  To optimize the bulk-insulating behavior in the topological insulator
materials having the tetradymite structure, we have synthesized and
characterized single-crystal samples of Bi_(2-x)Sb_(x)Te_(3-y)Se_(y) (BSTS)
solid solution at various compositions. We have elucidated that there are a
series of "intrinsic" compositions where the acceptors and donors compensate
each other and present a maximally bulk-insulating behavior. At such
compositions, the resistivity can become as large as several Ohmcm at low
temperature and one can infer the role of the surface-transport channel in the
non-linear Hall effect. In particular, the composition of Bi1.5Sb0.5Te1.7Se1.3
achieves the lowest bulk carrier density and appears to be best suited for
surface transport studies.
