Effects of a grain boundary on the photoluminescence spectrum of silicon: expansion of electron-hole droplet cloud

Abstract

Spatially resolved photoluminescence (PL) experiments on Sigma =25 silicon bicrystals have been performed at very low temperatures ( approximately 4.2 K) for the first time. Owing to the focusing mechanism required for high spatial resolution, the concomitant increase of the pumping power, P, has led to the emergence of an electron-hole droplet (EHD) PL band besides the usual free-exciton (FE) line. The effects of the grain boundary (GB) of both as-delivered and heat-treated specimens were investigated by additional scanning of the bicrystal by the exciting laser beam across the interface. For the annealed as-received or deliberately contaminated samples (with Cu), the scan profiles of the maximum intensities emitted by the EHDs and FEs were symmetrical with respect to the GB and have revealed a smooth increase followed by an abrupt drop upon approaching the interface. While the PL enhancement is interpreted in terms of the existence of a denuded zone, exempt from non-radiative channels, on either side of the boundary, the built-in electric field due to GB precipitates is thought to dissociate the condensed and free excitons leading to the above-mentioned steep decrease around the boundary. A complete modelling of these effects has been carried out for the case of EHD and the reproduction of the related scan profiles has allowed the determination of the effective lateral expansion, Leff, of the EHD cloud, as well as a rough estimate of the average drift velocity, vd, of the droplets. The reported variation of Leff with P, exhibiting a fairly good P1/3 dependence as established earlier by Bagaev and co-workers and explained in terms of a phonon wind mechanism, signifies a nearly linear expansion of the cloud volume with laser power and, therefore, an almost constant filling factor of the cloud by the generated droplets. Furthermore, the evolution of vd from 1.4*104 to 3.5*104 cm s-1 when P varies from 7 to 500 mW provides further confirmation for the droplet transport being mainly due to the so-called phonon wind. Finally, the behaviour of EHD and FE scan profiles in the as-grown samples is interpreted in terms of stresses and strains developed by the lattice distortion in the close vicinity of the GB.