KEYWORDS: Interfaces, Tellurium, Crystals, Optical discs, Optical storage, Clocks, Digital video recorders, Germanium, Digital video discs, Multilayers
We have developed phase change optical media with appropriate write strategy for 1X-2X dual speed Blu-ray Disc system and for the future high speed phase change optical data storage. For recording layer, eutectic-based Ge(Sb70Te30)+Sb material was used and Sb/Te ratio and Ge content were optimized to obtain proper erasability and archival stability of recorded amorphous marks. The recording layer is wrapped up in GeN interface layers to obtain reasonable overwrite cyclability and higher crystallization speed. In addition, we designed appropriate write strategy so-called time-shifted multipulse(TSMP) where the starting positions of multipulse parts are shifted from reference clock. With this write strategy, the overall jitter characteristics of the disc was improved and especially we found that trailing edge jitter was improved much more than leading edge jitter. Thus feasibility of 1X-2X dual speed applicable Blu-ray Disc and its write strategy was demonstrated.
For the phase change optical disk, the jitter value of recorded signal during the early several times of direct overwrite is usually much larger than that of initial writing.
Phase-change optical data storage is basically based on the difference in optical properties between amorphous and crystalline phases of a recording medium. It has been known that the physical properties including reflectance and thermal conductivity of a material basically depend on the chemical composition of materials and their microstructure. Lately, the manipulation of chemical composition in GeSbTe-based chalcogenide that is one of the most promising candidate materials for phase-change recording medium was tried by doping a small amount of nitrogen in Ge-Sb-Te phase-change optical disks. From the earlier work, an enhanced media cyclability was achieved in PPM recording at the wavelength of around 780 nm. The goal of this research is to investigate the variation of thermal, optical and dynamic properties, and their correlation to microstructure in nitrogen-doped Ge2Sb2Te5 phase-change recording media. Primary endeavors focused on the dependence of nitrogen doping on crystallization temperature, reflectance, initializing power level, CNR and overwrite jitter at the wavelength of 650 nm and pulse-width modulation (PWM) recording, leading to higher storage density.
The complex refractive index of phase-change Ge2Sb2+xTe5 media fabricated by DC magnetron sputtering has been determined by using spectroscopic ellipsometry and atomic force microscopy. The composition of Ge2Sb2+xTe5 analyzed by Inductively Coupled Plasma/Atomic Emission Spectrometer and X-ray Fluorescence was in the range 0.08 less than or equal to x less than or equal to 0.58. The complex refractive index and the reflectivity have been verified nearly constant with Sb-addition at 780 nm, 650 nm, and 410 nm, respectively.
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