Influence of ionization and cumulative effects on laser-induced crystallization in multilayer dielectrics

Crystallization of amorphous layers has been demonstrated under various radically different laser-exposure conditions, including continuous wave (cw) and pulsed lasers. Here, we investigate the specific role of ionization in the crystallization of dielectric SiO2/SiNx multilayers. This choice is motivated by the technological relevance of the interaction between lasers and a multilayer thin films system in distributed Bragg reflectors. Specimens were exposed to both femtosecond laser pulses, triggering bulk ionization thanks to nonlinear absorption processes, and a cw infrared laser, whose photon energy is lower than the dielectrics band gap, resulting in a purely linear absorption mechanism rather than ionization. While femtosecond laser exposure leads to the formation of Si nanocrystals homogeneously confined within the focal volume and extending to adjacent SiNx layers, cw laser exposure leads to the formation of polymorphic crystalline phases of Si3N4 in the SiNx layers, along with a Si phase within the SiO2 layers, which otherwise remain amorphous.