We have demonstrated for the first time the feasibility of using a rewritable dual-layer phase-change optical disk utilizing a blue-violet laser. For the first medium, we adopted a very thin recording layer with a new phase-change material Ge–Sn–Sb–Te, and a 10-nm-thick silver-alloy reflective layer to obtain a large transmittance and high-quality signals. For the second medium, we optimized the thickness of each layer to obtain both a large optical absorption of the recording layer and a small heat capacity. Carrier-to-noise ratios of more than 50 dB, erasability of more than 30 dB and recording powers of 8 mW for the first medium and 11 mW for the second medium were obtained under typical recording conditions corresponding to a capacity of 27 GB per one side of a 120 mm disk and a user data transfer rate of 33 Mbps.

A thin film of Sn-doped and GeTe-rich GeTe-Sb2Te3 shows characteristics that make it suitable for use in rewritable dual-layer optical disks employing a violet laser. By increasing the GeTe component from Ge2Sb2Te5 to Ge4Sb2Te7, Ge6Sb2Te9, and Ge8Sb2Te11, optical changes were increased. By substituting Sn for a proportion of Ge in these compositions, crystallization rates are greatly increased and even a 5 nm-thick film showed a very short laser-crystallization time of less than 50 ns. The material film was successfully applied to Layer 0 of a rewritable dual-layer disk: capacity of 27 GB and a 33 Mbps data transfer rate were confirmed for a disk using a conventional 0.6 mm substrate (NA 0.65),and 45 GB capacity and the same data transfer rate were obtained for another disk using a thin cover layer 0.1 mm thick (NA = 0.85).

It is found that a rapid cooling structure is effective to improve overwrite characteristics of phase change type optical disk media. For a rapid cooling structure, thin A1N dielectric layer, 30 nm, with high thermal conductivity is suitable. The layer is adopted between the active layer and the reflective metal layer in quadrilayer disk structure. The cooling rate of the disk is calculated to be 12°C/nsec. This disk structure shows clear amorphous marks. A thin active layer disk structure using GeTe–Sb2Te3–Sb ,alloy of 20 nm has produced two million cycle stable bit error rate characteristics.1) And the erasability of the disk having a rapid cooling structure goes up more than 30 dB and shows rather wide plateau region of power dependency.

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We have developed dual-layer write-once media with Te-O-Pd based recording films on Blu-ray Disc (BD) format. Reordmg CapaCity was 50GB with dual layers on a disk of 120mm in diameter. Rear and Front layers showed jitters of 5.8% and 7.7% at lx speed, and 6.0% and 8.0% at 2x speed, respetive1y, which were good enough to satisfy the BD format. Evaluations were Camed out with blue-violet laser of 405nm wavelength, objective lens NA of 0.85. Reoording linear veloifles were 4.92m/s at BD lx (36Mbps), and 9.84m/s at BD 2x (72Mbps). CharaCteriStiCS at 4x speed reoording were also examined, and it was revealed that ather to noise ratio at high recording linear velocity of 19.7mJs, whiCh COfleSpOfldS to BD 4x (144Mbps), was almost as same as those of lx and 2x. Recording mechanism was diseussed and proposed a model that Te-O-Pd films were not rystaThzod direCtly thrOUgh solid process, but rystalEzed through melting.