Crystallization kinetics and recording mechanism of a-Si∕Ni bilayer for write-once blue-ray recording

For the a-Si∕Ni bilayer with a thickness ratio of 4:1, the formation of NiSi and NiSi2 phases took place in the temperature range between 200 and 350°C and only ∼9% of unreacted a-Si would crystallize to c-Si, which will not cause appreciable reflectivity change. As the thickness ratio of a-Si to Ni was increased to 20:1, the formation of NiSi2 phase and subsequent crystallization of a-Si mediated by NiSi2 precipitates were clearly observed. The crystallization temperature of a-Si in the a-Si∕Ni bilayer with a thickness ratio of 20:1 was significantly reduced to around 350°C, which was 130°C lower than that in the a-Si∕Cu bilayer. The activation energies for NiSi2 phase formation and crystallizations of a-Si for the a-Si∕Ni bilayer with a thickness ratio of 20:1 were determined to be 1.12±0.09 and 2.19±0.08eV, respectively. The crystallization behavior of the a-Si(20nm)∕Ni(1nm) bilayer recording film under pulsed laser irradiation is similar to that under thermal annealing. During the recording process, the NiSi2 phase will precipitate first and serve as the nucleation sites for the following crystallization of the remaining amorphous Si. The maximum value of carrier to noise ratio for 3T could reach 43dB for the write-once blue-ray disk with layer structure of a-Si(20nm)∕Ni(1nm), demonstrating a high potentiality for practical use.