  Images of the steady-state luminescence of passivated GaAs self-standing
films under excitation by a tightly-focussed laser are analyzed as a function
of light excitation power. While unipolar diffusion of photoelectrons is
dominant at very low light excitation power, an increased power results in a
decrease of the diffusion constant near the center of the image due to the
onset of ambipolar diffusion. The results are in agreement with a numerical
solution of the diffusion equations and with a physical analysis of the
luminescence intensity at the centre of the image, which permits the
determination of the ambipolar diffusion constant as a function of electron
concentration.
