We have developed dual-layer write-once media with Te-O-Pd based recording films on Blu-ray (BD) format. Recording 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 1x speed, and 6.0% and 8.0% at 2x speed, respectively, which were good enough to satisfy the BD format. Evaluations were carried out with blue-violet laser of 405nm wavelength, objective lens NA of 0.85. Recording linear velocities were 4.92m/s at BD 1x (36Mbps), and 9.84m/s at BD 2x (72Mbps). Characteristics at 4x speed recording were also examined, and it was revealed that carrier to niose ratio at high recording linear velocity of 19.7m/s, which corresponds to BD 4x (144Mbps), was alomst as same as those of 1x and 2x. Recording mechanism was discussed and proposed a model that Te-O-Pd films were not crystallized directly through solid process, but crystallized through melting.
KEYWORDS: Modulation, Digital video discs, Crystals, Germanium, Optical discs, Tin, Phase shift keying, Computing systems, Laser marking, Chemical species
We demonstrated the possibility of high data rate recording on a DVD-RAM disk which utilizes Ge-Sb-Te phase-change materials. To ensure high transfer rate overwriting on the DVD, quadruple speed (44Mbps) recording at a linear velocity of 16.4 m/s was tested using a Sn-added Ge-Sn-Sb-Te material as the recording layer. Double speed (22Mbps) recording on the present 4.7GB DVD-RAM at a linear velocity of 8.2 m/s was also tested. A CNR of more than 53 dB and an erasability of more than 30 dB were obtained at each double, triple and quadruple speeds. In addition, by recording via 8-16 random modulation signals, a jitter of 9 percent or less and a direct overwrite performance of 100,000 cycles were confirmed.
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 form Ge2Sb2Te5 to Ge4Sb2Te7, and Ge8Sb2Te11, optical changes were increased. By substituting Sn for a proposition 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 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, and 45 GB capacity and the same data transfer rate were obtained for another disk using thin cover layer 0.1 mm thick.
By nitrogen doping into a Ge-Sb-Te phase change optical disk's recording layer, we were able to significantly increase its cyclability. For example, our PD attained, at the maximum, 800,000 overwrite cycles through accurate control of nitrogen concentration. We quantified the nitrogen concentration of recording layer using secondary ion mass spectrometry (SIMS) and determined, from the viewpoint of cyclability, signal amplitude and other parameters, the optimum concentration to be around 2 - 3 at.%. From analyses by thermal desorption mass spectrometry (TDMS) and X-ray diffraction (XD) using powder, we found: (1) nitrogen atoms are mainly bound with Ge to create an amorphous phase of Ge-N; (2) as long as the nitrogen concentration remains around 5 at.%, those Ge, Sb and Te atoms which are not bound with nitrogen form NaCl type crystals. We obtained the following model by combining the results of the above analysis. Nitrogen-doped Ge-Sb-Te recording layer is composed of Ge-Sb-Te grains intermingled with a small quantity of amorphous Ge-N, which exists in the form of a thin film penetrating the grain boundary of Ge-Sb-Te. The Ge-N composing this high-melting-point material layer appears to suppress any micro-material-flow that may occur during overwrite.
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