Optical and electrical properties. of pulsed Iaser annealed thin Si films J. Bischof, J. Boneberg, C.R. Dorfmiiller, M. Keil, M. Lux-Steiner, P. Leiderer Fakultat fiis Physik, Universitat Konstanz, 78431 Konstanz, Germany ABSTRACT Thin Si films used for solar energy purposes are commonly treated by relatively slow thermal annealing on a time scale of seconds to obtain the proper electrical behavior. We investiga~e a different approach, in which the films are annealed and/or molten by a frequency doubled Qswitched k'd:YAG Iaser pulse on a nanosecond time scale. We studied thin polycrystdline Si films of thickness between 43 nm and 250 nrn on fused silica and on sapphire substrates. The different thermal conductivikies of these substrates lead to different quench rates for the molten Si films. The optical and electrical properties of the Si films were systematically characterized during, respectively after the various annealing conditions. In addition we monitored the solidification process in situ by time-resolved optical measurements. At low energy densities the film is not completely molten by the laser pulse and resolidification takw place at the moving liquidsolid interface. Above a thicknpss-dependent threshold energy density complete melting is observed and nucleation in the supercooletl melt pr~vails. In the latter cze Samehima and Usui ' showed that amosphization can be observed far Si films on fused silica up to thicknesses of 36 nm. Mte found that Si films on sapphire even with a thickness of 80nm can be amorphized. The reproducible thteshold values sugg~t the possibilty of lateral structuring. 1. INTRODUCTION In thin film technology of semiconductars different approaches can be taken to remove defects in the crystal structure ', e.g. after production on substrates or ion implantation. This can be achieved for example by thermal annealing in a furnace, electron beam processing and laser annealing. In this work we report on pulsed laser annealing of thin Si films on fused silica and sapphire substrat-. Time resolved optical measurements provide a powerful tool to get insight into the dynamics during the annealing process. We show that depending on the laser energy density and substrate material difT~rent electrical, optical and structural properties can be achieved. These can be associatied with different dynamic scenarios. 2. EXPERIMENT Thc samples used were polycrystalline Si (pc-Si) films with various thicknesses in thc range between 40nm and 250nm on I mrn fused silica and O.5rnm sapphire substrates. The Si films tvere deposited by CVD technique at a substrate temperaturr of 600 'C. The reacting gas consisted of 99.5 % SiIIs and 0.5% PHs, the latter being addcd in order to shift the eleckrical conductivity into a morc suitable range. Ry spectral ellipsometric measurements the cry~tallinit~y was prover) to bc usually more than 65 %. The annealing laser was a frequency doubled Q-switched Nd:YAG laqer with a pulsc length of 'ins full width at half-maximum and a beam profile close to TEMoo. The Nd:YhG pulse, incident nearly perpendicular to the surface, was only mildly focused to a spot diameter of about I mm. To investigate th, dynamics during annealing we measured the reflectivity of two s-potarized continuous wave lasers " (Fig. 1). A ReNe laser (633nn-1, 8 mW) wiw used for measuring the reflectivity from the free surface of the Si film at an angle of incidence of 45 ', and an Ar+ laser (488nm, 15rnW) probed the reflectivity from the film-substrate interface, incident at 30 O onto the back of the substrate. Thae two probe lasers were focused to a l/e diameter smaller than 20 Irrn onto the surface of the Si film and on to filmsubstrate interface, respectively. There the overlap of both spots was better than 90%. In this manner variations 834 1 SPIE Vol. 2255