  We demonstrate that the charge-spin conversion efficiency of topological
insulators (TI) can be experimentally determined by injecting spin-polarized
tunneling electrons into a TI. Through a comparative study between bismuth
selenide and bismuth antimony telluride, we verified the
topological-surface-state origin of the observed giant spin signals. By
injecting energetic electrons into bismuth selenide, we further studied the
energy dependence of the effective spin polarization at the TI surface. The
experimentally verified large spin polarization, as well as our calculations,
provides new insights into optimizing TI materials for near room-temperature
spintronic applications.
