Magneto-transport behaviour of Bi₂Se_3-xTe_x: Role of disorder

Magneto-resistance and Hall resistance measurements have been carried out in fast-cooled single crystals of Bi₂Se_3-xTe_x(x = 0 to 2) in 4-300 K temperature range, under magnetic fields up to 15 T. The variation of resistivity with temperature that points to a metallic behaviour in Bi₂Se₃, shows an up-turn at low temperatures in the Te doped samples. Magneto-resistance measurements in Bi₂Se₃ show clear signatures of Shubnikov-de Hass (SdH) oscillations that gets suppressed in the Te doped samples. In the Bi₂SeTe₂ sample, the magneto-resistance shows a cusp like positive magneto-resistance at low magnetic fields and low temperatures, a feature associated with weak anti-localisation (WAL), that crosses over to negative magneto-resistance at higher fields. The qualitatively different magneto-transport behaviour seen in Bi₂SeTe₂ as compared to Bi₂Se₃ is rationalised in terms of the disorder, through an estimate of the carrier density, carrier mobility and an analysis in terms of the Ioffe-Regel criterion with support from Hall Effect measurements. We demonstrate that by introducing Te, in the strongly disordered samples a smooth crossover of SdH and WAL can be seen in the Bi₂Se_3-xTe_x series, both of which provide signatures for the presence of topological surface states.