in the range of reported value given by Zhang et.al. [45]. Since Topological insulators are insulating in bulk but conducting on surface, at high temperature bulk contribution dominates over surface contribution of the sample. As we know topological surface state is a complete quantum phenomenon, existence of quantum mechanical behavior is significant at very low temperature. As a matter of fact, at very low temperature surface state is dominating over bulk state and that is why the carrier concentration is low at low temperature (T≤20K) whereas it is very high at high temperature. Moreover, the rate of increment of carrier density is also increasing with the increase of temperature; this also 9
confirms that bulk insulating character is dominating over surface metallic character of the sample at higher temperature. We have also determined the mobility (μ) of the carriers from the Hall data. Calculated mobility as function of temperature is shown in the inset 2 of Fig. 4(a). Mobility for pure Bi2Te3 sample at the applied field of 2T is 271.48cm2/Vsec and 131.68cm2/Vsec at 200K and 300K respectively whereas at Field 5T it is 225.99cm2/Vsec and 126.71cm2/Vsec at 200K and 300K respectively. Hence it is clear that as we increase both the temperature and field, mobility decreases. This is as expected because with decreasing temperature, freezing out of phonons takes place and as a consequence, the carrier scattering and thus thermal vibration or the contribution of phonon decreases and high mobility prevails. Similar trend happens with the magnetic field. Figure 4(b) shows the variation of Hall resistivity with the applied magnetic field for Bi2Cu0.15Te2.85 sample at 200K and 300K. The non-linearity is observed at both the temperatures. We tried to fit the data with two-carrier model [46] (not shown), but the obtained parameters are not feasible. Therefore, the observed non-linearity might be the indication of Anomalous Hall Effect (AHE), the origin of which is due to the existence of ferromagnetism in Bi2Cu0.15Te2.85. In a magnetic sample the Hall resistivity depends upon applied field as well as magnetization of the materials and is expressed as ρH=R0B+RMM where R0 is the Hall coefficient, B is the applied magnetic field, RM is the anomalous Hall coefficient and M is the magnetization of the material. At high fields, the ordinary Hall Effect dominates as is seen by the linear dependence of ρH whereas at low fields Anomalous Hall effect dominates. The origin of the ferromagnetism in Bi2Cu0.15Te2.85 will be discussed later. The slope of the curve is positive which shows that carriers are p type supporting the observation in Seebeck coefficient. The carrier concentration is 10
2.20x1019 per cm3 and 2.96x1019 per cm3 at 200K and 300K respectively, which is of the same order to that of the pure sample. Carrier density increases with temperature similar to the undoped sample. The variation of mobility with the applied field is shown in the inset of fig. 4(b). Mobility decreases with increase in field and temperature as is observed in the Bi 2Te3 and the value is lower in Bi2Cu0.15Te2.85 than that of the undoped sample which has already been discussed above. It is worthwhile to mention that the presence of foreign element i.e. Cu may also produce extra center of scattering and results higher resistivity as is observed in [Fig. 3(a)]. Moreover, mobility, calculated from the Hall measurement in pure sample, is higher than that of doped one giving rise to the low resistivity. Therefore, both Hall data and thermopower data corroborate the resistivity data. Variation of magneto resistance (MR) as a function of applied magnetic field at different temperatures for pure and doped samples is shown in figure 5. Figure 5 shows a clear linear and non-saturating MR of both the Bi2Te3 and Bi2Cu0.15Te2.85. Also, the MR of Bi2Te3 increases with applied field but decreases with increasing temperature. Besides this linear magnetoresistance (LMR) at high field, the cusp like positive MR at low magnetic field is also observed in Bi2Cu0.15Te2.85 sample indicating weak anti-localization (WAL) supporting the existence of surface state [47-53], and at higher temperature (viz. at 300K) it shows quadratic behavior. Similar behavior without any sign of saturation at high temperature in Bi2Te3 has been reported by He et al. [14]. At high temperature the low field MR dip broadened as at higher temperature phase coherence length decreases [14] Hence WAL decreases with increasing temperature [Fig. 5(d)]. Since the origin of WAL effect is the spin momentum locking at the surface state, Dirac cone [14, 54] requires zero-gap Dirac fermions. Moreover, the maximum MR increases with Cu doping (from 375% for undoped sample to 1000% for doped sample) which enhances the
11
potentiality for application. Coupling of Cu mediated spin to the applied magnetic field may be a reason for the large MR value in doped sample. Xu et al. [9] observed giant positive and linear magnetoresistance in two nonmagnetic narrow band gap semiconductors Ag2+_δSe and Ag2+_δTe having highly disordered stoichiometry. Abrikosov [19] predicted that nonstoichiometry induced disorder can change a narrow band gap semiconductor to a zero band gap with a linear energy spectrum. According to this theory LMR appears in the system having zero band gap with linear energy spectrum similar to the surface state of the topological insulators. The quantum limit requires that applied magnetic field should be so large that only one Landau level participate in the conductivity in such type of gapless state induced by nonstoichiometric disorder. Moreover, According to generic quantum description of galvano-magnetic phenomenon, the longitudinal MR can be represented as 𝜌𝑥𝑥= 𝜌𝑦𝑦= 𝑁𝑖 𝐻 , where ρxx and ρyy are the transverse component of 𝜋𝑛2 𝑒𝑐
magnetoresistance, Ni is the concentration of static scattering centers, H is the applied field, e is the charge of electron, c is the velocity of light and n is the density of electrons. The quantum MR is positive, linear, temperature independent and nonsaturating down to very small fields in such type of systems. According to the above expression resistance is inversely related to the square of the density of electrons (n) which is also not well matched with our experimental data. It is obvious from Fig. 5(a) that MR is independent of temperature up to only 10K in pure Bi2Te3 whereas it is completely temperature dependent for the Cu doped sample and MR is decreasing as we increase the temperature [Fig. 5(b, c, d)], so on the basis of this fact we can say that quantum model praposed by Abrikosov [19] is not applicable in our system.
Parish and
Littlewood [55] presented a classical model using a random resistor network and argued that strongly inhomogeneous and macroscopically disordered semiconductors will show a nonsaturating MR. This model showed that inhomogeneity, disorderness and zero band gap is
12
essential condition for the presence of LMR. According to this classical model, magnitude of the fluctuations in mobility due to disorder is an important factor for the origin of MR, rather than the mobility. It is observed that both MR [Fig. 5(a, b, c, d)] and mobility decrease with increase of temperature. This behavior is consistent with the Parish-Littlewood model [59], hence we expect that the better explanation may be close to classical case. Moreover, it is observed that magnetic impurities break the TRS and hence disappearance of robust metallicity of the surface in TIs occurs which results in band gap opening [18]. As a matter of fact the LMR is due to the presence of surface state as already reported in bulk Bi 2Te3 [56]. Moreover, as we have already discussed that there is site and antisite defects i.e. the presence of disorderness in the studied samples, therefore, this disorderness might be a factor for the linearity in MR which is similar to the results obtained by Wang et al. [57] where it has been explained that magnetic field dependent disorder induced by spin fluctuation might be the reason of linear MR. In order to observe the magnetic state we have performed magnetic measurements. Ferromagnetic states have been confirmed by the M(H) curve in doped sample. Figure 6(a) and 6(b) show the M(H) curves at temperatures 5K and 50K for both the doped and undoped samples. It is clear from the M(H) curves that doping of Cu is inducing magnetic moment. The inset in Fig.s 6(a) and 6(b) at temperatures 5K and 50K clearly show the hysteresis loop indicating that the doping of Cu is tuning the sample from nonmagnetic to ferromagnetic state. The result is also consistent with Hall data of doped sample in which non linearity is observed [Fig.4(b)] which is a clear indication of ferromagnetism. Ferromagnetism has also been reported in magnetic ion (Fe, Cr) doped Bi2Se3 [18, 58]. Thus it can be mentioned that the occurrence of ferromagnetism in the present case is due to the non-magnetic Cu doping instead of doping of
13
any ferromagnetic material. In order to provide detail magnetic state of doped sample, we have plotted magnetization (M) vs. temperature (T) and magnetization (M) vs. applied magnetic field (H) for the doped sample in Fig.s 6(c) and 6(d) respectively. M(T) was measured in the temperature range of 2K ≤ T ≤ 300K at the applied field of 1000 Oe. M(H) measurement of the doped sample was carried out at different temperatures (viz. at 2K, 5K, 50K and 300K) under the applied field range of -2T to 2T. It is clear from the M(H) curve that sample shows ferromagnetic nature even at room temperature. But as the temperature increases the saturation magnetization value decreases from ~0.005 emu/gm (2K) to ~0.001 emu/gm (300K). To investigate the origin of ferromagnetism in our doped sample we have collected the detail information about the valence state and chemical bonding of the dopant ions in the parent lattice by X-ray photoemission spectroscopy (XPS) analysis. The spectra of samples were recorded from freshly prepared surfaces at room temperature (RT) where oxygen and carbon contamination were removed via Ar ion sputtering. The binding energies were calibrated with the C1s reference (284.2eV). No other metal ions other than Bi, Te and Cu can be detected in the samples which clearly indicate that as prepared single crystal is pure. The experimental data are fitted using XPS peak fit software. Fig 7(a) and 7(b) show the core level XPS spectra of Bi4f and Te3d in pure Bi2Te3 whereas fig.s 7(c) and 7(e) show the Bi4f, Te3d along with Cu 2p core level spectra of doped sample. Two peaks at 157.2 eV and 162.3 eV in both pure sample and doped samples are observed in high resolution scan of the Bi4f region which correspond to the binding energies of Bi4f7/2 and Bi4f5/2 spin states, respectively (shown in fig.7(a) and fig.7(c)), which are consistent with the reported data [59]. Similarly, the figures 7(b) and 7(d) reveal two 3d Te spectral peaks at 571.4 and 581.8 for pure and doped samples corresponding to the spin states of Te 3d5/2 and 3d3/2 respectively, consistent with the data observed in a Bi2Te3 single crystal [59]. Fig.7(e)
14
presents the high resolution XPS spectra of Cu2p levels in doped sample. Two peaks at 931.3 eV and 950.3 eV correspond to the Cu2p3/2 and Cu2p1/2 spin states respectively. Absence of any satellite peak around 943 eV rules out the presence of Cu2+ or Cu1+ state in the doped sample [60] indicating that Cu is in Cu0 state. Recently, it has been reported that defect may induce magnetic moment [65-68]. Gao et al. [61] reported that oxygen vacancy at the surface of ZnO nanoparticles are likely to be responsible for the ferromagnetism, Using first principle calculation Yazyev [62] found magnetism in graphene, induced by single carbon atom defects whereas Hong et al. [63] observed room temperature ferromagnetism (RTFM) in different type of semiconducting and insulating oxide thin films such as HfO2, TiO2 and In2O3 due to the defects and oxygen vacancies. In another study, Maiti [64] found that, the exchange splitting in the impurity states having B2p character which originates due to the Boron vacancy, leads to a ferromagnetic state in CaB6. The vacancy or defect phenomenon as discussed above might be a reason for the RTFM in our Cu doped sample, as there is strong probability of site-antisite defects introduced by Cu which also causes tuning of carrier type from n to p. Further study is required to establish the exact origin and mechanism for defect introducd ferromagnetism in doped sample.
Conclusion: The structural, AIPES, magnetotransport and magnetic properties of Cu doped and undoped Bi2Te3 topological insulators were investigated. With Cu doping, resistivity increases as Fermi level is shifted into valence band with extra scattering centers. It is also observed that Cu doping tunes the carrier from n to p type, which is attributed to the presence of TeBi and BiTe antisites. The observed LMR is believed to be of classical origin and associated with gapless linear energy
15
spectrum of surface Dirac Fermions along with disorderness in the sample. Moreover Cu doping induces room temperature ferromagnetism. By developing such type of magnetic TIs a new way may open to design and develop future spintronics and electronic devices where high carrier concentration and mobility, large magnetoresistance and room temperature ferromagnetism (RTFM) is required.
Acknowledgement The authors are grateful to UGC-DAE Consortium for Scientific Research, Indore, India for providing facility. Authors are also grateful to CIF, IIT(BHU) for providing facility for magnetic measurement.
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Figure Captions: Fig1. Room temperature X-ray diffraction patterns of Bi2Te3 and Bi2Cu0.15Te2.85, Inset: Laue pattern of Bi2Cu0.15Te2.85. Fig2. (a, b) Valence Band study of Bi2Te3 and Bi2Cu0.15Te2.85 by
Angle Integrated
Photoemission Spectroscopy. Fig3. (a) Temperature dependence of electrical resistivity for Bi2Te3 and Bi2Cu0.15Te2.85 and (b) Variation of Seebeck coefficient as a function of temperature. Inset represents the power factor (PF) of the Bi2Te3 and Bi2Cu0.15Te2.85. Fig4. (a)Magnetic field dependence of the Hall resistivity of the Bi2Te3 at 200K and 300K. Inset1 represents the variation of carrier concentration as a function of temperature whereas Inset 2 shows the variation of carrier mobility with temperature for Bi2Te3 and (b) Magnetic field dependence of the Hall resistivity of the Bi2Cu0.15Te2.85 at 200K and 300K, Inset is the variation of carrier mobility as a function of applied magnetic field at 200K and 300K. Fig5. (a) Magnetoresistance =
𝑅(𝐻)−𝑅(0) 𝑅(0)
𝑋100% as a function of magnetic field for Bi2Te3 at
different temperatures and (b, c, d) Magnetoresistance=
𝑅(𝐻)−𝑅(0) 𝑅(0)
𝑋100%
as a function
of magnetic field for Bi2Cu0.15Te2.85 at different temperatures. Fig6. (a)-(b) Field dependence of magnetization for Bi2Te3 and Bi2Cu0.15Te2.85 samples at 5K and 50K respectively. The inset represents hysteresis in Bi2Cu0.15Te2.85 sample. (c) Temperature dependence of magnetization for Bi2Cu0.15Te2.85 at an applied magnetic field of 1000 Oe showing ferromagnetic nature. (d) Field dependence of magnetization for Bi2Cu0.15Te2.85 at different temperatures (i.e. 2K, 5K 50K and 300K).
23
Fig7. X-ray Photoemission core level spectrum of Bi and Te in Bi2Te3 sample (a) & (b) whereas (c), (d) & (e) show core level spectrum of Bi, Te and Cu in Bi2Cu0.15Te2.85.
24
10
20
30 40 50 2(in deg.)
Fig1
25
(00 18)
(00 21)
(00 15)
(006)
-Bi2Cu0.15Te2.85
(003)
Intensity(a.u.)
-Bi2Te3
60
70
Bi2Te3
DOS (arb. units)
16 eV 27 eV
(a)
1.0 0.5 0.0 -0.5 -1.0
Bi2Cu0.15Te2.85
DOS (arb. units)
16 eV 27 eV
(b)
1.0 0.5 0.0 -0.5 -1.0
0.0
0.2
0.4
0.6
B.E. (eV)
Fig2
26
0.8
3
(a)
Bi2Te3
xx(m ohm-cm)
Bi2Cu0.15Te2.85 2
1
0
0
100
0
200
T(K)
300
Bi2Te3
150
Bi2Cu0.15Te2.85
-30
90
-90
60 8
Bi2Cu0.15Te2.85
6
15 4 10 2 5 0 0
-150
0
50
100
30
-4
-4
-120
2
2
20
Bi2Te3
PF (10 W/K m)
25
100
T(K)
200
150
200
T(K)
Fig3
27
0
300
250
300
Seebeck Cofficient(V/K)
(b)
-60
PF(10 W/K m)
Seebeck Cofficient(V/K)
120
5.0
N (10
19
cm
-3 )
Bi2Te3 200K 300K
-0.03 2
Mobility (cm / VS)
xy(m-cm)
0.00
-0.06
Bi2Te3 Carrier Concentration
4.5 4.0 Inset I
3.5 3.0
4000
0
200
300
T(K)
Bi2Te3
3000
Inset II
2000 1000 0
100
0
100
200
300
(a)
T(K)
0
1
2
3
4
5
H(T)
Bi2Cu0.15Te2.85 200K 300K Bi Cu0.15Te 2 2.85
150
2
0.06
Mobility (cm / VS)
xy(m-cm)
0.12
(b) 0.00 0
120
200K
Inset II
90
300K 60 10
20
30
40
50
H (KOe)
1
2
3 H(T)
Fig4 28
4
5
400
MR (%)
300
200 100
2K 3K 4K 5K 6K 7K 8K 9K 10K 20K 30K 40K 50K 100K 150K 200K 250K 300K
(a)
1000
MR (%)
Bi2Te3
Bi2Cu0.15Te2.85
500
-10
-5
0
5
-10
10
0
H(T)
80
(c)
Bi2Cu0.15Te2.85
44
MR (%)
Bi2Cu0.15Te2.85
100K 110K 120K 130K 140K 150K 160K 170K 180K 190K 200K
10
H(T) 55
MR (%)
(b)
0
0
160
2K 10K 20K 30K 40K 50K 60K 70K 80K 90K 100K 110K
33 22 11
210K 220K 230K 240K 250K 260K 270K 280K 290K 300K 310K 320K 330K
(d)
0
0 -10
0
10
-10
0
H(T)
H(T)
Fig5
29
10
Bi2Te3 Bi2Cu0.15Te2.85
0 -5
6
5K
Bi2Cu0.15Te2.85
0
50K
0.0 M( emu / g ) X 10 - 5
M(emu/g)X10
(a)
-2
Bi2Te3 Bi2Cu0.15Te2.85
-3
2
1.5
M( emu / g) X 10
M(emu/g)X10
-3
4
(b)
5K
-6 -200
-4
-20
-10
0
0 H(Oe)
10
100
200
-1.5
20
0
50K -100
-50
0
50
100
H(Oe)
-20
-10
0
10
20
H(KOe)
5
-4
M(emu/g)X10
(c)
6
ZFC 1000 Oe 5
4
0
2K 5K 50K 300K
-3
7
M(emu/g)X10
2
-2 -100
H(KOe)
3
Bi2Cu0.15Te2.85
4
100
200
300
T(K)
(d)
0
Bi2Cu0.15Te2.85 -5 -20
0
H(KOe)
Fig6
30
20
(a)
4f7/2
Experimental Data Background Fitted Data
Bi4f Intensity(a.u.)
Intensity(a.u.)
Experimental Data Background 4f5/2 Fitted Data
Bi2Te3
170
165
160
155
3d 3/2
590
585
580
170
165
(c)
4f7/2
4f5/2
570
565
160
155
(d)
Experimental Data Background Fitted
3d 5/2
3d 3/2
Intensity(a.u.)
Bi4f
Bi2Cu0.15Te2.85
175
575
B.E.
Bi2Cu0.15Te2.85
150
590
585
B.E.(eV)
Te 3d
580
575
B.E.(eV)
2p3/2
Experimental Data Background Fitted Data
Intensity(a.u.)
Intensity(a.u.)
Experimental Data Background Fitted Data
Te3d
Bi2Te3
B.E.
(b)
3d 5/2
Bi2Cu0.15Te2.85
980
970
960
Cu2p
2p1/2
950
940
B.E.
Fig7
31
(e)
930
920
910
570
565
S0025-5408(17)32792-7 https://doi.org/10.1016/j.materresbull.2017.09.060 MRB 9601
To appear in:
MRB
Received date: Revised date: Accepted date:
18-7-2017 21-9-2017 27-9-2017
Please cite this article as: Abhishek Singh, Rahul Singh, T.Patel, G.S.Okram, A.Lakhani, V.Ganeshan, A.K.Ghosh, S.N.Jha, Swapnil Patil, Sandip Chatterjee, Tuning of carrier type, enhancement of Linear magnetoresistance and inducing ferromagnetism at room temperature with Cu doping in Bi2Te3 Topological Insulators, Materials Research Bulletin https://doi.org/10.1016/j.materresbull.2017.09.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Tuning
of
carrier
type,
enhancement
of
Linear
magnetoresistance and inducing ferromagnetism at room temperature with Cu doping in Bi2Te3 Topological Insulators Abhishek Singh1, Rahul Singh1, T. Patel2, G. S. Okram2, A. Lakhani2, V. Ganeshan2, A. K. Ghosh3, S. N. Jha4, Swapnil Patil1, Sandip Chatterjee1,* 1
Department of Physics, Indian Institute of Technology (Banaras Hindu University), Varanasi-
221005, India 2
UGC-DAE Consortium for Scientific Research, Indore, Madhya Pradesh 452001, India
3
Department of Physics, Banaras Hindu University, Varanasi-221005, India
4
Raja-Ramanna center for advanced technology, Indore, Madhya Pradesh 452001, India
*corresponding author e-mail id: [email protected]
Graphical abstract
(a)
xx(m ohm-cm)
3
Bi2Te3 Bi2Cu0.15Te2.85
2
1
0
0
100
200
T(K)
0
300
Bi2Te3
(b)
150
Bi2Cu0.15Te2.85
-30
60
25
2
6
10 2 5 0 0
-150
30
-4
4
-4
-120
2
Bi2Cu0.15Te2.85
15
Power Factor(10 W/K m)
8
Bi2Te3 20
100
200
Seebeck Cofficient(V/K)
90
-90 PF(10 W/K m)
Seebeck Cofficient(V/K)
120
-60
0
300
T(K)
0
100
200
300
T(K)
Clear observation of charge carrier tuning and large thermoelectric power factor
1
Highlights
With Cu doping in Bi2Te3, resistivity increases as Fermi level is shifted into valence band with extra scattering centers.
Cu doping tunes the carrier from n to p type, which is attributed to the presence of TeBi and BiTe antisites.
The observed Linear Magneto resistance is believed to be of quantum origin and associated with gapless linear energy spectrum of surface Dirac Fermions along with disorderness in the sample.
Cu doping induces room temperature ferromagnetism.
Abstract Structural, resistivity, thermoelectric power, magneto-transport and magnetic properties of Cu doped Bi2Te3 topological insulators have been investigated. The tuning of charge carriers from n to p type by Cu doping at Te sites of Bi2Te3 is observed both from Hall effect and thermoelectric power measurements. Carrier mobility decreases with the doping of Cu which provides evidence of the movement of Fermi level from bulk conduction band to the bulk valence band. Thermoelectric power also increases with doping of Cu. Moreover, linear magnetoresistance (LMR) is observed at high magnetic field in pure Bi2Te3 which is associated to the gapless topological surface states protected by time reversal symmetry (TRS). Furthermore, doping of non-magnetic Cu induces ferromagnetism at room temperature and also enhances the magnetoresistance.
2
Introduction: Topological insulators (TIs) are a new class of materials, which are insulating in bulk but conducting at the surface. This is due to the gapless edge or spin resolved surface states (SS), which are topologically protected by time reversal symmetry. The spins are locked in the perpendicular direction of momentum due to the strong spin-orbit interaction. As a matter of fact, electrical conduction is robust against backscattering at the edge states or on the surfaces in TIs. These special helical spin properties of electrons make TIs interesting. Since the locking of spin and orbital states is protected by time reversal symmetry [1], the delocalized surface states are unaffected from nonmagnetic dopants and defects. The possibility of Majorana Fermions [2], Topological superconductivity [3,4], novel magnetoelecric quantum states [5], the absence of backscattering from nonmagnetic impurities [6], exciton condensation [7], magnetic monopole [8], and anomalous quantum Hall effect [9,10] types of exotic properties in TIs are very promising in the application of spintronic devices and quantum computing. Topological surface states in Bi2Te3 and Bi2Se3 with only one massless Dirac cone on each surface were studied using Angle-resolved photoemission spectroscopy (ARPES) [11,12]. Quantum magneto-transport phenomenon such as weak antilocalization [13-15], Aharonov-Bohm oscillations [16], and quantum conductance fluctuations [17], are associated with surface states. The time reversal symmetry protection of the Dirac point can be lifted by magnetic dopants, resulting in a band gap due to the separation in the upper and lower branches of the Dirac cone [18]. Moreover, materials with large magnetoresistance (MR) are of great interest from the application point of view as well as for fundamental research. According to Abrikosov's theory [19], a quantum linear magneto resistance, is expected in the materials having zero band gap with linear energy spectrum whereas Parish and Littlewood (PL) have proposed a classical model for
3
the explanation of MR in the inhomogeneous system having strong disorder such as nonmagnetic graphene, Ag2±δSe and Ag2±δTe, InSb system [20-22] which was based on two-dimensional random register network. Materials with linear MR might be used in magneto electronic applications. Recently, a giant and linear MR in TIs have been reported [23-30]. Zhang et al. showed that on ultra thin film of Bi2Se3 a surface state gap opened at the Dirac point because of tunneling between the top and bottom surface states [31]. Interestingly, while most of the reports in Bi2Te3 samples were on doping on Bi site [32-36], doping on Te site has not yet been reported to the best of our knowledge, which however might be equally interesting to study. Here we report a study of 5% Cu doping on Te site in Bi2Te3. It is observed from Hall resistivity and thermoelectric data that type of carrier changes from n type to p type with Cu doping. Furthermore, beside the interesting features in TIs, breaking of time reversal symmetry on the surface may open a new way for applications. Therefore, magnetically doped TIs have recently been attracted great interest for their promising applications in the emerging field of spintronics and many other novel phenomena discussed above. Jo et al. [37] reported study on Fe doped Bi2Te3 in paramagnetic state, Kulbachinskii [38] have observed that Bi2Te3 becomes ferromagnetic by Fe doping with c axis as the easy magnetization axis and Curie temperature (TC) below 12K. Watson et al.[32] found that Mn doped Bi2Te3 are soft ferromagnets with small coercive field and TC ≈ 9.4K whereas Hor et al. [33] demonstrated that very weak ferromagnetic exchange energy may responsible for the small coercivity and weak ferromagnetism in Mn doped Bi2Te3 which showed second order ferromagnetic transition with a TC of approximately 12K, but ferromagnetic ordering at room temperature is required for the spin based applications. In this present work, we investigate the structural, magneto-transport and magnetic properties of Cu doped Bi2Te3 which shows tuning of carrier type from n to p type, high carrier concentration
4
and mobility, large magnetoresistance and room temperature ferromagnetism which are important for quantum computing, spintronic devices, multifunctional electromagnetic application and disc reading heads.
Experiment: Single crystals of Bi2CuxTe3-x(x=0, 0.15) were grown by modified Bridgman method. High purity powder of Bi (99.999%), Te (99.999%) and Cu (99.999%) were mixed uniformly in their stoichiometric ratios. The mixture was sealed in quartz tube after evacuating down to ~10 -6 torr. The crystal growth involves cooling from 9500C to 5500C in 24 hours and annealing at this temperature for 72 hours. Thus obtained silver colored single crystals were then cooled down to room temperature slowly. For phase identification X-ray powder diffraction (XRD) measurements were carried out with Cu Kα radiation using a Rigaku (MiniFlex II DEXTOP) powder diffractometer (the cleaved surface is shiny and ~5 mm in size). Hall effect and magnetotransport measurements were performed on single crystals by standard four probe technique. These measurement were carried out as a function of magnetic field and temperature by using Quantum Design Physical Property Measurement system (PPMS). Thermopower was measured using a standard home-made set up. Sample was sandwiched between two cylindrical oxygenfree highly conducting copper blocks in vacuum. The voltage difference between the cold and the hot ends was measured at each chosen temperature (details are given in Ref. 39). Magnetic properties were measured using the Quantum Design Magnetic Property Measurement system (MPMS). To study the chemical states, X-ray photoelectron spectroscopy (XPS) investigation were carried out using AlKα radiation, the samples were sputtered with 1.5KeV argon ions.
5
Results & Discussion: The XRD pattern of Bi2CuxTe3-x (x=0, 0.15) samples are shown in Fig. 1. X-ray diffracted 5 beam directions showed only the (00L) with space group 𝐷3𝑑 (R-3m). All the peaks of the as
obtained XRD patterns are indexed using the standard JCPDS file (JCPDS#15-0863). Moreover, the peak positions are consistent with those of already reported data [37]. All peaks positions of pure and Cu doped Bi2Te3 well matched to the standard Bragg’s positions of Rhombohedral structure of Bi2Te3. It is obvious from the XRD patterns that the Cu doping does not lead to the presence of any extra peaks or absence of any peaks of the rhombohedral structure of pure Bi2Te3 indicating the structure of the Cu doped Bi2Te3 retains the rhombohedral crystal structure 5 belonging to the space group 𝐷3𝑑 (R-3m). Laue pattern of Bi2Cu0.15Te2.85 sample is shown in
inset of Fig. 1. The Full Width Half Maxima (FWHM) of the (003) peak is 0.1150 and 0.1250 whereas for (006) peak it is 0.1310 and 0.1370 for pure and Cu doped samples respectively, which are very small indicating that as prepared samples have excellent single crystallinity in long range. Around 2θ=540 and 640, a small splitting due to the CuKβ radiation is observed. An additional peak around 2θ=27.60 and 400 for both the samples is observed which might be due to the slight misalignment or slight tilt angle of the crystallinity of as cleaved single crystals. A small shift of ~0.150±0.020 in every peak position towards lower angle in 2θ value is seen in the doped sample. As a matter of fact, according to Bragg’s law, interplaner spacing (d) increases with Cu doping. The obtained lattice parameters for pure Bi2Te3 sample is a=b=4.55Å and c=30.52 Å and the cell volume V is 547.72 Å3 whereas the lattice parameters for Cu doped sample is a=b=4.46Å and c=30.61 Å and the obtained V is 528.27 Å3, which are consistent with the reported values [31]. Shift in Bragg’s angle and the change in the lattice parameters also support the doping of Cu at Te site. The values of a, b parameters and the cell volume of doped
6
sample are lower than those values for undoped sample which is attributed to the smaller ionic radius of Cu than that of Te, The slightly elongated c parameter in doped sample do not however compensate the decreased a and b parameters in maintaining same lattice volume, leading to smaller cell volume. The valence band spectra was collected for Bi2Te3 and Bi2Cu0.15Te2.85 for a range of photon energies from 16 eV to 27 eV in the angle integrated mode of photoelectron detection (AIPES). Such a range of photon energy was chosen for these compounds based on previous experiments [40]. Based on the inelastic mean free path (IMFP) of the photoelectrons [41] it is concluded that the surface probing depth for h=16 eV becomes almost twice as large as that for h=27 eV. Hence we demonstrate in fig.2 the evolution of the electronic structure across bulk and surface for both the compounds, by comparing the density of states (DOS) obtained at the two extreme photon energies. The DOS was obtained by the well known procedures of division of the valence band spectrum by the Fermi distribution function and the symmetrization procedure [42] (shown in the inset of fig.2(a) and fig. 2(b)). Both the procedures agree with each other in that and both show the surface is more metallic than the bulk which is expected from a topological insulator. This holds true for both the compounds studied. Thus the photoemission results allow us to characterize the surface metallicity of the topological insulator surface. Therefore, the photoemission results together with bulk measurements indicate a reliable topological insulating behavior of Bi2Te3 and Bi2Cu0.15Te2.85 single crystals. In order to see the electrical transport behavior of the materials, temperature variation of resistivity under zero magnetic field for pure and Cu doped Bi2Te3 samples were carried out [Fig. 3 (a)]. It is observed that the resistivity increases with temperature for both pure and doped Bi2Te3 samples indicating typical metallic behavior. Figure 3(b) shows the variation of Seebeck
7
coefficient with temperature for both the Bi2Te3 and Bi2Cu0.15Te2.85 samples in the temperature range of 10K-300K. Seebeck coefficient for pure Bi2Te3 sample shows negative slope whereas Bi2Cu0.15Te2.85 sample shows positive slope in the whole range of temperature indicating a change-over in carrier type from n to p with Cu doping in Bi2Te3. Moreover, the maximum absolute value of Seebeck coefficient in case of Bi2Te3 is 142.8µv/K whereas in doped sample it is around 159.8µV/K. The values are consistent with those reported by Cao et.al. [43]. Recently Seebeck coefficient of Bi2Te3/Cu composite has been reported. It is observed as the Cu content increases the value of the Seebeck coefficient increases but it remains negative [44]. In the present investigation doping of Cu is not only tuning the carrier type but also increasing absolute value of Seebeck coefficient. It has been reported [37] that if bulk single crystal Bi2Te3 are prepared using stoichiometric melting, Tellurium rich composition generates n type charge carriers because of TeBi antisite defects i.e. some of the Te atoms occupy the Bi vacancies. As a consequence, n type charge carriers are generated such that Fermi level touches the bulk conduction band. In the same way, when an excess Bi is incorporated in Te sites, then Bi Te antisite defects become dominant and Bi atoms occupy the Te sites. Bi rich composition thus becomes p type and Fermi level touches the valence band. It is clear that the Cu doped Bi2Cu0.15Te
2.85
is Te deficient. Furthermore, the valence states of Bi and Te is +3 and -2
respectively. Moreover, we have shown below from the XPS measurement that Cu is in Cu0 state. Therefore, Cu0 substitutes the Te which is in -2 states leading to hole doping and hence the Cu doped sample becomes p type. Also, Fermi level moves from the conduction band to bulk valence band. This gives rise to the suppression in carrier mobility and increment in electrical resistivity.
8
To determine the efficiency of thermoelectric power we have calculated the power factor [shown in the inset of Fig. 3(b)] of doped and undoped samples using the formula Power factor (PF) =S2/ρ where ρ is the electrical resistivity and S is the Seebeck coefficient. It has already been shown that with increase of the temperature electrical resistivity increases in both pure and doped samples but the increase in Seebeck coefficient (S) is much larger than the increase of electrical resistivity which gives rise to the increase of power factor with increasing temperature. In order to determine the carrier concentration (n or p) and mobility (μ) of the pure and Cu doped Bi2Te3, we have also carried out the Hall measurement. The carrier concentration has been calculated using the slope of the Hall resistivity. Using the data of carrier concentration and electrical resistivity we determined carrier mobility of the samples. Figure 4(a) shows the variation of Hall resistivity as a function of applied magnetic fields at temperatures 200K and 300K for pure Bi2Te3. Slope of the curve is negative which shows that carriers in pure Bi2Te3 are n type for the entire range of temperature of measurement which is also consistent with the negative Seebeck coefficient of the sample. Calculated carrier concentration (n) of the pure Bi2Te3 is shown in the inset 1 of Fig. 4(a) which comes in the range of reported value given by Zhang et.al. [45]. Since Topological insulators are insulating in bulk but conducting on surface, at high temperature bulk contribution dominates over surface contribution of the sample. As we know topological surface state is a complete quantum phenomenon, existence of quantum mechanical behavior is significant at very low temperature. As a matter of fact, at very low temperature surface state is dominating over bulk state and that is why the carrier concentration is low at low temperature (T≤20K) whereas it is very high at high temperature. Moreover, the rate of increment of carrier density is also increasing with the increase of temperature; this also 9
confirms that bulk insulating character is dominating over surface metallic character of the sample at higher temperature. We have also determined the mobility (μ) of the carriers from the Hall data. Calculated mobility as function of temperature is shown in the inset 2 of Fig. 4(a). Mobility for pure Bi2Te3 sample at the applied field of 2T is 271.48cm2/Vsec and 131.68cm2/Vsec at 200K and 300K respectively whereas at Field 5T it is 225.99cm2/Vsec and 126.71cm2/Vsec at 200K and 300K respectively. Hence it is clear that as we increase both the temperature and field, mobility decreases. This is as expected because with decreasing temperature, freezing out of phonons takes place and as a consequence, the carrier scattering and thus thermal vibration or the contribution of phonon decreases and high mobility prevails. Similar trend happens with the magnetic field. Figure 4(b) shows the variation of Hall resistivity with the applied magnetic field for Bi2Cu0.15Te2.85 sample at 200K and 300K. The non-linearity is observed at both the temperatures. We tried to fit the data with two-carrier model [46] (not shown), but the obtained parameters are not feasible. Therefore, the observed non-linearity might be the indication of Anomalous Hall Effect (AHE), the origin of which is due to the existence of ferromagnetism in Bi2Cu0.15Te2.85. In a magnetic sample the Hall resistivity depends upon applied field as well as magnetization of the materials and is expressed as ρH=R0B+RMM where R0 is the Hall coefficient, B is the applied magnetic field, RM is the anomalous Hall coefficient and M is the magnetization of the material. At high fields, the ordinary Hall Effect dominates as is seen by the linear dependence of ρH whereas at low fields Anomalous Hall effect dominates. The origin of the ferromagnetism in Bi2Cu0.15Te2.85 will be discussed later. The slope of the curve is positive which shows that carriers are p type supporting the observation in Seebeck coefficient. The carrier concentration is 10
2.20x1019 per cm3 and 2.96x1019 per cm3 at 200K and 300K respectively, which is of the same order to that of the pure sample. Carrier density increases with temperature similar to the undoped sample. The variation of mobility with the applied field is shown in the inset of fig. 4(b). Mobility decreases with increase in field and temperature as is observed in the Bi 2Te3 and the value is lower in Bi2Cu0.15Te2.85 than that of the undoped sample which has already been discussed above. It is worthwhile to mention that the presence of foreign element i.e. Cu may also produce extra center of scattering and results higher resistivity as is observed in [Fig. 3(a)]. Moreover, mobility, calculated from the Hall measurement in pure sample, is higher than that of doped one giving rise to the low resistivity. Therefore, both Hall data and thermopower data corroborate the resistivity data. Variation of magneto resistance (MR) as a function of applied magnetic field at different temperatures for pure and doped samples is shown in figure 5. Figure 5 shows a clear linear and non-saturating MR of both the Bi2Te3 and Bi2Cu0.15Te2.85. Also, the MR of Bi2Te3 increases with applied field but decreases with increasing temperature. Besides this linear magnetoresistance (LMR) at high field, the cusp like positive MR at low magnetic field is also observed in Bi2Cu0.15Te2.85 sample indicating weak anti-localization (WAL) supporting the existence of surface state [47-53], and at higher temperature (viz. at 300K) it shows quadratic behavior. Similar behavior without any sign of saturation at high temperature in Bi2Te3 has been reported by He et al. [14]. At high temperature the low field MR dip broadened as at higher temperature phase coherence length decreases [14] Hence WAL decreases with increasing temperature [Fig. 5(d)]. Since the origin of WAL effect is the spin momentum locking at the surface state, Dirac cone [14, 54] requires zero-gap Dirac fermions. Moreover, the maximum MR increases with Cu doping (from 375% for undoped sample to 1000% for doped sample) which enhances the
11
potentiality for application. Coupling of Cu mediated spin to the applied magnetic field may be a reason for the large MR value in doped sample. Xu et al. [9] observed giant positive and linear magnetoresistance in two nonmagnetic narrow band gap semiconductors Ag2+_δSe and Ag2+_δTe having highly disordered stoichiometry. Abrikosov [19] predicted that nonstoichiometry induced disorder can change a narrow band gap semiconductor to a zero band gap with a linear energy spectrum. According to this theory LMR appears in the system having zero band gap with linear energy spectrum similar to the surface state of the topological insulators. The quantum limit requires that applied magnetic field should be so large that only one Landau level participate in the conductivity in such type of gapless state induced by nonstoichiometric disorder. Moreover, According to generic quantum description of galvano-magnetic phenomenon, the longitudinal MR can be represented as 𝜌𝑥𝑥= 𝜌𝑦𝑦= 𝑁𝑖 𝐻 , where ρxx and ρyy are the transverse component of 𝜋𝑛2 𝑒𝑐
magnetoresistance, Ni is the concentration of static scattering centers, H is the applied field, e is the charge of electron, c is the velocity of light and n is the density of electrons. The quantum MR is positive, linear, temperature independent and nonsaturating down to very small fields in such type of systems. According to the above expression resistance is inversely related to the square of the density of electrons (n) which is also not well matched with our experimental data. It is obvious from Fig. 5(a) that MR is independent of temperature up to only 10K in pure Bi2Te3 whereas it is completely temperature dependent for the Cu doped sample and MR is decreasing as we increase the temperature [Fig. 5(b, c, d)], so on the basis of this fact we can say that quantum model praposed by Abrikosov [19] is not applicable in our system.
Parish and
Littlewood [55] presented a classical model using a random resistor network and argued that strongly inhomogeneous and macroscopically disordered semiconductors will show a nonsaturating MR. This model showed that inhomogeneity, disorderness and zero band gap is
12
essential condition for the presence of LMR. According to this classical model, magnitude of the fluctuations in mobility due to disorder is an important factor for the origin of MR, rather than the mobility. It is observed that both MR [Fig. 5(a, b, c, d)] and mobility decrease with increase of temperature. This behavior is consistent with the Parish-Littlewood model [59], hence we expect that the better explanation may be close to classical case. Moreover, it is observed that magnetic impurities break the TRS and hence disappearance of robust metallicity of the surface in TIs occurs which results in band gap opening [18]. As a matter of fact the LMR is due to the presence of surface state as already reported in bulk Bi 2Te3 [56]. Moreover, as we have already discussed that there is site and antisite defects i.e. the presence of disorderness in the studied samples, therefore, this disorderness might be a factor for the linearity in MR which is similar to the results obtained by Wang et al. [57] where it has been explained that magnetic field dependent disorder induced by spin fluctuation might be the reason of linear MR. In order to observe the magnetic state we have performed magnetic measurements. Ferromagnetic states have been confirmed by the M(H) curve in doped sample. Figure 6(a) and 6(b) show the M(H) curves at temperatures 5K and 50K for both the doped and undoped samples. It is clear from the M(H) curves that doping of Cu is inducing magnetic moment. The inset in Fig.s 6(a) and 6(b) at temperatures 5K and 50K clearly show the hysteresis loop indicating that the doping of Cu is tuning the sample from nonmagnetic to ferromagnetic state. The result is also consistent with Hall data of doped sample in which non linearity is observed [Fig.4(b)] which is a clear indication of ferromagnetism. Ferromagnetism has also been reported in magnetic ion (Fe, Cr) doped Bi2Se3 [18, 58]. Thus it can be mentioned that the occurrence of ferromagnetism in the present case is due to the non-magnetic Cu doping instead of doping of
13
any ferromagnetic material. In order to provide detail magnetic state of doped sample, we have plotted magnetization (M) vs. temperature (T) and magnetization (M) vs. applied magnetic field (H) for the doped sample in Fig.s 6(c) and 6(d) respectively. M(T) was measured in the temperature range of 2K ≤ T ≤ 300K at the applied field of 1000 Oe. M(H) measurement of the doped sample was carried out at different temperatures (viz. at 2K, 5K, 50K and 300K) under the applied field range of -2T to 2T. It is clear from the M(H) curve that sample shows ferromagnetic nature even at room temperature. But as the temperature increases the saturation magnetization value decreases from ~0.005 emu/gm (2K) to ~0.001 emu/gm (300K). To investigate the origin of ferromagnetism in our doped sample we have collected the detail information about the valence state and chemical bonding of the dopant ions in the parent lattice by X-ray photoemission spectroscopy (XPS) analysis. The spectra of samples were recorded from freshly prepared surfaces at room temperature (RT) where oxygen and carbon contamination were removed via Ar ion sputtering. The binding energies were calibrated with the C1s reference (284.2eV). No other metal ions other than Bi, Te and Cu can be detected in the samples which clearly indicate that as prepared single crystal is pure. The experimental data are fitted using XPS peak fit software. Fig 7(a) and 7(b) show the core level XPS spectra of Bi4f and Te3d in pure Bi2Te3 whereas fig.s 7(c) and 7(e) show the Bi4f, Te3d along with Cu 2p core level spectra of doped sample. Two peaks at 157.2 eV and 162.3 eV in both pure sample and doped samples are observed in high resolution scan of the Bi4f region which correspond to the binding energies of Bi4f7/2 and Bi4f5/2 spin states, respectively (shown in fig.7(a) and fig.7(c)), which are consistent with the reported data [59]. Similarly, the figures 7(b) and 7(d) reveal two 3d Te spectral peaks at 571.4 and 581.8 for pure and doped samples corresponding to the spin states of Te 3d5/2 and 3d3/2 respectively, consistent with the data observed in a Bi2Te3 single crystal [59]. Fig.7(e)
14
presents the high resolution XPS spectra of Cu2p levels in doped sample. Two peaks at 931.3 eV and 950.3 eV correspond to the Cu2p3/2 and Cu2p1/2 spin states respectively. Absence of any satellite peak around 943 eV rules out the presence of Cu2+ or Cu1+ state in the doped sample [60] indicating that Cu is in Cu0 state. Recently, it has been reported that defect may induce magnetic moment [65-68]. Gao et al. [61] reported that oxygen vacancy at the surface of ZnO nanoparticles are likely to be responsible for the ferromagnetism, Using first principle calculation Yazyev [62] found magnetism in graphene, induced by single carbon atom defects whereas Hong et al. [63] observed room temperature ferromagnetism (RTFM) in different type of semiconducting and insulating oxide thin films such as HfO2, TiO2 and In2O3 due to the defects and oxygen vacancies. In another study, Maiti [64] found that, the exchange splitting in the impurity states having B2p character which originates due to the Boron vacancy, leads to a ferromagnetic state in CaB6. The vacancy or defect phenomenon as discussed above might be a reason for the RTFM in our Cu doped sample, as there is strong probability of site-antisite defects introduced by Cu which also causes tuning of carrier type from n to p. Further study is required to establish the exact origin and mechanism for defect introducd ferromagnetism in doped sample.
Conclusion: The structural, AIPES, magnetotransport and magnetic properties of Cu doped and undoped Bi2Te3 topological insulators were investigated. With Cu doping, resistivity increases as Fermi level is shifted into valence band with extra scattering centers. It is also observed that Cu doping tunes the carrier from n to p type, which is attributed to the presence of TeBi and BiTe antisites. The observed LMR is believed to be of classical origin and associated with gapless linear energy
15
spectrum of surface Dirac Fermions along with disorderness in the sample. Moreover Cu doping induces room temperature ferromagnetism. By developing such type of magnetic TIs a new way may open to design and develop future spintronics and electronic devices where high carrier concentration and mobility, large magnetoresistance and room temperature ferromagnetism (RTFM) is required.
Acknowledgement The authors are grateful to UGC-DAE Consortium for Scientific Research, Indore, India for providing facility. Authors are also grateful to CIF, IIT(BHU) for providing facility for magnetic measurement.
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22
Figure Captions: Fig1. Room temperature X-ray diffraction patterns of Bi2Te3 and Bi2Cu0.15Te2.85, Inset: Laue pattern of Bi2Cu0.15Te2.85. Fig2. (a, b) Valence Band study of Bi2Te3 and Bi2Cu0.15Te2.85 by
Angle Integrated
Photoemission Spectroscopy. Fig3. (a) Temperature dependence of electrical resistivity for Bi2Te3 and Bi2Cu0.15Te2.85 and (b) Variation of Seebeck coefficient as a function of temperature. Inset represents the power factor (PF) of the Bi2Te3 and Bi2Cu0.15Te2.85. Fig4. (a)Magnetic field dependence of the Hall resistivity of the Bi2Te3 at 200K and 300K. Inset1 represents the variation of carrier concentration as a function of temperature whereas Inset 2 shows the variation of carrier mobility with temperature for Bi2Te3 and (b) Magnetic field dependence of the Hall resistivity of the Bi2Cu0.15Te2.85 at 200K and 300K, Inset is the variation of carrier mobility as a function of applied magnetic field at 200K and 300K. Fig5. (a) Magnetoresistance =
𝑅(𝐻)−𝑅(0) 𝑅(0)
𝑋100% as a function of magnetic field for Bi2Te3 at
different temperatures and (b, c, d) Magnetoresistance=
𝑅(𝐻)−𝑅(0) 𝑅(0)
𝑋100%
as a function
of magnetic field for Bi2Cu0.15Te2.85 at different temperatures. Fig6. (a)-(b) Field dependence of magnetization for Bi2Te3 and Bi2Cu0.15Te2.85 samples at 5K and 50K respectively. The inset represents hysteresis in Bi2Cu0.15Te2.85 sample. (c) Temperature dependence of magnetization for Bi2Cu0.15Te2.85 at an applied magnetic field of 1000 Oe showing ferromagnetic nature. (d) Field dependence of magnetization for Bi2Cu0.15Te2.85 at different temperatures (i.e. 2K, 5K 50K and 300K).
23
Fig7. X-ray Photoemission core level spectrum of Bi and Te in Bi2Te3 sample (a) & (b) whereas (c), (d) & (e) show core level spectrum of Bi, Te and Cu in Bi2Cu0.15Te2.85.
24
10
20
30 40 50 2(in deg.)
Fig1
25
(00 18)
(00 21)
(00 15)
(006)
-Bi2Cu0.15Te2.85
(003)
Intensity(a.u.)
-Bi2Te3
60
70
Bi2Te3
DOS (arb. units)
16 eV 27 eV
(a)
1.0 0.5 0.0 -0.5 -1.0
Bi2Cu0.15Te2.85
DOS (arb. units)
16 eV 27 eV
(b)
1.0 0.5 0.0 -0.5 -1.0
0.0
0.2
0.4
0.6
B.E. (eV)
Fig2
26
0.8
3
(a)
Bi2Te3
xx(m ohm-cm)
Bi2Cu0.15Te2.85 2
1
0
0
100
0
200
T(K)
300
Bi2Te3
150
Bi2Cu0.15Te2.85
-30
90
-90
60 8
Bi2Cu0.15Te2.85
6
15 4 10 2 5 0 0
-150
0
50
100
30
-4
-4
-120
2
2
20
Bi2Te3
PF (10 W/K m)
25
100
T(K)
200
150
200
T(K)
Fig3
27
0
300
250
300
Seebeck Cofficient(V/K)
(b)
-60
PF(10 W/K m)
Seebeck Cofficient(V/K)
120
5.0
N (10
19
cm
-3 )
Bi2Te3 200K 300K
-0.03 2
Mobility (cm / VS)
xy(m-cm)
0.00
-0.06
Bi2Te3 Carrier Concentration
4.5 4.0 Inset I
3.5 3.0
4000
0
200
300
T(K)
Bi2Te3
3000
Inset II
2000 1000 0
100
0
100
200
300
(a)
T(K)
0
1
2
3
4
5
H(T)
Bi2Cu0.15Te2.85 200K 300K Bi Cu0.15Te 2 2.85
150
2
0.06
Mobility (cm / VS)
xy(m-cm)
0.12
(b) 0.00 0
120
200K
Inset II
90
300K 60 10
20
30
40
50
H (KOe)
1
2
3 H(T)
Fig4 28
4
5
400
MR (%)
300
200 100
2K 3K 4K 5K 6K 7K 8K 9K 10K 20K 30K 40K 50K 100K 150K 200K 250K 300K
(a)
1000
MR (%)
Bi2Te3
Bi2Cu0.15Te2.85
500
-10
-5
0
5
-10
10
0
H(T)
80
(c)
Bi2Cu0.15Te2.85
44
MR (%)
Bi2Cu0.15Te2.85
100K 110K 120K 130K 140K 150K 160K 170K 180K 190K 200K
10
H(T) 55
MR (%)
(b)
0
0
160
2K 10K 20K 30K 40K 50K 60K 70K 80K 90K 100K 110K
33 22 11
210K 220K 230K 240K 250K 260K 270K 280K 290K 300K 310K 320K 330K
(d)
0
0 -10
0
10
-10
0
H(T)
H(T)
Fig5
29
10
Bi2Te3 Bi2Cu0.15Te2.85
0 -5
6
5K
Bi2Cu0.15Te2.85
0
50K
0.0 M( emu / g ) X 10 - 5
M(emu/g)X10
(a)
-2
Bi2Te3 Bi2Cu0.15Te2.85
-3
2
1.5
M( emu / g) X 10
M(emu/g)X10
-3
4
(b)
5K
-6 -200
-4
-20
-10
0
0 H(Oe)
10
100
200
-1.5
20
0
50K -100
-50
0
50
100
H(Oe)
-20
-10
0
10
20
H(KOe)
5
-4
M(emu/g)X10
(c)
6
ZFC 1000 Oe 5
4
0
2K 5K 50K 300K
-3
7
M(emu/g)X10
2
-2 -100
H(KOe)
3
Bi2Cu0.15Te2.85
4
100
200
300
T(K)
(d)
0
Bi2Cu0.15Te2.85 -5 -20
0
H(KOe)
Fig6
30
20
(a)
4f7/2
Experimental Data Background Fitted Data
Bi4f Intensity(a.u.)
Intensity(a.u.)
Experimental Data Background 4f5/2 Fitted Data
Bi2Te3
170
165
160
155
3d 3/2
590
585
580
170
165
(c)
4f7/2
4f5/2
570
565
160
155
(d)
Experimental Data Background Fitted
3d 5/2
3d 3/2
Intensity(a.u.)
Bi4f
Bi2Cu0.15Te2.85
175
575
B.E.
Bi2Cu0.15Te2.85
150
590
585
B.E.(eV)
Te 3d
580
575
B.E.(eV)
2p3/2
Experimental Data Background Fitted Data
Intensity(a.u.)
Intensity(a.u.)
Experimental Data Background Fitted Data
Te3d
Bi2Te3
B.E.
(b)
3d 5/2
Bi2Cu0.15Te2.85
980
970
960
Cu2p
2p1/2
950
940
B.E.
Fig7
31
(e)
930
920
910
570
565