Influence of Te Composition on Magneto-Transport Behavior of the Bi_xTe_y Thin Films Co-Sputtered on Si (100)

Topological insulators (TIs) are an emerging class of quantum matter that consists of non-trivial Dirac fermionic gapless states between the insulating bulk bandgap. These surface states are proposed to have real-world applications in quantum computing and spintronics. The materials such as Bi₂Te₃and Bi₂Se₃ are extensively studied as 3D TIs. However, most studies are focused either on single crystals or MBE grown samples which are not industry-friendly. Recently industry-friendly viable sputtering technique is also used to synthesize this class of materials. Unlike MBE, the microstructure of grown samples is nanocrystalline in nature. Moreover, due to the highly volatile nature of constituent elements namely Te, Se, and Bi, achieving desired stoichiometry is also extremely difficult. We have designed a systematic approach to tune composition in Bi_xTe_y using DC co-sputtering technique. The x-ray diffraction (XRD), Raman, and Energy dispersive x-ray analysis (EDAX) are used to classify the samples series as Te Deficient phase (BTD), Te stoichiometric phase (BTS), or Bi2Te3 and Te excessive phase (BTE). The sample microstructures are granular in nature. The magneto-transport properties are performed on these three samples. The temperature dependence of resistivity range between (400-229) μΩ-cm for the BTD sample, (1083-732) μΩ-cm for the BTS sample, and (9521-5826) μΩ-cm for the BTE sample. The extrinsic and intrinsic semiconductor-like behavior in temperature-dependent of resistivity measurement is observed between 20 to 300K. A sharp upturn is observed below 20K. The upturn has logarithmic temperature dependence and therefore it can only be explained by conductivity correction due to weak (anti)localization and electron-electron interaction (EEI). The sheet carrier concentration is found to decrease from 10¹⁵ cm^-2 to 10¹³cm^-2 with an increase in Te concentration while the order of mobility around 10cm²/Vs has remained equivalent in all the samples. Hence, it is verified that the Fermi level is lie in the conduction band and move toward the bandgap with the increase in Te composition. The weak antilocalization behavior is also observed in the perpendicular magnetoresistance curve. The data is fitted using Hikami-Larkin-Nagaoka (HLN) model and the parameter such as prefactor α and phase coherence length L_Φ is extracted. The range around -0.3 to -0.4 suggests an indirect coupling between the top and bottom surface state through conducting bulk. The L_Φ is sustainably decreased with Te composition. The temperature dependence of L_Φ follows a power law L_ΦαT^-p/2from which information about the dephasing mechanism and dimensionality are estimated. The dephasing mechanism in the BTD sample is found to be a combination of 2D Nyquist EEI and 3D inelastic phonon scattering, and the 3D inelastic phonon scattering contribution in dephasing is found to diminish with Te composition. The logarithmic temperature-dependent of conductivity at the varied magnetic fields is analyzed. From the temperature dependence of the slope, the number of transport channels and mass term or magnitude of any gap present in the Dirac type band are estimated. The weak antilocalization behavior is also observed in-plane magnetoresistance and data is fitted using Altshuler and Aronov formula. The surface state penetration depth (λ) is extracted by fitting the data with the model. Recently Banerjee <i>et al. </i>designed an equation λ=L/log(t₂/t₁) for granular TIs through which they have verified the topological phase in Bi₂Se₃ sample, here L is crystallite size and t₂, t₁ is intergrain and intragrain coupling between surfaces states. We have applied this equation and found that all the samples lie in a non-trivial (topological) regime. The procedure used in this work can be used to manufacture TIs based devices for industrial application.<br/>Ref:-<br/>(1) doi.org/10.1039/C7NR01355H<br/>(2) doi.org/10.1103/PhysRevLett.112.146601