PICOSECOND DECAY OF PHOTOINDUCED ABSORPTION IN UNDOPED AMORPHOUS AND POLYCRYSTALLINE SILICON THIN-FILMS

In this work we discuss picosecond photoinduced absorption (PA) decay curves obtained for amorphous and polycrystalline (furnace crystallised and laser crystallised) silicon thin films. The PA temporal decay curves are recorded using a fast modulation ''pump and probe'' technique, the pump and probe sources being two dye lasers driven synchronously by a mode-locked argon-ion laser. In order to have comparable decay characteristics, the PA curves have been transformed to da (change of the absorption coefficient) curves and normalised to constant charge-carrier densities. The data are found to be very sensitively dependent on the structural properties of the films investigated. In order to explain the phenomena observed we propose a tentative model whereby recombination occurs at the grain boundaries and is rate-limited by carrier diffusion from the crystallites.