Transport Studies On Topological Insulator Thin Films

Topological insulator is a new kind of semiconductors with topological surfacestates protected by time-reversal symmetry. The topological surface states showspin-polarized linear dispersion. Topological insulator has attracted intense studies forits potential applications in spintronics and quantum computation.The topological insulator family of Bi2Se3、Bi2Te3and Sb2Te3possess relativelysimple band structure with only one degenerate point of surface bands in the bulk bandgap, which is ideal for experimental studies. However, the carrier densities of thesebinary compounds are so high that both the surface states and the bulk states contributeto the transport properties. Thin films grown by molecular beam epitaxy are chosen inorder to limit the contribution from the bulk states.Weak antilocalization behavior of the topological surface states is expectedtheoretically. Magnetoconductance measurements on pure Bi2Se3thin films withmagnetic field perpendicular to the films are consistent with weak antilocalizationbehavior. The resistance-temperature curve shows insulating behavior at lowtemperatures, in contradiction with the expected metallic behavior for weakantilocalization. Electron-electron interaction is revealed in magnetoconductancemeasurement with magnetic field parallel to the films, explaining the emergence of theinsulating behavior.Magnetoconductance measurements on Cr-doped Bi2Se3thin films exhibit acrossover from weak antilocalization behavior to weak localization behavior withincreasing doping level. In situ ARPES measurements demonstrate that the band gap ofthe surface states increases with the doping level. These results are qualitativelyconsistent with related theoretical calculations, which show that a crossover oflocalization behavior takes place due to enlarged band gap of surface states band bymagnetic doping. More interestingly, the magnetoconductance of heavily-doped Bi2Se3films changes from weak localization to weak antilocalization with increasingtemperature. Similar behavior was observed in diluted magnetic semiconductorswithout topological surface states. The disappearance of the surface state band in theheavily-doped samples is also observed by ARPES. Thus the crossovers of localization behavior are intimately related to the topological phase transition of the band structureof topological insulator thin films. The accurate description, however, needs furthertheoretical and experimental investigations.Lower carrier density is achieved in Sb2Te3films doped with Bi. Quantumoscillations due to Landau levels of surface states are observed in transportmeasurements under ultrahigh magnetic field. Quantitative analysis shows that thecombination of linear dispersion and large Zeeman splitting leads to an unconventionalspectrum of Landau quantization of surface states.The systemic transport studies on topological insulator thin films uncover thespecial localization behavior and Landau levels of topological surface states, providinginsights into the exotic properties of topological insulator.