  We find that the total spectrum of electron states in a bounded 2D electron
gas with spin-orbit interaction contains two types of evanescent states lying
in different energy ranges. The first-type states fill in a gap, which opens in
the band of propagating spin-splitted states if tangential momentum is nonzero.
They are described by a pure imaginary wavevector. The states of second type
lie in the forbidden band. They are described by a complex wavevector. These
states give rise to unusual features of the electron transmission through a
lateral potential barrier with spin-orbit interaction, such as an oscillatory
dependence of the tunneling coefficient on the barrier width and electron
energy. But of most interest is the spin polarization of an unpolarized
incident electron flow. Particularly, the transmitted electron current acquires
spin polarization even if the distribution function of incident electrons is
symmetric with respect to the transverse momentum. The polarization efficiency
is an oscillatory function of the barrier width. Spin filtering is most
effective, if the Fermi energy is close to the barrier height.
