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.

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Quantum transport properties of a large bandgap In0.15Al0.79Ga0.06N/GaN quaternary GaN high electron mobility transistor (HEMT) heterostructure are studied at low temperatures up to 2 K. Herein, we report the first evidence of weak localization in a quaternary GaN two-dimensional electron gas (2DEG) system. We observe negative magnetoresistance behavior and extracted dephasing time (τΦ) using a Hikami–Larkin–Nagaoka model at 2.2 K. Linear dependency of dephasing rate with temperature (τΦ−1∝T) is established below 20 K. Furthermore, Shubnikov–de Haas quantum oscillation induced by 2DEG is observed using perpendicular magnetic (B⊥) field strengths up to 14 T. From the temperature-dependent oscillation amplitude, we extracted an effective mass m*≈0.237me. The dominance of small-angle scattering in the 2DEG channel is identified from less than unit ratio (τq/τt≪1) of quantum lifetime (τq) to the Hall transport lifetime (τt). In our study, we have demonstrated that the In0.15Al0.79Ga0.06N/GaN quaternary heterostructure possesses high dephasing time (τΦ=5.4 ps) and larger quantum lifetime (τq=0.102 ps) indicating better suitability and a way forward to high-power–high-frequency GaN HEMT development.

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