Giant Excitonic Transport in GaN/AlGaN Surface Quantum Wells*

A huge progressive spatial expansion of photoluminescence (PL) area over a few 100 $\mu $m with decreasing the GaN/Al$_{0.2}$Ga$_{0.8}$N surface quantum well width in the range of 2.9 - 1.51 nm has been observed by focusing 4.66 eV laser light (170 fs pulses) into the spot on the sample surface. The out-of-spot PL spectra reveal higher energy bands which are separated from the free exciton peak by multiples of optical phonon energy. The effect is explained as a dipolar excitonic explosion followed by nonequilibrium-optical-phonon-mediated lateral transport. The PL is observable due to radiative recombination of high-energy excitons accompanied by inelastic scattering with surface ionized acceptors.