Evidence of surface delocalization in ultrathin films of topological insulator in presence of intersurface hybridization and disorder

The study of surface transport in ultrathin films of few quintuple layers thick topological insulators (TIs) and its evolution with disorder is important for TI based device engineering. Here, we demonstrate the absence of Anderson localization in surface states of Bi2Se3 films on varying the disorder from the weak to moderate regime. On increasing the disorder, bulk of topological insulator transforms from diffusive to hopping transport while surface remains in quantum diffusive regime. The weak antilocalization (WAL) of surface states suppresses in thinner films, which could occur due to disorder or inter surface hybridization. Our analysis rules out the role of disorder in WAL suppression and shows that films crossover from decoupled surface states to a single coupled surface bulk channel and finally to hybridized surface states on reducing the film thickness. The dephasing mechanism of surface transport strongly depends on the nature of bulk transport. It is dominated by 2D electron–electron scattering for diffusive bulk transport while it is dominated by direct surface bulk charge puddle coupling and surface to hopping transport coupling for bulk in the variable range hopping regime. The surface to hopping transport coupling weakens with intersurface hybridization.