Surface state transport in MBE-grown topological insulator (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_3$ thin films and field effect transistors

Topological insulators feature spin-helical, Dirac fermion surface states, promising potential applications in both nanoelectronics and spintronics. However, experimental identification of a clear transport signal of the surface state conduction is still challenging. Here, we report a systematical study of the gate tunable magneto-transport in MBE grown (Bi$_{1-x}$Sb$_{x})_{2}$Te$_{3}$ (x$=$0.96) thin film on SrTiO$_{3}$ substrate. We observed an ambipolar field effect and a sign change in the Hall resistance as the gate voltage (V$_{g})$ crosses the Dirac point (V$_{D})$. Temperature (T) dependence of the resistance at different V$_{g}$ shows a transition from a metallic to an insulating bulk with 100{\%} surface conduction at low T. Weak antilocalization measurements indicate a $\pi $ Berry phase near V$_{D}$. We also performed spin valve measurements and observed a resistance asymmetry (which reverses with reversing current direction) between the positive and negative in-plane magnetic fields, demonstrating the predicted locking between spin and momentum for the surface state. We also studied the thermal-electric transport, demonstrating a sign change of the thermoelectric power across the V$_{D}$ as the carrier type switches from electron to hole.