  Under certain conditions, focused laser excitation in semiconductor quantum
well structures can lead to charge separation and a circular reaction front,
which is visible as a ring-shaped photoluminescence pattern. The
diffusion-reaction equations governing the system are studied here with the aim
of a detailed understanding of the steady state. The qualitative asymmetry in
the sources for the two carriers is found to lead to unusual effects which
dramatically affect the steady-state configuration. Analytic expressions are
derived for carrier distributions and interface position for a number of cases.
These are compared with steady-state information obtained from simulations of
the diffusion-reaction equations.
