Spatially resolved photoluminescence experiments were performed for the first time on  = 25 bi-crystals at about 4.2K. Due to the focusing mechanism which was required for a high spatial resolution, a concomitant increase in the pumping power led to the appearance of an electron-hole droplet photoluminescence band in addition to the usual free-exciton line. The effects of the grain boundary upon as-received and heat-treated specimens were investigated by additional scanning of the exciting laser beam across the bi-crystal interface. In the case of annealed as-received or deliberately Cu-contaminated samples, the scan profiles of the maximum intensities which were emitted by the electron-hole droplets and free-excitons were symmetrical with respect to the grain boundary and revealed a smooth increase, followed by an abrupt fall upon approaching the interface. The photoluminescence enhancement was interpreted in terms of the existence of a denuded zone (free from non-radiative channels) on each side of the boundary, while the built-in electric field (due to grain-boundary precipitates) was thought to dissociate the condensed and free excitons and lead to the observed rapid decrease towards the boundary. A model for these effects was developed for the case of electron-hole droplets, and the reproduction of the associated scan profiles permitted the determination of the effective lateral expansion of the electron-hole droplet cloud; thus providing a rough estimate for the average drift velocity of the droplets. A reported one-third power dependence of the effective lateral expansion upon the pumping power, which had been explained in terms of a phonon wind mechanism, reflected an almost linear expansion of the cloud volume as a function of the laser power. A change in average drift velocity, from 14000 to 35000cm2/s, when the pumping power was increased from 0.007 to 0.5W, further confirmed that droplet transport was due mainly to the so-called phonon wind. The behavior of electron-hole droplet and free-exciton scan profiles in as-grown samples was explained in terms of the stresses and strains which were produced by the lattice distortions in the immediate vicinity of the grain boundaries.

R.Rizk, R.Madelon, F.Cruège: Journal of Physics - Condensed Matter, 1995, 7[30], 6161-77