  The results of scanning tunneling microscopy experiments using iron-coated
tungsten tips and current-carrying bismuth selenide ($Bi_2Se_3$) samples are
reported. Asymmetry in tunneling currents with respect to the change in the
direction of bias currents through $Bi_2Se_3$ samples has been observed. It is
argued that this asymmetry is the manifestation of surface spin-polarized
electron accumulation caused by the ninety-degree electron spin-momentum
locking in the topologically protected surface current mode. It is demonstrated
that the manifestation of surface spin-polarized electron accumulation is
enhanced by tin doping of $Bi_2Se_3$ samples. Furthermore, the appearance of
spin-dependent density of states in current carrying $Bi_2Se_3$ samples has
also been observed.
