FUNDAMENTALS OF PULSED LASER IRRADIATION OF SILICON

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.