Kenichi Nishiuchi

Rapid‐phase transitions of GeTe‐Sb2Te3 pseudobinary amorphous thin films for an optical disk memory

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 ...

High Speed Overwritable Phase Change Optical Disk Material

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.

Rewritable Dual-Layer Phase-Change Optical Disk Utilizing a Blue-Violet Laser

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.

Overview and the Future of Phase-Change Optical Disk Technology

Phase-change optical disk technology in the early days dealt with three issues: cyclability, erase characteristics and sensitivity. These were resolved by materials development, and disk layer structure technologies. This paper shows the high (carrier to noise ratio) C/N recording characteristics of a multi-level phase-change optical disk. Multi-level mark radial width modulation (MRWM) recording experiments have demonstrated 4-level (level 0, level 1, level 2 and level 3) recording capability. Even level 1 marks with a radial width of 200 nm have a high C/N ratio of more than 50 dB. The laser pulse width and laser power assigned MRWM, M-level recording is expected the potential for marks storing density proportionally to log 2M. This technology offers the feature of enhanced density and compatibility with a digital versatile disk (DVD) pick-up.

TeOx thin films for an optical disc memory

Tellurium suboxide thin films TeOx were found to change in refractive index and extinction coefficient on thermal or optical heating, with accompanying changes in the reflectivity and transmission. The preparation method and thermal or optical properties of the TeOx thin films were investigated to obtain a stable and highly sensitive optical disc memory. A two‐source evaporation method using Te and TeO2 provided uniform and any desired composition. The properties of the film depended on the x value; increasing x to as large as x=1.2, which represents a Te‐poor composition, produced an excellent humidity and heat stability, and decreasing x to 0.8 made the film more sensitive to a laser diode, but susceptible to humidity. The TeO1.1 thin film was found to sufficiently satisfy all requirements for practical disc applications. A reflective optical disc was prepared using the TeO1.1 thin film deposited on a polymethylmethacrylate substrate with grooves for optical tracking. This disc is capable of recording v...

Land and Groove Recording for High Track Density on Phase-Change Optical Disks

We have proposed a high-track-density optical recording method, which records signals on both the land area and groove area. From a computer simulation based on the scalar diffraction theory, we found that crosstalk between recording tracks depends on the groove depth, and is most reduced at the groove depth of 80 nm~90 nm for a laser wavelength of 780 nm, a numerical aperture of 0.55 and a track pitch of 0.8 µm. Applying this method to a phase-change optical disk, we experimentally confirmed that crosstalk depends on the groove depth and obtained a crosstalk level of less than -32 dB for a groove depth of 64 nm to 78 nm and 0.9 µm length marks. In order to prove that this recording method can be used for rewritable media, we also measured change of carrier-to-noise ratio (CNR) when signals were repeatedly recorded on an adjacent track. The reduction of CNR after 1000 repetitions of writing was less than 2 dB for a linear velocity of 5 m/s.

Rewritable Dual-Layer Phase-Change Optical Disk

This paper describes the development of a rewritable phase-change optical disk with a dual-layer structure. To obtain equivalent signal amplitude and recording sensitivity from the medium located on the beam incident side (the first medium) and the medium beneath (the second medium), we developed a high transmittance first medium featuring a very thin recording film (7 nm) and no reflection layer, and a high recording sensitivity and high reflectivity second medium with a semi-transparent front layer and a reflection layer. We confirmed a jitter of approximately 10% for both media at a recording density corresponding to 8.5 GB on a 120 mm disk. The laser power required to record on the first medium was 12 mW, and the second medium, 13 mW. It was also confirmed that the new rewritable dual-layer phase-change optical disk possesses the potential for operation with a blue laser.

Phase-change material for use in rewritable dual-layer optical disk

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.

Dual-Layer Optical Disk with Te–O–Pd Phase-Change Film

A recordable dual-layer optical disk that uses a Te–O–Pd phase-change film was studied for high-density recording. In order to reduce thermal influence to adjacent tracks, we selected the optimum composition of Te42O46Pd12 that gives low thermal conductivity. To get equivalent signal amplitude and absorbance of layers, the thicknesses of the laser incident side recording layer (Layer 1) and the other layer (Layer 2) are designed to be 20 nm and 55 nm, respectively. We confirmed that there was less than 10% bottom jitter for both layers under the following recording densities: recording bit length of 0.41 µm and track pitch of 0.74 µm. The laser powers required for recording on Layer 1 and Layer 2 were typically 10 mW and 12 mW, respectively. The jitter was less than 13% for both layers with large power tolerance of 30%p-p.

New Optical Erasable Medium Using Tellurium Suboxide Thin Film

A suboxide thin film of TeOx ( x is smaller than the stoich.Lometric value ) containing a small amount of additives of such metals and semimetals as Sn and Ge has been found to have the property of showing a reversible change in its optical constants when the film was exposed in turn to two diode laser spots with different sizes, and this property can be utilized for an erasable disc. For recording, a TeOx thin film deposited on a PMMA substrate is irradiated by a diffraction limited laser spot, 0.8 pm in dia., modulated in response to an input electrical signal. This causes a micro-sized area of the film to be suddenly heated and rapidly quenched, resulting in a decrease in optical reflectivity. This recorded bit can be erased at real time by the irradiation of a diffused laser spot to an elliptical shape, 1 x 10 pm, by which the bit recovers its optical properties. An optical head having the two laser beams, one for recording/playback and one for erasing, has been developed. Real time erasing and recording at the same time has been achieved for a video signal. In excess of a million record/erase cycles have been demonstrated with no significant degradation in playback signal quality or erasability. Carrier-to-noise ratio, C/N, was more than 55 dB at 5 MHz, 30 kHz bandwidth.