| Summary: | 碩士 === 長庚大學 === 化學工程研究所 === 88 === In optical data storage, phase change materials have been chosen as the rewritable media by DVD consortium. From 650 Mbytes of CD-RW to 2.6 Gbytes or 4.7 Gbytes of DVD-RAM, the capacity of rewritable media grow substantially. In addition to the increase in capacity, the rewritability or overwrite cycle characteristics provides technological challenges to both academia and industry. Two phase change materials were investigated in this study, including one based on a three-element -alloy system of Ge-Sb-Te and the other one based on a four-element- alloy system of Ag-In-Sb-Te. The single-layer recording film was deposited on substrates by radio-frequency magnetron sputter method. We used atomic force microscope to find the average roughness of deposited films. The thermal behavior and phase transition temperatures were revealed using thermogravimetric analyzer (TGA) and differential scanning calorimetry (DSC). We also utilized ellipsometer to measured the change in optical constant due to the reversible phase transition in a recording layer between the amorphous and the crystalline states. The phase change behavior were further characterized by x-ray diffraction spectroscopy (XRD) on sputtered thin films of the phase change materials.
From experimental results showed that the average roughness of deposited films was obviously increased after annealed. For the Ge-Sb -Te alloy system, the melting point and crystallization temperature was around 625℃ and 167℃, respectively, and 521℃ and 181℃ for the Ag-In-Sb-Te alloy system. The activation energy for both materials was 2.69eV and 2.87eV, resspectively. The crystalline structure was confirmed as the annealing temperature increased, the fcc-GeTe phase in the Ge-Sb -Te film disappeared and changed into hex- Sb2Te3 phase. Similarly, for the Ag-In-Sb-Te film, the AgInTe2 phase disappeared and changed into AgSbTe2 phase as the annealing temperature increased. The maximun difference of reflectivity (ΔR) between the amorphous and the crysyalline state was more than 30％ for the Ge-Te-Sb alloy system and 50％ for the Ag-In-Sb-Te alloy system.
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