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

Abstract

Abslmct. Spatially resolved photoluminescence (PL) experiments on X = 25 silicon biqstals have been performed at very low temperatures (-4.2 K) for the first time. Owing10 the focusing mechanism required for high spatial resolution. the concomitanl increase of the pumping power, P, has led to the emergence of an electmn-hole droplet @€ID) PL band besides the usual freeexciton (FE) line. The effects of the p i n boundary (GB) of both as-delivered and heal-mted specimens were investigawl by additional scanning of the bicrystal by the exciting laser beam moss 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 F€s were symmetrical with respect to the GB and have revealed a smooth increase followed by an abrupt drop upon approaching the inlaface. 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 lo 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 oul for the case of EHD and the reproduction of the related scan profiles has allowed the determination of lhe effective lateral expansion, L.f/, of the EHD cloud, as well as a rough estimate of the average drift velocity. ud. of the droplets. The reported variation of Le/, with P, exhibiting a fakly good P1l3 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 Ud from 1.4 x lo4 to 3.5 x 10' cm s-' when P varies from 7 to 500 m W provides funher conhmtion for the droplet transport being mainly due to the so-called phonon wind. Finally, the behaviour of EHD and FE scam profiles in the as-grown samples is interpreted in terms of stresses and strains developed by the lanice distortion in lhe close viciniiy of the GB.

Molecular dynamics Monte Carlo simulations of grain boundary electron transport in n-silicon

“…Transmission electron micrographs of Si samples have clearly revealed the formation of granular precipitates at the GB, 15 while recent photoluminescence studies have directly confirmed the existence of spatially nonuniform internal electric fields. 16 These recent results underscore the discrete nature of GB charge distribution. However, many previous theoretical transport studies of GB simply assumed one-dimensional ͑1D͒ potential variations perpendicular to the grainboundary plane based on the depletion approximation.…”