Prof. Abdennaceur Karoui

To understand the dynamics of photogeneration of charge carriers, and to help the design of third generation nanostructured silicon solar cells, we have carried out pioneering work on the measurement and analysis of charge carrier recombination and the effects of submicron scale stress inhomogeneity in silicon at the nanoscale.

A pump-probe optical system and a delay line attached to a Near Field Optical Microscopy (NSOM) enabled mapping the minority carrier concentration as a function of time. The resolution of the concentration map were about 80 nm. The time dependent maps allowed visualization of the carrier motion in the vicinity of defects. Spatial variations of minority carrier lifetime were deduced from the decay of carrier concentration. The technique is dubbed τ-NSOM, in reference to carrier lifetime. Fig. 1 shows an example of carrier lifetime maps obtained as a function of time. The specimen was cut from an OSF ring defect structure found in annealed CZ silicon wafer.

Fig. 1: Sequence of carrier lifetime maps, measured with τ-NSOM, showing variations of generated free carriers by a laser pulse in heat-treated p-type CZ silicon. Yellow indicates low lifetime tight on the recombining defects, while red represents higher lifetimes in their vicinity. Neutral regions are black.[1]

Using another set-up, referred to as τ-NSOM, the local strain was mapped with a sub-micron resolution. The strain was obtained from the variations of polarization of an IR beam transmitted through the material. The beam is focused onto a silicon specimen that contained bands of O precipitates. Maps of carrier lifetime and residual strain acquired in the same region, showed that the mean far-field strain has little effect on carrier recombination, whereas the local stresses in the vicinity of oxygen precipitates appeared strongly correlated to lifetime variations. It is believed that gettered impurities produced a high concentration of recombination centers around the precipitates.

The correlated minority carrier lifetime and strain maps provide new means for improving the understanding of charge carrier dynamics in nanostructured semiconductors, electronic devices, and solar cells.

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[1] E. E. van Dyk, A. Karoui, A. H. La Rosa, G. Rozgonyi, Near-Field Scanning Optical Microscopy for Characterization of Photovoltaic Materials, Phys. Stat. Sol. (c) 1(9), 2292 (2004).