CO2 laser annealing of silicon

Boron doped silicon crystals, amorphized by 15N or 121Sb-ion implantation, were annealed with 100-ns CO2 laser pulses. 4He-ion backscattering and channeling spectra, and sychrotron x-ray topographs of heavily doped (1019...1020 atoms/cm3) samples show an almost complete recrystallization at peak intensities of about 40 MW/cm2, caused at 10.6 μm mainly by free carrier absorption. At intensities of about 20–35 MW/cm2, a rapid increase of the reflectivity to almost 100% at 10.6 μm, and a corresponding decrease in the transmitted intensity was observed. Time resolved transmission and reflection measurements at 10.6 μm and reflection measurements at 0.6.3 μm revealed that a liquid layer was formed at the surface of the crystal. Samples doped with 5×1015 and 3×1016 atoms/cm3 were not fully recrystallized. To estimate the annealing threshold at different doping levels, the maximum surface temperature of the sample was calculated as a function of the laser intensity. The calculations are in agreement with the observations. According to ( p,γ) measurements, some nitrogen segregates to the surface or moves out of the sample. Damage produced on the doped bulk silicon by a CO2 laser pulse at intensities of 20...100 MW/cm2 is also shown.