With high quality topological insulator Bi(2)Se(3) thin films, we report thickness-independent transport properties over wide thickness ranges. Conductance remained nominally constant as the sample thickness changed from 256 to ∼8 QL (where QL refers to quintuple layer, 1 QL≈1 nm). Two surface channels of very different behaviors were identified. The sheet carrier density of one channel remained constant at ∼3.0×10(13) cm(-2) down to 2 QL, while the other, which exhibited quantum oscillations, remained constant at ∼8×10(12) cm(-2) only down to ∼8 QL. The weak antilocalization parameters also exhibited similar thickness independence. These two channels are most consistent with the topological surface states and the surface accumulation layers, respectively.
We show that a number of transport properties in topological insulator (TI) Bi 2 Se 3 exhibit striking thickness-dependences over a range of up to five orders of thickness (3 nm -170 µm). Volume carrier density decreased with thickness, presumably due to diffusion-limited formation of selenium vacancies. Mobility increased linearly with thickness in the thin film regime and saturated in the thick limit. The weak anti-
Atomically sharp epitaxial growth of Bi 2 Se 3 films is achieved on Si (111) substrate with MBE (Molecular Beam Epitaxy). Two-step growth process is found to be a key to achieve interfacial-layer-free epitaxial Bi 2 Se 3 films on Si substrates. With a single-step high temperature growth, second phase clusters are formed at an early stage. On the other hand, with low temperature growth, the film tends to be disordered even in the absence of a second phase. With a low temperature initial growth followed by a high temperature growth, secondphase-free atomically sharp interface is obtained between Bi 2 Se 3 and Si substrate, as verified 2 by RHEED (Reflection High Energy Electron Diffraction), TEM (Transmission Electron Microscopy) and XRD (X-Ray Diffraction). The lattice constant of Bi 2 Se 3 is observed to relax to its bulk value during the first quintuple layer according to RHEED analysis, implying the absence of strain from the substrate. TEM shows a fully epitaxial structure of Bi 2 Se 3 film down to the first quintuple layer without any second phase or an amorphous layer.
Abstract:Topological insulators (TIs) are predicted to be composed of an insulating bulk state along with conducting channels on the boundary of the material. In Bi 2 Se 3 , however, the Fermi level naturally resides in the conduction band due to intrinsic doping by selenium vacancies, leading to metallic bulk states. In such non-ideal TIs it is not well understood how the surface and bulk states behave under environmental disorder. In this letter, based on transport measurements of Bi 2 Se 3 thin films, we show that the bulk states are sensitive to environmental disorder but the surface states remain robust.
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