FUNDAMENTALS OF PULSED LASER IRRADIATION OF SILICON

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Abstract
Most of the experimental work on pulsed laser processing of semiconductors is consistent with an optical heating model. Thermal equilibrium between the dense electron-hole plasma and the lattice of amorphous or crystalline silicon is established on a time scale of 2 ps or less, during the laser pulse. Nevertheless, a series of speculations on the nonthermal nature of laser annealing of ion-implanted silicon surfaces has persisted. These invoke a hot, dense carrier plasma, inducing a phase transformation in a much cooler silicon lattice. According to the thermal model the observed phase changes involve melting. Picosecond irradiation experiments provide a stringent test of the time scale on which the thermal model remains valid. In this paper we present our recent time-resolved measurements of refractive index changes of silicon on a picosecond time scale. During and after the irradiation with a single picosecond laser pulse, the reflectivity and transmission of thin silicon films exhibit characteristic transients, which allow a detailed insight into the plasma kinetics, energy transfer to the lattice and lattice heating, and subsequent melting of the surface. In agreement with recently-published results, obtained with subpicosecond resolution, the energy stored in the electron-hole plasma is found to be transferred to the lattice in a few picoseconds

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