  Magneto resistance measurements coupled with positron lifetime measurements,
to characterize the vacancy type defects, have been carried out on the
topological insulator (TI) system Bi2Se3, of varying Se/Bi ratio. Pronounced
Shubnikov de Haas (SdH) oscillations are seen in nominal Bi2Se3.1 crystals for
measurements performed in magnetic fields up to 15 T in the 4 K to 10 K
temperature range, with field applied perpendicular to the (001) plane of the
crystal. The quantum oscillations, characteristic of 2D electronic structure,
are seen only in the crystals that have a lower concentration of Se vacancies,
as inferred from positron annihilation spectroscopy.
