  The creation of free excitons by absorption of circularly polarized photons,
and their subsequent fast capture by donors, is at the origin of the spin
polarization of donor-bound electrons. The sign of the electronic spin
polarization at low density of excitation is, as expected, fixed by the
helicity of the exciting light; but at high density of excitation we show that
the spin polarization is of the opposite sign. This striking inversion is
explained, here, by the contribution of dark excitons to mechanisms of spin
polarization of localized electrons.
