Electrical transport study of Bi$_{2}$Se$_{3}$ and Bi$_{1-x}$Sb$_{x}$ crystals

Bi$_{2}$Se$_{3}$ and Bi$_{1-x}$Sb$_{x}$ are predicted and recently confirmed by ARPES experiments to be 3D topological insulators. We have successfully grown Bi$_{2}$Se$_{3}$ and Bi$_{1-x}$Sb$_{x}$ single crystals using a two-step melting method. Both X-ray diffraction (XRD) and electron back-scatter diffraction (EBSD) show excellent structural quality of the crystals on the micro- and macro-scales. The undoped Bi$_{2}$Se$_{3}$ is metallic, but Bi$_{1-x}$Sb$_{x}$ is insulating for x=0.09 and 0.10. We have also fabricated devices with a wide range of thicknesses and measured the electrical transport properties. The longitudinal resistance R$_{xx}$ and Hall resistance R$_{xy}$ measurements are conducted from 1.5K to room temperature in a magnetic field up to 8 Tesla. The sample thickness dependence of the 2D carrier density indicates that there is a large surface carrier density contributing to the transport properties. The devices are then exposed to several dilute gases (e.g. H2, Cl2, etc.), and the effect of the chemical treatment is studied by comparing to the transport properties measured before the gas exposure. We will discuss the correlation between the surface carrier density and the chemical treatment.