  The Dirac surface states of topological insulators offer a unique possibility
for creating spin polarized charge currents due to the spin-momentum locking.
Here we demonstrate that the control over the bulk and surface contribution is
crucial to maximize the charge-to-spin conversion efficiency. We observe an
enhancement of the spin signal due to surface-dominated spin polarization while
freezing out the bulk conductivity in semiconducting Bi1.5Sb0.5Te1.7Se1.3 below
100K. Detailed measurements up to room temperature exhibit a strong reduction
of the magnetoresistance signal between 2 and 100K, which we attribute to the
thermal excitation of bulk carriers and to the electron-phonon coupling in the
surface states. The presence and dominance of this effect up to room
temperature is promising for spintronic science and technology.
