KEYWORDS: Crystals, Amorphous silicon, Reflectivity, Nickel, Annealing, Silicon, Pulsed laser operation, Temperature metrology, Transmission electron microscopy, Chemical species
The crystallization characteristics and recording mechanism of the a-Si (20nm)/Ni (5 nm) bilayer recording film have been studied. Upon heating, the formation of the metastable NiSi phase occurred at about 200oC, followed by the formation of the thermodynamically favored NiSi2 phase at ~ 400oC, and the crystallization of a-Si mediated by the Ni silicides below 600oC, indicating that inserting a thin Ni layer could effectively reduce the crystallization temperature of a-Si. As Si atoms of approximately 92 at% were consumed by NiSi2 phase formation, the reflectivity increase of the a-Si/Ni bilayer recording film was contributed to the formation of NiSi2 phase. The activation energy for Ni silicide formation was determined to be 1.98 eV. The a-Si/Ni bilayer exhibited an optical contrast of 20% and abosrptance of 54%, which can provide high enough CNR and adequate recording sensitivity. After irradiated by a 405 nm blue laser pulse, a recording mark consisting of NiSi2 was clearly formed in the a-Si/Ni bilayer. Under the dynamic test, CNR of 37 dB was achieved at a recording power of 4 mW, demonstrating that the a-Si/Ni bilayer recording film shows a great potential for use in write-once Blu-ray disk.
The addition of Al increases the temperature and activation energy of crystallization of the fast-growth type Sb70Te30
recording film so that the archival stability will be improved. Meanwhile, the addition of Al increases the temperature
and activation energy of melting of the Sb70Te30 recording film so that the local melting of the crystalline Sb and Sb2Te3
phases will be suppressed. The pulsed laser powers required to trigger crystallization, melting, and ablation of Sb70Te30
recording film increase with increasing Al content indicating the increased stability of the non-crystalline phase, in the
meantime, the increased melting and ablation powers indicating the Al addition could increase the melting temperature
and prevent ablation effect of the Sb70Te30 recording film.
The crystallization kinetics and recording characteristics of the Cu/a-Si bilayer recording film have been studied. Under thermal annealing, the crystallization temperature and activation energy for crystallization of a-Si with a thin Cu metal layer were reduced to about 485°C and 3.3±0.1 eV, respectively, indicating that inserting a thin Cu underlayer could effectively reduce the crystallization temperature and activation energy for crystallization of a-Si. The reaction exponent was determined to be around 1.8, corresponding to a crystallization process in which grain growth occurs with nucleation, and the nucleation rate decreases with the progress of the grain growth process. This is consistent with the hypothesis that the Cu3Si precipitates act as the nucleation sites for the subsequent crystallization of a-Si in the Cu/a-Si bilayer recording film. Additionally, the maximum data-transfer-rates that can be achieved by the Cu/a-Si bilayer recording film at recording powers of 6, 8, and 10 mW were estimated to be about 23, 46, and 223 Mb/s, respectively.
The effects of the film thickness and foreign element addition on the crystallization kinetics and optical properties of eutectic Sb70Te30 phase change recording film have been studied. The crystallization temperature and activation energy for crystallization of the non-doped Sb70Te30 recording films were decreased from 161 to 145, 144, 141, and 125°C, and from 3.0 to 2.6, 2.3, 2.0, and 1.9 eV/atom, respectively, when the thickness was increased from 15 to 20, 25, 30 and 100 nm. For the Ag-doped Sb70Te30 recording film of 20 nm in thickness, the crystallization temperature was found to increase from 145 to 146 and 156°C, and the activation energy was found to decrease from 2.6 to 2.5 and 2.1 eV/atom, respectively, as the concentration of Ag was increased from 0 to 3.8 and 11.8 at%, respectively. Meanwhile, the crystallization temperature and activation energy of the 20-nm-thick In-doped Sb70Te30 film was found to increase from 145 to 153 and 168°C, and increased from 2.6 to 2.8 and 3.4 eV/atom, respectively, as the concentration of In was increased from 0 to 1.4 and 4.8 at%.
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