Hisashi Andoh

Reversible phase‐change optical data storage in InSbTe alloy films

Some characteristics of reversible phase‐change optical data storage based on an amorphous‐crystalline transformation in InSbTe alloys are given. The reversible phase change was observed in a wide region of composition. The laser amorphized spot of a ternary compound In3SbTe2 film could be crystallized using a diode laser pulse of less than 100 ns with an incident laser power of more than 10 mW. The crystallization temperature of the amorphized spot was 280 °C and the activation energy was about 1.8 eV which shows that long‐term data retention at room temperature is possible. The repetition number of static write and erase using the pulse of 50 ns reached above 105. These data show that the ternary compound film has potential for reversible optical data storage media with high‐speed erasing and long‐term data retention.

Single‐beam overwriting with melt‐erasing process in an InSbTe phase‐change optical disk

Single‐beam overwriting with melt‐erasing process was made in a 5.25‐in.‐diam phase‐change optical disk using an In22Sb37Te41 recording film. In the overwriting between 2 and 3 MHz signals at the linear velocity of 3–11 m/s, a carrier to noise ratio (C/N) more than 46 dB and an erasability less than −35 dB could be obtained. This high erasability was found to be due to the melt‐erasing process. This disk presents highly erasable overwriting and long data retention time supported by an activation energy of 2.3 eV and a temperature of 230 °C for crystallization of the amorphized part.

400 dpi integrated contact type linear image sensors with poly-Si TFT's analog readout circuits and dynamic shift registers

Four-hundred-dots-per-inch (dpi) sensors, including poly-Si thin-film-transistor (TFT) scanning circuits, and a-Si photodiodes fabricated on borosilicate glass have been developed. This contact-type image sensor contains TFT analog buffer amplifiers in the readout circuits. The scanning circuits can operate in a frequency range between 200 kHz and 1 MHz. The readout circuits incorporating TFT analog impedance converters decrease photodiode impedance by more than three orders of magnitude and improve the linearity between illumination intensity and the sensor output. High-resolution reading is achieved by the new contact-type linear image sensors with a storage time of 2 ms/line. >

Three‐dimensional analysis of overwritable phase‐change optical disks

Three‐dimensional computer calculations which analyze time‐transient behavior of heat conduction and phase‐change kinetics in an InSbTe overwritable disk were made. To improve the calculation accuracy, thermal conductivities of thin‐film materials and critical cooling rate for crystallizing the recording film were estimated. For realization of high‐performance overwrite, it was found that the cooling conditions of writing and erasing processes should be controlled to match the critical cooling rate in a given linear velocity range. This could be achieved by optimal design of the disk structure.

Crystallizing mechanism and recording properties of In3SbTe2 phase-change optical disks

The crystallization mechanism and its application to improvement of the carrier-to-noise ratio (CNR) for In3SbTe2 phase-change optical disks were examined by that transmission electron microscopy (TEM). The TEM images indicated: (1) mark shapes were distorted when the interval between laser irradiations was short; (2) following dc laser irradiation onto the marks, crystallization proceeded only along the periphery of the amorphous phase; and (3) mark shapes were varied corresponding to the dc laser power. We assumed that the crystallization mechanism for In3SbTe2 was dominated by crystalline growth rather than nucleation. Then we simulated the mark shapes after dc irradiation. By controlling the thermal distribution on the marks, the CNR was improved promoted. We found that dc irradiation was a simple way of improving recording properties.

Reversible colour change in Cu-Al-Ni alloy ribbon associated with phase transformation

The colour change behaviour and its relation to phase transformation of Cu-14 wt% Al-4 wt% Ni alloy ribbon produced by the twin-roller type melt-quenching method were investigated by spectral reflectivity and X-ray diffraction, respectively. This ribbon turns copper-coloured around room temperature on quenching it from a temperature above 1020 K, and turns gold-coloured on ageing it between 670 and 970 K. By repeating these heat treatments, either of the two colours can be acquired interchangeably. The spectral reflectivity also changes with respect to the colour change. The copper-coloured alloy shows a DO3 structure which is theβ1, phase. The gold-coloured alloy shows a mixed phase ofγ2 (cubic, Cu9Al4 compound), andα (fcc, Cu-Al-Ni solid solution) and/orγ1′ which is a martensite of theβ1 phase having a (1 21) twinning structure. Therefore, the colour change between copper and gold is due to the solid-state phase transformation betweenβ1 andγ2 + (α and/orγ1′) on heat treatment.

Reversible color changes in sputter-deposited Ag-Zn alloy films

Reversible color‐change behavior and its relation to phase transformations in sputter‐deposited Ag‐50 at. % Zn alloy films were investigated by spectral reflectivity measurements, x‐ray diffractometry, and transmission electron microscopy. The color of the alloy film is pink at room temperature when produced by quenching above 560 K, and is silver by quenching between 425 and 545 K. By alternately repeating these heat treatments, the two colored states can be reversibly produced. This color change is observed as reflectivity changes at wavelengths below 600 nm and in the near‐infrared range for spectral reflectivity. The pink‐colored film has a CsCl‐type ordered structure (β’ phase), while the silver colored has a hexagonal one (ζ phase). Therefore, the reversible color change is due to the solid‐state phase transformations between the β’ and ζ phases. By irradiation with semiconductor laser beam, the color change can be induced in a microarea (about 1 μm) of the film.

Thermophotometric study of phase transformations in silver-zinc alloy thin films

The transformations between β1, ζ, and β phases in a sputter‐deposited Ag‐50 at. % Zn alloy thin film were examined by a developed thermophotometric apparatus. Good observations were made using the change in reflectivity of the thin film with temperature and time, and some characteristic transformation behaviors in the thin film were made clear. Kinetics of the β1→ζ transformation were analyzed using the Johnson–Mehl–Avrami equation. The activation energy obtained from the relaxation time measured at different temperatures was 220.6 kJ/mol. It was much larger than that in the bulk. The time exponent of the equation was 1.51. This shows that the ζ phase nucleates at the grain edge and/or the boundary of the β1 phase after site saturation under a zero nucleation rate. The activation entropy was a large positive value, and the transformation interface was expected to have a complicated structure.

Overwritable phase-change optical disk using an In3SbTe2 ternary compound

By employing transmission electron microscopy and thermal ana1yis, we examined overwrite repeatability of optical disks using In-Sb-Te alloys, which have high C/N ratio and excellent overwrite modulation characteristics. The overwrite repeatability of more than 105 was achieved by using an In3SbTe2 single-phase ternary compound and heat-flow balanced disk structure.

Melt-Erasing Method Of Single Beam Overwrite For Phase-Change Optical Disk

A new single beam overwrite method for a phase-change optical disk is proposed to obtain both high carrier to noise ratio (CNR) and high overwrite erasing ratio. Contrary to ordinary method is, both writing and erasing process are controlled over the melting point of a phase-change recording film. To realize new overwriting method, we discuss desired thermal characteristics of a disk and crystallization properties of recording film theoretically. Necessary conditions to realize this method are (1) optical disks must have high thermal conductivity to control cooling rate, (2) recording material may have adequate crystallizing tine within dynamic range of disks cooling rate. Examination on this method was carried out by using newly developed In-Sb-Te phase-change optical disk. CNR higher than 50dB and erasing ratio higher than 40dB were obtained.