  We show both experimentally and theoretically that the recently observed
optically induced in-plane charge separation in quantum well (QW) structures
and the exciton ring emission pattern at this charge separation boundary have
an extremely long lifetime. The oppositely charged carriers remain separated
and provide a reservoir of excitons at their boundary with a persistent
emission which lasts hundreds of microseconds (orders of magnitude longer than
their recombination time) after the external excitation is removed. This long
lifetime is due to an interplay between the slow in-plane carrier diffusion and
slow carrier tunnelling perpendicular to the QW plane.
