INHOMOGENEOUS PULSED-LASER MELTING OF HIGH-DOSE GE-IMPLANTED SILICON

Pulsed laser melting of Si substrates implanted either with Ge-74+ ions at a nominal dose of 10(17) cm-2 or Si-28+ ions at a dose of 10(16) cm-2 has been investigated by time-resolved reflectivity using visible and infrared probe laser beams. In Si-implanted samples one reflectivity peak is observed during irradiation with 25 ns ruby laser pulses due to the formation and solidification of a liquid layer nucleated at the sample surface. Instead, Ge-implanted Si samples show different reflectivity spectra during irradiation in the same experimental conditions. Two reflectivity peaks have been observed in Ge-implanted samples for laser energy densities below 0.5 J/cm2 whose origin cannot be explained with the same melting dynamics as in pure Si samples. The results are explained assuming that during laser irradiation an inhomogeneous melting process occurs in Ge-implanted samples. On the basis of time-resolved reflectivity, electron microscopy, and Rutherford backscattering measurements, a melting model is suggested for the melting process of Ge-implanted samples. According to this model the liquid phase is nucleated just below the sample surface because of the reduced melting temperature caused by the Gaussian Ge implantation profile. The two reflectivity peaks originate from a time modulation of the liquid-solid ratio at the sample surface.