Charge emission and precursor accumulation in the multiple-pulse damage regime of silicon

To contribute to further understanding of damage mechanisms in silicon induced by picosecond Nd:YAG 1.06-μm laser pulses, the damage at laser intensities below the one-shot damage threshold has been studied. By using a biased charge-collection wire or electron-multiplier tube, one can detect the charge emitted during small pit formation, which may be considered as the initial damage morphology. The investigation of the incubation period has been conducted by observing the first charge emission, which indicates its termination. This incubation time is analogous to the lifetime of the solids subject to a repeated mechanical load and is characterized by processes that are irreversible for at least 3 sec. This suggests that the damage precursors are long-lived excitations or an accumulation of permanent states. The heterogeneous nature of the nucleation of damage suggests that these precursor states act as nucleation seeds to the laser-induced damage. Charge emission after the incubation period was also studied. Positive and negative particles are emitted equally after damage initiation, probably because of thermal evaporation of silicon from small regions. The charge emission follows the Arrhenius relation. Experimentally, this result was independent of pulse-repetition period up to at least 10 sec. Damage morphology was compared to charge emission and suggests that charges are emitted mainly from pits, grooves, and holes within the damaged area.