The resistance across a step on ultrathin films of three different topological insulators, Bi2Te3, Bi2Se3, and (Bi1-xPbx)2Te3, was measured through anisotropy in two-dimensional resistivity by using the in situ square four-point probe method in ultrahigh vacuum. The step resistance was much larger for Bi2Te3 than for Bi2Se3 in the range of 1-10 quintuple-layer thickness, due to the smaller critical thickness for isolation of topological surface states in Bi2Te3. The transmission probability of carriers across a step is much higher for the bulk-insulating (Bi1-xPbx)2Te3 than bulk-metallic Bi2Te3, due to prevention of scattering of surface-state carriers into the bulk states. We were able to deduce microscopic information concerning the transmission probability at individual steps from the resistance data obtained macroscopically.
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