Tuning of charge carrier density by deposition pressure in Sb-doped Bi2Se3 thin films

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Tuning of charge carrier density by deposition pressure in Sb-doped Bi2 Se3 thin films S. Abhirami , Shilpam Sharma , E.P. Amaladass , R. Rajitha , P. Magudapathy , R. Pandian , Awadhesh Mani PII: DOI: Reference:

S0040-6090(19)30716-3 https://doi.org/10.1016/j.tsf.2019.137689 TSF 137689

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Thin Solid Films

Received date: Revised date: Accepted date:

5 August 2019 14 October 2019 6 November 2019

Please cite this article as: S. Abhirami , Shilpam Sharma , E.P. Amaladass , R. Rajitha , P. Magudapathy , R. Pandian , Awadhesh Mani , Tuning of charge carrier density by deposition pressure in Sb-doped Bi2 Se3 thin films, Thin Solid Films (2019), doi: https://doi.org/10.1016/j.tsf.2019.137689

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Highlights    

With increase in Ar pressure, carrier density of Sb-doped Bi2Se3 film decreases. Quantum coherence length increases with increase in Ar pressure. Solely electron-electron interaction based de-phasing in 50 Pa deposited films.  of 50 Pa deposited film is -1, indicating two independent transport channels.

Tuning of charge carrier density by deposition pressure in Sb-doped Bi2Se3 thin films Abhirami S1,4, Shilpam Sharma5, E P Amaladass1,4, Rajitha R1,4, Magudapathy P2, Pandian R3,4, and Awadhesh Mani1,4 1

Condensed Matter Physics Division, Materials Science Group, Indira Gandhi Centre for Atomic Research,

Kalpakkam 603102, India 2

Metal Physics Division, Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102,

India 3

Surface and Nanoscience Division, Materials Science Group, Indira Gandhi Centre for Atomic Research,

Kalpakkam 603102, India 4

Homi Bhabha National Institute, IGCAR, Kalpakkam 603102, India

5

Field Emission Laser and Utilization Section, Materials Science Group, Raja Ramanna Centre for Advanced

Technology, Indore 452013, India

Abstract We report the tunability of charge carrier density in a pulsed laser deposition grown Bi1.95Sb0.05Se3 thin film system as a function of the deposition pressure. Dynamic partial pressure of argon maintained in the chamber during deposition suppresses Se escape, effectively regulating the stoichiometry, which in turn modulates the charge carrier density. With pressure variation, a striking three order change in carrier density is recorded and with additional control through the substrate temperature a carrier density value as low as 2×1018 cm-3 is found to be achievable. We observe a systematic evolution of structural, electronic, and magneto-transport properties of the system with variation of deposition pressure. The magnetotransport properties of the films reveal weak antilocalization behavior,

analyzing which we have been able to determine the influence of deposition pressure on properties like the de-phasing mechanism involved in the system, the number of effective transport channels, and the coherence length. Key words: Topological Insulator, carrier density tuning, pulsed laser deposition, thin films, bismuth selenide, weak antilocalization

1.

Introduction

Topological insulators (TI) owe the immense interest that they have garnered to the robust linearly-dispersive surface states residing in their bulk band gap [1, 2]. The Dirac electrons occupying these surface states are spin-momentum locked, a property protected by time reversal symmetry. This spin texturing suppresses backscattering of Dirac electrons causing multifold surface mobility enhancement. Such quantum signatures make TI viable candidates for futuristic technology like quantum computation [3] and spintronics [4]. One of the early entrants into the 3DTI candidate database is Bi2Se3, which possesses a textbook-perfect single Dirac cone in the bulk band gap [5]. Bi2Se3 has a layered structure, wherein the quintuple layers (QL) are bound together weakly through van der Waals (VdW) interactions. Consequently, the selenium (Se) atoms terminating a QL are readily prone to escape from the crystal, leaving behind Se vacancy defects (V Se) which act as electron donors. The overwhelming contribution of n-type charge carriers from the VSe defect states eclipses the intrinsic semiconducting behaviour of the bulk and

renders the metallicity of the surface states indiscernible [6]. Much research has been carried out by groups working on Bi2Se3 in efforts to tune the Fermi level of Bi2Se3 into its bulk band gap. Charge doping, gating, and alloying/isoelectronic substitution are among the more effectual approaches that have been employed. Hor et al. [7] observed lowering of Fermi level into the valence band on doping with Ca and Zhang et al. [8] observed a significant depletion of conduction electrons on doping with Pb. Both Ca and Pb being divalent in Bi2Se3, introduce holes into the system and compensate for the n-type charges present. Electrical gating technique has been found to steer the Fermi level all across the band gap from the conduction band minimum to the valence band maximum [9-12]. The most extensively researched method of reducing bulk contribution is perhaps alloying. Insulating behaviour has been observed in a system of Bi2Se3 alloyed, in an optimized ratio, with Bi2Te3 [13, 14]. Partial substitution of Se with Te atoms suppresses VSe formation and also contributes p-type carriers through Te in Bi antisite defects, reducing the overall carrier concentration. Similarly, isoelectronic substitution of Bi with Sb has also been observed to definitively influence the carrier density as a function of Sb concentration [15]. The smaller atomic radius of Sb compared to Bi strengthens Se bonding to the crystal, thus suppressing the formation of VSe.

The present paper reports the structural and electronic properties of Sbdoped Bi2Se3 system with the nominal composition Bi1.95Sb0.05Se3 (BSS) in thin film form. The choice of composition has been made based on our earlier work [16] which established the single crystal form of this composition to exhibit appreciable Shubnikov-De Haas oscillations [17], which is considered to be the signature of quantized Landau levels in a system under a magnetic field, identifying the system as possessing topological surface states. Studying the system in thin film form is in the interest of further reducing the carrier concentration by increasing the surface to bulk ratio, and with the motivation to fabricate the TI in a form that is suitable for large scale device making. The growth of oriented Bi2Se3 thin films through techniques like molecular beam epitaxy [18-20], magnetron sputtering [21, 22], and pulsed laser deposition (PLD) [23-26] have been documented in literature, with each technique possessing unique advantages and disadvantages. We identify PLD to be well suited for the fabrication of tailored heterostructures, a necessity for TI device making [23]. Furthermore, as we report in this work, PLD allows for a surprisingly wide range of control over parameter tuning. Change in the deposition pressure is observed to cause an astounding three order variation in charge carrier concentration. It is also seen to dramatically alter the average length scale (i.e., coherence length) over which backscattering is suppressed and the mechanism through which decoherence

occurs in the system. Despite the significant number of reports on PLD grown Bi2Se3 thin films, the physics of the system deposited under low-pressure conditions has been largely unexplored or unreported. This article investigates the system grown in three different pressure ranges (0.002 pa vacuum, 4 and 50 Pa argon (Ar)), establishing the systematic evolution of structural and electronic properties with increase in the ambient Ar pressure during film growth. The reproducible control that pressure is found to have on the charge carrier concentration, in particular, promises a simple way of tuning the Fermi level of Bi2Se3 based TI systems.

2. Experimental details Thin films of Bi1.95Sb0.05Se3 were grown on Si substrates coated with 300 nm thick amorphous SiO2, in a PLD system. The target was prepared by grinding and pelletizing a crystal of Bi1.95Sb0.05Se3. UV LASER of wavelength 248 nm and pulse duration 25 ns was used to ablate the target and create a plasma plume. LASER fluence and pulse frequency were kept constant at 3.25 J/cm2 and 5 Hz respectively and all films were deposited for a period of 600 s, with a total of 3000 LASER shots. The substrate to target distance was maintained at 30 mm. Films were grown in 0.002 Pa vacuum, 4 Pa Ar, and in 50 Pa Ar environments, at different substrate temperatures of 250 oC, 300 oC and 350 oC. Glancing angle incidence X-ray

diffraction (GI-XRD) was performed on all the films using Cu-k wavelength in an Inel Equinox 2000 instrument with a glancing angle of 0.6 o. Raman spectra of the films were recorded using a micro Raman spectrometer (Renishaw, model inVia) with an excitation wavelength of 514 nm from an Ar ion laser. A scanning electron microscope (SEM) with a field emission gun (SUPRA 55 by Carl Zeiss, Germany) was used in the low kV mode (2-3 kV) to analyze the surface morphology of the samples. Energy dispersive X-ray spectroscopy (EDS) was performed with a 15 kV accelerating voltage within the same SEM incorporated with an INCA EDS detector (by Oxford Instruments, UK) to identify the elemental composition of the samples. Resistivity measurements were carried out in the four probe Van der Pauw geometry in a dipper cryostat within the temperature range of 4.2 K and 300 K. Rutherford backscattering studies (RBS) were performed using 3.4 MeV He2+ ions from a 1.7 MV Tandetron accelerator under normal incidence to determine the thickness of the deposited films and also to substantiate the composition determined by EDS. RBS spectrum was simulated using the SIMNRA software [27]. Linear magneto-resistance and Hall measurements were performed between temperatures of 2 and 20 K in the magnetic field range of ±2 T (±8 T for Hall) in a commercial Heliox cryostat, Oxford instruments, UK, with the magnetic field applied perpendicular to the films’ surface.

3. Results For ease of reference BSS films grown at 350 oC substrate temperature in 0.002 Pa vacuum, 4 Pa Ar, and 50 Pa Ar are labeled as P1, P2, and P3 respectively. Similarly, films grown in 50 Pa Ar environment at 250 oC, 300 oC and 350 oC substrate temperatures are labeled as T1, T2, and T3, respectively. Note that P3 and T3 refer to the same growth conditions addressed differently based on the variable parameter. All films discussed were grown on amorphous SiO2 (300 nm) coated Si substrates. The weak VdW force that characterizes the interactions between the substrate and the QLs of BSS greatly reduces the structural dependence of the film on the substrate [28]. Hence the lack of imposed constraints by the amorphous SiO 2 substrate aids rather than hinders the growth of the layered BSS compound. Highly c-axis oriented films of BSS were found to grow on amorphous SiO 2 under conducive environmental conditions.

3.1. Effects of deposition pressure. 3.1.1. Structural effects. Thicknesses of the films grown in all the pressure conditions were found to be ~ 250 nm. Comparing the GI-XRD of P1, P2, and P3 with the XRD pattern of the BSS target material (Fig. 1(a)), we find that P 1 and P2 have shifted peaks while peaks of P3 mimic that of Bi2Se3 [16]. The corresponding Raman spectra are given in Fig. 1(b). SEM images of the films P 1, P2, and P3 are

compiled in Fig. 1(c)-(e). The average grain size was calculated by performing a lognormal fit of the size distribution obtained through ImageJ software. As seen in Fig. 1(f), pressure has a deteriorating effect on the size and structure of the grains. Grains of P1 and P2 show a tendency for hexagonal structure formation, while those of P3 show no such behaviour. This is explained by considering the larger scattering of ablated species in higher pressure environments in the PLD chamber. Scattering causes a reduction in the energy of the species reaching the substrate, and hence the energy available for the grain growth process is also reduced. To further probe the various effects of deposition in a low-pressure environment, the films were subjected to Raman studies. The Raman spectrum of P 3 mimics that of Bi2Se3. The 1% substitution of Bi with Sb does not alter the Raman spectrum significantly enough to be perceivable. Fig. 1(b) shows P3 exhibiting the expected modes

and

at 70.8, 129.3, and 173.4 cm-1 respectively [29, 30]. Raman

bands of P1 and P2 on the other hand are significantly shifted to lower energies. The most striking feature in the Raman spectrum of the low-pressure deposited films is the presence of an additional band at 92 cm-1 in P1 which shifts to 97 cm-1 in P2, which may correspond to A1g mode of Bi-Bi bond [31]. This band suggests the presence of segregated Bi layers [32] in the system. The large shift in the bands and the additional Bi band in P1 and P2 motivated the study of the systems’ stoichiometries, which were determined independently through EDS and RBS

techniques. Table 1 summarizes the atomic compositions of all the films discussed in this report. A substantial deficiency of Se and a corresponding overstoichiometry of Bi are seen to be present in P1. Se has a melting point of 220 oC and is known to be highly volatile. At 350 oC substrate temperature, it is presumable that the system is susceptible to large Se escape. The observation of reduction in Se deficiency with increasing pressure indicates that the Ar background suppresses the escape of Se atoms. The compositional trend shows that stoichiometric BSS is achievable in 50 Pa Ar pressure environment. Presence of Sb in the system has been detected by EDS but the concentration of Sb being comparable with the detection limits of the EDS, we refrain from reporting Sb concentration values. The results obtained in the Raman analysis of P 1 and P2 can be explained as follows: A significant proportion of the abundantly present VSe sites are occupied by Bi atoms, causing the shift in Raman peaks of P 1 and P2. The lack of Se atoms in the atomic planes terminating the QLs may encourage Bi atoms to occupy the VdW gap, forming segregated Bi layers which reveal themselves as the additional peak at 92 cm-1(97 cm-1) in P1(P2). 3.1.2. Electronic effects. Temperature dependent resistivity measurements, summarized in Fig. 2(a), show that all films deposited at 350 oC exhibit metallic behaviour. While the resistance of P1 saturates at low temperatures, the resistances of P2 and P3 show upturns at 9 K and 14.5 K, respectively. The upturn is indicative

of suppression of charge carriers of the defect states and revelation of the intrinsic semiconducting nature of the bulk [33, 34]. Even though the insulating behaviour is due to localization of charge carriers from defect states, the insulating region does not fit to either the activation model or the variable range hopping model. This is indicative of the competition between the localization driven insulating nature and the metallicity of the highly mobile surface state electrons. We also notice that the upturn temperature of P3 is higher than that of P2. The onset of the upturn arguably depends on the number of defect states in the system. As inferred from the stoichiometry table (Table 1), the reduced Se deficiency (in P3) is expected to create a lesser number of defect states in P 3 than in P2 and therefore cause a higher upturn temperature in P3. 3.1.3. Charge carrier tuning. Of the four prominent point defects in a Bi2Se3 system (Bi and Se vacancies, Bi and Se antisites), V Se has the least formation energy

in a Bi rich environment [14]. Therefore it is pertinent to assume that

VSe are the major contributors of charge carriers in the system and suppression of Se escape can be expected to directly influence the defect carrier density. In order to find the charge carrier concentration, Hall measurements were carried out within a magnetic field range of

8 T at different temperatures. Representative 2 K data

of P1, P2, and P3 are shown in Fig. 2(b). Electrons were identified to be the majority carriers, as should be the case where defect states are due to V Se [15]. The obtained

carrier concentration was plotted as a function of P and was found to decrease monotonically with increasing pressure, as shown in Fig. 2(c). This observation indicates that variation of deposition pressure provides us a simple tool to tune the charge carrier concentration of a Bi2Se3 based thin film system. The volume carrier concentration (N) for P1, P2, and P3 were found to be 2 × 1021 cm-3, 3.5 × 1020 cm-3, and 2 × 1019 cm-3 respectively.

3.2. Effects of Substrate temperature. 3.2.1. Structural effects. XRD pattern of BSS thin films grown in 50 Pa Ar environment at 250 oC, 300 oC, and 350 oC (T1, T2, and T3 respectively) show phase purity, as can be seen in Fig. 3(a). T1 and T2 are polycrystalline, with all their XRD peaks indexed to that of BSS. The 015 peak, which is the maximum intensity peak of polycrystalline BSS, is suppressed with increase in substrate temperature, eventually being superseded by the [0 0 6] peak in P 3. The film P3 shows a predominantly [0 0 l] oriented XRD pattern with the additional presence of the [0 1 5] peak of appreciable intensity. The inference drawn is that the majority of grains are [0 0 l] oriented while a smaller fraction has grown along the [0 1 5] orientation. The corresponding Raman spectra are given in Fig. 3(b). Fig. 3(c)-(e) show the SEM images of T1, T2, and T3. The full width at half maximum of the XRD peaks decreases with increase in substrate temperature suggesting an increase

in crystallite size, which is corroborated by the SEM images. Comparison of the average grain size of the films is brought out in Fig. 3(f). As seen in Fig. 3(b), Raman studies performed on these films show that all films possess the and

modes of Bi2Se3. While the peaks of T3 are at the expected positions- 70.8

cm-1 (

), 129.3 cm-1 (

shifted modes (

), and 173.4 cm-1(

modes by 1.3 cm-1 and

), T1 and T2 have marginally redby 3 cm-1). These shifts are possibly

due to stresses in the films because of the presence of multiple orientations of grains. T1 and T2 also show a broad peak at around 250 cm-1 which may correspond to Sb-Sb Mode [35]. This suggests segregation of Sb from the compound. The band at 250 cm-1 is suppressed with increase in temperature, eventually vanishing at 350 oC (T3). It is noteworthy that the Raman band indicating Bi segregation which was observed in low-pressure films P1 and P2 is absent in all films deposited in 50 Pa Ar pressure. The stoichiometry table (Table 1) shows that T 1, T2, and T3 are all close to the stoichiometric state of Bi2Se3 (Sb ignored due to its small concentration). 3.2.2. Electronic effects. Temperature dependent (4 K-300 K) resistivity measurements showed that T1 has negative temperature coefficients (NTC) of resistance in the entire measured temperature range, T2 possesses NTC for most part of the measured temperature range (Fig. 4(a)), while T3 is metallic with an

upturn at low temperature (Inset in Fig. 4(a)). The small grain sizes of T 1 and T2 entail the presence of a large number of grain boundaries. This enhanced concentration of grain boundary scattering centers could be the reason for the observed NTC. At low temperatures (23 K in T1 and 20 K in T2), a sharp change in the magnitude of the slope is observed. Similar to the case of upturn in P 2 and P3(T3), this change in slope can also be ascribed to the localization of defect state charge carriers and the non-conformity to activated fit or hopping fit can be related to the competition between surface metallicity and localization of carriers. Charge carrier concentration values obtained from Hall measurements reveal a carrier concentration as low as 3.8×1018 cm-3 in T1. With increasing substrate temperature, the carrier concentration increases (Fig. 4(b)), reaching 2.04×10 19 cm-3 in T3.

3.3. Magneto-transport Studies. Magnetoresistance measurements of the BSS thin films were performed to identify and study the topological surface states of the system. These studies have been carried out in a perpendicular magnetic field configuration, within a field range of ±2 T, at different temperatures between 2 and 20 K. The magneto-conductance term (∆

) which originates from the cyclotronic motion of the charge carriers

is defined as the difference of zero field conductance ( at field B (

).

) from the conductance

∆ The ∆

–Eq. 1

of the BSS films are plotted against magnetic field (B) in Fig. 5 (a)-(e).

On observing the data we found that the ∆

of all the films exhibit weak-

antilocalization (WAL) behaviour in the low magnetic field range of

1 T. WAL,

which appears as a sharp positive cusp, is a quantum correction to the conductance and is characteristic of 2D states in the strong spin-orbit coupling limit, which is the regime where the spin-orbit scattering time is much greater than the de-phasing time [36]. In the absence of magnetic impurities and fields, the time-reversed (TR) electron paths with a phase difference , in a strongly spin-orbit coupled 2D system, destructively interfere with each other. The destructive interference of the TR backscattering paths significantly reduces the probability amplitude of backscattering, thereby increasing the conductance. With application of magnetic field, the phase difference is disturbed and the backscattering probability amplitude increases, giving rise to a sharp dip in conductance on either side of 0 T (WAL). The average length scale over which a coherent loop of TR paths is sustained by the system is called the coherence length

and the corresponding time scale is

termed as the de-phasing time. The ∆

versus B data were analyzed within the purview of the Hikami-Larkin-

Nagaoka (HLN) theory [37], which captures the essential physics of the systems

displaying WAL behaviour in the low magnetic field regime. It was found that ∆

of all the films fit well to the HLN equation given in Eq. 2, in the field

range of ±1T.

∆

* ((

is the digamma function, charge, and

and

)

)

(

)+

–Eq. 2

the reduced Planck’s constant, e the electronic

the parameters obtained through the fitting. The

term

contains information about the effective number of coherent transport channels present in the system. Each coherent channel contributes a value of -0.5 to . In a system with two completely de-coupled channels, each channel contributes -0.5, resulting in an value of -1 [38].We observed that all the BSS films deposited in 50 Pa Ar background (T1, T2, and T3) possess an with -1 value, indicating the presence of two completely de-coupled channels involved in coherent charge transport. The top surface, bottom surface, and the 2DEG states in the bulk are the three available transport channels in the system. Due to possible disorder and strain in the bottom surface owing to its interface with the amorphous SiO 2 substrate, the probability of the bottom surface supporting topological surface states is negligible. Therefore it seems plausible that the two de-coupled channels involved

in coherent transport in T1, T2, and T3 are the top surface and the 2DEG states in the bulk, created due to band bending [39]. The other fitting parameter

is

expected to decrease rapidly with increase in temperature (Fig. 6 (a)-(e)), as at higher temperatures the quantum interference in the system is destroyed due to thermal fluctuations. The powerlaw fit:

, is expected to yield an exponent

value of  = -0.5 for a system with a purely electron-electron interaction based dephasing mechanism [40]. The  value of the BSS films was found to approach -0.5 with increasing deposition pressure (Fig. 6 (f)). P3 was seen to have a  value of 0.44, indicating that electron-electron interactions are the predominant reason for decoherence in the system. The larger magnitude of  observed in P1 and P3 are indicative of additional de-phasing mechanisms like electron-phonon interactions being present in the system [40]. Presence of multiple de-phasing mechanisms causes a sharper fall in

as a function of temperature. A system with a large

average value of coherence loop circumference (~ 2 ) will naturally also have a long de-phasing time. This means that the number of de-phasing and backscattering events in such a system is lesser than in a system with a shorter dephasing time. Therefore the mobility of surface state carriers is high in a system with a long found that the

and such a system qualifies as possessing superior TI states. We values of the BSS films increases with increasing deposition

pressure. We ascribe the smaller values of

in P1 and P2 to the presence of

multiple de-phasing mechanisms in these films, as inferred from their  ≠ -0.5 values. P3 was seen to possess the longest coherence length of 223 nm which is close to the magnitude of its average grain size of 216 nm. Due to the desirably high coherence length value, de-coupled transport channels, and a solely electronelectron interaction based de-phasing mechanism, 50 Pa Ar pressure and 350 oC are found to be the most favorable of the studied deposition conditions for the growth of BSS TI films.

4. Conclusion Thin films of Bi1.95Sb0.05Se3 were grown using a PLD system in different permutations of deposition pressures and substrate temperatures. Systematic evolution of structural, electronic, and magneto-transport properties has been observed with variation of deposition pressure. Increasing deposition pressure has been found to desirably influence the phase purity of the system. Pressure is found to regulate the stoichiometry through suppression of Se escape and bring about a three order variation in charge carrier density. A power-law decay in charge carrier density (with exponent -0.26) is observed as a function of pressure. This approach can function as a simple and dependable method of tuning the charge carrier density of a Bi2Se3 based system. Manipulation of substrate temperature while

maintaining 50 Pa deposition pressure is seen to reduce the carrier density to a value as low as 2×1018 cm-3. Magneto-transport studies carried out on the films reveal that in the best of the films the de-phasing mechanism is electron-electron interaction based and that it possesses exactly two de-coupled coherent transport channels: top surface states and the bulk 2DEG states.

Acknowledgements The authors thank B. Sundaravel, PIF, IGCAR, for performing RBS measurements. One of the authors (Abhirami S) would like to acknowledge the funding from the Department of Atomic Energy, India.

Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Tables: 1 Table1. Atomic percentage of Bi and Se obtained from EDS and RBS analysis. Note that the atomic percentage of Bi and Se in a stoichiometric Bi2Se3 crystal are 40% and 60% respectively. Growth condition

350 oC, 0.002 Pa vacuum (P1) 350 oC, 4 Pa Ar (P2) 250 oC, 50 Pa Ar (T1) 300 oC, 50 Pa Ar (T2) 350 oC, 50 Pa Ar (P3/T3)

Atomic percentage of Bi (%) EDS RBS 52.8 43.5 39.8 40.1 39.6

52.5 43.79 39.0 39.2 39.9

Atomic percentage of Se (%) EDS RBS 46.45 55.2 58.8 58.7 59.9

45.5 55.21 60.0 59.8 59.1

Figure captions: Fig. 1. (a) GI-XRD of Bi1.95Sb0.05Se3 thin films deposited at 350 oC in varying pressure conditions (0.002 Pa vacuum (P1), 4 Pa Ar (P2), and 50 Pa Ar (P3)). Intensity is represented in log-scale. (b) Raman spectra of films P1, P2, and P3. (c)-(e) SEM images of P1, P2, and P3 with the grain size distribution fit to lognormal function in the inset. (f) Variation of grain size with pressure. Fig. 2. (a) Temperature dependent resistance of Bi1.95Sb0.05Se3 thin films deposited at 350 oC in 0.002 Pa vacuum (P1), 4 Pa Ar (P2), and 50 Pa Ar (P3) environments. Upturn temperature increases with increasing pressure. (b) 2 K Hall data of P 1, P2, and P3 with linear fit within magnetic field range of ±8 T. (c) Monotonic decrease of carrier density (N) with increasing deposition pressure (P) (Line is guide to eye). Fig. 3. (a) GI-XRD of Bi1.95Sb0.05Se3 thin films deposited in 50 Pa Ar pressure at varying substrate temperatures (250 oC (T1), 300 oC (T2), and 350 oC (T3)). Intensity is represented in log-scale. (b) Raman spectra of films T 1, T2, and T3. (c)-(e) SEM images of T1, T2, and T3 with the grain size distribution fit to lognormal function in inset. (f) Variation of grain size with substrate temperature. Fig. 4. (a) Temperature dependent resistance of Bi1.95Sb0.05Se3 thin films deposited in 50 Pa Ar pressure at varying substrate temperatures (250 oC (T1), 300 oC (T2), and 350 oC (T3)) within the temperature range of 4 and 300 K (T3 is in inset). Negative temperature coefficient of resistance observed in entire measured temperature range of T 1 and in most part of T2. (b) Charge carrier density (N) variation with substrate temperature (Line is guide to eye). Fig. 5. Magneto-conductance (G) in units of e2/h versus magnetic field (B) in the temperature range of 2 to 10 K plotted for (a)-(b) films deposited at 350 oC in 0.002 Pa vacuum (P1) and 4 Pa Ar (P2), (c)-(d) films deposited in 50 Pa Ar at 250 oC (T1) and 300 oC (T2), and (e) films deposited at 350 oC in 50 Pa Ar (P3/T3). Representative HLN fit at 2K of the films is included as inset in each data set. Fig. 6. Coherence length (l) as a function of temperature obtained for (a)-(b) films deposited at 350 oC in 0.002 pa vacuum (P1) and 4 pa Ar (P2), (c)-(d) films deposited in 50 Pa Ar at 250 oC (T1) and 300 oC (T2), and (e) films deposited at 350 oC in 50 Pa Ar (P3/T3). (f) Variation of  with deposition pressure.

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