  One of the most fundamental and exotic properties of 3D topological
insulators (TIs) is spin-momentum-locking (SML) of their topological surface
states (TSSs), promising for potential applications in future spintronics.
However, other possible conduction channels, such as a trivial two-dimensional
electron gas (2DEG) with strong Rashba type spin-orbit interaction (SOI) and
bulk conducting states that may possess a spin Hall effect (SHE), can coexist
in 3D TIs, making determining the origin of the current induced spin
polarization (CISP) difficult. In this work, we directly compared the CISP
between bulk-insulating Bi2Te2Se (BTS221) and bulk-metallic Bi2Se3 thin flakes
using spin potentiometry. In the bulk insulating BTS221, the observed CISP has
a sign consistent with the expected helicity of the SML of the TSS, but an
opposite sign to its calculated bulk spin Hall conductivity (SHC). However,
compared to BTS221, an opposite CISP is observed in the bulk metallic Bi2Se3,
consistent instead with both the expectations of its Rashba-Edelstein effect of
the band-bending induced 2DEG and bulk spin Hall Effect (SHE). Our results
provide an electrical way to distinguish the TSS from other possible conducting
channels in spin transport measurements on 3D TIs, and open ways for the
potential applications in charge-spin conversion devices.
