Crystal defects in monocrystalline silicon induced by spot laser melting

Laser processing of monocrystalline silicon has become an important tool for a wide range of applications. Here, we use microsecond spot laser melting as a model experiment to investigate the generation of crystal defects and residual stress. Using Micro-Raman spectroscopy, defect etching, and transmission electron microscopy, we find no dislocations in the recrystallized volume for cooling rates exceeding |dT/dt|=2×107 K/s, and the samples remain free of residual stress. For cooling rates less than |dT/dt|=2×107 K/s, however, the experiments show a sharp transition to a defective microstructure that is rich in dislocations and residual stress. Moreover, transmission electron microscopy indicates dislocation loops, stacking-fault tetrahedra, and voids within the recrystallized volume, thereby indicating supersaturation of intrinsic point defects during recrystallization. Complementing photoluminescence spectroscopy indicates even three regimes with decreasing cooling rate. Spectra of regime 1 do not contain any defect related spectral lines. In regime 2, spectral lines appear related to point defect clusters. In regime 3, the spectral lines related to point defect clusters vanish, but dislocation-related ones appear. We propose a quantitative model explaining the transition from dislocation-free to dislocation-rich recrystallization by means of the interaction between intrinsic point defects and dislocations.