Amorphous films having a component of the stoichiometric GeTe-Sb2Te3 pseudobinary alloy system, GeSb2Te4 or Ge2Sb2Te5 representatively, were found to have featuring characteristics for optical memory material presenting a large optical change and enabling high-speed one-beam data rewriting. The material films being sandwiched by heat-conductive ZnS layers can be crystallized (low power) or reamorphized (high power) by laser irradiation of very short duration, less than 50 ns. The cooling speed of the sandwiched film is extremely high: more than 1010 deg/s, which permits the molten material to convert to the amorphous state spontaneously; whereas, a low-power pulse irradiation of the same duration changed the exposed portion into the crystalline state. The optical constant changes between the amorphous state and the crystalline state of them were measured to be large: from 4.7+i1.3 to 6.9+i2.6 and from 5.0+i1.3 to 6.5+i3.5, respectively. The crystallized portion was known to have a GeTe-like fcc structure by an analytical experiment using transmission electron microscopy, differential scanning calorimetry, and x-ray and electron diffraction methods. The high crystallization speed is ascribed to (1) the pseudobinary system which can form crystalline compositions without any phase separation, (2) the high symmetry of the fcc structure which is the nearest to the random amorphous structure, (3) the high-energy difference between the amorphous state and the fcc crystal state.

It was found that GeTe-Sb2Te3 pseud-binary amorphous alloy films showed remarkably fast switching properties to laser irradiation. By the static laser irradiation test, the film whose composition corresponded to stoichiometric compound of GeSb2Te4 were crystallized within 50ns of pulse duration at power of 8mW, whilst they could be amorphized with the same pulse duration at power of 20mW. Direct overwriting cycle test was performed on the revolving disk system for 105 times using single laser beam. CNR of more than 50dB and erasability of -22dB were obtained for linear velocity of 22m/s and overwriting frequencies of 5 and 7 MHz. The laser powers were 22 mW for recording and 10 mW for erasing. These materials will be applicable to high data rate direct overwritable disk media.

If a large amount of polymer radicals remain trapped after the irradiation of ultrahigh molecular weight polyethylene (UHMWPE), the radicals may result in a significant alteration of its physical properties during long‐term shelf storage and implantation. An electron spin resonance spectroscopic study was undertaken to investigate the remaining free radicals in UHMWPE after electron beam irradiation up to 500 kGy in air and an N2 environment. Heat treatment was employed at 110 and 145°C for various periods of time to decay the free radicals. The free radicals were rapidly decayed for 1 h and gradually decayed as a function of time with the heat treatment. The decay of the free radicals was completed more rapidly with a heat treatment at 145°C than at 110°C. Therefore, a longer heat treatment time is required to scavenge all the free radicals formed in UHMWPE at 110°C. The oxidation profiles showed that the oxidation index of the heat‐treated UHMWPE was lower than the oxidation index of the non‐heat‐treated UHMWPE. The heat treatment of irradiated UHMWPE can substantially reduce the concentration of free radicals; therefore, UHMWPE has resistance against long‐term oxidative degradation. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 103–116, 2005

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The nitrogen doping effect on the Ge–Sb–Te recording layer was quantitatively examined. We succeeded in the quantitative analysis of the nitrogen concentration in the Ge–Sb–Te–(N) recording layer by secondary ion mass spectrometry (SIMS) observation. The nitrogen concentration could be finely controlled at a high deposition rate of 4.7 nm/s. The addition of a small amount of nitrogen remarkably improved the overwrite cycle numbers. We found that the most suitable nitrogen concentration was from 2 to 3 at%. We proposed a model to explain the nitrogen atom function in the recording layer. The nitrogen atoms produced nitrides, which are condensed near the grain boundaries of Ge–Sb–Te microcrystals. This resulted in the formation of very thin wrappings, which wrap the crystal grain in a manner similar to that of the peel of a peach and suppressed the micro-material flow. We achieved 8×105 overwrite cycles at λ=790 nm, N A=0.50 and using the pit position modulation (PPM) recording method where the minimum bit length is 0.87 µm.