  We determine the diffusion length of excess carriers in GaN by spatially
resolved cathodoluminescence spectroscopy utilizing a single quantum well as
carrier collector or carrier sink. Monochromatic intensity profiles across the
quantum well are recorded for temperatures between 10 and 300 K. A classical
diffusion model accounts for the profiles acquired between 120 and 300 K, while
for temperatures lower than 120 K, a quantum capture process has to be taken
into account in addition. Combining the diffusion length extracted from these
profiles and the effective carrier lifetime measured by time-resolved
photoluminescence experiments, we deduce the carrier diffusivity as a function
of temperature. The experimental values are found to be close to theoretical
ones for the ambipolar diffusivity of free carriers limited only by intrinsic
phonon scattering. This agreement is shown to be fortuitous. The high
diffusivity at low temperatures instead originates from an increasing
participation of excitons in the diffusion process.
